Probing anomalous Higgs couplings at an <Emphasis Type="Italic">eγ</Emphasis> collider using unpolarised beams

We examine the sensitivity of eγ colliders (based on e ⁺ e ⁻ linear colliders of c.m. energy 500 GeV) to the anomalous couplings of the Higgs to W-boson via the process e ⁻ γ → νWH. This has the advantage over e ⁺ e ⁻ collider in being able to dissociate WWH vertex from ZZH. We are able to construct several dynamical variables which may be used to constrain the various couplings in the WWH vertex.      

Effective-Lagrangian approach to γγ→WW II: Results and comparison with e+e-→WW

We present a study of anomalous electroweak gauge-boson couplings that can be measured in e⁺e^- and γγ collisions at a future linear collider like ILC. We consider the gauge-boson sector of a locally SU(2)×U(1) invariant effective Lagrangian with ten dimension-six operators added to the Lagrangian of the standard model. These operators induce anomalous three-gauge-boson and four-gauge-boson couplings and an anomalous γγH coupling. We calculate the reachable sensitivity for the measurement of the anomalous couplings in γγ→WW. We compare these results with the reachable precision in the reaction e⁺e^-→WW on the one hand and with the bounds that one can obtain from high-precision observables in Z decays on the other hand. We show that one needs both the e⁺e^- and the γγ modes at an ILC to constrain the largest possible number of anomalous couplings and that the Giga-Z mode offers the best sensitivity for certain anomalous couplings.     

Associated single photons and doubly-charged scalars at linear <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> colliders

Doubly-charged scalars, predicted in many models having exotic Higgs rep-resentations, can in general have lepton-number violating (LFV) couplings. We show that by using an associated monoenergetic final state photon seen at a future linear e ⁻ e ⁻ collider, we can have a clear and distinct signature for a doubly-charged resonance. The strength of the ΔL = 2 coupling can also be probed quite effectively as a function of the recoil mass of the doubly-charged scalar.      

Recent results from LEP

Recent results from the LEP collider at CERN are presented: on the identification of e ⁺ e ⁻ → W ⁺ W ⁻ and the determination of the W mass and width and limits on its anomalous couplings; the search for the Standard Model and non-minimal Higgs; search for SUSY and other new particles. Fits to all electroweak data leading to predictions of the Higgs mass within the Standard Model are presented.     

Prospects for the measurement of the Higgs boson masswith a linear <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> collider

The potential of a linear e ⁺ e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. Received: 30 May 2005, Revised: 6 July 2005, Published online: 6 October 2005     

Anomalous gauge-boson couplings and the Higgs-boson mass

We study anomalous gauge-boson couplings induced by a locally SU(2) × U(1) invariant effective Lagrangian containing ten operators of dimension six built from boson fields of the standard model (SM) before spontaneous symmetry breaking (SSB). After SSB some operators lead to new three- and four-gauge-boson interactions, some contribute to the diagonal and off-diagonal kinetic terms of the gauge bosons, to the kinetic term of the Higgs boson and to the mass terms of the W and Z bosons. This requires a renormalisation of the gauge-boson fields, which, in turn, modifies the charged- and neutral-current interactions, although none of the additional operators contain fermion fields. Also the Higgs field must be renormalised. Bounds on the anomalous couplings from electroweak precision measurements at LEP and SLC are correlated with the Higgs-boson mass m_H. Rather moderate values of anomalous couplings allow m_H up to 500 GeV. At a future linear collider the triple-gauge-boson couplings and ZWW can be measured in the reaction . We compare three approaches to anomalous gauge-boson couplings: the form-factor approach, the addition of anomalous-coupling terms to the SM Lagrangian after and, as outlined above, before SSB. The translation of the bounds on the couplings from one approach to another is not straightforward. We show that it can be done for the process by defining new effective and ZWW couplings.Received: 8 June 2004, Revised: 26 January 2005, Published online: 8 June 2005     

Probing anomalous <Emphasis Type="Italic">Wtb</Emphasis> couplings in top pair decays

We investigate several quantities, defined in the decays of top quark pairs, which can be used to explore non-standard Wtb interactions. Two new angular asymmetries are introduced in the leptonic decay of top (anti)quarks. Both are very sensitive to anomalous Wtb couplings, and their measurement allows for a precise determination of the W helicity fractions. We also examine other angular and energy asymmetries, the W helicity fractions and their ratios, as well as spin correlation asymmetries, analysing their dependence on anomalous Wtb couplings and identifing the quantities which are most sensitive to them. It is explicitly shown that spin correlation asymmetries are less sensitive to new interactions in the decay of the top quark; therefore, when combined with the measurement of other observables, they can be used to determine the tt̄ spin correlation even in the presence of anomalous Wtb couplings. We finally discuss some asymmetries which can be used to test CP violation in tt̄ production and complex phases in the effective Wtb vertex.     

ATLAS sensitivity to Wtb anomalous couplings in top quark decays

We study the sensitivity of the ATLAS experiment to Wtb anomalous couplings in top pair production with semileptonic decay, pp→tt̄→W⁺bW^-b̄ with one of the W bosons decaying leptonically and the other hadronically. Several observables are examined, including the W helicity fractions and new quantities recently introduced, such as the ratios of helicity fractions and some angular asymmetries defined in the W rest frame. The dependence on anomalous couplings of all these observables has been previously obtained. In this work we show that some of the new observables also have smaller systematic uncertainties than the helicity fractions, with a similar or stronger dependence on anomalous couplings. Consequently, their measurement can significantly improve the limits on anomalous couplings. Moreover, the most sensitive measurements can be combined. In this case, the precision achieved in the determination of Wtb anomalous couplings can be of a few percent in the semileptonic channel alone.     

CMS overview

We discuss the production of γπ⁰ and π⁰ π⁰ pairs with a large invariant mass at collider energies. We present a study based on a perturbative QCD calculation at full next-to-leading order accuracy, implemented in the computer programme DIPHOX. We give estimations for various observables, which concern the reducible background to the Higgs boson search in the channel H → γγ, in the mass range 80-140 GeV at the LHC. We critically discuss the reliability of these estimates due to our imperfect knowledge of fragmentation functions at high z and a subtle interplay between higher order corrections and realistic experimental cuts. Whereas the invariant mass spectrum of photon-pion pairs is theoretically better under control, in the dipion case large uncertainties remain. Finally we comment on the impact of our findings on Higgs boson searches at the LHC. We conclude that the qualitative statement that the pion backgrounds should not be dangerous for the H → γγ search channel remains true at the next-to-leading order level.     

Measurement of the top-Higgs Yukawa coupling at a linear e<Superscript>+</Superscript>e<Superscript>-</Superscript> collider

Understanding the mechanism of electroweak symmetry breaking and the origin of boson and fermion masses is among the most pressing questions raised in contemporary particle physics. If these issues involve one (several) Higgs boson(s), a precise measurement of all its (their) properties will be of prime importance. Among those, the Higgs coupling to matter fermions (the Yukawa coupling). At a linear collider, the process e⁺e^-→tt̄H will allow a direct measurement of the top-Higgs Yukawa coupling. We present a realistic feasibility study of the measurement in the context of the TESLA collider. Four channels are studied and the analysis is repeated for several Higgs mass values within the range 120–200 GeV/c ². PACS 13.66.Jn; 14.65.Ha; 14.80.Bn     

WWψ couplings from singleW-production in polarizede + e − collisions

Using the processeψ→Wv we study theWWψ vertex ine ⁺ e ^? collisions with polarized beams at a future 500 GeV collider. The equivalent photon approximation and its accuracy are discussed for the case of polarized electron beams. The resulting photon spectrum is then used to compute the total cross section as well as verious differential distributions for the processe ⁺ e ^?→e ⁺ W ^? v. Special emphasis is put on the effects of using polarized electron beams and their influence on the sensitivity to anomalous couplings.     

Identifying new physics contributions in the Higgs sector at linear <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> colliders

Loop-driven decay modes of the Higgs are sensitive to new physics contributions because of new particles in the loops. To highlight this we look at the dilepton-dijet signal in the dominant Higgs production channel at a linear e ⁺ e ⁻ collider. We show that by taking a simple ratio between cross-sections of two different final states such contributions can be very easily identified.      

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.     

Probing top-quark couplings indirectly at Higgs factories

We perform a global effective-field-theory analysis to assess the combined precision of Higgs couplings,triple gauge-boson couplings, and top-quark couplings, at future circular e~+e~- colliders, with a focus on runs below the tt production threshold. Deviations in the top-quark sector entering as one-loop corrections are consistently taken into account in the Higgs and diboson processes. We find that future lepton colliders running at center-of-mass energies below the tt production threshold can still provide useful information on top-quark couplings, by measuring virtual top-quark effects. With rate and differential measurements, the indirect individual sensitivity achievable is better than at the high-luminosity LHC. However, strong correlations between the extracted top-quark and Higgs couplings are also present and lead to much weaker global constraints on top-quark couplings. This implies that a direct probe of top-quark couplings above the tt production threshold is also helpful for the determination of Higgs and triple-gauge-boson couplings. In addition, we find that below the e~+e~-→tth production threshold, the top-quark Yukawa coupling can be determined by its loop corrections to all Higgs production and decay channels. Degeneracy with the ggh coupling can be resolved, and even a global limit is competitive with the prospects of a linear collider above the threshold. This provides an additional means of determining the top-quark Yukawa coupling indirectly at lepton colliders.     

Discovery potential of Higgs boson pair production through 4■+■ final states at a 100 TeV collider

We explore the discovery potential of Higgs pair production at a 100 Te V collider via full leptonic mode.The same mode can be explored at the LHC when Higgs pair production is enhanced by new physics. We examine two types of fully leptonic final states and propose a partial reconstruction method, which can reconstruct some useful kinematic observables. It is found that the m T2 variable determined by this reconstruction method and the reconstructed visible Higgs mass are crucial to discriminate the signal and background events. It is also noticed that a new variable, denoted as ?m, which is defined as the mass difference of two possible combinations, is very useful as a discriminant. To examine the detector effects, we consider seven detector setups for a 100 Te V collider and investigate the changes in the sensitivity, and we find that lepton isolation and the minimal lepton P t cut are crucial in order to reduce the integrated luminosity.     

Hadronic vacuum polarization contribution to the Lamb shift in muonic hydrogen

We report on the accuracy of the measurement of the total cross section of the process e⁺e^– ZH and of the Higgs boson mass that would be achieved in a linear collider operating at a centre-of-mass energy of 350 GeV, assuming an integrated luminosity of 500 fb^–1. For that we have exploited the recoil mass off the Z using its leptonic decays into electron and muon pairs. The Higgs mass is determined with 150 MeV accuracy, the recoil mass resolution is about 1.5 GeV and the cross section is obtained with a statistical error of 3%.     

Probe of the Wtb coupling in t \bar{t} pair production at linear colliders

The Wtb vertex can be probed on future colliders in the processes of single top production (LHC, pp mode, NLC, mode) and of top pair production (NLC, mode). We analyze observables sensitive to anomalous Wtb couplings in the top pair production process of collisions. In particular, forward–backward and spin–spin asymmetries of the top decay products and the asymmetry of the lepton energy spectrum are considered. Possible bounds on anomalous couplings obtained are competitive to those expected from the upgraded Tevatron and LHC. The validity of the infinitely small width approximation for the three-body top decay is also studied in detail. Received: 20 January 2000 / Revised version: 12 April 2000 / Published online: 18 May 2000     

Search for Higgs Bosons at LEP2 and Hadron Colliders

The search for the Higgs boson was one of the most relevant issues of the final years of LEP running at high energies. An excess of 3σ beyond the background expectation has been found, consistent with the production of the Higgs boson with a mass near 115 GeV/c². At the upgraded TeVatron and at LHC the search for the Higgs boson will continue. At TeVatron Higgs bosons can be detected with masses up to 180 GeV with an assumed total integrated luminosity of 20 fb^—1. LHC has the potential to discover the Higgs boson in many different decay channels for Higgs masses up to 1 TeV. It will be possible to measure Higgs boson parameters, such as mass, width, and couplings to fermions and bosons. The results from Higgs searches at LEP2 and the possibilities for searches at hadron colliders will be reviewed.     

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.     

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.