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Register a new userMeasurements of $W$ and $Z/gamma^*$ cross sections and their ratios in $p+p$ collisions at RHIC
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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.
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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.
Particle production sensitive to non-factorizable and non-perturbative processes that contribute to the underlying event associated with a high transverse momentum ($p_{T}$) jet in proton+proton collisions at $sqrt{s}$=200 GeV is studied with the STAR detector. Each event is divided into three regions based on the azimuthal angle with respect to the highest-$p_{T}$ jet direction: in the leading jet direction (Toward), opposite to the leading jet (Away), and perpendicular to the leading jet (Transverse). In the Transverse region, the average charged particle density is found to be between 0.4 and 0.6 and the mean transverse momentum, $langle p_{T}rangle$, between 0.5-0.7 GeV/$c$ for particles with $p_{T}$$>$0.2 GeV/$c$ at mid-pseudorapidity ($|eta|$$<$1) and jet $p_{T}$$>$15 GeV/$c$. Both average particle density and $langle p_{T}rangle$ depend weakly on the leading jet $p_{T}$. Closer inspection of the Transverse region hints that contributions to the underlying event from initial- and final-state radiation are significantly smaller in these collisions than at the higher energies, up to 13 TeV, recorded at the LHC. Underlying event measurements associated with a high-$p_{T}$ jet will contribute to our understanding of QCD processes at hard and soft scales at RHIC energies, as well as provide constraints to modeling of underlying event dynamics.
We present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at $sqrt{s} = 500~text{GeV}$ by the STAR experiment at RHIC. The measured observable is sensitive to the Sivers function, one of the transverse momentum dependent parton distribution functions, which is predicted to have the opposite sign in proton-proton collisions from that observed in deep inelastic lepton-proton scattering. These data provide the first experimental investigation of the non-universality of the Sivers function, fundamental to our understanding of QCD.
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.
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.
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