Search for a light <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>Z</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>L</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>−</mml:mo><mml:msub><mml:mi>L</mml:mi><mml:mi>τ</mml:mi></mml:msub></mml:math> scenario with the NA64-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>e</mml:mi></mml:math> experiment at CERN
Yu. M. Andreev, D. Banerjee, B. Banto Oberhauser, J. Bernhard, P. Bisio, M. Bondí, V.E. Burtsev, A. Celentano, N. Charitonidis, А. Г. Чумаков, D. Cooke, P. Crivelli, E. Depero, A. Dermenev, S.V. Donskov, R. R. Dusaev, T. Enik, V. Frolov, A. Gardikiotis, S. Gerassimov, S. Gninenko, M. Hösgen, M. Jeckel, Vassili Kachanov, A. Karneyeu, G. D. Kekelidze, B. Ketzer, D. Kirpichnikov, M. Kirsanov, V. N. Kolosov, I. Konorov, S. G. Kovalenko, V. A. Kramarenko, L. Kravchuk, Nikolai Krasnikov, S. Kuleshov, Valery E. Lyubovitskij, V. Lysan, L. Marsicano, V. Matveev, Yu.V. Mikhailov, L. Molina Bueno, D. V. Peshekhonov, V. A. Polyakov, B. Radics, R. A. Rojas, A. Rubbia, K. Salamatin, V. D. Samoylenko, H. Sieber, Dmitry G. Shchukin, V. O. Tikhomirov, I. Tlisova, A. Toropin, A. Yu. Trifonov, P. Ulloa, B. I. Vasilishin, G. Vasquez Arenas, P. Volkov, V. Yu. Volkov, I. V. Voronchikhin
Abstract
The extension of Standard Model made by inclusion of additional $U(1)$ gauge ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light ${Z}^{\ensuremath{'}}$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the ${Z}^{\ensuremath{'}}$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits touch the muon $g\ensuremath{-}2$ preferred band for values of the ${Z}^{\ensuremath{'}}$ mass of order of 1 MeV, while the sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.