Search for the Higgs Boson at the LHC. Karl Jakobs Physikalisches Institut Universität Freiburg

Transcription

1 Search for the Higgs Boson at the LHC Karl Jakobs Physikalisches Institut Universität Freiburg

2 Outline of the talk:. Introduction, the Higgs boson in the Standard Model. The Standard Model Higgs boson search - ATLAS and CMS results in various channels (with some focus on H à WW à lν lν) - The combination of searches 3. Prospects and outlook Disclaimer: I will try to highlight the most important results on searches for the Higgs boson. The coverage is not complete, i.e. not all results available will be presented.

3 The Standard Model of Particle Physics γ (i) Constituents of matter: quarks and leptons (ii) Four fundamental forces (described by quantum field theories, except gravitation) (iii) The Higgs field (problem of mass)

4 Why do we need the Higgs Boson? The Higgs boson enters the Standard Model to solve two fundamental problems: Masses of the vector bosons W and Z: Experimental results: M W = ± 0.03 GeV / c M Z = ± 0.00 GeV / c A local gauge invariant theory requires massless gauge fields Divergences in the theory (scattering of W bosons) + + s im( W W W W ) ~ M W for s

5 The Higgs mechanism Spontaneous breaking of the SU() x U() gauge symmetry Scalar fields are introduced φ = + φ + iφ φ = 0 φ3 + iφ4 φ Potential : V (") = µ (" *") + #(" *") For µ < 0, λ > 0, minimum of potential: " + " + " 3 + " 4 = v v = #µ /$ Perturbation theory around ground state: 0 3 massive vector fields: φ0( x) = v + h( x) massless vector field: m W ± = vg m Z = m W cos! W m! = 0 massive scalar field: The Higgs boson H m H =!v v = vacuum expectation value

6 The Higgs mechanism (cont.) Coupling terms of W- and Z-bosons and fermions to the Higgs field: The introduced scalar fields can also be used to generate fermion masses (where g f is the coupling of the Higgs field to the fermion) Higgs boson self-coupling and finally Higgs boson regulates divergences in the WW scattering cross section

7 The Higgs boson as a UV regulator Scattering of longitudinally polarized W bosons "im(w + W " # W + W " ) ~ s m W for s # $ Higgs boson guarantees unitarity (if its mass is < ~ TeV) "im(w + W " # W + W " ) ~ m H for s # $

8 The decay properes of the Higgs boson are fixed, if the mass is known: Higgs Boson Decays H W +, Z, t, b, c, τ +,..., g, γ W -, Z, t, b, c, τ,..., g, γ Total width

9 Constraints on the Higgs boson mass. Constraints from theory. Indirect limits from electroweak precision data (theory and experiment) 3. Limits from Direct Searches (LEP, Tevatron)

10 (i) Tighter Higgs mass constraints from theory: Stronger bounds on the Higgs-boson mass result from the energy dependence of the Higgs coupling λ (Q ) (if the Standard Model is assumed to be valid up to some scale Λ) λ( Q ) 3λ0 Q 3g Q = λ0 + log ( ) + λ π v 3π v 4 t log ( ) + where 0 = m v h Upper bound: Lower bound: diverging coupling (Landau Pole) stability of the vacuum (negative contribution from top quark dominates) Mass bounds depend on scale Λ up to which the Standard Model should be valid Hambye, Risselmann et al.

11 (ii) Indirect limits from electroweak precision data (m W and m t ) Sensitivity to the Higgs boson and other new particles via quantum corrections: experimental errors 68% CL: LEP/Tevatron (today) light SUSY M W [GeV] M H = 4 GeV SM M H = 400 GeV SM MSSM both models MSSM heavy SUSY Heinemeyer, Hollik, Stockinger, Weber, Weiglein m t [GeV] m H = GeV/c m H < 6 GeV/c (95 % CL)

12 (iii) Constraints from direct searches m H > 4.4 GeV from direct searches at LEP m H < 56 GeV.or. m H > 77 GeV from direct searches at the Tevatron Tevatron Run II Preliminary, L 8.6 fb 95% CL Limit/SM LEP Exclusion SM= Expected Observed! Expected! Expected Tevatron Exclusion Tevatron Exclusion July 7, m H (GeV/c )

13 Begin of a new era in particle physics CMS LHCb ALICE 3 ATLAS

14 Data taking in 0 Original goal to collect fb already surpassed in June 0 World record on instantaneous luminosity on. April 0: cm - s (Tevatron record: cm - s ) meanwhile: > 33 cm - s Collect per day as much luminosity as in 0 Data taking efficiency is high Pile-up is high (high intensity bunches)

15 Production Cross Sections at the LHC Rates for L = 34 cm - s : (LHC) Inelastic proton-proton reactions: 9 / s bb pairs 5 6 / s tt pairs 8 / s W e ν Z e e 50 / s 5 / s Higgs (50 GeV) 0. / s Gluino, Squarks ( TeV) 0.03 / s

16 An impressive number of processes has already been measured [pb] total fb 0.7 fb ATLAS Preliminary L dt = fb Theory Data 0 (~35 pb ) Data 0 fb s = 7 TeV fb fb W Z W Z tt t WW WZ ZZ Excellent agreement with the Standard Model predications

17 Higgs boson producon at the LHC Gluon Fusion Vector boson fusion tt associated production WH/ZH associated production

18 Higher order corrections: - Spira, Djouadi, Graudenz, Zerwas (99) - Dawson (99) - Harlander, Kilgore (00) - Anastasiou, Melnikov (00) - Ravindran, Smith, van Neerven (003) Independent variation of renormalization and factorization scales (with 0.5 m H < µ F, µ R < m H )

19 Useful Higgs Boson Decays at Hadron Colliders at high mass: Lepton final states (via H WW, ZZ) at low mass: Lepton and Photon final states (via H WW*, ZZ*) Tau final states The dominant bb decay mode is only useable in the associated production mode (tth, W/Z H) (due to the huge QCD jet background, leptons from W/Z or tt decays)

20 H WW lν lν Large H WW BR for m H ~ 60 GeV/c Neutrinos no mass peak, use transverse mass MC SIMULATION Large backgrounds: WW, Wt, tt Two main discriminants: (i) Lepton angular correlation (ii) Jet veto: no jet activity in central detector region Channel with highest sensitivity! Sensitive to a Standard Model Higgs boson already now, with fb!

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22 Search for H WW lν lν L int =.70 fb Select events with two opposite sign leptons (e, µ ) p T (l ) > 5 GeV p T (l ) > 0 GeV (e), 5 GeV (µ) First look at invariant mass distributions à good agreement for all channels Entries / 5 GeV Data / MC 9 Data SM (sys stat) 8 ATLAS Preliminary WW WZ/ZZ/W s = 7 TeV, L dt =.70 fb t t Single Top H WW e e Z+jets W+jets H [50 GeV] M ee [GeV] Entries / 5 GeV Data / MC 00 Data SM (sys stat) WW WZ/ZZ/W ATLAS Preliminary s = 7 TeV, H WW e! L dt =.70 fb t t Single Top Z+jets W+jets H [50 GeV] M e! [GeV] ee, µµ: large Z/γ* production background (Drell-Yan), can be rejected by Z mass veto eµ : residual contribution from Zà ττ eνν µνν and top production

23 Search for H WW lν lν (cont.) Additional discrimination: missing transverse energy Entries / 4 GeV Data / MC 9 Data SM (sys stat) 8 ATLAS Preliminary WW WZ/ZZ/W s = 7 TeV, L dt =.70 fb t t Single Top H WW e e Z+jets W+jets H [50 GeV] miss ET,rel [GeV] Entries / 4 GeV Data / MC ATLAS Preliminary s = 7 TeV, L dt =.70 fb H WW e! Data SM (sys stat) WW WZ/ZZ/W t t Single Top Z+jets W+jets H [50 GeV] miss ET,rel [GeV] Apply additional cuts on: Z-mass veto (previous slide) m ll m Z < 5 GeV (ee, µµ) E T,rel miss > 40 GeV (ee,µµ), > 5 GeV (eµ)

24 Search for H WW lν lν (cont.) Split sample according to jet multiplicity (E T > 5 GeV) (different production modes, different background compositions) Entries 000 Data SM (sys stat) ATLAS Preliminary s = 7 TeV, H WW l l L dt =.70 fb WW WZ/ZZ/W t t Single Top Z+jets W+jets (data driven) H [50 GeV] - Jet distribution well described # observed events: 405 # expected background: 4000 ± 500 Data / MC à Select 0- and -jet events N jets

25 Search for H WW lν lν (cont.) Further discrimination using m ll and Δφ between the two leptons: Entries / GeV 60 ATLAS Preliminary Data SM (sys stat) 40 WW WZ/ZZ/W s = 7 TeV, L dt =.70 fb t t Single Top H WW l l + 0 jets Z+jets W+jets (data driven) H [50 GeV] Entries / ATLAS Preliminary Data SM (sys stat) 80 WW WZ/ZZ/W s = 7 TeV, L dt =.70 fb t t Single Top H WW l l + 0 jets Z+jets W+jets (data driven) H [50 GeV] jet 0 Data / MC [GeV] M ll Data / MC (ll) [rad] Entries / GeV ATLAS Preliminary s = 7 TeV, L dt =.70 fb H WW l l + jet Data SM (sys stat) WW WZ/ZZ/W t t Single Top Z+jets W+jets (data driven) H [50 GeV] Entries / ATLAS Preliminary Data SM (sys stat) WW WZ/ZZ/W 80 s = 7 TeV, L dt =.70 fb t t Single Top 70 H WW l l + jet Z+jets W+jets (data driven) H [50 GeV] jet Data / MC [GeV] M ll Data / MC (ll) [rad]

26 Search for H WW lν lν (cont.) Transverse mass distributions: Entries / GeV 45 Data SM (sys stat) ATLAS Preliminary L dt =.70 fb s = 7 TeV, H WW l l + 0 jets WW WZ/ZZ/W t t Single Top Z+jets W+jets (data driven) H [50 GeV] Entries / GeV 8 ATLAS Preliminary Data SM (sys stat) 6 WW WZ/ZZ/W s = 7 TeV, t t Single Top 4 L dt =.70 fb H WW l l + jet Z+jets W+jets (data driven) H [50 GeV] Data / MC M T [GeV] Data / MC M T [GeV] No evidence for an excess above Standard Model backgrounds Use statistical methods to quantify this and extract a limit on the production cross section

27 Entries / GeV Data / MC 45 ATLAS Preliminary Data SM (sys stat) 40 WW WZ/ZZ/W s = 7 TeV, t t Single Top 35 L dt =.70 fb H WW l l + 0 jets Z+jets W+jets (data driven) H [50 GeV] [GeV] M T Search for H WW lν lν (cont.) - number of events at various cut stages- 0 jet Entries / GeV Data / MC 8 ATLAS Preliminary Data SM (sys stat) 6 WW WZ/ZZ/W s = 7 TeV, t t Single Top 4 L dt =.70 fb H WW l l + jet Z+jets W+jets (data driven) H [50 GeV] [GeV] M T jet

28 How are the backgrounds normalized? Most important backgrounds are WW continuum production and tt production Both are well described by the Standard Model calculations / Monte Carlos Cross-check / normalization is performed in defined control regions * WW control region: remove cuts on Δφ cut and require: m ll > 80 GeV/c (eµ) m ll > m Z + 5 GeV/c (ee,µµ) * tt control region via b-tag selections * Z+jets / Drell-Yan (suffers from E T miss / lepton mismodelling) Number of events in WW control region Entries / GeV Data / MC ATLAS Preliminary s = 7 TeV, L dt =.70 fb H WW l l + jet Data SM (sys stat) WW WZ/ZZ/W t t Single Top Z+jets W+jets (data driven) H [50 GeV] [GeV] M ll

29 Systematic uncertainties

30 Sensitivity and exclusion Calculate cross sections that can be excluded with a 95% C.L. = σ 95 Normalize them to the Standard Model cross sections for a given Higgs mass 95% CL Limit on / SM ATLAS Preliminary Observed Expected!! CLs Limits H WW (*) l l Ldt =.7 fb s = 7 TeV Excluded mass regions (95% C.L.): 54 < m H < 86 GeV Expected exclusion: 35 < m H < 96 GeV m H [GeV] - The observed limits at neighboring mass points are highly correlated due to the limited mass resolution in this final state; - Jump in the expected and observed limits at 0 GeV is caused by a change in the selection at that point

31 Significance for H à WW Statistical significance ATLAS Preliminary Observed Expected!! (*) H WW l l Ldt =.7 fb m H [GeV] s = 7 TeV 0 p ATLAS Preliminary (*) H WW l l Observed Expected Ldt =.7 fb m H [GeV] s = 7 TeV The expected (dashed) and observed (solid) signal significances The expected (dashed) and observed (solid) probabilities for the background-only scenario, P 0

32 Significance for H à WW Statistical significance ATLAS Preliminary Observed Expected!! (*) H WW l l Ldt =.7 fb m H [GeV] s = 7 TeV 0 p ATLAS Preliminary (*) H WW l l Observed Expected Ldt =.7 fb m H [GeV] s = 7 TeV The expected (dashed) and observed (solid) signal significances The expected (dashed) and observed (solid) probabilities for the background-only scenario, P 0

33 What does the CMS experiment see? L int =.55 fb A very similar analysis Good agreement between data and expectations from Standard Model processes without a Higgs boson events 6 CMS, s = 7 TeV, L =.55 fb int data H(30) WW W+jets di-boson top Z+jets WW p l/l T E T >0/ miss Z veto cuts p cut anti b-tag jet veto l T p l T cut cut m ll cuts m T Δ Φ ll cut

34 Results from CMS on the H WW lν lν search: L =.55 fb (large fraction of 0 data) entries / 5 degrees data m H =60 WW Z+jets top WZ/ZZ W+jets CMS preliminary L =.55 fb entries / 5 degrees data m H =60 WW Z+jets top WZ/ZZ W+jets CMS preliminary L =.55 fb "! ll [degrees] "! ll [degrees] ll + E T miss + 0 jets ll + E T miss + jet Data are in reasonable agreement with expectations from Standard Model processes; Important background normalized using control regions in data (like ATLAS)

35 CMS sensitivity and exclusion 95% CL Limit on σ/σ SM H WW lν + 0// jets (CLs) 95% CL exclusion: median 95% CL exclusion: 68% band 95% CL exclusion: 95% band observed Excluded mass regions (95% C.L.): Higgs mass, m [GeV/c ] H 47 < m H < 94 GeV Expected exclusion: 35 < m H < 00 GeV - Very similar results as the ATLAS collaboration: Similar sensitivity, similar excluded mass range, σ like excess at low mass

36 What can be the cause of this excess? H à WW is a difficult channel! - no resonant structure / mass reconstruction not possible Excess can be due to: - First indications of a signal? - A statistical fluctuation? - systematic uncertainties in modelling of the Standard Model backgrounds (low mass) correlated systematic uncertainties between the two experiments, since they use the same Monte Carlo generators More data needed to perform many more systematic checks Support from other channels needed, e.g. H à ZZ* or H à γγ

37 H ZZ (*) llll Signal: σ BR = 5.7 fb (m H = 0 GeV) Background: Top production tt Wb Wb lν clν lν clν σ BR 300 fb Associated production Z bb Z bb ll clν clν P T (,) > 0 GeV P T (3,4) > 7 GeV η <.5 Isolated leptons M(ll) ~ M(l l ) ~ < M z MC SIMULATION M Z Background rejection: Leptons from b-quark decays non isolated do not originate from primary vertex (B-meson lifetime: ~.5 ps) Dominant background after isolation cuts: ZZ continuum Discovery potential in mass range from ~30 to ~600 GeV/c

38 m = 90.6 GeV m 34 = 47.4 GeV H ZZ (*) µµ µµ candidate event

39 ATLAS results on H à ZZ* à 4 l searches Data corresponding to.8 fb (taken up to August 0) included No excess above Standard Model expectations visible L int = fb Events/ GeV DATA Background Signal (m =50 GeV) H Signal (m =0 GeV) H Signal (m =480 GeV)! H (*) 4l ATLAS H ZZ Ldt = fb s = 7 TeV [GeV] m 4l 95% CL limit on / SM ATLAS (*) H ZZ 4l Ldt = fb s=7 TeV Observed CL s Expected CL!! [GeV] m H s Rare decay à still limited by the available integrated luminosity Challenging for m H < 30 GeV, requires low p T leptons However, Standard Model sensitivity already reached at high mass

40 ATLAS and CMS results for H à ZZ* à 4 l over the full mass range 95% CL limit on / SM ATLAS (*) H ZZ 4l Ldt = fb s=7 TeV Observed CL s Expected CL!! s 95% CL limit on / SM 5 Observed Limit CL S 5 Expected Limit CL S CL S Expected! CL S Expected! L int =.66 fb CMS preliminary 0.66 fb at s = 7 TeV [GeV] m H M H [GeV/c ]

41 Main backgrounds: γγ irreducible background q γ q γ γ-jet and jet-jet (reducible) Decay modes at low mass: H γγ MC SIMULATION CMS q g q γ π 0 γ σ γj+jj ~ 6 σ γγ with large uncertainties need R j > 3 for ε γ 80% to get σ γj+jj «σ γγ Signal expectation x, for fb Main exp. tools for background suppression: - photon identification - γ / jet separation (calorimeter + tracker) Sensitivity in the low mass region, however, higher integrated luminosities required

42 ATLAS results on H à γγ searches Data corresponding to.08 fb analyzed, more to come soon L int =.08 fb Sensitivity to Standard Model Higgs boson cross section not yet reached, as expected Events /.0 GeV s = 7 TeV, Ldt =.08 fb Inclusive diphoton sample Data 0 Background exp. fit Bkg + MC signal m = 0 GeV, 5xSM H 95% CL limit on / SM Observed CL s limit Expected CL limit!! s ATLAS H Data 0, s = 7 TeV Ldt =.08 fb ATLAS m [GeV] m H [GeV] Sensitivity is stable / flat in the low mass region, access to the 5 GeV region; Channel is less sensitive to pile-up

43 More channels at low mass? With higher integrated luminosities more channels are expected to contribute: (i) Tau decay mode via Vector Boson Fusion: qq H à qq τ τ (challenging) (ii) bb decay mode via the associated production with vector bosons or top quarks: WH, ZH or tth with H à bb (very challenging)

44 Higgs boson search at high mass? - Search at high mass is easier at the LHC; - Higgs boson decays nearly 0% into WW or ZZ pairs, which give charged leptons, neutrinos, b-jets, - Important search channels: H à WW à lν lν H à WW à lν qq H à ZZ à ll ll H à ZZ à ll qq H à ZZ à ll νν

45 Results from ATLAS on various high mass search channels: L =.04.8 fb (up to data taken shortly before Lepton-photon conference) Events / GeV Data 0, ATLAS Preliminary s = 7 TeV, Ldt = fb Data m H =0 GeV, xsm Total background (*) H ZZ 4l Events / 5 GeV Data 0, ATLAS Preliminary s = 7 TeV, Ldt =.04 fb Data m H =300 GeV, 5xSM Total background H ZZ llbb [GeV] m 4l [GeV] m lljj Events / 50 GeV Data 0, ATLAS Preliminary s = 7 TeV, Ldt =.04 fb Data m H =300 GeV, xsm Total background H ZZ ll m T [GeV] Also in these channels: data are consistent with expectations from Standard Model background processes à work out significances / statistics

46 Current status of the Higgs boson search at the LHC -ATLAS and CMS- 95% CL limit on / SM Exp. Obs. Exp. Obs. H H (.08 (.08 fb ) fb ) H ZZ H ZZ llll llll ( fb fb) H WW H WW l l l l (.70 (.70 fb fb) ) H ZZ H ZZ llqq llqq (.04 (.04 fb fb) ) W/Z H, W/Z H H, H bb bb (.04 (.04 fb fb) ) H ZZ H ZZ ll ll (.04 (.04 fb fb) ) H H (.06 (.06 fb ) fb ) ) ATLAS Preliminary L dt ~.0-.3 fb, s=7 TeV CLs limits m H [GeV] The two collaborations show similar performance - in terms of analysis power in the collaboration (many channels) - in terms of sensitivity - in terms of conclusions on the existence of the Higgs boson

47 Putting everything together The grand combination of channels V. Sharma So far: ATLAS and CMS combinations available (summer conferences) Combination ATLAS + CMS is also ready; will be released this Friday, at HCP conference

48 Current status of the Higgs boson search at the LHC -ATLAS and CMS combinations- 95% CL Limit on / SM ATLAS Preliminary Observed Expected!! CLs Limits Ldt =.0-.3 fb s = 7 TeV m H [GeV] Excluded mass regions (95% C.L.): 46 < m H < 3 GeV 56 < m H < 8 GeV 96 < m H < 466 GeV Excluded mass regions (95% C.L.): 45 < m H < 6 GeV 6 < m H < 88 GeV 3 < m H < 400 GeV

49 Prospects (new future) What can be done with 5 fb of data? Will most likely be shown in December this year, council meeting at CERN How much luminosity is needed for a 3σ evidence or a 5σ discovery? (in particular at low mass) Can the Higgs be ruled out? or can it be discovered with the 0/0 data?

50 Expectations for higher integrated luminosities -95% C.L. exclusion limits- CMS, V. Sharma, Lepton-Photon 0 95% C.L. exclusion can be reached with 5 fb in one experiment However, needs the combination of all low-mass channels

51 Expectations for higher integrated luminosities -discovery significances- For a 3σ effect in one experiment for a Higgs boson with a mass of 5 GeV more than fb will be needed Significant gain by increasing the energy from 7 to 8 TeV; 3σ with ~ fb Combined result of the two experiments will be interesting!

52 What, if no Higgs boson is found Well, this would be a big discovery! The Standard Model would be proven to be wrong Maybe Nature is not as simple as thought? In this case: the LHC is a unique facility to investigate further - It might only take more running time (e.g modified couplings in more complicated Higgs scenarios, like composite Higgs bosons) - We all might get many years older before WW scattering is precisely measured - Invisibly decaying Higgs sceanrios are very very difficult to be measured at the LHC

53 What, if the Higgs boson will be found soon Well, this would be a big discovery! à Measure its parameters and prove that it is a Higgs boson Example: Precision on ratio of couplings to W coupling at the 0% level, needs high luminosity Higgs boson self coupling very difficult to measure; maybe not even possible at the slhc (needs further studies) A discovery of a light Higgs boson would also indicate new physics

54 Summary and Conclusions LHC machine and detectors are running extremely well! An impressive list of high quality physics measurements has already been performed The search for the Higgs boson has started very well - So far: data consistent with the Standard Model background hypothesis without a Higgs boson signal - Important new mass ranges have been excluded; however, the most interesting one not yet covered, but sensitivity will be reached soon Exiting times ahead of us!