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The HADES (High Acceptance DiElectron Spectrometer) is an experimental
apparatus installed at the heavy-ion synchrotron SIS-18 at GSI, Darmstadt.
The main physics motivation of the HADES experiment is the measurement
of e+e− pairs in the invariant-mass range up to 1 GeV/c2 in heavy-ion collisions
as well as in pion and proton-induced reactions.
The HADES physics program is focused on in-medium properties of the light
vector mesons ρ(770), ω(783) and φ(1020), which decay with a small branching
ratio into dileptons. Dileptons are penetrating probes which allow to study
the in-medium properties of hadrons. However, in heavy-ion collisions, the
measurement of such lepton pairs is difficult because they are rare and have a
very large combinatorial background.
Recently, HADES has been upgraded with new detectors and new electronics
in order to handle higher intensity beams and reactions with heavy nuclei up
to Au.
HADES will continue for a few more years its rich physics program at its
current place at SIS-18 and then move to the upcoming international Facility
for Antiproton and Ion Research (FAIR) accelerator complex. In this context
the physics results presented in this work are important prerequisites for the investigation
of in-medium vector meson properties in p + A and A+A collisions.
This work consists of five chapters. The first chapter introduces the physics
motivation and a review of recent physics results. In the second chapter, the
HADES spectrometer is described and its sub-detectors are presented. Chapter
three deals with the issue of lepton identification and the reconstruction of
the dielectron spectra in p + p collisions is presented. Here, two reactions
are characterized: inclusive and exclusive dilepton production reactions. From
the spectra obtained, the corresponding cross sections are presented with the
respective statistical and systematical errors. A comparison with theoretical
models is included as well. Conclusions are given in chapter four.
The final part of this work is dedicated to the HADES upgrade, whose goal
is among others the achievement of a reliable and fast data acquisition of the
Multiwire Drift Chambers (MDCs). Chapter five presents my contribution to
this successful project during the three years of my stay at GSI.
A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at sNN−−−√=2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η|<0.5. The transverse momentum pT of charged particles is measured down to 0.15 GeV/c which gives access to the low pT fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R=0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high pT leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R=0.2 and R=0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R<0.3.
Partial wave analysis of the reaction p(3.5 GeV) + p → pK +Λ to search for the "ppK−" bound state
(2015)
Employing the Bonn–Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5 GeV) + p → pK +Λ. This reaction might contain information about the kaonic cluster “ppK −” (with quantum numbers J P = 0− and total isospin I = 1/2) via its decay into pΛ. Due to interference effects in our coherent description of the data, a hypothetical KNN (or, specifically “ppK −”) cluster signal need not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectrum like pΛ. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a KNN cluster. At a confidence level of CLs = 95% such a cluster cannot contribute more than 2–12% to the total cross section with a pK +Λ final state, which translates into a production cross-section between 0.7 μb and 4.2 μb, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.
Many QCD based and phenomenological models predict changes of hadron properties in a strongly interacting environment. The results of these models differ significantly and the experimental determination of hadron properties in nuclear matter is essential. In this paper we present a review of selected physics results obtained at GSI Helmholtzzentrum für Schwerionenforschung GmbH by HADES (High-Acceptance Di-Electron Spectrometer). The e+e− pair emission measured for proton and heavy-ion induced collisions is reported together with results on strangeness production. The future HADES activities at the planned FAIR facility are also discussed.
The knowledge of baryonic resonance properties and production cross sections plays an important role for the extraction and understanding of medium modifications of mesons in hot and/or dense nuclear matter. We present and discuss systematics on dielectron and strangeness production obtained with HADES on p+p, p+A and A+A collisions in the few GeV energy regime with respect to these resonances.
The High Acceptance DiElectron Spectrometer HADES [1] is installed at the Helmholtzzentrum für Schwerionenforschung (GSI) accelerator facility in Darmstadt. It investigates dielectron emission and strangeness production in the 1-3 AGeV regime. A recent experiment series focusses on medium-modifications of light vector mesons in cold nuclear matter. In two runs, p+p and p+Nb reactions were investigated at 3.5 GeV beam energy; about 9·109 events have been registered. In contrast to other experiments the high acceptance of the HADES allows for a detailed analysis of electron pairs with low momenta relative to nuclear matter, where modifications of the spectral functions of vector mesons are predicted to be most prominent. Comparing these low momentum electron pairs to the reference measurement in the elementary p+p reaction, we find in fact a strong modification of the spectral distribution in the whole vector meson region.
The transverse momentum (pT) spectrum and nuclear modification factor (RAA) of reconstructed jets in 0–10% and 10–30% central Pb–Pb collisions at √sNN = 2.76 TeV were measured. Jets were reconstructed using the anti-kT jet algorithm with a resolution parameter of R = 0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet pT spectra are reported in the pseudorapidity interval of |ηjet| < 0.5 for 40 < pT, jet < 120 GeV/c in 0–10% and for 30 < pT, jet < 100 GeV/c in 10–30% collisions. Reconstructed jets were required to contain a leading charged particle with pT > 5 GeV/c to suppress jets constructed from the combinatorial background in Pb–Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and Pb–Pb collisions had a negligible effect on the RAA. The nuclear modification factor RAA was found to be 0.28 ± 0.04 in 0–10% and 0.35 ± 0.04 in 10–30% collisions, independent of pT, jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.
We have performed the first measurement of the coherent ψ(2S) photo production cross section in ultraperipheral Pb–Pb collisions at the LHC. This charmonium excited state is reconstructed via the ψ(2S) → l +l − and ψ(2S) → J/ψπ+π− decays, where the J/ψ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 μb−1. The cross section for coherent ψ(2S) production in the rapidity interval −0.9 < y < 0.9 is dσcoh ψ(2S)/dy = 0.83±0.19 stat+syst mb. The ψ(2S) to J/ψ coherent cross section ratio is 0.34+0.08 −0.07(stat + syst). The obtained results are compared to predictions from theoretical models.
A measurement of dijet correlations in p–Pb collisions at √sNN = 5.02 TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, kTy = pch+ne T,jet sin(ϕdijet) with ϕdijet the azimuthal angle between a full and charged jet and pch+ne T,jet the transverse momentum of the full jet, is used to study nuclear matter effects in p–Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p–Pb collisions with respect to pp collisions. Measurements of the dijet kTy as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of kTy due to nuclear matter effects in p–Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.
The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons1,2. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories3, but cannot yet be directly derived from quantum chromodynamics. Here we report a measurement of the difference between the ratios of the mass and charge of deuterons (d) and anti-deuterons (), and 3He and nuclei carried out with the ALICE (A Large Ion Collider Experiment)4 detector in Pb–Pb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. Our direct measurement of the mass-over-charge differences confirms CPT invariance to an unprecedented precision in the sector of light nuclei5,6. This fundamental symmetry of nature, which exchanges particles with anti-particles, implies that all physics laws are the same under the simultaneous reversal of charge(s) (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).