Exclusive production of $\rho^0$ mesons was studied at the COMPASS experiment by scattering 160 GeV/$c$ muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured as a function of $Q^2$, $x_{Bj}$, or $p_{T}^{2}$. The $\sin \phi_S$ asymmetry is found to be $-0.019 \pm 0.008(stat.) \pm 0.003(syst.)$. All other asymmetries are also found to be of small magnitude and consistent with zero within experimental uncertainties. Very recent calculations using a GPD-based model agree well with the present results. The data is interpreted as evidence for the existence of chiral-odd, transverse generalized parton distributions.
Single-spin azimuthal asymmetries for a transversely (T) polarised target and unpolarised (U) beam.
Single-spin azimuthal asymmetries for a transversely (T) polarised target and unpolarised (U) beam.
Single-spin azimuthal asymmetries for a transversely (T) polarised target and unpolarised (U) beam.
The PHENIX experiment has measured open heavy-flavor production via semileptonic decay muons over the transverse momentum range 1 < pT < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in d+Au and p+p collisions at ?sNN = 200 GeV. In central d+Au collisions an enhancement (suppression) of heavy-flavor muon production is observed at backward (forward) rapidity relative to the yield in p+p collisions scaled by the number of binary collisions. Modification of the gluon density distribution in the Au nucleus contributes in terms of anti-shadowing enhancement and shadowing suppression; however, the enhancement seen at backward rapidity exceeds expectations from this effect alone. These results, implying an important role for additional cold nuclear matter effects, serves as a key baseline for heavy-quark measurements in A+A collisions and in constraining the magnitude of charmonia breakup effects at the Relativistic Heavy Ion Collider and the Large Hadron Collider.
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
We report the beam energy (\sqrt s_{NN} = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (\sigma), skewness (S), and kurtosis (\kappa) of the net-proton multiplicity distributions in Au+Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| < 0.5) and within the transverse momentum range 0.4 < pT < 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the Quantum Chromodynamic (QCD) phase diagram. The products of the moments, S\sigma and \kappa\sigma^{2}, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and anti-proton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation, and also to a hadron resonance gas model.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=7.7$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=11.5$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=19.6$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
The azimuthal anisotropy coefficients v_2 and v_4 of pi^0 and eta mesons are measured in Au+Au collisions at sqrt(s_NN)=200 GeV, as a function of transverse momentum p_T (1-14 GeV/c) and centrality. The extracted v_2 coefficients are found to be consistent between the two meson species over the measured p_T range. The ratio of v_4/v_2^2 for pi^0 mesons is found to be independent of p_T for 1-9 GeV/c, implying a lack of sensitivity of the ratio to the change of underlying physics with p_T. Furthermore, the ratio of v_4/v_2^2 is systematically larger in central collisions, which may reflect the combined effects of fluctuations in the initial collision geometry and finite viscosity in the evolving medium.
$\eta$ meson $v_2, 0-20\%$ centrality
$\eta$ meson $v_2, 20-60\%$ centrality
$\pi^0$ meson $v_4, 0-20\%$ centrality
Spectra of $K^0_S$ mesons and $\Lambda$ hyperons were measured in p+C interactions at 31 GeV/c with the large acceptance NA61/SHINE spectrometer at the CERN SPS. The data were collected with an isotropic graphite target with a thickness of 4% of a nuclear interaction length. Interaction cross sections, charged pion spectra, and charged kaon spectra were previously measured using the same data set. Results on $K^0_S$ and $\Lambda$ production in p+C interactions serve as reference for the understanding of the enhancement of strangeness production in nucleus-nucleus collisions. Moreover, they provide important input for the improvement of neutrino flux predictions for the T2K long baseline neutrino oscillation experiment in Japan. Inclusive production cross sections for $K^0_S$ and $\Lambda$ are presented as a function of laboratory momentum in intervals of the laboratory polar angle covering the range from 0 up to 240 mrad. The results are compared with predictions of several hadron production models. The $K^0_S$ mean multiplicity in production processes $<n_{K^0_S}>$ and the inclusive cross section for $K^0_S$ production were measured and amount to 0.127 $\pm$ 0.005 (stat) $\pm$ 0.022 (sys) and 29.0 $\pm$ 1.6 (stat) $\pm$ 5.0 (sys) mb, respectively.
$K^0_S$ production cross sections in the [0, 20] mrad polar interval.
$K^0_S$ production cross sections in the [20, 140] mrad polar interval.
$K^0_S$ production cross sections in the [140, 240] mrad polar interval.
The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 < {\eta} < 2.0 in polarized proton-proton collisions at sqrt{s} = 200 GeV are presented. Neutral pions were detected using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 < p_T < 16 GeV/c and is found to be within the scale uncertainty of a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, A_LL, is measured in the same pseudorapidity range. This quantity is sensitive to the gluonic contribution to the proton spin, {\Delta}g(x), at low Bjorken-x (down to x approx 0.01), where it is less constrained by measurements at central pseudorapidity. The measured A_LL is consistent with model predictions. The parity-violating asymmetry, A_L, is also measured and found to be consistent with zero. The transverse single-spin asymmetry, A_N, is measured within a previously unexplored kinematic range in Feynman-x and p_T. Such measurements may aid our understanding of the on-set and kinematic dependence of the large asymmetries observed at more forward pseudorapidity ({\eta} approx 3) and their underlying mechanisms. The A_N results presented are consistent with a twist-3 model prediction of a small asymmetry within the present kinematic range.
Distributions of x1 and x2 in two different bins of reconstructed $\pi^{0}$ pT for events at $\sqrt{s}$ = 200 GeV over 0.8 < $\eta$ < 2.
Comparison of data to Monte Carlo for the distributions of two-photon invariant mass (left) and energy for the higher (center) and lower (right) energy photon.
Comparison of data to Monte Carlo for the distributions of two-photon invariant mass (left) and energy for the higher (center) and lower (right) energy photon.
The production of $J/\psi$ mesons with rapidity $1.5<y<4.0$ or $-5.0<y<-2.5$ and transverse momentum $p_\mathrm{T}<14 \mathrm{GeV}/c$ is studied with the LHCb detector in proton-lead collisions at a nucleon-nucleon centre-of-mass energy $\sqrt{s_{NN}}=5 \mathrm{TeV}$. The analysis is based on a data sample corresponding to an integrated luminosity of about $1.6 \mathrm{nb}^{-1}$. For the first time the nuclear modification factor and forward-backward production ratio are determined separately for prompt $J/\psi$ mesons and $J/\psi$ from $b$-hadron decays. Clear suppression of prompt $J/\psi$ production with respect to proton-proton collisions at large rapidity is observed, while the production of $J/\psi$ from $b$-hadron decays is less suppressed. These results show good agreement with available theoretical predictions. The measurement shows that cold nuclear matter effects are important for interpretations of the related quark-gluon plasma signatures in heavy-ion collisions.
Single differential production cross sections of prompt J/PSI mesons and of J/PSI from B decay as a function of transverse momentum in the FORWARD region. The errors shown are statistical and the uncorrelated and correlated components of the systematic uncertainties.
Single differential production cross sections of prompt J/PSI mesons and of J/PSI from B decay as a function of transverse momentum in the BACKWARD region. The errors shown are statistical and the uncorrelated and correlated components of the systematic uncertainties.
Single differential production cross sections of prompt J/PSI mesons and of J/PSI from B decay as a function of rapidity in the FORWARD region. The errors shown are statistical and the uncorrelated and correlated components of the systematic uncertainties.
Measurements of $\gamma p \rightarrow K^{+} \Lambda$ and $\gamma p \rightarrow K^{+} \Sigma^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy ($W$) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark-diquark dynamics in the nucleon.
Excitation function at cos(Theta_K+)cm = -0.8
Excitation function at cos(Theta_K+)cm = -0.7
Excitation function at cos(Theta_K+)cm = -0.6
We report measurements of the inclusive transverse momentum pT distribution of centrally produced kshort, kstar(892), and phi(1020) mesons up to pT = 10 GeV/c in minimum-bias events, and kshort and lambda particles up to pT = 20 GeV/c in jets with transverse energy between 25 GeV and 160 GeV in pbar p collisions. The data were taken with the CDF II detector at the Fermilab Tevatron at sqrt(s) = 1.96 TeV. We find that as pT increases, the pT slopes of the three mesons (kshort, kstar, and phi) are similar, and the ratio of lambda to kshort as a function of pT in minimum-bias events becomes similar to the fairly constant ratio in jets at pT ~ 5 GeV/c. This suggests that the particles with pT >~ 5 GeV/c in minimum-bias events are from soft jets, and that the pT slope of particles in jets is insensitive to light quark flavor (u, d, or s) and to the number of valence quarks. We also find that for pT <~ 4 GeV relatively more lambda baryons are produced in minimum-bias events than in jets.
The inclusive invariant differential cross section for K0S production.
The inclusive invariant differential cross section for PHI production.
The inclusive invariant differential cross section for K*(892)+- production.
We report a high-statistics measurement of the differential cross section of the process gamma gamma --> K^0_S K^0_S in the range 1.05 GeV <= W <= 4.00 GeV, where W is the center-of-mass energy of the colliding photons, using 972 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e^+ e^- collider operated at and near the Upsilon-resonance region. The differential cross section is fitted by parameterized S-, D_0-, D_2-, G_0- and G_2-wave amplitudes. In the D_2 wave, the f_2(1270), a_2(1320) and f_2'(1525) are dominant and a resonance, the f_2(2200), is also present. The f_0(1710) and possibly the f_0(2500) are seen in the S wave. The mass, total width and product of the two-photon partial decay width and decay branching fraction to the K bar{K} state Gamma_{gamma gamma}B(K bar{K}) are extracted for the f_2'(1525), f_0(1710), f_2(2200) and f_0(2500). The destructive interference between the f_2(1270) and a_2(1320) is confirmed by measuring their relative phase. The parameters of the charmonium states chi_{c0} and chi_{c2} are updated. Possible contributions from the chi_{c0}(2P) and chi_{c2}(2P) states are discussed. A new upper limit for the branching fraction of the P- and CP-violating decay channel eta_c --> K^0_S K^0_S is reported. The detailed behavior of the cross section is updated and compared with QCD-based calculations.
The W dependence of the cross section after integrating up to |cosTheta*|<0.8 and 0.6.
The cos(Theta*) dependence of the differential cross section for the W ranges 1.10-1.11, 1.11-1.12 and 1.12-1.13.
The cos(Theta*) dependence of the differential cross section for the W ranges 1.13-1.14, 1.14-1.15 and 1.15-1.16.
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