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Register a new userProbing resonance matter with virtual photons
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Tetyana Galatyuk
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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.
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Results on low-mass dileptons, covering the very broad energy range from the BEVALAC up to SPS are reviewed. The emphasis is on the open questions raised by the intriguing results obtained so far and the prospects for addressing them in the near future with the second generation of experiments, in particular HADES, NA60 and PHENIX.
We report on new measurements of the electric Generalized Polarizability (GP) of the proton $alpha_E$ in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have found that $alpha_E = (5.3 pm 0.6_{stat} pm 1.3_{sys})~10^{-4} fm^3$ at $Q^2=0.20~(GeV/c)^2$. The new measurements, when considered along with the rest of the world data, suggest that $alpha_E$ can be described by either a local plateau or by an enhancement in the region $Q^2=0.20~(GeV/c)^2$ to $0.33~(GeV/c)^2$. The experiment also provides the first measurement of the Coulomb quadrupole amplitude in the $N rightarrow Delta$ transition through the exploration of the $p(e,ep)gamma$ reaction. The new measurement gives $CMR = (-4.4 pm 0.8_{stat} pm 0.6_{sys})~%$ at $Q^2=0.20~(GeV/c)^2$ and is consistent with the results from the pion electroproduction world data. It has been obtained using a completely different extraction method, and therefore represents a strong validation test of the world data model uncertainties.
The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, where a proton is knocked-out of the nucleus with high momentum transfer and high missing momentum, show that in 12C the neutron-proton pairs are nearly twenty times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.
The heavy-ion programme at the CERN SPS, which started back in 86, has produced a wealth of very interesting and intriguing results in the quest for the quark-gluon plasma. The highlights of the programme on dilepton and direct photon measurements are reviewed emphasizing the most recent results obtained in Pb-Pb collisions at 158 A GeV. Prospects from RHIC are discussed.
Triple differential dijet cross-sections in e p interactions measured with the H1 and ZEUS detectors at HERA are presented. The data are compared to Monte Carlo simulations which differ in their assumptions about photon structure and parton evolution. Effects of the resolved processes of longitudinally polarized virtual photons at HERA are investigated for the first time.
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