Soft-gluon Resummation Research Articles

Higher-order threshold resummation for semi-inclusive [formula omitted] annihilation

The complete soft-enhanced and virtual-gluon contributions are derived for the quark coefficient functions in semi-inclusive e+e− annihilation to the third order in massless perturbative QCD. These terms enable us to extend the soft-gluon resummation for the fragmentation functions by two orders to the next-to-next-to-next-to-leading logarithmic (N3LL) accuracy. The resummation exponent is found to be the same as for the structure functions in inclusive deep-inelastic scattering. This finding, together with known results on the higher-order quark form factor, facilitates the determination of all soft and virtual contributions of the fourth-order difference of the coefficient functions for these two processes. Unlike the previous (N2LL) order in the exponentiation, the numerical effect of the N3LL contributions turns out to be negligible at LEP energies.

On the soft-gluon resummation in top quark pair production at hadron colliders

We uncover a contribution to the NLO/NLL threshold resummed total cross section for top quark pair production at hadron colliders, which has not been taken into account in earlier literature. We derive this contribution – the difference between the singlet and octet hard (matching) coefficients – in exact analytic form. The numerical impact of our findings on the Sudakov resummed cross section turns out to be large, and comparable in size to the current estimates for the theoretical uncertainty of the total cross section. A rough estimate points toward a few percent decrease of the latter at the LHC.

Total photoproduction cross section at very high energy

In this paper we apply to photoproduction total cross-section a model we have proposed for purely hadronic processes and which is based on QCD mini-jets and soft gluon re-summation. We compare the predictions of our model with the HERA data as well as with other models. For cosmic rays, our model predicts substantially higher cross-sections at TeV energies than models based on factorization but lower than models based on mini-jets alone, without soft gluons. We discuss the origin of this difference.

Two-Loop Soft Anomalous Dimensions and Next-to-Next-to-Leading-Logarithm Resummation for Heavy Quark Production

I present results for two-loop soft anomalous dimensions for heavy quark production which control soft-gluon resummation at next-to-next-to-leading-logarithm accuracy. I derive an explicit expression for the exact result and study it numerically for top quark production via e(+)e(-)-->tt, and I construct a surprisingly simple but very accurate approximation. I show that the two-loop soft anomalous dimensions with massive quarks display a simple proportionality relation to the one-loop result only in the limit of vanishing quark mass. I also discuss the extension of the calculation to single top and top pair production in hadron colliders.

Renormalization-group improved prediction for Higgs production at hadron colliders

We use renormalization-group methods in effective field theory to improve the theoretical prediction for the cross section for Higgs-boson production at hadron colliders. In addition to soft-gluon resummation at N3LL, we also resum enhanced contributions of the form (C A π α s ) n , which arise in the analytic continuation of the gluon form factor to time-like momentum transfer. This resummation is achieved by evaluating the matching corrections arising at the Higgs-boson mass scale at a time-like renormalization point μ 2<0, followed by renormalization-group evolution to μ 2>0. We match our resummed result to NNLO fixed-order perturbation theory and give numerical predictions for the total production cross section as a function of the Higgs-boson mass. Resummation effects are significant even at NNLO, where our improved predictions for the cross sections at the Tevatron and the LHC exceed the fixed-order predictions by about 13% and 8%, respectively, for m H =120 GeV. We also discuss the application of our technique to other time-like processes such as Drell–Yan production, e + e −→hadrons, and hadronic decays of the Higgs boson.

Higgs production through gluon fusion: Updated cross sections at the Tevatron and the LHC

We present updated predictions for the total cross section for Higgs boson production by gluon–gluon fusion in hadron collisions. Our calculation includes the most advanced theoretical information available at present for this observable: soft-gluon resummation up to next-to-next-to-leading logarithmic accuracy, the exact treatment of the bottom-quark contribution up to next-to-leading order, and two-loop electroweak effects. We adopt the most recent parametrization of parton distribution functions at next-to-next-to-leading order, and we evaluate the corresponding uncertainties. In comparison with our previous central predictions, at the Tevatron the difference ranges from +9% for mH=115 GeV to −9% for mH=200 GeV. At the LHC the cross section is instead significantly increased. The effect goes from +30% for mH=115 GeV to +9% for mH=300 GeV, and is mostly due to the new parton distribution functions. We also provide new predictions for the LHC at s=10 TeV.

Top-quark pair production near threshold at LHC

The next-to-leading order analysis for the cross section for hadroproduction of top-quark pairs close to threshold is presented. Within the framework of non-relativistic QCD a significant enhancement compared to fixed-order perturbation theory is observed which originates from the characteristic remnant of the 1S peak below production threshold of top-quark pairs. The analysis includes all color-singlet and color-octet configurations of top-quark pairs in S-wave states and, for the dominant configurations, it employs all-order soft-gluon resummation for the hard parton cross section. Numerical results for the Large Hadron Collider at $\sqrt{s}=14~$ TeV and $\sqrt{s}=10~$ TeV and also for the Tevatron are presented. The possibility of a top-quark mass measurement from the invariant-mass distribution of top-quark pairs is discussed.

Higher order QCD corrections to charged-lepton deep-inelastic scattering and global fits of parton distributions

We study the perturbative QCD corrections to the heavy-quark structure functions of charged-lepton deep-inelastic scattering and their impact on global fits of parton distributions. We include the logarithmically enhanced terms near threshold due to soft gluon resummation in the QCD corrections at next-to-next-to-leading order. We demonstrate that this approximation is sufficient to describe the available HERA data in most parts of the kinematic region. The threshold-enhanced next-to-next-to-leading order corrections improve the agreement between predictions based on global fits of the parton distribution functions and the HERA collider data even in the small-x region.

Inclusive measure of |V ub | with the analytic coupling model

By analyzing B→X u ℓ ν ℓ spectra with a model based on soft-gluon resummation and an analytic time-like QCD coupling, we obtain $$|V_{ub}|=(3.76\pm0.13\pm0.22)\times10^{-3},$$ where the first and the second error refer to experimental and theoretical error, respectively. This model successfully describes the accurate experimental data in beauty fragmentation, which has similar soft-gluon effects. The |V ub | value is obtained from the available measured semileptonic branching fractions in limited regions of the phase space. The distributions in the lepton energy E ℓ , the hadron invariant mass m X , the light-cone momentum \(P_{+}\equiv E_{X}-|\vec{p}_{X}|\) , together with the double distributions in (m X ,q 2) and (E ℓ ,s max h ), are used to select the phase-space regions. q 2 is the dilepton squared momentum and s maxh is the maximal m 2 X at fixed q 2 and E ℓ . The |V ub | value obtained is in complete agreement with the value coming from exclusive B decays and from an over-all fit to the standard model parameters. We show that the slight disagreement (up to +2σ) with respect to previous inclusive measurements is not related to different choices for the b (and c) masses, but to a different modeling of the threshold (Sudakov) region.

Formula omitted] extraction using the Analytic Coupling model

We summarize the main characteristics and recent results on B → X u ℓ ν ℓ decays of a model based on soft–gluon resummation and an analytic time–like QCD coupling.

Minijets, soft gluon resummation and photon cross-sections

We compare the high energy behaviour of hadronic photon-photon cross-sections in different models. We find that the photon-photon cross-section appears to rise faster than the purely hadronic ones (proton-proton and proton-antiproton).

Theoretical status and prospects for top-quark pair production at hadron colliders

We present an update of the theoretical predictions for the cross section of top-quark pair production at Tevatron and LHC. In particular, we employ improvements due to soft-gluon resummation at next-to-next-to-leading logarithmic accuracy. We expand the resummed results and derive analytical finite-order cross sections through next-to-next-to-leading order which are exact in all logarithmically enhanced terms near threshold. These results are the best present estimates for the top-quark pair production cross section. We investigate the scale dependence as well as the sensitivity on the parton luminosities.

Revisiting the QCD Corrections to the R-Parity Violating Processes pp̄/pp → eμ + X

We present the theoretical predictions up to QCD next-to-leading order for the cross section of high-mass electron-muon pair production at the Tevatron and at the Large Hadron Collider (LHC), considering only the dominant contributions from the third-generation sneutrino. The dependence of the renormalization and factorization scales on the total cross section, and the effects on the K-factor due to the uncertainty of parton distribution function are carefully investigated. By considering soft-gluon resummation effects to all orders in αs of leading logarithm, we present the transverse momentum distributions of the final eμ pair.

Total cross section and rapidity gap survival probability at the LHC through an eikonal with soft gluon resummation

New results are presented for total pp/p¯p cross sections, in the framework of our QCD based model (GGPS). This is an improved eikonal mini-jet model, where soft gluon radiation tames the fast energy rise normally present in mini-jet models. We discuss the variability in our predictions and provide a handy parametrization of our results for the LHC. We find that our model predictions span the range σtotLHC=100−13+10 mb. While this matches nicely with the range of most other models, it does not agree with recent ones which include a “hard” pomeron, even though our model does include hard scattering. We compute the survival probability for large rapidity gap (LRG) events at the LHC and at the Tevatron. These events are relevant, for example, for Higgs signal in the WW fusion process. We also explore whether measurements of the total cross sections at the LHC can help us sharpen the model parameters and hence estimates for these survival probabilities, further.

Soft-gluon resummation for pseudoscalar Higgs boson production at hadron colliders

We compute the threshold-resummed cross section for pseudo-scalar MSSM Higgs boson production by gluon fusion at hadron colliders. The calculation is performed at next-to-next-to leading logarithmic accuracy. We present results for both the LHC and Tevatron Run II. We analyze the factorization and renormalization scale dependence of the results, finding that after performing the resummation the corresponding cross section can be computed with an accuracy better than 10%.

Photon pairs: Quantum chromodynamics continuum and the Higgs boson

A new QCD calculation is summarized for the transverse momentum distribution of photon pairs produced by QCD subprocesses, including all-orders soft-gluon resummation valid at next-to-next-to-leading logarithmic accuracy. Resummation is needed to obtain reliable predictions in the range of transverse momentum where the cross-section is the largest. Results are compared with data from the Fermilab Tevatron and predictions are made for the large hadron collider. The QCD continuum is shown to have a softer spectrum than the Higgs boson signal at the LHC.

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.

A new prescription for soft gluon resummation

We present a new prescription for the resummation of the divergent series of perturbative corrections, due to soft gluon emission, to hard processes near threshold in perturbative QCD (threshold resummation). This prescription is based on Borel resummation, and contrary to the commonly used minimal prescription, it does not introduce a dependence of resummed physical observables on the kinematically unaccessible x→0 region of parton distributions. We compare results for resummed deep-inelastic scattering obtained using the Borel prescription and the minimal prescription and exploit the comparison to discuss the ambiguities related to the resummation procedure.

Transversely Polarized Drell-Yan Process and Soft Gluon Resummation in QCD

We calculate the transverse-momentum $Q_T$ spectrum of the dilepton in the transversely polarized Drell-Yan process on the basis of the factorization theorem in QCD. We take into account universal logarithmically enhanced corrections in edge region of phase space by resumming multiple soft-gluon emissions to all orders in the small $Q_T$ region.

QCD corrections to single slepton production at hadron colliders

We evaluate the cross section for single slepton production at hadron colliders in supersymmetric theories with R-parity violating interactions to the next-to-leading order in QCD. We obtain fully differential cross section by using the phase space slicing method. We also perform soft-gluon resummation to all order in $\alpha_s$ of leading logarithm to obtain a complete transverse momentum spectrum of the slepton. We find that the full transverse momentum spectrum is peaked at a few GeV, consistent with the early results for Drell-Yan production of lepton pairs. We also consider the contribution from gluon fusion via quark-triangle loop diagrams dominated by the $b$-quark loop. The cross section of this process is significantly smaller than that of the tree-level process induced by the initial $b\bar{b}$ annihilation.