Searcharxiv⌕ Search

arXiv subjects

A. M. Stasto

Publications and source records attributed to A. M. Stasto.

At least 37 records · Page 2Linked to original sources
arXiv2003

Physics of ultrahigh energy neutrinos

Ultrahigh energy neutrinos can provide important information about the distant astronomical objects and the origin of the Universe. Precise knowledge about their interactions and production rates is essential for estimating background, expected fluxes and detection probabilities. In this paper we review the applications of the high energy QCD to the calculations of the interaction cross sections of the neutrinos. We also study the production of the ultrahigh energy neutrinos in the atmosphere due to the charm and beauty decays.

astro-ph↗
arXiv2003

On solutions of the Balitsky-Kovchegov equation with impact parameter

We numerically analyze the Balitsky-Kovchegov equation with the full dependence on impact parameter b. We show that due to a particular b-dependence of the initial condition the amplitude decreases for large dipole sizes r. Thus the region of saturation has a finite extension in the dipole size r, and its width increases with rapidity. We also calculate the b-dependent saturation scale and discuss limitations on geometric scaling. We demonstrate the instant emergence of the power-like tail in impact parameter, which is due to the long range contributions. Thus the resulting cross section violates the Froissart bound despite the presence of a nonlinear term responsible for saturation.

hep-ph↗
arXiv2003

Renormalisation group improved small-x Green's function

We investigate the basic features of the gluon density predicted by a renormalisation group improved small-x equation which incorporates both the gluon splitting function at leading collinear level and the exact BFKL kernel at next-to-leading level. We provide resummed results for the Green's function and its hard Pomeron exponent $ω_s(α_s)$, and for the splitting function and its critical exponent $ω_c(α_s)$. We find that non-linear resummation effects considerably extend the validity of the hard Pomeron regime by decreasing diffusion corrections to the Green's function exponent and by slowing down the drift towards the non-perturbative Pomeron regime. As in previous analyses, the resummed exponents are reduced to phenomenologically interesting values. Furthermore, significant preasymptotic effects are observed. In particular, the resummed splitting function departs from the DGLAP result in the moderate small-x region, showing a shallow dip followed by the expected power increase in the very small-x region. Finally, we outline the extension of the resummation procedure to include the photon impact factors.

hep-ph↗
arXiv2003

The q_T spectrum of the Higgs boson at the LHC in QCD perturbation theory

We consider the transverse-momentum (q_T) distribution of Higgs bosons produced at hadron colliders. We use a formalism that uniformly treats both the small-q_T and large-q_T regions in QCD perturbation theory. At small q_T (q_T << M_H, M_H being the mass of the Higgs boson), we implement an all-order resummation of logarithmically-enhanced contributions up to next-to-next-to-leading logarithmic accuracy. At large q_T (q_T \gtap M_H), we use fixed-order perturbation theory up to next-to-leading order. The resummed and fixed-order approaches are consistently matched by avoiding double-counting in the intermediate-q_T region. In this region, the introduction of unjustified higher-order terms is avoided by imposing unitarity constraints, so that the integral of the q_T spectrum exactly reproduces the perturbative result for the total cross section up to next-to-next-to-leading order. Numerical results at the LHC are presented. These show that the main features of the q_T distribution are quite stable with respect to perturbative QCD uncertainties.

hep-ph↗
arXiv2002

Running coupling and BFKL pomeron

We show that in the case of the BFKL pomeron with running coupling the diffusion pattern is strongly modified and is characterised by the sudden tunneling transition to the non-perturbative regime. We suggest that by using the b - expansion method one can suppress the non-perturbative Pomeron and isolate purely perturbative part of the gluon Green's function.

hep-ph↗
arXiv2002

Geometric scaling and QCD evolution

We study the impact of the QCD DGLAP evolution on the geometric scaling of the gluon distributions which is expected to hold at small x within the saturation models. To this aim we solve the DGLAP evolution equations with the initial conditions provided along the critical line Q^2=Q_s^2(x) where Q_s^2(x) = Q_0^2 x^{-λ}and satisfying geometric scaling. Both fixed and running coupling cases are studied. We show that in the fixed coupling case the geometric scaling at low x is stable against the DGLAP evolution for sufficiently large values of the parameter λand in the double logarithmic approximation of the DGLAP evolution this happens for λ> 4N_cα_s/π. In the running coupling case geometric scaling is found to be approximately preserved at very small x. The residual geometric scaling violation in this case can be approximately factored out and the corresponding form-factor controlling this violation is found.

hep-ph↗
arXiv2002

Saturation at low x and nonlinear evolution

In this talk the results of the analytical and numerical analysis of the nonlinear Balitsky-Kovchegov equation are presented. The characteristic BFKL diffusion into infrared regime is suppressed by the generation of the saturation scale. We identify the scaling and linear regimes for the solution. We also study the impact of subleading corrections onto the nonlinear evolution.

hep-ph↗
arXiv2002

Expanding running coupling effects in the hard Pomeron

We study QCD hard processes at scales of order k^2 > Lambda^2 in the limit in which the beta-function coefficient - b is taken to be small, but alphas(k) is kept fixed. The (nonperturbative) Pomeron is exponentially suppressed in this limit, making it possible to define purely perturbative high-energy Green's functions. The hard Pomeron exponent acquires diffusion and running coupling corrections which can be expanded in the b parameter and turn out to be dependent on the effective coupling b alphas^2 Y. We provide a general setup for this b-expansion and we calculate the first few terms both analytically and numerically.

hep-ph↗
arXiv2001

Diffusion into infra-red and unitarization of the BFKL pomeron

The BFKL pomeron in perturbative QCD is plagued by the lack of unitarity and diffusion into the infra-red region of gluon virtualities. These two problems are intimately related. We perform numerical studies of the evolution equation proposed by Balitsky and Kovchegov which unitarizes the BFKL pomeron. We show how diffusion into the infra-red region is suppressed due to the emergence of a saturation scale and scaling behaviour. We study universality of this phenomenon as well as its dependence on subleading corrections to the BFKL pomeron such as the running coupling and kinematic constraint. These corrections are very important for phenomenological applications.

hep-ph↗
arXiv2001

Impact parameter dependent S-matrix for dipole-proton scattering from diffractive meson electroproduction

We extract the S-matrix element for dipole-proton scattering using the data on diffractive electroproduction of vector mesons at HERA. By considering the full t dependence of this process we are able to reliably unfold the profile of the S-matrix for impact parameter values b>0.3 fm. We show that the results depend only weakly on the choice of the form for the vector meson wave function. We relate this result to the discussion about possible saturation effects at HERA.

hep-ph↗
arXiv2000

Geometric scaling for the total gamma^* p cross section in the low x region

We observe that the saturation model of deep inelastic scattering, which successfully describes inclusive and diffractive data at small x, predicts a geometric scaling of the total gamma^* p cross section in the region of small Bjorken variable x. The geometric scaling in this case means that the cross section is a function of only one dimensionless variable tau = Q^2 R_0^2(x), where the function R_0(x) (called saturation radius) decreases with decreasing x. We show that the experimental data from HERA in the region x<0.01 confirm the expectations of this scaling over a very broad region of Q^2. We suggest that the geometric scaling is more general than the saturation model.

hep-ph↗
arXiv2000

Parametrisation of F_2^gamma at low Q^2 and of sigma(gamma,gamma) and sigma(gamma^*,gamma) at high energies

A parametrisation of the real photon structure function F_2^gamma in the low Q^2, low x region is formulated. It includes both the VMD and the QCD components, the latter suitably extrapolated to the low Q^2 region and based on arbitrary parton distributions in the photon. The parametrisation used together with the GRV and GRS' parton densities describes reasonably well the existing high energy data on F_2^gamma, sigma(gamma,gamma) and the low Q^2 data on sigma(gamma^*, gamma). Predictions for sigma(gamma,gamma) and for sigma(gamma^* ,gamma) for energies which may become accessible in future linear colliders are also given.

hep-ph↗
arXiv2000

The unintegrated gluon distribution from the CCFM equation

The gluon distribution f(x, k_t^2,mu^2), unintegrated over the transverse momentum k_t of the gluon, satisfies the angular-ordered CCFM equation which interlocks the dependence on the scale k_t with the scale μof the probe. We show how, to leading logarithmic accuracy, the equation can be simplified to a single scale problem. In particular we demonstrate how to determine the two-scale unintegrated distribution f(x,k_t^2,mu^2) from knowledge of the integrated gluon obtained from a unified scheme embodying both BFKL and DGLAP evolution.

hep-ph↗
arXiv2000

Ultrahigh energy neutrino physics

We discuss a problem concerning the ultrahigh energy neutrino propagation through the Earth. We present calculation of the neutrino-nucleon cross section at high energies, based on the unifed evolution equation at small x. We also show the solution of the transport equation for different neutrino fluxes originating from active galactic nuclei, gamma ray bursts and top-down model.

hep-ph↗
arXiv1999

Penetration of the Earth by ultrahigh energy neutrinos and the parton distributions inside the nucleon

In this talk we would like to present recent calculations of the cross sections for the ultrahigh energy neutrinos interacting with nucleons. We briefly present the framework of unifed BFKL/DGLAP equations which resums all the leading log(1/x) effects as well as the subleading terms. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. We then use these parton distributions to calculate the cross section for the neutrino-nucleon interactions at very high energies, up to 10^12 GeV. We do also investigate the attenuation of neutrinos when traversing through the Earth. We use the transport equation which we solve for different fluxes of high energy neutrinos (Active Galactic Nucleai, Gamma Ray Bursts, and top-down models). We study the effects of the regeneration of the neutrino flux via neutral current interaction.

hep-ph↗
arXiv1999

Predictions for dijet production in DIS using small x dynamics

We study the properties of dijet production in deep inelastic scattering using a unified BFKL/DGLAP framework, which includes important subleading ln (1/x) contributions. We calculate the azimuthal decorrelation between the jets. We compute the cross section for dijet production as a function of Q^2 and the jet transverse momentum, as well as calculate the total dijet rate. We compare the predictions with HERA data.

hep-ph↗
arXiv1998

The description of $F_2$ at low $Q^2$

We analyse the data for the proton structure function $F_2$ over the entire $Q^2$ domain, including especially low $Q^2$, in terms of perturbative and non-perturbative QCD contributions. The small distance configurations are given by perturbative QCD, while the large distance contributions are given by the vector dominance model and, for the higher mass $q \bar{q}$ states, by the additive quark approach. The interference between states of different $q\bar{q}$ mass (in the perturbative contribution) is found to play a crucial role in obtaining an excellent description of the data throughout the whole $Q^2$ region, including photoproduction.

hep-ph↗