Narrow Results

It is argued that while the scale dependence of the parton distributions in the fixed flavor factorization scheme is governed by three active flavors, the scale dependence of the running coupling should nevertheless be better governed by a variable number of active flavors.

The perturbative QCD approach is based on k_T factorization, including the Sudakov form factors so that to avoid the endpoint singularity. In this approach, we calculate the charmless B decays like B → ππ decays etc. to produce the right number of branching ratios and also CP asymmetry parameters. For final states with at least one charmed meson, like B → Dπ decays, our results also agree with the experiments.

After the final analyses of the H1 and ZEUS collaborations for the diffractive photoproduction of dijets have appeared, we have recalculated these cross sections in next-to-leading order (NLO) of perturbative QCD to see whether they can be interpreted consistently. The results of these calculations are compared to the data of both collaborations. We find that at NLO the cross sections disagree with the data, showing that factorization breaking occurs at this order. If direct and resolved contributions are both suppressed by the same amount, the global suppression factor depends on the transverse-energy cut and is 0.42 for the H1 and 0.71 for the ZEUS analysis. However, by suppressing only the resolved contribution by a factor of approximately three, also reasonably good agreement with all the data is found. The size of the factorization breaking effects for resolved photons agrees with absorptive-model predictions.

We analyze small-x deeply virtual Compton scattering data using flexible generalized parton distribution models to pin down both the skewness and t dependence. We compare our outcome at t = 0 with the full Shuvaev transformation. We point out that this integral transform is a model which is equivalent to a conformal generalized parton distribution and a minimalist "dual" parameterization. Some mathematical subtleties of conformal representations are recalled.

The standard theoretical description of Polarized DIS is based on the use of the DGLAP evolution equations. This approach controls the Q² -evolution of the DIS structure functions but cannot describe the x -evolution and therefore should not be used at small x. We complement this approach, accounting for the x-evolution of the spin structure function g₁ in the leading logarithmic approximation.

Measurements of double-spin asymmetries for single-inclusive heavy flavor production and for heavy quark correlations are expected to provide valuable information about the spin structure of the nucleon, in particular, the elusive gluon polarization. We discuss progress towards a versatile parton-level Monte-Carlo code for the production and decay of heavy quarks in longitudinally polarized hadron-hadron and photon-hadron collisions at next-to-leading order accuracy of QCD. Phenomenological studies are presented for BNL-RHIC and COMPASS.

Starting from the divergent character of the perturbative expansions in QCD and using the technique of series acceleration by the conformal mappings of the Borel plane, I define a novel, non-power perturbative expansion for the Adler function, which simultaneously implements renormalisation-group summation and has a tamed large-order behaviour. The new expansion functions, which replace the standard powers of the coupling, are singular at the origin of the coupling plane and have divergent perturbative expansions, resembling the expanded function itself. Confronting the new perturbative expansions with the standard ones on specific models investigated recently in the literature, I show that they approximate in an impressive way the exact Adler function and the spectral function moments. Applied to the τ hadronic width, the contour-improved and the renormalisation-group summed non-power expansions in the scheme lead to the prediction , which translates to .

For processes involving structure functions and/or fragmentation functions, arguments that over a range of a proper kinematic variable, there is a part that dominates the next-to-leading order (NLO) corrections, are briefly reviewed. The arguments are tested against more recent NLO and in particular complete next-to-next-to-leading order (NNLO) calculations. A critical examination of when these arguments may not be useful is also presented.

According to a rederivation — due to Collins and Qiu — the DGLAP equation can be reinterpreted (in leading order) in a probabilistic way. This form of the equation has been used indirectly to prove the bound |Δ f(x, Q)| < f(x, Q) between polarized and unpolarized distributions, or positivity of the helicity distributions, for any Q. We reanalyze this issue by performing a detailed numerical study of the positivity bounds of the helicity distributions. To obtain the numerical solution we implement an x-space based algorithm for polarized and unpolarized distributions to next-to-leading order in α_s, which we illustrate. We also elaborate on some of the formal properties of the Collins–Qiu form and comment on the underlying regularization, introduce a Kramers–Moyal expansion of the equation and briefly analyze its Fokker–Planck approximation. These follow quite naturally once the master version is given. We illustrate this expansion both for the valence quark distribution q_V and for the transverse spin distribution h₁.

We predict the shape of the transverse momentum p_T spectrum of ϒ production. The distribution at low p_T is dominated by the region of small impact parameter b and may be computed reliably in perturbation theory. We resum to all orders in the strong coupling α_s the process-independent large logarithmic contributions that arise from initial-state gluon showers in the small p_T(≤ M_ϒ) region. The cross section at large p_T is represented by the lowest-order non-vanishing perturbative contribution.

The status of RHIC theory and phenomenology is reviewed with an emphasis on the indications for the creation of a new deconfined state of matter. The critical role of high energy nuclear physics in the development of theoretical tools that address various aspects of the QCD many body dynamics is highlighted. The perspectives for studying nuclear matter under even more extreme conditions at the LHC and the overlap with high energy physics is discussed.

Next-to-leading order QCD corrections, order 1 GeV mass corrections and the role of a strangeness asymmetry and isospin violation in the x dependence of parton distributions are evaluated in the context of the neutrino-nucleon cross section. Their contributions to evaluations of the weak mixing angle using the Paschos-Wolfenstein relation are discussed.

The recent BaBar measurements of the γγ*→π⁰ transition form-factor show spectacular deviation from perturbative QCD prediction for large spacelike Q² up to 34 GeV². When plotted against Q², Q²F(Q²) shows steady increase with Q² in contrast with the flat Q² behavior predicted by perturbative QCD, and at 34 GeV² is more than 50% larger than the QCD prediction. Stimulated by the BaBar measurements, we revisit our previous paper on the cancellation of anomaly effects in high energy processes Z⁰→π⁰γ, e⁺e^- → π⁰γ and apply our results to the γ*γ→π⁰ transition form-factor measured in the e⁺e^- → e⁺e^-π⁰ process with one highly virtual photon. We find that, the transition form-factor F(Q²) behaves as and produces a striking agreement with the BaBar data for Q²F(Q²) with m = 132 MeV which also reproduces very well the CLEO data at lower Q².

In this paper we summarize some recent developments in perturbative QCD and their application to particle physics phenomenology.

In this review we present our model which is an example of the self-consistent approach that incorporates our theoretical understanding of long distance physics, based both on N = 4 SYM for strong coupling and on the matching with the perturbative QCD approach. We demonstrate how important and decisive the LHC data were on strong interactions which led us to a set of the phenomenological parameters that fully confirmed our theoretical expectations, and produced a new picture of the strong interaction at high energy. We also show how far we have come towards creating a framework for the description of minimal bias events for high energy scattering without generating Monte Carlo codes.

Heavy quarkonium production is a powerful implement to study the strong interaction dynamics and QCD theory. Fragmentation is the dominant production mechanism for heavy quarkonia with large transverse momentum. With the large heavy quark mass, the relative motion of the heavy quark pair inside a heavy quarkonium is effectively nonrelativistic and it is also well known that their fragmentation functions can be calculated in the perturbative QCD framework. Here, we analytically calculate the process-independent fragmentation functions for a gluon to split into the spin-singlet and spin-triplet $S$-wave heavy quarkonia using three different scenarios. We will show that the fragmentation probability of the gluon into the spin-triplet bound-state is the biggest one.

We review the recent progress on the calculations on the inclusive forward hadron production within the saturation formalism. After introducing the concept of perturbative parton saturation and nonlinear evolution we discuss the formalism for the forward hadron production at high energy in the leading and next-to-leading order. Numerical results are presented and compared with the experimental data on forward hadron production in $dA$ and $pA$. We discuss the problem of the negativity of the NLO cross-section at high transverse momenta, study its origin in detail and present possible improvements which include the corrected kinematics and the suitable choice of the rapidity cutoff.

We review what is currently known about the transverse spin structure of hadrons, in particular from observables that can be analyzed within a collinear framework. These effects have been around for 40 years and represent a critical test of perturbative QCD. We look at both proton–proton and lepton–nucleon collisions for various final states. While the main focus is on transverse single-spin asymmetries, we also discuss how longitudinal-transverse double-spin asymmetries offer a complimentary, yet equally important, source of information on the quark–gluon content of hadrons. We also summarize some recent progress in solidifying the theoretical formalism behind these observables and give an outlook on future directions of research.

The $\mathrm{{\rm Y}}(1S)\to B_c{D}_s^{\ast }$ weak decay is studied with the perturbative QCD approach firstly. It is found that (1) main contributions to branching ratio come from the longitudinal and parallel helicity amplitudes, (2) branching ratio, longitudinal and parallel polarization fractions are sensitive to the wave functions of the $\mathrm{{\rm Y}}(1S)$ meson, (3) branching ratio for the $\mathrm{{\rm Y}}(1S)\to B_c{D}_s^{\ast }$ decay can reach up to $10^{-9}$, which might be promisingly measured by the future experiments.

Inspired by the recent measurements on two-body nonleptonic $J/\psi$ weak decay at BESIII, the charm-changing $J/\psi \to D_{s,d}V$ weak decays are studied with perturbative QCD approach, where $V$ denotes $\rho$ and $K^{\ast }$ vector mesons. It is found that branching ratio for $J/\psi \to D_s\rho$ decay can reach up to ${𝒪}(10^{-9})$, which is within the potential measurement capability of the future high-luminosity experiments.