arXiv subjects

Kai Hencken

Publications and source records attributed to Kai Hencken.

At least 19 recordsLinked to original sources

arXiv2009

Improved radiative corrections and proton charge form factor from the Rosenbluth separation technique

We investigate whether the apparent discrepancy between proton electric form factor from measurements using the Rosenbluth separation technique and polarization transfer method is due to the standard approximations employed in radiative correction procedures. Inaccuracies due to both the peaking approximation and the soft-photon approximation have been removed in our simulation approach. In contrast to results from (e,e'p) experiments, we find them in this case to be too small to explain the discrepancy.

hep-ph↗

arXiv2008

Improved radiative corrections for (e,e'p) experiments - A novel approach to multi-photon bremsstrahlung

Radiative processes lead to important corrections to (e,e'p) experiments. While radiative corrections can be calculated exactly in QED and to a good accuracy also including hadronic corrections, these corrections cannot be included into data analyses to arbitrary orders exactly. Nevertheless consideration of multi-photon bremsstrahlung above the low-energy cut-off is important for many (e,e'p) experiments. To date, higher-order bremsstrahlung effects concerning electron scattering experiments have been implemented approximately by employing the soft-photon approximation (SPA). In this paper we propose a novel approach to multi-photon emission which partially removes the SPA from (e,e'p) experiments. In this combined approach one hard photon is treated exactly; and additional softer bremsstrahlung photons are taken into account resorting to the soft-photon approximation. This partial removal of the soft-photon approximation is shown to be relevant for the missing-energy distribution for several kinematic settings at MAMI and TJNAF energies.

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arXiv2007

Electron-Positron Pair Production in Relativistic Heavy Ion Collisions

In recent years, a large number of papers have appeared that dealt with $\EPEM$ pair production in heavy ion collisions at high energies. To a large extent these studies were motivated by the existence of relativistic heavy ion accelerators all over the world. There pair production can be studied in so called ``ultra-peripheral collisions'', where the ions do not come close enough to interact strongly with each other. Various different methods have been used and it is the purpose of this review to present a unified picture of the present status of the field.

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arXiv2007

Improved radiative corrections for (e,e'p) experiments: Beyond the peaking approximation and implications of the soft-photon approximation

Analysing (e,e'p) experimental data involves corrections for radiative effects which change the interaction kinematics and which have to be carefully considered in order to obtain the desired accuracy. Missing momentum and energy due to bremsstrahlung have so far always been calculated using the peaking approximation which assumes that all bremsstrahlung is emitted in the direction of the radiating particle. In this article we introduce a full angular Monte Carlo simulation method which overcomes this approximation. The angular distribution of the bremsstrahlung photons is reconstructed from H(e,e'p) data. Its width is found to be underestimated by the peaking approximation and described much better by the approach developed in this work.

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arXiv2005

Transverse momentum distribution of vector mesons produced in ultraperipheral relativistic heavy ion collisions

We study the transverse momentum distribution of vector mesons produced in ultraperipheral relativistic heavy ion collisions (UPCs). In UPCs there is no strong interaction between the nuclei and the vector mesons are produced in photon-nucleus collisions where the (quasireal) photon is emitted from the other nucleus. Exchanging the role of both ions leads to interference effects. A detailed study of the transverse momentum distribution which is determined by the transverse momentum of the emitted photon, the production process on the target and the interference effect is done. We study the total unrestricted cross section and those, where an additional electromagnetic excitation of one or both of the ions takes place in addition to the vector meson production, in the latter case small impact parameters are emphasized.

hep-ph↗

arXiv2005

Ultraperipheral Collisions

Ultraperipheral collisions at heavy ion colliders use the strong Coulomb fields surrounding the ions to study photon-photon and photon-hadron processes at high energy. A number of processes of interest are discussed here.

hep-ph↗

arXiv2004

Coulomb corrections for quasielastic (e,e') scattering: eikonal approximation

We address the problem of including Coulomb distortion effects in inclusive quasielastic (e,e') reactions using the eikonal approximation. Our results indicate that Coulomb corrections may become large for heavy nuclei for certain kinematical regions. The issues of our model are presented in detail and the results are compared to calculations of the Ohio group, where Dirac wave functions were used both for electrons and nucleons. Our results are in good agreement with those obtained by exact calculations.

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arXiv2004

Production of QED pairs at small impact parameter in relativistic heavy ion collisions

The STAR collaboration at RHIC is measuring the production of electron-positron pairs at small impact parameters, larger than but already close to the range, where the ions interact strongly with each other. We calculate the total cross section, as well as, differential distributions of the pair production process with the electromagnetic excitation of both ions in a semiclassical approach and within a lowest order QED calculation. We compare the distribution of electron and positron with the one coming from the cross section calculation without restriction on impact parameter. Finally we give an outlook of possible results at the LHC.

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arXiv2004

A cluster version of the GGT sum rule

We discuss the derivation of a ``cluster sum rule'' from the Gellmann-Goldberger-Thirring (GGT) sum rule as an alternative to the Thomas-Reiche-Kuhn (TRK) sum rule, which was used as the basis up to now. We compare differences in the assumptions and approximations. Some applications of the sum rule for halo nuclei, as well as, nuclei with a pronounced cluster structure are discussed.

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arXiv2003

Multiphoton Exchange Processes in Ultraperipheral Relativistic Heavy Ion Collisions

The very strong electromagnetic fields present in ultraperipheral relativistic heavy ion collisions lead to important higher order effects of the electromagnetic interaction. These multiphoton exchange processes are studied using perturbation theory and the sudden or Glauber approximation. In many important cases, the multi-photon amplitudes factorize into independent single-photon amplitudes. These amplitudes have a common impact parameter vector, which induces correlations between the amplitudes. Impact-parameter dependent equivalent-photon spectra for simultaneous excitation are calculated, as well as, impact-parameter dependent gammagamma-luminosities. Excitations, like the multiphonon giant dipole resonances, vector meson production and multiple e+e- pair production can be treated analytically in a bosonic model, analogous to the emission of soft photons in QED.

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arXiv2003

Electromagnetic Dissociation as a Tool for Nuclear Structure and Astrophysics

Coulomb dissociation is an especially simple and important reaction mechanism. Since the perturbation due to the electric field of the (target) nucleus is exactly known, firm conclusions can be drawn from such measurements. Electromagnetic matrixelements and astrophysical S-factors for radiative capture processes can be extracted from experiments. We describe the basic elements of the theory of nonrelativistic and relativistic electromagnetic excitation with heavy ions. This is contrasted to electromagnetic excitation with leptons (electrons), with their small electric charge and the absence of strong interactions. We discuss various approaches to the study of higher order electromagnetic effects and how these effects depend on the basic parameters of the experiment. The dissociation of neutron halo nuclei is studied in a zero range model using analytical methods. We also review ways how to treat nuclear interactions, show their characteristics and how to avoid them (as far as possible). We review the experimental results from a theoretical point of view. Of special interest for nuclear structure physics is the appearence of low lying electric dipole strength in neutron rich nuclei. Applications of Coulomb dissociation to some selected radiative capture reactions relevant for nuclear astrophysics are discussed. The Coulomb dissociation of 8B is relevant for the solar neutrino problem. The potential of the method especially for future investigations of (medium) heavy exotic nuclei for nuclear structure and astrophysics is explored. We conclude that the Coulomb dissociation mechanism is theoretically well understood, the potential difficulties are identified and can be taken care of. Many interesting experiments have been done in this field and many more are expected in the future.

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arXiv2002

Study of Postacceleration Effects in the Coulomb Dissociation of Neutron Halo Nuclei

We investgate the breakup of one-neutron halo nuclei in the Coulomb field of a target nucleus. In the post-form distorted wave Born approximation theory of this reaction, with only Coulomb distortions in the entrance and the outgoing channels, an analytic solution for the breakup $T$-matrix is known. We study this $T$-matrix and the corresponding cross-sections numerically. This formula can be related to the first order semiclassical treatment of the electromagnetic dissociation. This theory contains the electromagnetic interaction between the core and the target nucleus to all orders. We show that higher order effects (including postacceleration) are small in the case of higher beam energies and forward scattering. We investigate the beam energy dependence of the postacceleration effects. They are found to be quite important for smaller beam energies (slow collisions), but almost negligible at larger ones.

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arXiv2001

Coherent electromagnetic processes in relativistic heavy ion collisions

Using the strong electromagnetic fields in peripheral heavy ion collisions gives rise to a number of interesting possibilities of applications in both photon-photon and photon-hadron physics. We look at the theoretical foundations of the equivalent photon approximation and the specific problems in the heavy ion case. The interesting physics processes that can be studied in this way are outlined. Electron positron pair production plays a special role. We look at multiple pair production and Coulomb corrections as typical strong field effects. But electron positron pair production is also an important loss process and has some practical applications.

hep-ph↗

arXiv2001

Coherent gamma-gamma and gamma-A interactions in very peripheral collisions at relativistic ion colliders

Due to coherence, there are strong electromagnetic fields of short duration in very peripheral collisions. They give rise to photon-photon and photon-nucleus collisions with a high flux up to an invariant mass region hitherto unexplored experimentally. After a general survey of the field equivalent photon numbers and photon-photon luminosities, especially for relativistic heavy ion collisions, are discussed. Special care needs to be taken to include the effects of the strong interaction and nuclear size in this case. Photon-photon and photon-hadron physics at various invariant mass scales are then discussed. The maximum equivalent photon energy in the lab-system (collider frame) are typically of the order of 3 GeV for RHIC and 100 GeV for LHC. Diffractive processes are an important background process. Lepton-pair, especially electron-positron pair production is copious. Due to the strong fields there will be new phenomena, like multiple e+e- pair production. The experimental techniques to select gamma-gamma-processes are finally discussed together with important background processes.

hep-ph↗

arXiv2001

A Realistic Solvable Model for the Coulomb Dissociation of Neutron Halo Nuclei

As a model of a neutron halo nucleus we consider a neutron bound to an inert core by a zero range force. We study the breakup of this simple nucleus in the Coulomb field of a target nucleus. In the post-form DWBA (or, in our simple model CWBA (``Coulomb Wave Born Approximation'')) an analytic solution for the T-matrix is known. We study limiting cases of this T-matrix. As it should be, we recover the Born approximation for weak Coulomb fields (i.e., for the relevant Coulomb parameters much smaller than 1). For strong Coulomb fields, high beam energies, and scattering to the forward region we find a result which is very similar to the Born result. It is only modified by a relative phase (close to 0) between the two terms and a prefactor (close to 1). A similar situation exists for bremsstrahlung emission. This formula can be related to the first order semiclassical treatment of the electromagnetic dissociation. Since our CWBA model contains the electromagnetic interaction between the core and the target nucleus to all orders, this means that higher order effects (including postacceleration effects) are small in the case of high beam energies and forward scattering. Our model also predicts a scaling behavior of the differential cross section, that is, different systems (with different binding energies, beam energies and scattering angles) show the same dependence on two variables x and y.

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arXiv2000

Electromagnetic Dissociation as a Tool for Nuclear Structure and Astrophysics

Coulomb dissociation is an especially simple and important reaction mechanism. Since the perturbation due to the electric field of the nucleus is exactly known, firm conclusions can be drawn from such measurements. Electromagnetic matrix elements and astrophysical S-factors for radiative capture processes can be extracted from experiments. We describe the basic theory, new results concerning higher order effects in the dissociation of neutron halo nuclei, and briefly review the experimental results obtained up to now. Some new applications of Coulomb dissociation for nuclear astrophysics and nuclear structure physics are discussed.

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arXiv2000

Bound-Free Electron-Positron Pair Production in Relativistic Heavy Ion Collisions

We study the bound-free electron-positron pair production in relativistic heavy ion collisions to different bound states in a full Plane Wave Born Approximation (PWBA) calculation. Exact Dirac wave functions are used for both the bound electron and the free positron in the final states. Results for the Relativistic Heavy Ion Collider RHIC as well as the forthcoming Large Hadron Collider (LHC) are given. This process is one of the dominant beam loss processes and can become critical for the operation with heavy ions. A simple parameterization is given as well. We compare our results with calculations of other groups.

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arXiv1999

Production of Low Mass Electron Pairs Due to the Photon-Photon Mechanism in Central Collisions

We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the gamma-gamma mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at RHIC are also given.

hep-ph↗