Experimental tests of the Standard Model - PDF document

Experimental tests of the Standard Model Experimental tests of the Standard ModelDiscovery of the and bosonsPrecision tests of the sector Precision tests of the sectorElectro-weak unification at HERARadiative corrections andprediction of the top and Higgs massTop discovery at the TevatronHiggs searches at the LHC 1. Discovery of the and boson1983 at CERN SppS accelerator, 540 GeV, UA-1/2 experiments1.1 Boson production in pp interactions p p u d W sˆ q,-
q,
n q,-
q,+
p p u sˆ Z u XWpp+®®

n ff Similar to Drell-Yan: (photon instead of W) 10 100 1 ZWMs,ˆ )(ZWss nb1.0 nb nb10 12.0 mit GeV65(014.0 Cross section is small ! 1.2 UA-1 Detector 1.3 Event signature: p p +
-
High-energy lepton pair:

ff GeV91 1.4 Event signature: XWpp+®®

n p p n
High-energy lepton –Large transverse momentum Undetected n -Missing momentumMissing p vector How can the mass be reconstructed ? mass measurement In the rest frame: 2WMpp==n
2WTMp£
Tp
dpdN 2WM In the lab system: system boosted only along axis distribution is conservedJacobian Peak: Trans. Movement of the Finite decay width decay is not isotropic GeV80 dN The Nobel Prize in Physics 1984 Simon van der MeerCarlo Rubbia "for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction" One of the achievements to allow high-intensity pp collisions, is stochastic cooling of thep beams before inserting them into SPS. S. van der Meer 1.5 Production of and bosons in annihilation Precision tests of the sector Tests of the W sector qqW® 2. Precision tests of the Z sector (LEP and SLC)2.1 Cross section for ff + =2M 2 g Z iMcosnr for propagator considering a finite width~4.5M decays / experiment ]qqqqqmmmmcoscos)()(cossin16(cos ]qqqqqmmmgcoscos1(cossin(cos Differential cross section (cos(cos(coscospa gg /Z interference cos1(cos1((cos Vanishes at for with At the -pole contribution is dominant interference vanishes () 12 []cossin12 åG=GiiZ With partial and total widths: Cross sections and widths can be calculated within the Standard Model if all parameters are known [][] cossin16tot 12 Resonance looks the same, independent of final state: Propagator is the same hadrons -+-+®mmee -e -e +e +e k k¢ p p¢ -e +e -e +e k p p¢ k¢ + channel channel -+-+®eeee t channel contribution forward peak = 91.1876 0.0021 GeV= 2.4952 0.0023 GeVhad = 1.7458 0.0027 GeV = 0.08392 0.00012 GeV= 0.08399 0.00018 GeV = 0.08408 0.00022 GeV= 2.4952 0.0023 GeVhad = 1.7444 0.0022 GeV = 0.083985 0.000086 GeV line shape parameters (LEP average) Assuming lepton universality: e = 3 leptons are treated independently= ±23 ppm (*)0.09 %*) error of the LEP energy determination: 1.7 MeV (19 ppm) http://lepewwg.web.cern.ch/ (Summer 2005) test of lepton universality LEP energy calibration: Hunting for ppm effects Changes of the circumference of the LEP ring changes the energy of the electrons: tide effects water level in lake Geneva 1992 1993 Changes of LEP circumference C=1…2 mm/27km (4…8x10-8 ppm100 Effect of moonEffect of lake Effect of the French “Train a Grande Vitesse” (TGV)In conclusion:Measurements at the ppm level are difficult to perform. Many effects must be considered! Vagabonding currents (~1A) from trains 2.3 Number of light neutrino generations In the Standard Model: inv :invisible eeZeenn®®-+ mmnn®®-+Zee ttnn®®-+Zee To determine the number of light neutrino generations: SMinvexp GeV0015.04990.0inv =1.9910.001 (small theo. uncertainties from top 5.94310.0163 = 2.9840 0.0082No room for new physics: ® new had Forward-backward asymmetry -+-+®®mmZee coscos1(cosFB FB with coscos)0(1)1 Fs Fs At the -pole contribution is dominant interference vanishes FB Forward-backward asymmetry -+-+®®mmZee Away from the resonance FB is large interference term dominatesAt the Z pole: Interference = 0 very small because gl small in SM FB FB Fermion couplings Away from the resonance FB is large interference term dominatesAt the pole: Interference = 0 very small because gl small in SM FB FB Asymmetries together with cross sections allow the determination of the fermion couplings and Confirms lepton universalityHigher order corrections seen Lowest order SM prediction: sin decays LeptonNeutrinoJetJet dq,-
uq,
n W branching ratios Lepton universality tested to 2% )%28.077.67(Br Invariant mass recontruction indirect More difficult: pairing ambiguities 4. Electro-weak unification, as visible at HERA NC dQ ... ... Q2MZ22 CC dQ Q2MW22 g /Z Z 4. Electro-weak unification, as visible at HERA NC dQ ... ... Q2MZ22 CC dQ Q2MW22 5. Higher order corrections and the Higgs mass Lowest order SM predictions Including radiative corrections The measurement of the radiative corrections: effeffsin1(sin1(sin allows an indirectdetermination of theunknown parameters m and MH. Direct measurement of m Top mass prediction from radiative corrections Good agreement between the indirect prediction of m and the value obtained in direct measurements confirm the radiative corrections of the SM Prediction of mt by LEP before the discovery of the top at TEVATRON. Observation of the top quark at TEVATRON (1995) q Top decay q

n Channel used for mass reconstruction: jetjetjetinv TeV q Higgs mass prediction from radiative corrections � 114 GeV (from direct searches) 144 GeV (from EW fits) Awaiting the discovery of the Higgs at the LHC Status end of 2007