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M. A. Braun

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

At least 109 records · Page 6Linked to original sources
arXiv2001

Cumulative particle production and percolation of strings

Calculations of the production rate of particles with $x>1$ in nuclear collisions due to the interaction of colour strings are presented. Momentum and colour sum rules are used to determine the fragmentation functions of fused strings. Mechanisms of the string interaction are considered with total and partial overlapping in the transverse plane. The results reveal strong dependence of the chosen mechanism. In the percolation scenario with partial overlapping the $x$-dependence of the production rates agrees well with the existing data. The magnitude of the rates for $π^+$ production is in agreement with experiment. However the rates for the protons are substantially below the data.

hep-ph↗
arXiv2001

Nuclear Structure Functions and Heavy Flavour Leptoproduction Off the Nucleus at Small x in Perturbative QCD

Nuclear structure functions and cross-sections for heavy flavour production in lepton-nucleus collisions are investigated in the low $x$ region accessible now or in the near future. The scattering on a heavy nucleus is described by the sum of fan diagrams of BFKL pomerons, which is exact in the high-colour limit. The initial condition for the evolution at $x=0.01$ is taken from a saturation model, which reproduces the experimental data on the proton. The $A$ dependence of the structure functions is well described by a power factor $A^α$, with $α$ reaching values as low as 1/2 at extremely low $x$. The total cross-sections for heavy flavour production reach values of the order of mb, and the corresponding transverse momentum distributions are sizeable up to transverse momenta larger than the initial large scale $\sqrt{Q^2+4m_f^2}$.

hep-ph↗
arXiv2001

Parton densities and dipole cross-sections at small x in large nuclei

Unintegrated gluon densities in nuclei, dipole-nucleus cross-sections and quark densities are numerically investigated in the high-colour limit, with the scattering on a heavy nucleus exactly described by the sum of fan diagrams of BFKL pomerons. The initial condition for the evolution in rapidity is quickly forgotten, and the gluon density presents a "supersaturation" pattern, as previous studies indicated. Both dipole-nucleus cross-sections and quark densities present the expected saturation features. Identifying the position in transverse momentum $l$ of the maximum of the gluon distribution with the saturation momentum $Q_s(y,b)$, at large rapidities all distributions depend only on one variable $[l/Q_s(y,b)]$ or $[rQ_s(y,b)]$.

hep-ph↗
arXiv2001

Diffractive eta_c photo-and electroproduction with the perturbative QCD Odderon

Using a set of new odderon states, we calculate their contribution to the diffractive eta_c photo- and electroproduction process. Compared to previous simple 3-gluon exchange calculations we find an enhancement of about one order of magnitude in the cross section. The t-dependence of the cross section exhibits a dip structure in the small t region.

hep-ph↗
arXiv2000

Final state interaction in inclusive and exclusive quasi-elastic processes

We discuss a new approach to final state interactions, that keeps explicitly into account the virtuality of the ejected nucleon in quasi-elastic $A(e,e'p)X$ scattering at very large $Q^2$, and we present some recent results, at moderately large $Q^2$ values, for the nuclear transparency in $^4He$, $^{16}O$ and $^{40}Ca$ and for the momentum distributions of $^4He$.

nucl-th↗
arXiv2000

Comments on parton saturation at small x in large nuclei

It is argued that the gluon density introduced by A.Mueller in relation to the saturation phenomenon is different from the standardly defined one, which turned out to have a soliton-like form in the numerical calculations. It is shown that A.Mueller's densities obtained from the latter possess the same saturation properties as originally predicted.

hep-ph↗
arXiv2000

On the forward-backward correlations in a two-stage scenario

It is demonstrated that in a two-stage scenario with elementary Poissonian emitters of particles (colour strings) arbitrarily distributed in their number and average multiplicities, the forward- backward correlations are completely determined by the final distribution of the forward particles. The observed linear form of the correlations then necessarily requires this distribution to have a negative binomial form. For emitters with a negative binomial distribution of the produced particles distributed so as to give the final distribution also of a negative binomial form, the forward-backward correlations have an essentially non-linear form, which disagrees with the experimental data.

hep-ph↗
arXiv2000

Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach

The forward-backward correlations in the $p_T$ distributions, which present a clear signature of non-linear effects in particle production, are studied in the model of percolating colour strings. Quantitative predictions are given for these correlations at SPS, RHIC and LHC energies. Interaction of strings also naturally explains the flattening of $p_T$ distributions and increase of $ $ with energy and atomic number for nuclear collisions

hep-ph↗
arXiv2000

Chaotic sources and Percolation of strings

It is shown that different ways of interacting strings formed in high energy nucleus-nucleus collisions cause a different strength of the chaoticity parameter lambda of Bose-Einstein correlations. In particular, in the case of percolation of strings, lambda shows a peculiar dependence on the string density, very similar to the dependence of the fractional average cluster size. In both, the derivative on the string density is maximum at the critical point. The reasonable agreement with the existing experimental data indicates that percolation of strings can actually occurs.

hep-ph↗
arXiv2000

Inclusive jet production on the nucleus in the perturbative QCD with $N_c\to\infty$

Using the recently found hA amplitude, single and double inclusive jet production rates on the nucleus are studied in the perturbative QCD with $N_c\to\infty$. Jet multiplicities are found to grow with $A$ and energy as $A^{2/9}s^Δ$ where $Δ$ is the BFKL intercept. Long range correlations are found to behave as $A^{4/9}s^{2Δ}$ and change from negative to positive at a certain high energy.

hep-ph↗
arXiv2000

Nucleus-nucleus scattering in perturbative QCD with $N_c\to\infty$

In the perturbative QCD with $N_c\to\infty$ equations for the amplitude of the nucleus-nucleus scattering are derived by the effective field method. The asymptotic form of the solution is discussed. It is argued that in the high-energy limit the total nucleus-nucleus cross-sections become constant and purely geometrical.

hep-ph↗
arXiv2000

Structure function of the nucleus in the perturbative QCD with $N_c\to\infty$ (BFKL pomeron fan diagrams)

Equation for the sum of BFKL pomeron fan diagrams is rederived by direct summation and solved numerically for rapidities $y\leq 50$. At high rapidities y>20 the resulting cross-sections for the scattering of a longitudinally polarized $q\bar q$ pair on the nucleus cease to depend on its transverse dimension and tend to a constant limit 0.1768 $R_A^2$, which corresponds to scattering of a colour dipole on a black disk. Thus the unitarity is restored and the singularity in the j plane is reduced to a simple pole at j=1.The nuclear structure function at small x behaves as $Q^2\ln(1/x)$. The found gluon density has a soliton-like form in the $\log k$ space: its form is close to Gaussian, independent of rapidity, the centermoving towards higher $\log k$ with a nearly constant velocity as rapidity increases.

hep-ph↗
arXiv1999

Inplication of percolation of colour strings on multiplicities, correlations and the transverse momentum

In the colour string model the impact of string percolation on multiplicities, their long-range correlations and average transverse momentum is studied. The multiplicities are shown to be damped by a simple factor which follows from the percolation theory. A clear signature of the phase transition is found to be a behaviour of the correlations for intensive observables, such as average transverse momentum, which can be detected in the high-energy heavy ion collisions.

hep-ph↗
arXiv1999

Cumulative particle production as a rare event

The generalization of the Glauber formula for cumulative production events is derived. On its basis the multiplicity distribution in such events is related to the one in the minimum bias events. As compared to the rare events of type $C$, the formula involves a shift in the arguments determined by the multiplicity from a collision with a cluster of several nucleons.

hep-ph↗
arXiv1999

Comments on the 2nd order bootstrap relation

The 2nd order bootstrap relation is discussed in view of the recent critics by F.Fadin, R.Fiore y A.Papa. It is shown that the strong bootstrap condition and the anzatz to solve it used in our earlier paper are valid at least for the quark part of the next-to-leading contribution.

hep-ph↗