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Bolometric direct dark matter search at the University of Tokyo
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SUPPLEMENTS
ELSEVIER
Nuclear
Physics
B (Proc.
Suppl.)
51B (1996)
314-317
Bolometric direct dark matter search at the University of Tokyo Makoto MINOWAab, Yutaka ITOa, Wataru OOTANIa, Takayuki WATANABEa, Yoshizumi INOUEbC and Youiti OOTUKAd *Department Japan
of Physics, School of Science, University
Keiji NISHIGAKIa,
Yasuhiro KISHIMOTOa,
of Tokyo, 7-3-l Hongo, Bunkyoku,
Tokyo 113,
bResearch Center for the Early Universe, School of Science, University of Tokyo, 7-3-l Hongo, Bunkyo-ku, Tokyo 113, Japan ‘International Center for Elementary Tokyo 113, Japan dCryogenic Center, University
Particle Physics, University
of Tokyo, 7-3-l Hongo, Bunkyoku,
of Tokyo, 2-11-16 Yayoi, Bunkyc+ku, Tokyo 113, Japan
A direct dark matter search experiment is now under preparation at the University of Tokyo. An array of LiF bolometers with a total mass of roughly one kilogram is to be used aiming at a direct detection of spin-dependently interacting supersymmetric neutralinos. A dedicated low-radioactivity dilution refrigerator for the bolometer has been constructed along with a small on-line helium requefier to help the detector operation in the underground laboratory where liquid helium supply is not always available. A long term test operation of a bolometer at the University of Tokyo proved the stability of the detector for more than 45 days. The detector will be installed in the Nokogiri-yama underground laboratory of the Institute for Cosmic Kay Research of the University of Tokyo. The laboratory is 200 m w.e. deep and located about 60 km to the south of Tokyo.
1. Dark
Matter
Candidates
The observation of a flat rotation curve of our Milky Way galaxy suggests that the dark matter exists in our neighbourhood with a mass density of p - 0.3 GeV/cm3. The velocity of the dark matter is usually assumed to have a Maxwellian distribution with an rms velocity of uc - 320 km/s. The primordial nucleosynthesis theory of the cosmology implies the dark matter be mostly non-baryonic although micro lensing searches for baryonic dark matter are ongoing recently by some groups. Many kinds of non-baryonic dark matter candidates have been proposed so far, ranging from massive neutrinos to odd particles like cosmions introduced to fix the solar neutrino deficit problem. As a result of studies with and without particle accelerators to detect such particles, there remain only three candidates, supersymmetric neu0920-5632/96/$15.00 PZZSO920-5632(96)00522-l
Copyright
@I1996
Elsevier
Science
B.V.
tralinos, axions and light neutrinos. Our aim is to detect nuclear recoils caused by the elastic scattering of the possible neutralino dark matter with a cryogenic detector installed underground. The neutralino is a generic name of supersymmetric partners of electrically neutral bosons. It is represented as a superposition of four such partners; two gaugincs B and I@s, and two higgsinos I$’ and @. There are four independent neutralinos among which the lightest one is stable and considered to be the dark matter candidate. The minimal super symmetric models have four basic parameters, the gaugino mass parameters Mi , M2, higgsino mixing parameter p and vacuum expectation value ratio of two Higgs doublets tan j3 = (uz/q). If one assumes the grand unified theories, then another constraint is given to them; MI = i tan2 &,vMs. We are, in this case, left with three parameters, Mz, ,u and tanp. Once theses three parameters are given, many All rights
reserved.
M. Minowa
et al. /Nuclear
Physics
things are fixed. The mass of the neutralino, interaction nature with matter, and so on. We are going to search for the neutralinos in this parameter space. We should be aware of the existing restrictions on the mass of the neutralino, Mg > 20GeV made by accelerator searches of super symmetric particles. 2. Direct
Detection
The detection rates of the neutralinos with a certain kind of detector material have been calculated by several authors. The rate R can be separated into two independent parts. Using spindependent rate Rs.-,. and spin-independent rate RsJ.,R is written as[l]
where pz and MN are the local mass density of the neutralinos and the nuclear mass of the de tector, respectively. Rs.1.is proportional to the square of the number of nucleons in a nuclus of the detector material. Heavy nuclei, therefore, are favourable to the spin-independently interacting neutralinos. On the other hand, Rs.D,is a function of the spin and so called Lande factor of the nucleus used in the detector. The latter measures the contribution of unpaired nucleon spin to the nuclear spin. It is also a function of the quark spin contents of the unpaired nucleon. Ellis and Flores[l] calculated key factors in Rs.D,for various nuclides. It is now a common understanding that the best material is “F for the detection of the spin-dependently interacting neutralinos. In order to make the detector sensitive to the neutralinos with spin dependent interaction, We adopted lithium fluoride(LiF) as the detection material. We became to learn that the spin dependently interacting neutralino is favourable to the observed CDF event as reported by Kane in this symposium[2]. As implied by the mean velocity of the dark matter particles, expected kinetic energy of the nuclei recoiled by the dark matter particles is calculated to be order of 10 keV[3]. Conventional
B (Pmt.
Suppl.)
5lB
(1996)
314-317
315
ionization particle detectors suffer from quenching, resulting in relatively high effective energy threshold for the nuclear recoil detection. The bolometer has no such shortcomings. 3. LIF Bolometers
at Tokyo
In 1993, we constructed a prototype bolometer with a 2.8-gram LiF absorber with an energy threshold of 4 keV operated at 12 mK[4]. Since then, several improvements have been done with the detector system. Home-brewed high-sensitivity neutron transmutation doped germanium thermistors (NTD Ge) have been developed at the University of Tokyo[5]. With these NTD Ge thermistors we will be able to increase the mass of the absorber by a factor of ten. Construction of a larger bolometer is just completed, and the test operation is ongoing now. We plan to make an array of 27 pieces of 30-gram LiF absorbers with a total mass of roughly one kilogram. A part of y-ray background could be rejeted if multiple Compton scatterings occur in the array. A dedicated low-radioactivity dilution refrigerator has been also constructed. To avoid obvious radioactivity in common materials like aluminum, stainless steel and fibre reinforced plastics used in ordinary refrigerators, our dewar is made mostly of oxigen free copper which is known to be the best metal material from the view point of the radioactivity. Every raw material for the refrigerator was radioassayed by a low-background yray spectrometer prior to the assembly. No detectable radio activity is allowed especially for the material around the absorber. We also got a small on-line helium requefier with a requefying rate of 500 cm3/hour. It will be used when the detector is installed in the underground laboratory, and should help the detector operation even if liquid helium supply in not routinely available. 4. Detector
Performance
With the dedicated low-activity refrigerator, a long term test operation of a small bolometer is ongoing at the University of Tokyo. The absorber
M. Minowa et al. /Nuclear Physics B (Proc. SuppI.) 51B (1996) 314-317
316
is a 0.5-gram CdTe crystal, which we intend to use for the double /3+ decay experiment [6], a positron version of the double p decay experiments. By the time of this symposium, it had been running more than 45 days without any interruption. The temperature of the refrigerator and the detector gain had been periodically monitored. The detector response to various standard y ray sources are plotted as a function of time in Fig.1. We found the gain changes in this period were less than 0.5%. The temperature drift in the mixing chamber stage, to which the bolometer is attached, was also less than 0.25 mK in this period as is seen in Fig.2.
h
t
I
Time ( days )
v) 2.5
.:..::.T... ... . ... .. . -Y
i
E .-m
..m (&Ii0
and l&.1
kev)
_
2 l _ _
=ti
,$
1.5
.I.. :....
‘Co
_
_
_
_
_
_
_
_
.
.
_
Figure 2. Temperature a function of time.
of the mixing chamber as
17173.21 and 1332.48 keV1
................... .... ............. ............. .......... ..b5znfi.,,s.s*ke v,.................: ..
located about 60 km to the south of Tokyo. We : I,,,, i,;,,i,,,;i,.,;;i ,,,,-i.;-,;-,.i-;,j are going to try engineering runs for a couple of OS E,,,, ,,,, I,,,, years to make experiences to operate the cryo0 5 1015202530354045 genic detectors underground. We studied environmental neutron background in the laboratory, Time ( days ) and estimated that the existing sensitivity limit for the spin-dependently interacting particle dark matter be overcome with 100 kgdays of running there. Figure 1. Mean pulse height for standard y-ray A measurement in a still deeper undergresources as a function of time. ound site, Kamioka for example, will enable us to reach the required sensitivity to the spindependently interacting SUSY neutralinos with apropriate shieldings around the detector. 5. Prospects There is a considerable possibility that the Our next step is to install the detector at the lightest neutralino is a spin-dependently interactNokogiri-yama underground laboratory of the Ining neutralino[2]. If this is the case our bolometer stitute for Cosmic Ray Research of the University would be one of the most sensitive detectors for of Tokyo. The laboratory is 200 m w.e. deep and the dark matter neutralinos.
M. Minowa et al. /Nuclear Physics B (Proc. Suppl.) 5IB (1996) 314-317
ACKNOWLEDGEMENTS This work is supported by the Iwatani Naoji Foundation’s Research Grant, the Grant-in-Aid in Scientific Research (A) and the Grant-in-Aid for COE Research by the Japanese Ministry of Education, Science and Culture, and the Yamada Science Foundation. REFERENCES 1. J. Ellis and R. A. Flores, Phys. Lett. B263 (1991) 259, and CERN Preprint CERN-TH6588/92. 2. G. Kane, Talk in this symposium. 3. P. F. Smith and J. D. Lewin, Phs. Rep. 187 (1990) 203. 4. M. Minowa et al., J.of Low Temp. Phys. 93 (1993) 803, and M. Minowa et al., Nucl. Instr. Methods Phys. Res. A327 (1993) 612. 5. W. Ootani et al., preprint RESCEU 9511(1995), (to be published in Nucl. Instr. Methods Phys. Res. A). 6. Y. Ito et al., preprint RESCEU 96-13(1996).
317
SUPPLEMENTS
ELSEVIER
Nuclear
Physics
B (Proc.
Suppl.)
51B (1996)
314-317
Bolometric direct dark matter search at the University of Tokyo Makoto MINOWAab, Yutaka ITOa, Wataru OOTANIa, Takayuki WATANABEa, Yoshizumi INOUEbC and Youiti OOTUKAd *Department Japan
of Physics, School of Science, University
Keiji NISHIGAKIa,
Yasuhiro KISHIMOTOa,
of Tokyo, 7-3-l Hongo, Bunkyoku,
Tokyo 113,
bResearch Center for the Early Universe, School of Science, University of Tokyo, 7-3-l Hongo, Bunkyo-ku, Tokyo 113, Japan ‘International Center for Elementary Tokyo 113, Japan dCryogenic Center, University
Particle Physics, University
of Tokyo, 7-3-l Hongo, Bunkyoku,
of Tokyo, 2-11-16 Yayoi, Bunkyc+ku, Tokyo 113, Japan
A direct dark matter search experiment is now under preparation at the University of Tokyo. An array of LiF bolometers with a total mass of roughly one kilogram is to be used aiming at a direct detection of spin-dependently interacting supersymmetric neutralinos. A dedicated low-radioactivity dilution refrigerator for the bolometer has been constructed along with a small on-line helium requefier to help the detector operation in the underground laboratory where liquid helium supply is not always available. A long term test operation of a bolometer at the University of Tokyo proved the stability of the detector for more than 45 days. The detector will be installed in the Nokogiri-yama underground laboratory of the Institute for Cosmic Kay Research of the University of Tokyo. The laboratory is 200 m w.e. deep and located about 60 km to the south of Tokyo.
1. Dark
Matter
Candidates
The observation of a flat rotation curve of our Milky Way galaxy suggests that the dark matter exists in our neighbourhood with a mass density of p - 0.3 GeV/cm3. The velocity of the dark matter is usually assumed to have a Maxwellian distribution with an rms velocity of uc - 320 km/s. The primordial nucleosynthesis theory of the cosmology implies the dark matter be mostly non-baryonic although micro lensing searches for baryonic dark matter are ongoing recently by some groups. Many kinds of non-baryonic dark matter candidates have been proposed so far, ranging from massive neutrinos to odd particles like cosmions introduced to fix the solar neutrino deficit problem. As a result of studies with and without particle accelerators to detect such particles, there remain only three candidates, supersymmetric neu0920-5632/96/$15.00 PZZSO920-5632(96)00522-l
Copyright
@I1996
Elsevier
Science
B.V.
tralinos, axions and light neutrinos. Our aim is to detect nuclear recoils caused by the elastic scattering of the possible neutralino dark matter with a cryogenic detector installed underground. The neutralino is a generic name of supersymmetric partners of electrically neutral bosons. It is represented as a superposition of four such partners; two gaugincs B and I@s, and two higgsinos I$’ and @. There are four independent neutralinos among which the lightest one is stable and considered to be the dark matter candidate. The minimal super symmetric models have four basic parameters, the gaugino mass parameters Mi , M2, higgsino mixing parameter p and vacuum expectation value ratio of two Higgs doublets tan j3 = (uz/q). If one assumes the grand unified theories, then another constraint is given to them; MI = i tan2 &,vMs. We are, in this case, left with three parameters, Mz, ,u and tanp. Once theses three parameters are given, many All rights
reserved.
M. Minowa
et al. /Nuclear
Physics
things are fixed. The mass of the neutralino, interaction nature with matter, and so on. We are going to search for the neutralinos in this parameter space. We should be aware of the existing restrictions on the mass of the neutralino, Mg > 20GeV made by accelerator searches of super symmetric particles. 2. Direct
Detection
The detection rates of the neutralinos with a certain kind of detector material have been calculated by several authors. The rate R can be separated into two independent parts. Using spindependent rate Rs.-,. and spin-independent rate RsJ.,R is written as[l]
where pz and MN are the local mass density of the neutralinos and the nuclear mass of the de tector, respectively. Rs.1.is proportional to the square of the number of nucleons in a nuclus of the detector material. Heavy nuclei, therefore, are favourable to the spin-independently interacting neutralinos. On the other hand, Rs.D,is a function of the spin and so called Lande factor of the nucleus used in the detector. The latter measures the contribution of unpaired nucleon spin to the nuclear spin. It is also a function of the quark spin contents of the unpaired nucleon. Ellis and Flores[l] calculated key factors in Rs.D,for various nuclides. It is now a common understanding that the best material is “F for the detection of the spin-dependently interacting neutralinos. In order to make the detector sensitive to the neutralinos with spin dependent interaction, We adopted lithium fluoride(LiF) as the detection material. We became to learn that the spin dependently interacting neutralino is favourable to the observed CDF event as reported by Kane in this symposium[2]. As implied by the mean velocity of the dark matter particles, expected kinetic energy of the nuclei recoiled by the dark matter particles is calculated to be order of 10 keV[3]. Conventional
B (Pmt.
Suppl.)
5lB
(1996)
314-317
315
ionization particle detectors suffer from quenching, resulting in relatively high effective energy threshold for the nuclear recoil detection. The bolometer has no such shortcomings. 3. LIF Bolometers
at Tokyo
In 1993, we constructed a prototype bolometer with a 2.8-gram LiF absorber with an energy threshold of 4 keV operated at 12 mK[4]. Since then, several improvements have been done with the detector system. Home-brewed high-sensitivity neutron transmutation doped germanium thermistors (NTD Ge) have been developed at the University of Tokyo[5]. With these NTD Ge thermistors we will be able to increase the mass of the absorber by a factor of ten. Construction of a larger bolometer is just completed, and the test operation is ongoing now. We plan to make an array of 27 pieces of 30-gram LiF absorbers with a total mass of roughly one kilogram. A part of y-ray background could be rejeted if multiple Compton scatterings occur in the array. A dedicated low-radioactivity dilution refrigerator has been also constructed. To avoid obvious radioactivity in common materials like aluminum, stainless steel and fibre reinforced plastics used in ordinary refrigerators, our dewar is made mostly of oxigen free copper which is known to be the best metal material from the view point of the radioactivity. Every raw material for the refrigerator was radioassayed by a low-background yray spectrometer prior to the assembly. No detectable radio activity is allowed especially for the material around the absorber. We also got a small on-line helium requefier with a requefying rate of 500 cm3/hour. It will be used when the detector is installed in the underground laboratory, and should help the detector operation even if liquid helium supply in not routinely available. 4. Detector
Performance
With the dedicated low-activity refrigerator, a long term test operation of a small bolometer is ongoing at the University of Tokyo. The absorber
M. Minowa et al. /Nuclear Physics B (Proc. SuppI.) 51B (1996) 314-317
316
is a 0.5-gram CdTe crystal, which we intend to use for the double /3+ decay experiment [6], a positron version of the double p decay experiments. By the time of this symposium, it had been running more than 45 days without any interruption. The temperature of the refrigerator and the detector gain had been periodically monitored. The detector response to various standard y ray sources are plotted as a function of time in Fig.1. We found the gain changes in this period were less than 0.5%. The temperature drift in the mixing chamber stage, to which the bolometer is attached, was also less than 0.25 mK in this period as is seen in Fig.2.
h
t
I
Time ( days )
v) 2.5
.:..::.T... ... . ... .. . -Y
i
E .-m
..m (&Ii0
and l&.1
kev)
_
2 l _ _
=ti
,$
1.5
.I.. :....
‘Co
_
_
_
_
_
_
_
_
.
.
_
Figure 2. Temperature a function of time.
of the mixing chamber as
17173.21 and 1332.48 keV1
................... .... ............. ............. .......... ..b5znfi.,,s.s*ke v,.................: ..
located about 60 km to the south of Tokyo. We : I,,,, i,;,,i,,,;i,.,;;i ,,,,-i.;-,;-,.i-;,j are going to try engineering runs for a couple of OS E,,,, ,,,, I,,,, years to make experiences to operate the cryo0 5 1015202530354045 genic detectors underground. We studied environmental neutron background in the laboratory, Time ( days ) and estimated that the existing sensitivity limit for the spin-dependently interacting particle dark matter be overcome with 100 kgdays of running there. Figure 1. Mean pulse height for standard y-ray A measurement in a still deeper undergresources as a function of time. ound site, Kamioka for example, will enable us to reach the required sensitivity to the spindependently interacting SUSY neutralinos with apropriate shieldings around the detector. 5. Prospects There is a considerable possibility that the Our next step is to install the detector at the lightest neutralino is a spin-dependently interactNokogiri-yama underground laboratory of the Ining neutralino[2]. If this is the case our bolometer stitute for Cosmic Ray Research of the University would be one of the most sensitive detectors for of Tokyo. The laboratory is 200 m w.e. deep and the dark matter neutralinos.
M. Minowa et al. /Nuclear Physics B (Proc. Suppl.) 5IB (1996) 314-317
ACKNOWLEDGEMENTS This work is supported by the Iwatani Naoji Foundation’s Research Grant, the Grant-in-Aid in Scientific Research (A) and the Grant-in-Aid for COE Research by the Japanese Ministry of Education, Science and Culture, and the Yamada Science Foundation. REFERENCES 1. J. Ellis and R. A. Flores, Phys. Lett. B263 (1991) 259, and CERN Preprint CERN-TH6588/92. 2. G. Kane, Talk in this symposium. 3. P. F. Smith and J. D. Lewin, Phs. Rep. 187 (1990) 203. 4. M. Minowa et al., J.of Low Temp. Phys. 93 (1993) 803, and M. Minowa et al., Nucl. Instr. Methods Phys. Res. A327 (1993) 612. 5. W. Ootani et al., preprint RESCEU 9511(1995), (to be published in Nucl. Instr. Methods Phys. Res. A). 6. Y. Ito et al., preprint RESCEU 96-13(1996).
317