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Christine T. H. Davies

Publications and source records attributed to Christine T. H. Davies.

At least 19 recordsLinked to original sources
arXiv2026

New high-precision $b$, $c$, and $s$ masses from pseudoscalar-pseudoscalar correlators in $n_f=4$ lattice QCD

We extend an earlier lattice QCD analysis of heavy-quark current-current correlators to obtain new values for the $\overline{\mathrm{MS}}$ masses of the $b$, $c$, and $s$~quarks. The analysis uses gluon configurations from the MILC collaboration with vacuum polarization contributions from $u$, $d$, $s$, and~$c$ quarks ($n_f=4$), and lattice spacings down to~0.032~fm. We find that $\overline{m}_b(\overline{m}_b, n_f=5)=4.1923(63)$~GeV, $\overline{m}_c(3~\mathrm{GeV}, n_f=4)=0.9813(34)$~GeV, and $\overline{m}_s(3~\mathrm{GeV}, n_f=4)=83.39(26)$~MeV. These results are corrected for QED by including (quenched) QED in the simulations. They are among the most accurate values by any method to date. We give a detailed analysis of finite lattice-spacing errors that shows why the HISQ discretization of the quark action is particularly useful for $b$-quark simulations even for lattices where~$am_b\approx1$. We also calculate QED and isospin corrections to the (fictitious) $η_s$-meson mass, which is used to tune $s$-quark masses in lattice simulations.

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arXiv2025

Precise prediction of the decay rate for $η_b\to γγ$ from lattice QCD

We calculate the decay rate for $η_b \to γγ$ in lattice QCD for the first time, providing a precise prediction for the Belle II experiment. Our calculation includes $u$, $d$, $s$ and $c$ quarks in the sea, using gluon field configurations generated by the MILC collaboration, at three values of the lattice spacing from $0.06\;\mathrm{fm}$ to $0.03\;\mathrm{fm}$. All quarks are treated in the Highly Improved Staggered Quark formalism, which enables us to reach the $b$ quark mass for our valence quarks on these fine lattices. We calculate quark-line connected correlation functions only. By working at additional heavy quark masses between those of $c$ and $b$ we map out the behaviour of the ratio $f_{η_h}/(M_{η_h}^2F_{η_h}(0,0))$, where $f$ is the decay constant, $M$, the mass and $F(0,0)$, the form factor for decay to two on-shell photons for the pseudoscalar heavyonium meson, $η_h$. This ratio takes the approximate value 0.5 in leading-order non-relativistic QCD (NRQCD) but we are able to give a much more accurate analysis than this. Focussing on the $b$ quark mass, we find a ratio of $0.467(11)$, giving $Γ(η_b \to γγ) = 0.559(32)_{\text{fit}}(1)_{\text{syst}} \: \mathrm{keV}$. Combined with a value for the branching fraction from NRQCD, our result can be used to determine the total width of the $η_b$ with a $6\%$ uncertainty.

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arXiv2025

Constraints on axion-like particles using lattice QCD calculations of the rate for $J/ψ\to γa$

A key search mode for axion-like particles (ALPs) that couple to charm quarks is $J/ψ\to γa$. Here we calculate the form factor that allows the rate of this process to be determined using lattice QCD for the first time. Our calculations use the relativistic Highly Improved Staggered Quark (HISQ) action for the valence charm quarks on gluon field configurations generated by the MILC collaboration that include $u$, $d$, $s$ and $c$ HISQ quarks in the sea at four values of the lattice spacing and both unphysical and physical sea quark masses. We determine the form factor as a function of ALP mass with an uncertainty of less than 2\% across our full range of ALP masses from zero up to 95\% of the $J/ψ$ mass. This represents a substantial improvement in accuracy of the theoretical picture of this decay compared to the previously used tree-level and $\mathcal{O}(α_s)$ perturbation theory. We use our form factor to determine constraints on ALP masses and couplings to charm quarks and photons in several different scenarios using recent experimental data from BESIII. Our calculation paves the way for further lattice QCD input on new physics constraints from radiative decays.

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arXiv2025

Heavy-light meson decay constants and hyperfine splittings with the heavy-HISQ method

We compute ratios between the vector and pseudoscalar, and tensor and vector decay constants, and between hyperfine splittings for $D_{(s)}^{(*)}$ and $B_{(s)}^{(*)}$ mesons. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks, paired with the second generation MILC $n_f = 2+1+1$ HISQ gluon field configurations. These include light sea quarks with $m_u = m_d \equiv m_l$ going down to the physical values, as well as physically tuned strange and charm sea quarks. We also use a HISQ valence heavy quark, with mass ranging from that of the $c$-quark up to very nearly that of the physical $b$-quark on the finest lattices, allowing us to map out the heavy-quark mass dependence of the decay constant and hyperfine splitting ratios.

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arXiv2024

$B \rightarrow D^*$ vector, axial-vector and tensor form factors for the full $q^2$ range from lattice QCD

We compute the complete set of SM and tensor $B_{(s)}\to D_{(s)}^*\ell\barν$ semileptonic form factors across the full kinematic range of the decay using second generation MILC $n_f=2+1+1$ HISQ gluon field configurations and HISQ valence quarks, with the heavy-HISQ method. Lattice spacings range from $0.09\mathrm{fm}$ to $0.044\mathrm{fm}$ with pion masses from $\approx 300\mathrm{MeV}$ down to the physical value and heavy quark masses ranging between $\approx 1.5 m_c$ and $4.1 m_c \approx 0.9 m_b$; currents are normalised nonperturbatively. Using the recent $B_{(s)}\to D^*_{(s)}\ell\barν_\ell$ data from Belle and LHCb together with our form factors we determine a model independent value of $V_{cb}=39.03(56)_\mathrm{exp}(67)_\mathrm{latt}\times 10^{-3}$, in agreement with previous exclusive determinations and in tension with the inclusive result at the level of $3.6σ$. We observe a $\approx 1σ$ tension between the shape of the differential decay rates computed using our form factors and those measured by Belle. We compute a lattice-only SM value for the ratio of semitauonic and semimuonic decay rates, $R(D^*)=0.273(15)$, which we find to be closer to the recent Belle measurement and HFLAV average than theory predictions using fits to experimental differential rate data for $B\to D^*\ell\barν_\ell$. Determining $V_{cb}$ using the total rate for $B\to D^*\ellν$ gives a value in agreement with inclusive results. We compute the longitudinal polarisation fraction for the semitauonic mode, $F_L^{D^*}=0.395(24)$, which is in tension at the level of $2.2σ$ with the recent Belle measurement. Our calculation combines $B\to D^*$ and $B_s\to D_s^*$ lattice results, and we provide an update which supersedes our previous lattice computation of the $B_s\to D_s^*$ form factors. We also give the chiral perturbation theory needed to analyse the tensor form factors.

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arXiv2023

Precise determination of decay rates for $η_c \to γγ$, $J/ψ\to γη_c$ and $J/ψ\to η_c e^+e^-$ from lattice QCD

We calculate the decay rates for $η_c \to γγ$, $J/ψ\to γη_c$ and $J/ψ\to η_c e^+e^-$ in lattice QCD with $u$, $d$, $s$ and $c$ quarks in the sea for the first time. We improve significantly on previous theory calculations to achieve accuracies of 1--2\%, giving lattice QCD results that are now more accurate than the experimental values. In particular our results transform the theoretical picture for $η_c\toγγ$ decays. We use gluon field configurations generated by the MILC collaboration that include $n_f=2+1+1$ flavours of Highly Improved Staggered (HISQ) sea quarks at four lattice spacing values from 0.15 fm to 0.06 fm and with sea u/d masses down to their physical value. We also implement the valence $c$ quarks using the HISQ action. We find ${Γ(η_c \to γγ) = 6.788(45)_{\text{fit}}(41)_{\text{syst}} \: \mathrm{keV}}$, in good agreement with experimental results using $γγ\to η_c \to K\overline{K}π$ but in 4$σ$ tension with the Particle Data Group global fit result; we suggest this fit is revisited. We also calculate $Γ(J/ψ\to γη_c) = 2.219(17)_{\text{fit}}(18)_{\text{syst}}(24)_{\text{expt}}(4)_{\text{QED}} \; \mathrm{keV}$, in good agreement with results from CLEO, and predict the Dalitz decay rate $Γ(J/ψ\to η_c e^+ e^-) = 0.01349(21)_{\text{latt}}(13)_{\text{QED}} \; \mathrm{keV}$. We use our results to calibrate other theoretical approaches and to test simple relationships between the form factors and $J/ψ$ decay constant expected in the nonrelativistic limit.

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arXiv2022

A lattice QCD perspective on weak decays of b and c quarks Snowmass 2022 White Paper

Lattice quantum chromodynamics has proven to be an indispensable method to determine nonperturbative strong contributions to weak decay processes. In this white paper for the Snowmass community planning process we highlight achievements and future avenues of research for lattice calculations of weak $b$ and $c$ quark decays, and point out how these calculations will help to address the anomalies currently in the spotlight of the particle physics community. With future increases in computational resources and algorithmic improvements, percent level (and below) lattice determinations will play a central role in constraining the standard model or identifying new physics.

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arXiv2022

Form factors for the processes $B_c^+ \to D^0 \ell^+ ν_{\ell}$ and $B_c^+ \to D_s^+ \ell^+ \ell^- (ν\overlineν)$ from lattice QCD

We present results of the first lattice QCD calculations of the weak matrix elements for the decays $B_c^+ \to D^0 \ell^+ ν_{\ell}$, $B_c^+ \to D_s^+ \ell^+ \ell^-$ and $B_c^+ \to D_s^+ ν\overlineν$. Form factors across the entire physical $q^2$ range are then extracted and extrapolated to the continuum limit with physical quark masses. Results are derived from correlation functions computed on MILC Collaboration gauge configurations with three different lattice spacings and including 2+1+1 flavours of sea quarks in the Highly Improved Staggered Quark (HISQ) formalism. HISQ is also used for all of the valence quarks. The uncertainty on the decay widths from our form factors is similar in size to that from the present value for $V_{ub}$. We obtain the ratio $Γ(B_{c}^{+} \rightarrow D^0 μ^{+} ν_μ) /\left|η_{\mathrm{EW}} V_{u b}\right|^{2}=4.43(63) \times 10^{12} \mathrm{~s}^{-1}$. Combining our form factors with those found previously by HPQCD for $B_{c}^{+} \rightarrow J / ψμ^{+} ν_μ$, we find $\left|V_{cb}/V_{ub} \right|^2 Γ( B_c^+ \to D^0 μ^+ ν_μ)/Γ(B_{c}^{+} \rightarrow J / ψμ^{+} ν_μ) = 0.257(36)_{B_c \to D}(18)_{B_c \to J/ψ}$. We calculate the differential decay widths of $B_c^+ \to D_s^+ \ell^+ \ell^-$ across the full $q^2$ range, and give integrated results in $q^2$ bins that avoid possible effects from charmonium and $u \overline{u}$ resonances. For example, we find that the ratio of differential branching fractions integrated over the range $q^2 = 1 \; \mathrm{GeV}^2 - 6 \; \mathrm{GeV}^2$ for $B_c^+ \to D_s^+ μ^+ μ^-$ and $B_{c}^{+} \rightarrow J / ψμ^{+} ν_μ$ is $5.23{\tiny }(73)_{B_c \to D_s}(54)_{B_c \to J/ψ} \times 10^{-6}$. We also give results for the branching fraction of $B_c^+ \to D_s^+ ν\overlineν$. Prospects for reducing our errors in the future are discussed.

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arXiv2022

$B_s \rightarrow D_s^*$ Form Factors for the full $q^2$ range from Lattice QCD

We compute the Standard Model semileptonic vector and axial-vector form factors for $B_s\to D_s^*$ decay across the full $q^2$ range using lattice QCD. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks, enabling us to normalise weak currents nonperturbatively. We use gluon field configurations including $u$, $d$, $s$ and $c$ HISQ sea quarks and multiple HISQ heavy quarks with masses from the $c$ mass up to that of the $b$ on our finest lattices. We determine the physical form factors, with which we construct the differential and total rates for $Γ(B_s^0\to D_s^{*-}\ell^+ν_\ell)$. We find $Γ_{\ell=e}/|η_\mathrm{EW}V_{cb}|^2=2.07(17)_\mathrm{latt}(2)_\mathrm{EM}\times 10^{13} ~\mathrm{s}^{-1}$, $Γ_{\ell=μ}/|η_\mathrm{EW}V_{cb}|^2=2.06(16)_\mathrm{latt}(2)_\mathrm{EM}\times 10^{13} ~\mathrm{s}^{-1}$ and $Γ_{\ell=τ}/|η_\mathrm{EW}V_{cb}|^2=5.14(37)_\mathrm{latt}(5)_\mathrm{EM}\times 10^{12} ~\mathrm{s}^{-1}$, where $η_\mathrm{EW}$ contains the electroweak correction to $G_F$, the first uncertainty is from our lattice calculation, and the second allows for long-distance QED effects. We compute the ratio $R(D_s^{*-})\equiv Γ_{\ell=τ}/Γ_{\ell=μ}=0.2490(60)_\mathrm{latt}(35)_\mathrm{EM}$ and obtain a value for the ratio of decay rates $Γ_{\ell=μ}(B_s\to D_s)/Γ_{\ell=μ}(B_s\to D_s^*)=0.443(40)_\mathrm{latt}(4)_\mathrm{EM}$, which agrees well with recent LHCb results. We determine $|V_{cb}|=42.2 (1.5)_\mathrm{latt}(1.7)_\mathrm{exp}(0.4)_\mathrm{EM} \times 10^{-3}$ by combining our lattice results across the full q^2 range with experimental results from LHCb. A comparison of our results to the normalised differential decay rate from LHCb shows good agreement. We also test the impact of new physics couplings on observables sensitive to lepton flavor universality violation.

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arXiv2021

$B_c \to B_{s(d)}$ form factors from lattice QCD

We present results of the first lattice QCD calculations of $B_c \to B_s$ and $B_c \to B_d$ weak matrix elements. Form factors across the entire physical $q^2$ range are then extracted and extrapolated to the physical-continuum limit before combining with CKM matrix elements to predict the semileptonic decay rates $Γ(B_c^+ \to B_s^0 \overline{\ell} ν_{\ell}) = 26.2(1.2) \times 10^9 \,\text{s}^{-1}$ and $Γ(B_c^+ \to B^0 \overline{\ell} ν_{\ell}) = 1.65(10) \times 10^9 \,\text{s}^{-1}$. The lattice QCD uncertainty is comparable to the CKM uncertainty here. Results are derived from correlation functions computed on MILC Collaboration gauge configurations with a range of lattice spacings including 2+1+1 flavours of dynamical sea quarks in the Highly Improved Staggered Quark (HISQ) formalism. HISQ is also used for the propagators of the valence light, strange, and charm quarks. Two different formalisms are employed for the bottom quark: non-relativistic QCD (NRQCD) and heavy-HISQ. Checking agreement between these two approaches is an important test of our strategies for heavy quarks on the lattice. From chained fits of NRQCD and heavy-HISQ data, we obtain the differential decay rates $dΓ/ d q^2$ as well as integrated values for comparison to future experimental results.

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arXiv2020

$R(J/ψ)$ and $B_c^- \rightarrow J/ψ\ell^-\barν_\ell$ Lepton Flavor Universality Violating Observables from Lattice QCD

We use our lattice QCD computation of the $B_c\rightarrow J/ψ$ form factors to determine the differential decay rate for the semitauonic decay channel and construct the ratio of branching fractions $R(J/ψ) = \mathcal{B}(B_c^- \rightarrow J/ψτ^-\barν_τ)/\mathcal{B}(B_c^- \rightarrow J/ψμ^-\barν_μ)$. We find $R(J/ψ) = 0.2582(38)$ and give an error budget. We also extend the relevant angular observables, which were recently suggested for the study of lepton flavor universality violating effects in $B\rightarrow D^*\ellν$, to $B_c \rightarrow J/ψ\ellν$ and make predictions for their values under different new physics scenarios.

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arXiv2020

$B_c \rightarrow J/ψ$ Form Factors for the full $q^2$ range from Lattice QCD

We present the first lattice QCD determination of the $B_c \rightarrow J/ψ$ vector and axial-vector form factors. These will enable experimental information on the rate for $B_c$ semileptonic decays to $J/ψ$ to be converted into a value for $V_{cb}$. Our calculation covers the full physical $q^2$ range of the decay and uses non-perturbatively renormalised lattice currents. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks on the second generation MILC ensembles of gluon field configurations including $u$, $d$, $s$ and $c$ HISQ sea quarks. Our HISQ heavy quarks have masses ranging upwards from that of $c$; we are able to reach that of the $b$ on our finest lattices. This enables us to map out the dependence on heavy quark mass and determine results in the continuum limit at the $b$. We use our form factors to construct the differential rates for $B_c^- \rightarrow J/ψμ^- \barν_μ$ and obtain a total rate with $7\%$ uncertainty: $Γ(B_c^-\rightarrow J/ψμ^-\barν_μ)/|η_{\mathrm{EW}}V_{cb}|^2 = 1.73(12)\times 10^{13} ~\mathrm{s}^{-1}$. Including values for $V_{cb}$, $η_{\mathrm{EW}}$ and $τ_{B_c}$ yields a branching fraction for this decay mode of 0.0150(11)(10)(3) ~with uncertainties from lattice QCD, $η_\mathrm{EW}V_{cb}$ and $τ_{B_c}$ respectively.

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arXiv2019

Lattice QCD matrix elements for the ${B_s^0-\bar{B}_s^0}$ width difference beyond leading order

Predicting the $B_s^0-\bar{B}_s^0$ width difference $ΔΓ_s$ relies on the heavy quark expansion and on hadronic matrix elements of $ΔB=2$ operators. We present the first lattice QCD results for matrix elements of the dimension-7 operators $R_{2,3}$ and linear combinations $\tilde{R}_{2,3}$ using nonrelativistic QCD for the bottom quark and a highly improved staggered quark (HISQ) action for the strange quark. Computations use MILC ensembles of gauge field configuations with $2+1+1$ flavors of sea quarks with the HISQ discretization, including lattices with physically light up/down quark masses. We discuss features unique to calculating matrix elements of these operators and analyze uncertainties from series truncation, discretization, and quark mass dependence. Finally we report the first Standard Model determination of $ΔΓ_s$ using lattice QCD results for all hadronic matrix elements through $\mathcal{O}(1/m_b)$. The main result of our calculations yields the $1/m_b$ contribution $ΔΓ_{1/m_b} = -0.022(10)~\mathrm{ps}^{-1}$. Adding this to the leading order contribution, the Standard Model prediction is $ΔΓ_s = 0.092(14)~\mathrm{ps}^{-1}$.

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arXiv2018

Improving the Kinetic Couplings in Lattice Non-Relativistic QCD

We improve the non-relativistic QCD (NRQCD) action by comparing the dispersion relation to that of the continuum through $\mathcal{O}(p^6)$ in perturbation theory. The one-loop matching coefficients of the $\mathcal{O}(p^4)$ kinetic operators are determined, as well as the scale at which to evaluate $α_s$ in the $V$-scheme for each quantity. We utilise automated lattice perturbation theory using twisted boundary conditions as an infrared regulator. The one-loop radiative corrections to the mass renormalisation, zero-point energy and overall energy-shift of an NRQCD $b$-quark are also found. We also explore how a Fat$3$-smeared NRQCD action and changes of the stability parameter $n$ affect the coefficients. Finally, we use gluon field ensembles at multiple lattice spacing values, all of which include $u$, $d$, $s$ and $c$ quark vacuum polarisation, to test how the improvements affect the non-perturbatively determined $Υ(1S)$ and $η_b(1S)$ kinetic masses, and the tuning of the $b$ quark mass.

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arXiv2018

Higher-order hadronic-vacuum-polarization contribution to the muon g-2 from lattice QCD

We introduce a new method for calculating the ${\rm O}(α^3)$ hadronic-vacuum-polarization contribution to the muon anomalous magnetic moment from ${ab-initio}$ lattice QCD. We first derive expressions suitable for computing the higher-order contributions either from the renormalized vacuum polarization function $\hatΠ(q^2)$, or directly from the lattice vector-current correlator in Euclidean space. We then demonstrate the approach using previously-published results for the Taylor coefficients of $\hatΠ(q^2)$ that were obtained on four-flavor QCD gauge-field configurations with physical light-quark masses. We obtain $10^{10} a_μ^{\rm HVP,HO} = -9.3(1.3)$, in agreement with, but with a larger uncertainty than, determinations from $e^+e^- \to {\rm hadrons}$ data plus dispersion relations.

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arXiv2018

The Search for Beauty-fully Bound Tetraquarks Using Lattice Non-Relativistic QCD

Motivated by multiple phenomenological considerations, we perform the first search for the existence of a $\bar{b}\bar{b}bb$ tetraquark bound state with a mass below the lowest non-interacting bottomonium-pair threshold using the first-principles lattice non-relativistic QCD methodology. We use a full $S$-wave colour/spin basis for the $\bar{b}\bar{b}bb$ operators in the three $0^{++}$, $1^{+-}$ and $2^{++}$ channels. We employ four gluon field ensembles at multiple lattice spacing values ranging from $a = 0.06 - 0.12$ fm, all of which include $u$, $d$, $s$ and $c$ quarks in the sea, and one ensemble which has physical light-quark masses. Additionally, we perform novel exploratory work with the objective of highlighting any signal of a near threshold tetraquark, if it existed, by adding an auxiliary potential into the QCD interactions. With our results we find no evidence of a QCD bound tetraquark below the lowest non-interacting thresholds in the channels studied.

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arXiv2017

Pion electromagnetic form factor from full lattice QCD

We present the first calculation of the pion electromagnetic form factor at physical light quark masses. This form factor parameterises the deviations from the behaviour of a point-like particle when a photon hits the pion. These deviations result from the internal structure of the pion and can thus be calculated in QCD. We use three sets (different lattice spacings) of $n_f = 2+1+1$ lattice configurations generated by the MILC collaboration. The Highly Improved Staggered Quark formalism (HISQ) is used for all of the sea and valence quarks. Using lattice configurations with $u$/$d$ quark masses very close to the physical value is a big advantage, as we avoid the chiral extrapolation. We study the shape of the vector ($f_+$) form factor in the $q^2$ range from $0$ to $-0.15$~GeV$^2$ and extract the mean square radius, $\langle r^2_v\rangle$. The shape of the vector form factor and the resulting radius is compared with experiment. We also discuss the scalar form factor and radius extracted from that, which is not directly accessible to experiment. We have also calculated the contributions from the disconnected diagrams to the scalar form factor at small $q^2$ and discuss their impact on the scalar radius $\langle r^2_s\rangle$.

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