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K. V. Dharmawardane

Publications and source records attributed to K. V. Dharmawardane.

46 records · Page 3Linked to original sources
arXiv2012

Cold-nuclear-matter effcts on heavy-quark production in d+Au collisions at sqrt(s_NN)=200 GeV

The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt(s_NN)=200 GeV at the Relativistic Heavy Ion Collider, in the transverse-momentum range 0.85 < pT < 8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R_dA at 1.5 < pT < 5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at RHIC extends to the heavy-D-meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the pi0 and heavy-flavor-electron nuclear modification factor R_AA.

nucl-ex↗
arXiv2011

Suppression of back-to-back hadron pairs at forward rapidity in d+Au Collisions at sqrt(s_NN)=200 GeV

Back-to-back hadron pair yields in d+Au and p+p collisions at sqrt(s_NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |eta|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<eta<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p_T, and eta points to cold nuclear matter effects arising at high parton densities.

nucl-ex↗
arXiv2011

Ground and excited charmonium state production in p+p collisions at sqrt(s)=200 GeV

We report on charmonium measurements [J/psi(1S), psi'(2S), and chi_c(1P)] in p+p collisions at sqrt(s)=200 GeV. We find that the fraction of J/psi coming from the feed-down decay of psi' and chi_c in the midrapidity region ($|η|<0.35$) is 9.6+/-2.4% and 32+/-9%, respectively. We also report new, higher statistics p_T and rapidity dependencies of the J/psi yield via dielectron decay in the same midrapidity range and at forward rapidity (1.2<|eta|<2.4) via dimuon decay. These results are compared with measurements from other experiments and discussed in the context of current charmonium production models.

hep-ex↗
arXiv2010

Cold Nuclear Matter Effects on J/psi Yields as a Function of Rapidity and Nuclear Geometry in Deuteron-Gold Collisions at sqrt(s_NN) = 200 GeV

We present measurements of J/psi yields in d+Au collisions at sqrt(s_NN) = 200 GeV recorded by the PHENIX experiment and compare with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/psi rapidity (-2.2 < y < 2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. To remove model dependent systematic uncertainties we also compare the data to a simple geometric model. We find that calculations where the nuclear modification is linear or exponential in the density weighted longitudinal thickness are difficult to reconcile with the forward rapidity data.

nucl-ex↗
arXiv2010

Measurement of Transverse Single-Spin Asymmetries for J/psi Production in Polarized p+p Collisions at sqrt(s) = 200 GeV

We report the first measurement of transverse single-spin asymmetries in $J/ψ$ production from transversely polarized $p+p$ collisions at $\sqrt{s} = 200$ GeV with data taken by the PHENIX experiment in 2006 and 2008. The measurement was performed over the rapidity ranges $1.2 < |y| < 2.2$ and $ |y| < 0.35$ for transverse momenta up to 6 GeV/$c$. $J/ψ$ production at RHIC is dominated by processes involving initial-state gluons, and transverse single-spin asymmetries of the $J/ψ$ can provide access to gluon dynamics within the nucleon. Such asymmetries may also shed light on the long-standing question in QCD of the $J/ψ$ production mechanism. Asymmetries were obtained as a function of $J/ψ$ transverse momentum and Feynman-$x$, with a value of $-0.086 \pm 0.026^{\rm stat} \pm 0.003^{\rm syst}$ in the forward region. This result suggests possible nonzero trigluon correlation functions in transversely polarized protons and, if well defined in this reaction, a nonzero gluon Sivers distribution function.

hep-ex↗
arXiv2010

Cross Section and Parity Violating Spin Asymmetries of W^+/- Boson Production in Polarized p+p Collisions at sqrt(s)=500 GeV

Large parity violating longitudinal single spin asymmetries A^{e^-}_L= -0.86^{+0.14}_{-0.30} and A^{e^+}_L= 0.88^{+0.12}_{-0.71} are observed for inclusive high transverse momentum electrons and positrons in polarized pp collisions at a center of mass energy of \sqrt{s}=500\ GeV with the PHENIX detector at RHIC. These e^{+/-} come mainly from the decay of W^{+/-} and Z^0 bosons, and the asymmetries directly demonstrate parity violation in the couplings of the W^{\pm} to the light quarks. The observed electron and positron yields were used to estimate W^\pm boson production cross sections equal to σ(pp \to W^+ X) \times BR(W^ \to ν_e)= 144.1+/-21.2(stat)^{+3.4}_{-10.3}(syst) +/- 15%(norm) pb, and σ(pp \to W^{-}X) \times BR(W^\to e^-\bar{ν_e}) = 31.7+/-12.1(stat)^{+10.1}_{-8.2}(syst)+/-15%(norm) pb.

hep-ex↗
arXiv2009

Moments of the Spin Structure Functions g_1^p and g_1^d for 0.05 < Q^2 < 3.0 GeV^2

The spin structure functions g_1 for the proton and the deuteron have been measured over a wide kinematic range in x and Q2 using 1.6 and 5.7 GeV longitudinally polarized electrons incident upon polarized NH_3 and ND_3 targets at Jefferson Lab. Scattered electrons were detected in the CEBAF Large Acceptance Spectrometer, for 0.05 < Q^2 < 5 GeV^2 and W < 3 GeV. The first moments of g_1 for the proton and deuteron are presented -- both have a negative slope at low Q^2, as predicted by the extended Gerasimov-Drell-Hearn sum rule. The first result for the generalized forward spin polarizability of the proton gamma_0^p is also reported. This quantity shows strong Q^2 dependence at low Q^2, while Q^6γ_0^p seems to flatten out at the highest Q^2 accessed by our experiment. Although the first moments of g_1 are consistent with Chiral Perturbation Theory (ChPT) calculations up to approximately Q^2 = 0.06 GeV^2, a significant discrepancy is observed between the γ_0^p data and ChPT for gamma_0^p, even at the lowest Q2.

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arXiv2006

Quark-Hadron Duality in Spin Structure Functions g1p and g1d

New measurements of the spin structure functions of the proton and deuteron g1p(x,Q2) and g1d(x,Q2) in the nucleon resonance region are compared with extrapolations of target-mass-corrected next-to-leading-order (NLO) QCD fits to higher energy data. Averaged over the entire resonance region (W<2 GeV), the data and QCD fits are in good agreement in both magnitude and Q2 dependence for Q2>1.7 GeV2. This global duality appears to result from cancellations among the prominent local resonance regions: in particular strong sigma{3/2} contributions in the Delta(1232) region appear to be compensated by strong sigma{1/2} contributions in the resonance region centered on 1.5 GeV. These results are encouraging for the extension of NLO QCD fits to lower W and Q2 than have been used previously.

hep-ph↗
arXiv2006

Measurement of the $x$- and $Q^2$-Dependence of the Asymmetry $A_1$ on the Nucleon

We report results for the virtual photon asymmetry $A_1$ on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ($^{15}$NH$_3$) and deuteron ($^{15}$ND$_3$) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present paper, we concentrate on our results for $A_1(x,Q^2)$ and the related ratio $g_1/F_1(x,Q^2)$ in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer $Q^2$ from 0.05 to 5.0 GeV$^2$ and in final-state invariant mass $W$ up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong $Q^2$--dependence of $A_1(x,Q^2)$ for $W$ below 2 GeV. At higher $W$, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but $A_1(x,Q^2)$ is not strictly $Q^2$--independent. We add significantly to the world data set at high $x$, up to $x = 0.6$. Our data exceed the SU(6)-symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative $d$-quark polarization up to our highest $x$. This data setshould improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.

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