1. Dibosons @ CDF: Some Experience and Lessons Mark Neubauer University of Illinois at Urbana-Champaign ATLAS Electroweak Meeting October 30, 2008

2. Why am I giving this? EWK Meeting Oct. 30, 2008Mark Neubauer2 Short answer: Troels asked me to A little more informative: Multiboson production is central to the ATLAS physics program (SM cross sections and backgrounds searches, Higgs, aTGC, VBS, SUSY, LED, GKW…) Diboson physics has been a primary focus of the Tevatron experiments, with extraordinary progress made in this area, especially in the last few years. Improved σ(WW) (CDF ‘06) WZ First Observation (CDF ‘06) ZZ First Observation in hadron collisions (Dzero ‘08) World’s Best SM Higgs limits for m H > 135 GeV/c 2 February 2006 Now

3. Why am I giving this? (cont) EWK Meeting Oct. 30, 2008Mark Neubauer3 It is easy to both overstate and understate the importance of the Tevatron experience to what we are doing here Clearly, the Tevatron experience is important because involves confronting the (often sobering) realities of hadron collision data Albeit, the measurements just described used a much better understood detector than we’ll have in early LHC data There are, of course, substantial differences in the production physics (QCD at higher scale) and detectors which makes some comparisons difficult. e.g. much high ratio of QCD jets to isolated leptons @ LHC e.g. gg processes much move relevant (x100) @ LHC compared to Tevatron e.g. LAr with accordian geometry vs. scintillator projective tower for Ecal Still, there are lessons to be learned, especially for heavy (W or Z) diboson physics where the experiments operate near the edge of their sensitivity to these processes (same will be true for early diboson analyses of LHC data, Higgs, and possible NP on the longer horizon).

4. What I will talk about EWK Meeting Oct. 30, 2008Mark Neubauer4 Not a seminar on diboson physics, so I won’t try to give full Tevatron experience Instead, I’ll give a specific “case study” of the quest to observe WZ production from the CDF perspective, which may have some relevance to pushing our sensitivity to rare multi-lepton processes for ATLAS. My area of recent interest has been in heavy diboson physics for Higgs and NP searches, with particular focus on the SM processes to push our sensitivity to rare multi-lepton signatures. The CDF effort I describe involved a number of collaborators, including some that have found their way to ATLAS (S.C. Hsu, E. Lipeles, C. Hays)

5. WZ Search in Trileptons EWK Meeting Oct. 30, 2008Mark Neubauer5 WZ decays WZ→lllν selection: 3 leptons (20, 10, 10 GeV) MET > 25 GeV At least one opposite-sign, same- flavor pair in Z mass window [76, 106] Backgrounds: Z+X (jet, Z, or γ) Z+γZ+jet Estimated using MC (Baur w/ min E T (photon) @ gen- level) Estimated from jet and dilepton data At √s = 2 TeV, σ(WZ) = 3.6 ± 0.3 pb (NLO)

6. Our Original WZ Search (March 2006) EWK Meeting Oct. 30, 2008Mark Neubauer6 Using 0.8 fb -1, we observed 2 events (eee) with an expected background of 0.9 ± 0.2 events and WZ signal of 3.7 ± 0.3 events

7. Dzero WZ First Evidence (Summer 2006) EWK Meeting Oct. 30, 2008Mark Neubauer7 Using 0.8 fb -1, they observed 12 events with an expected background of 3.6 ± 0.2 events and WZ signal of 7.5 ± 1.2 events

8. CDF Detector Electromagnetic Calorimeter Muon Chambers Solenoid Tracking Chamber Silicon Vertex Detectors Beamline Hadron Calorimeter η =0 η =1 pseudorapidity: η =-ln[tan( θ /2)]

9. “Standard” Lepton Selection EWK Meeting Oct. 30, 2008Mark Neubauer9 Central Forward (w/ Si-based track)‏ Central Muons Central MIP All leptons required to be calorimeter isolated

10. Our New Charged Lepton Selection EWK Meeting Oct. 30, 2008Mark Neubauer10 Central Forward (w/ Si-based track)‏ Central Muons Central MIP Forward (w/o Si-based track)‏ Track Try to exploit all of the available event information. Which leptons make Z? Z(e + + e - ) + W(f s ) dirty (large Z γ bkg), but Z(e + + f s ) + W(e ± ) very clean! Z(e + + e - ) + W(t ± ) dirty (large Zj bkg), but Z(e + + t - ) + W(e ± ) very clean! New lepton strategy nearly tripled our WZ acceptance! Forward w/o Si-based track≡f s, charge neutral; Track≡t, flavor neutral

11. Control Regions in WZ EWK Meeting Oct. 30, 2008Mark Neubauer11 Signal Low-E T Z-Veto q q ℓ+ Z0Z0 ℓ- γ q q γ Z0Z0 ℓℓ ℓℓ and Z+jet(s)‏ FSR ISR WZ→lllν + MET Signal Region Dilepton (Drell-Yan) Region Trilepton Control Regions Z 0 mass region [76, 106] MET > 25 GeV Tests efficiency and acceptance calculations Z 0 mass region [76, 106] No MET cut Tests background modeling Low-MET Region Invert MET cut Z-Veto Region Invert Z 0 mass cut Invert MET cut

12. Some Control Region Plots EWK Meeting Oct. 30, 2008Mark Neubauer12 Z→ ee reconstructed as ee and et Z→ μμ reconstructed as μμ and μt N-1 Z-mass Region Z-mass Region Low-MET Region DY Region DY Region

13. First WZ Observation (October 2006) EWK Meeting Oct. 30, 2008Mark Neubauer13 Using 1.1 fb -1, our new lepton ID, and an additional MET-based trigger, we observed 16 events with an expected background of 2.7 events and WZ signal of 9.8 events > 6σ first observation! (two bin MET fit)

14. WZ Update (August 2007) EWK Meeting Oct. 30, 2008Mark Neubauer14 Using 1.9 fb -1, we observe 25 events with an expected background of 5 events and WZ signal of 21 events

15. W ± Z 0 → e ± ν e e + e - Candidate EWK Meeting Oct. 30, 2008Mark Neubauer15

16. W ± Z 0 → e ± ν e e + e - Candidate EWK Meeting Oct. 30, 2008Mark Neubauer16

17. WZ Hiding in Plain Sight! EWK Meeting Oct. 30, 2008Mark Neubauer17

18. ATLAS and Multi-leptons EWK Meeting Oct. 30, 2008Mark Neubauer18 We have a student (Hannah Deberg) working on dileptons + MET (WW, ZZ) and student (Scott Wolin) working on trileptons (WZ, etc), both in fully-leptonic decay channels We’ve basically reproduced the CSC results in these channels and are working through other lepton and high-level selections to improve the sensitivity to interesting processes, using the SM multi-lepton states as a testbed. The increase lepton acceptance studies have just started. Some thoughts: - We can pick up extra acceptance using isolated tracks in the electron and muon crack regions (next slide has some very preliminary plots) - Troels points out that some of these isolated “crack tracks” can be reasonably well ID’d as electrons by the TRT alone for extra bkg rejections - Investigate the use of very forward electrons (2.5<|η|<3.2 ECAL, 3.2<|η|<4.9 FCAL) where the tracking acceptance turns off but there is still good calorimeter coverage (working with Mohamed Aharrouche who is looking at this for AFB in Z→ee) - Also revisit medium and loose electron selections and think about lepton MVTs

19. Quick study of e, μ eta acceptance with DY EWK Meeting Oct. 30, 2008Mark Neubauer19 electrons in Z→ee Tracks not matched to electrons muons in Z→ μμ Tracks not matched to muons Tight leptons, pt(electron)>15 GeV, pt(muon)>15 GeV, pt(track)>20 GeV

20. Extras HETEP Seminar 09/08/08Mark Neubauer20

21. ZZ Production Before ICHEP08, never observed in hadron collisions Search in two decay modes: – ZZ → llll: 4 leptons Tiny cross section (1.4 pb) × BR (0.5%) Only 14 decays expected in 2 fb -1 SM backgrounds very very tiny ⇒ Count events over background – ZZ → llνν: 2 leptons + MET Large WW backgrounds ⇒ Use kinematic properties to statistically separate ZZ from WW (ME techniques) Probability density for x obs = (p 1, p 2, MET x, MET y ): ☺ ☹ ZZ WW

22. ZZ Production (CDF) two μμμμ eeee ZZ→llll (top) and ZZ→llνν (bottom) “Trackless electron”: A shower in forward Ecal but beyond tracking acceptance

23. ZZ Strong Evidence (CDF) First ZZ signal w/ a significance exceeding 4σ (“strong evidence”) and measurement of the ZZ cross section at a hadron collider Results published as PRL 100, 201801 (2008) At ICHEP08, D0 announced ZZ observation (exp 4.8σ, obs 5.7σ) ZZ→μ + μ - μ + μ - candidate Highest P(x obs ) ZZ→llνν candidate Combined Significance and measured cross section

24. ZZ Observation (Dzero)

25. Triple Gauge Couplings HETEP Seminar 09/08/08Mark Neubauer25