Measurement of masses in the tt system by kinematic endpoints in pp collisions at √s=7TeV

Eu . Phys. J. C (2013) 73:2494 DOI 10.1140/epjc/s10052-013-2494-7 Regula A icle - Expe imen al Physics Measu emen o masses in he sys em by kinema ic endpoin s in pp collisions a √s=7TeV The CMS Collabo a ion∗ CERN, Gene a, Swi ze land Recei ed: 21 Ap il 2013 / Re ised: 18 June 2013 / Published online: 16 July 2013 © CERN o he bene i o he CMS collabo a ion 2013. This a icle is published wi h open access a Sp inge link.com Abs ac A simul aneous measu emen o he op-qua k, W-boson, and neu ino masses is epo ed o e en s se- lec ed in he dilep on inal s a e om a da a sample co e- sponding o an in eg a ed luminosi y o 5.0 b−1collec ed by he CMS expe imen in pp collisions a √s=7TeV. The analysis is based on endpoin de e mina ions in kine- ma ic dis ibu ions. When he neu ino and W-boson masses a e cons ained o hei wo ld-a e age alues, a op-qua k mass alue o M =173.9±0.9(s a .) +1.7 −2.1(sys .) GeV is ob ained. When such cons ain s a e no used, he h ee pa - icle masses a e ob ained in a simul aneous i . In his uncon- s ained mode he s udy se es as a es o mass de e mina- ion me hods ha may be used in beyond s anda d model physics scena ios whe e se e al masses in a decay chain may be unknown and unde ec ed pa icles lead o unde con- s ained kinema ics. 1 In oduc ion The de e mina ion o he op-qua k mass se s a undamen- al benchma k o he s anda d model (SM), and is one o he p ecision measu emen s ha de ines elec oweak con- s ain s on possible new physics beyond he SM [1]. Wi h he ecen obse a ions [2,3] o a Higgs boson candida e a a mass o app oxima ely 125 GeV, exis ing da a can now o e cons ain he SM. The op qua k plays an impo an ole in such cons ain s because i s la ge mass, appea ing quad a ically in loop co ec ions o many SM obse ables, domina es o he con ibu ions. I is also key o he qua - ic e m in he Higgs po en ial a high ene gy, and he e- o e o he ques ion o s abili y o he elec oweak acuum [4,5]. Fo hese easons, p ecise op-qua k mass de e mi- na ions a e essen ial o cha ac e ize and p obe he SM. Re- cen esul s ob ained a he La ge Had on Collide (LHC) ∗e-mail: [email p o ec ed] o he op-qua k mass in e en s include hose epo ed by ATLAS [6], M =174.5±0.6(s a .)±2.3 (sys .) GeV, and by he Compac Muon Solenoid (CMS) [7], M = 173.49 ±0.43 (s a .) ±0.98 (sys .) GeV, using he semilep- onic decay channel o he pai . The CMS Collabo a ion has also epo ed a measu emen [8] in he dilep on chan- nel, M =172.5±0.4(s a .)±1.5 (sys .) GeV. A ecen summa y o op-qua k mass measu emen s conduc ed by he CDF and D0 Collabo a ions [9] epo s a combined esul M =173.18 ±0.56 (s a .) ±0.75 (sys .) GeV. In pa allel wi h ecen measu emen s o he p ope ies o he op qua k a he LHC, he e has been a g ea deal o heo e ical p og ess on me hods using endpoin s o kine- ma ic a iables o measu e pa icle masses wi h minimal in- pu om simula ion. These me hods a e gene ally aimed a measu ing he masses o new pa icles, should hey be dis- co e ed, bu can also be applied o measu e he masses o s anda d model pa icles such as he op qua k. Such an ap- plica ion ac s as bo h a es o he me hods and a measu e- men o he op-qua k mass u ilizing echnique e y di e en om hose used in p e ious s udies. Indeed, op-qua k pai p oduc ion p o ides a good ma ch o hese new me hods, as dilep on decays o op-qua k pai s ( →(b+ν)(b−¯ν)) p o ide challenges in mass measu e- men e y simila o he ones ha hese me hods we e de- signed o sol e. A key ea u e o many cu en heo ies o physics beyond he s anda d model is he exis ence o a can- dida e o da k ma e , such as a weakly in e ac ing massi e pa icle (WIMP). These pa icles a e usually s abilized in a heo y by a conse ed pa i y, o en in oduced ad hoc, unde which SM pa icles a e e en and new-physics pa icles a e odd. Examples include R-pa i y in supe symme y (SUSY) and T-pa i y in li le-Higgs models. One consequence o his pa i y is ha new physics pa icles mus be p oduced in pai s. Each o he pai -p oduced pa icles will hen decay o a cascade o SM pa icles, e mina ing wi h he ligh es odd-pa i y pa icle o he new heo y. In such cases, he e Page 2 o 28 Eu . Phys. J. C (2013) 73:2494 Fig. 1 Top-qua k pai dilep on decays, wi h wo je s, wo lep ons, and wo unobse ed pa icles (le ) exhibi a signa u e simila o some SUSY modes ( igh ). In he igu e, u,χ±,ν,andχ0deno e he u-squa k, cha gino, sneu ino, and neu alino espec i ely; an as e isk indica es he an ipa icle o he co esponding SUSY pa icle will be wo pa icles which do no in e ac wi h he de ec o , yielding e en s whe e he obse able kinema ics a e unde - cons ained. Mass measu emen s in hese e en s a e u he complica ed by he p esence o mul iple new pa icles wi h unknown masses. The dilep on decays o e en s a he LHC o e a ich sou ce o symme ic decay chains e mina ing in wo neu i- nos. Wi h hei combina ion o je s, lep ons, and unde ec ed pa icles, hese e en s bea close kinema ic and opologi- cal esemblance o new-physics scena ios such as he supe - symme ic decay chain illus a ed in Fig. 1. This co espon- dence has mo i a ed [10] he idea o use he abundan sam- ples o he LHC as a es bed o he new me hods and no el obse ables ha ha e been p oposed o handle mass mea- su emen in new-physics e en s [11]. A simul aneous mea- su emen o he op-qua k, W-boson, and neu ino masses in dilep on decays closely mimics he s a egies needed o s udies o new physics. The analysis p esen ed he e ocuses on he MT2 a iable and i s a ian s [11,12]. These kinema ic obse ables a e mass es ima o s ha will be de ined in Sec . 4. The goals o his analysis a e wo- old: o demons a e he pe o mance o a new mass measu emen echnique, and o make a p ecise measu emen o he op-qua k mass. To demons a e he pe - o mance o he me hod, we apply i o he sys em assum- ing no knowledge o he W-boson o neu ino masses. This allows us o measu e he masses o all h ee unde ec ed pa - icles in ol ed in he dilep on decay: he op qua k, W bo- son, and neu ino. This “uncons ained” i p o ides a es o he me hod unde condi ions simila o wha one migh ex- pec o ind when a emp ing o measu e he masses o new pa icles. In o de o make a p ecise measu emen o he op- qua k mass, on he o he hand, we assume he wo ld-a e age alues o he W boson and neu ino masses. This “doubly- cons ained” i achie es a p ecision in he op-qua k mass de e mina ion simila o ha ob ained by adi ional me h- ods. The MT2 obse able has been p e iously sugges ed [13] o used [14] in op-qua k mass measu emen s. In conside ing any op-qua k mass measu emen , how- e e , i is c i ical o con on he ac ha deep heo e ical p oblems complica e he in e p e a ion o he measu emen . The issues a ise because a op qua k is a colo ed objec while he W boson and had onic je obse ed in he inal s a e a e no . In he ansi ion →Wb, a single colo cha ge mus come om elsewhe e o neu alize he inal-s a e b je , wi h he ine i able consequence ha he obse ed ene gy and mo- men um o he inal s a e di e om ha o he o iginal op qua k. The esul ing di e ence be ween measu ed mass and op-qua k mass is he e o e a leas a he le el a which so colo exchanges occu , i.e. ∼ΛQCD [15,16]. In he cu en s a e o he a , a Mon e Ca lo (MC) gene a o is no mally used o ix a ela ionship be ween he expe imen ally mea- su ed mass o he inal s a e and a op-qua k mass pa am- e e o he simula ion; bu model assump ions upon which he simula ion o nonpe u ba i e physics depend u he limi he p ecision o such in e p e a i e s a emen s o abou 1GeV[17]. We he e o e ake ca e in his measu emen o dis inguish be ween he in e p e i e use o MC simula ion desc ibed abo e, which is inhe en ly model dependen , and expe i- men al p ocedu es, which can be made clea and model in- dependen . A dis inc i e ea u e o he op-qua k mass mea- su emen epo ed he e is i s limi ed dependence on MC simula ion. The e is no eliance on MC empla es [14], and he endpoin me hod gi es a esul which is consis en wi h he kinema ic mass in MC wi hou u he uning o co ec- ion. Fo his eason, he measu emen ou lined he e comple- men s he se o con en ional op-qua k mass measu emen s, and is applicable o new-physics scena ios whe e MC simu- la ion is used spa ingly. 2 The CMS de ec o and e en econs uc ion The cen al ea u e o he CMS appa a us is a supe conduc - ing solenoid o 6 m in e nal diame e , p o iding a magne ic ield o 3.8 T. Inside he supe conduc ing solenoid olume a e silicon pixel and s ip acke s, a lead ungs a e c ys al elec omagne ic calo ime e , and a b ass/scin illa o had on calo ime e . Muons a e measu ed in gas-ioniza ion de ec o s embedded in he s eel lux e u n yoke. Ex ensi e o wa d calo ime y complemen s he co e age p o ided by he ba - el and endcap de ec o s. A mo e de ailed desc ip ion o he CMS de ec o can be ound in Re . [18]. Je s, elec ons, muons, and missing ans e se momen- um a e econs uc ed using a global e en econs uc ion echnique, also called pa icle- low e en econs uc ion [19, 20]. Had onic je s a e clus e ed om he econs uc ed pa icles wi h he in a ed and collinea -sa e an i-kTalgo- i hm [21], using a size pa ame e 0.5. The je momen um is de e mined as he ec o ial sum o all pa icle momen a in his je , and is ound in he simula ion o be wi hin 5 % o 10 % o he ue momen um o e he whole ans e se mo- men um (pT) spec um and de ec o accep ance. Je ene gy co ec ions a e de i ed om he simula ion, and a e con- i med in measu emen s on da a wi h he ene gy balance o Eu . Phys. J. C (2013) 73:2494 Page 3 o 28 dije and pho on +je e en s [22]. The je ene gy esolu- ion amoun s ypically o 15 % a je pTo 10 GeV, 8 % a 100 GeV, and 4 % a 1 TeV. The missing ans e se mo- men um ec o is de ined by /pT≡−pTwhe e he sum is aken o e all pa icle- low objec s in he e en ; and missing ans e se “ene gy” is gi en by Emiss T≡|/pT|. 3 E en selec ion The da a se used o his analysis co esponds o an in e- g a ed luminosi y o 5.0 b−1o p o on-p o on collisions a √s=7 TeV eco ded by he CMS de ec o in 2011. We apply an e en selec ion o isola e a dilep on sample ha is la gely ee o backg ounds. We equi e wo well- iden i ied and isola ed opposi e-sign lep ons (elec ons o muons) passing dilep on igge equi emen s; he minimum pT equi emen s o he igge s a e 17 GeV and 8 GeV o he leading and sub-leading lep ons. In addi ion we e- qui e a leas wo b- agged je s, subsequen ly used in he op- qua k econs uc ion, and missing ans e se ene gy. He e and h oughou his pape , we use (and “lep on”) o de- no e an elec on o muon; he signal decays o in e es a e →bν. Lep ons mus sa is y pT>20 GeV and he e en is e oed i he lep ons ha e he same la o and hei dilep- on in a ian mass is wi hin 15 GeV o he Z boson mass. I h ee lep ons a e ound, he wo highes -pTlep ons o m- ing an opposi e-sign pai a e selec ed. Je s mus sa is y pT> 30 GeV a e co ec ing o addi i e e ec s o pileup (mul- iple p o on collisions in a single c ossing) and mul iplica- i e e ec s o je ene gy scale calib a ion. Je s a e u he equi ed o lie wi hin |η|<2.5, whe e ηis he pseudo apid- i y a iable, η≡−ln[ an(θ/2)]. The b- agging algo i hm is he Combined Seconda y Ve ex (CSV) agge o Re . [23], deployed he e wi h an ope a ing poin ha yields a agging e iciency o 85 % and mis ag a e o 10 %. The mis ag a e measu es he p obabili y o a ligh qua k o gluon je o be misiden i ied as a b je . In he subsample o e en s passing all selec ion equi emen s o his analysis he b-je pu i y is 91 %. Je masses a e equi ed o sa is y a e y loose equi e- men mje <40 GeV o assu e he exis ence o kinema ic solu ions and ejec poo ly econs uc ed je s. The missing ans e se ene gy mus sa is y Emiss T>30 GeV o e+e− and μ+μ−e en s and Emiss T>20 GeV o e±μ∓e en s, whe e D ell–Yan backg ounds a e smalle . Wi h he excep- ion o he b- agging c i e ia and he b-je mass equi emen , all selec ion equi emen s summa ized he e a e discussed in mo e de ail in [24,25]. The sample o e en s in da a mee ing all o he signal selec ion c i e ia con ains 8700 e en s. 4 Kinema ic a iables The endpoin me hod o mass ex ac ion is based on se e al a iables ha a e designed o use in he kinema ically com- plex en i onmen o e en s wi h wo cascade decays, each ending in an in isible pa icle. The challenge he e is wo- old, combining he complica ions o a many-body decay wi h he limi a ions o an unde cons ained sys em. In a wo- body decay A→BC, he momen um o ei he daugh e in he pa en es ame exhibi s a simple and di ec ela ion- ship o he pa en mass. In a h ee-body decay, A→BCD, he ela ionship is less di ec , encoded no in a del a unc ion o momen um bu in he kinema ic bounda y o he daugh- e s’ phase space. In gene al, he pa en mass may be de e - mined om he endpoin s o he obse able daugh e mo- men a in he pa en es ame. To ca y ou his p og am, howe e , he daugh e masses mus be known and enough o he momen a be measu able o cons ained by conse a- ion laws o sol e he kinema ic equa ions. Applying his p og am o he measu emen o he op- qua k mass in he decay →bν, one immedia ely encoun- e s a numbe o obs acles. A a had on collide , he sys em is p oduced wi h unknown cen e -o -mass ene gy and has an e en -dependen pT-boos due o ecoil om he ini ial- s a e adia ion (ISR). Fu he mo e, in pp collisions we can apply cons ain s o momen um conse a ion only in he wo dimensions ans e se o he beam di ec ion. Since op qua ks a e no mally p oduced in pai s, he indi idual neu- ino momen a a e inde e mina e, adding u he complica- ion. These obs acles seem daun ing bu can be o e come by he use o “designe ” kinema ic a iables MT2 [12] and MCT [26], which, by cons uc ion, add ess p ecisely hese issues. In his pape we use MT2. Because he ans e se momen um o he sys em a ies om e en o e en , he pT-insensi i e e sion [27,28], MT2⊥, is pa icula ly use- ul. To measu e he masses o he op-qua k, W-boson, and neu ino, we measu e he endpoin s o h ee kinema ic dis- ibu ions, μ,μbb, and Mb, as discussed in he ollowing subsec ions. 4.1 MT2 and subsys em a iables 4.1.1 The MT2 obse able The a iable MT2 is based on he ans e se mass, MT, which was i s in oduced o measu e he W-boson mass in he decay W →ν. In his case, MTis de ined by M2 T≡m2 ν+m2 +2Eν TE T−pν T·p T.(4.1) The obse able MT ep esen s he smalles mass he W bo- son could ha e and s ill gi e ise o he obse ed ans e se momen a p Tand pν T=/pT. The u ili y o MTlies in he ac ha MT≤MWis gua an eed o W bosons wi h low ans- e se momen um. Fo a single W →ν decay such a lowe limi is only ma ginally in o ma i e, bu in an ensemble o e en s, he maximum alue achie ed, i.e. he endpoin o he MTdis ibu ion, di ec ly e eals he W boson mass. This Page 4 o 28 Eu . Phys. J. C (2013) 73:2494 obse a ion sugges s a “min-max” s a egy which is gene - alized by he in en ion o MT2. The MT2 obse able is use ul o inding he minimum pa en mass ha is consis en wi h obse ed kinema ics when wo iden ical decay chains aand beach e mina e in a missing pa icle. Figu e 1shows bo h a SM and a new physics example. I one knew he wo missing ans e se momen a sepa a ely, a alue o MTcould be calcula ed o ei he o bo h o he win decay chains and he pa en mass Mwould sa is y he ela ionship max(Ma T,Mb T)≤M.In p ac ice he wo missing momen a canno be known sepa- a ely, and a e obse able only in he combina ion pa T+pb T= /pT. This compels one o conside all possible pa i ions o /pTin o wo hypo he ical cons i uen s pa Tand pb T, e alua ing wi hin his ensemble o pa i ions he minimum pa en mass Mconsis en wi h he obse ed e en kinema ics. Wi h his ex ension o he MTconcep , he a iable is now called MT2: MT2 ≡min pa T+pb T=/pTmaxMa T,Mb T.(4.2) As wi h MT, he endpoin o he MT2 dis ibu ion has a quan i iable ela ionship o he pa en mass, and he endpoin o an MT2 dis ibu ion is he e o e a measu e o he unseen pa en mass in e en s wi h wo iden ical decay chains. The obse able MT2 equi es some ca e in i s use. The p esence o ET=p2 T+m2in Eq. (4.1) implies ha one mus ei he know (as in he case o W →ν)o assume(as in he case o unknown new physics) a alue o he mass m o he unde ec ed pa icle(s). In his pape we will e e o an assumed mass as he “ es mass” and dis inguish i wi h a ilde (i.e. m); he ac ual mass o he missing pa icle, whe he known o no , will be e e ed o as he “ ue mass”, and w i en wi hou he ilde. Bo h he alue o MT2 in any e en and he alue o he endpoin o he MT2 dis ibu ion in an ensemble o e en s a e in he end unc ions o he es mass. E en when a es mass has been chosen, howe e , he endpoin o he MT2 dis ibu ion may no be unique because i is in gene al sensi i e o ans e se momen um PT=|PT| o he unde lying wo-pa en sys em, which a ies om e en o e en . The sensi i i y anishes i he es mass can be se equal o he ue mass, bu such an op ion will no be immedia ely a ailable in a s udy o new physics whe e he ue mass is no known. The PTp oblem is ins ead add essed by in oducing MT2⊥[27,28], which uses only momen um componen s ans e se o he PTboos di ec ion. In his way, MT2⊥ achie es in a iance unde PTboos s o he unde lying wo- pa en sys em. The cons uc ion o MT2⊥is iden ical o ha o MT2 excep ha pT alues ha appea explici ly o im- plici ly in Eq. (4.1) a e e e ywhe e eplaced by pT⊥ alues, whe e pT⊥is de ined o be he componen o pTin he di- ec ion pe pendicula o he PTo he wo-pa en sys em. Fo mally, pT⊥≡ˆ nT×(pT׈ nT), (4.3) whe e ˆ nT=PT/|PT|is he uni ec o pa allel o he ans- e se momen um o he wo-pa en sys em. 4.1.2 Subsys em a iables A u he in es iga ion o MT2 and MT2⊥ e eals he ull ange o kinema ic in o ma ion con ained in mul is ep decay chains by spli ing and g ouping he elemen s o he decay chain in independen ways. The MT2 a iable classi ies he pa icles in an e en in o h ee ca ego ies: “ups eam”, “ isible”, and “child”. The child pa icles a e hose a he end o he decay chain ha a e unobse able o simply ea ed as unobse able; he isible pa icles a e hose whose ans e se momen a a e measu ed and used in he calcula ions; and he ups eam pa icles a e hose om u he up he decay chain, including any ISR accompanying he ha d collision. In gene al, he child, isible, and ups eam objec s may ac ually be collec ions o objec s, and he subsys em obse - ables in oduced in Re . [10] pa cel ou he kinema ic in o - ma ion in as many independen g oupings as possible. Fig- u e 2shows wo o he h ee possible ways o classi ying he daugh e s o MT2 calcula ions. The μ a iable, known Fig. 2 A dilep on decay wi h he wo subsys ems o compu ing μ and μbb indica ed. The “ups eam” and “child” objec s a e enclosed in dashed ec angles, while he isible objec s, which en e in o he com- pu a ion, a e enclosed in solid ec angles.Theμ and μbb a iables used he e a e iden ical o M210 T2⊥and M221 T2⊥o Re . [10] Eu . Phys. J. C (2013) 73:2494 Page 5 o 28 as M210 T2⊥in Re . [10], uses he wo lep ons o he dilep on decays, ea ing he neu inos as los child pa icles (which hey a e), and combining he b je s wi h all o he “ups eam” momen um in he e en . The μbb a iable, known as M221 T2⊥ in Re . [10], uses he b je s, and ea s he W bosons as los child pa icles (igno ing he ac ha hei cha ged daugh e lep ons a e in ac obse able). I conside s only ISR je s as gene a o s o ups eam momen um. Fo comple eness, we no e ha a hi d MT2⊥subsys em can be cons uc ed by combining he b je and he lep on as a single isible sys em. This a iable, known as M220 T2⊥ in he nomencla u e o Re . [10], exhibi s signi ican co e- la ion wi h Mb, he in a ian mass o he b je and lep on. A hi d obse able is needed o sol e he unde lying sys em o equa ions, and o his we choose Mb. 4.2 Obse ables used in his analysis This analysis is based on wo MT2⊥ a iables, μ and μbb as desc ibed abo e, and one in a ian mass, Mb, hein a i- an mass o a b je and lep on om he same op-qua k de- cay. These h ee quan i ies ha e been selec ed om a la ge se o possibili ies based on he low co ela ion we obse e among hem and he gene ally a o able shapes o he dis- ibu ions in hei endpoin egions. The obse ables can be summa ized by he unde lying kinema ics om which hey a e de i ed, and he endpoin ela ions which include he op-qua k, W-boson, and neu ino masses. Fo he μ a iable, he shape o he dis ibu ion is known analy ically [27]. In e ms o he alue x=μ and i s kinema ic endpoin xmax, he no malized dis ibu ion can be w i en: dN dx=αδ(x) +(1−α) 4x x2 max ln xmax x,(4.4) whe e he pa ame e αis ea ed as an empi ical quan i y o be measu ed. In p ac ice, α∼0.6, and he ze o bin o μ his og ams will be supp essed o be e show he ea u es o he endpoin egion. The o igin o he del a unc ion is geome ic: o massless lep ons, μ anishes when he wo lep on pT⊥ ec o s lie on opposi e sides o he axis de ined by he ups eam PT ec o , and is equal o 2(p+ T⊥p− T⊥)1/2 o he wise. Fo a es mass o he child pa icle mν, he endpoin is ela ed o he masses ia [10,27]: μmax  ≡xmax =MW 21−m2 ν M2 W +M2 W 41−m2 ν M2 W2 +m2 ν.(4.5) In he case, we se he es mass o mν=0. We hen ex- pec he endpoin a μmax  =MW(1−m2 ν/M2 W)=MW= 80.4 GeV. No e ha mνis he ue mass o he child and MW is he ue pa en mass; hese should be iewed as a iables in a unc ion o which mνis a pa ame e . In a new-physics applica ion, he analogs o MWand mνa e no known; bu gi en Eq. (4.5), he measu emen o he endpoin , and an a - bi a y choice o child mass mν, one can ix a ela ionship be ween he wo unknown masses. We emphasize ha he equali y exp essed by Eq. (4.5) holds ega dless o he alue o he es mass, because he es mass en e s in o bo h sides o he equa ion (see discussion in Sec . 4.1.1). This applies below o Eq. (4.6)also. In he case o μbb, he isible pa icles a e he wo b je s, he child pa icles a e he cha ged lep ons and neu i- nos (combined), and ISR adia ion gene a es he ups eam ans e se momen um. We ake he isible pa icle masses o be he obse ed je masses, which a e ypically ∼10 GeV. The endpoin is una ec ed by nonze o je masses p o ided he es mass is se o he ue mass, and is a ec ed only a he ±0.1 GeV le el o e a la ge ange o es masses, 0< MW<2MW. Fo an assumed child mass  MW, he end- poin is gi en by [10,27]: μmax bb =M 21−M2 W M2 +M2 41−M2 W M2 2 + M2 W.(4.6) In he case, we se he es mass o  MW=MW= 80.4 GeV. We hen expec he endpoin a μmax bb =M .As in he p e ious case, in a new-physics applica ion whe e he analogs o M and MWa e no known, he measu emen o he endpoin oge he wi h an a bi a y choice o he child mass  MWyields a ela ionship be ween he wo unknown masses. As no ed abo e, a hi d a iable is needed, and we adop Mb, he in a ian mass o med ou o je -lep on pai s eme ging om he op-qua k decay. Two alues o Mbcan be compu ed in a e en , one o each op decay. In p ac- ice ou a e calcula ed because one does no know a p i- o i how o associa e he b je s and lep ons; we discuss la e an algo i hm o mi iga ing he combina o ial e ec s on he endpoin . The shape o he dis ibu ion is known o co ec combina ions bu is no used he e since co ec combina ions canno be gua an eed (see Sec . 5.3). The endpoin is gi en by: Mmax b=m2 b+1−m2 ν M2 WE∗ W+p∗E∗ b+p∗,(4.7) whe e E∗ W,E∗ b, and p∗a e ene gies and momen a o he daugh e s o →bW in he op-qua k es ame. In hese o mulae he cha ged-lep on mass is neglec ed bu he ob- se ed b-je mass mbis ini e and a ies e en - o-e en . Page 6 o 28 Eu . Phys. J. C (2013) 73:2494 Fig. 3 Dis ibu ions o he h ee kinema ic dis ibu ions μ,μbb, and Mb.Da a(5.0 b −1) a e shown wi h e o ba s. MC simula ion is o e laid in solid colo o illus a e he app oxima e signal and backg ound con en o he dis ibu ions. The backg ounds con ained in “O he ” a e lis ed in Table 1. The ze o-bin o he μ plo is sup- p essed o cla i y. The Mbplo con ains mul iple en ies pe e en (see Sec . 5.3 o de ails). In all cases, he simula ion is no malized o an in eg a ed luminosi y o 5.0 b−1wi h nex - o-leading-o de (NLO) c oss sec ions as desc ibed in he ex We can now summa ize he mass measu emen s a egy. I he masses M ,MW, and mνwe e unknown, one would measu e he wo endpoin s and he in a ian mass ha ap- pea on he le -hand sides o Eqs. (4.5)–(4.7), using a bi- a y es mass alues o he i s wo, o ob ain h ee inde- penden equa ions o he h ee unknown masses. Then, in p inciple, one sol es o he h ee masses. In p ac ice, he measu emen s ca y unce ain ies and an op imum solu ion mus be de e mined by a i . In he case when one o mo e o he masses is known, a cons ained i can imp o e he de e mina ion o he emaining unknown mass(es). In Fig. 3we show dis ibu ions o he h ee obse ables μ,μbb, and Mb. He e and h oughou his pape , he ze o bin o he μ dis ibu ion, co esponding o he del a unc- ion o Eq. (4.4), is supp essed o emphasize he kinema - ically in e es ing componen o he shape. In he μbb plo shown he e, he p ominen peak ha domina es he igu e is an analog o he del a unc ion in μ, i s subs an ial wid h being due o he a iable mass o he je s ha en e in o he μbb calcula ion. As wi h he μ del a unc ion, he peak a ises om e en s whe e he axis o he ups eam PT alls be ween he wo isible-objec pT ec o s. In la e plo s his μbb peak will be supp essed o be e e eal he beha io o he dis ibu ion in he endpoin egion. The ag eemen be ween da a and MC is gene ally good, bu he compa isons a e o illus a ion only and he analysis and esul s ha ollow do no depend s ongly on he MC simula ion o i s ag eemen wi h obse a ion. 5 Backg ounds The wo-lep on equi emen a he co e o he e en selec- ion ensu es an excep ionally clean sample. Ne e heless a Table 1 Es ima e o signal and backg ound composi ion in MC sim- ula ion, no malized o an in eg a ed luminosi y o 5.0 b−1and NLO c oss sec ions as desc ibed in he ex P ocess Numbe o e en s signal (no τ) 7000 signal(τ→ν) 1100 Single op ( W,¯ W) 270 D ell–Yan 77 Had onic/Semilep onic wi h mis econs uc ed lep on(s) 55 Dibosons (WW, ZZ, WZ) 14 W+je s 9 ew ypes o backg ound mus be conside ed, including op- qua k decays wi h τ-lep on daugh e s, pp → W e en s, and sub-pe cen con ibu ions om o he sou ces. 5.1 Physics backg ounds The physics backg ounds consis o decays ha do no con o m o he dilep on opology o in e es , as well as non- decays. Table 1shows he es ima ion o signal and back- g ound e en s in MC simula ion. The MC gene a o s used h oughou his s udy a e MC@NLO 3.41 [29] o all sam- ples, PYTHIA 6.4 [30] o he diboson samples, and MAD- GRAPH 5.1.1.0 [31] o all o he s. The simula ed da a sam- ples a e no malized o 7 TeV NLO c oss sec ions and an in eg a ed luminosi y o 5.0 b−1. E en s in which a op qua k decays h ough a τlep on (e.g. →bτ+ντ→b+ν¯ντντ), cons i u e abou 13 % o he e en s su i ing all selec ion equi emen s. F om he poin o iew o e en selec ion, hese e en s a e back- Eu . Phys. J. C (2013) 73:2494 Page 7 o 28 g ound. The unobse ed momen um ca ied by he ex a neu inos, howe e , ensu es ha hese e en s econs uc o MT2 and Mb alues below hei ue alues and hence all below he endpoin o signal e en s wi h di ec decays o e o μ inal s a es. We he e o e include hese e en s among he signal sample. This lea es in p inciple a small dis o ion o he kinema ic shapes, bu he dis o ion is a om he endpoin and i s impac on he mass ex ac ion is negligible. 5.2 Modelling he mis ag backg ound In addi ion o he backg ounds discussed abo e, which all wi hin he bulk he dis ibu ions, i is essen ial also o ea e en s ha lie beyond he nominal signal endpoin . In his analysis, he main sou ce o such e en s comes om gen- uine e en s whe e one o he je s no o igina ing om a op-qua k decay is mis agged as a b je . An e en in which a ligh -qua k o gluon je is ea ed as coming om a op qua k can esul in e en s beyond he endpoin in he μbb and Mbdis ibu ions, as can be seen in Fig. 4. The mea- su emen o μ, on he o he hand, depends p ima ily on he wo lep ons and is una ec ed by mis ags. To de e mine he shape o he mis ag backg ound in μbb and Mb, we selec a con ol sample wi h one b- agged je and one an i agged je , whe e he an i agging iden i ies je s ha a e mo e likely o be ligh -qua k o gluon je s han b je s. An i agging uses he same algo i hm as combined seconda y e ex algo i hm, bu selec s je s wi h a low disc imina o alue o ob ain a sample domina ed by ligh -qua k and gluon je s. We classi y e en samples by he b- ag alues o he wo selec ed je s, and iden i y h ee samples o in e es : a signal sample whe e bo h je s a e b- agged; a backg ound sample whe e one je is b- agged and he o he an i agged; and an- o he backg ound sample whe e bo h je s a e an i agged. Ta- ble 2shows he composi ion o hese samples as de e mined in MC simula ion. We selec he sample consis ing o pai s wi h one agged and one an i agged je o be he con ol sam- ple and use i o de e mine he shape o he backg ound lying beyond he signal endpoin . I con ains a signi ican ac ion o signal e en s, 27 %, bu hese all lie below he endpoin and ca ego izing hem as backg ound does no change he endpoin i . The con ol sample is used o gene a e dis ibu ions in μbb and Mb, whose shapes a e hen cha ac e ized wi h an adap i e ke nel densi y es ima ion (AKDE) me hod [32]. The unde lying KDE me hod is a non-pa ame ic shape cha ac e iza ion ha uses he ac ual con ol sample o es- ima e he p obabili y dis ibu ion unc ion (PDF) o he backg ound by summing e en -by-e en Gaussian ke nels. In he AKDE algo i hm, on he o he hand, he Gaussian wid hs depend on he local densi y o e en s; empi ically his algo i hm yields lowe bias in he inal mass de e mi- na ion han al e na i e algo i hms. Figu e 5shows he pe - o mance o he backg ound shape de e mina ion; he se o Fig. 4 Composi ion o MC e en samples, illus a ing ha signal e en s wi h ligh -qua k and gluon je con amina ion domina e he e- gion beyond he endpoin . The op and bo om Mbdis ibu ions con- ain he same in o ma ion plo ed wi h di e en e ical scales. The backg ounds con ained in “O he ” a e lis ed in Table 1 Table 2 Composi ion o b- agged, dije samples as de e mined in MC simula ion. Each column is an independen ly selec ed sample; columns sum o 100 % 2 b- ags b- ag, an i ag 2 an i ags bje ,bje 86% 27% 7.1% b je , non b je 14 % 70 % 53 % nonbje ,nonbje 0.3% 3% 40% con ol sample e en s a e aken om MC simula ion in o de o illus a e he composi ion o he backg ound and signal. 5.3 Supp essing he combina o ial backg ound E en i he b- agging algo i hm selec ed only b je s, he e would emain a combina o ics p oblem in dilep on e en s. In he case o he Mbdis ibu ion he ma ching p ob- Page 8 o 28 Eu . Phys. J. C (2013) 73:2494 Fig. 5 Backg ound PDF shapes de e mined by he AKDE me hod, on MC samples. All e en s pass he signal selec ion c i e ia. Top:Mb; bo om:μbb.Thehea y black cu e is he AKDE shape lem a ises in pai ing he b je o he lep on: o b je s j1 and j2, and lep ons +and −, wo pai ings a e possible: j1+,j2−and j1−,j2+. Fou alues o Mbwill hus be a ailable in e e y e en , bu only wo o hem a e co ec . The wo inco ec pai ings can (bu do no ha e o) gene - a e alues o Mbbeyond he kinema ic endpoin o Mbin op-qua k decay. To minimize he unwan ed backg ound o inco ec pai ings while maximizing he chance o e aining he highes alues o Mbin co ec bpai ings, which do espec he endpoin , we employ he ollowing algo i hm. Le Aand adeno e he wo Mb alues calcula ed om one o he wo possible bpai ings, and le Band bdeno e he Mb alues calcula ed om he o he pai ing. Choose he labeling such ha a<Aand b<B. Wi hou making any assump ions abou which pai ing is co ec , one can o de he Mb alues om smalles o la ges ; he e a e six pos- sible o de ings. Fo example he o de ing b,B,a,A means ha he bB pai ing has Mb alues which a e bo h smalle han he Mb alues in he aA pai ing. In his case, while we Table 3 Mbo de ings: in each column he le - o- igh sequencing o he a,A,b,B labels is om lowes Mb alue o highes . The le column lis s he six possible Mbo de ings; he igh column indica es o each o de ing which alues a e selec ed o inclusion in he Mb plo O de ing Selec ion bBaA b,B aAbB a,A baBA b,a,B baAB b,a,A abBA a,b,B abAB a,b,A do no know which pai ing is co ec , we can be ce ain ha bo h Mb alues o he bB pai ing mus espec he ue end- poin since ei he (a) bB is a co ec pai ing, in which case i s Mb alues na u ally lie below he endpoin , o (b) aA is he co ec pai ing, so i s Mb alues lie below he ue end- poin , wi h he bB alues alling a ye lowe alues. Simila a gumen s apply o each o he o he possible o de ings. Table 3shows he six possibili ies. Fo each mass o de - ing shown in he le column, he igh column shows he mass alues ha will be selec ed o use in he Mb i . Fo any gi en e en only one ow o he able applies. Fo an e en alling in one o he i s wo ows, wo alues o Mb en e in he subsequen i s; o an e en alling in he las ou ows, h ee alues en e he i s. This selec ion algo i hm ensu es ha all masses used in he i s ha can be gua an eed o be below he endpoin will be used, while any ha could exceed he endpoin because o w ong pai ings will be igno ed. No e ha i does no gua - an ee ha he masses ha a e used a e all om co ec b pai ings; in p ac ice, howe e , we ind ha 83 % o he en- ies in he i egion a e co ec bpai ings, and ha his ac ion ises o o e 90 % wi hin 10 GeV o he endpoin . 6 Fi s a egy The kinema ic obse ables μ,μbb, and Mb, along wi h hei endpoin ela ions (Sec . 4.2) and backg ound mi iga- ion echniques (Sec s. 5.2,5.3), a e combined in an un- binned e en -by-e en maximum likelihood i . The likeli- hood unc ion is gi en by a p oduc o e all e en s o in- di idual e en likelihoods de ined on each o he kinema ic a iables: L(M)= N  i=1Lμ i(ui|M)·Lμbb i(ui|M)·LMb i(ui|M). (6.1) The ec o M=(M ,MW,m2 ν)con ains he mass pa am- e e s o be de e mined by he i , and each uicomp ises Eu . Phys. J. C (2013) 73:2494 Page 9 o 28 he se o ans e se momen um ec o s, econs uc ed ob- jec masses, and missing-pa icle es masses om which he kinema ic obse ables μ,μbb, and Mbo he e en ia e compu ed. We i o m2 ν a he han mνbecause only m2 νap- pea s in he endpoin o mulae (Eqs. (4.5) and (4.7)); we do no cons ain m2 ν o be posi i e. As will be desc ibed mo e ully below, only he endpoin egion o each a iable is used in he i . I an e en idoes no all wi hin he endpoin e- gion o a gi en a iable, he co esponding likelihood com- ponen (Lμ i,Lμbb i,o LMb i) de aul s o uni y. Fo each obse able x∈{μ,μbb,Mb}, he likelihood componen Liin Eq. (6.1) can be exp essed in e ms o he alue o he obse able i sel , xi=x(ui), and i s kinema ic endpoin , xmax =xmax(M). Explici o mulae o xmax(M) a e gi en in Eqs. (4.5), (4.6), and (4.7); in he i s wo cases he e is addi ional dependence on he missing-pa icle es mass. Le ing he label a ∈{, bb,b}index he h ee la- o s o obse ables, we can w i e he signal, backg ound, and esolu ion shapes as S(x|xa max),Ba(x), and Ra i(x). While he o m o he signal shape S(x) iscommon oall h ee i s, he backg ound shape Ba(x) is speci ic o each ob- se able and he esolu ion unc ion Ra i(x) is speci ic o bo h he obse able and he indi idual e en . Then each unc ion La iappea ing on he igh -hand side o Eq. (6.1) is gi en by he gene al o m: La ixi|xa max=βSy|xa maxRa i(xi−y)dy+(1−β)Ba(xi). (6.2) The i pa ame e βde e mines he ela i e con ibu ion o signal and mis ag backg ound. Fo he common signal shape S(x|xa max)we use an ap- p oxima ion consis ing o a kinked-line shape, cons uc ed piecewise om a descending s aigh line in he egion jus below he endpoin and a cons an ze o alue abo e he end- poin . The kinked-line unc ion is de ined o e a ange om xlo o xhi. The gene ic o m is: S(x|xmax)≡N(xmax −x) xlo ≤x≤xmax; 0xmax ≤x≤xhi.(6.3) The pa ame e Nis ixed by no maliza ion. The ideli y o his i s -o de app oxima ion o he unde lying shape de- pends on bo h he shape and he alue o xlo. The ange o he i , (xlo,xhi), is chosen o minimize he dependence o he i esul s on he ange, and hen he alues o xlo and xhi a e subsequen ly a ied o es ima e he co esponding sys- ema ic unce ain ies. The ollowing pa ag aphs discuss he o ms o Ba(x) and Ra(x) o each o he h ee kinema ic dis ibu ions. 6.1 μ In he case o μ, he isible pa icles a e he wo lep ons, which a e well measu ed. The p ojec ion o hei ec o s on o he axis o hogonal o he ups eam PT, howe e , nec- essa ily in ol es he di ec ion o he ups eam PT, which is no nea ly as well de e mined. The esolu ion unc ion is he e o e wholly domina ed by he angula unce ain y in PT, and i a ies subs an ially om e en o e en de- pending on he pa icula con igu a ion o je s ound in each e en . Al hough je esolu ions a e known o ha e small non- Gaussian ails, hei impac on he μ esolu ion unc ion and he subsequen i p ocedu e is small and we ea only he Gaussian co e. A a mo e impo an ea u e o he eso- lu ion a ises when he PTdi ec ion unce ain y is p opaga ed in o he μ a iable o de i e R i(x). In his p ocedu e a sha p Jacobian peak appea s whe e e he PTsmea ing can cause μ o pass h ough a local maximum o minimum alue. These peaks depend only on azimu hal angles and oc- cu a any alue o μ. The de ailed shape o he highly non- Gaussian μ esolu ion and i s con olu ion wi h he unde - lying signal shape, as speci ied in Eq. (6.2), a e handled by exac o mulae de i ed analy ically (see he Appendix). The backg ound in he μ dis ibu ion is anishingly small, so we se B(x) =0. 6.2 μbb Fo μbb, he isible pa icles a e he b je s, and since he es- olu ion smea ing o bo h he b je s and he ups eam je s de ining PTa e la ge and o compa able magni udes, he e en -by-e en esolu ion is mo e complica ed han in he μ case. As a esul , no analy ic calcula ion is possible and we ins ead de e mine he μbb esolu ion unc ion, Rbb i(x), nume ically in each e en , using he known pTand φ es- olu ion unc ions o he je s. As wi h he μ esolu ions, Jacobian peaks appea in he μbb esolu ions. The mis ag backg ound is included by scaling he shape Bbb(x) ob- ained om he AKDE p ocedu e as discussed in Sec . 5.2. 6.3 Mb In he Mbcase, he heo e ical shape S(x) is well-known, bu he combina o ics o bma ching, oge he wi h he me hod o selec ing bpai s om he a ailable choices (see Sec . 5.3), sculp he dis ibu ion o he deg ee ha he he- o e ical shape is no longe use ul. The e o e we use he kinked-line shape o Eq. (6.3) o model he signal nea he endpoin . In con as o he μ and μbb a iables, nume i- cal s udies con i m ha linea ly p opaga ed Gaussian esolu- ions accu a ely e lec he smea ing Rb i(x) o Mb, as one expec s in his case. The backg ound shape Bb(x) is gi en by he AKDE p ocedu e as discussed in Sec . 5.2. Page 16 o 28 Eu . Phys. J. C (2013) 73:2494 sha ply peaked. In he la e case he del a unc ion R0δ(μ) is no plo ed. In he op panel he Φis midway be ween he ex emes ±1 2φ and π/2 and he σΦis ela i ely na ow; in he bo om panel, Φis close o π/2 and has a la ge alue o σΦ ha allows smea ing in o he −1 2φ < Φ < 1 2φ e- gion. The high bin a −45 GeV in he his og am compo- nen o he bo om panel con ains e en s in which he eso- lu ion smea ing o he ups eam momen um ec o pushed he μ alue in o he del a unc ion a μ =0. In he an- aly ic o m, he co esponding ea u e would be he del a unc ion R0δ(μ); bu , as no ed abo e, his has no been ex- plici ly d awn. Re e ences 1. H. 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Bencze, C. Hajdu, P. Hidas, D. Ho a h19, B. Radics, F. Sikle , V. Veszp emi, G. Vesz e gombi20, A.J. Zsigmond Ins i u e o Nuclea Resea ch ATOMKI, Deb ecen, Hunga y N. Beni, S. Czella , J. Molna , J. Palinkas, Z. Szillasi Page 20 o 28 Eu . Phys. J. C (2013) 73:2494 Uni e si y o Deb ecen, Deb ecen, Hunga y J. Ka ancsi, P. Raics, Z.L. T ocsanyi, B. Uj a i Panjab Uni e si y, Chandiga h, India S.B. Be i, V. Bha naga , N. Dhing a, R. Gup a, M. Kau , M.Z. Meh a, M. Mi al, N. Nishu, L.K. Saini, A. Sha ma, J.B. Singh Uni e si y o Delhi, Delhi, India Ashok Kuma , A un Kuma , S. Ahuja, A. Bha dwaj, B.C. Choudha y, S. Malho a, M. Naimuddin, K. Ranjan, P. Saxena, V. Sha ma, R.K. Shi pu i Saha Ins i u e o Nuclea Physics, Kolka a, India S. Bane jee, S. Bha acha ya, K. Cha e jee, S. Du a, B. Gombe , Sa. Jain, Sh. Jain, R. Khu ana, A. Modak, S. Mukhe jee, D. Roy, S. Sa ka , M. Sha an Bhabha A omic Resea ch Cen e, Mumbai, India A. Abdulsalam, D. Du a, S. Kailas, V. Kuma , A.K. Mohan y2, L.M. Pan , P. Shukla, A. Topka Ta a Ins i u e o Fundamen al Resea ch - EHEP, Mumbai, India T. Aziz, R.M. Cha e jee, S. Ganguly, M. Guchai 21,A.Gu u 22, M. Mai y23, G. Majumde , K. Mazumda , G.B. Mohan y, B. Pa ida, K. Sudhaka , N. Wick amage Ta a Ins i u e o Fundamen al Resea ch - HECR, Mumbai, India S. Bane jee, S. Dugad Ins i u e o Resea ch in Fundamen al Sciences (IPM), Teh an, I an H. A aei24, H. Bakhshiansohi, S.M. E esami25,A.Fahim 24, H. Hesa i, A. Ja a i, M. Khakzad, M. Mohammadi Naja abadi, S. Pak ina Mehdiabadi, B. Sa a zadeh26, M. Zeinali Uni e si y College Dublin, Dublin, I eland M. G unewald INFN Sezione di Ba ia, Uni e si à di Ba ib, Poli ecnico di Ba ic, Ba i, I aly M. Abb esciaa,b, L. Ba bonea,b, C. Calab iaa,b,2, S.S. Chhib aa,b, A. Colaleoa, D. C eanzaa,c, N. De Filippisa,c,2, M. De Palmaa,b,L.Fio e a,G.Iaselli a,c, G. Maggia,c, M. Maggia, B. Ma angellia,b,S.My a,c, S. Nuzzoa,b, N. Paci icoa, A. Pompilia,b,G.Pugliese a,c, G. Sel aggia,b, L. Sil es isa, G. Singha,b, R. Vendi ia,b, P. Ve willigena, G. Zi oa INFN Sezione di Bolognaa, Uni e si à di Bolognab, Bologna, I aly G. Abbiendia, A.C. Ben enu ia, D. Bonaco sia,b, S. B aiban -Giacomellia,b, L. B igliado ia,b, R. Campaninia,b, P. Capiluppia,b,A.Cas o a,b, F.R. Ca alloa, M. Cu iania,b, G.M. Dalla allea, F. Fabb ia, A. Fan ania,b, D. Fasanellaa,b, P. Giacomellia, C. G andia, L. Guiduccia,b, S. Ma cellinia, G. Mase ia, M. Meneghellia,b,2, A. Mon ana ia,F.L.Na a ia a,b, F. Odo icia, A. Pe o aa, F. P ima e aa,b,A.M.Rossi a,b,T.Ro elli a,b,G.P.Si oli a,b,N.Tosi a,b,R.T a aglini a,b INFN Sezione di Ca aniaa, Uni e si à di Ca aniab, Ca ania, I aly S. Albe goa,b, M. Chio bolia,b,S.Cos a a,b, R. Po enzaa,b, A. T icomia,b,C.Tu e a,b INFN Sezione di Fi enzea, Uni e si à di Fi enzeb, Fi enze, I aly G. Ba baglia, V. Ciullia,b, C. Ci ininia, R. D’Alessand oa,b, E. Foca dia,b,S.F osali a,b, E. Galloa, S. Gonzia,b, P. Lenzia,b, M. Meschinia, S. Paole ia, G. Sguazzonia, A. T opianoa,b INFN Labo a o i Nazionali di F asca i, F asca i, I aly L. Benussi, S. Bianco, F. Fabb i, D. Piccolo INFN Sezione di Geno aa, Uni e si à di Geno ab, Geno a, I aly P. Fabb ica o ea, R. Musenicha,S.Tosi a,b INFN Sezione di Milano-Bicoccaa, Uni e si à di Milano-Bicoccab, Milano, I aly A. Benagliaa,F.DeGuio a,b,L.DiMa eo a,b,2, S. Fio endia,b, S. Gennaia,2, A. Ghezzia,b, P. Go onia,b, M.T. Lucchinia,b,2, S. Mal ezzia, R.A. Manzonia,b, A. Ma ellia,b,A.Massi oni a,b, D. Menascea, L. Mo onia, M. Paganonia,b, D. Ped inia, S. Ragazzia,b, N. Redaellia, T. Taba elli de Fa isa,b Eu . Phys. J. C (2013) 73:2494 Page 21 o 28 INFN Sezione di Napolia, Uni e si à di Napoli ’Fede ico II’b, Uni e si à della Basilica a (Po enza)c,Uni e - si à G. Ma coni (Roma)d, Napoli, I aly S. Buon empoa, N. Ca alloa,c,A.DeCosa a,b,2, F. Fabozzia,c, A.O.M. Io ioa,b, L. Lis aa, S. Meolaa,d,2, M. Me olaa, P. Paoluccia,2 INFN Sezione di Pado aa, Uni e si à di Pado ab, Uni e si à di T en o (T en o)c,Pado a,I aly P. Azzia, N. Bacche aa,2,P.Bellan a,b, D. Biselloa,b, A. B ancaa,b,R.Ca lin a,b, P. Checchiaa,T.Do igo a, M. Galan ia,b, F. Gaspa inia,b, U. Gaspa inia,b, P. Giubila oa,b, A. Gozzelinoa, K. Kanishche a,c, S. Lacap a aa, I. Lazzizze aa,c, M. Ma gonia,b, A.T. Meneguzzoa,b, M. Michelo oa, F. Mon ecassianoa, M. Nespoloa, J. Pazzinia,b, M. Pego a oa, N. Pozzobona,b, P. Ronchesea,b, F. Simone oa,b,E.To assa a,M.Tosi a,b,P.Zo o a,b, G. Zume lea,b INFN Sezione di Pa iaa, Uni e si à di Pa iab,Pa ia,I aly M. Gabusia,b, S.P. Ra ia,b, C. Ricca dia,b, P. Vi uloa,b INFN Sezione di Pe ugiaa, Uni e si à di Pe ugiab, Pe ugia, I aly M. Biasinia,b, G.M. Bileia, L. Fanòa,b, P. La icciaa,b, G. Man o ania,b, M. Menichellia, A. Nappia,b,†, F. Romeoa,b, A. Sahaa, A. San occhiaa,b, A. Spieziaa,b INFN Sezione di Pisaa, Uni e si à di Pisab, Scuola No male Supe io e di Pisac, Pisa, I aly K. And oso a,27,P.Azzu i a, G. Bagliesia, T. Boccalia, G. B occoloa,c, R. Cas aldia, R.T. D’Agnoloa,c,2, R. Dell’O soa, F. Fio ia,c,2,L.Foà a,c, A. Giassia, A. K aana, F. Ligabuea,c, T. Lom adzea,L.Ma ini a,27, A. Messineoa,b, F. Pallaa, A. Rizzia,b, A.T. Se bana, P. Spagnoloa, P. Squillacio ia, R. Tenchinia, G. Tonellia,b, A. Ven u ia, P.G. Ve dinia, C. Ve nie ia,c INFN Sezione di Romaa, Uni e si à di Romab, Roma, I aly L. Ba onea,b,F.Ca alla i a,D.DelRe a,b,M.Diemoz a, C. Fanellia,b, M. G assia,b,2, E. Longoa,b,F.Ma ga oli a,b, P. Me idiania,2, F. Michelia,b, S. Nou bakhsha,b, G. O gan inia,b, R. Pa ama ia, S. Raha loua,b, L. So ia,b INFN Sezione di To inoa, Uni e si à di To inob, Uni e si à del Piemon e O ien ale (No a a)c, To ino, I aly N. Amapanea,b, R. A cidiaconoa,c,S.A gi o a,b, M. A neodoa,c,C.Biino a, N. Ca igliaa,S.Casasso a,b,M.Cos a a,b, P. De Remigisa, N. Dema iaa, C. Ma io ia,2, S. Masellia, E. Miglio ea,b, V. Monacoa,b,M.Musich a,2,M.M.Obe ino a,c, N. Pas onea, M. Pelliccionia, A. Po enzaa,b, A. Rome oa,b, M. Ruspaa,c, R. Sacchia,b, A. Solanoa,b, A. S aianoa, U. Tamponia INFN Sezione di T ies ea, Uni e si à di T ies eb, T ies e, I aly S. Bel o ea, V. Candelisea,b, M. Casa saa,F.Cossu i a,2,G.DellaRicca a,b, B. Gobboa, C. La Lica aa,b, M. Ma onea,b,2, D. Mon aninoa,b, A. Penzoa, A. Schizzia,b, A. Zane ia Kangwon Na ional Uni e si y, Chunchon, Ko ea T.Y. Kim, S.K. Nam Kyungpook Na ional Uni e si y, Daegu, Ko ea S. Chang, D.H. Kim, G.N. Kim, J.E. Kim, D.J. Kong, Y.D. Oh, H. Pa k, D.C. Son Chonnam Na ional Uni e si y, Ins i u e o Uni e se and Elemen a y Pa icles, Kwangju, Ko ea J.Y. Kim, Z.J. Kim, S. Song Ko ea Uni e si y, Seoul, Ko ea S. Choi, D. Gyun, B. Hong, M. Jo, H. Kim, T.J. Kim, K.S. Lee, D.H. Moon, S.K. Pa k, Y. Roh Uni e si y o Seoul, Seoul, Ko ea M. Choi, J.H. Kim, C. Pa k, I.C. Pa k, S. Pa k, G. Ryu Sungkyunkwan Uni e si y, Suwon, Ko ea Y. Choi, Y.K. Choi, J. Goh, M.S. Kim, E. Kwon, B. Lee, J. Lee, S. Lee, H. Seo, I. Yu Vilnius Uni e si y, Vilnius, Li huania I. G igelionis, A. Juodagal is Cen o de In es igacion y de Es udios A anzados del IPN, Mexico Ci y, Mexico H. Cas illa-Valdez, E. De La C uz-Bu elo, I. He edia-de La C uz, R. Lopez-Fe nandez, J. Ma ínez-O ega, A. Sanchez- He nandez, L.M. Villaseno -Cendejas Page 22 o 28 Eu . Phys. J. C (2013) 73:2494 Uni e sidad Ibe oame icana, Mexico Ci y, Mexico S. Ca illo Mo eno, F. Vazquez Valencia Beneme i a Uni e sidad Au onoma de Puebla, Puebla, Mexico H.A. Salaza Iba guen Uni e sidad Au ónoma de San Luis Po osí, San Luis Po osí, Mexico E. Casimi o Lina es, A. Mo elos Pineda, M.A. Reyes-San os Uni e si y o Auckland, Auckland, New Zealand D. K o check Uni e si y o Can e bu y, Ch is chu ch, New Zealand A.J. Bell, P.H. Bu le , R. Doesbu g, S. Reuc o , H. Sil e wood Na ional Cen e o Physics, Quaid-I-Azam Uni e si y, Islamabad, Pakis an M. Ahmad, M.I. Asgha , J. Bu , H.R. Hoo ani, S. Khalid, W.A. Khan, T. Khu shid, S. Qazi, M.A. Shah, M. Shoaib Na ional Cen e o Nuclea Resea ch, Swie k, Poland H. Bialkowska, B. Boimska, T. F ueboes, M. Gó ski, M. Kazana, K. Naw ocki, K. Romanowska-Rybinska, M. Szlepe , G. W ochna, P. Zalewski Ins i u e o Expe imen al Physics, Facul y o Physics, Uni e si y o Wa saw, Wa saw, Poland G. B ona, K. Bunkowski, M. Cwiok, W. Dominik, K. Do oba, A. Kalinowski, M. Konecki, J. K olikowski, M. Misiu a, W. Wolszczak Labo a ó io de Ins umen ação e Física Expe imen al de Pa ículas, Lisboa, Po ugal N. Almeida, P. Ba gassa, A. Da id, P. Faccioli, P.G. Fe ei a Pa acho, M. Gallina o, J. Seixas2, J. Va ela, P. Vischia Join Ins i u e o Nuclea Resea ch, Dubna, Russia P. Bunin, M. Ga ilenko, I. Golu in, I. Go buno , A. Kamene , V. Ka ja in, V. Konoplyaniko , G. Kozlo , A. Lane , A. Malakho , P. Moisenz, V. Palichik, V. Pe elygin, S. Shma o , V. Smi no , A. Volodko, A. Za ubin Pe e sbu g Nuclea Physics Ins i u e, Ga china (S . Pe e sbu g), Russia S. E s yukhin, V. Golo so , Y. I ano , V. Kim, P. Le chenko, V. Mu zin, V. O eshkin, I. Smi no , V. Sulimo , L. U a o , S. Va ilo , A. Vo obye , An. Vo obye Ins i u e o Nuclea Resea ch, Moscow, Russia Yu. And ee , A. De mene , S. Gninenko, N. Golube , M. Ki sano , N. K asniko , V. Ma ee , A. Pashenko , D. Tliso , A. To opin Ins i u e o Theo e ical and Expe imen al Physics, Moscow, Russia V. Epsh eyn, M. E o ee a, V. Ga ilo , N. Lychko skaya, V. Popo , G. Sa ono , S. Semeno , A. Spi idono , V. S olin, E. Vlaso , A. Zhokin P.N. Lebede Physical Ins i u e, Moscow, Russia V. And ee , M. Aza kin, I. D emin, M. Ki akosyan, A. Leonido , G. Mesya s, S.V. Rusako , A. Vinog ado Skobel syn Ins i u e o Nuclea Physics, Lomonoso Moscow S a e Uni e si y, Moscow, Russia A. Belyae , E. Boos, V. Buniche , M. Dubinin6, L. Dudko, A. E sho , A. G ibushin, V. Klyukhin, I. Lokh in, A. Ma kina, S. Ob az so , M. Pe ilo , V. Sa in, N. Tsi o a S a e Resea ch Cen e o Russian Fede a ion, Ins i u e o High Ene gy Physics, P o ino, Russia I. Azhgi ey, I. Bayshe , S. Bi iouko , V. Kachano , A. Kalinin, D. Kons an ino , V. K ychkine, V. Pe o , R. Ryu in, A. Sobol, L. Tou chano i ch, S. T oshin, N. Tyu in, A. Uzunian, A. Volko Uni e si y o Belg ade, Facul y o Physics and Vinca Ins i u e o Nuclea Sciences, Belg ade, Se bia P. Adzic28, M. Ekmedzic, D. K pic28,J.Milose ic Cen o de In es igaciones Ene gé icas Medioambien ales y Tecnológicas (CIEMAT), Mad id, Spain M. Aguila -Beni ez, J. Alca az Maes e, C. Ba ilana, E. Cal o, M. Ce ada, M. Chamizo Lla as2, N. Colino, B. De La C uz, A. Delgado Pe is, D. Domínguez Vázquez, C. Fe nandez Bedoya, J.P. Fe nández Ramos, A. Fe ando, J. Flix, M.C. Fouz, P. Ga cia-Abia, O. Gonzalez Lopez, S. Goy Lopez, J.M. He nandez, M.I. Josa, G. Me ino, E. Na a o De Ma ino, J. Pue a Pelayo, A. Quin a io Olmeda, I. Redondo, L. Rome o, J. San aolalla, M.S. Soa es, C. Willmo Eu . Phys. J. C (2013) 73:2494 Page 23 o 28 Uni e sidad Au ónoma de Mad id, Mad id, Spain C. Albaja , J.F. de T ocóniz Uni e sidad de O iedo, O iedo, Spain H. B un, J. Cue as, J. Fe nandez Menendez, S. Folgue as, I. Gonzalez Caballe o, L. Llo e Iglesias, J. Pied a Gomez Ins i u o de Física de Can ab ia (IFCA), CSIC-Uni e sidad de Can ab ia, San ande , Spain J.A. B oche o Ci uen es, I.J. Cab illo, A. Calde on, S.H. Chuang, J. Dua e Campde os, M. Fe nandez, G. Gomez, J. Gonzalez Sanchez, A. G aziano, C. Jo da, A. Lopez Vi o, J. Ma co, R. Ma co, C. Ma inez Ri e o, F. Ma o as, F.J. Munoz Sanchez, T. Rod igo, A.Y. Rod íguez-Ma e o, A. Ruiz-Jimeno, L. Scodella o, I. Vila, R. Vila Co abi a e CERN, Eu opean O ganiza ion o Nuclea Resea ch, Gene a, Swi ze land D. Abbaneo, E. Au ay, G. Auzinge , M. Bach is, P. Baillon, A.H. Ball, D. Ba ney, J. Benda id, J.F. Beni ez, C. Be ne 7, G. Bianchi, P. Bloch, A. Bocci, A. Bona o, O. Bondu, C. Bo a, H. B euke , T. Campo esi, G. Ce mina a, T. Ch is iansen, J.A. Coa asa Pe ez, S. Cola anceschi29, D. d’En e ia, A. Dab owski, A. De Roeck, S. De Vissche , S. Di Guida, M. Dobson, N. Dupon -Sago in, A. Ellio -Peise , J. Eugs e , W. Funk, G. Geo giou, M. Gi els, D. Gigi, K. Gill, D. Gio dano, M. Gi one, M. Giun a, F. Glege, R. Gomez-Reino Ga ido, S. Gowdy, R. Guida, J. Hamme , M. Hansen, P. Ha is, C. Ha l, B. Hegne , A. Hinzmann, V. Innocen e, P. Jano , K. Kaadze, E. Ka a akis, K. Kousou is, K. K ajcza , P. Lecoq, Y.-J. Lee, C. Lou enço, N. Magini, M. Malbe i, L. Malge i, M. Mannelli, L. Mase i, F. Meije s, S. Me si, E. Meschi, R. Mose , M. Mulde s, P. Musella, E. Nes old, L. O sini, E. Palencia Co ezon, E. Pe ez, L. Pe ozzi, A. Pe illi, A. P ei e , M. Pie ini, M. Pimiä, D. Pipa o, G. Polese, L. Que enmon , A. Racz, W. Reece, J. Rod igues An unes, G. Rolandi30, C. Ro elli31, M. Ro e e, H. Sakulin, F. San anas asio, C. Schä e , C. Schwick, I. Segoni, S. Sekmen, A. Sha ma, P. Sieg is , P. Sil a, M. Simon, P. Sphicas32, D. Spiga, M. S oye, A. Tsi ou, G.I. Ve es20, J.R. Vliman , H.K. Wöh i, S.D. Wo m33, W.D. Zeune Paul Sche e Ins i u , Villigen, Swi ze land W. Be l, K. Dei e s, W. E dmann, K. 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