asymmetries in w ± and z 0 / g * production at the tevatron

Asymmetries in W ± and Z0/ * Production

at the TeVatronICHEP ‘04

August 17, 2004Eva Halkiadakis

University of RochesterFor the CDF and D0

Collaborations

CDF

August 17, 2004 Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

2

EWK “Spectroscopy”• Lots of interesting physics in production

of W ± and Z0 bosons at the Tevatron! Rapidity Spectrum: d/dy

o quark PDF constraints Direct impact on MW

Polar Angle Spectrum: d/dcoso In Z’s vs. Mee

AFB, sin2W

quark, lepton couplingso In W’s vs. pT

Tests QCD predictions Boson pT Spectrum: d/dpT

o Tests QCD predictions Direct impact on MW

W Charge W Charge AsymmetrAsymmetryy


3

W ± Charge Asymmetry

)2()1()2()1()2()1()2()1(

////

)(

xuxdxdxuxuxdxdxu

dyddyddyddyd

yA

WW

WW

W

u quark carries higher fraction of p momentum!

←anti-proton direction proton direction→

ud

ud–

–

log(x)

xf(x

,Q2

)

yW

Measurement of the W charge asymmetry constrains PDF’s of the proton.

[http://durpdg.dur.ac.uk/hepdata/pdf3.html]


4

Lepton Charge Asymmetry

)()(~

////

)(

xuxd

dddddddd

A

ll

ll

l

• Leptonic W decay involves → pz

is unmeasured.• Use experimentally more direct l

± direction to measure A(l). This convolves W production

asymmetry with V-A decay distribution.

Sensitivity to the ratio of PDFs for u and d quarks, u(x)/d(x).

Least constrained at high !

CDF Run I: PRL 81, 5754, CDF Run I: PRL 81, 5754, (1998)(1998)


5

Event Selection• W ± →e ± Candidates

[ET, MET > 25GeV, 50 GeV < MT < 100 GeV] Central: 49214 events Forward: 28806 events

• Charge ID in forward region is key! Use new silicon tracker and

forward calorimeters. Align forward calorimeters

with tracks from central tracker.o Global offsets, rotations.o Internal misalignments.

CDF

l

Out to ~2.8!

1pb 170L

Residuals after alignment look very good.Global

(mRad)


6

Raw Asymmetry• Corrections to

extract true asymmetry: Charge

misidentification rate.

Background subtraction.o Both bias the

asymmetry low → dilution.

o Measured in each bin.

o Uncertainties in corrections go directly in A.Curve is just to guide the

eye.

< 1 ~ linear

Shape is convolution of A(yW) and V-A

Sign switch @ > 2


7

Corrected W Charge Asymmetry

CDF• Gain sensitivity to W production

asymmetry A(yW) with ET dependence. Higher ET: electron direction closer to W

direction. Production asymmetry enhanced. Lower ET: decay asymmetry enhanced

Data lower than existing CTEQ prediction.


8

AFB in Z0 / * → e+e- Decays

• Vector (V) and Axial-vector (A) couplings give rise to AFB.

• Interference between Z0 and* exchanges.

• Different combinations of V and A couplings contribute to d/dcosdMee. AFB direct probe of relative strengths of coupling between Z0 and quarks. Mass dependence is sensitive to u and d quarks separately.

ff

fA

fV

Zf

QJ

fggfJ

~

)(~*

5

Z0 / * f = e, quarks

f

f f ’

f ’

f

f ’

f

f ’

ABA

ddddA

BAdd

BF

FB

83

)0(cos)0(cos)0(cos)0(cos

cos)cos1(cos/ 2

AFB


9

AFB Beyond LEPII

eeuu

• Interesting for Mee above LEP II energies.

• New interactions → deviations of AFB and d/dM from SM predictions. Various models predict new

neutral, heavy bosons: Z´s• New resonance could interfere with

and Z.• Complementary

to direct searches → excess in total cross section.

AFB

500 GeV/c2 Z’

Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) Rosner, J.L.: Phys. Rev. D 54, 1078 (1996)

Z

Zx

Mee(GeV/c2)

2.2 s deviation from SM

CDF Run I (~110 CDF Run I (~110 pbpb-1-1) )


10

Calculating AFB• 5211 Candidate Z0→ e+e- events

2 isolated electrons ET>20 GeV 1892 Central-Central 3319 Central-Forward

• Backgrounds: Central-Central: 1% Central-Forward: 5% Dijet background dominant

1pb 72L

AFB is largest

CDF

1pb 72L

)0*(cos)0*(cos)0*(cos)0*(cos

ddddAFB

Low mass

Pole

High mass

CDF II Preliminary

Backward

Forward


11

Acceptance Corrections• Unconstrained unfolding analysis

Acceptance and event migration parameterized to transform AFB

phys to AFB

raw. Use maximum log likelihood method to

compare to data at the detector level.

40GeV/c 2

600GeV/c 2

600 GeV/c 2

Low mass

Pole

High mass

Kin. and geom. cuts sculpt the cos* distrib., esp. in forward region.

Large correlations

at pole

High mass ok!

CDF

Detector resolution and radiative effects → event migration.


12

Unconstrained Fit with Smoothing • Systematic uncertainties small

compared to statistical: Energy Scale, Resolution Detector Material Backgrounds

CDF II Preliminary

CDF II Preliminary

CDF• Result fitting AFB

Large statistical uncertainties No SM assumptions about AFB!

• All results consistent with SM.• Not useful for non-SM physics near Z pole.• Nothing new above the pole yet.


13

Couplings ResultsCDF

Run II2 fb-1

Uncert.

Experimental values

(PDG)

SM Prediction

uL 0.41 0.14

0.028

0.330 0.016

0.3459 0.0002

ur 0.01 0.12

0.024

-0.176 0.008

-0.1550 0.0001

dL -0.32 0.26

0.057

-0.439 0.011

-0.4291 0.0002

dr -0.02 0.34

0.088

-0.023 0.058

0.0776 0.0001

Quark Couplings:

Electron Couplings:CDF Run II SLD+LEP SM prediction

eV -0.056 0.018

-0.03816 0.00047

-0.03816 0.00047

eA -0.54 0.19

-0.50111 0.00035

-0.5064 0.0001

CDF

13/5.12ndf/χ 2

11/66.10ndf/χ 2


14

Summary• CDF has a new measurement

of the lepton charge asymmetry in W→e decays. Look at data at large ET

and large . Uncertainty on PDFs could

be reduced by inclusion of this data in global fits.

First Run II measurement of W charge asymmetry!

• We also measure AFB vs. Mee in Z→ee events. Unfolded AFB with out SM

assumptions. Fit for Z couplings. Nothing new above the Z

pole yet.

CDF

Future• d/dy measurements in

Z’s will further constrain PDF uncertainties.

• d/dpT in both W’s and Z’s will further test QCD predictions.

1

*0

pb 148

/

L

Z

Important for MW!


15

Backup


16

Phoenix Phoenix (Calorimeter-Seeded) Tracking

Phoenix SiTrackCal. Seed Tracks

Zoom into Silicon:

CDFUse both central and forward electrons! |< 2.8

Two points and a curvature define a helix:• Primary collision vertex position.• Fitted position of calorimeter shower maximum.


17

Charge Misidentification Rate• Charge mis-ID rate, fQ:

Residual misalignments in silicon and plug shower maximum detectors.

Measured in data using Z0→ e+e- events, with one leg in the central tracker (COT).

Measured in each bin. Uncertainties in fQ

directly go in A. Monte Carlo predicts

fQ<1% even at =2 → naïve.

Q

meastrue f

AA21

totalwrongfQ

CDF


18

Background Corrections• We correct A for backgrounds:

W → → e , Z → e eo Measured from Monte

Carloo Asymmetric

QCD Jetso Measured from data

Use Calorimeter Isolation (e) and MET distributions () projected into signal region.

Isolation correlated with other selection criteria.

Estimate is upper limit.o Biases A toward 0.

QCD background is upper limit → use 0.5% ± 0.25% → gives full coverage.

CDF


19

Past Results from e+e-• Previous experiments have done very precise measurements (LEP, SLC, etc.)• Need > 10 fb-1 to compete on sin2(W)

• Difficult to compete with LEPII 120<s<207 GeV


20

Calculating AFB

Candidates ckwardForward/Ba:N

Efficiency & Acceptance ckwardForward/Ba:a

aNN

aNN

aNN

aNN

ABkgrndBkgrnd

BkgrndBkgrnd

FB

• cos* in Collins-Soper frame (minimize ambiguity in the incoming quark pT)

• cos*>0 Forward • cos*<0 Backward

)0*(cos)0*(cos)0*(cos)0*(cos

ddddAFB

Z0/*pp

p p

Z0/* PZ =0Z-Axis

lab frame

IDradreskingeom aaaa )()()()()(a

Sculpt cos* dist.

Mee bin migration

Correct AFB raw to obtain AFBphys →compare

to theory:

CDF


21

Mee and for Z→ee


22

Backgrounds for Z/* • Dijet dominant

Source # of events

C-C C-PDijet 9 128

We+X 1.8 25Z 5.6 7.2WZ 1.4 1.7WW 1.5 1.8Top 1.1 0.7

Background Summary

CDF II Preliminary

CDF II Preliminary


23

Fitting for Z Couplings• Fit at Raw AFB

Level Test AFB is smeared

• 2 show good agreement with SM

2=10.66/11


24

Acceptance*Efficiency• Correction Factors:

Energy Resolution Kinematic and

Fiducial cuts Radiation from FSR

and Brems Electron ID

efficiency• Assumes SM

Allow Z couplings to float Couplings fit

used as input AFB

CDF II Preliminary


25

Theory Band• Different MC’s:• VPB

g resummation• ZGRad

O() EW corr.• Pythia

Parton shower (QED+QCD)

• Nothing w/ NLO QCD & O() EW

• Indicates size of effects PDF’s, ISR…


26

SM with Measured Z0 couplings

• All results consistent with SM.• Not useful for non-SM physics near Z

pole• Nothing new above the pole yet.

CDFUsing couplings fit as input AFB gives:

asymmetries in w ± and z 0 / g * production at the tevatron

Documents

production of w

w direction

w production asymmetry

zdrellyan production

halkiadakis afb

true asymmetry

z0 bosons

e e decaysvector v