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Register a new userCharmonium properties from lattice QCD + QED: hyperfine splitting, $J/psi$ leptonic width, charm quark mass and $a_{mu}^c$
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We have performed the first $n_f = 2+1+1$ lattice QCD computations of the properties (masses and decay constants) of ground-state charmonium mesons. Our calculation uses the HISQ action to generate quark-line connected two-point correlation functions on MILC gluon field configurations that include $u/d$ quark masses going down to the physical point, tuning the $c$ quark mass from $M_{J/psi}$ and including the effect of the $c$ quarks electric charge through quenched QED. We obtain $M_{J/psi}-M_{eta_c}$ (connected) = 120.3(1.1) MeV and interpret the difference with experiment as the impact on $M_{eta_c}$ of its decay to gluons, missing from the lattice calculation. This allows us to determine $Delta M_{eta_c}^{mathrm{annihiln}}$ =+7.3(1.2) MeV, giving its value for the first time. Our result of $f_{J/psi}=$ 0.4104(17) GeV, gives $Gamma(J/psi rightarrow e^+e^-)$=5.637(49) keV, in agreement with, but now more accurate than experiment. At the same time we have improved the determination of the $c$ quark mass, including the impact of quenched QED to give $overline{m}_c(3,mathrm{GeV})$ = 0.9841(51) GeV. We have also used the time-moments of the vector charmonium current-current correlators to improve the lattice QCD result for the $c$ quark HVP contribution to the anomalous magnetic moment of the muon. We obtain $a_{mu}^c = 14.638(47) times 10^{-10}$, which is 2.5$sigma$ higher than the value derived using moments extracted from some sets of experimental data on $R(e^+e^- rightarrow mathrm{hadrons})$. This value for $a_{mu}^c$ includes our determination of the effect of QED on this quantity, $delta a_{mu}^c = 0.0313(28) times 10^{-10}$.
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We calculate the mass difference between the $Upsilon$ and $eta_b$ and the $Upsilon$ leptonic width from lattice QCD using the Highly Improved Staggered Quark formalism for the $b$ quark and including $u$, $d$, $s$ and $c$ quarks in the sea. We have results for lattices with lattice spacing as low as 0.03 fm and multiple heavy quark masses, enabling us to map out the heavy quark mass dependence and determine values at the $b$ quark mass. Our results are: $M_{Upsilon} -M_{eta_b} = 57.5(2.3)(1.0) ,mathrm{MeV}$ (where the second uncertainty comes from neglect of quark-line disconnected correlation functions) and decay constants, $f_{eta_b}=724(12)$ MeV and $f_{Upsilon} =677.2(9.7)$ MeV, giving $Gamma(Upsilon rightarrow e^+e^-) = 1.292(37)(3) ,mathrm{keV}$. The hyperfine splitting and leptonic width are both in good agreement with experiment, and provide the most accurate lattice QCD results to date for these quantities by some margin. At the same time results for the time moments of the vector-vector correlation function can be compared to values for the $b$ quark contribution to $sigma(e^+e^- rightarrow mathrm{hadrons})$ determined from experiment. Moments 4--10 provide a 2% test of QCD and yield a $b$ quark contribution to the anomalous magnetic moment of the muon of 0.300(15)$times 10^{-10}$. Our results, covering a range of heavy quark masses, may also be useful to constrain QCD-like composite theories for beyond the Standard Model physics.
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.
In calculations of the hyperfine splitting in charmonium, the contributions of the disconnected diagrams are considered small and are typically ignored. We aim to estimate nonperturbatively the size of the resulting correction, which may eventually be needed in high precision calculations of the charmonium spectrum. We study this problem in the quenched and unquenched QCD cases. On dynamical ensembles the disconnected charmonium propagators contain light modes which complicate the extraction of the signal at large distances. In the fully quenched case, where there are no such light modes, the interpretation of the signal is simplified. We present results from lattices with $aapprox 0.09$ fm and $aapprox 0.06$ fm.
We determine the mass of the charm quark ($m_c$) from lattice QCD with two flavors of dynamical quarks with a mass around the strange quark. We compare this to a determination in quenched QCD which has the same lattice spacing (0.1 fm). We investigate different formulations of the quark mass, based on the Vector Ward Identity, PCAC relation and the FNAL heavy quark formalism. Based on these preliminary results we find no effects due to sea quarks with a mass around strange.
We use lattice QCD simulations, with MILC gluon configurations and HISQ c-quark propagators, to make very precise determinations of moments of charm-quark pseudoscalar, vector and axial-vector correlators. These moments are combined with new four-loop results from continuum perturbation theory to obtain several new determinations of the MSbar mass of the charm quark and of the MSbar coupling. We find m_c(3GeV)=0.986(10)GeV, or, equivalently, m_c(m_c)=1.268(9)GeV, both for n_f=4 flavors; and alpha_msb(3GeV,n_f=4)=0.251(6), or, equivalently, alpha_msb(M_Z,n_f=5)=0.1174(12). The new mass agrees well with results from continuum analyses of the vector correlator using experimental data for e+e- annihilation (instead of using lattice QCD simulations). These lattice and continuum results are the most accurate determinations to date of this mass. Ours is also one of the most accurate determinations of the QCD coupling by any method.
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