The search for the (SM) Higgs Boson

Transcription

1 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 1 Lecture 9: The search for the (SM) Higgs Boson theoretical basics Higgs production and decay Higgs search in e + e annihilation direct search indirect mass limits from electroweak radiation corrections Higgs searches in hadron collisions Tevatron LHC (-> next lecture )

2 The Standard Model Higgs Boson: theoretical basics and expectations gauge field theory with gauge symmetry in weak isospin/hyper charge [SU(2) x U(1) ] to describe electromagnetic and weak interactions of quarks and leptons: includes massless gauge bosons (γ, Z 0, W +, W ) and fermions any attempt to include mass terms breaks gauge symmetry and destroys renormalizabilty of the theories Englert, Brout and Higgs (1964): spontaneous symmetry breaking (generates mass, keeps renormalizabilty): introduction of complex SU(2) doublets of scalar fields with a potential # of V(φ) = λ (φ + φ) 2 - μ 2 φ + φ ; with λ, μ 2 > 0 ; φ = φ 1 +iφ 2 & % ( $ φ 3 +iφ 4 ' V does not have minimum at φ = 0, but at φ = µ2 2λ v 2 3 of the 4 real degrees of freedom are used to generate the longitudinal spin d.o.f. of Z 0 and W ± ; 4. d.o.f. > physical Higgs particle! Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 2

3 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 3 theoretical basis and expectations inserting φ in Lagrange function results in 3 massive vector fields, 1 massless vectorfield, plus one massive scalar field with M W = 1 gv v = 246 GeV 2 M Z = M W /cosθ w (g = e/sinθ w ) M γ = 0 M H = 2µ 2 = 2λv 2 introduction of Yukawa-couplings g f between φ and the fermion fields: generates fermion masses m f = g f v/ 2 fundamental fermion-higgs couplings: f f H g ff H = e m f 2M w sinθ w W +, Z 0 W, Z 0 H g WWH = e M w sinθ w g ZZH = e M Z sinθ w cos θ w

4 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 4 theoretical basis and expectations theoretical bounds for M H from self-consistency arguments of the Standard-Model: upper bounds: perturbativity lower bounds: vacuum stability n.b.: if SM is valid only up to Λ = O(1 TeV), then M H = GeV n.b.: if SM is valid up to Λ = O(M Planck ) then M H ~ GeV Λ: energy scale up to which SM is valid

5 Higgs: production and decays e e + Z * Z H q q W (Z) W (Z) q' H q' Higgs-radiation W- (Z-) fusion g g Gluon - Fusion Higgs-decay: t H q Z,W Z *,W * q H Higgs-radiation ( associate production ) q, l +, W +,Z 0 predominantly into heaviest, kinematically accessible pair of leptons or bosons Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 5 H q, l, W,Z 0

6 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 6 Higgs: decays Br(H γγ) ~ 10-3 M H < ~135 GeV: dominanter Zerfallskanal H > b b M H > ~135 GeV: dominanter Zerfallskanal H > W + W -

7 Higgs: decays Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 7

8 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 8 Higgs: production e + e annihilation

9 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 9 Higgs: production Standard Model Higgs Tevatron cross section (pb)

10 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 10 Higgs: production Standard Model Higgs LHC (7 TeV)

11 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) Higgs: production Standard Model Higgs LHC (14 TeV) cross section (pb)

12 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 12 Higgs production cross-sections 10 9 proton - (anti)proton cross sections ! tot Tevatron LHC !!!!"nb# ! b! jet (E T jet > "s/20)! W!Z! jet (E T jet > 100 GeV)! t events / sec for L = cm -2 s ! Higgs (M H =120 GeV) WJS GeV 500 GeV "s (TeV)

13 Higgs search Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 13

14 Higgs-search in e + e annihilation: direct production: decay channel (e+e > HZ): background: e e + Z * Z H e + e W + W ν H ν search includes ~ 80% of all final states with ~ 40-50% selection efficiency Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 14

15 Higgs-search in e + e annihilation: direct L3: Candidate event for ee --> HZ --> eeqq LEP: from direct search M H > GeV Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 15

16 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 16 Higgs-search in e + e annihilation: indirect radiation corrections in SM: photonic corrections: γ corrections ~ 100%, selection dependent; factorisable: (1 + δ rad )! non-photonic corrections: f + Z + W,Z + + H g +... sin 2 θ eff (s) $ N c,f 1+ α s π +1.4 α s & π % ( ) M W 2 M Z 2 = ρ cos2 θ w mit ρ = 1 corrections ~ 10%, selection independent; can be absorbed into running coupling constants: α α(s) = 1 Δα ; Δα = bei s = M Z ' ) (für Quarks) ( 1-Δρ ; Δρ = M 2 t 2 M ln ' M * H ), z ( + M w

17 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 17 Higgs-search in e + e annihilation: indirect insertion of running couplings into Born -approximation : partial decay widths of Z: Γ f = G 3 f M z 6π 2 g a,f [ gv,f ] (and thus, also the cross sections) become dependent on: M t M H α s ==> indirect determination (fit) of M t, M H, und α s from combination of all available electro-weak observables (differential cross sections, partial decay widths, forwardbackward asymmetries, τ-polarisation, ) g a, f = I 3, f (3. Komponente schw. Isospin; = ±1/2) g v,f = I 3, f 2Qsin 2 θ w

18 indirect (adjusting radiative corrections): M H = GeV (68% c.l.) Higgs-search in e + e annihilation: indirect July 2008 Theory uncertainty α (5) α had = ± ± incl. low Q 2 data m Limit = 154 GeV χ 2 3 direct searches (exclusion) M H > GeV 2 1 Excluded Preliminary m H [GeV] GeV < M H < 154 GeV (1-sided 95% c.l.) M H < 185 GeV n.b.: at the end of LEP (2000), indication for few events with M H ~ 115 GeV (~2.3 std. dev.) Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) (incl. 114 GeV lower limit) 18

19 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 19 Higgs-Search at Hadron colliders: Tevatron M H < ~135 GeV: dominant decay H > b b (~90%) H > τ + τ - (~ 8%) M H > ~135 GeV: dominant decay H > W + W - hadron collider: b b background from QCD processes dominates; irreducible; => g g > H > b b cannot be used therefore: cross section (pb) focus on associate production (ZH, WH) and analyse e.g. Z > l + l ; H > bb ττ decay suitable for all production channels

20 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) Higgs-Search at Hadron colliders: Tevatron example: M H = 120 GeV and 30 fb -1 (model study!) very difficult measurement; background must be known extremely well!

21 (delivered) Tevatron was shut down on Sept. 29, 2011, after 26 years of colliding p and p Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 21

22 definition of: significance of signal Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 22

23 Expected Tevatron sensititvity Tevatron at end-of-run (Sept. 2011): < 12 fb -1 / experiment > exclusion (3 σ) expected: und GeV; 5 σ discovery possible to be reached at GeV. Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 23

24 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 24

25 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 25

26 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 26

27 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 27

28 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) CDF/D0 combined conclusion (July 2012): SM Higgs exclusion in the range (and ) CL Expected exclusion range GeV 2.5 σ excess in region GeV (3.0 σ at M H =125 GeV) observed signal strength:

29 Summary SM Higgs-search w/o LHC data (2012): Tevatron and und LHC WS16/17 WS12/13 TUM S.Bethke, F. Simon V9: V10: Search Higgs for Suche the Higgs Boson (1) 29

30 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 30 Higgs-search and discovery at LHC (preview): (see next lecture; )

31 Tevatron and LHC WS16/17 TUM S.Bethke, F. Simon V9: Search for the Higgs Boson (1) 31 Literatur: Higgs Particle: The Origin of Mass. Yasuhiro Okada (KEK, Tsukuba & Tsukuba, Graduate U. Adv. Studies). KEK-TH-1171, Aug pp. e-print: arxiv: [hep-ph] Higgs Boson Properties in the Standard Model and its Supersymmetric Extensions. John R. Ellis (CERN), Giovanni Ridolfi (INFN, Genoa & Genoa U.), Fabio Zwirner (Padua U. & INFN, Padua). CERN-PH-TH , Feb Published in Comptes Rendus Physique 8: ,2007. e-print: hep-ph/ The Tevatron Higgs exclusion limits and theoretical uncertainties: a critical appraisal. J. Baglio, A. Djouadi, S. Ferrag, R.M. Godbole,. CERN-PH-TH , LPT-ORSAY , Jan pp. Temporary entry e-print: arxiv: [hep-ph] Updated Combination of CDF and D0 Searches for Standard Model Higgs Boson Production with up to 10.0 fb-1 of Data,