p recision measurements of w/z asymmetry at dØ

Precision measurements of W/Z asymmetry at DØ

尹航Fermi National Accelerator Laboratory

USTC SeminarAugust 30th, 2012

Hang Yin, USTC seminar 2

OutlineIntroduction

W charge asymmetry (W e)

Z Forward-Backward charge asymmetry (AFB)

Conclusions

08/30/2012


Tevatron collider

08/30/2012

CDF

DØ

Chicago

Main injector& Recycler

Booster

pbar sourceTevatron


The DØ Collaboration

08/30/2012

September 2011 Collaboration Photo

429 physicists from 18 nationsInstitutions: 82 total

~120 students and postdocsPhysics:B, EW, QCD, TOP, Higgs


The DØ detector

08/30/2012

Drift chambers and scintillator counters 1.8 T toroidsCoverage: || < 2.0

Uranium Liquid Argon calorimetersCentral (CC) and Endcap (EC)Coverage: || < 4.2

Silicon Microstrip Tracker (SMT)Central Fiber Tracker (CFT)2 T magnetic fieldCoverage: || < 3.0

Hang Yin, USTC seminar 608/30/2012


W and Z physics

08/30/2012

W and Z boson production at Tevatron

Z (ee, ) events are often used for detector calibration W/Z are backgrounds for many measurements and searches Make precision measurements of electroweak parameters

(sin2W) Test high-order QED and QCD corrections (AFB) Constrain parton distribution functions (PDFs) (W charge

asymmetry) Search for physics beyond the SM (AFB for high mass region)

l = e, ,


W and Z events

08/30/2012

Z boson: Two high pT charged leptons Both charged leptons are detected and their momentum

measured W boson: One high pT charged lepton, one high pT neutrino

Charged lepton is detected and momentum measured, Neutrino cannot be detected

pT() is inferred by the “missing ET (MET)” in the detector


W and Z events in detector

08/30/2012

Electron

ElectronTrack

Electron

MET

W Z


W CHARGE ASYMMETRY

08/30/2012


u

d

gluon

W charge asymmetry

08/30/2012

u u

d

proton

Proton - antiproton collisions


Tevatron W productions

08/30/2012

u u

d

proton

u u

d

Anti-proton

- -

-


W and lepton asymmetry W charge asymmetry is sensitive to Patron distribution

functions (PDFs) Tevatron is pp collider, u quark intend to carry higher momentum than

d quark Lepton asymmetry is comes from a convolution of W

asymmetry and the W V-A decay: A(y) (V-A)

08/30/2012

-

PTYHIA with CTEQ6.6M


X-Q2 reach

08/30/2012

14

D0 W e

CDF/D0 central jets

CDF/D0 forward jets

x = momentum fraction of partonQ2 = square of momentum transfer

W asymmetry measurement:

This measurement: |yW| < 3.2 0.002 < x < 1.0 Previous measurements: |yW| < 2.5 0.003 < x < 0.5 Complementary to central and forward

jet measurements at D0 and CDF CTEQ and MRST groups Well constrained PDFs are essential for

many measurements and searches at hadron colliders Expect Tevatron Run II WM <15

MeV, currently 11 MeV due to PDFs

Q2 ≈ MW2, x = e±yW

MW

√s


W Asymmetry predictions

08/30/2012

VRAP generator @ NLO


Solutions of neutrino pZ

W boson asymmetry:

W rapidity: for a massive particle rapidity

To get the solutions for neutrino pZ

Fix the W mass at the world average one: 80.385 GeV

08/30/2012


Two solutionsWith given W mass

Two imaginary solutions: MET is not measured ‘properly’, we will scale the MET to make the imaginary part to be 0

Two solutions: give each solution a weight factor, according to the W generation properties (W pT, collins) and differential cross section (dσ/dy)

08/30/2012


Collins angle

08/30/2012

MC@NLO with CTEQ6.6M, 90 M


Sea-quark/valence quark ratio

08/30/2012

MC@NLO with CTEQ6.6M, 90 M


Fitting of W pT vs. rapidityFitting function

08/30/2012


Rapidity predictionsVRAP + NLO + PDF sets

08/30/2012


Closure test of method

08/30/2012

All events Lepton pT > 25 GeV

Good agreement between predictions and closure test results.

MC@NLO with CTEQ6.6M, 10 M as faked data


Input bias removeThe NNLO differential cross sections are took as

inputs, which may introduce the biasA iteration method will be performed to remove this

biasTake the measured cross section as inputs …

08/30/2012


Results …Cannot show you final results with full data-set, still

working on this

08/30/2012

Rapidity region extent to 3.2

With 10 fb-1 data

Test CT10 or even latest PDFs

ET>35 GeV


FORWARD BACKWARD CHARGE ASYMMETRY (AFB) OF Z

08/30/2012

Phys. Rev.D 84, 012007 (2011)Phys. Rev. Lett. 101, 191801 (2008)


Drell-Yan

08/30/2012

Coupling of a virtual photon to fermions: vector coupling

Couplings of a Z-boson to fermions: vector and axial-vector couplings

Drell-Yan Procedure: annihilation of and the production of di-lepton or pair via a Z-boson or virtual photon


AFB

08/30/2012

Forward-backward asymmetry :

The differential cross section:

The presence of both vector and axial vector couplings gives rise to non-zero AFB.

Forward Backward

*cos*cos

*,cos*cos

0

1

1

0

d

d

dd

d

dBF


Pure Z exchange

Primarily pure * exchange

Z/* interference

uu e+e-_

Asymmetry prediction

08/30/2012


Tevatron prediction

08/30/2012

PTYHIA prediction


Weak mixing angle

08/30/2012

AFB is sensitive to sin2ϴW

SM parameters: α, Gf, MZ, MW and sin2ϴW

Only depends on lepton couplings

Also depends on quark couplings


Standard model parameters

08/30/20120.2277 0.0016sin2W(N)

LEP and SLD most precise results are off by 3 in opposite direction

(sin2Weff includes higher

order corrections) Dzero 1.1 fb-1 publication.Phys. Rev. Lett. 101, 191801 (2008)

And CDF 72 pb-1 PRD publication


Z quark couplings

08/30/2012

LEP/SLD: Phys. Rep. 427, 5 (2006)


Mee (GeV)

New resonance search

08/30/2012

New resonance (Z’, LED etc) can interfere with Z and * AFB measurement is complementary to bump search

500 GeV Z’ boson

Zx

Z

Rosner, PRD 54, 1078 (1996)

AFB


Event selection

08/30/2012

RunII, L = 5.0 fb-1, with all diem triggers

Primary vertex |Z|< 40 cm

Electron selections: Transverse momentum > 25 GeV, isolated and shower shape cuts in calorimeter fiducial region

Only look at CCCC and CCEC events For CCCC events, require two opposite track matches For CCEC events, require CC electron must has a track match(use the charge of

CC cluster to determine forward/backward event)

Track matching requirements:


MC tuning

08/30/2012

Pythia Geant MC samples: Z/γ*->ee samples : 40-60 GeV, 60-130 GeV, 130-250 GeV, 250-500 GeV and > 500 GeV At generate level:

applying QCD NNLO corrections on Z-pT rapidity and Z Mass reweighting At reconstructed level:

EM scale and smearing Electron selection efficiencies corrections Trigger efficiencies VtxZ, Phi-mod and luminosity correction F/B efficiencies difference vs. InvMass

Before sample reweight After sample reweight


Background modeling

08/30/2012

EW: Z ττ, W+X, WW, WZ, γγ, ttbar

QCD background: jets are misidentified as electrons, estimated from data

Shape(Mee) of QCD background

measured from real data: inverting electron shower shaperequirement in CC and in EC

Fraction of QCD background: using minimum χ2 fitting


Data and MC comparison

08/30/2012


Using minimum χ2 fitting method

Measured value： sin2 ϴeff

lept = 0.2304±0.0008(Stat.)±0.0006(Syst.)

Extraction of weak mixing angle

08/30/2012

Pythia templates generated with 40 sin2 ϴefflept inputs:

From 0.22552 to 0.23722 with step size 0.0003


Corrections and uncertainties

08/30/2012

High order correction Measured from ZGrad2 Correction: +0.0005

Final results:

Systematic uncertainties:• Dominant systematic is from PDF

CTEQ6.1M


Weak mixing angle comparisons

08/30/2012


Mee

Unfolding: events migration

08/30/2012

Mee

Due to the finite detector resolution, events migrate into other mass bins

At generate level: and

At selected level: and

][][][ ,, jNRjNRiN Bsel

FBji

Fsel

FFji

Fgen

][][][ ,,B jNRjNRiN B

selBBji

Fsel

BFjigen


Unfolding: acceptance*eff

08/30/2012

Acc*eff for CCCC and CCEC events

To remove geometric and kinematic cuts effects


Unfolding: charge mis-ID

08/30/2012

Measured fQ vs. Mee in analysis

fQ : Charge mis-identification rate For CCCC events, we have

For CCEC events, we have

If fQ = 50%, cannot measure charge asymmetry


Unfolded AFB

08/30/2012


Z-quark couplings

08/30/2012

Use ResBos generate pure γ*, pure Z boson, and Z/γ* interference: 20M*44 (PDFs) events for each sample

Performed both 4-D, and 2-D fit of Z to light quark couplings

Fix weak mixing angle into world average one (0.23153)

ddee-


4-D fits

08/30/2012

Z-u quark couplings Z-d quark couplings


Compared with LEP, H1

08/30/2012

Z-u quark couplings Z-d quark couplings


Conclusion DØ collected ~10 fb-1 ppbar data

W charge asymmetry Measure both W asymmetry and lepton asymmetry Useful for future global PDFs fit

Z charge asymmetry Legacy measurement of weak mixing angle Z to light quark couplings

Full dataset analyses are on going: with better understanding of the detector, more high precision electroweak measurement expected!!

08/30/2012


Ongoing/future topicsZ Rapidity measurementZ angular coefficientZ pT measurementW pT measurementCross section of W and Z……

08/30/2012


BACKUP

08/30/2012


Collins-Soper frameCollins-Soper frame

08/30/2012


1


Strategy

08/30/2012

Unfold data and compare unfolded AFB with theoretical prediction

Extract sin2θlepteff using the distribution

Subtract backgrounds and measure distribution

Select Z/γ* candidates from data

After various Corrections:

Extract Z to light quark couplings using the unfolded AFB distribution


Unfolding bias

08/30/2012

MC with stw(3σ)

Response matrices ……..

Data raw AFB

Extract stw

Use Geant MC to get the response matrices and acc*eff corrections. Those corrections depends on the input AFB and may bias the unfolded results.

Do an iterative unfolding to remove this bias:1. Use response matrices measured from a MC sample(with different ) to

unfold the raw AFB(start with 3σ(3*0.001) away from the measured )

2. Compare unfolded AFB with AFB generated with difference input using χ2 fitting method to get best

3. Use corrections measured from a MC sample(with from step 2) to unfold the raw AFB

4. Repeat step 1 – 3 , until is stable


In order to quantify the deviation in the high mass region. A pseudo-experiment is done. Use PYTHIA generator, generated 158k events from 50-1000 GeV as one

pseudo-experiment 43 k independence samples are generated Calculate the Χ2 value for each samples

Compare the Χ2 get from data with pseudo-experiment.

For 5.0 fb-1 results, with Χ2 = 15.3, the p-value is 59%.

Signal based probability test on the high mass bin


Charge mis-ID

08/30/2012

The rate of same-sign events as a function of di-electron invariant mass

Very few events for Mee > 200 GeV

Mis-ID rate shape determined from MC

MC cannot describe the data for charge mis-ID rate:

normalized MC distribution to data in the Z peak region

• Data• MC

data MC


Apply same analysis procedure on GEANT MC and compare the unfolded AFB with truth

Three closure tests have been done:1. Raw AFB and all corrections measured using the same GEANT MC files

Closure tests


Closure tests(Cont.)2. Use half of GEANT files to measure raw AFB and other half to measure all

correction

3. Randomly flip the charge to make generator level AFB flat and then measure AFB and all corrections


Unfolding Source of systematic uncertainties

Theoretical: PDF, QCD and EW corrections Electrons: energy scale, energy smear, difference between forward

and backward efficiencies, charge mis-identification Other: background, Acc*eff, the input of , the unfolding

method


EM

EM

FHFH

FH CH

CH

CHCH3.2 A

3 A

22 X0

21 X0 4.4 A 4 .1A


W selectionRunIIb data, ~ 10 fb-1, with single EM triggerPrimary vertex |Z| < 40 cmElectron requirements:

Electron transverse momentum > 25 GeV, isolated, with shower shape cuts

Missing transverse momentum: MET > 25 GeVW mass windows: transverse mass MT> 50

GeVTrack matching requirements

08/30/2012


Electron types

08/30/2012

Type 1η=1.1

η=1.5

η=1.67 Type 2η=1.67

η=2.3Type 3

Type 4η=2.3

η=3.2


Recoil modeling

08/30/2012


Tevatron compared with LHC

08/30/2012


d

Tevatron vs. LHC

08/30/2012

uu

u-u-d

d

uu

u-u-d

d

uu

d

uu

d

uu

d

uu

Z/γ* e+, μ+

e-, μ-

-

-

W+ e+, μ+

ν

Z/γ* e+, μ+

e-, μ-

q

q-

W+ e+, μ+

ν

q

q-

prot

onAn

ti-pr

oton

prot

onAn

ti-pr

oton

prot

onpr

oton

prot

onpr

oton

With known incoming quark direction. In the forward region, the valence quarkhas higher momentum.

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