LEP Results - 1. Pippa Wells CERN

Transcription

1 LEP Results - 1 Pippa Wells CERN

2 1: Z Resonance The LEP machine, beam energy, detectors Z lineshape: cross-sections, luminosity Lepton Forward-Backward asymmetry, polarised asymmetries Number o light neutrinos, lepton couplings 2: LEP2 Results WW and ZZ physics at LEP2 b-tagging, electroweak physics with heavy lavours (b and c) Global electroweak its Standard Model Higgs boson - a hint?

3 LEP Time Line 1960 s Glashow-Weinberg-Salam SU(2) U(1) theory o electroweak interactions, prediction o W and Z gauge bosons SU(3) colour QCD theory o strong interactions 1976 CERN study group considers Large Electron-Positron storage ring, s = GeV, L cm 2 s Observation o gluon at PETRA. December: 27 km design approved by CERN council 1983 Chose LEP experiments. W and Z observed at CERN SPS 1989 Scooped! e + e collisions at Z in MARK II at SLC. First collisions in LEP with s M Z 1995 Gradual installation o LEP2 SC RF system starts Energy raised to s = 140 GeV at end o year. Top quark observation at Fermilab conirmed 1996 W pair threshold crossed at LEP Nobel Prize or t Hoot and Veltman or or elucidating the quantum structure o electroweak interactions in physics 2000 Last year o LEP running with s up to 209 GeV.

4 Electron-positron annihilation Cross-section (pb) Z e + e hadrons CESR DORIS PEP PETRA TRISTAN KEKB SLACB LEP I SLC W + W - LEP II Centre-o-mass energy (GeV) γ Z Z/γ LEP collected 4.5 million Z, 12 thousand WW per experiment W e - e + ν e W W W γ Z W W

5 Z resonance lineshape To measure the Z mass, total width and cross-section, partial widths (branching ratios) and couplings: LEP machine gives e + e collisions at a ew energies on and near the Z peak and precise measurement o E beam Detectors ALEPH, DELPHI, L3, OPAL distinguish Z inal states and measure the luminosity rom QED t-channel process e + e e + e (Bhabha scattering) σ( s) = (N observed N background )/ɛl Monte Carlo simulation o the signal eiciency and background. Theoretical prediction o the lineshape Match precision rom 4.5 million Z events per experiment - relative statistical error about Several thousand people involved σ(m Z ) 340 MeV rom UA2+CDF in Hoped to reduce to 10 MeV (limited by beam energy precision) Count the number o generations. 2.5 generations were known in 1989, top quark and ν τ not yet established. Number o light neutrinos limited by big bang nucleosynthesis to < 4. Expected precision o about ±0.2 on the number.

6 Pippa Wells July 2

7 The LEP Collider A good ill lasts around 10 h (LEP1 at Z) or 3 h (LEP2) E (GeV) or L(10 31 cm -2 s -1 ) A day at LEP (example rom 1999) Energy Luminosity Time (hours)

8 Beam energy - resonant depolarisation E beam = e 2π B dl Spin o electrons aligns with vertical B ield due to synchrotron radiation. Slow (hours) build up o transverse polarisation IF beam orbit suiciently smooth. Spins precess in B ield. Number o precessions per turn o LEP: ν s = g e 2 2 e 2πm e B dl = g e 2 2 E beam m e ν s 101.5, 103.5, at s = peak-2, peak, peak+2 Apply oscillating horizontal B ield, ν, at one place. Scan ν. I ν = ν s, polarisation is destroyed. s s Fast sweeping horizontal B ield b x ν scan.474 to to to to to to.477 Polarization (%) Instantaneous precision 100 kev. In 1986 expected limit rom magnetic stability δm Z 10 MeV 22:25 22:30 22:35 22:40 22:45 Daytime

9 Stability? Quadrupole movements... Earth Rotation Axis irst calibrations saw luctuations o order 10 MeV. Earth tides driven by moon and sun. R > 0 ε E R < 0 ε M Moon ecliptic Length o orbit ixed by RF system, but magnets move with ground. Beam no longer goes through centre o quadrupoles. Sensitive to 1mm change in 27 km, typical 10 MeV peak-to-peak. Beam Energy (MeV) Nov. 11th :00 2:00 6:00 10:00 14:00 18:00 22:00 2:00 Daytime Also see ground distortion due to lake level, heavy rain...

10 Stability? Dipole ields : Measured energy at the end o many ills 1995: Measurements o B ield in tunnel dipoles Equivalent Beam Energy (MeV) th August MeV Noisy period Quiet period 16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:00 Daytime Human activity increasing dipole ields during ill: BIAS 5 MeV Long investigation revealed cause - Vagabond electric currents rom nearby trains. Correct earlier years using model o average train behaviour. Final M Z systematic o 1.7 MeV

11 LEP1 data samples Approximate luminosity delivered per year. (Experiments collect 10 15% less) year centre-o-mass total o-peak energies luminosity luminosity [GeV] [pb 1 ] [pb 1 ] , 91.2, , 91.3, In , 6 o-peak points were measured. In 1993 and 1995 only 2 o-peak points were selected, to maximise the statistical precision. The exact values o the energies are chosen to allow resonant depolarisation at the end o each ill.

12 Cut-away view o OPAL Hadron calorimeters and return yoke Electromagnetic calorimeters Muon detectors Jet chamber Vertex chamber Microvertex detector y z θ ϕ x Forward detector Presampler Silicon tungsten luminometer Time o light detector Solenoid and pressure vessel Z chambers Overall size m

13 Hadronic event in ALEPH ALEPH DALI Run=9063 Evt=7848 Made on 9-Sep :43:03 by DREVERMANN with DALI_D1. This example has 3 jets e + e qqg Curved tracks in B ield (ALEPH and DELPHI have superconducting solenoids - B ield about 1.5 T compared to about 0.5 T in OPAL and L3) Many tracks and clusters in calorimeters

14 e + e e + e event in OPAL Run : even t 4093 : 1150 Da t e T ime C t r k (N= 2 Sump= ) Eca l (N= 9 SumE= ) Hca l (N= 0 SumE= 0. 0 ) Ebeam Ev i s Emi s s V t x ( , 0. 08, ) Muon (N= 0 ) Sec V t x (N= 0 ) Fde t (N= 1 SumE= 0. 0 ) Bz= Th r us t = Ap l an= Ob l a t = Sphe r = Y Z X 200. cm GeV Cen t r e o s c r een i s ( , , ) Lepton pair events have low multiplicity Electrons are identiied by a track in the central detector, and a large energy deposit in the electromagnetic calorimeter, E/p = 1.

15 e + e µ + µ event in L3 HCAL BGO Muon Chambers Tracking Muons penetrate the entire detector, and leave little energy in the calorimeters. L3 detector emphasizes lepton and photon id with a precise BGO crystal ECAL, and large muon spectrometer. The tracking volume is relatively small (radius 1m) ALL detectors inside 6m radius solenoid, ield 0.5T.

16 e + e τ + τ event in DELPHI DELPHI Interactive Analysis Beam: 45.6 GeV Proc: 8-Mar-1992 Run: Evt: 581 DAS : 18-Jun :22:19 Scan: 29-Apr-1992 TD TE TS TK TV ST PA Act ( 44) ( 48) ( 0) ( 9) ( 9) ( 0) ( 0) Deact ( 0) ( 4) ( 0) ( 6) ( 5) ( 0) ( 0) Barrel RICH Y Z X Tau lepton decays dominated by 1 and 3 charged tracks, with or without neutrals, missing neutrino(s), back-to-back very narrow jets. DELPHI has extra particle ID detectors, RICH.

17 Event selection A ew very simple cuts can distinguish hadronic, e + e, µ + µ and τ + τ events, and also background rom γγ, cosmic rays... The diicult task is to control systematic errors - how good is Monte Carlo description o data? Example 1: Hadronic event selection rom L e + e hadrons(γ) ( cos θ t 0.74) Events data 1994 e + e hadrons(γ) e + e τ + τ (γ) e + e e + e (γ) e + e µ + µ (γ) N cl

18 Event selection Example 2: Σ p tracks vs ΣE clusters or leptons p total / s µ + µ - τ + τ - OPAL e + e E total / s Representative values (vary rom experiment to experiment) Channel hadron e + e µ + µ τ + τ Eiciency % Background % Syst error %

19 Luminosity Measurement e - e + γ e - e + The t-channel contribution to e + e e + e dominates at small angles. Detectors typically 25 to 60 mrad rom beam. Very clear electron signal in orward detectors (calorimeters). E L /E Beam DELPHI E R /E Beam Accepted cross section at least 2 σ had. 1/θ 3 variation. Experimental diiculty: deine geometric edge o acceptance to give cross-section precision < 0.05%. Common theory error o 0.05% (c 1% in 1989). (BHLUMI program: S. Jadach, B.F.L. Ward et al.)

20 Standard Model relationships Masses o heavy gauge bosons and their couplings to ermions depend on SAME mixing angle cos θ W = M W /M Z SU(2) U(1) coupling constants, g, g, proportional to electric charge e: g = e sin θ W, g = e cos θ W γ Z W ieqγ µ ieγ µ (g v g a γ 5 ) 1 2 sin θ W cos θ W ieγ µ (1 γ 5 ) sin θ W where Q, g a and g v depend on ermion type, with g a = T 3 = ± 1 2 g v = (T 3 2Q sin 2 θ W ) = ± 1 2 (1 4 Q sin2 θ W ) g v /g a gives sin 2 θ W i you know Q.

21 Standard Model relationships Relate e, sin θ W and M W to the best measured parameters: α e2 4π = 1/ (50) G F πα 2M 2 W sin 2 θ W = (1) 10 5 GeV 2 M Z = (21) GeV G F measured rom muon decay; M Z rom LEP. These relations are true at tree level, but to check that they are valid, must take into account radiative corrections, which give sensitivity to virtual heavy particles, and possibly new physics! Aside: Other SM inputs needed are ermion masses, Higgs mass, CKM matrix (quark mass eigenstates are not weak eigenstates), strong coupling constant, α s

22 Radiative corrections Propagator corrections are the same or each ermion type. W H Z/W/γ Z/W/γ Z/W/γ Z/W/γ Z/W/γ Z/W Z/W Z/W QED, QCD and vertex corrections give ermion dependent terms. e - e + γ e - Z e + Electroweak corrections absorbed into eective couplings: Z g V g e V = (1 + ρ)(t 3 2Q sin 2 θ e ) g A g e A = (1 + ρ)t 3 sin 2 θ e = (1 + κ) sin 2 θ W q g q Z t t W b b ρ = 3G FM 2 W 8 2π 2 κ = 3G FM 2 W 8 2π 2 ( M 2 t M 2 W tan 2 θ W [ ln M 2 H ( cot 2 θ W M 2 t M 2 W ]) + MW 2 6 [ ln M H 2 5 ]) + MW 2 6 Extra M 2 t /M 2 W contributions or b quark

23 The value o G F is also modiied: Radiative corrections G F = πα 2M 2 W sin 2 θ W 1 1 r where r = α + r w = α κ + α term incorporates the running o the electromagnetic coupling due to ermion loops in the photon propagator. The diicult part o the calculation is to account or all the hadronic states. Use experimental measurement o e + e hadrons at low s. α(s) = α(0) 1 α α(0) = 1/ (50) ; α(m Z ) = 1/ (46) Quadratic dependence on M t Logarithmic dependence on M H Can it both M t and M H Use programs such as ZFITTER (D Bardin et al.) and TOPAZ0 (G Montagna et al.) or calculations to higher order. Leading order expressions above are or large M H.

24 QED corrections Dominant QED correction rom initial state radiation. Accounted or by radiator unction H. We want σ ew (s) e - e + γ Z σ(s) = 1 dzhqed(z, tot s)σ ew (zs). 4m 2 /s σ had [nb] ALEPH DELPHI L3 OPAL σ 0 20 Γ Z 10 measurements, error bars increased by actor 10 σ rom it QED unolded M Z E cm [GeV]

25 Dierential cross-section Improved Born Approximation or e + e (Ignoring ermion masses, QED/QCD ISR/FSR...) θ e - e + dσ ew d cos θ = πn c 2s 16 χ(s) 2 [ (g 2 Ve + gae)(g 2 V 2 + ga)(1 2 + cos 2 θ) + 8g Ve g Ae g V g A cos θ ] +[γ exchange] + [γz intererence] Where χ(s) = G FM 2 Z 8π 2 s s M 2 Z + isγ Z/M Z χ(s) 2 gives lineshape as a unction o s. Even term in cos θ gives total cross-section σ (g 2 Ve + g 2 Ae)(g 2 V + g 2 A) Odd term in cos θ leads to orward-backward asymmetry: A FB = σ F σ B σ F + σ B where σ F = 1 (dσ/d cos θ)d cos θ. At the Z peak: 0 A 0, FB = 3 4 2g Ve g Ae g 2 Ve + g2 Ae 2g V g A g 2 V + g2 A 3 4 A ea A FB depends on g V /g A, i.e. on sin 2 θ e Cross-section plus A FB allow g V and g A to be derived.

26 Polarised asymmetries Final state ermions in e + e are polarised. Polarisation can be measured or τ lepton inal states at LEP. P τ (σ + σ )/(σ + + σ ) where σ +( ) cross section or producing + (-) helicity τ leptons. Eg. τ πν, momentum o the π depends on the τ helicity Initial state: LEP beams are unpolarised (except or special energy calibration conditions) Stanord Linear Collider - longitudinally polarised electron beam to detector SLD. Electron beam 75% polarised rom e Damping Ring Thermionic Source Polarized e Source Electron Spin Direction Spin Rotation Solenoids (LTR Solenoid) e+ Return Line e+ Damping Ring e+ Source e Spin Vertical Linac Collider Final Arcs Focus IP e Extr. Line Spectrometer e+ Extr. Line Spectrometer Compton Polarimeter Knowing polarisation o inal (τ ) or initial (SLD) state, can construct let-right, let-right-orward-backward... asymmetries, and measure A e or A, eg. A LR (s) = N L N R N L + N R 1 P e, A0 LR A e

27 Cross-section and partial widths Cross-section as a unction o s (rom χ(s) 2 ): Z lineshape sγ 2 Z σ (s) = σ 0 (s M Z ) 2 + s 2 Γ 2 Z /M Z 2 where pole cross-section is σ 0 = 12π M 2 Z Γ ee Γ Γ 2 Z. with Γ /Γ Z = BR(Z ) and partial width is Γ = N c G F M 3 Z 6 2π ( g 2 A + g 2 V) + QED/QCD corrections eg. QCD: Γ q q Γ q q (1 + α s /π + ) Total width o Z Γ Z = Γ had + 3Γ ll + Γ inv = ΣΓ q q + 3Γ ll + N ν Γ νν Comparing total width to partial width gives N ν Cross-sections and widths correlated. Choose to it: M Z, Γ Z, σh 0 Ratios: Re 0 Γ had /Γ ee, Rµ 0 Γ had /Γ µµ, Rτ 0 Γ had /Γ ττ or R 0 l Γ had/γ ll Asymmetries: A 0, e FB, A0, µ FB and A0, τ FB or A0, l FB Extra inormation rom tagging some quark lavours (lecture 2).

28 Cross-sections vs s σ (nb) hadrons ee 1.4 ALEPH σ (nb) µµ ττ s(gev)

29 Lepton orward-backward asymmetries L3 e + e µ + µ (γ) d σ / d cos θ [nb] peak 2 peak peak+2 Forward-backward asymmetry or lepton pairs is straightorward to measure. Charge o lepton rom tracking Asymmetry cos θ varies with centre-o-mass energy. Asymmetry µ + µ - OPAL Ecm

30 Lepton Universality Plot A 0, l FB vs. R0 l = Γ had/γ ll. Contours contain 68% probability. Lepton universality OK. Results agree with SM (arrows) M t = ± 5.1 GeV M H = GeV (low M H preerred) α s (MZ) 2 = ± % CL A 0,l b α l + l e + e µ + µ τ + τ m t m H α s R 0 l=γ had /Γ l Next lecture: interpretation o asymmetries in terms o sin 2 θ lept e

31 LEP combined results Z resonance parameters - recall pre-lep hopes: σ(m Z ) 10 MeV (limited by beam energy precision) Number o generations σ(n ν ) 0.2 Fitted M Z [GeV] ± Γ Z [GeV] ± σ 0 h [nb] ± R 0 l ± A 0, l FB ± Derived Γ inv [MeV] ± 1.5 Γ had [MeV] ± 2.0 Γ ll [MeV] ± N ν ± Summary - Very precise measurements o Z mass, width, cross-sections, partial widths and lepton orward-backward asymmetries. High statistics data samples. Careul control o systematic errors.