BFKL equation with running QCD coupling and HERA data

In this paper we developed approach based on the BFKL evolution in \ln\Lb Q^2\Rb. We show that the simplest diffusion approximation with running QCD coupling is able to describe the HERA experimental data on the deep inelastic structure function with good $\chi^2/d.o.f. \approx 1.3$. From our description of the experimental data we learned several lessons; (i) the non-perturbative physics at long distances started to show up at $Q^2 = 0.25\,GeV^2$; (ii) the scattering amplitude at $Q^2 = 0.25\,GeV^2$ cannot be written as sum of soft Pomeron and the secondary Reggeon but the Pomeron interactions should be taken into account; (iii) the Pomeron interactions can be reduced to the enhanced diagrams and, therefore, we do not see any needs for the shadowing corrections at HERA energies; and (iv) we demonstrated that the shadowing correction could be sizable at higher than HERA energies without any contradiction with our initial conditions.Comment: 16pp,14 figures in eps.file

A heuristic description of high-pT hadron production in heavy ion collisions

Using a simplified model for in-medium dipole evolution accounting for color filtering effects we study production of hadrons at large transverse momenta $p_T$ in heavy ion collisions. In the framework of this model, several important sources of the nuclear suppression observed recently at RHIC and LHC have been analysed. A short production length of the leading hadron $l_p$ causes a strong onset of color transparency effects manifested themselves as a steep rise of the nuclear modification factor $R_{AA}(p_T)$ at large hadron $p_T$'s. A dominance of quarks with higher $l_p$ leads to a weaker suppression at RHIC than the one observed at LHC. In the RHIC kinematic region we include an additional suppression factor steeply falling with $p_T$, which is tightly related to the energy conservation constraints. The latter is irrelevant at LHC up to $p_T\lesssim 70$ GeV while it causes a rather flat $p_T$ dependence of the $R_{AA}(p_T)$ factor at RHIC c.m. energy $\sqrt{s} = 200$ GeV and even an increasing suppression with $p_T$ at $\sqrt{s} = 62$ GeV. The calculations contain only a medium density adjustment, and for an initial time scale $t_0$ = 1 fm we found the energy-dependent maximal values of the transport coefficient, $\hat{q}_0 = 0.7, 1.0$ and 1.3 GeV$^2$/fm corresponding to $\sqrt{s} = 62, 200$ GeV and 2.76 TeV, respectively. We present a broad variety of predictions for the nuclear modification factor and the azimuthal asymmetry which are in a good agreement with available data from experiments at RHIC and LHC.Comment: 14 pages, 17 figures; extra clarifications added in Sects. II and III (with additional Figs. 1-6) and in the extended Sect. V B (with additional Fig.11), references added, conclusions unchange

Diffractive Bremsstrahlung in Hadronic Collisions

Production of heavy photons (Drell-Yan), gauge bosons, Higgs bosons, heavy flavors, which is treated within the QCD parton model as a result of hard parton-parton collision, can be considered as a bremsstrahlung process in the target rest frame. In this review, we discuss the basic features of the diffractive channels of these processes in the framework of color dipole approach. The main observation is a dramatic breakdown of diffractive QCD factorisation due to the interplay between soft and hard interactions, which dominates these processes. This observation is crucial for phenomenological studies of diffractive reactions in high-energy hadronic collisions.Comment: 29 pages, 16 figures; typos corrected; references added; minor correction

"Buddha's Light" of Cumulative Particles

We show analytically that in the cumulative particles production off nuclei multiple interactions lead to a glory-like backward focusing effect. Employing the small phase space method we arrived at a characteristic angular dependence of the production cross section $d\sigma \sim 1/ \sqrt {\pi - \theta}$ near the strictly backward direction. This effect takes place for any number $n\geq 3$ of interactions of rescattered particle, either elastic or inelastic (with resonance excitations in intermediate states), when the final particle is produced near corresponding kinematical boundary. Such a behaviour of the cross section near the backward direction is in qualitative agreement with some of available data. Explanation of this effect and the angular dependence of the cross section near $\theta \sim \pi$ are presented for the first time.Comment: The proof of the azimuthal focusing for arbitrary polar angles of the momenta of rescattered light particle is given (section 4). Amendments made in the text and figures. Presented at the International seminar Quarks-2014, Suzdal', Russia, 2-8 June 2014. 26 pages, 7 figures. To the memory of Lyonya Kondratyuk, outstanding scientist and perso

Heavy flavor production in high-energy pppp collisions: color dipole description

We present a detailed study of open heavy flavor production in high-energy $pp$ collisions at the LHC in the color dipole framework. The transverse momentum distributions of produced $b$-jets, accounting for the jet energy loss, as well as produced open charm $D$ and bottom $B$ mesons in distinct rapidity intervals relevant for LHC measurements are computed. The dipole model results for the differential $b$-jet production cross section are compared to the recent ATLAS and CMS data while the results for $D$ and $B$ mesons production cross sections -- to the corresponding LHCb data. Several models for the phenomenological dipole cross section have been employed to estimate theoretical uncertainties of the dipole model predictions. We demonstrate that the primordial transverse momentum distribution of the projectile gluon significantly affects the meson spectra at low transverse momenta and contributes to the largest uncertainty of the dipole model predictions.Comment: 22 pages, 9 figure

Abundant radiation of soft photons: a puzzle lasting four decades

The observed enhancement of low-kT photons in comparison with incorrect calculations, should not be treated as a puzzle. The paper by Low considered a large rapidity gap process of diffractive excitation of a hadron, h -> h+\gamma, rather than multiple hadron production spanning all over the rapidity interval between colliding hadrons. The optical theorem connects these two processes, and what is inner bremsstrahlung, suppressed according to Low, corresponds to radiation from final state hadrons. Thus, the main result of the Low theorem, based on gauge invariance of the diffractive bremsstrahlung amplitude, supplemented with the optical theorem, contradicts the so-called bremsstrahlung model. The latter has been used for comparison with data, leading to the longstanding soft photon puzzle.Comment: 8 pages 8 figure

DGLAP evolution for DIS diffraction production of high masses

In this paper we develop the DGLAP evolution for the system of produced gluons in the process of diffractive production in DIS, directly from the evolution equation in Color Glass Condensate approach. We are able to describe the available experimental data with small value of the QCD coupling ($\bar{\alpha_S} \approx 0.1$). We conclude that in diffractive production, we have a dilute system of emitted gluons and in the order to describe them, we need to develop the next-to-leading order approach in perturbative QCD.Comment: 20 pp. 7 figs in eps files. arXiv admin note: text overlap with arXiv:1802.0634