62 research outputs found
Exploratory spectrum calculations using overlap valence quarks on a staggered sea
We present exploratory results for the hadron mass spectrum and pseudoscalar
meson decay constants using mixed actions. We use improved staggered sea quarks
and HYP-smeared overlap valence quarks. We obtain good signals on 10
configurations at one lattice spacing and two different sets of sea quark
masses.Comment: Lattice2004(spectrum), 3pages, 4 figure
Semi-leptonic decays of heavy mesons and the Isgur-Wise function in quenched lattice QCD
The form factors for the semi-leptonic B->D and B->D* decays are evaluated in
quenched lattice QCD at two different values of the coupling, beta=6.0 and 6.2.
The action and the operators are fully O(a) non-perturbatively improved. The
slope of the Isgur-Wise function is evaluated, and found to be
rho^2=0.83^{+15+24}_{-11-1} (quoted errors are statistical and systematic
respectively). Ratios of form factors are evaluated and compared to
experimental determinations.Comment: 21 pages, 10 figure
Heavy Quark Spin Symmetry and Heavy Baryons: Electroweak Decays
Heavy quark spin symmetry is discussed in the context of single and doubly
heavy baryons. A special attention is paid to the constraints/simplifications
that this symmetry imposes on the non-relativistic constituent quark model wave
functions and on the b->c semileptonic decays of these hadrons.Comment: Presented at the 21st European Conference on Few-Body Problems in
Physics, Salamanca, Spain, 30 August - 3 September 201
An Investigation of the Soft Pion Relation in Quenched Lattice QCD
A lattice determination of the form factor and decay constants for the
semileptonic decay of heavy pseudoscalar (PS) mesons at zero recoil is
presented from which the soft pion relation is satisfied. Chiral extrapolation
of the form factor is performed at constant . Pole dominance is used to
extrapolate the form factor in heavy quark mass. At the B mass, the form factor
at zero recoil lies somewhat below the ratio of decay constants; the relation
remains satisfied within error.Comment: 3 pages, 2 figure
Low-lying spectrum of the Y-string three-quark potential using hyper-spherical coordinates
We calculate the energies of three-quark states with definite permutation
symmetry (i.e. of SU(6) multiplets) in the N=0,1,2 shells, confined by the
Y-string three-quark potential. The exact Y-string potential consists of one,
so-called three-string term, and three angle-dependent two-string terms. Due to
this technical complication we treat the problem at three increasingly accurate
levels of approximation: 1) the (approximate) three-string potential expanded
to first order in trigonometric functions of hyper-spherical angles; 2) the
(approximate) three-string potential to all orders in the power expansion in
hyper-spherical harmonics, but without taking into account the transition(s) to
two-string potentials; 3) the exact minimal-length string potential to all
orders in power expansion in hyper-spherical harmonics, and taking into account
the transition(s) to two-string potentials. We show the general trend of
improvement %convergence of these approximations: The exact non-perturbative
corrections to the total energy are of the order of one per cent, as compared
with approximation 2), yet the exact energy differences between the
-plets are shifted to 2:2:0.9,
from the Bowler and Tynemouth separation rule 2:2:1, which is obeyed by
approximation 2) at the one per cent level. The precise value of the energy
separation of the first radial excitation ("Roper") -plet
from the -plet depends on the approximation, but does not become
negative, i.e. the "Roper" remains heavier than the odd-parity
-plet in all of our approximations.Comment: 19 pages, 6 figure
Lattice Study of the Decay B^0-bar -> rho^+ l^- nu_l-bar: Model-Independent Determination of |V_{ub}|
We present results of a lattice computation of the vector and axial-vector
current matrix elements relevant for the semileptonic decay B^0-bar -> rho^+
l^- nu_l-bar. The computations are performed in the quenched approximation of
lattice QCD on a 24^3 x 48 lattice at beta = 6.2, using an O(a) improved
fermionic action. Our principal result is for the differential decay rate,
dGamma/dq^2, for the decay B^0-bar -> rho^+ l^- nu_l-bar in a region beyond the
charm threshold, allowing a model-independent extraction of |V_{ub}| from
experimental measurements. Heavy quark symmetry relations between radiative and
semileptonic decays of B-bar mesons into light vector mesons are also
discussed.Comment: 22 pages LaTeX-209 (dependent on settings in a4.sty), 23 PostScript
figures included with epsf.sty. Complete PostScript file including figures
available at http://wwwhep.phys.soton.ac.uk/hepwww/papers/shep9518
Analysis of the Lambda_b -> Lambda_c + l nu_l decay within a light-front constituent quark model
We present an investigation of the Isgur-Wise form factor relevant for the
semileptonic decay Lambda_b -> \Lambda_c + l nu_l performed within a
light-front constituent quark model. Adopting different baryon wave functions
it is found that the Isgur-Wise form factor depends sensitively on the baryon
structure. It is shown however that the shape of the Isgur-Wise function in the
full recoil range relevant for the Lambda_b -> \Lambda_c + l nu_l decay can be
effectively constrained using recent lattice QCD results at low recoil. Then,
the Lambda_b -> \Lambda_c + l nu_l decay is investigated including both
radiative effects and first-order power corrections in the inverse heavy-quark
mass. Our final predictions for the exclusive semileptonic branching ratio, the
longitudinal and transverse asymmetries, and the longitudinal to transverse
decay ratio are: Br(Lambda_b -> \Lambda_c + l nu_l) = (6.3 +/- 1.6) % |V_bc /
0.040|**2 ~ tau(Lambda_b) / (1.24 ps), a_L = -0.945 +/- 0.014, a_T = -0.62 +/-
0.09 and R_L/T = 1.57 +/- 0.15, respectively. Moreover, both the longitudinal
asymmetry and the (partially integrated) longitudinal to transverse decay ratio
are found to be only marginally affected by the model dependence of the
Isgur-Wise form factor as well as by first-order power corrections; therefore,
their experimental determination might be a very interesting tool for testing
the SM and for investigating possible New Physics.Comment: revised version with inclusion of PQCD corrections and improved
discussion of power corrections, to appear in Physical Review
Chiral Symmetry Breaking and Cooling in Lattice QCD
Chiral symmetry breaking is calculated as a function of cooling in quenched
lattice QCD. A non-zero signal is found for the chiral condensate beyond one
hundred cooling steps, suggesting that there is chiral symmetry breaking
associated with instantons. Quantitatively, the chiral condensate in cooled
gauge field configurations is small compared to the value without cooling.Comment: 11 pages in REVTEX including 4 PS figures embedded using psfig.sty,
uuencode
Numerical study of O(a) improved Wilson quark action on anisotropic lattice
The improved Wilson quark action on the anisotropic lattice is
investigated. We carry out numerical simulations in the quenched approximation
at three values of lattice spacing (--2 GeV) with the
anisotropy , where and are
the spatial and the temporal lattice spacings, respectively. The bare
anisotropy in the quark field action is numerically tuned by the
dispersion relation of mesons so that the renormalized fermionic anisotropy
coincides with that of gauge field. This calibration of bare anisotropy is
performed to the level of 1 % statistical accuracy in the quark mass region
below the charm quark mass. The systematic uncertainty in the calibration is
estimated by comparing the results from different types of dispersion
relations, which results in 3 % on our coarsest lattice and tends to vanish in
the continuum limit. In the chiral limit, there is an additional systematic
uncertainty of 1 % from the chiral extrapolation.
Taking the central value from the result of the
calibration, we compute the light hadron spectrum. Our hadron spectrum is
consistent with the result by UKQCD Collaboration on the isotropic lattice. We
also study the response of the hadron spectrum to the change of anisotropic
parameter, . We find that the change
of by 2 % induces a change of 1 % in the spectrum for physical quark
masses. Thus the systematic uncertainty on the anisotropic lattice, as well as
the statistical one, is under control.Comment: 27 pages, 25 eps figures, LaTe
B-->pi and B-->K transitions in standard and quenched chiral perturbation theory
We study the effects of chiral logs on the heavy-->light pseudoscalar meson
transition form factors by using standard and quenched chiral perturbation
theory combined with the static heavy quark limit. The resulting expressions
are used to indicate the size of uncertainties due to the use of the quenched
approximation in the current lattice studies. They may also be used to assess
the size of systematic uncertainties induced by missing chiral log terms in
extrapolating toward the physical pion mass. We also provide the coefficient
multiplying the quenched chiral log, which may be useful if the quenched
lattice studies are performed with very light mesons.Comment: 33 pages, 8 PostScript figures, version to appear in PR
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