arXiv subjects

B. Colquhoun

Publications and source records attributed to B. Colquhoun.

12 recordsLinked to original sources

arXiv2026

A "MeerKAT-meets-LOFAR" Study of 2A 0335+096: Discovery of a Complex 500 kpc Radio Halo in a Disturbed Cool-Core Cluster

The emerging picture of radio haloes and mini-haloes in galaxy clusters is more blurred and far less distinct than once thought. While both trace cosmic ray electrons and magnetic fields in the intracluster medium (ICM), our knowledge of the mechanisms powering these sources, their properties and their connection to cluster dynamics, is incomplete. We present new deep MeerKAT L-band (856$-$1712\,MHz) observations of the sloshing cool-core cluster 2A~0335+096, performed as part of the ``MeerKAT-meets-LOFAR'' mini-halo census, as well as reprocessed LOFAR 120$-$168\,MHz LOFAR Two-metre Sky Survey data. We present highly sensitive radio continuum images, from which we perform a spectral analysis and a comparison of the thermal and non-thermal properties using reprocessed archival \textit{XMM-Newton} data. Our MeerKAT data reveal that the known mini-halo is embedded in a large-scale radio halo spanning up to 510\,kpc in this sloshing cluster, although only the inner mini-halo is detected by LOFAR. We find a typical steep spectrum of $\alpha = -1.20 \pm 0.09$ for the mini-halo; the non-detection of the larger-scale halo by LOFAR indicates a spectral index no steeper than $-1.2$. Our results are broadly consistent with an interpretation of turbulent (re-)acceleration powered by sloshing within the cluster potential well. This work emphasises the need for using deep, high-dynamic multi-frequency radio observations in conjunction with high-quality X-ray observations to complete our understanding of the connection between radio halo properties and cluster dynamics.

astro-ph.CO↗

arXiv2024

Quark mass dependence of doubly heavy tetraquark binding

The existence of bound doubly heavy tetraquark states was confirmed by the recent LHCb discovery of the doubly charmed $T_{cc}$, less than 1 MeV below the meson pair threshold. Others states with two heavy (bottom or charm) quarks could also be bound, perhaps more deeply. Here we discuss our previous work, and the improvements in our current, updated analysis of various heavy-heavy-light-light tetraquark candidates, including the light and heavy quark mass dependence of the binding.

hep-lat↗

arXiv2024

Improved analysis of strong-interaction-stable doubly-bottom tetraquarks on the lattice

We update earlier lattice results for the binding energies of the flavor antitriplet of strong-interaction-stable doubly bottom, $J^P=1^+$ tetraquarks, employing an extended sink construction which produces significantly improved ground-state effective-mass plateaus, as well as new, larger-volume ensembles which reduce possible finite-volume effects at lighter pion masses. The updated bindings are $115(17)$ MeV for the $I=0$ member of the antitriplet and $47(8)$ MeV for its $I=1/2$ partner. We also provide an update of our earlier study of the variable heavy mass dependence of binding in the $1^+$ channel and new results on this dependence for binding in the $0^+$ channel, accessible when the two heavy quarks have unequal masses. Implications of these results of potential relevance to experimental searches for signals of the production of doubly bottom tetraquarks and/or a possible bottom-charm partner of the $T_{cc}$ are also discussed.

hep-lat↗

arXiv2023

$B \to D^*\ell\nu_\ell$ semileptonic form factors from lattice QCD with M\"obius domain-wall quarks

We calculate the form factors for the $B \to D^*\ell\nu_\ell$ decay in 2+1 flavor lattice QCD. For all quark flavors, we employ the M\"obius domain-wall action, which preserves chiral symmetry to a good precision. Our gauge ensembles are generated at three lattice cutoffs $a^{-1} \sim 2.5$, 3.6 and 4.5 GeV with pion masses as low as $M_\pi \sim 230$ MeV. The physical lattice size $L$ satisfies the condition $M_\pi L \geq 4$ to control finite volume effects (FVEs), while we simulate a smaller size at the smallest $M_\pi$ to directly examine FVEs. The bottom quark masses are chosen in a range from the physical charm quark mass to $0.7 a^{-1}$ to control discretization effects. We extrapolate the form factors to the continuum limit and physical quark masses based on heavy meson chiral perturbation theory at next-to-leading order. Then the recoil parameter dependence is parametrized using a model independent form leading to our estimate of the decay rate ratio between the tau ($\ell = \tau$) and light lepton ($\ell = e,\mu$) channels $R(D^*) = 0.252(22)$ in the Standard Model. A simultaneous fit with recent data from the Belle experiment yields $|V_{cb}| = 39.19(91)\times 10^{-3}$, which is consistent with previous exclusive determinations, and shows good consistency in the kinematical distribution of the differential decay rate between the lattice and experimental data.

hep-lat↗

arXiv2021

$B\!\to\!D^{(*)}\ell\nu$ semileptonic decays in lattice QCD with domain-wall heavy quarks

We calculate the $B \to D^{(*)} \ell \nu$ form factors in 2+1 flavor relativistic lattice QCD by employing the M\"obius domain-wall action for all quark flavors. Our simulations are carried out at lattice cut-offs $a^{-1} \sim 2.5$, 3.6 and 4.5 GeV with the bottom quark masses up to 0.7 $a^{-1}$ to control discretization effects. We extrapolate the form factors to the continuum limit and physical quark masses, and discuss systematic uncertainties of the form factors.

hep-lat↗

arXiv2020

A lattice investigation of exotic tetraquark channels

We perform an $n_f=2+1$ lattice study of a number of channels where past claims exist in the literature for the existence of strong-interaction-stable light-heavy tetraquarks. We find no evidence for any such deeply-bound states, beyond the $J^P=1^+$, $I=0$ $ud\bar{b}\bar{b}$ and $I=1/2$ $ls\bar{b}\bar{b}$ states already identified in earlier lattice studies. We also describe a number of systematic improvements to our previous lattice studies, including working with larger $m_\pi L$ to better suppress possible finite volume effects, employing extended sinks to better control excited-state contamination, and expanding the number of operators used in the GEVP analyses. Our results also allow us to rule out several phenomenological models which predict significant tetraquark binding in channels where no such binding is found.

hep-lat↗

arXiv2019

$B\!\to\!D^{(*)}\ell\nu$ form factors from lattice QCD with relativistic heavy quarks

We report on our calculation of the B \to D^(*) \ell \nu form factors in 2+1 flavor lattice QCD. The M\"obius domain-wall action is employed for light, strange, charm and bottom quarks. At lattice cutoffs 1/a \sim 2.4, 3.6 and 4.5 GeV, we simulate bottom quark masses up to 0.7/a to control discretization errors. The pion mass is as low as 230 MeV. We extrapolate the form factors to the continuum limit and physical quark masses, and make a comparison with recent phenomenological analyses.

hep-lat↗

arXiv2015

$B \rightarrow \pi \ell \nu$ at zero recoil from lattice QCD with physical $u/d$ quarks

The exclusive semileptonic decay $B \rightarrow \pi \ell \nu$ is a key process for the determination of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$ from the comparison of experimental rates as a function of $q^2$ with theoretically determined form factors. The sensitivity of the form factors to the $u/d$ quark mass has meant significant systematic uncertainties in lattice QCD calculations at unphysically heavy pion masses. Here we give the first lattice QCD calculations of this process for u/d quark masses going down to their physical values, calculating the $f_0$ form factor at zero recoil to 3\%. We are able to resolve a long-standing controversy by showing that the soft-pion theorem result $f_0(q^2_{max}) = f_B/f_{\pi}$ does hold as $m_{\pi} \rightarrow 0$. We use the Highly Improved Staggered Quark formalism for the light quarks and show that staggered chiral perturbation theory for the $m_{\pi}$ dependence is almost identical to continuum chiral perturbation theory for $f_0$, $f_B$ and $f_{\pi}$. We also give results for other processes such as $B_s \rightarrow K \ell \nu$.

hep-lat↗

arXiv2015

B-meson decay constants: a more complete picture from full lattice QCD

We extend the picture of $B$-meson decay constants obtained in lattice QCD beyond those of the $B$, $B_s$ and $B_c$ to give the first full lattice QCD results for the $B^*$, $B^*_s$ and $B^*_c$. We use improved NonRelativistic QCD for the valence $b$ quark and the Highly Improved Staggered Quark (HISQ) action for the lighter quarks on gluon field configurations that include the effect of $u/d$, $s$ and $c$ quarks in the sea with $u/d$ quark masses going down to physical values. For the ratio of vector to pseudoscalar decay constants, we find $f_{B^*}/f_B$ = 0.941(26), $f_{B^*_s}/f_{B_s}$ = 0.953(23) (both $2\sigma$ less than 1.0) and $f_{B^*_c}/f_{B_c}$ = 0.988(27). Taking correlated uncertainties into account we see clear indications that the ratio increases as the mass of the lighter quark increases. We compare our results to those using the HISQ formalism for all quarks and find good agreement both on decay constant values when the heaviest quark is a $b$ and on the dependence on the mass of the heaviest quark in the region of the $b$. Finally, we give an overview plot of decay constants for gold-plated mesons, the most complete picture of these hadronic parameters to date.

hep-lat↗

arXiv2014

The $\Upsilon$ and $\Upsilon^{\prime}$ Leptonic Widths, $a_{\mu}^b$ and $m_b$ from full lattice QCD

We determine the decay rate to leptons of the ground-state $\Upsilon$ meson and its first radial excitation in lattice QCD for the first time. We use radiatively-improved NRQCD for the $b$ quarks and include $u$, $d$, $s$ and $c$ quarks in the sea with $u/d$ masses down to their physical values. We find $\Gamma(\Upsilon \rightarrow e^+e^-)$ = 1.19(11) keV and $\Gamma(\Upsilon^{\prime} \rightarrow e^+e^-)$ = 0.69(9) keV, both in good agreement with experiment. The decay constants we obtain are included in a summary plot of meson decay constants from lattice QCD given in the Conclusions. We also test time-moments of the vector current-current correlator against values determined from the $b$ quark contribution to $\sigma(e^+e^- \rightarrow \mathrm{hadrons})$ and calculate the $b$-quark piece of the hadronic vacuum polarisation contribution to the anomalous magnetic moment of the muon, $a_{\mu}^b = 0.271(37) \times 10^{-10}$. Finally we determine the $b$-quark mass, obtaining in the $\overline{MS}$ scheme, $\overline{m}_b(\overline{m}_b, n_f=5)$ = 4.196(23) GeV, the most accurate result from lattice QCD to date.

hep-lat↗

arXiv2013

Bottomonium and B results from full lattice QCD

We have developed two methods for handling $b$ quarks in lattice QCD. One uses NRQCD (now improved to include radiative corrections) and the other uses Highly Improved Staggered Quarks (HISQ), extrapolating to the $b$ quark from lighter masses and using multiple lattice spacings to control discretisation errors. Comparison of results for the two different methods gives confidence in estimates of lattice QCD systematic errors, since they are very different in these two cases. Here we show results for heavyonium hyperfine splittings and vector current-current correlator moments using HISQ quarks, to add to earlier results testing the heavy HISQ method with pseudoscalar mesons. We also show the form factor for $B \rightarrow \pi l \nu$ decay at zero recoil using NRQCD $b$ quarks and $u/d$ quarks with physical masses. This allows us to test the soft pion theorem relation ($f_0(q^2_{max})=f_B/f_{\pi}$) accurately and we find good agreement as $M_{\pi} \rightarrow 0$. }

hep-lat↗

arXiv2011

The Upsilon spectrum and the determination of the lattice spacing from lattice QCD including charm quarks in the sea

We give results for the Upsilon spectrum from lattice QCD using an improved version of the NRQCD action for b quarks which includes radiative corrections to kinetic terms at O(v^4) in the velocity expansion. We also include for the first time the effect of up, down, strange and charm quarks in the sea using 'second generation' gluon field configurations from the MILC collaboration. Using the {\Upsilon} 2S - 1S splitting to determine the lattice spacing, we are able to obtain the 1P - 1S splitting to 1.4% and the 3S - 1S splitting to 2.4%. Our improved result for M (Upsilon) - M (eta_b) is 70(9) MeV and we predict M(Upsilon) - M(eta_b') = 35(3) MeV. We also calculate pi, K and eta_s correlators using the Highly Improved Staggered Quark action and perform a chiral and continuum extrapolation to give values for M(eta_s) (0.6893(12) GeV) and f_{eta_s} (0.1819(5) GeV) that allow us to tune the strange quark mass as well as providing an independent and consistent determination of the lattice spacing. Combining the NRQCD and HISQ analyses gives mb/ms = 54.7(2.5) and a value for the heavy quark potential parameter of r_1 = 0.3209(26) fm.

hep-lat↗