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تسجيل مستخدم جديدContributions of charm anihilation to the hyperfine splitting in charmonium
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In calculations of the hyperfine splitting in charmonium, the contributions of the disconnected diagrams is considered small and is typically ignored. We aim to estimate nonperturbatively the size of the resulting error, which could potentially affect the high precision calculations of the charmonium spectrum. Following our work on the effects of the disconnected diagrams in unquenched QCD presented at Lattice 2007, we study the same problem in the quenched case. 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 $aapprox 0.063$ fm.
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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 b
e 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.
Experimentally the charmonium hyperfine splitting is $M_{J/psi}-M_{eta_c}=117$ MeV and current lattice results are generally below this value. The difference could be due to the effects of the disconnected flavor singlet diagrams which have not been
included in these calculations. Previous attempts to determine the disconnected flavor singlet corrections have led just to rough estimates in the range of $pm 20$ MeV. We present preliminary results for these corrections calculated on fine ($aapprox 0.09$ fm) Asqtad 2+1 flavor lattices provided by the MILC Collaboration.
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, $delta a_{mu}^c = 0.0313(28) times 10^{-10}$.
We present preliminary results for strange and charm contributions to nucleon charges and moments. The scalar, axial and tensor charges, and unpolarized first moments are calculated using clover-on-HISQ formulation and cover four lattice spacings, $a
={0.06,0.09,0.12, 0.15}$~fm, and three pion masses, $M_pi={310,220,130}$~MeV. The renormalization factors are calculated nonperturbatively using the RI-sMOM scheme. We carry out a chiral and continuum extrapolation to obtain physical results.
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 Pad{e} approximants, from its der
ivatives 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 $a_mu^s = 53.41(59) times 10^{-10}$, and from charm to be $a_mu^c = 14.42(39)times 10^{-10}$. 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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