W/Z properties
description
Transcript of W/Z properties
Emily Nurse W/Z properties at the Tevatron 1
(for the DØ and CDF (for the DØ and CDF collaborations)collaborations)
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W/Z production at the Tevatron
(ppW l) ~ 2700 pb (ppZ0 l+l-) ~ 250 pb
• Use electron and muon decay channels to measure W/Z properties, due to their clean experimental signature. • The large W/Z masses give their decay products large pT.
Proton-antiproton collider : s = 1.96 TeV TeVatron
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W/Z detection at DØ/CDF
Z events: • 2 high pT charged leptons (+- or e+e-).• Both charged leptons are detected and their momenta measured.• Muons: central tracker, muon detectors• Electrons: central tracker, calorimeter
W events: • 1 high pT charged lepton, 1 high pT
neutrino ( or e).• Charged lepton is detected and momentum measured.• Neutrino cannot be detected! pT
is inferred by the “missing ET (ET
miss)” in the detector.
DØ
CDF
|e(trk)|<3.2, ||<2
|e(trk)|<2.8, ||<1
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W/Z analyses
W/Z events can be used to:a) Constrain the QCD part of the production mechanism:
• PDF constraints Forward cross-sections W charge asymmetry Z rapidity
• Higher order QCD constraints Z pT and W pT
Z+jets and W+jets (see talk by Chris Neu)b) Make precision measurements of EWK parameters:
Z FB asymmetry (sin2weff)
W width W mass (see talk by Ilija Bizjak)
Note: All results are fully corrected for detector acceptance and smearing effects.
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Leading Order :
€
dσpp →W / Z →ll
= [ f iq (x p ) f j
q (x p )i, j= u,d ,s,(c,b )∑ +∫ f i
q (x p ) f jq (x p )] × dσ
qq →W / Z →ll dx pdx p
PDF constraints from W/Z data:1) Z rapidity2) W charge asymmetry
PDF constraints with W/Z eventsParton Distribution Functions (PDFs) describe the momentum distribution of partons in the (anti-)proton. They are obtained from parameterized fits to data (fits performed by CTEQ and MRST groups).Well constrained PDFs are essential for many measurements and searches at hadron colliders.
probability of quark i to carry proton momentum fraction x
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Z rapidity
• Z rapidity (yZ) is dependant on x1,2
• A measurement of d/dy constrains PDFs
x1
x2
New 2.1 fb-1 CDF measurement (~170,000 Zee events with |e|<2.8 )
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Z rapidity
statistical errors only
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W charge asymmetry: introduction x
f(x,
Q2)
x
Q2 = 6400 GeV2
MRST2004NLO
u
pW+u d
u uudp
e+
e
d
W- W+
yprotonantiproton
Fraction of proton momentum carried by quark
)2()1()2()1(
)2()1()2()1(
//
//
)(
xuxdxdxu
xuxdxdxu
dyddyd
dyddyd
yA
WW
WW
W
+−
≈+−
=
−+
−+
helps constrain fu(x)/fd(x)
W/Z properties at the Tevatron
udW+ duW
10-3 10-2 10-1 1
http://durpdg.dur.ac.uk/hepdata/pdf3.html
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W charge asymmetry: from lepton charge asymmetry • Since pL
is not known the W rapidity cannot be reconstructed traditionally the lepton charge asymmetry is measured:• But the V-A structure of the W+(-) decay favors a backward (forward) l+(-).
0.3 fb-1 DØ measurement
(~190,000 W events with ||<2 ):
PRD 77, 011106(R) (2008)
= spin
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W charge asymmetry: direct measurement
• pL determined by constraining MW = 80.4 GeV two possible yW solutions.
• Each solution receives a weight probability according to:– V-A decay structure
– W cross-section: (yW)
• Process iterated since (yW) depends on asymmetry
It is also possible to extract A(yW) directly:
1 fb-1 CDF measurement(~715,000 We events
with |e|<2.8 ):
Analysis method: arXiv:hep-ph/0711.2859
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QCD constraints with Z events
• Measuring the Z pT distribution tests QCD predictions for initial state gluon radiation tune and validate Monte Carlo generators.
• High Z pT dominated by a single hard emission (pQCD reliable).• Low Z pT dominated by multiple soft emissions (resummation techniques/parton shower
Monte Carlos with non-perturbative models required).
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• Z pT < 30 GeV region agrees well with ResBos (NLO QCD + CSS resummation with BNLY non-perturbative form factor).
• The Z pT distribution is predicted to broaden at small x (large |yZ |) - important for the LHC!
• Broadening modelled with an additional “small-x” form factor.
• Data with |yZ| > 2 prefers ResBos with-out “small-x” form factor (NOTE: non-perturbative parameters have not been retuned with additional form factor!).
Z pT
2/dof= 11/11
2/dof= 32/11
2/dof= 9/11
New 0.98 fb-1 DØ measurement
(~64,000 Zee events with |e|<3.2 )
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Z pT
• In Z pT > 30 GeV region a NNLO k-factor is required. • Even then the theory is too low.• The NNLO shape agrees if normalized at Z pT = 30 GeV.
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Precision measurements
1) Extract sin2weff from Z0/* forward-backward asymmetry
2) Direct measurement of W width
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Z/* forward-backward asymmetry
e+
e-
p p* p p*
FORWARD (F) : BACKWARD (B) :
e-
e+
Z and Z/* couplings to fermions have vector : d/dcos* ~ 1 + cos2* and axial-vector : d/dcos* ~ cos* components.
AFB = (F - B) / (F + B)
AFB depends on MZ/*
AFB sensitive to sin2weff
cos* : in Collins-Soper frame (W rest frame)]
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Z/* forward-backward asymmetry
• Measurement consistent with the SM prediction (note: large MZ/* region sensitive to a new Z’ boson).
• sin2weff extracted from fit to AFB:
– 0.2327 0.0019 (DØ 1.1 fb-1)– 0.23152 0.00014 (current world average)
New 1.1 fb-1 DØ measurement (~36,000 Zee events with |e|<2.5 )
arXiv:hep-ph/0804.3220
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W width
• Measuring W tests the very precise Standard Model prediction:
WSM = 2091 ± 2 MeV [PDG: J. Phys. G 33, 1]
€
mT = 2pTl pT
ν (1− cosφlν )
• Ideally W would be extracted from the decay products invariant mass.
• Since the neutrino isn’t detected we construct the transverse mass:
• Fit for W in the high MT tail.• Require a very accurate, fast parameterized detector simulation, tuned to Z and W data.
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W width
W = 2032 73 (stat + syst) MeV
350 pb-1 CDF measurement (~109,000 W and ~127,000 W e events)
PRL 100, 071801
World’s most precise measurement! (reduces uncertainty by 20%)
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W Width: Indirect measurement
R = W
Z (Zll)
(Wl)
(W)
(Z) X X
Rexp = BR (Wl)
BR (Zll) Precision LEP Measurements
SM CalculationNNLO Calculation
CDF Run II INDIRECT width :
2092 ± 42 MeVJ. Phys. G 34 2457-2544, PRL 94, 091803
CDF Run II DIRECT width :
2032 ± 73 MeVPRL 100, 071801
72 pb-1 350 pb-1
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Summary
• Lots of interesting W/Z results from the Tevatron.• New measurements providing constraints for PDF fits:
– Z rapidity [CDF]– W charge asymmetry [CDF]
• New Z pT measurement, testing inclusive and “small x” region [DØ].
• New sin2weff measurement from AFB [DØ].
• W width measurement (world’s most precise measurement!) - consistent with SM and indirect measurement [CDF].
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Back-up slides
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Backup : W/Z detection at DØ/CDF
• Silicon detectors (||< ~2.8)• Central drift chambers (||< ~1.5)• Tracking resolution : (pT)/pT ~ 0.05%pT
• Segmented sampling calorimeters (||< ~3.6) EM: (E)/E ~ 13.5% / ET 1.5%• Muon detectors (||< ~1.0)
• Silicon detectors (||< ~3.2)• Central fiber tracker (||< ~2)• Tracking resolution : (pT)/pT ~ 0.2%pT
• Segmented sampling calorimeters (||< ~4.2) EM: (E)/E ~ 15% / E 4%• Muon detectors (||< ~2.0)
CDF
DØ
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Backup: Z rapidity
PDF constraint: ~0.75 < y < ~2
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Backup: Z rapidity
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Backup: W charge asymmetry
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e asymmetry
€
pp → W → eν
||
||
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Backup: Z/* forward-backward asymmetry
AFB = 0.2327 0.0018 (stat.) 0.0006 (syst.)
Primarily PDFs, and EM energy scale/resolution
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Backup: W width