Hidden sector effects on double Higgs production near threshold at the LHC

In this Letter we study a novel effect of a hidden sector coupling to the standard model Higgs boson: an enhancement of the Higgs pair production cross section near threshold due to bound state effects. After summing the ladder contributions of the hidden sector to the effective ggHH coupling, we find the amplitude for gluon-gluon scattering via a Higgs loop. We relate this amplitude to the double Higgs production cross section via the optical theorem. We find that enhancements of the O(100) for the partonic cross section near the threshold region can be obtained for a hidden sector strongly coupled to the Higgs boson. The corresponding cross section at the LHC can be as large as O(10) times the SM result for extreme values of the coupling. The detection of such an effect could in principle lead to important information about the hidden sector.     

Distinguishing a SM-like MSSM Higgs boson from SM Higgs boson at muon collider

We explore the possibility of distinguishing the SM-like MSSM Higgs boson from the SM Higgs boson via Higgs boson pair production at future muon collider. We study the behavior of the production cross-section in SM and MSSM with Higgs boson mass for various MSSM parameters tan β and m _A . We observe that at fixed CM energy, in the SM, the total cross-section increases with the increase in Higgs boson mass whereas this trend is reversed for the MSSM. The changes that occur for the MSSM in comparison to the SM predictions are quantified in terms of the relative percentage deviation in cross-section. The observed deviations in cross-section for different choices of Higgs boson masses suggest that the measurements of the cross-section could possibly distinguish the SM-like MSSM Higgs boson from the SM Higgs boson.      

Higgs boson production and decay in little Higgs models with T-parity

We study Higgs boson production and decay in a certain class of little Higgs models with T-parity in which some T-parity partners of the Standard Model (SM) fermions gain their masses through Yukawa-type couplings. We find that the Higgs boson production cross section of a 120 GeV Higgs boson at the CERN LHC via gg fusion process at one-loop level could be reduced by about 45%, 35% and 20%, as compared to its SM prediction, for a relatively low new particle mass scale f=600, 700 and 1000 GeV, respectively. On the other hand, the weak boson fusion cross section is close to the SM value. Furthermore, the Higgs boson decay branching ratio into di-photon mode can be enhanced by about 35% in small Higgs mass region in certain case, for the total decay width of Higgs boson in the little Higgs model is always smaller than that in the SM.     

h --> mutau at Hadron colliders

We study the observability for a lepton flavor-changing decay of a Higgs boson h--> mutau at Hadron colliders. Flavor-changing couplings of a Higgs boson exist at tree level in models with multiple Higgs doublets. The hmutau coupling is particularly motivated by the favorable interpretation of nu(mu)-nu(tau) oscillation. We find that at the Tevatron run II the unique mutau signature could serve as the Higgs discovery channel, surpassing expectations for Higgs boson searches in the SM and in a large parameter region of the MSSM. The sensitivity will be greatly improved at the LHC, beyond the coverage at a muon collider Higgs factory.     

Measuring the top Yukawa coupling to a heavy Higgs Boson at future e(+)e(-) linear colliders

If the Higgs boson mass is greater than 350 GeV, the top quark Yukawa coupling can be determined using the Higgs resonant contribution to t&tmacr; production from W+W- fusion at high energy e(+)e(-) linear colliders. We have evaluated the significance with which the signal of a Higgs decaying to t&tmacr; pairs could be observed at future e(+)e(-) colliders, with center of mass energies of 800 GeV and 1 TeV, and an integrated luminosity of 1 ab(-1). We find that a signal significance greater than 5sigma and a relative error in the top Yukawa measurement better than 10% can be achieved at these facilities, for Higgs boson masses in the ranges of 350-500 GeV and 350-650 GeV, respectively.     

Enhancement of “CP-odd” Higgs boson production in the minimal supersymmetric Standard Model with explicit CP-violation

We calculate the production cross section of the “CP-odd” Higgs boson via gluon fusion in the minimal supersymmetric Standard Model with explicit CP-violation in the stop sector. We show that there is a parameter region in which the cross section is enhanced by a factor of about 1000, as compared to the case without CP-violation in the stop sector. In the parameter region where the “CP-odd” Higgs boson can decay into a stop pair, the stop pair events will be the important signature of the enhanced “CP-odd” Higgs boson production. In the case where the “CP-odd” Higgs boson cannot decay into any superparticles, the γγ and ττ decay channels could become important for discovering the “CP-odd” Higgs boson. We also discuss the constraints from electric dipole moments of electron, neutron and mercury on the viable parameter space mentioned above.     

SUSY QCD Residual Effects in pp→bbh Process

If all the supersymmetry particles (sparticles) except a light Higgs boson are too heavy to be directly produced at the Large Hadron Collider (LHC), a possible way to reveal evidence for supersymmetry is through their virtual effects in other processes. We examine such supersymmetric QCD effects in bottom pair production associated with a light Higgs boson at the LHC. We find that if the relevant sparticles (gluinos and squarks) are too heavy to be directly produced well above the TeV scale, they can still have sizable virtual effects in this process. For large tanβ, such residual effects can alter the production rate by over 40 percent,which should be observable in future measurements of this process at the LHC.     

Search strategies for non-standard Higgs bosons at future ee colliders

Already in the simplest two-Higgs-doublet model with CP violation in the Higgs sector, the 3×3 mixing matrix for the neutral Higgs bosons can substantially modify their couplings, thereby endangering the “classical” Higgs search strategies. However, there are sum rules relating Yukawa and Higgs–Z couplings which ensure that the ZZ, and couplings of a given neutral 2HDM Higgs boson cannot all be simultaneously suppressed. This result implies that any single Higgs boson will be detectable at an e⁺e⁻ collider if the Z+Higgs, Higgs and Higgs production channels are all kinematically accessible and if the integrated luminosity is sufficient. We explore, as a function of Higgs mass, the luminosity required to guarantee Higgs boson detection, and find that for moderate tanβ values the needed luminosity is unlikely to be available for all possible mixing scenarios. Implications of the sum rules for Higgs discovery at the Tevatron and LHC are briefly discussed.     

Can the Higgs be seen in rapidity gap events at the Tevatron or the LHC?

Double diffractive Higgs production at pp (or ) colliders continues to attract attention as a potential signal in the search for the boson. We present improved perturbative QCD estimates of the event rates for both the exclusive and inclusive double diffractive Higgs processes, paying particular attention to the survival probability of the rapidity gaps. We find that the major uncertainty is in the prediction for the survival probability associated with soft rescattering. We show that an analogous process, the double diffractive production of a pair of jets with large values of , has an event rate which makes it accessible at the Tevatron. Observation of this process can therefore be used as a luminosity monitor for two-gluon exchange processes, such as the production of a Higgs boson with rapidity gaps on either side. Received: 8 February 2000 / Published online: 14 April 2000     

Searching for a light Higgs boson

The Lindé-Weinberg bound [1] on the mass of the Higgs boson does not apply if one of the fermions of the standard model is very massive [2] or in non-standard models with multiple Higgs particles [1]. We consider both the standard model and a common extension thereof to two or more Higgs doublets. If the Higgs responsible for lepton masses is very light, one reliable method for indirectly detecting it would be a more careful measurement of g − 2 for the muon. More direct is the search for Higgs particles detected in association with τ pairs in existing or defunct e⁺e⁻ colliders operating well below the Z mass. We analyze both methods in detail, and find that data from several existing colliders could eliminate large portions of parameter space - or, perhaps, find the Higgs boson.     

On the NLO QCD corrections to Higgs production and decay in the MSSM

We present explicit analytic results for the two-loop top/stop/gluino contributions to the cross section for the production of CP-even Higgs bosons via gluon fusion in the MSSM, under the approximation of neglecting the Higgs boson mass with respect to the masses of the particles circulating in the loops. The results are obtained employing the low-energy theorem for Higgs interactions adapted to the case of particle mixing. We discuss the validity of the approximation used by computing the first-order correction in an expansion in powers of the Higgs boson mass. We find that, for the lightest CP-even Higgs boson, the gluino contribution is very well approximated by the result obtained in the limit of vanishing Higgs mass. As a byproduct of our calculation, we provide results for the two-loop QCD contributions to the photonic Higgs decay.     

Determining the chirality of Yukawa couplings via single charged Higgs boson production in polarized photon collisions

When the charged Higgs boson is too heavy to be produced in pairs, the predominant production mechanism at linear colliders is via the single charged Higgs boson production processes, such as e(-)e(+)-->bcH+,taunuH+ and gammagamma-->bcH+,taunuH+. We show that the yield of a heavy charged Higgs boson at a gammagamma collider is typically 1 or 2 orders of magnitude larger than that at an e(-)e(+) collider. Furthermore, a polarized gammagamma collider can determine the chirality of the Yukawa couplings of fermions with charged Higgs boson via single charged Higgs boson production and, thus, discriminate models of new physics.     

Measuring the Higgs boson self-coupling at the Large Hadron Collider

Inclusive standard model Higgs boson pair production and subsequent decay to same-sign dileptons via weak gauge W+/- bosons at the CERN Large Hadron Collider (LHC) has the capability to determine the Higgs boson self-coupling, lambda. The large top quark mass limit is found not to be a good approximation for the signal if one wishes to utilize differential distributions in the analysis. We find that it should be possible at the LHC with design luminosity to establish that the standard model Higgs boson has a nonzero self-coupling and that lambda/lambda(SM) can be restricted to a range of 0-3.7 at 95% confidence level if its mass is between 150 and 200 GeV.     

Unintegrated gluon distributions and Higgs boson productionin proton-proton collisions

Inclusive cross sections for Higgs boson production in proton-proton collisions are calculated in the formalism of unintegrated gluon distributions (UGDFs). Different UGDFs from the literature are used. Although they were constructed in order to describe the HERA deep-inelastic scattering F₂ data, they lead to surprisingly different results for Higgs boson production. We present both the two-dimensional invariant cross section as a function of Higgs boson rapidity and transverse momentum, as well as the corresponding projections on rapidity or transverse momentum. We quantify the differences between different UGDs by applying different cuts on interrelations between the transverse momentum of the Higgs and the transverse momenta of both fusing gluons. We focus on the large rapidity region. The interplay of the gluon-gluon fusion and weak-boson fusion in rapidity and transverse momentum is discussed. We find that above p_t ∼ 50–100 GeV the weak-gauge boson fusion dominates over gluon-gluon fusion. PACS. 12.38.Bx,12.38.Cy,13.85.Qk,14.70.Hp,14.80.Bn     

Three-body decays of Higgs bosons at LEP2 and application to a hidden fermiophobic Higgs

We study the decays of Higgs bosons to a lighter Higgs boson and a virtual gauge boson in the context of the non-supersymmetric two-Higgs doublet model (2HDM). We consider the phenomenological impact at LEP2 and find that such decays, when open, may be dominant in regions of parameter space and thus affect current Higgs boson search techniques. Three-body decays would be a way of producing light neutral Higgs bosons which have so far escaped detection at LEP due to suppressed couplings to the Z, and are of particular importance in the 2HDM (Model I) which allows both a light fermiophobic Higgs and a light charged scalar.     

Higgs and Z<Superscript>′</Superscript> phenomenology in B–L extension of the standard model at LHC

The phenomenology of the low scale U(1)_B–L extension of the standard model and its implications at LHC energies is presented. In this model, an extra gauge boson corresponding to B–L gauge symmetry and an extra SM singlet scalar (heavy Higgs boson) are predicted. We show a detailed analysis of both heavy and light Higgs bosons decay and production in addition to the possible decay channels of the new gauge boson. We find that the cross sections of the SM-like Higgs production are reduced by ∼20–30%, while its decay branching ratios remain intact. The extra Higgs boson has relatively small cross sections and the branching ratios of Z^′→l⁺l^- are of order ∼20% to be compared to ∼3% of the SM results. Hence, the search for Z^′ is accessible via a clean dilepton signal at LHC.     

Searching for Higgs decays to four bottom quarks at LHCb

We discuss the feasibility of seeing a Higgs boson which decays to four bottom quarks through a pair of (pseudo-)scalars at the LHCb experiment to argue that the use of b-physics triggers and off-line vertex reconstruction, as opposed to jet triggers with b tagging, may be more effective for this signal. Focusing on inclusive production for the Higgs, we find that for light scalar masses below 20 GeV, signal reconstruction efficiencies of order a few percent may be enough for LHCb to find evidence for a Higgs with a dominant 4b decay channel.     

The Higgs Boson Mass in MSM and the New Data

In this paper we study the implications of the top quark mass measurement at CDF for the Higgs boson mass M_H. We find that the present data are most likely in favor of heavy Higgs boson, M_H = 340_-270⁺⁷⁷⁰ GeV for M_t = 174 GeV, but no realistic constraints on M_H could be derived until the measurement of M_t reaches high precision.     

Ways to detect a light Higgs boson at the LHC

We summarize the possible processes which may be used to search for a Higgs boson, of mass in the range 114-130 GeV, at the LHC. We discuss, in detail, two processes with rapidity gaps: exclusive Higgs production with tagged outgoing protons and production by Weak Boson Fusion, in each case taking as the signal. We make an extensive study of all possible backgrounds, and discuss the relevant experimental issues. We emphasize the special features of these signals, and of their background processes, and show that they could play an important role in identifying a light Higgs boson at the LHC. Received: 5 July 2002 / Published online: 30 August 2002     

$e^{+}e^{-}\to t\bar{t}H$ including decays: on the size of background contributions

We present results for the lowest order cross sections, calculated with the complete set of standard model Feynman diagrams, of all possible detection channels of the associated production of the top quark pair and the light Higgs boson, which may be used for determination of the top–Higgs Yukawa coupling at the future e ⁺ e ⁻ linear collider. We show that, for typical particle identification cuts, the background contributions are large. In particular, the QCD background contributions are much bigger than could be expected when taking into account a possibly low virtuality of exchanged gluons. Moreover, we include the initial state radiation effects and discuss the dependence of the cross sections on the Higgs boson and top quark masses.