P u b l i s h e d f o r SISSA b y S p r i n g e r R e c e i v e d: A ugust 17, 2015 R e v i s e d: Novem ber 4, 2015 A c c e p t e d: Novem ber 9, 2015 P u b l i s h e d: N ovem ber 30, 2015
Search for lepton-flavour-violating H -> μτ decays of the Higgs boson with the ATLAS detector
T h e A T LA S collaboration
E - m a i l : a t l a s . p u b l i c a t i o n s @ c e r n .c h
A b s t r a c t : A direct search for lepton-flavour-violating H ^ decays of th e recently discovered Higgs boson w ith th e ATLAS d e te c to r a t th e LHC is presented. T he analysis is perform ed in th e H ^ ^Thad channel, w here Thad is a hadronically decaying T-lepton. T he search is based on th e d a ta sam ple of p ro to n -p ro to n collisions collected by th e ATLAS ex
perim ent corresponding to an in teg rated lum inosity of 20.3 fb -1 a t a centre-of-m ass energy of y f s = 8 TeV. No statistically significant excess of d a ta over th e predicted background is observed. T h e observed (expected) 95% confidence-level u p p er lim it on th e branching fraction, B r ( H ^ ^ t ) , is 1.85% (1.24%).
Ke y w o r d s: H adron-H adron S cattering, Beyond S tan d ard M odel, Higgs physics, L epton pro duction
ArXiv ePr in t: 1508.03372
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C o n te n ts
1 I n tr o d u c tio n 1
2 T h e A T L A S d e t e c t o r a n d o b je c t r e c o n s tr u c tio n 2
3 E v e n t s e le c tio n a n d c a te g o r iz a tio n 3
4 B a c k g r o u n d e s t im a t io n 6
5 S y s t e m a t ic u n c e r ta in t ie s 10
6 R e s u lts 10
7 S u m m a r y 12
T h e A T L A S c o lla b o r a tio n 17
1 In tr o d u c tio n
T he observation of th e Higgs boson [1, 2] w ith a m ass of ab o u t 125 GeV [3] by th e ATLAS and CMS experim ents is a great success of th e Large H adron Collider (LHC) physics program at C E R N . T he next im p o rta n t step in this program are searches for signs of new physics beyond th e S tan d ard M odel (SM) and detailed studies of th e Higgs boson p ro p erties. D irect evidence for physics beyond th e SM could be indicated via lepton- flavour-violating (LFV) Higgs boson decays. If th e SM is replaced w ith an effective field theory, which has a single Higgs boson and is required to be renorm alizable only to a finite m ass scale, th e n L FV couplings m ay be introduced [4]. L FV decays can also occur n atu rally in m odels w ith m ore th a n one Higgs doublet [5- 8], com posite Higgs m odels [9 , 10], m odels w ith flavour sym m etries [11], R andall-S undru m m odels [12] and m any others [13- 18].
T here are th re e possibilities for L FV effects m ediated via v irtu a l Higgs bosons: ^-e,
t-^, and t-e tran sitio n s. Ind irect experim ental co n strain ts are reviewed and tra n sla te d into co n strain ts on B r ( H ^ e ^ , ^ T , e T) in recent papers [4, 19]. Searches for ^ ^ ey [20] place a very string ent co n strain t on H ^ e ^ decays: B r ( H ^ e^) < O (10- 8 ) [4, 19]. T he indi
rect co n strain ts on H ^ ^ t , eT decays m ostly come from searches for t ^ ^ y , ey [21- 23]
or o th er rare t -lepton decays [24], as well as from m easurem ents of th e anom alous m ag
netic m om ent of th e m uon and th e electron [25] and are m uch less stringent: B r ( H ^
^ T , e T) < 0(10 % ) [4 , 19]. A relatively large B r(H ^ ^ t ) can be achieved w ith ou t any p a rticu la r tu n in g of th e effective couplings, while a large B r ( H ^ eT) is possible only at th e cost of some fine-tuning of th e corresponding couplings [19]. It is also im p o rta n t to note th a t th e presence of a H ^ ^ t signal would essentially exclude th e presence of a
J H E P 1 1 (2 0 1 5 )2 1 1
H ^ e r signal, and vice versa, a t an experim entally observable level at th e LHC due to stro ng experim ental bounds on y ^ ey decays [19]. T he CMS C o llaboration has recently perform ed th e first d irect search for L FV H ^ yT decays [26] and rep orted a slight excess (2.4 sta n d a rd deviations) of d a ta over th e predicted background. T h eir results give a 1.57%
up p er lim it on B r(H ^ y r) a t th e 95% confidence level (CL).
T his p a p e r presents a search for lepton-flavour-violating H ^ y r decays of th e recently discovered Higgs boson w ith one m uon and one hadronically decaying T-lepton (Thad) in th e final sta te . All Higgs boson p ro d uction processes are considered. T he analysis is based on th e d a ta sam ple of p p collisions which was collected a t a centre-of-m ass energy of
y f s = 8TeV and corresponds to an in tegrated lum inosity of 20.3 fb -1.
2 T h e A T L A S d e te c to r and o b je c t r e c o n str u c tio n
T he ATLAS d e te c to r1 is described in d etail in ref. [27]. ATLAS consists of an inner tracking d e te c to r (ID) covering th e range |n| < 2.5, surrounded by a superconducting solenoid providing a 2 T m agnetic field, electrom agnetic (|n| < 3.2) and hadronic calorim eters (|n| < 4.9) and a m uon sp ectrom eter (MS) (|n| < 2.7) w ith a to roidal m agnetic field.
T he signature of L FV H ^ yT decays used in this search is characterised by th e presence of an energetic m uon, originating directly from a Higgs boson decay and carrying roughly half of its energy, and th e hadronic decay p ro d u cts of a T-lepton. T he d a ta were collected w ith a single-m uon trig g er w ith a transverse m om entum , px = p sin 9, threshold of p x = 24 GeV. T h e H ^ t t and th e L FV H ^ yT signatures w ith a m uon and Thad in th e final sta te share m any com m on features. Therefore, th e object definitions and d a ta quality cuts used in this analysis are th e sam e as those in th e recently published ATLAS search for H ^ t t decays [28]. A brief description of th e ob ject definitions is provided below.
M uon candidates are reco n stru cted using an algorithm [29] th a t com bines inform ation from th e ID and th e MS. M uon quality c riteria such as inner d e te c to r hit requirem ents are applied to achieve a precise m easurem ent of th e m uon m om entum and to reduce th e m isidentification rate. M uons are required to have p x > 10 GeV and to be w ithin |n| < 2.5.
T ypical recon struction and identification efficiencies for m uons satisfying these selection c riteria are above 95% [29]. E x actly one identified m uon is required in this analysis.
E lectro n cand id ates are recon structed from energy clusters in th e electrom agnetic calorim eters m atched to tracks in th e ID. T hey are required to have a transverse en
ergy, E x = E sin 9, g reater th a n 15 GeV, to be w ithin th e pseudorapidity range |n| < 2.47, and to satisfy th e m e d i u m shower shape and tra c k selection c riteria defined in ref. [30].
C an did ates found in th e tra n sitio n region betw een th e end-cap and b arrel calorim eters (1.37 < |n| < 1.52) are not considered. Typical reconstru ction and identification efficien
cies for electrons satisfying these selection criteria range betw een 80% and 90% depending 1ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, 0) are used in the transverse plane, 0 being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle 6 as n = — lntan(6/2).
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on E t and p . Since electrons do not ap p e ar in th e H ^ p + Thad decay m ode, events w ith identified electrons are rejected.
Je ts are reco n stru cted using th e an ti-kt je t clustering algorithm [31] w ith a radius p a ra m ete r R = 0.4, tak in g clusters of calorim eter cells w ith deposited energy as inputs.
Fully calib rated je ts [32] are required to be reconstru cted in th e range |n| < 4.5 and to have p T > 30 GeV. To reduce th e co ntam in ation of je ts by ad ditio nal interactions in th e sam e or neighbouring bunch crossings (pile-up), track s originating from th e p rim ary vertex m ust co n trib u te a large fraction of th e je t p t w hen sum m ing th e scalar p t of all tracks in th e je t. T he p rim ary vertex is chosen as th e p ro to n -p ro to n collision v ertex can did ate w ith th e highest sum of th e squared transverse m om enta of all associated tracks. This je t vertex fraction is required to be a t least 50% for je ts w ith |n| < 2 .4 and p T < 50 GeV (no cut is applied to je ts w ith p T > 50 GeV). J e ts w ith no associated tracks are retained.
In th e pseudorapidity range |n| < 2.5, je ts containing b-hadrons (b-jets) are selected using a tagging algorithm [33], which has an efficiency of ^7 0 % for b-jets in t t events. T he corresponding light-flavour je t m isidentification prob ab ility is 0.1-1%, depending on th e p T and n of th e jet. Only a very small fraction of signal events have b-jets, therefore events w ith identified b-jets are vetoed in th e selection of signal events.
H adronically decaying T-leptons are identified by m eans of a m ultiv ariate analysis technique [34] based on boosted decision trees, which exploits inform ation ab o u t ID tracks and clusters in th e electrom agnetic and hadronic calorim eters. T he Thad can did ates are required to have charge qT = ± 1 in units of electron charge, and m ust be 1- or 3-track (1- or 3-prong) candidates. E vents w ith exactly one Thad can d id ate satisfying th e m edium identification c riteria [34] w ith p T > 20 GeV and |n| < 2.47 are considered in th is analysis.
T he identification efficiency for Thad candidates satisfying these requirem ents is 55-60%.
D edicated criteria [34] to sep arate Thad candidates from m isidentified electrons are also applied, w ith a selection efficiency for tru e Thad decays of 95%. To reduce th e con tam in atio n due to backgrounds w here a m uon fakes a Thad signature, events w here an identified m uon w ith p T (p) > 4 GeV overlaps w ith an identified Thad are rejected [28]. T he probab ility to m isidentify a je t w ith p T > 20 GeV as a Thad can d id ate is typically 1-2%.
T he m issing transverse m om entum (w ith m agn itud e ETpiss) is reco nstru cted using th e energy deposits in calorim eter cells calibrated according to th e reco n structed physics ob
jects (e, y , Thad, jets and p) w ith which th ey are associated [35]. T h e energy from calo
rim eter cells not associated w ith any physics object is included in th e E™ ss calculation. It is scaled by th e ratio of th e scalar sum of p t of tracks which originate from th e prim ary v ertex b u t are not m atched to any objects and th e scalar sum of p T of all tracks in th e event which are not m atched to objects. T he scaling procedure achieves a m ore accurate recon stru ction of E™ ss in high pile-up conditions. In this search, E T ^iss is a signature of neutrinos, and it is used to select and reco nstruct signal events, as described below.
3 E v en t s e le c tio n an d c a te g o r iz a tio n
Signal H ^ pT events in th e pThad final s ta te are ch aracterised by th e presence of an energetic m uon and a Thad of opposite charge as well as m o d erate E™ ss, which tends to be
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aligned w ith th e Thad direction. B ackgrounds for th is sig natu re can be broadly classified into two m ajo r categories:
• Events w ith tru e m uon and Thad signatures, d o m in ated by irreducible Z /y * ^ t t p rod uctio n w ith some co n trib utio ns from th e V V ^ ^ t + X (where V = W, Z ), tt, single-top and SM H ^ t t production processes; these events exhibit a very strong charge correlation betw een th e m uon and th e Thad; therefore, th e expected num ber of events w ith opposite-sign (OS) charges (N o s) is m uch larger th a n th e num ber of events w ith sam e-sign (SS) charges (N s s ).
• Events w ith a fake Thad signature, dom inated by W + je ts events w ith some con tri
b u tio n from m ulti-jet (m any m ulti-jet background events have genuine m uons from sem ileptonic decays of heavy-flavour hadrons), diboson (V V ), t t and single-top events w ith some charge asy m m etry N OS > N s s ; Z ^ ^ + j e t s events, w here a Thad signa
tu re can be faked by eith er a je t (no charge correlation) or a m uon (strong charge correlation), also co n trib u te to th is category.
E vents w ith a fake Thad ten d to have a m uch softer p T (Thad) sp ectrum and a larger angu lar sep aratio n betw een th e Thad and E™ ss directions. These p roperties are exploited to suppress such backgrounds and define signal and control regions. E vents w ith exactly one m uon and exactly one Thad w ith p T (tt) > 26 GeV, pT (Thad) > 45 GeV and |y(tt) — n(Thad)| <
2 form a baseline sam ple. T he |y(tt) — n(Thad)| cu t has ^9 9 % efficiency for signal and rejects a considerable fraction of m ulti-jet and W + je ts events. At th is stage of th e event selection, th e identified m uon is also required to be isolated [28] in th e calorim eters and in th e track ing d e te c to r in order to reduce co ntam ination from th e m ulti-jet background. Two signal regions are defined using th e transverse m ass, mT ,2 of th e ^-E ™ ss and Thad-Em iss system s: OS events w ith m T (^ ,E lp iss) > 40 GeV and m T (Thad, ETpiss) < 30 GeV form th e signal region-1 (SR1), while OS events w ith m T ( p , E™ ss) < 40 GeV and m T (Thad, Eipiss) <
60 GeV form th e signal region-2 (SR2). B o th signal regions have sim ilar sensitivity to signal (see section 6) . T he d o m in an t backgrounds in SR1 and SR2 are W + je ts and Z / y * ^ t t events, respectively. T he m odelling of th e W + je ts background is checked in a dedicated control region (W C R ) form ed by events w ith m T (^, Eepiss) > 60 GeV and m e (Thad, Eepiss) >
40 GeV. As it is discussed in detail in section 4 , th e m odelling of th e Z / y * ^ t t background is checked in SR2. T he choice of th e m T cuts to define SR1, SR2 and W C R is m otivated by correlations betw een m T (^,E ep iss) and m T (Thad, E™ ss) in H ^ ^ t signal and m ajor background (W + je ts and Z /y * ^ t t ) events, as illu strated in figure 1. No events w ith identified b-jets are allowed in SR1, SR2 and W C R . T he m odelling of th e t t and single-top backgrounds is checked in a ded icated control region (T C R ), form ed by events th a t satisfy th e baseline selection and have a t least two jets, w ith at least one being b-tagged. Table 1 provides a sum m ary of th e event selection cuts used to define th e signal and control regions.
T he L FV signal is searched for by perform ing a fit to th e m ass d istrib u tio n in d a ta , mMTMC, reco nstru cted from th e observed m uon, Thad and Eepiss objects by m eans of th e 2mT = \ /2P tEmlss(1 — cos Ay), where £ = y, r had and Ay is the azimuthal separation between the directions of the lepton (y or Thad) and E™lss vectors.
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F ig u re 1. Two-dimensional distributions of the transverse mass of the y-Ei
plss
system, mx
(y, E™ss
), and th at of the Thad
-£T?lss
system, mT
(Thad
, Emlss
), in simulated Z/y* ^ t t (top left plot), W +jets (top right plot), H ^ yT signal (bottom left plot) and data (bottom right plot) events. Magenta, red and yellow boxes on the bottom right plot illustarte locations of SR1, SR2, and WCR, respectively. All events are required to have a well-identified muon and Thad
of opposite charge with pT
(Thad
) > 20GeV and pT
(y) > 26GeV.C ut SR1 SR2 W C R T C R
PT(y) PT (Thad) m T (y, E™ ss) mT(Thad,i^miSS)
|n (y ) - n (Thad)|
Njet Nb-jet
>26 GeV
>45 GeV
>40 GeV
<30 GeV
<2
0
>26 GeV
>45 GeV
<40 GeV
<60 GeV
<2
0
> 26 GeV
>45 GeV
> 60 GeV
> 40 GeV
<2
0
>26 GeV
>45 GeV
<2
>1
>0
T a b le 1. Summary of the event selection criteria used to define the signal and various control regions (see text).
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M issing M ass C alcu lato r [36] (M M C). C onceptually, th e M M C is a m ore sophisticated version of th e collinear appro x im ation [37]. T he m ain im provem ent comes from requir
ing th a t th e relative orientations of th e neutrinos and o th er T-lepton decay p ro ducts are consistent w ith th e m ass and kinem atics of a T-lepton decay. This is achieved by m ax
im ising a prob ability defined in th e kinem atically allowed phase space region. T he M MC used in th e H ^ t t analysis [28] is modified to take into account th a t th ere is only one n eu trin o from a hadronic T-lepton decay in L FV H ^ ^t events. For a Higgs boson w ith m H = 125 GeV, th e recon stru cted mMrMC d istrib u tio n has a roughly G aussian shape w ith a full w idth a t half m axim um of ~ 1 9 GeV. T he analysis is perform ed “blinded” in th e 110 G eV < mMMC <150 GeV regions of SR1 and SR2, which contains ~94% of th e expected signal events. T he event selection and th e analysis stra te g y are defined w itho ut looking at th e d a ta in these blinded regions.
4 B a ck g ro u n d e stim a tio n
T he background estim atio n m eth o d takes into account th e background properties and com position discussed in section 3 . I t also relies on th e assum ption th a t th e shape of th e mMrMC d istrib u tio n for th e m ulti-jet background is th e sam e for OS and SS events.
T his assum ption was verified in th e published H ^ t t search [38]. In addition, it was confirm ed using a d ed icated control region, M JC R , w ith an enhanced c o n trib u tio n from th e m ulti-jet background. E vents in th is control region are required to pass all c riteria for SR1 and SR2 w ith th e exception of th e requirem ent on | n ( 7 — n(Thad)|, which is reversed:
InGu) — n(Thad)| > 2. Therefore, th e num ber of th e to ta l OS background events, N<°Sg in each bin of th e mMrMC (or any other) d istrib u tio n in SR1 and SR2 can be obtain ed according to th e following form ula:
N osg = r QCD ■ N Ssta + n os- ss + n os- ss + N os—sS + N OS-SS + n os- ss + N & - S , (4.1) w here th e individual term s are described below. NSSta is th e num ber of SS d a ta events, which are dom in ated by W + je ts events b u t also contain contrib utio ns from m ulti-jet and o th er backgrounds. T he fractions of m ulti-jet background in SS d a ta events inside th e 1 10 G eV < mMTMC < 150 GeV m ass window are ~17% and ~44% in SR1 and SR2, re
spectively. T he co n trib utio n s N ^ S ^ -s = N<°sg -i — rQCD ■N ° k g - t are a d d - o n term s for th e different backgrounds com ponents (where bkg-i indicates th e ith background source:
Z ^ t t, Z ^ ^ , W + je ts, V V , H ^ t t and events w ith t-q uark s), which also account for com ponents of these backgrounds already included in SS d a ta even ts.3 T he factor rQCD = Nmsultl- je7 N sr tl-jet accounts for p o ten tial differences in flavour com position (and, as a consequence, in je t ^ Thad fake rates) of final-state je ts introduced by th e same-sign or opposite-sign charge requirem ents. T he value of rQCD = 1.10 ± 0.14 is obtained from a
3The rQCD • N^g8-® correction in the add-on term is needed because same-sign data events include multi-jet as well as electroweak events (Z ^ t t, Z ^ y y , W +jets, VV, H ^ t t and events with t-quarks) and their contributions cannot be separated.
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m ulti-jet-enriched control region in d a ta , as discussed in detail in ref. [38]. It was verified in th e M JC R in th is search.
T he largely irreducible Z /y * ^ t t background is m odelled by Z /y * ^ y y d a ta events, w here th e m uon tracks and associated energy deposits in th e calorim eters are replaced by th e corresponding sim ulated signatures of th e final-state particles of th e t-lepton decay. In th is approach, essential features such as th e m odelling of th e kinem atics of th e produced boson, th e m odelling of th e hadronic activ ity of th e event (jets and underlying event) as well as co ntrib utio n s from pile-up are tak en from d a ta . Therefore, th e dependence on th e sim ulation is m inim ized and only th e T-lepton decays and th e d etecto r response to th e T-lepton decay p ro d ucts are based on sim ulation. T his hybrid sam ple is referred to as em bedded d a ta in th e following. A detailed description of th e em bedding procedure can be found in ref. [39]. T he Z / y * ^ t t norm alization is a free-floating p a ra m ete r in th e final fit to d a ta and it is m ainly co n strained by events w ith 6 0 G eV < m M MC< 1 1 0 G eV in SR2.
T he W + je ts and Z ^ y y backgrounds are m odelled by th e A L P G E N [40] event g en erato r interfaced w ith P Y T H IA8 [41] to provide th e p a rto n showering, had ronization and th e m odelling of th e underlying event. In all W + je ts events, th e Thad sig n atu re is faked by jets. T he W C R region is used to check th e m odelling of th e W + je ts kinem atics and to o b tain norm alizations for OS and SS W + je ts events. An ad ditio nal overall norm alization factor for th e term in eq. (4.1) is introduced as a free-floating p a ra m ete r in th e final fit in SR1. By studying W C R events and SR1 events w ith mMTMC > 150 GeV (dom inated by W + je ts background), it is also found th a t a mMTMC shape correction, which depends on th e num ber of jets, p T (Thad) and |n(y) — n(Thad)|, needs to be applied in SR1. This correction is derived from SR1 events w ith mMMC > 150 GeV and it is applied to events w ith all values of m MTMC. A 50% difference betw een th e SR1-based correction and th a tI-11 o b tain ed in W C R is tak e n as a corresponding m odelling u n certain ty on th e mMMC shape for th e W + je ts background in SR1. T he size of this u n certain ty depends on mMTMC and it is as large as ±10% for W + je ts events w ith mMMC < 150 GeV. In th e case of SR2, a good m odelling of th e Njet, p T (Thad) and |n(y) — n(Thad)| d istrib u tio n s suggests th a t such a correction is not needed. However, a m odelling u n certain ty on th e mMTMC shape of th e W + je ts background in SR2 is assigned based on th e 50% difference betw een th e default m MrMC shape and th e one o b tained after applying th e correction derived for SR1 events.
T he size of th is u n certain ty is below 5% in th e 1 1 0 G eV < mMMC < 150 GeV region, th a t contains m ost of th e signal events. It was also checked th a t applying th e sam e correction in SR2 as in SR1 had a negligible effect on th e final result and th e e x tra cte d branching ratio B r ( H ^ yT) (see section 6) would only be affected a t a level below 3%. T he m odelling of je t frag m en tatio n and th e underlying event has a significant effect on th e estim ate of th e je t ^ Thad fake ra te in different regions of th e phase space and has to be accounted for w ith a corresponding sy stem atic uncertainty. To e stim ate th is effect, th e analysis was repeated using a sam ple of W + je ts events m odelled by A L P G E N interfaced w ith th e H ER W IG [42]
event g enerator. Differences in th e W + je ts predictions in SR1 and SR2 are found to be
±9% and ± 2% , respectively, and are taken as corresponding system atic un certainties.
In th e case of th e Z ^ y y background, th ere are two com ponents: events w here a m uon fakes a Thad and events w here a je t fakes a Thad. P rediction s for th e shape and norm alization of th e Z ^ y y (where y ^ T ^ d 3) background are o b tained from sim ulation. For th e Z ^
J H E P 1 1 (2 0 1 5 )2 1 1
p p background w here a je t is m isidentified as a Thad cand id ate, th e n orm alization factor and shape corrections, which d epend on th e num ber of jets, p T (Thad) and |n(p) — n(Thad)|, are derived by using events w ith two identified OS m uons w ith an invariant mass, m w , in th e range of 80-100 GeV. Since th is background does not have an OS-SS charge asym m etry, a single correction factor is derived for OS and SS events. A 50% difference betw een th e mMTMC shape w ith and w ith ou t th is correction is tak en as a corresponding system atic uncertainty.
T he backgrounds w ith to p quarks are m odelled by th e P O W H E G [43- 45] (for t t , W t
and s-channel single-top production) and AcerM C [46] (t-channel single-top production) event generators interfaced w ith P Y T H IA8 to provide th e p a rto n showering, had ronization and th e m odelling of th e underlying event. T he T C R is used to check th e m odelling and to o b tain norm alizations for OS and SS events w ith to p quarks. T he norm alization factors o b tained in th e T C R are ex tra p o la ted into SR1 and SR2, w here ttt and single-top events m ay have different p roperties. To e stim ate th e u n certain ty associated w ith such an ex trap o latio n , th e analysis is rep eated using th e MC@NLO [47] event g en erator instead of P O W H E G for t t p ro d u ctio n.4 T his u n certain ty is found to be ±7.2% (±3.7% ) for backgrounds w ith to p quarks in SR1 (SR2).
T he background due to diboson (W W , Z Z and W Z) prod uctio n is estim ated from sim ulation, norm alized to th e cross sections calculated at next-to-leading o rder (NLO) in QCD [48]. T he A L P G E N event g en erator interfaced w ith H ER W IG is used to m odel th e W W process, and H ER W IG is used for th e Z Z and W Z processes.
Finally, events w ith Higgs bosons produced via gluon fusion or vector-boson fusion (V BF) processes are g enerated at NLO accuracy w ith P O W H E G [49] event generator interfaced w ith P Y T H IA8 to provide th e p a rto n showering, hadron ization and th e m od
elling of th e underlying event. T he associated prod uctio n ( Z H and W H ) sam ples are sim ulated using P Y T H IA8. All events w ith Higgs bosons are produced w ith a m ass of m H = 125 GeV and norm alized to cross sections calculated a t next-to -nex t-to -lead ing or
d er in QCD [50- 52]. T h e SM H ^ t t decays are sim ulated by P Y T H IA8. T he L FV Higgs boson decays are m odelled by th e E v tG en [53] event g enerato r according to th e phase-space m odel. In th e H ^ pT decays, th e T-lepton decays are tre a te d as unpolarised because th e left- and rig ht-hand ed T-lepton p o larisation sta te s are produced a t equal rates.
All sim ulated sam ples are passed th ro u g h th e G E A N T 4-based ATLAS d e te c to r sim u latio n [54, 55]. T he sim ulated events are overlaid w ith additio nal m inim um -bias events to account for th e effect of m ultiple pp interactions (pile-up) occurring in th e sam e and neighbouring bunch crossings.
F igure 2 shows th e mMMC d istrib u tio n s for d a ta and th e predicted backgrounds in each of th e signal regions. T he backgrounds are estim ated using th e m etho d described above. T he signal efficiencies for passing th e SR1 or SR2 selection requirem ents are 2.1%
and 1.5%, respectively, and th e com bined efficiency is 3.6%. T he num bers of observed events in th e d a ta as well as th e signal and background predictions in th e m ass region 1 10 G eV < m MI-1TMC <150 GeV can be found in ta b le 2 .
1
4The same extrapolation uncertainty is assumed for tt and single-top backgrounds.
J H E P 1 1 (2 0 1 5 )2 1 1
F ig u re 2. Distributions of the mass reconstructed by the Missing Mass Calculator, m j ^ 0 , in SR1 (left) and SR2 (right). The background distributions are determined in a global fit. The signal distribution corresponds to B r(H ^ yT)=25%. The bottom panel of each sub-figure shows the ratio of the observed data and the estimated background. The grey band for the ratio illustrates post-fit systematic uncertainties on the background prediction. The statistical uncertainties for data and background predictions are added in quadrature for the ratios. The last bin in each distribution contains overflow events.
SR1 SR2
Signal 69.1 ± 0.8 ± 9.2 48.5 ± 0.8 ± 7.5 Z ^ TT
W + je ts Top
Sam e-Sign events V V + Z ^ y y
H ^ TT T otal background
133.4 ± 6.9 ± 9.1 619 ± 54 ± 55 39.5 ± 5.3 ± 4.7
335 ± 19 ± 47 90 ± 21 ± 16 6.82 ± 0.21 ± 0.97
1224 ± 62 ± 63
262.6 ± 9.7 ± 18.6 406 ± 42 ± 34 19.6 ± 3.1 ± 3.3
238 ± 16 ± 34 81 ± 22 ± 17 13.7 ± 0.3 ± 1.9
1021 ± 51 ± 49
D a ta 1217 1075
T ab le 2. Data yields, signal and post-fit OS-SS background predictions (see eq. (4.1)) for the 110GeV< mjj^ 0 <150GeV region. The signal predictions are given for B r(H ^ yT)=0.77%.
The background predictions are obtained from the combined fit to SR1, SR2, WCR and TCR. The post-fit values of systematic uncertainties are provided for the background predictions. For the total background, all correlations between various sources of systematic uncertainties and backgrounds are taken into account. The quoted uncertainties represent the statistical (first) and systematic (second) uncertainties, respectively.
J H E P 1 1 (2 0 1 5 )2 1 1
5 S y s te m a tic u n c e r ta in tie s
T he largest system atic uncertainties arise from th e norm alization (±10% u n certainty ) and m odelling5 of th e W + je ts background. T he uncertainties on rQCD (±12.7% ) and on th e norm alization ( ±6% u n certain ty ) and m odelling of Z ^ t t also play an im p o rta n t role.
T he o th er m ajor sources of experim ental uncertainty, affecting b o th th e shape and nor
m alization of signal and backgrounds, are th e u n certain ty on th e Thad energy scale [34]
(m easured w ith ± (2-4)% precision) and un certainties on th e em bedding m ethod used to m odel th e Z ^ t t background [28]. Less significant sources of experim ental uncertainty, affecting th e shape and norm alization of signal and backgrounds, are th e uncertain ty on th e je t energy scale [32, 56] and resolution [57]. T he uncertainties in th e Thad energy resolution, th e m om entum scale and resolution of m uons, and th e scale u n certain ty on E™ ss due to th e energy in calorim eter cells not associated w ith physics objects are tak en into account, however, th ey are found to be relatively small. T he following experim ental uncertainties prim arily affect th e norm alization of signal and backgrounds: th e ±2.8% u n certain ty on th e in tegrated lum inosity [58], th e u n certain ty on th e Thad identification efficiency [34], which is m easured to be ± (2-3)% for 1-prong and ± (3 -5 )% for 3-prong decays, th e ±2.1%
u n certain ty for triggering, reco n stru cting and identifying m uons [29, 59], and th e ±2%
u n certain ty on th e b-jet tagging efficiency [33].
T heoretical uncertainties are estim ated for th e signal and for th e H ^ t t, V V and Z ^ ^ (w ith ^ ^ f id O backgrounds, which are m odelled w ith th e sim ulation and are not norm alized to d a ta in d edicated control regions. U ncertainties due to m issing higher-order QCD corrections on th e p ro d u ctio n cross sections are found to be [60] ±10.1% (±7.8% ) for th e Higgs boson p ro d u ctio n via gluon fusion in SR1 (SR2), ±1% for th e Z ^ ^ background and for V B F and V H Higgs boson production, and ±5% for th e V V background. T he system atic uncertain ties due to th e choice of p a rto n d istrib u tio n functions used in th e sim ulation are evaluated based on th e prescription described in ref. [60] and th e following values are used in th is analysis: ±7.5% for th e Higgs boson p rod uction via gluon fusion,
±2.8% for th e V B F and V H Higgs boson production, and ±4% for th e Z ^ ^ and V V backgrounds. Finally, an ad d itio nal ±5.7% sy stem atic u n certain ty on B r ( H ^ t t) is applied to th e SM H ^ t t background.
6 R e su lts
A sim ultaneous binned m axim um -likelihood fit is perform ed on th e mMMC distrib u tio n s in SR1 and SR2 and on event yields in W C R and T C R to e x tra ct th e L FV branching ratio B r(H ^ ^t). T he fit exploits th e control regions and th e d istin ct shapes of the W + je ts and Z ^ t t backgrounds in th e signal regions to co n strain some of th e sy stem atic uncertainties. T his leads to an im proved sensitivity of th e analysis. T he post-fit mMMC d istrib u tio n s in SR1 and SR2 are shown in figure 2, and th e com bined mMrMC d istrib u tio n for b o th signal regions is presented in figure 3 . F igure 2 illu strates good agreem ent betw een
5Some of these uncertainties (e.g., uncertainties due to rn M ^0 shape corrections and extrapolation uncertainties) are discussed in the text above.
J H E P 1 1 (2 0 1 5 )2 1 1
F ig u re 3. Post-fit combined mM4 0 distribution obtained by adding individual distributions in SR1 and SR2. In the lower part of the figure, the data are shown after subtraction of the estimated backgrounds. The grey band in the bottom panel illustrates the post-fit systematic uncertainties on the background prediction. The statistical uncertainties for data and background predictions are added in quadrature on the bottom part of the figure. The signal is shown assuming B r(H ^ pT)=0.77%, the central value of the best fit to B r(H ^ pT). The last bin of the distribution contains overflow events.
SR1 SR2 Com bined
E x p ected lim it on B r(H ^ pT) [%]
O bserved lim it on B r ( H ^ pT) [%]
B est fit B r(H ^ pT ) [%]
1 60+0'64 1 .60- 0.45
1.55 0 07+0.81
° . ° ‘ -0.86
1 7 5 + 0.71 1 75—0.49
3.51 1 94+0.92
94—0.89
1 24+0.50 1 4—0.35
1.85 0.77±0.62
T ab le 3. The expected and observed 95% confidence level (CL) upper limits and the best fit values for the branching fractions for the two signal regions and their combination.
d a ta and background expectatio ns in SR1. A small excess of th e d a ta over th e predicted background is observed in th e 120 G eV < mMTMC <140 GeV region in SR2. T his small excess in SR2 has a local significance of 2.2 sta n d a rd deviations and a com bined significance for b o th signal regions of 1.3 sta n d a rd deviations. T his corresponds to a best fit value for th e branching fraction of B r(H ^ pT )= (0 .7 7 ± 0.62)%. D ue to th e low significance of th e observed excess, an u p p er lim it on th e L FV branching ratio B r ( H ^ pT) for a Higgs boson w ith m H = 125 GeV is set using th e C Ls m odified frequentist form alism [61] w ith th e profile likelihood-ratio te s t sta tistic s [62]. T he observed and th e m edian expected 95%
CL u p p er lim its are 1.85% and 1.24-q30%, respectively. Table 3 provides a sum m ary of all results.
J H E P 1 1 (2 0 1 5 )2 1 1
7 S u m m a ry
A direct search for lepton-flavour-violating H ^ yT decays of th e recently discovered Higgs boson is perform ed in th e Thad decay m ode of T-leptons using a d a ta sam ple of p ro to n -p ro to n collisions recorded by th e ATLAS d etecto r at th e LHC corresponding to an in teg rated lum inosity of 20.3 fb -1 a t a centre-of-m ass energy of a/s = 8TeV. T he observed and th e m edian expected u p p er lim its at 95% CL on th e branching fraction, B r ( H ^ yT ), are 1.85% and 1.24+035%, respectively. T his search places significantly m ore stringent co n strain ts on B r ( H ^ yT) com pared to earlier indirect estim ates. T he result of th is analysis is found to be consistent w ith th e one published by th e CMS C ollaboration [26].
A c k n o w le d g m e n ts
We th a n k C E R N for th e very successful op eratio n of th e LHC, as well as th e su p p o rt staff from our in stitu tio n s w ith o u t whom ATLAS could not be o p erated efficiently.
We acknowledge th e su p p o rt of A N P C yT , A rgentina; Y erPhI, A rm enia; ARC, A us
tralia; B M W F W and F W F , A ustria; ANAS, A zerbaijan; SSTC, Belarus; C N P q and FA PESP, Brazil; NSERC , NRC and C FI, C anada; CERN; C O N IC Y T , Chile; CAS, M O ST and NSFC, C hina; C O LC IEN C IA S, Colombia; M SM T CR, M PO C R and VSC CR, Czech Republic; D N R F, DN SRC and L undbeck F oundation, D enm ark; E P L A N E T , ER C and N SR F, E u ro p ean Union; IN 2P3-C N R S, C E A -D S M /IR F U , France; G N SF, Georgia;
B M B F, D FG , H G F, M PG and AvH F oundation, G erm any; GSRT and N SR F, Greece;
R G C , H ong Kong SAR, China; ISF, MINERVA, G IF, I-C O R E and Benoziyo C enter, Is
rael; IN FN , Italy; M E X T and JS P S , Ja p an ; C N R ST, Morocco; FO M and N W O , N eth er
lands; B R F and RCN, Norway; M NiSW and NCN, Poland; G R IC ES and F C T , P ortugal;
M N E /IF A , R om ania; M ES of R ussia and NRC KI, R ussian Federation; JIN R ; M STD, Serbia; M SSR, Slovakia; A RRS and MIZS, Slovenia; D S T /N R F , South Africa; M IN ECO , Spain; SRC and W allenberg F oundation, Sweden; SER, SNSF and C antons of B ern and Geneva, Sw itzerland; NSC, Taiwan; TA EK , Turkey; STFC , th e Royal Society and Lever- hulm e T rust, U nited K ingdom ; D O E and N SF, U nited S tates of A m erica.
T he crucial com puting su p p o rt from all W LC G p a rtn e rs is acknowledged gratefully, in p a rticu la r from C E R N and th e ATLAS Tier-1 facilities at T R IU M F (C anad a), N D G F (D enm ark, Norway, Sweden), CC -IN 2P3 (France), K IT /G rid K A (G erm any), IN FN -C N A F (Italy ), NL-T1 (N etherlands), P IC (Spain), ASGC (Taiw an), RA L (U.K.) and BNL (U.S.A.) and in th e Tier-2 facilities worldwide.
O p e n A c c e s s . This article is d istrib u ted under th e term s of th e C reative Com m ons A ttrib u tio n License (CC -B Y 4.0) , which perm its any use, d istrib u tio n and reprodu ction in any m edium , provided th e original au th o r(s) and source are credited.
R e fe r e n c e s
[1] ATLAS collaboration, O b se rv a tio n o f a n e w p a rtic le in th e search f o r the S ta n d a r d M o d el H iggs boson w ith th e A T L A S d e te c to r a t th e L H C , P h y s. L ett. B 716 (2012) 1
[arX iv:1207.7214] [inSPIRE].
J H E P 1 1 (2 0 1 5 )2 1 1
[2] CMS collaboration, O b se r v a tio n o f a n ew boson a t a m a ss o f 125 G e V w ith th e C M S e x p e r im e n t a t th e L H C , P h y s. L e tt. B 716 (2012) 30 [arX iv:1207.7235] [inSPIRE].
[3] ATLAS, CMS collaboration, C o m b in e d M e a s u r e m e n t o f th e H iggs B o s o n M a ss in pp
C o llisio n s a t a
/
s = 7 a n d 8T e V w ith the A T L A S a n d C M S E x p e r im e n ts , P h y s. R ev. L ett.114 (2015) 191803 [arX iv:1503.07589] [inSPIRE].
[4] R. Harnik, J. Kopp and J. Zupan, F la v o r V io la tin g H iggs D ecays, J H E P 03 (2013) 026 [arX iv:1209.1397] [inSPIRE].
[5] J.D. Bjorken and S. Weinberg, A M e c h a n ism f o r N o n c o n s e r v a tio n o f M u o n N u m b e r, P h ys.
R ev. L ett. 38 (1977) 622 [inSPIRE].
[6] J.L. Diaz-Cruz and J.J. Toscano, L e p to n fla v o r v io la tin g decays o f H iggs bosons beyond the sta n d a r d m odel, P h y s. R ev . D 62 (2000) 116005 [hep-ph/9910233] [inSPIRE].
[7] T. Han and D. Marfatia, h ^ yT a t h a d ro n colliders, P h y s. R ev. L ett. 86 (2001) 1442 [hep-ph/0008141] [inSPIRE].
[8] A. Arhrib, Y. Cheng and O.C.W. Kong, C o m p re h e n siv e a n a ly sis o n le p to n fla v o r v io la tin g H iggs boson to y^T± decay in s u p e r s y m m e tr y w ith o u t R p a rity , P h y s. R ev. D 87 (2013) 015025 [arX iv:1210.8241] [inSPIRE].
[9] K. Agashe and R. Contino, C o m p o site H ig g s-M e d ia te d F C N C, P h ys. R ev. D 80 (2009) 075016 [arX iv:0906.1542] [inSPIRE].
[10] A. Azatov, M. Toharia and L. Zhu, H iggs M ed ia ted F C N C ’s in W arped E x tra D im e n sio n s, P h y s. R ev. D 80 (2009) 035016 [arX iv:0906.1990] [inSPIRE].
[11] H. Ishimori, T. Kobayashi, H. Ohki, Y. Shimizu, H. Okada and M. Tanimoto, N o n -A b e lia n D isc re te S y m m e tr ie s in P a r tic le P h y s ic s, Prog. T heor. P h y s. Suppl. 183 (2010) 1
[arX iv:1003.3552] [inSPIRE].
[12] G. Perez and L. Randall, N a tu ra l N e u tr in o M a sse s a n d M ix in g s fr o m W arped G eo m etry, J H E P 01 (2009) 077 [arX iv:0805.4652] [inSPIRE].
[13] M. Blanke, A.J. Buras, B. Duling, S. Gori and A. Weiler, A F = 2 O bservables a n d
F in e -T u n in g in a W arped E x tra D im e n s io n w ith C u sto d ia l P r o te c tio n, J H E P 03 (2009) 001 [arX iv:0809.1073] [inSPIRE].
[14] G.F. Giudice and O. Lebedev, H ig g s-d e p e n d e n t Y ukaw a couplings, P h y s. L e tt. B 665 (2008) 79 [arX iv:0804.1753] [inSPIRE].
[15] J.A. Aguilar-Saavedra, A M in im a l s e t o f top-H iggs a n o m a lo u s cou p lin g s, N u cl. P h ys. B 821 (2009) 215 [arX iv:0904.2387] [inSPIRE].
[16] M.E. Albrecht, M. Blanke, A.J. Buras, B. Duling and K. Gemmler, E lectro w ea k a n d F la v o u r S tr u c tu r e o f a W arped E x tra D im e n s io n w ith C u sto d ia l P ro te c tio n , J H E P 09 (2009) 064 [arX iv:0903.2415] [inSPIRE].
[17] A. Goudelis, O. Lebedev and J.-h. Park, H ig g s-in d u ce d le p to n fla v o r vio la tio n , P h y s. L e tt. B 707 (2012) 369 [arX iv:1111.1715] [inSPIRE].
[18] D. McKeen, M. Pospelov and A. Ritz, M o d ified H iggs branching ra tio s v e rsu s C P a n d le p to n fla v o r v io la tio n , P h y s. R ev . D 86 (2012) 113004 [arX iv:1208.4597] [inSPIRE].
[19] G. Blankenburg, J. Ellis and G. Isidori, F la v o u r-C h a n g in g D ecays o f a 125 G e V H iggs-like P a rtic le , P h y s. L e tt. B 712 (2012) 386 [arX iv:1202.5704] [inSPIRE].
J H E P 1 1 (2 0 1 5 )2 1 1
[20] M EG collaboration, J. Adam et al., N e w c o n s tra in t on th e e x iste n c e o f th e ^ e+y decay, P h y s. R ev. L ett. 110 (2013) 201801 [arX iv:1303.0754] [inSPIRE].
[21] B a B a r collaboration, B. Aubert et al., Searches f o r L e p to n F la v o r V io la tio n in the D ecays
t ± ^ e±y a n d t± ^ y ± y , P h y s. R ev. L e tt. 104 (2010) 021802 [arX iv:0908.2381] [inSPIRE].
[22] S. Kanemura, T. Ota and K. Tsumura, L e p to n fla v o r v io la tio n in H iggs boson decays u n d e r th e rare ta u decay results, P h y s. R ev. D 73 (2006) 016006 [hep-ph/0505191] [inSPIRE].
[23] S. Davidson and G.J. Grenier, L e p to n fla v o u r vio la tin g H iggs a n d t ^ y y , P h y s. R ev. D 81 (2010) 095016 [arX iv:1001.0434] [inSPIRE].
[24] A. Celis, V. Cirigliano and E. Passemar, L e p to n fla v o r v io la tio n in th e H iggs se c to r a n d the role o f h a d ro n ic t -le p to n decays, P h y s. R ev. D 89 (2014) 013008 [arX iv:1309.3564]
[inSPIRE].
[25] P a r t i c l e D a ta G ro u p collaboration, K.A. Olive et al., R e v ie w o f P a r tic le P h y sic s, C hin.
P h y s. C 38 (2014) 090001 [inSPIRE].
[26] CMS collaboration, S ea rch f o r L e p to n -F la v o u r -V io la tin g D ecays o f th e H iggs B o so n , P h ys.
L ett. B 749 (2015) 337 [arX iv:1502.07400] [inSPIRE].
[27] ATLAS collaboration, T h e A T L A S E x p e r im e n t a t th e C E R N Large H a d ro n C ollider, 2008
J I N S T 3 S08003 [inSPIRE].
[28] ATLAS collaboration, E v id e n c e f o r th e H igg s-b o so n Y u k a w a coupling to ta u le p to n s w ith the A T L A S detecto r, J H E P 04 (2015) 117 [arX iv:1501.04943] [inSPIRE].
[29] ATLAS collaboration, M e a s u r e m e n t o f th e m u o n re c o n str u c tio n p e r fo rm a n c e o f the A T L A S d e te c to r u sin g 2011 a n d 2012 L H C p r o to n -p r o to n co llisio n data, E u r. P h y s. J. C 74 (2014) 3130 [arX iv:1407.3935] [inSPIRE].
[30] ATLAS collaboration, E le c tro n re c o n str u c tio n a n d id e n tific a tio n e ffic ie n c y m e a su r e m e n ts w ith th e A T L A S d e te c to r u sin g th e 2011 L H C p r o to n -p r o to n co llisio n data, E u r. P h y s. J. C 74 (2014) 2941 [arX iv:1404.2240] [inSPIRE].
[31] M. Cacciari, G.P. Salam and G. Soyez, T h e a n ti- k t j e t clu ste rin g algo rith m , J H E P 04 (2008) 063 [arX iv:0802.1189] [inSPIRE].
[32] ATLAS collaboration, S in g le h a d ro n response m e a s u r e m e n t a n d c a lo rim e te r j e t energy scale u n c e r ta in ty w ith th e A T L A S d e te c to r a t the L H C , E u r. P h y s. J. C 73 (2013) 2305
[arX iv:1203.1302] [inSPIRE].
[33] ATLAS collaboration, C a lib ra tio n o f th e p e r fo rm a n c e o f b-tagging f o r c a n d lig h t-fla v o u r je ts in th e 2012 A T L A S data, ATLAS-CONF-2014-046 (2014).
[34] ATLAS collaboration, Id e n tific a tio n a n d energy ca libration o f h a d ro n ic a lly decaying ta u le p to n s w ith th e A T L A S e x p e r im e n t in pp co llisio n s a t a/s = 8 TeV, E u r. P h y s. J. C 75 (2015) 303 [arX iv:1412.7086] [inSPIRE].
[35] ATLAS collaboration, P e r fo r m a n c e o f M is s in g T ra n sve rse M o m e n tu m R e c o n s tr u c tio n in P r o to n -P r o to n C o llisio n s a t 7 T e V w ith A T L A S , E u r. P h y s. J. C 72 (2012) 1844
[arX iv:1108.5602] [inSPIRE].
[36] A. Elagin, P. Murat, A. Pranko and A. Safonov, A N e w M a s s R e c o n s tr u c tio n T echnique f o r R e so n a n c e s D ecaying to d i-ta u , N u c l. In s tr u m . M e th . A 654 (2011) 481 [arX iv:1012.4686]
[inSPIRE].
J H E P 1 1 (2 0 1 5 )2 1 1
[37] R.K. Ellis, I. Hinchliffe, M. Soldate and J.J. van der Bij, H iggs D ecay to t + t : A P o ssib le S ig n a tu r e o f In te r m e d ia te M a ss H iggs B o s o n s a t th e S S C , N u c l. P h y s. B 297 (1988) 221 [inSPIRE].
[38] ATLAS collaboration, S ea rch f o r th e S ta n d a rd M o d el H iggs boson in th e H to t + t - decay m ode in f s = 7 T e V p p co llisio n s w ith A T L A S , J H E P 09 (2012) 070 [arX iv:1206.5971]
[inSPIRE].
[39] ATLAS collaboration, M o d ellin g Z ^ t t p ro cesses in A T L A S w itht -em bedded Z ^ pp
data, 2015 J I N S T 10 P09018 [arX iv:1506.05623] [inSPIRE].
[40] M.L. Mangano, M. Moretti, F. Piccinini, R. P ittau and A.D. Polosa, A L P G E N , a g e n e ra to r f o r hard m u ltip a r to n p ro cesses in h a d ro n ic co llisions, J H E P 07 (2003) 001 [hep-ph/0206293]
[inSPIRE].
[41] T. Sjostrand, S. Mrenna and P.Z. Skands, A B r i e f In tr o d u c tio n to P Y T H I A 8.1, C o m p u t.
P h y s. C o m m u n . 178 (2008) 852 [arX iv:0710.3820] [inSPIRE].
[42] G. Corcella et al., H E R W I G 6: A n E v e n t g e n e ra to r f o r h a d ro n e m is s io n re a ctio n s w ith in te r fe r in g g lu o n s (in c lu d in g s u p e r s y m m e tr ic pro c esse s), J H E P 01 (2001) 010
[hep-ph/0011363] [inSPIRE].
[43] P. Nason, A N e w m e th o d f o r co m b in in g N L O Q C D w ith sh o w e r M o n te C arlo a lg o rith m s,
J H E P 11 (2004) 040 [hep-ph/0409146] [inSPIRE].
[44] S. Frixione, P. Nason and C. Oleari, M a tc h in g N L O Q C D c o m p u ta tio n s w ith P a r to n S h o w e r s im u la tio n s: the P O W H E G m eth o d , J H E P 11 (2007) 070 [arX iv:0709.2092] [inSPIRE].
[45] S. Alioli, P. Nason, C. Oleari and E. Re, A g en e ra l fr a m e w o r k f o r im p le m e n tin g N L O c a lcu la tio n s in sh o w e r M o n te C arlo program s: the P O W H E G B O X , J H E P 06 (2010) 043 [arX iv:1002.2581] [inSPIRE].
[46] B.P. Kersevan and E. Richter-Was, T h e M o n te C arlo e v e n t g e n e ra to r A c e r M C v e rsio n s 2.0 to 3 .8 'with in te rfa c e s to P Y T H I A 6.4, H E R W I G 6 .5 a n d A R I A D N E 4.1, C o m p u t. P h ys.
C o m m u n . 184 (2013) 919 [hep-ph/0405247] [inSPIRE].
[47] S. Frixione and B.R. Webber, M a tc h in g N L O Q C D c o m p u ta tio n s a n d p a r to n sh o w er sim u la tio n s, J H E P 06 (2002) 029 [hep-ph/0204244] [inSPIRE].
[48] J.M. Campbell, R.K. Ellis and C. Williams, V ector boson p a ir p ro d u c tio n a t th e L H C, J H E P 07 (2011) 018 [arX iv:1105.0020] [inSPIRE].
[49] S. Alioli, P. Nason, C. Oleari and E. Re, N L O H iggs boson p r o d u c tio n via g lu o n fu s io n m a tc h e d w ith sh o w e r in P O W H E G , J H E P 04 (2009) 002 [arX iv:0812.0578] [inSPIRE].
[50] C. Anastasiou and K. Melnikov, H iggs boson p ro d u c tio n a t h a d ro n colliders in N N L O Q CD , N u cl. P h y s. B 646 (2002) 220 [hep-ph/0207004] [inSPIRE].
[51] V. Ravindran, J. Smith and W.L. van Neerven, N N L O co rrectio n s to th e to ta l c ro ss-sec tio n f o r H iggs boson p r o d u c tio n in h a d ro n h a d ro n co llisions, N u c l. P h y s. B 665 (2003) 325
[hep-ph/0302135] [inSPIRE].
[52] P. Bolzoni, F. Maltoni, S.-O. Moch and M. Zaro, H iggs p r o d u c tio n via v e cto r-b o so n fu s io n a t N N L O in Q C D , P h y s. R ev . L e tt. 105 (2010) 011801 [arX iv:1003.4451] [inSPIRE].
[53] D.J. Lange, T h e E v tG e n p a rtic le decay s im u la tio n package, N u cl. In s tr u m . M eth . A 462 (2001) 152 [inSPIRE].
J H E P 1 1 (2 0 1 5 )2 1 1
[54] ATLAS collaboration, T h e A T L A S S im u la tio n In fra str u c tu re , E u r. P h y s. J. C 70 (2010) 823 [arX iv:1005.4568] [inSPIRE].
[55] G E A N T4 collaboration, S. Agostinelli et al., G E A N T 4 : A S im u la tio n toolkit, N ucl.
In s tr u m . M eth . A 506 (2003) 250 [inSPIRE].
[56] ATLAS collaboration, J e t energy m e a s u r e m e n t a n d its s y s te m a tic u n c e r ta in ty in
p r o to n -p r o to n co llisio n s a t a
/
s = 7 T e V w ith th e A T L A S detecto r, E u r. P h y s. J. C 7 5 (2015) 17 [arX iv:1406.0076] [inSPIRE].[57] ATLAS collaboration, J e t energy re so lu tio n in p r o to n -p r o to n co llisio n s a t a
/
s = 7 T e Vrecorded in 2010 w ith the A T L A S detecto r, E u r. P h y s. J. C 73 (2013) 2306 [arX iv:1210.6210] [inSPIRE].
[58] ATLAS collaboration, Im p ro v e d lu m in o s ity d e te r m in a tio n in p p co llisio n s a t ^ fs = 7 T e V
u sin g th e A T L A S d e te c to r a t the L H C , E u r. P h y s. J. C 73 (2013) 2518 [arX iv:1302.4393]
[inSPIRE].
[59] ATLAS collaboration, P e r fo r m a n c e o f th e A T L A S m u o n trig g er in p p co llisio n s a t
V = 8 TeV, E u r. P h y s. J. C 7 5 (2015) 120 [arX iv:1408.3179] [inSPIRE].
[60] LHC H iggs C ro s s S e c tio n WoRkiNg G ro u p collaboration, S. Dittmaier et al., H andbook o f L H C H iggs C ross S e c tio n s : 1. In c lu siv e O bservables, arX iv:1101.0593 [inSPIRE].
[61] A.L. Read, P r e s e n ta tio n o f search results: T h e C L s technique, J. P h y s. G 28 (2002) 2693 [inSPIRE].
[62] G. Cowan, K. Cranmer, E. Gross and O. Vitells, A s y m p to tic fo r m u la e f o r likelihood-based te sts o f n ew p h y sic s, E u r. P h y s. J. C 71 (2011) 1554 [E rra tu m ibid. C 7 3 (2013) 2501]
[arX iv:1007.1727] [inSPIRE].
J H E P 1 1 (2 0 1 5 )2 1 1
T h e A T L A S c o lla b o r a tio n
G. Aad85, B. A bbott113, J. Abdallah151, O. Abdinov11, R. Aben107, M. Abolins90, O.S. AbouZeid158, H. Abramowicz153, H. Abreu152, R. Abreu116, Y. Abulaiti146a,146b, B.S. Acharya164a,164b,“, L. Adamczyk38a, D.L. Adams25, J. Adelman108, S. Adomeit100,
T. Adye131, A.A. Affolder74, T. Agatonovic-Jovin13, J. Agricola54, J.A. Aguilar-Saavedra126a,126f, S.P. Ahlen22, F. Ahmadov65,b, G. Aielli133a,133b, H. Akerstedt146a,146b, T.P.A. Akesson81,
A.V. Akimov96, G.L. Alberghi20a,20b, J. A lbert169, S. Albrand55, M.J. Alconada Verzini71, M. Aleksa30, I.N. Aleksandrov65, C. Alexa26b, G. Alexander153, T. Alexopoulos10, M. Alhroob113, G. Alimonti91a, L. Alio85, J. Alison31, S.P. Alkire35, B.M.M. Allbrooke149, P.P. Allport74, A. Aloisio104a,104b, A. Alonso36, F. Alonso71, C. Alpigiani76, A. Altheimer35,
B. Alvarez Gonzalez30, D. Alvarez Piqueras167, M.G. Alviggi104a,104b, B.T. Amadio15,
K. Amako66, Y. Amaral Coutinho24a, C. Amelung23, D. Amidei89, S.P. Amor Dos Santos126a,126c, A. Amorim126a,126b, S. Amoroso48, N. Amram153, G. Amundsen23, C. Anastopoulos139,
L.S. Ancu49, N. Andari108, T. Andeen35, C.F. Anders58b, G. Anders30, J.K. Anders74,
K.J. Anderson31, A. Andreazza91a,91b, V. Andrei58a, S. Angelidakis9, I. Angelozzi107, P. Anger44, A. Angerami35, F. Anghinolfi30, A.V. Anisenkov109,c, N. Anjos12, A. Annovi124a,124b,
M. Antonelli47, A. Antonov98, J. Antos144b, F. Anulli132a, M. Aoki66, L. Aperio Bella18, G. Arabidze90, Y. Arai66, J.P. Araque126a, A.T.H. Arce45, F.A. Arduh71, J-F. Arguin95, S. Argyropoulos63, M. Arik19a, A.J. Armbruster30, O. Arnaez30, V. Arnal82, H. Arnold48,
M. A rratia28, O. Arslan21, A. Artamonov97, G. Artoni23, S. Asai155, N. Asbah42, A. Ashkenazi153, B. Asman146a,146b, L. Asquith149, K. Assamagan25, R. Astalos144a, M. Atkinson165, N.B. Atlay141, K. Augsten128, M. Aurousseau145b, G. Avolio30, B. Axen15, M.K. Ayoub117, G. Azuelos95,d, M.A. Baak30, A.E. Baas58a, M.J. Baca18, C. Bacci134a’134b, H. Bachacou136, K. Bachas154, M. Backes30, M. Backhaus30, P. Bagiacchi132a,132b, P. Bagnaia132a,132b, Y. Bai33a, T. Bain35, J.T. Baines131, O.K. Baker176, E.M. Baldin109’c, P. Balek129, T. Balestri148, F. Balli84,
W.K. Balunas122, E. Banas39, Sw. Banerjee173, A.A.E. Bannoura175, H.S. Bansil18, L. Barak30, E.L. Barberio88, D. Barberis50a,50b, M. Barbero85, T. Barillari101, M. Barisonzi164a,164b, T. Barklow143, N. Barlow28, S.L. Barnes84, B.M. B arnett131, R.M. B arnett15, Z. Barnovska5, A. Baroncelli134a, G. Barone23, A.J. B arr120, F. Barreiro82, J. Barreiro Guimaraes da Costa57, R. Bartoldus143, A.E. Barton72, P. Bartos144a, A. Basalaev123, A. Bassalat117, A. Basye165, R.L. Bates53, S.J. B atista158, J.R. Batley28, M. Battaglia137, M. Bauce132a,132b, F. Bauer136, H.S. Bawa143,e, J.B. Beacham111, M.D. Beattie72, T. Beau80, P.H. Beauchemin161,
R. Beccherle124a,124b, P. Bechtle21, H.P. Beck17,f, K. Becker120, M. Becker83, M. Beckingham170, C. Becot117, A.J. Beddall19b, A. Beddall19b, V.A. Bednyakov65, C.P. Bee148, L.J. Beemster107, T.A. Beermann30, M. Begel25, J.K. Behr120, C. Belanger-Champagne87, W.H. Bell49, G. Bella153, L. Bellagamba20a, A. Bellerive29, M. Bellomo86, K. Belotskiy98, O. Beltramello30, O. Benary153, D. Benchekroun135a, M. Bender100, K. Bendtz146a,146b, N. Benekos10, Y. Benhammou153, E. Benhar Noccioli49, J.A. Benitez Garcia159b, D.P. Benjamin45, J.R. Bensinger23, S. Bentvelsen107, L. Beresford120, M. Beretta47, D. Berge107, E. Bergeaas K uutm ann166, N. Berger5, F. Berghaus169, J. Beringer15, C. Bernard22, N.R. Bernard86, C. Bernius110, F.U. Bernlochner21, T. Berry77, P. B erta129, C. Bertella83, G. Bertoli146a,146b,
F. Bertolucci124a,124b, C. Bertsche113, D. Bertsche113, M.I. Besana91a, G.J. Besjes36,
O. Bessidskaia Bylund146a,146b, M. Bessner42, N. Besson136, C. Betancourt48, S. Bethke101, A.J. Bevan76, W. Bhimji15, R.M. Bianchi125, L. Bianchini23, M. Bianco30, O. Biebel100, D. Biedermann16, S.P. Bieniek78, M. Biglietti134a, J. Bilbao De Mendizabal49, H. Bilokon47, M. Bindi54, S. Binet117, A. Bingul19b, C. Bini132a,132b, S. Biondi20a,20b, D.M. Bjergaard45, C.W. Black150, J.E. Black143, K.M. Black22, D. Blackburn138, R.E. Blair6, J.-B. Blanchard136,
J H E P 1 1 (2 0 1 5 )2 1 1
J.E. Blanco77, T. Blazek144a, I. Bloch42, C. Blocker23, W. Blum83’*, U. Blumenschein54,
G.J. Bobbink107, V.S. Bobrovnikov109,c, S.S. Bocchetta81, A. Bocci45, C. Bock100, M. Boehler48, J.A. Bogaerts30, D. Bogavac13, A.G. Bogdanchikov109, C. Bohm146a, V. Boisvert77, T. Bold38a, V. Boldea26b, A.S. Boldyrev99, M. Bomben80, M. Bona76, M. Boonekamp136, A. Borisov130, G. Borissov72, S. Borroni42, J. Bortfeldt100, V. Bortolotto60a,60b,60c, K. Bos107, D. Boscherini20a, M. Bosman12, J. Boudreau125, J. Bouffard2, E.V. Bouhova-Thacker72, D. Boumediene34,
C. Bourdarios117, N. Bousson114, S.K. Boutle53, A. Boveia30, J. Boyd30, I.R. Boyko65, I. Bozic13, J. Bracinik18, A. Brandt8, G. Brandt54, O. Brandt58a, U. Bratzler156, B. Brau86, J.E. B rau116, H.M. B raun175’*, S.F. Brazzale164a,164c, W.D. Breaden Madden53, K. Brendlinger122,
A.J. Brennan88, L. Brenner107, R. Brenner166, S. Bressler172, K. Bristow145c, T.M. Bristow46, D. Britton53, D. Britzger42, F.M. Brochu28, I. Brock21, R. Brock90, J. Bronner101,
G. Brooijmans35, T. Brooks77, W.K. Brooks32b, J. Brosamer15, E. Brost116, J. Brown55, P.A. Bruckman de Renstrom39, D. Bruncko144b, R. Bruneliere48, A. Bruni20a, G. Bruni20a, M. Bruschi20a, N. Bruscino21, L. Bryngemark81, T. Buanes14, Q. B u at142, P. Buchholz141, A.G. Buckley53, S.I. Buda26b, I.A. Budagov65, F. Buehrer48, L. Bugge119, M.K. Bugge119, O. Bulekov98, D. Bullock8, H. Burckhart30, S. Burdin74, C.D. Burgard48, B. Burghgrave108, S. Burke131, I. Burmeister43, E. Busato34, D. Biischer48, V. Biischer83, P. Bussey53, J.M. Butler22, A.I. B u tt3, C.M. B uttar53, J.M. Butterworth78, P. B u tti107, W. Buttinger25, A. Buzatu53,
A.R. Buzykaev109,c, S. Cabrera Urban167, D. Caforio128, V.M. Cairo37a,37b, O. Cakir4a, N. Calace49, P. Calafiura15, A. Calandri136, G. Calderini80, P. Calfayan100, L.P. Caloba24a, D. Calvet34, S. Calvet34, R. Camacho Toro31, S. Camarda42, P. Cam arri133a,133b, D. Cameron119, R. Caminal Armadans165, S. Campana30, M. Campanelli78, A. Campoverde148,
V. Canale104a,104b, A. Canepa159a, M. Cano Bret33e, J. Cantero82, R. Cantrill126a, T. Cao40, M.D.M. Capeans Garrido30, I. Caprini26b, M. Caprini26b, M. Capua37a,37b, R. Caputo83, R. Cardarelli133a, F. Cardillo48, T. Carli30, G. Carlino104a, L. Carminati91a’91b, S. Caron106, E. Carquin32a, G.D. Carrillo-Montoya30, J.R. Carter28, J. Carvalho126a,126c, D. Casadei78,
M.P. Casado12, M. Casolino12, E. Castaneda-M iranda145a, A. Castelli107, V. Castillo Gimenez167, N.F. Castro126a,g, P. Catastini57, A. Catinaccio30, J.R. Catmore119, A. C attai30, J. Caudron83, V. Cavaliere165, D. Cavalli91a, M. Cavalli-Sforza12, V. Cavasinni124a’124b, F. Ceradini134a,134b, B.C. Cerio45, K. Cerny129, A.S. Cerqueira24b, A. Cerri149, L. Cerrito76, F. C erutti15, M. Cerv30, A. Cervelli17, S.A. Cetin19c, A. Chafaq135a, D. Chakraborty108, I. Chalupkova129, P. Chang165, J.D. Chapman28, D.G. Charlton18, C.C. Chau158, C.A. Chavez Barajas149, S. Cheatham 152, A. Chegwidden90, S. Chekanov6, S.V. Chekulaev159a, G.A. Chelkov65,h, M.A. Chelstowska89, C. Chen64, H. Chen25, K. Chen148, L. Chen33dć S. Chen33c, S. Chen155, X. Chen33f, Y. Chen67, H.C. Cheng89, Y. Cheng31, A. Cheplakov65, E. Cheremushkina130, R. Cherkaoui El Moursli135e, V. Chernyatin25’*, E. Cheu7, L. Chevalier136, V. Chiarella47, G. Chiarelli124a,124b, G. Chiodini73a, A.S. Chisholm18, R.T. Chislett78, A. Chitan26b, M.V. Chizhov65, K. Choi61, S. Chouridou9, B.K.B. Chow100, V. Christodoulou78, D. Chromek-Burckhart30, J. Chudoba127, A.J. Chuinard87, J.J. Chwastowski39, L. Chytka115, G. C iapetti132a,132b, A.K. Ciftci4a, D. Cinca53, V. Cindro75, I.A. Cioara21, A. Ciocio15, F. Cirotto104a,104b, Z.H. Citron172, M. Ciubancan26b, A. Clark49, B.L. Clark57, P.J. Clark46, R.N. Clarke15, W. Cleland125, C. Clement 146a’146b, Y. Coadou85, M. Cobal164a,164c, A. Coccaro49, J. Cochran64, L. Coffey23, J.G. Cogan143, L. Colasurdo106, B. Cole35, S. Cole108, A.P. Colijn107, J. Collot55, T. Colombo58c, G. Compostella101, P. Conde Muino126a’126b, E. Coniavitis48, S.H. Connell145b, I.A. Connelly77, V. Consorti48, S. Constantinescu26b, C. Conta121a,121b, G. Conti30, F. Conventi104aj, M. Cooke15,
B.D. Cooper78, A.M. Cooper-Sarkar120, T. Cornelissen175, M. Corradi20a, F. Corriveau87,k, A. Corso-Radu163, A. Cortes-Gonzalez12, G. Cortiana101, G. Costa91a, M.J. Costa167, D. Costanzo139, D. Cote8, G. Cottin28, G. Cowan77, B.E. Cox84, K. Cranmer110, G. Cree29,
