1 c. mills (Harvard U.) 20 September, 2010 W and Z Physics at ATLAS Corrinne Mills Harvard DOE Site Visit 20 September 2010

2 c. mills (Harvard U.) 20 September, 2010 W and Z at the LHC 5 months of 7 TeV collisions 5 months of coherent effort by Harvard group on muon-focused analysis Results presented at PLHC, ICHEP, HCP/SUSY conferences Work shown here to be submitted for publication event selection lepton charge asymmetry cross section calculations muon definition & efficiency muon definition & efficiency data quality QCD background Kashif Mills Belloni Smith Kagan Martinez-Outschoorn Prasad Zevi della Porta Jeanty

3 c. mills (Harvard U.) 20 September, 2010 Combined muon: matched inner detector (ID) and muon spectrometer (MS) track Selection:  p T (combined) > 15 GeV  p T (MS) > 10 GeV  |p T (MS) – p T (ID)| < 15 GeV Muons in ATLAS  |  | < 2.4 (trigger geometry) Trigger: L1 (hardware)  p T > 6 GeV reject decays in flight

4 c. mills (Harvard U.) 20 September, 2010 Muon Quality Criteria Leverage knowledge from studies of cosmic ray data Consistency requirement for combined muon kinematics: |p T (MS) – p T (ID)| < 15 GeV

5 c. mills (Harvard U.) 20 September, 2010 Refine muon selection: p T > 20 GeV and relative track isolation < 0.2  Sum  p T of tracks in cone around muon of  R < 0.4, divided by the muon p T Reduce backgrounds by requiring E T miss > 25 GeV Selecting the W signal (I) electron channel muon channel

6 c. mills (Harvard U.) 20 September, 2010 Selecting the W signal (II) Clean up sample with M T > 40 GeV Transverse mass electron channel muon channel

7 c. mills (Harvard U.) 20 September, 2010 W Cross Section Measure cross section times branching ratio BR(W→ l ) Theoretical prediction:  ± 0.02 nb Luminosity uncertainty is 11% channelint. lumi.N cand N background acceptance x efficiency electron315 nb ± ± muon310 nb ± ± channelcross section (nb) electron ± 0.34 (stat) ± 0.81 (sys) ± 1.16 (lum) muon9.58 ± 0.30 (stat) ± 0.50 (sys) ± 1.05 (lum) combined 9.96 ± 0.23 (stat) ± 0.50 (sys) ± 1.10 (lum) S. Prasad thesis: graduation ~ May 2011

8 c. mills (Harvard U.) 20 September, 2010 W Cross Section in Context

9 c. mills (Harvard U.) 20 September, 2010 Charge Asymmetry W + favored in proton-proton collisions Sensitive to valence quark PDFs electron muon integral result0.200 ± (stat) ± (sys) theory prediction0.20 V. Martinez-Outschoorn thesis: graduation ~ May 2011

10 c. mills (Harvard U.) 20 September, 2010 Oppositely-charged muon candidates p T > 20 GeV,  range, quality requirements as with W analysis, including track isolation 66 GeV < M ll < 116 GeV Selecting the Z →  signal muon channel muon

11 c. mills (Harvard U.) 20 September, 2010 Z Cross Section Measure cross section times branching ratio BR(W→ l ) Theoretical prediction:  ± nb Luminosity uncertainty is 11% channelint. lumi.N cand N background acceptance x efficiency electron316 nb ± ± muon331 nb ± ± channelcross section (nb) electron 0.75 ± 0.09 (stat) ± 0.08 (sys) ± 0.08 (lum) muon0.87 ± 0.08 (stat) ± 0.05 (sys) ± 0.10 (lum) combined 0.83 ± 0.06 (stat) ± 0.04 (sys) ± 0.09 (lum) L. Kashif thesis: graduation ~ Dec. 2010

12 c. mills (Harvard U.) 20 September, 2010 Z Cross Section in Context

13 c. mills (Harvard U.) 20 September, 2010 More Data in the Pipeline muon channel

14 c. mills (Harvard U.) 20 September, 2010Conclusion Establishing the W and Z samples at ATLAS Rapidly increasing dataset  Better precision  W/Z properties, differential cross sections  W p T (next talk) W and Z data at the LHC will illuminate the Standard Model in a new momentum regime And pave the way to find what may lie beyond it  key to validation of high-p T leptons and E T miss Harvard role  Developing baseline muon selection for high-p T muon analysis  Driving W and Z cross section analyses, W lepton charge asymmetry in muon channel  Major contributor to 310 nb -1 paper, to be submitted soon

15 c. mills (Harvard U.) 20 September, 2010 Backup

16 c. mills (Harvard U.) 20 September, 2010 W   event selection good run list, filled bunch crossing, jet cleaning (data only) vertex with ≥ 3 matched tracks and |z| < 150 mmexists passed trigger (via p T cut on matched L1 trigger object)L1_MU6 at least one combined muon with p T > 15, |  | < 2.4 exists muon spectrometer p T > 10 GeV/c | p T (spectrometer) – p T (ID) |< 15 GeV/c combined muon |z0 - z(pv)|< 10 mm muon combined p T > 20 GeV/c muon |  | < 2.4 (track iso (cone 0.4))/p T (  ) < 0.2 MET> 25 GeV transverse mass> 40 GeV P RESELECTION W SELECTION

17 c. mills (Harvard U.) 20 September, 2010 Backgrounds to W →  Z → , W → , Z → , ttbar: 77.6 ± 5.4 (stat+sys) events  From simulation QCD: 21.1 ± 9.8 (stat+sys) events  “Matrix Method”  Solve for N QCD using number of candidates with and without isolation req. (N loose = 1272, N isol = 1181)  Measure  non-QCD = ± 0.01 from Z’s  Measure  QCD in data with 15 < p T  < 20 GeV (get ± 0.004)  extrapolate to p T  > 20 GeV by scaling based on simulated dijet events (get 0.227) Cosmics: 1.7 ± 0.8 event  Consideration of empty and unpaired bunch crossings

18 c. mills (Harvard U.) 20 September, 2010 QCD BG: Matrix Method (1) Solve for N QCD in isolated candidate sample N isol (1181) and N loose (1272) are number of W candidates with and without isolation cut  QCD and  non-QCD are efficiency of isolation cut for QCD and prompt muons  Measure  non-QCD = ± 0.01 in tag-and-probe with Z’s  Measure  QCD in QCD-dominated data: candidate events with 15 < p T  < 20 GeV  extrapolate to p T  > 20 GeV by scaling by  (p T  > 20 GeV)/  (15 < p T  < 20 GeV) as measured in the MC (more on next slide)

19 c. mills (Harvard U.) 20 September, 2010 QCD BG: Matrix Method (2) Measure  QCD in QCD-dominated data: candidate events with 15 < p T  < 20 GeV (get ± 0.004)  extrapolate to p T  > 20 GeV by scaling by  (p T  > 20 GeV)/  (15 < p T  < 20 GeV) as measured in the MC  (0.238 ± 0.005)/(0.307 ± 0.003) = ± Uncertainties  systematic from 100% uncertainty on extrapolation  stat. uncert. from  non-QCD also significant Bottom line 21.1 ± 4.5 (stat) ± 8.7 (sys)

20 c. mills (Harvard U.) 20 September, 2010 Backgrounds to Z Predicted total backgrounds:  electron: 1.18 ± 0.11 (stat) ± 0.41 (syst)  muon: 0.25 ± 0.01 (stat) ± 0.04 (syst)  compare to 3 (0) same-sign events in electron (muon) channel  2.8 same-sign events from Z → ee signal are expected Magnitude is small (<1% relative to expected signal) ttbar Z →  W → e /  QCD (muon channel) QCD (electron channel)  Sideband subtraction for loose-loose electron-positron pairs  Apply loose  medium “rejection factor” measured in data from simulation

21 c. mills (Harvard U.) 20 September, 2010 Electrons in ATLAS EM calorimeter cluster matched to inner detector (ID) track E T > 20 GeV, |  | < 2.47  exclude gap between barrel and endcap 1.37 < |  | < 1.52 “Loose” selection  shower shape in middle layer of calorimeter “Medium” selection  add fine-granularity shower shape and track match  “Tight” selection  add E/p, more track quality, high-threshold TRT hits, conversion veto Trigger: Level 1 (hardware) requires coarse-granularity cluster with |  | 5 GeV

22 c. mills (Harvard U.) 20 September, 2010 More on Electrons Trigger: sliding-window algorithm using reduced-granularity clusters  x  = 0.1 x 0.1 Offline reconstruction: sliding window of 3x5 cells or x in  x   Electron = cluster with E T > 2.5 GeV and matched track with p T > 0.5 GeV Reconstruction: exact requirements vary with E T and |  |, but three categories: Loose electrons  Fiducial: |  | < 2.37 and exclude 1.37 < |  | < 1.52  Shower shape in middle (largest) layer of calorimeter: cluster width in   Hadronic leakage: E T (innermost later of HCAL) / cluster E T Medium electrons: loose +=  Shower shape in innermost (finely segemented in  ) layer of calorimeter  Track match (  )  Track quality (pixel, SCT hits and impact parameter) Tight electrons: medium +=  High-threshold hits in transition-radiation tracker (TRT); hit in innermost pixel layer  E/p