Search for charged Higgs bosons in the $H^{\pm }\rightarrow \mathit{tb}$ decay channel in $\mathit{pp}$ collisions at $\sqrt{s}=8$ TeV using the ATLAS detector

P u b l i s h e d f o r S IS S A b y S p r i n g e r R e c e i v e d: December 14, 2015

R e v i s e d: February 1, 2016 A c c e p t e d: February 2, 2016 P u b l i s h e d: March 17, 2016

Search for charged Higgs bosons in the H ± tb decay channel in pp collisions at √ s = 8 TeV using the ATLAS detector

T h e A T LA S collaboration

E-m ail: atlas.publications@cern.ch

Ab s t r a c t:

C harged Higgs bosons heavier th a n th e to p q u ark and decaying via H ± ^

√ s = 8 T eV corresponding to an integrated lum inosity of 20.3 fb- 1 . T he prod uction of a

charged Higgs boson in association w ith a to p quark, gb ^ t H ±, is explored in th e m ass range 200 to 600 GeV using m ulti-jet final sta te s w ith one electron or m uon. In order to sep arate th e signal from th e S tan d ard M odel background, analysis techniques com bining several kinem atic variables are employed. An excess of events above th e background- only hypothesis is observed across a wide m ass range, am ounting to up to 2.4 sta n d a rd deviations. U p per lim its are set on th e gb ^ tH ± p ro d u ction cross section tim es th e branching fraction B R (H ± ^ tb). A dditionally, th e com plem entary s-channel production,

sta te s w ith one electron or m uon are relevant for H ± m asses from 0.4 to 2.0 TeV, w hereas th e all-hadronic final s ta te covers th e range 1.5 to 3.0 TeV. In these search channels, no significant excesses from th e predictions of th e S ta n d ard M odel are observed, and u p p er lim its are placed on th e qq' ^ H ± p ro d uctio n cross section tim es th e branching fraction B R (H ± ^ tb).

Ke y w o r d s:

H adron-H adron scattering, Higgs physics

1512.03704

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C o n te n ts

1 I n t r o d u c t i o n 1

2 D a t a a n d s i m u l a t e d e v e n t s 3

2.1 ATLAS d e te c to r and d a ta sam ple 3

2.2 B ackground and signal m odelling 3

3 O b j e c t r e c o n s t r u c t i o n a n d i d e n t i f i c a t i o n 6

4 S e a r c h f o r a c h a r g e d H i g g s b o s o n i n a s s o c i a t i o n w i t h a t o p q u a r k 8

4.1 E vent selection and categorisation 8

4.2 Analysis stra te g y 8

4.3 System atic un certainties 10

4.4 R esults 13

5 S e a r c h f o r a c h a r g e d H i g g s b o s o n p r o d u c e d i n t h e s - c h a n n e l 1 8

5.1 L e p to n + je ts final s ta te 18

5.2 A ll-hadronic final s ta te 19

5.3 R esults and in te rp re ta tio n s 21

6 C o n c l u s i o n s 2 2

T h e A T L A S c o l l a b o r a t i o n 31

1 In tr o d u c tio n

T he discovery of a n e u tra l scalar particle H at th e Large H adron Collider (LHC) in 2012 [1, 2], w ith a m easured m ass of 125.09 ± 0.21(stat.) ± 0.11(syst.) GeV [3], raises th e question of w h ether th is new particle is th e Higgs boson of th e S ta n d ard M odel (SM) or one physical s ta te of an extended Higgs sector. T he observation of a heavy charged scalar p a r­

ticle would clearly indicate physics beyond th e SM. Charged Higgs bosons 1 are predicted by several non-m inim al Higgs scenarios, such as tw o-H iggs-doublet M odels (2HDM ) [4] and m odels containing Higgs trip le ts [5- 9].

T he pro duction m echanism s and decay m odes of a charged Higgs boson depend on its mass, m H +. For light charged Higgs bosons ( m H + < m top, w here m top is th e top- q u ark m ass), th e prim ary pro du ction m echanism is th ro u g h th e decay of a to p quark,

to be in association w ith a to p quark, as illu strated by th e left-hand and central plots

T n the following, charged Higgs bosons are denoted H +, with the charge-conjugate H - always implied.

Similarly, generic quark symbols are used for q and g.

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F ig u re 1. L eading-order F eynm an diagram s for th e p ro d u c tio n of a charged Higgs boson w ith a m ass m H+ > m top, in association w ith a to p q u ark (left in th e 5FS, a n d centre in th e 4FS) an d in th e s-channel (right).

of figure 1. W hen calculating th e corresponding cross section in a four-flavour scheme (4FS), b-quarks are dynam ically produced, w hereas in a five-flavour scheme (5FS), th e b- q u ark is also considered as an active flavour inside th e proton. T h e 4FS and 5FS cross sections are averaged according to ref. [10]. In th e 2HDM, th e p ro d u ction and decay of th e charged Higgs boson also depend on th e p a ra m ete r ta n fi, defined as th e ratio of th e vacuum e x p ectatio n values of th e two Higgs doublets, and th e m ixing angle a betw een th e CP-even Higgs bosons. For m H + > m top and in th e case of co s(^

a )

0, th e dom inant decay is H + ^ tb, w ith a su b sta n tia l co n trib u tio n from H +

for large values of ta n [11].

A com plem entary H + pro d u ctio n m ode, shown in th e righ t-h an d plot of figure 1, is th e s-channel process, qq1 ^ H +.

T he L E P experim ents placed u p p e r lim its on th e p ro duction of H + in th e m ass range of 40-100 GeV [12], and th e Tevatron experim ents set up p er lim its on B R (t

b H +) for mH + in th e range 8 0 -1 5 0 GeV [13, 14]. T he D0 experim ent also searched for a charged Higgs boson w ith a m ass in th e range 180-300 GeV using th e H +

tb decay channel [15].

Light charged Higgs bosons have been searched for in th e

decay m ode at th e LHC by CMS (2 fb - 1 , -\fs = 7T eV [16]) and ATLAS (4.7 fb- 1 ,

s = 7T eV [17, 18]). Searches for charged Higgs bosons were also perform ed in p ro to n -p ro to n (pp) collisions at yfs = 8 TeV, by ATLAS using th e

decay m ode [19] and by CMS using final sta te s originating from b o th th e

and tb decay m odes [20]. CMS set an u p p er lim it of 2 .0 -0 .1 3 pb on th e p ro d u ctio n cross section tim es branching fraction for H +

tb in th e m ass range 180­

600 GeV. V ector-boson-fusion H + productio n was also searched for by ATLAS using th e W Z final s ta te [21]. No evidence for a charged Higgs boson was found in any of these searches.

T his p a p e r describes searches for charged Higgs bosons decaying into tb. In th e H + m ass range of 200-600 GeV, th e pro d uction m ode in association w ith a to p q u ark is stu d ­ ied. T he 5FS process is generated. Cross sections averaging 4FS and 5FS are used for m odel-dependent predictions. T h e search is based on selecting two to p quarks, w ith th eir decays producing one charged lepton (electron or m uon), and a t least one additio nal je t containing a b-flavoured hadron. In th e com plem entary s-channel pro du ctio n m ode, H + m asses betw een 0.4 and 2.0 TeV are explored in a final s ta te containing one charged lepton and jets (referred to as le p to n + je ts in th e following), while th e all-hadronic final sta te is

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used for very high H + masses, 1.5 to 3.0 TeV, w ith a je t su b stru c tu re technique to re­

co n stru ct th e to p -q u a rk decay p ro d ucts in one single large-radius je t. T he two s-channel analyses are rein terp re ta tio n s of recent searches for W ' ^ tb in ATLAS [22, 23]. Based on dedicated sim ulations of th e H + ^ tb signal and a rein terp re ta tio n of th e d a ta , u pper lim its are derived for th e s-channel p ro d uction of a charged scalar particle decaying to tb.

T he pap er is organised as follows. Section 2 describes briefly th e ATLAS detecto r, th e n sum m arises th e d a ta and th e sam ples of sim ulated events used for th e analyses.

Section 3 describes th e reco n stru ctio n of objects in ATLAS. Section 4 presents th e event selection and analysis stra te g y of th e search for H + ^ tb produced in association w ith a to p quark. System atic un certain ties are also discussed, before exclusion lim its in term s of cross section tim es branching fraction are presented, to g eth er w ith th eir in te rp re ta tio n in benchm ark scenarios of th e M inim al Supersym m etric S tan d ard M odel (MSSM) [24- 28].

T he rein terp re ta tio n s of W ' ^ tb analyses as searches for th e p ro d u ction of H + ^ tb in th e s-channel, including a discussion of th e H + signal shapes and uncertainties, are presented in section 5. Finally, a sum m ary is given in section 6 .

2 D a ta an d sim u la te d e v en ts

2 .1 A T L A S d e t e c t o r a n d d a t a s a m p l e

T he ATLAS d e te c to r [29] consists of an inner tracking system w ith coverage in pseudora­

p id ity 2 up to |n| = 2.5, surrounded by a th in 2 T superconducting solenoid, a calorim eter system extending up to |n| = 4.9 and a m uon sp ectro m eter extending up to |n| = 2.7 th a t m easures th e deflection of m uon tracks in th e field of th re e su perconducting toroid m agnets. A three-level trigger system is used to select events of interest. T he first-level trigg er (L1) is im plem ented in hardw are, using a subset of d etecto r inform ation to reduce th e event ra te to no m ore th a n 75 kHz. T his is followed by two softw are-based trigger levels (L2 and E F ), which to g eth e r fu rth e r reduce th e event ra te to less th a n 400 Hz.

S tringent d a ta -q u ality requirem ents are applied, resulting in an in teg rated lum inosity of 20.3 fb-1 for th e 2012 d a ta -ta k in g period. T he integ rated lum inosity has an un certain ty of 2.8%, m easured following th e m ethodology described in ref. [30]. E vents are required to have a p rim ary v ertex w ith a t least five associated tracks, each w ith a transverse m om entum

g reater th a n 400 MeV. If an event has m ore th a n one recon stru cted vertex satisfying these criteria, th e p rim ary vertex is defined as th e reconstructed vertex w ith th e largest sum of squared tra c k tran sv erse m om enta.

2 .2 B a c k g r o u n d a n d s i g n a l m o d e l l i n g

T he background processes for th e searches in this pap er include SM pair p ro d u ctio n of to p q uarks (w ith ad d itio n al jets, or in association w ith a vector boson V = W, Z or th e SM

2ATLAS 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 upwards. Cylindrical coordinates (r, 0) are used in the transverse plane, 0 being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle 6 as n = — ln tan (6 /2 ).

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Higgs boson), as well as th e p ro d u ction of single-top-quark, W + je ts, Z /y * + je ts , diboson

pairs w ith ad dition al je ts in th e final state.

In th e analyses w ith an electron or a m uon in th e final sta te , all backgrounds are tak en from sim ulation, except for th e m ulti-jet events. These m ostly c o n trib u te via th e presence of a no n-prom pt electron or m uon, e.g. from a sem ileptonic b- or c-flavoured h adron decay, or th ro u g h th e m isidentification of a je t. T he norm alisation of th e m ulti-jet events and th e shape of th e relevant d istrib u tio n s are determ ined w ith a data-d riv en technique known as th e m a trix m etho d [31]. In th e search for H + ^ tb in th e s-channel p ro d uction m ode w ith an all-hadronic final sta te , all backgrounds are estim ated using a d ata-d riv en m ethod based on a com bined fit to th e d a ta under th e SM background plus H + signal hypothesis.

T he m odelling of t t events is perform ed w ith P o w H E g -B o x v2.0 [32, 33], using th e CT10 [34, 35] p a rto n d istrib u tio n function (P D F ) set. It is interfaced to

v6.425 [36], w ith th e P erug ia P2011C [37] set of tu n e d param eters (tune) for th e underlying event.

T he tt cross section a t 8 TeV is

¾ = 253+H pb for a to p -q u a rk m ass of 172.5 GeV. It is calculated a t next-to-n ext-to -leadin g order (NNLO) in QCD including resum m atio n of next-to -n ex t-to-lead in g logarithm ic (NNLL) soft gluon term s w ith

+ + v2.0 [38- 44].

In th e search for H + p ro d u ctio n in association w ith a to p quark, sim ulated t t events are classified according to th eir flavour content a t p a rto n level, using th e sam e m ethodology as in ref. [45]. E vents are labelled as tt+ b 6 if th ey contain a t least one particle je t th a t is m atched to a b-flavoured h adron not originating from th e decay of th e tti system . E vents w here a t least one particle je t is m atched to a c-flavoured hadron, and not already labelled as ti+ bb, are labelled as ti+ c c . E vents labelled as eith er tb+bb or ti+ c c are generically referred to as tt+ h eav y -flav o ur (H F) events. T he rem aining events, including those w ith no additio n al jets, are labelled as tt+ lig h t-flav o u r (LF). In th e following, a sequential rew eighting is applied a t th e g en erato r level for all tt+ L F and ti+ c ć events produced w ith Pow H E g+ PY T H iA . Two correction factors are used, based on th e values of th e transverse m om enta of th e to p q u ark and th e tti system , tak in g th e correlation betw een these two param eters into account. T his rew eighting procedure was originally im plem ented in order to m atch sim ulation to d a ta in th e m easurem ent of to p -q u ark -p air differential cross sections a t yfs = 7 TeV [46]. It was verified th a t this procedure is also reasonable a t yfs = 8 TeV.

T he tb+bb com ponent is rew eighted to m atch th e NLO th eo ry calculation provided w ithin

w ith th e

fram ew ork [47, 48]. For th is rew eighting, th e sam e settings as in ref. [45] are used in th is paper. T he rew eighting is perform ed a t th e g en erato r level using several kinem atic variables such as th e transverse m om enta of th e to p quark, th e tt system and th e dijet system not com ing from th e to p -q u ark decay, as well as th e d ista n c e3

gen erated w ith th e MADgRAPH5 v1.5.11 LO g en erato r [49], using th e CT10 P D F set and interfaced to

v6.425 for p a rto n shower and fragm entation.

Sam ples of tiV events are generated using MADgRAPH5 v1.3.33, w ith th e CTEQ 6L1 [50]

P D F , interfaced to

v6.425 for th e showering and h adronisation, w ith th e A U ET2B

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underlying-event tu n e [51]. T hey are norm alised to th e next-to-leading-order (NLO) cross section [52, 53].

Single-top-quark pro du ctio n in th e s- and W t-channels are sim ulated w ith

B o x v2.0, using th e CT10 P D F , interfaced to

v6.425 w ith th e underlying-event tu n e P2011C. T he sam e procedure is used for th e single-top-quark p ro d uctio n in th e t- channel, except in th e search for qq' ^ H + ^ tb in th e le p to n + je ts final sta te , w here th e leading-order (LO) g en erato r

v3.8 [54] w ith th e CTEQ 6L1 P D F , interfaced to

v6.425 w ith th e underlying-event tu n e P2011C , is used instead. O verlaps betw een th e t t and W t final sta te s are handled using inclusive diagram removal [55]. T he single-top- q u ark sam ples are norm alised to th e approx im ate NNLO th eoretical cross sections [56- 58]

using th e M STW 2008 NNLO [59- 61] P D F .

Sam ples of W /Z + je ts events are generated using th e

v2.14 [62] generator, w ith th e CTEQ 6L1 P D F , interfaced to

v6.425 w ith th e underlying-event tu n e P2011C.

T he W + je ts events are generated w ith up to five ad ditio nal p arto ns, sep arately for th e W + L F , W bb+ jets, W c c+ jets and W c + je ts processes. Similarly, th e Z + je ts background is gen erated w ith up to five ad dition al parto n s separated in different flavours. T he samples of W /Z + je ts events are norm alised to th e inclusive NNLO th eo retical cross sections [63].

Finally, th e W /Z + je ts events are rew eighted to account for differences in th e W /Z p T sp ectru m betw een d a ta and sim ulation [64].

In th e searches for H + ^ tb w ith a le p to n + je ts final sta te , diboson events are generated w ith th e requirem ent of having a t least one boson decaying leptonically.

v2.14 is used, w ith th e CTEQ 6L1 P D F , and it is interfaced to

v6.520 [65] for showering and hadronisation , to g eth e r w ith Jim m y v4.31 [66] for th e underlying event, using th e AU ET2 tu n e [67]. T he diboson backgrounds are norm alised to th e pro du ction cross sections calculated at NLO [68].

T he p ro d uctio n of th e SM Higgs boson in association w ith a to p -q u a rk p air (tb H ) is m odelled using NLO m atrix elem ents obtain ed from th e

package [69].

Po w h e g- Bo x

is used as an interface to shower sim ulation program s. T he sam ples created using th is approach are referred to as

sam ples [70]. T hey are inclusive in Higgs boson decays and are produced for a Higgs boson m ass of 125 GeV, using th e CT10 P D F , and interfaced to

v8.1 [71] w ith th e AU2 underlying-event tu n e [72]. As in th e generation of t t background events, th e to p -q u a rk m ass is set to 172.5 GeV. T he tbH cross section and th e decay branching fractions of th e Higgs boson are tak en from th e (N)NLO th eoretical calculations collected in ref. [73].

In th e search for H + produced in association w ith a to p quark, signal sam ples are g enerated w ith

, using th e CT10 P D F , interfaced to

v8.1 w ith th e AU2 underlying-event tu n e. For th e m H+ range of 200-300 GeV, th e sam ples are produced in steps of 25 GeV, th e n in intervals of 50 GeV up to 600 GeV. T he sam ples are generated a t NLO using th e 5FS and w ith a zero w idth for H +.

In th e search for H + in th e s-channel, signal events are g enerated using MAdGRAPH5 v1.5.12, w ith th e CTEQ 6L1 P D F , interfaced to

v8.1 w ith th e AU2 underlying- event tu ne, for b o th th e le p to n + je ts and all-hadronic final states. In th e form er (la tte r) case, sam ples are produced in m H + steps of 200 (250) GeV, betw een 0.4 and 2.0 TeV (1.5

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and 3.0 TeV). A narrow -w idth ap proxim ation is used for b o th final states. T his is justified as th e experim ental resolution is m uch larger th a n th e H + n a tu ra l w idth.

In all background sim ulations,

v1.20 [74] is used for th e t decays and

t o s

v2.15 [75] is em ployed for p h oton rad iatio n from charged leptons. For th e signal sim ulations,

v3.51 [76] is used. All signal and background events are overlaid w ith ad d itio n al m inim um -bias events g enerated using

v8.1 w ith th e M STW 2008 LO P D F and th e AU ET2 underlying-event tun e, in order to sim ulate th e effect of m ultiple pp collisions per bunch crossing (pile-up). Finally, all background sam ples and all-hadronic signal sam ples are processed th ro u g h a sim ulation [77] of th e d e te c to r geom etry and re­

sponse using

[78]. T he signal sam ples w ith leptons in th e final s ta te are passed th ro u g h a fast sim ulation of th e calorim eter response [79]. All sam ples from sim ulation are processed th ro u g h th e sam e reconstruction software as th e d a ta .

3 O b je c t r e c o n str u c tio n and id e n tific a tio n

T he m ain objects used for th e searches rep o rted in th is pap er are electrons, m uons, jets (possibly identified as originating from b-quarks), and m issing transverse m om entum . A brief sum m ary of th e m ain reco n stru ction and identification c riteria used for each of these ob jects is given below.

E lectron cand id ates [80] are reconstructed from energy deposits (clusters) in th e elec­

tro m ag n etic calorim eter which are associated w ith a recon stru cted tra c k in th e inner de­

te c to r system . T h eir transverse energy, E T = E cius/co sh (n track), is com puted using the electrom agnetic cluster energy E cius and th e direction of th e electron tra c k ntrack, and is required to exceed 25 GeV. T he p seudorapidity range for th e electrom agnetic clu ster covers th e fiducial volum e of th e d etecto r, |n| < 2.47 (the tra n sitio n region betw een th e b arrel and end-cap calorim eters, 1.37 < |n| < 1.52, is excluded). T he longitudinal im pact p aram eter

|z0| of th e electron tra c k relative to th e p rim ary vertex m ust be sm aller th a n 2 m m. In or­

d er to reduce th e co n tam in atio n from m isidentified hadrons, electrons from heavy-flavour decays and p h oton conversions, th e electron can didates are also required to satisfy E T- and n-dependent calorim eter (and tracker) isolation requirem ents im posed in a cone w ith a fixed size A R = 0.2 (0.3) aro un d th e electron position.

M uon cand id ates are reco n stru cted from tra c k segm ents in th e m uon spectrom eter, and m atched w ith track s found in th e inner d e te c to r system [81]. T he final m uon can did ates are refitted using th e com plete tra c k inform ation from b o th d e te c to r system s, and th ey are required to satisfy p T > 25 GeV, |n| < 2.5 and |z0| < 2 m m. Furth erm ore, m uons m ust fulfil a p T-dependent track-based isolation requirem ent th a t has good perform ance under high pile-up conditions a n d /o r when th e m uon is close to a je t. For th a t purpose, th e scalar sum of th e tra c k p T in a cone of a variable size, defined by A R = 1 0 G e V /p T , around th e m uon position (while excluding th e m uon tra c k itself) m ust be less th a n 5% of th e muon transverse m om entum .

Je ts are recon stru cted from topological energy clusters [82] in th e calorim eters, using th e a n ti- k algorithm [83, 84]. Tw o radius param eters are used, R = 0.4 ( ’sm all-radius je t s ’) or R = 1.0 ( ’large-radius je ts ’). T he large-radius je ts are only used w hen recon-

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stru c tin g high-pT to p quarks as single objects in th e search for H + ^ tb produced in th e s-channel and decaying into an all-hadronic final sta te , as described below. W hen no je t ty p e is specified, sm all-radius je ts are im plied. Small- and large-radius je ts are calib rated using energy- and n-dependent correction factors derived from sim ulation and w ith residual corrections from in situ m easurem ents [85]. Only sm all-radius jets th a t have p T > 25G eV and |n| < 2.5 are considered in th is paper. Je ts originating from pile-up interaction s are suppressed by requiring th a t a t least 50% of th e scalar sum of th e p T of th e associated track s is due to tracks originating from th e p rim ary vertex [86]. This is referred to as th e je t v ertex fraction (JV F ) and is only applied to je ts w ith p T < 50 GeV and |n| < 2.4.

J e ts are identified as originating from th e h adron isatio n of a b-quark (b-tagged) via an algorithm th a t uses m u ltiv ariate techniques to com bine inform ation from th e im pact p aram eters of displaced track s w ith topological properties of secondary and te rtia ry decay vertices reconstructed w ithin th e je t [87]. T he nom inal w orking point used here is chosen to correspond to a 70% efficiency to ta g a b-quark je t, w ith a light-jet m istag ra te of 1%

and a c-jet m istag ra te of 20%, as determ ined w ith b-tagged je ts w ith p T > 20 GeV and

|n| < 2.5 in sim ulated t t events. T he tagging efficiencies from sim ulation are corrected based on th e results of flavour-tagging calibrations perform ed w ith th e d a ta [88].

In th e search for H + ^ tb produced in th e s-channel and decaying into an all-hadronic final sta te (section 5.2) , hadronically decaying high-pT to p quarks are reconstructed as single o bjects th ro u g h ’to p -ta g g in g ’. Large-radius jets are used as in p u t to th e top-tagger.

In order to m inim ise th e effects of pile-up [89], th e large-radius je ts are trim m ed [90].

T he trim m in g is perform ed by reclustering th e large-radius je t using th e inclusive kt al­

gorithm [91] w ith a je t radius p a ra m ete r R = 0.3, and by rem oving soft su b jets w ith a p T sm aller th a n 5% of th e original je t p T . Trim m ed large-radius je ts are required to have p T > 350 GeV and |n| < 2.0. L arge-radius jets are top-tagg ed if th ey have a su b stru c tu re com patible w ith a three-p ron g decay. T he to p -tag ger used in th e search of section 5.2 was developed for th e search for W ' ^ tb in ATLAS [23]. It uses th e kt sp littin g scale [91]

v d 2 and th e N -su b je ttin e ss [92, 93] variables t 21 and t 32. T he kt algorithm clusters th e hard est o bjects last, which m eans th a t a tw o-body decay (such as t ^ bW ) typically gets a larger value of th a n light jets. T he T j d istrib u tio n peaks closer to 0 for i-subjet-like je ts and closer to 1 for j-su b jet-lik e jets. T he top-tag ged je t is required to pass th e cuts

v d 2 > 40 GeV, t 32 < 0.65, and 0.4 < t 21 < 0.9, as in th e search for W 1 ^ tb [23].

W hen several selected objects overlap geom etrically, th e following procedures are ap ­ plied. In th e searches w ith a le p to n + je ts final sta te , m uons are rejected if found to be A R < 0.4 from any je t w ith nom inal

, n and JV F selections. In order to avoid double­

counting of electrons as jets, th e closest je t to an electron is th e n removed if lying A R < 0.2 from an electron. Finally, electrons are rejected if found to be A R < 0.4 from any rem ain ­ ing je t w ith nom inal

, n and JV F selections. In th e search for s-channel pro du ction of H + ^ tb in th e all-hadronic final sta te , large-radius je ts are required to be separated by A R > 2.0 from th e sm all-radius b-tagged je ts used to recon struct th e invariant m ass of H + candidates. E vents w ith electrons (muons) fulfilling E T > 30 GeV (pT > 30 GeV) are vetoed in th is p a rticu la r search channel.

T he m ag n itu d e Em iss of th e m issing transverse m om entum is reco nstru cted from th e negative vector sum of tran sv erse m om enta of reconstru cted objects, as well as from un­

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

m atched topological clusters and tracks (collected in a so-called soft term ). T he ETpiss is fu rth e r refined by using object-level corrections for th e identified electrons, m uons and jets, and th e effects of pile-up in th e soft term are m itigated [94].

4 S earch for a ch a rg ed H ig g s b o so n in a s so c ia tio n w ith a to p quark

4 .1 E v e n t s e l e c t i o n a n d c a t e g o r i s a t i o n

In this section, th e search for a charged Higgs boson produced in association w ith a top quark, gb ^ t H + w ith H + ^ tb, is described. In th e events selected for th is analysis, th e to p quarks b o th decay via t ^ Wb, w here one W boson decays hadronically and th e o th er decays into an electron or a m uon, eith er directly or th ro u g h a T-lepton decay, and th e corresponding neutrino(s). T he signal event sig natu re is therefore characterised by th e presence of exactly one high-pT charged lepton (electron or m uon) and five or m ore jets, a t least th re e of th em being b-tagged.

E vents collected using eith er an isolated or non-isolated single-lepton trig ger are consid­

ered. Isolated triggers have a threshold of 24 GeV on pT for m uons and on E T for electrons, while non-isolated triggers have higher thresholds a t 36 GeV (muons) and 60 GeV (elec­

tro n s). T he isolated triggers have a loss of efficiency a t high p T or E T , which is recovered by th e triggers w ith higher thresholds. E vents accepted by th e trigger are th e n required to have exactly one identified electron or m uon, and a t least four jets, of which at least two m ust be identified as b-tagged jets. T he selected lepton is required to m atch, w ith A R < 0.15, a lepton reco n structed by th e trigger.

A t th is stage, th e sam ples contain m ostly t t events. T he selected events are fu rth er categorised into different regions, depending on th e num ber of je ts and b-tagged jets. T he categories are inclusive in th e lepton flavour. In th e following, a given category w ith m jets, of which n are b-tagged, is referred to as m j(n b ). A to ta l of five independent categories are considered: four control regions (CR) w ith little sensitivity to signal, 4j(2b), 5j(2b),

> 6 j(2 b ), 4 j(> 3 b ), and one signal-rich region (SR), > 5 j(> 3 b ). T he CR are used to control th e backgrounds and to co n strain system atic uncertainties (section 4.3) . For each category, th e expected event yields of all processes and th e num ber of events observed in th e d a ta are given in tab le 1. T he d om inant background process in every category is tt+ L F . In th e signal-rich region, contrib ution s from ttt + H F are also sizeable. In all categories except

> 6 j(2 b ), th e d a ta exceed th e SM prediction, b u t th ey are consistent w ithin th e large un certainties on th e background. In tab le 2, th e expected am ount of signal is listed for a few points of th e mm°d - b enchm ark scenario of th e MSSM [95]. T he th eoretical predictions are tak en from refs. [11, 96- 98].

4 .2 A n a l y s i s s t r a t e g y

In order to sep arate th e H + signal from th e SM background, and to co n strain th e large un certainties on th e background, different discrim inants are used depending on th e event category, and are th e n com bined in a binned m axim um -likelihood fit. In th e four CR, th e d iscrim inating variable is th e scalar sum of th e p T of th e selected je ts (HTad) and

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

Process 4j(2b) 5j(2b) >6j(2b) 4j(>3b) >5j(>3b) tt+ L F 80 300 ± 9900 38 700 ± 7400 19 300 ± 5300 6300 ± 1000 5600 ± 1600

tf+cc 5200 ± 2900 4500 ± 2600 3800 ± 2300 740 ± 410 1800 ± 1000

tt + bb 1720 ± 940 1550 ± 830 1390 ± 820 660 ± 370 2300 ± 1200

ttH 33.7 ± 4.6 44.6 ± 5.4 68.9 ± 9.1 15.5 ± 2.5 87 ± 11

ttV 128 ± 40 151 ± 47 189 ± 59 17.6 ± 5.7 85 ± 27

Single-top 5020 ± 770 1970 ± 420 880 ± 270 360 ± 83 330 ± 110

W +jets 3400 ± 1700 1270 ± 720 640 ± 400 190 ± 100 170 ± 100

Z+jets 1330 ± 670 400 ± 220 150 ± 95 53 ± 31 49 ± 39

VV 232 ± 69 108 ± 41 52 ± 25 10.7± 3.6 13.7 ± 6.0

Multi-jets 2160 ± 870 670 ± 260 330 ± 150 160 ± 67 150 ± 100

Total bkg 100 000 ± 11000 49 300 ± 8600 27100 ± 6600 8500 ± 1300 10 600 ± 2500

Data 102 462 51 421 26 948 9102 11 945

T a b le 1. E x p ected event yields of th e SM b ackground processes a n d observed d a ta in th e five categories. T he first four colum ns show th e event yields in th e CR, th e last colum n shows th e event yields in th e SR. T he u n c e rtain tie s include s ta tistic a l an d sy stem atic com ponents (system atic u n c e rtain ties are discussed in section 4.3) .

m H + [GeV] ta n d 4j(2b) 5j(2b) >6j(2b) 4j(>3b) > 5j(> 3b)

0.5 2580 ± 420 1670 ± 190 1050 ± 300 730 ± 190 1750 ± 200

200 0.7 1290 ± 210 834 ± 93 520 ± 150 366 ± 95 880 ± 100

0.9 760 ± 120 493 ± 55 309 ± 88 216 ± 56 518 ± 59

0.5 397 ± 69 406 ± 44 390 ± 100 211 ± 56 756 ± 76

400 0.7 200 ± 35 204 ± 22 197 ± 51 106 ± 28 380 ± 38

0.9 119 ± 21 121 ± 13 117 ± 31 63 ± 17 226 ± 23

0.5 71 ± 14 85 ± 12 107 ± 29 36 ± 11 183 ± 23

600 0.7 34.7 ± 6.9 41.5 ± 5.6 52 ± 14 17.4 ± 5.3 89 ± 11

0.9 19.8 ± 3.9 23.7 ± 3.2 29.8 ± 8.1 10.0 ± 3.0 50.9 ± 6.5

T a b le 2. N um ber of ex p ected signal events in th e five categories for a few rep resen tativ e p o in ts of th e mmod- scenario of th e MSSM. T h e la st colum n shows th e event yields in th e SR. T he expected u n c e rtain ties co n tain s ta tistic a l an d sy stem atic co m ponents (sy stem atic u n c e rtain ties are discussed in section 4.3) . U n certa in ties on th e cross sections a n d bran ch in g fractions for th e m ™ 0 - scenario are n o t included.

in th e SR, th e o u tp u t of a boosted decision tre e (B D T ) is used. T he Toolkit for M ulti­

v ariate D a ta Analysis (T M V A ) [99] is used for th e tra in in g and evaluation of th e B D T responses. T he B D T is tra in ed to specifically discrim inate th e H + signal from th e tt+ b b background process. This m eth o d reduces correlations and anti-correlations betw een th e signal n orm alisation and th e p aram eters connected to th e d om inant system atic u n c e rtain ­ ties, in p a rticu la r for H + m asses below 350 GeV, w here those correlations are sizeable. T he largest correlation a t low m ass is th a t betw een th e tt+ b b cross section and th e signal nor­

m alisation, which is -5 0 % at 200 GeV. Consequently, this specific B D T is m ore sensitive th a n a B D T tra in ed against th e sum of all backgrounds when uncertainties are included.

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

T he variables entering th e B D T tra in in g are:

• th e scalar sum of th e pT of all selected je ts (HTad),

• th e pT of th e leading je t,

• th e invariant m ass of th e two b-tagged je ts th a t are closest in A R ,

• th e second Fox-W olfram m om ent [100], calculated from th e selected jets,

• th e average A R betw een all pairs of b-tagged je ts in th e event.

M any o th er kinem atic and event shape variables were teste d before th is set of variables was selected. T he variables listed above provide th e best sep aratio n betw een signal and background across all m ass hypotheses. T he B D T tra in in g is perform ed independently for each H + m ass hypothesis, and only for events in th e SR. T he B D T in p u t variables were validated in th e CR by com paring th e ir distrib u tio n s in th e d a ta and sim ulation, and th ey were fu rth e r validated by evaluating th e B D T responses in th e four C R for every m ass point. T he d a ta and expected SM backgrounds were found to be com patible a t all tim es. T he sta tistic a l analysis was perform ed after th e selection and th e B D T train in g were finalised.

T he pre-fit d istrib u tio n s of HTad in th e four control regions are displayed in figure 2.

Good agreem ent betw een d a ta and th e SM expectatio n is found, given th e large u n certain ­ ties. T he pre-fit B D T o u tp u t d istrib u tio n s for two m ass hypotheses are shown in figure 3.

In th e SR, th e d a ta exceed th e expected background, b u t th ey are consistent given th e large u ncertainties. T he d iscrim ination betw een signal and background significantly improves for larger signal masses.

4 .3 S y s t e m a t i c u n c e r t a i n t i e s

Several sources of sy stem atic uncertainty, affecting th e no rm alisation of signal and back­

ground processes or th e shape of th eir distrib u tio n s, are considered. T he individual sources of system atic u n certain ty are assum ed to be uncorrelated, b u t correlations of a given sys­

tem a tic effect are m ain tain ed across categories and processes, w hen applicable. All vari­

ations, except those from uncertain ties on th e th eoretical cross section, are sym m etrised w ith respect to th e nom inal value. T he uncertainties arising from th e reco nstru cted objects and th e background m odelling, in p a rticu la r th e ttt background m odelling, receive th e same tre a tm e n t as in ref. [45].

T he following un certainties on th e reconstru cted objects are considered. T he system ­ atic uncertain ties associated w ith th e electron or m uon selection arise from th e trigger, reco n struction and identification efficiency, isolation criteria, as well as from th e m om en­

tu m scale and resolution [80, 81]. In to ta l, th e system atic uncertainties associated w ith electrons (m uons) include five (six) com ponents. T he system atic uncertain ties associated w ith th e je t selection arise from th e je t energy scale (JE S ), th e JV F requirem ent, th e je t energy resolution and th e je t reconstruction efficiency. Am ong these, th e JE S un certain ty has th e largest im pact on th e search. It is derived by com bining inform ation from test-b eam

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

F ig u re 2. P re-fit d istrib u tio n s of th e scalar sum of th e p t of all selected je ts, HTad, for th e four control regions: (a) 4j(2b), (b) 5j(2b), (c) > 6 j(2 b ), (d) 4 j(> 3 b ). E ach b ackground process is norm alised according to its cross section. A signal w ith m # + = 300 GeV, norm alised to a p ro d u ctio n cross section tim es b ran ch in g fraction for H + ^ tb (a x BR ) of 1 pb, is show n in pink, stacked on to p of th e background. T w o signal shapes are show n su p erim p o sed as d ash ed lines norm alised to th e d a ta . T he la st b in includes th e overflow. T h e h a tc h e d b an d s show th e pre-fit uncerta in tie s, which are d o m in ated by sy stem atic u n ce rtain ties (discussed in section 4.3) . T h e lower panels display th e ra tio of th e d a ta to th e to ta l p red ic ted background.

d a ta , LHC collision d a ta and sim ulation [85]. T he JE S u n certain ty is split into 22 uncorre­

lated sources, which can have different je t

- and n-dependencies. Six (four) independent sources of system atic u n certain ty affecting th e b(c)-tagging efficiency are considered [88].

A n ad d ition al u n certain ty is assigned due to th e e x tra p o la tio n of th e m easurem ent of th e b-tagging efficiency to th e high-p

region. Twelve uncertain ties are considered for th e light-jet m istagging rate, w ith dependencies on th e je t

and n.

T he u n certain ty on th e inclusive tt p ro d u ction cross section is + 5 % /- 6 % [38- 44].

It accounts for un certainties from th e choice of P D F , a

and th e to p -q u ark m ass. T he

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

F i g u r e 3. P re-fit d istrib u tio n s of th e B D T o u tp u t in th e signal-rich region tra in e d for tw o signal m ass hypotheses: (a) 300 G eV an d (b) 500 GeV. E ach b ackground process is norm alised according to its cross section. A signal, norm alised to a p ro d u c tio n cross section tim es bran ch in g fraction for H + ^ tb (a x B R ) of 1 pb, is show n in pink, stacked on to p of th e background. T he signal sh ap e is show n su perim posed as d ash ed line norm alised to th e d a ta . T he h a tc h e d b an d s show th e pre-fit unce rtain ties, w hich are d o m in ated by sy stem atic u n c e rta in ties (discussed in section 4.3) . T he lower panels display th e ra tio of th e d a ta to th e to ta l p red icted background.

P D F and a S un certain ties were calculated using th e PD F 4L H C prescription [101] w ith th e M STW 2008 68% CL NNLO, CT10 NNLO and N N PD F2.3 NNLO [102] P D F sets, added in q u a d ra tu re to th e scale uncertainty . System atic uncertainties due to th e choice of par- to n shower and h ad ron isatio n m odels are derived by com paring t t events produced w ith P o w h e g - B o x interfaced to eith er P y t h i a or H e r w ig . Nine uncertainties associated w ith th e experim ental m easurem ent of th e p T of th e to p q u ark and th e t t system are considered as sep arate sources of system atic u n certain ty in th e rew eighting procedure [46]. Two ad ­ ditio nal uncorrelated un certain ties are assigned specifically to tf+ c c events, consisting of th e full difference betw een applying and not applying th e p T rew eighting procedure for th e to p q uark and th e t t system , respectively. A conservative system atic u n certain ty of 50% is applied to tt+ bb events to account for differences betw een th e cross sections obtained w ith P o w h e g + P y t h i a and th e NLO prediction based on S h e r p a w ith O p e n L o o p s [47, 48].

In th e absence of an NLO prediction for tt+ cc, th e sam e u n certain ty of 50% is applied to th is com ponent of th e t t background. Four additio n al system atic uncertain ties are considered for th e tt+ c e background, derived from th e sim ultaneous variation of factori­

sation and renorm alisation scales, thresh old of th e p a rto n -je t m atching scheme [103], and c-quark m ass v ariations in th e sim ulation of t t events w ith MAdGRAPH+PYTHiA, as well as th e difference betw een sim ulations of th e tt+ ca process w ith MAdGRAPH+PYTHiA and P o w h e g + P y t h i a . For th e tt+ bb background, eight add ition al system atic uncertainties are considered: th re e arise from scale uncertainties, one from th e shower recoil model, two

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

from th e choice of P D F in th e NLO prediction from

w ith

and two from th e uncertainties on m u lti-p arto n in teractio n and final-state rad iation , which are not present in

w ith

A n u n certain ty of + 5 % /- 4 % is assum ed for th e cross section of single-top-quark pro­

d u ctio n [56, 57], corresponding to th e weighted average of th e th eoretical uncertain ties on th e s-, t- and W t-channel p ro d uction modes. One ad ditio nal system atic u n certain ty is considered to account for different ways of handling th e interference betw een t t and W t events [55]. For t t V , an u n certain ty of 30% on th e cross section is assum ed [52, 53] and an a d ditional u n certain ty arises from variations in th e am ount of rad iation . T he un certain ty on th e t t H cross section is + 8 .9 % /- 1 2 % [11]. T he uncertain ties on th e V + je ts and diboson backgrounds are 48% and 25%, respectively [63, 68]. For events w ith 5 (> 6 jets), one (two) a d ditional un certain ties of 24% are added in q u a d ra tu re to account for th e ex trap o latio n to higher je t m ultiplicities. In addition, th e full difference betw een applying and not applying th e p T rew eighting for th e vector boson is taken as a system atic uncertainty. U ncertainties on th e estim ate of th e m ulti-jet background come from th e lim ited num ber of events in th e d a ta , especially at high je t and b-tagged je t m ultiplicities, from th e uncertainties on th e m easured lepton m isidentification rates (assum ed to be 50%, b u t un correlated betw een events w ith an electron or m uon), as well as from th e su b tra c tio n of sim ulated events w ith a p ro m p t lepton w hen estim atin g th e m isidentification rates.

T hree sources of system atic u n certain ty are considered w hen m odelling H + ^ tb events. U ncertainties arising from th e choice of P D F are estim ated using sam ples generated w ith M C @ N L O v4.6 [104] interfaced to

v2.5.2 [105], by tak in g th e envelope of th e M STW 2008 68% CL NLO, CT10 NLO and N N PD F2.3 NLO P D F sets, and by nor­

m alising to th e nom inal cross section [101]. T he uncertain ties observed across th e charged Higgs boson m ass range are of th e order of 5-10% and increase slightly w ith th e H + mass.

T his sy stem atic u n certain ty affects b o th shape and norm alisation. U ncertainties from th e choice of th e event g en erator are estim ated from a com parison of th e signal acceptances betw een events produced using eith er

or

v2.1.1 [106], b o th interfaced to

v8.1, w ith a charged Higgs boson m ass of 400 GeV. In th e SR, th is u n certain ty is found to be ab o u t 1%, while it increases to as m uch as 20% in th e CR. It is applied to all signal m ass points as a norm alisation-only system atic uncertainty. U ncer­

tain tie s o riginating from initial- and final-state p a rto n rad iatio n , which can m odify th e je t pro d uction rate, are evaluated by varying fac to risatio n /re n o rm alisa tio n scale p aram eters in th e p ro d u ction of signal sam ples. T his system atic u n certain ty is found to be below 2%

in all five event categories.

4 .4 R e s u l t s

A binned m axim um likelihood fit to th e d a ta is perform ed sim ultaneously in th e five event categories, and each m ass hypothesis is teste d separately. T he in p u ts to th e sim ultaneous fit are th e d istrib u tio n s of H ^ ad in th e four CR, and th e B D T o u tp u t histogram s in th e SR. T he procedures for quantifying how well th e d a ta agree w ith th e background-only hypothesis and for determ inin g exclusion lim its are based on th e profile likelihood ratio te s t [107].

T he p a ra m ete r of interest is th e p ro d uction cross section a(gb ^ t H +) m ultiplied by th e

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

Source of un certain ty Fractional u n certain ty [%]

m H+ = 300 GeV m H+ = 500 GeV

t l m odelling 31 33

Je ts 21 9.5

Flavour tagging 19 24

O th e r background m odelling 9.6 12

Signal m odelling 8.0 3.5

Lepton 1.2 0

Lum inosity 1.1 0.4

S tatistics 8.9 18

T a b le 3. P ercentage of th e to ta l u n c e rta in ty on th e signal stre n g th th a t is induced from various sys­

te m a tic uncerta in ties. T he values are o b tain ed after fits to th e background-plus-signal hypothesis.

T he largest co n trib u tio n to th e to ta l u n c e rta in ty comes from th e t t m odelling.

b ranching fraction B R (H + ^ tb), also referred to as th e signal stren g th . All system atic u ncertainties, eith er from th eo retical or experim ental sources, are im plem ented as nuisance p aram eters w ith log-norm al co n strain t term s. T here are ab o u t 100 nuisance p aram eters considered in th e fit, th e num ber varying slightly across th e range of m ass hypotheses.

T he largest uncertain ties for any tested m ass point are those arising from th e m odelling of th e t l processes. For m H+ < 350 GeV, th e u n certain ty on th e tt+bb cross section has th e largest im pact on th e result. For higher m ass hypotheses, th e uncertainties on th e shape of th e d istrib u tio n s for tt+bb from th e rew eighting to th e NLO prediction are dom inant.

T he fractional contribu tio ns of various sources of u n certain ty to th e to ta l u n certain ty on th e p a ra m ete r of in terest are presented in tab le 3 , for two hypothesised H + masses. T he un certainties decrease for higher m ass hypotheses because of th e larger signal acceptance and th e im proved sep aratio n betw een signal and background. T he pulls of th e nuisance p aram eters after profiling to th e d a ta are alm ost all w ithin + 1 a and never exceed ± 1 .5 a for all teste d m ass hypotheses. T he pulls th a t are larger th a n ± 1 a in a t least one of th e teste d m ass hypotheses are those associated w ith uncertainties on th e ti+ H F cross sections, on th e p a rto n shower m odelling of th e tl+ c c process, and on th e tt+bb NLO m odelling, derived from variations of th e functional form of th e renorm alisation scale .

T he post-fit d istrib u tio n s of th e H ^ ad variable in th e four CR for th e fit un d er th e background-only hypothesis are shown in figure 4 , w hereas th e background-only post-fit d istrib u tio n s of th e B D T o u tp u t in th e SR are presented in figure 5. T h e background com ­ p onent of a fit under th e background-plus-signal hypothesis is overlayed. T he post-fit event yields for th e fit un d er th e background-plus-signal hypothesis for m H + = 300 GeV are given in tab le 4. T he fit prefers a positive signal stre n g th for all tested m ass hypotheses, except a t 600 GeV. T he post-fit event yields for th e t l+ H F process are higher in background-only fits th a n those obtain ed in fits w here th e signal hypothesis is included.

T he m odified frequentist m eth o d (CLs) [108] and asy m p to tic form ulae [109] are used to calculate up p er lim its on a(gb ^ t H +) x B R (H + ^ tb). T he 95% confidence level (CL) u p p er lim its are presented in figure 6. T he m ass hypotheses are tested in 25 GeV steps

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

Process 4j(2b) 5j(2b) >6j(2b) 4j(>3b) >5j(>3b)

tt+ L F 83 600 ± 1900 41 800 ± 1400 21000 ±1000 6750 ± 270 6650 ± 390

tt+cc 3200 ± 1700 2600 ± 1400 2100 ±1200 490 ± 230 1260 ± 570

tt+bb 1500 ± 530 1300 ± 440 1050 ± 450 600 ± 210 2040 ± 550

ttH 34.6 ± 3.8 44.6 ± 4.9 66.7 ± 7.8 16.2 ± 1.9 87 ± 10

ttV 132 ± 39 153 ± 46 186 ± 57 18.5 ± 5.4 87 ± 26

Single-top 5030 ± 530 1970 ± 270 860 ± 170 386 ± 55 342 ± 70

W + jets 4500 ± 1100 1660 ± 470 750 ± 270 250 ± 62 220 ± 69

Z +jets 1330 ± 560 370 ± 190 137 ± 80 56 ± 23 36 ± 27

VV 223 ± 63 103 ± 39 47 ± 23 10.4 ± 3.1 15.0 ± 5.3

Multi-jets 2230 ± 590 690 ± 180 330 ± 100 160 ± 46 208 ± 88

Total bkg 101 800 ± 2200 50 700 ± 1600 26 600 ±1100 8730 ± 330 10950 ± 490

H + 700 ± 310 600 ± 260 430 ± 190 370 ± 160 990 ± 440

D ata 102462 51 421 26 948 9102 11 945

T a b le 4. E v en t yields of SM backgrounds, signal an d d a ta in all categories, after th e fit to th e d a ta u n d er th e background-plus-signal h ypothesis w ith a signal m ass of 300 GeV. T he la st colum n shows th e event yields in th e SR. T he u n certain ties tak e in to account correlations an d co n strain ts of th e nuisance p aram eters.

betw een 200 and 300 GeV, and in 50 GeV steps up to 600 GeV. At 250 GeV, th e local p 0- value for th e observation to be in agreem ent w ith th e background-only hypothesis reaches its sm allest value of 0.9% (corresponding to 2.4 sta n d a rd deviations). A t m H + values of 300 and 450 GeV, th e excess of th e d a ta w ith respect to th e background-only hypothesis corresponds to 2.3 sta n d a rd deviations.

For com parison, th e expected u p p er lim it is com puted w ith a signal injected a t m H+ = 300 GeV, w ith a p ro d u ctio n cross section tim es branching fraction of 1.65 pb, corresponding to th e best-fit value of th e signal stre n g th at this m ass point. T his results in an excess th a t is m ore localised a t th e injected m ass value, i.e. extends less to lower and higher masses th a n th e tre n d seen in th e observed u p p er lim it, as shown in figure 6. T he H + signal is g enerated w ith a zero w idth. T he experim ental m ass resolution ranges from approxim ately 30 GeV (for m H+ = 200 GeV) up to 100 GeV (for m H + = 600 GeV) and is 50 GeV for th e m ass hypothesis of 300 GeV. A system atic background m ism odelling is considerably more likely to give rise to th e observed excess th a n a hypothesised signal a t a specific m ass. T he cross sections of th e tt+ H F backgrounds and th e shape of th e tt+bb com ponent have large un certainties which are co rrelated w ith th e signal norm alisation. T ogether w ith th e pre-fit excess of d a ta com pared to th e SM prediction (tab le 1) , th is can result in a post-fit excess over a wide H + m ass range. T he fits were rep eated using two alternativ e, less sensitive, discrim inants in th e SR: (a) a B D T tra in ed against th e sum of all backgrounds or (b) th e variable HTad. Sim ilar excesses were observed w ith these two a ltern ativ e m ethods. T he teste d m ass points are co rrelated w ith each other, since no m ass-dependent event selections are applied in th e analysis and th e d a ta s e t is th e sam e regardless of th e hypothesised H + m ass.

J H E P 0 3 ( 2 0 1 6 ) 1 2 7

F ig u re 4. D istrib u tio n s of H Tad after th e fit to th e d a ta u n d er th e background-only h ypothesis in th e four control regions: (a) 4j(2b), (b) 5j(2b), (c) > 6 j(2 b ), (d) 4 j(> 3 b ). E ach background is norm alised according to its p ost-fit cross section. T h e signal shape is show n as a su perim posed d ash ed blue line norm alised to th e d a ta . T he la st b in includes th e overflow. T he h a tc h e d b an d s show th e post-fit u n c ertain ties ta k in g in to account th e c o n stra in ts an d correlations of th e nuisance p a ram eters. T he lower panels display th e ra tio of th e d a ta to th e to ta l p red icted background.

In ad d itio n , th e solid red line shows th e to ta l background after an u n co n d itio n al fit u n d er th e background-plus-signal h ypothesis w ith a signal m ass of 300 GeV.

T he lim its in figure 6 are presented to g eth er w ith th e signal prediction in th e mm°d - benchm ark scenario of th e MSSM [95]. M odel points w ith 0.5 < ta n [ < 0.6 and ta n [ w 0.5 are excluded in th e H + m ass ranges of 200-300 GeV and 350-400 GeV, respectively,4 while th e expected lim its in th e m ass range of 200-400 GeV reach ta n [ = 0.7. T he mm°d - scenario is chosen as a reference model, b u t sim ilar exclusions are o b tained in o th er relevant scenarios of th e MSSM [95], i.e. mm°d+, mmax-up, tau-phobic, light stau and light stop. It has been verified th a t th e w id th predicted by these m odels does not have a notable im pact on th e exclusions.

4No reliable theoretical predictions exist for tan < 0.5.

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F ig u re 5. D istrib u tio n s of th e B D T o u tp u t in th e signal-rich region after th e fit to th e d a ta u nder th e background-only hypothesis. T he B D T was tra in e d for tw o signal m ass hypotheses: (a) 300 G eV an d (b) 500 GeV. E ach background is norm alised according to its p o st-fit cross section. T he signal sh ap e is show n as a su perim posed d ash ed blue line norm alised to th e d a ta . T he h atc h e d b an d s show th e post-fit u n c ertain ties ta k in g in to account th e c o n stra in ts an d correlations of th e nuisance p a ram eters. T he lower panels display th e ra tio of th e d a ta to th e to ta l p red icted background.

In ad d itio n , th e solid red line shows th e to ta l background after an u n co n d itio n al fit u n d er th e background-plus-signal h ypothesis w ith a signal m ass of (a) 300 G eV and (b) 500 GeV.

F ig u re 6. E x p ec ted an d observed lim its for th e p ro d u c tio n of H + ^ tb in association w ith a to p q uark, as well as b an d s for 68% (in green) an d 95% (in yellow) confidence intervals. T he red dash- d o tte d line shows th e ex p ected lim it o b ta in e d in th e case w here a sim u lated signal is in jected a t m H+ = 300 GeV, w ith a p ro d u c tio n cross section tim es b ran ch in g fraction of 1.65 p b (corresponding to th e b est-fit signal stre n g th a t th a t m ass hypothesis), yielding a d ev iatio n from th e ex p ecta tio n th a t ex ten d s less to higher an d lower m ass values th a n th e observed u p p e r lim it. T h eo ry predictions are show n for th re e rep resen tativ e values of ta n ft in th e b en ch m a rk scenario of th e MSSM.

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5 S earch for a ch a rg ed H ig g s b o so n p r o d u ce d in th e s-c h a n n el

In this section, two searches for qq' ^ W ' ^ tb recently published by ATLAS [22, 23]

are rein terp reted as searches for th e s-channel p ro d u ctio n 5 of charged Higgs bosons, i.e.

qq' ^ H + ^ tb, based on final sta te s w ith one charged lepton (electron or m uon) and jets, or hadronic jets only.

5 .1 L e p t o n + j e t s f i n a l s t a t e

In th e search for H + ^ tb ^ (£vb)b produced in th e s-channel, w here th e charged lepton

£ is an electron or m uon (from a pro m p t W -boson decay or a leptonic t decay), only events collected using a single-electron or single-m uon trigg er are considered, w ith th e sam e com bination of thresholds as in section 4.1. E x actly one charged lepton is required, which m ust m atch, w ith A R < 0.15, a lepton reco nstructed by th e trigger. T he electron or m uon is th e n required to have E T or pT g reater th a n 30 GeV. T he selected events m ust th e n have two or th ree jets, w ith exactly two of th em b-tagged. In addition, th e Emiss m ust exceed 35 GeV, and th e sum E™ ss + m T , w here m T is th e transverse m ass6 of th e W boson, is required to be g reater th a n 60 GeV in o rder to reduce th e co n trib utio n from th e m ulti-jet background. A ssum ing th a t th e m issing transverse m om entum arises solely from th e n eu trin o in th e W -boson decay, its tran sv erse m om entum is given by th e x- and y-com ponents of th e Em iss vector, while th e unm easured z-com ponent of th e n eutrin o m om entum p vz is inferred by im posing th e W -boson m ass co n strain t on th e lepton-neutrino system . This leads to a q u a d ra tic equation for pV. In th e case of two real solutions, th e one w ith th e sm aller pV is chosen. If th e solutions are complex, a real estim ate of th e p vz is obtained by a kinem atic fit th a t rescales th e n eu trin o m om entum com ponents pX and py such th a t th e im aginary term vanishes. T he corrected m issing transv erse m om entum of th e n eu trin o is kept as close as possible to th e m easured Em iss [110].

H aving determ ined th e four-m om entum of th e leptonically decaying W boson, th e to p q u ark is th e n recon stru cted. T he b-tagged je t for which th e invariant m ass of th e W b system is closest to m top is assum ed to originate from th e to p -q u a rk decay, th e oth er b- tagged je t being in tu rn assigned to th e H + decay. T he selected events are th e n classified into one signal-rich and one signal-depleted region, separately for events w ith two or th ree jets. T he signal-rich region is th e subset of th e sam ple w ith two b-tagged je ts and an invariant m ass > 330 GeV. T h e signal-depleted region is th e com plem entary subset, w ith two b-tagged jets and < 330 GeV.

T he shape and n o rm alisation of th e m ulti-jet background w ith a m isidentified lepton are determ ined w ith th e m atrix m ethod [31]. All o th er backgrounds are tak e n from sim u­

lation. For W + je ts events, th e sam ple com position in th e signal-rich and signal-depleted regions w ith two b-tagged jets are sim ilar, hence an overall renorm alisation of th e W + je ts background, based on th e event yield m easured in th e signal-depleted region, is applied to th e events w ith two jets. In th e events w ith th re e jets, th e con trib u tio n of th e W + je ts

5 While the process generated is qq' ^ H +, the most commonly occuring reaction is cs ^ H +.

6The transverse mass is defined as m T = y / 2pTEmlss(l — cos A0£,mlss), where A0£,mlss is the azimuthal separation between the reconstructed lepton and the missing momentum in the transverse plane.

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