Measurement of fiducial differential cross sections of gluon-fusion production of Higgs bosons decaying to $WW^{\ast}\rightarrow e\nu \mu \nu$ with the ATLAS detector at $\sqrt{s}=8$ TeV

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 pril 12, 2016

R e v i s e d: June 15, 2016 A c c e p t e d: A ugust 4, 2016 P u b l i s h e d: A ugust 17, 2016

Measurement of fiducial differential cross sections of gluon-fusion production of Higgs bosons decaying to W W *→eνμν with the A T L A S detector at

√ s = 8 T eV

T h e A T L A S collaboration

E - m a i l : a tla s .p u b lic a tio n s @ c e r n .c h

A b s t r a c t : T h is p a p e r d esc rib es a m e a su re m e n t of fiducial a n d d iffe ren tial cross sectio n s o f gluo n -fu sio n H iggs b o so n p ro d u c tio n in th e H → W W *→e ν μ ν ch a n n el, u sing 20.3 fb -1 o f p ro to n -p ro to n collision d a ta . T h e d a t a w ere p ro d u c e d a t a ce n tre-o f-m ass en e rg y of / s = 8 T e V a t th e C E R N L arg e H a d ro n C o llid er a n d re co rd e d by th e A T L A S d e te c to r in 2012. C ross sectio n s a re m e a su re d from th e o b serv ed H→ W W *→e ν μ ν signal yield in ca te g o ries d istin g u ish e d by th e n u m b e r of a sso c ia te d je ts . T h e t o ta l cross sec tio n is m e a su re d in a fiducial region defined by th e k in e m a tic p ro p e rtie s of th e ch a rg ed le p to n s an d n e u trin o s . D ifferen tial cross sectio n s a re re p o rte d as a fu n c tio n of th e n u m b e r of je ts , th e H iggs b oson tra n s v e rs e m o m e n tu m , th e d ile p to n ra p id ity , a n d th e tra n s v e rs e m o m e n tu m o f th e lead in g je t. T h e je t-v e to efficiency, o r fra c tio n of ev e n ts w ith no je ts abov e a given tra n s v e rs e m o m e n tu m th re s h o ld , is also re p o rte d . All m e a su re m e n ts are c o m p a re d to Q C D p re d ic tio n s from M o n te C arlo g e n e ra to rs an d fix ed -o rd er c a lc u la tio n s, a n d a re in a g reem en t w ith th e S ta n d a rd M o d el p re d ic tio n s.

Ke y w o r d s: H a d ro n -H a d ro n s c a tte rin g (e x p e rim e n ts)

ArXiy ePr i n t: 1604.02997

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Contents

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 3

3 S ig n a l a n d b a c k g r o u n d m o d e ls 4

4 E v e n t s e le c tio n 5

4.1 O b je c t re c o n s tru c tio n a n d id e n tific a tio n 5

4.2 S ignal region selectio n 6

5 B a c k g r o u n d e s t im a t io n 9

6 R e c o n s t r u c t e d y ie ld s a n d d is tr ib u tio n s 15

7 F id u c ia l r e g io n a n d c o r r e c tio n for d e t e c t o r e ffe c ts 15

7.1 D efin itio n o f th e fiducial reg ion 17

7.2 C o rre c tio n for d e te c to r effects 18

8 S t a tis tic a l a n d s y s t e m a t ic u n c e r ta in tie s 19

8.1 S ta tis tic a l u n c e rta in tie s 20

8.2 E x p e rim e n ta l sy s te m a tic u n c e rta in tie s 20

8.3 S y ste m a tic u n c e rta in tie s in th e sig nal m od el 20

8.4 S y ste m a tic u n c e rta in ty in th e c o rre c tio n p ro c e d u re 21 8.5 S y ste m a tic u n c e rta in tie s in th e b a c k g ro u n d m o del 21

9 T h e o r y p r e d ic tio n s 24

10 R e s u lts 25

10.1 D ifferen tial fidu cial cross sectio n s 27

10.2 N o rm a lised d iffe ren tial fiducial cross sectio n s 27

10.3 J e t-v e to efficiency 29

11 C o n c lu s io n 39

T h e A T L A S c o lla b o r a tio n 46

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

Since th e o b se rv a tio n of a new p a rtic le by th e A T L A S [ 1] a n d C M S [2] c o lla b o ra tio n s in th e search for th e S ta n d a rd M odel (SM ) H iggs b oso n [3- 8], th e m ass, spin, a n d ch arg e

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c o n ju g a tio n tim e s p a rity of th e new p a rtic le have b ee n m e a su re d by b o th c o lla b o ra tio n s [9­

11]. Its m ass h as b een m e a su re d to b e m H = 125.09 ± 0.24 GeV [9] by co m b in in g A T L A S a n d C M S m e a su re m e n ts. T h e s tre n g th s o f its co u p lin g s to g au g e b o so ns a n d ferm io ns have also b een ex p lo re d [12, 13]. In all cases th e re su lts are c o n siste n t w ith SM p re d ic tio n s.

D ifferen tial cro ss-sectio n m e a su re m e n ts have re cen tly b ee n m a d e by th e A T L A S a n d CM S c o lla b o ra tio n s in th e Z Z ^ 41 [14, 15] a n d 7 7 [16, 17] final s ta te s . T h e re s u lts of th e A T L A S c o lla b o ra tio n have b ee n co m b in ed in ref. [18].

In th is p a p e r, m e a s u re m e n ts of fiducial a n d d iffe ren tial cross sectio n s for H iggs b oso n p ro d u c tio n in th e H ^ W W e v p v final s ta te are p re se n te d . T h e se m e a su re m e n ts use 20.3 fb-1 of p ro to n -p ro to n collision d a t a a t a ce n tre-o f-m ass en e rg y of y f s = 8 TeV reco rd ed by th e A T L A S e x p e rim e n t a t th e C E R N L arg e H a d ro n C o llid er (L H C ). T h e p re se n te d m e a su re m e n ts c h a ra c te ris e th e gluo n -fu sio n p ro d u c tio n m o d e (g g F ), w hich is th e d o m in a n t signal c o n trib u tio n to th e H ^ W W e v p v ev en t sam p le. T h e re s u lts a re co m p a re d to q u a n tu m c h ro m o d y n a m ic s (Q C D ) p re d ic tio n s of th is p ro d u c tio n m ech an ism . S m all c o n trib u tio n s from th e v e c to r-b o so n fusion (V B F ), a n d v e c to r-b o so n asso c ia te d p ro d u c tio n ( V H ) m odes are s u b tra c te d a ssu m in g th e SM e x p e c ta tio n . C o n trib u tio n s fro m asso c ia te d H iggs b o so n p ro d u c tio n v ia t t H a n d bbH a re e x p e c te d to be negligible a fte r a p p ly in g th e e x p e rim e n ta l ev e n t-selectio n c rite ria . To m in im ise th e m od el d ep e n d en cies of th e c o rre c tio n for th e d e te c to r ac ce p ta n c e , a n d to allow d ire c t c o m p a riso n w ith th e o re tic a l p re d ic tio n s, all cross sectio n s p re se n te d in th is p a p e r are fid ucial cross sectio n s c o rre c te d for d e te c to r effects. H ere, th e cross sectio n s are given in a fidu cial regio n d efined usin g p article-lev el o b je c ts w h ere m o st of th e ev e n t-selectio n re q u ire m e n ts o f th e an aly sis are ap p lied .

T h e d iffe ren tial g gF H iggs b o so n p ro d u c tio n cross sectio n s are chosen to p ro b e several d ifferen t p h y sical effects:

• H ig h e r-o rd e r p e r tu r b a tiv e Q C D c o n trib u tio n s to th e g g F p ro d u c tio n are p ro b e d by m e a su rin g th e n u m b e r of je ts , N je t, a n d tra n s v e rs e m o m e n tu m , p T , of th e h ig h e st-p T ( “le a d in g ” ) je t, p ÿ .

• M u ltip le so ft-g lu o n em ission, as m o d elled by re s u m m a tio n c a lc u la tio n s, a n d non- p e r tu r b a tiv e effects a re p ro b e d by m e a su rin g th e tra n s v e rs e m o m e n tu m o f th e re co n ­ s tru c te d H iggs boson, p H .

• P a r to n d is trib u tio n fu n c tio n s (P D F s) are p ro b e d by m e a su rin g th e a b s o lu te value of th e ra p id ity of th e re c o n s tru c te d d ile p to n sy stem , |y ^ |.

T h e d ile p to n ra p id ity , y u , is hig h ly c o rre la te d to th e ra p id ity o f th e re c o n s tru c te d Higgs boson, yH , w hich is know n to b e sen sitiv e to P D F s . Since it is n o t p ossib le to re c o n s tru c t y H e x p e rim e n ta lly in th e H ^ W W * ^ e v p v final s ta te , th e d iffe ren tial cross sec tio n is m e a su re d as a fu n c tio n o f |y ^ |. A n a d d itio n a l im p o rta n t te s t o f Q C D p re d ic tio n s is th e p ro d u c tio n cross sec tio n o f th e H iggs b o so n w ith o u t a d d itio n a l je ts ( H + 0 -jet), w hich is also a significant so urce of u n c e rta in ty in m e a su re m e n ts o f th e t o ta l H ^ W W * p ro d u c tio n ra te . L arg e u n c e rta in tie s arise from u n re su m m e d lo g a rith m s in fix ed -o rd er p re d ic tio n s o r fro m u n c e rta in tie s assigned to re su m m ed p re d ic tio n s for th e H + 0-jet cross sectio n . T h e H + 0-jet cross section, a 0 ( p T Lresh), ca n b e c a lc u la te d fro m th e p ro d u c t o f th e to ta l cross

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section, atot, and the jet-veto efficiency for H + 0-jet events, e0(pîhresh), which is defined as the fraction of events with the leading jet below a given threshold, pîpresh:

In addition to the measurement of the Njet distribution, a measurement of the jet-veto efficiency for H + 0 -jet events, e0, is presented for three different values of pT“resh. All results are compared to a set of predictions from fixed-order calculations and Monte Carlo (MC) generators.

Differential cross-section measurements are performed for the first time in the H ^ W W * ^ evßv final state. This analysis is an extension of the ggF coupling mea­

surement performed using the Run-1 dataset [19], and uses the same object definitions, background-estimation techniques, and strategies to evaluate the systematic uncertain­

ties. In contrast to the couplings measurement, in which the results were obtained using a likelihood-based approach to simultaneously fit several signal regions and background­

dominated control regions, the analysis presented here utilizes a simplified approach. First the dominant backgrounds are estimated using control regions in data, and then the pre­

dicted backgrounds are subtracted from the observed data in the signal region to obtain the signal yield. Another difference is that events with two leptons of the same flavour (ee/ßß) are not considered due to the large Drell-Yan (pp ^ Z / y * ^ ££) background.

Using an iterative Bayesian method, the distributions are corrected for detector efficien­

cies and resolutions. Statistical and systematic uncertainties are propagated through these corrections, taking correlations among bins into account.

2 T h e A T L A S d e t e c t o r

The ATLAS detector [20] at the LHC covers nearly the entire solid angle around the colli­

sion point. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorpo­

rating three large superconducting toroid magnets. The inner-detector system (ID) is immersed in a 2 T axial magnetic field and provides charged-particle tracking in the range

|nl <

.

Closest to the interaction point, the silicon-pixel detector forms the three innermost layers of the inner detector. The silicon-microstrip tracker surrounding it typically provides four additional two-dimensional measurement points per track. The silicon detectors are complemented by the transition-radiation tracker, which enables radially extended track reconstruction up to |n| = 2.0 and provides electron identification information based on the fraction of hits above a higher energy-deposit threshold indicating the presence of transition radiation.

1ATLAS uses a right-handed coordinate system w ith its origin a t th e nom inal in teractio n point (IP) in th e centre of th e detecto r and th e z-axis along th e beam pipe. T he x-axis points from th e IP to th e centre of th e LHC ring, and th e y-axis points upw ards. C ylindrical coordinates (r, 0) are used in th e transverse plane, 0 being th e azim uthal angle around th e z-axis. T he pseudorapidity is defined in term s of th e polar angle 0 as y = — ln ta n ( 0 /2 ). A ngular separation is m easured in u n its of A R = \J(A y )2 + (A 0 )2.

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a c (p tThresh) = eo(ptThresh) ■a to t. (1.1)

T h e c a lo rim e te r sy ste m covers th e ra n g e |n| < 4.9. W ith in th e regio n |n| < 3.2, e le c tro ­ m a g n e tic c a lo rim e try is p ro v id ed by a h ig h -g ra n u la rity le a d /liq u id -a rg o n (L A r) sa m p lin g c a lo rim e te r. T h e h a d ro n ic c a lo rim e te r co n sists o f steel a n d s c in tilla to r tile s in th e cen ­ tr a l region a n d tw o c o p p e r/L A r h a d ro n ic e n d c a p c a lo rim e te rs. T h e so lid -an gle coverage is c o m p le te d w ith fo rw ard c o p p e r/L A r a n d tu n g s te n /L A r c a lo rim e te r m o d u les o p tim ise d for e le c tro m a g n e tic a n d h a d ro n ic m e a su re m e n ts respectively.

T h e m u o n s p e c tro m e te r (M S) covers th e regio n |n| < 2.7 w ith precise p o s itio n m ea­

su re m e n ts from th re e layers of m o n ito re d d rift tu b e s (M D T s). C a th o d e -s trip ch a m b e rs p ro v id e a d d itio n a l h ig h -g ra n u la rity coverage in th e fo rw ard (2 < |n| < 2.7) region. T h e m u o n trig g e r sy ste m covers th e ra n g e |n| < 2.4 w ith re sistiv e -p la te c h a m b e rs in th e b a rre l a n d th in -g a p c h a m b e rs in th e e n d c a p regions, b o th of w h ich also p ro v id e p o s itio n m e a su re ­ m e n ts in th e d ire c tio n n o rm a l to th e b e n d in g p lan e, c o m p le m e n ta ry to th e precisio n h its from th e M D T s.

A th ree -lev el trig g e r sy ste m re d u ces th e ev en t r a te to a b o u t 400 H z [21]. T h e Level-1 trig g e r is im p le m e n te d in h a rd w a re a n d uses a su b se t o f d e te c to r in fo rm a tio n to red u ce th e ev en t ra te to a d esign value of a t m o st 75 kH z. T h e tw o s u b se q u e n t trig g e r levels, collectively referred to as th e H igh-L evel T rig g er (H L T ), are im p le m e n te d in softw are.

3 S ig n a l a n d b a c k g r o u n d m o d e ls

Signal a n d b a c k g ro u n d processes a re m o delled by M o n te C arlo sim u la tio n , u sin g th e sam e sam p les a n d co n fig u ratio n s as in ref. [19], w hich a re su m m a riz e d here. E v e n ts re p re s e n t­

ing th e g g F a n d V B F H ^ W W * sign al processes are p ro d u c e d from c a lc u la tio n s a t n e x t-to -le a d in g o rd e r (N L O ) in th e s tro n g co u p lin g a s as im p le m e n te d in th e P o w h e g M C g e n e ra to r [22- 25], in terfa ced w ith P y t h i a8 [26] (version 8.165) for th e p a r to n show er, h a d ro n is a tio n , a n d u n d e rly in g ev en t. T h e C T 1 0 [27] P D F set is used a n d th e p a ra m e te rs of th e P y t h i a8 g e n e ra to r co n tro llin g th e m o d ellin g o f th e p a r to n show er a n d th e u n d e rly in g ev en t a re th o se co rre sp o n d in g to th e AU2 set [28]. T h e H iggs b o so n m ass set in th e g e n e ra ­ tio n is 125.0 GeV, w hich is close to th e m e a su re d value. T h e P o w h e g g g F m od el ta k e s in to ac c o u n t fin ite q u a rk m asses a n d a ru n n in g -w id th B re it-W ig n e r d is trib u tio n t h a t in clu des electro w eak c o rre c tio n s a t N L O [29]. To im p ro ve th e m o d ellin g of th e H iggs b o so n p x d is tri­

b u tio n , a rew eig h tin g schem e is a p p lie d to re p ro d u c e th e p re d ic tio n of th e n e x t-to -n e x t-to - le a d in g -o rd e r (N N L O ) a n d n e x t-to -n e x t-to -le a d in g -lo g a rith m (N N L L ) d y n am ic-sc ale cal­

c u la tio n given by th e H R e s 2.1 p ro g ra m [30]. E v e n ts w ith > 2 je ts are f u r th e r rew eigh ted to re p ro d u c e th e pH s p e c tru m p re d ic te d by th e N L O P o w h e g sim u la tio n of H iggs b o ­ son p ro d u c tio n in a sso c ia tio n w ith tw o je ts ( H + 2 je ts ) [31]. In te rfe re n c e w ith c o n tin u u m W W p ro d u c tio n [3 2 , 33] h as a negligible im p a c t on th is an aly sis d u e to th e tra n sv e rse -m a ss selectio n c rite ria d e sc rib e d in sec tio n 4 a n d is n o t in clu d ed in th e signal m odel.

T h e inclusive cross sectio n s a t y f s = 8 TeV for a H iggs b o so n m ass of 125.0 GeV, c a lc u la te d a t N N L O + N N L L in Q C D a n d N L O in th e electro w eak cou pling s, are 19.3 p b a n d 1.58 p b for g g F a n d V B F re sp ectiv ely [34]. T h e u n c e rta in ty o n th e gg F cross sectio n has a p p ro x im a te ly eq u a l c o n trib u tio n s from Q C D scale v a ria tio n s (7.5% ) an d P D F s (7.2% ).

F o r th e V B F p ro d u c tio n , th e u n c e rta in ty o n th e cross sec tio n is 2.7% , m a in ly from P D F

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v a ria tio n s. T h e W H a n d Z H processes are m od elled w ith P y t h i a8 a n d n o rm alised to cross sections of 0.70 p b a n d 0.42 p b respectively, c a lc u la te d a t N N L O in Q C D a n d N LO in th e electro w eak co upling s [34]. T h e u n c e rta in ty is 2.5% on th e W H cross sec tio n an d 4.0% o n th e Z H cross sectio n.

F o r all of th e b a c k g ro u n d processes, w ith th e ex c e p tio n of W + je ts a n d m u ltije t ev ents, M C sim u la tio n is u sed to m o d el ev e n t k in e m a tic s a n d as a n in p u t to th e b a c k g ro u n d n o rm a l­

isa tio n . T h e W + je ts a n d m u ltije t b a c k g ro u n d m od els are d eriv e d from d a t a as d esc rib ed in sec tio n 5 . F or th e d o m in a n t W W a n d to p -q u a r k b ac k g ro u n d s, th e M C g e n e ra to r is P o w H E g + P y t h i a6 [35] (version 6.426), also w ith C T 1 0 for th e in p u t P D F s . T h e P e ru g ia 2011 p a r a m e te r set is u sed for P y t h i a6 [36]. F o r th e W W b a c k g ro u n d w ith N jet > 2, to b e t t e r m odel th e a d d itio n a l p a rto n s , th e S h e r p a [37] p ro g ra m (versio n 1.4.3) w ith th e C T 1 0 P D F set is u sed. T h e D rell-Y an b ac k g ro u n d , in clu d in g Z / q * ^ t t , is sim u la te d w ith th e ALPgEN [38] p ro g ra m (version 2.14). I t is in terfa ced w ith HERW ig [39] (version 6.520) w ith p a ra m e te rs set to th o se of th e A TLA S U n d e rly in g E v e n t T u n e 2 [40] a n d uses th e C T E Q 6 L 1 [41] P D F set. T h e sam e c o n fig u ra tio n is ap p lied for W q ev en ts. E v e n ts in th e Z /q * sam p le a re rew eig h ted to th e M R S T m c a l P D F set [42]. F o r th e W q * a n d Z / q b ac k g ro u n d s, th e S h e r p a p ro g ra m is u sed, w ith th e sam e version n u m b e r a n d P D F set as th e W W b ac k g ro u n d w ith > 2 je ts . A d d itio n a l d ib o so n b a c k g ro u n d s, from W Z a n d Z Z , a re m o d elled u sin g P o w H E g + P y t h i a8.

F o r all M C sam ples, th e A T L A S d e te c to r re sp o n se is s im u la te d [43] u sin g e ith e r G e a n t 4 [44] or G e a n t 4 co m b in ed w ith a p a ra m e te rise d GEAN T4-based c a lo rim e te r sim ­ u la tio n [45]. M u ltip le p ro to n -p ro to n (p ile-u p ) in te ra c tio n s are m o d elled by overlay ing m in im u m -b ia s in te ra c tio n s g e n e ra te d u sin g P y t h i a8.

4 E v e n t s e l e c t i o n

T h is sec tio n d esc rib es th e re c o n stru c tio n -le v e l d efin itio n of th e sig nal region. T h e d efin itio n o f physics o b je c ts re c o n s tru c te d in th e d e te c to r follows t h a t o f ref. [19] e x a c tly an d is su m m a rise d here. All o b je c ts a re d efined w ith re s p e c t to a p rim a ry in te ra c tio n v erte x , w hich is re q u ire d to have a t le a st th re e a sso c ia te d tra c k s w ith p T > 400 MeV. If m ore th a n one such v e rte x is p re se n t, th e one w ith th e la rg e st value o f XXPt), w h ere th e sum is over all tra c k s asso c ia te d w ith t h a t v erte x , is selected as th e p rim a ry v ertex .

4 .1 O b je c t r e c o n s t r u c tio n a n d id e n tific a tio n

E le c tro n c a n d id a te s are b u ilt from c lu ste rs o f en e rg y d e p o sitio n s in th e E M c a lo rim e te r w ith a n a sso c ia te d w e ll-re c o n stru c te d tra c k . T h e y a re re q u ired to have E T > 10 GeV, w here th e tra n s v e rs e en e rg y E T is defined as E s in ( 0 ) . E le c tro n s re c o n s tru c te d w ith | n | < 2.47 a re used, ex c lu d in g 1.37 < | n | < 1.52, w hich c o rre sp o n d s to th e tra n s itio n regio n b etw een th e b a rre l an d th e e n d c a p c a lo rim e te rs. A d d itio n a l id e n tific a tio n c r ite ria a re a p p lie d to re je ct b a c k g ro u n d , u sin g th e c a lo rim e te r show er sh ap e , th e q u a lity o f th e m a tc h b etw een th e tra c k a n d th e c lu ste r, a n d th e a m o u n t of tra n s itio n ra d ia tio n e m itte d in th e ID [46­

48]. F or ele c tro n s w ith 10 GeV < E T < 25 GeV, a lik elih o o d -b ased e le c tro n selection a t th e “v ery t i g h t” o p e ra tin g p o in t is u sed for its im p ro v ed b a c k g ro u n d reje ctio n . F o r

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Et > 25 GeV, a m ore efficient “m e d iu m ” selectio n is u sed b e c au se b a c k g ro u n d is less o f a co n cern . T h e efficiency of th e s e re q u ire m e n ts varies s tro n g ly as a fu n c tio n o f Et, s ta rtin g from 6 5-70% for Et < 25 GeV, ju m p in g to a b o u t 80% w ith th e ch a n g e in id e n tific a tio n c r ite ria a t Et = 25 GeV, a n d th e n ste a d ily in cre asin g as a fu n c tio n of Et [47].

M u o n c a n d id a te s a re selected from tra c k s re c o n s tru c te d in th e ID m a tc h e d to tra c k s re c o n s tru c te d in th e m u o n s p e c tro m e te r. T racks in b o th d e te c to rs a re re q u ire d to have a m in im u m n u m b e r of h its to en su re ro b u s t re c o n stru c tio n . M u on s a re re q u ire d to have

| n | < 2.5 a n d p T > 10 GeV. T h e re c o n s tru c tio n efficiency is b etw e en 96% a n d 98%, an d s ta b le as a fu n c tio n of p T [49].

A d d itio n a l c r ite ria are a p p lie d to e le c tro n s a n d m u on s to re d u ce b a c k g ro u n d s from n o n -p ro m p t le p to n s an d e le c tro m a g n e tic sig n a tu re s p ro d u c e d by h a d ro n ic ac tiv ity . L e p to n iso la tio n is defined u sin g tra c k -b a se d a n d c a lo rim e te r-b a se d q u a n titie s . All iso latio n v aria b les used are n o rm alised re la tiv e to th e tra n s v e rs e m o m e n tu m of th e le p to n , an d a re o p tim ise d for th e H ^ W W * ^ e v ^ v an aly sis, re s u ltin g in s tric te r c r ite ria for b e tte r b a c k g ro u n d re je c tio n a t low er p x a n d looser c r ite ria for b e tte r efficiency a t h ig h er p x . Sim ilarly, re q u ire m e n ts on th e tra n s v e rs e im p a c t-p a ra m e te r significance d0/ ^ do an d th e lo n g itu d in a l im p a c t p a r a m e te r z 0 a re m ad e. T h e efficiency o f th e iso la tio n an d im p a c t-p a ra m e te r re q u ire m e n ts for elec tro n s satisfy in g all of th e id e n tific a tio n c rite ria re q u ire m e n ts ra n g es from 68% for 10 GeV < E t < 15 GeV to g re a te r th a n 90% for e le c tro n s w ith E t > 25 GeV. F or m uons, th e eq u iv ale n t efficiencies are 60-96 % .

J e ts are re c o n s tru c te d from to p o lo g ic al c lu ste rs of c a lo rim e te r cells [50- 52] usin g th e a n ti-k t a lg o rith m w ith a ra d iu s p a r a m e te r of R = 0.4 [53]. J e t energies a re c o rre c te d for th e effects of c a lo rim e te r n o n -c o m p e n sa tio n , signal losses d u e to noise th re s h o ld effects, en erg y lost in n o n -in s tru m e n te d regions, c o n trib u tio n s fro m in -tim e a n d o u t-o f-tim e p ile-u p, an d th e p o sitio n of th e p rim a ry in te ra c tio n v e rte x [50, 54]. S u b sequ ently , th e je ts a re c a lib ra te d to th e h a d ro n ic en e rg y scale [50, 55]. To re d u ce th e ch an ce of u sin g a j e t p ro d u c e d by a p ile-u p in te ra c tio n , je ts w ith w ith p T < 50 GeV a n d |n| < 2.4 are re q u ire d to have m ore th a n 50% of th e sc a la r sum o f th e p T o f th e ir asso c ia te d tra c k s com e from tra c k s asso c ia te d w ith th e p rim a ry v erte x . J e ts used for d e fin itio n of th e sig nal reg ion a re re q u ire d to have p T > 25 GeV if | n | < 2.4 a n d p T > 30 GeV if 2.4 < | n | < 4.5.

J e ts c o n ta in in g b -h ad ro n s are iden tified u sin g a m u ltiv a ria te b-tag g in g a lg o rith m [56, 57]

w hich com bines im p a c t-p a ra m e te r in fo rm a tio n of tra c k s a n d th e re c o n s tru c tio n of ch a rm - a n d b o tto m - h a d ro n d ecays. T h e w orkin g p o in t, chosen to m ax im ise to p -q u a r k b a c k g ro u n d re je ctio n , h as a n efficiency of 85% for b-jets a n d a m is-ta g r a te for lig h t-flav o u r je ts (ex­

clu d in g je ts from c h a rm q u a rk s) of 10.3% in s im u la te d t t events.

M issing tra n s v e rs e m o m e n tu m ( p p iss) is p ro d u c e d in sign al ev en ts by th e tw o n e u trin o s from th e W b o so n decays. It is re c o n s tru c te d as th e n eg a tiv e v e c to r su m o f th e tra n s v e rs e m o m e n ta of m uons, elec tro n s, p h o to n s, je ts , a n d tra c k s w ith p T > 0.5 GeV a sso c ia te d w ith th e p rim a ry v e rte x b u t u n a s so c ia te d w ith an y o f th e p re v io u s o b je c ts.

4 .2 S ig n a l r e g io n s e le c tio n

E v e n ts a re selected from th o s e w ith e x a c tly one e le c tro n a n d o ne m u o n w ith o p p o site ch arg e, a d ile p to n in v a ria n t m ass m u g re a te r th a n 10 GeV, a n d pîpiss > 20 GeV. A t least

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

one of th e tw o lep to n s is re q u ire d to have p T > 22 GeV a n d th e le p to n w ith h ig h er p T is referred to as th e lead in g le p to n . T h e o th e r ( “s u b le a d in g ” ) le p to n is re q u ired to have p T > 15 GeV. All ev en ts a re re q u ired to p ass a t least one sin g le-lep to n o r d ile p to n trig g e r.

T h e Level-1 p T th re sh o ld s for th e sin g le-lep to n trig g e rs a re 18 GeV a n d 15 GeV fo r e lec tro n s a n d m uons, respectively. T h e H L T uses o b je c t re c o n s tru c tio n a n d c a lib ra tio n s close to th o se used offline, a n d th e e le c tro n a n d m u o n trig g e rs b o th have th re s h o ld s a t 24 GeV a n d a n iso latio n re q u ire m e n t. To reco ver efficiency, a s u p p o rtin g trig g e r w ith no iso latio n re q u ire m e n t b u t h ig h er p T th re sh o ld s, 60 GeV for e lec tro n s a n d 36 GeV for m uo ns, is used.

T h e d ile p to n trig g e r re q u ires a n e le c tro n a n d a m u o n abo ve a th re s h o ld of 10 GeV an d 6 GeV, respectively, a t Level-1, a n d 12 GeV a n d 8 GeV in th e H L T . T h is increases th e signal efficiency by in clu d in g ev e n ts w ith a lead in g le p to n below th e th re s h o ld im p o sed by th e sin g le-lep to n trig g e rs b u t still o n th e p la te a u o f th e d ile p to n trig g e r efficiency. T h e re c o n s tru c te d lep to n s a re re q u ire d to m a tc h th o s e firing th e trig g e r. T h e to ta l p er-ev e n t trig g e r efficiencies for ev e n ts w ith N jet = 0 a re 96% for ev e n ts w ith a lead in g e le c tro n an d 84% for ev e n ts w ith a lead in g m uon. T h e efficiency increases w ith in cre asin g je t m u ltip licity , u p to 97% for ev e n ts w ith a lead in g e le c tro n a n d 89% for ev en ts w ith a lead in g m uon.

T h re e n o n -o v e rlap p in g signal reg ion s a re defined, d is tin g u is h e d by th e n u m b e r of re c o n s tru c te d je ts : N jet = 0, N jet = 1, o r N jet > 2. T h ese s e p a ra te th e d a t a in to signal regions w ith d ifferen t b ac k g ro u n d c o m p o sitio n s, w h ich im p ro ves th e se n sitiv ity of th e an aly sis. T h e d o m in a n t b a c k g ro u n d processes are W W p ro d u c tio n for N jet = 0 , to p -q u a rk p ro d u c tio n for N jet > 2, a n d a m ix tu re of th e tw o for N jet = 1. F o r je t m u ltip lic itie s above tw o, th e n u m b e r of ev e n ts d ec reases w ith in cre asin g n u m b e r of je ts b u t th e b a c k g ro u n d co m p o sitio n re m a in s d o m in a te d by to p -q u a r k p ro d u c tio n , so th e s e ev e n ts are all co llected in th e N jet > 2 sign al region.

T h e signal regions are b ase d on th e selectio n u sed for th e g g F an a ly sis of ref. [19], w ith m o d ificatio n s to im prove th e sig n a l-to -b a c k g ro u n d ra tio , a n d to ac c o u n t for th e tr e a tm e n t o f V B F a n d V H as b ac k g ro u n d s. T h e fo rm e r includ es th e in crease in th e su b le a d in g le p to n p T th re s h o ld a n d th e exclu sion o f sam e-flav our ev ents, to re d u ce b a c k g ro u n d from W + je ts a n d D rell-Y an ev en ts, respectively.

T h e selectio n c r ite ria a re su m m a rise d in ta b le 1. T h e 6-je t v e to uses je ts w ith p T >

20 GeV a n d |nje t| < 2.4, an d re je c ts to p -q u a rk b a c k g ro u n d in th e N jet = 1 a n d N jet > 2 ca te g o ries. B a c k g ro u n d from Z /y * ^ t t a n d m u ltije t ev e n ts is re d u ced in th e N jet = 0 c a te g o ry w ith a re q u ire m e n t on th e tra n s v e rs e m o m e n tu m o f th e d ile p to n sy stem , p T >

30 GeV. In th e N jet = 1 category, th is is acco m p lish ed in p a r t by re q u ire m e n ts o n th e single­

le p to n tra n s v e rs e m ass m T , defined for each le p to n as m T = ^ 2(p™sspT — p T ' P™iss). A t le a st on e of th e tw o le p to n s is re q u ire d to have m T > 50 GeV. F o r Z / y * ^ t t b a c k g ro u n d ev e n ts in th e N jet = 1 a n d N jet > 2 ca teg o ries, th e p T of th e t t sy stem is larg er, so th e co llin ear a p p ro x im a tio n is u sed to c a lc u la te th e t t in v a ria n t m ass m TT [58]. A re q u ire m e n t t h a t m TT a t m Z — 25 GeV su p p resses m o st b a c k g ro u n d from Z / y * ^ t t . S electio n t h a t re je c ts Z /y * ^ t t ev en ts also re je c ts H ^ t t ev en ts, w hich are k in e m a tic a lly sim ilar. T h e V B F v eto in th e N jet > 2 signal reg ion rem oves ev e n ts in w hich th e tw o lead in g je ts have a n in v a ria n t m ass m j j > 600 GeV a n d a ra p id ity s e p a ra tio n A y jj > 3.6, w hich re je c ts a b o u t 40% of V B F ev en ts b u t o n ly 5% of gg F events.

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Category CD O Njet = 1 Njet > 2

Preselection

Two isolated leptons ( t = e, p) with opposite charge pTad > 22 GeV, pT blead > 15 GeV

m u > 10 GeV p !fi33 > 20 GeV

Background rejection - Nb-jet = 0 Nb-jet = 0

A ^ (£ £ ,p !fi33) > 1.57 max(mT) > 50 GeV -

p T > 30 GeV mTT < m Z — 25 GeV mTT < m Z — 25 GeV

VBF veto - - m jj < 600 GeV or A y j j < 3.6

H ^ W W £ v £ v m u < 55 GeV

topology A0££ < 1.8

85 GeV < m T < 125 GeV

T a b le 1. Event selection criteria used to define the signal regions in the H ^ W W * ^ evp,v dif­

ferential cross section m easurem ents. The preselection and signal-topology selection criteria are identical across all signal regions. The background rejection and VBF-veto selection depend on Njet, and a dash (‘- ’) indicates th a t no selection is applied. Definitions including the p T thresholds for je t counting are given in the text.

U p p e r b o u n d s o n m u a n d th e a z im u th a l ang le b etw e en th e lep to n s A ÿ u ta k e a d v a n ta g e of th e u n iq u e k in e m a tic s o f th e H ^ W W * d ecay to d isc rim in a te b etw een th e s e signal ev e n ts a n d th e c o n tin u u m W W b a c k g ro u n d . T h e sp in -zero n a tu re of th e H iggs boson, to g e th e r w ith th e s tr u c tu r e o f th e w eak in te ra c tio n in th e W b o son decays, p re fe re n tia lly p ro d u c e s le p to n s p o in tin g in to th e sam e h em isp h ere o f th e d e te c to r. T h e sm all d ile p to n in v a ria n t m ass is a co n seq u en ce of t h a t a n d th e fa ct t h a t m H < 2m W , w hich forces one of th e tw o W b o so ns off-shell, re s u ltin g in low er le p to n m o m e n ta in th e cen tre-o f-m ass fram e of th e H iggs b o so n decay.

S ignal ev e n ts are p ea k ed in th e d is trib u tio n o f th e tra n s v e rs e m ass m T , defined as

m T = ^ j( E T + P^ 188) 2 - | P t + P miss|2, (4.1) w here

E t = ^ / | P t!2 + m ^ . (4.2)

F ig u re 1 show s th e m T d is trib u tio n a fte r a p p lic a tio n of all o th e r selectio n c r ite ria in each o f th e signal regions. S electing ev en ts w ith 85 GeV < m T < 125 GeV in creases th e signal regio n p u rity a n d m in im ises th e to ta l u n c e rta in ty of th is m e a su re m e n t of th e g g F cross sectio n . R em o v in g ev en ts w ith m T > m H also red u ces th e effect of in terfe ren c e w ith th e c o n tin u u m W W p rocess to negligible levels co m p a re d to th e o b serv ed ev en t yield [32].

T h e d is trib u tio n s to be m e a su re d a re b u ilt u sin g th e sam e lep to n s, je ts , a n d pipiss t h a t e n te r th e ev en t selection . T h e pT of th e H iggs bo so n (pH ) is re c o n s tru c te d as th e v e c to r sum of th e m issing tra n s v e rs e m o m e n tu m a n d th e p t of th e tw o lep to n s:

pH = |pTad + pTublead + p miss |. (4.3)

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

pH [ GeV]: [0-20], [20-60], [60-300]

|y « |: [0.0-0.6], [0.6-1.2], [1.2-2.5]

pT1 [ GeV]: [0-30], [30-60], [60-300]

T a b le 2. Bin edges for the reconstructed and unfolded distributions.

T h e ra p id ity of th e d ile p to n sy stem |y ^ | is re c o n s tru c te d from th e ch a rg ed le p to n fo u r m o ­ m e n ta . T h e re c o n s tru c te d a n d u n fo ld ed d is trib u tio n s are b in n e d u sin g th e b in edges defined in ta b le 2 . T h e bin edges a re d e te rm in e d by b a la n c in g th e e x p e c te d s ta tis tic a l a n d sy ste m ­ a tic u n c e rta in tie s in each bin. T h e re so lu tio n of th e v aria b les is sm aller th a n th e b in size an d do es n o t affect th e b in n in g choice. F or each d is trib u tio n , th e u p p e r edge of th e h ig h est b in is chosen so t h a t less th a n 1% of th e e x p e c te d ev en t yield in th e fiducial reg ion is ex clu ded.

5 B a c k g r o u n d e s t i m a t i o n

I m p o r ta n t b a c k g ro u n d pro cesses for th is an a ly sis are W W , ti , single to p -q u a rk , Z /y * ^ t t , W + j e t s , a n d d ib o so n processes o th e r th a n W W , co llectiv ely re ferred to as “O th e r V V ” a n d in clu d in g Wy*, Wy, W Z , a n d Z Z ev en ts. T h e b a c k g ro u n d e s tim a tio n te c h n iq u e s are d e sc rib e d in d e ta il in ref. [19] a n d briefly here. T h e n o rm a lis a tio n s tra te g y is su m m arised in ta b le 3 . As m uch as possible, b ac k g ro u n d s a re e s tim a te d u sin g a c o n tro l regio n (C R ) en ric h ed in th e ta r g e t b a c k g ro u n d a n d o rth o g o n a l to th e signal regio n (S R ), b ec au se th e s ta tis tic a l a n d e x tra p o la tio n u n c e rta in tie s a re sm aller th a n th e ty p ic a l u n c e rta in tie s asso ­ c ia te d w ith ex p licit p re d ic tio n of th e yield s in exclusive N jet ca te g o ries. T h e b a c k g ro u n d e s tim a te s d o n e in th e C R s a re e x tra p o la te d to th e S R u sin g e x tra p o la tio n fa c to rs ta k e n from sim u la tio n . T h e c o n tro l region d efin itio n s are su m m a rise d in ta b le 4 , a n d in clu d e th e low er su b le a d in g le p to n p T th re sh o ld o f 10 GeV for all c o n tro l regions ex c e p t th e one for W W . T h is is d o n e b ec au se th e g ain in s ta tis tic a l precisio n o f th e re su ltin g b a c k g ro u n d e s tim a te s is la rg e r th a n th e in cre ase of th e s y s te m a tic u n c e rta in tie s on th e e x tra p o la tio n fa cto rs, p a rtic u la rly for th e Z /y * ^ t t a n d V V processes.

F o r all k in e m a tic d is trib u tio n s , ex c e p t N je t, th e sh ap e s a re d eriv ed from d a t a for th e W + je ts a n d m u ltije t b ac k g ro u n d s, a n d from th e M C -sim u la te d b ac k g ro u n d sam p les fo r all o th e r processes. B eca u se th e signal regions are d efined in te rm s of N je t, th e N jet d is trib u tio n is d e te rm in e d d ire c tly in each b in by th e sum of th e b a c k g ro u n d p re d ic tio n s. T h e o re tic a l a n d e x p e rim e n ta l u n c e rta in tie s a re e v a lu a te d for all M C -sim u la tio n -d e riv ed sh a p e s an d in clu d ed in th e an aly sis, as d e sc rib e d in sec tio n 8.

T h e c o n trib u tio n to th e sig n al regio n from th e V B F a n d V H H iggs b o so n p ro d u c tio n m odes, a n d all c o n trib u tio n s from H ^ t t decays, a re tr e a te d as a b ac k g ro u n d a ssu m in g th e S ta n d a rd M odel cross sectio n, b ra n c h in g ra tio , a n d a c c e p ta n c e for m H = 125 GeV.

T h e c o n trib u tio n o f H ^ t t ev en ts is negligible d u e to th e selectio n c r ite ria re je c tin g t t

ev en ts. T h e la rg e st c o n trib u tio n from all n o n -g g F H iggs b o so n p ro cesses is in th e N jet > 2 category , in w hich ev en ts from V B F a n d V H c o n trib u te a b o u t h a lf th e n u m b e r o f events

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J H E P 0 8 ( 2 0 1 6 ) 1 0 4

250 eV]

F ig u r e 1. Observed distributions of m T with signal and background expectations after all other selection criteria have been applied for the Njet = 0 (top left), Njet = 1 (top right) and Njet > 2 (bottom ) signal regions. The background contributions are normalised as described in section 5.

The SM Higgs boson signal prediction shown is summed over all production processes. The hatched band shows the sum in q uadrature of statistical and system atic uncertainties of the sum of the backgrounds. The vertical dashed lines indicate the lower and upper selection boundaries on m T at 85 and 125 GeV.

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

C h an n el W W T op Z /y* ^ TT Z /y* ^ e e / ^ W + j e t s / m u lt i j e t O th e r V V

N jet = 0 C R C R C R M C D a ta C R

N jet = 1 C R C R C R M C D a ta C R

Njet > 2 M C C R C R M C D a ta M C

T a b le 3. Summ ary of background-estim ation procedures for the three signal regions. Each back­

ground is categorised according to w hether it is norm alised using a control region (CR), a fully data-derived estim ate (D ata), or the theoretical cross section and acceptance from sim ulation (MC).

C R N jet = 0 N jet = 1 Njet > 2

W W 55 < m u < 110 GeV A i f i u < 2.6

psubtead > 15 GeV

m u > 80 GeV

|m TT — m Z | > 25 GeV psubiead > 15 GeV 6-je t v eto

max(mSS) > 50 GeV T op q u a rk N o N jet re q u ire m e n t

A ^ u < 2 .8

> 16-je t re q u ired m u > 80 GeV 6-je t v eto T op q u a rk aux. N o N jet re q u ire m e n t Njet = 2

> 1 6-je t re q u ired > 16-je t re q u ired - O th e r V V S am e-sign lep to n s

All SR c u ts

S am e-sign lep to n s A ll SR c u ts

-

z/y * ^ t t m u < 80 GeV m u < 80 GeV m u < 70 GeV A ^ u > 2 .8 m TT > m Z — 25 GeV

6-je t v eto

A ^ u > 2 .8 6-je t v eto

T a b le 4. Event selection criteria used to define the control regions. Every control region sta rts from the same basic charged lepton and pmiss selection as the signal regions except th a t the subleading lepton p T threshold is lowered to 10 GeV unless otherwise stated. Jet-m ultiplicity requirem ents also m atch the corresponding signal region, except where noted for some top-quark control regions.

The “top quark aux.” lines describe auxiliary d a ta control regions used to correct the norm alisation found in the m ain control region. Dashes indicate th a t a particular control region is not defined.

The definitions of m TT, m ^, and the je t counting p T thresholds are as for the signal regions.

t h a t g g F does, a n d c o n s titu te a b o u t 3% of th e to ta l b a c k g ro u n d . T h e N jet d is trib u tio n a n d o th e r sh a p e s are ta k e n from sim u la tio n .

F o r th e N jet = 0 a n d N jet = 1 ca te g o ries, th e W W b a c k g ro u n d is n o rm alised u sin g co n ­ tro l regions d istin g u is h e d from th e SR p rim a rily by m ^ , a n d th e s h a p e is ta k e n from sim u ­ la te d ev e n ts g e n e ra te d usin g P o w h e g + P y t h i a6 as d e sc rib e d in sec tio n 3 . F or th e N jet > 2 category , W W is n o rm alised usin g th e N L O cross sec tio n c a lc u la te d w ith M C F M [59]. T h e efficiency for th e N jet > 2 re q u ire m e n t a n d o th e r SR selectio n s is ta k e n from M C sim ula-

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J H E P 0 8 ( 2 0 1 6 ) 1 0 4

Figure 2. Observed distributions of (a) |y«| in the Njet = 0 W W CR and (b) pH in the Njet = 1 W W CR, w ith signal and background expectations. Relevant background norm alisation factors have been applied. The SM Higgs boson signal prediction shown is summed over all production processes. The hatched band in the upper panel and the shaded band in the lower panel show the sum in quadrature of statistical and system atic uncertainties of the prediction.

tion, for which the

generator is used. It is LO in QCD but has matrix elements implemented for WW + N jets, for 0 < N < 3. For all N

categories, WW ^ ^v^v back­

ground events produced by double parton scattering are normalised using the predicted cross section times branching ratio of 0.44 ± 0.26 pb [19]. The acceptance is modelled at LO using events generated by P

8. The |y^| distribution in the N

= 0 WW CR and the pH distribution in the N

= 1 W W CR are shown in figure 2.

The top-quark background normalisation is estimated using control regions for all N

, and the shapes of the distributions other than N

are taken from MC simulation.

The tt and single-top (i.e. Wt) backgrounds are treated together and the normalisation factor determined from the CR yield is applied to their sum. In the N

= 0 category, the normalisation is derived from an inclusive sample of events meeting all of the lepton and p

preselection criteria but with no requirements on the number of jets, in which the majority of events contain top quarks. The efficiency of the N

= 0 signal region selection is modelled using MC simulation. To reduce the uncertainty on the efficiency of the jet veto, the fraction of b-tagged events which have no additional jets is measured in a data sample with at least one b-tagged jet and compared to the fraction predicted by simulation. The efficiency of the jet veto is corrected by the square of the ratio of the measured fraction over the predicted one to account for the presence of two jets in rt production. In the N

= 1 category, the normalisation of the top-quark background is determined from a control region distinguished from the signal region by requiring that the jet is b-tagged. To reduce the effect of b-tagging systematic uncertainties, the extrapolation factor from the CR to the SR is corrected using an effective b-jet tagging scale factor derived from a control

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

Figure 3. Observed distributions of (a) pT in the Njet = 1 top-quark CR and (b) p T in the Njet > 2 top-quark CR, w ith signal and background expectations. Relevant background norm alisation factors have been applied. The SM Higgs boson signal prediction shown is summed over all production processes. The hatched band in the upper panel and the shaded band in the lower panel show the sum in quadrature of statistical and system atic uncertainties of the prediction.

region with two jets, at least one of which is 6-tagged. In the Njet > 2 category, the number of top-quark events is sufficiently large th at a CR with a 6-jet veto can be defined using

in the Njet > 2 top-quark CR are shown in figure 3.

The W + je ts background contribution is estimated using a control sample of events in which one of the two lepton candidates satisfies the identification and isolation criteria used to define the signal sample (these lepton candidates are denoted “fully identified”), and the other ( “anti-identified”) lepton fails to meet the nominal selection criteria but satisfies a less restrictive one. Events in this sample are otherwise required to satisfy all of the signal-region selection criteria. The W + jets contamination in the SR is determined by scaling the number of events in the control sample by an extrapolation factor measured in a Z + jets data sample. The extrapolation factor is the ratio of the number of fully identified leptons to the number of anti-identified leptons, measured in bins of anti-identified lepton pT and n. To account for differences between the composition of jets associated with W- and Z-boson production, the extrapolation factors are measured in simulated W + jets and Z + je ts events. The ratio of the two extrapolation factors is applied as a multiplicative correction to the extrapolation factor measured in the Z + jets data. The background due to multijet events is determined similarly to the W + je ts background, using a control sample that has two anti-identified lepton candidates, but otherwise satisfies the SR selection criteria. The extrapolation factor is constructed from data events dominated by QCD-produced jet activity, and is applied to both anti-identified leptons.

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Figure 4. Observed distributions of (a) |y«| in the Nj e t = 0 same-sign (V V ) CR and (b) pH in the N^{j e t} > 2 ^{Z / y}* ^ ^{t t} CR, with signal and background expectations. Relevant background norm alisation factors have been applied. The SM Higgs boson signal prediction shown is summed over all production processes. The hatched band in the upper panel and the shaded band in the lower panel show the sum in qu adrature of statistical and system atic uncertainties of the prediction.

The background from diboson processes other than W W , primarily from

*,

, and W Z events, is normalised in the N

= 0 and N

= 1 categories using a control region identical to the signal region except th at the leptons are required to have the same sign.

The number and properties of same-sign and opposite-sign dilepton events produced by

W y

^ and W Z are almost identical. In the N

> 2 analysis, this same-sign sample is too small to be used as a control region, and the background is estimated from the predicted inclusive cross sections and MC acceptance alone. For all N

, the MC simulation is used to predict the shapes of the distributions to be unfolded. Figure 4(a) shows the distribution of |y^| in the N

= 0 same-sign control region.

The

* ^

background normalisation is derived from control regions, and the shape is derived from MC, for all three signal regions. The small contributions from

Z /y

* ^ ee and

* ^ ^ , including

, are estimated from MC simulation and the predicted cross sections, as described in section 3. Figure 4(b) shows the distribution of pH in the

* ^

control region with N

> 2.

Each control region is designed for the calculation of a normalisation factor (NF) for a particular target process, The NF is defined as (N — B ')/B , where N is the number of data events observed in the control region, B is the expected background yield in the CR for the target process based on the predicted cross section and acceptance from MC simulation, and B 1 is the predicted yield from other processes in the control region. The CRs have a small contribution from the signal process, which is normalised to the SM expectation.

The effect of this choice is negligible. The normalisation of each background associated with a CR is scaled by the corresponding NF. All NFs used are given in table 5, along

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

C o n tro l R egions W W T op z / y * ^ t t O th e r V V N jet = 0 1.22 ± 0.03 1.08 ± 0.02 0.99 ± 0.02 0.92 ± 0.07 N jet = 1 1.05 ± 0.05 1.06 ± 0.02 1.06 ± 0.04 0.96 ± 0.12

Njet > 2 - 1.05 ± 0.03 1.00 ± 0.09 -

T a b le 5. Background norm alisation factors (NFs) obtained from the control regions, for different background contributions and N jet categories. The uncertainty quoted is the statistical uncertainty;

system atic uncertainties on the predicted yield, not shown, restore com patibility of the NF with unity b u t do not directly enter the analysis because th ey are replaced by extrapolation uncertainties.

A dash (‘- ’) indicates th a t there is no control region corresponding to th a t background.

w ith th e ir s ta tis tic a l u n c e rta in tie s. T h ese are in clu d ed in th e s ta tis tic a l u n c e rta in tie s of th e final re su lts. T h e value of th e N jet = 0 W W N F h as b een s tu d ie d in d e ta il [19]; its d e v ia tio n fro m u n ity is d u e to th e m o d e llin g o f th e je t v e to a n d h ig h e r-o rd e r c o rrec tio n s o n th e p re d ic tio n o f th e W W cross sectio n . A new er c a lc u la tio n of th e inclusive W W cross sectio n , w ith N N L O precisio n in a S [60], m oves th e N F clo ser to u nity, c o m p a re d to th e one show n here, as d e sc rib e d in ref. [61].

6 R e c o n s t r u c t e d y i e l d s a n d d i s t r i b u t i o n s

T h e n u m b e rs of e x p e c te d a n d o b serv ed ev e n ts sa tisfy in g all of th e sig nal regio n selectio n c r ite ria are show n in ta b le 6. T h e n u m b e rs of e x p e c te d sig nal a n d b a c k g ro u n d events a re also show n, w ith all d a ta -d riv e n c o rre c tio n s a n d n o rm a lisa tio n fa c to rs ap p lied . In each category, th e b a c k g ro u n d -s u b tra c te d n u m b e r o f ev en ts, co rre sp o n d in g to th e o b serv ed yield o f signal ev en ts, is sig n ifican tly d ifferen t from zero. T ak in g in to ac c o u n t th e t o ta l s ta tis tic a l a n d s y s te m a tic u n c e rta in tie s, th e s e yields a re in a g reem en t w ith th o se re p o rte d in ref. [19]

a n d w ith e x p e c ta tio n s from S M H iggs b o so n p ro d u c tio n th ro u g h glu o n fusion.

T h e fo u r d is trib u tio n s u n d e r stu d y : N jet, pH (re c o n s tru c te d as p T (£tpm iss)), |y « |, an d pH a re show n in figure 5 . F o r p re s e n ta tio n p u rp o se s, th e re c o n s tru c te d d is trib u tio n s are co m b in ed over th e th re e signal regions, w ith th e u n c e rta in tie s co m b in ed a c c o u n tin g for c o rre la tio n s. In th e pT1 d is trib u tio n , N jet = 0 ev e n ts a re all in th e first bin, pT1 < 30 GeV, by c o n s tru c tio n b ec au se of th e d e fin itio n o f th e je t c o u n tin g . T h e c o m p o sitio n of th e b ack ­ g ro u n d is show n, to illu s tra te how it varies as a fu n c tio n of th e q u a n titie s b ein g m easu red . T h e W W b a c k g ro u n d d ecreases as a fu n c tio n of th e n u m b e r of je ts , a n d th e to p -q u a rk b a c k g ro u n d in creases, as ca n also b e seen in ta b le 6. F o r th e p H a n d pT1 d istrib u tio n s , th e W W b a c k g ro u n d d ec reases w ith pT w hile th e to p -q u a rk b a c k g ro u n d in creases. T h e b a c k g ro u n d co m p o sitio n does n o t v ary s u b s ta n tia lly as a fu n c tio n of |y ^ |.

7 F i d u c i a l r e g io n a n d c o r r e c t i o n fo r d e t e c t o r e f f e c t s

E a c h o f th e re c o n s tru c te d d is trib u tio n s is c o rre c te d for d e te c to r effects a n d re so lu tio n to e x tr a c t th e d iffe ren tial cross sectio n s for th e g gF H iggs b o so n signal. A ll d iffe ren tial cross sectio n s a re show n in a fiducial region defined b ase d on o b je c ts a t p a rtic le level, to red u ce

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Figure 5. Observed distributions of (a) Njet, (b) p H , (c) \v u \, and (d) pT w ith signal and background expectations, combined over the Njet = 0 , = 1 , and > 2 signal-region categories. The background processes are normalised as described in section 5. The SM Higgs boson signal pre­

diction shown is summed over all production processes. In the pT distribution, Njet = 0 events are all in the first bin by construction because of the definition of the je t thresholds used to define the signal regions. The hatched band shows the sum in quad rature of statistical and system atic uncertainties of the sum of the backgrounds.

J H E P 0 8 ( 2 0 1 6 ) 1 0 4

Njet = 0 Njet = 1 Njet > 2 N o n -g g F H 2.2 ± 0.2 ± 0.2 7.1 ± 0.3 ± 0.5 8.2 ± 0.3 ± 0.4

W W 686 ± 19 ± 43 153 ± 7 ± 13 44 ± 1 ± 11

O th e r V V 88 ± 3 ± 12 44 ± 3 ± 11 21.6 ± 1.6 ± 3.3

Top 60.2 ± 1.5 ± 3.8 111.2 ± 2.7 ± 8.2 164 ± 2 ± 16

Z / 7 * 8.7 ± 2.3 ± 2.3 6.2 ± 1.3 ± 2.2 7.3 ± 1.5 ± 2.2

W + je ts 90 ± 2 ± 21 33.5 ± 2.0 ± 7.6 16.9 ± 1.2 ± 3.9

M u ltije t 1.3 ± 0.5 ± 0.5 0.7 ± 0.2 ± 0.3 0.9 ± 0.1 ± 0.4

T o ta l b ac k g ro u n d 936 ± 21 ± 41 355 ± 9 ± 12 263 ± 6 ± 9

O bserved 1107 414 301

O bserv ed — b a c k g ro u n d 171 ± 39 ± 41 59 ± 22 ± 12 38 ± 18 ± 9

g gF H 125.9 ± 0.4 ± 5.7 43.4 ± 0.2 ± 1.7 17.6 ± 0.2 ± 1.4

T a b le 6. Predicted and observed event yields in the three signal regions. Predicted num bers are given w ith their statistical (first) and system atic (second) uncertainties evaluated as described in section 8. The “Non-ggF H ” row includes the contributions from VBF and V H w ith H ^ W W* and from H ^ t t . The to tal background in the third-from -last row is the sum of these and of all other backgrounds.

th e m od el d e p e n d e n c e of th e re su lts. T h e p a rtic le o b je c ts an d th e d e fin itio n o f th e fid ucial region are d e sc rib e d in sec tio n 7 .1 . In sec tio n 7 .2 , th e c o rre c tio n p ro c e d u re is discu ssed . 7.1 D e f in it io n o f t h e fid u c ia l r e g io n

T h e fiducial selectio n is desig n ed to re p lic a te th e an a ly sis selectio n d esc rib ed in sec tio n 4 as closely as p ossible a t p a rtic le level, befo re th e s im u la tio n o f d e te c to r effects. In th is analy sis, m e a su re m e n ts are p erfo rm ed in th re e sig n al-reg io n c a te g o ries differing in th e n u m b e r of je ts in th e ev en t. In o rd e r to p re se n t re su lts w ith ev e n ts fro m all ca te g o ries, th e fidu cial selectio n o n ly app lies a selectio n co m m o n to all ca te g o ries a n d u sin g th e le p to n s a n d m issing tra n s v e rs e m o m e n tu m in th e final s ta te . T h e c r ite ria are su m m a rise d in ta b le 7 .

T h e fiducial selectio n is ap p lied to each p article-lev el le p to n , defined as a fin a l-sta te e le c tro n or m uon. H ere, e le c tro n s o r m u on s from h a d ro n d ecays a n d t d ecay s a re re je cted . T h e le p to n m o m e n ta are c o rre c te d by a d d in g th e m o m e n ta of p h o to n s, n o t o rig in a tin g from h a d ro n decays, w ith in a cone o f size A R = 0.1 a ro u n d each lep to n ; th e s e p h o to n s a rise p re d o m in a n tly from fin a l-s ta te -ra d ia tio n . S elected lep to n s are re q u ire d to satisfy th e sam e k in e m a tic re q u ire m e n ts as re c o n s tru c te d lep to n s. A selected ev en t h as e x a c tly tw o d ifferen t-flav o u r lep to n s w ith o p p o s ite charge.

T h e m issing tra n s v e rs e m o m e n tu m p p iss is defined as th e v e c to r sum of all fin a l-s ta te n e u trin o s ex c lu d in g th o se p ro d u c e d in th e d ecay s o f h a d ro n s a n d t’s.

P a rticle-le v el je ts are re c o n s tru c te d u sin g th e a n t i - k alg o rith m , im p le m e n te d in th e Fa s tJe t package [62], w ith a ra d iu s p a r a m e te r o f R = 0.4. F o r th e c lu ste rin g , all s ta b le p a rtic le s w ith a m e a n lifetim e g re a te r th a n 30 ps a re used, ex c e p t fo r elec tro n s, p h o to n s,

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J H E P 0 8 ( 2 0 1 6 ) 1 0 4

O b je c t s e le c tio n

E le c tro n s Pt > 15 GeV, |n| < 1.37 o r 1.52 < |n| < 2.47 M uons p T > 15 GeV, |n| < 2.5

J e ts p T > 25 GeV if |n| < 2.4, p T > 30 GeV if 2.4 < |n| < 4.5 E v e n t s e le c tio n

pT adW > 22 GeV P re se le c tio n m u > 10 GeV

pipiss > 20 GeV A f y u < 1.8 T opology

m u < 55 GeV

T a b le 7. Sum m ary of the selection defining th e fiducial region for the cross-section measurements.

The m om enta of the electrons and muons are corrected for radiative energy losses by adding the m om enta of nearby photons, as described in the text.

m uons, a n d n e u trin o s n o t o rig in a tin g from h a d ro n decays. S elected je ts are re q u ire d to have p T > 25 GeV if |n| < 2.4 or p T > 30 GeV if 2.4 < |n| < 4.5.

S elected ev e n ts p ass all p re se le c tio n re q u ire m e n ts in tro d u c e d in sec tio n 4 a n d th e H ^ W W e v p v to p o lo g y selectio n on a n d mu . T h e m T th re s h o ld s a re n o t a p ­ p lied in th e fiducial region since th e sh a p e o f th e m T d is trib u tio n a t re c o n s tru c tio n level differs sig n ifican tly from th e sh a p e o f th e d is trib u tio n a t p a rtic le level. All selectio n re ­ q u ire m e n ts a p p lie d a re su m m a rise d in ta b le 7. F o r a SM H iggs b o so n th e a c c e p ta n c e of th e fidu cial region w ith re sp e c t to th e full p h a se sp ace of H ^ W W e v p v is 11.3%.

7 .2 C o r r e c tio n for d e t e c t o r e ffe c ts

To e x tr a c t th e d iffe ren tial cross section s, th e m easu red d is trib u tio n s , show n in figure 5 , are c o rre c te d for d e te c to r effects a n d e x tra p o la te d to th e fidu cial region. F o r th e co rrec tio n s, th e re c o n s tru c te d d is trib u tio n s of th e d ifferent je t-b in n e d sign al-reg io n ca te g o ries a re n o t com b in ed , b u t in s te a d are s im u lta n e o u sly c o rre c te d for d e te c to r effects as a fu n c tio n of th e v a ria b le u n d e r s tu d y a n d th e n u m b e r of je ts . T h u s, th e c o rre la tio n o f th e v aria b le u n d e r s tu d y w ith N jet is c o rre c tly ta k e n in to ac c o u n t. F in a l re su lts a re p re se n te d in te g ra te d over all values of N jet for th e p H, |y « | a n d p T v ariables.

In th e follow ing, each b in of th e re c o n s tru c te d d is trib u tio n is re ferred to by th e in d ex j , w hile each bin of th e p artic le -lev el d is trib u tio n is re ferred to by th e in d ex i. T h e c o rre c tio n itse lf is d o n e as follows:

N part = ^ ^ M - ^ ^ f reco-only ^ ( j c o _ , (7.1)

3

w h ere N part is th e n u m b e r of p artic le -lev el ev e n ts in a given b in i o f th e p article-lev el d is trib u tio n in th e fiducial region. T h e q u a n tity N j eco is th e n u m b e r of re c o n stru c te d ev e n ts in a given b in j of th e re c o n s tru c te d d is trib u tio n in th e sign al region, a n d N Jbkg

J H E P 0 8 ( 2 0 1 6 ) 1 0 4