Página 7 dos resultados de 19478 itens digitais encontrados em 0.030 segundos

Nuclear modification of valence-quark distributions and its effects on NuTeV sin^2 theta_W anomaly

Hirai, M.; Kumano, S.; Nagai, T. -H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We investigated a nuclear modification difference between up- and down-valence quark distributions by analyzing structure function F_2 and Drell-Yan cross-section ratios. Although nuclear modifications of the valence-quark distributions themselves are rather well determined, it is difficult to find their difference from the present data. We estimated such an effect on the NuTeV sin^2 theta_W value and its uncertainty by the Hessian method. At this stage, it is not large enough to explain the whole NuTeV anomaly. However, the modification difference cannot be precisely determined, so that further studies are needed.; Comment: 3 pages, LaTeX, 1 eps file, to be published in Proceedings of the 6th International Workshop on Neutrino Factories and Superbeams (NuFact04)

Prospects for precision measurements in nuclear beta decay at the LHC era

González-Alonso, Martín; Naviliat-Cuncic, Oscar
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Precision measurements in nuclear beta decay offer a sensitive window to search for new physics beyond the standard electroweak model and allow also the determination of the fundamental weak vector coupling in processes involving the lightest quarks. Searches for new physics are also a strong motivation for experiments carried out at the high energy frontier reached at the most powerful particle colliders. It is instructive to confront results from the low energy and the high energy frontiers in order to look for possible complementarities and orient new avenues for experiments at low energies. We review here the status of constraints on new physics obtained from nuclear and neutron decays and compare them to those from other semi-leptonic processes and from the LHC. We stress the requirements of new precision experiments in beta decay in order to impact the search for new physics at the light of current and projected LHC results. We describe recent experimental results and ongoing developments in nuclear and neutron beta decay, with emphasis on their planned goals to improve present limits on exotic weak couplings.; Comment: Submitted to Annalen der Physik for the Special Issue "Precision experiments and fundamental physics at low energies"

Micro-physics simulations of columnar recombination along nuclear recoil tracks in high-pressure Xe gas for directional dark matter searches

Nakajima, Y.; Goldschmidt, A.; Long, M.; Nygren, D.; Oliveira, C.; Renner, J.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Directional sensitivity is one of the most important aspects of WIMP dark matter searches. Yet, making the direction of nuclear recoil visible with large target masses is a challenge. To achieve this, we are exploring a new method of detecting directions of short nuclear recoil tracks in high-pressure Xe gas, down to a few micron long, by utilizing columnar recombination. Columnar recombination changes the scintillation and ionization yields depending on the angle between a track and the electric field direction. In order to realize this, efficient cooling of electrons is essential. Trimethylamine(TMA) is one of the candidate additives to gaseous Xe in order to enhance the effect, not only by efficiently cooling the electrons, but also by increasing the amount of columnar recombination by Penning transfer. We performed a detailed simulation of ionization electrons transport created by nuclear recoils in a Xe + TMA gas mixture, and evaluated the size of the columnar recombination signal. The results show that the directionality signal can be obtained for a track longer than a few micrometers in some ideal cases. Although more studies with realistic assumptions are still needed in order to assess feasibility of this technique, this potentially opens a new possibility for dark matter searches.; Comment: 8 pages...

Measurements of non-photonic electrons with the STAR experiment

Rusňáková, Olga; Collaboration, for the STAR
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Non-photonic electrons (NPE), produced by semileptonic decays of D and B mesons, are good probes to study the properties of hot and dense medium created in relativistic heavy ion collisions at RHIC. Studies of heavy quark production in p+p collisions can test the validity of perturbative QCD. They also provide a baseline to study the effects of nuclear matter on the production of heavy quarks in heavy ion collisions. In this paper, we present recent results of NPE spectra measured in p+p collisions at $\sqrt{s}=200$ GeV in mid-rapidity. We also report NPE nuclear modification factor $R_{AA}$ and elliptic flow $v_2$ in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.; Comment: Proceedings of Hot Quarks 2014 conference

D meson nuclear modification factors in Pb-Pb collisions at {\surd}sNN = 2.76 TeV, measured with the ALICE detector

Rossi, Andrea; Collaboration, for the ALICE
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
The ALICE experiment has measured the D meson production in pp and Pb-Pb collisions at the LHC at {\surd}s = 7 and 2.76 TeV and {\surd}sNN = 2.76 TeV respectively, via the exclusive reconstruction of hadronic decay channels. The analyses of the D0{\to}K-pi+ and D+{\to}K-pi+pi+ channels will be described and the preliminary results for the D0 and D+ nuclear modification factor will be presented.; Comment: Proceedings of Quark Matter 2011 conference. 4 pages, 4 figures. The slides of the talk can be found at the link: http://indico.cern.ch/materialDisplay.py?contribId=591&sessionId=53&materialId=slides&confId=30248

Measurements of Cold Nuclear Matter Effects on J/psi in the PHENIX Experiment via Deuteron-Gold Collisions

Wysocki, Matthew; Collaboration, for the PHENIX
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
A new calculation of R_{dAu} has been performed using the 2003 d+Au data and the higher-statistics 2005 p+p data. These nuclear modification factors are compared to calculations using nuclear-modified PDFs and a J/psi breakup cross section is extracted. These values are then used to project the cold nuclear matter effects in Au+Au collisions. Additionally, a more data-driven projection is performed.; Comment: 4 pages, 6 figures, proceedings for Quark Matter 2008

Electron-Positron Production in Ultra-Peripheral Heavy-Ion Collisions with the STAR Experiment

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
This thesis presents a measurement of the cross-section of the purely electromagnetic production of $e^+e^-$ pairs accompanied by mutual nuclear Coulomb excitation $AuAu\to Au^*Au^*+e^+e^-$, in ultra-peripheral gold-gold collisions at RHIC at the center-of-mass collision energy of $\sqrt{s_{NN}} = 200$ GeV per nucleon. These reactions were selected by detecting neutron emission by the excited gold ions in the Zero Degree Calorimeters. The charged tracks in the $e^+e^-$ events were reconstructed with the STAR Time Projection Chamber. The detector acceptance limits the kinematical range of the observed $e^+e^-$ pairs; therefore the measured cross-section is extrapolated to $4\pi$ with the use of Monte Carlo simulations. We have developed a Monte Carlo simulation for ultra-peripheral $e^+e^-$ production at RHIC based on the Equivalent Photon Approximation, the lowest-order QED $e^+e^-$ production cross-section by two real photons and the assumption that the mutual nuclear excitations and the \ee production are independent (EPA model). %We present cross-section predictions from this model in various kinematic regions. We compare our experimental results to two models: the EPA model and a model based on full QED calculation of the \ee production...

Correlated Strength in Nuclear Spectral Function

Rohe, D.; Armstrong, C. S.; Asaturyan, R.; Baker, O. K.; Bueltmann, S.; Carasco, C.; Day, D.; Ent, R.; Fenker, H. C.; Garrow, K.; Gasparian, A.; Gueye, P.; Hauger, M.; Honegger, A.; Jourdan, J.; Keppel, C. E.; Kubon, G.; Lindgren, R.; Lung, A.; Mack, D. J
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We have carried out an (e,e'p) experiment at high momentum transfer and in parallel kinematics to measure the strength of the nuclear spectral function S(k,E) at high nucleon momenta k and large removal energies E. This strength is related to the presence of short-range and tensor correlations, and was known hitherto only indirectly and with considerable uncertainty from the lack of strength in the independent-particle region. This experiment confirms by direct measurement the correlated strength predicted by theory.; Comment: 4 pages, 2 figures, accepted by Phys. Rev. Lett

Measurement of nuclear effects in neutrino interactions with minimal dependence on neutrino energy

Lu, X. -G.; Pickering, L.; Dolan, S.; Barr, G.; Coplowe, D.; Uchida, Y.; Wark, D.; Wascko, M. O.; Weber, A.; Yuan, T.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We present a phenomenological study of nuclear effects in neutrino charged-current interactions, using transverse kinematic imbalances in exclusive measurements as a direct probe. Novel observables with minimal dependence on neutrino energy are proposed to study quasielastic scattering, and especially resonance production. They should be able to provide direct constraints on nuclear effects in neutrino- and antineutrino-nucleus interactions.; Comment: 5 pages, 5 figures

Cold-nuclear-matter effcts on heavy-quark production in d+Au collisions at sqrt(s_NN)=200 GeV

Adare, A.; Afanasiev, S.; Aidala, C.; Ajitanand, N. N.; Akiba, Y.; Al-Bataineh, H.; Alexander, J.; Angerami, A.; Aoki, K.; Apadula, N.; Aramaki, Y.; Atomssa, E. T.; Averbeck, R.; Awes, T. C.; Azmoun, B.; Babintsev, V.; Bai, M.; Baksay, G.; Baksay, L.; Bar
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt(s_NN)=200 GeV at the Relativistic Heavy Ion Collider, in the transverse-momentum range 0.85 < pT < 8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R_dA at 1.5 < pT < 5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at RHIC extends to the heavy-D-meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the pi0 and heavy-flavor-electron nuclear modification factor R_AA.; Comment: 383 authors, 7 pages, 3 figures. Submitted to Phys. Rev. Lett. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.html

Nuclear-Modification Factor for Open-Heavy-Flavor Production at Forward Rapidity in Cu+Cu Collisions at sqrt(s_NN)=200 GeV

Adare, A.; Afanasiev, S.; Aidala, C.; Ajitanand, N. N.; Akiba, Y.; Al-Bataineh, H.; Alexander, J.; Aoki, K.; Aphecetche, L.; Armendariz, R.; Aronson, S. H.; Asai, J.; Atomssa, E. T.; Averbeck, R.; Awes, T. C.; Azmoun, B.; Babintsev, V.; Baksay, G.; Baksay
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Background: Heavy-flavor production in p+p collisions tests perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p+p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Purpose: Determine transverse-momentum, pt, spectra and the corresponding R_AA for muons from heavy-flavor mesons decay in p+p and Cu+Cu collisions at sqrt(s_NN)=200 GeV and y=1.65. Method: Results are obtained using the semi-leptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte-Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p+p collisions at sqrt{s}=200 GeV, integrated over pt and in the rapidity range 1.4

On the Nuclear Modification Factor at RHIC and LHC

Kormilitzin, Andrey; Levin, Eugene; Rezaeian, Amir H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We show that pQCD factorization incorporated with pre-haronization energy-loss effect naturally leads to flatness of the nuclear modification factor R_{AA} for produced hadrons at high transverse momentum p_T. We consider two possible scenarios for the pre-hadronization: In scenario 1, the produced gluon propagates through dense QCD medium and loses energy. In scenario 2, all gluons first decay to quark-antiquark pairs and then each pair loses energy as propagating through the medium. We show that the estimates of the energy-loss in these two different models lead to very close values and is able to explain the suppression of high-p_T hadrons in nucleus-nucleus collisions at RHIC. We show that the onset of the flatness of R_{AA} for the produced hadron in central collisions at midrapidity is about p_T\approx 15 and 25 GeV at RHIC and the LHC energies, respectively. We show that the smallness (R_{AA}<0.5) and the high-p_T flatness of R_{AA} obtained from the k_T factorization supplemented with the Balitsky-Kovchegov (BK) equation is rather generic and it does not strongly depend on the details of the BK solutions. We show that energy-loss effect reduces the nuclear modification factor obtained from the k_T factorization about 30\div 50% at moderate p_T.; Comment: 14 pages...

Nuclear beams in HERA

Arneodo, M.; Bialas, A.; Krasny, M. W.; Sloan, T.; Strikman, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
A study has been made of the physics interest and feasibility of experiments with nuclear beams in HERA. It is shown that such experiments widen considerably the horizon for probing QCD compared to that from free nucleon targets. In addition there is some sensitivity to physics beyond the standard model. Hence the option to include circulating nuclear beams in HERA allows a wide range of physics processes to be studied and understood.; Comment: 40 pages, 20 figures, Latex, Convenor's report of the Working Group on "Light and Heavy Nuclei in HERA" of the Workshop on Future Physics at HERA

Measurement of the Nucleon Structure Function F2 in the Nuclear Medium and Evaluation of its Moments

Osipenko, M.; Collaboration, CLAS
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We report on the measurement of inclusive electron scattering off a carbon target performed with CLAS at Jefferson Laboratory. A combination of three different beam energies 1.161, 2.261 and 4.461 GeV allowed us to reach an invariant mass of the final-state hadronic system W~2.4 GeV with four-momentum transfers Q2 ranging from 0.2 to 5 GeV2. These data, together with previous measurements of the inclusive electron scattering off proton and deuteron, which cover a similar continuous two-dimensional region of Q2 and Bjorken variable x, permit the study of nuclear modifications of the nucleon structure. By using these, as well as other world data, we evaluated the F2 structure function and its moments. Using an OPE-based twist expansion, we studied the Q2-evolution of the moments, obtaining a separation of the leading-twist and the total higher-twist terms. The carbon-to-deuteron ratio of the leading-twist contributions to the F2 moments exhibits the well known EMC effect, compatible with that discovered previously in x-space. The total higher-twist term in the carbon nucleus appears, although with large systematic uncertainites, to be smaller with respect to the deuteron case for n<7, suggesting partial parton deconfinement in nuclear matter. We speculate that the spatial extension of the nucleon is changed when it is immersed in the nuclear medium.; Comment: 37 pages...

Quantification of nuclear uncertainties in nucleosynthesis of elements beyond Iron

Rauscher, T.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Nucleosynthesis beyond Fe poses additional challenges not encountered when studying astrophysical processes involving light nuclei. Generally higher temperatures and nuclear level densities lead to stronger contributions of transitions on excited target states. This may prevent cross section measurements to determine stellar reaction rates and theory contributions remain important. Furthermore, measurements often are not feasible in the astrophysically relevant energy range. Sensitivity analysis allows not only to determine the contributing nuclear properties but also is a handy tool for experimentalists to interpret the impact of their data on predicted cross sections and rates. It can also speed up future input variation studies of nucleosynthesis by simplifying an intermediate step in the full calculation sequence. Large-scale predictions of sensitivities and ground-state contributions to the stellar rates are presented, allowing an estimate of how well rates can be directly constrained by experiment. The reactions 185W(n,gamma) and 186W(gamma,n) are discussed as application examples. Studies of uncertainties in abundances predicted in nucleosynthesis simulations rely on the knowledge of reaction rate errors. An improved treatment of uncertainty analysis is presented as well as a recipe for combining experimental data and theory to arrive at a new reaction rate and its uncertainty. As an example...

Pion production in the MiniBooNE experiment

Lalakulich, Olga; Mosel, Ulrich
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
Background: Charged current pion production gives information on the axial formfactors of nucleon resonances. It also introduces a noticeable background to quasi-elastic measurements on nuclear targets. Purpose: Understand pion production in neutrino interactions with nucleons and the reaction mechanism in nuclei. Method: The Giessen Boltzmann--Uehling--Uhlenbeck (GiBUU) model is used for an investigation of neutrino-nucleus reactions. Results: Theoretical results for integrated and differential cross sections for the MiniBooNE neutrino flux are compared to the data. Two sets of pion production data on elementary targets are used to obtain limits for the neutrino-nucleus reactions. Conclusions: The MiniBooNE pion production data are consistent with the BNL elementary data if a small flux renormalization is performed while the ANL input data lead to significantly too low cross sections. A final determination of in-medium effects requires new data on elementary (p,D) targets.; Comment: 21 pages, 13 figures, minor text changes, 1 footnote added. version as published in Phys.Rev. C87 (2013) 014602

Perspectives to measure neutrino-nuclear neutral current coherent scattering with two-phase emission detector

RED Collaboration; Akimov, D. Yu.; Alexandrov, I. S.; Aleshin, V. I.; Belov, V. A.; Bolozdynya, A. I.; Burenkov, A. A.; Chepurnov, A. S.; Danilov, M. V.; Derbin, A. V.; Dmitrenko, V. V.; Dolgolenko, A. G.; Egorov, D. A.; Efremenko, Yu. V.; Etenko, A. V.;
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
We propose to detect and to study neutrino neutral current coherent scattering off atomic nuclei with a two-phase emission detector using liquid xenon as a working medium. Expected signals and backgrounds are calculated for two possible experimental sites: Kalinin Nuclear Power Plant in the Russian Federation and Spallation Neutron Source at the Oak Ridge National Laboratory in the USA. Both sites have advantages as well as limitations. However the experiment looks feasible at either location. Preliminary design of the detector and supporting R&D program are discussed.; Comment: 16 pages, 10 figures, RED collaboration

Quantifying Correlations Between Isovector Observables and the Density Dependence of Nuclear Symmetry Energy away from Saturation Density

Fattoyev, F. J.; Newton, W. G.; Li, Bao-An
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
According to the Hugenholtz-Van Hove theorem, the nuclear symmetry energy $S(\rho)$ and its slope $L(\rho)$ at arbitrary densities can be decomposed in terms of the density and momentum dependence of the single-nucleon potentials in isospin-asymmetric nuclear matter which are potentially accessible to experiment. We quantify the correlations between several well-known isovector observables and $L(\rho)$ to locate the density range in which each isovector observable is most sensitive to the density dependence of the $S(\rho)$. We then study the correlation coefficients between those isovector observables and all the components of the $L(\rho)$. The neutron skin thickness of $^{208}$Pb is found to be strongly correlated with the $L(\rho)$ at a subsaturation density of $\rho = 0.59 \rho_0$ through the density dependence of the first-order symmetry potential. Neutron star radii are found to be strongly correlated with the $L(\rho)$ over a wide range of supra-saturation densities mainly through both the density and momentum dependence of the first-order symmetry potential. Finally, we find that although the crust-core transition pressure has a complex correlation with the $L(\rho)$, it is strongly correlated with the momentum derivative of the first-order symmetry potential...

Photonuclear vector meson production in ultra-peripheral Pb-Pb collisions studied by the ALICE experiment at the LHC

Nystrand, Joakim
Tipo: Artigo de Revista Científica