NLO QCD Research Articles

NLO corrections to squark-squark production and decay at the LHC

We present the analysis of the signature 2j + ETmiss (+X) via squark-squark production and direct decay into the lightest neutralino, pp -> squark squark -> j j chi_1^0 chi_1^0 (+X), in next-to-leading order QCD within the framework of the minimal supersymmetric standard model. In our approximation the produced squarks are treated on shell. Thus, the calculation of production and decay factorizes. In this way, we provide a consistent, fully differential calculation of NLO QCD factorizable corrections to the given processes. Clustering final states into partonic jets, we investigate the experimental inclusive signature 2j + ETmiss for several benchmark scenarios. We compare resulting differential distributions with leading-order approximations rescaled by a flat K-factor and examine a possible impact for cut-and-count searches for supersymmetry at the LHC.

SusHi: A program for the calculation of Higgs production in gluon fusion and bottom-quark annihilation in the Standard Model and the MSSM

This article describes the code SusHi (for “Supersymmetric Higgs”) [108] which calculates the cross sections pp/pp¯→ϕ+X in gluon fusion and bottom-quark annihilation in the SM and the MSSM, where ϕ is any of the neutral Higgs bosons within these models. Apart from inclusive cross sections up to NNLO QCD, differential cross sections with respect to the Higgs transverse momentum pT and (pseudo-)rapidity y(η) can be calculated through NLO QCD. In the case of gluon fusion, SusHi contains NLO QCD contributions from the third family of quarks and squarks, NNLO corrections due to top-quarks, approximate NNLO corrections due to top-squarks, and electro-weak effects. It supports various renormalization schemes for the sbottom sector and the bottom Yukawa coupling, as well as resummation effects of higher order tanβ-enhanced sbottom contributions. SusHi provides a link to FeynHiggs for the calculation of the Higgs masses. Program SummaryProgram title: SusHiCatalogue identifier: AEOY_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOY_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 47725No. of bytes in distributed program, including test data, etc.: 338380Distribution format: tar.gzProgramming language: Fortran 77.Computer: Personal computer.Operating system: Unix/Linux, Mac OS.RAM: A few 100 MBClassification: 11.1.External routines: LHAPDF (http://lhapdf.hepforge.org), FeynHiggs (http://www.feynhiggs.de)Nature of problem:Calculation of inclusive and exclusive Higgs production cross sections in gluon fusion and bottom-quark annihilation in the Standard Model and the MSSM through next-to-leading order QCD, includes next-to-next-to-leading order top-(s)quark contributions and electro-weak effectsSolution method:Numerical Monte Carlo integrationAdditional comments:Code includes ggh@nnlo (http://www.robert-harlander.de/software/ggh@nnlo), bbh@nnlo (http://www.robert-harlander.de/software/bbh@nnlo) and evalcsusy.f (http://www.robert-harlander.de/software)Running time:All the examples provided take a minute or less to run.

Combination and QCD analysis of charm production cross section measurements in deep-inelastic ep scattering at HERA

Measurements of open charm production cross sections in deep-inelastic ep scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections \(\sigma_{\rm red}^{c\bar{c}}\) for charm production are obtained in the kinematic range of photon virtuality 2.5≤Q2≤2000 GeV2 and Bjorken scaling variable 3⋅10−5≤x≤5⋅10−2. The combination method accounts for the correlations of the systematic uncertainties among the different data sets. The combined charm data together with the combined inclusive deep-inelastic scattering cross sections from HERA are used as input for a detailed NLO QCD analysis to study the influence of different heavy flavour schemes on the parton distribution functions. The optimal values of the charm mass as a parameter in these different schemes are obtained. The implications on the NLO predictions for W± and Z production cross sections at the LHC are investigated. Using the fixed flavour number scheme, the running mass of the charm quark is determined.

NLO QCD corrections to tW ′ and tZ ′ production in forward-backward asymmetry models

We consider Z' and W' models recently proposed to explain the top forward-backward asymmetry at the Tevatron. We present the next-to-leading order QCD corrections to associated production of such vector bosons together with top quarks at the Large Hadron Collider, for centre-of-mass energies of 7 and 8 TeV. The corrections are significant, modifying the total production cross-section by 30-50%. We consider the effects of the corrections on the top and vector-boson kinematics. The results are directly applicable to current experimental searches, for both the ATLAS and CMS collaborations.

Evaluation of [formula omitted] using the ATLAS inclusive jet cross-section data

We present a determination of the strong coupling constant using ATLAS inclusive jet cross section data at s=7 TeV, with their full information on the bin-to-bin correlations. Several procedures for combining the statistical information from the data inputs are compared. The theoretical prediction is obtained using NLO QCD with non-perturbative corrections. Our determination uses inputs with transverse momenta between 45 and 600 GeV, the running of the strong coupling being also tested in this range. Good agreement is observed when comparing our result with the world average at the Z-boson scale, as well as with the most recent results from the Tevatron.

Photon structure functions with heavy particle mass effects

In the framework of the perturbative QCD we investigate heavy particle mass effects on the unpolarized and polarized photon structure functions, F2γ and g1γ, respectively. We present our basic formalism to treat heavy particle mass effects to NLO in perturbative QCD. We also study heavy quark effects on the QCD sum rule for the first moment of g1γ, which is related to axial anomaly. The photon structure function in supersymmetric QCD is also briefly discussed.

NLO QCD matrix elements + parton showers in e+e− → hadrons

Abstract We present a new approach to combine multiple NLO parton-level calculations matched to parton showers into a single inclusive event sample. The method provides a description of hard multi-jet configurations at next-to leading order in the perturbative expansion of QCD, and it is supplemented with the all-orders resummed modelling of jet fragmentation provided by the parton shower. The formal accuracy of this technique is discussed in detail, invoking the example of electron-positron annihilation into hadrons. We focus on the effect of renormalisation scale variations in particular. Comparison with experimental data from LEP underlines that this novel formalism describes data with a theoretical accuracy that has hitherto not been achieved in standard Monte Carlo event generators.

Production cross section of b-hadrons from decays to [formula omitted] final states

The production of b-hadrons has been recently studied by ATLAS at the LHC from partially reconstructed final states containing a μ− and a D⁎+. The production cross section and its dependence for |η|<2.5 and pT>9GeV are discussed and compared to NLO QCD predictions, which are lower than data, but not incompatible with them.

Uncertainties on low-x parton densities and the corresponding high-energy neutrino cross sections

A brief review of how parton densities are determined within the conventional DGLAP formalism of NLO QCD will be given with particular reference to the low-x region. Predictions for high energy neutrino and antineutrino charged and neutral current cross-sections using several different modern PDF fits will be compared. PDF uncertainties from model assumptions and parametrization bias are considered in addition to the experimental uncertainties. Particular attention is paid to assumptions and biases which could signal the need for extension of the conventional formalism to include effects such as ln(1/x) resummation or non-linear effects of high gluon density. See the published paper [1].

NLO QCD corrections to multi-jet production at the LHC with a centre-of-mass energy of [formula omitted

We study three and four jet production in hadronic collisions at next-to-leading order accuracy in massless QCD. We cross check results previously obtained by the BlackHat Collaboration for the LHC with a centre-of-mass energy of s=7 TeV and present new results for the LHC operating at 8 TeV. We find large negative NLO corrections reducing the leading order cross sections by about 40–50%. Furthermore we observe an important reduction of the scale uncertainty. In addition to the cross sections we also present results for differential distributions. The dynamical renormalization/factorization scale used in the calculation leads to a remarkably stable K-factor. The results presented here were obtained with the NJet package (Badger et al., 2012) [1], a publicly available library for the evaluation of one-loop amplitudes in massless QCD.

HELAC-NLO

Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program that is capable of numerically evaluating QCD virtual corrections to scattering amplitudes. A detailed presentation of the algorithm is given, along with instructions to run the code and benchmark results. The program is part of the HELAC-NLO framework that allows for a complete evaluation of QCD NLO corrections. Program summaryProgram title:HELAC-1LOOPCatalogue identifier: AEOC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOC_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 290945No. of bytes in distributed program, including test data, etc.: 3013326Distribution format: tar.gzProgramming language: Fortran (gfortran(http://gcc.gnu.org/fortran/), lahey95 (http://www.lahey.com), ifort3(http://software.intel.com)).Computer: Any.Operating system: Linux, Unix, Mac OS.Classification: 11.1.Nature of problem:The evaluation of virtual one-loop amplitudes for multi-particle scattering is a long-standing problem [1]. In recent years the OPP reduction technique [2] opened the road for a fully numerical approach based on the evaluation of the one-loop amplitude for well-defined values of the loop momentum.Solution method:By using HELAC [3–5] and CutTools [6], HELAC-1LOOP is capable of evaluating QCD virtual corrections [7]. The one-loop n-particle amplitudes are constructed as part of the n+2 tree-order ones, by using the basic recursive algorithm used in HELAC. A Les Houches Event (LHE) file is produced, combining the complete information from tree-order and virtual one-loop contributions. In conjunction with real corrections, obtained with the use of HELAC-DIPOLES [8], the full NLO corrections can be computed. The program has been successfully used in many applications.Running time:Depending on the number of particles and generated events from seconds to days.

SANC integrator in the progress: QCD and EW contributions

Modules and packages for the one-loop calculations at the partonic level represent the first level of SANC output computer product. The next level represents Monte Carlo integrator mcsanc, realizing fully differential hadron level calculations (convolution with PDF) for the HEP processes at LHC. In this paper we describe the implementation into the framework mcsanc first set of processes: DY NC, DY CC, $$f_1 \bar f'_1 \to HW^ \pm (Z)$$ and single top production. Both EW and QCD NLO corrections are taken into account. A comparison of SANC results with those existing in the world literature is given.

WZ production beyond NLO for high-[formula omitted] observables

We use the LoopSim and VBFNLO packages to investigate a merged sample of partonic events that is accurate at NLO in QCD simultaneously for the WZ and WZ+jet production processes. In certain regions of phase space such a procedure is expected to account for the dominant part of the NNLO QCD corrections to the WZ production process. For a number of commonly used experimental observables, we find that these corrections are substantial, in the 30–100% range and in some cases their inclusion can reduce scale uncertainties by a factor of two. As in the underlying VBFNLO calculations, we include the leptonic decays of the vector bosons and all off-shell and finite-width effects.

O(α s ) corrections to the decays of polarized W ± and Z bosons into massive quark pairs

We present O(\alpha_s) results on the decays of polarized W^+- and Z bosons into massive quark pairs. The NLO QCD corrections to the polarized decay functions are given up to the second order in the quark mass expansion. We find a surprisingly strong dependence of the NLO polarized decay functions on finite quark mass effects even at the relatively large mass scale of the W^+- and Z bosons. As a main application we consider the decay t -> b + W^+ involving the helicity fractions \rho_{mm} of the W^+ boson followed by the polarized decay W^+(\uparrow) -> q_1 qbar_2 for which we determine the O(\alpha_s) polar angle decay distribution. We also discuss NLO polarization effects in the production/decay process e^+e^- -> Z(\uparrow) -> q qbar.

Standard SANC modules for NLO QCD radiative corrections to single top-quark production

In this paper we present the results obtained with the newly created standard SANC modules for calculation of the NLO QCD corrections to single top-quark-production processes in s and t channels at the partonic level, as well as to top-decays. The main aim of these results is to prove the correct work of modules. A comprehensive comparison with the results of the CompHEP system is given, where possible. These modules are intended to be used in Monte Carlo generators for single top-quark-production processes at the LHC. As in our recent paper, devoted to the electroweak corrections to these processes, we study the regularization of the top-legs associated infrared divergences with the aid of the complex mass of the top quark. A comparison of QCD corrections with those computed by the conventional method is presented both for top production and for decays. For s-channel production we give an analytic proof of equivalence of the two methods in the limit of low top width.

Top quark and leptonic charge asymmetries for the Tevatron and LHC

We compute, for $t \bar t$ production at the LHC and at the Tevatron, several charge asymmetries to NLO QCD, including also the electromagnetic and weak-interaction corrections. We calculate these asymmetries both inclusively and with additional kinematic cuts and compare our results, where possible, with recent experimental results and with SM predictions. The $t \bar t$ asymmetries induce also corresponding asymmetries for the charged leptons from semileptonic top-quark decay. Although these asymmetries are, in the SM, smaller than the corresponding ones for top quarks, they are expected to be measurable quite precisely. In fact, measurement of a lepton asymmetry in $\ell$ + jets events was reported by the D$\emptyset$ and CDF experiments. We analyze and compute to NLO in the gauge couplings leptonic charge asymmetries for dileptonic and semileptonic $\ttbar$ events, with and without acceptance cuts, at the Tevatron and the LHC.

QCD correction to gluino decay to in the MSSM

We calculate the complete next-to-leading order QCD corrections to the three-body decay of gluino into top pair associated with a lightest neutralino in the minimal supersymmetric standard model. We obtain that the LO and NLO QCD-corrected decay widths of at the SPS6 benchmark point are 0.1490 and 0.1069 GeV respectively, and the relative correction is −28.2%. We investigate the dependence of the QCD correction to on tan β and the masses of gluino, scalar top quarks and the lightest neutralino around the SPS6 benchmark point, separately. We find that the NLO QCD corrections suppress the LO decay width, and the absolute relative correction can exceed 30% in some parameter space. Therefore, the QCD corrections to the three-body decay should be taken into account for the precise experimental measurement at future colliders. Moreover, we study the distributions of the top-pair invariant mass () and the missing energy (Emiss) and find that the line shapes of the LO distributions of and Emiss are not obviously distorted by the NLO QCD corrections.

Gluon fusion contribution to W + W − + jet production

We describe the computation of the $gg \to W^+W^-g$ process that contributes to the production of two $W$-bosons and a jet at the CERN Large Hadron Collider (LHC). While formally of next-to-next-to-leading order (NNLO) in QCD, this process can be evaluated separately from the bulk of NNLO QCD corrections because it is finite and gauge-invariant. It is also enhanced by the large gluon flux and by selection cuts employed in the Higgs boson searches in the decay channel $ H \to W^+W^-$, as was first pointed out by Binoth {\it et al.} in the context of $gg \to W^+W^-$ production. For cuts employed by the ATLAS collaboration, we find that the gluon fusion contribution to $pp \to W^+W^-j$ enhances the background by about ten percent and can lead to moderate distortions of kinematic distributions which are instrumental for the ongoing Higgs boson searches at the LHC. We also release a public code to compute the NLO QCD corrections to this process, in the form of an add-on to the package {\tt MCFM}.

Inclusive measurement of diffractive deep-inelastic scattering at HERA

The diffractive process ep \rightarrow eXY, where Y denotes a proton or its low mass excitation with MY < 1.6 GeV, is studied with the H1 experiment at HERA. The analysis is restricted to the phase space region of the photon virtuality 3 \leq Q2 \leq 1600 GeV2, the square of the four-momentum transfer at the proton vertex |t| < 1.0 GeV2 and the longitudinal momentum fraction of the incident proton carried by the colourless exchange xIP < 0.05. Triple differential cross sections are measured as a function of xIP, Q2 and beta = x/xIP where x is the Bjorken scaling variable. These measurements are made after selecting diffractive events by demanding a large empty rapidity interval separating the final state hadronic systems X and Y . High statistics measurements covering the data taking periods 1999-2000 and 2004-2007 are combined with previously published results in order to provide a single set of diffractive cross sections from the H1 experiment using the large rapidity gap selection method. The combined data represent a factor between three and thirty increase in statistics with respect to the previously published results. The measurements are compared with predictions from NLO QCD calculations based on diffractive parton densities and from a dipole model. The proton vertex factorisation hypothesis is tested.

NLO QCD corrections to the production of [formula omitted] plus two jets at the LHC

We present the full NLO QCD corrections to the production of a W+W− pair in association with two jets in hadronic collisions, which is an important background to New Physics and Higgs boson searches. We include leptonic decays of the W-bosons with full spin correlations. We find NLO corrections of the order of 10% for standard cuts at the LHC. The scale dependence is considerably reduced by the NLO corrections.