Tau mass measurement at KEDR A. G. Shamov a for KEDR Collaboration ∗ V. V. Anashin a , V. M. Aulchenko a , E. M. Baldin a , A. K. Barladyan a , A. Yu. Barnyakov a , M. Yu. Barnyakov a , S. E. Baru a , I. V. Bedny a , O. L. Beloborodova a , A. E. Blinov a , V. E. Blinov a , A. V. Bobrov a , V. S. Bobrovnikov a , A. V. Bogomyagkov a , A. E. Bondar a , D. V. Bondarev a , A. R. Buzykaev a , S. I. Eidelman a , Yu. M. Glukhovchenko a , V. V. Gulevich a , S. E. Karnaev a , G. V. Karpov a , S. V. Karpov a , V. A. Kiselev a , S. A. Kononov a , T. A. Kozlova a , K. Yu. Kotov a , E. A. Kravchenko a , V. F. Kulikov a , G. Ya. Kurkin a , E. A. Kuper a , E. B. Levichev a , D. A. Maksimov a , V. M. Malyshev a , A. L. Maslennikov a , A. S. Medvedko a , O. I. Meshkov a , S. I. Mishnev a , I. I. Morozov a , N. Yu. Muchnoi a , V. V. Neufeld a , S. A. Nikitin a , I. B. Nikolaev a , I. N. Okunev a , A. P. Onuchin a , S. B. Oreshkin a , I. O. Orlov a , A. A. Osipov a , S. V. Peleganchuk a , S. S. Petrosyan a , S. G. Pivovarov a , P. A. Piminov a , V. V. Petrov a , A. O. Poluektov a , I. N. Popkov a , G. E. Pospelov a , V. G. Prisekin a , A. A. Ruban a , V. K. Sandyrev a , G. A. Savinov a , A. G. Shamov a , D. N. Shatilov a , B. A. Shwartz a , E. A. Simonov a , S. V. Sinyatkin a , Yu. I. Skovpen a , A. N. Skrinsky a , V. V. Smaluk a , A. M. Sukharev a , E. V. Starostina a , A. A. Talyshev a , V. A. Tayursky a , V. I. Telnov a , Yu. A. Tikhonov a , K. Yu. Todyshev a , G. M. Tumaikin a , Yu. V. Usov a , A. I. Vorobiov a , A. N. Yushkov a , V. N. Zhilich a , V. V. Zhulanov a , A. N. Zhuravlev a a Budker Institute of Nuclear Physics Lavrentiev prospect 11, Novosibirsk, 630090 Russia The status of the experiment on the precise τ lepton mass measurement running at the VEPP-4M collider with the KEDR detector is reported. The mass value is evaluated from the τ + τ - cross section behavior around the production threshold. The preliminary result based on 15.2 pb -1 of data is mτ = 1776.69 +0.17 -0.19 ± 0.15 MeV. 1. Introduction The τ lepton mass m τ is one of the fundamental characteristics of the Standard Model. Together with the lifetime and the decay probability to e ¯ ν e ν τ this value can be used to test the lepton uni- versality principle which is one of the postulates of the modern Electroweak theory. The world average value m τ = 1776.99 +0.29 −0.26 MeV [1] is dom- inated by the result of the BES collaboration [2] which statistical analysis and uncertainty estima- tions were discussed in Refs. [3] and [4]. Thus, additional measurements are desirable to improve the mass accuracy and ensure future progress in the lepton universality tests. The direct method of the τ mass determination is a study of the threshold behavior of the τ + τ − * Partially supported by the Russian Foundation for Basic Research, Grant 07-02-00426-a production cross section in e + e − collisions as it was done in the experiments [5] and then [2]. The key question of such experiments is the precision of the beam energy determination. The impor- tant feature of the present work is an application of two independent methods of the beam energy measurement, while the previous experiments re- lied on the extrapolation based on the J/ψ and ψ(2S ) mesons as reference points. The detailed description of the KEDR measure- ment of the τ mass is presented in [6] where the first 6.7 pb −1 of data have been analyzed. Below we remind the principal features of the VEPP-4M collider and the KEDR detector and present the preliminary result on the τ mass obtained using the full statistics of 15.2 pb −1 . Nuclear Physics B (Proc. Suppl.) 189 (2009) 21–23 0920-5632/$ – see front matter © 2009 Published by Elsevier B.V. www.elsevierphysics.com doi:10.1016/j.nuclphysbps.2009.03.005