Physics Highlights from 12 Years at LEP

Transcription

1 Physics Highlights from 12 Years at LEP Colloquium Frascati,, Dieter Schlatter CERN / Geneva 1

2 Standard Model In 1989 ingredients of Standard Model were known: Matter particles: u,d,s,c,b,t quarks (top not yet seen) e,µ,τ charged leptons and 3 neutrinos (ν τ indirect)? Interactions Electroweak gauge theory, SU(2)xU(1). W and Z bosons (CERN 1983) and γ QCD, theory of strong interaction, SU(3) Gluon (DESY 1979) 2

3 Boson Masses (before LEP) M Z (GeV) (20) M W (GeV) (4) M Higgs Precision(2000):? Fine structure const. 0.04ppm Fermi constant 10ppm M Z 23ppm M W 460ppm Weak mix.angle 1000ppm Strong coupling 20000ppm 3

4 1. LEP e e Collider at CERN 27 km circumference Beam energy: LEP I : 45 GeV LEP II: GeV Operation: July 1989-November2000 4

5 Distribution of cavity gradients (96 to 104 GeV) Number of cavities 96 GeV 100 GeV 104 GeV 96 GeV: Mean Nb/Cu 6.1 MV/m 100 GeV: 3500MV Mean Nb/Cu 6.9 MV/m 104 GeV: 3666MV Mean Nb/Cu 7.5 MV/m design In situ conditioning... pulse processing, He processing (only a few times) Accelerating field [MV/m] 5

6 LEP superconducting RF cavities 4 out of 288 SC cavities 6

7 2. Electroweak Precision Measurements 7

8 Number of light Neutrino Families Z boson lineshape Invisible width of Z: Γ inv = Γ Z Γ hadron 3Γ lepton Γ inv /Γ lepton = N ν Γ ν /Γ lepton SM: Γ ν /Γ lepton = 1.991±0.001 N ν = ±0.012 Experiment: Small Γ Z σ(e cm ) Lumi E cm 8

9 Luminosity Measurement e + e e + e Small angle X-section σ ee ~1/θ 2 min Luminosity measurement: SiW calorimeters 20µm position resolution at r min =6cm Error pre-lep 1993 Experimental 3% 0.07% Theory 2% 0.06% 9

10 Z Boson Mass σ ff (s) = Z 2 + (γ-z) + γ 2 Z 2 = σ ff peak BreitWigner(s,M Z ) observed X-section smaller due to initial state radiation (-30% at peak) Dominant error is from LEP energy calibration using Resonance Depolarisation (±1.7 MeV) γ-z interference term from fit with high energy X-section M Z = GeV 10

11 LEP Energy Calibration by Resonance Depolarisation Intrinsic error: E=0.2 MeV, in physics fills E=1.1 MeV 11

12 LEP Beam Energy Surprises Tide effects Return currents of nearby railtrack of TGV influences LEP bending magnet NMR s. Max. energy variation: 10 MeV 12

13 W Boson Mass WW 4 quarks qq e ν qq µ ν qq τ ν 4 leptons Direct mass reconstruction + kinematic fit: σ sys (MeV) hadr. 24 LEP 17 FSI 13 13

14 Z and W Masses with time Z Boson Mass Progress in time Final! Aim: ±30MeV 14

15 b-quark fraction, R b = bb / had Z bb R b = Z hadrons From double tagging of b quarks, with precision Si vertex detectors. 15

16 Leptonic Z Couplings, g A, g V Z g A,g V e +,µ +,τ + e,µ,τ M top =174±5GeV, 113 < M Higgs <700 GeV LEP Before LEP SM g Vµ /g Ve =0.96(6) g Aτ /g Ae =1.002(2) 16

17 Summary Electroweak Parameters Global SM fit results Input: Z lineshape lepton asymmetries quark asymmetries lepton fraction b-quark fraction Z,W,top mass χ 2 = 21 / 15 d.o.f (12%) 17

18 3. Strong Interaction, QCD Hard gluon radiation of quarks e + e - qqg 3 hadronic jets Perturbative QCD (α s =0.12) 18

19 Quark-Antiquark Hadronic Final States E cm =200GeV Jet resolution 19

20 Strong Coupling Constant q α s q Running of strong coupling constant from tau mass to Z mass g From hadronic tau decays R τ = Γ(τ ν τ hadrons)/γ(τ ν τ e - ν ε ) QCD evolution Data: R τ =3.484(24) QCD: R τ =3.058(1+α s /π+5.2 (α s /π) (α s /π) 3 +..) + non pert. terms Tau Z From Z decays: three-jet rate = "qqg" / "qq" or hadronic Z width, Γ(Z hadrons) QCD: R had =R 0 (1+ α s /π+1.4 (α s /π) (α s /π) 3 +..) E cm α s (m Z )=0.119±0.003 theory 20

21 Color Factors and QCD Gauge group 2 s C F Color = extra degree of freedom in QCD 2 2 s s C A n f T F C A C F 2 = = 2 gg - coupling qg - coupling T F * N A = C F * N F # Gluons # Quark colors T F C F = # Quark-colors # Gluons =

22 QCD Gauge Group from four-jet angular distr. SU(3) SU(3) 22

23 Running b-quark mass m b, similar to α s (s), is effective parameter, m b (s) one of the few parameters of the QCD Lagrangian usually measured close to threshold independent measurement at much larger scale is interesting basic input for test of m b -m τ unification (GUT) R b/l = b uds Pert. QCD: R 2 mb s X ) = 1 + b0 ( m, X ) + b ( m, ) 2 b b m 2 b / l ( 1 X Z e.g 3-jet rate R b/l = 1 + few % 23

24 m b (m Z ) Dominant systematic error is hadronisation uncertainty Starting with m τ =m b at GUT scale, MSSM predicts m b (m b ) 4.5 to 5 GeV 24

25 4.. Heavy Flavour Results 25

26 Quark Mixing B d,s Transitions between up- and down-type quarks Example: B d,s t c u b s d Cabibbo-Kobayashi-Maskawa mass matrix (CKM) quark mass eigenstates weak eigenstates Unitarity four independent parameters: λ=sin C θ=0.22 (from K e3 decays), A=0.8 (from B 0 l + ρ and η from many results 26

27 B 0 d,s Oscillations Observed B 0 and B 0 mesons have mass eigenstates: B 1 = 1/ 2 ( B 0 + B 0 ) B 2 = 1/ 2 (B 0 - B 0 ) with slightly different mass (and life-time). Pure B 0 state decays as B 0 or B 0 with probability: P(t) = - e 2 t/ ( ) 1± cos( m t) Experiments (Z bb): Select B 0 d, D*l ν (B0 s, D s l ν) candidates l=e,µ b-tagging exploiting b lifetime or high b mass (high p t leptons) identify Ds φπ, ΚΚ, φ lν decays. Lepton charge B 0 or B 0 Measure decay length ( decay time t) m = mass difference τ = proper time 27

28 b-quark tagging Exploit finite lifetime of b hadrons (decay length of a few mm) Impact parameter signals presence of long-lived particles. Precision Si-strip tracking detector B s lifetime: τ=1.5 ps Track position measured to 15µm 28

29 Z Bs bb display B 0 s B0 s 29

30 B d Oscillation Β d D *+ l Β d D * l + Time m d =0.486± 0.015ps -1 Mass difference= ev 30

31 B s Oscillation Propertime frequency ( m) P(t) ~ 1±A cos ( m s t) m s > 15 ps 1 or m s = 17.7 ± 1.4 ps Best SM estimate: 14.6 m s 31 ps -1 31

32 B decay results m d =0.49(2), m s >15ps -1, Vcb =0.041(2), Vub Vub 32

33 Progress since 1988 Future: BABAR/SLAC BELLE/Tokyo A. Stocchi,

34 5.. Virtual mass effects: m top, m Higgs 34

35 Masses in the SM [SU(2) [SU(2)xU(1)] Boson masses: M W 2 = _ g w 2 v 2 M Z 2 = _ (g w 2 + g' w 2 )v 2 Μγ 2 = 0 Ground state value of Higgs field: v = [ 2 G F ] -_ 246 GeV Electroweak mixing angle: tanθ W = g' w /g w (Expt: sinθ W ) Fermion masses m f = g f v/ 2 Yukawa coupling g f unknown Higgs mass: M 2 Higgs = λv2 SM does not predict couplings masses unknown... BUT 35

36 Heavy Mass Constraints Top quark mass and Higgs boson mass enter SM calculations through higher order, virtual quantum corrections ( radiative corrections ). Loops modify the electroweak relations of the SM, for example the W propagator: m top 2 log(m Higgs2 ) Using W and Z boson precision measurements one can predict top quark mass: m t = 170±8 GeV direct measurement (CDF+D0) : m t = 174±5 GeV Higgs mass prediction is less precise (log(m H2 ) )! 36

37 Radiative ative Corrections (cont. cont.) For the W mass: m 2 W 1 m m 2 W 2 Z = C 1 r W C=π/ 2GF Measurement of and ± α ± Electroweak corrections: α ρ top + Higgs α: shift of the fine structure constant α α M Z ρ top : top quark mass correction ~ m 2 top +... Higgs : Higgs mass correction ~ log m H 2 /m 2 W + Similarly for effective EW mixing angle: sin 2 l eff = _ (1-g V /g A ) 2 l 2 l sin eff cos eff = C 2 m (1 r ) Z Z 37

38 Higgs Boson Mass Limits (90% CL) Using 16 million Z decays and 20,000 WW events: J. Erler hep-ph/ Indirect limit on m H from: Z boson lineshape Z asymmetries W mass νν scattering m H < 200 GeV (95% CL) Direct limit (LEP 2) m H > GeV (95% CL) Higgs Mass (GeV) m W Z line. Top mass (GeV) 38

39 Higgs mass in SM fits 39

40 6. Direct Search for SM Higgs Boson At LEP, Higgs boson production in association with Z boson (Higgsstrahlung) e + e - H Z Four final states: qq νν (i.e. missing energy), Ηl + l - (l, µ Ητ + τ Light Higgs boson (<120 GeV) decays dominantly in bb (90%) and τ + τ Expected number of Higgs bosons produced falls rapidly approaching the kinematic limit! m max 206GeV GeV Γ Z = 2.5 GeV Higgs search is NOT background free! ΣBkg( ZZ, WW, qqg) ~ 100 events 12 H =114GeV 40

41 WW and ZZ production cross sections WW ZZ 41

42 Likelihood Ratio L s+ b Q = L b Backgroundlike Signallike Expected from background only Minimum is M h =115 GeV s+b Observed s+b hypothesis is 32 times more likely than background only hypothesis! Stat. significance is 2.9 σ 42

43 Likelihood Results by Experiment Observed Background Signal + background (m H =115 GeV) Probability density Log Likelihood 43

44 Likelihood Results by Channel Observed Background Signal + background (m H =115 GeV) Probability density H Log Likelihood 44

45 Summary Higgs Search The combined result of 4 LEP experiments show a hint for the SM Higgs with a mass of M H =115 ± 1.3 GeV but statistical significance is only 2.9 sigma. Future experiments will decide, if first direct sign for Higgs boson or fluctuation! ATLAS ~ 1000 events in the peak LEP2 5σ 100 fb -1 45

46 7. Hints for Physics beyond SM Grand Unified Theory (GUT) : unification of coupling constants at high energy at Planck mass GeV Standard GUTs, or below ( E GeV) SUSY GUTs α s (M Z ) Prediction of strong coupling constant (from sin 2 θ W and α QED (M Z ) 46

47 What if M H =115 GeV? If M H =115 GeV then new physics must appear at a scale of 10 6 GeV If not, and SM valid up to GeV: 135 < M H < 180 GeV 47

48 Conclusion Unfortunately no discovery of signs for new physics at LEP BUT very rich exploitation of SM physics Three families with light neutrinos Precision electroweak results (~10-3 ) constrain SM m Z, m W,, Z-lineshape, quark and lepton asymmetries, α s (m Z ), running of α s electroweak quantum corrections have been demonstrated ( m top ) SM Higgs must be light: 113 GeV m Higgs 200 GeV (95% CL) quark mass mixing, CKM matrix more complete B d oscillations measured, hint for B s oscillations Direct searches SUSY no signal SM Higgs: a hint (~ 3sigma) for m Higgs = 115±1.3 GeV? Future good chances for TEVATRON and/or LHC to answer Higgs question, find SUSY particles or. 48

49 SM Physical Constants 1987 PDG 2000 Strong coupling constant: s : 0.12(2) (20) Weak mixing angle, sin 2 MS (m Z ) 0.230(6) (16) W boson mass: m W (GeV) (56) Z boson mass, m Z (GeV) (22) 49