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The Standard Model Higgs Searches at LEP

The Standard Model Higgs Searches at LEP. Second Lecture Outline: 1. Brief Theory Reminder: Mass, Decays, Production 2. The Situation before LEP 3. Higgs Boson Searches at LEP 1 4. Higgs Boson Searches at LEP 2 5. 2001, A Spoilt Odyssey. Higgs.

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The Standard Model Higgs Searches at LEP

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  1. The Standard Model Higgs Searches at LEP Second Lecture Outline: 1. Brief Theory Reminder: Mass, Decays, Production 2. The Situation before LEP 3. Higgs Boson Searches at LEP 1 4. Higgs Boson Searches at LEP 2 5. 2001, A Spoilt Odyssey Higgs Probes of Electroweak Symmetry Breaking at LEP and SLC

  2. “Theory”: SM Higgs Boson Mass and Couplings From the Higgs Mechanism … and from Gauge Invariance : mZ = mW/cosW ; mW = gv/2; ( v  250 GeV). vacuum expectation value m2 /v Empty vacuum mf/v W,Z -v4/24 (Our vacuum) All couplings predicted (unknown) m2/v m2/v2 H H m2 = v2/3 H Mass unknown Probes of Electroweak Symmetry Breaking at LEP and SLC

  3. “Theory”: SM Higgs Boson Mass The mass mH is mostly unknown, but … (In the Standard Model)   m2/v2 H In the Standard Model: As for any other coupling constant, the particle content of the standard model determines the running of  up to a scale , at which the model is no longer valid. The following conditions must be realized: 115 Vacuum Stability Triviality R.G.E. Probes of Electroweak Symmetry Breaking at LEP and SLC

  4. “Theory”: SM Higgs Boson Decays The decay branching ratios depend only on mH: • mH 2me: H   + large lifetime; • mH 2mb up to 1000 GeV/c2: • mH 2mm: H  e+e- dominates; • mH 2mp: H  m+m- dominates; • mH 3 - 4 GeV: H  gg dominates; p0p0, p+p-, KK, , … etc - • mH 2mb: H  t+t- and cc dominate; Probes of Electroweak Symmetry Breaking at LEP and SLC

  5. “Theory”: SM Higgs Boson Production at LEP Dominant at LEP: The Higgs-strahlung process (The production cross section depends only on mH) LEP 2: s  mZ + mH LEP 1: s  mZ (Large coupling to the Z  Only sizeable cross section) Probes of Electroweak Symmetry Breaking at LEP and SLC

  6. The situation before LEP (I) Quite a few searches in hadron decays: NA31, 1990 CLEO, 1989 No excess seen, but… No unambiguous limit set SINDRUM, 1989 CUSB, 1989 (Large theoretical uncertainties on the predicted decay rates.) Probes of Electroweak Symmetry Breaking at LEP and SLC

  7. The situation before LEP (II) Only one unambiguous limit: M. Davier and H. Nguyen Ngoc, 1990 1) Production: 2) Decay (for mH between 2me and 2mm): me/v 103 102 10 1 0 Dump 2 1016 e- (2 GeV) No counts above 750 MeV Calorimeter 2m Decay Distance 1.2 MeV/c2 mH 52 MeV/c2 Excluded at 95% C.L. 0 1 2 3 E (GeV) Probes of Electroweak Symmetry Breaking at LEP and SLC

  8. Direct Searches at LEP 1 Acoplanar lepton pairs Events expected at LEP1 Monojets (among 2 107 Z) Acoplanar pairs Acoplanar jets - BR(Z  Hff) Very little background expected Probes of Electroweak Symmetry Breaking at LEP and SLC

  9. - Search for acoplanar jets (e+e- H) - 20 Hnn events to be looked for (4 expts, if mH = 65 GeV/c2) 400,000 Full Data Sample Mass  mH 4.5 million Acoplanar Jets 200,000 Missing Energy and Momentum Within more than 20 million other events from Z decays (or from other processes) Probes of Electroweak Symmetry Breaking at LEP and SLC

  10. - Search for acoplanar jets (e+e- H) • Two main subsamples: • High Multiplicity (Selected) • Low Multiplicity (Rejected) 70,000 Events with MVIS 70 GeV/c2 : CUT 4.5 Million Z  hadrons ALEPH Lots of Z  hadrons with missing energy - + 5 Hnn events ? (mH = 65 GeV/c2) Visible Mass (GeV/c2) Lots of gg interactions A few Z  t+t- With high multiplicity Z  e+e- Z  m+m- Z  t+t- A few gg interactions Visible Mass (GeV/c2) - Hnn signalexpected ( 100) Origin of missing energy in Z  hadrons ? Probes of Electroweak Symmetry Breaking at LEP and SLC

  11. Energy Losses in the Beam Pipe (Not instrumented) ALEPH Data (70,736 events) - Z  qq events: Two back-to-back jets Beam Cut - - Hnn simulation (starts from 10,000 events) Hnn signal Beam - Expected in the data: n Cut 5  67.09%  3.4 events n X30 60% X30 = Fraction of measured energy above 30 degrees from the beam axis Probes of Electroweak Symmetry Breaking at LEP and SLC

  12. Energy Losses in Semi-Leptonic b decays - Z  bb events: The n is in the jet ALEPH Data (26,041 events) n Cut Acollinearity  180 degrees - Hnn signal - - Hnn simulation (2.8 events expected) n Acollinearity << 180 degrees n Cut Acoll.  165deg. Acollinearity Angle (Degrees) Probes of Electroweak Symmetry Breaking at LEP and SLC

  13. Energy Losses due to I.S.R. - (Initial State Radiation) e+e- qq events: The pmisis along the beam pmis Beam ALEPH Data (1466 events) g Cut - Hnn signal Beam - Hnn simulation (2.4 events “left”) - a n n pmis Cut tan a  0.4 tan a Probes of Electroweak Symmetry Breaking at LEP and SLC

  14. Energy Losses due to I.S.R. + Semi-Leptonic b decay Acoplanarity angle  180 degrees - Z  bb events: ALEPH Data (1001 events) g Cut pmis n n - - Hnn signal: Hnn simulation (2.3 events “left”) - n Acoplanarity << 180 degrees Cut n Acoplanarity Angle (Degrees) Acop.  175deg. Probes of Electroweak Symmetry Breaking at LEP and SLC

  15. - A Semi-Leptonic decay in bbg (3-jet) events - Z  bbg events: The pmisis not isolated ALEPH Data (824 events) pmis Cut n - Hnn signal: The pmisis isolated - Hnn simulation (2.1 events “left”) - n pmis Cut n ECONE (GeV) = Energy contained in a cone of half-angle 30 degrees around pmis ECONE 1 GeV Probes of Electroweak Symmetry Breaking at LEP and SLC

  16. - Two Semi-Leptonic decays in bbg (3-jet) events - Z  bbg events: ALEPH Data (365 events) n S = 12+ 23+ 13 S  360 deg. 12 pmis (6 events) 13 Cut 23 n - - Hnn signal: - Hnn simulation (1.8 events “left”) n 23 (1.5 events “left”) 13 12 Cut S << 360 deg. - n S = 12+ 23+ 13 S  342 degrees Probes of Electroweak Symmetry Breaking at LEP and SLC

  17. Two Semi-Leptonic decays + Three Jets + I.S.R. (!!) - e+e- bbg(g) events: E1, E2, E3, Eg = Energies Recomputed with energy-momentum conservation constraint EMIN = MIN ( E1, E2, E3) n E1 n E2 - Hnn signal: Eg E1 - E3 E2 bbg(g) events are 4-body Compatible: EMIN is positive n - - n n Signal simulation - n E3 - Hnn: The three jets are in the same hemisphere. One of the Ei tend to be negative No events left in the data; Still 1.3 event expected fromHnn - Probes of Electroweak Symmetry Breaking at LEP and SLC

  18. Higgs Boson Searches at LEP 1: Result With the 4 LEP expts combined, 4.0 signal events were expected. None were observed. Saturation was being reached: 70 60 50 40 30 20 10 0 1995 1994 1993 1992 1991 1990 1989 10-2 10-1 1 10 102 Million Hadronic Z decays 0.0  mH 65.6 GeV/c2 Excluded at 95% C.L. GO FOR LEP 2 ! Probes of Electroweak Symmetry Breaking at LEP and SLC

  19. 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; Probes of Electroweak Symmetry Breaking at LEP and SLC

  20. Signal vs Background (I) Must evaluate the “signal-ness”, s/b, of the candidate events e+e- ZZ s ~ 2 pb e+e- HZ s = 0.1 pb • Reconstructed Higgs boson mass; • Other kinematic variables; • b-tagging (lifetime, leptons, …); Background Signal - e+e-W+W- s ~ 20 pb e+e-qq s ~ 100 pb Background++ Background+ Zoom of 1cm around the interaction point Probes of Electroweak Symmetry Breaking at LEP and SLC

  21. Elements of b tagging (I) 2 decrease with a Secondary vertex Impact Parameter with respect to Primary Vertex Combine with Neural Networks Likelihood's, … Lepton Transverse Momentum with respect to b-jet axis Other jet-shape variables (multiplicity, mass, sphericity) Probes of Electroweak Symmetry Breaking at LEP and SLC

  22. Elements of b tagging (II) Result on jets from hadronic Z decays (collected every year for calibration) Result on jets from hadronic W decays (from semi-leptonic e+e- W+W- events ) No b-jets in W decays • b-jets and light-quark jets are well separated; • Simulation reproduces well the data. (Essential) Probes of Electroweak Symmetry Breaking at LEP and SLC

  23. Elements of b tagging (III) - Separation of HZ, W+W- and qq events at LEP 2 energies (b tagging only) (Jets 1 and 2 are chosen to be the jet-pairing most compatible with originating from a Z decay, according to the di-jet invariant mass, the decay angle, … etc.) Probes of Electroweak Symmetry Breaking at LEP and SLC

  24. Signal vs Background (II) Combine b-tagging and kinematics in a single Neural Network / Likelihood / … : 103 102 10 1 Data e+e- Z Z e+e- qq e+e- W+W- Higgs Signal (mH = 100 GeV/c2) Backgrounds Higgs Signal (mH = 115 GeV/c2) In each bin, one can define the probability for an event i to come from signal(si) or from background(bi), and the event weight wi = si/bi 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 GLOBAL NEURAL NETWORK OUTPUT Probes of Electroweak Symmetry Breaking at LEP and SLC

  25. Signal vsBackground (III) N • Overall Likelihood of a given event sample: Q =(sibi)/bi; • Larger in presence of signal; • Negative Log-Likelihood L = -2 Log Q(Smaller in presence of signal). i = 1 Signal + Background Background only What if mH 115 GeV/c2? 100 GeV/c2 (Expected) 107 GeV/c2 115 GeV/c2 Signal? (Expected) Four Experiments s = 206 GeV One Experiment Probes of Electroweak Symmetry Breaking at LEP and SLC

  26. LEP Optimization: Luminosity or Energy ? Higgs mass 3s sensitivity = f(Lumi, E) • A typical (and realistic) example: • Beam Energy: 102 GeV; • 200 pb-1 / experiment. • The 3s sensitivity of the Higgs boson • search is about112 GeV/c2,i.e., only • 1 GeV/c2 away from the “kinematic • threshold” mH = s – mZ. • To gain 2 GeV/c2 of sensitivity, two • possibilities : • Increase the beam energy by 1 GeV; • Multiply the luminosity by 4. mH = s - mZ 1 GeV of beam energy (feasible)  A factor of 4 in luminosity (just a dream) Probes of Electroweak Symmetry Breaking at LEP and SLC

  27. 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. Probes of Electroweak Symmetry Breaking at LEP and SLC

  28. LEP Improvements in 1999/2000 1) Increase RF Gradient & Upgrade Cryogenics 200 GeV: Mean Nb/Cu 7.0 MV/m • 272 Nb/Cu cavities in 1998; • 2850 MV available, 189 GeV • 288 Nb/Cu cavities in 1999; • 3000 MV available, 192 GeV • Condition all cavities, damp the • oscillations, install part of LHC • cryogenics, improve the phasing… • 3500 MV available (end 1999) • 3650 MV available (2000) 192 GeV: Mean Nb/Cu 6.0 MV/m 204 GeV: Mean Nb/Cu 7.5 MV/m Design: 5.5 MV/m E: 192  200  204 GeV; mH: 100  108  112 GeV/c2 Distributions of all Nb/Cu cavity gradient (MV/m) Probes of Electroweak Symmetry Breaking at LEP and SLC

  29. Improvements in 1999/2000 (Cont’d) 2) Improve stability & Decrease security margin 3s sensitivity optimization with 0 or 1 miniramp 1 Start with a margin of • Two- to one-klystron margin • (Fill duration 2h30  1h30): 2 klystron 0 mH 3s Sensitivity (GeV/c2) E: 204  205.5 GeV; mH: 112  113 GeV/c2 Turnaround: 45mins • Mini-ramp to no margin at all • (Fill duration 15 minutes!) • Turnaround time reduced to 45 mins: Turnaround: 1h30 No Miniramp. Turnaround 1h30 E: 205.5  207 GeV; mH: 113  114 GeV/c2 Starting Energy (GeV) Probes of Electroweak Symmetry Breaking at LEP and SLC

  30. Improvements in 1999/2000 (Cont’d) 3) Re-install 8 Cu cavities 5) Decrease the RF frequency Adds 30 MV (0.8% of total RF Voltage) f = 0 Hz E: 207  207.4 GeV; mH: 114  114.25 GeV/c2 More dipolar magnetic field seen in the quadrupoles ! LEP: f=350 MHz 4) Use orbit correctors as magnetic dipoles f = -50 Hz Increases LEP radius ! But … 10-20% luminosity reduction E: 207.4  207.8 GeV; mH: 114.25  114.5 GeV/c2 E: 207.8  209.2 GeV; mH: 114.5  115.1 GeV/c2 Probes of Electroweak Symmetry Breaking at LEP and SLC

  31. Improvements in 1999/2000: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. Probes of Electroweak Symmetry Breaking at LEP and SLC

  32. 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. Probes of Electroweak Symmetry Breaking at LEP and SLC

  33. 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 Probes of Electroweak Symmetry Breaking at LEP and SLC

  34. 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 Probes of Electroweak Symmetry Breaking at LEP and SLC

  35. A Few Consistency Checks (I) Comparison of the four channels: Comparison of the four experiments: (115 GeV/c2) Good consistency with s+b according to expected separation; Not a “4-jet excess”, but a Higgs excess Good consistency with s+b distribution; Not an “ALEPH excess”, but a LEP excess Probes of Electroweak Symmetry Breaking at LEP and SLC

  36. A Few Consistency Checks (II) 0.3 Distribution of s/b for the whole data sample: Excess visible all the way down to ~ 0.1 Cutting tighter or looser on s/b: 0.3 Good Consistency with Signal+Background Hypothesis For Any Purity Cut Not a “cut-around-the data” effect Probes of Electroweak Symmetry Breaking at LEP and SLC

  37. A Few Consistency Checks (III) • 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 Probes of Electroweak Symmetry Breaking at LEP and SLC

  38. A Few Consistency Checks (IV) Question: Why is there a 2s excess all the way down to 100-105 GeV Answer: It is as expected for a 115 GeV/c2 Higgs signal because of mass resolution. Question: Is the background understood at the kinematic limit (115+91 = 206 GeV)? Answer: Let’s have a look at lower energy data (500 pb-1 between 183 and 205 GeV) Observed Observed below 205 GeV Expected with a 115 GeV/c2 Higgs What would have been observed if the excess above 205 GeV were due to a systematic effect !!! Probes of Electroweak Symmetry Breaking at LEP and SLC

  39. 2001: A Spoilt Odyssey 2001 expected mass spectrum: (S/B = 2.0 around 115 GeV/c2) • With six more months in 2001; • With a integrated luminosity of 200 pb-1; • With an energy of 208.5  210 GeV; • (made possible with add’l cavities and a few tricks) The 3s evidence could have been turned into A 5.5 s discovery +0.6 -0.9 Background subtracted: 2000 observed mass spectrum: ~28 signal events Probes of Electroweak Symmetry Breaking at LEP and SLC

  40. Ongoing Combination (still preliminary) (3%) mH = 115.6 GeV/c2 + 0.8 1) 2) DELPHI data being reprocessed (deficit?) 3) L3 final, ALEPH and OPAL quasi-final 4) ALEPH, L3 and OPAL (all in excess with respect to the background) combination still leads to a 2.9s excess. - 0.7 (1-CLb = 0.3%) Probes of Electroweak Symmetry Breaking at LEP and SLC

  41. Conclusions of the 2nd Lecture After 12 years of outstanding Physics at LEP and SLC: • Precision electroweak measurements: • Direct Searches (~2-3s effect) mH = 108 GeV/c2 15 GeV/c2 with 5 more years (two at the Z pole, three at high energy) + 57 - 38 Impressive Consistency ! mH = 115.6 GeV/c2 + 0.8 Could have been confirmed in 2001 DmH = 100 MeV/c2 with three years - 0.7 About 5-10 years needed for a confirmation • Lots of upgrades still to be done to reach 15 fb-1 in 2007 at the Tevatron; • Lots of things still to be done to make LHC start in 2007; • The end of the decade might be hot. Probes of Electroweak Symmetry Breaking at LEP and SLC

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