1,344 research outputs found
Positivity and topology in lattice gauge theory
The admissibility condition usually used to define the topological charge in
lattice gauge theory is incompatible with a positive transfer matrix.Comment: 6 pages, revtex; revision has some clarifications and additional
references, representing the final version to appear in Physical Revie
Non perturbative determination of the running coupling constant in quenched SU(2)
Through a finite size renormalization group technique we calculate the
running coupling constant for quenched SU(2) with a few percent error over a
range of energy varying by a factor thirty. The definition is based on ratio of
correlations of Polyakov loops with twisted boundary conditions. The
extrapolation to the continuum limit is governed by corrections due to lattice
artifacts which are proportional to the square of the lattice spacing and
appears rather smooth.Comment: 18 pages of ps fil
Leading Quenching Effects in the Proton Magnetic Moment
We present the first investigation of the extrapolation of quenched nucleon
magnetic moments in quenched chiral effective field theory. We utilize
established techniques in finite-range regularisation and compare with standard
dimensional regularisation methods. Finite-volume corrections to the relevant
loop integrals are also addressed. Finally, the contributions of dynamical sea
quarks to the proton moment are estimated using a recently discovered
phenomenological link between quenched and physical QCD.Comment: 9 pages, 11 figs; v2: revised finite volume discussio
Dark Matter at Boulby Mine
The Boulby Dark Matter Collaboration (BDMC) is running a WIMP Dark Matter
research programme in the underground laboratory of Boulby Mine. The two axes
of the programme are based on (1) liquid Xenon (LXe) as the WIMP target and (2)
directional detection in low pressure gas detectors. ZEPLIN-1 is a 3.1kg of LXe
scintillation detector with a background discrimination based on Pulse Shape
Analysis. A new limit from a preliminary analysis is presented. Further
generation setups with improved background discrimination tools (as the
ionisation is also recorded) are in construction. ZEPLIN-2 and ZEPLIN-3, are
predicted to be sensitive to rate of 0.1-0.01 events/kg/day within 2 years of
data, with fiducial masses of 30kg and 6kg, respectively. These are important
steps towards the design of a tonne-scale LXe Dark Matter detector array. The
DRIFT programme relies on discrimination by tracking and directionnal
detection. This enables the measurement of any sidereal modulation, as the
earth's rotation changes the flux angle of Galactic WIMPs. A low pressure
1m chamber (DRIFT-1) is currently taking data underground. A setup for a
bigger volume with increased spatial resolution (enabling the pressure, and
hence the target mass to be increase) is in the designing process.Comment: 6 pages, 5 figure
On the discretization of physical momenta in lattice QCD
The adoption of two distinct boundary conditions for two fermions species on
a finite lattice allows to deal with arbitrary relative momentum between the
two particle species, in spite of the momentum quantization rule due to a
limited physical box size. We test the physical significance of this
topological momentum by checking in the continuum limit the validity of the
expected energy-momentum dispersion relations.Comment: 9 pages, 3 figures; references added; no changes in text or results;
version accepted for pubblication in Phys. Lett.
Heavy quark masses in the continuum limit of quenched Lattice QCD
We compute charm and bottom quark masses in the quenched approximation and in
the continuum limit of lattice QCD. We make use of a step scaling method,
previously introduced to deal with two scale problems, that allows to take the
continuum limit of the lattice data. We determine the RGI quark masses and make
the connection to the MSbar scheme. The continuum extrapolation gives us a
value m_b^{RGI} = 6.73(16) GeV for the b-quark and m_c^{RGI} = 1.681(36) GeV
for the c-quark, corresponding respectively to m_b^{MSbar}(m_b^{MSbar}) =
4.33(10) GeV and m_c^{MSbar}(m_c^{MSbar}) = 1.319(28) GeV. The latter result,
in agreement with current estimates, is for us a check of the method. Using our
results on the heavy quark masses we compute the mass of the Bc meson, M_{Bc} =
6.46(15) GeV.Comment: 29 pages, 9 figures, version accepted for publication in Nucl. Phys.
Hybrid meson decay from the lattice
We discuss the allowed decays of a hybrid meson in the heavy quark limit. We
deduce that an important decay will be into a heavy quark non-hybrid state and
a light quark meson, in other words, the de-excitation of an excited gluonic
string by emission of a light quark-antiquark pair.
We discuss the study of hadronic decays from the lattice in the heavy quark
limit and apply this approach to explore the transitions from a spin-exotic
hybrid to and where is a scalar meson. We obtain a
signal for the transition emitting a scalar meson and we discuss the
phenomenological implications.Comment: 18 pages, LATEX, 3 ps figure
APENet: LQCD clusters a la APE
Developed by the APE group, APENet is a new high speed, low latency,
3-dimensional interconnect architecture optimized for PC clusters running
LQCD-like numerical applications. The hardware implementation is based on a
single PCI-X 133MHz network interface card hosting six indipendent
bi-directional channels with a peak bandwidth of 676 MB/s each direction. We
discuss preliminary benchmark results showing exciting performances similar or
better than those found in high-end commercial network systems.Comment: Lattice2004(machines), 3 pages, 4 figure
Quenched lattice calculation of the vector channel B --> D* l nu decay rate
We calculate, in the continuum limit of quenched lattice QCD, the form factor
that enters the decay rate of the semileptonic decay B --> D* l nu. By using
the step scaling method (SSM), previously introduced to handle two scale
problems in lattice QCD, and by adopting flavor twisted boundary conditions we
extract F(w) at finite momentum transfer and at the physical values of the
heavy quark masses. Our results can be used in order to extract the CKM matrix
element Vcb by the experimental decay rate without model dependent
extrapolations. The value of Vcb agrees with the one obtained from the B --> D
l nu channel and makes us confident that the quenched approximation well
applies to these transitions.Comment: 11 pages, 8 figure
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