Search for supersymmetry in events with b-tagged jets and missing transverse momentum in $\mathit{pp}$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

P u b l i s h e d f o r S IS SA b y £ } S p r i n g e r R e c e i v e d: August 31, 2017

R e v i s e d: October 20, 2017

A c c e p t e d: November 21, 2017

P u b l i s h e d: November 29, 2017

Search for supersymmetry in events with b-tagged jets and missing transverse momentum in p p collisions at

√ s = 13 T eV with the A T L A S detector

T h e A T L A S collaboration

E -m a il: a t l a s . p u b l i c a t i o n s @ c e r n . c h

Ab s t r a c t: A search fo r th e su p ersy m m etric partn ers o f the S tan dard M o d e l b o tt o m and to p quarks is presented. T h e search uses 36.1 f b- 1 o f pp collision d a ta at √ s = 13 T e V co lle cte d b y th e A T L A S exp erim en t at th e L arge H a d ro n C ollid e r. D ire ct p ro d u ctio n o f pairs o f b o tt o m and to p squarks (b1 and i 1) is searched for in final states w ith b-tagged je ts and m issing transverse m om en tu m . D istin ctiv e selection s are defin ed w ith eith er n o ch arged lep ton s (electron s or m u on s) in th e final state, o r on e ch arged lep ton . T h e z e ro -le p to n selection targets m od els in w h ich th e b1 is th e lightest squark and d eca ys via b1 ^ 6x0, w here x⁰ is th e lightest n eu tralin o. T h e o n e -le p to n final state targets m od els w here b o t to m o r to p squarks are p ro d u ce d and ca n d e ca y in to m u ltiple channels, b¹ ^ ⁶X¹ and b¹ ^ t x f , o r b¹ ^ t x⁰ and b¹ ^ b x f , w here x f is th e lightest ch a rg in o and th e m ass differen ce m x± — m^{x 0} is set t o 1 G e V . N o excess a b o v e th e e x p e cte d Stan dard M o d e l b ack g rou n d is

X 1

ob served . E x clu sio n lim its at 95% co n fid e n ce level o n th e m ass o f th ird -g en era tion squarks are d erived in various su p ersy m m etry-in sp ired sim plified m od els.

Ke y w o r d s: H a d ro n -H a d ro n sca tterin g (exp erim en ts)

ArXiy ePr i n t: 1708.09266

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Contents

1 In trod u ction 1

2 A T L A S d etector 2

3 D a ta and sim ulated event sam ples 3

4 Event reconstruction 5

5 Event selection 7

5.1 D iscrim in a tin g variables 7

5.2 Z e ro -le p to n ch an n el selection s 10

5.3 O n e -le p to n channel selection s 11

6 B ackground estim ation 12

6.1 B a ck grou n d estim a tion in th e z e ro -le p to n signal regions 13 6.2 B a ck grou n d estim a tion in th e o n e -le p to n signal regions 16

6.3 V a lid a tion regions 16

7 S ystem atic uncertainties 18

8 R esu lts and interpretation 21

9 C onclusion 26

T h e A T L A S collaboration 33

1 Introduction

S u p ersym m etry (S U S Y ) [1- 6] provid es an exten sion o f th e Stan dard M o d e l (S M ) th a t solves th e h ierarch y p rob lem [7- 10] b y in tro d u cin g partners o f th e know n b o so n s and ferm ions.

In th e fram ew ork o f R -p a rity -co n s e rv in g m od els, S U S Y p articles are p ro d u ce d in pairs and th e lightest su p ersy m m etric particle (L S P ) is stable, p ro v id in g a p ossib le ca n d id a te for d ark m a tter [11, 12]. In a large variety o f m od els the L S P is th e lightest n eu tra lin o (x 1 ).

N aturalness con sid eration s [13, 14] suggest th at th e su p ersy m m etric partners o f th e th ird - gen eration S M quarks are th e lightest co lo u re d su p ersy m m etric p articles. T h is m a y lead t o th e lightest b o t t o m squark (b 1) and to p squark ( i 1) m ass eigen states1 b e in g significan tly lighter th an th e o th e r squarks and the glu in os. A s a con seq u en ce, b1 and i1 co u ld b e p a ir-p ro d u ce d w ith relatively large cross-section s at th e L arge H a d ron C ollid er (L H C ).

1Scalar partners of the left-handed and right-handed chiral components of the bottom quark (bL,R) or top quark (£l,r) mix to form mass eigenstates for which b1 and q are defined as the lighter of the two.

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T h is p ap er presents a search for th e d irect pair p ro d u ctio n o f b o tto m and to p squarks d e ca y in g in to final states w ith je ts, tw o o f th em origin a tin g fro m th e fra gm en ta tion o f b- quarks (b -je ts), and m issing transverse m om e n tu m (p!piss, w h ose m a gn itu d e is referred to as ETpiss). T h e d ata set analysed co rre sp o n d s to 36.1 f b^{- 1} o f p r o to n -p r o to n (pp) collision s d a ta at ^/s = 13 TeV co lle cte d b y th e A T L A S exp erim en t d u rin g R u n 2 o f the L H C in 2015 and 2016. T h e th ird -g en era tion squarks are assum ed to d e ca y t o th e lightest n eu tralin o (L S P ) d ire ctly o r th rou g h on e interm ed iate stage. T h e search is based o n sim plified m odels inspired b y th e m in im al su p ersy m m etric exten sion o f th e SM (M S S M ) [15- 17], w here the b1 e xclu siv ely d eca ys as b1 ^ bx? o r w here tw o d e ca y m od es fo r th e b o tto m (to p ) squark are allow ed and d irect d eca ys t o th e LSP, b1 ^ bx? (% ^ t%?) co m p e te w ith d eca ys via an in term ed ia te ch a rg in o ( x ± ) state, b1 ^ t x ± (% ^ b% ±). In this case it is assum ed th at the X ± is th e n e x t-to -lig h te st su p ersy m m etric p article (N L S P ) and is alm ost d egen era te w ith X¹, such th a t o th e r d e ca y p ro d u cts are t o o low in m om e n tu m to b e efficien tly recon stru cted . T h e first set o f m od els lead t o fin a l-state events fro m b o t t o m squark pair p ro d u ctio n ch ar­

acterized b y th e presence o f tw o b-jets, E ™ ss and n o ch arged lep ton s (I = e, p ), referred to as the z e ro -le p to n ch an n el (figu re 1a) . F or m ixed d eca ys m od els (in ten d ed as m od els w here b o th d irect d eca ys and d eca ys th rou g h an interm ed iate stage are k in em a tica lly allow ed), th e final state o f b o t t o m o r t o p squark pair p ro d u ctio n d e p e n d s o n th e b ra n ch in g ratios o f the c o m p e tin g d e ca y m od es. I f th e d e ca y m od es are eq u a lly p ro b a b le , a large fra ction o f th e signal events are ch a ra cterized b y th e presen ce o f a to p quark, a b o t to m quark, and neu tralin os. H a d ro n ic d eca ys o f th e to p quark are ta rgeted b y th e ze ro -le p to n channel, w hilst novel d e d ica te d selection s requ irin g on e ch arged lepton , tw o b-jets and ETpiss are d e v e lo p e d fo r sem i-le p to n ic d eca ys o f the to p quark, referred to as the o n e -le p to n channel (figu re 1b ) . A statistical co m b in a tio n o f th e tw o channels is p erform ed w h en interp retin g th e results in term s o f e x clu sion lim its o n th e th ird -g e n e ra tio n squark masses.

P rev iou s searches fo r th e exclu siv e d e ca y b¹ ^ bx? w ith th e y fs = 13 T eV L H C R u n -2 d ata set at A T L A S and C M S have set e x clu sion lim its at 95% co n fid e n ce level (C L ) on b¹ m asses in such scenarios [18, 19] . Searches in th e co n te x t o f m ix e d -d e ca y m od els were p erform ed o n ly b y A T L A S using th e R u n^{- 1} y/s = ⁸ T eV d ata set and resulted in exclu sion lim its o n th e th ird -g en era tion squark m ass th at d e p e n d o n the b ra n ch in g ratios o f the c o m p e tin g d e ca y m od es [^{2 0}] .

2 A T L A S detector

T h e A T L A S d e te cto r [2 1] is a m u lti-p u rp ose particle physics d e te cto r w ith a forw a rd - backw ard sy m m e tric cy lin d rica l g e o m e try and n early 4n cov era ge in solid a n g le^{. 2} T h e

2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector. The positive x-axis is defined by the direction from the interaction point to the centre of the LHC ring, with the positive y-axis pointing upwards, while the beam direction defines the z-axis. Cylindrical coordinates (r, 4>) are used in the transverse plane, 0 being the azimuthal angle around the z-axis. The pseudorapidity n is defined in terms of the polar angle ⁶ by n = — lntan(6/2). Rapidity is defined as y = 0.5ln[(E + pz)/(E — pz)] where E denotes the energy and pz is the component of the momentum along the beam direction.

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Figure 1. Diagrams illustrating the most relevant signal scenarios considered for the pair pro­

duction o f b ottom and top squarks targeted by the (a) zero-lepton and (b) one-lepton channel selections. In (a) b ottom squarks decay to a b ottom quark and the lightest neutralino. In (b), decays via intermediate charginos are kinematically available and com pete. If the mass difference Am(x ± , X ?) is small, the W bosons from chargino decays are off-shell.

inner tracking detector consists o f pixel and silicon m icrostrip detectors covering the pseu­

dorapidity region |n| < 2.5, surrounded by a transition radiation tracker which enhances electron identification in the region |n| < 2.0. Between Run 1 and R un 2, a new inner pixel layer, the insertable B-layer [22], was added at a mean sensor radius o f 3.3 cm . T he inner detector is surrounded by a thin superconducting solenoid providing an axial 2 T m agnetic field and by a fine-granularity lead/liquid-argon (L A r) electrom agnetic calorim eter cover­

ing |n| < 3.2. A steel/scintillator-tile calorim eter provides hadronic coverage in the central pseudorapidity range (|n| < 1.7). T he endcap and forward regions (1.5 < |n| < 4.9) o f the hadronic calorim eter are made o f L A r active layers with either copp er or tungsten as the absorber material. A n extensive muon spectrom eter with an air-core toroidal magnet system surrounds the calorimeters. Three layers o f high-precision tracking chambers pro­

vide coverage in the range |n| < 2.7, while dedicated fast chambers allow triggering in the region |n| < 2.4. T he A T L A S trigger system consists o f a hardware-based level-1 trigger followed by a software-based high-level trigger [23] .

3 D ata and simulated event samples

The data used in this analysis were collected by the A T L A S detector in pp collisions at the LH C with a centre-of-m ass energy o f 13 TeV and a 25 ns proton bunch crossing interval during 2015 and 2016. T he full dataset corresponds to an integrated lum inosity o f 36.1 fb -1 after requiring that all detector subsystems were operational during data recording. T he uncertainty in the com bined 2015+2016 integrated lum inosity is 3.2%. It is derived following a m eth odology similar to that detailed in ref. [24] from a preliminary calibration o f the lum inosity scale using x - y beam -separation scans perform ed in August 2015 and M ay 2016. Each event includes on average 13.7 and 24.9 inelastic pp collisions ( “pile-up” ) in the same bunch crossing in the 2015 and 2016 dataset, respectively. In the zero-lepton channel, events are required to pass an ETpiss trigger [25]. This trigger is

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fu lly efficient fo r events passing th e p reselection defin ed in section 5 , w h ich requires the offline re co n stru cte d ETpiss t o e xceed 200 G eV . E ven ts in th e o n e -le p to n channel, as well as events used fo r co n tro l regions, are selected on lin e b y a trigger requ irin g th e presence o f on e e le ctro n o r m uon. T h e on lin e selection th resh olds are such th a t a plateau o f the efficien cy is reached for ch a rg e d -le p to n transverse m om en ta o f 27 G eV and a bove.

M o n te C a rlo (M C ) sam ples o f sim u lated events are used t o m o d e l th e signal and to aid in the estim a tion o f SM b ack g rou n d processes, e x ce p t m u ltijet processes, w h ich are estim ated fro m d a ta only.

A ll sim ulated sam ples w ere p ro d u ce d using th e A T L A S sim u lation in fra stru ctu re [26]

using G E A N T 4 [27] , o r a faster sim u lation [28] based o n a p a ra m eteriza tion o f th e ca lorim e­

ter respon se and G E A N T 4 fo r the o th e r d e te cto r system s. T h e sim ulated events are re­

co n s tru cte d w ith th e sam e algorith m as th at used for d ata.

S U S Y signal sam ples w ere gen erated w ith M A D G R A P H 5 _ a M C @ N L O [29] v2.2 .3 at lead in g ord er (L O ) and interfaced t o Py t h i a v8 .1 86 [30] w ith th e A 1 4 [31] set o f tu n ed param eters (tu n e) fo r th e m o d e llin g o f th e p a rto n show ering (P S ), h a d ro n iza tio n and un­

d erlyin g event. T h e m a trix elem ent (M E ) ca lcu la tio n was p erform ed at tree level and includes th e em ission o f up t o tw o a d d ition a l parton s. T h e M E -P S m a tch in g was d on e using th e C K K W -L [32] p rescrip tion , w ith a m a tch in g scale set to on e q u a rter o f th e th ird - gen eration squark m ass. T h e N N P D F 2 3 L O [33] p a rto n d istrib u tio n fu n ctio n (P D F ) set was used. T h e cross-section s used t o evalu ate th e signal yields are ca lcu la ted t o n e x t-to - lea d in g-ord er (N L O ) a ccu ra cy in th e stron g co u p lin g con sta n t, a d d in g the resu m m ation o f soft glu on em ission at n e x t-to -le a d in g -lo g a rith m ic a ccu ra cy (N L O + N L L ) [34- 36] . T h e n om in a l cro ss-se ctio n and u n certain ty are taken as th e m id p o in t and h alf-w id th o f an en velop e o f cro ss-se ctio n p re d ictio n s using different P D F sets and fa cto riz a tio n and ren or­

m a lization scales, as d e scrib e d in ref. [37].

SM b ack g rou n d sam ples were sim ulated using different M C event gen erator p rogram s d e p e n d in g o n th e p rocess. T h e gen eration o f tt was p erform ed b y th e P o w h e g - B o x [38]

v2 gen erator w ith th e C T 1 0 [39] P D F set fo r th e m a trix elem ent ca lcu la tion s. Single- to p -q u a rk events in th e W t , s-, and t — channels w ere gen erated using the P o w h e g - B o x v1 gen erator. F or all processes in v olv in g to p quarks, to p quark spin correlation s were preserved. T h e p a rto n shower, fra gm en ta tion and th e u n derlyin g event were sim ulated using P y t h i a v6.428 [40] w ith th e C T E Q 6 L 1 P D F set and th e P eru gia 2012 [41] tu n e for th e u n derlyin g event. T h e h damp param eter in P o w h e g , w h ich co n tro ls th e p T o f th e first ad d itio n a l em ission b e y o n d th e B o rn level and thus regulates th e p T o f th e recoil em ission against th e tt system , was set t o the m ass o f th e to p quark (m t = 172.5 G e V ). A ll events w ith at least on e le p to n ica lly d e ca y in g W b o s o n are retained. Fully h a d ro n ic ttt and single­

to p events d o n ot con ta in sufficient E m iss to co n trib u te significan tly t o the b ack g rou n d . T h e tt sam ples are n orm a lized using th eir n e x t-to -N L O (N N L O ) cro ss-se ctio n in clu d in g th e resu m m a tion o f soft glu on em ission at n e x t-to -N L L a ccu ra cy using T o p + —+ 2 .0 [42].

Sam ples o f sin gle-top -q u a rk events are n orm a lized using th e N L O cross-section s rep orted in refs. [43- 45] for th e s-, t- and W t-ch a n n e ls, respectively.

E vents co n ta in in g W o r Z b o so n s w ith a ssociated je ts, in clu d in g je ts from th e frag­

m en ta tion o f b- and c-quarks, were sim ulated using th e Sh e r p a v2.2 .1 [46] gen erator.

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M a tr ix elem ents w ere ca lcu la te d fo r up t o tw o a d d itio n a l p arton s at N L O and fo u r par- ton s at L O using th e C o m ix [47] and O p e n L o o p s [48] m a trix elem ent event generators and m erged w ith th e S h e r p a P S [49] using th e M E + P S @ N L O p rescrip tion [50] . T h e N N P D F 3 0 N N L O [33] P D F set was used in co n ju n c tio n w ith a d e d ica te d P S tu n e d e v e lo p e d b y th e S h e r p a authors. A d d itio n a l S h e r p a Z + je t s sam ples w ere p ro d u ce d w ith sim ilar settings b u t w ith u p t o fo u r p arton s L O , fo r th e Y + je ts studies d etailed in section 6. T h e W / Z + j e t s events are n orm a lized using th eir N N L O Q C D th e o re tica l cross-section s [51] .

D ib o s o n processes were also sim ulated using th e S h e r p a gen erator using the N N P D F 3 0 N N L O P D F set in c o n ju n c tio n w ith a d e d ica te d P S tu n e d e v e lo p e d b y the S h e r p a authors. T h e y w ere ca lcu la ted fo r u p to on e ( Z Z ) or zero (W W , W Z ) a d d i­

tion a l p arton s at N L O and up t o three a d d ition al p arton s at L O . A d d itio n a l co n trib u tio n s t o th e SM b ack g rou n d s in the signal regions arise from the p ro d u ctio n o f to p quark pairs in a ssocia tion w ith W / Z / h b o so n s and p o ssib ly a d d ition a l je ts . T h e p ro d u ctio n o f t o p quark pairs in a sso cia tio n w ith electrow eak v e c to r b o so n s (W , Z ) o r H iggs b o so n s was m od eled b y sam ples gen erated at N L O using M A D G R A P H 5 _ a M C @ N L O v2 .2 .3 and show ered w ith P y t h i a v8.212. O th e r p o te n tia l sources o f b ack grou n ds, such as th e p ro d u ctio n o f three o r fo u r to p quarks o r th ree gau ge b oson s, are fou n d to b e n egligible.

F or all sam ples, e x ce p t th e ones gen erated using Sh e r p a, th e Ev tGe n v1 .2 .0 p ro ­ gram [52] was used t o sim ulate th e p rop erties o f th e b o tto m - and ch a rm -h a d ron decays.

In -tim e and o u t-o f-tim e p ile-u p in teraction s from the sam e or n earby bu n ch -crossin gs were sim u lated b y overla yin g a d d ition al pp collision s gen erated b y Py t h i a v8.186, w ith the M S T W 2 0 0 8 L O [53] P D F set, su p erim p osed o n to th e h a rd -sca tterin g events t o rep rod u ce th e o b served d istrib u tio n o f th e average n u m ber o f interaction s p er b u n ch crossin g [54].

Several sam ples p ro d u ce d w ith o u t d e te cto r sim u lation are e m p loy ed t o estim ate sys­

te m a tic u n certain ties a ssociated w ith th e sp ecific co n fig u ra tio n o f th e M C event gen erators used for th e n om inal S M b a ck g rou n d sam ples. T h e y in clu d e variation s o f th e ren orm aliza­

tio n and fa cto riz a tio n scales, th e C K K W -L m a tch in g scale, as well as different P D F sets and fra g m e n ta tio n /h a d r o n iz a tio n m od els. D eta ils o f th e M C m o d e llin g u n certain ties are discu ssed in section 7 .

4 Event reconstruction

T h e search for p air p ro d u ctio n o f b o t to m and to p squarks is based o n tw o d istin ct selections o f events w ith b-jets and large m issing transverse m om en tu m , w ith either n o ch arged lepton s in th e final state, o r requiring e x a ctly on e electron o r m u on (fo r details, see section 5). For th e z e ro -le p to n channel selection , events co n ta in in g ch arged lep ton s are e x p licitly v e to e d in th e signal and va lid a tion regions. E ven ts ch a ra cterized b y th e presen ce o f e x a ctly on e e le ctro n o r m u on w ith transverse m om en tu m a b o v e 27 G eV are retained in th e o n e -le p to n selection and are also used to define co n tro l regions fo r th e z e ro -le p to n channel. Finally, sam e-flavou r o p p o s ite -s ig n (S F O S ) tw o -le p to n (e le ctro n o r m u on ) events w ith d ile p to n invariant m ass near th e Z b o s o n m ass are used for co n tro l regions e m p loy ed to aid in th e estim a tion o f th e Z + j e t s b ack g rou n d fo r th e ze ro -le p to n channel. T h e d etails o f the re co n stru ctio n and selection , as w ell as th e overlap rem oval p ro ce d u re are given below .

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S elected events are required to have a recon stru cted p rim ary v e rte x con sisten t w ith th e b e a m s p o t en velop e and t o con sist o f at least tw o tracks in th e inner d e te cto r w ith p T

> 0.4 G eV . W h e n m ore th a n on e such v e rte x is fou n d , th e on e w ith th e largest sum o f the squares o f transverse m om en ta o f a ssociated tracks [55] is chosen.

Jet ca n d id ates are re co n stru cte d from th ree-d im en sion a l en ergy clusters [56] in the ca lo rim e te r using th e a nti-k t je t algorith m [5 7 , 58] w ith a radius param eter o f 0.4. T h e re co n stru cte d je ts are th en ca lib ra te d t o th e p article level b y th e a p p lica tio n o f a je t en ergy scale (JE S ) d erived from y/s = 13 T eV d a ta and sim u lation [59]. Q u a lity criteria are im p o se d to iden tify je ts arising fro m n o n -co llisio n sources o r d e te cto r noise, and any event co n ta in in g such a je t is rem oved [60]. F urther tra ck -b a sed selection s are a pp lied t o reject je ts w ith p T < 60 G eV and |n| < 2.4 th at o rigin ate from p ile-u p interaction s [61], and th e je t m om en tu m is co rre cte d b y s u b tra ctin g th e e x p e cte d average en ergy co n trib u tio n fro m p ile-u p using th e je t area m e th o d [62] . Jets are classified as “baselin e” and “ signal” . B aselin e je ts are required t o have p T > 20 G eV and |n| < 4.8. Signal je ts, selected after resolvin g overlaps w ith electron s and m uons, are required t o pass th e stricter requirem ent o f p T > 35 G eV and |n| < 2.8.

Jets are identified as b-jets if ta gged b y a m u ltivariate algorith m w h ich uses in form a tion a b o u t th e im p a ct param eters o f inner d e te cto r tracks m a tch ed t o the je t, th e presence o f d isp la ced se co n d a ry vertices, and th e re co n stru cte d flight p ath s o f b- and c-h a d ron s inside th e je t [63]. T h e b -taggin g w ork in g p oin t w ith a 77% efficiency, as d eterm in ed in a sam ple o f sim u lated t t events, was ch osen as part o f th e o p tim iz a tio n p ro ce d u re . T h e co rre sp o n d in g re je ctio n fa ctors against je ts origin a tin g from c-quarks and from light quarks and glu on s at this w ork in g p oin t are 6.2 and 134, resp ectively [64] . T o co m p e n s a te for differences b etw een d a ta and M C sim u lation in th e b -taggin g efficiencies and m is-ta g rates, co r re ctio n factors are derived from d a ta and a pp lied to the sam ples o f sim u lated events [63] . C a n d id a te b-jets are required t o have p T > 20 G eV and |n| < 2.5.

E le ctro n ca n d id ates are recon stru cted fro m en ergy clu sters in th e e le ctro m a g n e tic ca lo rim e te r m a tch ed to a track in th e inner d e te cto r and are required t o satisfy a set o f “lo o s e ” q u a lity criteria [65- 67]. T h e y are also required t o lie w ith in th e fidu cial volu m e

|n| < 2.47. M u o n ca n d id a tes are re co n stru cte d b y m a tch in g tracks in th e inner d e te cto r w ith tracks in th e m u on sp ectrom eter. E vents co n ta in in g on e o r m ore m u on ca n d id ates th a t have a transverse (lo n g itu d in a l) im p a ct param eter w ith resp ect t o th e p rim ary vertex larger th a n 0.2 m m (1 m m ) are rejected to suppress m uons from co s m ic rays. M u o n ca n ­ d id ates are also required t o satisfy “m e d iu m ” q u a lity criteria [^{6 8}] and have |n| i 2.5. A ll e le ctro n and m u on ca n d id a tes m ust have p T > 10 G eV . L e p to n ca n d id ates rem ain in g after resolvin g overlaps w ith baseline je ts (see n ext p aragrap h ) are called “ b aselin e” lepton s. In th e co n tro l and signal regions w here le p to n id en tifica tion is required, “ signal” lep ton s are ch osen from th e baseline set w ith p T > 27 G eV t o ensure full efficien cy o f th e trigg er and are required t o b e isolated from o th e r a ctiv ity in the d e te cto r using a criterion d esign ed to a cce p t at least 95% o f lep ton s from Z b o s o n d eca ys as d etailed in ref. [69]. T h e angular sepa ra tion betw een the le p to n and th e b-jet arising fro m a sem i-le p to n ica lly d e ca y in g to p quark narrow s as th e to p q u a rk ’ s p T increases. T h is increased co llim a tio n is a cco u n te d fo r b y varyin g the radius o f th e isolation co n e as m a x (0.2, 1 0 G e V /p T P), w here pTP is the

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le p to n p t . Signal electron s are fu rth er required t o satisfy “tig h t” q u a lity criteria. E lectron s (m u on s) are m a tch ed t o the p rim ary ve rte x b y requ irin g the transverse im p a ct param eter (d °) t o satisfy \d°\/a(d°) < 5 (3 ), and th e lon g itu d in a l im p a ct param eter (z ° ) t o satisfy

\z° sin d\ < 0.5 m m for b o th th e electron s and m uons. T h e M C events are co rre cte d to a cco u n t for differences in th e le p to n trigger, re co n stru ctio n and id en tifica tion efficiencies b etw een d a ta and M C sim ulation.

T h e sequ en ce t o resolve overla p p in g electron s, m uons and je ts begin s b y rem ovin g e le ctro n ca n d id a tes sharing an inner d e te cto r track w ith a m u on ca n d id a te. N ext, je t ca n d id a tes w ith in A R = ^ / ( A y) 2 + ( A^ ) 2 = 0.2 o f an electron ca n d id a te are d isca rd ed , unless th e je t is b-tagged, in w h ich case th e e le ctro n is d isca rd ed since it is likely to origin ate fro m a sem ilep ton ic b-h adron decay. E lectron s are d isca rd ed if th e y lie w ith in A R = 0.4 o f a je t. M u on s w ith p T b e lo w (a b o v e ) 50 G eV are d isca rd ed if th e y lie w ith in A R = 0.4 ( A R = 0.04 + 10 G e V /p t ) o f any rem ain in g je t , e x ce p t fo r th e case w here th e n u m ber o f tracks a ssociated w ith th e je t is less th a n three.

T h e m issing transverse m om en tu m is d efin ed as th e n egative v e c to r sum o f th e p t o f all selected and ca lib ra te d physics o b je c ts (electron s, m uons and je ts ) in th e event, w ith an extra term a dd ed t o a cco u n t for soft en ergy in the event w h ich is n ot a ssociated w ith any o f th e selected o b je c ts . T h is soft term is ca lcu la ted from inner d e te cto r tracks w ith p T a b o v e 0.4 G eV m a tch ed t o th e p rim ary v e rte x t o m ake it m ore rob u st against p ile-u p co n ta m in a tio n [7 0 , 71].

R e co n s tru cte d p h o to n s are n ot used in th e m ain signal event selection s b u t are selected in th e regions e m p loy ed in on e o f th e a ltern ative m e th o d s used t o estim ate the Z + je t s b a ck g rou n d , as exp la in ed in section ⁶. P h o to n ca n d id ates are required t o have p T >

145 G eV and \n\ < 2.37, w hilst b ein g ou tsid e th e tran sition region 1.37 < \n\ < 1.52, to satisfy the tigh t p h o to n show er shape and e le ctro n re je ctio n criteria [72] , and t o b e isolated.

5 Event selection

T w o sets o f signal regions (S R s) are defin ed and o p tim iz e d t o ta rget different th ird - gen eration squark d e ca y m od es and m ass hierarchies o f th e p articles involved. T h e zero- le p to n ch an n el SRs (b 0 L ) are d esign ed t o m a xim ize th e efficien cy t o retain b o tto m -sq u a rk p air p ro d u ctio n events w here b¹ ^ b x ?. T h e o n e -le p to n channel selection s (b 1 L ) ta rget S U S Y m od els w here b o t to m squarks d e ca y w ith a significant bra n ch in g ra tio as b1 ^ t% ± and th e lightest ch a rg in o is alm ost d egen era te w ith th e lightest n eu tralin o. W ith these as­

su m ption s, th e final d e ca y p ro d u cts o f th e off-shell W b o s o n from x ± ^ X ° W * are t o o soft t o b e d e te cte d . I f th e bra n ch in g ratios o f the tw o c o m p e tin g d e ca y m od es (b x ° , t x ± ) are arou n d 50% , th e final state for th e largest fra ctio n o f signal events is ch a ra cterized b y the presen ce o f a t o p quark, a b o t to m quark, and n eu tralin os esca p in g the d e te cto r. Sim ilarly, i¹ p air p ro d u ctio n can lead to an equivalent final state if th e i¹ ^ t x ° and i¹ ^ b x ± d eca y m o d e s co m p e te .

5.1 D iscrim inating variables

Several k in em a tic variables and angular correlation s, bu ilt from th e physics o b je c ts defined in th e p reviou s section , are e m p loy ed t o d iscrim in ate S U S Y from S M b ack g rou n d events

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and are re p o rte d b elow . In th e follow in g , signal je ts are used and are ord ered a cco rd in g to d ecreasin g p T .

• A ^ i n , m in [A0 ( je t 1 -4 , ETpiss)], m in [A 0 ( je t 1 -2 , ETpiss)]: these variables are th e m in­

im um A ^ betw een any o f th e leadin g je ts and th e m issing transverse m om en tu m v e cto r. T h e b ack g rou n d from m u ltijet p rocesses is ch a ra cterized b y sm all values o f this variable. D e p e n d in g o n th e signal regions, all, fo u r or tw o je ts are used.

• H t : this is defin ed as th e scalar sum o f the p T o f all je ts in the event

h t = ^ ( p T ? ) ^ i

w here th e n u m ber o f je ts involved d ep en d s o n the signal region. In a d d ition , the m od ified form o f H T , referred t o as th e HT 4 variable, is used t o reject events w ith e x tra -je t a ctiv ity in signal regions ta rg etin g m od els ch a ra cterized b y sm all m ass- sp littin g betw een th e b o t to m squark and th e n eu tralin o. In H^{T 4} th e sum starts w ith th e fo u rth je t (if an y).

• m eff: this is defin ed as th e scalar sum o f th e p T o f th e je ts and the ETpiss, i.e.:

meff = ^ ( p JTet)i + E m iss.

T h e m eff ob serva ble is correla ted w ith th e m ass o f th e p a ir-p ro d u ce d S U S Y particles and is e m p loy ed as a d iscrim in atin g variable in som e o f the ze ro -le p to n and o n e -le p to n channel selection s, as well as in the co m p u ta tio n o f o th e r c o m p o s ite observables.

• E “ iss/ m eff, E™iss/ V H T : th e first ra tio is th e E “ iss d iv id e d b y th e m eff, w hile the secon d em ulates th e g lo b a l ETpiss significan ce, given th at th e ETpiss resolu tion scales a p p ro x im a te ly w ith th e square ro o t o f th e to ta l h a d ro n ic en ergy in th e event. E vents w ith low values for these variables are rejected as it is m ost p ro b a b le th a t ETpiss arises from je ts m ism easurem ents, cau sed b y instru m en tal and resolu tion effects.

• m j j : this variable is ca lcu la ted as th e invariant m ass o f the lead in g tw o je ts . In events w here at least on e o f th e lead in g je ts is b-tagged, this variable aids in red u cin g th e co n ta m in a tio n from tt events. It is referred t o as m bb fo r events w here th e tw o leadin g je ts are b-tagged.

• m T : th e event transverse m ass m T is defin ed as m T = \J2pTPETpiss — 2p T P • Pmiss and is used in th e o n e -le p to n co n tro l and signal regions t o red u ce th e W + je t s and ttt b ackgrou n ds.

• m ™ n: th e m in im u m invariant m ass o f th e le p to n and on e o f th e tw o b-jets is defin ed as:

m ™ n = m in i= i, 2 ( m ^ ) .

T h is variable is b o u n d fro m a b o v e b y — m W for t t p ro d u ctio n , and it is used t o distin gu ish ttt co n trib u tio n s from W t-ch a n n e l sin gle-top -q u a rk events in th e on e- le p to n co n tro l regions.

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C on tran sverse m ass (t o -c t ) [73]: this is th e m ain d iscrim in atin g variable in som e o f th e z e ro -le p to n ch an n el signal regions [74]. It is used t o m easure th e m asses o f pair- p ro d u ce d sem i-in visib ly d e ca y in g h eavy particles. F or iden tica l d eca ys o f tw o heavy p articles (e.g. th e b o tto m squarks d e ca y in g e xclu siv ely as b¹ ^ b%°) in to tw o visible p articles v1 and v2 (th e b-quarks), and tw o invisible p articles X1 and X2 (th e x ° for th e signal), m CT is defin ed as

m C T(V¹, V²) = [E T (V¹) + E t ( V^{2 ) ] 2} - [ p t ( V¹) - p t ( V²)]2,

w ith E t = ^ /pT + m 2, and it has a k in em atical e n d p o in t at m ^ T = ( m² — m X ) / m ; w here i is th e initially p a ir-p ro d u ce d particle. T h is variable is effective in suppressin g th e to p -q u a rk p air p ro d u ctio n b ack g rou n d (i = t , X = W ), fo r w h ich th e e n d p o in t is at 135 G eV .

mmin( je t 1 -4 , E “ lss): this is the m in im u m o f th e transverse m asses ca lcu la ted using any o f th e leadin g fou r je ts and th e E ™ ss in th e event. For signal scenarios w ith low values o f m jm p, this kin em a tic variable is an altern ative d iscrim in atin g variable to red u ce th e t t b ack g rou n d .

a m T 2: th e a sy m m etric transverse m ass [75 , 76] is a k in em a tic variable w h ich ca n be used t o separate processes in w h ich tw o d eca ys givin g m issing transverse m om en tu m o ccu r, and it is th e m ain d iscrim in atin g ob serva ble in th e o n e -le p to n ch an n el signal regions. T h e a mT 2 d efin ition is based on th e stransverse m ass (m T2) [77] :

m T²( x ) = m in m a x { m T (pT ( v¹), q£ } ; x ) , m T (pT (V²), qT °; x ) j

1,-.,(2) .- .miss L (. J

qT + qT —pT

w here pT (v j) are recon stru cted transverse m om en ta v ectors and qT represent the m issing transverse m om en ta from th e tw o deca ys, w ith a to ta l m issing transverse m om en tu m , p™ ss; X is a free p aram eter representing the u n kn ow n m ass o f the in­

visib le p articles — here assum ed t o b e zero. T h e a in a m^{T 2} in d ica tes th a t th e tw o visib le d e ca y legs are asy m m etric, i.e. n ot co m p o s e d o f th e sam e particles.

In th e case o f events w ith on e le p to n (electron o r m u on ) and tw o b-jets, th e m^{T 2} variable is ca lcu la te d for different values o f pT ( v 1) and pT (v 2), b y gro u p in g the le p to n and th e tw o b-jets in to tw o visib le o b je c t s v1 and v 2. T h e le p to n needs t o be paired w ith on e o f th e tw o b-jets and th e ch o ice is d riven b y th e value o f m b^(n) — th e invariant mass o f th e n th b-tagged je t and th e lep ton . If th e tw o p articles are co r re c tly associated , this value has an u p p er b o u n d given b y th e to p quark mass.

T h e value o f a mT 2 is thus co m p u te d a cco rd in g ly :

- If m b^(1) and m b^(2) are b o th > 170 G eV , neither o f th e tw o a ssociation s is c o m ­ p a tib le w ith th e b-jet and th e le p to n origin a tin g from a to p decay, so th e event is rejected since all co n tro l, v a lid ation and signal regions require th e sm aller value o f m be t o b e < 170 G eV .

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- If m ^ ( 1 ) is < 170 G eV and m ^ ( 2 ) is > 170 G eV , a m^{T 2} is ca lcu la te d w ith v1 = b1 + 1 and v2 = b2. T h is is d o n e b eca u se o n ly th e first p airin g is co m p a tib le w ith a t o p quark decay.

- Sim ilarly, if m ^ ( 1 ) is > 170 G eV and m b^(2) is < 170 G eV , a m^{T 2} is ca lcu la ted w ith v1 = b1 and v2 = b2 + G

- If m b^(1) and m b^(2) are b o th < 170 G eV , a mT 2 is ca lcu la te d in b o th co n fig u ­ ration s and its value is taken to b e th e sm aller o f th e tw o. T h is m ust b e d on e beca u se, a cco rd in g t o the m be check, b o th pairings w ou ld b e a ccep ta b le.

• A : this is th e p T a sy m m e try o f th e lead in g tw o je ts and is defin ed as:

^ = p t ( j1) — p T ( j2) p t ( j¹) + p T ( j²)

T h e A variable is e m p loy ed in scenarios w here the m a ss-sp littin g betw een th e b o t to m squark and th e n eu tra lin o is sm all ( < 20 G eV ) and the selection e x p lo its the presence o f a h ig h -m om en tu m je t from in itial-sta te ra d ia tion (IS R ).

5.2 Z ero -lep to n channel selections

T h e selection criteria for th e z e ro -le p to n channel SR s are su m m arized in ta b le 1 and have th e m ain requirem ent o f n o baseline lepton s w ith p T > 10 G eV and tw o b-tagged je ts. T o e x p lo it th e k in em a tic p rop erties over th e large range o f b¹ and X¹ m asses exp lo re d , three sets o f SRs are defin ed.

T h e b 0 L -S R A regions are o p tim iz e d t o b e sensitive t o m od els w ith large m a ss-sp littin g betw een th e b¹ and th e X i, A m ( b¹, X1) > 250 G eV . In crem en ta l th resh olds are im p o se d on th e m ain d iscrim in atin g variable, m CT, resulting in th ree o verla p p in g regions ( m CT > 3 5 0 , 450 and 550 G e V ). O n ly events w ith ETpiss > 250 G eV are retained to ensure full efficien cy o f th e trigg er and c o m p ly w ith th e e x p e cte d signal to p o lo g y . T h e tw o leadin g je ts are required t o b e b -tagged w hilst co n ta m in a tio n from b ack g rou n d s w ith high je t m ultiplicity, p a rticu la rly t t p ro d u ctio n , is suppressed b y v e to in g events w ith a fo u rth je t w ith p T >

50 G eV . T o d iscrim in ate against m u ltijet b a ck g rou n d , events w here ETpiss is aligned w ith a je t in the transverse plane are rejected b y requiring m in [A ^ ( je t¹-⁴, ETpiss)] > 0.4, and ETpiss/ m eff > 0.25. A selection on th e invariant m ass o f th e tw o b-jets (m bb > 200 G eV ) is a pp lied t o fu rth er en h an ce the signal yield over th e SM b ack g rou n d con trib u tion s.

T h e b 0 L -S R B region targets in term ed ia te m a ss-sp littin g betw een b¹ and X 1, 50 <

A m ( b¹,X i ) < 250 G eV . In these scenarios, th e selection s based on th e m CT and m bb vari­

ables are n o longer effective and the variable m m in( je t1- 4, ETpiss) is e m p loy ed to red u ce SM ba ck g rou n d co n trib u tio n s fro m tt p ro d u ctio n , w ith events selected if mmin( je t¹-⁴, ETpiss) >

250 G eV . N o m ore th an fo u r signal je ts are allow ed, t o red u ce a d d itio n a l h a d ro n ic a ctiv ity in th e selected events. A s o p p o s e d t o the b 0 L -S R A criteria, n o v e to based o n the fo u rth je t p T is app lied. A series o f selection s o n the a zim u th al angle betw een th e tw o b-tagged je ts and th e ETpiss are im plem en ted ( | A 0(b¹, ETpiss)I < ^{2 . 0} and |A 0(b²,ETpiss)| < 2.5) t o red u ce Z + je t s b ack g rou n d events.

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b 0 L -S R A x b0L -S R B b0L -S R C L ep ton veto N o e / p w ith p e > 1 0 GeV after overlap rem oval

Njets (p t > 35 GeV) 2 -4 2 -4 —

Njets (p t > 20 GeV) — — 2 -5

P T ( j1) [G eV ] > 130 > 50 > 500

P T ( j²) [GeV] > 50 > 50 > ^{2 0}

P T ( j 4) [GeV] < 50 — —

H t4 [GeV] — — < 70

6-jets j 1 and ^{j 2} any ² j2 and ( j 3 or j 4 or j 5)

ET?iss [GeV] > 250 > 250 > 500

E ^ iss/m eff > 0.25 - —

m in [A ^ (je t1 -4 , E ™ ss)] > 0.4 > 0.4 —

m in [A ^ (je t1_ 2, Elj?iss)] — - > 0 . 2

A ^ (^{6 1} ,ET?iss) — < 2 . 0 —

A ^ (b2 ,ET?iss) — < 2.5 —

A⁰ ( j¹, E ^ iss) — - > 2.5

m j j [GeV] > 2 0 0 — > 2 0 0

m c T [GeV] > 3 5 0 , 450, 550 — —

m min(je t1 -4 , ET?iss) [GeV] — > 250 —

m eff [GeV/’] — — > 1300

A — — > 0 . 8

T a b le 1. Summary o f the event selection in each signal region for the zero-lepton channel. For SRA, the “x” denotes the selection used. The term lepton is used in the table to refer to baseline electrons and muons. Jets ( j i , j 2, j ’3, j4 and j s ) are labelled with an index corresponding to their decreasing order in p T .

Finally, th e b 0 L -S R C region targets events w here a b o t t o m squark p air is p ro d u ce d in a ssocia tion w ith a je t fro m IS R . T h is selection provid es sen sitivity t o m od els w ith a sm all m ass differen ce betw een th e b1 and th e x ? , A m ( b 1,x 1 ) < 50 G eV , such th at a b o o s te d b o t t o m squark pair w ou ld satisfy the trigger requirem ents. T o efficien tly suppress t t and W + je t s b ack g rou n d s, events are selected w ith on e h igh -p T n on -b -tagg ed je t and E>miss > ^{5 0 0} G eV such th a t A ^ ( j 1,E m iss) > 2.5. S tringent requirem ents o n th e m in im u m azim u th al angle betw een the je ts and Eepiss are n ot suited fo r these scen arios w here b- je ts have softer m om en ta and are p o ssib ly aligned w ith Eepiss. A large a sy m m etry A is required to red u ce the m u ltijet b ack g rou n d w hile loosen in g th e selection o n th e m in im u m azim u th al angle betw een th e je ts and Eepiss t o m in [A 0 ( je t 1_ 2, ETpiss)j > 0 .2 , and relaxing th e p T th resh old on signal je ts t o 20 GeV .

5.3 O n e -lep to n channel selections

T h e selection criteria fo r th e o n e -le p to n channel SR s are su m m arized in ta b le 2 . E ven ts are required t o have e x a ctly on e signal e le ctro n or m u on and n o a d d itio n a l baseline lepton s, tw o b -tagged je ts and a large Eepiss. Sim ilarly t o th e z e ro -le p to n channel, th ree sets o f SRs are defin ed t o m a xim ize th e sen sitivity d e p e n d in g o n th e m ass hierarchy betw een b1( t 1)

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b 1 L -S R A x b 1 L -S R A 3 0 0 -2 j b 1 L -S R B

N u m b er o f lep ton s (e, ^ ) 1 1 1

Njets (Pt > 35 G eV ) > ² = ² > ²

6-jets any ² j1 and j2 any ²

E jplss [GeV] > ^{2 0 0} > ^{2 0 0} > ^{2 0 0}

ETplss/ ^ H T [G eV 1/2] > 8 > 8 > 8

m ^ [GeV] < 170 < 170 < 170

Ad>j •rmin > 0.4 — > 0.4

m in [A 0 ( je t 1 -2 , E™iss)] — > 0.4 —

a mT 2 [GeV] > 250 > 250 > 2 0 0

m T [GeV] > 140 > 140 > ^{1 2 0}

m bb [GeV] > 2 0 0 > 2 0 0 < 2 0 0

meff [GeV] > 6 0 0 ,7 5 0 > 300 > 300

m mln(6- je t 1 -2 , ETplss) [GeV] — — > 2 0 0

A⁰ (⁶i,E T plss) — — > 2 . 0

T a b le 2. Summary o f the event selection in each signal region for the one-lepton channel. For SRA, the “x ” denotes the m eff selection used. The term lepton is used in the table to refer to signal electrons and muons. Jets ( ji , j 2) are labelled with an index corresponding to their decreasing order in p T .

T h e b 1 L -S R A regions are o p tim iz e d fo r m od els w ith large A m (61,% 0): events are re­

quired t o have large ETpiss and E m lss/\ /H T and A ^ m a b o v e 0.4 t o red u ce th e m u ltijet ba ck g rou n d co n trib u tio n s t o n egligible levels. R equ irem en ts o n th e m T and a mT 2 variables t o b e a b o v e 140 G eV and 250 G eV , respectively, are set t o reject W + je t s and tt events w hilst th e selection o n the invariant m ass o f th e tw o b-jets (m bb > 200 G eV ) is app lied to fu rth er en h an ce th e signal yield over th e SM b ack g rou n d co n trib u tio n s. T w o in crem en ­ tal th resh olds are finally im p o se d on m eff (600 and 750 G eV ) t o define tw o overla p p in g signal regions.

T h e b 1 L -S R B region is d esign ed t o b e sensitive t o com p ressed m ass spectra , h en ce low m bb is e x p e cte d , and the selection s o n th e m T and a mT 2 variables m ust b e relaxed to avoid loss o f signal events. T h e m in [m T (b-jet, E m lss)] is e m p loy ed t o d iscrim in ate signal fro m ti events, w h ich is th e d om in a n t S M b a ck g rou n d co n trib u tio n .

A th ird region, referred to as b 1 L -S R A 3 0 0 -2 j, is defin ed sim ilarly t o th e b lL - S R A s but requ irin g n o e x tra je ts b eside th e tw o ⁶-jets and m eff a b o v e 300 G eV . Such a selection also targets S U S Y m od els ch a ra cterized b y com p ressed m ass spectra . It is k in em a tica lly sim ilar t o th e signal region in th e R u n -1 analysis [20] w ith a v e to requirem ent o n th e n u m ber o f je ts w ith p T > 50 G eV .

6 Background estimation

M o n te C a rlo sim u lation is used t o estim ate th e b a ck g rou n d yield in th e signal regions.

T h e M C p re d ictio n fo r th e m a jo r b ack g rou n ds is n orm a lized to d a ta in co n tro l regions

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( C R ) co n stru cte d t o en h an ce a p articu la r b ack g rou n d and to b e k in em a tica lly sim ilar bu t m u tu a lly exclu siv e t o th e signal regions. T h e co n tro l regions are d efin ed b y e x p licitly requ irin g th e presen ce o f on e or tw o lepton s (electron s o r m u o n s ) in th e final state to g e th e r w ith fu rth er selection criteria sim ilar t o th ose o f th e co rre sp o n d in g signal region. T o ensure th a t th e b 0L and b1L analyses ca n b e sta tistica lly co m b in e d , th e C R s a ssociated w ith b0L and b1L SRs are m u tu ally exclu sive, w ith th e e x ce p tio n o f th e sin g le-top C R , w here the sam e C R is used for b o th channels.

T h e e x p e cte d SM b ack g rou n ds are d eterm in ed separately fo r each S R w ith a profile lik elih ood fit [78] , referred t o as th e b a ck g ro u n d -o n ly fit. T h e fit uses as a con strain t the o b served event yields in a set o f a ssociated C R s t o adju st th e n orm a liza tion o f th e m ain b ack g rou n d s, assum ing th at n o signal is present. T h e inpu ts to th e fit fo r each S R inclu de th e n u m ber o f events ob served in its a ssociated C R s and th e n u m ber o f events p re d icte d by sim u lation in each region fo r all b ack g rou n d processes. T h e latter are d e scrib e d b y P oisson statistics. T h e system a tic u n certain ties in th e e x p e cte d values are in clu d ed in th e fit as nuisance param eters. T h e y are con strain ed by G aussian d istrib u tion s w ith w idth s co rre ­ s p o n d in g t o th e sizes o f th e u n certain ties and are treated as correla ted , w h en ap p rop riate, b etw een th e various regions. T h e p ro d u ct o f th e various p ro b a b ility d en sity fu n ction s form s th e lik elih ood , w h ich th e fit m axim izes b y a d ju stin g th e b a ck g rou n d n orm a liza tion and the nuisance param eters. Finally, th e reliability o f th e M C e x tr a p o la tio n o f th e SM b ack g rou n d estim ate ou tsid e o f the co n tro l regions is evalu ated in several v a lid ation regions (V R s ).

6 .1 B ackground estim ation in the zero -lep to n signal regions

T h e m ain SM b ack g rou n d in th e b 0L signal regions is from th e p ro d u ctio n o f Z + je t s follow ed b y invisible d eca ys o f th e Z b o so n . T h e p ro d u ctio n o f to p quark pairs, single to p quarks and W + je t s also results in im p o rta n t b ack g rou n d s, w ith th eir relative co n trib u tio n s d e p e n d in g o n th e sp ecific S R co n sid ered . Full details o f th e C R defin ition s are given in ta bles ³ and ⁴ .

T h ree sam e-flavou r o p p o s ite -s ig n (S F O S ) tw o -le p to n (e le ctro n o r m u on ) co n tro l regions w ith d ile p to n invariant m ass near th e Z b o so n m ass (76 < m u < 106 G eV ) and tw o b- ta gged je ts p ro v id e d a ta sam ples d o m in a te d b y Z b o s o n p ro d u ctio n . Signal lep ton s are con sid ered , w ith th e th resh old for th e secon d le p to n p T loosen ed t o 20 G eV . For these co n tro l regions, labelled in th e fo llo w in g as b 0 L -C R z A , b 0 L -C R z B and b 0 L -C R z C , th e p e o f the lep ton s is a dd ed v e cto ria lly t o th e p™ ss t o m im ic th e e x p e cte d m issing transverse m om e n tu m sp ectru m o f Z ^ v t events, and is in d ica ted in th e fo llo w in g as E m iss,cor (le p to n co r re c te d ). In a d d ition , a selection is app lied t o th e u n co rre cte d Eepiss o f th e event, in ord er t o fu rth er en h an ce th e Z b o s o n co n trib u tio n .

E vents w ith on e ch arged le p to n in th e final state are used t o define co n tro l regions d o m in a te d b y W + je t s and to p quark p ro d u ctio n b y requ irin g eith er on e o r tw o b-tagged je ts , respectively. Selections o n th e variable m T are used t o ensure th a t th e le p to n origin ates fro m a W decay. F or th e C R s co rre sp o n d in g to b 0 L -S R A , th e co n trib u tio n from ttt and single to p quark p ro d u ctio n are separated b y a p p ly in g th e selection m bb < 200 G eV and m bb > 200 G eV , respectively. T o fu rth er en h an ce th e sin gle-top -q u a rk co n trib u tio n , a selection o n the m in im u m invariant m ass o f th e lep ton and on e o f th e b-jets, m ™ >

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b0L- CRzA CRttA CRstA CRwA CRzB CRttB CRwB

Number of leptons (1 = e, p) 2 SFOS 1 1 1 2 SFOS 1 1

px(ti) [GeV] > 90 > 27 > 27 > 27 > 27 > 27 > 27

Pt(G) [GeV] > 20 — — — > 20 — —

mu [GeV] [76 -106] — — — [76-106] —

—

Njets (pt > 35 GeV) 2-4 2-4 2-4 2-4 2-4 2-4 2-4

Pt ( ji) [GeV] > 50 > 130 - > 130 > 50 > 50 > 50 Pt (j2) [GeV] > 50 > 50 > 50 > 50 > 50 > 50 > 50

Pt (j'4) [GeV] < 50 < 50 < 50 < 50 — — —

b-jets ji and j2 ji and j ji and j2 j i any 2 any 2 any 2

E5?iss [GeV] < 100 > 200 > 200 > 200 < 100 > 100 > 100

£miss’cor [GeV] > 100 — — — > 200 — —

E ^iss/meff > 0.25 > 0.25 > 0.25 > 0.25 — — —

min[A^(jeti-4 , Emiss)] — > 0.4 > 0.4 > 0.4 > 0.4 > 0.4 > 0.4

mT [GeV] — — — > 30 — > 30 > 30

mbb [GeV] > 200 < 200 > 200 mbj > 200 — — —

mcT [GeV] > 250 > 250 > 250 > 250

—

—

mmin [GeV] — — > 170 —

—

—

mm?in(jeti _4, E miss) [GeV] — — — — > 200 > 200 > 250

A^(bi,ETpiss) — — — — — < 2.0 < 2.0

A^(b2, E miss) — — — — — < 2.5 —

T a b le 3. Summary o f the event selection in each control region corresponding to b0L-SR A and b0L-SRB. The term lepton is used in the table to refer to signal electrons and muons. Jets ( j 1, j 2, j³ and j 4) and leptons (¹i and l 2) are labelled with an index corresponding to their decreasing order in p T .

b0L- CRzC CRttC CRwC

Number o f leptons (1 = e, p) 2 SFOS 1 1

P T (fi) [GeV] > 27 > 27 > 27

Pt(12) [GeV] > 2 0 — —

m « [GeV] [76 -106] — —

Njets (Pt > 20 GeV) 2-5 2-5 2-5

Leading jet p t [GeV] > 250 > 500 > 500

b-jets j2 and (j' 3 or j4) j2 and (j' 3 or j'4) j²

E miss [GeV] < ^{1 0 0} > ^{1 0 0} > ^{1 0 0}

E miss,c°r [GeV] > 2 0 0 — —

mT [GeV] — > 30 [30-120]

meff [GeV] > 500 > 1300 > 500

mj j [GeV] > 2 0 0 > 2 0 0 > 2 0 0

H^{T 4} [GeV] < 70 < 70 < 70

A > 0.5 > 0.5 > ^{0 . 8}

A j Emiss) > 2.5 > 2.5 > 2.5

T a b le 4. Summary o f the event selection in each control region corresponding to b0L-SRC. The term lepton is used in the table to refer to signal electrons and muons. Jets ( j i , j 2, j3 and j 4) and leptons (¹i and £2) are labelled with an index corresponding to their decreasing order in p T .

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170 G eV is a pp lied. F or th e C R s co rre sp o n d in g t o b 0 L -S R B , selection s o n th e azim uthal angle betw een the 6-jets and th e £Tpiss value are app lied t o en h an ce the tt and W + je t s co n trib u tio n s, w hile th e sin gle-top -q u a rk b ack g rou n d is estim ated fro m M C sim ulation.

T h e C R s co rre sp o n d in g t o th e b 0 L -S R C are defin ed w ith on e o r tw o ⁶-je ts t o en h an ce th e t t and W + je t s co n trib u tio n s, respectively. Finally, th e single to p quark p ro d u ctio n is estim ated using th e M C n orm a lization .

T h e co n trib u tio n s from d ib o so n s ( W W , W Z , Z Z ), tt p ro d u ctio n a ssociated w ith W and Z b oson s, and o th e r rare b ack g rou n ds are estim ated from M C sim u lation for b o th th e signal and th e co n tro l regions and in clu d ed in th e fit p roced u re, and are allow ed to vary w ith in th eir n orm a liza tion uncertainty. T h e b ack g rou n d from m u ltijet p ro d u ctio n is estim ated fro m d a ta using a p ro ce d u re d e scrib e d in detail in ref. [79] and m o d ified t o a cco u n t for the h eavy flavour o f th e je ts . T h e co n tr ib u tio n from m u ltijet p ro d u ctio n in all regions is fou n d t o b e n egligible.

In to ta l, fou r C R s are defin ed fo r th e b 0 L -S R A t o estim ate th e co n trib u tio n s from W + je t s , Z + je t s , tt and single to p quark p ro d u ctio n indep end en tly, w hile th ree C R s are defin ed fo r each o f th e b 0 L -S R B and b 0 L -S R C to estim ate W + je ts , Z + je t s and tt. T h e

£Tpiss d istrib u tio n in b 0 L -C R w A and b 0 L -C R z C is sh ow n in figures 2a and 2 b , w here g o o d agreem ent w ith th e S M p re d ictio n is achieved after th e b a ck g ro u n d -o n ly fit. T h e yields in all these C R s are show n in figure 3 and co m p a re d to th e M C p re d ictio n s b e fo re the lik elih ood fit is p e rform ed , in clu d in g o n ly th e statistical u n certain ty o f th e M C sam ples.

T h e b o t to m panel shows th e value o f th e n orm a liza tion factors, p , used for each o f the b ack g rou n d s fitted and given ta k in g in to a cco u n t statistical and d e te cto r-re la te d system a tic uncertainties.

A s a fu rth er va lid ation , tw o altern ative m e th o d s are used t o estim ate th e Z + j e t s c o n ­ trib u tio n . T h e first m e th o d e x p lo its th e sim ilarity o f th e Z + j e t s and 7 + je t s processes [79].

F or a p h o to n w ith p x significan tly larger th a n th e m ass o f th e Z b o so n , the k in em atics o f Y + je ts events stron gly resem ble th ose o f Z + je t s events. A set o f d e d ica te d co n tro l regions is defin ed b y requ irin g on e isolated p h o to n w ith p T > 145 G eV . T h e p T o f th e p h o to n is v e cto ria lly a d d ed to th e p™ ss, and th e m a gn itu d e o f this sum is used t o replace th e E ™ ss- based selection s. T h e yield s are th en p ro p a g a te d to th e SRs using a rew eightin g fa cto r d erived using th e M C sim u lation . T h is fa c to r takes in to a cco u n t th e different kin em atics o f th e tw o processes and residual effects arising from th e different geo m e trica l a cce p ta n ce and re co n stru ctio n efficien cy fo r p h o to n s. In the secon d altern ative m e th o d , app lied to b 0 L -S R A only, th e M C sim u lation is used t o verify th a t th e sh ap e o f th e m CT d istrib u tio n fo r events w ith n o ⁶-ta g g e d je ts is co m p a tib le w ith th e sh ap e o f th e m Ct d istrib u tio n for events w here tw o 6-ta g g e d je ts are present. A new h igh ly p o p u la te d Z + je t s C R is d e ­ fined, selectin g Z ^ H events w ith n o 6-ta g g e d je ts . T h e m CT d istrib u tio n in this C R is co n s tru cte d using th e tw o lead in g je ts and is used t o estim ate th e shape o f th e m CT d istri­

b u tio n in the b 0 L -S R A , w h ilst th e n orm a liza tion in S R A is rescaled based on th e ratio in d a ta o f Z ^ H events w ith n o 6-ta g g e d je ts t o events w ith tw o 6-ta g g e d je ts. A d d itio n a l M C -b a s e d co rre ctio n s are a pp lied to take in to a cco u n t th e tw o -le p to n selection in this C R . T h e tw o altern ative m e th o d s are in agreem ent w ith in u n certain ties w ith th e estim ates o b ­ ta in ed w ith the p rofile lik elih ood fit t o th e co n tro l regions. E xp e rim e n ta l and th eoretica l

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sy ste m a tic u n certain ties in th e estim ates from th e n om inal and altern ative m e th o d s are taken in to a cco u n t (see section 7) .

6 .2 B ackground estim ation in the on e-lep ton signal regions

T h e m ain SM b a ck g rou n d in th e b 1L signal regions is th e p ro d u ctio n o f t t and single- to p -q u a rk events in th e W t channel. T w o co n tro l regions (b 1 L -C R t t A and b 1 L -C R ttB ) w here the tt p ro d u ctio n is en h an ced are defin ed by inverting th e a m T2 selection . In th e case o f b 1 L -C R t t A th e m bb selection is also inverted, w h ile fo r b 1 L -C R ttB the m in [m x (b -je t, ETplss)] requirem ent is inverted. T o allow a statistical co m b in a tio n o f the results fro m th e b 0 L -S R A and b 1 L -S R A regions the co rre sp o n d in g tt C R s are defined t o b e o rth o g o n a l via th e m Ct selection . T h e sin gle-top -q u a rk co n trib u tio n is estim ated w ith th e sam e C R e m p loy ed b y th e b 0L analysis. In th e case o f b 1 L -S R B th e p ro d u ctio n o f W + je t s is n o lon ger n egligible, and is estim ated b y using a d e d ica te d co n tro l region b 1 L -C R w B , w here o n ly on e b-tagged je t is required. In to ta l, tw o C R s are used t o estim ate th e event yield s in b 1 L -S R A and th ree C R s t o estim ate th e yields in b 1 L -S R B . Full details o f th e C R selection s are given in ta b le 5 . T h e d istrib u tio n o f m bb in b 1 L -C R s tA and o f m T in b 1 L -C R ttB are presented in figures 2c and 2d t o sh ow th e level o f agreem ent achieved after the b a ck g ro u n d -o n ly fit.

T h e yield s in all these C R s are also show n in figure 3 and co m p a re d t o the d irect M C p re d ictio n b e fo re th e lik elih ood fit is p e rform ed . T h e n orm a liza tion param eters re­

p o rte d fo r each S R and SM b ack g rou n d p rocess in clu d e th e statistical and d e te cto r-re la te d sy ste m a tic u n certain ties. T h e decrease o f th e param eter from S R A t o S R C is related t o m ism od ellin g in th e d e scrip tio n o f tt processes b y P o w h e g + P y t h i a 6 M C sam ples.

P re v io u s analyses [80] also fou n d n orm a liza tion factors co n sid e ra b ly sm aller th a n u n ity fo r tt b a ck g rou n d p rocesses in sim ilar regions o f phase space. T h e W + je t s and Z + je t s n orm a liza tion fa ctors are larger th an unity. T h is is p o ss ib ly related t o th e fa ct th a t in the defa u lt S h e r p a 2.2.1 th e h eavy-flavou r p ro d u ctio n fra ction s are n ot con sisten t w ith the m easured values [81] .

6 .3 V alidation regions

T h e results o f th e b a ck g ro u n d -o n ly fit to th e C R s are e x tra p o la te d t o a set o f V R s defined t o b e sim ilar t o th e SRs, w ith som e o f the selection criteria m od ified t o en h an ce the b a ck g rou n d co n trib u tio n , w hile m ain tain in g a sm all signal co n trib u tio n . For each SR , on e o r m ore V R s are defin ed startin g from th e S R d efin ition and invertin g or ch a n gin g som e o f th e selection s as su m m arized in ta b le ⁶.

T h e n u m ber o f events p re d icte d b y th e b a ck g ro u n d -o n ly fit is co m p a re d t o th e d a ta in th e u p p er pan el o f figure 4 . T h e pull, defin ed b y the differen ce betw een th e ob served n u m ber o f events (n obs) and th e p re d icte d b ack g rou n d yield ( n pred) d iv id e d b y th e to ta l u n certain ty (a tot), is sh ow n fo r each region in th e low er panel. N o e vid en ce o f significant b a ck g rou n d m ism od ellin g is o b served in th e V R s.

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