1. Prospects for Early Measurements of W and Z Bosons with CMS at the LHC
Jeffrey Berryhill (Fermilab)
The CMS Collaboration
PANIC ’08, Eilat, Israel
November 11, 2008
W and Z inclusive cross sections (10 pb-1)
W lepton rapidity asymmetry (10, 100 pb-1)
Z + bb cross section (100 pb-1)

2. Conclusion “Everyone has a plan… until they get punched in the face.”
--old boxing maxim

3. Electroweak Bosons as a Precision Calibration Tool
For Standard Model physics:
Inclusive cross sections compared against initial 10% precision (lumi)
…or estimate parton luminosity itself to ~5% (theory)
Cross section ratios estimated at ~1% (PDF)
…or further constrain PDFs themselves once ~% precision achieved
For detector performance:
Commissioning leptons for triggering, reconstruction, and identification
for PT from 20 to 60 GeV
measuring performance at the % level
First look at efficacy of missing ET for analysis (real or instrumental)
Associated production with jets an ubiquitous background for searches

4. Inclusive W and Z Production Cross Sections
At TeV, initiated by an admixture of both valence-sea and sea-sea quark annihilation at HERA-like x (10-5 to 10-2)
Compared to Tevatron:
Cross section 4-6X larger
Acceptance for |hl| < 2.5 is 2X better for W, 1X for Z
100k W’s and 10k Z’s
for 10 TeV
W → l n
s×BR (pb)
N×A/pb-1
1.96 TeV pp
2700
2*600
10 TeV pp
11800
2*6000
14 TeV pp
17200
2*9000
Z→ l l
250
2*80
1200
2*400
1800
MRST, Eur.Phys.J. C14 (2000)

5. Inclusive W and Z Production Cross Sections : Z Selection
Trigger, reconstruction, and ID efficiency for single leptons can be measured directly from data with
Z → ll “tag and probe” samples
Electron trigger efficiency
Z →ee
Single electron trigger w/isolation
2 high ET electrons (ET>20.0 GeV, |h| < 2.5)
70 GeV < Μee < 110 GeV
Track Isolated: Σ(PT/PTe)2<0.02 in a cone of 0.6 Electron Id: Had/EM energy (Δη, Δφ) track/cluster matching , σηη shower shape
Z→ mm
Single muon trigger
2 high PT muons (PT>20.0 GeV, |η|<2.0)
Mμμ > 40 GeV
Hits from Tracker + Muon Chambers
Track Isolated: ΣPT<3 GeV in a cone of 0.3
Muon trigger efficiency
NOT using precision tracking cuts
(e.g. ds) in 10 pb-1 startup era

6. Inclusive W and Z Production Cross Sections: Z results
4600 ee (1.5% stat), 5300 mm (1.4% stat) selected
Backgrounds < sqrt(S) (i.e. any subtraction error is subleading)
Mean lepton efficiencies known to 1% (stat) or better
Efficiency corrected signal precision comfortably below 5% (useful for lumi)

7. Inclusive W and Z Production Cross Sections: W selection
W →mn
One muon (PT>25.0 GeV, |η|<2.0)
Veto 2nd muon w/ PT> 20 GeV
Track Isolated: ΣPT/PTμ < 0.09, ΔR<0.3
Acoplanarity and multijet vetos
Background is primarily bb dijet and Z →mm
Transverse mass separates W signal from bb above ~50 GeV
Non-isolated muons an unbiased template for background transverse mass
Z →mm sample used to describe missing ET → an unbiased template for W signal MT
Use simple “four-square” (iso/noniso vs. low/high MT) calculation,
or a multi-bin likelihood fit, to extract signal yield from MT.

8. Inclusive W and Z Production Cross Sections: W selection
W →en
One electron (ET>30 GeV, |h|<2.5)
Veto 2nd electron w/ ET> 20 GeV
Tighter Track Isolation:
no tracks with PT>1.5 GeV in a cone of ΔR<0.6
Tighter electron Id
QCD background is admixture of bb,
light dijet, and photon+jet
Non-isolated leptons give unbiased estimator of missing ET for isolated leptons, Z→ ee gives missing ET for W signal
Four-square method (iso/noniso vs.
low/high missing ET) gives marginally unbiased signal yield
Need tight ID/Iso cuts to minimize S/B at expense of S/sqrt(S+B)

9. Inclusive W and Z Production Cross Sections : W Results
28k e, 64k m selected, B/S ~ 25% without missing ET cut, <13% with cut
Systematics from QCD background subtraction biases comparable to sqrt(S+B)/S
Efficiency corrected signal yield precision ~1% for muons, ~3% for electrons
Background rejection from missing ET and lepton ID crucial to performance

10. W Lepton Rapidity Charge Asymmetry
W production in pp collisions is globally charge asymmetric
(pp has more ud than du collisions due to valence quark content of p):
(W+- W-)/(W+ + W-) = total cross section asymmetry
Also differentially charge asymmetric in lepton rapidity due to d/u pdf asymmetry vs. x and V-A decay asymmetry of W
For leptons in CMS W acceptance,
A varies from +10 to +20%
Precision at 1% or less, sampled
in several rapidity bins, provides
PDF constraint tighter than
global fits

11. W Lepton Rapidity Charge Asymmetry
Select same W →mn events as for the inclusive cross section:
PT(m) > 25 GeV, |h(m)| < 2.0, MT > 50 GeV
Muon triggering, reconstruction, and ID asymmetry known to <1% from tag-and-probe
Major backgrounds (QCD, DY) symmetric, w/uniform dilution (20%) vs. h(m)
Detector misalignment/miscalibration biases reduced to 1% by 100 pb-1

12. W Lepton Rapidity Charge Asymmetry
For 20 bins in 0.1 of eta, differential asymmetry known nearly uniformly
to 2%(stat) per pb-1, an interesting precision cross check
<1%(stat) per pb-1 would improve the PDFs
Further studies: do electrons help (esp. for |h| > 2.0)?
Can W boson rapidity be measured reliably?

13. Z + bb Production Cross Section
MCFM cross
is 46 pb, LO diagrams via
gg fusion and quark-initiated
Z+split g
Important background to SM heavy Higgs→ 4l and light SUSY Higgs searches
Jumping off point for W+bb, Z+b, et al.
Lepton selection: lepton pairs selected as in the inclusive Z analysis
Jet selection: >= 2 jets with corrected ET > 30 GeV and |h| < 2.4
Missing ET > 50 GeV veto to reduce top quark background
Dilepton mass, no b-tagging

14. Z + bb Production Cross Section: Selection
b-tagging: 2 jets with a good value
wrt a simple discriminator (3D IP significance of 3rd highest IP sig. track)
Remaining background is predominantly top, which is estimated using dilepton mass sidebands
b-tagging efficiency and udsg mistagging from m+jet control samples, c-mistagging from MC
b jet signal
Signal yield corrected for b-tag efficiency and Z+jj,cc backgrounds

15. Z + bb Production Cross Section : Results
46 Z+bb events selected in 100 pb-1, with top background at 30% level
Biggest systematics are b-tagging efficiency (±16%), JES (±8%),
lumi (10%), MC model (-10%), missing ET model (7%)
Significant but systematics limited signal with 100 pb-1

16. Conclusion With first 10 pb-1 of pp collision data:
W and Z production cross section measured and dominated by luminosity error (~10%) in electron and muon channels.
Z counting suitable as ~5% estimator of luminosity,
dominated by theory error. W a bit more challenging due to
QCD background.
Lepton trigger/reconstruction/ID tuned and measured to 1% level
With 100 pb-1 of pp collision data:
Novel PDF constraints from W asymmetry
Abundant W and Z + jets signals, including Z + heavy flavor