Parton-parton Cross Sections Research Articles

Charged particle elliptic flow in collisions at LHC energies in a parton and hadron cascade model PACIAE

The parton and hadron cascade model PACIAE based on PYTHIA was used to investigate the charged particle elliptic flow in minimum bias pp collisions at the LHC energies. The strings were distributed randomly in the transverse ellipsoid of the pp collision system with major axis of $R$ (proton radius) and minor axis of $R(1-\xi)$ before parton rescattering. The charged particle elliptic flow as a function of the random number $\xi$ and transverse momentum $p_T$ were investigated. The calculated $v_2/\epsilon$ as a function of reaction energy increases monotonously with increasing reaction energy up to $\sqrt{s}\sim$7 TeV and then turns to saturation. With the parton-parton cross section enlarges three times in parton rescattering, the rapidity integrated charged particle elliptic flow may reach 0.025 at $p_T \sim$2 GeV/c in the minimum bias pp collisions at $\sqrt{s}$=7 TeV.

Instanton-induced inelastic collisions in QCD

We show that the instanton-induced inelastic processes, leading to multi-gluon production in in high-energy parton-parton scattering, are considerably enhanced over the quasi-elastic ones, by a factor of 100. The basic instanton-induced inelastic contribution cause the parton-parton cross section to increase as ${\rm ln} {s}$, and their Poisson resummation in hadron-hadron scattering yield a regge-type cross section. The pomeron slope and intercept due to instanton-induced contributions are evaluated. We show that the small intercept is due to the diluteness of the instantons in the QCD vacuum, while the small slope is related to the smallness of the instanton sizes.

QCD phenomenology of nucleon-nucleon cross sections

The authors calculate the inclusive parton-parton cross section in p+p and p+p collisions in the framework of perturbative QCD and snow that the measured non-diffractive cross section requires a transverse momentum cut-off pTmin of several GeV c-1 at high energy. They consider the implications for nuclear collisions at RHIC and LHC energies.

Simple formulae for the order αs3 QCD corrections to the reaction [formula omitted

The order α s 3 QCD corrections to the parton-parton cross sections contributing to heavy flavour production in hadron-hadron collisions are discussed. We construct simple formulae which should yield reasonable approximations to the exact order α s 3 results. Then we examine the differences between the predictions of the exact and approximate formulae in the case of the reaction p + p → Q + Q + X where Q is either the b or the t quark.

Constraints on parton-parton cross sections from large p⊥ angular correlations

Recent data on rapidity distributions opposite a large p ⊥ trigger are analyzed. These data are found to constrain the angular dependence of the hard collision subprocess strongly. Estimates are given for quark-meson and quark-quark scattering. Using these results the behaviour of the position of the maximum of the opposite-side rapidity distribution is studied for larger values of the trigger χ ⊥ and for different trigger and beam particles.

Hadronic correlations at large transverse momenta in the parton model

Distributions in rapidity and transverse momentum of particles in the hemisphere opposite to a large p T trigger particle are studied in the framework of the parton model in order to get information on the parton-parton cross section. The partons are taken to be the same as those in deep inelastic lepton-nucleon scattering and e +e − annihilation. The transverse momentum spread in the fragmentation of the jet opposite to the trigger side is taken into account. It is shown that the presently available correlation data measured at CERN-ISR give rather strong restrictions on the energy and angular dependences of the differential cross section for parton-parton scattering. For models with no quantum number exchange in the basic collision, charge ratios for the opposite side particles and for single particle spectra are investigated which provide additional constraints on the parton-parton cross section.

Can We Measure Parton-Parton Cross Sections?

Multiple-core production in hadron-hadron collisions directly measures (in the context of the parton model) differential cross sections for parton-parton collisions. To obtain \ensuremath{\sim}20% accuracy in reconstruction of the parton-parton kinematics requires transverse momentum of the cores \ensuremath{\gtrsim} 5 GeV/c. Experiments at NAL as well as the CERN ISR appear feasible. What might be learned from such studies is discussed.

New effects in the ISR region

We suggest that the increase of total cross sections and large P T effects observed over the ISR range have a common origin in the hard collision of parton pairs. The observed magnitude of these effects suggest a parton-parton cross section ∼ 0.2 mb. The data is consistent with vector gluon exchange mechanisms. These mechanisms can also cause inhibited shrinkage and energy dependent dips in differential cross sections.