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Register a new userQCD determination of the leading order hadronic contribution to the muon g-2
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The leading order hadronic contribution to the muon magnetic moment anomaly, $a^{HAD}_mu$, is determined entirely in the framework of QCD. The result in the light-quark sector, in units of $10^{-10}$, is $a^{HAD}_mu|_{uds} =686 pm 26$, and in the heavy-quark sector $a^{HAD}_mu|_{c} =14.4 pm 0.1$, and $a^{HAD}_mu|_{b} =0.29 pm 0.01$, resulting in $a^{HAD}_mu = 701 pm 26$. The main uncertainty is due to the current lattice QCD value of the first and second derivative of the electromagnetic current correlator at the origin. Expected improvement in the precision of these derivatives may render this approach the most accurate and trustworthy determination of the leading order $a^{HAD}_mu$.
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We calculate the leading-order hadronic correction to the anomalous magnetic moments of each of the three charged leptons in the Standard Model: the electron, muon and tau. Working in two-flavor lattice QCD, we address essentially all sources of systematic error: lattice artifacts, finite-size effects, quark-mass extrapolation, momentum extrapolation and disconnected diagrams. The most significant remaining systematic error, the exclusion of the strange and charm quark contributions, will be addressed in our four-flavor calculation. We achieve a statistical accuracy of 2% or better for the physical values for each of the three leptons and the systematic errors are at most comparable.
The evaluation of the hadronic contribution to the muon magnetic anomaly $a_mu$ is revisited, taking advantage of new experimental data on $e^+e^-$ annihilation into hadrons: SND and CMD-2 for the $pi^+pi^-$ channel, and babar for multihadron final states. Discrepancies are observed between KLOE and CMD-2/SND data, preventing one from averaging all the $e^+e^-$ results. The long-standing disagreement between spectral functions obtained from $tau$ decays and $e^+e^-$ annihilation is still present, and not accounted by isospin-breaking corrections, for which new estimates have been presented. The updated Standard Model value for $a_mu$ based on $e^+e^-$ annihilation data is now reaching a precision better than experiment, and it disagrees with the direct measurement from BNL at the 3.3$sigma$ level, while the $tau$-based estimate is in much better agreement. The $tau$/$e^+e^-$ discrepancy, best revealed when comparing the measured branching fraction for $tau^- to pi^- pi^0 u_tau$ to its prediction from the isospin-breaking-corrected $e^+e^-$ spectral function, remains a serious problem to be understood.
The persistent discrepancy of about 3.5 standard deviations between the experimental measurement and the Standard Model prediction for the muon anomalous magnetic moment, $a_mu$, is one of the most promising hints for the possible existence of new physics. Here we report on our lattice QCD calculation of the hadronic vacuum polarisation contribution $a_mu^{rm hvp}$, based on gauge ensembles with $N_f=2+1$ flavours of O($a$) improved Wilson quarks. We address the conceptual and numerical challenges that one encounters along the way to a sub-percent determination of the hadronic vacuum polarisation contribution. The current status of lattice calculations of $a_mu^{rm hvp}$ is presented by performing a detailed comparison with the results from other groups.
We calculate the leading order hadronic contribution to the muon anomalous magnetic moment using twisted mass lattice QCD. The pion masses range from 330 MeV to 650 MeV. We use two lattice spacings, a=0.079 fm and 0.063 fm, to study lattice artifacts. Finite-size effects are studied for two values of the pion mass, and we calculate the disconnected contributions for four ensembles. Particular attention is paid to the dominant contributions of the vector mesons, both phenomenologically and from our lattice calculation.
We propose a new experiment to measure the running of the fine-structure constant in the space-like region by scattering high-energy muons on atomic electrons of a low-Z target through the process $mu e to mu e$. The differential cross section of this process, measured as a function of the squared momentum transfer $t=q^2<0$, provides direct sensitivity to the leading-order hadronic contribution to the muon anomaly $a^{rm{HLO}}_{mu}$. By using a muon beam of 150 GeV, with an average rate of $sim1.3times 10^7$ muon/s, currently available at the CERN North Area, a statistical uncertainty of $sim 0.3%$ can be achieved on $a^{rm{HLO}}_{mu}$ after two years of data taking. This direct measurement of $a^{rm{HLO}}_{mu}$ will provide an independent determination, competitive with the time-like dispersive approach, and consolidate the theoretical prediction for the muon $g$-2 in the Standard Model. It will allow therefore a firmer interpretation of the measurements of the future muon $g$-2 experiments at Fermilab and J-PARC.
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