Searcharxiv⌕ Search

arXiv subjects

E. Levin

Publications and source records attributed to E. Levin.

At least 109 records · Page 6Linked to original sources
arXiv2002

Towards a new global QCD analysis: low x DIS data from non-linear evolution

A new approach to global QCD analysis is developed. The main ingredients are two QCD-based evolution equations. The first one is the Balitsky-Kovchegov nonlinear equation, which sums higher twists while preserving unitarity. The second equation is linear and it is responsible for the correct short distance behavior of the theory, namely it includes the DGLAP kernel. Our approach allows extrapolation of the parton distributions to very high energies available at the LHC as well as very low photon virtualities, $Q^2\ll 1 {\rm GeV^2}$. All existing low $x$ data on the $F_2$ structure function is reproduced using one fitting parameter. The resulting $χ^2/df=1$. Analyzing the parameter $λ\equiv \partial\ln F_2/\partial(\ln 1/x)$ at very low $x$ and $Q^2$ well below $1 {\rm GeV^2}$ we find $λ\simeq 0.08 - 0.1$. A result which agrees with the "soft pomeron" intercept without involving soft physics.

hep-ph↗
arXiv2002

Classical gluodynamics in curved space--time and the soft Pomeron

QCD at the classical level possesses scale invariance which is broken by quantum effects. This ``dimensional transmutation'' phenomenon can be mathematically described by formulating classical gluodynamics in a curved, conformally flat, space-time with non-vanishing cosmological constant. We study QCD high-energy scattering in this theory. We find that the properties of the scattering amplitude at small momentum transfer are determined by the energy density of vacuum fluctuations. The approach gives rise to the power growth of the total hadron-hadron cross section with energy, i.e. the Pomeron. The intercept of the Pomeron and the multiplicity of produced particles are evaluated. We also speculate about a possible link between conformal anomaly and TeV scale quantum gravity.

hep-ph↗
arXiv2002

Recent experimental data and the size of the quark in the Constituent Quark Model

We use the Constituent Quark Model (CQM) to describe CDF data on double parton cross section and HERA data on the $ J / Ψ$ ratio cross section of elastic and inelastic diffractive productions. Our estimate shows that the radius of the constituent quark turns out to be rather small, $R^2_{quark} \approx 0.1 Gev^{-2}$, in accordance with the assumption on which CQM is based.

hep-ph↗
arXiv2002

Momentum Transfer Dependence of the Differential Cross Section for $J/ψ$ Production

We discuss the $t$ dependence of $J/ψ$ production in the region of $0< -t < M^2_ψ$. The forward slope of the elastic differential cross section is calculated assuming a dipole-type dependence for the gluon ladder-proton form factor. The $t$ dependence of $dσ/dt$ is obtained using DGLAP evolution both for the elastic channel at low $-t$ and for the inelastic channel at large values of $-t$, up to $-t\simeq M^2_ψ$. Results are presented and compared with the relevant experimental data.

hep-ph↗
arXiv2001

The onset of classical QCD dynamics in relativistic heavy ion collisions

The experimental results on hadron production obtained recently at RHIC offer a new prospective on the energy dependence of the nuclear collision dynamics. In particular, it is possible that parton saturation -- the phenomenon likely providing initial conditions for the multi--particle production at RHIC energies -- may have started to set in central heavy ion collisions already around the highest SPS energy. We examine this scenario, and make predictions based on high density QCD for the forthcoming 22 GeV run at RHIC.

hep-ph↗
arXiv2001

Manifestations of high density QCD in the first RHIC data

We derive a simple analytical scaling function which embodies the predictions of high density QCD on the energy, centrality, rapidity, and atomic number dependences of hadron multiplicities in nuclear collisions. Both centrality and rapidity dependences of hadron multiplicity in $Au-Au$ collisions as measured at RHIC at $\sqrt{s}= 130 {GeV}$ are well described in this approach. The centrality and rapidity dependences of hadron multiplicity at $\sqrt{s}= 200 \ {GeV}$ run at RHIC are predicted; the variation of these dependences with energy appear different from other approaches, and can be used as an important test of the ideas based on parton saturation and classical Chromo-Dynamics.

nucl-th↗
arXiv2001

Non-linear evolution and high energy diffractive production

The ratio of the diffractive production to the total cross section in DIS is computed as a function of the produced mass. The analysis is based on the solution to the non-linear evolution equation for the diffraction dissociation in DIS. The obtained ratios almost do not depend on the central mass energy in agreement with the HERA experimental data. This independence is argued to be a consequence of the scaling phenomena displayed by the cross sections. As a weakness point a significant discrepancy between the data and the obtained results is found in the absolute values of the ratios. Several explanatory reasons are discussed.

hep-ph↗
arXiv2001

Diffractive dissociation and saturation scale from non-linear evolution in high energy DIS

This paper presents the first numerical solution to the non-linear evolution equation for diffractive dissociation processes in deep inelastic scattering. It is shown that the solution depends on one scaling variable $τ= Q^2/Q^{D 2}_s(x,x_0)$, where $Q^D_s(x,x_0)$ is the saturation scale for the diffraction processes. The dependence of the saturation scale $Q^D_s(x,x_0)$ on both $x$ and $x_0$ is investigated, ($Y_0 = \ln(1/x_0)$ is a minimal rapidity gap for the diffraction process). The $x$ - dependence of $Q^D_s$ turns out to be the same as of the saturation scale in the total inclusive DIS cross section. In our calculations $Q^D_s(x,x_0)$ reveals only mild dependence on $x_0$. The scaling is shown to hold for $x \ll x_0$ but is violated at $ x \sim x_0$.

hep-ph↗
arXiv2001

Higher Twist Corrections and Maxima for DIS on a Proton in the High Density QCD Region

We show that the ratio of different structure functions have a maximum which depends on energy. We argue, using Golec-Biernat and Wusthoff model, that these maxima are the certain functions of the saturation scale. The analysis of leading and higher twist contributions for different observables is given with the aim of determining the kinematic region where high parton density effects could be seen experimentally.

hep-ph↗
arXiv2001

Saturation at low $x$

This talk is an attempt to review all our knowledge on saturation at low $x$ both theoretical and experimental, to stimulate a search for saturation effects at THERA. The main goals of this presentation are 1. To discuss an intuitive picture of the deep inelastic scattering that leads to the saturation of the parton densities; 2. To show that the saturation hypothesis has solid theoretical proof; 3. To report on the theoretical progress that has been made over the past two years in high parton density QCD, and on the property of the saturation phase that emerges from the theory that has been developed; 4. To collect all that we know theoretically and experimentally about the saturation scale $Q_s(x)$ .

hep-ph↗
arXiv2001

Parton densities and saturation scale from non-linear evolution in DIS on nuclei

We present the numerical solution of the non-linear evolution equation for DIS on nuclei for $x = 10^{-2} ÷10^{-7}$. We demonstrate that the solution to the non-linear evolution equation is quite different from the Glauber - Mueller formula which was used as the initial condition for the equation. We illustrate the energy profit for performing DIS experiments on nuclei. However, it turns out that the gain is quite modest: $x_{Au} \simeq 5 x_{\rm proton} $ for the same parton density. We find that the saturation scale $Q^2_s \propto A^{1/3}$. For gold the saturation scale $Q_{s,Au} \simeq 1.5 GeV$ at $x= 10^{-3}$. Such a large value leads to considerable contribution of the high density QCD phase to RHIC data and reveals itself in essential damping for both $xG_A$ and $F_{2A}$.

hep-ph↗
arXiv2001

Nonlinear evolution and saturation for heavy nuclei in DIS

The nonlinear evolution equation for the scattering amplitude of colour dipole off the heavy nucleus is solved in the double logarithmic approximation. It is found that if the initial parton density in a nucleus is smaller then some critical value, then the scattering amplitude is a function of one scaling variable inside the saturation region, whereas if it is greater then the critical value, then the scaling behaviour breaks down. Dependence of the saturation scale on the number of nucleons is discussed as well.

hep-ph↗
arXiv2001

Non-linear evolution and parton distributions at LHC and THERA energies

We suggest a new procedure for extrapolating the parton distributions from HERA energies to higher energies at THERA and LHC. The procedure suggested consists of two steps: first, we solve the non-linear evolution equation which includes the higher twists contributions, however this equation is deficient due to the low ($log(1/x)$) accuracy of our calculations. Second, we introduce a correcting function for which we write a DGLAP type linear evolution equation. We show that this correcting function is small in the whole kinematic region and decreases at low $x$. The nonlinear evolution equation is solved numerically and first estimates for the saturation scale, as well as for the value of the gluon density at THERA and LHC energies are made. We show that non-linear effects lead to damping of the gluon density by a factor of $2 ÷3$ at $x \approx 10^{-7}$.

hep-ph↗
arXiv2001

High density QCD at THERA

These notes are a summary of our predictions for the new THERA project, related to deep inelastic scattering in the region of ultra low $x$ ($x \to 10^{-7}$). We collect here predictions that satisfy two criteria (i) they do not depend on specific features of the model that we have to use to estimate a possible effect; and (ii) they do not contradict the HERA data.

hep-ph↗
arXiv2001

New Scaling at High Energy DIS

We develop a new approach for solving the non-linear evolution equation in the low $x_B$ region and show that the remarkable "geometric" scaling of its solution holds not only in the saturation region, but in much wider kinematical region. This is in a full agreement with experimental data (Golec-Biernat, Kwiecinski and Stasto).

hep-ph↗
arXiv2001

Screening Corrections in Photo and DIS Production of $J/Ψ$

Photo and DIS production of $J/Ψ$ are investigated and compared with calculations based on pQCD in the LLA approximation without and with screening corrections. The calculation includes corrections induced by the real part of the production amplitude, the skewed (off diagonal) gluon structure function and the relativistic Fermi motion within the charmonium system. Our pQCD results are also compared with the predictions obtained from a Regge type two Pomeron model. Our results suggest some preference for our screened model, but the available data does not enable us to exclude any of the three models we have examined. The properties of these models, when extrapolated to both low and very high energies may provide a more effective discrimination between the different parameterizations.

hep-ph↗
arXiv2000

Scaling Violations in the $Q^2$ Logarithmic Derivative of $F_2$

We examine the latest HERA experimental measurements of the $Q^2$ logarithmic derivative of the proton structure function. We analyze the characteristics of DGLAP with and without screening, as well as a Regge type model, and compare their predictions to all available data on $\slope$ including those for fixed W. Our results show that the present data can be described in pQCD taking into account shadowing corrections. However, such a description cannot be considered a conclusive signal of gluon saturation since the experimental data on $\slope$ do not allow one to discriminate between the various approaches, in spite of their very different construction.

hep-ph↗