arXiv subjects

S. Simula

Publications and source records attributed to S. Simula.

At least 163 records · Page 9Linked to original sources

arXiv2004

Extracting excited states from lattice QCD: the Roper resonance

We present a new method for extracting excited states from a single two-point correlation function calculated on the lattice. Our method simply combines the correlation function evaluated at different time slices so as to ``subtract'' the leading exponential decay (ground state) and to give access to the first excited state. The method is applied to a quenched lattice study (volume = 24^3 x 64, beta = 6.2, 1/a = 2.55 GeV) of the first excited state of the nucleon using the local interpolating operator O = [uT C gamma5 d] u. The results are consistent with the identification of our extracted excited state with the Roper resonance N'(1440). The switching of the level ordering with respect to the negative-parity partner of the nucleon, N*(1535), is not seen at the simulated quark masses and, basing on crude extrapolations, is tentatively expected to occur close to the physical point.

hep-lat↗

arXiv2004

Light Hadron Spectrum, Renormalization Constants and Light Quark Masses with Two Dynamical Fermions

The results of a preliminary partially quenched (N_f=2) study of the light hadron spectrum, renormalization constants and light quark masses are presented. Numerical simulations are carried out with the LL-SSOR preconditioned Hybrid Monte Carlo with two degenerate dynamical fermions, using the plaquette gauge action and the Wilson quark action at beta = 5.8. Finite volume effects have been investigated employing two lattice volumes: 16^3 x 48 and 24^3 x 48. Configurations have been generated at four values of the sea quark mass corresponding to M_{PS}/M_V ~ 0.6 - 0.8.

hep-lat↗

arXiv2004

Self-Consistent Data Analysis of the Proton Structure Function g1 and Extraction of its Moments

The reanalysis of all available world data on the longitudinal asymmetry A|| is presented. The proton structure function g1 was extracted within a unique framework of data inputs and assumptions. These data allowed for a reliable evaluation of moments of the structure function g1 in the Q2 range from 0.2 up to 30 GeV2. The Q2 evolution of the moments was studied in QCD by means of Operator Product Expansion (OPE).

hep-ex↗

arXiv2004

Semileptonic Hyperon Decays on the Lattice: an Exploratory Study

We present preliminary results of an exploratory lattice study of the vector form factor $f_1(q^2 = 0)$ relevant for the semileptonic hyperon decay $Σ^{-} --> n l ν$. This study is based on the same method used for the extraction of $f^+(0)$ for the decay $K^0 --> π^{-} l ν$. The main purpose of this study is to test the method for hyperon form factors in order to estimate the precision that can be reached and the importance of SU(3)-breaking effects.

hep-lat↗

arXiv2004

The Neutron Electric Dipole Moment in the Instanton Vacuum: Quenched Versus Unquenched Simulations

We investigate the role played by the fermionic determinant in the evaluation of the CP-violating neutron electric dipole moment (EDM) adopting the Instanton Liquid Model. Significant differences between quenched and unquenched calculations are found. In the case of unquenched simulations the neutron EDM decreases linearly with the quark mass and is expected to vanish in the chiral limit. On the contrary, within the quenched approximation, the neutron EDM increases as the quark mass decreases and is expected to diverge as (1/m)**Nf in the chiral limit. We argue that such a qualitatively different behavior is a parameter-free, semi-classical prediction and occurs because the neutron EDM is sensitive to the topological structure of the vacuum. The present analysis suggests that quenched and unquenched lattice QCD simulations of the neutron EDM as well as of other observables governed by topology might show up important differences in the quark mass dependence, for mq < Lambda(QCD).

hep-ph↗

arXiv2004

Width of the J^P=1/2^+ pentaquark in the quark-diquark model

We analyse the width of the $θ(\frac12^+)$ pentaquark assuming that it is a bound state of two extended spin-zero $ud$-diquarks and the $\bar s$ antiquark (the Jaffe-Wilczek scenario). The width obtained when the size parameters of the pentaquark wave function are taken to be close to the parameters of the nucleon is found to be $\simeq 150$ MeV, i.e. it has a normal value for a $P$-wave hadron decay with the corresponding energy release.However, we found a strong dynamical suppression of the decay width if the pentaquark has an asymmetric "peanut" structure with the strange antiquark in the center and the two diquarks rotating around. In this case a decay width of $\simeq$ 1 MeV is a natural possibility.

hep-ph↗

arXiv2004

Evaluation of the Higher Twist Contribution to the Moments of Proton Structure Functions F2 and g1

We performed the measurement of the inclusive electron scattering off the proton in the resonance region (W<2.5 GeV) at momentum transfer Q2 below 4.5 (GeV/c)^2 with the CLAS detector. The large acceptance of CLAS provided an access to a large, continuous two-dimensional kinematic domain in Q2 and x, allowing therefore an integration of the data at fixed Q2 over x-interval. The covered x-interval at each measured Q2 value is sufficient for an evaluation of the higher moments (n>2). From these data we extracted the structure function F2 and, by including other world data, we studied the Q2 evolution of its moments, Mn(Q2), in order to estimate the higher twist contributions. A similar experiment with polarized proton target is completed at CLAS. These new data allow an accurate determination of higher moments of the proton structure function g1. A preliminary phenomenological analysis indicates an excess of the higher twist contribution in the spin-dependent structure function with respect to the spin-independent one.

hep-ph↗

arXiv2003

Possible evidence of extended objects inside the proton

Recent data on the Nachtmann moments of the unpolarized proton structure function F2p, obtained at low momentum transfer with the CLAS detector at Jefferson Lab, are interpreted in terms of the dominance of the elastic coupling of the virtual photon with extended substructures inside the proton. The CLAS data exhibit a new type of scaling behavior and the resulting scaling function can be interpreted as a constituent form factor consistent with the elastic nucleon data. A constituent size of ~ 0.2 - 0.3 fm is obtained.

nucl-th↗

arXiv2003

New extraction of alpha_s(MZ) from proton DIS data

An exploratory study for a new determination of the strong coupling constant alpha_s(MZ) from existing world data on the proton structure function F2p in the Q**2-range from ~ 5 to ~ 120 (GeV/c)**2 is presented. The main features of our approach are: 1) the use of low-order Nachtmann moments evaluated with a direct contribution from data larger than 70% of the total; 2) the inclusion of high-order perturbative effects through the soft gluon resummation technique; 3) a direct control over higher-twist effects; and 4) the independence from any specific choice of the x-shape of the input parton distributions. At next-to-leading order we get alpha_s(MZ) = 0.1209 +/- 0.0010 (stat.) +/- 0.0015 (syst.) with a significant dependence upon the order of the moment used. Including soft gluon effects we obtain alpha_s(MZ) = 0.1188 +/- 0.0010 (stat.) +/- 0.0014 (syst.) with a remarkable better stability against the order of the moment. Our findings are compared with recent DIS determinations of alpha_s(MZ) and directions for future improvements are discussed.

hep-ph↗

arXiv2003

Analysis of the Axial Anomaly on the Lattice with O(a)-improved Wilson Action

Flavor singlet and non-singlet axial Ward identities are investigated using the Wilson formulation of lattice QCD with Clover O(a)-improvement, which breaks explicitly chiral symmetry. The matching at one-loop order of all the relevant renormalization constants with the continuum \bar{MS} scheme is presented. Our calculations include: 1) the contributions arising from the Clover term of the action; 2) the complete mixing of the gluon operator G \tilde{G} with the divergence of the singlet axial current; 3) the use of both local and extended definitions of the fermionic bilinear operators. A definition of the gluon operator G \tilde{G} on the lattice outside the chiral limit is proposed. Our definition takes into account the possible power-divergent mixing with the pseudoscalar density, generated by the breaking of chiral symmetry. A non-perturbative procedure for the evaluation of such mixing constant is worked out. Finally, the renormalization properties of the composite insertion of the topological charge operator relevant for the lattice calculation of the neutron electric dipole moment, induced by the strong CP-violating term of the QCD Lagrangian, are discussed.

hep-lat↗

arXiv2003

A Kinematically Complete Measurement of the Proton Structure Function F2 in the Resonance Region and Evaluation of Its Moments

We measured the inclusive electron-proton cross section in the nucleon resonance region (W < 2.5 GeV) at momentum transfers Q**2 below 4.5 (GeV/c)**2 with the CLAS detector. The large acceptance of CLAS allowed for the first time the measurement of the cross section in a large, contiguous two-dimensional range of Q**2 and x, making it possible to perform an integration of the data at fixed Q**2 over the whole significant x-interval. From these data we extracted the structure function F2 and, by including other world data, we studied the Q**2 evolution of its moments, Mn(Q**2), in order to estimate higher twist contributions. The small statistical and systematic uncertainties of the CLAS data allow a precise extraction of the higher twists and demand significant improvements in theoretical predictions for a meaningful comparison with new experimental results.

hep-ph↗

arXiv2003

Neutron Electric Dipole Moment on the Lattice: a Theoretical Reappraisal

We present a strategy for a lattice evaluation of the neutron electric dipole moment induced by the strong CP violating term of the QCD Lagrangian. Our strategy is based on the standard definition of the electric dipole moment, involving the charge density operator J0, in case of three flavors with non-degenerate masses. We present a complete diagrammatic analysis showing how the axial chiral Ward identities can be used to replace the opological charge operator with the flavor-singlet pseudoscalar density PS. Our final result is characterized only by disconnected diagrams, where the disconnected part can be either the single insertion of PS or the separate insertions of both PS and J0. The applicability of our strategy to the case of lattice formulations that explicitly break chiral symmetry, like the Wilson and Clover actions, is discussed.

hep-lat↗

arXiv2003

Possible evidence of extended objects inside the proton

Recent experimental determinations of the Nachtmann moments of the inelastic structure function of the proton F2p(x, Q**2), obtained at Jefferson Lab, are analyzed for values of the squared four-momentum transfer Q**2 ranging from ~ 0.1 to ~ 2 (GeV/c)**2. It is shown that such inelastic proton data exhibit a new type of scaling behavior and that the resulting scaling function can be interpreted as a constituent form factor consistent with the elastic nucleon data. These findings suggest that at low momentum transfer the inclusive proton structure function originates mainly from the elastic coupling with extended objects inside the proton. We obtain a constituent size of ~ 0.2 - 0.3 fm.

hep-ph↗

arXiv2003

End-point singularities of Feynman graphs on the light cone

We show that some Lorentz components of the Feynman integrals calculated in terms of the light-cone variables may contain end-point singularities which originate from the contribution of the big-circle integral in the complex k_ plane. These singularities appear in various types of diagrams (two-point functions, three-point functions, etc) and provide the covariance of the Feynman integrals on the light-cone. We propose a procedure for calculating Feynman integrals which guarantees that the end-point singularities do not appear in the light-cone representations of the invariant amplitudes.

hep-ph↗

arXiv2002

Proton and neutron polarized structure functions from low to high Q**2

Phenomenological parameterizations of proton and neutron polarized structure functions, g1p and g1n, are developed for x > 0.02 using deep inelastic data up to ~ 50 (GeV/c)**2 as well as available experimental results on photo- and electro-production of nucleon resonances. The generalized Drell-Hearn-Gerasimov sum rules are predicted from low to high values of Q**2 and compared with proton and neutron data. Furthermore, the main results of the power correction analysis carried out on the Q**2-behavior of the polarized proton Nachtmann moments, evaluated using our parameterization of g1p, are briefly summarized.

nucl-th↗

arXiv2002

A nuclear physicist on the lattice

Nuclear and lattice physicists share several topics of common interest, like hadronic masses, electroweak form factors and structure functions of hadrons. The main physics issues that a new collaboration between a nuclear physicist and the lattice group of the SPQcdR collaboration is going to address, are summarized and preliminary results obtained for meson and baryon masses are presented. The importance of adopting non-perturbative approaches based on the fundamental theory of the strong interaction, like lattice formulations of QCD, is stressed.

nucl-th↗

arXiv2002

Neutron structure function and inclusive DIS from H-3 and He-3 at large Bjorken-x

A detailed study of inclusive deep inelastic scattering (DIS) from mirror A = 3 nuclei at large values of the Bjorken variable x is presented. The main purpose is to estimate the theoretical uncertainties on the extraction of the neutron DIS structure function from such nuclear measurements. On one hand, within models in which no modification of the bound nucleon structure functions is taken into account, we have investigated the possible uncertainties arising from: i) charge symmetry breaking terms in the nucleon-nucleon interaction, ii) finite Q**2 effects neglected in the Bjorken limit, iii) the role of different prescriptions for the nucleon Spectral Function normalization providing baryon number conservation, and iv) the differences between the virtual nucleon and light cone formalisms. Although these effects have been not yet considered in existing analyses, our conclusion is that all these effects cancel at the level of ~ 1% for x < 0.75 in overall agreement with previous findings. On the other hand we have considered several models in which the modification of the bound nucleon structure functions is accounted for to describe the EMC effect in DIS scattering from nuclei. It turns out that within these models the cancellation of nuclear effects is expected to occur only at a level of ~ 3%, leading to an accuracy of ~ 12 % in the extraction of the neutron to proton structure function ratio at x ~ 0.7 -0.8$. Another consequence of considering a broad range of models of the EMC effect is that the previously suggested iteration procedure does not improve the accuracy of the extraction of the neutron to proton structure function ratio.

nucl-th↗

arXiv2002

Leading and higher twists in the proton polarised structure function g1p at large Bjorken-x

Power corrections to the Q**2 behavior of the Nachtmann moments of the proton polarized structure function g1p are investigated at large Bjorken-x by developing a phenomenological fit of both the resonance (including the photon point) and deep inelastic data up to Q**2 ~ 50 (GeV/c)**2. The leading twist is treated at NLO in the strong coupling constant and the effects of higher orders of the perturbative series are estimated using soft-gluon resummation techniques. In case of the first moment higher-twist effects are found to be quite small for Q**2 > 1 (GeV/c)**2, and the singlet axial charge is determined to be a0[10 (GeV/c)**2] = 0.16 +/- 0.09. In case of higher order moments, which are sensitive to the large-x region, higher-twist effects are significantly reduced by the introduction of soft gluon contributions, but they are still relevant at Q**2 ~ few (GeV/c)**2 at variance with the case of the unpolarized transverse structure function of the proton. This finding suggests that spin-dependent correlations among partons may have more impact than spin-independent ones. It is also shown that the parton-hadron local duality is violated in the region of polarized electroproduction of the Delta(1232) resonance.

hep-ph↗