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E. Levin

Publications and source records attributed to E. Levin.

At least 127 records · Page 7Linked to original sources
arXiv2000

Has HERA reached a new QCD regime?

These notes are a summary of our efforts to answer the question in the title. Our answer is in the affirmative as: (i) HERA data indicate a large value of the gluon structure function; (ii) no contradictions with the asymptotic predictions of high density QCD have been observed; and (iii) the numerical estimates of our model give a natural description of the size of deviation from the routine DGLAP explanation. We discuss the alternative approaches and possible new experiments.

hep-ph↗
arXiv2000

Energy Dependence of σ^{DD}/σ_{tot} in DIS and Shadowing Corrections

We gereralize the Kochegov-McLerran formula for the ratio $σ^{DD}/σ_{tot}$ in perturbative QCD, using Mueller-Glauber approach for shadowing corrections and AGK cutting rules. We investigate several phenomenological approaches with the goal of obtaining results consisent with experimental data. We fail to reproduce the observed weak energy dependence of the ratio, and conclude that the soft nonperturbative contribution present at short distances must also be included.

hep-ph↗
arXiv2000

Higher twists and maxima for DIS on nuclei in high density QCD region

We show that the ratio of different structure functions have a maximum which depends on $x_B$ and $A$. We argue that these maxima are proportional to 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↗
arXiv2000

Screening corrections in DIS at low Q^2 and x

In this investigation we present a detailed study of $\partial F_2/\partial \ln Q^2$ which is supported by a coupled analysis of $J/Ψ$ photo and DIS production. Both channels are directly proportional to $xG(x,Q^2)$, and as such serve as excellent discriminators between different approaches and models. We then show that taking GRV98NLO as input and correcting it for SC, we can reproduce the recent HERA data well. The calculation depends on one parameter $R^2=8.5GeV^{-2}$ which is directly determined from the $J/Ψ$ photoproduction forward differential slope. With this input we obtain an excellent fit to the $J/Ψ$ photo and DIS production data. Our calculations made in the LLA of pQCD take into account the corrections implied by the real part of the production amplitude, off diagonal (skewed) gluon distributions and the Fermi motion of the charm quarks within the bound Charmonium system.

hep-ph↗
arXiv2000

Soft Double-Diffractive Higgs Production at Hadron Colliders

We evaluate the non-perturbative contribution to the double-diffractive production of the Higgs boson, which arises due to the QCD scale anomaly if the mass of the Higgs M_H is smaller than the mass of the top quark M_T, M_H < M_T. The cross section appears to be larger than expected from perturbative calculations; we find σ_H = 0.019 - 0.14 pbarn at the Tevatron energy, and σ_H = 0.01 - 0.27 pbarn at the energy of the LHC.

hep-ph↗
arXiv2000

Shadowing Corrections and Diffractive Production in DIS on Nuclei

We calculate, in the Glauber-Mueller approach, the ratio of the diffractive dissociation cross section to the total cross section on nuclei. We observe that shadowing corrections (SC) are significant, in the calculated $\ddt$ ratio but they do not lead to a smooth energy dependence of the ratio for a proton target as was observed at HERA.

hep-ph↗
arXiv2000

A Pomeron Approach to Hadron-Nucleus and Nucleus-Nucleus "Soft" Interactions at High Energy

We formulate a generalization of the Glauber formalism for hadron-nucleus and nucleus-nucleus collisions based on the Pomeron approach to high energy interactions. Our treatment is based on two physical assumptions (i.e. two small parameters) : (i) that only sufficiently small distances contribute to the Pomeron structure; and (ii) the triple Pomeron vertex $G_{3P}/g_{P-N} \ll 1$ (where $g_{P-N}$ is the Pomeron-nucleon vertex) is small. A systematic method is developed for calculating the total, elastic and diffractive dissociation cross sections as well as the survival probability of large rapidity gap processes and inclusive observables, both for hadron - nucleus and nucleus-nucleus collisions. Our approach suggests saturation of the density of the produced hadrons in nucleus-nucleus collisions, the value of the saturation density turns out to be large and depends on the number of nucleons in the lightest nucleus.

hep-ph↗
arXiv2000

The Components of the $γ^*γ^*$ Cross Section

We extend our previous treatment of $γ^*p$ cross section based on Gribov's hypothesis to the case of photon-photon scattering. With the aid of two parameters, determined from experimental data, we separate the interactions into two categories corresponding to short ("soft") and long ("hard") distance processes. The photon-photon cross section, thus, receives contributions from three sectors, soft-soft, hard-hard and hard-soft. The additive quark model is used to describe the soft-soft sector, pQCD the hard-hard sector, while the hard-soft sector is determined by relating it to the $γ^{*}$p system. We calculate and display the behaviour of the total photon-photon cross section and it's various components and polarizations for different values of energy and virtuality of the two photons, and discuss the significance of our results.

hep-ph↗
arXiv1999

Scale Anomaly and "Soft" Pomeron in QCD

We propose a new non-perturbative approach to hadronic interactions at high energies and small momentum transfer, which is based on the scale anomaly of QCD and emphasizes the role of semi-classical vacuum fields. We find that the hadron scattering amplitudes exhibit Regge behavior and evaluate the intercept alpha(0) of the corresponding trajectory. Both the intercept and the scale for the slope of the trajectory appear to be determined by the energy density of non-perturbative QCD vacuum (the gluon condensate). Numerically, we find Delta = alpha(0) - 1 = 0.08 -0.1, consistent with the values ascribed phenomenologically to the ``soft'' Pomeron. For arbitrary numbers of colors N_c and flavors N_f, Delta is found to be proportional to (N_f/N_c)^2; however, in the large N_c (N_f fixed) limit, Delta \sim N_c^0.

hep-ph↗
arXiv1999

Screening Effects on $F_2$ at Low $x$ and $Q^2$

This is a talk given at Ringberg Workshop:"New Trends in HERA Physics", 30 May - 4 June, 1999.In this talk we discuss how deeply the region of high parton densities has been studied experimentally at HERA. We show that the measurements of deep inelastic structure functions at HERA confirm our theoretical expectation that at HERA we face a challenging problem of understanding a new system of partons: quarks and gluons at short distances with so large densities that we cannot treat this system perturbatively. We collect all experimental indications and manifestations of specific properties of high parton density QCD.

hep-ph↗
arXiv1999

Total $γ^*$ p Cross Section

This paper shows that the approach based on Gribov's ideas for the photon-proton interaction, is able to describe the experimental data over a wide range of photon virtualities $Q^2 = 0 - 100 GeV^2$ and energies $\sqrt{s} = W = 3 - 300 GeV$. A simple model is suggested which provides a quantitative way of describing the matching between short and long distances (between "soft" and "hard" processes) in photon-proton interaction at high energy. The main results of our analysis are: (i) the values of the separation parameters differentiating the "soft" and "hard" interactions are determined; (ii) the additive quark model can be used to calculate the "soft" contribution to the photon - proton interaction; (iii) a good description of the ratio $σ_L/σ_T$ is obtained ; and (iv) a considerable contribution of the "soft" process at large $Q^2$, and of "hard" processes at small $Q^2$ was found.

hep-ph↗
arXiv1999

The survival probability of large rapidity gaps in a three channel model

The values and energy dependence for the survival probability $< \mid S\mid^2 >$ of large rapidity gaps (LRG) are calculated in a three channel model. This model includes single and double diffractive production, as well as elastic rescattering. It is shown that $< \mid S \mid^2 >$ decreases with increasing energy, in line with recent results for LRG dijet production at the Tevatron. This is in spite of the weak dependence on energy of the ratio $ (σ_{el}+ σ_{SD})/σ_{tot}$.

hep-ph↗
arXiv1999

A Two Channel Calculation of Screening Corrections

We present a two channel eikonal calculation in which the rescattering through a diffractive channel is included in addition to the elastic channel. Considering the spread of the experimental data, we find that we can obtain a very good description of $\s_{tot}$, $\s_{el}$ and $B_{el}$ in the ISR - Tevatron energy range. In this range of energy the diffractive channel, that was included in our calculation, leads to a ratio of $\s_{SD}/\s_{el}$ which varies between 1 and 0.5 for $20 GeV \leq \sqrt{s} \leq 14 TeV$ in agreement with the experimental data. The calculated survival probability of dijet production with a large rapidity gap is consistent with the data.

hep-ph↗
arXiv1998

25 years with the Pomeron

This paper is written for "Gribov Memorial Volume", Heavy Ion Physics, Acta Physica Hungarica. This is a report on my 25 years activity in understanding of the Pomeron structure. Since I was involved in the Pomeron business moreless from the beginning, I hope, that this report shows the development of the main ideas in their historical perspective from the first enthusiastic attempts to find a simple solution to understanding of the complexity and difficulty of the problem. In other words, this is a story about a young guy who wanted to understand everything in high energy interaction, who did his best but who is still in the beginning, but who has not lost his temper and considers the Pomeron structure as the beautiful and difficult problem, which deserves his time and efforts to be solved.

hep-ph↗
arXiv1998

An introduction to Pomerons

This talk is an attempt to clarify for experimentalists what we (theorists) mean when we are saying "Pomeron". I hope, that in this talk they will find answers to such questions as what is Pomeron, what we have learned about Pomeron, both experimentally and theoretically, what is correct strategy to study Pomeron experimentally and etc. I also hope that this talk could be used as a guide in the zoo of Pomerons: "soft" Pomeron, "hard" Pomeron, the BFKL Pomeron, the Donnachie-Landshoff Pomeron and so on. The large number of different Pomerons just reflects our poor understanding of high energy asymptotics in out microscopic theory -QCD. The motto of my talk, which gives you my opinion on the subject in short is:``Pomeron is still unknown but needed for 25 year to describe experimental data".

hep-ph↗
arXiv1998

The effect of screening on the $x$ and $Q^2$ bhaviour of $F_2$ slopes

A systematic study of $\frac{\partial F_2(x,Q^2)}{\partial \ln Q^2}$ and $\frac{\partial \ln F_2(x,Q^2)}{\partial \ln (1/x)}$ is carried out in pQCD taking screening corrections into account. The result of calculations, which are different from the non screened DGLAP prediction, are compared and shown to agree with the available experimental data as well as a pseudo data base generated from the ALLM'97 parameterization. This pseudo data base allows us to study in detail our predictions over a wider kinematic region than is available experimentally, and allows us to make suggestions for future experiments. Our results are compared with the GRV'94 parameterization (which is used as an input for our calculations) as well as the recently proposed MRST structure functions.

hep-ph↗