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C. Michael

Publications and source records attributed to C. Michael.

At least 55 records · Page 3Linked to original sources
arXiv2005

Twisted mass fermions: neutral pion masses from disconnected contributions

Twisted mass fermions allow light quarks to be explored but with the consequence that there are mass splittings, such as between the neutral and charged pion. Using a direct calculation of the connected neutral pion correlator and stochastic methods to evaluate the disconnected correlations, we determine the neutral pion mass. We explore the dependence on lattice spacing and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the splitting which is linked, via chiral PT, to the nature of the phase transition at small quark mass.

hep-lat↗
arXiv2005

Hadronic decays

Hadronic decays and transitions are a key ingredient of hadronic physics. I discuss how hadronic decays can be explored in lattice gauge theory and review studies undertaken. I also discuss the impact of decays on masses and how lattice studies can explore the nature of a hadronic state: namely whether it is a molecular or quark-antiquark state. A brief discussion of lattice exploration of pentaquark states is presented.

hep-lat↗
arXiv2005

Flavour Breaking Effects of Wilson twisted mass fermions

We study the flavour breaking effects appearing in the Wilson twisted mass formulation of lattice QCD. In this quenched study, we focus on the mass splitting between the neutral and the charged pion, determining the neutral pion mass with a stochastic noise method to evaluate the disconnected contributions. We find that these disconnected contributions are significant. Using the Osterwalder-Seiler interpretation of the connected piece of the neutral pion correlator, we compute the corresponding neutral pion mass to study with more precision the scaling behaviour of the mass splitting.

hep-lat↗
arXiv2004

P-wave Radial distributions of a Heavy-light meson on a lattice

This is a follow-up to our earlier work for the charge (vector) and matter (scalar) distributions for S-wave states in a heavy-light meson, where the heavy quark is static and the light quark has a mass about that of the strange quark. The calculation is again carried out with dynamical fermions on a 16^3x24 lattice with a lattice spacing of about 0.14 fm. It is shown that several features of the S- and P-wave distributions are in qualitative agreement with what one expects from a simple one-body Dirac equation interpretation.

hep-lat↗
arXiv2004

Searching for chiral logs in the static-light decay constant

Using the clover fermion action in unquenched QCD with pion masses as low as 420 MeV, we look for evidence for chiral logs in the static-light decay constant. There is some evidence for a chiral log term, if the original static theory of Eichten and Hill is used. However, the more precise data from the static action of the ALPHA collaboration do not show any evidence for non-linear dependence of the static-light decay constant on the light quark mass. We make some comments on the connection between chiral perturbation theory for decay constants of the pion and static-light meson.

hep-lat↗
arXiv2004

An unquenched lattice QCD calculation of the mass of the bottom quark

We compute the b quark mass from dynamical lattice QCD with clover quarks. The calculation is done at a fixed lattice spacing with sea quark masses as low as half the strange quark mass. Our final result is m_b(m_b} = 4.25(2)(11) GeV, where the first error is statistical and the last error is the systematic uncertainty.

hep-lat↗
arXiv2004

Hadronic decay of a scalar B meson from the lattice

We explore the transitions B$(0^+)$ to B $π$ and B$_s(0^+)$ to B K from lattice QCD with $N_f=2$ flavours of sea quark, using the static approximation for the heavy quark. We evaluate the effective coupling constants, predicting a B$(0^+)$ to B $π$ width of around 160 MeV. Our result for the coupling strength adds to the evidence that the B$_s(0^+)$ meson is not predominantly a molecular state (BK).

hep-lat↗
arXiv2003

Excited B mesons from the lattice

We determine the energies of the excited states of a heavy-light meson $Q\bar{q}$, with a static heavy quark and light quark with mass approximately that of the strange quark from both quenched lattices and with dynamical fermions. We are able to explore the energies of orbital excitations up to L=3, the spin-orbit splitting up to L=2 and the first radial excitation. These $b \bar{s}$ mesons will be very narrow if their mass is less than 5775 MeV -- the $BK$ threshold. We investigate this in detail and present evidence that the scalar meson (L=1) will be very narrow and that as many as 6 $b \bar{s}$ excited states will have energies close to the $BK$ threshold and should also be relatively narrow.

hep-lat↗
arXiv2003

Energy spectrum of a Heavy-Light meson

The energies of different angular momentum states of a heavy-light meson were measured on a lattice by Michael and Peisa in Phys. Rev. D 58, 34506 (1998). We have now repeated this study using a larger, quenched lattice to check that there the finite size effects are under control. Also the effect of quenching is studied by using dynamical fermion configurations. In the latter case the lattice spacing a is smaller, which allows one to make an estimate of a -> 0 corrections. The new measurements confirm the earlier calculated energy spectrum. From some theoretical considerations it is expected that, for high angular momentum states, the multiplets should be inverted compared with the Coulomb spectrum, i.e. L- should lie higher than L+ (see H. J. Schnitzer Phys. Lett. B 226, 171 (1989)). This happens when a pure scalar confining potential is present. Here the notation L+(-) means that the light quark spin couples to angular momentum L giving the total j=L+/- 1/2. Experimentally this inversion is not seen for P-wave mesons, and lattice measurements now suggest that there is no inversion in the D-wave either. Indeed we find that the spin-orbit splitting is very small for both values L=1 and 2. This almost exact cancellation need not be a coincidence, and could probably be understood by using straightforward symmetry arguments (P.R. Page, T. Goldman and J.N. Ginocchio, Phys. Rev. Lett. 86, 204 (2001)).

hep-lat↗
arXiv2003

Hadronic decays from the lattice

We discuss strategies to determine hadronic decay couplings from lattice studies. As an application, we explore the decay of a vector meson to two pseudoscalar mesons with $N_f=2$ flavours of sea quark. Although we are working with quark masses that do not allow a physical decay, we show how the transition rate can be evaluated from the amplitude for $ρ\to ππ$ and from the annihilation component of $ππ\to ππ$. We explore the decay amplitude for two different pion momenta and find consistent results. The coupling strength we find is in agreement with experiment. We also find evidence for a shift in the $ρ$ mass caused by mixing with two pion states.

hep-lat↗
arXiv2003

Hadronic decay of a vector meson from the lattice

We explore the decay of a vector meson to two pseudoscalar mesons on the lattice with $N_f=2$ flavours of sea quark. Although we are working with quark masses that do not allow a physical decay, we show how the transition rate can be evaluated from the amplitude for $ρ\to ππ$ and from the annihilation component of $ππ\to ππ$. We explore the decay amplitude for two different pion momenta and find consistent results. The coupling strength we find is in agreement with experiment. We also find evidence for a shift in the $ρ$ mass caused by mixing with two pion states.

hep-lat↗
arXiv2003

Exotics

We review lattice QCD results for glueballs (including a discussion of mixing with scalar mesons), hybrid mesons and other exotic states (such as $B_s B_s$ molecules).

hep-lat↗
arXiv2002

The measurement, fitting and interpretation of the radial distributions of Heavy-Light mesons calculated on a lattice with dynamical fermions

In our earlier work, the charge and matter radial distributions of heavy-light mesons were measured on a 16^3 times 24 lattice with a lattice spacing of a = 0.17 fm and a light quark mass about that of the strange quark. Several major improvements have now been made: 1) Dynamical fermions are used with a approximately 0.14 fm; 2) More gauge configurations are included (78 vs 20); 3) Off-axis, in addition to on-axis, insertions are made; 4) The data analysis is much more complete. In particular, distributions involving excited states are extracted.

hep-lat↗
arXiv2002

The eta and eta' mesons in QCD

We study the flavour singlet pseudoscalar mesons from first principles using lattice QCD. We explore the quark content of the $η$ and $η'$ mesons and we discuss their decay constants.

hep-lat↗
arXiv2002

The radial distributions of a heavy-light meson on a lattice

In an earlier work, the charge (vector) and matter (scalar) radial distributions of heavy-light mesons were measured in the quenched approximation on a 16^3 times 24 lattice with a quark-gluon coupling of 5.7, a lattice spacing of 0.17 fm, and a hopping parameter corresponding to a light quark mass about that of the strange quark. Several improvements are now made: 1) The configurations are generated using dynamical fermions with a quark-gluon coupling of 5.2 (a lattice spacing of 0.14 fm); 2) Many more gauge configurations are included (78 compared with the earlier 20); 3) The distributions at many off-axis, in addition to on-axis, points are measured; 4) The data-analysis is much more complete. In particular, distributions involving excited states are extracted. The exponential decay of the charge and matter distributions can be described by mesons of mass 0.9+-0.1 and 1.5+-0.1 GeV respectively - values that are consistent with those of vector and scalar qqbar-states calculated directly with the same lattice parameters.

hep-lat↗
arXiv2002

Beyond the quark model of hadrons from lattice QCD

Lattice QCD can give direct information on OZI-violating contributions to mesons. Here we explore the contributions that split flavour singlet and non-singlet meson masses. I discuss in detail the spectrum and decays for scalar mesons (ie including glueball effects). I also review the status of hybrid mesons and their decays.

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