Rare and Exotic Decays of Higgs boson

Transcription

1 Rare and Exotic Decays of Higgs boson Kajari Mazumdar Tata Institute of Fundamental Research Mumbai Plan of talk Status of measurements for standard model Higgs boson Example of a rare decay Search for invisible decay of Higgs boson, produced in vector boson fusion process Indian Association for Cultivation of Sciences, Kolkata

2 Interaction processes at LHC Cross section in nano-barn Event rate : dn/dt = σ. L Maximum L = 7.54 *10 33 /cm 2 /s σ ~ 20 pb = 2*10 35 cm 2 for 125 GeV Higgs boson ~ 1500 Higgs events produced /sec.! Efficiency of actual measurement <<1 2

3 Total weight 1400 t Overall diameter 15m Overall length 28.7m ECAL 76k scintillating PbWO 4 crystals HCAL Scintilator/brass interleaved 3.8T Solenoid CMS IRON YOKE MUON ENDCAPS Cathode Strip Ch. (CSC) and RPC Tracker Pixels (100x150 µm 2 ) ~ 1m 2 66M channels Silicon Microstrips ~ 207m 2 9.6M channels MUON BARREL 7/10/2013 Drift Tubes (DT) and 3

4 Basic Principles of experiments Need general-purpose experiment covering as much of the solid angle as possible ( 4π ),since we don t know how New Physics will manifest itself. Detectors must be able to detect as many particles and signatures as possible: e, µ, τ, ν, γ, jets, b-quarks,. Momentum / charge of tracks and secondary vertices (e.g. from b-quark decays) are measured in central tracker (Silicon layers). Energy and positions of electrons and photons measured in high resolution electromagnetic calorimeters. (~ E T ~ 50 GeV) Energy and position of hadrons and jets measured mainly in hadronic calorimeters Muons identified and momentum measured in external muon spectrometer (+central tracker) 100GeV and <10%@1 TeV Neutrinos detected and measured through measurement of missing 4 transverse energy (E miss T ) in calorimeters (hermeticity good resolution)

5 A typical photo November/December 2009 first LHC collisions 57/10/2013

6 Standard Model Higgs boson production at LHC and decays 6

7 Minimal recapitulation of statistical jargons Define σ obs /σ SM = µ measure of signal strength compared to SM expectation Background distribution mostly Gaussian stability of result expressed in terms of width σ of the Gaussian. Greater the significance (σ) minor the p-value lower is the chance that the observed excess is due to background fluctuation. 5σ, or p-value = 2.87 x 10-7 probability that the observed excess is a statistical fluctuation of background only 1 time of each 3 million and half experiments [1/p]. 7/10/2013 7

8 Current Higgs results from bosonic decays H ZZ 4l H γγ, Br=0.228 H ZZ+γγ combined mass ATLAS: ± 0.2 ± 0.6 GeV CMS : ± 0.23 ± 0.3 GeV H WW* lνlν 8

9 Search for H Ζγ process Branching ratio = 0.154% Similar to H γγ analysis, but only one photon + 2 muons compatible with Z ATLAS: µ = 11(9) observed (expected) CMS: µ = 10(10) observed (expected) Even rarer decay H γ γ µµγ Dalitz decay. Production rate ~ 1 fb. 1,2 dominant 3 FSR at high Mµµ 6 can be identified through resonance structure in Mµµ 4,5,7 negligible Backgrounds to worry about pp Z γ, H Z γ Observed Upper limit at m H =125 GeV and M µµ < 20 GeV 10 σ SM 9

10 Current Higgs results from fermionic decays Crucial to establish the resonance as standard model Higgs boson Γ ff : ( 2G F /8π) m f 2 m H LHC studies of fermionic modes have less sensitivity suffer from high backgrounds Used discriminants: m bb, m ττ, MVA tth, H bb VH, H bb H ττ H µµ /ee Combining all final States of tth µ = VH, H bb, V=W,Z Observation of gg tth direct access to top-higgs Yukawa coupling CMS: µ = 1.0 ± 0.5(stat. + syst.) observed(expected) significance 2.1 σ (2.1 σ) ATLAS: µ = 0.4 ± 0.5 (stat.) ± 0.5(syst.) does not observe any excess tth coupling consistent with SM 10

11 H µµ, H ee Significantly low branching ratios: Br(H µµ)= , Br(H ee)= 4*10-9 Compare with Br (Z µµ) = 3.37 and Br (H γγ) = 2.1 *10-3 H µµ : one of the rarest decay modes but also one of the cleanest signals. Gives access to the Higgs decay into 2 nd generation fermion pair probes into Yukawa coupling of Higgs to fermions. Look for narrow peak on steep falling background due to Z/γ* µµ Fit M µµ in data with analytical shapes for signal and background. Not yet reached the sensitivity for observation of SM process If observed would indicate huge contribution from a non-sm particle CMS upper limits on σ*br H ee: 0.38 pb H µµ: 0.34 pb Some of the details of CMS analysis for H µµ follows 11

12 Search strategy Focus on GF and VBF, but include associated production modes as well. GF: largest signal, but also has largest irreducible DY background. VBF: smaller signal, but has better discrimination against background. Higgs natural width very small: ~4 MeV experimental resolution for measuring resonance is very important. ( M mm ~ 2 GeV, varies with η) Mass measurement dominated by resolution in p T apply state-of-the art momentum scale correction Need to also account for final state radiation off muon 12

13 Key points of H µ + µ analysis Dominant production process, gluon-fusion (GF) produces longitudinally boosted H. Higher order quantum corrections result in i) non-zero transverse momentum (p T ) of Higgs ii) Additional jet(s) accompanying Higgs in the final state The irreducible background Drell-Yan process can also have dimuon system with non-zero p T, due to higher order corrections. But the nature of initial state effect of corrections more for Higgs than DY. Vector-boson fusion (VBF) process produce Higgs (i) accompanied by 2 jets with distinct topology (ii) with significant p T 13

14 Categorize events to increase sensitivity of analysis Barrel central region of the detector which detects particles produced at high azimuthal angle/ high p T Resolution is better in this region. 14

15 Invariant mass distributions of dimuon system in different categories 15

16 Systematic uncertainties in H µµ analysis Shape uncertainties: signal model shape. Non-shape uncertainties: affects signal yield. Effects studied as a fn. of energy/production mechanism/mass/category. independent of mass at a given energy, category, production process. MC statistics < 1-8% depending on category. For non-shape systematics, signal efficiency calculated every time. PDF uncertainty, UE tunes, Jet energy resolution, jet energy scale. Main uncertainty in signal shape is related to muon pt scale (Muscle appld.) Mean of Gaussian varied by 0.2% Signal width allowed to vary in fit by 3(2)% at 7(8) TeV Uncertainty in luminosity of LHC 2.2 (4)% at 7(8) TeV (remember N = σ ʃl dt ) Total experimental signal systematics ~ 1% 16

17 Final results Signal modeling: double Gaussian with different sets of mean and width All parameters floating in the fit performed in mass range Mµµ = GeV.. Parametric background modeling (by fitting data): exp(p 2 M)/(p 1 -M) 2 CMS upper limit on σ*br H ee: 0.38 pb H µµ: 0.34 pb 17

18 Evidence for H ττ Crucial for establishing non-zero Higgs coupling to down-type fermions Branching ratio ~ 0.06 Searched in multiple production mode and various decay channels of τ Further categories based on jet multiplicity and transverse momentum (exploit transverse boost of the produced H to reduce backgrounds) ATLAS: µ = 1.5 ± 0.5(stat. ) ± 0.4 ( syst.) Observed (expected) significance 4.1 σ (3.2 σ) CMS: µ = 0.78 ± 0.27 Observed (expected) significance 3.2 σ (3.7 σ) Evidence significance More than 3σ for H ττ 18

19 Measurement of Higgs properties Ultimate precision on Higgs mass expected in combining ATLAS and CMS : ~ 0.2% Spin parity measurement Distribution of production angle of daughters sensitive to spin/parity of the parent Higgs. a) For H ff b) H ZZ 4 l Extremely rich in angular information best suited to study spin, parity No real measurement of spin, parity. Experiments have tested alternative hypothesis: O + or something different? Data favours standard model hypothesis BEH boson! Higgs width can t be measured in LHC experiment directly, too small (~4 MeV), compared to experimental resolution. Indirect ways have put an upper limit: 3.4(3.7) GeV from γγ (4l) decay mode 19

20 Couplings of Higgs boson Within errors, couplings are equal to standard model expectations. 20

21 Invisible decay of Higgs boson Higgs can decay invisibly in SM via process H ZZ* νννν (~0.1%) LHC results do not exclude the possibility of a sizeable branching ratio to invisible particles of the SM Higgs boson candidate of mass 125 GeV. eg., i) H 2LSPs in SUSY ii) H graviscalar in the ADD model Use global fit to measured decay modes while fixing unmeasured modes to SM predictions. Interprete in terms of Br(H inv.) for mh = 125 GeV Look for additional new physics contribution to gg and γγ loops Modification of width from the loops are kept free ATLAS: Br(H inv.) < 0.41@ 95% CL (exp.<0.55) CMS: Br(H inv.) < 0.52@ 95% CL (exp.<0.67) 21

22 Where to look for invisible Higgs boson LEP experiments excluded an invisibly decaying Higgs boson for m H < GeV assuming it is produced in association with Z(e + e - Z* Z+H inv ) and that it decays predominantly to invisible particles. Direct search for a narrow scalar boson decaying to invisible particles also assumes that it decays predominantly to invisible particles. Search in mass range between 115 to 300 GeV. Exploited channels so far: ATLAS: Z(ll)+ E T mis CMS: Z(ll, bb)+ E T mis, CMS NEW: VBF jets + E T mis Some details of CMS analysis of VBF mode The event can t be triggered is Higgs is produced single & decays invisibly. VBF process has higher rate than associated Higgs (VH, tth) production Events are tagged with 2 jets and large missing transverse energy Perform simple counting experiment Excess over expected SM background in a suitable distribution would indicate contribution due to invisible decays. 22

23 Candidate event display Final state of VBF process: Only 2 jets and missing transverse energy Estimate main backgrounds using data-driven methods, since monte carlo may not model well the corner of phase space occupied by signal. Major backgrounds: Z ( νν)+jets, W+jets and QCD multijets Estimate minor backgrounds (tt, WW, ZZ, WZ, etc.) using monte carlo. 23

24 Trigger & offfline selection of events To start with look for inclusive dijet + E miss T final state daunting QCD multijet process (about 8 orders of magnitude higher than signal) Apply loose VBF-like criteria at trigger and more stringent conditions at analysis level. 2 jets p T >40 (50) GeV, η <4.7 η1*η2<0 Event reconstruction: anti-kt jets η jj >3.5 (4.2) M jj > 800 (1000) GeV Missing transverse energy > 65 (130) GeV Veto events with leptons if p T > 10 GeV Central jet Veto: p T >30 GeV, η1<η <η2 Final signal region φ jj < 1.0 rad 24

25 Estimation of Z ( νν) + 2jets background Use Z ( µµ /ee )+ 2jets background control sample to estimate rate of Z νν + 2jets background Select dimuon events (no 3 rd hard lepton) with invariant mass between GeV. Define missing transverse energy without counting the muons Apply VBF criteria on jets MET > 130 GeV N C obs : Observed dimuon + dijet events satisfying above criteria N C bkg : backgrounds in dimuon + dijet events satisfying above criteria Other than Z µµ events, e.g., tt, WW, WZ, ZZ. estimated from monte carlo From MC estimate efficiency factors ε µµ :dimuon selection efficiency N s ε = 68.2± 9.2 ε s VBF : Z νν in signal region ε c VBF Z µµ in control region N s µ = 67.2±

26 Estimation of W ( lν)+2jets background Number of W lν (l= e, µ) events in the signal region, where the lepton is not identified. Keep only one lepton in control region, veto additional leptons W τ ν, where τ decays through hadrons to be treated differently. Define a hard tau in central region. Do not apply central jet veto Efficiencies estimated from W τ had MC τ selection 16%, CJV eff. ~ 42% N s τ = 54± 16 Add 20% theoretical uncertainty on the estimates of all V+jets backgrounds 26

27 Estimation of QCD multijet background from data ABCD method based on MET and CJV selections, Assuming these 2 variables are uncorrelated. To gain statistics of QCD multijet events, choose φ > 2.6 rad. (signal has φ< 1 rad.) A: fail MET, fail CJV B: pass MET, fail CJV C: fail MET pass CJV D: pass MET, pass CJV (signal region) MET> 130 GeV, no 3 rd jet above pt>30 GeV Total QCD background in signal region ~ 31 events, N_D = N_B*N_C /N_A Systematic uncertainty ~ 40% Estimate of signal and background events in signal region 27

28 Full selection: signal and control regions Signal region : φ< 1 rad. Z ll control region 28

29 Limit setting CMS: 95% confidence level upper limit on branching ratio to invisible decay H invisible, by combining ZH (H ll or bb) and VBF production modes: 58(44)% obsd.(expected) Indirect limit from total width:< 52(56)% ATLAS: 75(62)% obsd.(expected) in Z( ll)h Combining direct and indirect analyses Br(H inv.) < 37 (39)% 29

30 Dark matter interpretation of invisible width Higgs portal model Higgs acts as mediator between SM particles and DM particles Dark matter candidate couple mainly to standard model Higgs boson Complementary search for DM through invisible decay of Higgs Look for H χχ using invisible branching ratio There is no sensitivity to these models once the mass of the DM candidate exceeds m H /2. Partial width Γ inv depends on spin (single DM candidate is considered and is either a scalar, a vector or a Majorana fermion, eg.. In direct detection expt. DM-nucleon scattering takes place via Higgs exchange For a give mass of dark matter an upper bound on invisible decay of Higgs puts a limit on the upper bound on the DM-nucleon cross section. m N : nucleon mass, f N : Higgs-nucleon coupling parameter = from lattice calculation 30

31 Upper limit on spin-independent DM-nucleon cross section Note: 90% CL UL is used for all experimental measurements. CMS limit: < 51% 31

32 conclusion Run1 of LHC was a success! Higgs boson is discovered Higgs boson observed in multiple channels. Properties of the resonance are being established. Most likely it is the standard model Higgs boson No significant deviation observed so far from SM predictions. Higgs bosons predicted in beyond standard model scenarios are still being searched with accumulated data at centre of mass energies7 and 8 TeV LHC Run2 starts in 2015 at CM energy 13 TeV and higher luminosity. It will be an even more interesting time! Stay tuned! 32

33 Backup 33

34 Monte carlo samples 34

35 35

36 LHC motto: explore, search, measure Collision of the protons occasionally collates sufficient energy so that heavy particles could be produced in the lab ( E = mc 2 ) Subprocess energy : s E 2 ~ s = s x 1 x 2 x i = p i z / Eb Eb = beam energy Each constituent of proton carries only a fraction of the proton s energy effective energy ( s ) and hence the inelasticity of the event varies. possibility of producing various new particles of different masses Higgs of any mass within the allowed range could be produced at LHC Higgs boson discovered on July 4,