We present the centrality dependent measurement of multiplicity and pseudorapidity distributions of charged particles and photons in Au + Au collisions at sqrt{s_NN} = 62.4 GeV. The charged particles and photons are measured in the pseudorapidity region 2.9 < eta < 3.9 and 2.3 < eta < 3.7, respectively. We have studied the scaling of particle production with the number of participating nucleons and the number of binary collisions. The photon and charged particle production in the measured pseudorapidity range has been shown to be consistent with energy independent limiting fragmentation behavior. The photons are observed to follow a centrality independent limiting fragmentation behavior while for the charged particles it is centrality dependent. We have carried out a comparative study of the pseudorapidity distributions of positively charged hadrons, negatively charged hadrons, photons, pions, net protons in nucleus--nucleus collisions and pseudorapidity distributions from p+p collisions. From these comparisons we conclude that baryons in the inclusive charged particle distribution are responsible for the observed centrality dependence of limiting fragmentation. The mesons are found to follow an energy independent behavior of limiting fragmentation while the behavior of baryons seems to be energy dependent.
We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0's in their parent jets was found to be around 0.7 for electromagnetically triggered events.
Transverse momentum (p^e_T) spectra of electrons from semileptonic weak decays of heavy flavor mesons in the range of 0.3 < p^e_T < 9.0 GeV/c have been measured at mid-rapidity (|eta| < 0.35) by the PHENIX experiment at the Relativistic Heavy Ion Collider in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The nuclear modification factor R_AA with respect to p+p collisions indicates substantial energy loss of heavy quarks in the produced medium. In addition, the azimuthal anisotropy parameter v_2 has been measured for 0.3 < p^e_T < 5.0 GeV/c in Au+Au collisions. Comparisons of R_AA and v_2 are made to various model calculations.
We report the first measurement of the longitudinal double-spin asymmetry $A_{LL}$ for mid-rapidity di-jet production in polarized $pp$ collisions at a center-of-mass energy of $\sqrt{s} = 200$ GeV. The di-jet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. $A_{LL}$ results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. The measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-$x > 0.05$.
A search for physics beyond the standard model (SM) in the final state with a hadronically decaying tau lepton and a neutrino is presented. This analysis is based on data recorded by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to a total integrated luminosity of 138 fb$^{=1}$. The transverse mass spectrum is analyzed for the presence of new physics. No significant deviation from the SM prediction is observed. Limits are set on the production cross section of a W' boson decaying into a tau lepton and a neutrino. Lower limits are set on the mass of the sequential SM-like heavy charged vector boson and the mass of a quantum black hole. Upper limits are placed on the couplings of a new boson to the SM fermions. Constraints are put on a nonuniversal gauge interaction model and an effective field theory model. For the first time, upper limits on the cross section of $t$-channel leptoquark (LQ) exchange are presented. These limits are translated into exclusion limits on the LQ mass and on its coupling in the $t$-channel. The sensitivity of this analysis extends into the parameter space of LQ models that attempt to explain the anomalies observed in B meson decays. The limits presented for the various interpretations are the most stringent to date. Additionally, a model-independent limit is provided.
Two-particle correlation functions were measured for $\rm p\overline{p}$, $\rm p\overline{\Lambda}$, $\rm \overline{p}\Lambda$, and $\Lambda\overline{\Lambda}$ pairs in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV and $\sqrt{s_{\rm NN}}=5.02$ TeV recorded by the ALICE detector. From a simultaneous fit to all obtained correlation functions, real and imaginary components of the scattering lengths, as well as the effective ranges, were extracted for combined $\rm p\overline{\Lambda}$ and $\rm \overline{p}\Lambda$ pairs and, for the first time, for $\Lambda\overline{\Lambda}$ pairs. Effective averaged scattering parameters for heavier baryon-antibaryon pairs, not measured directly, are also provided. The results reveal similarly strong interaction between measured baryon-antibaryon pairs, suggesting that they all annihilate in the same manner at the same pair relative momentum $k^{*}$. Moreover, the reported significant non-zero imaginary part and negative real part of the scattering length provide motivation for future baryon-antibaryon bound state searches.
The production of c and b quarks in gamma-gamma collisions is studied with the L3 detector at LEP with 410 pb^-1 of data, collected at centre-of-mass energies from 189 GeV to 202 GeV. Hadronic final states containing c and b quarks are identified by detecting electrons or muons from their semileptonic decays. The cross sections sigma(e+e- -> e+e- c c~ X) and sigma(e+e- -> e+e- b b~ X) are measured and compared to next-to-leading order perturbative QCD calculations. The cross section of b production is measured in gamma-gamma collisions for the first time. It is in excess of the QCD prediction by a factor of three.
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity $|y_{ee}|<1$ in minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $\rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For $\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.
A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy-ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical $K\pi$, $p\pi$, and $Kp$ fluctuations as measured by the STAR experiment in central 0-5\% Au+Au collisions from center-of-mass collision energies $\rm \sqrt{s_{NN}}$ = 7.7 to 200 GeV are presented. The observable $\rm \nu_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $K\pi$, $p\pi$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5\% Au+Au collisions all demonstrate a smooth evolution with collision energy.
We present first measurements of the pseudorapidity and azimuth $(\eta,\phi)$ bin-size dependence of event-wise mean transverse momentum $<p_{t} >$ fluctuations for Au-Au collisions at $\sqrt{s_{NN}} = 200$ GeV. We invert that dependence to obtain $p_t$ autocorrelations on differences $(\eta_\Delta,\phi_\Delta)$ interpreted to represent velocity/temperature distributions on ($\eta,\phi$). The general form of the autocorrelations suggests that the basic correlation mechanism is parton fragmentation. The autocorrelations vary strongly with collision centrality, which suggests that fragmentation is strongly modified by a dissipative medium in the more central
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