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Register a new userMeasurement of the $W to e u$ and $Z/gamma^* to e^+e^-$ Production Cross Sections at Mid-rapidity in Proton-Proton Collisions at $sqrt{s}$ = 500 GeV
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We report measurements of the charge-separated $W^{+(-)} to e^{+(-)} + u_e(bar{ u}_e)$ and $Z/gamma^* to e^+e^-$ production cross sections at mid-rapidity in proton-proton collisions at $sqrt{s}$ = 500 GeV. These results are based on 13.2 pb$^{-1}$ of data recorded in 2009 by the STAR detector at RHIC. Production cross sections for W bosons that decay via the $e u$ channel were measured to be $sigma(pp to W^+ X) cdot BR(W^+ to e^+ u_e)$ = 117.3 pm 5.9(stat) pm 6.2(syst) pm 15.2(lumi) pb, and $sigma(pp to W^- X) cdot BR(W^- to e^- bar{ u}_e)$ = 43.3 pm 4.6(stat) pm 3.4(syst) pm 5.6(lumi) pb. For $Z/gamma^*$ production, $sigma(pp to Z/gamma^* X) cdot BR(Z/gamma^* to e^+ e^-)$ = 7.7 pm 2.1(stat) $^{+0.5}_{-0.9}$(syst) pm 1.0(lumi) pb for di-lepton invariant masses $m_{e^+e^-}$ between 70 and 110 GeV/$c^2$. First measurements of the W cross section ratio, $sigma(pp to W^+ X) / sigma(pp to W^- X)$, at $sqrt{s}$ = 500 GeV are also reported. Theoretical predictions, calculated using recent parton distribution functions, are found to agree with the measured cross sections.
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We present measurements of the differential cross sections of inclusive $J/psi$ meson production as a function of transverse momentum ($p_{T}^{J/psi}$) using the $mu^{+}mu^{-}$ and $e^{+}e^{-}$ decay channels in proton+proton collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the $mu^{+}mu^{-}$ channel is for 0 $< p_{T}^{J/psi} <$ 9 GeV/$c$ and rapidity range $|y^{J/psi}| < $ 0.4, and that from the $e^{+}e^{-}$ channel is for 4 $< p_{T}^{J/psi} <$ 20 GeV/$c$ and $|y^{J/psi}| < $ 1.0. The $psi(2S)$ to $J/psi$ ratio is also measured for 4 $< p_{T}^{rm meson} <$ 12 GeV/$c$ through the $e^{+}e^{-}$ decay channel. Model calculations, which incorporate different approaches toward the $J/psi$ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.
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We report results on the total and elastic cross sections in proton-proton collisions at $sqrt{s}=200$ GeV obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section was measured in the squared four-momentum transfer range $0.045 leq -t leq 0.135$ GeV$^2$. The value of the exponential slope parameter $B$ of the elastic differential cross section $dsigma/dt sim e^{-Bt}$ in the measured $-t$ range was found to be $B = 14.32 pm 0.09 (stat.)^{scriptstyle +0.13}_{scriptstyle -0.28} (syst.)$ GeV$^{-2}$. The total cross section $sigma_{tot}$, obtained from extrapolation of the $dsigma/dt$ to the optical point at $-t = 0$, is $sigma_{tot} = 54.67 pm 0.21 (stat.) ^{scriptstyle +1.28}_{scriptstyle -1.38} (syst.)$ mb. We also present the values of the elastic cross section $sigma_{el} = 10.85 pm 0.03 (stat.) ^{scriptstyle +0.49}_{scriptstyle -0.41}(syst.)$ mb, the elastic cross section integrated within the STAR $t$-range $sigma^{det}_{el} = 4.05 pm 0.01 (stat.) ^{scriptstyle+0.18}_{scriptstyle -0.17}(syst.)$ mb, and the inelastic cross section $sigma_{inel} = 43.82 pm 0.21 (stat.) ^{scriptstyle +1.37}_{scriptstyle -1.44} (syst.)$ mb. The results are compared with the world data.
We report first measurements of $e^{+}e^{-}$ pair production in the mass region 0.4 $<M_{ee}<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_T<$ 0.15 GeV/$c$) in non-central Au$+$Au collisions at $sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $sqrt{s_{NN}}$ = 193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40-80% centrality of these collisions. The excess yields peak distinctly at low-$p_T$ with a width ($sqrt{langle p^2_Trangle}$) between 40 to 60 MeV/$c$. The absolute cross section of the excess depends weakly on centrality while those from a theoretical model calculation incorporating an in-medium broadened $rho$ spectral function and radiation from a Quark Gluon Plasma or hadronic cocktail contributions increase dramatically with increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the $p^{2}_{T}$ distributions.
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