We report a measurement of the reaction γγ→K+K−π+π− in both tagged and untagged events at PEP. The cross section rises with invariant γγ mass to about 15 nb at 2 GeV and falls slowly at higher masses. We find clear evidence for the processes γγ→φπ+π− and γγ→K*0(892)Kπ. Upper limits (95% C.L.) of 1.5 and 5.7 nb in the mass range from 1.7 to 3.7 GeV are obtained for φρ0 and K*0K¯*0 production, respectively.
In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, $v_1$ and $v_2$, of light nuclei ($d$, $t$, $^{3}$He, $^{4}$He) produced in $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured $v_1$ slopes of light nuclei at mid-rapidity. For the measured $v_2$ magnitude, a strong rapidity dependence is observed. Unlike $v_2$ at higher collision energies, the $v_2$ values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.
Results are presented on the exclusive production of four-prong final states in photon-photon collisions from the TPC/Two-Gamma detector at the SLAC e+e− storage ring PEP. Measurement of dE/dx and momentum in the time-projection chamber (TPC) provides identification of the final states 2π+2π−, K+K−π+π−, and 2K+2K−. For two quasireal incident photons, both the 2π+2π− and K+K−π+π− cross sections show a steep rise from threshold to a peak value, followed by a decrease at higher mass. Cross sections for the production of the final states ρ0ρ0, ρ0π+π−, and φπ+π− are presented, together with upper limits for φρ0, φφ, and K*0K¯ *0. The ρ0ρ0 contribution dominates the four-pion cross section at low masses, but falls to nearly zero above 2 GeV. Such behavior is inconsistent with expectations from vector dominance but can be accommodated by four-quark resonance models or by t-channel factorization. Angular distributions for the part of the data dominated by ρ0ρ0 final states are consistent with the production of JP=2+ or 0+ resonances but also with isotropic (nonresonant) production. When one of the virtual photons has mass (mγ2=-Q2≠0), the four-pion cross section is still dominated by ρ0ρ0 at low final-state masses Wγγ and by 2π+2π− at higher mass. Further, the dependence of the cross section on Q2 becomes increasingly flat as Wγγ increases.
Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While naïve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.
We report on the measurements of directed flow $v_1$ and elliptic flow $v_2$ for hadrons ($\pi^{\pm}$, $K^{\pm}$, $K_{S}^0$, $p$, $\phi$, $\Lambda$ and $\Xi^{-}$) from Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV and $v_{2}$ for ($\pi^{\pm}$, $K^{\pm}$, $p$ and $\overline{p}$) at 27 and 54.4 GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3 GeV the $v_{2}$ at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the $v_1$ slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative $v_2$ and positive $v_1$ slope at 3 GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions.
Data on multiplicities of charged particles produced in proton-nucleus and nucleus-nucleus collisions at 200 GeV per nucleon are presented. It is shown that the mean multiplicity of negative particles is proportional to the mean number of nucleons participating in the collision both for nucleus-nucleus and proton-nucleus collisions. The apparent consistency of pion multiplicity data with the assumption of an incoherent superposition of nucleon-nucleon collisions is critically discussed.
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 analyze a sample of W + jet events collected with the Collider Detector at Fermilab (CDF) in ppbar collisions at sqrt(s) = 1.8 TeV to study ttbar production. We employ a simple kinematical variable "H", defined as the scalar sum of the transverse energies of the lepton, neutrino and jets. For events with a W boson and four or more jets, the shape of the "H" distribution deviates by 3.8 standard deviations from that expected from known backgrounds to ttbar production. However this distribution agrees well with a linear combination of background and ttbar events, the agreement being best for a top mass of 180 GeV/c^2.
The reaction e^+e^- -> e^+e^- proton antiproton is studied with the L3 detector at LEP. The analysis is based on data collected at e^+e^- center-of-mass energies from 183 GeV to 209 GeV, corresponding to an integrated luminosity of 667 pb^-1. The gamma gamma -> proton antiproton differential cross section is measured in the range of the two-photon center-of-mass energy from 2.1 GeV to 4.5 GeV. The results are compared to the predictions of the three-quark and quark-diquark models.
We report a high-statistics measurement of differential cross sections for the process gamma gamma -> pi^0 pi^0 in the kinematic range 0.6 GeV <= W <= 4.0 GeV and |cos theta*| <= 0.8, where W and theta* are the energy and pion scattering angle, respectively, in the gamma gamma center-of-mass system. Differential cross sections are fitted to obtain information on S, D_0, D_2, G_0 and G_2 waves. The G waves are important above W ~= 1.6 GeV. For W <= 1.6 GeV the D_2 wave is dominated by the f_2(1270) resonance while the S wave requires at least one additional resonance besides the f_0(980), which may be the f_0(1370) or f_0(1500). The differential cross sections are fitted with a simple parameterization to determine the parameters (the mass, total width and Gamma_{gamma gamma}B(f_0 -> pi^0 pi^0)) of this scalar meson as well as the f_0(980). The helicity 0 fraction of the f_2(1270) meson, taking into account interference for the first time, is also obtained.
We have made a precise measurement of the central inclusive jet cross section at sqrt(s) = 1.8 TeV. The measurement is based on an integrated luminosity of 92 pb-1 collected at the Fermilab Tevatron pbar-p Collider with the D-Zero detector. The cross section, reported as a function of jet transverse energy (ET >= 60 GeV) in the pseudorapidity interval |eta| <= 0.5, is in good agreement with predictions from next-to-leading order quantum chromodynamics.
This paper presents the first measurement of the inclusive J/Psi production cross section in the forward pseudorapidity region 2.5<|eta|<3.7 in ppbar collisions at sqrt(s)=1.8TeV. The results are based on 9.8 pb-1 of data collected using the D0 detector at the Fermilab Tevatron Collider. The inclusive J/Psi cross section for transverse momenta between 1 and 16 GeV/c is compared with theoretical models of charmonium production.
The dielectron yield in p + d and p + p collisions at a beam kinetic energy of 4.9 GeV has been measured using the Dilepton Spectrometer (DLS) at the Bevalac. The measured ratio of the yield in p + d to that in p + p collisions, 1.92±0.06, is in disagreement with the assumptions of model calculations applied to our ealier p +Be data, where it was found that p + n bremsstrahlung dominated other sources. While the measured ratio is consistent with a hadron-like origin of the dielectrons, the contributions of known hadronic decays are smaller than the measured yield from p + p collissions.
We present results from a search for anomalous WW and WZ production in ppbar collisions at sqrt(s) = 1.8 TeV. We used ppbar->evjjX events observed during the 1992-1993 run of the Fermilab Tevatron collider, corresponding to an integrated luminosity of 13.7 +- 0.7 pb^-1. A fit to the transverse momentum spectrum of the W boson yields direct limits on the CP-conserving anomalous WWgamma and WWZ coupling parameters of -0.9 < delta kappa < 1.1 (with lambda = 0) and -0.6 < lambda < 0.7 (with delta kappa = 0) at the 95% confidence level, for a form factor scale Lambda = 1.5 TeV, assuming that the WWgamma and WWZ coupling parameters are equal.
We have searched for first generation scalar leptoquark (LQ) pairs in the enu+jets channel using ppbar collider data (integrated luminosity= 115 pb^-1) collected by the DZero experiment at the Fermilab Tevatron during 1992-96. The analysis yields no candidate events. We combine the results with those from the ee+jets and nunu+jets channels to obtain 95% confidence level (CL) upper limits on the LQ pair production cross section as a function of mass and of beta, the branching fraction to a charged lepton. Comparing with the next-to-leading order theory, we set 95% CL lower limits on the LQ mass of 225, 204, and 79 GeV/c^2 for beta=1, 1/2, and 0, respectively.
We present results on transverse momentum ($p_{\rm T}$) and rapidity ($y$) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/$\psi$ and $\psi(2S)$ at forward rapidity ($2.5<y<4$) as well as $\psi(2S)$-to-J/$\psi$ cross section ratios. These quantities are measured in pp collisions at center of mass energies $\sqrt{s}=5.02$ and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at $\sqrt{s}=2.76$, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full $p_{\rm T}$ range, provided that both contributions are summed. In particular, it is found that for $p_{\rm T}>15$ GeV/$c$ the non-prompt contribution reaches up to 50% of the total charmonium yield.
We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion Collider. A significant $v_3$ signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at mid-rapidity, $dv_3/dy|_{(y=0)}$, opposite in sign compared to the slope for directed flow. No significant $v_3$ signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity ($y$) and transverse momentum ($p_{\rm T}$) within $-0.5 < y<0$ and $0.4 < p_{\rm T} <2.0 $ GeV/$c$. In the most central 0--5% collisions, a proton cumulant ratio is measured to be $C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.82 ~(\rm syst.)$, which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our $C_4/C_2$ in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in $C_4/C_2$ is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV.
The differential cross section for the process $\gamma \gamma \to \eta \pi^0$ has been measured in the kinematic range $0.84 \GeV < W < 4.0 \GeV$, $|\cos \theta^*|<0.8$, where $W$ and $\theta^*$ are the energy and $\pi^0$ (or $\eta$) scattering angle, respectively, in the $\gamma\gamma$ center-of-mass system. The results are based on a 223 fb$^{-1}$ data sample collected with the Belle detector at the KEKB $e^+ e^-$ collider. Clear peaks due to the $a_0(980)$ and $a_2(1320)$ are visible. The differential cross sections are fitted in the energy region $0.9 \GeV < W < 1.46 \GeV$ to obtain the parameters of the $a_0(980)$. Its mass, width and $\Gamma_{\gamma \gamma} \B (\eta \pi^0)$ are measured to be $982.3 ^{+0.6}_{-0.7} ^{+3.1}_{-4.7} \MeV/c^2$, $75.6 \pm 1.6 ^{+17.4}_{-10.0} \MeV$ and $128 ^{+3}_{-2} ^{+502}_{-43} \eV$, respectively. The energy and angular dependences above 3.1 GeV are compared with those measured in the $\pi^0 \pi^0$ channel. The integrated cross section over $|\cos \theta^*|<0.8$ has a $W^{-n}$ dependence with $n = 10.5 \pm 1.2 \pm 0.5$, which is slightly larger than that for $\pi^0 \pi^0$. The differential cross sections show a $\sin^{-4} \theta^*$ dependence similar to $\gamma \gamma \to \pi^0 \pi^0$. The measured cross section ratio, $\sigma(\eta \pi^0)/\sigma(\pi^0 \pi^0) = 0.48 \pm 0.05 \pm 0.04$, is consistent with a QCD-based prediction.
We present measurements of the b-bbar production cross section and angular correlations using the D0 detector at the Fermilab Tevatron p-pbar Collider operating at sqrt(s) = 1.8 TeV. The b quark production cross section for |y(b)|<1.0 and p_T(b)>6 GeV/c is extracted from single muon and dimuon data samples. The results agree in shape with the next-to-leading order QCD calculation of heavy flavor production but are greater than the central values of these predictions. The angular correlations between b and bbar quarks, measured from the azimuthal opening angle between their decay muons, also agree in shape with the next-to-leading order QCD prediction.
We report the first multi-differential measurements of strange hadrons of $K^{-}$, $\phi$ and $\Xi^{-}$ yields as well as the ratios of $\phi/K^-$ and $\phi/\Xi^-$ in Au+Au collisions at ${\sqrt{s_{\rm NN}} = \rm{3\,GeV}}$ with the STAR experiment fixed target configuration at RHIC. The $\phi$ mesons and $\Xi^{-}$ hyperons are measured through hadronic decay channels, $\phi\rightarrow K^+K^-$ and $\Xi^-\rightarrow \Lambda\pi^-$. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The $4\pi$ yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the $\phi/K^-$ and $\phi/\Xi^-$ ratios while the result of canonical ensemble (CE) calculations reproduce $\phi/K^-$, with the correlation length $r_c \sim 2.7$ fm, and $\phi/\Xi^-$, $r_c \sim 4.2$ fm, for the 0-10% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at $\rm{3\,GeV}$ implies a rather different medium property at high baryon density.
The ALICE Collaboration has measured inclusive J/psi production in pp collisions at a center of mass energy sqrt(s)=2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges |y|<0.9 and 2.5<y<4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The integrated luminosities for the two channels are L^e_int=1.1 nb^-1 and L^mu_int=19.9 nb^-1, and the corresponding signal statistics are N_J/psi^e+e-=59 +/- 14 and N_J/psi^mu+mu-=1364 +/- 53. We present dsigma_J/psi/dy for the two rapidity regions under study and, for the forward-y range, d^2sigma_J/psi/dydp_t in the transverse momentum domain 0<p_t<8 GeV/c. The results are compared with previously published results at sqrt(s)=7 TeV and with theoretical calculations.
Using the DZero detector at the 1.8 TeV pbarp Fermilab Tevatron collider, we have measured the inclusive dijet mass spectrum in the central pseudorapidity region |eta_jet| < 1.0 for dijet masses greater than 200 Gev/c^2. We have also measured the ratio of spectra sigma(|eta_jet| < 0.5)/sigma(0.5 < |eta_jet| < 1.0). The order alpha_s^3 QCD predictions are in good agreement with the data and we rule out models of quark compositeness with a contact interaction scale < 2.4 TeV at the 95% confidence level.
The charged-particle fractional momentum distribution within jets, D(z), has been measured in dijet events from 1.8-TeV p¯p collisions in the Collider Detector at Fermilab. As expected from scale breaking in quantum chromodynamics, the fragmentation function D(z) falls more steeply as dijet invariant mass increases from 60 to 200 GeV/c2. The average fraction of the jet momentum carried by charged particles is 0.65±0.02(stat)±0.08(syst).
The yields and average transverse momenta of pions, kaons, and antiprotons produced at the Fermilab p¯p collider at s=300, 540, 1000, and 1800 GeV are presented and compared with data from the energies reached at the CERN collider. We also present data on the dependence of average transverse momentum 〈pt〉 and particle ratios as a function of charged particle density dNcdη; data for particle densities as high as six times the average value, corresponding to a Bjorken energy density 6 GeV/fm3, are reported. These data are relevant to the search for quark-gluon phase of QCD.
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