We present the first measurement of the Michel parameter <math><msup><mi>ξ</mi><mo>′</mo></msup></math> in the <math><msup><mi>τ</mi><mo>−</mo></msup><mo>→</mo><msup><mi>μ</mi><mo>−</mo></msup><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub><msub><mi>ν</mi><mi>τ</mi></msub></math> decay using the full data sample of <math><mn>988</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> collected by the Belle detector operating at the KEKB asymmetric energy <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> collider. The method is based on the reconstruction of the <math><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><msup><mi>e</mi><mo>−</mo></msup><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>e</mi></msub><msub><mi>ν</mi><mi>μ</mi></msub></math> decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain <math><msup><mi>ξ</mi><mo>′</mo></msup><mo>=</mo><mrow><mn>0.22</mn><mo>±</mo><mn>0.94</mn><mo>±</mo><mn>0.42</mn></mrow></math> where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of <math><msup><mi>ξ</mi><mo>′</mo></msup><mo>=</mo><mn>1</mn></math>.

D. Bodrov

P. Pakhlov

I. Adachi

H. Aihara

S. Al Said

D. M. Asner

H. Atmacan

T. Aushev

R. Ayad

V. Babu

Sw. Banerjee

P. Behera

K. Belous

J. Bennett

M. Bessner

B. Bhuyan

T. Bilka

D. Biswas

A. Bobrov

A. Bondar

J. Borah

A. Bozek

M. Bračko

P. Branchini

T. E. Browder

A. Budano

M. Campajola

D. Červenkov

M.C. Chang

B. G. Cheon

K. Chilikin

K. Cho

S.J. Cho

S.K. Choi

Y. Choi

S. Choudhury

D. Cinabro

S. Das

G. De Nardo

G. De Pietro

R. Dhamija

F. Di Capua

J. Dingfelder

Z. Doležal

T. V. Dong

D. Epifanov

T. Ferber

D. Ferlewicz

B. G. Fulsom

V. Gaur

A. Garmash

A. Giri

P. Goldenzweig

E. Graziani

D. Greenwald

T. Gu

Y. Guan

K. Gudkova

C. Hadjivasiliou

S. Halder

K. Hayasaka

H. Hayashii

M. T. Hedges

D. Herrmann

W.S. Hou

C.L. Hsu

T. Iijima

K. Inami

N. Ipsita

A. Ishikawa

R. Itoh

M. Iwasaki

W. W. Jacobs

E.J. Jang

Q. P. Ji

S. Jia

Y. Jin

K. K. Joo

D. Kalita

A. B. Kaliyar

K. H. Kang

T. Kawasaki

C. Kiesling

C. H. Kim

D. Y. Kim

K.H. Kim

Y.K. Kim

H. Kindo

K. Kinoshita

P. Kodyš

A. Korobov

S. Korpar

P. Križan

P. Krokovny

T. Kuhr

M. Kumar

R. Kumar

K. Kumara

Y.J. Kwon

J. S. Lange

Study of the muon decay-in-flight in the <math><msup><mi>τ</mi><mo>−</mo></msup><mo>→</mo><msup><mi>μ</mi><mo>−</mo></msup><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub><msub><mi>ν</mi><mi>τ</mi></msub></math> decay to measure the Michel parameter <math><msup><mi>ξ</mi><mo>′</mo></msup></math>

We report the first measurement of the Michel parameter <math><msup><mi>ξ</mi><mo>′</mo></msup></math> in the <math><mrow><msup><mrow><mi>τ</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><msub><mrow><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mi>μ</mi></mrow></msub><msub><mrow><mi>ν</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></math> decay with a new method proposed just recently. The measurement is based on the reconstruction of the <math><msup><mi>τ</mi><mo>−</mo></msup><mo>→</mo><msup><mi>μ</mi><mo>−</mo></msup><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub><msub><mi>ν</mi><mi>τ</mi></msub></math> events with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of <math><mn>988</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> collected by the Belle detector corresponds to approximately <math><mrow><mn>912</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>6</mn></mrow></msup></mrow></math> <math><msup><mi>τ</mi><mo>+</mo></msup><msup><mi>τ</mi><mo>−</mo></msup></math> pairs. We measure <math><mrow><msup><mrow><mi>ξ</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>=</mo><mn>0.22</mn><mo>±</mo><mn>0.94</mn><mo>(</mo><mrow><mi>stat</mi></mrow><mo>)</mo><mo>±</mo><mn>0.42</mn><mo>(</mo><mrow><mi>syst</mi></mrow><mo>)</mo></mrow></math>, which is in agreement with the standard model prediction of <math><msup><mi>ξ</mi><mo>′</mo></msup><mo>=</mo><mn>1</mn></math>. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

H. E. Cho

S. C. Lee

First Measurement of the Michel Parameter <math><mrow><msup><mrow><mi>ξ</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math> in the <math><mrow><msup><mrow><mi>τ</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><msub><mrow><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mi>μ</mi></mrow></msub><msub><mrow><mi>ν</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></math> Decay at Belle

We analyse the effects of Dirac and Majorana neutrinos on leptonic decays of muon and tau leptons using the most general four-lepton effective interaction Hamiltonian of dimension six. We calculate the specific energy and angular distribution of the final charged lepton, including the polarizations of the initial and final charged leptons. We discuss the new generalized Michel parameters and focus on the effects of the heavy neutrino masses that would lead to sizable contributions on scenarios where the new sterile neutrinos have non-negligible mixing. Specifically, the most promising scenario is found for the case of τ decay with one heavy final-state neutrino with a mass around 102 − 103 MeV, where the linear term suppression could be of order 10 −4, low enough to be measured in current and forthcoming experiments.

Mexico

Juan Márquez

Gabriel Castro

Pablo Roig

Michel parameters in the presence of massive Dirac and Majorana neutrinos

This paper provides a detailed description of the method for the first direct measurement of all Michel parameters in the τ  −  → μ  −    <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$   μ  ν  τ  decay related to the polarization of the daughter muon. An application of the suggested method in the existing and future experiments at e + e  −  colliders is considered. We have performed a feasibility study for the future Super Charm-Tau Factory and Belle II experiments. For the first one, the sensitivity to the Michel parameters ξ′, ξ′′, η′′, α′/A, and β′/A is estimated. For the latter, only one Michel parameter, ξ′, for which the sensitivity is maximum, is considered.

Russia

A new method for the measurement of the Michel parameters that describe the daughter muon polarization in the τ  −  → μ  −    <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$   μ  ν  τ  decay

We present a measurement of the Michel parameters of the lepton, and , in the radiative leptonic decay using 711 f of collision data collected with the Belle detector at the KEKB collider. The Michel parameters are measured in an unbinned maximum likelihood fit to the kinematic distribution of or . The measured values of the Michel parameters are and , where the first error is statistical and the second is systematic. This is the first measurement of these parameters. These results are consistent with the Standard Model predictions within their uncertainties, and constrain the coupling constants of the generalized weak interaction.