Proceedings of RIKEN BNL Research Center Workshop, Volume 91, RBRC Scientific Review Committee Meeting

The ninth evaluation of the RIKEN BNL Research Center (RBRC) took place on Nov. 17-18, 2008, at Brookhaven National Laboratory. The members of the Scientific Review Committee (SRC) were Dr. Dr. Wit Busza (Chair), Dr. Miklos Gyulassy, Dr. Akira Masaike, Dr. Richard Milner, Dr. Alfred Mueller, and Dr. Akira Ukawa. We are pleased that Dr. Yasushige Yano, the Director of the Nishina Institute of RIKEN, Japan participated in this meeting both in informing the committee of the activities of the Nishina Institute and the role of RBRC and as an observer of this review. In order to illustrate the breadth and scope of the RBRC program, each member of the Center made a presentation on his/her research efforts. This encompassed three major areas of investigation, theoretical, experimental and computational physics. In addition the committee met privately with the fellows and postdocs to ascertain their opinions and concerns. Although the main purpose of this review is a report to RIKEN Management (Dr. Ryoji Noyori, RIKEN President) on the health, scientific value, management and future prospects of the Center, the RBRC management felt that a compendium of the scientific presentations are of sufficient quality and interest that they warrant a wider distribution. Therefore we have made this compilation and present it to the community for its information and enlightenment

A lattice determination of light quark masses

A fully non-perturbative lattice determination of the up/down and strange quark masses is given for quenched QCD using both, O(a) improved Wilson fermions and ordinary Wilson fermions. For the strange quark mass with O(a) improved fermions we obtain m_s"M"S (#mu# = 2 GeV) = 105(4) MeV, using the interquark force scale r_0. Due to quenching problems fits are only possible for quark masses larger than the strange quark mass. If we extrapolate our fits to the up/down quark mass we find for the average mass m_l"M"S (#mu# = 2 GeV) = 4.4(2) MeV. (orig.)23 refs.Available from TIB Hannover: RA 2999(99-097) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The polarized structure function g_2: a lattice study revisited

A recent lattice calculation of the spin-dependent structure function g_2 is revisited. It has been recognized that the twist-three operator, which gives rise to d_2, mixes non-perturbatively with operators of lower dimensions under renormalization. This changes the results substantially. (orig.)8 refs.Available from TIB Hannover: RA 2999(99-129) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A lattice determination of the second moment of the polarised valence quark distribution

We perform a Monte Carlo calculation of the second moment of the polarised valence quark distributions in the nucleon, using quenched Wilson fermions. The special feature of this moment is that it is directly accessible experimentally. At a scale of #mu##approx#2 GeV we find #DELTA#"("1")u=0.198(8), #DELTA#"("1")d=-0.0477(33). We compare these numbers with recent experimental results of the SMC colla boration. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

New results for non-perturbative O(a) improvement in light hadrons

We have results from light hadron simulations in quenched QCD at #beta# 6.0 and 6.2 using non-perturbatively improved Sheikholeslami-Wohlert fermions in an effort to remove all O(a) effects. From looking at hadron masses and splittings and the RG-invariant quark masses (where we have one point at #beta# = 5.7) we suggest this is plausible even with the limited data set. The interpretation of the decay constants appears to be less clear. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(97-198) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Renormalization of four-fermion operators for higher twist calculations

The evaluation of the higher twist contributions to deep inelastic scattering amplitudes involves a non trivial choice of operator bases for the higher orders of the OPE expansion of the two hadronic currents. In this talk we discuss the perturbative renormalization of the four-fermion operators that appear in the above bases. (orig.)7 refs.Available from TIB Hannover: RA 2999(98-132) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Scaling of non-perturbative O(a) improved Wilson fermions: Hadron spectrum, quark masses and decay constants

We compute the hadron mass spectrum, the quark masses and the meson decay constants in quenched lattice QCD with non-perturbatively O(a) improved Wilson fermions. The calculations are done for two values of the coupling constant, #beta# = 6.0 and 6.2, and the results are compared with the predictions of ordinary Wilson fermions. We find that the improved action reduces lattice artifacts as expected. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Accelerating the Hybrid Monte Carlo algorithm

An algorithm for separating the high- and low-frequency molecular dynamics modes in Hybrid Monte Carlo (HMC) simulations of gauge theories with dynamical fermions is presented. The separation is based on splitting the pseudo-fermion action into two parts, as was initially proposed by Hasenbusch. We propose to introduce different evolution time-scales for each part. We test our proposal in realistic simulations of two-flavor O(a) improved Wilson fermions. A speed-up of more than a factor of three compared to the standard HMC algorithm is observed in a typical run. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(03-040) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Higher-twist contributions to the structure functions coming from 4-fermion operators

We evaluate the contribution of a class of higher-twist operators to the lowest moment of the structure functions, by computing appropriate matrix elements of six four-fermion operators in the quenched approximation. Their perturbative renormalization constants and mixing coefficients are calculated in the 't Hooft-Veltman scheme of dimensional regularization, using codes written in the algebraic manipulation computer language FORM. (orig.)11 refs.Available from TIB Hannover: RA 2999(99-116) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman