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G. C. Donald

Publications and source records attributed to G. C. Donald.

9 recordsLinked to original sources
arXiv2023

Nonperturbative comparison of clover and HISQ quarks in lattice QCD and the properties of the phi meson

We compare correlators for pseudoscalar and vector mesons made from valence strange quarks using the clover quark and highly improved staggered quark (HISQ) formalisms in full lattice QCD. We use fully nonperturbative methods to normalise vector and axial vector current operators made from HISQ quarks, clover quarks and from combining HISQ and clover fields. This allows us to test expectations for the renormalisation factors based on perturbative QCD, with implications for the error budget of lattice QCD calculations of the matrix elements of clover-staggered $b$-light weak currents, as well as further HISQ calculations of the hadronic vacuum polarisation. We also compare the approach to the (same) continuum limit in clover and HISQ formalisms for the mass and decay constant of the $ϕ$ meson. Our final results for these parameters, using single-meson correlators and neglecting quark-line disconnected diagrams are: $m_ϕ =$ 1.023(6) GeV and $f_ϕ = $ 0.238(3) GeV in good agreement with experiment. The results come from calculations in the HISQ formalism using gluon fields that include the effect of $u$, $d$, $s$ and $c$ quarks in the sea with three lattice spacing values and $m_{u/d}$ values going down to the physical point.

hep-lat↗
arXiv2014

High-precision quark masses and QCD coupling from $n_f=4$ lattice QCD

We present a new lattice QCD analysis of heavy-quark pseudoscalar-pseudoscalar correlators, using gluon configurations from the MILC collaboration that include vacuum polarization from $u$, $d$, $s$ and $c$~quarks($n_f=4$). We extract new values for the QCD coupling and for the $c$ quark's $\overline{\mathrm{MS}}$ mass: $α_{\overline{\mathrm{MS}}}(M_Z,n_f=5) = 0.11822(74)$ and $m_c(3\mathrm{GeV}, n_f=4) = 0.9851(63)$GeV. These agree well with our earlier simulations using $n_f=3$ sea quarks, vindicating the perturbative treatment of $c$ quarks in that analysis. We also obtain a new nonperturbative result for the ratio of $c$~and $s$~quark masses: $m_c/m_s=11.652(65)$. This ratio implies $m_s(2\,\mathrm{GeV}, n_f=3)=93.6(8)$MeV when it is combined with our new~$c$~mass. Combining $m_c/m_s$ with our earlier $m_b/m_c$ gives $m_b/m_s=52.55(55)$, which is several standard deviations (but only 4%) away from the Georgi-Jarlskop prediction from certain GUTs. Finally we obtain an $n_f=4$ estimate for $m_b/m_c=4.528(54)$ which agrees well with our earlier $n_f=3$ result. The new ratio implies~$m_b(m_b,n_f=5)=4.162(48)$GeV.

hep-lat↗
arXiv2014

Strange and charm quark contributions to the anomalous magnetic moment of the muon

We describe a new technique to determine the contribution to the anomalous magnetic moment of the muon coming from the hadronic vacuum polarization using lattice QCD. Our method reconstructs the Adler function, using Padé approximants, from its derivatives at $q^2=0$ obtained simply and accurately from time-moments of the vector current-current correlator at zero spatial momentum. We test the method using strange quark correlators on large-volume gluon field configurations that include the effect of up and down (at physical masses), strange and charm quarks in the sea at multiple values of the lattice spacing and multiple volumes and show that 1% accuracy is achievable. For the charm quark contributions we use our previously determined moments with up, down and strange quarks in the sea on very fine lattices. We find the (connected) contribution to the anomalous moment from the strange quark vacuum polarization to be $a_μ^s = 53.41(59) \times 10^{-10}$, and from charm to be $a_μ^c = 14.42(39)\times 10^{-10}$. These are in good agreement with flavour-separated results from non-lattice methods, given caveats about the comparison. The extension of our method to the light quark contribution and to that from the quark-line disconnected diagram is straightforward.

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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 $B \rightarrow πl ν$ 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 ($f_0(q^2_{max})=f_B/f_π$) accurately and we find good agreement as $M_π \rightarrow 0$. }

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arXiv2013

Prediction of the $D_s^*$ width from a calculation of its radiative decay in full lattice QCD

We determine the rate for $D_s^* \rightarrow D_s γ$ for the first time from lattice QCD and include the full effect of $u$, $d$ and $s$ sea quarks. The valence quarks are implemented using the Highly Improved Staggered Quark (HISQ) formalism and we normalise the vector current nonperturbatively. We obtain $M(D_s^*)-M(D_s)$ of 148(4) MeV, in good agreement with experiment. The value of the decay constant of the $D_s^*$ we find to be 274(6) MeV, so that $f_{D_s^*}/f_{D_s} = 1.10(2)$. For the radiative decay we find $Γ(D_s^* \rightarrow D_s γ)$ = 0.066(26) keV. Given the experimental branching fraction for this decay we predict a total width for the $D_s^*$ of 0.070(28) keV, making this the longest lived charged vector meson.

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arXiv2013

V_cs from D_s to ϕlν semileptonic decay and full lattice QCD

We determine the complete set of axial and vector form factors for the Ds to ϕlν decay from full lattice QCD for the first time. The valence quarks are implemented using the Highly Improved Staggered Quark action and we normalise the appropriate axial and vector currents fully nonperturbatively. The q^2 and angular distributions we obtain for the differential rate agree well with those from the BaBar experiment and, from the total branching fraction, we obtain Vcs = 1.017(63), in good agreement with that from D to Klν semileptonic decay. We also find the mass and decay constant of the ϕ meson in good agreement with experiment, showing that its decay to K{\bar{K}} (which we do not include here) has at most a small effect. We include an Appendix on nonperturbative renormalisation of the complete set of staggered vector and axial vector bilinears needed for this calculation.

hep-lat↗
arXiv2013

The shape of the D -> K semileptonic form factor from full lattice QCD and V_cs

We present a new study of the form factors for D -> K semileptonic decay from lattice QCD that allows us to compare the shape of the vector form factor to experiment and, for the first time, to extract V_cs using results from all experimental q^2 bins. The valence quarks are implemented with the Highly Improved Staggered Quark action on MILC configurations that include u, d and s sea quarks. The scalar and vector currents are nonperturbatively normalised and, using phased boundary conditions, we are able to cover the full q^2 range accessible to experiment. Our result is V_cs = 0.963(5)_{expt}(14)_{lattice}. We also demonstrate that the form factors are insensitive to whether the spectator quark is u/d or s, which has implications for other decay channels.

hep-lat↗
arXiv2012

Precision tests of the J/ψ from full lattice QCD: mass, leptonic width and radiative decay rate to η_c

We calculate the J/ψ mass, leptonic width and radiative decay rate to γη_c from lattice QCD including u, d and s quarks in the sea for the first time. We use the Highly Improved Staggered Quark formalism and nonperturbatively normalised vector currents for the leptonic and radiative decay rates. Our results are: M_{J/ψ} -M_{η_c} = 116.5(3.2) MeV; Γ(J/ψ to e^+e^-) = 5.48(16) keV; Γ(J/ψ to γη_c) = 2.49(19) keV. The first two are in good agreement with experiment, with Γ(J/ψ to e^+e^-) providing a test of a decay matrix element in QCD, independent of CKM uncertainties, to 2%. At the same time results for the time moments of the correlation function can be compared to values from the charm contribution to Γ(e^+e^- to hadrons), giving a 1.5% test of QCD. Our results show that an improved experimental error would enable a similarly strong test from Γ(J/ψ to γη_c).

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