Infrared singularities of QCD amplitudes with massive partons

A formula for the two-loop infrared singularities of dimensionally regularized QCD scattering amplitudes with an arbitrary number of massive and massless legs is derived. The singularities are obtained from the solution of a renormalization-group equation, and factorization constraints on the relevant anomalous-dimension matrix are analyzed. The simplicity of the structure of the matrix relevant for massless partons does not carry over to the case with massive legs, where starting at two-loop order new color and momentum structures arise, which are not of the color-dipole form. The resulting two-loop three-parton correlations can be expressed in terms of two functions, for which some general properties are derived. This explains observations recently made by Mitov et al. in terms of symmetry arguments.Comment: 7 pages, 1 figure; v2: minor changes, reference added; v3: note added, correcting some statements regarding F1 and f2 in light of the recent calculations in [45,46], references update

Nonleptonic Weak Decays of B to D_s and D mesons

Branching ratios and polarization amplitudes for B decaying to all allowed pseudoscalar, vector, axial-vector, scalar and tensor combinations of D_s and D mesons are calculated in the Isgur Scora Grinstein Wise (ISGW) quark model after assuming factorization. We find good agreement with other models in the literature and the limited experimental data and make predictions for as yet unseen decay modes. Lattice QCD results in this area are very limited. We make phenomenological observations on decays in to D_s(2317) and D_s(2460) and propose tests for determining the status and mixings of the axial mesons. We use the same approach to calculate branching ratios and polarization fraction for decays in to two D type mesons.Comment: 21 pages, 9 figures. v3: updated to reflect changes in published paper, conclusions unchanged (see source file for details). Added comments on factorization. v2: experimental data updated, references added, tables of results added, more on axial D_s mixing, added section on D D decay modes and typos correcte

Two-Loop Renormalization of Heavy--Light Currents at Order 1/m_Q in the Heavy-Quark Expansion

We present exact results, at next-to-leading order in renormalization-group improved perturbation theory, for the Wilson coefficients appearing at order 1/m_Q in the heavy-quark expansion of heavy-light current operators. To this end, we complete the calculation of the corresponding two-loop anomalous dimension matrix. Our results are important for determinations of |V_{ub}| using exclusive and inclusive semileptonic B decays. They are also relevant to computations of the decay constant f_B based on a heavy-quark expansion.Comment: 16 pages, 2 figures; third author added and one reference updated, results unchange

Higgs-Boson Production at Small Transverse Momentum

Using methods from effective field theory, we have recently developed a novel, systematic framework for the calculation of the cross sections for electroweak gauge-boson production at small and very small transverse momentum q_T, in which large logarithms of the scale ratio m_V/q_T are resummed to all orders. This formalism is applied to the production of Higgs bosons in gluon fusion at the LHC. The production cross section receives logarithmically enhanced corrections from two sources: the running of the hard matching coefficient and the collinear factorization anomaly. The anomaly leads to the dynamical generation of a non-perturbative scale q_* ~ m_H e^{-const/\alpha_s(m_H)} ~ 8 GeV, which protects the process from receiving large long-distance hadronic contributions. We present detailed numerical predictions for the transverse-momentum spectrum of the Higgs boson, finding that it is quite insensitive to hadronic effects.Comment: 18 pages, 5 figures; v2: published version, includes a correction in (8) and (22

Factorization and N^3LL_p+NNLO Predictions for the Higgs Cross Section with a Jet Veto

We have recently derived a factorization formula for the Higgs-boson production cross section in the presence of a jet veto, which allows for a systematic resummation of large Sudakov logarithms of the form alpha_s^n ln^m(p_T^veto/m_H), along with the large virtual corrections known to affect also the total cross section. Here we determine the ingredients entering this formula at two-loop accuracy. Specifically, we compute the dependence on the jet-radius parameter R, which is encoded in the two-loop coefficient of the collinear anomaly, by means of a direct, fully analytic calculation in the framework of soft-collinear effective theory. We confirm the result obtained by Banfi et al. from a related calculation in QCD, and demonstrate that factorization-breaking, soft-collinear mixing effects do not arise at leading power in p_T^veto/m_H, even for R=O(1). In addition, we extract the two-loop collinear beam functions numerically. We present detailed numerical predictions for the jet-veto cross section with partial next-to-next-to-next-to-leading logarithmic accuracy, matched to the next-to-next-to-leading order cross section in fixed-order perturbation theory. The only missing ingredients at this level of accuracy are the three-loop anomaly coefficient and the four-loop cusp anomalous dimension, whose numerical effects we estimate to be small.Comment: 43 pages, 12 figures; minor changes, references updated; version published in JHE

Dynamical Threshold Enhancement and Resummation in Drell-Yan Production

Partonic cross sections for the production of massive objects in hadronic collisions receive large corrections when the invariant mass of the initial-state partons is just above the production threshold. Since typically the center-of-mass energy of the hadronic collision is much higher than the mass of the heavy objects, it is not obvious that these contributions translate into large corrections to the hadronic cross section. Using a recent approach to threshold resummation based on effective field theory, we quantify to which extent the fall-off of the parton densities at high x leads to a dynamical enhancement of the partonic threshold region. With the example of Drell-Yan production, we study the emergence of an effective physical scale characterizing the soft emissions in the process. We derive compact analytical expressions for the resummed Drell-Yan cross section and rapidity distribution directly in momentum space. They are free of Landau-pole singularities and are trivially matched onto fixed-order perturbative calculations. Evaluating the resummed cross sections at NNNLL order and matching onto NNLO fixed-order calculations, we perform a detailed numerical analysis of the cross section and rapidity distribution in proton-proton collisions.Comment: 43 pages; references added; small numerical changes at NNLO due to improved fixed-order cod

Massive Boson Production at Small q_T in Soft-Collinear Effective Theory

We study the differential cross sections for electroweak gauge-boson and Higgs production at small and very small transverse-momentum q_T. Large logarithms are resummed using soft-collinear effective theory. The collinear anomaly generates a non-perturbative scale q_*, which protects the processes from receiving large long-distance hadronic contributions. A numerical comparison of our predictions with data on the transverse-momentum distribution in Z-boson production at the Tevatron and LHC is given.Comment: PDF LaTeX, 4 pages, 7 pdf figures. To appear in the proceedings of the 16th International Conference in Quantum ChromoDynamics (QCD12), 2-6 July 2012, Montpellie

Threshold Resummation in Momentum Space from Effective Field Theory

Methods from soft-collinear effective theory are used to perform the threshold resummation of Sudakov logarithms for the deep-inelastic structure function F_2(x,Q^2) in the endpoint region x->1 directly in momentum space. An explicit all-order formula is derived, which expresses the short-distance coefficient function C in the convolution F_2=C*phi_q in terms of Wilson coefficients and anomalous dimensions defined in the effective theory. Contributions associated with the physical scales Q^2 and Q^2(1-x) are separated from non-perturbative hadronic physics in a transparent way. A crucial ingredient to the momentum-space resummation is the exact solution to the integro-differential evolution equation of the jet function, which is derived. The methods developed in this Letter can be applied to many other hard QCD processes.Comment: 4 pages, 1 figure. Version to appear in PR

Magnetization of Planar Four-Fermion Systems

We consider a planar system of fermions, at finite temperature and density, under a static magnetic field parallel to the two-dimensional plane. This magnetic field generates a Zeeman effect and, then, a spin polarization of the system. The critical properties are studied from the Landau's free energy. The possible observable consequences of the magnetization of planar systems such as polymer films and graphene are discussed.Comment: 10 pages, two-column, revtex style, 6 eps figures. Published versio