Index
We report current status of global analyses on nuclear parton distribution functions (NPDFs). The optimum NPDFs are determined by analyzing high-energy nuclear reaction data. Due to limited experimental measurements, antiquark modifications have large uncertainties at x>0.2 and gluon modifications cannot be determined. A nuclear modification difference between u and d quark distributions could be an origin of the long-standing NuTeV sin^2 theta_w anomaly. There is also an issue of nuclear modification differences between the structure functions of charged-lepton and neutrino reactions. Next, nuclear clustering effects are discussed in structure functions F_2^A as a possible explanation for an anomalous result in the Be-9 nucleus at the Thomas Jefferson National Accelerator Facility (JLab). Last, tensor-polarized quark and antiquark distribution functions are extracted from HERMES data on the polarized structure function b_1 of the deuteron, and they could be used for testing theoretical models and for proposing future experiments, for example, the one at JLab. Such measurements could open a new field of spin physics in spin-one hadrons.
An anomalous nuclear modification was reported by JLab measurements on the beryllium-9 structure function F_2. It is unexpected in the sense that a nuclear modification slope is too large to be expected from its average nuclear density. We investigated whether it is explained by a nuclear clustering configuration in Be-9 with two \alpha nuclei and surrounding neutron clouds. Such clustering aspects are studied by using antisymmetrized molecular dynamics (AMD) and also by a simple shell model for comparison. We consider that nuclear structure functions F_2^A consist of a mean conventional part and a remaining one depending on the maximum local density. The first mean part does not show a significant cluster effect on F_2. However, we propose that the remaining one could explain the anonymous JLab slope, and it is associated with high densities created by the cluster formation in Be-9. The JLab measurement is possibly the first signature of clustering effects in high-energy nuclear reactions. A responsible physics could be an internal nucleon modification, which is caused by the high densities due to the cluster configuration.
The Lambdac(2940)+ baryon with quantum numbers J(P) = 1/2(+) is considered as a hadronic molecule composed of a nucleon and D* meson. We give predictions for the width of the strong three-body decay processes Lambdac(2940)+ to Lambdac(2286)+ pi(+) pi(-) and Lambdac(2940)+ to Lambdac(2286)+ pi(0) pi(0) in this interpretation. Upcoming experimental facilities like a Super B factory at KEK or LHCb might be able to provide data on these decay modes.
Semi-inclusive hadron-production processes are becoming important in high-energy hadron reactions. They are used for investigating properties of quark-hadron matters in heavy-ion collisions, for finding the origin of nucleon spin in polarized lepton-nucleon and nucleon-nucleon reactions, and possibly for finding exotic hadrons. In describing the hadron-production cross sections in high-energy reactions, fragmentation functions are essential quantities. A fragmentation function indicates the probability of producing a hadron from a parton. Its $Q^2$ dependence is described by the standard DGLAP (Dokshitzer-Gribov-Lipatov-Altarelli-Parisi) evolution equations, which are often used in theoretical and experimental analyses of the fragmentation functions and in calculating semi-inclusive cross sections. The DGLAP equations are complicated integro-differential equations, which cannot be solved in an analytical method. In this work, a simple method is employed for solving the evolution equations by using Gauss-Legendre quadrature for evaluating integrals, and a useful code is provided for calculating the Q^2 evolution of the fragmentation functions in the leading order (LO) and next-to-leading order (NLO) of the running coupling constant $\alpha_s$. The renormalization scheme is \overline{MS} in the NLO evolution. Our evolution code is explained for using it in one's studies on the fragmentation functions.
Foreword:
The study of the fundamental structure of nuclear matter is a central
thrust of physics research in the United States. As indicated
in Frontiers of Nuclear Science, the 2007 Nuclear Science Advisory Committee
long range plan, consideration of a future Electron-Ion Collider (EIC)
is a priority and will likely be a significant focus of discussion at the next long
range plan. We are therefore pleased to have supported the ten week program in fall 2010 at
the Institute of Nuclear Theory which examined at length the science case for the EIC. This
program was a major effort; it attracted the maximum allowable attendance over ten weeks.
This report summarizes the current understanding of the physics and articulates important
open questions that can be addressed by an EIC. It converges towards a set of “golden”
experiments that illustrate both the science reach and the technical demands on such a
facility, and thereby establishes a firm ground from which to launch the next phase in
preparation for the upcoming long range plan discussions. We thank all the participants in
this productive program. In particular, we would like to acknowledge the leadership and
dedication of the five co-organizers of the program who are also the co-editors of this report
1. The spin and flavor structure of the proton
2. Three-dimensional structure of the proton and nuclei:
transverse momentum
3. Three-dimensional structure of the proton and nuclei: spatial imaging
4. Input from lattice QCD
5. QCD matter under extreme conditions
6. Electroweak physics
7. Experimental aspects
Dijet anomaly reported by the CDF (Collider Detector at Fermilab) collaboration in 1.96 TeV p-pbar collisions is investigated within the standard model by considering effects of parton distribution functions on various processes: W+dijet, Z+dijet, WW, ZW, and top production. Since the anomalous peak exists in the dijet-mass region of 140 GeV with the p-pbar center-of-mass energy sqrt{s}=1.96 TeV, a relevant momentum fraction x of partons is roughly 0.1. In this x region, recent HERMES semi-inclusive charged-lepton scattering experiment indicated that the strange-quark distribution could be very different from a conventional one, which has been used for many years, based on opposite-sign dimuon measurements in neutrino-induced deep inelastic scattering. We investigated effects of such variations in the strange-quark distribution s(x) on the anomaly. We found that distributions of W+dijets and other process are affected by the strange-quark modifications in wide dijet-mass regions including the 140 GeV one. Since the CDF anomaly was observed in the shoulder region of the dijet-mass distribution, a slight modification of the distribution shape could explain at least partially the CDF excess. Therefore, it is important to consider such effects within the standard model for judging whether the CDF anomaly indicates new physics beyond the standard model. We also show modification effects of the strange-quark distribution in the LHC (Large Hadron Collider) kinematics, where cross sections are sensitive to a smaller-x region of s(x).
We report recent studies on structure functions of the nucleon and nuclei. First, clustering effects are investigated in the structure function F_2 of Be-9 for explaining an unusual nuclear correction found in a JLab experiment. We propose that high densities created by formation of clustering structure like 2*alpha+neutron in Be-9 is the origin of the unexpected JLab result by using the antisymmetrized molecular dynamics (AMD). There is an approved proposal at JLab to investigate the structure functions of light nuclei including the cluster structure, so that much details will become clear in a few years. Second, tensor-polarized quark and antiquark distributions are obtained by analyzing HERMES measurements on the structure function b_1 for the deuteron. The result suggests a finite tensor polarization for antiquark distributions, which is an interesting topic for further theoretical and experimental investigations. An experimental proposal exists at JLab for measuring b_1 of the deuteron as a new tensor-structure study in 2010's. Furthermore, the antiquark tensor polarization could be measured by polarized deuteron Drell-Yan processes at hadron facilities such as J-PARC and GSI-FAIR. Third, the recent CDF dijet anomaly is investigated within the standard model by considering possible modifications of the strange-quark distribution. We find that the shape of a dijet-mass spectrum changes depending on the strange-quark distribution. It indicates that the CDF excess could be partially explained as a PDF effect, particularly by the strangeness in the nucleon, within the standard model if the excess at m_{jj}~140 GeV is not a sharp peak.
ドイツ・ベルリン郊外のドイツ電子シンクロトロン研究所 （DESY, Zeuthen支所）で、2011年10月3日〜5日に開催された 日独ワークショップ「量子色力学の最近の動向」について報告する。