w physics at lep e.barberio southern methodist university dallas (usa) september 2003

W physics at LEP

E.Barberio

Southern Methodist UniversityDallas (USA)

September 2003

Nikhef, September 12th, 2003 E.Barberio

the LEP program

LEP1: 18 Million Z boson decays (89-95)LEP2: 36 Thousand W pairs (96-00)

• W pair production• triple and quartic gauge couplings• W mass and width measurements • final state interactions

this talk:


WW events

semileptonic channel

43.8%missing energylow background

hadronic channel

45.6%large backgroundambiguity in assigning jets to W

leptonic channel 10.6%large missing energy

WWll

WWqql

WWqqqq


W branching fractions

= 0.997 0.021

= 1.058 0.028

= 1.061 0.028

test of lepton universality at 3% (less precise than LEP1)

SM: 67.51%

SM: Wl and Wqq couplings are equal, but QCD correction enhance hadronic branching fraction:Br(Wqq’) = 67.8 0.28%

SM: 10.83%


CKM unitarity and Vcs

ji,

2

ijs

ji V)πα

(1)qq'BR(W

|Vcs| = 0.989 ± 0.014dominated by the error on the Br

measurement of Vcs the least know CKM element before LEP2 (11%):

flavour changing transitionsW on-shell

b)s,(d,j

c)(u,i 0.0252.039V2

ij

CKM unitarity for elements not involving the top quark

2V2

ij

dominated by the error on the Br

∝|Vqq|2

qW

q’


W pair cross section

1% measurement

clear evidence of WW and WWZ vertices: probe of the non-Abelian structure of the Standard Model

+ +

theo

WW

σ

σ=0.9780.006(stat)0.007(syst)

preliminary LEP


triple gauge couplings WW WWZ

W

W

W

W

Zgeneral WW and WWZinteraction: 14 parameters

electric quadrupole moment

magnetic dipole moment

1W

m2e

W

2W

m

eW

q

applying C and P invariance& use low-energy constraints we are left with 3 parameters

relation with the static W properties:

SM values

0

1z1

g

1


measuring the coupling at LEP2

sensitive observables

WW production:most constraining

We-

e+

W+

W-

W

f

f

W decay angles (helicity)

W+W- production angle cosW

W rest frame and of W decay products


WW production/decay angular distributions

W1


Single W

single W production +

8% precision

but it is very constraining for k

smaller cross section than WW:

OPAL preliminary

- single W- WW angles- WW

- combined

k


TGC 1-parameter fit results

(almost final)

- ALEPH- DELPHI- L3- OPAL - LEP

g1Z, k 2-5% measurement

dominant systematics O(em)

g1Z,: 0.015 :0.039


TGC 3-D parameter fit results

2D contour: 3rd parameterat the minimum


W polarisationin the SM W boson longitudinally polarised

spin density matrix

evidence for WL at 5 level !

OPAL

cosW

LL=00d/dcosWdcosW

T=(+++--)d/dcosWdcosW

L/ =0.2430.0270.012

SM: 0.240 at s=197 GeV

cosh*

L/ =0.2100.0330.016

unfold decay angle distribution


Quartic Gauge Couplingin SM these couplings exist but too small to be seen at LEP

look for anomalous contributionsparameterised by additional termsin the Lagrangian

couplings a0, ac, an; physics scale

-0.020 < a0/2 < 0.020 GeV-

2

-0.053 < ac/2 < 0.037 GeV-

2

-0.16 < an/2 < 0.15 GeV-2

e.g. OPAL

s GeV


Standard Model parameterse.w. process at tree level are computed from three parameters , GF , mZ and the CKM matrix elements Vij

contrary to ‘exact gauge symmetry’ theories (QED or QCD) the effect of heavy particles do not decouple: mtop was predicted by LEP1/SLD

sensitivity to mHiggs or to any kind of “heavy new physics” at energies not accessible

vacuum fluctuations modify the value of the observables -> when higher orders are included, observables are predicted as:

O(em,s,mW, mZ, mHiggs, mtop ,Vij) on-shell renormalization scheme

em = 0.004 ppmG= 9 ppmmZ = 23 ppm

very well measured!


measurement of the W mass

measure mW and mtop prediction of mH or new heavy objects which couple with the W as the Higgs does

r radiative corrections

r = - + rew 3%

Δr)1() θsin1(G 2

)(mα π m

W2

F

ZEM2W

from data + theoryfrom decay from LEP

tree level mW= 80.937 GeV wrong by 10


excellent mass resolution comes from

kinematic fit:

constrain total (E,p) to (s,0)

need for precise knowledge of the beam energy from LEP

mass of the W boson

direct reconstruction :

mW from the invariant mass calculated using the W decay

products

WW qqqq and WW qql (ALEPH and OPAL also WW ll)

raw mass


reconstructed mass distributions

DELPHIeqq

ALEPH 4q

L3 qq

OPAL qq


mW spectrum

mW extraction calibrated

with Monte Carlo simulation

had

ron

isat

ionW

p

rod

ucti

on

an

d d

ecay

Pert

.QC

Dd

eca

y

W

ob

serv

ati

on

(D

ETEC

TO

R)

reconstructed mass distorted! - initial state radiation E0<Ebeam

- mW(jet/recon. lepton) mW(quark/lepton)


LEP: latest results

mW(GeV)

mWworld=80.4260.034 GeVW constrained to SM

relationship with mW: direct measurements

mH<210 GeV @ 95% C.L. SM fitmH > 114 GeV direct limit


Systematic errors

WWqqqq weight channel in the combination: 9%

experimentschannelsyears

qqlv qqqq comb. corr.e c y

CR - 90 9 e - y

BE - 35 3 e - y

other 4 5 4 - - -

rad. corrections 8 8 8

fragmentation 19 18 18 e c y

detector 14 10 14 - c y

LEP energy 17 17 17 e c y

systematics 31 101 31statistical 32 35 29

total 44 107 43

cross-LEP effort in progress to address these errorsderive them from data whenever is possible


radiative corrections

a new OPAL analysis tries to estimate on data the contribution of real production using WW events

mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included:

estimated mass shift dueto real photon production from data ~ 6-8 MeV


final state interactions (only 4q)

possible interaction between the two W decays products not in the simulation apparent shift in mw

only phenomenological modelsfm

Colour Reconnection (CR):

• W decay~0.1fm<< hadronization scale~1fm colour flow between Ws also at the hadronization phase

• seen at ep,pp colliders (rapidity gaps)

and in heavy meson decays

Bose Einstein Correlation (BEC):

• favours production of pairs/multiplets of identical particles close together

• well established in single Z and W


expected effects of color reconnection

effects:

- change in particle particle multiplicity

- depletion of soft momenta particles

- anomalies in the particle flow /string effect modified

- rapidity gaps

- change in the reconstructed value of mW : the most sensitive observable unfortunately

It affects:

- interaction between decay products at the parton level

- final hadronic color singlets do not correspond to the initial W bosons

the effect should be present in the data, but how strong it is ?


CR: particle flow in 4-jet events at LEP2

L330%

RN=(A+C)/(B+D) is

used to compare with models:

various models and parameters! one experiment can exclude only extreme cases LEP combination

CR: modifies particle flow between Ws:

W W


particle flow: LEP combination

r=RNdata/RN

no-CR

r=0 no CR, r0 CR

preferred value in data Precmin

~49%r

between various models SK1 gives the largest mW

bias: vary reconnection fraction

mass bias calculated from Prec

min+1 used in the mW

combination: mass shift increases (90 MeV) but data driven


using mw for CR?

mW is the most sensitive observable and we can use it

to measure/limit CR

CR affects more particles in the interjet region

variable used mass difference: e.g. mW(k<0)-mW(K>0)this allows to use the qqqq channel to measure mW

exclude/change the weight of soft inter-W particles from jets!

strategies to reduce CR bias: - hybrid cone jet cone

algorithm- remove low energy particle pcut

- jet direction from pk : K>0 decreases sensitivity; K<0 enhance it


mW and CR

SK1 parameter

most probably LEP will use these strategies for the final mW

trade statistics for systematics:

all CR model used behave as SK1!

it also reduces BEC systematics!systematics are under study

~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics

Delphi (this summer): cone and pcut


CR with mW

combination with colour flow (almost uncorrelated)

mW(no-CR)–mW

CR to study CR

- higher sensitivity than colour flow- mass difference still use the qqqq channel to measure mW!

use this combination to get the CR systematics for the W mass:the exact procedure is under discussion

all experiments are working on similar analyses

it will be difficult to achieve a 5discovery for CR in WW events


Bose Einstein Correlations

hadronic parts of qqln

rotate/boost

mix ‘WW’ event

measure BEC between W comparing (Q) (2-particle density) in 4q and ‘mixed’ WW events:

R2(Q)=ρ(4q) /ρ(mix WW)noBE

Δρ = ρ(4q)- ρ(mix WW)

ALEPH, L3: no sign of BEC between WsDELPHI: small BEC between Ws

propagate results on BEC between Ws into mW systematics: work in progresshowever mass shift due to BEC is expected to be smaller than CR


measuring the W widthfit simultaneously for mW and W direct measurement of W

SM 2.095 GeV

wworld=2.139 0.069 GeV


conclusions and outlookmeasurements at the Z peak demonstrate that the SM is a quantum field theorymeasurements above the WW threshold demonstrate that the SM is a non-abelian gaunge theory

and as for the Z, measurements of the W properties at LEP has brought the quantitative test of the SM to a high level of accuracy:no deviation are observed within that accuracy

LEP2 achievements were better than foreseen:• triple gauge coupling are now well determined: 5% measurement!

• 5 evidence of the longitudinal polarisation of the W

• measurement the W mass 42 MeV and 91 MeV for the width, with good prospects to improve mW to meet the 35 MeV error

…BUT LEP did not see the Higgs….


global fit of the SM to data

limit from direct searchesmH> 114.4 GeV

deduce mH which gives best 2

radiative corrections~ log mH mH

ew < 219 GeV 95% C.L.

GeV96m 6038H

mH

largest discrepancy: 3 P(2) ~ 4.4% allP(2) ~ 27.3% without NuTeV


LHC and the electroweak interaction

5

full mass range accessible in1 year ( 5) final word

~1 year ~3 years

~ 4 years

LHC pp, s=14 TeV, start 2007?

LEPlimit~50% ~ 35% ~25% ~10%

2002 LEP2+Run15.1 GeV33 MeV

2006 LEP2+Run22.5 GeV25 MeV

2009 ? LHC1.5 GeV15 MeV

??? LC ? 0.2 GeV 7 MeV

mtop

mW

H

H

mm

if Higgs discovered comparison of measured mH with indirect measurement


mW at hadron colliders:Tevatron

pTv is inferred from the recoil

system balancing the W

the non-zero pT is due to gluon

radiation from quarks

single W production through qq annihilation:

)cos1(pp2m TlT

TW

mW measurement is performed in the leptonic channels using the transverse mass:

p = Ebeam=s/2sx1 p x2 p

p p

W

l

s sxx s 21


Systematics: key issues

pTW distribution Z bosons (fully reconstruct) plus models/theory

for difference between Z and W (different initial state quarks)

recoil pT distribution Z bosons with study of underlying event ET distributions from proton remnants and multiple interactions

HENCE major limitation on systematics from Z statistics…

calibration, energy scales and resolutions:challenge for detector alignment and calibration,use Z, , J/ mass peaks


Tevatron results

error source CDF CDFe D0lepton E scale 85 75 56lepton E resl 20 25 19

PTW distrib. 20 15 15

recoil model 35 37 35selection bias 18 - 12backgrounds 25 5 9PDFs / lumi 15 15 8radiative corrn 11 11 12statisitcs 100 65 60total 144 113 84

RunI (~100 pb-1, 15-30k events per channel): CDF W and e, D0 We

Tevatron (+UA2): mW= 80.454 0.059 GeV main systematics ‘almost’ uncorrelated


mW at hadron machines: LHC

mtop~2 GeV requires mW ~ 15 MeV systematics mW(MeV

)statistics 2E-p scale 15?energy resolution

5?

recoil model 5?lepton id 5pT

W 5

parton distr.func.

10?

W width 7radiative decays 10background 5total 25

statistical error for 10 fb-1

mW<2 MeVW l: 3 x 108 eventsZ ll: 3 x 107 events

plus unknown effects ..…

one LHC experiment


conclusions and outlook• LEP gave a very solid ground to the Standard Model

of electroweak interactions

• however: the Higgs is still missing……

• Tevatron is exploring a higher energy region and will

reduce the uncertainties on mtop and mW

(measurement uncorrelated with LEP) but has little chances to see the Higgs

• LHC will explore a higher energy region: it will cover the full allowed range for the Higgs

• if we find the Higgs at LHC we will need another e+e- machine for precision measurements


event rate and particle multiplicity

• L = luminosity = 1034 cm-2 s-1

• bunch spacing = 25 ns• 22 events / bunch

LHC events previous machines in 1 year total statistics

Z 108 LEP: 107 in ~10 yrsW 109 FNAL: 107 in ~7 yrstop 108 FNAL: 105 in ~7 yrs

w physics at lep e.barberio southern methodist university dallas (usa) september 2003

Documents

w pairs

sm w boson

w mass measure mw

quartic gauge couplings

static w properties

w decay productsww qqqq

w pair production triple

invariant mass