Status of Higgs boson measurements at the LHC

Transcription

1 Status of Higgs boson measurements at the LHC Kajari Mazumdar Tata Institute of Fundamental Research Mumbai Disclaimer: not a full review of all the available public results. Could not avoid a bit of personal bias towards CMS! For more information visit: DAE-BRNS symposium, IIT, Guwahati December 8, 2014

2 Higgs boson is for the mass (of elementary particles), by the mass (creations by artists inspired by science), of the mass (science is part of human culture!) Higgs decay candidates to various final states 12/5/2014 2

3 Triumph of human intelect Discovery of Higgs boson in 2012, conceived ~50 years earlier human understanding of Nature reaches new height Elegance of the idea of unification and other hall marks achieved in the field during the last century Gigantic efforts in many fronts led to the success at the LHC, in first phase of operation ( ) : a Higgs like new particle : a Higgs boson Physics Nobel After the discovery: remarkable progress in understanding the Higgs sector Is it the Higgs boson of standard model (SM)? Is there any other Higgs particle? Is the resonance a window to new physics? 12/5/2014 3

4 LHC played the perfect role for the discovery 2 major multipurpose experiments A Toroidal LHC Apparatus (ATLAS) Compact Muon Solenoid (CMS) High efficiency of experiments and fast computing quick and grand harvest Higgs production at LHC: main modes Integrated luminosity (L ) delivered by LHC machine during Run1 ~ 30/fb s = 7 TeV, data used L ~5 /fb s = 8 TeV.. L ~20 /fb ~ 0.5 Million Higgs produced/expt ~ 500 per hour! gluon-gluon fusion Vector boson fusion Associated productions with W, Z, top-pair One of the rare modes of production being studied: thq Total ~ 22 pb 12/5/2014 4

5 Main decay channels of the Standard Model Higgs boson Statistical analysis Background distribution mostly Gaussian stability of result expressed in terms of width s of the Gaussian. Characterization of excess using test statistic Significance where Greater the significance (s) minor the p-value lower is the chance that the observed excess is due to background fluctuation. 12/5/2014 5

6 Example of categorization of production process Higgs Hunt Comprehensive Higgs pursuit : survey of each accessible production mechanism for each primary decay mode Categorization provided better sensitivity Tagging a search channel via production mode Tagging done for decay mode as well. 0, and 1 jet boosted 12/5/2014 6

7 Individual analyses Illustration s obs /s SM = m measure of signal strength compared to SM expectation for Higgs mass at the fitted value. Significance of observation of 125 GeV Higgs boson: CMS summary 12/5/2014 7

8 Higgs 2 photons Tiny peak above a falling background distribution for invariant mass of 2 energetic, isolated g s. Branching ratio: 2*10-3 Huge SM background estimated by fitting distributions on the side bands with polynomials. Enormous effort to understand the detector response to photons. Signal strength 12/5/2014 8

9 H ZZ* 4 leptons Use matrix element calculations for signal and background processes to evaluate probability densities on event-by-event basis. Increased sensitivity to discriminate background against signal Used for discovery & determination of mass, width, spin, parity, couplings. Signal: 4 energetic isolated leptons Discovery based on 25 events (18 expected) Backgrounds: Irreducible backgrounds: qq ZZ 4l + gg ZZ 4l (at NNLO) Reducible background form Z+X, tt, Zbb Z 4l H 4l 4l continuum Kinematic information utilized from MEM to construct discriminant improves significance by 20%. Interference of diagrams for off-shell resonance & continuum background must be taken into account. 12/5/2014 9

10 H WW Decay leptons closer compared to background 2 (3) leptons + missing energy+ 0/1/2 jets V = W/Z CMS: measured rate less than that expected from SM m = Recent from ATLAS ggh production m ggf = /5/2014 VBF + VH production m VBF =

11 Higgs decay to fermions The fermionic decay channels of Higgs need to be observed to confirm the nature of coupling of H to fermions. Yukawa coupling G ff : ( 2G F /8p) m 2 f m H H gg : mixed coupling: both vector boson and fermions. H tt : evidence 3.2 s significant excess above background. H bb: 2.1 s observation, m = 1.0 ± 0.5 Search in W( ln) H, Z ( ll)h, (Z nn) H modes tth : excess, but presently large error Higgs decay to fermions established by combining results from direct decay channels WH, ZH, with H bb & H tt Combined significance obs.(exp.) = 3.8 (4.4) s m = 0.83 ±.24 12/5/

12 H tt 3 production modes, combined with different decays t e/m/ hadrons Invariably missing energy in final state event by event estimator for di t mass likelihood background only hypothesis includes H(125GeV) WW for each value of m H ATLAS: m = significance obs.(exp.) = 4.5(3.5) s, mass = 125 GeV CMS: m = 0.78 ± 0.27, m H = 125 GeV, significance obs.(exp.) = 3.2(3.7) s best fit for m H = 122 ±7 GeV Results consistent with SM 12/5/

13 H mm, ee Probes 2 nd and 3 rd generation couplings, lepton non-universality of H decay. Similar to H gg search, but even rarer, though clean. Br(H mm) = 2.2 *10-4, Br(H ee) = 5 *10-9 Huge background due to Drell-Yan Don t expect observation of SM signal in current data CMS, mm channel Observed significance < 1s signal strength = 0.8 ± 3.5 s/s SM Br (H ee) Br (H mm) upper limit = 7.4 * SM Br (H ee) upper limit = 3.7*10 5 SM Measured Br (H tt) = ± /5/

14 Higgs and the top quark Higgs coupling to top quark appears crucially in multiple aspects. tth coupling has been probed by searching in tth production with H bb Direct probe for Yukawa coupling Note top quark in the loop can be replaced by new particles suitably Search in events with 2 b-tagged jets and H hadrons, leptons, photons Production rate too low to be observed. Best fit for signal strength m = 2.8 ± 1.0 at 68% CL 12/5/

15 One-loop decay probing couplings complementary to H gg and H gg Rare and Exotic: H Z g SM Branching ratio ~0.1% In composite Higgs model, H Z g rate can be large, though H gg & H gg remain small Experimental advantage: highest resolution photons give usable S/B Run1: limited sensitivity in the 5-20X range/expt. No evidence ~10X SM limit observed/expt. 15

16 Compatibility of Higgs signal with standard model Production & decay tagged ATLAS: m = 1.3 ± 0.20 NNLO calculation of Higgs production test of theory framework For LO, m = 2.6 ± 0.4, NLO, m = 1.9 ± 0.3 CMS: m = 1.0 ± 0.09(stat. ) (theo.)± 0.07(syst) m (VBF+VH)/ m(ggh+tth) = Theo. Includes QCD scales, PDF + a s UEPS, BR 12/5/

17 Measurement of mass Fundamental property, not predicted by theory Once measured, SM predictions are completely determined Use resonance structure in high resolution channels H gg, H 4leptons CMS : ± 0.26 (stat.) ± 0.14 (syst.) GeV ATLAS: ± 0.37 (stat.) ± 0.18 (syst.) GeV CMS and ATLAS results compatible within errors Off-shell Higgs production diagrams interfere with the diagrams for continuum backgrounds cause a mass shift, significant effect for 2 photon mode. CMS observes opposite trend of ATLAS! Dm H = GeV ATLAS Dm H =1.47 ± 0.72 GeV 12/5/

18 Higgs couplings Cross section measurement mixes the couplings In Narrow width approximation Higgs Coupling to P at production Higgs coupling to X for decay If actual couplings are modified compared to SM SM K I =1 V = vev =246 GeV Eg. for gg H gg, assuming SM particles in the loop only, the rate can be modified. Or there could be new particles in the loop. Disentangle the couplings using multiple final states

19 Framework for fitting couplings Leading order tree-level motivated: K W, K Z, K t, K b, K t, Effective couplings for the loop-induced processes : K g, K g ie., H gg, H Zg, gg H Cross section rescaled using couplings Quantify possible small deviations Assumptions: Single resonance of mass 125 GeV Narrow-width approximation SM tensor structure of the Lagrangian (spin0, CP even) Current measurements insufficient to simultaneously fit all couplings with precision assume universality of vector and fermion couplings though actual couplings are still different. K W = K Z = K V, and K t = K b = K t = K f Invisible Higgs decay inferred from 12/5/

20 Measurement of couplings Bosonic couplings with custodial symmetry fermionic couplings Loop-induced couplings Top coupling from tth measurement Effective couplings Contribution from new physics Regge Plot: scale couplings according to mass For SM, vev = M= 246 GeV, e = 0 consistent with minimal scalar sector (only 1 Higgs ). Current precision of some couplings (W/Z, g, g) ~ % 12/5/

21 Global measurement of Higgs width Availability of decay channels for Higgs depend on its mass Fortuitous value of m H! fermionic decays are possible to observe. chance to study the Yukawa couplings. At 125 GeV, total decay width G H ~ 4.2 MeV Experimental resolution for 2 photon and 4 leptonic channels ~ 1-3 GeV! A direct measurement of width can indicate a wider resonance or mix of 2. iven the mass, use global fit to estimate total decay idth, fixing unmeasured modes to SM predictions. G H total = S i G i H + G H BSM Even if Higgs couples to any new massive particles total decay width may or may not be affected. Direct measurement of Higgs width H ZZ 4l and H gg combined. 12/5/2014 G H obs.(exp.) < 1.7 (2.3) GeV. 21

22 Measurement of width using interferometry qq ZZ and gg ZZ (continuum) gg H ZZ (resonance production) interference gg initiated processes. Analysis strategy: larger G H produces more events for large m ZZ >> m H Off-shell Simultaneous fit to mass distribution of on-shell and off-shell regions. Use H ZZ 4l and 2l 2n CMS: G H < 22 MeV ~ 5G H SM ATLAS: G H < ( )G H SM More details in parallel session 12/5/

23 Determination of Spin-Parity (J P ) state Higgs is the only particle known so far to have spin = 0 scalar boson Theoretical arguments forbids it to have spin =1, since it decays to 2g It could have spin = 2! Spin-parity and tensor structure probed with kinematic information no real measurement. experiments have tested alternative hypotheses utilizing kinematics of decay products: 0 + or something different. 12/5/2014 All exotic states excluded at 99.9% CL. J P hypothesis of 0 + favoured 23

24 Are there 2 degenerate states? Non-zero spin state scattering amplitude contains large no. of free parameters There may be 2 Higgs particles, of slightly different other properties, eg, J P eg: 2 m + : Kaluza Klein graviton like with minimum coupling 2 b + : Kaluza Klein graviton like with standard model in the bulk 2 h + : BSM tensor with higher dimension operators. 2 h - : BSM pseudo-tensor with higher dimension operators. Not possible to determine with H ZZ 4l mass But presence of a 2 nd state would modify the decay kinematics. The fraction of the 2 nd state determined using Assume: widths of SM or alternative states are all much smaller than experimental mass resolution. 12/5/2014 All fractions consistent with J P =

25 Heavy Higgs in H W(ln) W(jj) final state l = e, m Use angular correlation among final state particles as discriminant. standard model-like Higgs boson excluded for mass ranges GeV and GeV at 95% CL. No possibility of 2 Higgs-like particles jointly ensuring unitarization of WW scattering: a light (125 GeV) and a second heavy Higgs Invariant mass distributions for various mass hypothesis in m final state 12/5/

26 Higgs production in association with a single top quark Probes thq coupling destructive interference in SM sensitive to relative sign of Higgs boson coupling to fermions & bosons. Assume Yukawa coupling of Higgs boson to top quark Y t = -1 rate increases by a factor of ~13 ~ 234 pb. Final state studied 4b quarks, 1 lepton, 1 light jet Multivariate analysis technique Observation agrees with b only and s+ b hypothesis. Exclude production rate larger than 1.77 pb at 95% CL. 12/5/

27 Higgs self-coupling Higgs self-coupling: key aspect of electroweak symmetry breaking mechanism Observing two Higgs boson in the event is the only way to probe it. Accurate measurement may indicate the extension of Higgs sector, if any. Higgs potential SM production rate of double Higgs is small, signal interfere with background destructively. Enhancement possible through resonant production of H hh in MSSM, NMSSM, 2HDM, Higgs portal model etc. Very good prospect for HL-LHC, with L=3000 fb -1, ~ HH events bbgg : small rate but relatively clean signature bbww: large rate but large background as well 12/5/

28 Di-Higgs production via narrow resonance and decay to 4b final state Spin0 radion 8 TeV data after b-tagging and kinematic selections Spin2 KK graviton 12/5/

29 Invisible decay of Higgs boson Higgs can decay invisibly in SM via process Br(H ZZ* νννν) ~0.1% Current LHC results do not exclude the possibility of a sizeable decay branching ratio to invisible particles of the discovered Higgs boson. eg.1.: H to stable or long-lived particle(s) which can t be detected in expt. 2: H decays to a pair of dark matter candidate particles which have very low interaction probability with SM particles. Search performed in vector boson fusion and associated production modes Z (bb) H(inv.) overlaps with Z(nn) H(bb)! Combined Br(H invisible) < 58 (44)% obs.(exp.) 12/5/2014 Details in Parallel session 29

30 Search for Higgs bosons invoked in SUSY General 2Higgs doublet model with MSSM as the most popular scenario. 5 Higgs bosons: h, H, A, H ±, all can have massed at or below TeV access at LHC Various possibilities for flavour conserving couplings for the lightest neutral, h For high masses of H, A, H ± (decoupling), h is essentially SM Higgs For light H, A, H ±, cos (b-a) 0 little sensitivity to tanb Search for the H is extension of SM search in high mass region measurement of cross section complements limit on couplings. Additional scalar singlet is motivated by dark matter, may mix with SM Higgs various implications, not covered here. Due to lack of time, can hardly discuss any result. Several talks in parallel sessions BSM Higgs Invisible Higgs Search for Neutral MSSM Higgs Bosons in the Di-Tau channel Search for a low mass charged Higgs boson decaying into cs quarks. 12/5/ CMS results, just as a sample.

31 Search for H hh and A Zh 1. Gluon fusion to produce heavy H, decay to 2 SM like h Search for decays WWWW, WWZZ, WWtt, ZZZZ, ZZtt, ZZbb, tttt, ggww, ggzz,ggtt 2. A Zh in ll WW, ll ZZ, ll tt, ll gg (can probe upto 1.5 pb) Observed and expected limits with 1 and 2 s bands in mulyileptoon and gg channels No contribution from A Zh 95% CL Upper limit s (gg H) < 7 pb No contribution from H hh 95% CL Upper limit s* Br(A hh) < 2 pb Constrain on tanb vs. cos(b-a) plane, in the context of 2HDM 12/5/

32 Pseudoscalar boson A Zh llbb Narrow width approx. G A = 30 GeV 12/5/

33 Future 12/5/

34 Projections for measurements of Higgs in high luminosity LHC Higgs couplings can be measured with uncertainties ~ 5% with HL-LHC Expected deviation from SM O(v 2 /TeV 2 ) k parameters get small contribution but pattern may indicate specific physics. Set to explore new territory! 12/5/

35 ds/dm(vv) Scattering of longitudinal vector bosons Fundamental probe to test the nature of Higgs boson and its role in EWSB Each diagram ~ s 2 s(pp WW) > s(pp anything)! Unitarity restored by scalar Higgs Cancellation also requires Higgs < 800 GeV Taming the rate could be managed by alternative EWSB mechanism Search for possible resonances VV Scattering spectrum, σ(vv VV) vs M(VV) SB < 1TeV SB > 1TeV Eg.,sensitivity to anomalous WW SB sector SB sector resonances in Vector boson scattering weakly coupled strongly coupled 12/5/

36 Summing it up LHC experiments have moved from discovery to precision physics era in Higgs sector. 125 GeV Higgs has been extensively characterized with Run1 data. Current measurements are in agreement with minimal Higgs mechanism. No exotic discovery as yet Established : Origin of mass (scalar field BEH mechanism) of particles in a quantum field theory with local (point-like) gauge interaction. Starting from a reductionism strategy: question of structure of matter evolved into the question of origin of interactions (local gauge symmetries) and matter (interaction with Higgs field) The rise in centre of mass energy at LHC in next run, gives access to new territory for the search of the unexpected. Actually miles to go before we sleep! Stay tuned! However, we shall always manage to know only a drop of the ocean! 12/5/

37 Backup 12/5/

38 Precision for determination of signal strength 12/5/

39 Diboson production CMS Preliminary (Aug.2014) measurement of WW Cross section being finalized Latest from ATLAS 12/5/

40 Upper limit on spin-independent DM-nucleon cross section Note: 90% CL Upper Limit is used for all experimental measurements. CMS limit: < 95% CL 12/5/

41 Foundation for Higgs search: stairways to heaven Yesterday s discovery is today s background AND signal as well! 12/5/

42 Run III Run I Run II Run IV cm -2 s ns bunch high pile up cm -2 s ns bunch pile up cm -2 s ns bunch pile up 60 ~5(7.5!) cm -2 s ns bunch pile up ns 12/5/

43 Precision on Higgs coupling 12/5/

44 Lepton flavour violation in Higgs decays? If SM fermions mix with vector like fermions not all mass originate from Higgs mechanism. Higgs dceay can have lepton flavour violation Search for H mt in electron and hadronic decays of t Has to compete with H tt mode Transverse momentum distribution of m in LFV decay will have a Jacobian peak at 0.5* m H Expect small peak in visible mass distribution beyond Z tt decay 95% CL UL on B(H mt) <1.7% Utilizing ATLAS limit on H tt derived limits on LFV Yukawa couplings, from Br (H t m) Theoretical naturalness limit Y ij Y ji m i m j / v 2 12/5/

45 Parallel talk Charged Higgs search in fully hadronic final state Light Higgs(m = GeV) Heavy Higgs (m = GeV) 4-flavour scheme 5-flavout scheme t(b) H ± H ± t n t b H ± H ± t n In (m H±, tanb) plane large regions excluded for light Higgs, except tanb 9, m H± > 150 GeV No luck till now, score cards 0 for BSM Higgs Heavy Higgs has also been searched in H ± tb mode via inclusive dilepton and single lepton final state. 12/5/

46 MSSM Higgs to tau pair 12/5/

47 Improvement in searches for future MSSM Higgs h, H, A Small, moderate tan b NOW large tan b Future LHC 13 TeV 300/fb Cross sections scaled by expected parton luminosity 12/5/

48 ATLAS coupling 4 Independent signal strengths + SM decay branchings 12/5/

49 Experiments almost ready 12/5/