Investigation of the production of electron-positron pairs in nucleon-nucleon interactions with the HADES detector
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EMENTARY PARTICLES AND FIELDS Theory
Investigation of the Production of Electron–Positron Pairs in Nucleon–Nucleon Interactions with the HADES Detector K. O. Lapidus (for the HADES collaboration) Institute for Nuclear Research, Russian Academy of Sciences, pr. Shestidesyatiletiya Oktyabrya 7a, Moscow, 117312 Russia Received August 21, 2009
Abstract—The inclusive production of electron–positron pairs in proton–proton and deuteron–proton collisions at a beam kinetic energy of 1.25 GeV per nucleon was studied with the HADES (High Acceptance Dielectron Spectrometer) detector. The main objective of the deuteron–proton experiment was to investigate pair production in quasifree neutron–proton collisions. In the invariant-mass region M > 140 MeV/c2, the invariant-mass spectrum determined in this channel shows a significant enhancement of the pair yield in relation to that in the case of proton–proton collisions. DOI: 10.1134/S1063778810060128
INTRODUCTION The HADES (High Acceptance Dielectron Spectrometer) detector [1] is deployed at the GSI research center (Darmstadt, Germany). This detector is a magnetic spectrometer that ensures the detection of electrons and positrons in the polar-angle range 18◦ –85◦ and provides an almost complete azimuthal coverage. The main part of the experimental program for this facility is aimed at studying the production of electron–positron pairs (dielectrons) in nucleon–nucleon, nucleon–nucleus, and nucleus– nucleus collisions at energies of 1 to 2 GeV per nucleon. Experiments with pion beams are also planned. In 1997, the DLS Collaboration presented data on dielectron production in nucleus–nucleus collisions at the energy of 1 GeV per nucleon [2]. The results obtained by the DLS Collaboration are indicative of a considerable excess of the dielectron yield in relation to the so-called hadron cocktail—that is, the total contribution to the dielectron spectrum from hadron decays after the chemical freeze-out. In order to test independently and to study systematically this discrepancy, dielectron production in 12 С + 12 С collisions at the beam energies of 1 and 2 GeV per nucleon was investigated with the HADES detector in 2002– 2004 [3, 4]. At the energy of 1 GeV per nucleon and in the invariant-mass interval between 150 and 500 MeV/c2 , the ratio of the measured dielectron yield to the contribution from the Dalitz decays of η mesons is Ytot /Yη = 6.8 [4]. The results obtained by the HADES Collaboration agree well with the DLS Data [2].
In order to understand the nature of the discovered “anomalous” yield of electron–positron pairs in nucleus–nucleus collisions, it is necessary to obtain information about the dielectron yield in elementary nucleon–nucleon collisions. Investigations of dielectron production in the neutron–proton channel are of particular interest since bremsstrahlung, which has not yet received a conclusive study in the theory of the process, is predicted to play a significant role there. The investigation performed within the model of oneboson exchange [5] predicts cross-secti
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