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What we Learnt from LEP

What we Learnt from LEP. (and SLC too! ). Gigi Rolandi CERN/EP. COMPLEMENTARY to LEP. Conventional collider e + e - ring; Energy upgradeable; Energy measurable; Four detectors (A,L,D,O); Large luminosity;. “Linear” e + e - Collider (with arcs); Electron polarized;

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What we Learnt from LEP

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  1. What we Learnt from LEP (and SLC too!) Gigi Rolandi CERN/EP COMPLEMENTARY to LEP • Conventional collider e+e- ring; • Energy upgradeable; • Energy measurable; • Four detectors (A,L,D,O); • Large luminosity; • “Linear” e+e- Collider (with arcs); • Electron polarized; • Small beam transverse dimensions Credits: most of the slides from Patrick Janot Electromagnetic probes of Fundamental Physics

  2. LEP and SLDOverview Total Luminosity: 1000 pb-1 550,000 Z decays 20 Million Z’s 1992-1998 SLD Run Precision: 0.1% 1989-2000 LEP Run 40,000 W+W- Average e- Polarization: 73% • Small transverse beam sizes; • Small beam pipe; Energy: 88 209.2 GeV A few Higgses? Electromagnetic probes of Fundamental Physics

  3. A Little Bit ofHistory 1967: Electroweak unification, with W, Z and H (Glashow, Weinberg, Salam); 1973: Discovery of neutral currents in e scattering (Gargamelle, CERN) 1983: W and Z discovery (UA1, UA2); LEP and SLC construction start; First Z detected ever : UA2 - qq  Z  e+e- 1989: First collisions in LEP and SLC; Precision tests of the SM (mtop); 1995: Discovery of the top (FNAL); Precision tests of the SM (mH); 2000:First hints of the Higgs boson? 1974: Complete formulation of the standard model (Illiopoulos) 1981: CERN SpS becomes a pp collider; LEP and SLC approved before W/Z discovery; - Electromagnetic probes of Fundamental Physics

  4. LEP: Luminosity,Energy,Precision Why isPrecisionNeeded? Electroweak Observables (i.e., related to W and Z) sensitive to vacuum polarization effects: L Couplings (v+a)  R Couplings (v-a) 0.1% Precision needed! with • Determine  and sin2W from LEP/SLD data; • Predict mtop and mW; • Compare with direct measurements; • Predict mH; • Compare with direct measurements. eff Electromagnetic probes of Fundamental Physics

  5. PrecisionElectroweakObservables (I) LEP1 LEP2 Heavy Flavour Rates Electromagnetic probes of Fundamental Physics

  6. Dependence on mtop, mHof mW and Rb (mW/mZ)2 (mW /mZ)2(1 + Dr) 220 200 180 160 140 mW mtop (GeV/c2) Rb Rb Rb (1 + dVb) Dr Combined 10 102 103 mH (GeV/c2) 0.5% Precision needed Electromagnetic probes of Fundamental Physics

  7. PrecisionElectroweakObservables (II) Precision of the Detectors LEP eff 1-4|Qi|sin2qW eff sin2qW e.g. Precise Energy Zm+m- to High Luminosity 5.10-4 + b-Tagging t-selection mW ... ... Polarized Beam (SLC only) Consistency Checks! tpnt eff sin2W = 1 – mW2/mZ2(1+) Electromagnetic probes of Fundamental Physics

  8. The Z Lineshape at LEP At tree-level:  (1+)2 • Measure s and s • Correct for QED and QCD -30% for s0 +200 MeV for mZ +4% for Gqq • Fit for the Z parameters • (mass, total width, peak cross • section and partial widths) s = Electromagnetic probes of Fundamental Physics

  9. Measurement of the LEP Beam Energy (I) 1) The electrons get transversally polarized (i.e., their spin tends to align with B), but Approximation: LEP is a circular ring immersed in a uniform magnetic field: • Process very sensitive to imperfections • ( slow, typically hours, and limited • to o(10%) polarization) • Process very sensitive to beam-beam • interactions ( one beam, no polarization • in collisions) Bdipole  p e- R LEP (L = 2pR = 27km) E  p = e B R = (e/2p) B L In real life: B non-uniform, ring not circular To be measured Electromagnetic probes of Fundamental Physics

  10. Measurement of the LEP Beam Energy (II) 2) The spin precesses around B with a frequency proportional to B. The number of revolutions for each LEP turn is thus proportional to BL (in fact, to  B dl, and then to Ebeam) 3) Measure ns : 0 B 1 2 Bx: oscillating field with frequency n, in one point. 3 -Bx Bx Vary n until Polarization = 0 101.5 Peak-2 103.5 Peak 105.5 Peak+2 1993 Precision  210-6 DEbeam  100 keV ! Electromagnetic probes of Fundamental Physics

  11. Measurement of the LEP Beam Energy (III) A dispersion of 10 MeV is observed ( 100 keV) in the same machine conditions. Correlation with the moon found on 1992, Nov 11th: • At midnight, the electrons see • less magnetic field, E is smaller; • At noon, they see more magnetic • field, and E is larger. LEP at midnight Longer by 300 mm LEP at noon Shorter by 300 mm  S  U  N Prediction and data fit perfectly … However, the electron orbit length is fixed by the RF frequency: L = c  t Electromagnetic probes of Fundamental Physics

  12. Measurement of the LEP Beam Energy (IV) • Other 10 MeV-ish effects understood even later: • Geological deformation due to the level of the lake or rain change LEP circumference; (controlled with the BOM’s) • Effect of the TGV: currents induced on the LEP beam pipe induce changes in the magnetic field (controlled by 16 NMR probes) Understood after one-day strike Understood after three rainy months Now: DEbeam 2 MeV Electromagnetic probes of Fundamental Physics

  13. Z Lineshape: Final State Identification (I) Electromagnetic probes of Fundamental Physics

  14. Z Lineshape: Final State Identification (II) - e) Z->nn - • Z  nn : • Not detectable. • Z  t+t- : Two low multiplicity jets + missing • energy carried by the decay neutrinos - • Z  qq :Two jets, large particle multiplicity. • Z  e+e-, m+m- :Two charged particles (e or .) Electromagnetic probes of Fundamental Physics

  15. Z Lineshape: Final State Identification (III) Leptonic decays: Low multiplicity, with (t) or without (e, m) missing energy 600,000 gg Collisions: Low multiplicity, Low mass 16 million • Selections with • High Efficiency; • High Purity; 600,000 Count events : Easy? Hadronic decays: High multiplicity High mass Note: Also need precise Luminosity determination Systematic Uncertainty  0.1% Electromagnetic probes of Fundamental Physics

  16. Z Lineshape: Results (III) (1998) 10-3 0 0 shad/sl 500 MeV in 1989 With this measurement alone: mtop  165  25 GeV/c2 GZ (1 + )2 (+small sensitivity to mH) Electromagnetic probes of Fundamental Physics

  17. Heavy Flavour Rates: Identification (I) b- and c-hadrons decay weakly towards c- and s-hadrons, with a macroscopic lifetime (1.6 ps for b’s), corresponding to few mm’s at LEP 3d-vertexing determines secondary and tertiary vertices. High resolution is crucial. Impact parameters of reconstructed tracks allow b quarks to be tagged with very good purity. Mass of secondary vertex tracks is a very powerful discriminator of flavour (b, c, and light quarks): mb5 GeV/c2, and mc1.5 GeV/c2.  10 cm  1 cm Electromagnetic probes of Fundamental Physics

  18. Heavy Flavour Rates: Identification (II) • Vertex detectors (Si m-strips, CCD’s, pixels): • At LEP: inner radius 6 cm, good resolution; • At SLC: inner radius 2.3 cm, superior resolution. SLD can do both b- and c-tagging with good purity. Data Vertex Mass for Events With a Secondary Vertex In SLD b (MC) c (MC) uds (MC) Vertex Mass (GeV/c2) Electromagnetic probes of Fundamental Physics

  19. Heavy Flavour Rates: Results Use double-tag method to reduce uncertainties from the simulation, e.g., in bb events: Take ec, euds, and rb (all small) from simulation. Solve for eb and Rb ! - Rb =Gbb/ Ghad 3 10-3 With this measurement alone: mtop  150  25 GeV/c2 (dependence on mH, as, … cancel in the ratio) Electromagnetic probes of Fundamental Physics

  20. Prediction of mtopfrom EW Measurements A top mass of 177 GeV/c2 was predicted by LEP & SLC with a precision of 10 GeV/c2 in March 1994. / SLD One month later, FNAL announced the first 3 evidence of the top. / SLD / SLD In 2001: / SLD (actually 2.9s, cross section three times the SM’s) Perfect consistency between prediction and direct measurement. Allows a global fit of the SM (with mH) to be performed. Electromagnetic probes of Fundamental Physics

  21. Asymmetries: Measurement of ALRat SLD (I) (e- beam polarization) • ALR( Ae ~ 14% if Pe = 100%) is 10 times more sensitive to sin2W than AFB( AeAl ~ 1-2%); • ALR is independent of the final state (Z  hadrons, +-, +-); • ALR is independent of the detector acceptance; • Most of the theoretical corrections, uncertainties (QED, QCD, …) cancel in the ALR ratio. lept Statistical & Systematic uncertainties compete with LEP asymmetry measurements (unpolarized) Electromagnetic probes of Fundamental Physics

  22. Asymmetries: Measurement of ALRat SLD (II) • Condition # 1: Have a longitudinally 100% polarized electron beam. • Get longitudinally polarized e- by illuminating a GaAs photo cathode with circularly polarized Lasers (frequency: 2  60 = 120 Hz) • In principle, Pe  100% can be reached. In practice, 80% was achieved. • Change the sign of the polarization on a random basis to ensure that equal amount of data are taken with both signs, and that the luminosity is not tied to any periodic effects in SLC. • Transport, accelerate and collide the polarized electrons, with enough care to keep the same polarization at the interaction point. SLC was designed to do so from the beginning (unlike LEP). Electromagnetic probes of Fundamental Physics

  23. Asymmetries: Measurement of ALRat SLD (III) Condition # 2: Measure the e- polarization Pe with 0.5% accuracy. • Collide 45.6 GeV long. polarized e- with 2.33 eV(532 nm)circularly polarized photons every 7th bunch(17 Hz); • Detect Compton back-scattered e- as a function of their energy after a bend magnet; • s(E) =s0 [1 + A(E) PePg] s0 A(E) s0and A(E) are theoretically well known (pure QED process) Cerenkov Detectors Electromagnetic probes of Fundamental Physics

  24. Asymmetries: Measurement of ALRat SLD (IV) • Reverse on a random basis the sign of the photon polarization Pg (close to 100%, optically measured with filters); • Count the number of e- detected in each of the Cerenkov channels (about a hundred electrons per channel at each beam crossing) • Deduce the e- beam polarization from the asymmetry Electromagnetic probes of Fundamental Physics

  25. Asymmetries: Measurement of ALRat SLD (V) • Cross-checks: • Count Compton-scattered gammas at the kin. threshold with two additional calorimeters. Agree with the main measurement to 0.4%. • Measure the positron polarization to be 0.0% with an accuracy better than 0.1%. Condition # 3: Count events in SLD Complete data set: • e.g., in 1997-98: • NL = 183,355; NR = 148,259 • Pe = 72.92%  Ae = 0.1491  0.0024 (stat.)  0.0010 (syst.) Electromagnetic probes of Fundamental Physics

  26. eff Asymmetries: Results for sin2W Leptons: 0.23100.0002 ??? Quarks: 0.23230.0003 Still statistically acceptable, but a couple of add’l years at LEP and SLD would have helped… 5 10-4 ALL: 0.231520.00017 Electromagnetic probes of Fundamental Physics

  27. Asymmetries: Results for al and vl Axial and vector couplings (al, vl) from ALR (SLC) and Z  l+l-(LEP) Before LEP&SLC After LEP&SLC Errors 10  200 Precision on sin2W: 5 10-4, adequate to become sensitive to mH. Electromagnetic probes of Fundamental Physics

  28. Asymmetries: Prediction of mW PredictmW in the SM: mW2 = mZ2(1+ )cos2W eff Direct Measurements* Precision Measurements mH dependence in the SM through quantum corrections (see later) Electromagnetic probes of Fundamental Physics

  29. W mass at LEP 2: Production and Decay s  2mW 45.6% 10.8% 43.8% - - - - W+W- q1q2ln: Two hadronic jets, One lepton, missing energy. - W+W- l1n1l2n2: Two leptons, missing energy W+W- q1q2q3q4: Four well separated jets. Electromagnetic probes of Fundamental Physics

  30. W mass at LEP 2: Threshold cross section mW (thresh.) = (80.40  0.22) GeV/c2 Systematics: Beam energy Electromagnetic probes of Fundamental Physics

  31. W mass at LEP 2: Direct measurement 0 unknowns, 5C fit 80.448  0.043 GeV 5 Constraints: 3 unknowns, 2C fit 80.457  0.062 GeV Fitted Mass (GeV/c2) Electromagnetic probes of Fundamental Physics

  32. W mass at LEP 2: Result and Uncertainties mW(LEP) = 80.450  0.025  0.030 (stat.) (syst.) Systematics: Beam energy, FSI • Good consistency between experiments; • Good consistency with hadron colliders • Fair consistency with Z data (LEP/SLD). Electromagnetic probes of Fundamental Physics

  33. W mass at LEP 2: Final State Interactions • Interactions between W hadronic decay products may • cause a shift between the mass from fully hadronic and • semileptonic events: • Colour Reconnection: QCD interaction between • quarks from different W’s; • Bose-Einstein Correlations between identical • hadrons (pions, kaons), well established in single • W or Z decays. q q SMALL q SMALL q Use the data to constrain them ! + = Without FSI Electromagnetic probes of Fundamental Physics

  34. aem(MZ) (I) Electromagnetic probes of Fundamental Physics

  35. aem(MZ) (II) Electromagnetic probes of Fundamental Physics

  36. Global Fit of the Standard Model to mH(I) Knowing mtop, most electroweak observables have a sensitivity to mH through Dr Electromagnetic probes of Fundamental Physics

  37. Global Fit of the Standard Model to mH(II) Global fit of mH and mtop: Electromagnetic probes of Fundamental Physics

  38. Global Fit of the Standard Model to mH(III) Internal Consistency of the Standard Model? Pull distribution = Normal Gaussian? Mean: 0.22  0.28 Sigma: 1.1  0.4 Largest discrepancy (-2.9s) well inside statistical expectation; 2 probability = 8%. Just fine. Electromagnetic probes of Fundamental Physics

  39. Global Fit of the Standard Model to mH(IV) What Next? • LEP and SLD tested the SM internal consistency with great precision; • LEP and SLD checked the predictions of the Electroweak Symmetry Breaking mechanism (e.g., mW = mZcosqW); • LEP and SLD allowed the mass of the top quark to be predicted years before it was discovered in 1995 at FNAL; • LEP and SLD measurements led to the prediction of a relatively small Higgs boson mass (around 100 GeV/c2); • A few more (5?) years of LEP and SLD would have allowed the prediction to be better than 15 GeV/c2. Precision EW Physics at Linear and Muon Colliders will be crucial (Z and WW production). • Now: dmtop =  5.1 GeV, dmW =  34 MeV • With dmtop =  2 GeV and dmW =  15 MeV • With a few more years of LEP and SLD Electromagnetic probes of Fundamental Physics

  40. Direct Searches at LEP 1 Acoplanar lepton pairs Events expected at LEP1 Monojets (among 2 107 Z) Acoplanar pairs Acoplanar jets - BR(Z  Hff) 0.0  mH 65.6 GeV/c2 Excluded at 95% C.L. Very little background expected Electromagnetic probes of Fundamental Physics

  41. Direct Searches at LEP 2 - Hnn He+e- s = mZ Z  Hff - s  mH+mZ Hm+m- - Hqq • 5ssensitivity for 200 pb-1: • s = 192 GeV for mH = 100 GeV/c2; • s = 210 GeV for mH = 115 GeV/c2; Electromagnetic probes of Fundamental Physics

  42. Beam Energy increases in LEP Energy Loss per Turn E4/ (Synchrotron Radiation) Maximum Beam Energy [RF VoltageBending Radius]1/4 • Increase RF Voltage; (130 MV for E = 45.6 GeV;  3 GV for E = 100 GeV;  Go for SC RF Cavities) • Increase Bending Radius! • Or increase both. Electromagnetic probes of Fundamental Physics

  43. Improvements in 99/00: From 192 to 209 GeV 1) Increase gradient & Cryogenics upgrade 3) Re-install 8 Cu cavities E: 207  207.4 GeV; mH: 114  114.25 GeV 200 GeV 7.0 MV/m 4) Use orbit correctors as magnetic dipoles 192 GeV 6.0 MV/m 204 GeV 7.5 MV/m E: 207.4  207.8 GeV; mH: 114.25  114.5 GeV 5) Decrease the RF frequency E: 207.8  209.2 GeV; mH: 114.5  115.1 GeV Accelerating field (MV/m) f = 0 Hz E: 192  204 GeV; mH: 100  112 GeV 2) Improve stability & Decrease security margin More dipolar magnetic field seen in the quadrupoles! • Two- to one-klystron margin (1h30): LEP: f=350 MHz E: 204  205.5 GeV; mH: 112  113 GeV • Mini-ramp to no margin at all (15 mins): f = -50 Hz E: 205.5  207 GeV; mH: 113  114 GeV Electromagnetic probes of Fundamental Physics

  44. Improvements in 99/00:Results • 220 pb-1 delivered in 2000: • starting at 204-205 GeV • (April-May) • Regularly above 206 GeV • (from June onwards) • Only above 206.5 GeV • (September to November) (288) (272) • Notes: • 372 cavities: •  E = 220 GeV; • 4 straight sections • E = 240 GeV. (240) 206.5 GeV (176) 205 GeV (144 cavities) 208+ GeV mH 114.1 GeV/c2 Excluded at 95% C.L. Electromagnetic probes of Fundamental Physics

  45. First pb-1’s above 206 GeV: First thrills at 115 GeV/c2 First Candidate Event (14-Jun-2000, 206.7 GeV) _ _ e+e-  bbqq • Mass 114.3 GeV/c2; • Good HZ fit; • Poor WW and ZZ fits; • P(Background) : 2% • s/b(115) = 4.7 Missing Momentum The purest candidate event ever! High pT muon • b-tagging • (0 = light quarks, 1 = b quarks) • Higgs jets: 0.99and 0.99; • Z jets: 0.14and 0.01. Electromagnetic probes of Fundamental Physics

  46. A few candidate events at 115 GeV/c2 27-Jun-2000 Mass: 113 GeV s/b115 = 0.52 31-Jul-2000 Mass: 112 GeV s/b115 = 2.0 _ _ ALEPH DELPHI 21-Aug-2000 Mass: 110 GeV s/b115 = 0.9 e+e-  bbnn 14-Oct-2000 Mass: 114 GeV s/b115 = 2.0 L3 21-Jul-2000 Mass: 114 GeV s/b115 = 0.4 Electromagnetic probes of Fundamental Physics

  47. The 14 Most Significant Events s/b > 0.3: Expected signal-to-noise ratio of ~1 Expected: 7 Observed: 14 Number of events compatible with s+b 0.7 115 In ALEPH: 6 In L3: 3 In OPAL: 3 In DELPHI: 2 Number of events in each experiment compatible with being democratic (~1.6 bkg expected) 0.7 In Hqq: 9 (70%) In Hnn: 3 (20%) In Hl+l-: 2 (10%) - Number of events in each Z decay compatible with HZ predictions - Values as of Nov 5th, 2000 Electromagnetic probes of Fundamental Physics

  48. Results as of 05-Nov-2000 • Regular increase of the significance; • Overall compatibility with mH = 115 GeV/c2. Minimum of the log-likelihood as deep as could have been a priori expected PRELIMINARY 2.9s 2.6s 2.2s PRELIMINARY 1.1s When interpreted as a Higgs signal: Reduced by  half a sigma on July 10, 2001 mH = 115.0 GeV/c2 + 0.7 - 0.3 Increased by half a GeV/c2 on July 10, 2001 Electromagnetic probes of Fundamental Physics

  49. Small hints of discrepancies ?? Altarelli, Carvaglios, Giudice, Gambino, Ridolfi This also moves up But there is no experimental reason to discard some measurements nor theoretical interpretation of the discrepancy Mh < 109 GeV in disagreement with direct searches. Agreement is restored by Supersymmetry Leptons: 0.23100.0002 ??? Quarks: 0.23230.0003 Mh < 212 GeV in agreement with direct searches 5 10-4 ALL: 0.231520.00017 Electromagnetic probes of Fundamental Physics

  50. Why we do believe in Supersymmetry:Experimental Hints • New Physics at a scale below 106 GeV would modify the predictions for the EW precision measurements, with radiative corrections parameterized according to e1, e2, e3(or S,T,U) • In the Standard Model, for instance • Supersymmetry does not change much these predictions (better agreement with measured values ?) • Model building with any other New Physics has not yet allowed such an agreement. 39% CL e3 103 Electromagnetic probes of Fundamental Physics

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