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In this paper we propose an improvement of the EKS nuclear parton distributions for the small x region of high energy processes, where the perturbative high parton density effects cannot be disregarded. We analyze the behavior of the ratios and and verify that at small x they are strongly modified when compared to the EKS predictions. The implications of our results for heavy ion collisions in RHIC and LHC are discussed. Received: 18 January 2001 / Revised version: 6 March 2001 / Published online: 3 May 2001  相似文献   

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Multiple parton scatterings inside a large nucleus generally involve higher-twist nuclear parton matrix elements. The gluon bremsstrahlung induced by multiple scattering depends not only on direct parton matrix elements but also on momentum-crossed ones, due to the Landau–Pomeranchuk–Migdal interference effect. We show that both types of twist-four nuclear parton matrix elements can be factorized approximately into the product of twist-two nucleon matrix elements in the limit of extremely large nuclei, A→∞, as assumed in previous studies. Due to the correlative nature of the twist-four matrix elements under consideration, it is actually the off-forward parton distributions that appear naturally in this decomposition, rather than the ordinary diagonal distributions probed in deeply-inelastic scattering. However, we argue that the difference between these two distribution classes is small in certain kinematic regimes. In these regions, the twist-four nuclear parton matrix elements are evaluated numerically and compared to the factorized form for different nuclear sizes within a schematic model of the two-nucleon correlation function. The nuclear size dependence is found to be A4/3 in the limit of large A, as expected. We find that the factorization is reasonably good when the momentum fraction carried by the gluon field is moderate. The deviation can be more than a factor of 2, however, for small gluon momentum fractions, where the gluon distribution is very large.  相似文献   

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The scale dependence of the ratios of parton distributions in a proton of a nucleus A and in the free proton, , is studied within the framework of the lowest order leading-twist DGLAP evolution. By evolving the initial nuclear distributions obtained with the GRV-LO and CTEQ4L sets at a scale , we show that the ratios are only moderately sensitive to the choice of a specific modern set of free parton distributions. We propose that to a good first approximation, this parton distribution set-dependence of the nuclear ratios can be neglected in practical applications. With this result, we offer a numerical parametrization of for all parton flavours i in any , and at any and any for computing cross sections of hard processes in nuclear collisions. Received: 10 August 1998 / Published online: 27 April 1999  相似文献   

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In order to overcome the shortcoming of nonconservation of nuclear momentum existing in the original -rescaling model (ORM) and avoid introducing nuclear shadowing factor, we proposed a double -rescaling model (DRM) for the parton distributions of the bound nucleon. Using the experimental data of lepton-nucleus deep inelastic scattering (DIS) and the condition of the nuclear momentum conservation, the -rescaling parameters of various partons for Sn, Fe, Ca and C nuclei are determined. The rescaling parameters of valence quark distributions are larger than unity and gradually increase with atomic number A, on the contrary, the rescaling parameters of sea quark distributions and gluon distributions are smaller than unity, and slowly decrease with A. By using this model, the experimental data of the DIS process, the nuclear Drell-Yan process and ph otoproduction process are consistently and quite satisfactorily explained. Received: 18 August 1997 / Published online: 20 February 1998  相似文献   

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We investigate parton distributions at small x using moments of the Gribov-Lipatov-Altarelli-Parisi (GLAP) evolution equation with respect to x. In this representation the kernel of the GLAP equation contains singularities in the moment variable ω at ω = 0. We show that the importance of these singularities at small x depends on the form of the starting distributions. We examine the range of x in which the GLAP equation is valid. The restrictions on the range of x depend on the form of the starting distributions. We investigate whether the GLAP equation can be used to interpolate data in the HERA region. Results are given for the structure function F2 at small x. A possible method of determining the gluon distribution from F2 is discussed.  相似文献   

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刘建业  左维  李希国  邢永忠 《物理学报》2007,56(3):1339-1346
利用同位旋相关的量子分子动力学(IQMD)对中子晕核,8He和10He引起核反应中重要的同位旋效应和松散的中子晕结构影响的平均特征进行了研究.因为IQMD中的互作用势和介质中核子-核子碰撞截面灵敏地依赖于碰撞系统的密度分布.而扩展的中子晕密度分布包含了中子晕核的同位旋效应和松散的中子晕结构的平均特征,从而将这些信息通过动力学碰撞带入到反应机理中. 为了清楚地鉴别中子晕核带入反应机理重要的同位旋效应和松散中子晕结构的影响,通过比较中子晕核和相等质量稳定弹核在相同入射道条件下,所得物理观测量之间的差别加以确定.计算结果确实发现具有初始晕核信息的中子扩展密度分布将重要的同位旋效应和松散中子晕结构带入到各种物理观测量中.例如与相等质量稳定相比,中子晕核的晕特征引起了原子核阻止的降低;并明显地增加了核子发射中子-质子比和同位旋分馏比. 关键词: 中子晕核 原子核阻止 核子发射中子-质子比 同位旋分馏比  相似文献   

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It has been argued that high-multiplicity proton-proton collisions at the LHC may exhibit collective phenomena usually studied in the context of heavy-ion collisions, such as elliptic flow. We study this issue using DIPSY—a Monte Carlo event generator based on the QCD dipole model. We calculate the eccentricity of the transverse area defined by the spatial distribution of produced gluons. The resulting elliptic flow is estimated to be about 6%, comparable to the value in nucleus-nucleus collisions at RHIC and the LHC. Experimentally, elliptic flow is inferred from the azimuthal correlation between hadrons, which receives contributions from collective flow, and from various other effects referred to as “nonflow”. We discuss how to identify in experiments the signal of flow in the presence of large nonflow effects.  相似文献   

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《Physics letters. A》1999,259(2):83-90
We consider the effects of a massive homogeneous body on the state vector of a molecule passing nearby. The interchange of virtual photons between body and molecule cause the latter to fail a proposed criterion of isolation even for molecules consisting of as little as 104 particles. The calculation contains no free parameters but furnishes only a lower bound to the effects. The fact that matter consists of electrically charged particles is essential for a smooth transition from the quantum to the classical behavior as the system grows larger.  相似文献   

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The propagation of hard parton jets through an expanding quark-gluon plasma is investigated in the framework of hydrodynamical approach. The influence of viscosity and transverse expansion of plasma on the intensity of jet rescattering is estimated for initial conditions, predicted to be achieved at LHC. The sensitivity of jet quenching and dijet acoplanarity to the space-time evolution of hot parton matter created in ultrarelativistic heavy ion collisions is analyzed.  相似文献   

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Representing the semi-hard partonic interactions by the exchange of Lipatov's perturbative Pomeron, we express the semi-hard nuclear cross section as a self shadowing cross section. With the help of a generating functional technique, we obtain average numbers of multiple semi-hard partonic collisions without any need of using explicit expressions for the multi-parton distributions. The average number of semi-hard interactions of a given projectile parton against a target nucleus is estimated quantitatively and it is shown to grow very rapidly above one with increasing the c.m. energy.  相似文献   

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The shadowing corrections to gluon and quark distributions in nuclei in the region of small values ofx are discussed. They are related to parton distributions in a pomeron which are in principle measurable in hard diffractive processes on the nucleon target. Multiple scattering corrections to shadowing are considered in a model dependent way. The perturbative QCD evolution of shadowing is also taken into account. Various possibilities of the partonic content of a pomeron are considered. It is shown in particular that the conventional parametrizations of parton distributions in a pomeron which are based on the assumption that it consists mostly of gluons imply substantial nuclear shadowing in gluon distributions in heavy nuclei. Possible phenomenological implications of shadowing corrections in nuclear parton distributions for various semi-hard processes with nuclear targets are briefly discussed.  相似文献   

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We have studied transverse momentum distributions for exclusive 0 muoproduction on protons and heavier nuclei at 2Q 2<25>2. TheQ 2 dependence of the slopes of thep t 2 andt distributions is discussed. The influence of the non-exclusive background is investigated. Thep t 2 -slope for exclusive events is 4.3±0.6±0.7 GeV–2 at largeQ 2. Thep t 2 spectra are much softer than inclusivep t 2 spectra of leading hadrons produced in deep inelastic scattering.Supported by Bundesministerium für Forschung und Technologie  相似文献   

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Xin-Nian Wang 《Physics Reports》1997,280(5-6):287-371
A pQCD-based model for parton production and equilibration in ultrarelativistic heavy-ion collisions is reviewed. The model combines pQCD processes including initial and final state radiations together with string phenomenology for nonperturbative soft processes. Nuclear effects on the initial parton production, such as multiple parton scattering and nuclear shadowing of parton distribution functions are considered. Comparisons with existing data are made and further tests of the model to constrain model parameters are proposed. With the obtained space-time history of the parton production, evolution of the minijet gas toward a fully equilibrated parton plasma is studied. Direct probes of the early parton dynamics, such as pre-equilibrium photon and dilepton production, open charm production, J/ψ suppression and jet quenching are also reviewed.  相似文献   

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By globally analyzing nuclear Drell-Yan data including all incident energies,the nuclear effects of nuclear parton distribution functions(nPDFs)and initial-state parton energy loss are investigated.Based on the Landau-Pomeranchuk-Migdal(LPM)regime,the calculations are carried out by means of analytic parametrizations of quenching weights derived from the Baier-Dokshitzer-Mueller-Peigné-Schiff(BDMPS)formalism and using the new EPPS16 nPDFs.It is found that the results are in good agreement with the data and the role of the energy loss effect in the suppression of Drell-Yan ratios is prominent,especially for low-mass Drell-Yan measurements.Thenuclear effects of nPDFs become more obvious with increasing nuclear mass number A,the same as the energy loss effect.By a global fit,the transport coefficient extracted is q=0.26±0.04 GeV2/fm.In addition,to avoid diminishing the QCD NLO correction to the data form of Drell-Yan ratios,separate calculations of the Compton differentialcross section ratios RFe(W)/C(xF)at 120 GeV are performed,which provides a feasible way to better distinguish the gluon energy loss in Compton scattering.It is found that the role of the initial-state gluon energy loss in the suppression of Compton scattering ratios is not very important and disappears with the increase of xF.  相似文献   

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The transverse spin asymmetry of a quark in a baryon and the linear polarization of a gluon in a vector meson are studied from thet-channel point of view. Using the Altarelli-Parisi approach, they are shown to obey independent evolution equations and to decrease with increasingQ 2. We investigate the possibility to measure them at leading twist, to leading order in α and αs and without analyzing the final polarizations. This requires simultaneous polarization of the beam and the target; the observable effect is in the azimuthal distribution of the highP T particle or jet. Assuming a simple (quark+scalar diquark) model for the baryon, a large asymmetry is expected inp \(\bar p\) Drell-Yan collisions, a smaller one in highP T pp collisions, from the interference term in the scattering of two identical quarks.  相似文献   

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