arXiv subjects
Publications and source records attributed to Alexander Mitov.
We review recently derived NNLO QCD results for top-pair production at hadron colliders. We compare the size of the NNLO corrections in $q\bar q\to t\bar t+X$ with the LO and NLO ones, and address the question of convergence of perturbative series. We compare the NLO and NNLO K-factors for the Tevatron.
This is a second paper in our ongoing calculation of the next-to-next-to-leading order (NNLO) QCD correction to the total inclusive top-pair production cross-section at hadron colliders. In this paper we calculate the reaction $q\bar q \to t\bar t + q\bar q$ which was not considered in our previous work on $q\bar q \to t\bar t +X$ due to its phenomenologically negligible size. We also calculate all remaining fermion-pair-initiated partonic channels $qq', q\bar q'$ and $qq$ that contribute to top-pair production starting from NNLO. The contributions of these reactions to the total cross-section for top-pair production at the Tevatron and LHC are small, at the permil level. The most interesting feature of these reactions is their characteristic logarithmic rise in the high energy limit. We compute the constant term in the leading power behavior in this limit, and achieve precision that is an order of magnitude better than the precision of a recent theoretical prediction for this constant. All four partonic reactions computed in this paper are included in our numerical program Top++. The calculation of the NNLO corrections to the two remaining partonic reactions, $qg\to t\bar t+X$ and $gg\to t\bar t+X$, is ongoing.
We compute the Next-to-Next-to-Leading Order (NNLO) QCD corrections to the partonic reaction that dominates top-pair production at the Tevatron. This is the first ever NNLO calculation of an observable with more than two colored partons, and/or massive fermions, at hadron colliders. Augmenting our fixed order calculation with soft-gluon resummation through Next-to-Next-to-Leading Logarithmic (NNLL) accuracy, we observe that the predicted total inclusive cross-section exhibits a very small perturbative uncertainty, estimated at +-2.7%. We expect that once all sub-dominant partonic reactions are accounted for, and work in this direction is ongoing, the perturbative theoretical uncertainty for this observable could drop below +-2%. Our calculation demonstrates the power of our computational approach and proves it can be successfully applied to all processes at hadron colliders for which high-precision analyses are needed.
Incorporating all recent theoretical advances, we resum soft-gluon corrections to the total $t\bar t$ cross-section at hadron colliders at the next-to-next-to-leading logarithmic (NNLL) order. We perform the resummation in the well established framework of Mellin $N$-space resummation. We exhaustively study the sources of systematic uncertainty like renormalization and factorization scale variation, power suppressed effects and missing two- and higher-loop corrections. The inclusion of soft-gluon resummation at NNLL brings only a minor decrease in the perturbative uncertainty with respect to the NLL approximation, and a small shift in the central value, consistent with the quoted uncertainties. These numerical predictions agree with the currently available measurements from the Tevatron and LHC and have uncertainty of similar size. We conclude that significant improvements in the $t\bar t$ cross-sections can potentially be expected only upon inclusion of the complete NNLO corrections.
We calculate the one-loop correction to the soft-gluon current with massive fermions. This current is process independent and controls the singular behavior of one-loop massive QCD amplitudes in the limit when one external gluon becomes soft. The result derived in this work is the last missing process-independent ingredient needed for numerical evaluation of observables with massive fermions at hadron colliders at the next-to-next-to-leading order.
The existing predictions for the like-sign dimuon charge asymmetry at the Tevatron are expressed in terms of parameters related to B mesons' mixing and inclusive production fractions. We show that in the realistic case when phase-space cuts are applied, the asymmetry depends also on the details of the production mechanism for the B mesons. In particular, it is sensitive to the difference in the fragmentation functions of B^0_d and B^0_s mesons. We estimate these fragmentation effects and find that they shift the theory prediction for this observable by approximately 10%. We also point out the approximately 20% sensitivity of the asymmetry depending on which set of values for the B meson production fractions is used: as measured at the Z pole or at the Tevatron. The impact of these effects on the extraction of A^s_{SL} from the DØmeasurement is presented.
We provide a recursive diagrammatic prescription for the exponentiation of gauge theory amplitudes involving products of Wilson lines and loops. This construction generalizes the concept of webs, originally developed for eikonal form factors and cross sections with two eikonal lines, to general soft functions in QCD and related gauge theories. Our coordinate space arguments apply to arbitrary paths for the lines.
We complete the coordinate space calculation of the three-parton correlation in the two-loop massive soft anomalous dimension matrix. The full answer agrees with the result found previously by a different approach. The coordinate space treatment of renormalized two-loop gluon exchange diagrams exhibits their color symmetries in a transparent fashion. We compare coordinate space calculations of the soft anomalous dimension matrix with massive and massless eikonal lines and examine its nonuniform limit at absolute threshold.
We study two-loop anomalous dimension matrices in QCD and related gauge theories for products of Wilson lines coupled at a point. We verify by an analysis in Euclidean space that the contributions to these matrices from diagrams that link three massive Wilson lines do not vanish in general. We show, however, that for two-to-two processes the two-loop anomalous dimension matrix is diagonal in the same color-exchange basis as the one-loop matrix for arbitrary masses at absolute threshold and for scattering at ninety degrees in the center of mass. This result is important for applications of threshold resummation in heavy quark production.
We present a new method for the calculation of differential distributions directly in Mellin space without recourse to the usual momentum-fraction (or z-) space. The method is completely general and can be applied to any process. It is based on solving the Integration-by-Parts identities when one of the powers of the propagators is an abstract number. The method retains the full dependence on the Mellin variable and can be implemented in any program for solving the IBP identities based on algebraic elimination, like Laporta. General features of the method are: 1) faster reduction, 2) smaller number of master integrals compared to the usual z-space approach and 3) the master integrals satisfy difference instead of differential equations. This approach generalizes previous results related to fully inclusive observables like the recently calculated three-loop space-like anomalous dimensions and coefficient functions in inclusive DIS to more general processes requiring separate treatment of the various physical cuts. Many possible applications of this method exist, the most notable being the direct evaluation of the three-loop time-like splitting functions in QCD.
We present a general discussion of collider processes with not-completely inclusive production of heavy flavors. We review the Perturbative Fragmentation Functions formalism as the appropriate tool for studying such processes and detail the extension of this formalism at Next-to-Next-to-Leading order. We conclude that the prospects for the future in this field are bright and at present are limited by the available experimental data. Hopefully, the future ILC will be able to fill this gap.
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.
We derive the gluon initiated contribution to the initial condition for the perturbative fragmentation function of a heavy quark through order O(α_s^2) in the MS-bar scheme. This result is needed for the resummation with next-to-next-to-leading logarithmic accuracy of quasi-collinear logarithms ln^k(m^2) in heavy quark differential distributions by solving the complete DGLAP equation. Together with the previously evaluated fermion initiated components, this result completes the derivation of the initial condition for the perturbative fragmentation function at next-to-next-to-leading order.
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.
In this thesis we study the b-quark fragmentation in top decay as well as the effect of the threshold resummation on heavy quark production in charged-current DIS. To predict the spectrum of b-flavored hadrons in top decay, we calculate at NLO the QCD corrections for bottom production in top decay with the b-mass fully taken into account. Using the perturbative fragmentation function formalism we then resum with NLL accuracy large collinear logs of the ratio of bottom-to-top mass, which leads to an essential improvement of the result. Next we perform the threshold resummation for the coefficient function for top decay with NLL accuracy. That resummation leads to an important improvement of the b-spectrum in the large energy fraction region. Finally, we extract information for the non-perturbative b-fragmentation into hadrons from e^+e^- data and make a prediction for the spectrum of those b-flavored hadrons produced in top-quark decay. Our second application is to charm production in charged-current DIS. We first calculate with NLL accuracy the soft-gluon resummed coefficient function for heavy quark production (initiated by a light quark) in DIS. Our result is applicable for the case of low momentum transfer that is of the order of the mass of the heavy quark. We also make a connection of this result to the known result for massless quark production. We then apply this result for charm quark production at NuTeV and HERA for a wide range of the transferred momentum, and present the effect of the resummation on the charm structure functions.
We systematically study the question of identification and consistent inclusion of the radion, within the Lagrangian approach, in a two brane Randall-Sundrum model. Exploiting the symmetry properties of the theory, we show how the radion can be identified unambiguously and give the action to all orders in the radion field and the metric. Using the background field method, we expand the theory to quadratic orders in the fields. We show that the most general classical solutions, for the induced metric on the branes in the case of a constant radion and a factorizable 4D metric, correspond to Einstein spaces. We discuss extensively the diagonalization of the quadratic action. Furthermore, we obtain the 4-dimensional effective theory from this and study the question of the spectrum as well as the couplings in these theories.