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W physics at LEP. E.Barberio Southern Methodist University Dallas (USA) September 2003. 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
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W physics at LEP E.Barberio Southern Methodist University Dallas (USA) September 2003
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: E.Barberio
WW events WWlnln leptonic channel 10.6% large missing energy semileptonic channel 43.8% missing energy low background WWqqln hadronic channel 45.6% large background ambiguity in assigning jets to W WWqqqq E.Barberio
= 0.997 0.021 = 1.058 0.028 = 1.061 0.028 W branching fractions test of lepton universality at 3% (less precise than LEP1) SM: Wln and Wqq couplings are equal, but QCD correction enhance hadronic branching fraction: Br(Wqq’) = 67.8 0.28% SM: 10.83% SM: 67.51% E.Barberio
q ∝|Vqq|2 W q’ CKM unitarity and Vcs CKM unitarity for elements not involving the top quark flavour changing transitions W on-shell dominated by the error on the Br measurement of Vcs the least know CKM element before LEP2 (11%): |Vcs| = 0.989 ± 0.014 dominated by the error on the Br E.Barberio
=0.9780.006(stat)0.007(syst) W pair cross section + + preliminary LEP clear evidence of WWg and WWZ vertices: probe of the non-Abelian structure of the Standard Model 1%measurement E.Barberio
W W g Z W W SM values triple gauge couplings WWg WWZ general WWg and WWZ interaction: 14 parameters applying C and P invariance & use low-energy constraints we are left with 3 parameters relation with the static W properties: magnetic dipole moment electric quadrupole moment E.Barberio
W- qW e- e+ f W+ q W f measuring the coupling at LEP2 WW production: most constraining sensitive observables W+W- production angle cosW W decay angles (helicity) W rest frame q and of W decay products E.Barberio
f1 qW WW production/decay angular distributions E.Barberio
+ OPAL preliminary • - single W • WW angles • sWW • combined 8% precision kg Single W single W production smaller cross section than WW: but it is very constraining for kg: E.Barberio
TGC 1-parameter fit results - ALEPH - DELPHI - L3 - OPAL - LEP (almost final) g1Z, kg 2-5% measurement dominant systematics O(em) g1Z,lg: 0.015 kg:0.039 E.Barberio
TGC 3-D parameter fit results 2D contour: 3rd parameter at the minimum E.Barberio
sL/s =0.2430.0270.012 SM: 0.240 at s=197 GeV cosqh* OPAL L sL=r00ds/dcosqWdcosqW sT=(r+++r--)ds/dcosqWdcosqW cosqW W polarisation in the SM W boson longitudinally polarised unfold decay angle distribution spin density matrix sL/s =0.2100.0330.016 evidence for WL at 5s level ! E.Barberio
e.g. OPAL • 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 s GeV Quartic Gauge Coupling in SM these couplings exist but too small to be seen at LEP look for anomalous contributions parameterised by additional terms in the Lagrangian E.Barberio
aem = 0.004 ppm • Gm = 9 ppm • mZ = 23 ppm very well measured! on-shell renormalization scheme O(em,s,mW, mZ, mHiggs, mtop ,Vij) Standard Model parameters e.w. process at tree level are computed from three parameters , GF , mZ and the CKM matrix elements Vij vacuum fluctuations modify the value of the observables -> when higher orders are included, observables are predicted as: 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 E.Barberio
from data + theory from LEP from decay tree level mW= 80.937 GeV wrong by 10s measurement of the W mass Dr radiative corrections r = -Da + Drew 3% measure mW and mtop prediction of mH or new heavy objects which couple with the W as the Higgs does E.Barberio
excellent mass resolution comes from kinematic fit: constrain total (E,p) to (s,0) need for precise knowledge of the beam energy from LEP raw mass mass of the W boson direct reconstruction: mW from the invariant mass calculated using the W decay products WW qqqq and WW qqln (ALEPH and OPAL also WW lnln) E.Barberio
L3 tnqq ALEPH 4q OPAL mnqq DELPHI enqq reconstructed mass distributions E.Barberio
mW spectrum W observation (DETECTOR) W production and decay Pert.QCD hadronisation decay reconstructed mass distorted! - initial state radiation E0<Ebeam - mW(jet/recon. lepton) mW(quark/lepton) mW extraction calibrated with Monte Carlo simulation E.Barberio
mH<210 GeV @ 95% C.L. SM fit mH > 114 GeV direct limit LEP: latest results mWworld=80.4260.034 GeV GW constrained to SM relationship with mW: direct measurements mW(GeV) E.Barberio
qqlv qqqq comb. corr. e c y rad. corrections 8 8 8 e c y fragmentation 19 18 18 - c y detector 1 4 10 1 4 e c y LEP energy 17 17 17 e - y CR - 90 9 BE - 35 3 e - y - - - other 4 5 4 systematics 31 101 31 statistical 3 2 3 5 29 total 44 107 4 3 Systematic errors experiments channels years WWqqqq weight channel in the combination: 9% cross-LEP effort in progress to address these errors derive them from data whenever is possible E.Barberio
radiative corrections mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included: a new OPAL analysis tries to estimate on data the contribution of real production using WWg events estimated mass shift due to real photon production from data ~ 6-8 MeV E.Barberio
final state interactions (only 4q) possible interaction between the two W decays products not in the simulation apparent shift in mw 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 fm only phenomenological models E.Barberio
expected effects of color reconnection the effect should be present in the data, but how strong it is ? It affects: - interaction between decay products at the parton level - final hadronic color singlets do not correspond to the initial W bosons 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 E.Barberio
W W L3 30% CR: particle flow in 4-jet events at LEP2 CR: modifies particle flow between Ws: RN=(A+C)/(B+D) is used to compare with models: various models and parameters! one experiment can exclude only extreme cases LEP combination E.Barberio
r particle flow: LEP combination between various models SK1 gives the largest mW bias: vary reconnection fraction preferred value in data Precmin~49% mass bias calculated from Precmin+1s used in the mW combination: mass shift increases (90 MeV) but data driven r=RNdata/RNno-CR r=0 no CR, r0 CR E.Barberio
using mw for CR? mW is the most sensitive observable andwe can use it to measure/limit CR CR affects more particles in the interjet region 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 Spk : • K>0 decreases sensitivity; • K<0 enhance it variable used mass difference: e.g. mW(k<0)-mW(K>0) this allows to use the qqqq channel to measure mW E.Barberio
mW and CR Delphi (this summer): cone and pcut all CR model used behave as SK1! it also reduces BEC systematics! systematics are under study SK1 parameter most probably LEP will use these strategies for the final mW trade statistics for systematics: ~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics E.Barberio
CR with mW - higher sensitivity than colour flow - mass difference still use the qqqq channel to measure mW! mW(no-CR)–mWCR to study CR combination with colour flow (almost uncorrelated) 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 5s discovery for CR in WW events E.Barberio
Δρ = ρ(4q)- ρ(mix WW) hadronic parts of qqln rotate/boost Bose Einstein Correlations measure BEC between W comparing r(Q) (2-particle density) in 4q and ‘mixed’ WW events: R2(Q)=ρ(4q) /ρ(mix WW)noBE mix ‘WW’ event ALEPH, L3: no sign of BEC between Ws DELPHI: small BEC between Ws propagate results on BEC between Ws into mW systematics: work in progress however mass shift due to BEC is expected to be smaller than CR E.Barberio
measuring the W width fit simultaneously for mW and GW direct measurement of GW Gwworld=2.139 0.069 GeV SM 2.095 GeV E.Barberio
conclusions and outlook • measurements at the Z peak demonstrate that the SM is a quantum field theory • measurements 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! • 5s 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…. E.Barberio
global fit of the SM to data deduce mH which gives best 2 limit from direct searches mH> 114.4 GeV mH radiative corrections ~ log mH mHew < 219 GeV 95% C.L. largest discrepancy: 3 P(2) ~ 4.4% all P(2) ~ 27.3% without NuTeV E.Barberio
LHC pp, s=14 TeV, start 2007? mW 2002 LEP2+Run1 5.1 GeV 33 MeV 2006 LEP2+Run2 2.5 GeV 25 MeV 2009 ? LHC 1.5 GeV 15 MeV ??? LC ? 0.2 GeV 7 MeV ~ 4 years ~1 year ~3 years 5 mtop full mass range accessible in 1 year ( 5) final word LEP limit ~50% ~ 35% ~25% ~10% LHC and the electroweak interaction if Higgs discovered comparison of measured mH with indirect measurement E.Barberio
x2 p x1 p W p p n l mW at hadron colliders:Tevatron single W production through qq annihilation: p = Ebeam=s/2 mW measurement is performed in the leptonic channels using the transverse mass: pTv is inferred from the recoil system balancing the W the non-zero pT is due to gluon radiation from quarks E.Barberio
Systematics: key issues calibration, energy scales and resolutions: challenge for detector alignment and calibration, use Z, , J/ mass peaks 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… E.Barberio
Tevatron results 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 E.Barberio
mW at hadron machines: LHC mtop~2 GeV requires mW ~ 15 MeV statistical error for 10 fb-1 DmW<2 MeV W l: 3 x 108 events Z ll: 3 x 107 events one LHC experiment plus unknown effects ..… E.Barberio
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 E.Barberio
LHC events previous machines in 1 year total statistics Z 108 LEP: 107 in ~10 yrs W 109 FNAL: 107 in ~7 yrs top 108 FNAL: 105 in ~7 yrs event rate and particle multiplicity • L = luminosity = 1034 cm-2 s-1 • bunch spacing = 25 ns • 22 events / bunch E.Barberio