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We study central collision of Pb + Pb at 20, 40, 80 and 160 A·GeV within the UrQMD transport approach and compare rapidity distributions of ,K+,K and with the recent measurements from the NA49 Collaboration at 40, 80 and 160 A·GeV. It is found that the UrQMD model reasonably describes the data, however, systematically overpredicts the yield by < 20%, whereas the K+ yield is underestimated by < 15%. The K yields are in a good agreement with the experimental data, the yields are also in a reasonable correspondence with the data for all energies. We find that hadronic flavour exchange reactions largely distort the information about the initial strangeness production mechanism at all energies considered. PACS: 25.75.+r
We discuss the possibility of producing a new kind of nuclear system by putting a few antibaryons inside ordinary nuclei. The structure of such systems is calculated within the relativistic mean field model assuming that the nucleon and antinucleon potentials are related by the G parity transformation. The presence of antinucleons leads to decreasing vector potential and increasing scalar potential for the nucleons. As a result, a strongly bound system of high density is formed. Due to the significant reduction of the available phase space the annihilation probability might be strongly suppressed in such systems.
The recently published experimental dependence of the J/psi suppression pattern in Pb+Pb collisions at the CERN SPS on the energy of zero degree calorimeter EZDC are analyzed. It is found that the data obtained within the minimum bias analysis (using theoretical Drell-Yan ) are at variance with the previously published experimental dependence of the same quantity on the transversal energy of neutral hadrons ET . The discrepancy is related to the moderate centrality region: 100 << Np << 200 (Np is the number of nucleon participants). This could result from systematic experimental errors in the minimum bias sample. A possible source of the errors may be contamination of the minimum bias sample by o -target interactions. The data obtained within the standard analysis (using measured Drell-Yan multiplicity) are found to be much less sensitive to the contamination.
The measured particle ratios in central heavy-ion collisions at RHIC-BNL are investigated within a chemical and thermal equilibrium chiral SU(3) theta - omega approach. The commonly adopted noninteracting gas calculations yield temperatures close to or above the critical temperature for the chiral phase transition, but without taking into account any interactions. Contrary, the chiral SU(3) model predicts temperature and density dependent e ective hadron masses and e ective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. Three di erent parametrizations of the model, which show di erent types of phase transition behaviour, are investigated. We show that if a chiral phase transition occured in those collisions, freezing of the relative hadron abundances in the symmetric phase is excluded by the data. Therefore, either very rapid chemical equilibration must occur in the broken phase, or the measured hadron ratios are the outcome of the dynamical symmetry breaking. Furthermore, the extracted chemical freeze-out parameters di er considerably from those obtained in simple noninteracting gas calculations. In particular, the three models yield up to 35 MeV lower temperatures than the free gas approximation. The in-medium masses turn out di er up to 150 MeV from their vacuum values.
We calculate p, ±,K± and (+ 0) rapidity distributions and compare to experimental data from SIS to SPS energies within the UrQMD and HSD transport approaches that are both based on string, quark, diquark (q, ¯q, qq, ¯q ¯q) and hadronic degrees of freedom. The two transport models do not include any explicit phase transition to a quark-gluon plasma (QGP). It is found that both approaches agree rather well with each other and with the experimental rapidity distributions for protons, s, ± and K±. In- spite of this apparent agreement both transport models fail to reproduce the maximum in the excitation function for the ratio K+/ + found experimen- tally between 11 and 40 A·GeV. A comparison to the various experimental data shows that this failure is dominantly due to an insu cient description of pion rapidity distributions rather than missing strangeness . The modest di erences in the transport model results on the other hand can be attributed to di erent implementations of string formation and frag- mentation, that are not su ciently controlled by experimental data for the elementary reactions in vacuum.
A canonical partition function for the two-component excluded volume model is derived, leading to two di erent van der Waals approximations. The one is known as the Lorentz-Berthelot mixture and the other has been proposed recently. Both models are analysed in the canonical and grand canonical ensemble. In comparison with the one-component van der Waals excluded volume model the suppression of particle densities is reduced in these two-component formulations, but in two essentially di erent ways. Presently used multi-component models have no such reduction. They are shown to be not correct when used for components with di erent hard-core radii. For high temperatures the excluded volume interaction is refined by accounting for the Lorentz contraction of the spherical excluded volumes, which leads to a distinct enhancement of lighter particles. The resulting e ects on pion yield ratios are studied for AGS and SPS data.
The amount of proton stopping in central Pb+Pb collisions from 20–160 A GeV as well as hyperon and antihyperon rapidity distributions are calculated within the UrQMD model in comparison to experimental data at 40, 80, and 160 A GeV taken recently from the NA49 collaboration. Furthermore, the amount of baryon stopping at 160A GeV for Pb+Pb collisions is studied as a function of centrality in comparison to the NA49 data. We find that the strange baryon yield is reasonably described for central collisions, however, the rapidity distributions are somewhat more narrow than the data. Moreover, the experimental antihyperon rapidity distributions at 40, 80, and 160 A GeV are underestimated by up to factors of 3—depending on the annihilation cross section employed—which might be addressed to missing multimeson fusion channels in the UrQMD model. Pacs-Nr.: 25.75.2q, 24.10.Jv, 24.10.Lx
The lightest supersymmetric particle, most likely the lightest neutralino, is one of the most prominent particle candidates for cold dark matter (CDM). We show that the primordial spectrum of density fluctuations in neutralino CDM has a sharp cut-off, induced by two different damping mechanisms. During the kinetic decoupling of neutralinos, non-equilibrium processes constitute viscosity effects, which damp or even absorb density perturbations in CDM. After the last scattering of neutralinos, free streaming induces neutralino flows from overdense to underdense regions of space. Both damping mechanisms together define a minimal mass scale for perturbations in neutralino CDM, before the inhomogeneities enter the non- linear epoch of structure formation. We find that the very first gravitationally bound neutralino clouds ought to have masses above 10-6M , which is six orders of magnitude above the mass of possible axion miniclusters.
We calculate open charm and charmonium production in Au + Au reac- tions at ps = 200 GeV within the hadron-string dynamics (HSD) transport approach employing open charm cross sections from pN and N reactions that are fitted to results from PYTHIA and scaled in magnitude to the available experimental data. Charmonium dissociation with nucleons and formed mesons to open charm (D + ¯D pairs) is included dynamically. The comover dissociation cross sections are described by a simple phase-space model including a single free parameter, i.e. an interaction strength M2 0 , that is fitted to the J/ suppression data for Pb + Pb collisions at SPS energies. As a novel feature we implement the backward channels for char- monium reproduction by D ¯D channels employing detailed balance. From our dynamical calculations we find that the charmonium recreation is com- parable to the dissociation by comoving mesons. This leads to the final result that the total J/ suppression at ps = 200 GeV as a function of centrality is slightly less than the suppression seen at SPS energies by the NA50 Collaboration, where the comover dissociation is substantial and the backward channels play no role. Furthermore, even in case that all di- rectly produced J/ mesons dissociate immediately (or are not formed as a mesonic state), a sizeable amount of charmonia is found asymptotically due to the D + ! J/ + meson channels in central collisions of Au + Au at ps = 200 GeV which, however, is lower than the J/ yield expected from f pp collis ns.
A significant drop of the vector meson masses in nuclear matter is observed in a chiral SU(3) model due to the e ects of the baryon Dirac sea. This is taken into account through the summation of baryonic tadpole diagrams in the relativistic Hartree approximation. The appreciable decrease of the in-medium vector meson masses is due to the vacuum polarisation e ects from the nucleon sector and is not observed in the mean field approximation.
We introduce a model for the real-time evolution of a relativistic fluid of quarks coupled to non-equilibrium dynamics of the long wavelength (classical) modes of the chiral condensate. We solve the equations of motion numerically in 3+1 spacetime dimensions. Starting the evolution at high temperature in the symmetric phase, we study dynamical trajectories that either cross the line of first-order phase transitions or evolve through its critical endpoint. For those cases, we predict the behavior of the azimuthal momentum asymmetry for highenergy heavy-ion collisions at nonzero impact parameter.
The J/psi yield at midrapidity at the top RHIC (relativistic heavy ion collider) energy is calculated within the statistical coalescence model, which assumes charmonium formation at the late stage of the reaction from the charm quarks and antiquarks created earlier in hard parton collisions. The results are compared to the new PHENIX data and to predictions of the standard models, which assume formation of charmonia exclusively at the initial stage of the reaction and their subsequent suppression. Two versions of the suppression scenario are considered. One of them assumes gradual charmonium suppression by comovers, while the other one supposes that the suppression sets in abruptly due to quark-gluon plasma formation. Surprisingly, both versions give very similar results. In contrast, the statistical coalescence model predicts a few times larger J/psi yield in the most central collisions.
The SENECA model, a new hybrid approach to air shower simulations, is presented. It combines the use of efficient cascade equations in the energy range where a shower can be treated as one-dimensional, with a traditional Monte Carlo method which traces individual particles. This allows one to reproduce natural fluctuations of individual showers as well as the lateral spread of low energy particles. The model is quite efficient in computation time. As an application of the new approach, the influence of the low energy hadronic models on shower properties for AUGER energies is studied. We conclude that these models have a significant impact on the tails of lateral distribution functions, and deserve therefore more attention.
Accelerating cavities exchange HOM power through interconnecting beam pipes in case of signal frequencies above the cut-off of their propagating waveguide modes. This may lead either to improved HOM damping or - in the case most severe - to unwanted phase coherence of fields to the beam. Therefore the knowledge of the scattering properties of a cavity as a line element is needed to analyse all kinds of RF cavity-cavity interaction. Since there is a lack of measurement tools capable to provide a multidimensional scattering matrix at a given frequency point, we have been developing a method for this purpose. It uses a set of 2-port S-parameters of the device under test, embedded in a number of geometrically different RF environments. The application of the method is demonstrated with copper models of TESLA cavities.
A new method for the determination of S-matrices of devices in multimoded waveguides and first experimental experiences are presented. The theoretical foundations are given. The scattering matrix of a TESLA copper cavity at a frequency above the cut-off of the second waveguide mode has been measured.
The influence of high and low energy hadronic models on lateral distribution functions of cosmic ray air showers for Auger energies is explored. A large variety of presently used high and low energy hadron interaction models are analysed and the resulting lateral distribution functions are compared. We show that the slope depends on both the high and low energy hadronic model used. The models are confronted with available hadron-nucleus data from accelerator experiments.
We discuss the new data for the production of the psi meson in pA collisions at 450 GeV at CERNSPS (of the NA50-collaboration) [1]. We extract from the CERN data sigma(psi'N) 8 mb under the assumption that the psi is produced as a result of the space-time evolution of a point-like c¯c pair which expands with time to the full size of the charmonium state. In the analysis we assume the existence of a relationship between the distribution of color in a hadron and the cross section of its interaction with a nucleon. However, our result is rather sensitive to the pattern of the expansion of the wave packet and significantly larger values of sigma(psi'N)are not ruled out by the data. We show that recent CERN data confirm the suggestion of ref. [2] that color fluctuations of the strengths in charmonium-nucleon interaction are the major source of suppression of the J/psi yield as observed at CERN in both pA and AA collisions.
We evaluate the in-medium D and -meson masses in hot hadronic matter induced by interactions with the light hadron sector described in a chiral SU(3) model. The e ective Lagrangian approach is generalized to SU(4) to include charmed mesons. We find that the D-mass drops substantially at finite temperatures and densities, which open the channels of the decay of the charmonium states ( 2, c, J/ ) to D pairs in the thermal medium. The e ects of vacuum polarisations from the baryon sector on the medium modification of the D-meson mass relative to those obtained in the mean field approximation are investigated. The results of the present work are compared to calculations based on the QCD sum-rule approach, the quark-meson coupling model, chiral perturbation theory, as well as to studies of quarkonium dissociation using heavy quark potential from lattice QCD.
Abstract: The e ect of vacuum fluctuations on the in-medium hadronic properties is investigated using a chiral SU(3) model in the nonlinear realization. The e ect of the baryon Dirac sea is seen to modify hadronic properties and in contrast to a calculation in mean field approximation it is seen to give rise to a significant drop of the vector meson masses in hot and dense matter. This e ect is taken into account through the summation of baryonic tadpole diagrams in the relativistic Hartree approximation (RHA), where the baryon self energy is modified due to interactions with both the non-strange ( ) and the strange ( ) scalar fields.
The N/Z ratio of free nucleons from collisions of neutron-rich nuclei as a function of their momentum is studied by means of Isospin dependent Quantum Molecular Dynamics. We find that this ratio is not only sensitive to the form of the density dependence of the symmetry potential energy but also its strength determined by the symmetry energy coe cient. The uncertainties about the symmetry energy coe cient influence the accuracy of probing the density dependence of the symmetry energy by means of the N/Z ratio of free nucleons of neutron-rich nuclei.
Dynamics of strange, charm and high momentum hadrons in relativistic nucleus nucleus collisions
(2003)
We investigate hadron production and attenuation of hadrons with strange and charm quarks (or antiquarks) as well as high transverse momentum hadrons in relativistic nucleus-nucleus col- lisions from 2 A·GeV to 21.3 A·TeV within two independent transport approaches (UrQMD and HSD). Both transport models are based on quark, diquark, string and hadronic degrees of freedom, but do not include any explicit phase transition to a quark-gluon plasma. From our dynamical calculations we find that both models do not describe the maximum in the K+/ + ratio at 20 - 30 A·GeV in central Au+Au collisions found experimentally, though the excitation functions of strange mesons are reproduced well in HSD and UrQMD. Furthermore, the transport calculations show that the charmonium recreation by D + J/ + meson reactions is comparable to the dissociation by comoving mesons at RHIC energies contrary to SPS energies. This leads to the final result that the total J/ suppression as a function of centrality at RHIC should be less than the suppression seen at SPS energies where the comover dissociation is substantial and the backward channels play no role. Furthermore, our transport calculations in comparison to exper- imental data on transverse momentum spectra from pp, d+Au and Au+Au reactions show that pre-hadronic e ects are responsible for both the hardening of the hadron spectra for low transverse momenta (Cronin e ect) as well as the suppression of high pT hadrons. The mutual interactions of formed hadrons are found to be negligible in central Au+Au collisions at s = 200 GeV for pT e 6 GeV/c and the sizeable suppression seen experimentally is attributed to a large extent to the interactions of leading pre-hadrons with the dense environment.
Abstract: The measured particle ratios in central heavy-ion collisions at RHIC-BNL are investigated within a chemical and thermal equilibrium chiral SU(3) Ã É approach. The commonly adopted non-interacting gas calculations yield temperatures close to or above the critical temperature for the chiral phase transition, but without taking into account any interactions. In contrast, the chiral SU(3) model predicts temperature and density dependent effective hadron masses and effective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. Three different parametrizations of the model, which show different types of phase transition behaviour, are investigated. We show that if a chiral phase transition occured in those collisions, freezing of the relative hadron abundances in the symmetric phase is excluded by the data. Therefore, either very rapid chemical equilibration must occur in the broken phase, or the measured hadron ratios are the outcome of the dynamical symmetry breaking. Furthermore, the extracted chemical freeze-out parameters differ considerably from those obtained in simple non-interacting gas calculations. In particular, the three models yield up to 35 MeV lower temperatures than the free gas approximation. The inmedium masses turn out to differ up to 150 MeV from their vacuum values.
Abstract: The medium modification of kaon and antikaon masses, compatible with low energy KN scattering data, are studied in a chiral SU(3) model. The mutual interactions with baryons in hot hadronic matter and the e ects from the baryonic Dirac sea on the K( ¯K ) masses are examined. The in-medium masses from the chiral SU(3) e ective model are compared to those from chiral perturbation theory. Furthermore, the influence of these in-medium e ects on kaon rapidity distributions and transverse energy spectra as well as the K, ¯K flow pattern in heavy-ion collision experiments at 1.5 to 2 A·GeV are investigated within the HSD transport approach. Detailed predictions on the transverse momentum and rapidity dependence of directed flow v1 and the elliptic flow v2 are provided for Ni+Ni at 1.93 A·GeV within the various models, that can be used to determine the in-medium K± properties from the experimental side in the near future.
Charmonium production and suppression in heavy-ion collisions at relativistic energies is investigated within di erent models, i.e. the comover absorption model, the threshold suppression model, the statistical coalescence model and the HSD transport approach. In HSD the charmonium dissociation cross sections with mesons are described by a simple phase-space parametrization including an e ective coupling strength |Mi|2 for the charmonium states i =Xc,J/psi, psi'. This allows to include the backward channels for charmonium reproduction by DD channels which are missed in the comover absorption and threshold suppression model employing detailed balance without introducing any new parameters. It is found that all approaches yield a reasonable description of J/psi suppression in S+U and Pb+Pb collisions at SPS energies. However, they di er significantly in the psi'/J/psi ratio versus centrality at SPS and especially at RHIC energies. These pronounced differences can be exploited in future measurements at RHIC to distinguish the hadronic rescattering scenarios from quark coalescence close to the QGP phase boundary.
By replacing the irises in an electron linac by a slit one gets a structure capable of focussing/defocussing an electron beam (rf-quadrupoles). Therefore one can think of a combination of rf- and conventional magnetic quadrupoles for transversal focussing in linear-colliders. Furthermore they can meet the demands of BNS-damping without initial energy spread. Considering multibunch-operation of a collider, the long-range wake behaviour of this kind of structure has to be investigated. A three-cell structure has been built and investigated for dipole-type transversal long-range wakes. The experimental results are compared to numerical simulations done with MAFIA.
The effect of a single HOM-damper cell within a channel of undamped cells is described theoretically using an equivalent circuit model. From this a simple equation can be derived which relates the Q-value of the single damping-cell, the bandwidth of the passband under consideration, and the additional phase shift which is introduced by the damper cell to provide energy flow into the damper cell. This equation immediately shows the limitations of such single cell damping systems. Comparisons with experimental results are shown.
Damping cells for the higher order modes are necessary for the S-band linear collider to minimize BBU (Beam-Break-Up). The construction of the damper cells has to take into account the different field geometries of the higher order modes. So two different types of dampers have been designed: a wall slotted an an iris slotted cell. In order to optimize the two types of damping cells with respect to damping strength, impedance matching between coupling system and waveguide dampers and between damping cell and undamped cells and the tuning system, damping cells of both types have been built and examinated.
Antibaryons bound in nuclei
(2004)
We study the possibility of producing a new kind of nuclear systems which in addition to ordinary nucleons contain a few antibaryons (B = p, , etc.). The properties of such systems are described within the relativistic mean field model by employing G parity transformed interactions for antibaryons. Calculations are first done for infinite systems and then for finite nuclei from 4He to 208Pb. It is demonstrated that the presence of a real antibaryon leads to a strong rearrangement of a target nucleus resulting in a significant increase of its binding energy and local compression. Noticeable e ects remain even after the antibaryon coupling constants are reduced by factor 3 4 compared to G parity motivated values. We have performed detailed calculations of the antibaryon annihilation rates in the nuclear environment by applying a kinetic approach. It is shown that due to significant reduction of the reaction Q values, the in medium annihilation rates should be strongly suppressed leading to relatively long lived antibaryon nucleus systems. Multi nucleon annihilation channels are analyzed too. We have also estimated formation probabilities of bound B + A systems in pA reactions and have found that their observation will be feasible at the future GSI antiproton facility. Several observable signatures are proposed. The possibility of producing multi quark antiquark clusters is discussed. PACS numbers: 25.43.+t, 21.10.-k, 21.30.Fe, 21.80.+a
A new method for measuring quality factors in cavities is presented. This method is capable of measuring Q-factors in heavily damped as well as in undamped cavities. In addition, the possibility of separating overlapping modes and measuring their Q-factors is provided. Measurements on HOM (higher order mode) damped cavities for the DESY/THD linear collider project are presented
We study the phase diagram of a generalized chiral SU(3)-flavor model in mean-field approxi- mation. In particular, the influence of the baryon resonances, and their couplings to the scalar and vector fields, on the characteristics of the chiral phase transition as a function of temperature and baryon-chemical potential is investigated. Present and future finite-density lattice calculations might constrain the couplings of the fields to the baryons. The results are compared to recent lattice QCD calculations and it is shown that it is non-trivial to obtain, simultaneously, stable cold nuclear matter.
A critical discussion of the present status of the CERN experiments on charm dynamics and hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A·GeV: here the hydrodynamic model has predicted the collapse of the v1-flow and of the v2-flow at 10 A·GeV; at 40 A·GeV it has been recently observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as potential evidence for a first order phase transition at high baryon density B. A detailed discussion of the collective flow as a barometer for the equation of state (EoS) of hot dense matter at RHIC follows. Here, hadronic rescattering models can explain < 30% of the observed elliptic flow, v2, for pT > 2 GeV/c. This is interpreted as evidence for the production of superdense matter at RHIC with initial pressure far above hadronic pressure, p > 1 GeV/fm3. We suggest that the fluctuations in the flow, v1 and v2, should be measured in future since ideal hydrodynamics predicts that they are larger than 50 % due to initial state fluctuations. Furthermore, the QGP coe cient of viscosity may be determined experimentally from the fluctuations observed. The connection of v2 to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the awayside jet suppression can only partially (< 50%) be due to hadronic rescattering. We, finally, propose upgrades and second generation experiments at RHIC which inspect the first order phase transition in the fragmentation region, i.e. at µB 400 MeV (y 4 5), where the collapse of the proton flow should be seen in analogy to the 40 A·GeV data. The study of Jet-Wake-riding potentials and Bow shocks caused by jets in the QGP formed at RHIC can give further information on the equation of state (EoS) and transport coe cients of the Quark Gluon Plasma (QGP).
We study the collective flow of open charm mesons and charmonia in Au + Au collisions at s = 200 GeV within the hadron-string-dynamics (HSD) transport approach. The detailed studies show that the coupling of D, mesons to the light hadrons leads to comparable directed and elliptic flow as for the light mesons. This also holds approximately for J/ mesons since more than 50% of the final charmonia for central and midcentral collisions stem from D + induced reactions in the transport calculations. The transverse momentum spectra of D, mesons and J/ s are only very moderately changed by the (pre-)hadronic interactions in HSD, which can be traced back to the collective flow generated by elastic interactions with the light hadrons. PACS-Nr. 25.75.-q, 13.60.Le, 14.40.Lb, 14.65.Dw
A scenario of heavy resonances, called massive Hagedorn states, is proposed which exhibits a fast (t H 1 fm/c) chemical equilibration of (strange) baryons and anti-baryons at the QCD critical temperature Tc. For relativistic heavy ion collisions this scenario predicts that hadronization is followed by a brief expansion phase during which the equilibration rate is higher than the expansion rate, so that baryons and antibaryons reach chemical equilibrium before chemical freeze-out occurs. PACS-Nr.: 12.38.Mh
To reach high luminosities in future linear colliders short range wakes havea to be controlled in the range of X-band frequencies or higher. Rectangular irises can be used to introduce strong focusing quadrupole-like rf-fields. Even circular irises in iris-loaded accelarator structures have the capability of focusing if the particle velocity differs from phase velocity. Theoretical investigations concerning the focusing strength to be expected are presented. Their applicability for linear colliders is discussed.
Mode propagation in an iris type accelerator section loaded with single heavily HOM-damped cells
(1994)
The wakefield effects in accelerator sections for future linear colliders will be reduced either by damping by detuning or by a combination of both. For the DESY/THD linac [1] it is forseen to employ heavily HOM-damped cells to provide a strong coupling to the TE/TM11-dipole passband as well as to the TM/TE11-dipole passband. For our experiments we have used wall-slotted damping cells. This leads to several problems concerning the propagation of fundamental and HOM-modes. Experimental investigations have been done. Results are presented.
A new method of measuring quality factors in cavities is presented. This method is well suited to measure quality factors in undamped cavities as well as in heavily damped cavities, and in addition this method provides a possibility of separating modes and measuring quality factors especially in cases of overlapping modes. Measurements have been carried out on HOM-damped cavities for the DESY/THD linear collider project. Results are presented.
Due to the additional need of very short bunches for the FEL operation with the TESLA-machine strong wakefield effects are expected. One third of the total wakefield energy per bunch is radiated into the frequency region above the energy gap of Cooper pairs in superconducting niobium. The energy of the cooper pairs in superconducting niobium at 2 K corresponds to a frequency of 700 GHz. An analytical and experimental estimation for the overall energy loss of the FEL bunch above energy gap is presented. The analytical method is based on a study from R. B. Palmer [1]. The results of the wakefield estimations are used to calculate possible quality factor reduction of the TESLA cavities during FEL operation. Results are presented.
The operation of a Free Electron Laser (FEL) in the ultraviolet or in the X-ray regime requires the acceleration of electron bunches with an rms length of 25 to 50 mikrometer. The wakefields generated by these sub picosecond bunches extend into the frequency range well beyond the threshold for Cooper pair breakup (about 750 GHz) in superconducting niobium at 2 K. It is shown, that the superconducting cavities can indeed be operated with 25 mikrometer bunches without suffering a breakdown of superconductivity (quench), however at the price of a reduced quality factor and an increased heat transfer to the superfluid helium bath. This was first shown by wakefield calculations based on the diffraction model [1]. In the meantime a more conventional method of computing wake fields in the time domain by numerical methods was developed and used for the wakefield calculations [2]. Both methods lead to comparable results: the operation of TESLA with 25 mikrometer bunches is possible but leads to an additional heat load due to the higher order modes (HOMs). Therefore HOM dampers for these high frequencies are under construction [3]. These dampers are located in the beam pipes between the 9-cell cavities. So it is of interest, if there are trapped modes in the cavity due to closed photon orbits. In this paper we investigate the existence of trapped modes and the distribution of heat load over the surface of the TESLA cavity by numerical photon tracking.
Robotic gesture recognition
(1998)
Robots of the future should communicate with humans in a natural way. We are especially interested in vision-based gesture interfaces. In the context of robotics several constraints exist, which make the task of gesture recognition particularly challenging. We discuss these constraints and report on progress being made in our lab in the development of techniques for building robust gesture interfaces which can handle these constraints. In an example application, the techniques are shown to be easily combined to build a gesture interface for a real robot grasping objects on a table in front of it.
The study of hidden charm production is an important part of the heavy ion program. The standard approach to this problem [1] assumes that c¯c bound states are created only at the initial stage of the reaction and then partially destroyed at later stages due to interactions with the medium [2, 3, 4].
The binding problem is regarded as one of today's key questions about brain function. Several solutions have been proposed, yet the issue is still controversial. The goal of this article is twofold. Firstly, we propose a new experimental paradigm requiring feature binding, the "delayed binding response task". Secondly, we propose a binding mechanism employing fast reversible synaptic plasticity to express the binding between concepts. We discuss the experimental predictions of our model for the delayed binding response task.
Nuclear collisions at intermediate, relativistic, and ultra-relativistic energies offer unique opportunities to study in detail manifold fragmentation and clustering phenomena in dense nuclear matter. At intermediate energies, the well known processes of nuclear multifragmentation -- the disintegration of bulk nuclear matter in clusters of a wide range of sizes and masses -- allow the study of the critical point of the equation of state of nuclear matter. At very high energies, ultra-relativistic heavy-ion collisions offer a glimpse at the substructure of hadronic matter by crossing the phase boundary to the quark-gluon plasma. The hadronization of the quark-gluon plasma created in the fireball of a ultra-relativistic heavy-ion collision can be considered, again, as a clustering process. We will present two models which allow the simulation of nuclear multifragmentation and the hadronization via the formation of clusters in an interacting gas of quarks, and will discuss the importance of clustering to our understanding of hadronization in ultra-relativistic heavy-ion collisions.
Entropy production in the compression stage of heavy ion collisions is discussed within three distinct macroscopic models (i.e. generalized RHTA, geometrical overlap model and three-fluid hydrodynamics). We find that within these models \sim 80% or more of the experimentally observed final-state entropy is created in the early stage. It is thus likely followed by a nearly isentropic expansion. We employ an equation of state with a first-order phase transition. For low net baryon density, the entropy density exhibits a jump at the phase boundary. However, the excitation function of the specific entropy per net baryon, S/A, does not reflect this jump. This is due to the fact that for final states (of the compression) in the mixed phase, the baryon density \rho_B increases with \sqrt{s}, but not the temperature T. Calculations within the three-fluid model show that a large fraction of the entropy is produced by nuclear shockwaves in the projectile and target. With increasing beam energy, this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the entropy of the newly produced particles around midrapidity. Hadron ratios are calculated for the entropy values produced initially at beam energies from 2 to 200 AGeV.