TOP-QUARK MASS MEASUREMENTS AT THE LHC

Transcription

1 TOP-QUARK MASS MEASUREMENTS AT THE LHC S. Blyweer, on behalf of he ATLAS and CMS Collaboraions Vrije Universiei Brussel - Ineruniversiy Insiue for High Energies Pleinlaan 2, 1050 Brussel, Belgium The op-quark mass m is one of he fundamenal parameers of he sandard model (SM) of paricle physics. The CDF and D0 collaboraions already performed very precise measuremens of m. Since he ATLAS and CMS collaboraions already have a very large sample of op quark pairs available for analysis, hey are producing resuls wih increasing precision. An overview of he mos recen measuremens of m by ATLAS and CMS is given, using up o 4.7fb 1 of daa in differen decay channels. The measuremen of he mass difference beween op and aniop quarks is also shown. Finally, he combinaion of he individually measured m values is discussed, ogeher wih an oulook for fuure m measuremens. 1 Inroducion The op quark, which was discovered in 1995 a he Tevaron, is he heavies currenly known fundamenal paricle. Is mass is an imporan parameer of he sandard model, since i is an imporan inpu for he global Elecro-eak fis. These fis can be used o consrain he mass of he SM Brou-Engler-Higgs boson, and are also a consisency check of he sandard model. m has been measured already by he CDF and D0 collaboraions wih grea precision, resuling in m = ± 0.9 GeV as he curren world average 1,2. A precise measuremen of m by he ATLAS and CMS collaboraions a he Large Hadron Collider (LHC) would provide an independen cross-check of his value and would help o furher reduce he oal uncerainy on he world average of m. In proon-proon collisions a s = 7 TeV op quarks are dominanly produced in pairs. In 2011 he LHC delivered more han 5 fb 1 o boh he ATLAS and CMS experimens, corresponding o abou pairs per experimen. This gave boh ATLAS and CMS he opporuniy o perform precise measuremens of m in he differen decay channels. Since op quarks decay mos of he ime ino a b quark and a boson, evens can be caegorized, according o he decay of he bosons, in dileponic ( b blν l l ν l ), all-jes ( b bqq q q ) and l+jes ( b blν l qq ) evens. More informaion abou he producion and decay of evens can be found in Ref Direc measuremens of he op-quark mass 2.1 Measuremen of he op-quark mass in he dilepon channel The m measuremen from CMS in he dilepon channel is performed wih 2.3 fb 1 of daa 4. Evens in his channel are seleced by requiring exacly wo lepons (elecrons (e) or muons (µ)) wih p T > 20 GeV and η < 2.4, a leas 2 jes wih p T > 30 GeV and η < 2.4,

2 missing ransverse energy E T > 30 GeV, and a leas one b-agged je. In he ee and µµ channels evens wih 76 GeV < m ll < 106 GeV are rejeced. The even-by-even op-quark mass m KINb is reconsruced wih he KINb algorihm 5. For each of he possible je-quark assignmens he kinemaic equaions are solved muliple imes per even, each ime varying he reconsruced kinemaics wihin heir resoluions. The je-quark assignmen wih he larges number of soluions is seleced. Finally, m KINb is exraced by aking he mean of a Gaussian fi o he disribuion of he reconsruced op quark mass for all he differen soluions of he kinemaic equaions, for he chosen je-quark assignmen. The exracion of m is hen performed by applying he emplae mehod. Templaes which are sensiive o m are consruced for differen op-quark mass hypoheses. The value of m is hen exraced by doing a maximum likelihood fi of hese emplaes o he disribuion of m KINb observed in daa. By applying his echnique, CMS measures m = ± 1.2(sa) (sys) GeV, where he sysemaic uncerainy is dominaed by he global Je Energy Scale (JES) uncerainy and he uncerainy on he flavour-dependen JES. This is he mos precise measuremen of m in he dilepon channel, wih similar precision as he mos recen resul from D Measuremen of he op-quark mass in he all-jes channel ATLAS performed he firs measuremen a he LHC of m in he all-jes channel, using 2.04 fb 1 of daa 7. Evens are seleced by asking 5 jes wih p T > 55 GeV and η < 4.5, and a 6 h je wih p T > 30 GeV and η < 4.5. Two of hese jes, wih p T > 55 GeV and inside he inner deecor accepance ( η < 2.5), mus be b-agged. A cu on he E T significance is also applied: E T / H T < 3, where H T is he scalar sum of he p T of all he jes in he evens. The even opology is reconsruced using a mass χ 2 : χ 2 = (m j 1,j 2 m ) 2 σ 2 + (m j 1,j 2,b 1 m ) 2 σ 2 + (m j 3,j 4 m ) 2 σ 2 + (m j 3,j 4,b 2 m ) 2 σ 2, (1) where σ = 10.2 GeV and σ = 17.4 GeV are he respecive mass resoluions from simulaion. For every even, he je-quark assignmen wih he lowes χ 2 is aken, requiring 50 GeV < m j1,j 2 < 110 GeV and 50 GeV < m j3,j 4 < 110 GeV. This χ 2 is minimized as a funcion of m and m. Only evens wih χ 2 min < 8 are considered in he exracion of m. The emplae mehod is applied o he disribuion of m jjb values from he seleced je-quark assignmen, as observed in daa. Each seleced even conribues wo values o his disribuion. ATLAS measures m = ± 2.1(sa) ± 3.8(sys) GeV. The sysemaic uncerainy is dominaed by he uncerainy on Iniial and Final Sae Radiaion (ISR/FSR), he uncerainy on he daa-driven mulije background and he JES uncerainy. 2.3 Measuremen of he op-quark mass in he l+jes channel Boh ATLAS and CMS have measured m in he l+jes channel 8,9. ATLAS selecs evens by asking exacly 1 isolaed elecron (E T > 25 GeV) or muon (p T > 20 GeV). The evens need o have 4 jes wih p T > 25 GeV and η < 2.5, of which a leas one is b-agged. Mulije evens are rejeced by asking E T > 35 GeV and m T > 25 GeV (e+jes), or E T > 20 GeV and E T +m T > 60 GeV (µ+jes). ATLAS is using wo differen approaches o measure m, designed o reduce he JES uncerainy. Boh analyses are based on he emplae mehod. In he 1d-analysis, R 32 mreco m reco is calculaed for every even, where m reco and m reco are he reconsruced invarian masses of he hadronically decaying op quark and boson, respecively. A kinemaic fi is used o selec a je-quark assignmen. The likelihood L of he kinemaic fi of he seleced je-quark assignmen has o pass: ln L > 50. The jes assigned o he bqq decay need o have p T > 40 GeV, and m reco has o fulfill 60 GeV < mreco < 100 GeV.

3 The 2d-analysis, a combined measuremen of m and a global Je energy Scale Facor (JSF), is performed by a emplae fi o m reco and m reco. In his analysis, he je riple assigned o he hadronic op decay is chosen as he one wih maximum p T, considering only riples wih 50 GeV < m reco < 110 GeV. Finally a kinemaic fi is performed o he chosen je riple. Boh analyses are applied on 1.04 fb 1 of daa. The 2d-analysis has a slighly smaller uncerainy: m = ± 0.6(sa) ± 2.3(sys) GeV. The sysemaic uncerainy is dominaed by he uncerainy on ISR/FSR and he uncerainy on he b-je energy scale. The CMS measuremen of m uses 4.7 fb 1 of daa in he µ+jes channel 9. Evens are seleced by asking exacly 1 isolaed muon wih p T > 30 GeV and η < 2.1, 4 jes wih p T > 30 GeV and η < 2.4, of which a leas 2 are b-agged. For every possible je-quark assignmen, a kinemaic fi is applied wih 3 mass consrains: an equal-mass consrain of m lep and m hadr, and 2 m consrains. The kinemaic fi reurns he fied op-quark mass m fi,i and he fi probabiliy Pfi i. rong je-quark assignmens are rejeced by asking P fi i > 0.2. To exrac m from he daa, he Ideogram mehod is used in a combined measuremen of m and he Je Energy Scale (JES). In his mehod, a likelihood is calculaed for every even: L(even m, JES) = ( n i=1 ( ) ) n i=1 P fi i Pfi i P m fi,i, m reco,i m, JES. (2) ( ) The disribuions P m fi,i, mreco,i m, JES for all possible je-quark assignmens (correc assignmens, wrong assignmens and unmached assignmens) are aken from simulaion. The individual even likelihoods are combined in a global likelihood, from which he measured m and JES values can be exraced. ih his mehod, CMS measures m = ± 0.6(sa) ± 1.2(sys) GeV. The sysemaic uncerainy is dominaed by he b-je energy scale uncerainy and he uncerainy on he facorizaion scale. The sysemaic uncerainy does no include he uncerainy on color reconnecion and on he underlying even. 3 Measuremen of he op-quark mass from he cross secion a s = 7 TeV One of he problems of he direc measuremens of m is ha hey use he mass definiion from Mone Carlo generaors, which is no relaed o m in a well-defined renormalizaion scheme (m pole or m MS ) in a sraighforward way. These masses can however be exraced from he measured cross secion of op quark pair producion σ, since he heoreical dependency of or m MS is known. ATLAS used σ measured from 35 pb 1 of daa in he l+jes channel, and obains m pole = GeV for he pole mass 10. CMS used σ measured in he dilepon final sae from 1.14 fb 1 of daa, and measures m pole = GeV and σ on m pole m MS = GeV for he pole mass and he MS mass, respecively 11. Boh he ATLAS and CMS resuls are in agreemen wih previous measuremen performed by CDF and D Measuremen of he mass difference beween op and aniop quarks One of he fundamenal symmeries in he sandard model, he invariance under CPT ransformaions, can be esed by measuring he difference in mass beween a paricle and he corresponding aniparicle. Since he op quark is he only quark ha decays before hadronizaion can ake place, his difference can be measured direcly. CMS performed a measuremen using 1.09 fb 1 of daa in he µ+jes channel 12. The evens were splied in wo disinc samples according o he charge of he lepon. In each of hese wo samples, he mass of he hadronically decaying op quarks was measured and finally boh masses were subraced from eachoher. This resuled in m = 1.20 ± 1.21(sa) ± 0.47(sys) GeV. The smallness of he sysemaic

4 uncerainy, when compared o m measuremens, can be explained by he cancellaion of mos sysemaics by aking he difference. 5 Conclusion and oulook Currenly he mos precise measuremens of m are performed by he Tevaron experimens, bu he ATLAS and CMS resuls are geing more and more precise. An overview of all hese resuls can be found in Fig. 1. The nex sep is o combine all he CMS and ATLAS measuremens, Figure 1: Overview of he m measuremens from ATLAS and CMS, including he laes CDF and D0 combinaion. on which work has already sared, and finally o combine hese also wih he CDF and D0 resuls. CMS already performed a combinaion of is measuremens, resuling in m = ± 0.4(sa) ± 1.2(sys) GeV. ih he huge amoun of daa which will be recorded by ATLAS and CMS, he precision on m is expeced o increase. The uncerainies are currenly dominaed by he je energy scale sysemaic uncerainy, which can be reduced once more daa is available o be analyzed. References 1. CDF and D0 Collaboraion, Combinaion of CDF and DO resuls on he mass of he op quark using up o 5.8 fb 1 of daa, (2011). arxiv: O. Brand, Measuremens of he op quark mass a he Tevaron, hese proceedings. arxiv: I. Aracena, Top Pair Producion a E cm = 7 TeV, hese proceedings. 4. CMS Collaboraion, Measuremen of he op-quark mass in he dilepon channel in pp collisions a s = 7 TeV, CMS Physics Analysis Summary CMS-PAS-TOP (2012). 5. CMS Collaboraion, Measuremen of he producion and he op-quark mass in he dilepon channel in pp collisions a s = 7 TeV, JHEP 07 (2011) 049. arxiv: D0 Collaboraion, Measuremen of he op quark mass in p p collisions using evens wih wo lepons, submied o Phys. Rev. Le. arxiv: ATLAS Collaboraion, Deerminaion of he Top Quark Mass wih a Templae Mehod in

5 he All-Hadronic Decay channel using 2.04 fb 1 of ATLAS daa, ATLAS Noe ATLAS- CONF (2012). 8. ATLAS Collaboraion, Measuremen of he op quark mass wih he emplae mehod in he op aniop lepon + jes channel using ATLAS daa, submied o Eur. Phys. J. C, arxiv: CMS Collaboraion, Measuremen of he op quark mass in he muon+jes channel, CMS Physics Analysis Summary CMS-PAS-TOP (2012). 10. ATLAS Collaboraion, Deerminaion of he Top-Quark Mass from he Cross Secion Measuremen in pp Collisions a s = 7 TeV wih he ATLAS deecor, ATLAS Noe ATLAS-CONF (2011). 11. CMS Collaboraion, Deerminaion of he Top Quark Mass from he Cross Secion a s = 7 TeV, CMS Physics Analysis Summary CMS-PAS-TOP (2011). 12. CMS Collaboraion, Measuremen of he mass difference beween op and aniop quarks, CMS Physics Analysis Summary CMS-PAS-TOP (2011).