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S. Leupold

Publications and source records attributed to S. Leupold.

32 records · Page 2Linked to original sources
arXiv2003

Ground state correlations in Deep Inelastic Scattering and the Drell-Yan process

Non-perturbative corrections to the LO cross section formulae for DIS and the Drell-Yan process were calculated. The interaction of partons was taken into account via dressing the incoming quark lines with spectral functions. We found the effect of ground state correlation in DIS to be large in the region of small Bjorken x and low momentum transfer Q. For a quark width of the order of 200 MeV, the deviation from parton model cross section reaches as much as 50% for Q^2=10 GeV^2. On the other hand, for the values of Q well above the resonance region, the effect of ISI turned out to be small in DIS, but still substantial for the triple differential Drell-Yan cross section. Our calculations show that the shape of the high mass Drell-Yan pair transverse momentum distribution is fully determined by quark off-shellness. From comparison to the resent data on the Drell-Yan cross section from NuSea collaboration, we obtained for the quark width in the nucleon the value about 200 MeV.

hep-ph↗
arXiv2002

Resonance lifetime in Boltzmann-Uehling-Uhlenbeck theory: observable consequences

Within the transport BUU theory we study the influence of the choice for the $Δ$ resonance lifetime on pion-nucleus reactions and on heavy-ion collisions at 1 A GeV. A quite small effect of these modifications on the pion absorption on nuclei is found provided that the absorption probability of the $Δ$ resonance is modified consistently. Observable effects in the case of heavy-ion collisions are demonstrated.

nucl-th↗
arXiv2001

Nuclear Matter Spectral Functions by Transport Theory

Quantum transport theory is used to calculate the nucleon spectral function in infinite nuclear matter. A self-consistent description is obtained by utilizing the relations between collision rates and correlation functions. Static and dynamical self-energies are taken into account in the single particle propagators. The real parts of the non-static self-energy contributions are calculated by dispersion theory thus conserving the analyticity of momentum distributions. The transport theoretical spectral functions, momentum distributions, occupation probabilities and responce functions are in close agreement with results of variational and other many-body theoretical calculations. The results indicate that the nucleon spectral functions are determined only by the average short-range correlation strength.

nucl-th↗
arXiv2001

Quenching of resonance production in heavy-ion collisions at 1-2 A GeV

We study the pion production in heavy-ion collisions at SIS energies within the coupled resonance BUU model. The standard calculation overpredicts the pion multiplicity in the central Au+Au collision at 1 A GeV by about a factor of 2, which is interpreted as a signature for the quenching of the resonance production/absorption in nucleon-nucleon collisions at high nuclear density. Results of the calculations taking into account a specific form of quenching are presented for pion spectra from light and heavy systems as well as for the pion in-plane and out-of-plane flow.

nucl-th↗
arXiv2001

Nuclear shadowing at low photon energies

We calculate the shadowing effect in nuclear photoabsorption at low photon energies (1-3 GeV) within a multiple scattering approach. We avoid some of the high energy approximations that are usually made in simple Glauber theory like the narrow width and the eikonal approximation. We find that the main contribution to nuclear shadowing at low energies stems from $ρ^0$ mesons with masses well below their pole mass. We also show that the possibility of scattering in non forward directions allows for a new contribution to shadowing at low energies: the production of neutral pions as intermediate hadronic states enhances the shadowing effect in the onset region. For light nuclei and small photon energies they give rise to about 30% of the total shadowing effect.

nucl-th↗
arXiv2001

The $ρ$ Spectral Function in a Relativistic Resonance Model

We calculate the spectral function $A_ρ$ of the $ρ$ meson in nuclear matter. The calculation is performed in the {\it low density} approximation, where the in-medium self energy $Σ_{med}$ is completely determined by the vacuum $ρN$ forward scattering amplitude. This amplitude is derived from a relativistic resonance model. In comparison to previous non-relativistic calculations we find a much weaker momentum dependence of $Σ_{med}$, especially in the transverse channel. Special attention is paid to uncertainties in the model. Thus, we compare the impact of different coupling schemes for the $RNρ$ interaction on the results and discuss various resonance parameter sets.

nucl-th↗
arXiv2000

Transport Theoretical Approach to the Nucleon Spectral Function in Nuclear Matter

The nucleon spectral function in infinite nuclear matter is calculated in a quantum transport theoretical approach. Exploiting the known relation between collision rates and correlation functions the spectral function is derived self-consistently. By re-inserting the spectral functions into the collision integrals the description of hard processes from the high-momentum components of wave functions and interactions is improved iteratively until convergence is achieved. The momentum and energy distributions and the nuclear matter occupation probabilities are in very good agreement with the results obtained from many-body theory.

nucl-th↗
arXiv2000

Low energy onset of nuclear shadowing in photoabsorption

The early onset of nuclear shadowing in photoabsorption at low photon energies has recently been interpreted as a possible signature of a decrease of the rho meson mass in nuclei. We show that one can understand this early onset within simple Glauber theory if one takes the negative real part of the rho N scattering amplitudes into account, corresponding to a higher effective mass of the rho meson in nuclear medium.

nucl-th↗
arXiv1999

Langevin Interpretation of Kadanoff-Baym Equations

We show that the nonperturbative quantum transport equations, the `Kadanoff-Baym equations', can be understood as the ensemble average over stochastic equations of Langevin type. For this we couple a free scalar boson quantum field to an environmental heat bath with some given temperature T. The inherent presence of noise and dissipation related by the fluctuation-dissipation-theorem guarantees that the modes or particles become thermally populated on average in the long-time limit. This interpretation leads to a more intuitive physical picture of the process of thermalization and of the interpretation of the Kadanoff-Baym equations.

hep-ph↗
arXiv1999

Neutron stars and quark phases in the NJL model

We study the possible existence of deconfined quark matter in the interior of neutron stars using the Nambu-Jona-Lasinio model to describe the quark phase. We find that typical neutron stars with masses around 1.4 solar masses do not possess any deconfined quark matter in their center. This can be traced back to the property of the NJL model which suggests a large constituent strange quark mass over a wide range of densities.

astro-ph↗
arXiv1997

The Spectral Function of the Rho Meson in Nuclear Matter

We calculate the modification of a rho meson in nuclear matter through its coupling to resonance-hole states. Starting from a recently proposed model, we include all four star resonances up to 1.9 GeV. In contrast to previous works, we include not only resonances that couple to the rho in a relative p-wave, but also those that couple to an s-wave state. In addition, we solve the equation for the rho spectral function self-consistently. We find that s-wave resonances affect the in medium spectral function of the rho strongly. In the transverse channel the rho meson is, especially at non zero momentum, completely washed out and can in the presence of nuclear matter no longer be viewed as a resonant excitation of the vacuum. Instead, our model shows a continuum of possible excitations with the quantum numbers of a transversely polarized rho. In the longitudinal channel, however, the rho retains its resonant character in our calculation. As a consequence of the self-consistent treatment we also find a strong enhancement of the widths of the included nucleon resonances in medium.

nucl-th↗
arXiv1997

Convergence properties of the equal-time connected Green function approach for temporal gauge SU(2)_{2+1} Yang-Mills theory

The hierarchy of equations of motion for equal-time Green functions in temporal gauge SU(N) Yang-Mills theory is truncated using an expansion in terms of connected Green functions. A second hierarchy of constraint equations arises from Gauss law and can be truncated in a similar way. Within this approximation scheme we investigate SU(2) Yang-Mills theory on a torus in 2+1 spacetime dimensions in a finite basis of plane wave states and focus on infrared and ultraviolet properties of the approach. We study the consequences of restoring the hierarchy of Gauss law constraints and of different momentum cutoffs for the 2- and the 3-point functions. In all truncation schemes considered up to the 4-point level the connected Green function approach is found to be UV divergent and either violating gauge invariance and/or energy conservation. The problems associated with adiabatically generating a perturbed ground state are discussed as well.

hep-th↗
arXiv1996

Thermalization and Lyapunov Exponents in the Yang-Mills-Higgs Theory

We investigate thermalization processes occurring at different time scales in the Yang-Mills-Higgs system at high temperatures. We determine the largest Lyapunov exponent associated with the gauge fields and verify its relation to the perturbatively calculated damping rate of a static gauge boson.

hep-th↗