Higgs boson measurements at ATLAS

Transcription

1 Higgs boson measurements at On behalf of the Collaboration Oxford University A comprehensive set of Higgs boson measurements has been performed in pp collisions produced by the Large Hadron Collider at centre-of-mass energies of 7 and 8 ev, and the results combined between the and CMS experiments. Recent results from at a centre-of-mass energy of 3 ev are consistent with expectations. With more data available, additional Higgs boson processes are on the cusp of observation, while measured processes promise improved precision. HEPMAD 6, 8th High-Energy Physics International Conference October 38, 6 Antananarivo, Madagascar Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

2 . Introduction Since the discovery of the Higgs boson in, the Large Hadron Collider (LHC) has produced more than ten times more Higgs bosons. hese bosons have been measured with increasing precision by the and CMS experiments, which have also provided first evidence of the rarer production and decay processes. hese proceedings first review the Higgs mechanism, then present the recent Higgs boson measurements, and finally discuss searches for rare Higgs boson processes. he focus is on results from the experiment [].. he Higgs mechanism In a quantum field theory, a scalar field can be represented by a complex number with dimensions of mass at each point in spacetime. he symmetry of the equations of motion under a particular gauge group transformation means that physical processes depend only on the magnitude of the field and not on its phase. he equations of motion can be derived from the Lagrangian of the scalar field, which can be represented as L (φ) = (D φ D φ + φ φ) λ(φ φ) 4 F νf ν, (.) where φ is the scalar field, D is the covariant derivative, F ν is the gauge field strength, and λ and are real-valued parameters. If both λ and are positive then the potential has a minimum at φ φ = /(λ). For the Higgs field this value is 46 GeV, and quantized oscillations about this minimum correspond to the physical Higgs boson. Because the covariant derivatives contain the gauge field, a non-zero magnitude of the scalar field corresponds to a potential term quadratic in the gauge field. his term results in massive gauge bosons. Massless gauge bosons have spin projections of ±, while a massive gauge boson can also have a spin projection equal to. he acquisition of this additional degree of freedom can be made explicit by choosing a gauge coordinate system where the scalar field has zero phase at every point in spacetime: φ = e iε/φ φ A = A + eφ ε, (.) where φ is the vacuum expectation value of the scalar field, A is the gauge field, and ε is the spacetime-dependent phase of a particular coordinate system of the scalar field. he removed phase of the scalar field appears as a spin- component on the gauge field. 3. Higgs boson measurements he increasing energy and luminosity of the LHC has resulted in progressively increasing Higgs boson yields. hese yields are shown in able for a Higgs boson mass of 5.9 GeV and the current integrated luminosity collected by. More than million Higgs bosons have been produced, though the majority of the events are not measurable because of the overwhelming strong production of b b pairs.

3 Process Produced events ( 3 ) Status 7 ev 8 ev 3 ev Production gg H Observed qq Hqq Evidence q q V H Evidence q q/gg t th Evidence Decay H b b > 95% C.L. signal H WW Observation H τ + τ Evidence H ZZ Observation H Observation able : he yields for various measured production processes and decay channels measured at the LHC. ggf and CMS LHC Run +CMS CMS ±σ ±σ γ γ and CMS LHC Run +CMS CMS ±σ ±σ VBF WH ZH ZZ WW tth ττ bb Parameter value Parameter value Figure : he ratios of measured cross sections to the SM predictions for various production processes (left) and Higgs boson decays (right), for the combined set of decay channels (left) or production processes (right) including both and CMS data at s = 7 and 8 ev []. he final combined and CMS results with s = 7 and 8 ev data [] are shown in Fig., expressed as a ratio of the measured cross section to the Standard Model prediction. he ratios can be recast as coupling multipliers κ to the Higgs boson interaction terms in the Lagrangian. If the measured multipliers are equal to one, the interaction strength has the mass dependence predicted by the SM. Figure shows the consistency of the measurements with the predictions as a function of particle mass. he first Higgs boson cross section measurements at s = 3 ev have been performed in its decays to ZZ and. Figure 3 shows the expected increase in production cross section in the 3

4 v V m κ V and CMS LHC Run W Z t v F m κ F or τ b 3 4 +CMS SM Higgs boson [M, ε] fit 68% CL 95% CL Particle mass [GeV] Figure : Combinations of Higgs boson coupling multipliers κ F (to fermions) or κ V (to gauge bosons) with the ratios of particle mass to the Higgs vacuum expectation value v []. [pb] σ pp H 8 Preliminary σ pp H m H = 5.9 GeV H γ γ H ZZ * 4l comb. data syst. unc. QCD scale uncertainty ot. uncert. (scale PDF+α s ) ZZ ggf ggf Preliminary m H =5.9 GeV s=3 ev, 3.3 fb (), 4.8 fb (ZZ) Observed 68% CL SM Prediction 6 ZZ VBF 4 VBF s = 7 ev, 4.5 fb s = 8 ev,.3 fb s = 3 ev, 3.3 fb (γ γ ), 4.8 fb (ZZ *) s [ev] VHhad VHlep top Parameter value norm. to SM value Figure 3: he measured Higgs boson production cross sections in its decays to and ZZ bosons, and the two decay channels combined. he production cross section as a function of energy (left) and for each production and decay process (right) are shown [3]. combined measurement and the measured cross sections of each production process [3]. Ratios to the SM cross sections are not given so that the uncertainties on the measurements can be separated from those on the predictions. Future measurements will subdivide the cross sections into various kinematic regions to produce simplified template cross sections. Measurements of the Higgs-boson interactions with third-generation quarks are important. he coupling strength between Higgs bosons and top quarks can be inferred from the measurement 4

5 tth(h ) (3 ev 3.3 fb ) tth(h WW/ττ/ZZ) (3 ev 3. fb tth(h bb) (3 ev 3. fb ) tth combination (3 ev) ) Preliminary total stat. s=3 ev, fb ( tot. ) ( stat., syst. ) (, ) (, ) (, ) (, ) tth combination ( 7-8eV, fb ) (, ) best fit for m H =5 GeV tth Figure 4: he ratio of measured cross sections to SM predictions for tth production in various decay channels, as well as their combination at s = 3 ev [4]. For comparison the combined result at s = 7 and 8 ev is also shown. of gluon fusion production, which is consistent with the SM prediction. However, to ensure that the gluon fusion process does not contain non-sm contributions, a direct measurement of the tth coupling is required, through tth production. he combined measurement at s = 7 and 8 ev showed a factor of excess over the SM prediction (Fig. ), though it corresponds to a < σ deviation. A combined measurement from at s = 3 ev [4] shows a somewhat smaller and less significant excess (Fig. 4). he sensitivity to a possible non-sm rate will continue to increase as more data are collected. he largest branching fraction of the Higgs boson is to b-quark pairs; a small change in this fraction has a substantial impact on other decay rates. he most promising process for measuring this decay is in V H production. Evidence for this production and decay was observed at the evatron and the s = 7 and 8 ev combination has more than σ significance, with the measured rate somewhat lower than expected (Fig. ). A first measurement at s = 3 ev from has been performed [5], with the result again showing a lower rate than expected but with larger uncertainties than the combination (Fig. 5). o add sensitivity to the H b b decay channel, has performed a first probe for this decay in vector-boson fusion production [6]. o reduce background from gluon-initiated jets, a photon is required in the event in addition to two b-quark jets and two forward jets resulting from the vector-boson radiation (Fig. 6). he analysis constructs a discriminant to separate signal from background using a boosted decision tree (BD). he inputs to the BD are the invariant mass and pseudorapidity separation of the forward jets; the width of the forward jets; the centrality of the photon between the jets; and the presence of low-p t jets (near the selection threshold). As a first step, the presence of Z-boson production is tested, with an expected significance of.3σ; the expected significance of Higgs boson production is.4σ. he observations are consistent with expectations, demonstrating a sufficient understanding of the background to contribute to an H b b measurement with more data. 5

6 Preliminary ot. Stat. s=3 ev, L dt= 3. fb ot. ( Stat. Syst. ) Events / 3 GeV Preliminary s = 3 ev Ldt = 3. fb lep., jets, tags V p 5 GeV Data VH(bb) (=.) Diboson tt Single top Z+(bb,bc,cc,bl) Uncertainty Pre-fit background ZH ( ) WH ( ) Combination ( ) Best fit =σ/σ for m H =5 GeV SM Data/Pred m bb [GeV] Figure 5: Left: he ratio of measured Higgs boson production cross sections to the SM expectation in its decays to b b and produced via WH, ZH, or the two processes combined. Right: he invariant mass of the b-quark pairs in the analysis category with two leptons, two identified b-quark jets, and no additional jets [5]. q q γ Events / GeV 7 Preliminary s = 3 ev,.6 fb 6 High BD 5 Data VBF H(5) + γ x Z + γ (QCD) Z + γ (EWK) NonRes Bkgd Uncertainty 4 W/Z b 3 W/Z H b q q (Data-Bkg)/Bkg m bb [GeV] Figure 6: Left: he Feynman diagram for vector-boson production of a Higgs boson in association with a photon, with the Higgs boson decaying to a pair of b-quarks. Right: he invariant mass of the b-quark pairs in events with high BD score to enhance the signal purity [6]. he overall consistency of the measured and predicted Higgs-boson decays to gauge bosons motivates a more detailed study of these channels. Differential measurements of the production kinematics improve sensitivity to non-sm processes, particularly in regions of high momentum transfer. First measurements have been performed in the ZZ and decay channels in s = 7 and 8 ev data; new measurements in the channel study kinematic distributions such as the jet multiplicity and p of the Higgs boson candidate using s = 3 ev data (Fig. 7) [7]. Additionally, measurements of Higgs boson decays to WW bosons have been recently performed with s = 8 ev data (Fig. 8) [8]. he constraints on new physics imposed by differential measurements can be quantified using an effective field theory (EF) [9] containing dimension-6 operators in the Lagrangian suppressed by factors of v /Λ, where v is the Higgs vacuum expectation value and Λ is the energy scale 6

7 [fb] σ fid Preliminary data, tot. unc. syst. unc. H γ γ, s = 3 ev, 3.3 fb 4 m H = 5.9 GeV gg H NNLOPS + XH k gg H =. XH = VBF + VH + tth anti k t R =.4, p > 3 GeV dσ fid /dp [fb/gev].5 Preliminary data, tot. unc. syst. unc. H γ γ, s = 3 ev, 3.3 fb m H = 5.9 GeV gg H NNLOPS + XH k gg H =. XH = VBF + VH + tth.5 data / prediction = = = 3 N jets data / prediction p Figure 7: he measured and predicted differential jet multiplicity (left) and Higgs-candidate p (right) distributions for gluon fusion production and H decay at s = 3 ev [7]. [fb] /dn jet dσ fid 3 5 data, tot. unc. gg H sys. unc. σ LHC-XS σ LHC-XS σ LHC-XS BLPW s = 8 ev,.3 fb H WW* eνν A NNLOPS+PY8 A MG5_aMC@NLO+PY8 A SHERPA.. [fb/gev] /dp dσ fid H data, tot. unc. gg H sys. unc. σ LHC-XS σ LHC-XS σ LHC-XS s = 8 ev,.3 fb H WW* eνν A NNLOPS+PY8 A MG5_aMC@NLO+PY8 A SHERPA.. 5 Ratio to NNLOPS = = N jet 3 [,] [,6] [6,3] Figure 8: he measured and predicted differential jet multiplicity (left) and Higgs-candidate p (right) distributions for gluon fusion production and H WW decay at s = 8 ev [8]. Ratio to NNLOPS ph [GeV] of physics beyond the Standard Model. Constraints on the coefficients of these operators c i have been determined using differential measurements of H production at s = 8 ev, as shown in Fig. 9 []. Within the context of an EF one can test for the existence of dimension-6 operators that describe CP-violating interactions between the Higgs and gauge bosons. he relative angles of the final-state jets in vector-boson fusion can be used to probe these operators. Using the full information from the matrix element, an optimal observable can be defined that quantifies the contribution of the interference between the CP-violating and SM matrix elements, relative to the 7

8 [fb] σ fid pp H γ γ, p [GeV] = - 8- s = 8 ev,.3 fb = - = 3 N jets π/6- π π/3-5π/6 π/3- π/3 - π / mjj [GeV] data Standard Model c g =. =.5 c HW φjj j p [GeV] 3 c g c ~ g pp H γ γ, s = 8 ev,.3 fb H γ γ, s = 8 ev,.3 fb internal N jets 3 = = pp Preliminary.8 ±. 4.7 ±. 7.7 ±.. ±. 7.3 ±. 4.3 ±. 5. ±. 9. ±. 3. ±. 5. ±. 9.7 ±. 9.4 ± ±..7 ±. 8.6 ±.9.4 ±..5 ±.9 3. ±.9.5 ±..6 ±. 9.4 ± Standard 3.7Model.3.3 = ±.5 ±. ±. ±. ±. ±. ±. ±. 95% CL % 6-8 CL p [GeV] c γ.9 ±..5 ±. 5.3 ± Statistical correlation [%] Figure 9: Left: he set of measured differential distributions for Higgs bosons decaying to two photons, compared to the SM prediction and to models with additional dimension-6 terms in the Lagrangian. Right: Constraints on the values of two dimension-6 operator coefficients []. Events / bin τ τ Signal Region lep lep s = 8 ev,.3 fb Data VBF H ( ~ d=) ggh/vh Z ττ tt+single-top Fake lepton Others Uncert. Events / bin τ τ Signal Region lep had 35 Data s = 8 ev,.3 fb VBF H ( 3 5 ~ d=) ggh/vh Z ττ tt+single-top Fake τ Others Uncert Optimal Observable Optimal Observable Figure : Left: he set of measured differential distributions for Higgs bosons decaying to two photons, compared to the SM prediction and to models with additional dimension-6 terms in the Lagrangian. Right: Constraints on the values of two dimension-6 operator coefficients []. square of the SM matrix element. his observable has been studied by in vector-boson fusion with the Higgs boson decaying to tau-lepton pairs (Fig. ) []. he coefficient of the CP-violating operator is constrained to be between -. and.5 at 68% confidence level if the contribution from new physics has a scale equal to the W boson mass. 4. Higgs boson searches here are many rare Higgs boson production and decay processes predicted by the Standard 8

9 Process Produced events Status 7 ev 8 ev 3 ev gg HH 3 3 σ < 9σ SM H Zγ σ < σ SM H 8 44 σ < 3.5σ SM H J/ψγ.. 5. σ < 54σ SM H φγ σ < 6σ SM H ϒγ.5.3. σ <. 6 σ SM able : he yields for various production processes and decay channels not yet measured at the LHC. Model that have yet to be observed. In many cases these provide the only access to the couplings described by the Lagrangian. An overview of the yields of a number of rare processes is given in able. A particularly important process is Higgs boson pair production, which gives the most direct access to the Higgs boson self-coupling. his self-coupling arises from the λ(φ φ) term in the Lagrangian in Eq.. that gives rise to a non-zero vacuum expectation value in the SM. Due to destructive interference between the self-coupling Feynman diagram and the diagram with the two Higgs bosons radiated by the top quarks in the loop, the rate for Higgs boson pair production is very small in the SM; however, non-sm contributions can significantly affect the rate. Searches for this process are challenging because of the low rate and the large number of possible decay channels. searches at s = 8 ev in the decays to b bb b, b bτ + τ, b b, and WW have set upper limits of 63, 6,, and 5, respectively, on the ratio of the production cross section to the SM prediction []. New results at s = 3 ev in the b bb b [3], b b [4], and WW [5] channels give corresponding limits of 9, and 75, respectively. Distributions from the b bb b and b b channels are shown in Fig.. In the b bb b channel, the sensitivity to HH production is primarily at high invariant mass of the Higgs-boson pair, where the diagram with two radiated Higgs bosons dominates. he b b channel is sensitive over the full region of invariant mass but has lower statistical sensitivity due to the low branching ratio of the Higgs boson to a photon pair. he next Higgs boson process within reach in the next few years is its decay to muon pairs. Such a measurement would provide the first verification that the second generation fermions follow the same mass generation mechanism as those of the third generation. A recent search from at s = 3 ev divided the data into six regions with different gluon-fusion purity, and into a vector-boson fusion region [6]. he dimuon mass distribution in the vector-boson fusion region is shown in Fig.. esting the Higgs-boson interactions with second-generation quarks is more challenging. One strategy is to search for decays to quark-antiquark mesons in association with a photon. However, this decay is dominated by the splitting of an off-shell photon in H decays rather than the direct Higgs-quark-quark coupling. Nonetheless a constraint can be made on the coupling by searching for decays to a meson and a photon. Recent searches have constrained the cross section for Higgs boson production and decay to J/ψγ [7] (φγ [8]) to be less than 54 (6) times the SM prediction, using s = 8 (3) ev data. 9

10 Events/ GeV 3 Data Preliminary s = 3 ev, 6,. fb Signal Region: Resolved Multijet tt G(3) G(8) SM hh 5 Stat+Syst Uncertainty Events /.5 GeV Preliminary s = 3 ev, 3. fb tag signal region Di Higgs Single Higgs Continuum Bkg. Sum Data Data/Bkgd m 4j [GeV] Data Fit m [GeV] Figure : Left: he invariant mass of the pair of Higgs-boson candidates in events with four jets identified to originate from b-quarks [3]. Right: he invariant mass of two photons in events containing two b-quark jets [4]. Entries / GeV Pull VBF Preliminary s = 3 ev, 3. fb Data Background model Signal [5] mll m [GeV] Figure : he invariant mass distributions of muon pairs in events consistent with the vector-boson fusion production of a Higgs boson [6].

11 5. Summary he prolific production of Higgs bosons at the LHC allows a rich experimental program of Higgs boson measurements. Production and decay processes with the highest rate have been used to study the production of Higgs bosons in detail. With the continued increase in collisions, rarer processes involving interactions that have never been observed will become accessible to the experiments. Current searches lay the groundwork for these potential observations, and have recently begun with data at the record centre-of-mass energy of s = 3 ev. Given that only % of the expected number of collisions have been produced, there is much to look forward to in Higgs boson measurements and searches at the LHC. References [] Collaboration, he Experiment at the CERN Large Hadron Collider, JINS 3 (8) S83. [] and CMS Collaborations, Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined and CMS analysis of the LHC pp collision data at s = 7 and 8 ev, J. High En. Phys. 8 (6), 45. [3] Collaboration, Combined measurements of the Higgs boson production and decay rates in H ZZ 4l and H final states using pp collision data at s = 3 ev in the experiment, -CONF-6-8, [4] Collaboration, Combination of the searches for Higgs boson production in association with top quarks in the, multilepton, and b b decay channels at s = 3 ev with the Detector, -CONF-6-68, [5] Collaboration, Search for the Standard Model Higgs boson produced in association with a vector boson and decaying to a b b pair in pp collisions at 3 ev using the detector, -CONF-6-9, [6] Collaboration, Search for Higgs boson production via weak boson fusion and decaying to b b in association with a high-energy photon in the detector, -CONF-6-63, [7] Collaboration, Measurement of fiducial, differential and production cross sections in the H decay channel with 3.3 fb of 3 ev proton-proton collision data with the detector, -CONF-6-67, [8] Collaboration, Measurement of fiducial differential cross sections of gluon-fusion production of Higgs bosons decaying to WW eνν with the detector at s = 8 ev, J. High En. Phys. 8 (6), 4. [9] D. de Florian et al., Handbook of LHC Higgs Cross Sections: 4. Deciphering the Nature of the Higgs Sector, arxiv:6.79 [hep-ph]. [] Collaboration, Constraints on non-standard Model Higgs boson interactions in an effective Lagrangian using differential cross sections measured in the H decay channel at s = 8 ev with the detector, Phys. Lett. B 753 (6), 69. [] Collaboration, est of CP Invariance in vector-boson fusion production of the Higgs boson using the Optimal Observable method in the ditau decay channel with the detector, arxiv:6.456 [hep-ex].

12 [] Collaboration, Searches for Higgs boson pair production in the hh bbττ,ww,bb,bbbb channels with the detector, Phys. Rev. D 9, 94 (5). [3] Collaboration, Search for pair production of Higgs bosons in the b bb b final state using proton proton collisions at s = 3 ev with the detector, -CONF-6-49, [4] Collaboration, Search for Higgs boson pair production in the b b final state using pp collision data at s = 3 ev with the detector, -CONF-6-4, [5] Collaboration, Search for Higgs boson pair production in the final state of WW ( lν j j) using 3.3 fb of pp collision data recorded at s = 3 ev with the detector, -CONF-6-7, [6] Collaboration, Search for Higgs bosons decaying into di-muon in pp collisions at s = 3 ev with the detector, -CONF-6-4, [7] Collaboration, Search for Higgs and Z Boson Decays to J/ψγ and ϒ(nS)γ with the Detector, Phys. Rev. Lett. 4, 8 (5). [8] Collaboration, Search for Higgs and Z Boson Decays to φ γ with the Detector, Phys. Rev. Lett. 7, 8 (6).