Search results for "Proton"
showing 10 items of 5886 documents
Leading order determination of the gluon polarisation from DIS events with high-pThadron pairs
2013
We present a determination of the gluon polarisation Delta g/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with Q(2) > 1 (GeV/c)(2) including a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised (LiD)-Li-6 target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction x(g) covering the range 0.04 < x(g) < 0.27. The values obtained at leading order in QCD do not show any significant dependence on x(g). Their average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (sy…
Measurement of the dijet invariant mass cross section in pp¯ collisions at s=1.96 TeV
2010
The inclusive dijet production double differential cross section as a function of the dijet invariant mass and of the largest absolute rapidity (y_{max}) of the two jets with the largest transverse momentum in an event is measured in proton anti-proton collisions at sqrt{s} = 1.96$ TeV using 0.7 fb^{-1} of data collected with the D0 detector at the Fermilab Tevatron Collider. The measurement is performed in six rapidity regions up to |y|=2.4. Next-to-leading order perturbative QCD predictions are found to be in agreement with the data.
Description of the spacelike and timelike proton form factors at large momentum transfer in the QCD factorization approach
2012
Abstract We propose QCD factorization approach for description of the soft spectator scattering contribution. It is expected that such contribution is very important for description of the many exclusive processes at moderate values of Q 2 . Assuming the dominance of this mechanism we provide qualitative consideration of the proton form factors in the spacelike and timelike regions. Using the same approach we also provide estimates of the cross sections for the processes γ γ → p p and p p → γ γ at large energy and momentum transfer.
Measurement of Proton Electromagnetic Form Factors in e+e−→pp¯ in the Energy Region 2.00–3.08 GeV
2020
The process of e+e−→pp¯ is studied at 22 center-of-mass energy points (s) from 2.00 to 3.08 GeV, exploiting 688.5 pb−1 of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section (σpp¯) of e+e−→pp¯ is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio (|GE/GM|) and the value of the effective (|Geff|), electric (|GE|), and magnetic (|GM|) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. |GE/GM| and |GM| are determined with high accuracy, providing uncertaintie…
Evidence of shadowing in inelastic nucleon-nucleon cross section
2021
The Glauber modeling plays a key role in centrality-dependent measurements of heavy-ion collisions. A central input parameter in Glauber models is the inelastic nucleon-nucleon cross section $\sigma_{\text{nn}}^{\text{inel}}$ which is nearly always taken from proton-proton measurements. At the LHC energies $\sigma_{\text{nn}}^{\text{inel}}$ depends on the QCD dynamics at small $x$ and low interaction scales where the shadowing/saturation phenomena are expected to become relatively more important for larger nuclei than for the proton. Thus, $\sigma_{\text{nn}}^{\text{inel}}$ e.g. in Pb+Pb collisions may well be lower than what is seen in proton-proton collisions. In this talk, we demonstrate…
Sivers Asymmetry for the proton and the neutron
2008
A formalism is presented to evaluate the Sivers function in constituent quark models. A non-relativistic reduction of the scheme is performed and applied to the Isgur-Karl model. The results obtained are consistent with a sizable Sivers effect and the signs for the u and d flavor contributions turn out to be opposite. The Burkardt Sum Rule is fulfilled to a large extent. After the estimate of the QCD evolution of the results from the momentum scale of the model to the experimental one, a reasonable agreement with the available data is obtained. A calculation of nuclear effects in the extraction of neutron single spin asymmetries in semi-inclusive deep inelastic scattering off 3He is also de…
Transition form factors of theN*(1535)as a dynamically generated resonance
2008
We discuss how electromagnetic properties provide useful tests of the nature of resonances, and we study these properties for the ${N}^{*}(1535)$ that appears dynamically generated from the strong interaction of mesons and baryons. Within this coupled-channels chiral unitary approach, we evaluate the ${A}_{1/2}$ and ${S}_{1/2}$ helicity amplitudes as a function of ${Q}^{2}$ for the electromagnetic ${N}^{*}(1535)\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}N$ transition. Within the same formalism we evaluate the cross section for the reactions $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\eta}N$. We find a fair agreement for the absolute values of the transition amplitudes, a…
Radiation from matter-antimatter annihilation in the quark nugget model of dark matter
2021
We revisit the properties of positron cloud in quark nugget (QN) model of dark matter (DM). In this model, dark matter particles are represented by compact composite objects composed of a large number of quarks or antiquarks with total baryon number $B\sim 10^{24}$. These particles have a very small number density in our galaxy which makes them "dark" to all DM detection experiments and cosmological observations. In this scenario, anti-quark nuggets play special role because they may manifest themselves in annihilation with visible matter. We study electron-positron annihilation in collisions of free electrons, hydrogen and helium gases with the positron cloud of anti-quark nuggets. We show…
Double parton correlations in constituent quark models
2013
Double parton correlations, having effects on the double parton scattering processes occurring in high-energy hadron-hadron collisions, for example at the LHC, are studied in the valence quark region by means of constituent quark models. In this framework, two particle correlations are present without any additional prescription, at variance with what happens, for example, in independent particle models, such as the MIT bag model in its simplest version. From the present analysis, conclusions similar to the ones obtained recently in a modified version of the bag model can be drawn: correlations in the longitudinal momenta of the active quarks are found to be sizable, while those in transver…
Higher Order Polarizabilities of the Proton
1999
Compton scattering results are used to probe proton structure via measurement of higher order polarizabilities. Values for $\alpha_{E2}^p,\beta_{E2}^p,\alpha_{E\nu}^p,$ $\beta_{E\nu}^p$ determined via dispersion relations are compared to predictions based upon chiral symmetry and from the constituent quark model. Extensions to spin-polarizabilities are also discussed.