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Southampton IOP: The Higgs discovered at LEP

Southampton IOP: The Higgs discovered at LEP. W. J. Murray RAL. Why do we need the Higgs?. Fermions. Gauge Symmetries. Bosons, Interactions. families, with leptons and quarks. U(1) Y : SU(2) L : SU(3) c :.  : QED Z, W  : Weak gluons : QCD. families, with leptons

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Southampton IOP: The Higgs discovered at LEP

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  1. Southampton IOP:The Higgs discovered at LEP W. J. Murray RAL

  2. Why do we need the Higgs? Fermions Gauge Symmetries Bosons, Interactions families, with leptons and quarks U(1)Y : SU(2)L: SU(3)c:  : QED Z, W : Weak gluons : QCD families, with leptons and quarks families, with leptons and quarks A mass term couples L & R and would violate SU(2)L solution: The Higgs Mechanism

  3. What is the Higgs mechanism? • Doublet of SU(2)L,F=(F1,F2) • Potential repects SU(2)L • But Vacuum does not! Fermions: Interact with Higgs field slows them down  generates mass 3 degress of freedom in Boson masses 4th becomes fundamental scalar Bosons: SU(2)L interact, gain mass U(1)g and SU(3)c do not, massless

  4. What does it predict? m = 0 mZ = mW/cosqW Direct consequences of the Higgs mechanism We can test them Tree level Z couplings: axial: a=±1/2 vector: v=a(1-4|Q|sin2qW) But the Higgs mass is not predicted

  5. Loop effect sensitive to Higgs Propagator corrections Vertex corrections Only sensitive to mtop 0.1% Precision needed! with (mZ = mW/cosW)

  6. Precision Electroweak Observables Asymmetries Z Lineshape Zm+m- mZ Dr sin2W mtop mW LEP1 eff - W+W- production Rate of Z  bb … to be compared to direct measurements of mW (LEP2) mtop (FNAL) • Find r=1+ from LEP/SLD data; • Predict mtop and mW; • Compare with direct measurements; • Predict mH; • Compare with direct measurements. LEP2

  7. Final state identification: Z - e) Z->nn - Z  nn: Not detectable. (weak and slow decays to lighter quarks) - - Z  bb:Like qq events, with detached vertices, measured in accurate vertex detectors - Z  qq:Two jets, large particle multiplicity. Z  e+e-, m+m-:Two charged particles (e or .) Z  t+t-: Two low multiplicity jets + missing energy carried by the decay neutrinos.

  8. Final state identification: W+W- - - - - W+W- q1q2ln: Two hadronic jets, One lepton, missing energy. - W+W- l1n1l2n2: Two leptons, missing energy W+W- q1q2q3q4: Four well separated jets.

  9. Did LEP improve measurements? v & a after LEP and SLD v & a before LEP and SLD eff sin2W Errors 10

  10. Compare with SM, technicolour sin2qW and r, leptons Discovery of the Higgs

  11. Fit of mtop and mW mW and mtop direct measurements Values agree! mZ,  and sin2W measurements Confirms Higgs Mechanism

  12. Comparison with SM and SUSY Light SUSY Heavy SUSY SM SUSY assumes Higgs mechanism

  13. Global Fit of SM: mH Prediction (with BES) (+ pQCD) 500 20 Satisfactory internal consistency? 4% probability From Dr and mtop: mH = 118 GeV/c2 + 63 - 42

  14. Closing in on the Higgs! Bayesian EW fits assume a Higgs Search looks for one Frequentist After A. Wagner, ICHEP 2000

  15. Direct Searches at LEP 1 • Great effort - which I have no time to describe • Many modes: • Stable,gg,ee,mm,pp,tt,bb • Clean Z decays (ll, nn) used • Prior to LEP only some patchy constraints • The mass range to 0 now excluded, no holes. Events expected at LEP1 0.0  mH 65 GeV/c2 Excluded at 95% C.L.

  16. LEP Higgs production: HZ Total events, 4 experiments `Higgstrahlung’ Ecms-mz `WW fusion’ no cut-off

  17. LEP Higgs channels Higgs decays Search channels

  18. Beam energy 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

  19. LEP machine in 2000 `Higgs discovery mode’ Results exclude last week or two of data

  20. First pb-1’s above 206 GeV: _ _ 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.

  21. Some 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

  22. 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 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) In Hqq: 9 (70%) In Hnn: 3 (20%) In Hl+l-: 1 (7%) In Htt: 1 (3%) Number of events in each Z decay compatible with HZ predictions Values still PRELIMINARY

  23. LEP mass distributions Little visible with loose selections Something appears with medium ones Tight cuts give reasonable agreement with 115GeV Higgs

  24. Signal vs Background 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

  25. How is significance assessed? • Maximum likelihood fit to observed distribution • Most channels work in 2D • Each bin needs signal and background estimates, from simulation, dependent upon Ecms, channel etc. `Qi’ • This is a weighted sum of events. • L is compared with distributions expected for background and signal to quantify probabilities

  26. LEP signal to background Only 3 events have s/b greater than 1 for MH=115 Was 4 for MH=114: cross-section dropped Most events buried under background

  27. LEP log-likelihood -2*ll~c2 minimum at 115GeV/c2 Probability of fluctuation: 4 in 1000 Rate median for signal

  28. Results by experiment Experiments scatter reasonably in the signal distribution

  29. Results by Physics channel Distribution by decay mode very good!

  30. More consistency checks: New data Tail at low mass Yes! Yes!

  31. What if you remove best results? ALEPH removed Yes, consistent, but not strong 4-jets removed

  32. The roadmap of 2000 At Sept 5th LEP seminar, we predicted 70pb-1 would give a 3s evidence for a MH=115 Actually, 55pb-1 gave 2.9s

  33. What did 2001 bring? • Six more months in 2001; • gave an integrated luminosity of 200 pb-1; • With an energy of 208.5  210 GeV; • (made possible with add’l cavities and a few tricks) Expected mass spectrum: The 3s evidence was turned into A 5.5s discovery Background subtracted: ~28 signal events

  34. What did 2001 REALLY bring? LEP had run its course LHC is sure to find a Higgs... DELPHI 2001

  35. Next-in-Line:The TEVATRON - • The Tevatron: • A pp collider, near Chicago; • Center-of-mass energy: 2 TeV; • Luminosity in Run I: 0.1 fb-1; • Two multi-purpose detectors. pp collider - s = 2 TeV D0: CDF: • Run II is starting • NOW: • 2 fb-1 in 2002; • 5 fb-1 in 2004; • 15 fb-1 in 2007.

  36. Tevatron Higgs Potential Same process as at LEP: 5s in 2007 3s in 2004 Produced: 50 events / fb-1 2s in 2002 - WH  bbln • Should confirm LEP hints; • Much more difficult • environment; • Somewhat later; • (3s from 2004 to 2007); • No detailed studies of the • Higgs mechanism… • Difficult above 115 GeV/c2. 10 fb-1 mH=120 GeV

  37. LHC Discovery Potential ATLAS H  ZZ eeee ATLAS H   CMS H  ZZ mmmm CMS +ATLAS • 115 GeV not too good; • 2007 may be very hot! • But… • LHC will cover the • whole mass range • in a year (5s) or in • a month (95% C.L.) 100 fb-1: 2010? 5s 10 fb-1: 2007?

  38. b g g IVB Fusion Discovery Potentialfor mH = 115 GeV/c2 - - - ttH  ttbb: ATLAS and CMS gg  H  gg: • 50 events / fb-1; • Branching Ratio 10-3. - b Minutes of 56th LEPC, 3rd November 2000 `The committee noted that there is unfortunately no single channel that is background-free.’ ATLAS 100 fb-1 ATLAS 100 fb-1 mH=120 GeV/c2 mH=120 GeV/c2 • s/b ~ 1/10 - 1/100: No detailed study of EWSB; • H  gg does not really test the Higgs mechanism; Equally Promising: gg

  39. LHC Potential: Supersymmetry 10fb-1 (End 2007) covers most of plane Some of it only by observing OTHER higgses

  40. Precision:e+e- Linear Colliders Similar to LEP, but s ~ 350-500 GeV, and L  1000 1) Fast Discovery (or confirmation) 2) Precision Measurements of all BR’s Precision Tests of the Higgs Mechanism Predictions At 115 GeV: 5s confirmation in half-a-day ! A week See D. Miller

  41. Precision Higgs Physicsmm Collider MUONS are heavy Large couplings to Higgs bosons; Very good energy resolution. 4 MW mA = 400 GeV GH = 4.0 MeV , W- s ~ mH mA = 300 GeV GH = 4.7 MeV - , W+ Higgs Lineshape Measurements! Standard Model GH = 3.2 MeV 4% 1% Statistics limited !

  42. Scan the H and A Resonances dE/E = 0 s ~ mA, mH tanb = 10 - dE/E = 3 10-4 H, A tanb = 8 s(m+m- H,A  bb), pb - mA = 400 GeV/c2 mh = 115 GeV/c2 mSUSY = 1 TeV/c2. One week of running tanb = 6 Background level • Determine mH, mA, and tanb to an excellent accuracy; • Fit for e.g., stop masses (mSUSY) and mixing (At, m). • Start precision tests of SUSY breaking through rad. corr. • to masses and widths; ( LEP for standard model and EWSB) • Tests of CP violation with muon polarization

  43. The Higgs discovered at LEP After 12 years of outstanding Physics: • Precision electroweak measurements DEMAND a Higgs: • MW agrees with Higgs predictions to 1 per mille, • Mtop agreement to 10% • Direct Searches (~3s effect) mH = 118 GeV/c2 + 63 - 42 No Need for Higgs Boson in the data? mW = mZ cosW is just a coincidence? mH = 115.0 GeV/c2 + 0.7 Just a statistical fluke? - 0.3 In about 5 years, Tevatron/LHC should decide Support your local linear Collider More Precision Measurements with Lepton Colliders will follow after 2010

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