231 research outputs found
Renormalization of HQET operators: O() improvement and matching with QCD
We determine a basis of dimension-7 operators which arise at O() in the
Symanzik expansion of the operators with static heavy quarks. We
consider both Wilson-like and Ginsparg-Wilson light quarks. Exact chiral
symmetry reduces the number of these O() counterterms by a factor of two.
Only a subset of these operators has previously appeared in the literature. We
then extend the analysis to the O() operators contributing beyond the
static approximation.Comment: 7 pages, presented at the 31st International Symposium on Lattice
Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, German
Combined Relativistic and static analysis for all Delta B=2 operators
We analyse matrix elements of Delta B=2 operators by combining QCD results
with the ones obtained in the static limit of HQET. The matching of all the QCD
operators to HQET is made at NLO order. To do that we have to include the
anomalous dimension matrix up to two loops, both in QCD and HQET, and the one
loop matching for all the Delta B=2 operators. The matrix elements of these
operators are relevant for the prediction of the B-\bar B mixing, B_s meson
width difference and supersymmetric effects in Delta B=2 transitions.Comment: 3 pages, 1 figure. Lattice2001(heavyquark
First Lattice QCD Study of the Sigma -> n Axial and Vector Form Factors with SU(3) Breaking Corrections
We present the first quenched lattice QCD study of the form factors relevant
for the hyperon semileptonic decay Sigma -> n l nu. The momentum dependence of
both axial and vector form factors is investigated and the values of all the
form factors at zero-momentum transfer are presented. Following the same
strategy already applied to the decay K0 -> pi- l nu, the SU(3)-breaking
corrections to the vector form factor at zero-momentum transfer, f1(0), are
determined with great statistical accuracy in the regime of the simulated quark
masses, which correspond to pion masses above ~ 0.7 GeV. Besides f1(0) also the
axial to vector ratio g1(0) / f1(0), which is relevant for the extraction of
the CKM matrix element Vus, is determined with significant accuracy. Due to the
heavy masses involved, a polynomial extrapolation, which does not include the
effects of meson loops, is performed down to the physical quark masses,
obtaining f1(0) = -0.948 +/- 0.029 and g1(0) / f1(0) = -0.287 +/- 0.052, where
the uncertainties do not include the quenching effect. Adding a recent
next-to-leading order determination of chiral loops, calculated within the
Heavy Baryon Chiral Perturbation Theory in the approximation of neglecting the
decuplet contribution, we obtain f1(0) = -0.988 +/- 0.029(lattice) +/-
0.040(HBChPT). Our findings indicate that SU(3)-breaking corrections are
moderate on both f1(0) and g1(0). They also favor the experimental scenario in
which the weak electricity form factor, g2(0), is large and positive, and
correspondingly the value of |g1(0) / f1(0)| is reduced with respect to the one
obtained with the conventional assumption g2(q**2) = 0 based on exact SU(3)
symmetry.Comment: final version to appear in Nucl. Phys.
A Tentative Description of Z_c,b States in Terms of Metastable Feshbach Resonances
We attempt a description of the recently discovered Z_{c,b} states in terms
of Feshbach resonances arising from the interaction between the `closed'
subspace of hadrocharmonium levels and the `open' one of open-charm/beauty
thresholds. We show how the neutrality of the X(3872) might be understood in
this scheme and provide a preliminary explanation of the pattern of the
measured total widths of X,Z_{c,b}.Comment: To appear in the proceedings of The 6th International Workshop on
Charm Physics (CHARM 2013
Scaling test of quenched Wilson twisted mass QCD at maximal twist
We present the results of an extended scaling test of quenched Wilson twisted
mass QCD. We fix the twist angle by using two definitions of the critical mass,
the first obtained by requiring the vanishing of the pseudoscalar meson mass
m_PS for standard Wilson fermions and the second by requiring restoration of
parity at non-zero value of the twisted mass mu and subsequently extrapolating
to mu=0. Depending on the choice of the critical mass we simulate at values of
beta in [5.7,6.45], for a range of pseudoscalar meson masses 250 MeV < m_PS < 1
GeV and we perform the continuum limit for the pseudoscalar meson decay
constant f_PS and various hadron masses (vector meson m_V, baryon octet m_oct
and baryon decuplet m_dec) at fixed value of r_0 m_PS. For both definitions of
the critical mass, lattice artifacts are consistent with O(a) improvement.
However, with the second definition, large O(a^2) discretization errors present
at small quark mass with the first definition are strongly suppressed. The
results in the continuum limit are in very good agreement with those from the
Alpha and CP-PACS Collaborations.Comment: 6 pages, Talk presented at Lattice 2005, Dublin, 25-30 July 200
Doubly Charmed Tetraquarks in B_c and Xi_bc Decays
The phenomenology of the so-called X, Y and Z hadronic resonances is hard to
reconcile with standard charmonium or bottomonium interpretations. It has been
suggested that some of these new hadrons can possibly be described as tightly
bound tetraquark states and/or as loosely bound two-meson molecules. In the
present paper we focus on the hypothetical existence of flavored, doubly
charmed, tetraquarks. Such states might also carry double electric charge, and
in this case, if discovered, they could univocally be interpreted in terms of
compact tetraquarks. Flavored tetraquarks are also amenable to lattice studies
as their interpolating operators do not overlap with ordinary meson ones. We
show that doubly charmed tetraquarks could significantly be produced at LHC
from B_c or Xi_bc heavy baryons.Comment: 12 pages, 8 figures. Comments and references added. Version to appear
in Phys.Rev.
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