Search Results (17 total)

Supersymmetric models with an additional singlet field offer the Higgs boson the possibility to decay to two pseudoscalars, a. If the mass of these pseudoscalars is above the bb̅ threshold, a→bb̅ is generically the dominant decay mode. The decay h→aa→bb̅ bb̅ may be seen above backgrounds at the Tevatron if the Higgs production cross section is enhanced relative to that of the standard model.

The reliability and validity of professionally written multiple-choice exams have been extensively studied for exams such as the SAT, graduate record examination, and the force concept inventory. Much of the success of these multiple-choice exams is attributed to the careful construction of each question, as well as each response. In this study, the reliability and validity of scores from multiple-choice exams written for and administered in the large introductory physics courses at the University of Illinois, Urbana-Champaign were investigated. The reliability of exam scores over the course of a semester results in approximately a 3% uncertainty in students’ total semester exam score. This semester test score uncertainty yields an uncertainty in the students’ assigned letter grade that is less than 1 / 3 of a letter grade. To study the validity of exam scores, a subset of students were ranked independently based on their multiple-choice score, graded explanations, and student interviews. The ranking of these students based on their multiple-choice score was found to be consistent with the ranking assigned by physics instructors based on the students’ written explanations (r>0.94 at the 95% confidence level) and oral interviews (r=0.94_−0.09^+0.06).

We study the quantum mechanical correlation between two identical neutralinos in the decays of minimal supersymmetric standard model (MSSM) scalar tau (stau) pair produced in e⁺e^- annihilation. Generally, the decay products of scalar (spinless) particles are not correlated. We show that a correlation between two neutralinos appears near pair production threshold, due to a finite stau width and mixing of the staus and/or neutralinos, and because the neutralinos are Majorana. Because the correlation is significant only in a specific kinematical configuration, it can be observed only in supersymmetric models where the neutralino momenta can be kinematically reconstructed, such as in models with R-parity violation.

We present a complete calculation of weak boson fusion production of colorless supersymmetric particles at the LHC, using the new matrix element generator susy-madgraph . The cross sections are small, generally at the attobarn level, with a few notable exceptions which might provide additional supersymmetric parameter measurements. We discuss in detail how to consistently define supersymmetric weak couplings to preserve unitarity of weak gauge boson scattering amplitudes to fermions, and derive sum rules for weak supersymmetric couplings.

We introduce a new color decomposition for multiparton amplitudes in QCD, free of fundamental-representation matrices and structure constants. This decomposition has a physical interpretation in terms of the flow of color, which makes it ideal for merging with shower Monte Carlo programs. The color-flow decomposition allows for very efficient evaluation of amplitudes with many quarks and gluons, many times faster than the standard color decomposition based on fundamental-representation matrices. This will increase the speed of event generators for multijet processes, which are the principal backgrounds to signals of new physics at colliders.

We study the production of the Higgs boson in association with a single top quark at hadron colliders. The cross sections for the three production processes (t channel, s channel, and W associated) at both the Fermilab Tevatron and the CERN Large Hadron Collider (LHC) are presented. We investigate the possibility of detecting a signal for the largest of these processes, the t-channel process at the LHC, via the Higgs boson decay into bb̅ . The QCD backgrounds are large and difficult to curb, hindering the extraction of the signal. Extensions of our analysis to the production of supersymmetric Higgs bosons are also addressed. The cross section is enhanced for large values of tan β, increasing the prospects for extracting a signal.

We present a calculation of two photon radiation in W and Z boson production in hadronic collisions, based on the complete matrix elements for the processes qq^′→l^±νγγ and qq̅ →l⁺l^-γγ, including finite charged lepton masses. In order to achieve stable numerical results over the full phase space, multiconfiguration Monte Carlo techniques are used to map the peaks in the differential cross section. Numerical results are presented for the Fermilab Tevatron.

The pole mass of a heavy quark is ambiguous by an amount of order Λ_QCD. We show that the heavy-quark potential, V(r), is similarly ambiguous, but that the total static energy, 2M_pole+V(r), is unambiguous when expressed in terms of a short-distance mass. This implies that the extraction of a short-distance mass from the quarkonium spectrum is free of an ambiguity of order Λ_QCD, in contrast with the pole mass.

We argue that the leading-order subprocess for Higgs-boson production in association with bottom quarks is bb̅ →H. This process is an important source of Higgs bosons with enhanced Yukawa coupling to bottom quarks. We calculate the corrections to this cross section at next-to-leading-order in 1/ln(m_H/m_b) and α_s and at next-to-next-to-leading order in 1/ln(m_H/m_b).

Single-top-quark production probes the charged-current weak interaction of the top quark, and provides a direct measurement of the CKM matrix element V_tb. We perform two independent analyses to quantify the accuracy with which the W-gluon fusion (gq→tb̅ q) and qq̅ →tb̅ signals can be extracted from the backgrounds at both the Fermilab Tevatron and the CERN LHC. Although perturbation theory breaks down at low transverse momentum for the W-gluon fusion b̅ differential cross section, we show how to obtain a reliable cross section integrated over low b̅ transverse momenta up to a cutoff. We estimate the accuracy with which V_tb can be measured in both analyses, including theoretical and statistical uncertainties. We also show that the polarization of the top quark in W-gluon fusion can be detected at the Fermilab Tevatron and the CERN LHC.

Single-top-quark production via W-gluon fusion at hadron colliders provides an opportunity to directly probe the charged-current interaction of the top quark. We calculate the next-to-leading-order corrections to this process at the Fermilab Tevatron, the CERN Large Hadron Collider, and DESY HERA. Using a b-quark distribution function to sum collinear logarithms, we show that there are two independent corrections, of order 1/ln(m_t²/m_b²) and α_s. This observation is generic to processes involving a perturbatively derived heavy-quark distribution function at an energy scale large compared with the heavy-quark mass.

Understanding the pattern of gluon radiation in tt̅ production and decay processes is important for making an accurate determination of the top-quark mass from the momenta of its decay products. The larger energy of the CERN LHC pp collider boosts the top-quark cross section by a factor of 100 compared to that at the Fermilab Tevatron, but it also increases the amount of additional gluon radiation. We calculate the cross section and distributions for gluon radiation in top-quark production and decay at the LHC.

We present a complete calculation of the matrix elements for the processes qq¯,gg→bW⁺b¯W^-g and qg→bW⁺b¯W^-q which are relevant for the study of events with an additional jet in tt¯ production at the Fermilab Tevatron pp¯ collider. Our calculation includes (i) the contributions from gluons emitted during the top quark production and decay stages and the interference between these, and (ii) the complete set of Feynman diagrams corresponding to both resonant and nonresonant top quark production. We study the distribution in phase space of the additional parton jet and make comparisons with previous studies based on the soft-gluon approximation and with results from parton-shower Monte Carlo simulations. The implications for top quark mass measurements are briefly discussed.

We calculate the production of two b-quark pairs in hadron collisions. Sources of multiple pairs are multiple interactions and higher order perturbative QCD mechanisms. We subsequently investigate the competing effects of multiple b-pair production on measurements of CP violation: (i) the increase in event rate with multiple b-pair cross sections which may reach values of the order of 1 b in the presence of multiple interactions and (ii) the dilution of b versus b¯ tagging efficiency because of the presence of events with four B mesons. The impact of multiple B-meson production is small unless the cross section for producing a single pair exceeds 1 mb. We show that even for larger values of the cross section the competing effects (i) and (ii) roughly compensate so that there is no loss in the precision with which CP-violating CKM angles can be determined.

We examine the structure of the underlying event in neutral Higgs-boson production at the Superconducting Super Collider (SSC). Gaps, regions of rapidity containing no soft particle production, can provide a clean signature for W boson fusion to the heavy Higgs boson. We first examine the physical basis of gap production and estimate the survival probability of gaps in the minijet model. Then, using pythia, and herwig, we compare gap events to W pair production from top quark decay and qq̅ fusion. We find that, if experimental problems can be overcome, gaps should provide a small, but clean, signal for heavy Higgs-boson production at the SSC.

Surface neutrino telescopes have the capability of detecting neutrinos and muons of similar energy. In a ν_μ oscillation experiment one can therefore tag the π progenitor of the neutrino by detecting the muon produced in the same decay. This eliminates the model dependence of the measurement inevitably associated with the calculation of the primary-cosmic-ray flux. We show that planned surface neutrino telescopes probe the parameter space Δm²≳10^-3 eV² and sin²2θ≳10^-3 using this technique.

The neutralino, the lightest superpartner in many supersymmetric theories, is arguably the leading dark-matter candidate from both the cosmological and particle-physics points of view. Its mass is bracketed by a minimum value of tens of GeV, determined from unsuccessful accelerator searches, and a maximum value of several TeV, above which neutralinos "overclose" the Universe. If neutralinos exist in our galactic halo, they will be gravitationally captured by scattering off elements in the Sun. Annihilation of neutralinos in the Sun will produce a neutrino flux which can be detected on Earth and thus provide indirect evidence for galactic dark matter. We show that a 1-km² area is the natural scale of a neutrino telescope capable of probing the GeV-TeV neutralino mass range by searching for high-energy neutrinos produced by their annihilation in the Sun.