Searches for muon-electron and muon-positron conversion in titanium

Abstract

Searches have been performed for neutrinoless muon-electron conversion and muon-positron conversion using a time projection chamber. An upper limit on the branching ratio for the coherent reaction R(jd ~+Ti-e~+Ti) <4.6xl0~1 2 [90% confidence level (C.L.)] relative to ordinary muon capture was obtained. For the process n ~ +Ti-* e + + Ca no events were observed for positron momenta p > 96 MeV/c leading to an upper limit on the partial branching ratio relative to ordinary muon capture r p>9 6(M~+Ti^e + + Ca)/rO*-+Ti-capture) <9xl0~1 2 (90% C.L.). With the assumption of a giant-resonance-excitation model the integrated limit would be < 1.7x 10 ~1 0 (90% C.L.).

State-by-state calculations for all channels of the exotic (μ-,e-) conversion process

“…has recently been set by SINDRUM II at PSI [1] by using 48 T i as target. This value is of the same order with the previous limit set at TRIUMF [2], i.e. R T i µe < 4.6×10 −12 .…”

“…This year [4], experimental data extracted at PSI by using 208 P b have yielded an upper limit R P b µe < 4.6 × 10 −11 . This experiment improved by an order of magnitude over the previous upper limit, R P b µe < 4.9 × 10 −10 , extracted from preliminary experimental data for the same target at TRIUMF [2].…”

“…The neutrinoless muon to electron conversion in the field of a nucleus, µ − + (A, Z) → e − + (A, Z) * (1) is forbidden in the standard model by lepton flavour conservation and plays an important role in the study of flavour changing neutral currents which violate muon and electron numbers [1]- [10]. Within the last decade, experiments at PSI [1,3,4] and TRIUMF [2] aiming at a search of µ − e conversion electrons have not yet observed such events. These experiments have, however, provided us with useful constraints for the violation of muon and electron numbers.…”

“…The experimental sensitivity is expected to be further improved by two to three orders of magnitude by on going experiments at PSI (to 10 −14 ) [1], at TRIUMF (to 10 −14 ) [2,8] and at INS (to 10 −14 −10 −16 ) [11]. Hopefully, such experiments will not only yield a still better limit, but they will detect some (µ − , e − ) events which will signal the break down of the muon number conservation revealing "new physics" beyond the standard model.…”

Exotic μ−−e− conversion in nuclei and R-parity violating supersymmetry

“…For technical reasons the MECO target has been chosen to be the light nucleus 27 Al. Traditionally the µ − e conversion process was searched by employing medium heavy (like 48 Ti and 63 Cu) [9,10] or very heavy (like 208 Pb and 197 Au) [9,11,12] targets (for a historical review on such experiments see Ref. [13]).…”