Energy loss in nuclear Drell–Yan process

By means of the nuclear parton distributions, which can be used to provide a good explanation for the EMC effect in the whole x range, we investigate the energy loss effect in the nuclear Drell–Yan (DY) process. When the cross section of lepton pair production is considered to vary with the center-of-mass energy of the nucleon–nucleon collision, we find that the nuclear DY ratio is suppressed because of the energy loss, which balances the overestimate of the DY ratio only when we consider the effect of nuclear parton distributions. Received: 11 December 1997 / Published online: 30 March 1998     

One-loop corrections to the Drell–Yan process in SANC

Radiative corrections to the neutral current Drell–Yan-like processes are considered. Complete one-loop electroweak corrections are calculated within the SANC system. Theoretical uncertainties are discussed. Numerical results are presented for typical conditions of LHC experiments. PACS 13.85.Qk; 12.15.Lk     

Parton distributions and new physics searches: the Drell–Yan forward–backward asymmetry as a case study

We present the first unquenched lattice-QCD calculation of the form factors for the decay \(B\rightarrow D^*\ell \nu \) at nonzero recoil. Our analysis includes 15 MILC ensembles with \(N_f=2+1\) flavors of asqtad sea quarks, with a strange quark mass close to its physical mass. The lattice spacings range from \(a\approx 0.15\) fm down to 0.045 fm, while the ratio between the light- and the strange-quark masses ranges from 0.05 to 0.4. The valence b and c quarks are treated using the Wilson-clover action with the Fermilab interpretation, whereas the light sector employs asqtad staggered fermions. We extrapolate our results to the physical point in the continuum limit using rooted staggered heavy-light meson chiral perturbation theory. Then we apply a model-independent parametrization to extend the form factors to the full kinematic range. With this parametrization we perform a joint lattice-QCD/experiment fit using several experimental datasets to determine the CKM matrix element \(|V_{cb}|\). We obtain \(\left| V_{cb}\right| = (38.40 \pm 0.68_{\text {th}} \pm 0.34_{\text {exp}} \pm 0.18_{\text {EM}})\times 10^{-3}\). The first error is theoretical, the second comes from experiment and the last one includes electromagnetic and electroweak uncertainties, with an overall \(\chi ^2\text {/dof} = 126/84\), which illustrates the tensions between the experimental data sets, and between theory and experiment. This result is in agreement with previous exclusive determinations, but the tension with the inclusive determination remains. Finally, we integrate the differential decay rate obtained solely from lattice data to predict \(R(D^*) = 0.265 \pm 0.013\), which confirms the current tension between theory and experiment.

     

Angular distributions of muon pairs produced by negative pions on deuterium and tungsten

We present the angular distributions of high-mass muon pairs produced in a high-statistics experiment by 140 and 194 GeV/c π^? beams impinging on a tungsten target, and by 286 GeV/c π^? beam on deuterium and tungsten targets. We find no evidence for a center-of-mass energy dependence or a nuclear dependence of the angular distribution parameters. The two parameters λ and μ are found to be essentially independent of any kinematical variable. In contrast, the parameterv increase with the dimuon transverse momentumP _T , at variance with recent perturbative QCD predictions. Our statistics at largex ₁ are insufficient to substantiate the highertwist prediction.     

Angular distributions of muon pairs produced by 194 GeV/c negative pions

We present the angular distributions of muon pairs obtained in a high-statistics experiment using a 194-GeV/c π^- impinging on a tungsten target. Our results are based on the analysis of 145,000 events with positive Feynmanx and mass above 4.07 GeV/c², excluding the ? region. Simple first-order QCD relations allow us to determine the ratio of annihilation with hard-gluon emission to the sum of annihilation with hard-gluon emission and hard-gluon Compton scattering, which is found to be about 58% to 75%. We determine the parton square intrinsic transverse momenta to be of the order of 0.6 (GeV/c)², and about 30% larger in the pion than in the nucleon. At largex ₁, our data agree with the higher-twist hypothesis, and support the interpretation of the relevant scale parameter as the dimuon square transverse momentum.     

One-loop corrections to the Drell–Yan process in SANC: the charged current case

Radiative corrections to the charged current Drell–Yan processes are revisited. Complete one-loop electroweak corrections are calculated within the automatic SANC system. Electroweak scheme dependence and the choice of the factorization scale are discussed. Comparisons with earlier calculations are presented. PACS 13.85.Qk, 12.15.Lk     

NLO semi-inclusive Drell–Yan cross-section in quantum chromodynamics as a factorization analyzer

We evaluate in perturbative QCD the semi-inclusive Drell–Yan cross-section for the production of a single hadron accompanying the lepton pair. We demonstrate to one loop level a collinear factorization formula within the fracture functions approach. We propose such a process as a factorization analyzer in hadronic collisions. Phenomenological implications at the hadron colliders are briefly discussed.     

Angular distributions in quasi-fission reactions

Angular distributions for fission-like fragments were measured in the systems ⁵⁰Ti, ⁵⁶Fe + ²⁰⁸Pb by applying an off-line K X-ray activation technique. The distributions d²σ/dθdZ exhibit forward-backward asymmetries that are strongly Z-dependent. They result from a process (quasi-fission) which yields nearly symmetric masses in times comparable to the rotational period of the composite system. A method for obtaining the variance of the tilting angular momentum, K₀², from these skewed, differential angular distributions is described. The results indicate that the tilting mode is not fully excited in quasi-fission reactions. The results are compared to the sum of the variances of all statistical spin components, measured via γ-multiplicities. Integration of the angular distributions d²σ/dθdZ over all values of Z yields the integral angular distributions dσ/dθ and dσ/dΩ symmetric around 90°. The associated unusually large anisotropies do not at all provide an adequate basis for tests or modifications of the transition-state theory. A deconvolution of d²σ/dθdZ is performed with gaussian distributions depending on rotational angles Δθ extending over a range of up to 540°. From the mean values 〈Δθ〉 a time scale for the evolution of K₀ is calculated.     

Angular distributions in pre-equilibrium reactions

A new model is proposed for calculating angular distributions in preequilibrium reactions. In this model, as in the model of Feshbach et al. the system consisting of target plus projectile initially branches into two sets of states with either no particle in the continuum (multistep compound states) or with at least one particle in the continuum (multistep direct states). The multistep compound emission is assumed to be isotropic while the angular distribution of the multistep direct emission is described using the fast particle model of Mantzouranis et al. A similar master equation is used for both chains of states differing only in the angular dependence of the emission rates. The two chains of states are treated independantly neglecting inter-branch transitions. The angular distributions for 14.6 MeV neutrons calculated using this model are found to be in better agreement with the data than the fast particle model.     

Angular distributions of penning electrons

Angular distributions of electrons ejected in thermal collisions of He(2¹ S, 2³ S)-metastables with Ar, Kr, Xe, Hg, CO, N₂ are measured with respect to the vector of relative velocity of the colliding particles. Except for preliminary results on the He(2₁S, 2³ S)-/Ar-system, these are the first such distributions reported. They are found to be strongly anisotropic and asymmetric in some cases, and approximately isotropic in those cases where the interaction potential between the metastable- and the target particle is strong. For the cases of weak interaction potential, the connection between the “internal” angular distribution in the frame fixed to the internuclear axis and the laboratory angular distribution, has been derived. Calculations of laboratory angular distributions involving an assumed “internal” distribution and the kinematics of a hard core collision show good qualitative agreement with the experimental results.     

Angular distribution of lepton pairs in Drell-Yan-like processes

We compute the angular distribution of lepton pairs at fixed transverse momentum using soft-gluon resummation. Theoretical predictions are compared with the recent NA10 experimental data. We also give predictions in the case ofZ ⁰ production.     

Angular distribution of lepton pairs in perturbative QCD

We compute to first order in perturbative QCD the decay angular distribution of the lepton pairs produced in high energy hadronic collisions. The results, integrated over the transverse momentum of the pair, are found in reasonable agreement with experimental data.     

Angular distributions in the photoelectron spectroscopy of carbon monoxide

The angular distributions of the photoelectron intensity for CO have been investigated using a photoelectron spectrometer that has a rotatable detector and an HeI light source. The asymmetry parameter β was carefully measured for 24 vibrational transitions spanning three ionization bands. Its variation with final-state vibrational quantum number was found to have an unusual behavior. It is concluded that an autoionizing state of CO should exist at 584 Å in order to account for this behavior, and that the value of β is very sensitive to the presence of such a state.     

Angular distributions of fast neutrons from the reaction

The angular distributions of neutrons from the reaction Co⁵⁹ (p, n) for 4 values of the proton energyE _p=5.23–4.05 MeV have been measured. The preliminary analysis of the results shows that in the reaction mechanism a certain role is played also by a direct process, most probably of a knock-out type, the relative contribution to the reaction yield of which decreases with decreasing value ofE _p.The authors would like to thank Prof. V. Petrílka for his constant interest in the work, the staff of the Cyclotron Laboratory of the Nuclear Research Institute of the Czechoslovak Academy of Sciences for their cooperation and the group of physicists of the Nuclear Reactions Department of the institute for lending most of the apparatus.     

Angular correlations of electron-positron pairs produced in heavy-ion collisions

We investigate the strength of angular correlations between coincident electron-positron pairs produced in Pb + Pb collisions at 5.7 MeV/u. The angular correlations are predicted to be observable at electron energies above 200 keV, almost independent of positron energy.