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W Physics at LEP Paolo Azzurri Pisa (ALEPH-CMS ) Hanoi August 7, th Rencontres du Vietnam New Views in Particle Physics

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Outline W-pair production at LEP2 W Gauge Couplings W Decay Couplings ¤ Other four fermion processes W mass

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Introduction a clear evidence of the presence of Standard Model triple gauge couplings and of their SU(2) U(1) structure the first direct measurements of the W decay branching ratios, and from these: ☆ a direct test of the W lepton family and lepton-quark universality to 1% and 0.5% a test of CKM unitarity and best current determination of the Vcs amplitude an independent W mass measurement at production threshold (200 MeV accuracy) ☼a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy The study of W-pair production at LEP2 was a nice and exciting work that provided:

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W-pair production at LEP2

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W-pair production CC03 diagrams (not gauge invariant) LEP2 produced about WW events/exp. with ~700pb -1 /exp. Expect: per experiment.

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W-pair event selection Clear event decay... … sophisticated selections qq qq,We qq qq,Zee , ,ll bkg 75-80%80-90%qqqq 80-85%50-80% qq ~95%75-90% qq ~90%75-90% e qq 80-90%50-80% l purity efficiencyChannel

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W-pair cross sections Test of the SM radiative corrections to the CC03 diagrams Clear proof of SU(2)xU(1) gauge couplings ! Precision better than 1% s (GeV) WW (pb)

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W Gauge Couplings

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Triple Gauge Couplings Standard electroweak theory U(1) SU(2) triple and quartic SU(2) gauge boson self couplings The signature of the non-abelian SU(2) electroweak structure !

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Triple Gauge Couplings The most general Lorentz Invariant WWV (V= ,Z) vertex has 7 complex couplings

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W-pair angular distributions The WWZ 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.

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Triple Gauge Couplings Standard measurements of three TGC couplings that conserve C and P, U(1) em and global SU(2) L ⊗ U(1) Y one dimensional fit results (LEP): Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings 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%) !

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W Decay Couplings

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W leptonic couplings Direct test of W lepton universality at the 1% precision level

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W hadronic couplings Direct test of W quark-lepton universality at the 0.6% precision level

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP ¤ Other four fermion processes

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Z-pairs In agreement with SM within 5% precision No gauge self couplings involved in Standard Model Z-pair production

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Single W and Z In agreement with SM within 8% precisionIn agreement with SM within 7% precision

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W mass

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W mass Improve the W invariant mass resolution: kinematic fit energy-momentum conservation (equal mass constraints) Direct kinematic reconstruction of the W mass in qqqq, qq l (and l l ) final states m W value extracted with different methods Breit-Wigner fit (with bias correction) Monte Carlo reweighting (with different observables M 1,δM) Probability Density function P(M 1,M 2,..) Statistical power of the LEP2 data: Δm W (stat)=21 MeV

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W mass systematics The fully hadronic channel is de-weighted to 0.10 for possible final state interconnection effects ! m W (qqqq)= ±107 MeV m W (qq l )= ± 44 MeV (ρ=0.18) Δm W (qqqq-qq l )= +22±43 MeV without CR and BE errors

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Hadronic final state interactions Bose-Einstein correlations: measure two particle correlations between W’s / inside W’s full effect Δm W =35 MeV measured fraction Δm W =15 MeV Colour Reconnection effects: measure particle flow in regions between W’s / inside W’s upper limits: k i <2.13 Prob(CR)<0.65 Δm W =90 MeV

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP CR insensitive jet reconstructions ? Ways to reduce the CR bias: 1.Remove low energy particles (p cut ) 2.Hybrid cone algorithm (R) Prospects Can reduce the CR mass shifts by a factor 2-3 deteriorating the statistical precision by 20% CR systematics : Δm W =90 40 MeV statistical error: Δm W =35 40 MeV total m W (qqqq) error:Δm W =110 60 MeV m W (qqqq) weight in combination : 0.10 0.30 combined m W error :42 38 MeV

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP W mass and width W measured with the same methods used for the m W extraction W = 2.150±0.91 GeV/c 2 m W = ±0.042 GeV/c 2

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Standard Model Fit All this and more in Roberto Tenchini’s talk tomorrow !

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5th Rencontres du VietnamP.Azzurri - W Physics at LEP Conclusions = Introduction a clear evidence of the presence of Standard Model triple gauge couplings and of their SU(2) U(1) structure the first direct measurements of the W decay branching ratios, and from these: ☆ a direct test of the W lepton family and lepton-quark universality to 1% and 0.5% a test of CKM unitarity and best current determination of the Vcs amplitude an independent W mass measurement at production threshold (200 MeV accuracy) ☼a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy The study of W-pair production at LEP2 was a nice and exciting work that provided:

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