Di-electron spectroscopy in HADES and CBM : from p + p and n + p collisions at GSI to Au + Au collisions at FAIR

The study of the electromagnetic structure of hadrons plays an important role in understanding the nature of matter. In particular the emission of lepton pairs out of the hot and dense collision zone in heavy-ion reactions is a promising probe to investigate in-medium properties of hadrons and in general the properties of matter under such extreme conditions. The first experimental observation of an enhanced di-electron yield in the invariant-mass region 0:3 - 0:7 GeV/c2 in p+Be collisions at 4:9 GeV/u beam energy [2] was announced by the DLS collaboration [1]. Recent results of the HADES collaboration show a moderate enhancement above n Dalitz decay contributions for 12C+12C at 1 and 2 GeV/u [3, 4] confirming the DLS results. There are several theoretical explanations of this observation, most of them focusing on possible in-medium modifications of the properties of vector mesons. At low beam energies the question whether the observed excess is related to any in-medium effects remains open because of uncertainties in the description of elementary di-electron sources. In this work the di-electron production in p+p and d+p reactions at a kinetic beam energy of 1:25 GeV/u measured by the HADES spectrometer is discussed. At Ekin = 1:25 GeV/u, i.e. below the n meson production threshold in proton-proton reactions, the delta Dalitz decay is expected to be the most abundant source above the pi 0 Dalitz decay region. The observed large difference in di-electron production in p+p and d+p collisions suggests that di-electron production in the d+p system is dominated by the n+p interaction. In order to separate delta Dalitz decays and np bremsstrahlung the di-electron yield observed in p+p and n+p reactions, both measured at the same beam energy, has been compared. The main interest here is the investigation of iso-spin effects in baryonic resonance excitations and the off-shell production of vector mesons [5]. We indeed observe a large difference in di-electron production in p+p and n+p reactions. Results of these studies will be compared to recent calculations. We will also present our experimentally defined cocktail for heavy-ion data. At much higher beam energies experimental results of the CERES [6] and NA60 [7] collaborations also show an enhancement in the invariant mass region 0:3 - 0:7 GeV/c2, in principle similar to the situation in DLS. A strong excess of lepton pairs observed by recent high energy heavy-ion dilepton experiments hint to a strong influence of baryons, however no data exist at highly compressed baryonic matter, achievable in heavy-ion collisions from 8 - 45 GeV/u beam energy. These conditions would allow to study the expected restoration of chiral symmetry by measuring in-medium modifications of hadronic properties, an experimental program which is foreseen by the future CBM experiment at FAIR. The experimental challenge is to suppress the large physical background on the one hand and to provide a clean identification of electrons on the other hand. In this work, strategies to reduce the combinatorial background in electron pair measurements with the CBM detector are discussed. The main goal is to study the feasibility of effectively reducing combinatorial background with the currently foreseen experimental setup, which does not provide electron identification in front of the magnetic field.Untersuchungen der elektromagnetischen Struktur von Hadronen spielen eine wichtige Rolle um die Eigenschaften von stark wechselwirkender Materie zu verstehen. Insbesondere die Emission von Leptonenpaare aus der heißen und dichten Phase in Schwerionenkollisionen stellt eine vielversprechende Probe in der Untersuchung von veränderten Eigenschaften der Hadronen im Medium und der Eigenschaften von Materie unter extremen Bedingungen dar. Das DLS Experiment [1] beobachtete einen Überschuss der Elektronenpaarausbeute über die Erwartungen im invarianten Massenbereich von 0:3 - 0:7 GeV/c2 in p+Be-Kollisionen bei 4:9 GeV/u Strahlenergie [2]. Neuere Ergebnisse des HADES Experimentes bestätigen diese DLS Resultate und zeigen einen Überschuss von Elektronenpaare über den erwarteten Beiträgen von n Dalitz-Zerfällen für 12C+12C-Reaktionen bei 1 und 2 GeV/u Strahlenergie [3, 4]. Diese Beobachtung wird in verschiedenen theoretischen Ansätzen unterschiedlich erklärt, die meisten Deutungen konzentrieren sich auf Veränderungen der Eigenschaften der Vektormesonen im Medium. Allerdings bleibt diese Interpretation insbesondere bei diesen niedrigen Strahlenergien wegen der großen Unsicherheiten in den elementaren Elektronenpaarquellen unsicher. In dieser Arbeit wurde die mit dem HADES Experiment gemessene Produktion von Elektronenpaare in p+p und in aus d+p extrahierten n+p-Reaktionen bei Strahlenergien von 1:25 GeV/u untersucht. Bei Energien von Ekin = 1:25 GeV/u befindet man sich in Proton-Proton Stößen noch unterhalb der Schwelle für n-Mesonproduktion, daher ist der delta Dalitz-Zerfall die dominante Quelle für Elektronenpaare oberhalb der pi 0 Dalitz-Zerfallsregion. Man beobachtet einen großen Unterschied in den Spektren aus p+p und n+p Reaktionen bei derselben Strahlenergie. Dieser Vergleich ermöglicht es, die Beiträge der delta Dalitz-Zerfälle von denen aus np-Bremsstrahlung zu trennen. Das wesentliche Interesse liegt hier auf der Untersuchung von Isospin-Effekten in der Anregung baryonischer Resonanzen Produktion von Vektormesonen jenseits der Massenschale [5]. Ergebnisse der hier vorgestellten Analyse werden mit neueren Rechnungen verglichen. Ein experimentell bestimmter Cocktail zum Vergleich mit der Elektronenpaar-Produktion in C+C wird vorgestellt. Auch Ergebnnisse der CERES [6] und NA60 [7] Kollaborationen bei höherer Strahlenergie einen Überschuss der Elektronenpaar-Produktion über den Erwartungen im invarianten Massenbereich von 0:3 - 0:7 GeV/c2 zeigen. Erklärungen dieses Überschusses weisen auf einen starken Einfluss von Baryonen auf die Leptonenpaar-Emission hin. Es existieren allerdings noch keine Messungen im Bereich höchster Kompression von baryonischer Materie bzw. höchster Netto-Baryonendichte, die mit Schwerionenkollisionen bei 8 - 45 GeV/u Strahlenergie erreicht werden können. Solche Bedingungen erlauben die Untersuchung der erwarteten Wiederherstellung der spontan gebrochenen Chiralen Symmetrie durch die Messung von Modifikationen der hadronischen Eigenschaften im Medium. Ein solches experimentelles Programm ist einer der Kernpunkte des geplanten CBM-Experimentes an FAIR. Die experimentelle Herausforderung besteht darin, den großen physikalischen Untergrund zu unterdrücken sowie eine saubere Elektronen Identifizierung zu erhalten. In dieser Arbeit wurden detaillierte Machbarkeitsstudien hierzu durchgeführt und es wurde gezeigt, dass mit der entwickelten Strategie der kombinatorische Untergrund mit der vorgeschlagenen Detektoranordnung hinreichend unterdrückt werden kann

Thermal Dileptons from Coarse-Grained Transport as Fireball Probes at SIS Energies

Utilizing a coarse-graining method to convert hadronic transport simulations of Au+Au collisions at SIS energies into local temperature, baryon and pion densities, we compute the pertinent radiation of thermal dileptons based on an in-medium $\rho$ spectral function that describes available spectra at ultrarelativistic collision energies. In particular, we analyze how far the resulting yields and slopes of the invariant-mass spectra can probe the lifetime and temperatures of the fireball. We find that dilepton radiation sets in after the initial overlap phase of the colliding nuclei of about 7 fm/c, and lasts for about 13 fm/c. This duration closely coincides with the development of the transverse collectivity of the baryons, thus establishing a direct correlation between hadronic collective effects and thermal EM radiation, and supporting a near local equilibration of the system. This fireball "lifetime" is substantially smaller than the typical 20-30 fm/c that naive considerations of the density evolution alone would suggest. We furthermore find that the total dilepton yield radiated into the invariant-mass window of $M=0.3-0.7$ GeV/$c^{2}$, normalized to the number of charged pions, follows a relation to the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion collisions, and thus corroborates the versatility of this tool. The spectral slopes of the invariant-mass spectra above the $\phi$ mass provide a thermometer of the hottest phases of the collision, and agree well with the maximal temperatures extracted from the coarse-grained hadron spectra.Comment: 9 pages, 6 figures; v2: extended discussion, matches the version accepted for publicatio

Probing resonance matter with virtual photons

In the energy domain of 1-2 GeV per nucleon, HADES has measured rare penetrating probes (e+e-) in C+C, Ar+KCl, d+p, p+p and p+Nb collisions. For the first time the electron pairs were reconstructed from quasi-free n+p sub-reactions by detecting the proton spectator from the deuteron breakup. An experimentally constrained NN reference spectrum was established. Our results demonstrate that the gross features of di-electron spectra in C+C collisions can be explained as a superposition of independent NN collisions. On the other hand, a direct comparison of the NN reference spectrum with the e+e- invariant mass distribution measured in the heavier system Ar+KCl at 1.76 GeV/u shows an excess yield above the reference, which we attribute to radiation from resonance matter. Moreover, the combined measurement of di-electrons and strangeness in Ar+KCl collisions has provided further intriguing results which are also discussed.Comment: 10 pages, 3 figures, proceedings of the International Nuclear Physics Conference - INPC 2010, Vancouver, Canada, July 4 - 9 201

Polarization of Thermal Dilepton Radiation

The spectra of dileptons radiated from the fireballs formed in high-energy heavy-ion collisions have been successfully used to investigate key properties of hot and dense QCD matter. In this paper we study polarization observables which have thus far received little attention. Microscopic calculations of in-medium electromagnetic spectral functions have thus far mostly focused on integrated yields which are proportional to the sum of the longitudinal and transverse components of the virtual photon's selfenergy. Photon polarization results from the difference of these components, which in general does not vanish for lepton pairs at finite three-momentum relative to the heat bath (and is maximal for fully transverse real photons). Using a model that successfully describes dilepton spectra in heavy-ion collisions, with hadronic emission via medium-modified vector mesons and quark-antiquark annihilation constrained by lattice QCD, we compute polarization observables in different dilepton mass bins and confront them with data of the HADES and NA60 experiments.Comment: 5 pages, 3 figure

Pluto: a Monte Carlo simulation tool for hadronic physics

Pluto is a Monte-Carlo event generator designed for hadronic interactions from Pion production threshold to intermediate energies of a few GeV per nucleon, as well as for studies of heavy ion reactions. The package is entirely based on ROOT, without the need of additional packages, and uses the embedded C++ interpreter of ROOT to control the event production. The generation of events based on a single reaction chain and the storage of the resulting particle objects can be done with a few lines of a ROOT-macro. However, the complete control of the package can be taken over by the steering macro and user-defined models may be added without a recompilation of the framework. Multi-reaction cocktails can be facilitated as well using either mass-dependent or user-defined static branching ratios. The included physics uses resonance production with mass-dependent Breit-Wigner sampling. The calculation of partial and total widths for resonances producing unstable particles is performed recursively in a coupled-channel approach. Here, particular attention is paid to the electromagnetic decays, motivated by the physics program of HADES. The thermal model supports 2-component thermal distributions, longitudinal broadening, radial blast, direct and elliptic flow, and impact-parameter sampled multiplicities. The interface allows angular distribution models (e.g. for the primary meson emission) to be attached by the user as well as descriptions of multi-particle correlations using decay chain templates. The exchange of mass sampling or momentum generation models is also possible. The first feature allows for consistent coupled-channel calculations, needed for a correct description of hadronic interactions. For elementary reactions, angular distribution models for selected channels are already part of the framework, based on parameterizations of existing data. This report gives an overview of the design of the package, the included models and the user interface

Status of the Micro Vertex Detector of the Compressed Baryonic Matter Experiment

The CBM experiment will investigate heavy-ion collisions at beam energies from 8 to 45 AGeV at the future accelerator facility FAIR. The goal of the experiment is to study the QCD phase diagram in the vincinity of the QCD critical point. To do so, CBM aims at measuring rare probes among them open charm. In order to identify those rare and short lived particles despite the rich combinatorial background generated in heavy ion collisions, a micro vertex detector (MVD) providing an unprecedented combination of high rate capability and radiation hardness, very light material budget and excellent granularity is required. In this work, we will discuss the concept of this detector and summarize the status of the R&D

Study of exclusive one-pion and one-eta production using hadron and dielectron channels in pp reactions at kinetic beam energies of 1.25 GeV and 2.2 GeV with HADES

We present measurements of exclusive ensuremathπ+,0 and η production in pp reactions at 1.25GeV and 2.2GeV beam kinetic energy in hadron and dielectron channels. In the case of π+ and π0 , high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance-corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of Δ (1232) and higher-lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross-sections are of special interest in the case of pp → pp η , since controversial data exist at 2.0GeV; we find \ensuremathσ=0.142±0.022 mb. Using the dielectron channels, the π0 and η Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance-corrected helicity angle distributions are found in good agreement with model predictions

The High-Acceptance Dielectron Spectrometer HADES

HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85% azimuthal coverage over a polar angle interval from 18 to 85 degree, a single electron efficiency of 50% and a vector meson mass resolution of 2.5%. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system