Charged-particle spectra associated with direct photon ($\gamma_{dir} $) and $\pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $\gamma_{dir}$ and $\pi^0$. Assuming no associated charged particles in the $\gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($\gamma_{frag}$), the associated charged-particle yields opposite to $\gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|\eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $\gamma_{dir}$ and $\pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|\eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $\gamma_{dir}$ and $\pi^0$, and independent of the $\gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.
The $z_{T}$ dependence of $\pi^{0}-h^{\pm}$ near side and away-side associated particle yields. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
The $z_{T}$ dependence of away-side associated-particle yields for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
The $z_{T}$ dependence $I_{AA}$ for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
Measurements of the differential cross-section and the transverse single-spin asymmetry, A_N, vs. x_F for pi0 and eta mesons are reported for 0.4 < x_F < 0.75 at an average pseudorapidity of 3.68. A data sample of approximately 6.3 pb^{-1} was analyzed, which was recorded during p+p collisions at sqrt{s} = 200 GeV by the STAR experiment at RHIC. The average transverse beam polarization was 56%. The cross-section for pi0 is consistent with a perturbative QCD prediction, and the eta/pi0 cross-section ratio agrees with previous mid-rapidity measurements. For 0.55 < x_F < 0.75, A_N for eta (0.210 +- 0.056) is 2.2 standard deviations larger than A_N for pi0 (0.081 +- 0.016).
Differential production cross-sections for $\pi^0$ and $\eta$ at average pseudorapidity of 3.68. Also shown are the previously published STAR results for similar kinematics [21] and a NLO pQCD calculation of the π0 cross-section [32]. The error band represents the uncertainty in the calculation due to scale variations.
The $\eta$ to $\pi^0$ cross-section ratio is shown in the bottom panel. The error bars indicate the total statistical and systematic uncertainties.
We present the measurement of non-photonic electron production at high transverse momentum ($p_T > $ 2.5 GeV/$c$) in $p$ + $p$ collisions at $\sqrt{s}$ = 200 GeV using data recorded during 2005 and 2008 by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The measured cross-sections from the two runs are consistent with each other despite a large difference in photonic background levels due to different detector configurations. We compare the measured non-photonic electron cross-sections with previously published RHIC data and pQCD calculations. Using the relative contributions of B and D mesons to non-photonic electrons, we determine the integrated cross sections of electrons ($\frac{e^++e^-}{2}$) at 3 GeV/$c < p_T <~$10 GeV/$c$ from bottom and charm meson decays to be ${d\sigma_{(B\to e)+(B\to D \to e)} \over dy_e}|_{y_e=0}$ = 4.0$\pm0.5$({\rm stat.})$\pm1.1$({\rm syst.}) nb and ${d\sigma_{D\to e} \over dy_e}|_{y_e=0}$ = 6.2$\pm0.7$({\rm stat.})$\pm1.5$({\rm syst.}) nb, respectively.
The electron pair invariant mass distributions for electrons at $2.5 < p_{T} < 3.0$ GeV/c
The electron pair invariant mass distributions for electrons at $8 < p_{T} < 10$ GeV/c
The simulated electron pair invariant mass distributions for electrons at $2.5 < p_{T} < 3$ GeV/c
We report measurements of charmed-hadron ($D^{0}$, $D^{*}$) production cross sections at mid-rapidity in $p$ + $p$ collisions at a center-of-mass energy of 200 GeV by the STAR experiment. Charmed hadrons were reconstructed via the hadronic decays $D^{0}\rightarrow K^{-}\pi^{+}$, $D^{*+}\rightarrow D^{0}\pi^{+}\rightarrow K^{-}\pi^{+}\pi^{+}$ and their charge conjugates, covering the $p_T$ range of 0.6$-$2.0 GeV/$c$ and 2.0$-$6.0 GeV/$c$ for $D^{0}$ and $D^{*+}$, respectively. From this analysis, the charm-pair production cross section at mid-rapidity is $d\sigma/dy|_{y=0}^{c\bar{c}}$ = 170 $\pm$ 45 (stat.) $^{+38}_{-59}$ (sys.) $\mu$b. The extracted charm-pair cross section is compared to perturbative QCD calculations. The transverse momentum differential cross section is found to be consistent with the upper bound of a Fixed-Order Next-to-Leading Logarithm calculation.
$c\bar{c}$ production cross section as inferred from D$^0$ and D$^\star$ production in p+p collisions at $\sqrt{s} = 200$ GeV at $|\eta| < 1$ compared with FONLL calculations. The D$^0$ and D$^\star$ data points were divided by the charm quark fragmentation ratios 0.565 ($c \rightarrow$ D$^0$) and 0.224 ($c \rightarrow$ D$^{\star +}$) [34], respectively, to convert to the $c\bar{c}$ production cross section.
We present results on strange and multi-strange particle production in Au+Au collisions at $\sqrt{s_{NN}}=62.4$ GeV as measured with the STAR detector at RHIC. Mid-rapidity transverse momentum spectra and integrated yields of $K^{0}_{S}$, $\Lambda$, $\Xi$, $\Omega$ and their anti-particles are presented for different centrality classes. The particle yields and ratios follow a smooth energy dependence. Chemical freeze-out parameters, temperature, baryon chemical potential and strangeness saturation factor obtained from the particle yields are presented. Intermediate transverse momentum ($p_T$) phenomena are discussed based on the ratio of the measured baryon-to-meson spectra and nuclear modification factor. The centrality dependence of various measurements presented show a similar behavior as seen in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
Strange particle production yields at mid-rapidity in central Au+Au and Pb+Pb collisions versus the center of mass energy √sNN. The top panel shows results for K0S and Λ. The AGS values are from E896 [1] (centrality 0 − 5 %). The SPS values are from NA49 [20] (centrality 0 − 7 %) and the RHIC values are from STAR [4, 15] (centrality 0 − 5 %). For the multi-strange baryons Ξ and Ω (bottom panel), the SPS results are from NA57 [2] (centrality 0 − 11 %) and the RHIC values are from STAR [15, 21] (centrality 0 − 20 %).
Strange particle production yields at mid-rapidity in central Au+Au and Pb+Pb collisions versus the center of mass energy √sNN. The top panel shows results for K0S and Λ. The AGS values are from E896 [1] (centrality 0 − 5 %). The SPS values are from NA49 [20] (centrality 0 − 7 %) and the RHIC values are from STAR [4, 15] (centrality 0 − 5 %). For the multi-strange baryons Ξ and Ω (bottom panel), the SPS results are from NA57 [2] (centrality 0 − 11 %) and the RHIC values are from STAR [15, 21] (centrality 0 − 20 %).
Temperature and baryon chemical potential obtained from thermal model fits as a function of √sNN (see Ref. [22]). The dashed lines correspond to the parametrizations given in Ref. [22]. The solid stars show the result for √sNN=62.4 and 200 GeV.
We report the first measurement of the parity violating single-spin asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized proton-proton collisions at $\sqrt{s}=500 $GeV by the STAR experiment at RHIC. The measured asymmetries, $A^{W^+}_{L}=-0.27\pm 0.10\/({\rm stat.})\pm 0.02\/({\rm syst.}) \pm 0.03\/({\rm norm.})$ and $A^{W^-}_{L}=0.14\pm 0.19\/({\rm stat.})\pm 0.02 \/({\rm syst.})\pm 0.01\/({\rm norm.})$, are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized DIS measurements.
$E^e_T$ for W+ (bottom) and W− (top) events showing the candidate histograms in black, the full background estimates in blue and the signal distributions in yellow.
$E^e_T$ for W+ (bottom) and W− (top) events showing the candidate histograms in black, the full background estimates in blue and the signal distributions in yellow.
Longitudinal single-spin asymmetry, AL, for W± events as a function of the leptonic pseudorapidity, $\eta_e$, for 25 < $E^e_T$ < 50 GeV in comparison to theory predictions
The longitudinal spin transfer, $D_{LL}$, from high energy polarized protons to $\Lambda$ and $\bar{\Lambda}$ hyperons has been measured for the first time in proton-proton collisions at $\sqrt{s} = 200 \mathrm{GeV}$ with the STAR detector at RHIC. The measurements cover pseudorapidity, $\eta$, in the range $|\eta| < 1.2$ and transverse momenta, $p_\mathrm{T}$, up to $4 \mathrm{GeV}/c$. The longitudinal spin transfer is found to be $D_{LL}= -0.03\pm 0.13(\mathrm{stat}) \pm 0.04(\mathrm{syst})$ for inclusive $\Lambda$ and $D_{LL} = -0.12 \pm 0.08(\mathrm{stat}) \pm 0.03(\mathrm{syst})$ for inclusive $\bar{\Lambda}$ hyperons with $<\eta> = 0.5$ and $<p_\mathrm{T}> = 3.7 \mathrm{GeV}/c$. The dependence on $\eta$ and $p_\mathrm{T}$ is presented.
The contribution of $B$ meson decays to non-photonic electrons, which are mainly produced by the semi-leptonic decays of heavy flavor mesons, in $p+p$ collisions at $\sqrt{s} =$ 200 GeV has been measured using azimuthal correlations between non-photonic electrons and hadrons. The extracted $B$ decay contribution is approximately 50% at a transverse momentum of $p_{T} \geq 5$ GeV/$c$. These measurements constrain the nuclear modification factor for electrons from $B$ and $D$ meson decays. The result indicates that $B$ meson production in heavy ion collisions is also suppressed at high $p_{T}$.
Distributions of the azimuthal angle between nonphotonic electrons and charged hadrons normalized per nonphotonic electron trigger. The trigger electron has (top) $2.5 < p_{T} < 3.5$ GeV/$c$ and (bottom) $5.5 < p_{T} < 6.5$ GeV/$c$. The curves represent PYTHIA calculations for $D$ (dotted curve) and $B$ (dashed curve) decays. The fit result is shown as the black solid curve.
(a) Background-subtracted invariant mass distribution of $K$ pairs requiring at least one nonphotonic electron trigger in the event. The solid line is a Gaussian fit to the data near the peak region. (b) Distribution of the azimuthal angle between nonphotonic electron (positron) trigger particles and $D^{0}$ ($\bar{D}^{0}$). The solid (dashed) line is a fit of the correlation function from PYTHIA (MC$@$NLO) simulations to the data points.
Transverse momentum dependence of the relative contribution from $B$ mesons ($r_{B}$) to the nonphotonic electron yields. Error bars are statistical and brackets are systematic uncertainties. The solid curve is the FONLL calculation [14]. Theoretical uncertainties are indicated by the dashed curves.
We report STAR measurements of the longitudinal double-spin asymmetry A_LL, the transverse single-spin asymmetry A_N, and the transverse double-spin asymmetries A_Sigma and A_TT for inclusive jet production at mid-rapidity in polarized p+p collisions at a center-of-mass energy of sqrt{s} = 200 GeV. The data represent integrated luminosities of 7.6 /pb with longitudinal polarization and 1.8 /pb with transverse polarization, with 50-55% beam polarization, and were recorded in 2005 and 2006. No evidence is found for the existence of statistically significant jet A_N, A_Sigma, or A_TT at mid-rapidity. Recent model calculations indicate the A_N results may provide new limits on the gluon Sivers distribution in the proton. The asymmetry A_LL significantly improves the knowledge of gluon polarization in the nucleon.
(color online) $A_{LL}$ for inclusive jet production versus corrected jet $p_{T}$ for 2005 data. For 2005 data we used a cone radius equal to 0.4 and a pseudorapidity range for the jet thrust axis of $0.2<\eta<0.8$. The error bars are statistical. The gray boxes show the systematic uncertainties.
(color online) $A_{LL}$ for inclusive jet production versus corrected jet $p_{T}$ for 2006 data. For 2006 data we used a jet cone radius equal to 0.7 and a pseudorapidity range of $−0.7<\eta<0.9$ to reflect the increased detector coverage. The error bars are statistical. The gray boxes show the systematic uncertainties.
We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.
Cross sections for inclusive $\pi^0$ production in p + p and d + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The solid lines correspond to NLO pQCD calculations. The measured $\pi^0$ cross sections were not corrected for feed-down contributions $\eta$ -> 3$\pi^0$, $\eta$ -> $\pi^+\pi^-\pi^0$, and $K_S^0$ -> $\pi^0\pi^0$, which were expected to be negligible. Normalization uncertainties of 11.7% for p+p and 5.3% for d+Au are not shown.
Nuclear modification factor $R_{CP}$ for $\pi^0$ measured in d + Au collisions, compared to STAR $\pi^{+-}$ measurements [24]. Peripheral d + Au collisions were used where $R_{CP}$ = ratio of particle production in 40-100% centrality (P) events and 0-20% centrality (C) events. Shaded bands in plot are $p_T$-correlated systematic uncertainties and the error bars are statistical uncertainties. Normalization uncertainty of 11.0% indicated by a shaded band around unity.
Direct photon yield in p + p collisions at $\sqrt{s_{NN}}$ = 200 GeV in terms of the double ration $R_{\gamma}$. Shaded bands in plot are $p_T$-correlated systematic uncertainties and the error bars are statistical uncertainties. The curves correspond to NLO pQCD calculations of the differential cross sections for direct photon [68] and $\pi^0$ [52] production in p + p collisions for different factorization scales $\mu$ (the upper pQCD curve corresponds to $\mu$ = $p_T$/2). The upper limit of the fractional neutral hadron contamination $C_0$ is shown as the shaded band at $R_{\gamma}$ = 0.
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