HIGGS Bosons at the LHC

Transcription

1 ATLAS HIGGS Bosons at the LHC Standard Model Higgs Boson - Search for a light Higgs at the LHC - Vector boson fusion - Comparison to the Tevatron potential Measurement of Higgs boson parameters The MSSM Higgs sector - heavy Higgs H/A in tt decay mode - charged Higgs large tanb

2 Revised LHC Schedule Dec Jan. - March 2006 April 2006 May - July 2006 Ring closed and cold Machine commissioning (1 beam) First collisions, pilot run L=5x10 32 to 2x10 33, 1 fb -1 Start detector commissioning ~ 10 5 Z fi ll, W fi ln, tt events Shutdown: continue det. installation Aug Physics run : L=2x10 33, 10 fb -1 - Feb Complete detector commissioning, start of physics fi 2008 L=2x10 34, 100 fb -1 per year (high luminosity LHC)

3 CMS Detector construction HCAL assembly

4 ATLAS detector construction Liquid Argon Calorimeter Superconducting solenoid ready

5 SM Higgs production at the LHC gg fusion WW/ZZ fusion associated tt H associated WH, ZH Events for 10 fb Leading order 10 K-factors ( higher-order corrections) = not included Some analyses very exlusive, NLO requires Monte Carlo implementation

6 Main search channels at the LHC Large QCD backgrounds: σ (H bb ) 20 pb direct production, m H =120 GeV σ ( bb ) 500 µb no hope to trigger / extract fully hadronic final states look for final states with l, γ (l = e,µ ) BR m H < 2 m Z : m H > 2 m Z : tth lbb + X, H γγ, H ZZ* 4l, H WW (*) lνlν H ZZ 4l qqh ZZ ll νν qqh ZZ ll jj qqh WW lνjj m H > 300 GeV forward jet tag Detector performance is crucial: b-tag, l/γ E-resolution, γ /j separation, E T miss resolution, forward jet tagging,...

7 Discovery potential for a SM Higgs boson S B Good sensitivity over the full mass range from ~100 GeV to ~ 1 TeV For most of the mass range at least two channels available

8 L per experiment M H = 115 GeV 10 fb -1 S/ B = 4.7 5σ LEP2 The Higgs boson discovery is possible over the full mass range already with ~10 fb -1 However: It requires the combination of both experiments and two channels ( H γγ and tth, H bb) in the low mass region It will take time to operate, understand and calibrate the detectors Higgs physics will not be done before 2007

9 The low-mass region m H < 130 GeV ATLAS+ CMS no K-factors H γγ tth ttbb S B S/B m H ~ 115 GeV ATLAS+CMS 10 fb -1 S/ B S/ B both channels together : 4.7 Two complementary channels robustness, interpretation

10 tt H tt bb ATLAS study σ x BR 300 fb Complex final state: H bb, t bjj, t blν Main backgrounds: -- combinatorial from signal (4b in final state) -- Wjjjjjj, WWbbjj, etc. -- ttjj (dominant, non-resonant) ATLAS 100 fb -1 m H =120 GeV S = 62 events B = 257 events S/B ~ 0.24 S/ B = 3.9 b-tagging performance is crucial ATLAS results for 2D-b-tag from full simulation (ε b =60% R j (uds)~ 100 at low L ) Shape of background must be known; 60% (from ttbb) can be measured from ttjj using anti-b tag

11 CMS Study (new) Use similar technique as ATLAS ttjj background generation done with CompHep + PYTHIA ISR/FSR Based on fast detector simulation only Likelihood method for reconstruction of top decays and event kinematics K-factors for signal included (1.5) S = 38 events B = 52 events S/B ~ 0.73 S/ B = 3.5 for K = 1.0 Comparable significance for K=1 (gain in significance by using likelihood method is compensated by larger background found with Comphep ttjj calculation) LHC experiments need a better understanding of the signal and the backgrounds (K-factors for signal and backgrounds)

12 The Tevatron competition Present Tevatron schedule : -- Run 2A : March 2001-end 2003 : ~ 2 fb -1 /expt. -- Run 2B : middle 2004? : ~ 15 fb -1 /expt by 2007 For m H ~ 115 GeV Tevatron needs : ~ 2 fb -1 for 95% CL exclusion end 2003? ~ 5 fb -1 for 3σ obervation beg. 2005? ~ 15 fb -1 for 5σ discovery end 2007? Higher masses : discovery more difficult Although these predictions are not on conservative side, Tevatron experiments will have lot of time to understand detectors and physics could observe a 5σ signal in

13 Comparison of the LHC and Tevatron potentials Higgs signal cross-section are ~10 times larger at LHC for qq W/Z + H and ~70-80 times larger for gg H (large g contribution to PDF s at LHC) PYTHIA Process σ BR σ BR 2 TeV pp 14 TeV pp LHC Tevatron WH lν bb m H =120 GeV 20 fb 210 fb 10 qq WH H WW lνlν 15 fb 1150 fb 77 m H = 150 GeV H γγ 0.3 fb 22 fb 73 m H = 150 GeV H ZZ 4l 0.07 fb 5.5 fb 78 m H = 150 GeV gg H gg H gg H s = 2 TeV: -- accessible channels: WH, ZH, WW (*) -- hopeless : H γγ, 4 l (best channels at LHC) (rate limited)

14 Backgrounds EW cross-section are ~10 times larger at LHC, QCD cross-sections ~ 100 times larger (gg and gq contributions strongly enhanced) PYTHIA Process σ (pb) σ (pb) pp 2 TeV pp 14 TeV LHC Tevatron qq' WZ WW W * tb t t QCD jets p hard T > 30 GeV Acceptance Acceptance of cuts for same detector and analysis is ~ 2 times larger at Tevatron: -- physics is more central higher efficiency of η cuts -- less initial state g radiation (smaller s) jet veto less harmful

15 For the same integrated luminosity, same detector, same analysis (e.g. ATLAS detector performance and analysis for pp s = 2 TeV and pp s = 14 TeV), after kinematic cuts : WH lν ZH ll bb bb H WW (*) lνlν (m H = 160 GeV) S (14 TeV) / S ( 2 TeV) 5 30 B (14 TeV) / B ( 2 TeV) 25 6 S/B (14 TeV) / S/B ( 2 TeV) S/ B (14 TeV) / S/ B (2 TeV) similar potential for WH and ZH, with larger S at s = 14 TeV and better S/B at s = 2 TeV -- larger potential at s = 14 TeV for H WW (*) (production dominated by gg H) Why are the WH and ZH channels (discovery channels for the Tevatron) not included in the LHC results?

16 m H = 120 GeV, 30 fb -1 s =2 TeV s =14 TeV WZ WH WZ WH Backgrounds (tt, Wjj, Wbb, WZ): -- large and with different shapes -- not all well known today (missing K-factors) -- not all can be measured precisely with data large background systematics in this channel and S/B ~ few % considered as marginal discovery channel at the LHC ZH ll bb, ZH ννbb have smaller sensitivity and/or even higher backgrounds hopeless

17 Higgs production via Weak Boson Fusion Motivation: Additional potential for Higgs boson discovery Important for the measurement of Higgs boson parameters (couplings to bosons, fermions (taus), total width) σ = 4 pb (20% of total cross section for m H = 120 GeV) however: distinctive signature of - two high P T forward jets - small jet activity in the central region Production process studied by D.Zeppenfeld et al.. Hep- Phys.Rev.D60:113004,1999; Phys.Lett.B454: ,1999; Phys.Rev.D59:014037,1999,... ATLAS and CMS studies have been performed, first, preliminary results available

18 qqh fiqq WW fi qq l n l n ATLAS: - Signal and background simulations with PYTHIA - El.weak backgrounds (t-channel vector boson exchange from matrix element calculation, D.Zeppenfeld et al.) - Initial and final state radiation included (PYTHIA) Basic cuts on isolated leptons: P T > 20 GeV η < 2.5 Basic cuts on tagging jets: P T > 20 GeV η > 4.4 Dominant background at that level: tt production H P T (tot) tt Additional rejection: M jj (inv. Mass of tag jets) P T (tot) = P T (l 1 ) + P T (l 2 ) + P t miss + P T (j 1 ) + P T (j 2 ) (less sensitive to pile-up than jet-veto over large rap.) Jet Veto ( no jets with P T > 20 GeV in η < 3.2 )

19 Expected event rates for 10 fb -1 : preliminary m H =160 GeV m H (GeV) Signal S/B Main background: remaining tt background (13.1 events) WW el. weak background ( 7.1 events) m H = 130 GeV e µ decays m H = 160 GeV e µ decays However: confirmed that WBF channel has a large discovery potential Still to be completed: estimate of forward jet tag efficiencies in a full simulation, combination with ee and µµ signature, optimization of cuts

20 qqh fiqq t t fi qq l n n l n n ATLAS: - Similar basic cuts as in WW analysis - Tau mass reconstruction using collinear approximation - Optimized cuts for eµ, ee and µµ channels m H = 115 GeV 30 fb -1 all channels (eµ best channel) S = 17.3 events B = 11.4 events S/B > 1 Combined significance (ee, µµ, eµ): m H (GeV) fb σ 30 fb σ Preliminary, no systematics yet, l-had channel to be added

21 Can VBF improve the significance at high mass? CMS Study

22 Measurement of the Higgs boson parameters Mass of Standard Model Higgs boson No theoretical error e.g. mass shift for large Γ H (interference resonant/non-resonant production) Dominant systematic uncertainty: γ /l E scale. Assumed 1 Goal 0.2 Scale from Z ll (close to light Higgs) Mass of MSSM Higgs bosons MSSM Higgs m/m (%) 300 fb -1 h, A, H γγ H 4 l H/A µµ h bb 1 2 Η hh bb γγ 1-2 Α Zh bb ll 1 2 H/A ττ 1-10

23 Measurements of Higgs couplings Without theoretical input only measurment of the ratios of couplings possible i) Ratio between couplings to bosons Direct measurement (QCD corrections cancel) Indirect measurement (Use proportionality between Γ W and Γ γ, needs theoretical input, 10% uncertainty assumed) Results for 30 fb-1 and 300 fb-1 per experiment

24 Ratios of boson/fermion couplings

25 Measurement of the Higgs width Direct measurement : from width of reconstructed mass peak for m H > 200 GeV (Γ H > Γ detector in SM) For lower masses, only indirect methods possible: from rates of qq qq H with H γγ, ττ, WW (Zeppenfeld et al., Phys. Rev. D62 (2000) )

26 MSSM HIGGS searches : h, H, A, H ± Large variety of channels: h γγ, tth ttbb, H ZZ (*) ± A/H µµ, ττ, tt, H τν, cs, tb H hh, A Zh 4l also in SM typical of MSSM A/H χ χ h χ 2 χ if SUSY particles accessible 2 steps: ❶ SUSY particles are heavy do not contribute to Higgs production / decay ❷ SUSY particles contribute Results are 5σ discovery contours on m A, tanβ plane for m top =175 GeV, M SUSY = 1 TeV, max mixing (for minimal mixing m h < GeV MSSM ~ fully explored by LEP). 2-loop calculations for masses and couplings (Carena et al., Phys. Lett. B355, 1995)

27 LHC discovery potential for MSSM Higgs bosons Assuming SUSY particles are heavy Not all channels shown Plane fully covered (no holes) at low L (30 fb -1 ) ± Main channels : h γγ, b b, A/H µµ, ττ, H τν Two or more Higgs can be observed over most of the parameter space disentangle SM / MSSM If LEP excess due to hz production (tanβ > 2, m Α >115GeV), LHC will observe: h for any tan β and m A A,H,H ± for large tan β and moderate m A

28 5 sigma contours Here only SM-like Higgs Only SM decay modes

29 Discovery potential for 10 fb -1 Large part of plane can be explored in 2007

30 A and and H bosons Bosons Large tanβ : bbh, bba strongly enhanced σ (MSSM) / σ (SM) 5000 tanβ =30, m =300 GeV H/A ττ, µµ observable and cover large part of parameter space Small tanβ : large number of channels measurement of many couplings (including Hhh, AZh)

31 A/H ττ h + ν h ν : Provides best reach for large m A. (CMS and ATLAS analyses) Signature: two stiff opposite-sign isolated tracks (P T > 40 GeV) P T miss or 1 b-tagged jet (bba/h) Main challenge: reject QCD jet backgr. (already at trigger-level) Feasible for m A > 300 GeV: (high P T hadrons, larger P T miss, larger rejection from isolation) R QCD ~ QCD background << 10% (tt + Z/γ* ττ) m A = 500 GeV tan β = 25 m A = 500 GeV tan β = 25 CMS 30 fb -1 P Τ miss analysis b-tag requirement improves S/B Mass resolution ~10% CMS: Additional studies on trigger acceptance performed

32 Charged Higgs Bosons Below top-quark mass: t -- > H+b; H+ - >τν decay mode (excess of events only) Above top-quark mass: gb --> H+ t production Large tanβ : both H+ --> t b and H+ --> τν enhanced Small tanβ : only H+ --> t b decay mode, almost 100% BR Tanβ cannot be determined from the ratio.

33 Charged Higgs Bosons For mh+ > mt, gb -> th+ -> ttb - require 3 b-tagged jet, - reconstruct both top-quarks and tb invariant mass Main backgrounds: ttb and signal combinatorial! ATLAS

34 Charged Higgs Bosons For mh > mt, gb -> th+ -> t τ ν Trigger on τ-jet. Hadronic decay mode of t-quark and τ lepton. - Reconstruct top-quark - Identify τ-jet, require pt > 100 GeV. - Take adventage of the distinctive polarisation of the τ lepton. Almost background free observability of excess in the transverse mass distribution (Etmiss, tau-jet)

35 Charged Higgs Bosons Determination of the Higgs mass looks promissing: - likelihood and fit method for H-->tb - only likelihood for H--> tn Precision better than 5 % for 100fb^-1 Precision better for H --> τν than H --> tb Higgs masses; also higher masses accessible Overall uncertainty dominated by statistics except low

36 Higgs decays via SUSY particles If SUSY exists : search for H/A χ 0 2 χ0 2 llχ0 1 llχ 0 1 5σ contours ATLAS 300 fb -1 ATLAS: SUGRA scan m 0 = GeV m 1/2 = GeV tan β = A 0 = 0 CMS: special choice in MSSM (no scan) M 1 = 60 GeV M 2 = 110 GeV µ = -500 GeV Exclusions depend on MSSM parameters (slepton masses, µ)

37 Light Higgs in SUSY cascade If SUSY exists : search for χ 0 2 qχ0 1 h(->bb) 5σ contours ATLAS: SUGRA scan m 0 = GeV m 1/2 = GeV tan β = A 0 = 0

38 Scan in SUGRA SUSY loops can enhance/suppress Higgs production (e.g. gg h) and decay (e.g. h γγ) A/H/H ± sparticles can compete with SM decays Study : msugra impact of SUSY on Higgs decays to SM particles is small : -- gg h γγ 10% smaller -- tth/wh γγ 30% smaller -- tth tt bb not affected -- BR (A/H/H ± SM particles) reduced by at most 40% SUSY ON ATLAS forbidden However : impact of mixing on couplings not studied for all possible mixing scenarios more work needed

39 Conclusions 1. LHC has a large discovery potential for a Standard Model and for MSSM Higgs Bosons -- SM Higgs can be discovered over full allowed mass range after ~ 1 year at cm -2 s -1 (provided detectors are well understood) -- MSSM Higgs sector can be fully explored. Two or more Higgs bosons should be observable in many cases. -- In addition, precise measurements of Higgs boson parameters (mass and couplings) can be performed 2. Vector Boson fusion channel seems to significantly enhance the discovery potential -- Tau tau channel in the low mass region -- Enhanced WW channels 3. New promising channels also in the MSSM section (Charged Higgs, had. Tau decays)