Angular distributions of dileptons in polarized hadronic collisions: Test of electroweak gauge models

The angular distributions of high-mass dileptons with large transverse momentum produced in longitudinally polarized hadronic collisions, hadron +hadron→(γ^*,Z ₁ ⁰ ,Z ₂ ⁰ , ...)+X→l ⁺ l ^?+X, are calculated in the QCD lowest order. The formulas presented in general form, are applicable to any gauge model of underlying electroweak interactions. Numerical results are obtained for the angular distributions in \(\bar p\) p-collisions at \(\sqrt s = 540GeV\) for the Weinberg-Salam,SU(2)×U(1)×U(1)′ andSU(2)×SU(2)′×U(1) models. Clearly distinguishable signals are revealed.     

Thermal dileptons in high-energy nuclear collisions

Clear signs of excess dileptons above the known sources have been found at the SPS for a long time. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region is consistent with a dominant contribution from π⁺π⁻→ρ→μ⁺μ⁻ annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. In the region , the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show a steep rise towards low masses characteristic of Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.     

Rescattering production of dileptons in nucleus-nucleus collisions

We present a space-time formulation for the rescattering of hard partons produced in primary QCD subcollisions within a nucleus-nucleus collision. As an application we calculate the rescattering rate for dilepton production in 100 GeV/A+100 GeV/A uranium-uranium collisions.     

Radiative corrections in hadronic collisions

A general treatment of radiative corrections to any two-body hadronic reaction is presented and applied to specific collisions of pions, kaons and protons with nuclei. The results are found to be significant compared to the accuracies of available data.     

Charge distributions in hadronic collisions

The distribution of charge versus rapidity is examined in the framework of available parton models for multiparticle production in hadron-hadron collisions. Fragmentation models with a twochain Pomeron contribution (based on the dual topological unitarization scheme) are shown to describe rather well the recently measured data on charge distributions in high energypp and πp collisions (140 GeV/c≦p _LAB≦360 GeV/c). Carefully measured charge distributions at still higher energies which will be accessible in the near future offer a possibility of distinguishing between some of the current theoretical approaches.     

Chiral symmetry and dileptons in heavy ion collisions

The cross section in terms of three independent radiative gluon correction form factors related to angular dependence and angular asymmetry is calculated as functions of the maximum recorded scaled gluon momentumX _g, with 0X _g 1. The effects of longitudinal and transversal polarization of the initial electron and positron are taken into account. For the same geometry the two independent thrust form factors are obtained as functions of the minimum thrustT _m recorded, with 2/3T _m <1. similarly=" the=" related=" invariant=" mass=" form=" factors=" are=" obtained.=" in=" particular,=" the=" regions=" of=" infrared="> , and of near infrared gluons, , are discussed.     

Strangeness distributions in hadronic collisions

The distribution of strangeness versus rapidity is studied in the multichain fragmentation model and in the one chain fragmentation model. It is shown that the multichain model describes rightly theK ⁺ p strangeness distribution data.     

Limiting fragmentation in hadronic collisions

I discuss the phenomenon of the limiting fragmentation in the hadronic collisions. The calculation of the multiplicities is presented, based on the inclusive gluon production within the kT$k_T$ factorization framework using the gluon distributions obtained from the nonlinear equation at small x.     

Prospects of technicolor-signatures in hadronic collisions

Within the context of the Dimopoulos' one family model, we consider possible technicolor signatures in hadronic reactions at high energies. For the single-production of light color-singlet states (P ⁰,P ³,P ^±) in \(p\bar p\) annihilation, we find the Compton-scattering processgc→P ⁰ c most promising. In case of PGB-pair-production, the reaction in \(p\bar p \to P^ + P^ - \) via the subprocess \(q\bar q\xrightarrow{{\gamma ,Z^0 }}P^ + P^ - \) , as well as the production of heavy color-triplets (P ₃) via \(p\bar p \to P_3 \bar P_3 \) via \(q\bar q\) andgg-fusion at Tevatron-energies, appear to be best suited for detecting these objects.     

Fluctuations in hadronic and nuclear collisions

We investigate several fluctuation effects in high-energy hadronic and nuclear collisions through the analysis of different observables. To introduce fluctuations in the initial stage of collisions, we use the interacting gluon model (IGM) modified by the inclusion of the impact parameter. The inelasticity and leading-particle distributions follow directly from this model. The fluctuation effects on rapidity distributions are then studied using Landau's hydrodynamic model in one dimension. To investigage further the effects of the multiplicity fluctuation, we use the longitudinal phase-space model, with the multiplicity distribution calculated within the hydrodynamic model, and the initial conditions given by the IGM. Forward-backward correlation is obtained in this way.     

Low-mass dileptons at high momenta in ultra-relativistic heavy-ion collisions

We calculate the emission rate of low-mass dileptons at high momenta from a quark-gluon plasma. We find a large enhancement compared with the naive Born rate. Numerical estimates for relativistic heavy ions collisions at RHIC and LHC suggest, however, that the thermal signal is dominated by the background coming from meson decays and from hadronic bremsstrahlung.     

Unravelling γγ dileptons in central relativistic heavy ion collisions

Using an impact parameter formulation, differential production probabilities for γγ dileptons in central relativistic heavy ion collisions at RHIC and LHC energies are calculated and compared to Drell-Yan and thermal ones. The angular distributions of the leptons give a handle for the discrimination. Nuclear stopping leads, apart from Bremsstrahlung pair production, to a modification of the pure γγ dilepton production. These modifications are studied in a simple model and are found to be of minor influence.     

Pion interferometry of ultra-relativistic hadronic collisions

Pion interferometry of ultra-relativistic hadronic collisions is described in the context of the inside-outside cascade model using a current ensemble method. In contrast to the usual gaussian and Kopylov parameterizations involving only geometrical variables, the correlation function in this model depends on a special combination of dynamical as well as geometrical parameters.     

Asymmetric di-jet production in polarized hadronic collisions

Using the collinear QCD factorization approach, we study the single-transverse-spin dependent cross section Δσ(S) for the hadronic production of two jets of momenta P₁=P+q/2 and P₂=−P+q/2. We consider the kinematic region where the transverse components of the momentum vectors satisfy PqΛ_QCD. For the case of initial-state gluon radiation, we show that at the leading power in q/P and at the lowest non-trivial perturbative order, the dependence of Δσ(S) on q decouples from that on P, so that the cross section can be factorized into a hard part that is a function only of the single scale P, and into perturbatively generated transverse-momentum dependent (TMD) parton distributions with transverse momenta .     

Seeing the B mesons in hadronic collisions

The weak decays B→ψ(J) + X are investigated. It is argued that these decays are dominated by B→ψ(J)Kπ (branching ratio of about 1%). Detailed predictions for these decays are made.