Higher Order QCD Corrections Research Articles

Dilepton production near partonic threshold in transversely polarizedp¯pcollisions

It has recently been suggested that collisions of transversely polarized protons and antiprotons at the GSI could be used to determine the nucleon's transversity densities from measurements of the double-spin asymmetry for the Drell-Yan process. We analyze the role of higher-order perturbative QCD corrections in this kinematic regime, in terms of the available fixed-order contributions as well as of all-order soft-gluon resummations. We find that the combined perturbative corrections to the individual unpolarized and transversely polarized cross sections are large. We trace these large enhancements to soft-gluon emission near partonic threshold, and we suggest that with a physically motivated cutoff enhancements beyond lowest order are moderated relative to resummed perturbation theory, but still significant. The unpolarized dilepton cross section for the GSI kinematics may therefore provide information on the relation of perturbative and nonperturbative dynamics in hadronic scattering. The spin asymmetry turns out to be rather robust, relatively insensitive to higher orders, resummation, and the cutoffs.

Pair production of scalar leptoquarks at the CERN LHC

Theoretical predictions for the production cross sections of leptoquarks at the CERN LHC are presented including higher-order QCD corrections. These corrections reduce the dependence on the renormalization/factorization scales significantly. Moreover, they are required to exploit the leptoquark mass reach of the LHC experiments. In this sequel to an earlier analysis performed for the Tevatron collider we extend the leptoquark analysis to the LHC energy.

EffectiveK-factors for gg H WW at the LHC

A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb^{-1}. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15 % smaller than the theoretical inclusive K-factor.

Precise prediction for theW-boson mass in the standard model

The presently most accurate prediction for the W-boson mass in the Standard Model is obtained by combining the complete two-loop result with the known higher-order QCD and electroweak corrections. The numerical impact of the different contributions is analysed in detail. A simple parametrisation of the full result is presented, which approximates the full result for M_W to better than 0.5 MeV for 10 GeV < M_H < 1 TeV if the other parameters are varied within their combined 2 sigma region around their experimental central values. The different sources of remaining theoretical uncertainties are investigated. Their effect on the prediction of M_W is estimated to be about 4 MeV for M_H < 300 GeV.

Summary of working group 3

The parallel sessions of the working group 3 were devoted to the discussions of short-baseline neutrino physics programme at neutrino factories. First, possible studies of parton distribution functions (PDFs), in particular nuclear and polarized PDFs, are discussed in this summary. Secondly, the extractions of αs from sum rules and structure functions of deep inelastic neutrino–nucleon scattering, higher-order perturbative QCD corrections and estimates of high-twist effects are summarized. Thirdly, the situation of the observed NuTeV sin2θW anomaly is discussed.

Photoproduction of W bosons at HERA: QCD corrections

W bosons can be produced in the channels e+- p -> W+- + X at HERA thus allowing to probe for anomalous trilinear couplings among the gauge bosons. We discuss the NLO QCD corrections to the photoproduction of W bosons with finite transverse momentum at HERA. The higher-order QCD corrections reduce the factorization scale dependence significantly and modify the leading order cross sections by +-O(10%).

Jets and produced particles in pp collisionsfrom SPS to RHIC energies for nuclear applications

Higher-order perturbative QCD corrections play animportant role in the reproduction of data at hightransverse momenta in the energy range 20 GeV⩽(s)1/2⩽200 GeV. Recent calculations of photon and pionproduction in pp collisions yield detailed information on thenext-to-leading-order (NLO) contributions. However, theapplication of these results in proton-nucleus andnucleus-nucleus collisions is not straightforward. Thestudy of nuclear effects requires a simplifiedunderstanding of the output of these computations. Herewe summarize our analysis of recent calculations, aimedat handling the NLO results by introducing process andenergy-dependent K factors.

Higher-order QCD corrections to γ*γ* → hadrons

I illustrate the techniques and the results of the computation of order-α s corrections to the production of hadrons in the collision of two virtual photons, that originate from the incoming leptons at e + e − colliders.

The QCD renormalization scale stability of high twists and αs in deep inelastic scattering

A sensitivity of twist-4 and α s values extracted in the NLO QCD analysis of non-singlet SLAC-BCDMS-NMC deep inelastic scattering data to the choice of QCD renormalization scale (RS) is analyzed. It is obtained that the high twist (HT) contribution to structure function F 2, is re-tuned with the change of RS. This re-tuning depends on the choice of starting QCD evolution point Q 0 and x. At Q 0≳10 GeV 2 the HT contribution to F 2 is re-tuned significantly at small x and weakly at large x; at small Q 0 the it exhibits approximate RS stability for all x in question. The HT contribution to F L is RS stable for all Q 0 and x. The RS sensitivity of α s obtained in the analysis also depends on the choice of Q 0: At large Q 0 this sensitivity is weaker than at small ones. For Q 0 2=9 GeV 2 the value α s M Z =0.1183±0.0021( stat+syst)±0.0013( RS) is obtained. Explanation of those effects based on the analysis of the QCD evolution equations properties is given and connection with the higher order QCD corrections is discussed.

Higher-order QCD corrections in prompt photon production

We exhibit a method for simultaneously treating recoil and threshold corrections in single-photon inclusive cross sections, working within the formalism of collinear factorization. This approach conserves both the energy and transverse momentum of resummed radiation. At moderate p(T), we find the potential for substantial enhancements from higher-order perturbative and power-law nonperturbative corrections.

FeynHiggs: a program for the calculation of the masses of the neutral [formula omitted]-even Higgs bosons in the MSSM

FeynHiggs is a Fortran code for the calculation of the masses of the neutral CP -even Higgs bosons in the MSSM up to two-loop order. It is based on the complete diagrammatic on-shell result at the one-loop level, the leading diagrammatic two-loop QCD contributions and further improvements taking into account leading electroweak two-loop and leading higher-order QCD corrections. The Higgs-boson masses are calculated as functions of the MSSM parameters for general mixing in the scalar top sector and arbitrary choices of the parameters in the Higgs sector of the model.

K-factors in parton cascades at RHIC and SPS

Different treatments for the inclusion of higher-order perturbative QCD corrections in parton based transport models of relativistic heavy-ion collisions are studied and their influence on experimental observables is investigated. At RHIC, particle multiplicities may vary by more than 30%, depending on the correction scheme. A detailed analysis of the squared relative momentum transfers Q 2 to be expected in parton (re)scatterings at RHIC casts doubt on the applicability of pQCD based transport approaches at the SPS and rules out the application of higher-order correction schemes based on a rescaling of Q 2 in the running coupling constant α s at all energies.

Relations among higher order QCD corrections

We study the scheme transformation of next to leading order QCD corrections to various processes. An interesting relation by Drell, Levy and Yan (DLY) among space like and time like processes is studied carefully in the next to leading order level. We construct factorisation scheme invariants and show that they are DLY-invariant.

QCD corrections to FCNC decays mediated by Z-penguins and W-boxes

QCD corrections are evaluated to FCNC processes like B→X sν ν ̄ , K→πν ν ̄ , B→ l + l − or K L → μ + μ −, i.e. to processes mediated by effective operators containing neutrino currents or axial leptonic currents. Such operators originate from W-box and Z-penguin diagrams in the Standard Model. QCD corrections to them are given by two-loop diagrams. We confirm results for those diagrams which are already present in the literature. However, our analytical expressions for the Wilson coefficients disagree, due to a subtlety in regulating spurious IR divergences. The numerical effect of the disagreement is rather small. The size of the perturbative QCD corrections compared to the leading terms depends on the renormalization scheme used at the leading order. It varies from 0 to around 15% for a reasonable class of schemes. The uncertainty originating from uncalculated higher-order (three-loop) QCD corrections is expected to be around 1%.

Double asymptotic scaling in Drell-Yan processes

Double scaling may be observed in the Drell-Yan process at the Tevatron and in the ALICE detector at the Large Hadron Collider in a window of masses. In the double scaling limit of the cross section, the higher order QCD corrections are quite large, and are driven by the rise in gluon densities in the double scaling regime. The naive parton model cannot be valid even at asymptotic energies in the region of phase space where double scaling holds.

Theoretical understanding of direct photon production

We review some theoretical developments concerning higher order QCD corrections to the bremsstrahlung cross-section and the problems raised by the definition of isolated cross sections. Then we discuss recent phenomenological analyses of direct photon fixed target data.

Vacuum Polarization Bubble Effects on the Triangle Relations for B Meson Charmless Decay Amplitudes

Assuming that factorization is valid and that contributions of the exchange, annihilation, and penguin-annihilation diagrams are negligible, we study the effects of the QCD vacuum polarization bubbles on the decay amplitudes of some B meson charmless decays into two light pseudoscalars. The bubbles have much less effects on than on thus the triangle relation is changed slightly while is altered significantly. This may indicate that in order to test or use these triangle relations other higher order QCD corrections should also be carefully taken into consideration.

Color-Singlet and Color-Octet Contributions to J/ψ Photoproduction

I discuss the impact of color-octet contributions and higher-order QCD corrections on the cross section for inelastic J/ψ photoproduction. The theoretical predictions are compared with recent experimental data obtained at HERA.

Dependence of Jet Photoproduction on Scales in Higher Order QCD Corrections at HERA

The dependencies of two-jet cross section on renormalization scale and factorization scales of resolved photon and proton are investigated up to next-to-leading order (NLO) QCD. It is shown that the NLO cross section is quite sensitive to the choice of the renormalization scale and the factorization scales of resolved photon and proton in large invariant mass and large negative rapidity of the two-jet system.

Erratum: Heavy-quark production by polarized and unpolarized photons in next-to-leading order [Phys. Rev. D 51, 4808 (1995)

Complete analytical results for the production of heavy-quark pairs by polarized and unpolarized photons in next-to-leading order are presented. Two-, three- and two plus three-jet cross sections for total photon spin $J_z=0,\pm 2$ are presented for $b\bar{b}(g)$ production. The 2-jet cross sections are considered as a background to $\gamma\gamma\rightarrow H^* \rightarrow b\bar{b}$ (standard model). Top production, not too far above threshold, is also considered for $J_z=0,\pm 2$. For both $b$- and $t$-quark production, the higher order QCD corrections are found to be significant.