W, Z and Drell-Yan Production II Asymmetries

Susan Blessing

Florida State University

For the DØ and CDF Collaborations

ICHEP 2006

July 27, 2006

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Outline

• W production charge asymmetry• W (DØ) and W e (CDF)

• direct method in W e (CDF)

• Z ee forward/backward asymmetry (CDF)

• Summary

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W boson production charge asymmetry

• u quarks typically carry more of a proton’s momentum than d quarks• W+ goes in the proton direction

• W in the antiproton direction

• Use this difference to get information about the proton’s u and d distributions – PDFs

Rapidity yd

σ/d

y

(nb

)

√s = 1.96 TeV

A(yW) =

ddy

(W)ddy

(W)

ddy

(W)ddy

(W)

≈

ud

(xp)ud

(xp)

ud

(xp)ud

(xp)

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Q2, x reach

Q2 ≈ MW2, x = e±yW

MW

√s

Traditionally, PDFs are measured in deep inelastic scattering – high energy electron-nucleon interactions.

W asymmetry measurements:

x = momentum fraction of partonQ2 = square of momentum transfer

for |yW| < 2.5 (W e, CDF 0.003 < x < 0.5

for |yW| < 2 (W , DØ)

0.005 < x < 0.3

DØ, W CDF, W e

Q2, x coverage for CDF, DØ,HERA and fixed target experiments

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Lepton charge asymmetry

• W asymmetry difficult to measure• neutrino longitudinal momentum

• Lepton pseudorapidity is available

• Lepton asymmetry is a combination of the W asymmetry and VA interaction from decay• at higher lepton transverse

momentum, VA contribution is smaller, A(yl) is larger

• at higher lepton rapidity, VA contribution is larger, A(yl) is smaller

For all muon momenta

A(l) = d (l)d

d (l)d

d (l)dd (l)d

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W (DØ) and W e (CDF)W W e Tight selections to reduce

charge and event misidentification

charge misid ≈104 for W ≈102 for W e

CDF charge misid vs pseudorapidity

PRD 71, 051104(R) (2005)

Analyses done bin-by-bin

L = 230 pb1

pT > 20 GeV < 2.0ET > 20 GeV MT > 40 GeV

/

L = 170 pb1

ET > 25 GeV < 2.5ET > 25 GeV50 < MT < 100 GeV

/

e

DØ transverse mass distribution+ Bkgd230 pb1

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Asymmetry vs pseudorapidity

PRD 71, 051104(R) (2005)

W

Pseudorapidity

L≈230 pb1

CTEQ6.1Muncertaintyband

MRST02pT > 20 GeV

Combineduncertainties

W e

Statistical andtotal uncertainties

W production and decay are CP invariant;fold the asymmetry to increase statistics.

Statistical uncertainties comparable to ormuch larger than systematic for || > 0.4~

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Asymmetry in ET bins (CDF)

25 < ET < 35 GeV35 < ET < 45 GeV

Improve correspondence between e and yW

pZ ambiguity is a smallereffect for high-ET electrons

For a given e, ET regionsprobe different ranges of yW (x); higher ET bin covers a narrower yW range

PRD 71, 051104(R) (2005)

Statistical andtotal uncertainties

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Direct W asymmetry method – CDF

Reconstruct yW distribution

W mass constraint

Weight the two solutions

Iterate since weight depends on yW

W+ events

cos*

valence-valence

sea-sea

cos*

W events

valence-valence

sea-sea

* = angle of charged lepton in W frame

1(y1)

e

u d_

2(y2)

weight takes production and decay into account

depends on cos*, y1,2, pT, (y1,2), yWW

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Projected uncertainties of direct method

400 pb1 Pythia eventsET > 25 GeVET > 25 GeV/

e

Compare expected statisticaluncertainties in W asymmetryand lepton asymmetry with CTEQ6M error sets

Direct method for W asymmetrymeasurement shows improvedstatistical uncertainty

Estimated systematics are small

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Z ee forward-backward asymmetry (CDF)

Interference between (*) and Z exchanges

= A(1+cos2) + Bcosddcos

AFB =(cos>0) (cos<0)

(cos>0) (cos<0)

=NF NB

NF NB

3B8A

A and B depend on gV, gA, Qq and Qland are related through the forward-backward asymmetry, AFB =

q,l q,l

Primarily * exchange below ZZ exchange at ZZ/ exchange above Z

)

e

e

q q_

Interference depends on Mee

+q

q

e

e

*) q

q

e

e

Z

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AFB distributions

Probe relative strengths of Z-q couplings;sensitive to u and d quarks separately

Exchange of new particle(s) would alter AFB

500 GeV Z´

J.L. Rosner, PRD 54, 1078 (1996)

Mee (GeV)

uu ee

ZX

Z

AF

B

AFB

Mee (GeV)

AF

B

dd ee

uu ee

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Determining AFB

AFB =

NF NF

aF

bkg NB NB

aB

bkg

NF NF

aF

bkg NB NB

aB

bkg

Correct for background,acceptance/efficiency

Since it’s a ratio, reduced systematics of luminosity, acceptances/efficiencies

Acceptance is symmetric; distribution is notshifts AFB within a mass bin

Event migration between binslarge correlations between bins near Z pole

Use a matrix based on MCevents to “unfold” raw AFB distribution (correct for acceptance/efficiency and smearing)ISR and FSR

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Unfolded AFB distributionAgreement with LO SM calculation is 2/dof = 10.9/12

No evidence for additional gauge bosons

Updating to 1 fb1

Can fit for quark and electron couplingsExample using 72 pb1 result

72 pb-1

CDF: PRD 71, 052002 (2005)Wichmann, HCP 2006

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Summary

Both DØ and CDF are measuring W and Z/ asymmetriesuseful for constraining PDF fits

for gaining information on quark and electron EW couplingsfor searching for additional gauge bosonsand, of course, for testing the Standard Model

Both experiments have significantly more data available, over 1fb1, and are making progress on extending theseanalyses to include this data.

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Backup Slides

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Lepton asymmetry and pT cut

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DØ overall efficiency ratio

Overall efficiency ratio = product of individual ratios

= 0.99 ± 0.01

2/dof = 0.71

L2 muon trigger efficiencyL3 track trigger efficiencyOffline muon reconstruction efficiencyOffline tracking efficiency

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DØ charge misidentification1. Only isolated muons

2. Plus hits in the tracker

3. Plus additional 2/dof

4. dca but no 2

ch

arg

e m

isid

ra

te

char

ge m

isid

ra

te

ch

arg

e m

isid

ra

te

cha

rge

mis

id r

ate

• Cross checks• no Z invariant mass cut• reduced muon pT cut• use other triggers• study misid in GEANT MC

• No difference

Sample of 10,000 ee events.After all selection cuts, only one same-sign event remains.

Charge misid rate = (0.010.01)% for ≤ 1.0 (0.01±0.05)% for 1.0

inflate uncertainty at high due to low statistics

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DØ systematic uncertainties

• Data• differences in efficiencies for

and

• take = 1.0 and use uncertainty as a systematic

• charge misidentification

• Background• PMCS modeling of EW

backgrounds• muon energy in calorimeter

• hadronic energy scale (ET)

• uncertainty in signal isolation efficiency

• uncertainty in background isolation efficiency

Vary everything by ±1,use change in asymmetry as uncertainty.

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DØ magnet polarities

Solenoid polarities

Toroid polaritiesData is 50.7% : 49.4%

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W (DØ) and W e (CDF)

W Z Z W QCD multijet

W e

PRD D 71, 051104(R) (2005)

Z eeW eQCD multijet

Pseudorapidity

Envelope of CTEQ6.1Merror sets

MRST02

Statistical - blackSystematic - red

≈230 pb1

pT > 20 GeV

Analyses done bin-by-bin

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DØ folded asymmetry plot

Red curve: CTEQ6.1M central valueBlue curve: MRST02Combined uncertainties.

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CDF W asymmetry weighting

P±(cos1, y1, pT) (y1)* WP±(cos2, y2, pT) (y2)* W+

F =±

1,2

P±(cos1,2, y1,2, pT) (y1,2)* W

P±(cos1,2, y1,2, pT ) = (1 cos*)2 + Q(yW, pT )(1 ± cos*)2* W ± W

valence-valence sea-searatio of twodistributions

W+ events

cos*

valence-valence

sea-sea

cos*

W events

valence-valence

sea-sea

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CDF Z → ee mass distributions

High mass region

Low mass region

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CDF AFB background