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11. Status of Higgs Boson Physics 1 11 11. Status of Higgs boson physics 1 11. STATUS OF HIGGS BOSON PHYSICS Revised May 2016 by M. Carena (Fermi National Accelerator Laboratory and the University of Chicago), C. Grojean (DESY, Hamburg, on leave from ICREA, Barcelona), M. Kado (Laboratoire de l’Acc´el´erateur Lin´eaire, Orsay), and V. Sharma (University of California, San Diego). I.Introduction . .3 II. The standard model and the mechanism of electroweak symmetry breaking . 6 II.1. The SM Higgs boson mass, couplings and quantum numbers ......8 II.2.TheSMcustodialsymmetry . 8 II.3.StabilityoftheHiggspotential . 9 II.4. Higgs production and decay mechanisms . 10 II.4.1. Production mechanisms at hadron colliders . 10 II.4.2. Production mechanisms at e+e− colliders . 16 II.4.3.SMHiggsbranchingratiosandtotalwidth . 16 III.TheexperimentalprofileoftheHiggsboson . 18 III.1. The principal discovery channels . 18 III.1.1. H γγ .................... 19 → III.1.2. H ZZ∗ ℓ+ℓ−ℓ′+ℓ′− ............... 21 → → III.2.MeasurementoftheHiggsbosonmass . 21 III.3. H W +W − ℓ+νℓ−ν ................ 23 → → III.4.Decaystofermions . 25 III.4.1. H τ +τ − ................... 25 → III.4.2. H bb .................... 26 → III.5. First results on the main production and decay channelsat13TeV . 28 III.6. Higgs production in association with top quarks or in topdecays . 29 III.6.1. The associated production with top quark pairs . ..... 29 III.6.2. The associated production with a single top quark . ...... 31 III.6.3. Flavor changing neutral current decays of the top quark . 31 III.7. Searches for other rare production modes . 32 III.7.1.SearchesforHiggsbosonpairproduction . 32 C. Patrignani et al. (Particle Data Group), Chin. Phys. C, 40, 100001 (2016) October 6, 2016 14:51 2 11. Status of Higgs boson physics III.8.SearchesforraredecaysoftheHiggsboson . 34 III.8.1. H Zγ .................... 34 → III.8.2. H µ+µ− ................... 35 → III.8.3. H e+e− ................... 35 → III.8.4. Lepton flavor violating (LFV) Higgs boson decays . 35 III.8.5. Probing charm- and light-quark Yukawa couplings . ...... 36 III.8.6.Raredecaysoutlook . 37 III.9. Searches for non-standard model decay channels . 37 III.9.1.InvisibledecaysoftheHiggsboson . 37 III.9.2.ExoticHiggsbosondecays . 38 IV.Combiningthemainchannels . 39 IV.1.Principlesofthecombination . 40 IV.2. Characterization of the main decay modes and observation of Higgs decays to taus 44 IV.3. Characterization of the main production modes and evidence for VBF production 45 V.MainquantumnumbersandwidthoftheHiggsboson . 48 V.1. Main quantum numbers J PC ............... 48 V.1.1.Chargeconjugation . 49 V.1.2.Spinandparity . 49 V.1.3. Probing fixed J P scenarios . 50 V.1.4.ProbinganomalousHVVcouplings . 52 V.2.Off-shellcouplingsoftheHiggsboson . 53 V.3.TheHiggsbosonwidth . 54 V.3.1.Directconstraints . 55 V.3.2. Indirect constraints from mass shift in the diphoton channel . 55 V.3.3. Indirect constraints from off-shell couplings . 56 VI. Probing the coupling properties of the Higgs boson . 57 VI.1.EffectiveLagrangianframework . 58 VI.2.Probingcouplingproperties . 60 VI.2.1. Combined measurements of the coupling properties of H ..... 61 October 6, 2016 14:51 11. Status of Higgs boson physics 3 VI.2.2.Differentialcrosssections . 70 VI.2.3. Constraints on non-SM Higgs boson interactions in an effective Lagrangian 71 VII. New physics models of EWSB in the light of the Higgs boson discovery . 72 VII.1. Higgs bosons in the minimal supersymmetric standard model (MSSM) . 74 VII.1.1.MSSMHiggsbosonphenomenology . 78 VII.2.HiggsbosonsinsingletextensionsoftheMSSM . 81 VII.3. Supersymmetry with extended gauge sectors . 85 VII.4.EffectsofCPviolation . 87 VII.5. Non-supersymmetric extensions of the Higgs sector . ...... 89 VII.5.1.Two-Higgs-doubletmodels . 90 VII.5.2.Higgstriplets . 92 VII.6.CompositeHiggsmodels. 94 VII.6.1.LittleHiggsmodels . 94 VII.6.2.Modelsofpartialcompositeness . 96 VII.6.3.MinimalcompositeHiggsmodels . 100 VII.6.4.TwinHiggsmodels . 101 VII.7.TheHiggsbosonasadilaton . 102 VII.8. Searches for signatures of extended Higgs sectors . 103 VII.8.1. Searches for non-standard production processes oftheHiggsboson . 114 VII.8.2.Outlookofsearchesforadditionalstates . 114 VIII.Summaryandoutlook . 114 I. Introduction Understanding the mechanism that breaks the electroweak symmetry and generates the masses of the known elementary particles1 has been one of the fundamental endeavors in particle physics. The discovery in 2012 by the ATLAS [1] and the CMS [2] Collaborations of a new resonance with a mass of approximately 125 GeV and the subsequent studies of its properties with a much larger data set have provided the first portrait of this mechanism. The mass of this boson has been precisely measured and its production 1 In the case of neutrinos, it is possible that the electroweak symmetry breaking mecha- nism plays only a partial role in generating the observed neutrino masses, with additional contributions at a higher scale via the so called see-saw mechanism. October 6, 2016 14:51 4 11. Status of Higgs boson physics and decay rates are found to be consistent, within errors, with the standard model (SM) predictions. Nevertheless, many theoretical questions remain unanswered and new conundrums about what lies behind the Higgs boson have come to fore. Four years since its discovery, the Higgs boson has turned into a new tool to explore the manifestations of the SM and to probe the physics landscape beyond it. In the SM [3] the electroweak interactions are described by a gauge field theory invariant under the SU(2)L U(1)Y symmetry group. The mechanism of electroweak symmetry breaking (EWSB)× [4] provides a general framework to keep untouched the structure of these gauge interactions at high energies and still generate the observed masses of the W and Z gauge bosons. The EWSB mechanism posits a self-interacting complex doublet scalar field, whose CP-even neutral component acquires a vacuum expectation value (VEV) v 246 GeV, which sets the scale of electroweak symmetry breaking. Three massless Goldstone≈ bosons are generated and are absorbed to give masses to the W and Z gauge bosons. The remaining component of the complex doublet becomes the Higgs boson – a new fundamental scalar particle. The masses of all fermions are also a consequence of EWSB since the Higgs doublet is postulated to couple to the fermions through Yukawa interactions. The true structure behind the newly discovered boson – including the exact dynamics that triggers the Higgs VEV– and the corresponding ultraviolet completion is, however, still unsolved. Even if the discovered boson has weak couplings to all known SM degrees of freedom, it is not excluded that it is part of an extended symmetry structure or that it emerges from a light resonance of a strongly coupled sector. It needs to be established whether the Higgs boson is solitary or whether other states populate the EWSB sector. Without the Higgs boson, the calculability of the SM would have been spoiled. In particular, perturbative unitarity [5, 6] would be lost at high energies as the longitudinal W/Z boson scattering amplitude would grow as the centre-of-mass energy increases. Moreover, the radiative corrections to the gauge boson self-energies would exhibit dangerous logarithmic divergences that would be difficult to reconcile with EW precision data. With the discovery of the Higgs boson, it has been experimentally established that the SM is based on a gauge theory that could a priori be consistently extrapolated to the Planck scale. The Higgs boson must have couplings to W/Z gauge bosons and fermions precisely as those in the SM to maintain the consistency of the theory at high energies, hence, formally there is no need for new physics at the EW scale. However, as the SM Higgs boson is a scalar particle, and therefore without a symmetry to protect its mass, at the quantum level it has sensitivity to the physics in the ultraviolet. Quite generally, the Higgs mass parameter m may be affected by the presence of heavy particles. Specifically, 2 2 2 in presence of fermion and boson particles with squared masses mi + λi φ /2, the running of the mass parameter from the scale µ to the scale Q reads m2(Q) = m2(µ) + δm2, (11.1) λ2m2 Q2 δm2 = g ( 1)2Si i i log( ), (11.2) i − 32π2 µ2 Xi where the sum is over all particles and gi and Si correspond to the number of degrees of freedom and the spin of the particle i. Therefore, particles that couple to the Higgs and October 6, 2016 14:51 11. Status of Higgs boson physics 5 2 have a large squared mass parameter mi would induce very large corrections to the Higgs mass parameter, demanding a large fine tuning to explain why m2 remains small. Hence, in general, light scalars like the Higgs boson cannot naturally survive in the presence of heavy states at the grand-unification, string or Planck scales. This is known as the hierarchy or naturalness problem [7]. There are two broad classes of models addressing the naturalness problem2: one is based on a new fermion-boson symmetry in nature called supersymmetry (SUSY) [9–11]. This is a weakly coupled approach to EWSB, and in this case, the Higgs boson remains elementary and the corrections to its mass are screened at the scale at which SUSY is broken and remain insensitive to the details of the physics at higher scales. These theories predict at least three neutral Higgs particles and a pair of charged Higgs particles [12]. One of the neutral Higgs bosons, most often the lightest CP-even Higgs, has properties that resemble those of the SM Higgs boson. It is referred to as a SM-like Higgs boson, meaning that its VEV is predominantly responsible for EWSB, and hence has SM-like couplings to the W and Z gauge bosons. The other approach invokes the existence of strong interactions at a scale of the order of a TeV or above and induces strong breaking of the electroweak symmetry [13].
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