Precision electroweak

physics

Roberto TenchiniINFN-Pisa

5th Rencontres du Vietnam Particle Physics and Astrophysics Hanoi August 5 to August 11, 2004

21 Years of W and Z Physics

W Mass History

77

78

79

80

81

82

83

84

1 2 3 4 5 6 7 8 9

Years

GeV

/c2

1986UA1UA2

1990UA2CDF

2000TeVLEP

=1.5 GeV

=400 MeV =100 MeV

=39 MeV

Only Published Results

(*) Preliminary average 2004 : =34 MeV, weight of LEP 2/3, Tevatron 1/3

(*)

21 Years of W and Z PhysicsZ Mass History

88

89

90

91

92

93

94

95

1 2 3 4 5 6 7 8 9

Years

GeV

/c2

1986UA1UA2

1990LEP

=1.7 GeV

=31 MeV

Only Published Results

91,08

91,1

91,12

91,14

91,16

91,18

91,2

=2.1 MeV

(2001)Pietrzyk Burkhardt,

046.0936.128

1)M( Z s

21 Years of W and Z Physics

Standard deviations

-2

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9

Years

Nu

mb

er o

f si

gm

as

W Mass vs Tree Level

77

78

79

80

81

82

83

84

1 2 3 4 5 6 7 8 9

Years

GeV

/c2

E.W. Tree level SM relation

F

Z

Z

W

W

G

M

M

MM

1

2

)(1 2

22

Strong Evidence of pure E.W.Higher Order Corrections

1986 20021986 2002

(with running QED)

Tree level not enough : more parameters

• Radiative corrections in function of three more parameters Mtop, Mhiggs, s

• An Observable is written as

sHiggstopZFZii MmMGMfO ,,,,,

Example :

rGM

MM

FZ

WW

1

1

21

2

22

one loop radiative

corrections

Outlook of the rest of the talk

• Z Couplings: asymmetries at the Z and measurement of sin2()W – (a crisis ?)

• W Couplings: W Branching Ratios – (three LEP exp are final)

• Measurement of the W mass – (some hot issues)

• Constraints on the SM Higgs

NEW

MEANS FIRST TIMEAT THIS CONFERENCE

eA

A

tot

rBrFlBlF

etot

rl

4

3,,,,polFB

LR

A

A

Asymmetries at the Z pole• Z decay into a ff pair. With unpolarized beam

from fermion direction and helicity 3

asymmetries

etot

LBLFRBRF

ftot

LBLFRBRF

fetot

A

A

AABF

4

3,,,,FBpol

,,,,pol

4

3 FB

A

A

A

Can measure for e,,,c,b

Can measure with ’s

• Z with beam polarization (SLC) :

Al

Vlleff

AfVf

AfVff

g

g

gg

ggA

14

1sin

)()(

2

2

22

Electroweak Couplings :from deep inelastic scattering to LEP-SLC

Huge increase in precision

Asymmetries at the Z and

• Consistency is at 6.2%

• Long standing difference between Alr and AFB(b)

leff

2sin

NEW FINAL

b anomalous couplings ?No independent evidence

Model) (Standard 0.2158 R

0.000650.21646 R

Model) (Standard 935.0

(SLD) 020.0923.0)(A

(b)A

b

b

FB

4

3 FB

pol

b

b

betot

A

bA

AABF

measuresAf

Vf

g

g

measures 22VfAf gg

Plots like this one includeb asymmetry !

NEW FINAL

Again on the b asymmetry

•Two techniques•use semileptonic b decays•use weighted charge of particles in the hemisphere

•Very different systematic effects•LEP average still statistically dominated

AFB(b)

STATISTICS 0.00156

UNCORRELATED SYSTEMATIC 0.00061

QCD CORRECTION 0.00030LIGHT QUARK FRAGMENTATION 0.00013SEMILEPTONIC DECAYS MODELLING 0.00013CHARM FRAGMENTATION 0.00006BOTTOM FRAGMENTATION 0.00003

TOTAL SYSTEMATIC ERROR 0.00073 Example: Stability of resultas a function of the b purity

NEW FINAL

The b asymmetry: results

0.00250.0996 (b)A0FB

0.00190.1000 (b)A0FB

0.00160.0998 (b)A0FB

LEP average

Leptons only

Inclusive

NEW FINAL

My conclusion on “sin2() crisis”

• There is no evidence of problems related to the b asymmetry measurement

• Will solve the ALR-b asymmmetry discrepancy with future linear colliders if– Polarization of both beams available– Very high statistics run at the Z will take place

WW Production at LEP2

Three diagrams contribute at Born level (CC03 diagrams) :

Actually look for subsequent W decay into lepton-(anti) neutrino and quark1- (anti) quark2.

Real process is defined by exp. cuts4321-- ffffWWee

WW production at LEP2: the backgrounds

•Selections needed to •Extract events from background •Classify WW decays to different channels (fully hadronic, fully leptonic, semileptonics) •(Semi)leptonic decays can further be separated in e,, channels

BR

Signatures - 2 energetic, aco-

planar leptons - large missing E, p

- 2 hadronic jets - missing E & p - isolated lepton

- 4 hadronic jets - low missing E, p

Backgrounds ZZ, Zee, Z We, qq() qq()

Efficiency ~35-65% ~65-90% ~85%

Purity ~80-95% ~94-99% ~85%

BR

Signatures - 2 energetic, aco-

planar leptons - large missing E, p

- 2 hadronic jets - missing E & p - isolated lepton

- 4 hadronic jets - low missing E, p

Backgrounds ZZ, Zee, Z We, qq() qq()

Efficiency ~35-65% ~65-90% ~85%

Purity ~80-95% ~94-99% ~85%

Event selection

q

q

q

q

q q

Semileptonic (qqSemileptonic (qqll)) Hadronic (4q) Hadronic (4q) LeptonicLeptonic

11% 44% 45%

Lepton=e,,

Cross Sections in all channels

Nj selected events in each selection (j)L total luminosity j expected events function of i

WW for each channel (i) ij efficiency matrix j

bkg background cross section j

4f four fermion interference correction

j

j

eN

Pj j

Nj

ij

j

!

)(

)( 4 fj

bkgj

WWiijj L

Likelihood

EXAMPLE OF PROCEDURE

Four Fermion Interference

ee- νeνeeefor diagrams f-4 two:Example

Small Correction taken from Monte Carlo (~ 1% relative)

SIGNAL (following the CC03 definition)

Single W

Most important(*) 4-f (non WW) processes can be directly measured

(*) for the interference

Total WW Cross Section

• Precision reached by LEP experiments a challenge to theoretical predictions– Predictions with enhanced

O() radiative corrections are needed !

87.042.97KORALW):(Theory

)LEP(R

89.032.99YFSWW):(Theory

)LEP(R

)O(without

)O(with

Strong evidence of Triple Gauge Couplings

NEW 3 exp FINAL

W decays : Branching Ratios

Standard Model : 10.8%Standard Model : 67.5%Test of lepton universality:

result higher than e+

NEW ADL FINAL

W Leptonic Couplings

NEW ADL FINAL

014.0037.1)g(

)g(

015.0034.1)g(

)g(

010.0997.0)g(

)g(

e

e

• If electron and muon couplings are assumed to be the same and combined the result is 3 higher

•was 2.3 in summer 2003•is 2.6if final results only are used

W Mass at LEP from direct reconstruction

• Above threshold the W mass is measured from direct reconstruction of the jet-jet invariant mass in the fully hadronic and semileptonic channels• Event reconstructed as 2 (semileptonic candidate) or 4 (hadronic candidate) jets with iterative procedure• In the hadronic channel 3 jet-jet combinations from 4 jet • WW boson pairs at LEP

– 161 – 209 GeV centre of mass energy– ~700 pb-1 by each experiments– ~4500 qqqq , ~4000 lqq events for

each experiments.

LEP2 EnergySystematics:Systematics:

• At LEP2: Ebeam~20MeV (E/E~10-4)

mW~17MeVmW~17MeV

– Error coming mainly from extrapolation.

Resonant depolarizationResonant depolarization:– only works up to 60GeV extrapolation

• Kinematic fit imposing energy-momentum conservation

RECENT FINAL

Reconstructed MW

L3 L3 qqqq

DELPHIDELPHIeeqqqq

OPALOPAL qqqq

LEP: Systematic Uncertainties for MW

• QED effects (ISR, FSR, etc.)• Fragmentation• Detector effects • Uncertainty on the LEP beam energy• Colour Reconnection• Bose-Einstein correlations

(Weight of qqqq in LEP combination: 0.09)

Effort to reduce this error by designing

4q analyses less affected by CR effects

Final State Interactions

Separation of W decays vertex at LEP2 ~0.1 fm small with respect to the hadronization scale ~1fm

Interconnection phenomena : Final State is no longer factorized into two separated W’s

can bias the W mass in the fully hadronic channel

)10ΛΓ ( QCDW

NEED DATA to control these non-perturbative effects

Present LEP result: Colour Reconnection constrained by the particle flow analysis

• Most CR models predict a modified particle flow in W+W- events:

CR:

No CR:

W-

W+

W-

W+

• The ratio of particle flow between the inter and intra-W regions is built:

(A + B) / (C + D)

A

B

C

D

•Data

-SK1(extreme parameter)-Jetset

• Measurement sensitive only to extreme scenarios,

Colour Reconnection Systematic error ~ 90 MeV

Final LEP result: Colour Rec. constrained by Mass variation when soft particles

are excluded

Mass Shift predicted by model

Example: difference between Mass measured in standard analysis

andMass measured using only particles in the core of the jet

Models

• This correlation is a general feature of All Models : CR is expected to affect mainly– low momentum particles– particles away from the jet

core

• By studying Mass Stability (or Measuring the W mass only with particles in the jet core ) expect to reduce CR Systematic Error to ~ 50 MeV

mass difference (no FSI syst)

MW(qqqq-lqq)= +2243 MeV

MW at LEP : 4q and lqq 80.411±0.032(stat) ±0.030(syst)GeV/c2

80.420±0.035(stat) ±0.101(syst)GeV/c2

• Before LEP: measurements at hadron colliders (SppS, Tevatron Run I)• After LEP : measurements at hadron colliders (Tevatron Run II, LHC)• Drell-Yan single W production (quark-antiquark annihilation)• W decay to leptons (e or ) + neutrino• Fit to MW

T , the transverse mass distribution

)cos1(2MTW T

leptoneT pp

W mass from hadron colliders: the past and the near future !

-

TeVatron RUN IIW and Z cross sections

Present MW at LEP and TeVatron

ElectroWeak fit results• Electroweak theory

tested at one loop level• Indications for a light

Higgs

Summer 2003

Winter 20042

Higgs

2top

GeV/c 219M

GeV/c 1.53.174M

2Higgs

2top

GeV/c 251M

GeV/c 3.40.178M

Conclusions

•Asymmetries at the Z are final (including quarks)

•W cross sections at LEP are essentially final

•The LEP W mass result is still PRELIMINARY

–LEP collaborations are still working on this measurement, final results for the end of this year

– Activities to reduce the uncertainty due to CR effects and gain information from the 4q channel

• Reducing the LEP final error (~ 35 MeV ) will be the challenge for Tevatron II and LHC

•In spite of the increased top mass there is still evidence for a light Higgs

Backup Slides

Impact parameter

Secondary vtx

Transverse momentum

Momentum

AFB(b) from semileptonic decays

• Can fit independently b and c asymmetries, mixing and background composition

AFB(b) QCD corrections : cross-check

No evidence of bias due to gluon emission

W Leptonic Branching Ratios

NEW ADL FINAL

028.0076.1)BR(W

)BR(W

029.0070.1)BR(W

)BR(W

020.0994.0)BR(W

)BR(W

e

e

• If the Branching Ratios to electron and muon are assumed to measure the same quantity and combined the result is 3 higher

•was 2.3 in summer 2003•is 2.6if final results only are used

Measuring BE in W+W- events

• Inter W, BEI confirmed

• Between W’s, BEB, disfavoured

MW down from ~35 to ~15 MeV

Final

BEBBEI

eg

Error on W mass (Run I)

Theory improvementsImprove PDF constraints with measurements (W charge asymmetry, Z rapidity distribution)

These errors are determined using CDF/D∅ data,scale with luminosityDetector improvements for Run II will also help

40 MeV per experiment with 2 fb-1 feasible

Running of QED:

')'('

)'(

3)(

)()()(1

)0()(

24

)5(

)5(

dssss

sRss

ssss

m

hadhad

tophadl

046.0936.128

1)M( Z s

)61(0359895.137)0(1

ldat threshoDominat βchannelt

βchannel s 3

GeV 22.040.80M

pb45.069.3

W

WW

dominating is productionW e

: dependence model Small

e

LEP average:

Mw from the threshold method at 161 GeV

Kinematic FitsKinematic fit used to improve reconstructed four momenta

pairingsjet -jet 3

fit-5Cor -4C a use

jets-4 force :qqqq mm4C :fit 5C

s)(0,E),P( :fit 4C

rec2rec1

fit P-5C2C

orfit P-4C1C

: qqor eqq

info. Mor

fit -EP-5C1C

: qq

qq

Spectrometer

Based on measurement of lepton bending angle

Spectrometer

Based on measurement of lepton bending angle

LEP Energy (2)• 3 methods used to cross-check extrapolation:

Systematics:Systematics:

Radiative returnRadiative returnEnergy lossBased on the frequency of the field provided to beam to compensate from syncroton radiation.

Energy lossBased on the frequency of the field provided to beam to compensate from syncroton radiation.

E loss by sync.

Jet velocity & massLast jet reconstruction(with ECAL cleaning)

Z peak jets boost

(T>0.8)

jetjet Mlog

)log(

P

MeV 20180M

%2.05.1)(: DATA/MC

jet MeV 205M %2.00.0)(: DATA/MC jet

Jet boost (and mass) now well calibrated !

Removing low energy neutrals(<1.5 GeV , and <2 GeV if mixed)

Radiative Z peak

MeV 54Total

MeV 24anglePolar

MeV 16 tracksForward

MeV 12stat MC

MeV 16background

MeV 7ISR

MeV 20methodFit

MeV 30rsCalorimete

MeV 19ionFragmentat

effect mSource 12

MeV (syst) 58 (stat) 3243E

MeV (syst) 54 (stat) 3040m

b

Z

Systematic uncertainties

Unbinned likelihood fit to a pdf built

with a fit to the MC reference

2GeV/cZm

Forward-backward Asymmetryof

The pull of individualpseudo-observablesin the global fit

bbZ

Deep Inelastic N scattering

Prob(2) 42% 14%

Prob(2 ) 14 % 1.7%

Observables vs top and Higgs mass

2tFmGr )/log( 222

WHWF MMMGr

Electroweak observables in the (mtop , mhiggs ) plane