Search for Higgs boson pair production in the $WW^{(*)}WW^{(*)}$ decay channel using ATLAS data recorded at $\sqrt{s}=13$ 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: N o v e m b e r 28, 2018

R e v i s e d: A p r il 3, 2019 A c c e p t e d: A p r il 27, 2019 P u b l i s h e d: M a y 21, 2019

Search for Higgs boson pair production in the W W (*)W W (*) decay channel using A T L A S data recorded at = 13 T eV

E X P E R I M E N T

T h e A T L A S collaboration

E - m a i l : atlas.publications@cern.ch

A b s t r a c t : A sea rch for a p a ir of n e u tra l, sc a la r b o son s w ith each d ec ay in g in to tw o W bo sons is p re se n te d usin g 36.1 f b- 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 ­ erg y o f 13 T e V re co rd e d w ith th e A T L A S d e te c to r a t th e L a rg e H a d ro n C o llid er. T h is sea rch uses th re e p ro d u c tio n m o d els: n o n -re so n a n t a n d re so n a n t H iggs b o so n p a ir p ro ­ d u c tio n a n d re so n a n t p ro d u c tio n of a p a ir o f h eav y sc a la r p a rtic le s. T h re e final s ta te s, classified by th e n u m b e r of lep to n s, are an aly sed : tw o sam e-sig n lep to n s, th re e lep to n s, a n d fo u r lep to n s. N o sig nificant excess over th e e x p e c te d S ta n d a rd M odel b a c k g ro u n d s is o b served. A n o b serv ed (e x p ected ) 95% confidence-level u p p e r lim it of 160 (120) tim e s th e S ta n d a rd M odel p re d ic tio n of n o n -re s o n a n t H iggs bo so n p a ir p ro d u c tio n cro ss-sectio n is set from a com b in ed an a ly sis o f th e th re e final sta te s . U p p e r lim its a re set o n th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h in g ra tio o f a h eav y sc a la r X d ec ay in g in to a H iggs b o so n p a ir in th e m ass ra n g e of 260 G eV < m X < 500 G eV a n d th e o b serv ed (e x p e c te d ) lim its ra n g e from 9.3 (10) p b to 2.8 (2.6) pb. U p p e r lim its are set on th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h in g ra tio of a h eav y sc a la r X d ec ay in g in to a p a ir of h ea v y scalars S for m ass ran g es o f 280 G eV < m X < 340 G eV a n d 135 G eV < m S < 165 G eV a n d th e o b serv ed (e x p ected ) lim its ra n g e from 2.5 (2.5) p b to 0.16 (0.17) pb.

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)

ArXiv ePr i n t: 1811.11028

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

C o n te n ts

1 I n tr o d u c tio n 1

2 D a t a a n d s im u la tio n s a m p le s 3

3 O b je c t s e le c tio n 4

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

5 B a c k g r o u n d e s tim a t io n 6

6 S y s t e m a t ic u n c e r ta in tie s 7

7 R e s u lts 8

8 C o n c lu s io n s 11

A F in a l s e le c tio n c r ite r ia 12

T h e A T L A S c o lla b o r a tio n 22

1 In tr o d u c tio n

A sc a la r b o so n w as d iscovered by th e A T L A S a n d C M S c o lla b o ra tio n s [1, 2] in 2 0 1 2. I t h as b ee n show n to have p ro p e rtie s co n siste n t w ith th o se p re d ic te d fo r th e S ta n d a rd M odel (SM ) H iggs boson, H , th ro u g h sp in a n d co u p lin g m e a su re m e n ts [3 , 3- 10]. T h e se m e a su re m e n ts a re b ase d on p ro d u c tio n o f th e H iggs b oso n v ia g lu o n -g lu o n fusion, v e c to r-b o so n fusion an d in asso c ia tio n w ith a W o r Z b o so n o r a to p q u a rk p air. T h e SM p re d ic ts n o n -re so n a n t H iggs boso n p a ir p ro d u c tio n v ia to p q u a rk loops as well as th ro u g h self-coupling. T h e SM H H p ro d u c tio n cro ss-sectio n is c o m p u te d to b e 3 3 .4 fb [11, 12] a t n e x t-to -n e x t-to -le a d in g o rd e r (N N L O ) in Q C D , in clu d in g re su m m a tio n of so ft-g lu o n em ission a t n e x t-to -n e x t-to - le a d in g -lo g a rith m ic (N N L L ) a c c u ra c y for m H = 125.09 G eV . T h e a c tu a l p ro d u c tio n ra te co u ld be la rg e r th a n t h a t p re d ic te d in th e SM d u e to a v a rie ty of B ey o n d th e S ta n d a rd M odel (B SM ) physics effects. O ne such e x te n sio n in clu d es a m o d ificatio n to th e SM H iggs self-coupling, AHHH , a n d a n o th e r th e ex isten c e o f a new h eav y re so n an ce w hich decays in to a p a ir of H iggs bosons. A n im p o rta n t H iggs b o so n d ecay ch a n n el is H ^ V V (*) in w hich V c a n b e e ith e r a W o r Z boson, o n o r off-shell, a n d th is p a p e r focuses o n th e 4 W final s ta te [13] in b o th SM a n d B SM H H p ro d u c tio n scenarios.

T h is w ork in v estig ates H H p ro d u c tio n th ro u g h th re e d ifferen t pro cesses. T h e first is ( 1.1) th e SM H H p ro d u c tio n (n o n -re so n a n t H H ). T h e second a n d th ir d a re b o th B SM p rocesses in sp ired by a n e x te n d e d H iggs secto r, such as a tw o -H ig g s-d o u b let m o d el [14], in

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

w hich a n e u tra l h eav y H iggs bo son, X [15] is p ro d u c e d a n d d ecay s e ith e r ( 1.2) d ire c tly in to tw o SM H iggs b o so n s (re so n a n t H H ) o r ( 1.3) in to a p a ir o f new sc a la r boso ns, S ( X ^ S S ), each of w hich in t u r n d ecays to o th e r SM p a rtic le s w ith th e sam e m a ss -d e p e n d e n t b ra n c h in g ra tio s of th e SM H . T h e re a c tio n s co n sid ered in th is w ork are:

T h e m e a su re d final s ta te s en c o m p a ss m u ltip le c o m b in a tio n s o f lep to n s a n d h ad ro n s:

w h ere £ is e ith e r an e le c tro n o r a m u on , q refers to q u a rk a n d a n ti-q u a rk d ecay p ro d u c ts from th e h a d ro n ic a lly d ecay in g W b o so n (s), a n d v re p re se n ts a n e u trin o , w hich re su lts in m issin g tra n s v e rs e m o m e n tu m . T h erefo re, th re e final s ta te s a re search ed for w ith tw o, th re e , o r fo u r le p to n s (p lu s m issin g en e rg y a n d m u ltip le je ts ), w h ich allow an y of th e m e n tio n e d p ro d u c tio n m odes to be p ro b e d .

T h e p ro d u c tio n of a new X sc a la r ( 1.2) w ould b e seen as a local excess in th e re co n ­ s tru c te d di-H iggs m ass sp e c tru m . I t is a ssu m ed in th is w ork t h a t m X > 2 m H such t h a t b o th H a re p ro d u c e d on th e ir m ass shell. In th e o th e r e x te n d e d H iggs se c to r m o del ( 1.3) X ^ S S is assu m ed to b e th e d o m in a n t X d ecay m od e. In th is scen ario , th e W W( *) W W (*) c h a n n e l is th e d o m in a n t d ecay m o d e for th e m ass ran g es 270 G eV < m X < 2 m t an d 135 G eV < m S < m X /2 , w h ere m t , m X an d m S a re th e m ass of th e to p q u a rk , X , a n d S scalars, respectively. T h e m ass ra n g e m X > 2 m t , w h ere X ^ t i is e x p e c te d to d o m in a te , is n o t co n sid ered . I t is a ssu m ed t h a t m S > 135 G eV such t h a t S ^ W W(*') is th e d o m in a n t d ecay m ode. I t is also a ssu m ed t h a t m S < mX / 2 such t h a t b o th S b o so ns a re p ro d u c e d o n th e ir m ass shell.

P re v io u s searches w ere p erfo rm ed fo r re so n a n t a n d n o n -re s o n a n t H H p ro d u c tio n u s­

ing v ario u s ch an n els, such as 6 6 7 7 [16, 17], bbbb [18- 20], bbVV [21], b b r r [2 2 , 23] an d W W7 7 [24], w ith d a t a from th e A TLA S a n d C M S ex p e rim e n ts. A d d itio n ally , a co m b i­

n a tio n o f ch a n n els h as b ee n p erfo rm ed u sin g d a t a from th e C M S e x p e rim e n t [25]. T h is p a p e r d esc rib es a search for re s o n a n t a n d n o n -re so n a n t H iggs b o so n p a ir p ro d u c tio n in th e H H ^ W W * W W * d ecay ch a n n e l a n d for a n e x te n d e d H iggs sec to r w ith th e d ecay o f X ^ S S ^ W W(*) W W (*). T h e an a ly sis is d iv id ed in to th re e in d e p e n d e n t ch an n els d e p e n d in g on th e n u m b e r o f light lep to n s (e o r ^ ) from le p to n ic d ecays of W bo so ns, an d th e n s ta tis tic a lly com b in ed to give th e final re su lt.

T h is p a p e r is o rg a n ise d as follows. D a ta a n d sim u la tio n sam p les are d e sc rib e d in sec tio n 2 . T h e o b je c t re c o n s tru c tio n a n d selectio n are o u tlin e d in sec tio n 3 . S ectio n 4 d e ta ils th e ev en t selectio n for each of th e th re e final s ta te s an a ly se d . T h e b a c k g ro u n d e s tim a tio n a n d th e s y s te m a tic u n c e rta in tie s a re d e sc rib e d in sec tio n 5 a n d sec tio n 6, respectively. T h e

pp ^ H H ^ W W (ł)W W(*) (n o n -re so n a n t, SM ),

p p ^ X ^ H H ^ W W (*>W W(*) (re so n a n t, B S M ), an d p p ^ X ^ S S ^ W W (*>W W(*) ( X ^ S S , B S M ).

(1.1) (1.2) (1.3)

W W ( ,) W W (*} ^ £v + £v + 4q,

W W (*) W W (*) ^ £v + £v + £v + 2q, or W W (*) W W (*) ^ £v + £v + £v + £v

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

re su lts of th is an a ly sis a re p re se n te d in sec tio n 7 a n d su m m a rise d in sec tio n 8. F in a lly , th e a p p e n d ix lists th e le p to n p a irin g s tra te g y used in each ch a n n el, th e final ev en t selectio n c r ite ria a n d th e co rre sp o n d in g a c c e p ta n c e a n d selectio n efficiencies.

2 D a ta an d sim u la tio n sa m p les

T h e d a t a w ere collected w ith th e A TLA S d e te c to r in 2015 a n d 2016 u sin g p p collisions p ro d u c e d a t y f s = 1 3 T eV a t th e L arg e H a d ro n C o llid er (L H C ), co rre sp o n d in g to a n in te ­ g ra te d lu m in o sity of 36.1 fb- 1 [26]. T h e A T L A S d e te c to r is d esc rib ed in d e ta il in ref. [27].

O n ly d a ta - ta k in g p e rio d s in w hich all re le v an t d e te c to r sy stem s a re o p e ra tio n a l a re used.

S am ples sim u la te d u sin g M o n te C arlo (M C ) te c h n iq u e s a re used to e s tim a te th e signal a c c e p ta n c e a n d selectio n efficiency. S im u la te d sam p les a re also used to e s tim a te th e accep ­ ta n c e a n d selectio n efficiency for vario u s b a c k g ro u n d p rocesses w h ich c o n trib u te p ro m p t le p to n s from W o r Z b o so n d ecay a n d le p to n s o rig in a tin g from p h o to n conversion. B ack ­ g ro u n d s d u e to elec tro n s w ith m isid en tified ch a rg e a n d je ts m isid en tified as le p to n s are e s tim a te d usin g d a ta -d riv e n te c h n iq u e s, as d e sc rib e d in sec tio n 5 .

T h e n o n -re so n a n t gg ^ H H a n d re so n a n t gg ^ X ^ H H sign al sam p les in w h ich H is c o n stra in e d to d ecay in to W W * are g e n e ra te d usin g M ADGRAPH5_aM C@ NLO [2 8 , 29] w ith th e C T 1 0 p a r to n d is trib u tio n fu n c tio n (P D F ) set [30] a n d th e p a r to n show er is m od elled by H e r w ig + + [31] w ith th e U E E E 5 set o f tu n e d p a ra m e te rs (tu n e ) for th e u n d e rly in g ev en t [32] a n d th e C T E Q 6 L 1 P D F set [33]. In re s o n a n t p ro d u c tio n , X d ecays in to a p a ir o f SM H iggs b o sons w ith a negligible w id th c o m p a re d to th e e x p e rim e n ta l m ass reso lu tio n . V arious re so n an ce m ass h y p o th e se s, m X , a re co n sid ered: 260, 300, 400, a n d 500 G eV . T h e b ra n c h in g ra tio B ( X ^ H H ) is a ssu m ed to be one. S am p les o f X ^ S S ^ W W (,)W W (*) ev e n ts p ro d u c e d by g lu o n -g lu o n fusion are g e n e ra te d a t lead in g o rd e r (L O ) u sin g P y t h i a 8

w ith th e N N P D F 2 .3 L O P D F set [34] such t h a t b o th th e X a n d S sca la rs are a ssu m ed to h av e n a rro w d ecay w id th s. T h e m ass h y p o th e se s are selected to scan a ra n g e of b o th m X a n d m S . In th e first scan, m S is fixed to 135 G eV for sam p les w ith m X = 280, 300, 320, a n d 340 G eV . In th e second scan, m X is fixed to 340 G eV for sam p les w ith m S = 135, 145, 155, a n d 165 G eV . T h e b ra n c h in g ra tio B ( X ^ S S) is a ssu m ed to b e one a n d th e b ra n c h in g ra tio B ( S ^ W W (*)) is a ssu m ed to b e th e m a s s -d e p e n d e n t e x p e c te d b ra n c h in g ra tio s of th e SM H iggs boson.

M u lti-b o so n ( V V / V V V ) a n d V y b a c k g ro u n d sam p les are g e n e ra te d a t n e x t-to -le a d in g - o rd e r (N L O ) u sin g S h e r p a 2.1 [35]. T h e V + je ts sam p les a re g e n e ra te d a t N L O w ith S h e r p a 2.2. T h e C T 1 0 P D F set is used for th e se sam p les. T h e V H b ac k g ro u n d sam p le is g e n e ra te d a t le a d in g -o rd e r (LO ) usin g P y t h i a 8 w ith th e N N P D F 2 .3 L O P D F set. T h e t t b a c k g ro u n d sam p le is g e n e ra te d a t N L O usin g P o w H E g - B o x 2.0 [36] in terfa ced w ith P y t h i a 8 w ith th e N N P D F 2 .3 L O P D F set. S in g le-to p b a c k g ro u n d sam p les a re g e n e ra te d a t N L O u sin g P o w H E g - B o x 2.0 in terfa ced w ith P y t h i a 6.4 [37] w ith th e C T 1 0 P D F set. T h e t t V b ac k g ro u n d sam p le is g e n e ra te d a t N L O u sin g M ADGRAPH5_aM C@ NLO in terfa ced w ith P y t h i a 8 w ith th e N N P D F 2 .3 L O P D F set. T h e t t H b a c k g ro u n d sam p le is g e n e ra te d a t N L O usin g M ADGRAPH5_aM C@ NLO in terfa ced w ith H e r w ig + + w ith th e

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N N P D F 3 .0 [38] P D F set. T h e sim u la te d sam ples of ti, t i H , t i V , a n d V V a re d esc rib ed in m o re d e ta il in refs. [39- 41].

T h e s ta n d a r d A T L A S d e te c to r s im u la tio n [42] b ase d o n G e a n t 4 [43] is u sed for b ac k ­ g ro u n d sim u la te d sam p les. F o r signal even ts, th e c a lo rim e te r sim u la tio n is re p la ced w ith th e fa st A T L A S c a lo rim e te r sim u la tio n [44] t h a t uses a p a ra m e te ris e d d e te c to r resp o n se.

Soft collisions g e n e ra te d u sin g P y t h i a 8 [45] w ith th e C T E Q 6 L 1 P D F set a n d th e A2 tu n e [46] a re overlaid on th e h a r d - s c a tte r processes. T h e n u m b e r of in -tim e a n d o u t-o f­

tim e collisions p e r b u n c h cro ssin g (p ileup ) is a d ju s te d to t h a t o b serv ed in d a ta .

3 O b je c t se le c tio n

E le c tro n c a n d id a te s are re c o n s tru c te d fro m en e rg y c lu s te rs in th e e le c tro m a g n e tic ca lo rim e­

t e r t h a t are a sso c ia te d w ith tra c k s re c o n s tru c te d in th e in n e r d e te c to r (ID ). E le c tro n s are id en tified usin g m ed iu m (tig h t) c rite ria [47] fo r th e fo u r le p to n ch a n n el (tw o a n d th re e le p to n ch a n n els). E le c tro n s are re q u ire d to have a tra n s v e rs e en e rg y E x > 10 G eV a n d b e w ith in th e d e te c to r fiducial v olum e of |n| < 2.47 ex c lu d in g th e tra n s itio n reg ion b etw e en th e b a rre l a n d e n d -c a p c a lo rim e te r, 1.37 < |n| < 1.52.1 M u o n c a n d id a te s are re c o n s tru c te d by co m b in in g tra c k s re c o n s tru c te d in th e ID w ith 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. M uons are identified usin g m ed iu m (tig h t) c rite ria [48] for th e fo u r le p to n c h a n n el (tw o a n d th re e le p to n ch a n n els). M uo ns a re re q u ire d to have a tra n s v e rs e m o m e n tu m p x > 10 G eV a n d |n| < 2.5. E le c tro n s are re q u ire d to satisfy c a lo rim e te r a n d tra c k iso la­

tio n c rite ria a n d m uons are re q u ire d to sa tisfy a tra c k iso la tio n crite rio n . T h e c a lo rim e te r (tra c k ) iso la tio n re q u ires t h a t th e to ta l sum of c lu s te r tra n s v e rs e energies (tra n s v e rs e m o ­ m e n ta of tra c k s w ith p x > 1 G eV ) in a s u rro u n d in g cone o f size A R = 0.2 a ro u n d th e le p to n , ex c lu d in g th e c lu s te r E x (tra c k p x ) of th e le p to n from th e sum , is less th a n 30%

(15% ) of th e p x of th e le p to n for th e fo u r le p to n selectio n a n d 6% for th e tw o a n d th re e le p to n selections.

J e ts a re re c o n s tru c te d from c a lib ra te d to p o lo g ic al c lu ste rs in th e c a lo rim e te rs [49] usin g th e a n t i - k a lg o rith m [50] w ith a ra d iu s p a r a m e te r R = 0.4. J e t energies a re c o rre c te d for effects from th e d e te c to r a n d from p ileu p [51] u sin g s im u la te d a n d i n s i t u te c h n iq u e s [51].

J e ts are re q u ire d to have p x > 25 G eV an d |n| < 2.5. J e ts w ith p x < 60 G eV a n d |n| < 2.4 a re re q u ire d to satisfy a d d itio n a l p ileu p re je c tio n c rite ria [52]. J e ts c o n ta in in g b-h ad ro n s a re id en tified (b-tagg ed) usin g th e M V 2c10 m u ltiv a ria te d is c rim in a n t [53]. T h e b-tag ging re q u ire m e n t re su lts in a n efficiency of 70% for je ts c o n ta in in g b-had ro ns, as d e te rm in e d in a sim u la te d sam p le of tti ev e n ts [54]. A n o v erla p rem oval p ro c e d u re is a p p lie d in o rd e r to resolve am b ig u itie s b etw e en re c o n s tru c te d physics o b je c ts . J e ts w ith in A R = 0.2 of a re c o n s tru c te d e le c tro n are rem oved. If th e n e a re s t re m a in in g je t is w ith in A R = 0.4 o f an elec tro n , th e e le c tro n is rem oved. S elected m uons w ith a n a n g u la r s e p a ra tio n of

"ATLAS 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 0 as y = — lntan(0/2). Angular distance is measured in units of AR = \ J(Ay)2 + (A0)2.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

A R < m in (0 .4 ,0.04 + 1 0 G e V /p T ) from th e n e a re st j e t a re rem oved if th e je t h as a t least th re e tra c k s o rig in a tin g from th e p rim a ry v erte x ; o th e rw ise th e je t is rem oved a n d th e m u o n is k ep t. T h e m issing tra n s v e rs e m o m e n tu m , E m iss, v e c to r is th e n e g a tiv e o f th e v e c to r su m of th e tra n s v e rs e m o m e n ta of all elec tro n s, m uo ns, a n d je ts . T racks from th e p rim a ry v e rte x2 t h a t are n o t a sso c ia te d w ith an y o b je c ts are also ta k e n in to ac c o u n t in th e

£>miss re c o n stru c tio n [55].

4 E v en t se le c tio n

E v e n ts a re re q u ired to pass sin g le-lep to n or d ile p to n trig g e rs [56] w ith m in im u m p T th r e s h ­ olds in th e ra n g e 2 0 -2 6 G eV , d e p e n d in g o n th e d a t a co llectio n p erio d , a n d to have a t least tw o le p to n s (e o r ^ ) . E v e n ts a re also re q u ire d to h ave a t le a st on e le p to n (tw o lep to n s) to be m a tc h e d to th e sin g le-lep to n (d ile p to n ) trig g e r sig n a tu re s. A h ig h er p x re q u ire m e n t th a n th e o n line trig g e r p x th re sh o ld is ap p lied to th e trig g e r-m a tc h e d le p to n . T h re e ch an n els a re defined ac co rd in g to th e n u m b e r o f re c o n s tru c te d le p to n s (tw o lep to n s, th re e lep to n s a n d fo u r le p to n s), a n d ev en ts a re fu r th e r classified ac co rd in g to th e ch arg e a n d flavour of th e lep to n s. In o rd e r to su p p re ss to p q u a rk b ac k g ro u n d s a n d to b e o rth o g o n a l to o th e r H iggs b o so n p a ir p ro d u c tio n search es ( b b y y [16], bbbb [18], a n d b b r r [2 2]) a t A T L A S, ev en ts c o n ta in in g b-tagged je ts are re je cted .

E v e n ts in th e tw o le p to n ch a n n e l a re re q u ire d to h ave e x a c tly tw o le p to n s w ith th e sam e elec tric charge, w hile th e th re e le p to n ch a n n e l ev e n ts a re re q u ired to have e x a c tly th re e le p to n s w ith a su m m ed elec tric ch a rg e ^ ie e qi = ± 1 . E v e n ts are re q u ire d to have N jets > 2 a n d ETpiss > 10 (30) G eV for th e tw o (th re e ) le p to n ch a n n el. In o rd e r to su p p re ss b a c k g ro u n d s c o n ta in in g a Z boso n in th e sam e-sig n ee ch a n n el (d u e to th e m isid e n tific atio n o f a n e le c tro n ’s ch arg e) a n d in th e th re e le p to n ch a n n el, ev e n ts are rem o ved if th e y c o n ta in e d a sam e-flav o u r le p to n p a ir w ith a n in v a ria n t m ass, m u , n e a r th e Z b o so n m ass: |m ^ — m Z | < 10 G eV . In o rd e r to re d u ce th e b a c k g ro u n d s from n o n -p ro m p t lep to n s, th e lead in g (su b lea d in g ) le p to n is re q u ire d to have p T > 30 (20) G eV in th e tw o le p to n ch a n n el. T h e tw o le p to n s w ith th e sam e ch arg e are b o th re q u ire d to have p T > 20 G eV in th e th re e le p to n ch a n n el. F o r n o n -re so n a n t p ro d u c tio n a n d re so n a n t p ro d u c tio n w ith m X > 300 G eV , signal ev e n ts te n d to have je ts w ith la rg e r p T c o m p a re d to low m X re so n a n t p ro d u c tio n scen arios a n d th u s N jets > 3 is re q u ire d in th e tw o le p to n c h a n n el to a c c o u n t for m o re je ts p assin g th e p T re q u ire m e n t. A d d itio n ally , ev en ts c o n ta in in g a sam e-flav ou r o p p o site -sig n (S F O S ) le p to n p a ir w ith a n in v a ria n t m ass m u < 15 G eV a re also rem ov ed in o rd e r to su p p re ss b a c k g ro u n d s from h a d ro n re so n an ces o r v irtu a l p h o to n s. Follow ing th is p re selectio n , a n u m b e r of ob serv ab les are co n sid ered a n d fo u r v aria b les are chosen b ase d o n th e ra n k in g o f th e g en eric a lg o rith m [57] a n d th e c o rre la tio n s b etw eeen v ariab les. T h e se fo u r v ariab les t h a t co n sist of th e a n g u la r s e p a ra tio n b etw e en each le p to n a n d th e n e a re s t je t as well as in v a ria n t m asses am o n g d ifferent c o m b in a tio n s of th e le p to n s a n d je ts are u sed for fu r th e r selectio n. T h e final selections o n th e se v aria b les are o p tim ise d in o rd e r to m ax im ise signal

2Proton-proton collision vertices are reconstructed by requiring that at least two tracks with p T >

0.4 GeV are associated with a given vertex. The primary vertex is defined as the vertex with the largest X;pT . t r a c k.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

(a) (b) (c)

F ig u r e 1. Distributions of the invariant mass of (a) two, (b) three, and (c) four leptons for the two, three, and four lepton channels after preselection. The charge misidentification background in the two lepton channel and the n on-Z Z backgrounds in the four lepton channel are non-zero bu t are too small to be seen in the distributions. The shaded band in the ratio plot shows the system atic uncertainty in the background estim ate. Resonant H H signal samples are denoted by m X . The integral of each signal sample distribution is scaled to th a t of the expected background.

significance. O n e o f th e se v aria b les is th e in v a ria n t m ass of tw o (th re e ) lep to n s in th e tw o (th re e ) le p to n c h a n n el a n d is show n in figure 1a (1b ) to illu s tra te its d isc rim in a tin g pow er. T h e o p tim is a tio n p ro c e d u re u sin g all fo ur v aria b les is p erfo rm ed s e p a ra te ly for each an a ly sis ch a n n el, each signal m ass p o in t, each le p to n flavour ca te g o ry (for th e tw o le p to n ch a n n e l), a n d each n u m b e r of sam e-flav o u r o p p o site -sig n (N SFOS) le p to n p a irs (for th e th re e le p to n ch a n n el). T h e o p tim ise d selectio n c rite ria are liste d in ta b le s 3- 9 in th e ap p e n d ix .

E v e n ts in th e fo u r le p to n c h a n n el a re re q u ire d to have e x a c tly fo ur le p to n s w ith qi = 0. A t le a st one of th e lep to n s is re q u ired to have p x > 2 2 G eV . E v e n ts t h a t c o n ta in a S FO S le p to n p a ir w ith m u < 4 G eV a re rem oved. Follow ing th is p re selectio n , selections o n th e in v a ria n t m asses a n d a n g u la r s e p a ra tio n of le p to n p a irs a re im p le m e n ted to re je ct b a c k g ro u n d s c o n ta in in g a Z b o so n o r n o n -p ro m p t le p to n s o r o th e r o b je c ts in co r­

re c tly identified as lep to n s, know n as fake lep to n s. A s u m m a ry of th e selectio n c rite ria u sed in th e fo u r le p to n c h a n n el is show n in ta b le s 10- 11 in th e ap p e n d ix . F ig u re 1c shows th e k in e m a tic d is trib u tio n of th e fo u r le p to n in v a ria n t m ass.

5 B a ck g ro u n d e stim a tio n

T h e b a c k g ro u n d s in th is search all have final s ta te s t h a t c o n ta in le p to n s t h a t c a n b e classi­

fied acco rd in g to th e ir orig in in to p ro m p t le p to n s, 3 le p to n s w ith m isid en tified ch arg es, an d fake lep to n s (in clu d in g n o n -p ro m p t a n d m isid en tified je ts ). T h e b a c k g ro u n d s in th e tw o a n d th re e le p to n ch a n n els are d o m in a te d by irre d u c ib le p ro m p t-le p to n processes, in c lu d ­ ing V V ( W Z a n d Z Z ), t i Z a n d V V V , w ith a sign ifican t c o n trib u tio n from fake lep to n s.

T h e b ac k g ro u n d in th e fo u r le p to n ch a n n el is a lm o st exclusively d u e to Z Z p ro d u c tio n (in clu d in g b o th on-shell a n d off-shell p ro d u c tio n ).

3Leptons not from hadron decays or photon conversions.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

P r o m p t-le p to n b a c k g ro u n d s are m o d elled usin g s im u la te d sam p les d e sc rib e d in sec­

tio n 2. C o n tro l regions c o n ta in in g on e p a ir (tw o p airs) of S FO S lep to n s w ith in v a ria n t m ass |m ^ — m Z | < 10 G eV in th e th re e (four) le p to n ch a n n el are u sed to check th e m o d ­ elling of W Z ( Z Z ) b ac k g ro u n d . A d a ta -d riv e n m e th o d [7 , 58] is u sed to e s tim a te th e charg e m isid e n tific a tio n r a te for elec tro n s from a sam p le o f Z ^ ee ev e n ts w ith m ee in a n arro w w indow a ro u n d m Z . T h e co rre sp o n d in g sam e-sig n ch a rg e m isid e n tific a tio n (Q m isID ) b ac k ­ g ro u n d is e v a lu a te d by scaling th e o p p o site -sig n ev e n ts by th is ra te . T h e p ro b a b ility of m isid e n tify in g th e ch a rg e o f a m u o n is checked in b o th d a t a a n d sim u la tio n , a n d fo u n d to b e negligible in th e k in e m a tic ra n g es re le v an t to th is an aly sis.

In th e tw o a n d th re e le p to n ch a n n els n o n -p ro m p t-le p to n c o n trib u tio n s from th e co nv er­

sion of p ro m p t p h o to n s a re e s tim a te d u sin g V7 sim u la te d sam p les. F a k e -le p to n a n d non- p ro m p t-le p to n c o n trib u tio n s from m isid e n tific a tio n o f h a d ro n ic je ts as lep to n s, sem ilep to n ic d ecay of heavy -flav o u r h a d ro n s a n d p h o to n conv ersion s from n e u tra l p io n d ecays are e s ti­

m a te d u sin g d a t a w ith a fa k e-fa cto r m e th o d [59]. T h e m e th o d defines “t ig h t” le p to n s as le p to n s p assin g all re q u ire m e n ts d e sc rib e d in sec tio n 3 a n d “a n ti- tig h t” le p to n s as lep to n s failing th e iso latio n o r id e n tific a tio n re q u ire m e n ts. T h e fake fa c to r is c a lc u la te d as th e ra tio o f ev en ts w ith tig h t lep to n s to ev e n ts w ith one tig h t le p to n re p la ced by a n a n ti-tig h t le p to n in th e d a t a c o n tro l sam p les. T h e c o n tro l sam p les o f th e tw o a n d th re e le p to n ch a n n els are e n su re d to b e larg ely o rth o g o n a l to co rre sp o n d in g p re selectio n sam p les by re q u irin g a lower je t m u ltip licity . A c o n tro l sam p le c o n ta in in g th re e lep to n s w ith en ric h ed Z + j e t s pro cesses is used in th e fo u r le p to n c h a n n el to e x tr a c t th e fake fa cto rs. All s im u la te d p ro m p t-le p to n c o n trib u tio n s are s u b tra c te d from th e d a t a before m e a su rin g th e fake fa c to r. T h e fake­

le p to n b a c k g ro u n d c o n trib u tio n s are e s tim a te d by a p p ly in g th e fake fa c to rs to ev e n ts w ith th e sam e selectio n as for th e signal regions b u t w ith a t le a st one a n ti-tig h t le p to n re p la cin g one of th e p ro m p t lep to n s. T h e fake fa c to rs in th e fo u r le p to n ch a n n el a re ap p lie d to even ts in tw o c o n tro l sam p les, one w ith th re e tig h t lep to n s a n d one a n ti-tig h t le p to n a n d th e o th e r w ith tw o tig h t le p to n s a n d tw o a n ti-tig h t lep to n s.

6 S y s te m a tic u n c e r ta in tie s

E x p e rim e n ta l sy ste m a tic u n c e rta in tie s a re ev a lu a te d . T h e y in clu d e u n c e rta in tie s re la te d to th e e le c tro n a n d je t en e rg y m e a su re m e n ts [51], m u o n m o m e n tu m m e a su re m e n t, ETpiss m o d ellin g [55], a n d le p to n re c o n stru c tio n , id en tificatio n , a n d iso la tio n efficiencies. T h e d o m in a n t sy s te m a tic u n c e rta in ty in th e fa k e-lep to n b a c k g ro u n d e s tim a tio n s arises from a clo su re te s t of th e fa k e-fa cto r m e th o d a n d th e re la tiv e c o n trib u tio n s from heavy -flav o u r h a d ro n decays a n d p h o to n conversions. P ile u p m odellin g, b -tagg in g efficiencies, a n d je t p ileu p re je c tio n m o d ellin g are also in clu d ed . T h e o re tic a l u n c e rta in tie s a re e v a lu a te d for all sim u la te d sam p les. T h e se in clu d e u n c e rta in tie s in P D F , Q C D scale, a n d p a r to n show er m o d ellin g t h a t im p a c t efficiency tim e s a c c e p ta n c e for sign al sam p les a n d u n c e rta in tie s in th e p ro d u c tio n cro ss-sectio n s for sim u la te d b a c k g ro u n d sam p les. T h e s ta tis tic a l u n c e rta in ­ tie s in M C sign al a n d b a c k g ro u n d sam p les as well as in d a t a c o n tro l regions a re in clu d ed as s y s te m a tic u n c e rta in tie s .

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

F ig u r e 2. Expected and observed yields in each channel after all selection criteria for the non­

resonant H H production searches. The label N SFOS indicates the num ber of same-flavour, opposite- sign lepton pairs in the channel. Low and high m 4£ indicates m 4£ < 180 GeV and m 4£ > 180 GeV, respectively. The shaded band in the ratio plot shows the system atic uncertainty in the background estim ate. The signal is scaled by a factor of 20.

T h e s y s te m a tic u n c e rta in tie s w ith th e la rg e st im p a c t on th e H H p ro d u c tio n cro ss­

sec tio n (tim es b ra n c h in g ra tio ) lim its com e from th e j e t en e rg y scale a n d re so lu tio n w ith a re la tiv e im p a c t c o m p a re d to th e to ta l sy s te m a tic plus s ta tis tic a l u n c e rta in ty of 45% (2 9 % - 55%) a n d fa k e-lep to n b a c k g ro u n d e s tim a tio n s w ith a re la tiv e im p a c t o f 42% (31% -54% ) for th e n o n -re so n a n t (re so n a n t) p ro d u c tio n searches. T h e o re tic a l u n c e rta in tie s are fou nd to have a re la tiv e im p a c t of 23% (24 % -3 6% ) for th e n o n -re so n a n t (re so n a n t) p ro d u c tio n searches. T h e re la tiv e im p a c t of je t en e rg y m e a su re m e n ts, fa k e-lep to n b a c k g ro u n d e s tim a ­ tio n s, a n d th e o re tic a l u n c e rta in tie s in th e X ^ S S an a ly sis a re 38 % -5 1% , 37% -52% an d 2 5% -32% , respectively . O th e r e x p e rim e n ta l u n c e rta in tie s d u e to lep to n , p ileup , b-tagging, p ileu p je t reje ctio n , p ro m p t-le p to n b a c k g ro u n d e s tim a tio n s, a n d ETpiss m o d ellin g are fo u n d to h ave a sm all im p a c t on th e re su lts. T h e u n c e rta in ty in th e co m b in ed 2 0 1 5 + 2 0 1 6 in te ­ g ra te d lu m in o sity is 2.1% . I t is deriv ed , follow ing a m e th o d o lo g y sim ilar to t h a t d e ta ile d in ref. [26], a n d u sin g th e L U C ID -2 d e te c to r for th e b ase lin e lu m in o sity m e a su re m e n ts [60], from c a lib ra tio n of th e lu m in o sity scale u sin g x - y b e a m -s e p a ra tio n scans. I t h as a 5% -10%

re la tiv e im p a c t d u e to its sim u lta n e o u s effect o n th e sign al a n d b a c k g ro u n d e stim a te s . All sim u la te d processes ex c e p t Z Z a re affected by th e u n c e rta in ty in th e lu m in o sity m e a su re ­ m e n t. T h e re la tiv e im p a c t of all sy s te m a tic u n c e rta in tie s is fo u n d to b e 71% (60% -79% ) for th e n o n -re so n a n t (re so n a n t) p ro d u c tio n searches. In a d d itio n to th e s y s te m a tic effects, th e s ta tis tic a l u n c e rta in tie s a re fo u n d to h av e a re la tiv e im p a c t of 71% (61 % -80% ) for th e n o n -re so n a n t (re so n a n t) p ro d u c tio n searches.

7 R e su lts

T h e e x p e c te d a n d o b served yields in each ch a n n e l a fte r all selectio n c r ite ria for th e n o n ­ re s o n a n t H H p ro d u c tio n searches are show n in figure 2 a n d ta b le 1.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

C h an n el C ateg o ry B ack gro und E x p e c te d Signal O bserved

ee 29 ± 1 0 0.028 ± 0.004 35

2 leptons eß 1 1 . 1 ± 2 .2 0.049 ± 0.005 18

ß ß 8 . 1 ± 2.5 0.034 ± 0.004 4

NS F O S = 0 1 . 0 ± 0.7 0 . 0 1 1 ± 0.005 3

3 leptons

N^{s f o s} = 1 , 2 4.3 ± 3.8 0.033 ± 0 . 0 1 0 8

NS F O S = 0, 1 2.3 ± 1.4 0.005 ± 0 . 0 0 1 2

4 lep to n s m u < 180 G eV N^{S F O S} = 2 ^{2 1} ± 5 0 . 0 0 2 ± 0 . 0 0 1 2 2

NS F O S = 0, 1 3.0 ± 1 .8 0 . 0 1 0 ± 0 . 0 0 2 3

4 lep to n s m u > 180 G eV N^{S F O S} = 2 7.9 ± 2 . 0 0.005 ± 0 . 0 0 1 4 T a b le 1. Expected and observed yields in each channel after all selection criteria and th e profile- likelihood fit for the non-resonant H H production searches. The expected signal refers to the SM non-resonant H H production, corresponding to its calculated cross-section at a/s = 13TeV of 33.4 fb. The label N SFOS indicates the num ber of same-flavour, opposite-sign lepton pairs in the channel. System atic uncertainties on the signal and background estim ates are shown.

A s ta tis tic a l an a ly sis u sin g a p ro file-lik e lih o o d -ratio te s t s ta tis tic [61] for th e tw o, th re e , a n d fo u r le p to n ch an n els, sep a rately , as well as th e c o m b in a tio n o f th e th re e ch a n n els is p e rfo rm e d . T h e e x p e c te d an d o b serv ed yields in each of th e n in e sign al regions show n in figure 2 as well as th e Z Z c o n tro l reg ion in th e fo u r le p to n ch a n n el are used as th e in p u t p a ra m e te rs to th e likelihood. N o sig nifican t excess over th e e s tim a te d b ac k g ro u n d s is o b serv ed in d a ta . U p p e r lim its a t 95% confidence level (C L ) are set o n th e p ro d u c tio n cro ss-sectio n for n o n -re so n a n t S M H H p ro d u c tio n a n d on th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h in g ra tio for re s o n a n t H H p ro d u c tio n as well as X ^ S S p ro d u c tio n . T h e e x p e c te d a n d o b serv ed lim its o n th e sig nal s tre n g th of n o n -re s o n a n t S M H H p ro d u c tio n , defined as th e ra tio of th e sign al cro ss-sectio n to th e S ta n d a rd M odel p re d ic tio n (ct/ctsm ), a re c a lc u la te d u sin g th e m odified fre q u e n tist C L s m e th o d [62] u sin g th e a s y m p to tic a p ­ p ro x im a tio n a n d a re show n in ta b le 2. All sy s te m a tic u n c e rta in tie s are in clu d ed in th e profile-likelihood fit as G a u ssia n n u isan ce p a ra m e te rs a n d are tr e a te d as c o rre la te d across all signal regions. T h e com b in ed o b serv ed (e x p e c te d ) u p p e r lim it o n th e n o n -re so n a n t SM H H p ro d u c tio n cro ss-sectio n is fo u n d to b e 5.3 (3.8) p b , w h ich c o rre sp o n d s to a lim it on th e signal s tre n g th of 160 (1 2 0).

U p p e r lim its a t 95% CL on th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h in g ra tio a re set for a sc a la r re so n an ce d ec ay in g in to e ith e r a p a ir of SM H iggs b o son s (show n in figure 3) o r in to a p a ir of h eavy sca la rs (show n in figure 4) . T h e o b serv ed (e x p ected ) u p p e r lim its o n re so n a n t H H p ro d u c tio n v ary w ith th e re so n an ce m ass m x a n d ra n g e from 9.3 (10) p b to 2.8 (2.6) pb, w ith th e sm allest lim it set for m x = 500 G eV . U p p e r lim its o n re so n a n t S S p ro d u c tio n v ary w ith th e re so n an ce m ass m x a n d th e sc a la r m ass m S . T h e o b served (e x p e c te d ) lim its ra n g e from 2.5 (2.5) p b to 0.16 (0.17) pb, w ith th e sm allest lim it set for m x = 340 G eV a n d m S = 165 G eV .

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

O b serv ed lim it o n ct/ctsm

E x p e c te d lim it on M e d ia n + 2ct + 1ct

ct/ctSM

—1ct —2ct

2 lep to n s 170 150 290 2 1 0 1 0 0 78

3 lep to n s 420 270 690 420 2 0 0 150

4 lep to n s 340 400 880 590 290 2 1 0

C o m bined 160 1 2 0 230 170 83 62

T a b le 2. Expected and observed 95% CL exclusion limits set on th e non-resonant H H signal strength. The SM non-resonant H H cross-section at y / s = 13TeV is calculated to be 33.4 fb.

Lim its are shown for each channel individually as well as for the com bination of the channels.

S tatistical and system atic uncertainties are included.

250 300 350 400 450 500

mx [GeV]

F ig u r e 3. Expected and observed 95% CL exclusion limits set on the cross-section times branching ratio of resonant H H production as a function of m x. Limits are shown for each channel individually as well as for the com bination of the channels. Statistical and system atic uncertainties are included.

(a) (b)

F ig u r e 4. Expected and observed 95% CL exclusion limits set on the cross-section times branching ratio of resonant X ^ S S production as a function of (a) m S and (b) m x . Lim its are shown for each channel individually as well as for the com bination of the channels. Statistical and system atic uncertainties are included.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

8 C o n c lu sio n s

A sea rch for re so n a n t a n d n o n -re so n a n t H iggs b o so n p a ir p ro d u c tio n as well as fo r a heav y sc a la r p a ir p ro d u c tio n h as b een p e rfo rm e d in th e W W (*) W W (*) d ecay ch a n n e l usin g 36.1 fb- 1 o f y f s = 1 3 T eV p ro to n -p ro to n collision d a t a co llected by th e A T L A S e x p e ri­

m e n t a t th e L H C in 2015 a n d 2016. T h e an aly sis is p erfo rm ed s e p a ra te ly in th re e ch an n els b ase d o n th e n u m b e r of le p to n s in th e final s ta te : tw o sam e-sig n lep to n s, th re e lep to n s, a n d fo u r lep to n s. N o significant excesses over th e e x p e c te d b a c k g ro u n d s a re o b serv ed in d a t a a n d th e re su lts from th e th re e ch a n n els are s ta tis tic a lly co m b in ed . A n o b serv ed (ex­

p e c te d ) 95% CL u p p e r lim it of 160 (120) is set on th e sig nal s tre n g th for th e n o n -re so n a n t H iggs b o so n p a ir p ro d u c tio n . U p p e r lim its are set on th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h in g ra tio of a h ea v y sc a la r X t h a t decays in to tw o H iggs b o son s for a m ass ra n g e o f 260 G eV < m X < 500 G eV a n d th e o b serv ed (e x p ected ) lim its ra n g e from 9.3 (10) p b to 2.8 (2.6) pb. U p p e r lim its are also set o n th e p ro d u c tio n cro ss-sectio n tim e s b ra n c h ­ ing ra tio o f a h eav y sc a la r X t h a t d ecays in to tw o h ea v y sca la rs S fo r m ass ra n g es of 280 G eV < m X < 340 G eV a n d 135 G eV < m S < 165 G eV a n d th e o b serv ed (e x p ected ) lim its ra n g e from 2.5 (2.5) p b to 0.16 (0.17) pb.

A c k n o w le d g m e n ts

W e th a n k C E R N for th e very successful o p e ra tio n o f th e L H C , as well as th e s u p p o rt sta ff fro m o u r in s titu tio n s w ith o u t w hom A T L A S co uld n o t b e o p e ra te d efficiently.

W e acknow ledge th e s u p p o rt of A N P C y T , A rg e n tin a ; Y e rP h I, A rm e n ia ; A R C , A u s­

tra lia ; B M W F W a n d F W F , A u stria ; A N A S, A z e rb a ija n ; S S T C , B elaru s; C N P q an d F A P E S P , B razil; N S E R C , N R C a n d C F I, C a n a d a ; C E R N ; C O N IC Y T , C hile; C A S, M O S T a n d N S F C , C h in a; C O L C IE N C IA S , C olom bia; M S M T C R , M P O C R a n d V S C C R , C zech R ep u b lic; D N R F a n d D N S R C , D e n m a rk ; IN 2 P 3 -C N R S , C E A -D R F /IR F U , F rance;

S R N S F G , G eorgia; B M B F , H G F , a n d M P G , G e rm an y ; G S R T , G reece; R G C , H on g K o ng SA R , C hina; IS F a n d B enoziyo C en ter, Israel; IN F N , Ita ly ; M E X T a n d J S P S , J a p a n ; C N R S T , M orocco; N W O , N e th e rla n d s; R C N , N orw ay; M N iS W a n d N C N , P o la n d ; F C T , P o rtu g a l; M N E /IF A , R o m a n ia ; M E S of R u ss ia a n d N R C K I, R u s sia n F e d e ra tio n ; JIN R ; M E S T D , S erbia; M SSR , Slovakia; A R R S a n d M IZS, Slovenia; D S T /N R F , S o u th A frica;

M IN E C O , S pain; SR C a n d W a lle n b erg F o u n d a tio n , Sw eden; S E R I, S N S F a n d C a n to n s of B e rn a n d G eneva, S w itz erla n d ; M O S T , T aiw an; T A E K , T urkey; S T F C , U n ite d K ing do m ; D O E a n d N S F , U n ite d S ta te s of A m erica. In a d d itio n , in d iv id u a l g ro u p s a n d m em b ers h ave received s u p p o rt from B C K D F , C A N A R IE , C R C a n d C o m p u te C a n a d a , C an a d a ; C O S T , E R C , E R D F , H o rizo n 2020, a n d M arie S k ło d ow ska-C u rie A c tio n s, E u ro p e a n U nion;

In v e stisse m e n ts d ’ A venir L a b e x a n d Id ex , A N R , F ran ce; D F G a n d A vH F o u n d a tio n , G e r­

m any; H erak leito s, T h a le s a n d A riste ia p ro g ra m m e s co -fin anced by E U -E S F a n d th e G reek N S R F , G reece; B S F -N S F a n d G IF , Israel; C E R C A P ro g ra m m e G e n e ra lita t d e C a ta lu n y a , Spain; T h e R o y al S ociety a n d L ev erh u lm e T ru st, U n ite d K in g d o m .

T h e c ru cial c o m p u tin g s u p p o rt from all W L C G p a r tn e rs is acknow ledged gratefully, in p a r tic u la r from C E R N , th e A T L A S T ier-1 facilities a t T R IU M F (C a n a d a ), N D G F

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

V ariab le D e sc rip tio n h L ea d in g le p to n

S u b -lead in g le p to n

A R £n j A n g u la r d is ta n c e b etw e en £ N a n d th e n e a re st je t m ££ In v a ria n t m ass of th e tw o lep to n s

m iN j j In v a ria n t m ass of £ N an d th e tw o n e a re s t je ts

mall In v a ria n t m ass of all o b je c ts t h a t p ass th e selectio n c rite ria T a b le 3. Description of the notation used in the two lepton analysis.

m x C h a n n e l A R £ lj m ££ [GeV] _{m £}lj j [GeV] mail [GeV]

ee [0.35, 1.85] < 1 0 0 < 145 < 1 1 0 0

260 G eV eß [0.25, 1.80] < 85 < 135 < 650

ß ß [0.25, 2.10] < 80 < 115 < 700

e e [0.35, 1.75] < 1 2 0 < 160 < 1400 300 G eV ^{e ß} [0.2 0, 1.80] < 135 < 160 < 800

ß ß [0.20, 1.75] < 115 < 185 < 1 0 0 0

T a b le 4. Optim ised selection criteria used in the two lepton channel in the X ^ H H search with m X = 260 GeV and m X = 300 GeV.

(D e n m a rk , N orw ay, S w eden), C C -IN 2 P 3 (F ra n ce), K I T /G r id K A (G e rm a n y ), IN F N -C N A F (Ita ly ), N L -T 1 (N e th e rla n d s), P IC (S p ain ), A S G C (T a iw an ), R A L (U .K .) a n d B N L (U .S .A .), th e T ier-2 facilities w orld w ide a n d large n o n -W L C G re so u rce p ro v id ers. M a­

jo r c o n trib u to rs of c o m p u tin g reso urces are liste d in ref. [63].

A F in a l se le c tio n c riter ia

T ables 3- 6 list th e final selectio n c r ite ria in th e tw o le p to n ch a n n el. T ables 7- 9 p re sen t th e final selectio n c rite ria in th e th re e le p to n ch a n n el. T ab le 10 defines th e v aria b les an d ta b le 1 1 lists th e selectio n c r ite ria in th e fou r le p to n ch an n el.

T h e le p to n p a irin g s tra te g y in th e fo u r lep to n s c h a n n el is desig n ed to id en tify th e d ecay o f a Z b o so n in o rd e r to efficiently re je ct th e d o m in a n t Z Z b ac k g ro u n d in ev e n ts w ith a t le a st one S FO S le p to n p air. E v e n ts a re classified b ase d on th e n u m b e r o f SFO S le p to n p a irs th e y c o n ta in in o rd e r to a c c o u n t for th e d ifferen t b a c k g ro u n d c o m p o sitio n in each sig nal region.

T ab le 12 show s th e final a c c e p ta n c e a n d selectio n efficiencies for th e sign al sam ples.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

m x C h an n el A R i 2j A R e1j m u [GeV] m £ l j j [GeV]

ee [0.35, 1.50] [0.30, 1.25] [45, 235] [40, 285]

400 G eV eß [0.20, 1.50] [0.20, 1.05] [35, 195] [30, 235]

ß ß [0.2 0, 1.2 0] [0.2 0, 1.2 0] [40, 215] [30, 260]

e e [0.20, 1.15] [0.20, 1.15] [100, 270] [40, 285]

500 G eV ^{e ß} [0.2 0, 1.0 0] [0.2 0, 0.80] [75, 250] [35, 350]

ß ß [0.20, 1.05] [0.20, 0.75] [60, 250] [30, 310]

e e [0.20, 1.40] [0.20, 1.15] [55, 270] [40, 285]

N on-res. ^{e ß} [0.20, 1.15] [0.2 0, 0.80] [75, 250] [35, 350]

ß ß [0.20, 1.05] [0.20, 0.75] [60, 250] [30, 310]

T a b le 5. Optim ised selection criteria used in the two lepton channel in the non-resonant H H search and the X ^ H H search w ith m X = 400 GeV and m X = 500 GeV.

M ass C h an n el A R £2j A R i 1 j m u [GeV] m j [GeV]

e e [0.35, 2.5] [0.4, 1.65] < 80 [50, 150]

m S = 135 G eV ^{e ß} [0.25, 1.7] [0.25, 1.65] < 95 [50, 150]

ß ß [0.25, 2.05] [0.2, 1.85] < 95 [50, 150]

e e [0.35, 1.85] [0.2, 1.65] < 130 [50, 190]

m x = 340 G eV ^{e ß} [0.25, 1.6] [0.25, 1.6] < 150 [50, 150]

ß ß [0.2, 2.0] [0.2, 1.65] < 115 [50, 185]

T a b le 6. Optim ised selection criteria used in the two lepton channel in the X ^ S S search. The selection criteria in the first row are used for m S = 135 GeV and m X = 280, 300, and 320 GeV.

The selection criteria in the second row are used for m X = 340 GeV and m S = 135, 145, 155, and 165 GeV.

V ariab le D e sc rip tio n

N SFOS N u m b e r of sam e-flavour o p p o site -sig n le p to n p airs

£i L e p to n w ith ch arg e o p p o s ite to t h a t o f th e sam e-sig n p a ir

£2 L e p to n from th e sam e-sig n p a ir t h a t is closest to £ \ in ß - f i space

£3 R e m a in in g le p to n

m u e In v a ria n t m ass of th e th re e lep to n s m eN j In v a ria n t m ass of £N a n d th e n e a re s t je t m eN j j In v a ria n t m ass of £N a n d th e tw o n e a re st je ts m ee+ ejj

T h e m in im u m sum of th e in v a ria n t m ass o f tw o o p p o site -sig n lep to n s a n d th e in v a ria n t m ass of th e re m a in in g le p to n a n d th e tw o lead in g je ts A R ee A n g u la r d is ta n c e b etw e en tw o lep to n s

T a b le 7. Description of the notation used in the three lepton analysis.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

m x V ariab le N SFOS = 0 N s f o s = 1 , 2

A R t2-0 [2.47, 5.85] [2.16, 3.50]

m i2i3 [GeV] [10, 70] [10, 70]

N on-res.

m e :ij j [GeV] [50, 110] [50, 115]

m N j [GeV] [15, 50] [15, 45]

m m [GeV] [30, 105] [20, 85]

m u + t,jj [GeV] [65, 200] [85, 360]

260 _{m £2j} [GeV] [20, 75] [1 0, 60]

A R £ l£2 [0.58, 1.66] [0.41, 1.77]

m m [GeV] [2 0, 1 1 0] [20, 130]

m u + i j j [GeV] [55, 195] [75, 175]

300 _{m i2j} [GeV] [35, 70] [15, 85]

A R , i ,2 [0.08, 1.49] [0.42, 1.14]

m t l i2 [GeV] [2 0, 60] [15, 45]

m ^ j [GeV] [15, 50] [15, 50]

400 m u + e j j [GeV] [50, 240] [80, 270]

A R t2t3 [1.97, 6.24] [2.09, 4.60]

m m [GeV] [130, 320] [150, 295]

A R NN [2.68, 3.47] [2.54, 6.19]

500 A R t l t2 [0.1 2, 0.6 8] [0.1 1, 1.08]

m t3j [GeV] [15, 90] [20, 50]

T a b le 8. Optim ised selection criteria for non-resonant and resonant H H searches in the three lepton channel. The selection criteria are chosen to ensure constant signal selection efficiency between the N sfo s = 0 and N SFOS = 1, 2 categories.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

m x / m s V ariab le N s f o s = 0 N SFOS = 1 2 m m [GeV] [55, 100] [25, 85]

280 135

m e :ij j [GeV] [50, 145] [50, 300]

m i2 j [GeV] [35, 75] [10, 65]

A R£ l £2 [0.51, 1.61] [0.19, 1.16]

m m [GeV] [55, 110] [20, 135]

300 135

m j j [GeV] [50, 190] [50, 135]

m i2 j [GeV] [20, 55] [20, 50]

A R e1e2 [0.1 0, 1.8 6] [0.46, 3.38]

m m [GeV] [25, 110] [25, 135]

320 135

m ts jj [GeV] [60, 2 1 0] [50, 135]

m £2j [GeV] [10, 55] [30, 60]

a r ^ 2 [0.24, 1.78] [0.15, 1.53]

m m [GeV] [50, 170] [25, 180]

340 135

m £:ij j [GeV] [50, 115] [50, 115]

m i2 j [GeV] [10, 40] [25, 65]

A R t l t2 [0.1 2, 1.6 8] [0.15, 1.10]

m m [GeV] [60, 1 1 0] [40, 130]

340 145

m £:ij j [GeV] [50, 350] [50, 140]

m i2 j [GeV] [10, 55] [10, 90]

A R t l t2 [0.19, 1.58] [0.41, 1.11]

m m [GeV] [30, 110] [35, 135]

340 155

m ts jj [GeV] [50, 205] [50, 140]

m £2j [GeV] [20, 55] [10, 85]

A R e1e2 [0.27, 2.24] [0.50, 1.15]

m m [GeV] [25, 110] [25, 135]

340 165

m ts jj [GeV] [50, 210] [50, 140]

m £2j [GeV] [15, 55] [2 0, 60]

A R e1e2 [0.2 0, 2.1 2] [0.39, 1.95]

T a b le 9. Optim ised selection criteria for the X ^ S S searches in the three lepton channel. The selection criteria are chosen to ensure constant signal selection efficiency between the N sfo s = 0

and Ns f o s = 1, 2 categories.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

Variable Description

Pt Pt of lepton i

G and G (Nsfos > 0) SFOS lepton pair w ith invariant mass closest to Z boson (p t,2 > Pt,3) G and G (Nsfos = 0) Different-flavour OS lepton pair with

invariant mass closest to Z boson (p t,2 > Pt,3) t 0 and G Remaining lepton pair (p t,0 > Pt,1)

Table 10. Description of the notation used in the four lepton analysis.

Table 11. Summ ary of the selection criteria used in the four lepton channel. All events are required to pass the common selection and then category-dependent selection criteria are applied according to the num ber of same-flavour opposite-sign lepton pairs in the event.

J H E P 0 5 ( 2 0 1 9 ) 1 2 4

E v e n t s e le c tio n in t h e fo u r le p to n c h a n n e l 4 le p to n s w ith p T > 10 G eV a n d ^ q = 0

T rigg er

T rig g er m a tc h e d le p to n

pTnatched > 22, 25, 27 G eV (depending on data period trigger) m gg > 4 G eV (for all SFOS pairs)

N b-tag = 0 m e 0e1 > 10 G eV Ns f o s = 0 ,1 s e le c tio n

\m £2£3 — m z > 5 G eV

m 4g < 180 G e V m 4g > 180 G e V

Nsf o s = 2 s e le c tio n

m g 2g3 < 70 G eV , m g 2g3 > 110 G eV

m 4g < 180 G e V m 4g > 180 G e V

A ^ g 2g3 < 2.6 ra d m g 0£1 < 70 G eV , m gogl > 110 G eV

Channel Category Non-resonant H H

[%]

Resonant H H m x G [280, 340] GeV

[%]

X ^ S S m x G [280, 340] GeV m s G [135,165] GeV

[%]

ee 0.60 0.30-0.55 0.41-0.82

Two lepton ^{e ß} 1.05 0.52-1.32 1.12-2.31

ß ß 0 .6 6 0.35-1.10 0.88-1.94

N s f o s = 0 0.32 0.07-0.24 0.09-0.5

Three lepton

N s f o s = 1 , 2 0.94 0.18-0.61 0.27-1.2

N s f o s = 0 , 1 2.94 2.08-3.32 2.65-3.66

Four lepton

N s f o s = 2 1.23 0.73-1.34 0.85-1.46

T a b le 12. The final acceptance times selection efficiencies in the 4 W channel for non-resonant, resonant, and S S signal samples after all selection criteria are applied. Acceptance times selection efficiency is defined as the ratio of reconstructed signal events passing all selection criteria to the num ber of generated signal events th a t are filtered for the corresponding channel. The generator filter efficiencies are 4.4 x 10- 3 for the two same-sign lepton channel, 4.2 x 10- 3 for the three lepton channel, and 5.1 x 10- 4 for the four lepton channel. All num bers are given as percentages.

O p e n A c c e s s . T h is a rtic le is d is trib u te d u n d e r th e te rm s o f th e C re a tiv e C o m m o ns A ttr ib u tio n L icense ( C C -B Y 4.0) , w hich p e rm its an y use, d is trib u tio n a n d re p ro d u c tio n in an y m ed iu m , p ro v id ed th e o rig in al a u th o r(s ) a n d so u rce are c re d ite d .

R e fer e n c es

[1] A T L A S collaboration, O b s e r v a tio n o f a n e w p a r tic le i n th e se a r c h f o r th e S t a n d a r d M o d e l H ig g s b o so n w it h th e A T L A S d e te c to r a t th e L H C , P h y s . L e tt. B 716 (2012) 1

[a rX iv :1 2 0 7 .7 2 1 4 ] [inSPIRE].

[2] CM S collaboration, O b s e r v a tio n o f a n e w b o so n a t a m a s s o f 125 G e V w it h th e C M S e x p e r i m e n t a t th e L H C , P h y s . L e tt. B 716 (2012) 30 [a rX iv :1 2 0 7 .7 2 3 5 ] [inSPIRE].

[3] A T L A S and CM S collaborations, M e a s u r e m e n t s o f th e H ig g s b o so n p r o d u c t io n a n d d eca y r a te s a n d c o n s tr a in ts o n it s c o u p lin g s f r o m a c o m b in e d A T L A S a n d C M S a n a ly s is o f th e L H C p p c o llis io n d a ta a t / s = 7 a n d 8 TeV, J H E P 08 (2016) 045 [a rX iv :1 6 0 6 .0 2 2 6 6 ] [inSPIRE].

[4] A T L A S collaboration, S t u d y o f th e s p i n a n d p a r i t y o f th e H ig g s b o so n i n d ib o s o n d e c a y s w ith th e A T L A S d e te c to r , E u r . P h y s . J. C 75 (2015) 476 [ E r r a tu m ib id . C 76 (2016) 152]

[a rX iv :1 5 0 6 .0 5 6 6 9 ] [inSPIRE].

[5] CM S collaboration, C o n s tr a in ts o n th e s p i n - p a r i ty a n d a n o m a lo u s H V V c o u p lin g s o f th e H ig g s b o so n i n p r o t o n c o llis io n s a t 7 a n d 8 TeV, P h y s . R e v . D 92 (2015) 012004

[a rX iv :1 4 1 1 .3 4 4 1 ] [inSPIRE].

[6] A T L A S collaboration, T e s t o f C P I n v a r ia n c e i n v e c to r - b o s o n f u s i o n p r o d u c t io n o f th e H ig g s b o so n u s in g th e O p tim a l O b se rv a b le m e th o d i n th e d ita u d e c a y c h a n n e l w it h th e A T L A S d e te c to r , E u r . P h y s . J . C 76 (2016) 658 [a rX iv :1 6 0 2 .0 4 5 1 6 ] [inSPIRE].

J H E P 0 5 ( 2 0 1 9 ) 1 2 4