NLO QCD Research Articles

J/ψ hadron interaction in vacuum and in QGP

Motivated by the recent lattice data that J/ψ will survive up to 1.6Tc, we calculate the thermal width of J/ψ at finite temperature in perturbative QCD. The inputs of the calculation are the parton quarkonium dissociation cross sections at the NLO in QCD, which were previously obtained by Song and Lee, and a Gaussian charmonium wavefunction, whose size was fitted to an estimate by Wong by solving the Schrödinger equation for charmonium in a potential extracted from the lattice at finite temperature. We find that the total thermal width above 1.4Tc becomes larger than 100 to 200 MeV, depending on the effective thermal masses of the quark and gluon, which we take it to vary from 600 to 400 MeV.

QCD corrections to triboson production

We present a computation of the next-to-leading order QCD corrections to the production of three $Z$ bosons at the Large Hadron Collider. We calculate these corrections using a completely numerical method that combines sector decomposition to extract infrared singularities with contour deformation of the Feynman parameter integrals to avoid internal loop thresholds. The NLO QCD corrections to $pp\ensuremath{\rightarrow}ZZZ$ are approximately 50% and are badly underestimated by the leading order scale dependence. However, the kinematic dependence of the corrections is minimal in phase space regions accessible at leading order.

NLO QCD corrections to inclusive jet and hadron production in DIS

We analyze the order a 2 corrections to the single inclusive jet and hadron cross sections in lepton-nucleon deep inelastic scattering. The full calculations are done analytically, obtaining finite NLO partonic level cross sections for these processes. We show that in both cases the dominant partonic mechanism starts at order a 2 , being effectively a lowest order estimate, with the consequent large factorization scale uncertainty, and the likelihood of non-negligible corrections at the subsequent order in perturbation theory.

Deeply virtual compton scattering in color dipole formalism

In this contribution we summarize recent investigations on the deeply virtual Compton Scattering (DVCS) within the color dipole approach. The color dipole cross section is implemented through the phenomenological saturation model. The role played by its QCD evolution and skewedness effects in the DVCS cross section are discussed. The results are compared with the recent H1 and ZEUS Collaborations data. The skewing factor, defined as the ratio of the imaginary parts of the amplitudes ImA(g*p ® g* p)/ImA(g*p ® gp) can be extracted from the data using recent DVCS and the inclusive inelastic cross section measurements at DESY-HERA. We report on this experimental extraction and compare the results to the theoretical predictions for NLO QCD and the color dipole approach.

Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q^2<0.01 GeV^2) and deep-inelastic scattering processes (DIS, 4<Q^2<80 GeV^2). The event topology is given by ep-> e X Y, in which the system X, containing at least two jets, is separated from a leading low-mass proton remnant system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5+-0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.

An extraction of the skewing factor from DESY-HERA data

In this contribution we present recent investigation on the extraction of the skewing factor, defined as the ratio of the imaginary parts of the amplitudes Im ${\cal A}$(gamma* p <FONT FACE=Symbol>® g</FONT>* p)/ Im ${\cal A}$(gamma* p <FONT FACE=Symbol>® g</FONT> p). This factor is extracted from the data using the recent DVCS and the inclusive inelastic cross section measurements at DESY-HERA. We compare the results to the theoretical predictions for NLO QCD and the color dipole approach.

Angular correlations of lepton pairs from vector boson and top quark decays in Monte Carlo simulations

We explain how angular correlations in leptonic decays of vector bosons and top quarks can be included in Monte Carlo parton showers, in particular those matched to NLO QCD computations. We consider the production of $n$ pairs of leptons, originating from the decays of $n$ electroweak vector bosons or of $n$ top quarks, in the narrow-width approximation. In the latter case, the information on the $n$ $b$ quarks emerging from the decays is also retained. We give results of implementing this procedure in MC@NLO

Measurement of [formula omitted] meson production in [formula omitted] scattering at low [formula omitted

The production of D * ± ( 2010 ) mesons in e ± p scattering in the range of exchanged photon virtuality 0.05 < Q 2 < 0.7 GeV 2 has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb −1. The decay channels D * + → D 0 π + with D 0 → K − π + and corresponding antiparticle decay were used to identify D * mesons and the ZEUS beampipe calorimeter was used to identify the scattered electron. Differential D * cross sections as functions of Q 2 , inelasticity, y, transverse momentum of the D * meson, p T ( D * ) , and pseudorapidity of the D * meson, η ( D * ) , have been measured in the kinematic region 0.02 < y < 0.85 , 1.5 < p T ( D * ) < 9.0 GeV and | η ( D * ) | < 1.5 . The measured differential cross sections are in agreement with two different NLO QCD calculations. The cross sections are also compared to previous ZEUS measurements in the photoproduction and DIS regimes.

Supersymmetric NLO QCD corrections to resonant slepton production and signals at the Fermilab Tevatron and the CERN LHC

We compute the total cross section and the transverse momentum distribution for single charged slepton and sneutrino production at hadronic colliders including NLO supersymmetric and nonsupersymmetric QCD corrections. The supersymmetric QCD corrections can be substantial. We also resum the gluon transverse momentum distribution and compare our results with two Monte Carlo generators. We compute branching ratios of the supersymmetric decays of the slepton and determine event rates for the like-sign dimuon final state at the Tevatron and at the LHC.

PION QUARK STRUCTURE IN QCD

We describe the present status of the pion distribution amplitude as it originated from two sources: (i) a nonperturbative approach, based on QCD sum rules with nonlocal condensates and (ii) a NLO QCD analysis of the CLEO data on Fγγ*π(Q2), supplemented by the recent high-precision lattice calculations of the second moment of the pion distribution amplitude.

The curvature of F2 p(x,Q2) as a probe of the range of validity of perturbative QCD evolutions in the small-x region

Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-x region, where they are in very good agreement with all recent precision measurements of F2 p(x,Q2). These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme (MS or DIS). A characteristic feature of perturbative QCD evolutions is a positive curvature of F2 p which increases as x decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD.

Anomalous Higgs boson couplings in vector boson fusion at the CERN LHC

Deviations from standard model expectations in the Higgs sector can be parametrized by an effective Lagrangian. The corresponding anomalous couplings have been implemented in a Monte Carlo program for Higgs production in vector boson fusion, at NLO QCD accuracy. It allows to study anomalous coupling effects for production and decay of the Higgs boson. We analyze deviations allowed by LEP data and study a new azimuthal angle variable which directly measures the interference between $CP$-even, $CP$-odd and standard model couplings.

J/ψ hadron interaction in vacuum and in QGP

I first discuss the NLO QCD calculation for the dissociation cross section of J/ψ into open charm, which shows a large second order correction near the threshold. The large correction softens when effective thermal masses in the order of the binding energy are introduced for the quarks and gluons. Hence, with the thermal masses, the method is reliably applied to calculate the dissociation cross section of J/ψ, which according to recent lattice results seems to remain bound even up to 1.6 × Tc. The dominant contribution to the dissociation comes from the thermal gluons, while the effects due to thermal quarks are suppressed due to suppressed phase space. The relevance of this result is discussed in the context of RHIC and signature of QGP.

PDF and scale uncertainties of various DY distributions in ADD and RS models at hadron colliders

In the extra dimension models of ADD and RS we study the dependence of the various parton distribution functions on observables of Drell–Yan processes to NLO in QCD at LHC and Tevatron energies. Uncertainties at LHC due to factorisation scales in going from leading to next-to-leading order in QCD for the various distributions get reduced by about 2.75 times for a μF range 0.5Q<μF<1.5Q. Further uncertainties arising from the error on the experimental data are estimated using the MRST parton distribution functions.

Measurement of thebjet cross section in events with aZboson inpp¯collisions ats=1.96 TeV

A measurement of the inclusive bottom jet cross section is presented for events containing a $Z$ boson in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV using the Collider Detector at Fermilab. $Z$ bosons are identified in their electron and muon decay modes, and $b$ jets with $E_T>20$ GeV and $|\eta|<1.5$ are identified by reconstructing a secondary decay vertex. The measurement is based on an integrated luminosity of about 330 ${\rm pb}^{-1}$. A cross section times branching ratio of $\sigma (Z+b {\rm jets}) \times {\cal B}(Z \to \ell^+ \ell^-)= 0.93 \pm 0.36$ pb is found, where ${\cal B}(Z\to \ell^+ \ell^-)$ is the branching ratio of the $Z$ boson or $\gamma^*$ into a single flavor dilepton pair ($e$ or $\mu$) in the mass range between 66 and 116 GeV$/c^2$. The ratio of $b$ jets to the total number of jets of any flavor in the $Z$ sample, within the same kinematic range as the $b$ jets, is $2.36 \pm 0.92%$. Here, the uncertainties are the quadratic sum of statistical and systematic uncertainties. Predictions made with NLO QCD agree, within experimental and theoretical uncertainties, with these measurements.

Non-singlet QCD analysis of [formula omitted] up to NNLO

The significance of NNLO (3-loop) QCD contributions to the flavor non-singlet sector of F 2 e p and F 2 e d has been studied as compared to uncertainties (different factorization schemes, higher twist and QED contributions) of standard NLO (and LO) QCD analyses. The latter effects turn out to be comparable in size to the NNLO contributions. Therefore the minute NNLO effects are unobservable with presently available (precision) data on non-singlet structure functions.

Single top-quark production in thet- channel at CERN LHC: A realistic test of electroweak models

We compute the complete electroweak one-loop effect on the process of $t$-channel single top production at LHC in the standard model and in the MSSM within the mSUGRA symmetry breaking scheme. We find that the one-loop electroweak SM effect is large, and decreases the cross section of an amount that is of the same size as that of the NLO QCD one. The genuine SUSY effect in the mSUGRA scheme, for a general choice of benchmark points, is rather small. It might become large and visible in more general scenarios around thresholds involving light stop and neutralino mass values.

Next-to-leading order QCD corrections toZboson pair production via vector-boson fusion

Vector-boson fusion processes are an important tool for the study of electroweak symmetry breaking at hadron colliders, since they allow to distinguish a light Higgs boson scenario from strong weak-boson scattering. We here consider the channels WW{yields}ZZ and ZZ{yields}ZZ as part of electroweak Z boson pair production in association with two tagging jets. We present the calculation of the NLO QCD corrections to the cross sections for pp{yields}e{sup +}e{sup -}{mu}{sup +}{mu}{sup -}+2 jets and pp{yields}e{sup +}e{sup -}{nu}{sub {mu}}{nu}{sub {mu}}+2 jets via vector-boson fusion at order {alpha}{sub s}{alpha}{sup 6}, which is performed in the form of a NLO parton-level Monte Carlo program. The corrections to the integrated cross sections are found to be modest, while the shapes of some kinematical distributions change appreciably at NLO. Residual scale uncertainties typically are at the few percent level.

Recent critical study of photon production in hadronic collisions

In the light of the new prompt photon data collected by PHENIX at RHIC and by D0 at the run II of the Tevatron, we revisit the world prompt photon data, both inclusive and isolated, in hadronic collisions, and compare them with the NLO QCD calculations implemented in the Monte Carlo programme JETPHOX.

NLO QCD Corrections to Drell-Yan in TeV-scale Gravity Models

In TeV scale gravity models, we present the NLO-QCD corrections for the double differential cross sections in the scattering angle for dilepton production at hadron colliders. The quantitative impact of QCD corrections for extra dimension searches at LHC and Tevatron are investigated for both ADD and RS models through K-factors. We also show how the inclusion of QCD corrections to NLO stabilises the cross section with respect to renormalisation and factorisation scale variations.