How To Find The Higgs Boson

Transcription

1 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 1/25 Search for Higgs bosons Balaton Summer School, Balatongyörök, Dezső Horváth MTA KFKI Research Institute for Particle and Nuclear Physics, Budapest and Institute of Nuclear Research, Debrecen

2 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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

3 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 3/25 pontaneous symmetry breaking mass Free fermion Higgs boson David J. Miller and CERN: djm/higgsa.html

4 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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 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 GeV: 130 GeV < < 190 GeV (GUT: Grand Unification Theory)

5 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 5/25 What is measured: cross section Bombarding particle beam Target σ = W/Φ transition probability/flux Unit: 1 barn = m 2 (1 pb = m 2 ) Flux = particle density velocity in beam: Φ = n b v b = particles/surface/sec

6 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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

7 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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) > GeV It was in 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)

8 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 8/25 Decay of SM Higgs boson Too large H mass too large width field distribution in negative mass region unitarity problem

9 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 9/25 SM fit Higgs mass (2005!) Γ Z σ 0 had [nb] A l (A 0,l FB) A 0,b FB R 0 l A 0,l FB A l (P τ ) A 0,c FB Measurement Measurement α (5) had = ± α s = ± m t = ± 4.3 GeV α (5) had = ± α s = ± m t = ± 4.3 GeV σ 0 lep [nb] A l (SLD)

10 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 10/25 What is the mass of the Higgs boson? Theoretical estimations Collection of Zoltán Trócsányi (Debrecen University)

11 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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 ).

12 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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?

13 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 13/25 σ (pb) 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 σ(e + e ZH) s Branching Ratio bb ττ gg cc WW ZZ Branching ratio: H XX tt

14 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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%

15 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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

16 ALEPH event (e + e HZ bbqq) Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 16/25

17 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 17/25 The Higgs boson is not seen 1-CL b ALEPH Observed Expected signal+background Expected background (GeV/c 2 ) 2σ 3σ 4σ 1-CL b DLO Observed Expected signal+background Expected background (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) ) Excess in ALEPH s 4-jet events at 115 GeV: E LEP2000 = 206 GeV (115) + m Z (91) = 206 GeV!!

18 L3 event (e + e HZ bbνν) Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 18/25

19 LEP: exclusion by experiment Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 19/25

20 LEP: exclusion by channel Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 20/25

21 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 21/25 Sensitivity of SM to Higgs mass SM fit (all data, 2009): +36 M H = GeV Lower limit (uncertainty of theory and expt): M H > 163 GeV (95 % CL) LEP search: M H > GeV Tevatron (2009): excl GeV If normalized to allowed region: < M H < 191 GeV (95% CL) χ March 2009 Theory uncertainty α had = α (5) ± ± incl. low Q 2 data Excluded Preliminary m Limit = 163 GeV LEP Electroweak Working Group (

22 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 22/25 SM fit Higgs mass: Theory uncertainty α (5) α had = ± ± incl. low Q 2 data Theory uncertainty α had = α (5) ± ± incl. low Q 2 data m Limit = 144 GeV χ 2 3 χ Excluded Excluded Preliminary : m t = 174 GeV 2007: m t = 172 GeV 2009: m t = 173 GeV LHC!!

23 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 23/25 SM Higgs: M H M t 200 March 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 Excluded evatron, 2004: m t = 178 GeV 2009: m t = 173 GeV

24 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, 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 Excluded LHC, : top physics!! 2009

25 Dezső Horváth: Search for Higgs bosons Balaton Summer School, Balatongyörök, p. 25/25 Acknowledgement National Office for Research and Development OTKA NK67974, K72172 and H07C EU FP6 MC-ToK and III Nice collaborating partners