1 TOP MASS MEASUREMENT WITH ATLAS A.-I. Etienvre, for the ATLAS Collaboration

2 EPS – A.-I. EtienvreTop mass with ATLAS Introduction Motivations for a precise top quark mass measurement: –EW precision observables depend on the value of the top mass  a precise top mass measurement is needed to: Perform consistency tests of the Standard Model constrain the Higgs mass within the Standard Model Search for new physics beyond the Standard Model July 2010

3 EPS – A.-I. EtienvreTop mass with ATLAS Top mass measurement with ATLAS Measurement : –performed in the l(e,µ)+jets channel Best compromise between BR and S/B –Based on template methods: Definition of an estimator sensitive to m top Produce (MC) templates of it for various m top Fit templates for signal and background  2 PDFs Likelihood fit to the data  top mass measurement Results shown today: –From 35 pb -1 recorded by ATLAS at  s = 7 TeV –Three complementary template methods reference: ATL-CONF – Alternative measurement: top mass extraction from ttbar cross section measurement : reference: ATL-CONF

4 EPS – A.-I. EtienvreTop mass with ATLAS Event selection Lepton + jets standard event selection: –Single lepton (e/µ) trigger fired –Exactly one isolated lepton: within good detector acceptance p T > 20 GeV –Transverse mass (l )/ E t miss (QCD rejection) e channel: –M T (l ) > 25 GeV –E T miss > 35 GeV µ channel: –M T (l ) + E T miss > 60 GeV – E T miss > 20 GeV –Jets: ≥ 4 jets, p T > 25 GeV, |  | < 2.5 ≥ 1 b-tagged jet –After all cuts, S/B is of the order of 5 Dominant background: QCD, W+ HF

5 EPS – A.-I. EtienvreTop mass with ATLAS R 32 template method Principle: –Observable chosen : Reduces sensitivity to Jet Energy Scale uncertainty –Hadronic top mass reconstruction: Jet triplet maximizing p T: Additional cuts: –W window : 60 < m W < 100 GeV –Veto 2b jets in triplet R 32 template parameterization –For signal (includes single top) : –For background Does not depend on m top e channelµ channel

6 EPS – A.-I. EtienvreTop mass with ATLAS Event yields –QCD estimate data driven –Good agreement between data and Monte Carlo

7 EPS – A.-I. EtienvreTop mass with ATLAS R 32 template method Top mass measurement –Likelihood fit performed on 35 pb -1 –Stat. uncertainties only in these plots –Systematic uncertainties: see next slide stat. syst. e channelµ channel

8 EPS – A.-I. EtienvreTop mass with ATLAS Systematic uncertainties Systematic uncertainties dominated by ISR/FSR, b-JES and JES Stat. uncertainy predicted by pseudo-exp, consistent with observation

9 EPS – A.-I. EtienvreTop mass with ATLAS 2D template method Simultaneous fit of m top and global Jet Scale Factor (JSF): –2 observables are chosen: m jj = m W reco = f(JSF) M top reco =f(JSF, m top ) –Final state reconstruction slightly different: Light jet pair (W) chosen by minimizing: –Light jet energies not rescaled for the W reconstruction –But rescaled for the top mass reconstruction Hadronic top reconstruction : jet triplet maximizing p T µ channel

10 EPS – A.-I. EtienvreTop mass with ATLAS 2D template method Measurement with 35 pb -1 – fitted Jet Scale Factor : e channel =, µ channel: stat. syst. µ channel

11 EPS – A.-I. EtienvreTop mass with ATLAS 1D template method with kinematic fit Kinematic likelihood fit –Information from the entire event used to reconstruct both sides of the ttbar decay with a kinematic likelihood fit: TF relate reconstructed objects energy and direction to those of their parent partons: –Lower stat. uncertainty but no constrain on JES Measurement: stat. syst.

12 EPS – A.-I. EtienvreTop mass with ATLAS Mass extraction from cross section Exploits the dependency of the top pair cross section wrt top mass (assuming m top MC = m top pole ): –Using most accurate ttbar cross section single measurement –Based on 3 different theoretical calculations 13% uncertainty on measured cross section  6 GeV uncertainty on m top theory uncertainty (scales + PDF)  4 GeV uncertainy on m top

13 EPS – A.-I. EtienvreTop mass with ATLAS Conclusion 3 template methods have been used for a direct top mass measurement: –Consistent results –Complementary Indirect measurement (extraction from ttbar cross section): – consistent with direct measurements With 1 fb -1 : stat. uncertainty ≤ 1 GeV –Systematic become crucial

14 EPS – A.-I. EtienvreTop mass with ATLAS Outlook First distributions of m jjb in the l+jets channel with 0.7 fb -1

15 BACK UP

16 EPS – A.-I. EtienvreTop mass with ATLAS ISR/FSR systematic uncertainty estimate ATLAS has adopted a very conservative approach: –6 samples have been generated (AcerMC + Herwig) enhanced ISR, or FSR, or both reduced ISR, or FSR, or both Parameters varied: –ISR: PARP(67) and PARP(64) –FSR: PARP(72) and PARJ(82) –Likelihood fit performed: For each dataset Maximum difference = systematic uncertainty ISR/FSR studies ongoing –Measurement in data foreseen