Search results for "Cross Section"
showing 10 items of 3111 documents
Enhancement of nucleon-nucleon cross section in sidewards flowing nuclear matter through zero sound mode excitation
1998
We investigate the modification induced on the nucleon-nucleon cross section by screening of the interaction inside nuclear matter. The interaction is described by the relativistic one boson exchange of $\sigma$, $\omega$ and $\pi$ mesons. Medium effects enter as loop corrections to the meson propagators and are characterized not only by density, but also by temperature and velocity with respect to the center of mass of the collision. The cross section displays peaks for some specific values of the velocity, corresponding to the excitation of zero-sound modes of the longitudinal $\omega$ field. The enhancement factor amounts to about 1.5 under reasonable conditions. It increases with densit…
Ab initio calculation of Li7 photodisintegration
2004
The Li7 total photoabsorption cross section is calculated microscopically. As nucleon-nucleon interaction the semi-realistic central AV4' potential with S- and P-wave forces is taken. The interaction of the final 7-nucleon system is fully taken into account via the Lorentz Integral Transform (LIT) method. For the calculation of the LIT we use expansions in hyperspherical harmonics (HH) in conjunction with the HH effective interaction (EIHH) approach. The convergence of the LIT expansion is discussed in detail. The calculated cross section agrees quite well with the available experimental data, which cover an energy range from threshold up to 100 MeV.
NUCLEAR EFFECTS IN NEUTRINO INDUCED REACTIONS
2008
We discuss the relevance of nuclear medium effects in the analysis of some low and medium energy neutrino reactions of current interest. In particular, we study the Quasi-Elastic (QE) process, where RPA correlations and Final State Interactions (FSI) are shown to play a crucial role. We have also investigated the neutrino induced coherent pion production. We find a strong reduction of the cross section due to the distortion of the pion wave function and the modification of the production mechanisms in the nucleus. The sensitivity of the results to the axial $N\Delta$ coupling $C_5^A(0)$ has been also investigated.
Mesonic and binding contributions to the nuclear Drell-Yan process
1997
We have evaluated the Drell-Yan cross section in nuclei paying special attention to the meson cloud contribution from pion and rho-meson, for which an accurate calculation using the meson nuclear spectral functions is used. Similarly, the nucleonic contribution is evaluated in terms of a relativistic nucleon spectral function. Fair agreement with experiment is found for different nuclei and the results show a sizeable contribution from the renormalized meson cloud. In order to reproduce the experiment a novel element is introduced, consisting of a gradual energy loss of the incoming proton in its pass through the nucleus which produces a strong A dependence at x_1 large.
ϕ meson width in the medium from proton induced ϕ production in nuclei
2004
We perform calculations for the production of $\phi$ mesons in nuclei at energies just above threshold and study the $A$ dependence of the cross section. We use results for the $\phi$ selfenergy in the medium obtained within a chiral unitary approach. We find a strong $A$ dependence which is tied to the distortion of the incident proton and to the absorption of the $\phi$ in its way out of the nucleus. The effect of this latter process reduces the cross section in about a factor two in heavy nuclei proving that the $A$ dependence of the cross section bears valuable information on the $\phi$ width in the nuclear medium. Calculations are done for energies which are accessible in an experiment…
Measurement of thee+e−→ηJ/ψcross section and search fore+e−→π0J/ψat center-of-mass energies between 3.810 and 4.600 GeV
2015
Using data samples collected with the BESIII detector operating at the BEPCII collider at 17 center-of-mass energies from 3.810 to 4.600 GeV, we perform a study of e(+)e(-) -> eta J/psi and pi(0)J/psi The Born cross sections of these two processes are measured at each center-of-mass energy. The measured energy-dependent Born cross section for e(+)e(-) -> eta J/psi shows an enhancement around 4.2 GeV. The measurement is compatible with an earlier measurement by Belle.
Precision predictions for the tt¯ production cross section at hadron colliders
2011
Abstract We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the top-quark pair production cross section at hadron colliders. In particular, we supplement the fixed-order NLO calculation with higher-order corrections from soft gluon resummation at NNLL accuracy. Uncertainties due to power corrections to the soft limit are estimated by combining results from single-particle inclusive and pair invariant-mass kinematics. We present our predictions as functions of the top-quark mass in both the pole scheme and the M S ¯ scheme. We also discuss the merits of using threshold masses as an alternative, and calculate …
Dynamical threshold enhancement and resummation in Drell-Yan production
2007
Partonic cross sections for the production of massive objects in hadronic collisions receive large corrections when the invariant mass of the initial-state partons is just above the production threshold. Since typically the center-of-mass energy of the hadronic collision is much higher than the mass of the heavy objects, it is not obvious that these contributions translate into large corrections to the hadronic cross section. Using a recent approach to threshold resummation based on effective field theory, we quantify to which extent the fall-off of the parton densities at high x leads to a dynamical enhancement of the partonic threshold region. With the example of Drell-Yan production, we …
Search for newπ0-like particles produced in association with aτ-lepton pair
2014
We report on a search in e^+e^− annihilations for new π^0-like particles produced in association with a τ-lepton pair. These objects, with a similar mass and similar decay modes to π^0 mesons, could provide an explanation for the non-asymptotic behavior of the pion-photon transition form factor observed by the BABAR Collaboration. No significant signal is observed, and limits on the production cross section at the level of 73 fb or 370 fb, depending on the model parameters, are determined at 90% confidence level. These upper limits lie below the cross section values needed to explain the BABAR form factor data.
On the model dependence of fiducial cross-section measurements
2019
Fiducial production cross-section measurements of Standard Model processes, in principle, provide constraints on new physics scenarios via a comparison of the predicted Standard Model cross-section and the observed cross-section. This approach received significant attention in recent years, both from direct constraints on specific models and the interpretation of measurements in the view of effective field theories. A generic problem in the reinterpretation of Standard Model measurements is the corrections application of to data to account for detector effects. These corrections inherently assume the Standard Model to be valid, thus implying a model bias of the final result. In this work, …