Quark Production Cross Section Research Articles

Heavy quark production from colour glass condensate at RHIC

I consider production of heavy quarks in pA and AA collisions in the framework of the colour glass condensate. I discuss the heavy quark production in pA collisions in a classical approximation which takes into account all multiple rescatterings of a proton in a nucleus. For peripheral collisions, the heavy quark production cross section can be written in the kT-factorized form. The kT-factorization is used to construct a simple model which takes into account both classical and quantum effects in pA and AA collisions. I review the main result of calculation based on that model: open charm in the central rapidity region at RHIC gets suppressed as a function of rapidity. Although the numbers obtained for the suppression factor are model dependent, the very fact of suppression is the general feature of the colour glass condensate at RHIC kinematical region. It indicates the onset of quantum evolution effects in a nucleus and is universal for all inclusive processes.

Heavy quark production by a quasi-classical color field in proton–nucleus collisions

We calculate the inclusive heavy quark production cross section for proton–nucleus collisions at high energies. We perform calculation in a quasi-classical approximation (McLerran–Venugopalan model) neglecting all low-x evolution effects. The derived expression for the differential cross section can be applied for studying the heavy quark production in the central rapidity region at RHIC.

Light Bottom Squark and Gluino Confront Electroweak Precision Measurements

We address the compatibility of a light sbottom (mass approximately 2-5.5 GeV) and a light gluino (mass approximately 12-16 GeV) with electroweak precision measurements. Such light particles have been suggested to explain the observed excess in the b quark production cross section at the Tevatron. The electroweak observables may be affected by the sbottom and gluino through the supersymmetric-QCD (SUSY-QCD) corrections to the Zbb vertex. We examine, in addition to the SUSY-QCD corrections, the gauge boson propagator corrections from the stop which are allowed to be light from the SU(2)(L) symmetry. We find that this scenario is strongly disfavored from electroweak precision measurements.

Measurement of the ratio ofbquark production cross sections inp¯pcollisions ats=630 GeVands=1800 GeV

We report a measurement of the ratio of the bottom quark production cross section in antiproton-proton collisions at 630 GeV to 1800 GeV using bottom quarks with transverse momenta greater than 10.75 GeV identified through their semileptonic decays and long lifetimes. The measured ratio sigma(630)/sigma(1800) = 0.171 +/- .024 +/- .012 is in good agreement with next-to-leading order (NLO) quantum chromodynamics (QCD).

Very light sbottom and gluino scenario confronting electroweak precision tests

The minimal supersymmetric model with light sbottom (∼ m b ) and light gluino (12∼16 GeV) can explain the excess of bottom quark production cross section at hadron colliders and has drawn much attention. We calculate one-loop contribution to Z-pole observable R b in this scenario when large CP-violating phases are allowed in the gluino mass and/or sbottom sector. We show that these large CP-violating phases can suppress the new contribution δR b safely within the experimental bounds even for heavier sbottom too heavy ( b ̃ 2≳200 GeV) to be produced at LEP2 in association with light sbottom b ̃ 1 .

QCD loop corrections to top quark production and decay ate+e−colliders

We present a computation of QCD next-to-leading order virtual corrections to the top quark production and decay process at linear colliders. The top quarks are allowed to be off-shell and the production and decay subprocesses are treated together, thus allowing for interference effects. The framework employed for our computation is the double pole approximation (DPA). We describe the implementation of this approximation for the top quark production and decay process and compare it with the implementation of DPA for the evaluation of QED corrections to the W pair production at CERN LEP II. Similarities and differences between the two cases are pointed out. One result of interest is the incomplete cancellation of interference corrections. Other results include values for the total NLO top quark production cross section, and the impact the nonfactorizable (interference) corrections have on the top quark invariant mass distribution.

Thresholdtt¯cross section at next-to-next-to-leading logarithmic order

The total cross section for top quark pair production close to threshold in e+e- annihilation is investigated. Details are given about the calculation at next-to-next-to-leading logarithmic order. The summation of logarithms leads to a convergent expansion for the normalization of the cross section, and small residual dependence on the subtraction parameter nu. A detailed analysis of the residual nu dependence is carried out. A conservative estimate for the remaining uncertainty in the normalization of the total cross section from QCD effects is $\lesssim \pm 3%$. This makes precise extractions of the strong coupling and top width feasible, and further studies of electroweak effects mandatory.

Sudakov resummation and finite order expansions of heavy quark hadroproduction cross sections

We resum Sudakov threshold enhancements in heavy quark hadroproduction for single-heavy quark inclusive and pair-inclusive kinematics. We expand these resummed results and derive analytical finite-order cross sections through next-to-next-to-leading order. This involves the construction of next-to-leading order matching conditions in color space. For the scale dependent terms we derive exact results using renormalization group methods. We study the effects of scale variations, scheme and kinematics choice on the partonic and hadronic cross sections, and provide estimates for top and bottom quark production cross sections.

Soft gluon resummation for polarized deep-inelastic production of heavy quarks

We present the threshold resummation for the cross section of heavy quark production in polarized deep-inelastic scattering to next-to-leading logarithmic accuracy and in single-particle inclusive kinematics. We expand our resummed result in α s to next-to-leading and next-to-next-to-leading order and study the impact of these higher order corrections on the charm structure function g 1 c in the kinematical range accessible to the HERMES and COMPASS experiments.

Quartic mass corrections to R at (α)

The total cross section for the production of massive quarks in electron positron annihilation can be predicted in perturbative QCD. After expansion in m^2/s the quartic terms, i.e. those proportional to m^4/s^2, are calculated up to order \alpha_s^3 for vector and axial current induced rates. Predictions relevant for charm, bottom and top quarks production are presented. The \alpha_s^3 corrections are shown to be comparable to terms of order \alpha_s and \alpha_s^2. As a consequence, the predictions exhibit a sizeable dependence on the renormalization scale. The stability of the prediction is improved and, at the same time, the relative size of the large order terms decreases by replacing the running mass \bar{m}(\mu) with the scheme independent invariant one \hat{m}. By combining these results with the prediction for massless case and the quadratic mass terms the cross section for massive quark production at electron positron colliders is put under control in order \alpha_s^3 from the high energy region down to fairly low energies.

The [formula omitted] production cross section and angular correlations in [formula omitted] collisions at [formula omitted

We present measurements of the b b production cross section and angular correlations using the DØ detector at the Fermilab Tevatron p p Collider operating at s = 1.8 TeV. The b quark production cross section for |y b| < 1.0 and p T b > 6 GeV/ c is extracted from single muon and dimuon data samples. The results agree in shape with the next-to-leading order QCD calculation of heavy flavor production but are greater than the central values of these predictions. The angular correlations between b and b quarks, measured from the azimuthal opening angle between their decay muons, also agree in shape with the next-to-leading order QCD prediction.

Search for scalar top and scalar bottom quarks in pp collisions at square root s=1.8 TeV.

We have searched for direct pair production of scalar top and scalar bottom quarks in 88 pb-1 of pp collisions at sqrt[s]=1.8 TeV with the CDF detector. We looked for events with a pair of heavy flavor jets and missing energy, consistent with scalar top (bottom) quark decays to a charm (bottom) quark and a neutralino. The numbers of events that pass our selections show no significant deviation from standard model expectations. We compare our results to the next-to-leading order scalar quark production cross sections to exclude regions in scalar quark-neutralino mass parameter space.

Soft-gluon resummation for heavy quark production in hadronic collisions

We discuss the heavy quark production cross section near partonic threshold in hadronic collisions, including the resummation of leading and next-to-leading logarithms arising from soft gluon emission. We show how to handle the complications due to the non-universal non-leading logarithms. We give analytical results for the $q {\bar q}$ partonic subprocess and numerical results in the DIS scheme for top quark production at the Fermilab Tevatron where the $q {\bar q}$ channel dominates.

Yukawa-coupling corrections to scalar quark decays

Heavy squark decays into top and charginos or neutralinos could be an unexpected source of top quarks at hadron colliders. A detailed treatment of these processes is necessary for a reliable calculation of both the top quark production cross-section and the standard top quark branching ratio in the MSSM. Along this line we compute the electroweak corrections to the sbottom decays b ̃ a→χ − i t within the Yukawa coupling approximation. The calculation of these higher order contributions requires renormalization of both the bottom squark mixing angle and of tan β. This type of corrections gives the leading order electroweak effects at low and high tan β.

Top quark production at threshold with O( αs2) accuracy

We calculate the next-to-next-to-leading order correction to the cross section for top quark pair production in e + e − annihilation in the threshold region, resumming all O[( α s / β) n × ( α s 2, β 2, α s β)] terms of perturbation series. We find that the magnitude of the NNLO correction is comparable to the size of the NLO corrections.

Extended Technicolor Dynamics and Single Top Quark Production at the Fermilab Tevatron

The large electron positron collider value of branching ratio Rb is larger than the standard model predicted value RbSM, this deviation δRb is treated as an allowed range for the correction to the single top quark production cross section σ(tb) arising from the extended technicolor dynamics. It is found that the relative correction δσ(tb)/σ0(tb) is quite large for a certain range of the free parameters in the model, and may be detected in the future proton-antiproton collision experiments.

Spatial Variation of Nuclear Structure Functions and Heavy Quark Production

We explore how nuclear modifications to the free nucleon structure functions (also known as shadowing) affect heavy quark production in collisions at different impact parameters. We assume that the nuclear modifications arise from a density dependent effect such as gluon recombination and are thus proportional to the local density. We calculate the dependence of charm and bottom quark production on impact parameter and show that density dependent modifications can lead to significant reductions in the heavy quark production cross sections in central relative to peripheral interactions.

Next-to-leading order QCD corrections to the polarized photoproduction of heavy flavors

We present a calculation of the next-to-leading order ( O(α s 2α)) QCD corrections to heavy flavor photoproduction with longitudinally polarized beams. We apply our results to study the longitudinal spin asymmetry for the total charm quark production cross section which will be utilized by the forthcoming COMPASS experiment at CERN to obtain first direct information on the polarized gluon density Δg. We also briefly discuss the main theoretical uncertainties inherent in this calculation. In particular we demonstrate that the factorization scale dependence is considerably reduced in next-to-leading order.

Slepton production as a probe of the squark mass scale

We investigate an important radiative correction to the slepton and chargino production processes. We consider a supersymmetric spectrum with a large splitting between the squark and slepton masses. In this case, in the effective theory below the squark mass threshold, the supersymmetric Slavnov-Taylor identities which enforce the equality of the gauge and gaugino couplings are violated. The gaugino propagators receive potentially large corrections due to virtual quark or squark loop effects. We compute the full one-loop (s)quark corrected slepton production cross sections. The $t$-channel one-loop scattering amplitudes are factorized into an effective chargino-neutralino mass matrix and an effective fermion-sfermion-gaugino coupling. The difference between the effective gaugino coupling and the gauge coupling is proportional to $\mathrm{ln}{(M}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{Q}}{/m}_{\mathcal{l}\ifmmode \tilde{}\else \~{}\fi{}})$ in the large squark mass limit. We find that the one-loop corrected slepton production cross sections can depend on the squark mass strongly, up to $9%\ifmmode\times\else\texttimes\fi{}{\mathrm{log}}_{10}{(M}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{Q}}{/m}_{\mathcal{l}\ifmmode \tilde{}\else \~{}\fi{}})$. We investigate the squark mass sensitivity of the slepton cross-section measurements at a future linear collider. For sneutrino production accessible at $\sqrt{s}=500$ GeV there can be sensitivity to squark masses at or larger than 1 TeV.

Threshold resummation of the total cross section for heavy quark production in hadronic collisions

We discuss calculations of the inclusive total cross section for heavy quark production at hadron collider energies within the context of perturbative quantum chromodynamics, including resummation of the effects of initial-state soft gluon radiation to all orders in the strong coupling strength. We resum the universal leading-logarithm contributions, and we restrict our integrations to the region of phase space that is demonstrably perturbative. We include a detailed comparison of the differences between ours and other methods. We provide predictions of the physical cross section as a function of the heavy quark mass in proton-antiproton reactions at center-of-mass energies of 1.8 and 2.0 TeV, and we discuss estimated uncertainties.