arXiv subjects

Laurence J. Cooper

Publications and source records attributed to Laurence J. Cooper.

3 recordsLinked to original sources

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗