Emily Nurse W/Z properties at the Tevatron 1

(for the DØ and CDF (for the DØ and CDF collaborations)collaborations)

Emily Nurse W/Z properties at the Tevatron 2

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