Next-to-leading Order Predictions Research Articles

NLO matching for [formula omitted] production with massive b-quarks

Theoretical uncertainties in the simulation of tt¯bb¯ production represent one of the main obstacles that still hamper the observation of Higgs-boson production in association with top-quark pairs in the H→bb¯ channel. In this Letter we present a next-to-leading order (NLO) simulation of tt¯bb¯ production with massive b-quarks matched to the Sherpa parton shower. This allows one to extend NLO predictions to arbitrary tt¯bb¯ kinematics, including the case where one or both b-jets arise from collinear g→bb¯ splittings. We find that this splitting mechanism plays an important role for the tt¯H(bb¯) analysis.

Ntuples for NLO events at hadron colliders

We present an event-file format for the dissemination of next-to-leading-order (NLO) predictions for QCD processes at hadron colliders. The files contain all information required to compute generic jet-based infrared-safe observables at fixed order (without showering or hadronization), and to recompute observables with different factorization and renormalization scales. The files also make it possible to evaluate cross sections and distributions with different parton distribution functions. This in turn makes it possible to estimate uncertainties in NLO predictions of a wide variety of observables without recomputing the short-distance matrix elements. The event files allow a user to choose among a wide range of commonly-used jet algorithms and jet-size parameters.We provide event files for a W or Z boson accompanied by up to four jets, and for pure-jet events with up to four jets. The files are for the Large Hadron Collider with a center of mass energy of 7 or 8 TeV.A C++ library along with a Python interface for handling these files is also provided and described in this article. The library allows a user to read the event files and recompute observables transparently for different pdf sets and factorization and renormalization scales. Program summaryProgram title: nTupleReaderCatalogue identifier: AESC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AESC_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.: 62761No. of bytes in distributed program, including test data, etc.: 442886Distribution format: tar.gzProgramming language: C++ with Python interface.Computer: Workstations.Operating system: Linux, MacOS.RAM: Varying bytesClassification: 11.1, 11.5.External routines: ROOT, LHAPDFNature of problem:NLO QCD predictions for vector boson + jets and jet processes for generic observables.Solution method:Event files containing all information required to compute generic jet-based infrared-safe observables at fixed order and to recompute observables with different factorization and renormalization scales are provided. The library provides an interface to access the information in these files.Running time:The runtime scales linearly with the file size.

Tt¯+largemissing energy from top-quark partners: A comprehensive study at next-to-leading order QCD

We perform a detailed study of top-quark partner production in the $t\overline{t}$ plus large missing energy final state at the LHC, presenting results for both scalar and fermionic top-quark partners in the semileptonic and dileptonic decay modes of the top quarks. We compare the results of several simulation tools: leading order matrix elements, next-to-leading order (NLO) matrix elements, leading order plus parton shower simulations, and merged samples that contain the signal process with an additional hard jet radiated. We find that predictions from leading order plus parton shower simulations can significantly deviate from NLO QCD or LO merged samples and do not correctly model the kinematics of the $t\overline{t}+{E}_{T,\mathrm{miss}}$ signature. They are therefore not a good framework for modeling this new physics signature. On the other hand, the acceptances obtained with a merged sample of the leading-order process together with the radiation of an additional hard jet are in agreement with the NLO predictions. We also demonstrate that the scale variation of the inclusive cross section, plus that of the acceptance, does not accurately reflect the uncertainty of the cross section after cuts, which is typically larger. We show the importance of including higher-order QCD corrections when using kinematic distributions to determine the spin of the top-quark partner.

NLO QCD corrections to off-shell top-antitop production with leptonic decays at hadron colliders

We present details of a calculation of the cross section for hadronic top-antitop production in next-to-leading order (NLO) QCD, including the decays of the top and antitop into bottom quarks and leptons. This calculation is based on matrix elements for \nu e e+ \mu- \bar{\nu}_{\mu}b\bar{b} production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Such contributions are formally suppressed by the top-quark width and turn out to be small in the inclusive cross section. However, they can be strongly enhanced in exclusive observables that play an important role in Higgs and new-physics searches. Also non-resonant and off-shell effects due to the finite W-boson width are investigated in detail, but their impact is much smaller than naively expected. We also introduce a matching approach to improve NLO calculations involving intermediate unstable particles. Using a fixed QCD scale leads to perturbative instabilities in the high-energy tails of distributions, but an appropriate dynamical scale stabilises NLO predictions. Numerical results for the total cross section, several distributions, and asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.

Next-to-leading order QCD predictions forW+1jet andW+2jet production with at least onebjet at the 7 TeV LHC

We provide predictions for the production of a $W$ boson in association with up to two jets including at least one $b$ jet at next-to-leading order (NLO) in QCD at the CERN Large Hadron Collider with 7 TeV center-of-mass energy. Both exclusive and inclusive event cross section and $b$-jet cross sections are presented. The calculation is performed consistently in the five-flavor-number scheme where both $q{\overline{q}}^{\ensuremath{'}}$ and $bq$ ($q\ensuremath{\ne}b$) initiated parton level processes are included at NLO QCD. We study the residual theoretical uncertainties of the NLO predictions due to the renormalization and factorization scale dependence, to the uncertainty from the parton distribution functions, and to the values of ${\ensuremath{\alpha}}_{s}$ and the bottom-quark mass.

Quantitative constraints on the gluon distribution function in the proton from collider isolated-photon data

The impact of isolated-photon data from proton–(anti)proton collisions at RHIC, Spp¯S, Tevatron and LHC energies, on the parton distribution functions of the proton is studied using a recently developed Bayesian reweighting method. The impact on the gluon density of the 35 existing isolated-γ measurements is quantified using next-to-leading order (NLO) perturbative QCD calculations complemented with the NNPDF2.1 parton densities. The NLO predictions are found to describe well most of the datasets from 200 GeV up to 7 TeV centre-of-mass energies. The isolated-photon spectra recently measured at the LHC are precise enough to constrain the gluon distribution and lead to a moderate reduction (up to 20%) of its uncertainties around fractional momenta x≈0.02. As a particular case, we show that the improved gluon density reduces the PDF uncertainty for the Higgs boson production cross section in the gluon-fusion channel by more than 20% at the LHC. We conclude that present and future isolated-photon measurements constitute an interesting addition to coming global PDF analyses.

Measurement of the Cross Section for Prompt Isolated Diphoton Production inpp¯Collisions ats=1.96 TeV

This letter reports a measurement of the cross section of prompt isolated photon pair production in p\bar p collisions at a total energy \sqrt{s} = 1.96 TeV using data of 5.36/fb integrated luminosity collected with the CDF II detector at the Fermilab Tevatron. The measured cross section, differential in basic kinematic variables, is compared with three perturbative QCD predictions, a Leading Order (LO) parton shower calculation and two Next-to-Leading Order (NLO) calculations. The NLO calculations reproduce most aspects of the data. By including photon radiation from quarks before and after hard scattering, the parton shower prediction becomes competitive with the NLO predictions.

Electric properties of the Beryllium-11 system in Halo EFT

We compute E1 transitions and electric radii in the Beryllium-11 nucleus using an effective field theory that exploits the separation of scales in this halo system. We fix the leading-order parameters of the EFT from measured data on the 1/2+ and 1/2− levels in 11Be and the B(E1) strength for the transition between them. We then obtain predictions for the B(E1) strength for Coulomb dissociation of the 11Be nucleus to the continuum. We also compute the charge radii of the 1/2+ and 1/2− states. Agreement with experiment within the expected accuracy of a leading-order computation in this EFT is obtained. We also discuss how next-to-leading-order (NLO) corrections involving both s-wave and p-wave 10Be–neutron interactions affect our results, and display the NLO predictions for quantities which are free of additional short-distance operators at this order. Information on neutron–10Be scattering in the relevant channels is inferred.

Single-inclusive production of large-p T charged particles in hadronic collisions at TeV energies and perturbative QCD predictions

The single inclusive spectrum of charged particles with transverse momenta pT=3-150 GeV/c measured at midrapidity by the CDF experiment in proton-antiproton (p-pbar) collisions at sqrt(s)=1.96 TeV is compared to next-to-leading order (NLO) perturbative QCD calculations using the most recent parametrizations of the parton distributions and parton-to-hadron fragmentation functions. Above pT~20 GeV/c, there is a very sizeable disagreement of the Tevatron data compared to the NLO predictions and to xT-scaling expectations, suggesting a problem in the experimental data. We also present the predictions for the pT-differential charged hadron spectra and the associated theoretical uncertainties for proton-proton (p-p) collisions at LHC energies (sqrt(s)=0.9-14 TeV). Two procedures to estimate the charged hadron spectra at LHC heavy-ion collision energies (sqrt(s)=2.76,5.5 TeV) from p-p measurements are suggested.

Multijet cross sections in charged currente±pscattering at HERA

Jet cross sections were measured in charged-current deep inelastic e(+/-)p scattering at high boson virtualities Q(2) with the ZEUS detector at HERA II using an integrated luminosity of 0.36 fb(-1). Differential cross sections are presented for inclusive-jet production as functions of Q(2), Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e(+/-)p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits.

The FMNR⊗Pythia interface for Heavy Quark production at HERA

A method to calculate heavy flavor visible-level cross sections at Next-to-Leading Order (NLO) in Quantum Chromodynamics (QCD), based on an interface of the FMNR program to PYTHIA, is described. It uses the NLO prediction at quark level provided by FMNR, with a statistical reduction procedure (REDSTAT) that allows a link to PYTHIA 6.2 to be made, from where the description of the full hadron fragmentation and decay chain is obtained. The method is applied to ep → bbX → D*μX and μ+μ−X final states at HERA. Comparisons of the data and NLO cross sections at visible and b-quark level were found to be consistent.

Resummation effects in the search of standard model Higgs boson at hadron colliders

We examine the soft-gluon resummation effects, including the exact spin correlations among the final state particles, in the search of the standard model Higgs boson, via the process $gg\ensuremath{\rightarrow}H\ensuremath{\rightarrow}WW/ZZ\ensuremath{\rightarrow}4$ leptons, at the Tevatron and the LHC. A comparison between the resummation and the next-to-leading order (NLO) calculation is performed after imposing various kinematics cuts suggested in the literature for the Higgs boson search. For the $H\ensuremath{\rightarrow}ZZ$ mode, the resummation effects increase the acceptance of the signal events by about 25%, as compared to the NLO prediction, and dramatically alter various kinematics distributions of the final state leptons. For the $H\ensuremath{\rightarrow}WW$ mode, the acceptance rates of the signal events predicted by the resummation and NLO calculations are almost the same, but some of the predicted kinematical distributions are quite different. Thus, to precisely determine the properties of the Higgs boson at hadron colliders, the soft-gluon resummation effects have to be taken into account.

Inclusive production of charged hadrons in photon–photon collisions

The inclusive production of charged hadrons in the collisions of quasi-real photons (e+e−→e+e−+X) has been measured using the OPAL detector at LEP. The data were taken at e+e− centre-of-mass energies from 183 to 209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant αs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

Threshold resummation for slepton-pair production at hadron colliders

We present a first and extensive study of threshold resummation effects for supersymmetric (SUSY) particle production at hadron colliders, focusing on Drell–Yan like slepton-pair and slepton–sneutrino associated production. After confirming the known next-to-leading order (NLO) QCD corrections and generalizing the NLO SUSY-QCD corrections to the case of mixing squarks in the virtual loop contributions, we employ the usual Mellin N-space resummation formalism with the minimal prescription for the inverse Mellin-transform and improve it by resumming 1 / N -suppressed and a class of N-independent universal contributions. Numerically, our results increase the theoretical cross sections by 5 to 15% with respect to the NLO predictions and stabilize them by reducing the scale dependence from up to 20% at NLO to less than 10% with threshold resummation.

Inclusive jet cross sections and dijet correlations in [formula omitted] photoproduction at HERA

Inclusive jet cross sections in photoproduction for events containing a D ∗ meson have been measured with the ZEUS detector at HERA using an integrated luminosity of 78.6 pb −1 . The events were required to have a virtuality of the incoming photon, Q 2 , of less than 1 GeV 2, and a photon–proton centre-of-mass energy in the range 130 < W γ p < 280 GeV . The measurements are compared with next-to-leading-order (NLO) QCD calculations. Good agreement is found with the NLO calculations over most of the measured kinematic region. Requiring a second jet in the event allowed a more detailed comparison with QCD calculations. The measured dijet cross sections are also compared to Monte Carlo (MC) models which incorporate leading-order matrix elements followed by parton showers and hadronisation. The NLO QCD predictions are in general agreement with the data although differences have been isolated to regions where contributions from higher orders are expected to be significant. The MC models give a better description than the NLO predictions of the shape of the measured cross sections.

Stability of NLO global analysis and implications for Hadron collider physics

The phenomenology of Standard Model and New Physics at hadron colliders depends critically on results from global QCD analysis for parton distribution functions (PDFs). The accuracy of the standard next-to-leading-order (NLO) global analysis, nominally a few percent, is generally well matched to the expected experimental precision. However, serious questions have been raised recently about the stability of the NLO analysis with respect to certain inputs, including the choice of kinematic cuts on the data sets and the parametrization of the gluon distribution. In this paper, we investigate this stability issue systematically within the CTEQ framework. We find that both the PDFs and their physical predictions are stable, well within the few percent level. Further, we have applied the Lagrange Multiplier method to explore the stability of the predicted cross sections for W production at the Tevatron and the LHC, since W production is often proposed as a standard candle for these colliders. We find the NLO predictions on sigma_W to be stable well within their previously-estimated uncertainty ranges.

Inclusive light-hadron production in photon-photon collisions at NLO

Abstract I review a recent global analysis of inclusive single-charged-hadron production in high-energy colliding-beam experiments. In the framework of the parton model of quantum chromodynamics at next-to-leading order (NLO), fragmentation functions (FFs) for charged pions, charged kaons, and (anti) protons are extracted from experimental data of e+e− annihilation at the Z-boson resonance and at centre-of-mass energy √s = 29 GeV. Comparisons of γγ data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem. Possible modifications of the theory in view of the found discrepancy of the NLO predictions with recent data from the L3 Collaboration are shortly discussed.

Theoretical aspects of inclusive light-hadron production

We summarize the key results of a recent global analysis of inclusive single-charged-hadron production in high-energy colliding-beam experiments. In the framework of the parton model of quantum chromodynamics at next-to-leading order (NLO), fragmentation functions (FFs) for charged pions, charged kaons and (anti)protons are extracted from the experimental data of e+e− annihilation at the Z-boson resonance and at centre-of-mass energy √s = 29 GeV. This fit also yields a new NLO value for the strong-coupling constant, namely α(5)s(MZ) = 0.1170 ± 0.0073. The scaling violations encoded in the FFs through the Altarelli–Parisi evolution equations are tested by confronting e+e−-annihilation data from DESY DORIS, DESY PETRA and CERN LEP2 with NLO predictions based on these FFs. Comparisons of pp̄ data from CERN Spp̄S and the Fermilab Tevatron, γp data from DESY HERA and γγ data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem.

Brodsky-Lepage-Mackenzie scale for the pion transition form factor

The next-to-leading order (NLO) Brodsky-Lepage-Mackenzie (BLM) scale for the pion transition form factor is determined. To achieve that, a consistent calculation up to ${n}_{f}$-proportional next-to-next-to-leading order contributions to both the hard-scattering amplitude and the perturbatively calculable part of the pion distribution amplitude is performed. By combining and matching the results obtained for these two amplitudes, a proper cancellation of collinear singularities is established and the ${\ensuremath{\gamma}}_{5}$ ambiguity problem (related to the use of the dimensional regularization method) is resolved by using the naive-${\ensuremath{\gamma}}_{5}$ as well as the 't Hooft--Veltman (HV) schemes. It is demonstrated that the prediction for the pion transition form factor is independent of the factorization scale ${\ensuremath{\mu}}_{F}^{2}$ at every order in the strong coupling constant, making it possible to use the simplest choice ${\ensuremath{\mu}}_{F}^{2}{=Q}^{2}$ at the intermediate steps of the calculation. Assuming the pion asymptotic distribution amplitude and working in the $\overline{\mathrm{MS}}$ scheme, we find the BLM scale to be ${\ensuremath{\mu}}_{R}^{2}={\ensuremath{\mu}}_{\mathrm{BLM}}^{2}\ensuremath{\approx}{Q}^{2}/9.$ Based on the same distribution, the complete NLO prediction for the pion transition form factor is calculated in the ${\ensuremath{\alpha}}_{V}$ definition of the QCD coupling renormalized at ${\ensuremath{\mu}}_{R}^{2}={\ensuremath{\mu}}_{V}^{2}{=e}^{5/3}{\ensuremath{\mu}}_{\mathrm{BLM}}^{2}\ensuremath{\approx}{Q}^{2}/2.$ It is in good agreement with the presently available experimental data.

Complete next-to-leading order perturbative QCD prediction for the pion electromagnetic form factor

We present the results of a complete leading-twist next-to-leading order (NLO) QCD analysis of the spacelike pion electromagnetic form factor at large momentum transfer Q. We have studied their dependence on the form of the pion distribution amplitude. For a given distribution amplitude, we have examined the sensitivity of the predictions to the choice of the renormalization and factorization scales. Compared to the renormalization scale, the factorization scale turns out to be of secondary importance. The renormalization scale dependence of the leading-order (LO) results has been significantly reduced by including the NLO corrections. Adopting the criteria according to which a NLO prediction is considered reliable if both the ratio of the NLO to LO contributions and the strong coupling constant are reasonably small, we find that reliable perturbative predictions for the pion electromagnetic form factor with all distribution amplitudes considered can already be made at a momentum transfer Q of the order $5\ensuremath{-}10$ GeV, with corrections to the LO results being up to $\ensuremath{\sim}30%.$ The theoretical uncertainty related to the renormalization scale ambiguity has been estimated to be less than $10%.$ To check our predictions and to discriminate between the distribution amplitudes, it is necessary to obtain experimental data extending to higher values of Q.