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Register a new userProduction cross sections of gamma-rays, electrons, and positrons in p-p collisions
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Because the production cross sections of gamma-rays, electrons, and positrons made in p-p collisions, $sigma_{pprightarrow gamma}$ and $sigma_{pprightarrow {e}^pm}$, respectively, are kinematically equivalent with respect to the parent pion-production cross section $sigma_{pprightarrow pi}$, we obtain $sigma_{pprightarrow {e}^pm}$ directly from the machine data on $sigma_{pprightarrow gamma}$. In Sato et al. (2012), we give explicitly $sigma_{pprightarrow gamma}$, reproducing quite well the accelerator data with LHC, namely $sigma_{pprightarrow {e}^pm}$ is applicable enough over the wide energy range from GeV to 20,PeV for projectile proton energy. We dicuss in detail the relation between the cross sections, and present explicitly $sigma_{pprightarrow {e}^pm}$ that are valid into the PeV electron energy.
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We report on the $W$ and $Z/gamma^*$ differential and total cross sections as well as the $W^+$/$W^-$ and $(W^+ + W^-)$/$(Z/gamma^*)$ cross-section ratios measured by the STAR experiment at RHIC in $p+p$ collisions at $sqrt{s} = 500$ GeV and $510$ GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the $W$ cross-section ratio is sensitive to the $bar{d}/bar{u}$ ratio. These measurements were taken at high $Q^2 sim M_W^2,M_Z^2$ and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR data sets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb$^{-1}$. We also assess the expected impact that our $W^+/W^-$ cross-section ratios will have on various quark distributions, and find sensitivity to the $bar{u}-bar{d}$ and $bar{d}/bar{u}$ distributions.
High-statistics measurements of differential cross sections and spin density matrix elements for the reaction $gamma p to phi p$ have been made using the CLAS detector at Jefferson Lab. We cover center-of-mass energies ($sqrt{s}$) from 1.97 to 2.84 GeV, with an extensive coverage in the $phi$ production angle. The high statistics of the data sample made it necessary to carefully account for the interplay between the $phi$ natural lineshape and effects of the detector resolution, that are found to be comparable in magnitude. We study both the charged- ($phi to K^+ K^-$) and neutral- ($phi to K^0_S K^0_L$) $Koverline{K}$ decay modes of the $phi$. Further, for the charged mode, we differentiate between the cases where the final $K^-$ track is directly detected or its momentum reconstructed as the total missing momentum in the event. The two charged-mode topologies and the neutral-mode have different resolutions and are calibrated against each other. Extensive usage is made of kinematic fitting to improve the reconstructed $phi$ mass resolution. Our final results are reported in 10- and mostly 30-MeV-wide $sqrt{s}$ bins for the charged- and the neutral-mode, respectively. Possible effects from $K^+ Lambda^ast$ channels with $p Koverline{K}$ final-states are discussed. These present results constitute the most precise and extensive $phi$ photoproduction measurements to date and in conjunction with the $omega$ photoproduction results recently published by CLAS, will greatly improve our understanding of low energy vector meson photoproduction.
We present strange particle spectra and yields measured at mid-rapidity in $sqrt{text{s}}=200$ GeV proton-proton ($p+p$) collisions at RHIC. We find that the previously observed universal transverse mass ($mathrm{m_{T}}equivsqrt{mathrm{p_{T}}^{2}+mathrm{m}^{2}}$) scaling of hadron production in $p+p$ collisions seems to break down at higher mt and that there is a difference in the shape of the mt spectrum between baryons and mesons. We observe mid-rapidity anti-baryon to baryon ratios near unity for $Lambda$ and $Xi$ baryons and no dependence of the ratio on transverse momentum, indicating that our data do not yet reach the quark-jet dominated region. We show the dependence of the mean transverse momentum (mpt) on measured charged particle multiplicity and on particle mass and infer that these trends are consistent with gluon-jet dominated particle production. The data are compared to previous measurements from CERN-SPS, ISR and FNAL experiments and to Leading Order (LO) and Next to Leading order (NLO) string fragmentation model predictions. We infer from these comparisons that the spectral shapes and particle yields from $p+p$ collisions at RHIC energies have large contributions from gluon jets rather than quark jets.
High-statistics differential cross sections for the reactions gamma p -> p eta and gamma p -> p eta-prime have been measured using the CLAS at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The eta-prime results are the most precise to date and provide the largest energy and angular coverage. The eta measurements extend the energy range of the worlds large-angle results by approximately 300 MeV. These new data, in particular the eta-prime measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states.
The production of $W^{pm}$ bosons in longitudinally polarized $vec{p}+vec{p}$ collisions at RHIC provides a new means of studying the spin-flavor asymmetries of the proton sea quark distributions. Details of the $W^{pm}$ event selection in the $e^{pm}$ decay channel at mid-rapidity are presented, along with preliminary results for the production cross section and parity-violating single-spin asymmetry, $A_L$, from the STAR Collaborations 2009 data at $sqrt{s}=500$ GeV.
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