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M. A. Braun

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

At least 127 records · Page 7Linked to original sources
arXiv1998

Direct Calculations of the Odderon Intercept in the Perturbative QCD

The odderon intercept is calculated directly, from its expression via an average energy of the odderon Hamiltonian, using both trial wave functions in the variational approach and the wave function recently constructed by R.A.Janik and J.Wosiek. The results confirm their reported value for the energy. The odderon intercept is calculated directly, from its expression via an average energy of the odderon Hamiltonian, using both trial wave functions in the variational approach and the wave function recently constructed by R.A.Janik and J.Wosiek.The results confirm their reported value for the energy. Variational calculations give energies some 30% higher. However they also predict the odderon intercept to be quite close to unity. In fact, for realistic values of $α_s$, the intercept calculated variationally is at most 2% lower than the exact one: 0.94 instead of 0.96. It is also found that the solution for $q_3=0$ does not belong to the odderon spectrum. The diffusion parameter is found to be of the order 0.6.

hep-ph↗
arXiv1998

On the 2nd Order Corrections to the Hard Pomeron and the Running Coupling Constant

It is shown that solutions to the 2nd order BFKL eigenvalue equation exist for arbitrary large real values of the complex angular momentum $j$. This corresponds to a cut in the complex $j$ plane along the whole real axis, and it makes the use of the complex angular momentum variable for the calculation of the high-energy behavior somewhat questionable. The eigenfunctions contain non-perturbative pieces which behave as $\exp(-1/α_s b)$ and have no counterpart in the leading-log BFKL equation. The high-energy behavior of the 2nd order BFKL Green function as found by other authors, is reproduced by excluding these non-perturbative pieces of the eigenfunctions.

hep-ph↗
arXiv1998

Odderon and QCD

A review is made of the odderon idea in high-energy physics, complemented by an outline of the recent calculations of the odderon properties in the perturbative QCD.

hep-ph↗
arXiv1998

On the odderon intercept in the perturbative QCD

The odderon intercept is calculated directly using the wave function recently constructed by R.A.Janik and J.Wosiek. The results confirm their reported value. It is also found that their solution for $q_3=0$ does not satisfy the Bose-symmetry requirements. Introduction of terms into the trial wave function with an asymptotical behavior similar to the Janik-Wosiek wave functions does not seem to improve variational estimates significantly. The diffusion parameter is found to be of the order 0.6.

hep-ph↗
arXiv1998

Double scattering on the nucleus in the perturbative QCD

In the hard pomeron model consequences are studied which follow from the recently obtained form of the diffractive amplitude for the double scattering on the nucleus and the related EMC effect at small $x$. It is shown that at large $Q^2$ the double scattering contribution to the latter falls as $Q^{-0.6338}$ and in all probability dominates the total effect.

hep-ph↗
arXiv1998

On the diffraction in the perturbative QCD

In the framework of the hard pomeron model it is shown that at large $Q^2$ the diffractive contribution to the structure functions rises as $Q^{0.3362}$ and much slowlier than the total contribution rising as $Q$.

hep-ph↗
arXiv1997

Triple pomeron vertex in the limit Nc to infinity

In the hard pomeron theory with the number of colours Nc in the infinity limit the diffractive amplitude obtained in [3] is compared with the results found for Nc=3 in [1] and in the dipole approach in [5]. It is shown that the double pomeron exchange contribution can be substituted by an equivalent triple pomeron interaction term. After such a substitution the triple pomeron vertices in [1,3,5] essentially coincide. It is demonstrated that, in any form, the triple pomeron vertex is conformal invariant. It is also shown that higher order densities in the dipole approach do not involve 1 to k pomeron verteces with k>2 but are rather given by a set of pomeron fan diagrams with only a triple pomeron coupling.

hep-ph↗
arXiv1997

Deuteron Disintegration in Quark-Parton Model

The deuteron disintegration process with the emission of fast proton in the vicinity of the kinematical boundary of the reaction, when Feynman variable $x\to 2$, is studied. The consideration is fulfilled in the framework of the quark-parton model of cumulative phenomena based on perturbative QCD calculations of the corresponding quark diagrams near the thresholds, at which some quarks ("donors") in the nuclear flucton transfer all their longitudinal momenta to the distinguished active quarks and become soft. The presence of the multi-quark $6q$-configuration in a deuteron is essentially exploited in the consideration. The different versions of hadronization mechanisms of the produced cumulative quarks into cumulative particles are analyzed. It is shown that in the case of the production of cumulative protons from deuteron the hadronization through the coalescence of three cumulative quarks is favorable and leads to the $(2-x)^5$ cross section threshold behavior whereas the usual hadronization through one cumulative quark fragmentation into proton the same as the calculations predicts for the deuteron structure function $F^d_2(x)$ at $x\to 2$ in DIS processes. The results of the calculations are compared with the available experimental data.

hep-ph↗
arXiv1997

On the odderon intercept in QCD

The odderon singularity is studied in perturbative QCD in the framework of the Bartels-Kwieciński-Praszalowicz (BKP) equation. Arguments for the odderon intercept being exactly equal to unity are given. Besides, a variational method based on a complete system of one-gluon functions is presented. For the odderon, the highest intercept calculated by this method is $1-(N_cα_{s}/π) 0.45$. Comparison to other calculations is shown.

hep-ph↗
arXiv1997

Probabilistic versus Field-Theoretical Description of Heavy Flavour Production off Nuclei

The absorptive corrections resulting from the rescattering of a heavy flavour (the so-called nuclear absorption) are usually calculated with a probabilistic formula valid only in the low energy limit. We extend this formula to all energies using a quantum field theoretical approach. For charmonium and bottonium we find that the absorptive corrections in the rigorous treatment are very similar to the ones in the probabilistic approach. On the contrary, at sufficiently high energy, open charm and bottom are absorbed as much as charmonium and bottonium - in spite of the fact that their absorptive cross-sections are zero, and therefore they are not absorbed in the probabilistic model. At high enough energies there are also absorptive corrections due to the shadowing of the nucleus structure function, which are present for all systems including Drell-Yan pair production. These shadowing corrections cancel in the low energy limit.

hep-ph↗
arXiv1997

Fusion of strings vs. percolation and the transition to the quark-gluon plasma

In most of the models of hadronic collisions the number of exchanged colour strings grows with energy and atomic numbers of the projectile and target. At high string densities interaction between them should melt them into the quark-gluon plasma state. It is shown that under certain assumptions about the the string interaction, a phase transition to the quark gluon plasma indeed takes place in the system of many colour strings. It may be of the first or second order (percolation), depending on the particular mechanism of the interaction. The critical string density is about unity in both cases. The critical density may have been already reached in central Pb-Pb collisions at 158 A GeV.

hep-ph↗
arXiv1996

Cumulative Phenomena through the Quark-Parton Diagram Summation at Thresholds

A microscopic treatment of cumulative phenomena based on perturbative QCD calculations of the corresponding quark diagrams near the threshold is presented. To sum all diagrams like these the special technique based on the recurrence relations was developed. The x-behaviour of the nuclear structure function $F_{2}(x)$ in the cumulative region x>1 was found to be roughly exponential, governed by an effective coupling constant, which depends on the QCD coupling constant and quark mass. Two mechanisms for cumulative particle production, direct and spectator ones, were analysed. It was shown that due to final state interactions the leading terms of the direct mechanism contribution are cancelled and the spectator mechanism is the dominant one. It leads to a smaller slope of the particle production rates compared to the slope of the nuclear structure function in the cumulative region, in agreement with the recent experimental data. The slope difference is due to additional multiple interactions between nuclear and projectile partons which enter the spectator mechanism for the cumulative production. The different versions of hadronization mechanisms of the produced cumulative quarks into hadrons are also discussed.

hep-ph↗
arXiv1996

Percolation approach to Quark Gluon Plasma and $J/ψ$ suppression

It is shown that the critical threshold for percolation of the overlapping strings exchanged in heavy ion collisions can naturally explain the sharp strong suppresion of $J/ψ$ shown by the experimental data on central Pb--Pb collisions, which does not occur in central O--U and S--U collisions.

hep-ph↗
arXiv1996

Diffraction dissociation, an important background to photon-photon collisions via heavy ion beams at LHC

The cross sections for the production of hadrons in quasi-real photon-photon collisions in proton-proton and heavy ion interactions are compared with the corresponding cross sections for central diffraction and for photon-pomeron collisions. The signatures, heavy ions or protons with only slightly changed momenta together with two large rapidity gaps and a cluster of produced hadrons in the central region, are nearly identical in all three processes. Therefore, it will be rather difficult to distinguish the reactions experimentally. It is found, that central diffraction is the dominant process in collisions of protons, light and medium-heavy ions. The photon-pomeron and photon-photon processes have quite similar cross sections in collisions of heavy ions like lead.

hep-ph↗
arXiv1996

Fusion of strings as a hadronic accelerator

It is shown that the fusion of strings is a source of particle production in nucleus--nucleus collisions outside the kinematical limits of nucleon--nucleon collisions. The spectrum of different particles is compared with the high energy data on p--A collisions obtaining a reasonable agreement. Results for A--B collisions at $\sqrt{s}=19.4$ AGeV and $\sqrt{s}=200$ AGeV are given. It is shown that the fusion of strings can accelerate particles up to the highest energy detected in cosmic rays without help of any additional cosmic accelerator. Also the rise of the average shower depth of maximun for cosmic rays in the energy range between $10^{16}$ eV and $10^{19}$ eV can be explained by the same mechanism without requiring any change in the chemical composition.

hep-ph↗
arXiv1996

The pomeron-pomeron interaction in the perturbative QCD

The pomeron-pomeron interaction is studied in the perturbative approach of BFKL-Bartels. The total pomeron-pomeron cross-section is proportional to $α_{s}^{4}s^Δ/\sqrt {t_{1}t_{2}}$ where $\sqrt{s}$ is the c.m. energy and $t_{1,2}$ are the virtualities of the colliding pomerons. Upon calculating the coefficient the cross-section is found to be of the order 2.2 mb at $\sqrt{s}= 6$ TeV and $\sqrt{-t_{1}}\sim\sqrt{-t_{2}}\sim 1$ GeV/c.

hep-ph↗
arXiv1996

On the triple pomeron contribution in the hard pomeron theory

Within the hard pomeron approach it is shown that the triple pomeron interaction gives no contribution to the cross-section for a projectile with large enough virtuality $g^{2}\ln Q^{2}\sim 1$. For small virtualities $g^{2}\ln Q^{2}<<1$ its contribution is essential.

hep-ph↗