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## Boson Gauge Couplings at LEP

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**Boson Gauge Couplings at LEP**Paolo SPAGNOLO INFN Pisa For the Four LEP Collaborations ADLO 40th Rencontres De Moriond On Electroweak Interactions And Unified Theories 8 Mar 2005, La Thuile, Aosta Valley, Italy Paolo SPAGNOLO – INFN PI**LEP collider performance**192-202 GeV 202-208 GeV 189 GeV 183 GeV 161-172 GeV • LEP: e+e- collider at CERN • LEP1: ECM ~ MZ from 1989 to 1995 • LEP2: ~ 700 pb-1 /exp above WW production threshold from 1996 to 2000 Paolo SPAGNOLO – INFN PI**Results from the LEP experiments**• Main analyses (cross section, W mass, TGC) by all experiments • Results combined by joint LEP Working Groups • Common systematics and correlations • Almost all the statistics analyzed • A lot of results still preliminary, many final publications this year !! Paolo SPAGNOLO – INFN PI**Physics processes at LEP2**~100k evts ~10k evts ~1k evts ~100 evts Paolo SPAGNOLO – INFN PI**Boson Gauge Couplings**• Standard Model : non-Abelian SU(2)xU(1) gauge symmetry • cTGC charged triple Gauge Couplings: WWg, WWZ • cQGC charged quartic Gauge Couplings: WWgg, WWZg, WWZZ, WWWW (small or negligible at LEP) • Search for anomalous Gauge Couplings • charged: beyond the SM • neutral: not in the SM: nTGC: ZZZ, ZZg, Zgg and nQGC: ZZgg • Gauge bosons production required • LEP2 energy allowed these studies for the “first time” Paolo SPAGNOLO – INFN PI**Sensitivity to TGCs in 3 processes**WW pairs production single W production single photon production Charged Triple Gauge Couplings Paolo SPAGNOLO – INFN PI**W-pair events**WW enqq (~15%) Channel efficiency purity bkg ,, ll ll11% 50-80% 80-90% eqq15% 75-90% ~90% qq, Zee qq15% 75-90% ~95% qq qq15% 50-80% 80-85% qq,We qqqq 45% 80-90% 75-80% qq Gauge Couplings ! Paolo SPAGNOLO – INFN PI**WW qqqq (~45%)**WW mnqq (~15%) Paolo SPAGNOLO – INFN PI**WW enmn (~4%)**WW tnqq (~15%) Paolo SPAGNOLO – INFN PI**Backgrounds in WW cross section**All 4f diagrams (enqq) Paolo SPAGNOLO – INFN PI**LEP combination**• sWW from each exp combined • Precision better than 1% • systematics correlation matrix • test the SM radiative correction to CC03 diagrams Paolo SPAGNOLO – INFN PI**W-pair cross sections**n exchange t channel ONLY Paolo SPAGNOLO – INFN PI**W-pair cross sections**n exchange and gWW vertex Paolo SPAGNOLO – INFN PI**W-pair cross sections**Clear proof of SU(2)xU(1) gauge couplings ! Paolo SPAGNOLO – INFN PI**Sensitivity to TGCs in 3 processes**WW pairs production single W production single photon production Charged Triple Gauge Couplings Paolo SPAGNOLO – INFN PI**Single W….**W Sensitive to the WWZ and WWg couplings In agreement with SM within 8% precision Paolo SPAGNOLO – INFN PI**… and single Z**In agreement with SM within 7% precision Paolo SPAGNOLO – INFN PI**Sensitivity to TGCs in 3 processes**WW pairs production single W production single photon production Charged Triple Gauge Couplings Paolo SPAGNOLO – INFN PI**Single g**n n Sensitive to the WWg couplings outside Z resonance !! Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings**Standard electroweak theory U(1) SU(2) triple and quartic SU(2) gauge boson self couplings are thesignature of the non-Abelian SU(2) electroweak structure ! The most general Lorentz Invariant WWV (V= + Z) vertex has 7+7 = 14 complex couplings Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings**C and P conservation only 6 real out of 14 complex parameters SU(2)L⊗U(1)Y only 3 free parameters Paolo SPAGNOLO – INFN PI**W Polar angle**fermion polar angle wrt the W direction fermion azimuthal angle wrt the WW plane Most important process for studying TGCs is TheWWZ and WW gauge couplings can be measured in W-pair events, fitting the W-pair event rates and the W production and decay angular distributions. Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings: Analysis**Use angular distributions from all event topologies: Do W charge tagging to obtain direction in events Use Monte Carlo to correct for detector effects and to implement radiative corrections Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings: 1d & 2d Fit**LEP Results 3 parameters fit with C and P cons. and SU(2)L⊗U(1)Y Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings: 3d Fit Results**3 parameters fit with C and P cons. and SU(2)L⊗U(1)Y Paolo SPAGNOLO – INFN PI**Triple Gauge Couplings**Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings g one-dimensional fit results for the SM non-zero TGC values (ALEPH data only) … all other 24 couplings are consistent with zero (within 5-20%) ! Proof of the Standard Model SU(2)L⊗U(1)YStructure Paolo SPAGNOLO – INFN PI**Investigated in ZZ and Zg processes**ZZ on shell: 4 parameters f4Z, f4g, f5Z, f5g Zg on shell: 8 parameters hiZ, hig (i=1,4) Neutral TGC (excluded by SM) Z angular distrib in bosted frame ZZ Paolo SPAGNOLO – INFN PI**nTGC combined results**Paolo SPAGNOLO – INFN PI**QGC at LEP: WWgg, WWZg, ZZgg**5 parameters: aW0, aWc, aWn, aZ0, aZc Total cross section Photon energy and angular distribution Quartic Gauge Couplings OPAL SM forbidden Paolo SPAGNOLO – INFN PI**95% C.L. Limits on quartic couplings in from**L3 DELPHI OPAL 95% C.L. Limits on quartic couplings in from ZZ gg Paolo SPAGNOLO – INFN PI**CONCLUSIONS**• The Standard Model non-Abelian SU(2)xU(1) gauge symmetry PROVED!!! • MEASURED cTGCs charged triple Gauge Couplings: WWg, WWZ • NEGLIGIBLE cQGC quartic Gauge Couplings: WWgg, WWZg, WWZZ, WWWW • EXCLUDED anomalous Gauge Couplings • charged: beyond the SM • neutral: not in the SM: nTGC: ZZZ, ZZg, Zgg and nQGC: ZZgg • one of the most important results of LEP2 Paolo SPAGNOLO – INFN PI**W Polarization in**Measure spin density matrices and extract polarized diff cross sec Use avg value of projection op (OPAL) Fit for (L3) Paolo SPAGNOLO – INFN PI**Form Factor for Vector Resonance Enhancement of**Map triangle onto plane Triangle correponds to 95% CL allowed region with ALEPH Preliminary 95%CL contour w/ center = (.96,-.15) For the technirho mass at 95% C.L. For the technirho mass at 95% C.L. Paolo SPAGNOLO – INFN PI**W Boson Production at LEP2**• e+e Wene • e+e W+W Paolo SPAGNOLO – INFN PI**Event selection: qqqq**Event properties Large multiplicity no missing momentum 4jet-like Main background: qq(g) Multidimensional techniques (NN, pdf, ...) Eff. ~ 90%, purity ~ 85 % • Main systematics ~1.3 ÷ 2.%: • hadronization models (signal and bkg) • correlated among experiments !! • detector simulation Paolo SPAGNOLO – INFN PI**Two jets + high energy and isolated lepton (or t jet) +**missing pt Main backgrounds: qq(g) and 4f (non-WW) Multivariable selections Eff. 60-90%, purity ~ 80-95% Main systematics ~1.4 ÷ 2.5%: lepton-id background subtraction Event selection: lnqq Paolo SPAGNOLO – INFN PI**Low multiplicity**two leptons or thin jets (t jets) Missing pt and acoplanarity cuts depend on lepton flavour Main background: two photons ,di-leptons, 4f non-WW Eff. ~50-80%, purity ~80-90% Main systematics ~1.3-4.5%: background subtraction lepton identification beam related detector noise Event selection: lnln Paolo SPAGNOLO – INFN PI**Not available at the beginning of LEP2**O(a) radiative correction to WW production TGC: ~1% change in WW production angle W mass: peak distortions and radiation: compensation: < 5-10 MeV No correction: ~2% higher cross section. No effect on acceptance Paolo SPAGNOLO – INFN PI