ewk cross-sections and widths

Aidan RobsonGlasgow University

for the CDF and D0 Collaborations

EWK Cross-sections and Widths

ICHEP, Philadelphia, 30 July 2008

Aidan Robson Glasgow University 2/24

CDF Zee

(from Stirling, ICHEP04)

2004, using < 100 pb–1


A high-pT physics programme


More challenging channels Differential cross-sections

High-precision measurements of Standard Model parameters

Now:


More challenging channels Differential cross-sections

High-precision measurements of Standard Model parameters

(ppZ) . Br(Z)

d / dyd / dpT

W

Z(invisible)

Outline


CDF and D0

= 1.0= 1.0

= 0.6= 0.6

= 2.0= 2.0muonchambers

D0

=2

=3

0 1 2 3 m

2

1

0

tracker had cal

hadronic calEM cal had

calsolenoid

pre-radiator shower max

silicon

EM cal

=1CDF

Fibre tracker to ||<1.8Calorimeter to ||<4Muon system to ||<2Drift chamber to ||<1

Further tracking from SiCalorimeter to ||<3Muon system to ||<1.5


W and Z selectionElectrons: good EM shower shape small hadronic energy isolated in calorimeter well-matching good track (except far forward)

Muons: MIP in calorimeter isolated hits in muon chamber well-matching good track

Z selection: 2 oppositely-charged electrons or muons invariant mass consistent with mZ

W selection: exactly one electron or muon energy imbalance in reconstructed event, associated with neutrino


Taus at D0

Neural net separator trainedon variables measuring:isolationshower shapecalorimeter–track correlations

Three D0 tau categories:Type 1: 1 track, no EM sub-clusterType 2: 1 track, ≥1 EM sub-clustersType 3: ≥2 tracks, ≥0 EM sub-clusters

Type 1 Type 2 Type 3

0 NN output score 1 0 NN output score 1 0 NN output score 1

The elements: calorimeter cluster (cone R<0.5) energy concentrated in inner cone R<0.3 tracks in cone R<0.3, mass<1.8GeV EM sub-clusters in finely segmented shower-max layer of calorimeter

ET measurement:For E<100GeV, track pT & calorimeter ET are combined for Types 2&3, and single ± energy corrections derived from special hadronic calorimeter simulation.Otherwise track pT used.


Z

Type 1 Type 2 Type 3

Select Zh from inclusive muon trigger. Additionally:

pT > 15 GeV

pT > 15 GeV

opposite charge

visible mass / GeV visible mass / GeV visible mass / GeV

D0 1fb–1 preliminary D0 1fb–1 preliminary D0 1fb–1 preliminary

scalar sum pT of tracks > 15GeV or 5 GeV (types 1&3/2) NN > 0.9 or 0.95 (types 1&3/2)

Backgrounds: QCD (bb) data-driven from same-sign events EWK backgrounds from MC

corrected for SS–OS ratio inQCD-enhanced sample

W+jets corrected for componentaccounted for in same-sign events

mvis = √(P + P + PT)2


Z

(ppZ) . Br(Z) = 247 ± 8(stat) ± 13(sys) ± 15(lumi) pb

Systematic source Value

Tau Energy Scale 1.0%NN 2.4%Tau track reco 1.5%QCD background 0.7%W background 0.5%Trigger 3.6%Muon track match 0.8%Muon identification 0.4%Z pT reweighting 1.6%PDF 1.7%

Total (excl lumi) 5.4%

Luminosity 6.1%

2.5%

3.5%2.3%

Values from previous published analysis (PRD 71 072004 (2005))

Total 1527 events observed, ~20% background


Z rapidity

rapidity y= lnE+pz

E–pz

1

2

closely related to parton x:

Z production:

m

√sx1,2 = e±y (LO)

P1

P2

fq(x1)

fq(x2)

x1P1

x2P2

*/Ze+

e–

CDF

e+

e–

CDF CDFx

y

-

Even

ts /

0.1


Z rapidity: shapes

Frac

tion

Acce

ptan

ce x

effi

cien

cy

Si tr

acki

ng e

ffici

ency

ener

gy s

calin

g fa

ctor

|’|

||<0.4 0.4<||<0.8

||<0.8 All

10 ET 60 10 ET 60

10 ET 60 10 ET 60

Calorimetry

Tracking

Total


Z rapidity

Isolation E

Events

Systematics:Detector material modelingBackground estimatesElectron identification efficienciesSilicon tracking efficiencyAcceptance


Z rapidityfr

actio

nal s

yste

mati

cun

cert

aint

y (%

)


Z pT

pT(Z) pT(Z)

pQCD reliable

resummation / parton shower with non-perturbative model

Measurement of Z pT tests QCD predictions for initial state gluon radiation

resummationrequired

multiple soft gluon radiation

300

RESBOS event generator implements NLO QCD and CSS resummation (with BNLY form-factor)

3-parameter function, global data fit Recent global fits suggest extra small-x form-factor


Z pT

pT (Z) / GeV

d/

dpT /

nb/

GeV

all y |y| > 2

d/

dpT /

pb/

GeV

Recent global fits suggest extra small-x form-factor – implies pT(Z) broadened at high y

D0

e+

e–

pT (Z) / GeV

d/

dpT /

nb/

GeV

d/

dpT /

pb/

GeV

ppZ0Xe+e–X, LHC:√s=14TeV ppW+Xe+, LHC:√s=14TeV

|y| < 2.5pT

e > 25 GeV

|ye| < 2.5pT

e > 25 GeVET

> 25 GeV

pT (Z) / GeV

ATLAS

e+

e–

pT (W) / GeV

No small-x broadening (BLNY)With small-x broadening

No small-x broadening (BLNY)With small-x broadening

0 35 0 35

0 35 0 35


Z pT

Regularized unfolding

pT(e1), pT(e2) > 25 GeV||<1.1 or 1.5<||<3.270 < mee < 110

Selection:

64k events, > 5k for |yZ|>2

Backgrounds: fit mee with templates

1.3–8.5%Unfolding method (input MC)Smearing parametersPDFUnfolding method (parameters)QCD backgroundpT dependence of eff. x acc.


Z pT

without small-x effect:2/dof = 11.1/11

including small-x effect:2/dof = 31.9/11

(curves normalised)

PRL 100 102002 (2008)

Data does not favour small-x broadening(but no refit of usual 3 parameters has been done)


Z pT

PRL 100 102002 (2008)

The complete spectrum:

Compare 4 models: Resbos with default parameters Resbos with additional NLO–NNLO K-factor NNLO (Melnikov and Petriello) NNLO rescaled at to data at 30GeV/c


W width

€

mT = 2pTl pT

ν (1− cosφlν )

W predicted in Standard Model: WSM = 2091±2 MeV (PDG)

Experimentally have access to transverse quantities:


W width Generator: LO MC matched with Resbos (QCD ISR) and Berends/Kleiss (QED FSR)

Fast simulation for templates: electron conversions + showering muon energy loss parametric model of recoil energy (QCD, underlying event + brem)

Tracking scale/resn

Calorimeter scale/resn

m (GeV)

mee (GeV)

Backgrounds

mT (GeV) mT (GeV)

21 MeV, 31 MeV

17 MeV, 26 MeV

32 MeV 33 MeV

54 MeV (ele), 49 MeV (mu)

2/dof=27.1/22

2/dof=18/22


W width

W = 2032 ± 73 (stat+sys) MeV

WSM = 2091 ± 2 MeV)

PRL 100 071801 (2008)

€

R =σ (pp →W )

σ (pp → Z)⋅

Γ(Z)

Γ(Z → ll )⋅Γ(W → l ν )

Γ(W )

Compare to CDF indirect measurement:

NNLO calc From LEP

SM value

W (indirect) = 2092 ± 42 MeVJ Phys G 34 2457

World most precisesingle measurement


Z invisible widthZ(invisible) measured very precisely indirectly from LEP: Z(invis) = 500.8 ± 2.6 MeV

However combined direct LEP measurement: Z(invis) = 503 ± 16 MeV

Tevatron measurement uncorrelated, relies less strongly on theoretical inputs

Z(inv)Z(ll)

(Z+1jet) . Br (Z inv) (Z+1jet) . Br (Z ll)=

Nobs – Nbck

(Zll+1jet) . L=

Must be corrected for different acceptance of ‘1-jet’ selection in Z versus Zll (applied after lepton removal)

Select single-jet events (ET trigger)Independently measure (Z+1jet) . Br (Z ll) from high-pT lepton trigger Selection:

ET > 80 GeV ET

jet > 80 GeVsecond jet ET<30 allowedbut ≥ 3 jets ET>20 rejected


Z invisible width

EWK ‘background’ includes Z prediction

W 2010 ± 69

W 1570 ± 54

We 824 ± 28

Zll 87 ± 3

QCD 708 ± 146

+jet 209 ± 41

non-collision 52 ± 52

Z 3203 ± 137

Total predicted 8663 ± 332

Data observed 8449

Measured (Zll+1jet) = 0.555 ± 0.024 pbZ(inv)Z(ll)

= 5.546 ± 0.506

Z(inv) = 466 ± 42 MeV

~ equal contributions from EWK bck, QCD bck and (Zll+1jet)

N = 2.79 ± 0.25


Summary

W and Z cross-section measurements underpin the Tevatron high-pT physics programme

Dedicated measurements are harnessing the high statistics datasets: improving tau identification testing higher-order calculations and PDFs and probing QCD making precision measurements of SM parameters


Backup

Backup


10-7 10-6 10-5 10-4 10-3 10-2 10-1 100100

101

102

103

104

105

106

107

108

109

fixedtarget

HERA

x1,2 = (M/1.96 TeV) exp(± )y=Q M

Tevatron parton kinematics

M10eV

M100eV

M1TeV

422 04=y

Q2(GeV

2)

x

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100100

101

102

103

104

105

106

107

108

109

fixedtarget

HERA

x1,2 = (M/14 TeV) exp(± )y=Q M

LHC parton kinematics

M10eV

M100eV

M1TeV

M10TeV

66y 40 224

Q2(GeV

2)

x

longer Q2

extrapolation

smaller x

(W James Stirling)


D0 Z rapidity


CDF d/dy


Z pT

pT (Z) / GeV

d/

dpT /

nb/

GeV

all y |y| > 2

ppZ0X, Tevatron:√s=1.96TeV ppZ0X, Tevatron:√s=1.96TeV

d/

dpT /

pb/

GeV

CDF

e+

e–

Nadolsky et al: global fits to HERA and fixed-target data suggest increased intrinsic pT carried by proton constituents, for interactions involving only a small fraction of proton’s momentum they insert extra factor in differential cross-section ; pT(Z) broadened at high y


Z pT

pT (Z) / GeVd/

dpT /

nb/

GeV

d/

dpT /

pb/

GeV

ppZ0Xe+e–X, LHC:√s=14TeV ppW+Xe+, LHC:√s=14TeV

|ye| < 2.5pT

e > 25 GeV|ye| < 2.5pT

e > 25 GeVET

> 25 GeV

ATLAS

e+

e–

LHC: beam energies ~7x higher than Tevatron Probing new part of phase space Tevatron forward detectors map on to LHC central detectors

Problem! W/Z production is a benchmark


Z pT

C-P Yuan


Indirect W


without small-x effect:pT < 5GeV/c: 2/dof = 0.8/1pT<30GeV/c: 2/dof = 11.1/11

including small-x effect:pT < 5GeV/c: 2/dof = 5.7/1pT<30GeV/c: 2/dof = 31.9/11


W±

p p

l±

duu

uud

ewk cross-sections and widths

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