1. Search for Narrow Resonance Decaying to Muon Pairs in 2.3 fb-1
Ye Li
on behalf of CDF Collaboration
University of Wisconsin-Madison

2. APS April Meeting, St. Louis - 14 April 2008
Motivation
Theory Driven
Standard Model successful but incomplete
Strong discovery potential in dimuon channel
New models predict narrow neutral resonance, e.g.
Additional U(1) symmetry: Z’
Extra space-time dimension: Randall-Sundrum graviton
Focus on Z’ →  channel
Ye Li
APS April Meeting, St. Louis - 14 April 2008

3. APS April Meeting, St. Louis - 14 April 2008
Motivation
Experiment Driven
Last Tevatron dimuon resonance searches performed with integrated luminosity 200 pb-1
→ Our search: L ≈ 2.3 fb-1 of CDF Run II data
Significant increase of sensitivity to dielectron and diphoton channels
Excellent tracking resolution (Central Outer Tracker, Drift Chamber)
Ye Li
APS April Meeting, St. Louis - 14 April 2008

4. APS April Meeting, St. Louis - 14 April 2008
Methodology
Model Drell-Yan background and signal resonance with PYTHIA + detector simulation
Use Z region for normalization
Remove uncertainty on luminosity
Easy accounting
Ye Li
APS April Meeting, St. Louis - 14 April 2008

5. APS April Meeting, St. Louis - 14 April 2008
Methodology
Inverse Mass (1/m) Scan
Excellent angular resolution → negligible
High PT track curvature (~1/PT) resolution constant → constant 1/m resonance width
CDF Run II Preliminary
1/m ≈ 0.16/TeV
Ye Li
APS April Meeting, St. Louis - 14 April 2008

6. APS April Meeting, St. Louis - 14 April 2008
Methodology
Fit for NZ’ (number of Z’ candidate)
Calculate Binned Poisson likelihood L(NZ’;MZ’) for region 1/m < 10/TeV
Construct the narrowest possible interval in NZ’ at 95% C.L.
Scan for resonance
Significance determined by Monte-Carlo pseudo-experiments
Ye Li
APS April Meeting, St. Louis - 14 April 2008

7. APS April Meeting, St. Louis - 14 April 2008
Dataset & Selection
Dataset from high PT muon trigger
Dimuon event selection
Two opposite-sign tight muons
At least one muon passing the trigger
CDF Run II Preliminary
Ye Li
APS April Meeting, St. Louis - 14 April 2008

8. APS April Meeting, St. Louis - 14 April 2008
Efficiency
Mass dependence
Track and muon-hit cuts independent of mass
Momentum dependence
Normalization of background expectation
Only consider P dependence
No P dependence of trigger eff. for PT > 30 GeV
Separate the sample into signal and normalization (Z-pole) regions
Ye Li
APS April Meeting, St. Louis - 14 April 2008

9. APS April Meeting, St. Louis - 14 April 2008
Acceptance
Comparison of muon  in W →  data vs. simulation for two muon detector regions
CDF Run II Preliminary
|| < 0.6 (CMUP)
0.6 < || < 1.0 (CMX)
Ye Li
APS April Meeting, St. Louis - 14 April 2008

10. APS April Meeting, St. Louis - 14 April 2008
Acceptance
Mass-dependent Acceptance
Larger mass → Lower boost → More central events → Larger acceptance
CDF Run II Preliminary
Ye Li
APS April Meeting, St. Louis - 14 April 2008

11. APS April Meeting, St. Louis - 14 April 2008
Background
Drell-Yan */Z → 
PYTHIA + Simulation
WW and tt-bar
Cosmic Rays
Identified Cosmic-ray samples
QCD Jets and Decays-in-Flight
Data
Ye Li
APS April Meeting, St. Louis - 14 April 2008

12. APS April Meeting, St. Louis - 14 April 2008
Drell-Yan
Dominant source for background
Dominant source for systematic uncertainty
Mass spectrum affected by higher-order corrections
Calculate up to next-to-next-to leading order (NNLO) correction → k-factor
Different Calculations give different k-factors
Average k-factor; Difference as uncertainty
Ye Li
APS April Meeting, St. Louis - 14 April 2008

13. APS April Meeting, St. Louis - 14 April 2008
Signal Scan
Pseudo-experiment: MZ’ = 250 GeV
CDF Run II Preliminary
Most likely distribution of NZ’
Ye Li
APS April Meeting, St. Louis - 14 April 2008

14. APS April Meeting, St. Louis - 14 April 2008
Signal Scan
Expected limits on NZ’ from 1000 pseudo-experiments on 50 Z’ masses
Data: to be implemented …!
CDF Run II Preliminary
Ye Li
APS April Meeting, St. Louis - 14 April 2008

15. APS April Meeting, St. Louis - 14 April 2008
Summary
Use 1/m distribution for constant resolution to search for Z’
Fitter and Simulation in place to study signal acceptance and identification efficiency
Systematic uncertainties to be determined
Signal scan performed on pseudo-experiments
Ready to unblind the data soon
Ye Li
APS April Meeting, St. Louis - 14 April 2008