Elliptic and Hexadecapole Flow of Charged Hadrons in Au plus Au Collisions at root s(NN)=200 GeV

Differential measurements of the elliptic (upsilon(2)) and hexadecapole (upsilon(4)) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p(T)) and collision centrality or number of participant nucleons (N(part)) for Au + Au collisions at root s(NN) = 200 GeV/ The upsilon(2,4) measurements at pseudorapidity vertical bar eta vertical bar <= 0.35, obtained with four separate reaction-plane detectors positioned in the range 1.0 < vertical bar eta vertical bar < 3.9, show good agreement, indicating the absence of significant Delta eta-dependent nonflow correlations. Sizable values for upsilon(4)(p(T)) are observed with a ratio upsilon(4)(p(T), N(part))/upsilon(2)(2)(p(T), N(part)) approximate to 0.8 for 50 less than or similar to N(part) less than or similar to 200, which is compatible with the combined effects of a finite viscosity and initial eccentricity fluctuations. For N(part) greater than or similar to 200 this ratio increases up to 1.7 in the most central collisions.

Transverse momentum dependence of eta meson suppression in Au plus Au collisions at root s(NN)=200 GeV

New measurements by the PHENIX experiment at the Relativistic Heavy Ion Collider for. production at midrapidity as a function of transverse momentum ((PT)) and collision centrality in root s(NN) = 200 GeV Au + Au and p + p collisions are presented. They indicate nuclear modification factors (R(AA)) which are similar in both magnitude and trend to those found in earlier pi(0) measurements. Linear fits to R(AA) as a function of (PT) in 5-20 GeV/c show that the slope is consistent with zero within two standard deviations at all centralities, although a slow rise cannot be excluded. Having different statistical and systematic uncertainties, the pi(0) and eta measurements are complementary at high (PT); thus, along with the extended (PT) range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.

Detailed measurement of the e(+)e(-) pair continuum in p plus p and Au plus Au collisions at root s(NN)=200 GeV and implications for direct photon production

PHENIX has measured the e(+)e(-) pair continuum in root s(NN) = 200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e(+)e(-) yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the phi and the J/psi meson, the yield is consistent with expectations from correlated c (c) over bar production, although other mechanisms are not ruled out. In the low-mass region, below the phi, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7 +/- 0.4(stat) +/- 1.5(syst) +/- 0.9(model). At low mass (m(ee) < 0.3 GeV/c(2)) and high p(T) (1 < p(T) < 5 GeV/c) an enhanced e(+)e(-) pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in p(T), with inverse slope T = 221 +/- 19(stat) +/- 19(syst) MeV. Hydrodynamical models with initial temperatures ranging from T(init) similar or equal to 300-600 MeV at times of 0.6-0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low p(T) < 1 GeV/c the low-mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T similar or equal to 100 MeV. Theoretical models underpredict the low-mass, low-p(T) enhancement.

Gluon-Spin Contribution to the Proton Spin from the Double-Helicity Asymmetry in Inclusive pi(0) Production in Polarized p plus p Collisions at root s=200 GeV

The double helicity asymmetry in neutral pion production for p(T) = 1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, Delta G, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of mu 2 = 4 GeV(2) a next to leading order QCD analysis gives Delta G([0.02,0.3]) = 0.2, with a constraint of -0.7 < Delta G([0.02,0.3]) < 0.5 at Delta chi(2) = 9 (similar to 3 sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta G constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

Suppression Pattern of Neutral Pions at High Transverse Momentum in Au plus Au Collisions at root S(NN)=200 GeV and Constraints on Medium Transport Coefficients

For Au + Au collisions at 200 GeV, we measure neutral pion production with good statistics for transverse momentum, p(T), up to 20 GeV/c. A fivefold suppression is found, which is essentially constant for 5 < p(T) < 20 GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parametrization for the transport coefficient of the medium, e. g., <(q) over cap > in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au + Au collisions.

Onset of pi(0) Suppression Studied in Cu plus Cu Collisions at root s(NN)=22.4, 62.4, and 200 GeV

Neutral pion transverse momentum (p(T)) spectra at midrapidity (|y| less than or similar to 0.35) were measured in Cu + Cu collisions at root s(NN) = 22.4, 62.4, and 200 GeV. Relative to pi(0) yields in p + p collisions scaled by the number of inelastic nucleon-nucleon collisions (N(coll)) the pi(0) yields for p(T) greater than or similar to 2 GeV/c in central Cu + Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu + Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.

Particle-species dependent modification of jet-induced correlations in Au plus Au collisions at root s(NN)=200 GeV

Measurements in Au + Au collisions at root s(NN) = 200 GeV of jet correlations for a trigger hadron at intermediate transverse momentum (p(T,trig)) with associated mesons or baryons at lower p(T,assoc) indicate strong modification of the away-side jet. The ratio of jet-associated baryons to mesons increases with centrality and p(T,assoc). For the most central collisions, the ratio is similar to that for inclusive measurements. This trend is incompatible with in-vacuum fragmentation but could be due to jetlike contributions from correlated soft partons, which recombine upon hadronization.

Identified charged hadron production in p plus p collisions at root s=200 and 62.4 GeV

Transverse momentum distributions and yields for pi(+/-), K(+/-), p, and (p) over bar in p + p collisions at root s = 200 and 62.4 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). These data provide important baseline spectra for comparisons with identified particle spectra in heavy ion collisions at RHIC. We present the inverse slope parameter T(inv), mean transverse momentum < p(T)>, and yield per unit rapidity dN/dy at each energy, and compare them to other measurements at different root s in p + p and p + (p) over bar collisions. We also present the scaling properties such as m(T) scaling and x(T) scaling on the p(T) spectra between different energies. To discuss the mechanism of the particle production in p + p collisions, the measured spectra are compared to next-to-leading-order or next-to-leading-logarithmic perturbative quantum chromodynamics calculations.

Spectra of identified high-p(T) pi(+/-) and p((p)over-bar ) in Cu + Cu collisions at root s(NN)=200 GeV

We report new results on identified (anti) proton and charged pion spectra at large transverse momenta (3 < p(T) < 10 GeV/c) from Cu + Cu collisions at root s(NN) = 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). This study explores the system size dependence of two novel features observed at RHIC with heavy ions: the hadron suppression at high-p(T) and the anomalous baryon to meson enhancement at intermediate transverse momenta. Both phenomena could be attributed to the creation of a new form of QCD matter. The results presented here bridge the system size gap between the available pp and Au + Au data, and allow for a detailed exploration of the onset of the novel features. Comparative analysis of all available 200 GeV data indicates that the system size is a major factor determining both the magnitude of the hadron spectra suppression at large transverse momenta and the relative baryon to meson enhancement.

Coherent Contributions to Isospin Mixing in the Mirror Pair (67)As and (67)Se

Isospin symmetry breaking has been investigated in mass A = 67 mirror nuclei through the experimental determination of the E1 strengths of analog electromagnetic transitions. Lifetimes of excited states have been measured in (67)Se and (67)As with the centroid shift method. Through the comparison of the B(E1) strengths of the mirror 9/2(+) -> 7/2(-) transitions, the isovector and the isoscalar components of the electromagnetic transition amplitude were extracted. The presence of a large isoscalar component provides evidence for coherent contributions to isospin mixing, probably involving the isovector giant monopole resonance.

Photoacoustic investigation of maghemite-based BSA nanocomposite

Maghemite-based bovine serum albumin (BSA) nanocomposites have been investigated using photoacustic spectroscopy (PAS). Features in the PAS spectra were analyzed in terms of three bands (band-C, band-S, and band-L). Though the observed PAS peak positions are insensitive to the nanoparticle concentration the PAS peak intensity displays a different behaviour. For band-C the PAS intensity scales almost linearly with the nanoparticle concentration in the hosting template. Nevertheless, it was found that the PAS intensity of band-L scales sub-linearly with the nanoparticle concentration.

Compact Stars and Magnetized CFL Matter

The stability of the color flavor locked phase in the presence of a strong magnetic field is investigated within the phenomenological MIT bag model. It is found that the minimum value of the energy per baryon in a color flavor locked state at vanishing pressure is lower than the corresponding one for unpaired magnetized strange quark matter and, as the magnetic field increases, the energy per baryon decreases. This implies that magnetized color flavor locked matter is more stable and could become the ground state inside neutron stars. The anisotropy of the pressures is discussed. The mass-radius relation for such stars is also studied.

Magnetic Field and Temperature Effects on Strangelets

The main properties of magnetized strangelets, namely, their energy per baryon, radius and electric charge, are studied in the unpaired strange quark matter phase. Temperature effects are taken into account in order to study their stability compared to the (56)Fe isotope and non-magnetized strangelets within the framework of the MIT bag model. It is concluded that the presence of a magnetic field tends to stabilize more the strangelets, even when temperature is considered. We find that the electric charge is modified in the presence of the magnetic field, leading to higher charge values for magnetized strangelets, when compared to the non-magnetized case.

Neutrinos and the Matter-Antimatters Asymmetry in the Universe

The discovery of neutrino oscillations provides a solid evidence for nonzero neutrino masses and leptonic mixing. The fact that neutrino masses are so tiny constitutes a puzzling problem in particle physics. From the theoretical viewpoint, the smallness of neutrino masses can be elegantly explained through the seesaw mechanism. Another challenging issue for particle physics and cosmology is the explanation of the matter-antimatter asymmetry observed in Nature. Among the viable mechanisms, leptogenesis is a simple and well-motivated framework. In this paper we briefly review these aspects, making emphasis on the possibility of linking neutrino physics to the cosmological bary asymmetry originated from leptogenesis.

Leptogenesis, CP violation and neutrino data: what can we learn?

A detailed analytic and numerical study of baryogenesis through leptogenesis is performed in the framework of the standard model of electroweak interactions extended by the addition of three right-handed neutrinos, leading to the seesaw mechanism. We analyze the connection between GUT-motivated relations for the quark and lepton mass matrices and the possibility of obtaining a viable leptogenesis scenario. In particular, we analyze whether the constraints imposed by SO(10) GUTs can be compatible with all the available solar, atmospheric and reactor neutrino data and, simultaneously, be capable of producing the required baryon asymmetry via the leptogenesis mechanism. It is found that the Just-So(2) and SMA solar solutions lead to a viable leptogenesis even for the simplest SO(10) GUT, while the LMA, LOW and VO solar solutions would require a different hierarchy for the Dirac neutrino masses in order to generate the observed baryon asymmetry. Some implications on CP violation at low energies and on neutrinoless double beta decay are also considered. (C) 2002 Elsevier Science B.V. All rights reserved.