τ dipole moments via radiative leptonic τ decays

Abstract

We propose a new method to probe the magnetic and electric dipole moments of the τ lepton using precise measurements of the differential rates of radiative leptonic τ decays at high-luminosity B factories. Possible deviations of these moments from the Standard Model values are analyzed in an effective Lagrangian approach, thus providing model-independent results. Analytic expressions for the relevant non-standard contributions to the differential decay rates are presented. Earlier proposals to probe the τ dipole moments are examined. A detailed feasibility study of our method is performed in the conditions of the Belle and Belle II experiments at the KEKB and Super-KEKB colliders, respectively. This study shows that our approach, applied to the planned full set of Belle II data for radiative leptonic τ decays, has the potential to improve the present experimental bound on the τ anomalous magnetic moment. On the contrary, its foreseen sensitivity is not expected to lower the current experimental limit on the τ electric dipole moment.

Radiativeτleptonic decays and the possibility to determine theτdipole moments

“…However the experimental sensitivity onã τ at the Belle II experiment, σã = 0.012 [61], can already be competitive with Delphi results in (11). On the other hand, the expected sensitivity on the τ EDM, σd = 6.1 × 10 −17 e·cm [61], is still worse than the most precise measurement ofd τ done at Belle in τ pair production [27].…”

“…Since a non-standard contribution to a τ spoils this radiation zero, precise measurements of this phasespace region could be used to set bounds on its value. However, studies with Belle data show no significant improvement of the existing limits: theã τ upper limit (UL) that can be achieved with the whole Belle statistics, about 0.9 × 10 9 τ pairs, is only UL(ã τ ) 2 [61].…”

τ−→ ℓi−ℓi+ℓj−ν̄jντ decays with a magnetic dipole term

“…3. Other approaches similarly yield stringent bounds on a τ through electroweak precision data (EWPD) and the experimental data for the e + e − → τ + τ − : −0.004 ≤ a τ ≤ 0.006 [16], −0.007 ≤ a τ ≤ 0.005 [17] and more recently −0.007 ≤ a τ ≤ 0.004 [18]. While on the theoretical side, other model extensions predict values for a τ of the order of O(10 −9 − 10 −6 ) [19]- [23], which could be useful in the case of a discrepancy between the experimental measurement and the SM prediction.…”

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.…”