We present a study for the measurement of τ radiative decays, τ → lγν¯ν, (l = e, μ) and τ → πγν using ∼ 500 fb−1 of e+e− annihilations recorded by the BaBar detector at the PEP-II asymmetric B-factory

Oberhof, B.

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DOI 10.1393/ncc/i2014-11722-6Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Measurement of radiative τ decays at BaBarB. OberhofUniversita` di Pisa e INFN, Sezione di Pisa - Pisa, Italyricevuto l’1 Ottobre 2013Summary. — We present a study for the measurement of τ radiative decays,τ → lγνν¯, (l = e, μ) and τ → πγν using ∼ 500 fb−1 of e+e− annihilations recordedby the BaBar detector at the PEP-II asymmetric B-factory.PACS 13.35.Dx – Decays of taus.PACS 14.60.Fg – Properties of taus.1. – IntroductionLeptonic radiative τ decays are sensitive to the Lorentz structure of the decay ver-tex and, in particular, to the anomalous magnetic moment of the τ [1]; their branchingfractions as well as the phase space distributions of the decay products have been calcu-lated with high precision in the SM. Up to date the only measurement of the branchingfraction for τ → eγνν¯ has been done by the CLEO Collaboration [2], which quotes theresult (1.75± 0.06± 0.17)× 10−2, using 4.68 fb−1 of electron positron annihilations. Thesame experiment also quotes the most precise value for the branching fraction for theτ → μγνν¯ decay, (3.61± 0.16± 0.35)× 10−3. Besides the leptonic radiative decays alsothe hadronic radiative decay τ → γπν is of great interest because its value, which de-pends on higher order resonance contributions, has been calculated with high precisionin chiral perturbation theory [3] while experimentally it has never been observed.The BaBar experiment ran from 2000 to 2008, mainly at the center of mass energyof the Υ(4S), with a τ+τ− production cross section of 0.919 nb which, together with thehigh luminosity, allowed to collect roughly 500 million τ -pairs, 2 orders of magnitudemore than what done by CLEO. The BaBar detector is described in detail elsewhere [4].2. – Analysis overviewTo search for radiative tau decays we look for events in which the first τ decaysto the selected mode while the other one decays to a charged lepton τ → lνν¯ or toa charged pion and (at most two) optional π0s, τ → π(n)π0ν, n = 0, 1, 2. Efficiencycalculation and background evaluation have been performed using Monte Carlo eventsc© Societa` Italiana di Fisica 311312 B. OBERHOFTable I. – Final selection efficiencies and purity for the three signal modes.Decay  S/(S+B)τ → eγνν¯ 0.44± 0.01% 0.79± 0.02τ → μγνν¯ 1.72± 0.03% 0.76± 0.03τ → πγν 1.68± 0.05% 0.20± 0.01and a complete simulation of the BaBar detector based on GEANT4. Generic τ eventsare selected mainly by their topology: each event is divided in two hemispheres, thesignal-side and the tag-side, by the vector which maximizes the sum of the longitudinalprojections of momenta in the CM frame, i.e. the Thrust. Hadronic events tend to havelow Thrust values while for QED backgrounds T ∼ 1. In the signal side we requireexactly one track identified either as an electron, muon or pion and a neutral depositwith an energy greater than 50MeV located between 10 and 50 cm from the track hiton the calorimeter. In the tag-side we require either exactly one electron, muon orpion, depending on mode, and a maximum of 5 photon candidates. If more than onephoton is present in tag side we require a reconstructed π0 for every photon pair. Sincesignal photons tend to be collinear to the track we set cos θmin = 0.99 for τ → eγνν¯and cos θmin = 0.96 for the other two signal modes. For τ → eγνν¯ the selection iscomplicated by the presence of external bremsstrahlung photons which have nearly thesame angular distribution as signal photons; for this reason we set an additional cut onthe minimum invariant mass of the electron-photon system meγ,min = 0.05GeV. Afterselection the major background source for τ → μγνν¯ is given by non-radiative decaysassociated to a photon from ISR while for τ → eγνν¯ external bremsstrahlung remains theonly significant background. The situation for the τ → πγν decay is more complicatedbecause of the lower branching fraction and a irreducible background from neutral “split-offs” i.e. clusters in the calorimeter which are caused by hadronic interactions betweenthe pion and the calorimeter material; an efficient background reduction technique is stillunder investigation. Final efficiencies and purity for the 3 signal modes are reported intable I.3. – ConclusionsThis preliminary study shows that the measurement of radiative leptonic decays atBaBar could be done with significant improvement with respect to CLEO’s measurement.Further study is necessary to understand if the measurement is suitable to extract usefulinformation on the Lorentz structure of the τ decay vertex and also to understand if andhow the systematics for the rare decay τ → πγν can be reduced.REFERENCES[1] Samuel M. A. et al., Phys. Rev. D, 29 (1984) 2652.[2] Bergfeld T. et al., Phys. Rev. Lett., 84 (2000) 830.[3] Guo Z. H. and Roig P., Phys. Rev. D, 82 (2010) 113.[4] Aubert B. et al., Nucl. Instrum. Methods A, 479 (2002) 1.

Measurement of radiative τ decays at BaBar

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Measurement of radiative τ decays at BaBar

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