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Register a new userExperimental Signature of Medium Modifications for rho and omega Mesons in the 12 GeV p + A Reactions
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The invariant mass spectra of e+e- pairs produced in 12-GeV proton-induced nuclear reactions are measured at the KEK Proton-Synchrotron. On the low-mass side of the omega meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.
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The inclusive production of omega and phi mesons is studied in the backward region of the interaction of 12 GeV protons with polyethylene, carbon, and copper targets. The mesons are measured in e^+ e^- decay channels. The production cross sections of the mesons are presented as functions of rapidity y and transverse momentum p_T. The nuclear mass number dependences (A dependences) are found to be A^{0.710 +/- 0.021(stat) +/- 0.037(syst)} for omega mesons and A^{0.937 +/- 0.049(stat) +/- 0.018(syst)} for phi mesons in the region of 0.9 < y < 1.7 and p_T < 0.75 GeV/c.
The use of nuclear transparency effect of pi^{-}-mesons in proton, and deuteron induced interactions with carbon nuclei at 4.2A GeV/c, to get information about the properties of nuclear matter, is presented in this work. Half angle (theta_{1/2}) technique is used to extract information on nuclear transparency effect. The theta_{1/2} divides the multiplicity of charged particles into two equal parts depending on their polar angle in the lab. frame in pp interactions. Particles with angle smaller than (incone particles) and greater than (outcone particles) theta_{1/2} are considered separate. The average values of multiplicity, momentum and transverse momentum of the pi^{-}-mesons are analyzed as a function of a number of identified protons in an event. We observed evidences in the data which could be considered as transparency effect. For quantitative analysis, the results are compared with cascade model. The observed effects are categorized into leading effect transparency and medium effect transparency. The transparency in the latter case could be the reason of collective interactions of grouped nucleons with the incident particles.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured omega meson production via leptonic and hadronic decay channels in p+p, d+Au, Cu+Cu, and Au+Au collisions at sqrt(s_NN) = 200 GeV. The invariant transverse momentum spectra measured in different decay modes give consistent results. Measurements in the hadronic decay channel in Cu+Cu and Au+Au collisions show that omega production has a suppression pattern at high transverse momentum, similar to that of pi^0 and eta in central collisions, but no suppression is observed in peripheral collisions. The nuclear modification factors, R_AA, are consistent in Cu+Cu and Au+Au collisions at similar numbers of participant nucleons.
The PHENIX experiment at RHIC has measured the invariant cross section for omega-meson production at mid-rapidity in the transverse momentum range 2.5 < p_T < 9.25 GeV/c in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV. Measurements in two decay channels (omega --> pi^0 pi^+ pi^- and omega --> pi^0 gamma) yield consistent results, and the reconstructed omega mass agrees with the accepted value within the p_T range of the measurements. The omega/pi^0 ratio is found to be 0.85 +/- 0.05(stat) +/- 0.09(sys) and 0.94 +/- 0.08(stat) +/- 0.12(sys) in p+p and d+Au collisions respectively, independent of p_T . The nuclear modification factor R_dA is 1.03 +/- 0.12(stat) +/- 0.21(sys) and 0.83 +/- 0.21(stat) +/- 0.17(sys) in minimum bias and central (0-20%) d+Au collisions, respectively.
Isospin violating mixing of rho- and omega-mesons is reconsidered in terms of propagators. Its influence on various pairs of (rho^0,omega)-decays to the same final states is demonstrated. Some of them, (rho^0,omega)topi^+pi^- and (rho^0,omega)topi^0gamma, have been earlier discussed in the literature, others (e.g., (rho^0,omega)toetagamma and (rho^0,omega)to e^+e^-) are new in this context. Changes in partial widths for all the decay pairs are shown to be correlated. The set of present experimental data, though yet inconclusive, provides some limits for the direct (rhoomega)-coupling and indirectly supports enhancement of rho^0topi^0gamma in comparison with rho^{pm}topi^{pm}gamma, though not so large as in some previous estimates.
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