We have searched for exclusive 2-photon production in proton-antiproton collisions at sqrt{s} = 1.96 TeV, using 532/pb of integrated luminosity taken by the Run II Collider Detector at Fermilab. The event signature requires two electromagnetic showers, each with transverse energy E_T > 5 GeV and pseudorapidity |eta|<1.0, with no other particles detected in the event. Three candidate events are observed. We discuss the consistency of the three events with gamma-gamma, pi0-pi0, or eta-eta production. The probability that other processes fluctuate to 3 events or more is 1.7x10^-4. An upper limit on the cross section of p+pbar --> p+gamma-gamma+pbar is set at 410 fb with 95% confidence level.
Upper limit on the cross section.
The cross section for e+e- to pi+ pi- psi(2S) between threshold and \sqrt{s}=5.5 GeV is measured using 673 fb^{-1} of data on and off the \Upsilon(4S) resonance collected with the Belle detector at KEKB. Two resonant structures are observed in the pi+ pi- psi(2S) invariant mass distribution, one at 4361\pm 9\pm 9 MeV/c2 with a width of 74\pm 15\pm 10 MeV/c2, and another at 4664\pm 11\pm 5 MeV/c2 with a width of 48\pm 15\pm 3 MeV/c2, if the mass spectrum is parameterized with the coherent sum of two Breit-Wigner functions. These values do not match those of any of the known charmonium states.
Measured cross section.
A measurement of the double-differential cross-section for the production of charged pions in proton--tantalum collisions emitted at large angles from the incoming beam direction is presented. The data were taken in 2002 with the HARP detector in the T9 beam line of the CERN PS. The pions were produced by proton beams in a momentum range from 3 \GeVc to 12 \GeVc hitting a tantalum target with a thickness of 5% of a nuclear interaction length. The angular and momentum range covered by the experiment ($100 \MeVc \le p < 800 \MeVc$ and $0.35 \rad \le \theta <2.15 \rad$) is of particular importance for the design of a neutrino factory. The produced particles were detected using a small-radius cylindrical time projection chamber (TPC) placed in a solenoidal magnet. Track recognition, momentum determination and particle identification were all performed based on the measurements made with the TPC. An elaborate system of detectors in the beam line ensured the identification of the incident particles. Results are shown for the double-differential cross-sections ${{\mathrm{d}^2 \sigma}} / {{\mathrm{d}p\mathrm{d}\theta}}$ at four incident proton beam momenta (3 \GeVc, 5 \GeVc, 8 \GeVc and 12 \GeVc). In addition, the pion yields within the acceptance of typical neutrino factory designs are shown as a function of beam momentum. The measurement of these yields within a single experiment eliminates most systematic errors in the comparison between rates at different beam momenta and between positive and negative pion production.
Double-differential cross section for PI+ production in the LAB system for PI+ polar angle from 0.35 to 0.55 radians.
Double-differential cross section for PI+ production in the LAB system for PI+ polar angle from 0.55 to 0.75 radians.
Double-differential cross section for PI+ production in the LAB system for PI+ polar angle from 0.75 to 0.95 radians.
We report the measurements of the t anti-t production cross section and of the top quark mass using 1.02 fb^-1 of p anti-p data collected with the CDFII detector at the Fermilab Tevatron. We select events with six or more jets on which a number of kinematical requirements are imposed by means of a neural network algorithm. At least one of these jets must be identified as initiated by a b-quark candidate by the reconstruction of a secondary vertex. The cross section is measured to be sigma_{tt}=8.3+-1.0(stat.)+2.0-1.5(syst.)+-0.5(lumi.) pb, which is consistent with the standard model prediction. The top quark mass of 174.0+-2.2(stat.)+-4.8(syst.) GeV/c^2 is derived from a likelihood fit incorporating reconstructed mass distributions representative of signal and background.
Total cross section measurement. The second DSYS error is the uncertainty on the luminosity.
We report the measurement of Lamda and Anti-Lamda yields and inverse slope parameters in d + Au collisions at sqrt(s_NN) = 200 GeV at forward and backward rapidities (y = +- 2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d + Au system. Comparisons to model calculations show that the baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side HIJING based models do not describe the measured particle yields while models with initial state nuclear effects and/or hadronic rescattering do. The Multi-Chain Model can provide a good description of the net baryon density in d + Au collisions at RHIC, and the derived parameters of the model agree with those from nuclear collisions at lower energies.
(Color online) a) Invariant mass distribution of $\Lambda$ candidates on the deuteron side with estimated $K_{s}^{0}$ background distribution, b) and c) show the background subtracted $\Lambda$ and $\overline{\Lambda}$ invariant mass distributions. The widths of the peaks are due to the limited momentum resolution of the detectors, and are reproduced by simulations.
(Color online) a) Invariant mass distribution of $\Lambda$ candidates on the deuteron side with estimated $K_{s}^{0}$ background distribution, b) and c) show the background subtracted $\Lambda$ and $\overline{\Lambda}$ invariant mass distributions. The widths of the peaks are due to the limited momentum resolution of the detectors, and are reproduced by simulations.
(Color online) a) Invariant mass distribution of $\Lambda$ candidates on the deuteron side with estimated $K_{s}^{0}$ background distribution, b) and c) show the background subtracted $\Lambda$ and $\overline{\Lambda}$ invariant mass distributions. The widths of the peaks are due to the limited momentum resolution of the detectors, and are reproduced by simulations.
We report on the observed differences in production rates of strange and multi-strange baryons in Au+Au collisions at sqrts = 200 GeV compared to pp interactions at the same energy. The strange baryon yields in Au+Au collisions, then scaled down by the number of participating nucleons, are enhanced relative to those measured in pp reactions. The enhancement observed increases with the strangeness content of the baryon, and increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at lower collision energy sqrts =17.3 GeV. The previous observations are for the bulk production, while at intermediate pT, 1 < pT< 4 GeV/c, the strange baryons even exceed binary scaling from pp yields.
Midrapidity E(i) as a function of $<N_{part}>$ for $\Lambda$, $\bar{\Lambda}$ ($|y| < 1.0$), $\Xi^{-}$, $\bar{\Xi}^{+}$ ($|y| < 0.75$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature. Grand Canonical Model arrows(values in brackets), for $\Lambda$ E(2.6) and T(165 MeV) for $\bar{\Lambda}$ E(2.2) and T(170 MeV), for $\Xi$ E(10.7) and T(165 MeV), for anti-$\Xi$ E(7.5) and T(170 MeV).
Midrapidity E(i) as a function of $<N_{part}>$ for $\Omega^{-}+\bar{\Omega}^{+}$ ($|y| < 0.75$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature.
Midrapidity E(i) as a function of $<N_{part}>$ for inclusive $p$ ($|y| < 0.5$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature.
The system created in non-central relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Due to spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of Lambda and anti-Lambda hyperon global polarization measurements in Au+Au collisions at sqrt{s_NN}=62.4 GeV and 200 GeV performed with the STAR detector at RHIC. The observed global polarization of Lambda and anti-Lambda hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, |P_{Lambda,anti-Lambda}| <= 0.02, is compared to the theoretical values discussed recently in the literature.
(Color online) Invariant mass distribution for the $\Lambda$ (filled circles) and $\overline{\Lambda}$ (open squares) candidates after the quality cuts for Au+Au collisions at $\sqrt{s_{NN}}$=62.4 GeV (centrality region 0-80%).
(Color online) Global polarization of $\Lambda$–hyperons as a function of $\Lambda$ transverse momentum $p^{\Lambda}_{t}$. Filled circles show the results for Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV (centrality region 20-70%) and open squares indicate the results for Au+Au collisions at $\sqrt{s_{NN}}$=62.4 GeV (centrality region 0-80%). Only statistical uncertainties are shown.
(Color online) Global polarization of $\Lambda$–hyperons as a function of $\Lambda$ pseudorapidity $\eta^{\Lambda}$. Filled circles show the results for Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV (centrality region 20-70%). A constant line fit to these data points yields $P_{\Lambda}=(2.8\pm 9.6)\times 10^{-3}$ with $\chi^{2}/ndf=6.5/10$. Open squares show the results for Au+Au collisions at $\sqrt{s_{NN}}$=62.4 GeV (centrality region 0-80%). A constant line fit gives $P_{\Lambda}=(1.9\pm 8.0)\times 10^{-3}$ with $\chi^{2}/ndf=14.3/10$. Only statistical uncertainties are shown.
We report the first measurement of the opening angle distribution between pairs of jets produced in high-energy collisions of transversely polarized protons. The measurement probes (Sivers) correlations between the transverse spin orientation of a proton and the transverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity ($-1 \leq \eta \leq +2$) coverage for jets permits separation of Sivers functions for the valence and sea regions. The resulting asymmetries are all consistent with zero and considerably smaller than Sivers effects observed in semi-inclusive deep inelastic scattering (SIDIS). We discuss theoretical attempts to reconcile the new results with the sizable transverse spin effects seen in SIDIS and forward hadron production in pp collisions.
Measured and calculated asymmetries vs. di-jet pseudorapidity sum for $+\hat{z}$ (left) and $−\hat{z}$ (right) beams. (a,b): Fraction of the calculated di-jet cross section with a quark (gluon) from the $+\hat{z}$ $(−\hat{z})$ beam. (c,d): Unweighted asymmetries compared with pQCD calculations [20] (histograms) for two models of quark Sivers functions fitted to SIDIS results [8]. (e,f): Asymmetries for $|\sin\zeta|$-weighted yields, compared with calculations [20, 21] based on twist-3 quark-gluon correlations. Vertical (horizontal) bars on the data indicate statistical uncertainties (bin widths). The systematic error bands exclude a $\pm12\%$ beam polarization normalization uncertainty.
Measured and calculated asymmetries vs. di-jet pseudorapidity sum for $+\hat{z}$ (left) and $−\hat{z}$ (right) beams. (a,b): Fraction of the calculated di-jet cross section with a quark (gluon) from the $+\hat{z}$ $(−\hat{z})$ beam. (c,d): Unweighted asymmetries compared with pQCD calculations [20] (histograms) for two models of quark Sivers functions fitted to SIDIS results [8]. (e,f): Asymmetries for $|\sin\zeta|$-weighted yields, compared with calculations [20, 21] based on twist-3 quark-gluon correlations. Vertical (horizontal) bars on the data indicate statistical uncertainties (bin widths). The systematic error bands exclude a $\pm12\%$ beam polarization normalization uncertainty.
Measured and calculated asymmetries vs. di-jet pseudorapidity sum for $+\hat{z}$ (left) and $−\hat{z}$ (right) beams. (a,b): Fraction of the calculated di-jet cross section with a quark (gluon) from the $+\hat{z}$ $(−\hat{z})$ beam. (c,d): Unweighted asymmetries compared with pQCD calculations [20] (histograms) for two models of quark Sivers functions fitted to SIDIS results [8]. (e,f): Asymmetries for $|\sin\zeta|$-weighted yields, compared with calculations [20, 21] based on twist-3 quark-gluon correlations. Vertical (horizontal) bars on the data indicate statistical uncertainties (bin widths). The systematic error bands exclude a $\pm12\%$ beam polarization normalization uncertainty.
Azimuthal angle \Delta\phi correlations are presented for charged hadrons from dijets for 0.4 < p_T < 10 GeV/c in Au+Au collisions at sqrt(s_NN) = 200 GeV. With increasing p_T, the away-side distribution evolves from a broad to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed 'head' region centered at Delta\phi ~ \pi, and an enhanced 'shoulder' region centered at Delta\phi ~ \pi +/- 1.1. The p_T spectrum for the 'head' region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the 'shoulder' region is independent of centrality and trigger p_T, which offers constraints on energy transport mechanisms and suggests that the 'shoulder' region contains the medium response to energetic jets.
<p>Per-trigger yield versus $\Delta\phi$ for various trigger and partner $p_T$ ($p_T^A$ $\otimes$ $p_T^B$), in $p$+$p$ and 0-20% Au+Au collisions.</p> <p><i>The systematic errors on $v_2$ are represented by "sys. corr." and are dominated by the reaction plane resolution. The fit that accounts for the overlap of the near- and away-side Gaussians at $\Delta\phi_{min}$ gives systematically lower $b_0$ values that that for ZYAM, and is assigned as the differences as one-sided systematic errors on $b_0$ represented by "sys. uncorr.".
<p>Per-trigger yield versus $\Delta\phi$ for various trigger and partner $p_T$ ($p_T^A$ $\otimes$ $p_T^B$), in $p$+$p$ and 0-20% Au+Au collisions.</p> <p><i>The systematic errors on $v_2$ are represented by "sys. corr." and are dominated by the reaction plane resolution. The fit that accounts for the overlap of the near- and away-side Gaussians at $\Delta\phi_{min}$ gives systematically lower $b_0$ values that that for ZYAM, and is assigned as the differences as one-sided systematic errors on $b_0$ represented by "sys. uncorr.".</p></i>
<p>$R_{HS}$ versus $p_T^B$ for $p$+$p$ and Au+Au collisions for four trigger selections.</p> <p><i>The systematic errors on $v_2$ are represented by "sys. corr." and are dominated by the reaction plane resolution. The fit that accounts for the overlap of the near- and away-side Gaussians at $\Delta\phi_{min}$ gives systematically lower $b_0$ values that that for ZYAM, and is assigned as the differences as one-sided systematic errors on $b_0$ represented by "sys. uncorr.".</p></i>
We have measured the polarizations of J/ψ and ψ(2S) mesons as functions of their transverse momentum pT when they are produced promptly in the rapidity range |y|<0.6 with pT≥5 GeV/c. The analysis is performed using a data sample with an integrated luminosity of about 800 pb−1 collected by the CDF II detector. For both vector mesons, we find that the polarizations become increasingly longitudinal as pT increases from 5 to 30 GeV/c. These results are compared to the predictions of nonrelativistic quantum chromodynamics and other contemporary models. The effective polarizations of J/ψ and ψ(2S) mesons from B-hadron decays are also reported.
Polarization parameter ALPHA for J/PSI production.
Polarization parameter ALPHA for PSI(2S) production.
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