Polarized Proton-proton Collisions Research Articles

Towards a global analysis of polarized parton distributions

We present a technique for implementing in a fast way, and without any approximations, higher-order calculations of partonic cross sections into global analyses of parton distribution functions. The approach, which is set up in Mellin-moment space, is particularly suited for analyses of future data from polarized proton-proton collisions, but not limited to this case. The usefulness and practicability of this method is demonstrated for the semi-inclusive production of hadrons in deep-inelastic scattering and the transverse momentum distribution of ``prompt'' photons in pp collisions, and a case study for a future global analysis of polarized parton densities is presented.

Lambda polarization in polarized proton-proton collisions at BNL RHIC

We discuss Lambda polarization in semi-inclusive proton-proton collisions, with one of the protons longitudinally polarized. The hyperfine interaction responsible for the $\Delta$-$N$ and $\Sigma$-$\Lambda$ mass splittings gives rise to flavor asymmetric fragmentation functions and to sizable polarized non-strange fragmentation functions. We predict large positive Lambda polarization in polarized proton-proton collisions at large rapidities of the produced Lambda, while other models, based on SU(3) flavor symmetric fragmentation functions, predict zero or negative Lambda polarization. The effect of $\Sigma^0$ and $\Sigma^*$ decays is also discussed. Forthcoming experiments at RHIC will be able to differentiate between these predictions.

Aspects of four-jet production in polarized proton-proton collisions.

We examine the intrinsic spin dependence of the dominant gg\ensuremath{\rightarrow}gggg subprocess contribution to four-jet production in polarized proton-proton collisions using helicity amplitude techniques. We find that the partonic level, longitudinal spin-spin asymmetry a${\mathrm{^}}_{\mathit{L}\mathit{L}}$ is intrinsically large in the kinematic regions probed in experiments detecting four isolated jets. Such events may provide another qualitative or semiquantitative test of the spin structure of QCD in planned polarized pp collisions at BNL RHIC.

Aspects of χ and ψ production in polarized proton-proton collisions

Several topics of relevance to low transverse momentum $\psi$ and $\chi_{1,2}(c\overline{c})$ production in polarized proton-proton collisions are discussed. The leading ${\cal O}(\alpha_S^3)$ contributions to the low $p_T$ $\chi_1$ production cross-sections via $gg$, $qg$, and $q\overline{q}$ initial states are calculated as well as the corresponding spin-spin asymmetries. We find that $\chi_1$ production increases relative to direct $\psi$ and $\chi_2$ production, providing up to $25\%$ of the observable $e^+e^-$ pairs arising from $\psi$ decays in $pp$ collisions at $\sqrt{s} = 500\,GeV$. The spin-dependence of $\chi_1$ production, however, is much smaller than for either direct $\psi$ or $\chi_2$ production and so will likely be far less useful than either process in probing the polarized gluon structure function of the proton. A subset of the ${\cal O}(\alpha_S^3)$ radiative corrections to $\chi_2$ production involving initial state quarks are also performed and compared to leading order $gg \rightarrow \chi_2$ predictions.

Associated J/ psi + gamma production as a probe of the polarized gluon distribution.

Associated production of $J/\psi$ and a $\gamma$ has recently been proposed as clean probe of the gluon distribution. The same mechanism can be used to probe the polarized gluon content of the proton in polarized proton-proton collisions. We study $J/\psi + \gamma$ production at both polarized fixed target and polarized collider energies.

Aspects of heavy quark production in polarized proton-proton collisions

We examine the spin-dependence of b-quark production at large p T in polarized pp collisions. The leading-order (LO) 2 → 2 subprocesses, g + g → Q + Q and q + q → Q + Q have a large ‘analyzing power’, with the average spin-spin asymmetry approaching −100% at large p T . Next-to-leading order (NLO) 2 → 3 processes have a large and positive asymmetry at large p T , leading to strong cancellations with the LO predictions. The results suggest that b-quark production might constitute an interesting test of the spin-dependence of NLO QCD but they also point up the need for the complete O (α 3 S spin-dependent corrections for extraction of the polarized gluon distribution.

Monte Carlo studies of photon production in polarized proton-proton collisions

We present first results obtained with a polarized version of the PYTHIA code, describing high-energy polarized hadron-hadron reactions. Such reactions are of great theoretical and experimental interest, as they give additional information on the internal spin structure of the hadrons, complementary to what can be obtained from polarized lepton-nucleon scattering. Experiments of this kind are under consideration both for the fixed target and for the collider mode. We demonstrate that corrections like e.g. showering influence the obtained asymmetries and enter the theoretical analyses necessary to deduce physically interesting quantities from the data. We also discuss the dominant background mechanisms, namely pion decay. We conclude that to determine the physics one can deduce from any kind of experimental setup a full-fledged numerical simulation is needed, towards which our present code is the first step.

Double photon production in polarized proton-proton collisions.

We derive the partonic level longitudinal spin-spin asymmetries for the processes relevant to the hadronic production of isolated double photon events, specifically $q\overline{q}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ and $\mathrm{gg}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$. We discuss production rates and observable asymmetries (${A}_{\mathrm{LL}}$) in polarized $\mathrm{pp}$ collisions at the BNL Relativistic Heavy Ion Collider and their dependence on the spin-dependent sea and gluon distributions. Because the decay ${H}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ is a possibly important signal for intermediate mass Higgs-scalar production, we also briefly discuss the spin dependence of standard model Higgs-boson production via $\mathrm{gg}\ensuremath{\rightarrow}{H}^{0}({\mathrm{low}}_{{P}_{T}})$ and $\mathrm{gg}\ensuremath{\rightarrow}g{H}^{0}$, $\mathrm{qg}\ensuremath{\rightarrow}q{H}^{0}$, and $q\overline{q}\ensuremath{\rightarrow}g{H}^{0}({\mathrm{high}}_{{P}_{T}})$.

Spin dependence of three-jet and two-jet plus photon production in polarized proton-proton collisions.

Using helicity-amplitude techniques, we derive the partonic level spin-spin asymmetries {ital {cflx a}}{sub {ital L}{ital L}} for all of the processes relevant to the hadronic production of three-jet and two-jet plus photon final states. We discuss the structure of the observable asymmetries in these processes in polarized proton-proton collisions and their dependence on the spin-dependent parton distributions.

Charmonium production at large transverse momentum in polarized proton-proton collisions.

We calculate the partonic asymmetries for the processes relevant to large-transverse-momentum quarkonium production in hadron collisions. We use these to evaluate the spin-spin asymmetries in large-${p}_{T}$ charmonium production in polarized proton-proton collisions using two different sets of polarized gluon distributions. The sensitivity of this process to the polarized gluon distribution is then compared to low-${p}_{T}$ $\ensuremath{\psi}$ production.

Low transverse momentum ψ and production in polarized proton-proton collisions

We present calculations of the spin-spin asymmetry in the low transverse momentum production of ψ's and ▪'s in polarized proton-proton collisions. A perturbative QCD approach using quarkonium wavefunctions derived from non-relativistic potential models is employed. The contributions from gg→ χ 0,2 and gg→g 3S 1 are known to dominate in this approach and we exhibit the sensitivity of the polarization asymmetry to various choices of polarized gluon distribution.

Higher power QCD mechanisms for largep T strange or charmed baryon production in polarized proton-proton collisions

We investigate potential higher power mechanisms for strange or charmed baryon production of large transverse momentum and calculate the corresponding helicity asymmetries, which turn out to be large and significantly different to those obtained from the leading power mechanism.

Polarization of prompt photons produced at large pT in polarized proton-proton collisions

Polarization transfer from a longitudinally polarized proton is shown to be particularly important in prompt photon production at large p T, and to provide a relatively unambiguous test of perturbative QCD.

Production of dimuons from high-energy polarized proton-proton collisions

The high-energy process p+p→ μ ++ μ −+X for dimuons with large invariant mass is studied in the case that the protons are polarized. In general, the cross section dσ/ dQ 2 dy dΩ is determined by nine structure functions, and several interesting correlations between the polarization direction and the observed cross section are possible. In the Drell-Yan parton model for the process, the structure of the cross section is much simpler: there are six linear relations among the structure functions. An experimental test of these relations would provide a stringent test of the Drell-Yan model.