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Direct search for mass of neutrino and anomaly in the tritium beta-spectrum
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277 www.elsevier.nl/locate/npe
DIRECT SEARCH FOR MASS OF NEUTRINO AND ANOMALY IN THE TRITIUM BETA-SPECTRUM V. M. Lobashev, V. N. Aseev, A. I. Belesev, A. I. Berlev, E. V. Geraskin, A. A. Golubev, O. V. Kazachenko, Yu. E. Kuznetsov, R. P. Ostroumov, L. A. Rivkis, B. E. Stern, N. A. Titov, C. V. Zadoroghny, Yu. I. Zakharov a aInstitute for Nuclear Research; Academy of Sciences of Russia; 60-th October Anniversary Prospect 7a; 117312 Moscow, Russia Results of the "Troitsk v-mass" experiment on the search for the neutrino rest mass in the tritium beta-decay are presented. Study of time dependence of anomaIious, bump-like structure at the end of beta spectrum reported earlier gives indication of periodic shift of the position of the bump with respect to end-point energy with period of 0.5 year. New upper limit for electron antineutrino rest mass my < 2.5eV/c 2 is derived after accounting for the bump.
I. I n t r o d u c t i o n .
3. A n o m a l i o u s s t r u c t u r e s in t h e s p e c t r u m
The direct or kinematic approach to the search for the neutrino rest mass is based on the study of neutrino momentum-energy balance in weak semileptonic decays. At present lowest limit for electron neutrino mass was achieved by the study of the shape of tritium beta spectrum near its end point on spectrometric facilities in Troitsk (Moscow) [2] and in Malnz [3].
Previous measurements of tritium integral spectrum in the vicinity of end-point, reported in [1] and [2] revealed existence of structure, which resembles small step superimposed on the regular spectrum. In differential mode such addendum would be seen as a bump-like structure with small width ( about resolution of the spectrometer). The size and position of the step with respect to end point of the spectrum turn out to vary from run to run but resulted in average for ANstepabout 6" 10-11 of total decay intensity and Eo - E , tep varying within 5 - 1 5 e V . The situation became more enigmatic when values of Eo - E , tep were plotted versus calendar time of corresponding run. The plot is given in Fig. 1. Very surprising feature of it turned out to be a possibility to describe time dependence of the step position by a sinusoid with a period 0,499 ± 0,003 year. Combining data of all the years in one year plot demonstrates that the variation of the step position have biseasonal character (see Fig. 2). Time dependence of the step sizevalues proved to be more peculiar one (see Fig. 3). The measurements, made at the end of December 1998 resulted in almost 3 times larger step size and E0 - Eaep somewhat below the sinusoid fittedto previous data.
2. T h e Troitsk u-mass set-up.
"Troitsk v-mass" set-up is an integral electrostatic spectrometer with a strong longitudinal inhomogeneous magnetic field providing adiabatic guiding and collimation of the electrons momenta. Spectrometer is coupled with the gaseous windowless tritium source also with strong magnetic field providing adiabatic transportation of the electrons.. Measurements were made in part of the spectrum from 18000 e V t o 18770 e V . Details of the set-up design and of the measurement and data analysis procedure may be found in [1] and [2].
- see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0920-5632(00)00678-2 0920-5632/00/$
V.M. Lobashev et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277
276
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Figure 1. The step position dependence on the calendar time of measurements. Parameters of the fitted sinusoid are: Period 0,499 + 0,003 year, mean value 10,4 4-0,4eV, amplitude 4,3 40, 55 eV, phase 2,8 4- 0, 16tad.
Next two measurements of Troitsk set-up were performed in the period April- May 1999• Both showed position of the step in accordance with the same sinusoid. (See Fig. 2, poins Run28-1 and Run28-2.) Existence of the short period fluctuations in Run26 may signify more complicated picture of step variations than single period sinusoid, but any way the half year period seems to be dominant in present set of the data. At the moment it seems to be impossible to propose any "customary" explanation of this phenomenon. The proximity of the oscillation period of the step (bump) to half period of Earth circulation around the Sun remind us of an old speculation about an effect produced by capture of the cosmological degenerated neutrino by tritium with emission of almost monochromatic electrons [4]. In order to produce the bump intensity, corresponding to 10- l ° of the total decay rate it is necessary to suppose existence of neutrino cloud with density as high as 0, 5.10ZSv/cm s • Observation of bump below end point of beta spectrum corresponds to capture of neutrino with
. . . . . . ~...; ................................... 4
Year
5
8
7
0
9
'i0
11
12
Time, month
Figure 2. The plot of step positions versus time of the year. Fitted sinusoid is the same as in Fig. 1, but with the period being 0.500 year. Horizontal bars are lengh of the run. Indices of the points indicate the number of the run. Indices ns and sh refer to sub run carried out biased (on +15V) tritium source, and such without it. Q3 and Q4 Mainz group data.
negative energy, and to assumption of binding of neutrino in the cloud• In the case of binding energy changing over the cloud,the Earth in its movement produces the periodical modulation of binding energy and correspondingly position of the step. The size of the neutrino cloud in this case must be comparable with Earth orbit and it does not contradict to average density of relic neutrino in the Universe. Of course this explanation of step phenomenon is extremely speculative and may be considered only for stimulation of further experiments. 4.
U p p e r l i m i t for n e u t r i n o m a s s
The procedure adopted for deduction of the neutrino mass consisted in addition to theoretical spectrum of the step function with two variable parameters assuming that such addition may describe in the first approximation local enhance-
V.M. Lobashev et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277
277
Final systematics error is obtained by averaging systematic errors of each run weighted by corresponding fit error. From here one may obtain a standard upper limit for my:
12,
10"
my < 2,5eV/c2; (95%C.L.)
8.
(2)
E
5. A c k n o w l e d g m e n t s
-+ !
!
!
i
i
|
|
i
i
i
|
,
This work was partially supported by the Russia~ Foundation for Basic Research (grants 3903 and 18401a), by Program for Fundamental Nuclear Physics. The authors are very thankful to leader of "Mainz neutrino" group Prof. E.W.Otten for financial support, which was crucial for carrying out measurements in April-May 1999.
i
Jan Feb Mar AIx May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 3. Step size versus time of the year. All the size values are reduced to the same intensity of the source.
ment in the beta-spectrum near end-point. The my2 effect unlike the local enhancement appears as an addition to (for negative m~) or deficiency (positive m~) of the spectrum that is linearly increasing with F_~ - E. This difference allows to separate both effects in fit procedure. Of course the size and position of the step being introduced as a free parameter, correlate with m~ and it increases the final error of neutrino mass thus acting as a kind of systematic error. This increase sufficiently compensates the uncertainty introduced by substitution of an a priory unknown anomaly shape by the step-like function. Others systematic errors come mostly from the uncertainties of parameters of the correction factors which are introduced in the spectrum before the fit. A remarkable property of the total systematic error from these factors is its reduction when lower energy of the fit interval comes nearer to end-point. Taking into account that fit error of m~ at the same time increases, one may select the optimal fit interval, when the total error, including both the fit and systematic error taken in quadrature, is minimal. The result for mg for the runs where correlation of m 2 and step is not large is: m "2 r = _ l , O = t = 3 , 0 f i t =l=2,1systeV2/c 4
(1)
REFERENCES 1. A.I. Belesev et al., Phys. Lett., B 350(1995) 263. 2. V. M. Lobashev et al., Phys.Lett.,B 460(1999)227. 3. Ch. Weinheimer et. al., Phys. Lett., B 460(1999) 219. 4. G. J. Stephenson Jr., T. Goldman and B.H.J.McKellar, Int. Jour. Mod. Phys. A, Vol. 13, No. 16, (1998) 2765-2790. 5. R.N. Mohapatra and S. Nussinov, Phys. Lett. B 395 (1997) 63-68.
Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277 www.elsevier.nl/locate/npe
DIRECT SEARCH FOR MASS OF NEUTRINO AND ANOMALY IN THE TRITIUM BETA-SPECTRUM V. M. Lobashev, V. N. Aseev, A. I. Belesev, A. I. Berlev, E. V. Geraskin, A. A. Golubev, O. V. Kazachenko, Yu. E. Kuznetsov, R. P. Ostroumov, L. A. Rivkis, B. E. Stern, N. A. Titov, C. V. Zadoroghny, Yu. I. Zakharov a aInstitute for Nuclear Research; Academy of Sciences of Russia; 60-th October Anniversary Prospect 7a; 117312 Moscow, Russia Results of the "Troitsk v-mass" experiment on the search for the neutrino rest mass in the tritium beta-decay are presented. Study of time dependence of anomaIious, bump-like structure at the end of beta spectrum reported earlier gives indication of periodic shift of the position of the bump with respect to end-point energy with period of 0.5 year. New upper limit for electron antineutrino rest mass my < 2.5eV/c 2 is derived after accounting for the bump.
I. I n t r o d u c t i o n .
3. A n o m a l i o u s s t r u c t u r e s in t h e s p e c t r u m
The direct or kinematic approach to the search for the neutrino rest mass is based on the study of neutrino momentum-energy balance in weak semileptonic decays. At present lowest limit for electron neutrino mass was achieved by the study of the shape of tritium beta spectrum near its end point on spectrometric facilities in Troitsk (Moscow) [2] and in Malnz [3].
Previous measurements of tritium integral spectrum in the vicinity of end-point, reported in [1] and [2] revealed existence of structure, which resembles small step superimposed on the regular spectrum. In differential mode such addendum would be seen as a bump-like structure with small width ( about resolution of the spectrometer). The size and position of the step with respect to end point of the spectrum turn out to vary from run to run but resulted in average for ANstepabout 6" 10-11 of total decay intensity and Eo - E , tep varying within 5 - 1 5 e V . The situation became more enigmatic when values of Eo - E , tep were plotted versus calendar time of corresponding run. The plot is given in Fig. 1. Very surprising feature of it turned out to be a possibility to describe time dependence of the step position by a sinusoid with a period 0,499 ± 0,003 year. Combining data of all the years in one year plot demonstrates that the variation of the step position have biseasonal character (see Fig. 2). Time dependence of the step sizevalues proved to be more peculiar one (see Fig. 3). The measurements, made at the end of December 1998 resulted in almost 3 times larger step size and E0 - Eaep somewhat below the sinusoid fittedto previous data.
2. T h e Troitsk u-mass set-up.
"Troitsk v-mass" set-up is an integral electrostatic spectrometer with a strong longitudinal inhomogeneous magnetic field providing adiabatic guiding and collimation of the electrons momenta. Spectrometer is coupled with the gaseous windowless tritium source also with strong magnetic field providing adiabatic transportation of the electrons.. Measurements were made in part of the spectrum from 18000 e V t o 18770 e V . Details of the set-up design and of the measurement and data analysis procedure may be found in [1] and [2].
- see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0920-5632(00)00678-2 0920-5632/00/$
V.M. Lobashev et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277
276
20
"°'l"''|••'|'''|'''r''°l'''l'''l'''l'''l°''l''"
4 18"1
SWpOm~ ~t
-,- T ~ . ~ s
,.-]
15-
•
T,o~d.,.
T'$ T
14
I~n20-,
Rim23
@ ~ 10" ~
--
]~T . . . . .
-~T
Run,7
J
"1
6-
2= i
1904
'
lge$
'
1gee
'
1997
'
19~e
'
Figure 1. The step position dependence on the calendar time of measurements. Parameters of the fitted sinusoid are: Period 0,499 + 0,003 year, mean value 10,4 4-0,4eV, amplitude 4,3 40, 55 eV, phase 2,8 4- 0, 16tad.
Next two measurements of Troitsk set-up were performed in the period April- May 1999• Both showed position of the step in accordance with the same sinusoid. (See Fig. 2, poins Run28-1 and Run28-2.) Existence of the short period fluctuations in Run26 may signify more complicated picture of step variations than single period sinusoid, but any way the half year period seems to be dominant in present set of the data. At the moment it seems to be impossible to propose any "customary" explanation of this phenomenon. The proximity of the oscillation period of the step (bump) to half period of Earth circulation around the Sun remind us of an old speculation about an effect produced by capture of the cosmological degenerated neutrino by tritium with emission of almost monochromatic electrons [4]. In order to produce the bump intensity, corresponding to 10- l ° of the total decay rate it is necessary to suppose existence of neutrino cloud with density as high as 0, 5.10ZSv/cm s • Observation of bump below end point of beta spectrum corresponds to capture of neutrino with
. . . . . . ~...; ................................... 4
Year
5
8
7
0
9
'i0
11
12
Time, month
Figure 2. The plot of step positions versus time of the year. Fitted sinusoid is the same as in Fig. 1, but with the period being 0.500 year. Horizontal bars are lengh of the run. Indices of the points indicate the number of the run. Indices ns and sh refer to sub run carried out biased (on +15V) tritium source, and such without it. Q3 and Q4 Mainz group data.
negative energy, and to assumption of binding of neutrino in the cloud• In the case of binding energy changing over the cloud,the Earth in its movement produces the periodical modulation of binding energy and correspondingly position of the step. The size of the neutrino cloud in this case must be comparable with Earth orbit and it does not contradict to average density of relic neutrino in the Universe. Of course this explanation of step phenomenon is extremely speculative and may be considered only for stimulation of further experiments. 4.
U p p e r l i m i t for n e u t r i n o m a s s
The procedure adopted for deduction of the neutrino mass consisted in addition to theoretical spectrum of the step function with two variable parameters assuming that such addition may describe in the first approximation local enhance-
V.M. Lobashev et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 275-277
277
Final systematics error is obtained by averaging systematic errors of each run weighted by corresponding fit error. From here one may obtain a standard upper limit for my:
12,
10"
my < 2,5eV/c2; (95%C.L.)
8.
(2)
E
5. A c k n o w l e d g m e n t s
-+ !
!
!
i
i
|
|
i
i
i
|
,
This work was partially supported by the Russia~ Foundation for Basic Research (grants 3903 and 18401a), by Program for Fundamental Nuclear Physics. The authors are very thankful to leader of "Mainz neutrino" group Prof. E.W.Otten for financial support, which was crucial for carrying out measurements in April-May 1999.
i
Jan Feb Mar AIx May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 3. Step size versus time of the year. All the size values are reduced to the same intensity of the source.
ment in the beta-spectrum near end-point. The my2 effect unlike the local enhancement appears as an addition to (for negative m~) or deficiency (positive m~) of the spectrum that is linearly increasing with F_~ - E. This difference allows to separate both effects in fit procedure. Of course the size and position of the step being introduced as a free parameter, correlate with m~ and it increases the final error of neutrino mass thus acting as a kind of systematic error. This increase sufficiently compensates the uncertainty introduced by substitution of an a priory unknown anomaly shape by the step-like function. Others systematic errors come mostly from the uncertainties of parameters of the correction factors which are introduced in the spectrum before the fit. A remarkable property of the total systematic error from these factors is its reduction when lower energy of the fit interval comes nearer to end-point. Taking into account that fit error of m~ at the same time increases, one may select the optimal fit interval, when the total error, including both the fit and systematic error taken in quadrature, is minimal. The result for mg for the runs where correlation of m 2 and step is not large is: m "2 r = _ l , O = t = 3 , 0 f i t =l=2,1systeV2/c 4
(1)
REFERENCES 1. A.I. Belesev et al., Phys. Lett., B 350(1995) 263. 2. V. M. Lobashev et al., Phys.Lett.,B 460(1999)227. 3. Ch. Weinheimer et. al., Phys. Lett., B 460(1999) 219. 4. G. J. Stephenson Jr., T. Goldman and B.H.J.McKellar, Int. Jour. Mod. Phys. A, Vol. 13, No. 16, (1998) 2765-2790. 5. R.N. Mohapatra and S. Nussinov, Phys. Lett. B 395 (1997) 63-68.