Showing papers by "Anthony W. Thomas published in 2002"

Chiral analysis of quenched baryon masses

Abstract: We extend to quenched QCD an earlier investigation of the chiral structure of the masses of the nucleon and the delta in lattice simulations of full QCD. Even after including the meson-loop self-energies which give rise to the leading and next-to-leading nonanalytic behavior (and hence the most rapid variation in the region of light quark mass), we find surprisingly little curvature in the quenched case. Replacing these meson-loop self-energies by the corresponding terms in full QCD yields a remarkable level of agreement with the results of the full QCD simulations. This comparison leads to a very good understanding of the origins of the mass splitting between these baryons.

Bethe-Salpeter equation and a nonperturbative quark-gluon vertex

Abstract: A Ward-Takahashi identity preserving Bethe-Salpeter kernel can always be calculated explicitly from a dressed-quark-gluon vertex whose diagrammatic content is enumerable. We illustrate that fact using a vertex obtained via the complete resummation of dressed-gluon ladders. While this vertex is planar, the vertex-consistent kernel is nonplanar and that is true for any dressed vertex. In an exemplifying model the rainbow-ladder truncation of the gap and Bethe-Salpeter equations yields many results; e.g., \ensuremath{\pi}- and \ensuremath{\rho}-meson masses, that are changed little by including higher-order corrections. Repulsion generated by nonplanar diagrams in the vertex-consistent Bethe-Salpeter kernel for quark-quark scattering is sufficient to guarantee that diquark bound states do not exist.

Determination of the pion-nucleon coupling constant and scattering lengths

Abstract: c =4 =0 :0786(11). This value is intermediate between that of indirect methods and the direct determination from backward np dierential scattering cross sections. We also use the pionic atom data to deduce the coherent symmetric and antisymmetric sums of the pion{proton and pion{ neutron scattering lengths with high precision, namely (a p + a n)= 2=( 172(statistical)8 (systematic))10 4 m 1 and (a p a n)=2 = (9003(statistical)13 (systematic))10 4 m 1 . Report-no: CERN/TH 2000-166,TSL/ISV-2000-0232, LPNHE/LPTPE 2709-0011, ADP-0036/T419

Nucleon mass and pion loops

Abstract: Poincar\'e covariant Faddeev equations for the nucleon and $\ensuremath{\Delta}$ are solved to illustrate that an internally consistent description in terms of confined-quark and non-point-like confined-diquark correlations can be obtained. $\ensuremath{\pi}N$-loop induced self-energy corrections to the nucleon's mass are analyzed and shown to be independent of whether a pseudoscalar or pseudovector coupling is used. Phenomenological constraints suggest that this self-energy correction reduces the nucleon's mass by up to several hundred MeV. That effect does not qualitatively alter the picture, suggested by the Faddeev equation, that baryons are quark-diquark composites. However, neglecting the $\ensuremath{\pi}$ loops leads to a quantitative overestimate of the nucleon's axial-vector diquark component.

Neutron polarizabilities investigated by quasifree Compton scattering from the deuteron.

Abstract: Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at ${\ensuremath{\theta}}_{\ensuremath{\gamma}}^{\mathrm{lab}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}136\ifmmode^\circ\else\textdegree\fi{}$ in an energy range from 200 to 400 MeV. From the data a polarizability difference of ${\ensuremath{\alpha}}_{n}\ensuremath{-}{\ensuremath{\beta}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.8\ifmmode\pm\else\textpm\fi{}3.6(\mathrm{stat}{)}_{\ensuremath{-}1.1}^{+2.1}(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}2.2(\mathrm{model})$ in units of ${10}^{\ensuremath{-}4}{\mathrm{fm}}^{3}$ has been determined. In combination with the polarizability sum ${\ensuremath{\alpha}}_{n}+{\ensuremath{\beta}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}15.2\ifmmode\pm\else\textpm\fi{}0.5$ deduced from photoabsorption data, the first precise results for the neutron electric and magnetic polarizabilities, ${\ensuremath{\alpha}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}12.5\ifmmode\pm\else\textpm\fi{}1.8(\mathrm{stat}{)}_{\ensuremath{-}0.6}^{+1.1}(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}1.1(\mathrm{model})$ and ${\ensuremath{\beta}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.7\ensuremath{\mp}1.8(\mathrm{stat}{)}_{\ensuremath{-}1.1}^{+0.6}(\mathrm{syst})\ensuremath{\mp}1.1(\mathrm{model})$, are obtained.

Complete analysis of spin structure function g 1 of 3 He

Abstract: We present a comprehensive analysis of the nuclear effects important in deep inelastic scattering on polarized ${}^{3}\mathrm{He}$ over a wide range of Bjorken x, ${10}^{\ensuremath{-}4}l~xl~0.8.$ Effects relevant for the extraction of the neutron spin structure function ${g}_{1}^{n}$ from the ${}^{3}\mathrm{He}$ data are emphasized.

Heavy-quark axial charges to nonleading order

Abstract: We combine Witten's renormalization group with the matching conditions of Bernreuther and Wetzel to calculate at next-to-leading order the complete heavy-quark contribution to the neutral-current axial-charge measurable in neutrino-proton elastic scattering. Our results are manifestly renormalization group invariant.

Simple quark model with chiral phenomenology

Abstract: We propose a new approach to the determination of hadronic observables in which the essential features of chiral symmetry are combined with conventional constituent quark models. To illustrate the approach, we consider the simple quark model in the limit of SU(3) flavor symmetry at the strange quark mass. The comparison with data is made after an analytic continuation which ensures the correct leading nonanalytic behavior of chiral perturbation theory. The approach not only gives an excellent fit for the octet baryon magnetic moments but the prediction for the ${\ensuremath{\Delta}}^{++}$ magnetic moment is also in good agreement with current measurements.

Phase transition from nuclear matter to color superconducting quark matter

Abstract: We construct the nuclear and quark matter equations of state at zero temperature in an effective quark theory (the Nambu-Jona-Lasinio model), and discuss the phase transition between them. The nuclear matter equation of state is based on the quark-diquark description of the single nucleon, while the quark matter equation of state includes the effects of scalar diquark condensation (color superconductivity). The effect of diquark condensation on the phase transition is discussed in detail.

Precise Determination of Electroweak Parameters in Neutrino-Nucleon Scattering

Abstract: A systematic error in the extraction of $\sin^2 \theta_W$ from nuclear deep inelastic scattering of neutrinos and antineutrinos arises from higher-twist effects arising from nuclear shadowing. We explain that these effects cause a correction to the results of the recently reported significant deviation from the Standard Model that is potentially as large as the deviation claimed, and of a sign that cannot be determined without an extremely careful study of the data set used to model the input parton distribution functions.

Moments of isovector quark distributions in lattice QCD

Abstract: We investigate the connection of lattice calculations of moments of isovector parton distributions to the physical regime through extrapolations in the quark mass. We consider the one-pion loop renormalisation of the nucleon matrix elements of the corresponding operators and thereby develop formulae with which to extrapolate the moments of the unpolarised, helicity and transversity distributions. These formulae are consistent with chiral perturbation theory in chiral limit and incorporate the correct heavy-quark limits. In the polarised cases, the inclusion of intermediate states involving the. Δ-isobar is found to be very important. The results of our extrapolations are in general agreement with the phenomenological values of these moments where they are known, and for the first time we make reliable predictions for the low moments of the isovector transversity distribution.

Convergence of chiral effective field theory

Abstract: We formulate the expansion for the mass of the nucleon as a function of pion mass within chiral perturbation theory using a number of different ultra-violet regularisation schemes; including dimensional regularisation and various finite-ranged regulators. Leading and next-to-leading order non-analytic contributions are included through the standard one-loop Feynman graphs. In addition to the physical nucleon mass, the expansion is constrained by recent, extremely accurate, lattice QCD data obtained with two flavors of dynamical quarks. The extent to which different regulators can describe the chiral expansion is examined, while varying the range of quark mass over which the expansions are matched. Renormalised chiral expansion parameters are recovered from each regularisation prescription and compared. We find that the finite-range regulators produce consistent, model-independent results over a wide range of quark mass sufficient to solve the chiral extrapolation problem in lattice QCD.

Quark-hadron duality and the nuclear EMC effect

Abstract: Recent data on polarized proton knockout reactions off 4He nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium Using model-independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium

Nucleon mass and pion loops: Renormalization

Abstract: Using Dyson-Schwinger equations, the nucleon propagator is analyzed nonperturbatively in a field-theoretical model for the pion-nucleon interaction. Infinities are circumvented by using pion-nucleon form factors which define the physical scale. It is shown that the correct, finite, on-shell nucleon renormalization is important for the value of the mass shift and the propagator. For physically acceptable forms of the pion-nucleon form factor the rainbow approximation together with renormalization is inconsistent. Going beyond the rainbow approximation, the full pion-nucleon vertex is modeled by its bare part plus a one-loop correction including an effective $\ensuremath{\Delta}.$ It is found that a consistent value for the nucleon mass shift can be obtained as a consequence of a subtle interplay between wave function and vertex renormalization. Furthermore, the bare and renormalized pion-nucleon coupling constants are approximately equal, consistent with results from the cloudy bag model.

Chiral Extrapolation of Hadronic Observables

Abstract: One of the great challenges of lattice QCD is to produce unambiguous predictions for the properties of physical hadrons. We review recent progress with respect to a major barrier to achieving this goal, namely the fact that computation time currently limits us to large quark mass. Using insights from the study of the lattice data itself, together with the general constraints of chiral symmetry, we demonstrate that it is possible to extrapolate accurately and in an essentially model independent manner from the mass region where calculations will be performed within the next five years to the chiral limit.

FLIC Fermions and Hadron Phenomenology

Abstract: A pedagogical overview of the formulation of the Fat Link Irrelevant Clover (FLIC) fermion action and its associated phenomenology is described. The scaling analysis indicates FLIC fermions provide a new form of nonperturbative O(a) improvement where near-continuum results are obtained at finite lattice spacing. Spin-1/2 and spin-3/2, even and odd parity baryon resonances are investigated in quenched QCD, where the nature of the Roper resonance and Lambda(1405) are of particular interest. FLIC fermions allow efficient access to the light quark-mass regime, where evidence of chiral nonanalytic behavior in the Delta-baryon mass is observed.

Enhanced direct CP violation in B ± , 0 → π + π − K ± , 0

Abstract: We investigate in a phenomenological way direct CP violation in the hadronic decays B 6,0 !p 1 p 2 K 6,0 where the effect of r-v mixing is included. If Nc ~the effective parameter associated with factorization! is constrained using the most recent experimental branching ratios ~to r 0 K 0 , r 6 K 7 , r 6 K 0 , r 0 K 6 and vK 6 ) from the BABAR, BELLE and CLEO Collaborations, we get a maximum CP violating asymmetry amax in the range 225% to 149% for B 2 !p 1 p 2 K 2 and 224% to 155% for B 0 !p 1 p 2 K ¯ 0 . We also find that CP violation is strongly dependent on the Cabibbo-Kobayashi-Maskawa matrix elements. Finally, we show that the sign of sin d is always positive in the allowed range of N c and hence, a measurement of direct CP violation in B 6,0 !p 1 p 2 K 6,0 would remove the mod(p) ambiguity in arg@2VtsV tb /VusV ub #. Direct CP violating asymmetries in B decays occur through the interference of at least two amplitudes with dif- ferent weak phase f and strong phase d. In order to extract the weak phase ~which is determined by the CKM matrix elements! through the measurement of a CP violating asym- metry, one must know the strong phase d and this is usually not well determined. In addition, in order to have a large signal, we have to appeal to some phenomenological mecha- nism to obtain a large d. The charge symmetry violating mixing between r 0 and v can be extremely important in this regard. In particular, it can lead to a large CP violation in B decays, such as B 6,0 !r 0 (v)K 6,0 !p 1 p 2 K 6,0 , because the strong phase passes through 90° at the v resonance @3-5#. We have collected the latest data for b to s transitions concentrating on the CLEO, BABAR and BELLE branching ratio results in our approach. The aim of the present work is multiple. The main one is to constrain the CP violating cal- culation in B 6,0 !r 0 (v)K 6,0 !p 1 p 2 K 6,0 , including r-v mixing and using the most recent experimental data for B !rK decays. The second one is to extract consistent con- straints for B decays into r( PS) where PS can be either p or K. In order to extract the strong phase d, we shall use the factorization approach, in which the hadronic matrix ele- ments of operators are saturated by vacuum intermediate states. Moreover, we approximate non-factorizable effects by introducing an effective number of colors, N c . In this paper we investigate five phenomenological mod- els with different weak form factors and determine the CP violating asymmetry, a, for B 6,0 !r 0 (v)K 6,0 !p 1 p 2 K 6,0 in these models. We select models which are consistent with all the data and determine the allowed range for N c @0.66(0.61),Nc ,2.84(2.82) #. Then, we study the sign of sin d in this range of N c for all these models. We also discuss the model dependence of our results in detail. The remainder of this paper is organized as it follows. In Sec. II, we present the form of the effective Hamiltonian which is based on the operator product expansion, together with the values of the corresponding Wilson coefficients. In Sec. III, we give the phenomenological formalism for the CP violating asymmetry in decay processes including r-v mixing, where all aspects of the calculation of direct CP violation, the CKM matrix, r-v mixing, factorization and form factors are discussed in detail. In Sec. IV we list all the numerical inputs which are needed for calculating the asym- metry, a ,i nB 6,0 !r 0 (v)K 6,0 !p 1 p 2 K 6,0 . Section V is devoted to results and discussions for these decays. In Sec. VI we calculate branching ratios for decays such as B 6,0

Chiral extrapolation of lattice data for the hyperfine splittings of heavy mesons

Abstract: Hyperfine splittings between the heavy vector ${(D}^{*}{,B}^{*})$ and pseudoscalar $(D,B)$ mesons have been calculated numerically in lattice QCD, where the pion mass (which is related to the light quark mass) is much larger than its physical value. Naive linear chiral extrapolations of the lattice data to the physical mass of the pion lead to hyperfine splittings which are smaller than experimental data. In order to extrapolate these lattice data to the physical mass of the pion more reasonably, we apply the effective chiral perturbation theory for heavy mesons, which is invariant under chiral symmetry when the light quark masses go to zero and heavy quark symmetry when the heavy quark masses go to infinity. This leads to a phenomenological functional form with three parameters to extrapolate the lattice data. It is found that the extrapolated hyperfine splittings are even smaller than those obtained using linear extrapolation. We conclude that the source of the discrepancy between lattice data for hyperfine splittings and experiment must lie in non-chiral physics.

Baryon Resonance Phenomenology

Abstract: The Japan Hadron Facility will provide an unprecedented opportunity for the study of baryon resonance properties. This talk will focus on the chiral nonanalytic behaviour of magnetic moments exclusive to baryons with open decay channels. To illustrate the novel features associated with an open decay channel, we consider the ``Access'' quark model, where an analytic continuation of chiral perturbation theory is employed to connect results obtained using the constituent quark model in the limit of SU(3)-flavour symmetry to empirical determinations.

Neutrinos and non-perturbative nucleon structure

Abstract: There are many vital questions concerning the non-perturbative structure of the nucleon for which neutrinos are uniquely suited to give answers. We outline some of the essential issues in hadronic physics which can and should be addressed with high intensity neutrino beams.

Quasi-free Compton Scattering and the Polarizabilities of the Neutron

Abstract: Differential cross sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48 cm $\oslash$ $\times$ 64 cm NaI(Tl) photon detector and the G\"ottingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $\theta^{LAB}_\gamma=136.2^\circ$. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction $p(\gamma,\pi^+ n)$. The "free" proton Compton scattering cross sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross section for free scattering from quasi-free data. Differential cross sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron have been obtained to be $\alpha-\beta= 9.8\pm 3.6(stat){}^{2.1}_1.1(syst)\pm 2.2(model)$ in units $10^{-4}fm^3$. In combination with the polarizability sum $\alpha +\beta=15.2\pm 0.5$ deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, $\alpha_n=12.5\pm 1.8(stat){}^{+1.1}_{-0.6}\pm 1.1(model)$ and $\beta_n=2.7\mp 1.8(stat){}^{+0.6}_{-1.1}(syst)\mp 1.1(model)$ are obtained. The backward spin polarizability of the neutron was determined to be $\gamma^{(n)}_\pi=(58.6\pm 4.0)\times 10^{-4}fm^4$.

Chiral behaviour of baryon masses in quenched lattice QCD

Abstract: We investigate the chiral behaviour of nucleon and delta masses as calculated in quenched lattice simulations. The perturbative chiral expansion of baryon masses can guide us to construct a functional form for extrapolating quenched lattice results. Through analysis of the different meson-loop self-energies in the quenched and unquenched theories we study quantitatively the errors associated with quenching over a range of pion mass. In the case of the delta we find that this error can be as much as 400–500 MeV in the chiral limit. This work, together with an analogous study for unquenched simulations, consistently demonstrates the success of this procedure in connecting lattice results with chiral physics.

Boundary of nuclear physics and QCD

Abstract: Recent progress in lattice QCD, combined with the imminent advent of a new generation of dedicated supercomputers and advances in chiral extrapolation mean that the next few years will bring quite novel insights into hadron structure. We review some of the recent highlights in this field, the questions which might be addressed and the experiments which may be expected to stretch that understanding to its limits. Only with a sound understanding of hadron structure can one hope to explore the fundamental issue of how that structure may change at finite density (or temperature). We explore potential future insights from lattice QCD into the phenomenon of nuclear saturation and a very important hint from recent data of a change in the structure of a bound nucleon.

Hadron mass extraction from lattice QCD

Abstract: The extraction of quantities from lattice QCD calculations at realistic quark masses is of considerable importance. Whilst physical quark masses are some way off, the recent advances in the calculation of hadron masses within full QCD now invite improved extrapolation methods. We show that, provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses, one can indeed obtain a fairly reliable determination of masses, the sigma commutator and the J parameter. We summarise these findings by presenting the nonanalytic behaviour of nucleon and rho masses in the standard Edinburgh plot.

Non-perturbative chiral corrections for lattice qcd

Abstract: We explore the chiral aspects of extrapolation of observables calculated within lattice QCD, using the nucleon magnetic moments as an example. Our analysis shows that the biggest effects of chiral dynamics occur for quark masses corresponding to a pion mass below 600 MeV. In this limited range chiral perturbation theory is not rapidly convergent, but we can develop some understanding of the behaviour through chiral quark models. This model dependent analysis leads us to a simple Pad\'e approximant which builds in both the limits $m_\pi \to 0$ and $m_\pi \to \infty$ correctly and permits a consistent, model independent extrapolation to the physical pion mass which should be extremely reliable.