Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 1/25 Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 Dezső Horváth MTA KFKI Research Institute for Particle and Nuclear Physics, Budapest and Institute of Nuclear Research, Debrecen
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 2/25 The SM Higgs boson The Higgs mechanism of the Standard Model 1 complex doublet field 4 degrees of freedom 3 masses (W ±, Z) + H V min at φ = v 246 GeV = vacuum expectation value Spin = 0, neutral, heavy particle, no quantum numbers ( 0 v ) Does it exist at all? According to theory it must exist as it creates mass (for the leptons, quarks and weak bosons) and helps to eliminate divergencies of the SM
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 3/25 pontaneous symmetry breaking mass Free fermion Higgs boson David J. Miller and CERN: http://www.hep.ucl.ac.uk/ djm/higgsa.html
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 4/25 The SM Higgs boson SM gives its production and decay probabilities, but not its mass. Everything depends on mass, e.g. for its decay to a fermion pair Γ(H ff) = N cg 2 m 2 f β 3 m 32πm 2 H prefers heavy particles W N c : nr. of colours (leptons: 1; quarks: 3) g 2 0.425 coupling constant β 2 = 1 4 m2 f m 2 H fermion velocity (c = 1). SM does not predict H mass, just limits 30 GeV < < 500 GeV (unitarity) If SM works up to E GUT 10 16 GeV: 130 GeV < < 190 GeV (GUT: Grand Unification Theory)
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 5/25 What is measured: cross section Bombarding particle beam Target σ = W/Φ transition probability/flux Unit: 1 barn = 10 28 m 2 (1 pb = 10 40 m 2 ) Flux = particle density velocity in beam: Φ = n b v b = particles/surface/sec
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 6/25 What is observed: resonance τ = Γ 1 lifetime exp. decay: N(t) = N 0 e Γt Probability distribution: χ(e) 2 = 1 (E M) 2 +Γ 2 /4 Breit-Wigner equation M Γ resonance centre width ( = 1, c = 1) Lorentz curve New particle discovery: resonance at decay energy corresponding to the particle mass
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 7/25 Where is the Higgs boson? By-product of spontaneous symmetry breaking of the SM Most wanted particle of physics as the only missing piece of the Standard Model. Experimentally not (yet?) observed, LEP: M(H) > 114.4 GeV It was in 1972... that my life as a boson really began Peter Higgs: My Life as a Boson: The Story of The Higgs, Int. J. Mod. Phys. A 17 Suppl. (2002) 86-88.
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 8/25 Decay of SM Higgs boson Too large H mass too large width field distribution in negative mass region unitarity problem
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 9/25 SM fit Higgs mass (2005!) 10 3 10 3 10 3 10 3 10 2 10 2 10 3 2.49 2.5 Γ Z 10 3 41.4 41.5 41.6 σ 0 had [nb] 10 2 10 2 10 3 0.137 0.147 0.157 A l (A 0,l FB) 0.096 0.102 0.108 A 0,b FB 10 3 10 2 10 2 10 2 10 2 20.7 20.75 20.8 R 0 l 10 3 0.014 0.017 0.02 A 0,l FB 10 3 0.137 0.147 0.157 A l (P τ ) 0.06 0.07 0.08 A 0,c FB Measurement Measurement α (5) had = 0.02758 ± 0.00035 α s = 0.118 ± 0.003 m t = 178.0 ± 4.3 GeV α (5) had = 0.02758 ± 0.00035 α s = 0.118 ± 0.003 m t = 178.0 ± 4.3 GeV 10 2 10 2 1.994 2 2.006 σ 0 lep [nb] 0.137 0.147 0.157 A l (SLD)
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 10/25 What is the mass of the Higgs boson? Theoretical estimations Collection of Zoltán Trócsányi (Debrecen University)
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 11/25 Hunting the Higgs boson Compose a complete SM background using Monte Carlo simulation taking all types of possible events normalized to their cross-sections. Higgs signal: simulation of all possible production and decay processes with all possible Higgs-boson masses Put all these through the detector simulation to get events analogous to the measured ones. Optimize the event selection: reduce background, enhance signal E.g. signal/sqrt(background) = max. Calculate at experimental luminosity expected nr. of events for signal and background at various conditions. SM background experiment? (YES / NO ).
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 12/25 Search for the Higgs boson Is the experiment signal-like? Compare properties of experimental events with simulation, get probabilities of signal- and background-likeness. Sum up for all events: whole experiment is how much signal-like or background-like. Combine the results of various measurements. Many candidate events signal-like; few ones background-like. Statistical (physical/philosophical) problem: What if there are fewer observed events than the expected background? And what if more than the expected background + signal?
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 13/25 σ (pb) 10 5 10 4 10 3 10 2 10 1 10-1 10-2 10-3 Higgs-strahlung SM prediction: Dominant formation at LEP: e + e Z 0 H 0 Dominant decay ( 0 < 2M W ±): H 0 bb (83 %) e + e - HZ e + e - hadrons Born e + e - µ + µ - 60 WW ZZ m h = 70 GeV 90 110 60 80 100 120 140 160 180 200 σ(e + e ZH) s Branching Ratio 1.5.2.1.05.02.01 bb ττ gg cc WW ZZ 100 200 300 500 0 Branching ratio: H XX tt
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 14/25 Higgs search at LEP: final states Z decay H decay channel fraction Z qq H 0 bb qqbb 61% Z νν H 0 bb bb + E mis 17% Z e + e H 0 bb bb + e + e 3% Z µ + µ H 0 bb bb + µ + µ 3% Z τ + τ H 0 bb bb + τ + τ 3% Z qq H 0 τ + τ bb + τ + τ 5% Total covered 92%
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 15/25 Identifying the b quark Cascade decay preferred: b W c; c W d; W lν or qq long lifetime secondary vertex, soft leptons
ALEPH event (e + e HZ bbqq) Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 16/25
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 17/25 The Higgs boson is not seen 1-CL b 1 10-1 10-2 10-3 10-4 10-5 ALEPH Observed Expected signal+background Expected background 100 102 104 106 108 110 112 114 116 118 120 (GeV/c 2 ) 2σ 3σ 4σ 1-CL b 1 10-1 10-2 10-3 10-4 10-5 DLO Observed Expected signal+background Expected background 100 102 104 106 108 110 112 114 116 118 120 (GeV/c 2 ) 2σ 3σ 4σ Expected and observed signal confidence level assuming background only (ALEPH, DELPHI, L3 and OPAL: Phys. Lett. B 565 (2003) 61-75.) Excess in ALEPH s 4-jet events at 115 GeV: E LEP2000 = 206 GeV (115) + m Z (91) = 206 GeV!!
L3 event (e + e HZ bbνν) Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 18/25
LEP: exclusion by experiment Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 19/25
LEP: exclusion by channel Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 20/25
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 21/25 Sensitivity of SM to Higgs mass SM fit (all data, 2009): +36 M H = 90 27 GeV Lower limit (uncertainty of theory and expt): M H > 163 GeV (95 % CL) LEP search: M H > 114.4 GeV Tevatron (2009): excl. 160 170 GeV If normalized to allowed region: 114.4 < M H < 191 GeV (95% CL) χ 2 6 5 4 March 2009 Theory uncertainty α had = α (5) 0.02758±0.00035 0.02749±0.00012 incl. low Q 2 data 3 2 1 Excluded Preliminary 0 30 100 300 m Limit = 163 GeV LEP Electroweak Working Group (http://lepewwg.web.cern.ch/)
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 22/25 SM fit Higgs mass: 2006 2007 6 5 4 Theory uncertainty α (5) α had = 0.02758±0.00035 0.02749±0.00012 incl. low Q 2 data 6 5 4 Theory uncertainty α had = α (5) 0.02758±0.00035 0.02749±0.00012 incl. low Q 2 data m Limit = 144 GeV χ 2 3 χ 2 3 2 2 1 1 Excluded Excluded Preliminary 0 0 30 100 300 30 100 300 2006: m t = 174 GeV 2007: m t = 172 GeV 2009: m t = 173 GeV LHC!!
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 23/25 SM Higgs: M H M t 200 March 2009 200 High Q 2 except m t 68% CL High Q 2 except m t 68% CL m t 180 m t (Tevatron) m t 180 m t (Tevatron) 160 Excluded 10 10 2 10 3 160 Excluded 10 10 2 10 3 evatron, 2004: m t = 178 GeV 2009: m t = 173 GeV
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 24/25 SM Higgs: M H M W 80.5 High Q 2 except m W /Γ W 68% CL 80.5 March 2009 High Q 2 except m W /Γ W 68% CL m W 80.4 m W (LEP2 prel., pp ) m W 80.4 m W (LEP2, Tevatron) 80.3 Excluded 10 10 2 10 3 80.3 Excluded 10 10 2 10 3 2005 LHC, 2009...: top physics!! 2009
Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 07.07.2009 p. 25/25 Acknowledgement National Office for Research and Development OTKA NK67974, K72172 and H07C-74153 EU FP6 MC-ToK 509252 and III 031688 Nice collaborating partners