Physics Letters B 300 ( 1993) 2 10-21 6 North-Holland PHYSICS LETTERS B Improved limit on the electron-antineutrino rest mass from tritium P-decay Ch. Weinheimer, M. Przyrembel, H. Backe, H. Barth, J. Bonn, B. Degen, Th. Edling, H. Fischer , L. Fleischmann, J.U. GrooB , R. Haid, A. Hermanni, G. Kube, P. Leiderer ', Th. Loeken, A. Molz, R.B. Moore2, A. Osipowicz, E.W. Otten, A. Picard, M. Schrader and M. Steininger Inrfzfut fur Physzk, UnzverszfafMaznz, W-6500 Maznz, FRG Received 4 November 1992 The endpoint region of the P-spectrum of tritium was remeasured by an electrostatic spectrometer with magnetic guiding field. It enabled the search for a rest mass of the electron-antineutrino with improved precision. The result is in: = -39 f 34,t,t f lSsysf (e~/c~)~, from which an upper limit of m, < 7.2 ev/c2 may be derived. The experiment yields the 'atomic mass difference m (T) - rn (3~e) =- 18 591 f 3 ev/c2. The cjuestion of a no*-vanishing neutrino rest mass is of fundamental importance in .particle and astro- physics. It therefore caused considerable excitement when Lubimov and collaborators [1',2] published a finite value for the electron-antineutrino rest mass m, obtained from a measurement of the endpoint region of the P-decay of tritium with a new, dedi- cated spectrometer of high resolution and luminos- ity [3]. It took a number of years until this result was checked with equivalent instrume~lts by groups in Ziirich [4] and Los Alamos [5] and disproved due to essential improvements of source and data anal- ysis [6,7]. At present, it seems that this generation of experiments is capable of reaching a precision of am: = 100 (ev/c2l2 (see table 3). A radically different type of P-spectrometer was proposed independently at several places [8,9]. It acts as an electrostatic filter guiding the electrons adiabat- ically along the,.lines of an inhomogeneous magnetic field. If the source is placed in a strong magnetic field Bo and the retarding potential Uo reaches its maxi- mum in a much weaker magnetic field B1, the full forward solid angle of electfons emitted with energy Present address: Fakultit fir Physik, Universitiit Konstanz, W-7750 Constance, FRG. Present address: McGill University, Montreal, Canada. E is analyzed with a filter width of AE = (BI /Bo) E. This is due to the adiabatic transformation of trans- verse cyclotron energy El around the B-lines into lon- gitudinal energy Ell along the B-lines by the magnetic field gradient reducing El (which cannot,be analyzed electrostatically) in proportion to the magnetic field. Based on this principle,~two somewhat different spec- trometers have been set up in recent years at INR in Troitzk [ 101 and at Mainz University. For details of the design, function and performance of the Mainz solenoid retarding spectrometer (SRS) and for fur- ther references see refs. [ l l, 121. The experiment described here was performed un- der the follo-wing conditions. The source is placed at a field Bs = 0.96B0,'slightly in front of the field max- imum of the source solenoid which is set to Bo = 2.4 T, limiting the accepted polar angles to 19 < 78". The magnetic field reaches its minimum BI = 8 x T in the symmetry plane of the spectrometer, where Uo maximizes. Retardation of the electrons and reac- celeration after the filter is provided by two sets of electrodes arranged symmetrically around the central one. Under these conditions the rise of the transmis- sion from 0 to 1 within the interval E (1 - BI /Bo) G eUd d E is given by [12] Elsevier Science Publishers B.V. First publ. in: Physics Letters B 300 (1993), 3, pp. 210-216 Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/3078/ URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-30787