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Latest results from the Mainz Neutrino Mass experiment
Nuclear Physics A721 (2003) 533c-536~ www.elsevier.com/locate/npe
Latest
results
from
the Mainz
Neutrino
Mass Experiment
Ch. Kraus”, J. Bonna, B. Bornscheinab, L. Bornscheinac, B. Flatt”, A. Kovalikd, B. Miiller”, E.W. Otten’, J.P. SchalP, Th. Thiimmler”, Ch. Weinheimere “Institute of Physics, Joh. Gutenberg University, 55099 Mainz/Germany bpresent address TLK Forschungszentrum Karlsruhe/Germany Cpresent address University of Karlsruhe/Germany don leave from JINR, Dubna/Russia epresent address HISKP, University Bonn/Germany The Maim Neutrino Mass experiment investigates the endpoint region of tritium /3 decay spectrum very precisely to extract the rest mass of the electron antineutrino. The measurements are performed with a MAC-E-Filter, combining Magnetic Adiabatic Collimation and an Electrostatic high pass Filter. After an optimal preparation very stable and high quality data have been taken in 2001, which do not show any residual problem. 1. The Mainz
Neutrino
Mass Experiment
Recent results from atmospheric and solar neutrinos give strong evidence that neutrinos oscillate from one flavor state into another. But v-oscillation experiments do not yield the values of the neutrino masses, they are only sensitive to differences between squared neutrino masses. The values Am:j from oscillation experiments only give lower limits on neutrino masses [I]. The kinematic of weak decays obtaining information on the neutrino mass directly and without further requirements. The Mainz Neutrino Mass Experiment is based on the principle of MAC-E-Filter [2], which combines high luminosity and high energy resolution at low background. These features are of considerable importance for the extraction of the neutrino mass from the presice measurement of the endpoint region of tritium /I spectrum. The principle of the MAC-E-filter is illustrated in figure 1: Two superconducting solonoids create a magnetic guiding field. The ,4 electrons, starting from the tritium source inside the left solenoid into the forward hemisphere, are guided on a cyclotron motion around the magnetic field lines into the spectrometer with an accepted solid angle of nearly 27r. The magnetic field strength drops from the center of the solonoid to the center of the spectrometer by several orders of magnitude. This leads to a transformation of the energy in the cyclotron motion El into energy in the longitudinal motion by the magnetic gradient force. In the center of the spectrometer, the analysing plane, the electron moments are almost perfectly aligned in the direction of the magnetic field lines. The energy in this motion El1is analysed energetically by applying an electrostatic potential formed by a system of cylindrical electrodes. All electrons with 0375-9474/03/% - see front matter 0 2003 Elsevier Science B.Y doi:lO.l016/S0375-9474(03)01117-S
All rights reserved.
Ch. Krws et al. /Nuclear Physics A721 (2QQ3)5330536~
534c
enough energy to pass the retarding potential barrier are reaccelerated onto the detector (in the right solenoid). Therefore the spectrometer works as an integrating high pass filter. The relativ energy resolution of the MAC-E-filter is given by the ratio of the minimal magnetic field Bmin in the analysing plane and the maximal magnetic field B,,, between source and spectrometer: Bmin 7na2
AE=EB=
18600 eV s
M 5.8 eV
By changing the retarding potential the p spectrum can be scanned. The Mainz setup uses a solid state source realized by a film of molecular tritium quench-candensed onto a graphite substrate (HOPG). Typical source parameters are: diameter 17 mm, thickness 45 nm (measured by Iaser ellipsometrie), activity 1 GBq. ELECTRODESDEiECT(
T*-SOURCE
N E W GUlDlNG MAGNETS
N E W HIGH FIELD ELECTRODES
Figure 1: The setup of Mainz II is shown schematically. detector is about 6 m and the diameter of the vessel is lm. 2. The
measurements
P
The distance between source and
of 2001
The Mainz II setup has a source solenoid consisting of two coils. The first coil houses the tritium film and the second one follows after a bend, so that tritium molecules evaporating from the source are trapped on the LHe cold tube. This eliminated source correlated background and allows to use a stronger source. The endpoint region of the Maim 1998 and 1999 data in comparison with the former data from 1994 is shown in figure 2. The signal to noise rate was improved by a factor of 10. Also shown are the data of 2001, which have a third of the statistic of the 98/99 data and even a lower background level. This further improvement is due to very careful preparation of the whole system. Especially all parts which need refreshment from time to time were replaced. In paticular: The graphite substrate for the tritium source, the oil for the high voltage divider, baking of all vacuum systems and reactivation of the non evaporable getter pumps. All these investigations lead to the most stable operation ever had. The background rate was about 12 mHz aver the whole periode (2 month) without the necessity of high voltage conditioning during the run. The results for fit on m:, of Mainz 2001 data as a function of the lower limit of fit interval are shown in figure 3. All values are in good agreement with each other and with the physically allowed range. To extract a limit on the neutrino mass the interval1 which
535c leads to the smallest combined statistical and systematical uncertainty (last 70 eV below endpoint) was choosen. This gives: mzc4 = +1.3 f 5.8,,,, f 2.2,y, eV2$/d.o.f. m2c4 A -1.0 f 6 . 1statf 1.7,, eV*x*/d.o.f.
for the first tritium film for the second tritium film
= 42136 = 41/36
(2)
(3)
Cimbining these measurements with the older measurements from 98/99 [3]: m2c4 = -1.6 f X,,,,
I!C2.1,
eV2x2/d.o.f.
= 125/121
mZc4 = -1.2 f 2.2,,, k 2.1,
eV2x2/d.o,f.
= 208/193
(4
one gets:
(5) which is compatible with a zero neutrino mass. This value corresponds to an upper limit on the electron neutrido mass of: n&2 4 2.2 ev
(95% CL,, uaif.uppr.)
(6) The limit on m, for 98/99 is the same but it is slightly pushed down by the negative mean value. 0.05
E:4 +
0.04
0 Moinz 94 data b Mainz 98j99 -
6
dota
fit for m.*=O
0 Mainl 2001 data
” o.I)55,.,, “ijifyyl 18.56
I%57
retarding
ahergy
18.58
Figtire 2; Averaged count rate of the 98399 data ~(filbd squares with fit (line) and the 2001 data 2open squares) in comparison with previous Mainz data from 94 (open circles) as foctioh of retarding energy near the endpoint Ee, and effective endpoint Eo,,fj. The position of the latter takes into account the width of the i’esolution function of the setup.
lkeVJ
In 2000 another measurement periode of 3 month took place, but under not very stable conditi’ons to study the behaviour of the system. Two weeks of simultaneous measurements with Qoitsk (t%.~13.1iLQ$.and 32.-2&12.02) were made. The spectrometer in %-oitsk is ak& a IvIAC~E-iliter W&I shghtly different ~dimensions, but with a windowless gaseous tritium source. J+om theit first measurement in 1994 on the TIoitsk group I’Wported about a small, but significant anomaly, the so called Troitsk anomaly [4j in their experimental spectrum. The,y &ah a rise of the count rate, starting a, few SV below the edpoint. This deep iik anmdy cormpeds to a mowekwrgetk Iine in the primary spectrum with a relative intensity of about 10-r’ of the total accepted decay rate.
Ch. Kraus et al. /Nuclear Physics A721 (2003) S33c-536~
536~
18.35 15
18.4 I.. m, ‘0
0
‘.
?
18.45 I’.
18.5 , , I.. ?ma
“ks0 0.8 0.6 0.4 0.2
-15
I . . ..I . . ..I . . . . 18.4 18.45 18.5 18.55 lower limit of fit interval
[keV1
18.35
if 1.4 1.2
0Td
n. -5 E -10
18.35
18.55 0
18.4 1.. m, ‘0
15 .y 12
lo 5
c.. -5 E -10
-
6
”
18.45 I
*
“.
18.5 1.. x’..
1.8 1.6 1.4 1.2
+++
‘1
-15 18.35
18.55 o
I . . . . . . . . . I 18.4 18.45 18.5 lower limit of fit intervol
’ 2 0.8 0.6 0.4 0.2
. . . 18.55
[keVl
Figure 3: Mainz fit results on rnt, (filled circles, left scale) as a function of the lower limit of fit interval (the upper bound is fixed at 18.66 keV, well above Ee) for the two different tritium films of 2001. The errorbars show the statistical uncertainties (inner bar) and the total uncertainties (outer bar). The corresponding values for the xred-x 2 - 2Jd.o. f. is given on the right scale (open circles). The position below the endpoint as well as the amplitude of this anomaly variies with time. There is no indication of such a step like anomaly in the Mainz data of 2000 and 2001 (nore in 98/99 data). To check this two more free parameters (position below endpoint and amplitude) are introduced in the fit procedure. If this is a better description of the measured spectrum one would expect a significant improvement in x2 by scanning it as function of the position below endpoint. The Mainz data show no significant improvement in x2, supporting clearly the assumption, that the Troitsk anomaly is caused by an unknown experimental artefact. 4. Summary The presice measurement of the endpoint region of the p decay spectrum of Tz by the Mainz Neutrino Mass Experiment lead to an upper limit for the electron neutrino mass of 2.2 eV (final (re-)analysis in preparation). Especially the synchronous measurements at Troitsk and Mainz show that the step like Troitsk anomaly is an experimental artefact. As the Mainz Experiment almost reached its sensitivity limit it was shut down and the spectrometer was changed for test experiments in view of KATRIN [5]. To determine the mass of the electron antineutrino with sub-eV sensitivity a large tritium p decay experiment using a MAC-E-Filter is being prepared by the KATRIN collaboration. This work was supported by the Deutsche Forschunsgemeinschaft Ot33/13 and y the German Bundesministerium fiir Bildung und Forschung 06M/8661/5. 5. References l] Q.R. Ahmad et. al., Phys. Rev. Lett. 8’7 (2001) 071301 21 A. Picard et al., Nucl. Inst. Meth. B63 (1992) 345 3] J. Bonn et al., Nucl. Phys. B (Proc. Suppl.) 91 (2001) 273. 41 V.M. Lobashev et al., Nucl. Phys. B (Proc. Suppl.) 91 (2001) 280. 51 KATRIN letter of intent, hep-ex/0109033, (2001).
Latest
results
from
the Mainz
Neutrino
Mass Experiment
Ch. Kraus”, J. Bonna, B. Bornscheinab, L. Bornscheinac, B. Flatt”, A. Kovalikd, B. Miiller”, E.W. Otten’, J.P. SchalP, Th. Thiimmler”, Ch. Weinheimere “Institute of Physics, Joh. Gutenberg University, 55099 Mainz/Germany bpresent address TLK Forschungszentrum Karlsruhe/Germany Cpresent address University of Karlsruhe/Germany don leave from JINR, Dubna/Russia epresent address HISKP, University Bonn/Germany The Maim Neutrino Mass experiment investigates the endpoint region of tritium /3 decay spectrum very precisely to extract the rest mass of the electron antineutrino. The measurements are performed with a MAC-E-Filter, combining Magnetic Adiabatic Collimation and an Electrostatic high pass Filter. After an optimal preparation very stable and high quality data have been taken in 2001, which do not show any residual problem. 1. The Mainz
Neutrino
Mass Experiment
Recent results from atmospheric and solar neutrinos give strong evidence that neutrinos oscillate from one flavor state into another. But v-oscillation experiments do not yield the values of the neutrino masses, they are only sensitive to differences between squared neutrino masses. The values Am:j from oscillation experiments only give lower limits on neutrino masses [I]. The kinematic of weak decays obtaining information on the neutrino mass directly and without further requirements. The Mainz Neutrino Mass Experiment is based on the principle of MAC-E-Filter [2], which combines high luminosity and high energy resolution at low background. These features are of considerable importance for the extraction of the neutrino mass from the presice measurement of the endpoint region of tritium /I spectrum. The principle of the MAC-E-filter is illustrated in figure 1: Two superconducting solonoids create a magnetic guiding field. The ,4 electrons, starting from the tritium source inside the left solenoid into the forward hemisphere, are guided on a cyclotron motion around the magnetic field lines into the spectrometer with an accepted solid angle of nearly 27r. The magnetic field strength drops from the center of the solonoid to the center of the spectrometer by several orders of magnitude. This leads to a transformation of the energy in the cyclotron motion El into energy in the longitudinal motion by the magnetic gradient force. In the center of the spectrometer, the analysing plane, the electron moments are almost perfectly aligned in the direction of the magnetic field lines. The energy in this motion El1is analysed energetically by applying an electrostatic potential formed by a system of cylindrical electrodes. All electrons with 0375-9474/03/% - see front matter 0 2003 Elsevier Science B.Y doi:lO.l016/S0375-9474(03)01117-S
All rights reserved.
Ch. Krws et al. /Nuclear Physics A721 (2QQ3)5330536~
534c
enough energy to pass the retarding potential barrier are reaccelerated onto the detector (in the right solenoid). Therefore the spectrometer works as an integrating high pass filter. The relativ energy resolution of the MAC-E-filter is given by the ratio of the minimal magnetic field Bmin in the analysing plane and the maximal magnetic field B,,, between source and spectrometer: Bmin 7na2
AE=EB=
18600 eV s
M 5.8 eV
By changing the retarding potential the p spectrum can be scanned. The Mainz setup uses a solid state source realized by a film of molecular tritium quench-candensed onto a graphite substrate (HOPG). Typical source parameters are: diameter 17 mm, thickness 45 nm (measured by Iaser ellipsometrie), activity 1 GBq. ELECTRODESDEiECT(
T*-SOURCE
N E W GUlDlNG MAGNETS
N E W HIGH FIELD ELECTRODES
Figure 1: The setup of Mainz II is shown schematically. detector is about 6 m and the diameter of the vessel is lm. 2. The
measurements
P
The distance between source and
of 2001
The Mainz II setup has a source solenoid consisting of two coils. The first coil houses the tritium film and the second one follows after a bend, so that tritium molecules evaporating from the source are trapped on the LHe cold tube. This eliminated source correlated background and allows to use a stronger source. The endpoint region of the Maim 1998 and 1999 data in comparison with the former data from 1994 is shown in figure 2. The signal to noise rate was improved by a factor of 10. Also shown are the data of 2001, which have a third of the statistic of the 98/99 data and even a lower background level. This further improvement is due to very careful preparation of the whole system. Especially all parts which need refreshment from time to time were replaced. In paticular: The graphite substrate for the tritium source, the oil for the high voltage divider, baking of all vacuum systems and reactivation of the non evaporable getter pumps. All these investigations lead to the most stable operation ever had. The background rate was about 12 mHz aver the whole periode (2 month) without the necessity of high voltage conditioning during the run. The results for fit on m:, of Mainz 2001 data as a function of the lower limit of fit interval are shown in figure 3. All values are in good agreement with each other and with the physically allowed range. To extract a limit on the neutrino mass the interval1 which
535c leads to the smallest combined statistical and systematical uncertainty (last 70 eV below endpoint) was choosen. This gives: mzc4 = +1.3 f 5.8,,,, f 2.2,y, eV2$/d.o.f. m2c4 A -1.0 f 6 . 1statf 1.7,, eV*x*/d.o.f.
for the first tritium film for the second tritium film
= 42136 = 41/36
(2)
(3)
Cimbining these measurements with the older measurements from 98/99 [3]: m2c4 = -1.6 f X,,,,
I!C2.1,
eV2x2/d.o.f.
= 125/121
mZc4 = -1.2 f 2.2,,, k 2.1,
eV2x2/d.o,f.
= 208/193
(4
one gets:
(5) which is compatible with a zero neutrino mass. This value corresponds to an upper limit on the electron neutrido mass of: n&2 4 2.2 ev
(95% CL,, uaif.uppr.)
(6) The limit on m, for 98/99 is the same but it is slightly pushed down by the negative mean value. 0.05
E:4 +
0.04
0 Moinz 94 data b Mainz 98j99 -
6
dota
fit for m.*=O
0 Mainl 2001 data
” o.I)55,.,, “ijifyyl 18.56
I%57
retarding
ahergy
18.58
Figtire 2; Averaged count rate of the 98399 data ~(filbd squares with fit (line) and the 2001 data 2open squares) in comparison with previous Mainz data from 94 (open circles) as foctioh of retarding energy near the endpoint Ee, and effective endpoint Eo,,fj. The position of the latter takes into account the width of the i’esolution function of the setup.
lkeVJ
In 2000 another measurement periode of 3 month took place, but under not very stable conditi’ons to study the behaviour of the system. Two weeks of simultaneous measurements with Qoitsk (t%.~13.1iLQ$.and 32.-2&12.02) were made. The spectrometer in %-oitsk is ak& a IvIAC~E-iliter W&I shghtly different ~dimensions, but with a windowless gaseous tritium source. J+om theit first measurement in 1994 on the TIoitsk group I’Wported about a small, but significant anomaly, the so called Troitsk anomaly [4j in their experimental spectrum. The,y &ah a rise of the count rate, starting a, few SV below the edpoint. This deep iik anmdy cormpeds to a mowekwrgetk Iine in the primary spectrum with a relative intensity of about 10-r’ of the total accepted decay rate.
Ch. Kraus et al. /Nuclear Physics A721 (2003) S33c-536~
536~
18.35 15
18.4 I.. m, ‘0
0
‘.
?
18.45 I’.
18.5 , , I.. ?ma
“ks0 0.8 0.6 0.4 0.2
-15
I . . ..I . . ..I . . . . 18.4 18.45 18.5 18.55 lower limit of fit interval
[keV1
18.35
if 1.4 1.2
0Td
n. -5 E -10
18.35
18.55 0
18.4 1.. m, ‘0
15 .y 12
lo 5
c.. -5 E -10
-
6
”
18.45 I
*
“.
18.5 1.. x’..
1.8 1.6 1.4 1.2
+++
‘1
-15 18.35
18.55 o
I . . . . . . . . . I 18.4 18.45 18.5 lower limit of fit intervol
’ 2 0.8 0.6 0.4 0.2
. . . 18.55
[keVl
Figure 3: Mainz fit results on rnt, (filled circles, left scale) as a function of the lower limit of fit interval (the upper bound is fixed at 18.66 keV, well above Ee) for the two different tritium films of 2001. The errorbars show the statistical uncertainties (inner bar) and the total uncertainties (outer bar). The corresponding values for the xred-x 2 - 2Jd.o. f. is given on the right scale (open circles). The position below the endpoint as well as the amplitude of this anomaly variies with time. There is no indication of such a step like anomaly in the Mainz data of 2000 and 2001 (nore in 98/99 data). To check this two more free parameters (position below endpoint and amplitude) are introduced in the fit procedure. If this is a better description of the measured spectrum one would expect a significant improvement in x2 by scanning it as function of the position below endpoint. The Mainz data show no significant improvement in x2, supporting clearly the assumption, that the Troitsk anomaly is caused by an unknown experimental artefact. 4. Summary The presice measurement of the endpoint region of the p decay spectrum of Tz by the Mainz Neutrino Mass Experiment lead to an upper limit for the electron neutrino mass of 2.2 eV (final (re-)analysis in preparation). Especially the synchronous measurements at Troitsk and Mainz show that the step like Troitsk anomaly is an experimental artefact. As the Mainz Experiment almost reached its sensitivity limit it was shut down and the spectrometer was changed for test experiments in view of KATRIN [5]. To determine the mass of the electron antineutrino with sub-eV sensitivity a large tritium p decay experiment using a MAC-E-Filter is being prepared by the KATRIN collaboration. This work was supported by the Deutsche Forschunsgemeinschaft Ot33/13 and y the German Bundesministerium fiir Bildung und Forschung 06M/8661/5. 5. References l] Q.R. Ahmad et. al., Phys. Rev. Lett. 8’7 (2001) 071301 21 A. Picard et al., Nucl. Inst. Meth. B63 (1992) 345 3] J. Bonn et al., Nucl. Phys. B (Proc. Suppl.) 91 (2001) 273. 41 V.M. Lobashev et al., Nucl. Phys. B (Proc. Suppl.) 91 (2001) 280. 51 KATRIN letter of intent, hep-ex/0109033, (2001).