Standard Model Cross Section Research Articles

Double neutral Higgs production in the two-Higgs doublet model at the LHC

In this work we study the production and decay of two CP-even Higgs bosons in the general CP-conserving two-Higgs doublet model at the LHC. We also study the limiting case of the decoupling scenario. For each Yukawa version of the model, we look for the region of the parameter space where a signal would be seen at the LHC. Taking into account theoretical and experimental constraints on the two Higgs doublet model parameters, we show that the cross section can be two orders of magnitude above the corresponding Standard Model cross section. We have also studied in detail the decay profile of the Higgs bosons and showed that interesting signatures may emerge for some particular values of the parameters. In some scenarios, specific triple Higgs could be measured.

Neutral Higgs boson pair production in photon-photon annihilation in the two Higgs doublet model

We study double Higgs production in photon-photon collisions as a probe of the new dynamics of Higgs interactions in the framework of two Higgs doublet models. We analyze neutral Higgs bosons production and decay in the fusion processes, $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{S}_{i}{S}_{j}$, ${S}_{i}={h}^{0}$, ${H}^{0}$, ${A}^{0}$, and show that both ${h}^{0}{h}^{0}$ and ${A}^{0}{A}^{0}$ production can be enhanced by threshold effects in the region ${E}_{\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\approx}2{m}_{H\ifmmode\pm\else\textpm\fi{}}$. Resonant effects due to the heavy Higgs ${H}^{0}$ can also play a role in the cross section enhancement when it is allowed to decay to two light $CP$-even ${h}^{0}$ or to two light $CP$-odd ${A}^{0}$ scalars. We have scanned the allowed parameter space of the two Higgs doublet model and found a vast region of the parameter space where the cross section is 2 orders of magnitude above the standard model cross section. We further show that the standard model experimental analysis can be used to discover or to constrain the two Higgs doublet model parameter space.

Analysis of the production of Higgs boson pairs at the one-loop level in the minimal supersymmetric standard model

Within theminimal supersymmetric standardmodel, the amplitudes and total cross sections for the processes e+e− → hh, e+e− → hH, e+e− → HH, and e+e− → AA are calculated in the first order of perturbation theory with allowance for a complete set of one-loop diagrams in the me → 0 approximation. Analytic expressions are obtained for the quantities under consideration; numerical results are presented in a graphical form. It is shown that the cross section for the process e+e− → hh is larger than those for the other processes (and is on the same order of magnitude as the cross section for the corresponding processes in the Standard Model). In the case of the collision energy equal to √s = 500 GeV, an integrated luminosity in the region ∫ ℒ ≥ 500 fb−1, and a longitudinal polarization of the e+e− beams used, 520, 320, and 300 production events are possible in the processes e+e− → hh (at Mh = 115 GeV), e+e− → HH, and e+e− → AA (at MH,A = 120 GeV), respectively. Even at MH,A ≈ 500 GeV and √s = 1.5 TeV, not less than 200 events for each of the processes can be accumulated. The cross section for the process e+e− → hH is small (about 10−2 fb), which complicates the detection of the sought signal significantly.

Search for a Higgs Boson Decaying to TwoWBosons at CDF

We present a search for a Higgs boson decaying to two W bosons in pp[over ] collisions at sqrt[s]=1.96 TeV center-of-mass energy. The data sample corresponds to an integrated luminosity of 3.0 fb;(-1) collected with the CDF II detector. We find no evidence for production of a Higgs boson with mass between 110 and 200 GeV/c;(2), and determine upper limits on the production cross section. For the mass of 160 GeV/c;(2), where the analysis is most sensitive, the observed (expected) limit is 0.7 pb (0.9 pb) at 95% Bayesian credibility level which is 1.7 (2.2) times the standard model cross section.

Implications of CTEQ global analysis for collider observables

The latest CTEQ6.6 parton distributions, obtained by global analysis of hard-scattering data in the framework of general-mass perturbative QCD, are employed to study theoretical predictions and their uncertainties for significant processes at the Fermilab Tevatron and CERN Large Hadron Collider. The previously observed increase in predicted cross sections for the standard-candle $W$ and $Z$ boson production processes in the general-mass scheme (compared to those in the zero-mass scheme) is further investigated and quantified. A novel method to constrain parton distribution function (PDF) uncertainties in LHC observables, by effectively exploiting PDF-induced correlations with benchmark standard model cross sections, is presented. Using this method, we show that the $t\overline{t}$ cross section can potentially serve as a standard-candle observable for the LHC processes dominated by initial-state gluon scattering. Among other benefits, precise measurements of $t\overline{t}$ cross sections would reduce PDF uncertainties in predictions for single top-quark and Higgs boson production in the standard model and minimal supersymmetric standard model.

Charged Higgs-boson production in association with an electron and a neutrino at electron-positron colliders

We present results of a calculation of the cross section for the production of a charged Higgs boson in association with an electron and a neutrino at electron-positron colliders (${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{H}^{+}{e}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{e}$, ${H}^{\ensuremath{-}}{e}^{+}{\ensuremath{\nu}}_{e}$). We study predictions for the cross section in the minimal supersymmetric standard model (MSSM) and the two Higgs doublet model (THDM), highlighting possible differences. The process is effectively loop-induced in both models. Hence, the cross section is expected to be strongly model-dependent. Most notably, due to the presence of superpartners, the MSSM amplitude contains Feynman graphs of pentagon-type, which are not present in the THDM. This is the first complete one-loop calculation of the cross section for this process in the THDM and the MSSM. For both models, so far, only approximate results with limited ranges of validity were available. Our main aim here is to clarify several open questions in the existing literature on this process. Specifically, we will discuss the validity of the heavy fermion loop approximation in both models, and of the fermion/sfermion loop approximation in the MSSM.

Higgs boson productions at the CERN LHC as a probe of different littlest Higgs models withTparity

Higgs boson productions at the LHC will serve as a sensitive probe of various little Higgs models. In this work we comparatively study two littlest Higgs models with different $T$-parity constructions through examining their effects in three production processes of the Higgs boson at the LHC, namely, the productions of a single Higgs, a Higgs-pair, as well as a Higgs boson associated with a pair of top and antitop quarks. The two models are characterized by predicting a top partner with, respectively, even and odd $T$-parity, which cancels the Higgs mass quadratic divergence caused by the top quark. We find that both models can alter the standard model cross sections sizably and their corrections also differ significantly. Therefore, the Higgs boson productions at the LHC might shed some light on these two models or even distinguish them.

Prospects for the measurement of the structure of the coupling of a Higgs boson to weak gauge bosons in weak boson fusion with the ATLAS detector

The prospects for the measurement of the tensor structure of the vertex between a standard model Higgs boson and two weak gauge bosons using the distribution of the azimuthal angles between the two tagging jets in the weak boson fusion channel are studied in a Monte Carlo analysis using the fast simulation of the ATLAS detector. The decay channels H→τ+τ-→ll+4ν, H→τ+τ-→lh+3ν at mH=120 GeV and H→W+W-→llνν at mH=160 GeV are used in the analysis. For a standard model Higgs boson it is found that purely anomalous couplings are expected to be excluded at a confidence level corresponding to 2σ or more at mH=120 GeV and more than 5σ at mH=160 GeV from 30 fb-1 of data. With a value of 1 roughly reproducing the standard model cross section for a purely anomalous coupling, the standard deviation in a measurement of a contribution of a CP even anomalous coupling in addition to the standard model coupling is estimated to be 0.20 at mH=120 GeV and 0.09 at mH=160 GeV.

Multi-Higgs boson production in the standard model and beyond

We present a calculation of the loop-induced processes gg -> HH and gg -> HHH, and investigate the observability of multi-Higgs boson production at the CERN Large Hadron Collider (LHC) in the Standard Model (SM) and beyond. While the SM cross sections are too small to allow observation at the LHC, we demonstrate that physics beyond the SM can lead to amplified, observable cross sections. Furthermore, the applicability of the heavy top quark approximation in two- and three-Higgs boson production is investigated. We conclude that multi-Higgs boson production at the SuperLHC is an interesting probe of Higgs sectors beyond the SM and warrants further study.

Cross Section Dependence of Event Rates at Neutrino Telescopes

We examine the dependence of event rates at neutrino telescopes on the neutrino-nucleon cross section for neutrinos with energy above 1 PeV, and contrast the results with those for cosmic ray experiments. Scaling of the standard model cross sections leaves the rate of upward events essentially unchanged. Details, such as detector depth and cross section inelasticity, can influence rates. Numerical estimates of upward shower, muon, and tau event rates in the IceCube detector confirm these results.

MEASURING THE NEUTRINO-NUCLEON CROSS SECTION WITH SALSA

These proceedings describe a study of the expected sensitivity of the SalSA experiment to the neutrino-nucleon cross section. We expect the measurement to be statistics limited for the events rates expected from SalSA. With 100 measured events, we expect to measure a standard model cross section with a 38% uncertainty that is dominantly statistical.

Distinguishing new physics scenarios at a linear collider with polarized beams

Numerous nonstandard dynamics dominated by very high mass exchanges are described at current and future accelerator energies by appropriate contactlike effective interactions among the standard model particles. Correspondingly, they can manifest themselves only through deviations of the cross sections from the standard model predictions. If one such deviation were observed, it would be important to definitely identify, to a given confidence level, the actual source among the various possible nonstandard interactions that, in principle, can explain it. Here we estimate the identification reach on different new physics effective interactions, obtainable from angular distributions of lepton pair production processes at the planned electron-positron International Linear Collider with polarized beams. For each nonstandard model, such an identification reach defines the range in the relevant heavy mass scale parameter where it can be unambiguously distinguished from the other nonstandard models as the source of corrections to the standard model cross sections, in case these are observed. The effective interactions for which we estimate the expected identification reach are the interactions based on gravity in large extra dimensions, in TeV{sup -1} extra dimensions and the compositeness-inspired four-fermion contact interactions. The availability of both beams polarized at the International Linear Collider turns out, in manymore » cases, to dramatically enhance the identification sensitivity.« less

Flavour independent search for neutral Higgs bosons at LEP

A flavour independent search for the CP-even and CP-odd neutral Higgs bosons h and A is performed in 624 pb−1 of data collected with the L3 detector at LEP at centre-of-mass energies between 189 and 209GeV. Higgs boson production through the e+e−→Zh and the e+e−→hA processes is considered and decays of the Higgs bosons into hadrons are studied. No significant signal is observed and 95% confidence level limits on the hZZ and hAZ couplings are derived as a function of the Higgs boson masses. Assuming the Standard Model cross section for the Higgs-strahlung process and a 100% branching fraction into hadrons, a 95% confidence level lower limit on the mass of the Higgs boson is set at 110.3 GeV.

Flavour and model-independent Higgs boson searches at LEP

Now the LEP experiments have finalized their results on the Standard-Model Higgs boson, and are finalizing results on the most common Supersymmetric scenari, a wealth of results are being released that do not rely on explicit model assumptions, but rather on particular final states. In the absence of any convincing excess in the data, results are then extracted in a quasi model-independent way, most often in the form of excluded signal cross-sections, normalized to Standard Model cross-sections. The aim of the experiments is to provide a catalog of results that can be used in a straightforward way for future studies of models of electroweak symmetry breaking. PACS: 13.66.Fg Gauge and Higgs boson production in e+e– intteractions – 13.66.Hk Production of non-standard model particles in e+e– interactions

Limits on extra dimensions and new particle production in the exclusive photon and missing energy signature in pp collisions at square root [s]=1.8 TeV.

The exclusive gammaE(T) signal has a small standard model cross section and is thus a channel sensitive to new physics. This signature is predicted by models with a superlight gravitino or with large extra spatial dimensions. We search for such signals at the Collider Detector at Fermilab, using 87 pb(-1) of data at square root [s]=1.8 TeV, and extract 95% C.L. limits on these processes. A limit of 221 GeV is set on the scale |F|(1/2) in supersymmetric models. For 4, 6, and 8 extra dimensions, model-dependent limits on the fundamental mass scale M(D) of 0.55, 0.58, and 0.60 TeV, respectively, are found. We also specify a "pseudo-model-independent" method of comparing the results to theoretical predictions.

Neutrino bounds on astrophysical sources and new physics

Ultrahigh energy cosmic neutrinos are incisive probes of both astrophysical sources and new TeV-scale physics. Such neutrinos would create extensive air showers deep in the atmosphere. The absence of such showers implies upper limits on incoming neutrino fluxes and cross sections. Combining the exposures of the Akeno Giant Air Shower Array, the largest existing ground array, with the exposure of the Fly's Eye fluorescence detector integrated over all its operating epochs, we derive 95% confidence level bounds that substantially improve existing limits. We begin with model-independent bounds on astrophysical fluxes, assuming standard model cross sections, and model-independent bounds on new physics cross sections, assuming a conservative cosmogenic flux. We then derive model-dependent constraints on new components of neutrino flux for several assumed power spectra, and we update bounds on the fundamental Planck scale ${M}_{D}$ in extra dimension scenarios from black hole production. For large numbers of extra dimensions, we find ${M}_{D}g2.0 (1.1)\mathrm{TeV}$ for ${M}_{\mathrm{BH}}^{\mathrm{min}}{=M}_{D}{(5M}_{D}),$ comparable to or exceeding the most stringent constraints to date.

Angular dependence of neutrino flux inKM3detectors in low scale gravity models

Cubic kilometer neutrino telescopes are capable of probing fundamental questions of ultra-high energy neutrino interactions. There is currently great interest in neutrino interactions caused by low-scale, extra dimension models. Above 1 PeV the cross section in low scale gravity models rises well above the total Standard Model cross section. We assess the observability of this effect in the 1 PeV - 100 PeV energy range of kilometer-scale detectors with several new points of emphasis that hinge on enhanced neutral current cross sections. A major point is the importance of ``feed-down'' regeneration of upward neutrino flux, driven by new-physics neutral current interactions in the flux evolution equations. Feed-down is far from negligible, and it is essential to include its effect. We then find that the angular distribution of events has high discriminating value in separating models. In particular the ``up-to-down'' ratio between upward and downward-moving neutrino fluxes is a practical diagnostic tool which can discriminate between models in the near future. The slope of the angular distribution, in the region of maximum detected flux, is also substantially different in low-scale gravity and the Standard Model. These observables are only weakly dependent on astrophysical flux uncertainties. We conclude that angular distributions can reveal a breakdown of the Standard Model and probe the new physics beyond, as soon as data become available.

Search for γγ decays of a Higgs boson in e+e− collisions at [formula omitted] up to 209 GeV

A search for events with a photon pair arising from the decay of a Higgs boson produced in association with a fermion pair, is performed in 893 pb−1 of data recorded by the ALEPH detector at LEP at centre-of-mass energies up to 209 GeV. No excess of such events is found over the expected background. An upper limit is derived on the product of the e+e−→HZ cross section and the H→γγ branching fraction as a function of the Higgs boson mass. A fermiophobic Higgs boson produced with the Standard Model cross section is excluded at 95% confidence level for all masses below 105.4 GeV/c2.

Pair production by neutrino bremsstrahlung

Standard model cross sections for electron-positron pair production by neutrino-bremsstrahlung in the nuclear field have been fitted to empirical formulas that can be used to estimate backgrounds to neutrino experiments and flux normalizations. A compact form of the matrix element obtained by analytic reduction is used to discuss the distributions and the physics. Predictions for typical supernovae neutrinos are compared with neutrino-electron scattering.

Neutralino-nucleus elastic cross section in the minimal supersymmetric standard model with explicitCPviolation

We study the elastic scattering of the lightest neutralino with a nucleus in the framework of the minimal supersymmetric standard model (MSSM) with explicit flavor preserving CP violation, including the one-loop CP-violating neutral Higgs-boson mixing effects induced dominantly by the CP phases in the top and bottom (s)quark sectors. We construct the most general form of the effective Lagrangian for the neutralino-nucleus scattering in the limit of vanishing momentum transfers and then we perform a comprehensive analysis of the effects of the complex CP phases on the mass spectra of the lightest neutralino, neutral Higgs bosons and top squarks, and on the the spin-dependent and spin-independent neutralino-nucleus scattering cross section for three neucleus targets F, Si and Ge. The CP phases can reduce or enhance the neutralino-nucleus cross sections significantly, depending on the values of the real parameters in the MSSM.