Enhanced J/Psi Production in Deconfined Quark Matter

In high energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider (LHC) at CERN, each central event will contain multiple pairs of heavy quarks. if a region of deconfined quarks and gluons is formed, a new mechanism for the formation of heavy quarkonium bound states will be activated. This is a result of the mobility of heavy quarks in the deconfined region, such that bound states can be formed from a quark and an antiquark which were originally produced in separate incoherent interactions. Model estimates of this effect for J/psi production at RHIC indicate that significant enhancements are to be expected. Experimental observation of such enhanced production would provide evidence for deconfinement unlikely to be compatible with competing scenarios.Comment: Added predictions with nonthermal charm quark distributions, also with gluon dissociation replaced by screening. Accepted for publication in Phys. Rev.

Satellite image-based generation of high frequency solar radiation time series for the assessment of solar energy systems

Solar energy is envisaged as a major pillar of the global transition to a climate-friendly energy system. Variability of solar radiation requires additional balancing measures to ensure a stable and secure energy supply. In order to analyze this issue in detail, solar radiation time series data of appropriate temporal and spatial resolution is necessary. Common weather models and satellites are only delivering solar surface irradiance with temporal resolutions of up to 15 min. Significant short-term variability in irradiances within seconds to minutes however is induced by clouds. Ground-based measurements typically used to capture this variability are costly and only sparsely available. Hence, a method to synthetically generate time series from currently available satellite imagery is of value for researchers, grid operators, and project developers. There are efforts to increase satellite resolution to 1 min, but this is not planned everywhere and will not change the spatial resolution. Therefore, the fundamental question remains if there are alternative strategies to obtain high temporal resolution observations at a pinpoint. This paper presents a method to generate 1 min resolved synthetic time series of global and direct normal irradiances for arbitrary locations. A neural network based on satellite image derived cloud structure parameters enables to classify high-frequency solar radiation variability. Combined with clear-sky radiation data, 1 min time series which reflect the typical variability characteristics of a site are reproduced. Testing and validation against ground observations (BSRN) show that the method can accurately reproduce characteristics such as frequency and ramp distributions. An application case demonstrates the usage in low-voltage grid studies

B_c Meson Production in Nuclear Collisions at RHIC

We study quantitatively the formation and evolution of B_c bound states in a space-time domain of deconfined quarks and gluons (quark-gluon plasma, QGP). At the Relativistic Heavy Ion Collider (RHIC) one expects for the first time that typical central collisions will result in multiple pairs of heavy (in this case charmed) quarks. This provides a new mechanism for the formation of heavy quarkonia which depends on the properties of the deconfined region. We find typical enhancements of about 500 fold for the B_c production yields over expectations from the elementary coherent hadronic B_c-meson production scenario. The final population of bound states may serve as a probe of the plasma phase parameters.Comment: 9 Pages, 11 Postscript Figure

New limits on the density of the extragalactic background light in the optical to the far-infrared from the spectra of all known TeV blazars

Aims: We derive limits on the EBL density from the energy spectra of distant sources of very high energetic gamma-rays (VHE gamma-rays). Methods: VHE gamma-rays are attenuated by the photons of the EBL via pair production, which leaves an imprint in the measured spectra from distant sources. In this paper we present a new method to derive constraints on the EBL. Hereby, we use only very basic assumptions about TeV blazar physics and no pre-defined EBL model, but rather a large number of generic shapes constructed from a grid in EBL density vs. wavelength. In our study we utilize spectral data from all known TeV blazars, making this the most complete study so far. Results: We derive limits on the EBL for three individual TeV blazar spectra (Mkn 501, H1426+428, 1ES1101-232) and for all spectra combined. Combining the results from individual spectra leads to significantly stronger constraints over a wide wavelength range from the optical (~ 1 micron) to the far-infrared (~ 80 microns). The limits are only a factor of 2 to 3 above the absolute lower limits derived from source counts. In the mid-infrared our limits are the strongest constraints derived from TeV blazar spectra so far. A high density of the EBL around 1 micron, reported by direct detection experiments, can be excluded. Conclusions: Our results can be interpreted in two ways: (i) The EBL is almost resolved by source counts, leaving only little room for additional components by e.g. the first stars or (ii) the assumptions about the underlying physics are not valid, which would require substantial changes in the standard emission models of TeV blazars.Comment: 16 pages, 15 figures, A&A in pres

High-energy neutrinos in the context of multimessenger physics

The field of astroparticle physics is currently developing rapidly, since new experiments challenge our understanding of the investigated processes. Three messengers can be used to extract information on the properties of astrophysical sources: photons, charged Cosmic Rays and neutrinos. This review focuses on high-energy neutrinos (E>100 GeV) with the main topics as follows. The production mechanism of high-energy neutrinos in astrophysical shocks. The connection between the observed photon spectra and charged Cosmic Rays is described and the source properties as they are known from photon observations and from charged Cosmic Rays are presented. High-energy neutrino detection. Current detection methods are described and the status of the next generation neutrino telescopes are reviewed. In particular, water and ice Cherenkov detectors as well as radio measurements in ice and with balloon experiments are presented. In addition, future perspectives for optical, radio and acoustic detection of neutrinos are reviewed. Sources of neutrino emission. The main source classes are reviewed, i.e. galactic sources, Active Galactic Nuclei, starburst galaxies and Gamma Ray Bursts. The interaction of high energy protons with the cosmic microwave background implies the production of neutrinos, referred to as GZK neutrinos. Implications of neutrino flux limits. Recent limits given by the AMANDA experiment and their implications regarding the physics of the sources are presented.Comment: accepted for publication by Physics Reports, 127 pages, 29 figure

The very high energy gamma-ray spectra of AGN

A total of six extragalactic objects have been detected so far at very high energies (VHE). They are BL Lac objects, a sub-group of active galactic nuclei characterized by intense nonthermal radiation. The VHE spectra of two of these, 1ES 1959 + 650 and 1ES 2344 + 514, were measured in this work. Similar to the other four BL Lacs detected, their VHE spectrum and flux level is highly variable and shows a broadband spectrum characterized by two emission peaks: one in X-ray, the other at GeV to TeV energies. For one of these, 1ES 1959 + 650, simultaneous observations were carried out at other wavelengths and for the first time, a VHE flare without increased X-ray flux level was recorded. For the other object, 1ES 2344 + 514, no simultaneous X-ray observations were taken, making further modeling impossible. VHE gamma-ray astronomy can establish important upper limits on the density of the extragalactic background light (EBL). If one can somehow guess what the source spectrum is, then one can infer the EBL density from the measured attenuation in the spectra. As the VHE spectra of BL Lac object are very similar, the zeroth order assumption was made (and justified) in this work that they are actually the same; differences in attenuation arising solely due to the different distances to the objects. The upper limits derived here are not very constraining, but they do question one particular set of EBL measurements that are very high in the near infrared waveband. Galaxy formation models are typically not able to reproduce this high density. The analysis of VHE spectra is still being developed; in part because of the new array of four telescopes, VERITAS, being built at the moment. Monte-Carlo simulations are used in this work and changes in the simulation software had not seen a comparison to the previous version until this work. Differences were identified that impact the energy reconstruction. A method was developed to calibrate the absolute energy scale by automatically identifying cosmic-ray muons recorded by the telescope