arXiv subjects

J. Huefner

Publications and source records attributed to J. Huefner.

17 recordsLinked to original sources

arXiv2002

Electroproduction of Charmonia off Protons and Nuclei

Elastic virtual photoproduction of charmonia on nucleons is calculated in a parameter free way with the light-cone dipole formalism and the same input: factorization in impact parameters, light-cone wave functions for the photons and the charmonia, and the universal phenomenological dipole cross section which is fitted to other data. The charmonium wave functions are calculated with four known realistic potentials, and two models for the dipole cross section are tested. Very good agreement with data for the cross section of charmonium electroproduction is found in a wide range of $s$ and $Q^2$. Using the ingredients from those calculations we calculate also exclusive electroproduction of charmonia off nuclei. Here new effects become important, (i) color filtering of the $c\bar c$ pair on its trajectory through nuclear matter, (ii) dependence on the finite lifetime of the $c\bar c$ fluctuation (coherence length) and (iii) gluon shadowing in a nucleus compared to the one in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as functions of $s$ are presented. The results can be tested with future electron-nucleus colliders or in the peripheral collisions of relativistic heavy ions.

hep-ph↗

arXiv2002

Electroproduction of Charmonia off Nuclei

In a recent publication we have calculated elastic charmonium production in ep collisions employing realistic charmonia wave functions and dipole cross sections and have found good agreement with the data in a wide range of s and Q^2. Using the ingredients from those calculations we calculate exclusive electroproduction of charmonia off nuclei. Here new effects become important, (i) color filtering of the c-cbar pair on its trajectory through nuclear matter, (ii) dependence on the finite lifetime of the c-cbar fluctuation (coherence length) and (iii) gluon shadowing in a nucleus compared to the one in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as functions of s and Q^2 are presented together with some differential cross sections. The results can be tested with future electron-nucleus colliders or in peripheral collisions of ultrarelativistic heavy ions.

hep-ph↗

arXiv2001

Scanning the Quark-Gluon Plasma with Charmonium

We suggest the variation of charmonium suppression with Feynman x_F in heavy ion collisions as a novel and sensitive probe for the properties of the matter created in such reactions. In contrast to the proton-nucleus case where nuclear suppression is weakest at small x_F, final state interactions with the comoving matter create a minimum at x_F=0, which is especially deep and narrow if a quark-gluon plasma is formed. While a particularly strong effect is predicted at SPS, at the higher RHIC energy it overlaps with the expected sharp variation with x_F of nuclear effects and needs comparison with proton-nucleus data. If thermal enhancement of J/Ψproduction takes over at the energies of RHIC and LHC, it will form an easily identified peak, rather than dip in x_F dependence. We predict a steep dependence on centrality and suggest that this new probe is complementary to the dependence on transverse energy, and is more sensitive to a scenario of final state interactions.

hep-ph↗

arXiv2001

Coherence Phenomena in Charmonium Production off Nuclei at the Energies of RHIC and LHC

In the energy range of RHIC and LHC the mechanisms of nuclear suppression of charmonia are expected to be strikingly different from what is known for the energy of the SPS. One cannot think any more of charmonium produced on a bound nucleon which then attenuates as it passes through the rest of the nucleus. The coherence length of charmonium production substantially exceeds the nuclear radius in the new energy range. Therefore the production amplitudes on different nucleons, rather than the cross sections, add up and interfere, i.e. shadowing is at work. So far no theoretical tool has been available to calculate nuclear effects for charmonium production in this energy regime. We develop a light-cone Green function formalism which incorporates the effects of the coherence of the production amplitudes and of charmonium wave function formation, and is the central result of this paper. We found a substantial deviation from QCD factorization, namely, gluon shadowing is much stronger for charmonium production than it is in DIS. We predict for nuclear effects $x_2$ scaling which is violated at lower energies by initial state energy loss which must be also included in order to compare with available data. In this paper only the indirect J/Psi originating from decay of P-wave charmonia are considered. The calculated x_F-dependence of J/Psi nuclear suppression is in a good accord with data. We predict a dramatic variation of nuclear suppression with x_F in pA and a peculiar peak at x_F=0 in AA collisions at RHIC.

hep-ph↗

arXiv2000

Nuclear Transparency in Heavy Ion Collisions at 14.6 GeV/nucleon

The probability of a projectile nucleon to traverse a target nucleus without interaction is calculated for central Si-Pb collisions and compared to the data of E814. The calculations are performed in two independent ways, via Glauber theory and using the transport code UrQMD. For central collisions Glauber predictions are about 30 to 50% higher than experiment, while the output of UrQMD does not show the experimental peak of beam rapidity particles.

nucl-th↗

arXiv2000

Photoproduction of Charmonia and Total Charmonium-Proton Cross Sections

Elastic virtual photoproduction cross sections gamma^*p -> J/psi(psi')p and total charmonium-nucleon cross sections for J/psi, psi' and chi states are calculated in a parameter free way with the light-cone dipole formalism and the same input: factorization in impact parameters, light-cone wave functions for the gamma^* and the charmonia, and the universal phenomenological dipole cross section which is fitted to other data. The charmonium wave functions are calculated with four known realistic potentials, and two models for the dipole cross section are tested. Very good agreement with data for the cross section of charmonium photoproduction is found in a wide range of s and Q^2. The inclusion of the Melosh spin rotation increases the psi' photoproduction rate by a factor 2-3 and removes previously observed discrepancies in the psi' to J/psi ratio in photoproduction. We also calculate the charmonium-proton cross sections whose absolute values and energy dependences are found to correlate strongly with the sizes of the states.

hep-ph↗

arXiv2000

Prompt Contributions to the Dilepton Yield in Heavy Ion Collisions

The mass spectrum is calculated for those dileptons which are produced in the early phase of a heavy ion collisions via the direct production NN -> (l^+)(l^-)X and via the Compton process GN -> (l^+)(l^-)X with prompt gluons radiated in preceding NN interactions. Both mechanisms produce a mass spectrum which decreases steeply with invariant mass of the l^+l^- pair and which is below the CERES data for Pb-Au collisions by about one order of magnitude.

nucl-th↗

arXiv1999

Nuclear Transparency and Effective NN Cross Section in Heavy Ion Collisions at 14.6 GeV/nucleon

The probability of a projectile nucleon to traverse a target nucleus without interaction is calculated for central Si-Cu and Si-Pb collisions. Special attention is given to the impact parameter range which contributes to events with large transverse energy. A fit to the data from E814 requires an effective NN cross section of sigma_eff=54.2+-5.0mb (compared to the free space value sigma_in=30mb) and is interpreted as one related to wounded target nucleons.

nucl-th↗

arXiv1999

J/Psi and Psi' total cross sections and formation times from data for charmonium suppression in $pA$ collisions

The recent data for E866 experiment on the x_F dependence for charmonium suppression in pA collisions at 800 GeV are analyzed using a time- and energy-dependent preformed charmonium absorption cross section σ_{abs}^ψ(τ,\sqrt{s}). For \sqrt{s}=10 GeV the initially (τ=0) produced premeson has an absorption cross section of σ_{pr}~3mb. At the same energy but for τ-> \infty one deduces for the total cross sections σ_{tot}^{J/Psi N}=(2.8\pm 0.3)mb, σ_{tot}^{J/Psi N}= (10.5\pm 3.6)mb. The date are compatible with a formation time τ_{1/2}=0.6 fm/c.

hep-ph↗

arXiv1999

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number of the prompt gluons are calculated perturbatively and the gluon-Psi inelastic cross section is estimated. The integrated cross sections for AB --> J/Psi (Psi')X for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter σ(gPsi).

hep-ph↗

arXiv1998

Charmonium Suppression in Heavy Ion Collisions by Prompt Gluons

In relativistic heavy ion collisions, also the bremsstrahlung of gluons in the fragmentation regions of the nuclei suppresses the produced charmonium states. In the energy range of the SPS, the radiation of semi-hard gluons occurs in the Bethe-Heitler regime and the density of gluons and therefore the suppression goes like $(AB)^{1\over3}$, where $A$ and $B$ are the nucleon numbers of the projectile and target nuclei. In contrast, the suppression via collisions with nucleons is proportional to $(A^{1\over3} + B^{1\over3})$. Parameter free perturbative QCD calculations are in a good agreement with the data on $J/Ψ$ and $Ψ'$ suppression in heavy ion collisions at SPS CERN. At higher energies (RHIC, LHC) the number of gluons which are able to break-up the charmonium substantially decreases and the additional suppression is expected to vanish.

hep-ph↗

arXiv1998

Spinodal and dynamical instabilities at the phase transition from the quark-gluon plasma to hadrons

The stability of an expanding parton plasma is analyzed within quasi-particle models. The effective mass of the parton is calculated self-consistently from a gap equation which is either obtained from the Nambu Jona-Lasinio Lagrangian or from a fit to observables calculated on the lattice. The latter shows an effective confinement. At thermal equilibrium the stability is studied within thermodynamics (mechanical stability) and via a linear response analysis of the Vlasov equation. The instabilities related to a first-order phase transition are found. For a plasma expanding in three and one dimensions far from equilibrium a new type of instability, called dynamical, appears. The relation to cluster formation is shown in a molecular dynamics calculation.

nucl-th↗

arXiv1996

Towards Study of Color Transparency with Medium Energy Electron Beams

Interference between vector mesons electroproduced at different longitudinal coordinates leads within Glauber approximation to a $Q^2$-dependence of nuclear effects similar to what is expected to be an onset of color transparency (CT). We suggest such a mapping of the photon energies and virtualities, which eliminates the undesirable $Q^2$-variation and allows to measure a net CT effect. We develop a multichannel evolution equation, which for the first time incorporates CT and the effects of the coherence length in the exclusive electroproduction of vector mesons.

nucl-th↗

arXiv1996

Inelastic $J/Ψ$ Photoproduction off Nuclei: Gluon Enhancement or Double Color Exchange?

The nuclear enhancement observed in inelastic photoproduction of $J/Ψ$ should not be interpreted as evidence for an increased gluon density in nuclei. The nuclear suppression of the production rate due to initial and final state interactions is calculated and a novel two-step color exchange process is proposed, which is able to explain the data.

nucl-th↗

arXiv1995

The Role of the Coherence Length in Exclusive Electroproduction of Vector Mesons off Nuclei

The variation of the coherence length $l_c=2ν/(Q^2+m_V^2)$ in virtual photoproduction of vector mesons off nuclei as a function of the photon energy $ν$ or virtuality $Q^2$ results in dramatic changes in the values of the nuclear transparency. Color transparency effects can be easily mixed up with the effects of the coherence length in incoherent photoproduction on nuclei.

hep-ph↗

arXiv1995

CHIRALLY INVARIANT TRANSPORT EQUATIONS FOR QUARK MATTER

Transport equations for quark matter are derived in the mean field approximation for the Nambu -- Jona-Lasinio Lagrangian. Emphasis is put on the chiral limit (zero current quark masses) and the consequences of the chiral symmetry are investigated in detail. Our approach is based on the spinor decomposition of the Wigner function. Kinetic equations for the quark densities and for the spin are derived. We find that the latter is constrained by the requirement of the axial current conservation.

hep-ph↗

arXiv1993

Colour Screening, Quark Propagation in Nuclear Matter and the Broadening of the Momentum Distribution of Drell-Yan Pairs

We calculate the broadening of the transverse momentum distribution of a quark propagating through nuclear matter. Colour screening plays a fundamental role in that it cuts off quark-nucleon interactions with soft gluons. The mean transverse momentum of the quark acquired along its trajectory, observed via Drell-Yan pairs, is related to it the ratio of the total inelastic meson-nucleon cross section it to the meson mean squared radius. Parameter-free calculations agree with the data.

hep-ph↗