Tau anomalous magnetic moment form factor at super B/flavor factories

Abstract

The proposed high-luminosity B/Flavor factories offer new opportunities for the improved determination of the fundamental physical parameters of standard heavy leptons. Compared to the electron or the muon case, the magnetic properties of the τ lepton are largely unexplored. We show that the electromagnetic properties of the τ , and in particular its magnetic form factor, may be measured competitively in these facilities, using unpolarized or polarized electron beams. Various observables of the τ 's produced on top of the Υ resonances, such as cross-section and normal polarization for unpolarized electrons or longitudinal and transverse asymmetries for polarized beams, can be combined in order to increase the sensitivity on the magnetic moment form factor. In the case of polarized electrons, we identify a special combination of transverse and longitudinal τ polarizations able to disentangle this anomalous magnetic form factor from both the charge form factor and the interference with the Z-mediating amplitude. For an integrated luminosity of 15 × 10 18 b −1 one could achieve a sensitivity of about 10 −6 , which is several orders of magnitude below any other existing high-or low-energy bound on the magnetic moment. Thus one may obtain a QED test of this fundamental quantity to a few % precision.

Tau spin correlations and the anomalous magnetic moment

“…Comparable sensitivities were found in Ref. [8] by using polarized electron beams and observables built on the polarization analysis of a single Tau.…”

“…The comparison with the results given in Ref. [8] shows that observables obtained with polarized electrons are better by a factor 3 than those given for unpolarized electron-beams. In addition, one must be aware that experimental uncertainties may be bigger for the detection in coincidence of two Taus, in order to get correlations, than for single Tau polarization, so that bounds from correlations may be looser that those for single Tau observables.…”

“…In Ref. [8] we already showed that the imaginary part of F 2 can be determined from the Normal Asymmetry of the decay products for a normal polarization of a single Tau. In this paper, we have shown that, for unpolarized electron beams, the real part of the form factor needs the measurement of correlations on the Tau decay products of both polarized Taus.…”

“…The polarization analyzer has been studied in detail in Ref. [8] and the results are shown in Table 1.…”

H →τ + τ − γ as a probe of the τ magnetic dipole moment

“…As explained above, this process is related through gauge invariance to the more traditional probes of the τ 's electroweak interactions via precision measurements of its production and/or decay [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], and we will see that Higgs decays provide both an opportunity to discover physics beyond the Standard model, or to provide some of the best constraints on an anomalous contribution to the τ magnetic moment. This work is structured as follows.…”