Higgs-like Particle Research Articles

The effective chiral Lagrangian for a light dynamical “Higgs particle”

We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector, to the case in which the Higgs-like particle is light. Gauge and gauge-Higgs operators are considered up to mass dimension five. This analysis completes the tool needed to explore at leading order the connection between linear realizations of the electroweak symmetry breaking mechanism — whose extreme case is the Standard Model — and non-linear realizations with a light Higgs-like particle present. It may also provide a model-independent guideline to explore which exotic gauge-Higgs couplings may be expected, and their relative strength to Higgsless observable amplitudes. With respect to fermions, the analysis is reduced by nature to the consideration of those flavor-conserving operators that can be written in terms of pure-gauge or gauge-Higgs ones via the equations of motion, but for the standard Yukawa-type couplings.

Dark matter and Higgs boson in a model with discrete gauge symmetry

In view of ongoing measurements of the Higgs-like boson at the LHC and direct searches for dark matter, we explore the possibility of accommodating the potential results in a simple new-physics model with discrete gauge symmetry as well as light neutrino masses. Specifically, we study collider and relic-density constraints on the new gauge coupling, predict the cross section of the dark matter scattering off nucleons, and compare it with current direct search data. We also discuss some of the implications if the dark matter is light. The new gauge sector of the model allows it to be compatible with the latest LHC information on the Higgs-like particle and simultaneously satisfy the requirements in its dark matter sector.

SO(10) GUT IN FOUR AND FIVE DIMENSIONS: A REVIEW

We review SO(10) grand unified theories (GUTs) in four and five dimensions (4D and 5D). The renormalizable minimal SO(10) SUSY GUT is the central theme of this paper. It is very predictive and makes it possible to construct all mass matrices including those of the Dirac and heavy right-handed Majorana neutrinos. Its predictions covers all ranges of particle phenomena. The explicit construction of the Higgs superpotential and the explicit display of a symmetry breaking pattern from GUT to the SM show that the naive desert from the SM to GUT in the minimal supersymmetric standard model (MSSM) is a too simplified concept and we have many definitely determined intermediate energy scales in general. This situation destroys the naive gauge coupling unification in the MSSM scheme. Also the precise measurements of neutrino oscillation data have revealed several small but manifest mismatches with our predictions. Also there are arguments that it is impossible to construct a GUT theory in 4D with a finite number of multiplets that leads to the MSSM with a residual R symmetry. If we try to solve all these pathologies comprehensively, it is very attractive for us to go into extra dimensions. Extra dimension may be either warped or flat. The fifth dimension, for simplicity, is compactified on the S1/(Z2) (warped) or on the S1/(Z2×Z′2) (flat) orbifold with two inequivalent branes at the orbifold fixed points. In the former warped case, intermediate energy scales are translated with the positions of Higgs fields in the bulk and the fundamental scheme of the MSSM is recovered. On the other hand, in the latter flat scenario, all matter and Higgs multiplets reside on the Pati–Salam (PS) brane where the PS symmetry is manifest. There the original renormalizability in Yukawa coupling is broken but its essential structures of mass matrices in minimal SO(10) GUT in 4D is promoted to the PS invariant action in 4D. In the gaugino mediation mechanism, the SO(10) gauge multiplet is transmitted to the PS brane through the gaugino mediation with bulk gauge multiplet. Further breaking of the PS gauge group to the SM group is realized by VEVs of the Higgs multiplets [Formula: see text]. We show that this model not only cures all pathologies in SO(10) GUT in 4D but also provides the consistent inflation scenario and dark matter candidate and leptogenesis. The gauge coupling unification is successfully realized after incorporating the threshold corrections of the Kaluza–Klein modes. Finally we add some comments on the impacts of the discovery of a Higgs-like particle by the Large Hadronic Collider at CERN (LHC) on SUSY GUTs.

MSSM interpretations of the LHC discovery: light or heavy Higgs?

A Higgs-like particle with a mass of about 126 GeV has been discovered at the LHC. Within the experimental uncertainties, the measured properties of this new state are compatible with those of the Higgs boson in the Standard Model (SM). While not statistically significant at present, the results show some interesting patterns of deviations from the SM predictions, in particular a higher rate in the γγ decay mode observed by ATLAS and CMS, and a somewhat smaller rate in the τ + τ − mode. The LHC discovery is also compatible with the predictions of the Higgs sector of the Minimal Supersymmetric Standard Model (MSSM), interpreting the new state as either the light or the heavy \(\mathcal{CP}\)-even MSSM Higgs boson. Within the framework of the MSSM with seven free parameters (pMSSM-7), we fit the various rates of cross section times branching ratio as measured by the LHC and Tevatron experiments under the hypotheses of either the light or the heavy \(\mathcal{CP}\)-even Higgs boson being the new state around 126 GeV, with and without the inclusion of further low-energy observables. We find an overall good quality of the fits, with the best fit points exhibiting an enhancement of the γγ rate, as well as a small suppression of the \(b \bar{b}\) and τ + τ − channels with respect to their SM expectations, depending on the details of the fit. For the fits including the whole dataset the light \(\mathcal{CP}\)-even Higgs interpretation in the MSSM results in a higher relative fit probability than the SM fit. On the other hand, we find that the present data also permit the more exotic interpretation in terms of the heavy \(\mathcal{CP}\)-even MSSM Higgs, which could give rise to experimental signatures of additional Higgs states in the near future.

Dynamical symmetry breaking, CP violation, and a Higgs-like particle

A pseudoscalar associated with dynamical symmetry breaking can mix with a heavier scalar when CP is violated. We study how such a mixed state that is mostly pseudoscalar can resemble the observed 125 GeV particle. The production rate from gg is enhanced by the new heavy quarks and this compensates for the reduced WW, ZZ and γγ branching ratios. The particle can appear to be a scalar from angular distributions in ZZ decay while its pseudoscalar couplings should become evident in other processes.

Neutralino-stop coannihilation into electroweak gauge and Higgs bosons at one loop

We compute the full O(alpha_s) supersymmetric QCD corrections for neutralino-stop co-annihilation into electroweak gauge and Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM). We show that these annihilation channels are phenomenologically relevant within the so-called phenomenological MSSM, in particular in the light of the observation of a Higgs-like particle with a mass of about 126 GeV at the LHC. We present in detail our calculation, including the renormalization scheme, the infrared treatment, and the kinematical subtleties to be addressed. Numerical results for the co-annihilation cross sections and the predicted neutralino relic density are presented. We demonstrate that the impact of including the corrections on the cosmologically preferred region of parameter space is larger than the current experimental uncertainty from WMAP data.

Exploring two Higgs doublet models through Higgs production

We discuss the connections between the recently observed Higgs-like particle and rare B decays in the context of two Higgs doublet models (2HDMs). The measured decays of the Higgs boson to fermions and gauge bosons, along with the observation of the decay B_s -> \mu+ \mu-, place stringent restrictions on the allowed parameter space of 2 Higgs doublet models. Future measurements of h0 -> \gamma \gamma can potentially exclude type I 2HDMs, while the parameters of other 2HDMs are already severely restricted. The recent observations of the h0 -> \tau+ \tau- and h0 -> b bbar decays further constrain the models.

LongitudinalWWscattering in light of the “Higgs boson” discovery

WW scattering is dominated at high energies by their longitudinal components, which are the most sensitive to the nature of the electroweak symmetry breaking. Prior to the discovery at the LHC of a Higgs-like particle, unitarization tools were extensively used to show that, in the absence of a light Higgs boson, new resonances resulting from the would-be strongly-interacting electroweak sector would appear, and furthermore these techniques would approximately predict their masses, widths, and signal strengths. With the discovery of a Higgs-like particle now firmly established, we re-investigate these techniques assuming this particle couples exactly as in the SM, but still being open to the possibility of an extended symmetry breaking sector. While the SM itself is free from problems with perturbative unitarity in the electroweak sector, "anomalous" self-couplings of the vector bosons -- low-energy remnants of such higher-energy symmetry breaking sectors -- are easily shown to re-introduce them. We demonstrate how new resonances should still appear in the scattering of EW vector bosons after imposing constraints from unitarity, and we discuss their ability to be probed with current and future LHC data.

TeV-scale B − L model with a flat Higgs potential at the Planck scale: In view of the hierarchy problem

The recent discovery of the Higgs-like particle at around 126 GeV has given us a big hint towards the origin of the Higgs potential. Especially the running quartic coupling vanishes near the Planck scale, which indicates a possible link between the physics in the electroweak and the Planck scales. Motivated by this and the hierarchy problem, we investigate a possibility that the Higgs has a flat potential at the Planck scale. In particular, we study the RG analysis of the B-L extension of the standard model with a classical conformality. The B-L symmetry is radiatively broken at the TeV scale via the Coleman-Weinberg mechanism. The electroweak symmetry breaking is triggered by a radiatively generated scalar mixing so that its scale 246 GeV is dynamically related with the B-L breaking scale at TeV. The Higgs boson mass is given at the border of the stability bound,which is lowered by a few GeV from the SM by the effect of the B-L gauge interaction.

Modified Higgs couplings and unitarity violation

Prompted by the recent observation of a Higgs-like particle at the CERN Large Hadron Collider (LHC), we investigate a quantitative correlation between possible departures of the gauge and Yukawa couplings of this particle from their Standard Model expectations and the scale of unitarity violation in the processes $WW \to WW$ and $t\bar t \to WW$.

Testing minimal universal extra dimensions using Higgs boson searches at the LHC

Large Hadron Collider (LHC) searches for the SM Higgs boson provide a powerful limit on models involving Universal Extra Dimensions (UED) where the Higgs production is enhanced. We have evaluated all one-loop diagrams for Higgs production from gluon fusion and decay to two photons within "minimal" UED (mUED), independently confirming previous results, and we have evaluated enhancement factors for Higgs boson production and decay over the mUED parameter space. Using these we have derived limits on the parameter space, combining data from both ATLAS and CMS collaborations for the most recent 7 TeV and 8 TeV LHC data. We have performed a rigorous statistical combination of several Higgs boson search channels which is important because mUED signatures from the Higgs boson are not universally enhanced. We have found that 1/R < 500 GeV is excluded at 95% CL, while for larger 1/R only a very narrow (\pm1-4 GeV) mass window around m_h = 125 GeV and another window (up to 2 GeV wide for 1/R > 1000 GeV) around m_h = 118 GeV are left. The latter is likely to be excluded as more data becomes available whereas the region around 125 GeV is where the recently discovered Higgs-like particle was observed and therefore where the exclusion limit is weaker. It is worth stressing that mUED predicts an enhancement for all channels for Higgs production by gluon fusion and decay while the vector boson fusion process WW/ZZ -> h -> AA is generically suppressed and WW/ZZ -> h -> WW*/ZZ* is standard. Therefore, as more 8 TeV LHC data becomes available, the information on individual Higgs boson production and decay processes provided by the CMS and ATLAS experiments can be effectively used to favour mUED or exclude it further.

Searches for exotic and rare higgs decays in CMS

The results on searches for Higgs-like particles beyond the Standard Model and rare Standard Model Higgs decays are presented. Searches are performed using the data collected by CMS experiment in proton-proton collision at LHC. Most recent results concerning light Higgses as predicted in the MSSM, NMSSM and fermiophobic models are discussed. Decays of the 125 GeV Higgs-like particle to Zγ channel is also presented.

Dynamical Electroweak Symmetry Breaking with a Heavy Fermion in Light of Recent LHC Results

The recent announcement of a discovery of a possible Higgs-like particle—its spin and parity are yet to be determined—at the LHC with a mass of 126 GeV necessitates a fresh look at the nature of the electroweak symmetry breaking, in particular if this newly-discovered particle will turn out to have the quantum numbers of a Standard Model Higgs boson. Even if it were a 0+scalar with the properties expected for a SM Higgs boson, there is still the quintessential hierarchy problem that one has to deal with and which, by itself, suggests a new physics energy scale around 1 TeV. This paper presents a minireview of one possible scenario: the formation of a fermion-antifermion condensate coming from a very heavy fourth generation, carrying the quantum number of the SM Higgs field, and thus breaking the electroweak symmetry.

Study of Higgs Production in Fermionic Decay Channels at CMS

In these proceedings to the LHCP conference 2013 results are presented on the study of the Higgs-like particle at a mass of 125 GeV decaying into final states consisting of either $\tau^+\tau^-$, or a $b\bar{b}$ quark pair, based on the full statistics of about 24 fb$^{-1}$, collected in 2011 and 2012 at 7 and 8 TeV respectively with the CMS experiment at the Large Hadron Collider. Leptonic and hadronic decay channels for the $\tau$-lepton are included in the search. Different production channels namely gluon fusion, vector boson fusion (VBF) and associated production with W/Z bosons have been studied $\tau^+\tau^-$ final states. The $b\bar{b}$ decay channel is studied in VBF as well as in associated production with W/Z and top quarks.

Higgs couplings and precision electroweak data

In light of the discovery of a Higgs-like particle at the LHC, we revisit the status of the precision electroweak data, focusing on two discrepant observables: 1) the long-standing 2.4 sigma deviation in the forward-backward asymmetry of the bottom quark A_{FB}^b, and 2) the 2.3 sigma deviation in R_b, the ratio of the Z \rightarrow b \bar b partial width to the inclusive hadronic width, which is now in tension after a recent calculation including new two-loop electroweak corrections. We consider possible resolutions of these discrepancies. Taking the data at face value, the most compelling scenario is that new physics directly affects A_{FB}^b and R_b, bringing the prediction into accord with the measured values. We propose a modified `Beautiful Mirrors' scenario which contains new vector-like quarks that mix with the b quark, modifying the Z b\bar b vertex and thus correcting A_{FB}^b and R_b. We show that this scenario can lead to modifications to the production rates of the Higgs boson in certain channels, and in particular a sizable enhancement in the diphoton channel. We also describe additional collider tests of this scenario.

A higgs-like dilaton: viability and implications

We re-examine the feasibility of the higgs-like particle discovered at the LHC being a dilaton: the Goldstone boson of spontaneous breaking of scale invariance. We review the expected phenomenological deviations from the SM higgs and compare with other Goldstone higgs scenarios, with particular emphasis on double higgs production.

Bootstrap Dynamical Symmetry Breaking

Despite the emergence of a 125 GeV Higgs-like particle at the LHC, we explore the possibility of dynamical electroweak symmetry breaking by strong Yukawa coupling of very heavy new chiral quarksQ. Taking the 125 GeV object to be a dilaton with suppressed couplings, we note that the Goldstone bosonsGexist as longitudinal modesVLof the weak bosons and would couple toQwith Yukawa couplingλQ. WithmQ≳700 GeV from LHC, the strongλQ≳4could lead to deeply boundQQ¯states. We postulate that the leading “collapsed state,” the color-singlet (heavy) isotriplet, pseudoscalarQQ¯mesonπ1, isGitself, and a gap equation without Higgs is constructed. Dynamical symmetry breaking is affected via strongλQ, generatingmQwhile self-consistently justifying treatingGas massless in the loop, hence, “bootstrap,” Solving such a gap equation, we find thatmQshould be several TeV, orλQ≳4π, and would become much heavier if there is a light Higgs boson. For such heavy chiral quarks, we find analogy with theπ−Nsystem, by which we conjecture the possible annihilation phenomena ofQQ¯→nVLwith high multiplicity, the search of which might be aided by Yukawa-boundQQ¯resonances.

Measurements of properties of the Higgs-like Particle at 125 GeV by the CMS collaboration

CMS results are presented on the measurement of properties of the Higgs-like particle discovered last summer with a mass in the range of 125–126 GeV, based on the full statistics of about 25 fb−1 , collected in 2011 and 2012 at 7 and 8 TeV respectively. Five decay channels are considered for these studies, namely the ZZ, γγ , WW, ττ , and bb modes. The mass of the new boson is measured to be 125.7 ± 0.4 GeV. The event yields measured by the different analyses, targeting specific decay modes and production mechanisms, are consistent with those expected for the standard model (SM) Higgs boson, with an overall best-fit signal strength of 0.80 ± 0.14 at the measured mass. A discussion on the measurement of the couplings and the spin-parity properties of this new particle is presented, using the most recent results.

Higgs boson Mass in GMSB with Messenger–Matter mixing

A Higgs-like particle with mass of order 125 GeV has been observed by both ATLAS and CMS experiments. This Higgs mass causes sparticle masses in the several to multi-TeV range in the simple version of minimal GMSB models. We consider the effects of messenger–matter mixing on the lightest CP–even Higgs boson mass in gauge–mediated supersymmetry breaking models. We find with such mixings a 125 GeV Higgs boson can be naturally obtained even with a sub–TeV SUSY spectrum, and when the gravitino has a cosmologically preferred sub–keV mass. In addition, when these models are embedded into a grand unification framework with a U(1) flavor symmetry they explain the fermion mass hierarchy and generate naturally large neutrino mixing angles accompanied with small quark mixing angles. While SUSY mediated flavor changing processes are sufficiently suppressed in such an embedding, it can resolve the apparent discrepancy in the CP asymmetry parameters sin2β and ϵK, and it predicts an observable μ→eγ decay rate.

D0 Results and Combined Tevatron Results on Standard Model Higgs

Results from the D0 experiment on the search for the Higgs boson of the Standard Model (SM) are presented with particular emphasis on testing the compatibility with recent LHC results on the observation of a Higgs-like particle. The D0 SM Higgs results, their combination and the Tevatron combination of CDF and D0 SM Higgs results are summarized. The article also puts recent developments in a historical context. The combined results from the Tevatron experiments give a local p-value of 0.4% for the background-only hypothesis at a mass of 125 GeV indicating that the combined Tevatron data strongly prefer the presence of a new particle with Standard Model Higgs-like properties in this mass region.