28 research outputs found
Evidence for charm-bottom tetraquarks and the mass dependence of heavy-light tetraquark states from lattice QCD
We continue our study of heavy-light four-quark states and find evidence from
lattice QCD for the existence of a strong-interaction-stable
tetraquark with mass in the range of 15 to 61 MeV below
threshold. Since this range includes the electromagnetic
decay threshold, current uncertainties do not allow us to
determine whether such a state would decay electromagnetically, or only weakly.
We also perform a study at fixed pion mass, with NRQCD for the heavy quarks,
simulating and tetraquarks with or
and variable, unphysical in order to investigate the heavy
mass-dependence of such tetraquark states. We find that the dependence of the
binding energy follows a phenomenologically-expected form and that, though
NRQCD breaks down before is reached, the results at higher
clearly identify the channel as the
most likely to support a strong-interaction-stable tetraquark state at
. This observation serves to motivate the direct
simulation. Throughout we use dynamical ensembles
with pion masses 415, 299, and 164 MeV reaching down almost to the
physical point, a relativistic heavy quark prescription for the charm quark,
and NRQCD for the bottom quark(s).Comment: 24 pages, 4 figure
Dark Matter from Strong Dynamics: The Minimal Theory of Dark Baryons
As a simple model for dark matter, we propose a QCD-like theory based on
gauge theory with one flavor of dark quark. The model is confining
at low energy and we use lattice simulations to investigate the properties of
the lowest-lying hadrons. Compared to QCD, the theory has several peculiar
differences: there are no Goldstone bosons or chiral symmetry restoration when
the dark quark becomes massless; the usual global baryon number symmetry is
enlarged to , resembling isospin; and baryons and mesons are
unified together in iso-multiplets. We argue that the lightest
baryon, a vector boson, is a stable dark matter candidate and is a composite
realization of the hidden vector dark matter scenario. The model naturally
includes a lighter state, the analog of the in QCD, for dark
matter to annihilate into to set the relic density via thermal freeze-out. Dark
matter baryons may also be asymmetric, strongly self-interacting, or have their
relic density set via cannibalizing transitions. We discuss some
experimental implications of coupling dark baryons to the Higgs portal.Comment: 26 pages, 16 figure
More on heavy tetraquarks in lattice QCD at almost physical pion mass
We report on our progress in studying exotic, heavy tetraquark states,
. Using publicly available dynamical
Wilson-Clover gauge configurations, generated by the PACS-CS collaboration,
with pion masses 164, 299 and 415 MeV, we extend our previous analysis
to heavy quark components containing heavier than physical bottom quarks or , charm and bottom quarks and also only charm quarks
. Throughout we employ NRQCD and relativistic heavy quarks for
the heavier than bottom, bottom and charm quarks. Using our previously
established diquark-antidiquark and meson-meson operator basis we comment in
particular on the dependence of the binding energy on the mass of the heavy
quark component , with heavy quarks ranging from . In the heavy flavor non-degenerate case, ,
and especially for the tetraquark channel , we extend our work
to utilize a GEVP to study the ground and threshold states thereby
enabling a clear identification of possible binding. Finally, we present
initial work on the system where a much
larger operator basis is available in comparison to flavor combinations with
NRQCD quarks.Comment: 8 pages, 5 figures, proceedings contribution to "Lattice 2017. 35th
International Symposium on Lattice Field Theory", 18th-24th June 2017,
Granada, Spai
Neutral kaon mixing beyond the Standard Model with n(f)=2+1 chiral fermions. Part 1: bare matrix elements and physical results
We compute the hadronic matrix elements of the four-quark operators relevant
for mixing beyond the Standard Model. Our results are from
lattice QCD simulations with flavours of domain-wall fermion, which
exhibit continuum-like chiral-flavour symmetry. The simulations are performed
at two different values of the lattice spacing ( and a\sim 0.11 \,
\fm ) and with lightest unitary pion mass \sim 300\, \MeV. For the first
time, the full set of relevant four-quark operators is renormalised
non-perturbatively through RI-SMOM schemes; a detailed description of the
renormalisation procedure is presented in a companion paper. We argue that the
intermediate renormalisation scheme is responsible for the discrepancies found
by different collaborations. We also study different normalisations and
determine the matrix elements of the relevant four-quark operators with a
precision of or better.Comment: 38 page
The charm-quark contribution to light-by-light scattering in the muon from lattice QCD
We compute the hadronic light-by-light scattering contribution to the muon
from the charm quark using lattice QCD. The calculation is performed on
ensembles generated with dynamical quarks at the SU(3)
symmetric point with degenerate pion and kaon masses of around 415 MeV. It
includes the connected charm contribution, as well as the leading disconnected
Wick contraction, involving the correlation between a charm and a light-quark
loop. Cutoff effects turn out to be sizeable, which leads us to use
lighter-than-physical charm masses, to employ a broad range of lattice spacings
reaching down to 0.039 fm and to perform a combined charm-mass and continuum
extrapolation. We use the meson to define the physical charm-mass
point and obtain a final value of , whose uncertainty is dominated by the systematics of the
extrapolation. Our result is consistent with the estimate based on a simple
charm-quark loop, whilst being free of any perturbative scheme dependence on
the charm mass. The mixed charm-light disconnected contraction contributes a
small negative amount to the final value.Comment: 21 pages, 8 figures, 9 table
Hadronic light-by-light contribution to from lattice QCD with SU(3) flavor symmetry
We perform a lattice QCD calculation of the hadronic light-by-light
contribution to at the SU(3) flavor-symmetric point
MeV. The representation used is based on
coordinate-space perturbation theory, with all QED elements of the relevant
Feynman diagrams implemented in continuum, infinite Euclidean space. As a
consequence, the effect of using finite lattices to evaluate the QCD four-point
function of the electromagnetic current is exponentially suppressed. Thanks to
the SU(3)-flavor symmetry, only two topologies of diagrams contribute, the
fully connected and the leading disconnected. We show the equivalence in the
continuum limit of two methods of computing the connected contribution, and
introduce a sparse-grid technique for computing the disconnected contribution.
Thanks to our previous calculation of the pion transition form factor, we are
able to correct for the residual finite-size effects and extend the tail of the
integrand. We test our understanding of finite-size effects by using gauge
ensembles differing only by their volume. After a continuum extrapolation based
on four lattice spacings, we obtain , where the first error results from the uncertainties on
the individual gauge ensembles and the second is the systematic error of the
continuum extrapolation. Finally, we estimate how this value will change as the
light-quark masses are lowered to their physical values.Comment: 19 figures, 39 pages; improved references, in particular concerning
the eta exchange; no figures or results change
Neutral kaon mixing beyond the Standard Model with nf=2+1 chiral fermions part II:Non Perturbative Renormalisation of the ΔF=2 four-quark operators
We compute the renormalisation factors (Z-matrices) of the
four-quark operators needed for Beyond the Standard Model (BSM) kaon mixing. We
work with nf=2+1 flavours of Domain-Wall fermions whose chiral-flavour
properties are essential to maintain a continuum-like mixing pattern. We
introduce new RI-SMOM renormalisation schemes, which we argue are better
behaved compared to the commonly-used corresponding RI-MOM one. We find that,
once converted to MS, the Z-factors computed through these RI-SMOM schemes are
in good agreement but differ significantly from the ones computed through the
RI-MOM scheme. The RI-SMOM Z-factors presented here have been used to compute
the BSM neutral kaon mixing matrix elements in the companion paper [1]. We
argue that the renormalisation procedure is responsible for the discrepancies
observed by different collaborations, we will investigate and elucidate the
origin of these differences throughout this work