arXiv subjects

Kirill Melnikov

Publications and source records attributed to Kirill Melnikov.

At least 145 records · Page 8Linked to original sources

arXiv2006

On the QCD corrections to Vainshtein's theorem for VVA correlator

We point out that, contrary to existing claims of the opposite, Vainshtein's theorem on the non-renormalization of the correlator of an axial and two vector currents, is only valid in the chiral limit. When quarks, that contribute to the correlator, are massive, the QCD corrections do not vanish and the transversal and longitudinal functions are renormalized differently. We compute those corrections and study their implications for QCD effects in electroweak corrections to the muon anomalous magnetic moment.

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arXiv2006

Combining Monte Carlo generators with next-to-next-to-leading order calculations: event reweighting for Higgs boson production at the LHC

We study a phenomenological ansatz for merging next-to-next-to-leading order (NNLO) calculations with Monte Carlo event generators. We reweight them to match bin-integrated NNLO differential distributions. To test this procedure, we study the Higgs boson production cross-section at the LHC, for which a fully differential partonic NNLO calculation is available. We normalize PYTHIA and MC@NLO Monte Carlo events for Higgs production in the gluon fusion channel to reproduce the bin integrated NNLO double differential distribution in the transverse momentum and rapidity of the Higgs boson. These events are used to compute differential distributions for the photons in the pp \to H \to γγdecay channel, and are compared to predictions from fixed-order perturbation theory at NNLO. We find agreement between the reweighted generators and the NNLO result in kinematic regions where we expect a good description using fixed-order perturbation theory. Kinematic boundaries where resummation is required are also modeled correctly using this procedure. We then use these events to compute distributions in the pp \to H \to W^+W^- \to l^+l^- ν\barν channel, for which an accurate description is needed for measurements at the LHC. We find that the final state lepton distributions obtained from PYTHIA are not significantly changed by the reweighting procedure.

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arXiv2006

The W boson production cross section at the LHC through O(alpha_s^2)

We compute the O(alpha_s^2) QCD corrections to the fully differential cross-section pp \to W X \to l νX, retaining all effects from spin correlations. The knowledge of these corrections makes it possible to calculate with high precision the W boson production rate and acceptance at the LHC, subject to realistic cuts on the lepton and missing energy distributions. For certain choices of cuts we find large corrections when going from next-to-leading order (NLO) to next-to-next-to-leading order (NNLO) in perturbation theory. These corrections are significantly larger than those obtained by parton-shower event generators merged with NLO calculations. Our calculation may be used to assess and significantly reduce the QCD uncertainties in the many studies of $W$ boson production planned at the LHC.

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arXiv2005

The gluon-fusion uncertainty in Higgs coupling extractions

We point out that the QCD corrections to the gluon-fusion Higgs boson production cross section at the LHC are very similar to the corrections to the Higgs decay rate into two gluons. Consequently, the ratio of these two quantities has a theoretical uncertainty smaller than the uncertainty in the cross section alone by a factor of two. We note that since this ratio is the theoretical input to analyses of Higgs coupling extractions at the LHC, the reduced uncertainty should be used; in previous studies, the full cross section uncertainty was employed.

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arXiv2005

One-loop matching of Delta S=2 four-quark operators with improved staggered fermions

We compute O(alpha_s) lattice-to-continuum perturbative matching coefficients for the Delta S=2 flavor changing four-quark operators for the Asqtad improved staggered action. In conjunction with lattice simulations with three flavors of light, dynamical quarks, our results yield an unquenched determination of B_K, the parameter that determines the amount of indirect CP violation in the neutral kaon system. Its value is an important input for the unitarity triangle analysis of weak decays.

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arXiv2005

The photon energy spectrum in B-> X_s + γin perturbative QCD through O(α_s^2)

We derive the dominant part of the O(α_s^2) correction to the photon energy spectrum in the inclusive decay B-> X_s+gamma. The detailed knowledge of the spectrum is important for relating the theoretical calculations of the B-> X_s + γdecay rate and the experimental measurements where a cut on the photon energy is applied. In addition, moments of the photon energy spectrum are used for the determination of the b-quark mass and other fundamental parameters of heavy quark physics. Our calculation reduces the theoretical uncertainty associated with uncalculated higher orders effects and shows that, for B-> X_s+γ, QCD radiative corrections to the photon energy spectrum are under theoretical control.

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arXiv2005

Fully differential Higgs boson production and the di-photon signal through next-to-next-to-leading order

We describe a calculation of the fully differential cross section for Higgs boson production in the gluon fusion channel through next-to-next-to-leading order (NNLO) in perturbative QCD. The decay of the Higgs boson into two photons is included. Technical aspects of the computation are discussed in detail. The implementation of the calculation into a numerical code, called FEHiP, is described. The NNLO K-factors for completely realistic photon acceptances and isolation cuts, including those employed by the ATLAS and CMS collaborations, are computed. We study various distributions of the photons from Higgs decay through NNLO.

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arXiv2005

The electron energy spectrum in muon decay through O(alpha^2)

We compute the complete O(alpha^2) QED corrections to the electron energy spectrum in unpolarized muon decay, including the full dependence on the electron mass. Our calculation reduces the theoretical uncertainty on the electron energy spectrum well below 10^{-4}, the precision anticipated by the TWIST experiment at TRIUMF, which is currently performing this measurement. For this calculation, we extend techniques we have recently developed for performing next-to-next-to-leading order computations to handle the decay spectra of massive particles. Such an extension enables further applications to precision predictions for b, t, and Higgs differential decay rates.

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arXiv2005

Perturbative Heavy Quark Fragmentation Function through O(alpha_s^2)

We derive the initial condition for the perturbative fragmentation function of a heavy quark through order O(alpha_s^2) in the MS-bar scheme. This initial condition is useful for computing heavy quark (or lepton, in case of QED) energy distributions from calculations with massless partons. In addition, the initial condition at O(alpha_s^2) can be used to resum collinear logarithms ln(Q^2/m^2) in heavy quark energy spectrum with next-to-next-to-leading logarithmic accuracy by solving the DGLAP equation.

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arXiv2004

Higgs boson production at hadron colliders: differential cross sections through next-to-next-to-leading order

We present a calculation of the fully differential cross section for Higgs boson production in the gluon fusion channel through next-to-next-to-leading order in perturbative QCD. We apply the method introduced in \cite{Anastasiou:2003gr} to compute double real emission corrections. Our calculation permits arbitrary cuts on the final state in the reaction $hh \to H + X$. It can be easily extended to include decays of the Higgs boson into observable final states. In this Letter, we discuss the most important features of the calculation, and present some examples of physical applications that illustrate the range of observables that can be studied using our result. We compute the NNLO rapidity distribution of the Higgs boson, and also calculate the NNLO rapidity distribution with a veto on jet activity.

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arXiv2004

High-precision QCD at hadron colliders: electroweak gauge boson rapidity distributions at NNLO

We compute the rapidity distributions of W and Z bosons produced at the Tevatron and the LHC through next-to-next-to leading order in QCD. Our results demonstrate remarkable stability with respect to variations of the factorization and renormalization scales for all values of rapidity accessible in current and future experiments. These processes are therefore ``gold-plated'': current theoretical knowledge yields QCD predictions accurate to better than one percent. These results strengthen the proposal to use W and Z production to determine parton-parton luminosities and constrain parton distribution functions at the LHC. For example, LHC data should easily be able to distinguish the central parton distribution fit obtained by MRST from that obtained by Alekhin.

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arXiv2004

A new method for real radiation at NNLO

We propose a new method of computing real emission contributions to hard QCD processes. Our approach uses sector decomposition of the exclusive final-state phase space to enable extraction of all singularities of the real emission matrix elements before integration over any kinematic variable. The exact kinematics of the real emission process are preserved in all regions of phase space. Traditional approaches to extracting singularities from real emission matrix elements, such as phase space slicing and dipole subtraction, require both the determination of counterterms for double real emission amplitudes in singular kinematic limits and the integration of these contributions analytically to cancel the resulting singularities against virtual corrections. Our method addresses both of these issues. The implementation of constraints on the final-state phase space, including various jet algorithms, is simple using our approach. We illustrate our method using electron-positron --> jets at NNLO as an example.

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arXiv2004

Real radiation at NNLO: e^+e^- to 2 jets through O(alpha_s^2)

We present a calculation of the differential two jet cross section in e^+e^- annihilation through next-to-next-to-leading order in the strong coupling constant alpha_s. The calculation is performed using a new method for dealing with real radiation suggested by us recently. For the first time, the two jet event rate is computed directly, without any reference to the inclusive cross-section e^+e^- to hadrons. We also calculate the energy distribution of the leading jet in e^+e^- to 2 jets, and find significant modifications of the shape of this distribution at NNLO.

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arXiv2003

Hadronic light-by-light scattering contribution to the muon anomalous magnetic moment revisited

We discuss hadronic light-by-light scattering contribution to the muon anomalous magnetic moment a_μ^{\rm lbl}, paying particular attention to the consistent matching between the short- and the long-distance behavior of the light-by-light scattering amplitude. We argue that the short-distance QCD imposes strong constraints on this amplitude overlooked in previous analyses. We find that accounting for these constraints leads to approximately 50 per cent increase in the central value of a_μ^{\rm lbl}, compared to existing estimates. The hadronic light-by-light scattering contribution becomes a_μ^{\rm lbl}=136(25) \times 10^{-11}, thereby shifting the Standard Model prediction closer to the experimental value.

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arXiv2003

Dilepton rapidity distribution in the Drell-Yan process at NNLO in QCD

We compute the rapidity distribution of the virtual photon produced in the Drell-Yan process through next-to-next-to-leading order in perturbative QCD. We introduce a powerful new method for calculating differential distributions in hard scattering processes. This method is based upon a generalization of the optical theorem; it allows the integration-by-parts technology developed for multi-loop diagrams to be applied to non-inclusive phase-space integrals, and permits a high degree of automation. We apply our results to the analysis of fixed target experiments.

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arXiv2003

The self-energy of improved staggered quarks

We calculate the fermion self-energy at O(alpha_s) for the Symanzik improved staggered fermion action used in recent lattice simulations by the MILC collaboration. We demonstrate that the algebraic approach to lattice perturbation theory, suggested by us recently, is a powerful tool also for improved lattice actions.

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arXiv2002

Continuum methods in lattice perturbation theory

We show how methods of continuum perturbation theory can be used to simplify perturbative lattice calculations. We use the technique of asymptotic expansions to expand lattice loop integrals around the continuum limit. After the expansion, all nontrivial dependence on momenta and masses is encoded in continuum loop integrals and the only genuine lattice integrals left are tadpole integrals. Using integration-by-parts relations all of these can be expressed in terms of a small number of master integrals. Four master integrals are needed for bosonic one loop integrals, sixteen in QCD with Wilson or staggered fermions.

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arXiv2002

NLO Higgs boson rapidity distribution at hadron colliders

We describe a new method, based on an extension of the unitarity cutting rules proposed earlier, which is very efficient for the algorithmic evaluation of phase-space integrals for various differential distributions. As a first application, we compute the next-to-leading order normalized rapidity distribution of the CP-even and the CP-odd Higgs boson produced in hadron collisions through gluon fusion. We work in the heavy top-quark approximation; we find that the NLO corrections at the LHC are approximately 5% in the zero rapidity region.

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