Higgs Searches at LHC Marco Pieri, UCSD – San Diego Hadron Collider Physics Symposium July 2005, Les Diablerets, Switzerland SM Higgs boson MSSM Higgs bosons Higgs bosons and SUSY particles Measurement of Higgs boson parameters

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 2 Current status of Higgs Searches INDIRECT CONSTRAINTS ON THE SM HIGGS BOSON  Electroweak fits to all high Q 2 measurements give:  M H = GeV  M H <285 95% CL  The central value and the upper limit have increased during the last few years DIRECT SEARCHES AT LEP GAVE NEGATIVE RESULTS  SM Higgs  M H >114.1 CL  MSSM neutral Higgs bosons  M h, M A >92.9, 93.3 CL  Charged Higgs Bosons  M H ± >89.6 CL for BR(M H ± → τν) =1  M H ± >78.6 CL for any BR

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 3  ATLAS and CMS are preparing for the search for the Higgs bosons of different models  Most of the studies presented are still carried out with fast simulation for the background. Full simulation has been used for the signal and for the estimation of the crucial aspects of the detectors  Studies with full simulation of signal and background are in progress  Most analyses shown in the following are optimized for the low luminosity phase LHC operation  Low luminosity phase:  ℒ ~ 2 x cm -2 s -1  Int ℒ ~ 30 fb -1  High luminosity phase:  ℒ ~ 1 x cm -2 s -1  Int ℒ ~ 300 fb -1 Introduction

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 4 SM Higgs production NLO Cross sections M. Spira et al. gg fusion IVB fusion

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 5 SM Higgs decays When WW channel opens up pronounced dip in the ZZ BR For very large mass the width of the Higgs boson becomes very large (Γ H >200 GeV for M H ≳ 700 GeV)

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 6 Most important SM search channels Production DECAY Inclusivegg fusionIVB fusion WH/ZHttH H  γγ YES H  bb YES H  ττ YES H  WW * YES H  ZZ *, Z  ℓ + ℓ -, ℓ=e,μ YES H  Zγ, Z  ℓ + ℓ -, ℓ=e,μ very low σ Low mass MH ≲ 160 GeV H → γγ and H → ZZ* → 4 ℓ are the only channels with a very good mass resolution ~1% Intermediate mass (160 GeV ≲ M H ≲ 700 GeV) High mass (M H ≳ 700 GeV) inclusive H → WW inclusive H → ZZ IVB fusion qqH → ZZ → ℓℓ νν IVB fusion qqH → WW → ℓ ν jj

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 7 H → γγ  Sigma x BR ~90 fb for M H = GeV  Irreducible backgrounds from gg → γγ, qq → γγ, pp → γ jet → γγ jet  Reducible background from fake photons from jets and isolated π 0 (isolation requirements)  Very good mass resolution ~1%  Vertex estimated from the underlying event and recoiling jet H → γγ M H = 115 GeV

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 8 H → ZZ* → 4 ℓ H → ZZ* → ℓ + ℓ - ℓ + ℓ - ℓ =e,μ  Irreducible background:  ZZ production  Reducible backgrounds  tt and Zbb  Very good mass resolution ~1%  In this channel (and in the H → γγ ) background can be easily estimated from data by fitting the sidebands  Above M H ~ 2M Z the two Z bosons are real and σxBR is larger  Golden channel for Higgs discovery at LHC Branching ratio dip due to opening of WW channel

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 9 H->bb, ttH production channel  Allows the measurement of the Higgs coupling to fermions  ttH → ℓ ν qqbbbb  Most useful for very light Higgs: M H ≲ 130 GeV  Fully reconstruct the top decays and estimate the right bb combination for the Higgs boson Mass resolution not excellent > 10 % b-tagging performances for CMS and ATLAS similar

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 10 forward jets Higgs decay products IVB Fusion qqH (low mass H) Tagging jets from qq are at high rapidity and large Δ η qqH → qqWW*, qq ττ (also qq γγ )  Much worse mass resolution or only transverse mass measurable  Background estimation from data much more difficult  Background is highly reduced by  tagging the two forward jets  requiring low activity in the central detector  Signal to BG ratio is increased reducing the effect of BG uncertainty Proposed by Dockshitzer, Khoze, Trojan and Rainwater, Zeppenfeld et al. qqH → qq γγ M H = 120 GeV ATLAS

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 11  ATLAS carried out a recent study in the qqH channels:  qqWW* → qq ℓ νν ℓ νν  qqWW* → qq ℓ νν jet jet  qq ττ → qq ℓ νν ℓ νν  qq ττ → qq ℓ ν had ν  with ℓ =e,μ  Main backgrounds  Z+jets, tt and WW+jets IVB fusion: qqH → qqττ signalWjj background τ reconstruction:  τ decay products are highly boosted, assume that they are collinear  From module and direction of the measured missing E t derive the neutrinos momenta  Mass resolution ~10% at M H =120 GeV ATLAS 30 fb -1 x τ i =fraction of τ energy carried by visible decay products

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 12 IVB fusion: qqH → qqWW*  H → WW → ℓ ν ℓ ν or ℓ ν qq  Trigger on the lepton(s) and on missing E t  Main backgrounds tt and tW  Higgs mass cannot be reconstructed, only transverse mass  Difficult to estimate the BG from the sidebands (syst BG ~ 10%) Plots for WW → e μνν channel One way to estimate the background: release lepton cuts use shape from MC M H =120 GeV M H =160 GeV ATLAS

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 13 Results for low mass ATLAS  VBF channels improve a lot discovery potential compared to previous results  No K-factors used, LO cross sections  With 30 fb -1 more than 5 sigma significance for M H >100 GeV  Higgs boson can be discovered in more than one channel, possible to measure its couplings

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 14 For low mass results updated with IVB fusion Results for the whole mass range All mass range accessible at 5σ significance with 10 fb -1 With a few fb -1 possible to discover the Higgs boson with mass between ~150 and ~500 GeV in the WW and ZZ channels  For mass larger than ~200 GeV use ZZ and WW leptonic decays  For mass larger than ~700 GeV use qqH, H → ZZ → ℓℓ νν and H → WW → ℓ ν qq

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 15 MSSM Higgs Searches  Two Higgs doublets model  5 Higgs bosons:  2 Neutral scalars h,H  1 Neutral pseudo-scalar A  2 Charged scalars H±  In the Higgs sector all masses and couplings are determined by two independent parameters  Most common choice:  tanβ – ratio of vacuum expectation values of the two doublets  M A – mass of pseudo- scalar Higgs boson In the MSSM: M h ≲ 135 GeV

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 16 Neutral MSSM Higgs bosons  Decoupling limit (M A ≳ 200 GeV)  h behaves like H SM  Standard model searches directly apply  M H ~M A ~M H ±  M A =O(M Z ) and large tanβ  H behaves similarly to SM Higgs (SM searches apply)  In other cases for large tanβ and M A <200 GeV  A → WW,ZZ never allowed at tree level,  h,H → WW,ZZ highly suppressed  h,H,A almost exclusively decay into bb and ττ  Large M A small tanβ  H,A decays almost 100% into tt  for lower masses ( GeV) also H → hh and A → Zh  If SUSY particles are light the Higgs bosons may decay into s- particles

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 17 h,H production and decay Decoupling region Large tanβ mainly bb, ττ decays Large tanβ hbb, Hbb (and Abb) production dominates h,H decaysh,H production tanβ = 30

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 18 Results from SM Higgs Searches  In a large part of the MSSM parameter space SM Higgs searches are effective to find the MSSM h boson  In the decoupling region if h observed hard to distinguish SM from MSSM  Search for H, A and H ±  For large tanβ exploit the large cross section of Higgs boson production in association with a bb pair  bbH,A → bbττ  bbH,A → bbμμ  bbH,A → bbbb (very difficult)  B-tagging (+ τ id and missing E t for the τ channel) are the key issues CMS 5 σ discovery contours

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 19 bbH,A → bb ττ  for M H ≲ 400 GeV  ττ → ℓ νν ℓ νν  ττ → ℓ νν had ν  Higher mass also add  ττ → had ν had ν  b-tagging, τ id and missing E t are the basic ingredients bbH,A → bb ττ  From the cross section measurement it is possible to extract the value of tanβ  tanβ uncertainty due to variation of SUSY parameters (M H MAX scenario considered) in a range ± 20% is 6%

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 20 bbH,A → bb μμ H,A → μμ  low rate, BR(H → μμ ) ~10 -3  high efficiency  precise mass measurement (μμ mass resolution ~1%)  Main backgrounds:  Z/γ* → μμ  tt → μμ X  Selection requires 2 muons, b-tagging and central jet veto CMS 20 fb -1 CMS 5σ discovery contours

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 21 Results on H,A 5σ discovery regions in the M H MAX scenario

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 22 Charged Higgs bosons H ± M H ± <m t -m b  Mainly produced in top decays tt → tH ± b  in the MSSM BR(H ± → τν )~100% includes top decays M H ± >m t +m b  Mainly produced in association with a t quark (gb → tH ± )  BR(H ± → tb)~100% for small tanβ  H ± → tb decay dominates but BR (H ± → τν) still sizeable for large tanβ T. Plehn et al. Analyses are in progress for the mass region M H ± ~ m top

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 23 M H ± >m t +m b  gb → tH ± with H ± → τν and t → bqq  Exploit helicity correlations  Similar endpoint of M T at M W for the background  M T can also be used for Higgs mass measurement (likelihood fit) M H ± <m t -m b  Main channel tt → bH ± bW → bτνb ℓ ν  ATLAS also considers tt → bτνbqq  Use transverse M T mass built with τ jet + missing E t  tt background has M T < M W Main search channel H ± → τν ATLAS 10 fb fb -1

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 24 Discovery regions for Charged Higgs Bosons ATLAS search in tt → bH ± bW → b τν bqq improves the sensitivity in this region 5σ discovery regions in the MSSM tanβ – M A plane for M H MAX scenario

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 25 MSSM scans  M H MAX scenario  M SUSY = 1 TeV  maximal m h < 133 GeV  No-mixing scenario  M SUSY = 1 TeV  m h < 116 GeV  Gluophobic scenario  suppressed coupling to gluons (cancellation of top+stop loops)  Small rate for : gg  H  M SUSY = 350 GeV, m h < 119 GeV  Small α scenario  coupling to b and τ suppressed for large tanβ, M A GeV  M SUSY = 800 GeV  m h < 123 GeV ATLAS studied the 4 benchmarks  With 30 fb -1 h or H can be seen in VBF channels in almost all parameter space ATLAS preliminary 4 CP conserving benchmarks suggested by Carena et al.

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 26 Results of MSSM scans  Smaller region covered by the ττ channel in the small α scenario (reduced coupling)  Covered by increased coupling to gauge bosons ATLAS preliminary covered by h → WW h → γγ (enhanced branching ratio to gauge bosons) 5σ discovery regions

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 27 Higgs Bosons visibility in the MSSM All the plane is covered but there is a large area where only h can be seen 4 Higgs observable 3 Higgs observable 2 Higgs observable 1 Higgs observable 5σ discovery regions in the MSSM tanβ – M A plane for M H MAX scenario

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 28 MSSM Higgs bosons and SUSY particles  If SUSY particles are heavier than the Higgs bosons Higgs bosons could be produced in gauginos decays:  χ 2 → h,H,A χ 1  χ 1 ± → H ± χ 1  Different cascades possible involving heavier gauginos  Search for h,H → bb  Neutralinos and charginos would be copiously produced in the decays of squarks and gluinos Possible to observe SUSY → h,H,A with h,H,A → bb  If SUSY particles are lighter than Higgs bosons we could have a rich variety of decays, some scenarios have been investigated:  H,A → χ 2 χ 2  using χ 2 → ℓ + ℓ - χ 1 decay (4 ℓ + missing E t events)  h → χ 1 χ 1  invisible Higgs decays

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 29 from chargino searches Invisible decays of the Higgs Boson  IVB fusion is the most sensitive process  Trigger on forward jets + missing E T  Selection:  forward jet tagging, central jet-veto, M(jet jet)  lepton veto, missing E t  Δφ jet-jet small  If we do not require gaugino mass unification and M 1 <<M 2 M χ can be rather small and BR(h → χχ ) can be very large ATLAS - Accessible region for 95% CL exclusion

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 30  After discovering the Higgs bosons we should measure their parameters  Studies for high luminosity (Int L = 300 fb -1 ) SM Higgs boson mass  direct reconstruction: 4 ℓ, γγ, bb  likelyhood fit WW Measurement of Higgs bosons parameters SM Higgs boson width  from ZZ → 4 ℓ ATLAS INT L = 300 fb -1

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 31 Measurement of Higgs couplings  From σ x BR measurements in all channels where the Higgs boson can be observed: Production cross section Decay BR SM framework D. Zeppenfeld et al.

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 32 Other studies Many other scenarios have been studied  CP Violating MSSM  Investigated by ATLAS, reduced discovery potential for small Higgs boson masses  Strongly interacting Higgs Sector: V L V L scattering  If no Higgs boson is found at LHC  Radions (Randall Sundrum model)  φ → hh ...  See Atlas Physics TDR and the soon coming CMS Physics TDR for details

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 33 Conclusions  ATLAS and CMS have studied the prospects of Higgs boson discovery for SM and MSSM  SM Higgs boson can be discovered with 5 sigma with 10 fb -1 at low luminosity in the whole mass range  At least 1 MSSM Higgs boson can be found for all investigated benchmarks  In some regions difficult to discriminate between SM and MSSM  WW and ZZ fusion process is very important both for SM and MSSM  Two years from the beginning of LHC, must continue to prepare the actual analyses  based on data  with minimal use of MC information  study of all the possible control samples needed to verify the performances of the detector  Studies with full simulation of signal and all the backgrounds are in progress  We are getting ready to find the Higgs boson(s) at LHC

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 34 EXTRA

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 35 H → ZZ → 4 ℓ

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 36 Higgs boson width

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 37  Above M H ~700 GeV the width of the Higgs boson becomes very large (>200 GeV), need higher rate  Use IVB fusion  H → ZZ → ℓℓ νν and  H → WW → ℓ ν jj High mass search ATLAS 100 fb -1

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 38 MSSM h,H decays Decoupling region Large tanβ bb, ττ decays Small tanβ H decays into tt when allowed

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 39 MSSM Production processes Large tanβ hbb, Hbb and Abb production dominates

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 40 bbH,A->bbbb CMS new analysis  Investigation of feasibility of the 4b channel  S/B ~5%, large effects of systematic error on BG estimation Seems extremely difficult to control the BG with the needed precision

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 41 Hadronic cannel: H ± → tb  CMS repeated the study with NLO cross section calculation  Old results showed some sensitivity  New results by indicate that with the current analysis the very small expected signal is washed out by systematic errors on the background estimation Effect of systematic error on BG

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 42 Discovery regions for MSSM Higgs bosons ATLAS MHMAX scenario 300 fb -1 1 boson 2 bosons 3 bosons All 4 bosons h only h,H,A,H +- h,H,A Excluded by LEP Similar results in the other 3 benchmarks 5σ discovery regions

Higgs Searches at LHC HCP 4-9 July 2005 Les Diablerets Marco Pieri - UCSD San Diego 43 Measurement of Higgs boson couplings