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M. A. Braun

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

At least 91 records · Page 5Linked to original sources
arXiv2004

Jet production in pA and AA collisions in the perturbative QCD pomeron model

Inclusive cross-sections for gluon jet production are studied numerically in the perturbative QCD pomeron model for pA and central AA collisions at high energies. Two forms for the inclusive cross-sections, with and without emission from the triple pomeron vertex, are compared. The difference was found to reduce to a numerical factor 0.7-0.8 for momenta below the saturation momentum Q_s. Above Q_s no difference was found at all. For pA collisions the jet spectrum was found to be \sim A^{0.7} at momenta below Q_s and \sim A^{0.9} above it. For central AA collisions it was found to be \sim A below Q_s and \sim A^{1.1} above it. At large momenta k the spectrum goes like 1/k^{2.7 - 3.3} flattenung with energy.

hep-ph↗
arXiv2004

Inclusive cross-sections for jet production in nucleus-nucleus collisions in perturbative QCD

Inclusive cross-sections for gluon jet production are studied numerically in the perturbative QCD pomeron model for central collisions of identical nuclei at high energies. Two forms for the inclusive cross-sections, with and without emission from the triple pomeron vertex, are compared. The difference was found to reduce to a numerical factor 0.8$÷$ 09 for momenta $k$ below the saturation momentum $Q_s$. Above $Q_s$ no difference was found at all. The gluon spectrum was found to be $\sim A$ for $k Q_s$. At large $k$ the spectrum falls as $1/k^{2.7÷3.3}$ flattening with energy. The multiplicities are proportional to $A$ with a good precision. Their values are high amd grow rapidly with energy.

hep-ph↗
arXiv2004

Colour strings vs. hard pomeron in perturbative QCD

Average multiplicities and transverse momenta in AA collisions are studied in the soft and hard regions, in fusing string and perturbative QCD scenarios respectively. Striking similarities are found between the predictions of the two approaches. Multiplicities per string and average p_t^2 are found to, respectively, drop and rise with A in a very similar manner, so that their product is nearly a constant. In both approaches total multiplicities grow as A, that is as the number of participants. The high tail of the p_t distribution is found to behave as A^1.1 in the perturbative QCD scenario.

hep-ph↗
arXiv2003

Rescattering and finite formation time effects in electro-disintegration of the deuteron in the cumulative region

The role of rescattering due to the final state interaction (FSI) and the influence of the finite formation time (FFT) on the inclusive $D(e,e')X$ and exclusive $D(e,e'p)n$ electro-disintegration of the deuteron are studied in the cumulative kinematical region $x>1$ and moderate values of the 4-momentum transfer $Q^2=2÷10$ (GeV/c)$^2$. The spins are averaged out. It is found that in the inclusive process the relative magnitude of rescattering steadily grows with $x$ and that at $x=1.7$ it has the same order as the plane wave impulse approximation (PWIA) contribution, with the finite formation time effects decreasing the rescattering contribution by $\sim 30$%. In the exclusive process, with increasing momentum transfer, FFT substantially reduces the effects from FSI, although the latter are still appreciable in the region of momentum transfer investigated.

nucl-th↗
arXiv2003

Nucleus-nucleus interaction in the perurbative QCD

Nucleus-nucleus interaction is studied in the framework of the perturbative QCD with large number of colours and a fixed coupling constant. The pomeron tree diagrams are summed by an effective field theory. The classical field equations are solved by iteration procedure, which is found convergent in a restricted domain of not too high energies and atomic numbers. The found gluon distributions do not scale, have their maxima close to 2 GeV/c independent of rapidity and fall towards the central rapidity region. The cross-sections slowly grow with energy due to the contribution from peripheral collisions, where evolution remains linear. Simple variational estimates at higher rapidities confirm this tendency.

hep-ph↗
arXiv2003

Subthreshold photoproduction of charm

Charm photoproduction rates off nuclei below the nucleon threshold are estimated using the phenomenologically known structure functions both for x>1 and x<1. The rates rapidly fall below the threshold from values of the order 10 pb for Pb close to the threshold (at 7.5 GeV) down to values of the order 1 pb at 6 GeV.

hep-ph↗
arXiv2003

Pomeron fan diagrams with an infrared cutoff and running coupling

By direct numerical calculations the influence of a physically relevant infrared cutoff and running coupling on the gluon density and structure function of a large nucleus is studied in the perturbative QCD approach. It is found that the infrared cutoff changes the solutions very little. Running of the coupling produces a bigger change, considerably lowering both the saturation momentum and values of the structure functions.

hep-ph↗
arXiv2003

Finite Formation Time in Electro-Disintegration of Few-Body Nuclei

Inclusive and exclusive electro-disintegration of 2-H, 3-He and 4-He are calculated within an approach where, besides the final state interaction (FSI), also the finite formation time (FFT) of the hit hadron to its asymptotic physical state is taken into account.

nucl-th↗
arXiv2003

Leptoproduction of nucleons in the cumulative region

Leptoproduction of nucleons into the backward hemisphere on nuclear targets is studied at relativistic subasymptotic energies and momenta. The relativistic internucleon potential is extracted from appropriate photoproduction data. Different production mechanisms are found to work together and interfere. Whenever rescattering is kinematically possible, it gives the bulk of the contribution, except at very high Q^2. Comparison with the existing data at E=2.4 GeV shows a reasonable agreement.

hep-ph↗
arXiv2003

Correlations between multiplicities and average transverse momentum in the percolating colour strings approach

Long range correlations multiplicity-multiplicity, $p_T^2$-multiplicity and $p^2 - p^2_T$ are studied in the percolating colour string picture under different assumptions of the dynamics of string interaction. It is found that the strength of these correlations is rather insensitive to these assumptions nor to the geometry of formed fused string clusters. Both multiplicity-multiplicity and $p_T^2$-multiplicity correlations are found to scale and depend only on the string density. The $p_T^2$-multiplicity correlations, which are absent in the independent string picture, are found to be of the order of 10% for central heavy ion collisions and can serve as a clear signature of string fusion. In contrast the $p^2_T - p^2_T$ correlations turned out to be inversely proportional to the number of strings and so very small for relaistic collisions.

hep-ph↗
arXiv2003

The Process gamma* + p -> eta_c + X: A Test for the Perturbative QCD Odderon

The rates of inclusive photo- and electroproduction of the eta_c meson: gamma* + p -> eta_c + X are calculated in the triple Regge region, integrated over the diffractive mass X. For the Regge exchanges we use the hard Pomeron and Odderon, both being calculated in the framework of perturbative QCD. The integrated cross-section depends upon the coupling of the BFKL pomeron to two $C=-1$ odderons, and it is found to be of the order 60 pb for photoproduction and 1.5 pb at Q^2=25 GeV^2.

hep-ph↗
arXiv2003

Rescattering and finite formation time effects in inclusive and exclusive electro-disintegration of nuclei within a relativistic approach: 1. The deuteron

The rescattering contribution to the inclusive and exclusive deuteron electro-disintegration at the values of the Bjorken scaling variable $x=1$, as well as in the so called {\it cumulative} region ($x>1$) is calculated within a relativistic approach based on the Feynman diagram formalism taking into account colour transparency effects by the inclusion of the {\it finite formation time} (FFT) of the ejected nucleon via the introduction of the dependence of the scattering amplitude of the ejectile upon its virtuality. In the cumulative region the FFT effects which result from the real part of the ejectile propagator are taken into account. It is found that the relative weight of the rescattering steadily grows with $x$ becoming of the order of unity at $x>1.4÷1.5$. At such values of $x$ the finite formation time effects become fairly visible, which may serve for their study at relatively small value of the four-momentum transfer$Q^2$. The relativistic rescattering contribution is compared with the Glauber rescattering, which is shown to be not valid in the cumulative region starting from $x>1.4$.

nucl-th↗
arXiv2002

Estimates of the subthreshold production of charm

Charm photoproduction rates off nuclei below the nucleon threshold are estimated using the phenomenologically known structure functions both for $x<1$ and $x>1$. The rates rapidly fall from values of the order 1 nb at the threshold to values of the order 1 pb at 6 GeV for Pb target.

hep-ph↗
arXiv2002

Cumulative structure function in terms of nucleonic wave function of the nucleus

The structure function of the nucleus in the cumulative region $x>1$ is studied in terms of nucleon degrees of freedom. At high $Q^2$ the resulting expressions are presented as a sum of contributions from few-nucleon correlations. Two-nucleon correlations are studied in some detail. Spin variables are averaged out. In the region $1<x<2$ the structure functions are calculated for the relativistic interaction proposed by F.Gross {\it et al}. They are found to fall with $x$ faster than the exponential. For Carbon at $x=1.05$, where the method is not rigorously applicable, they turn out to be rougly twice larger than the experimental data.

nucl-th↗
arXiv2002

Centrality dependence of the multiplicity and transverse momentum distributions at RHIC and LHC and the percolation of strings

The dependence of the multiplicity and the transverse momentum distribution on the number of collisions are studied for central and peripheral Au-Au collisions at SPS, RHIC and LHC energies in the framework of percolation of strings. A scaling law relating the multiplicity to the mean transverse momentum is obtained. Our results are in overall agreement with the SPS and RHIC data, obtaining a suppression on $p_T$ distribution even for $p_T$ larger than 1 GeV/c.

hep-ph↗
arXiv2002

On the jet spectrum in nucleus-nucleus interactions

We derive the inclusive transverse spectrum of minijets in nuclear collisions at very high energies by assuming that the nuclear S-matrix factorizes into a product of elastic S-matrices for elementary partonic collisions. Interference effects and, in particular, the contribution of loop diagrams are fully taken into account in the derivation of the spectrum, which is shown to coincide with the result earlier obtained by superposing the elementary interactions incoherently. A quantitative analysis confirms that the deformation induced by multiple collsions is a large effect at RHIC and LHC energies, for transverse momenta ~ 20 GeV.

hep-ph↗
arXiv2002

Genuine Three-Body Bose-Einstein Correlations and Percolation of Strings

Recent data show a large difference of the genuine three-body Bose-Einstein correlations in S-Pb collisions and in Pb-Pb central collisions being close to zero in the first case and to one in the second one. These results, unexpected from conventional approaches, are naturally explained by the percolation of colour strings produced in the collisions and subsequent incoherent fragmentation of the formed clusters.

hep-ph↗