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Physics with the large liquid-scintillator detector LENA
Progress in Particle and Nuclear Physics 64 (2010) 381–383
Contents lists available at ScienceDirect
Progress in Particle and Nuclear Physics journal homepage: www.elsevier.com/locate/ppnp
Review
Physics with the large liquid-scintillator detector LENA T. Lachenmaier a,b,∗ , F. von Feilitzsch a , M. Göger-Neff a , T. Lewke a , T. Marrodán Undagoitia a,c , Q. Meindl a , R. Möllenberg a , L. Oberauer a , J. Peltoniemi b , W. Potzel a , M. Tippmann a , J. Winter a , M. Wurm a a
Physik Department E15, Technische Universität München, Garching, Germany
b
Excellence Cluster Universe, Technische Universität München, Garching, Germany
c
Physik-Institut, Universität Zürich, Zürich, Switzerland
article
info
Keywords: Neutrino astronomy Liquid-scintillator detector Proton decay LENA
abstract A large liquid-scintillator detector with 50 kt target mass in an underground location of at least 4000 m.w.e. is considered as a unique tool for low-energy neutrino detection and the search for rare events from astrophysical sources and up to now unobserved processes beyond the Standard Model. In this contribution, the physics potential of LENA for selected topics is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A 50 kt detector for low-energy neutrino astronomy (LENA) has been proposed to address fundamental questions in particle astrophysics, elementary particle physics, and geophysics [1]. The physics goals of LENA include the search for proton decay with unprecedented sensitivity, detection of the diffuse supernova neutrino background, time-resolved flavorspecific detection of galactic supernova neutrinos, indirect WIMP detection, high-statistics solar neutrino spectroscopy, and detection of geo neutrinos. 2. Detector design The current detector design is based on a large volume of liquid scintillator with vertical cylindrical shape, with a diameter of 30 m and a height of approximately 100 m. A photo coverage of 30% is achieved if the inner volume of the detector is instrumented with e.g. 13,000 photomultiplier tubes of 20 inch diameter. Using 50 kt of a PXE or LAB (linear alkyl benzene) based scintillator with high light yield, this coverage corresponds to a photoelectron yield of at least 150 p.e./MeV. Both options have been characterized in terms of fluorescence decay-time constants, attenuation and scattering lengths as the exact light yield is dependent on the latter two [2,3]. The outer part of the detector is filled with water as shielding and is a Cherenkov detector for cosmic muons. To benefit from the full physics potential, the detector should be placed underground at a depth of 4000 m.w.e. or deeper. 3. Physics potential 3.1. Proton decay Among the primary goals of all large, liquid based, underground detectors is the search for proton decay. Liquidscintillator detectors offer one particular advantage over water Cerenkov detectors: the energy deposit of the charged
∗
Corresponding author at: Physik Department E15, Technische Universität München, Garching, Germany. E-mail address: [email protected] (T. Lachenmaier).
0146-6410/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2009.12.054
382
T. Lachenmaier et al. / Progress in Particle and Nuclear Physics 64 (2010) 381–383
kaon created in the decay channel p → K + ν¯ , which is favored by some supersymmetric theories, can be detected directly. The subsequent kaon decay provides a very fast coincidence signal which allows for a efficient suppression of the atmospheric neutrino background in the relevant energy range. Monte Carlo simulations have been performed and it has been estimated that with this suppression, less than one count in 10 years due to atmospheric neutrinos is expected while still maintaining 65% efficiency for proton decay events [4]. In the absence of a signal in 10 years the 90% C.L. lifetime limit could be increased to τ > 4 · 1034 y for this decay mode. This is an order of magnitude improvement over the current best limit by the Super-Kamiokande experiment [5]. 3.2. Neutrino signal from a galactic supernova LENA will allow for time-resolved and flavor-specific detection of supernova (SN) neutrinos in the case of a galactic core-collapse SN explosion. A total of 10,000–15,000 events is expected for an SN explosion of an 8 M progenitor star at a distance of 10 kpc, depending on the initial neutrino spectra and matter effects [6]. The dominant detection channel is the inverse beta decay (IBD) ν¯ e p → ne+ , but a variety of other possible reactions can be exploited with useful signal rates. These will provide invaluable information on the astrophysics of the core-collapse explosion and on the neutrino mixing parameters. The νe flux can be determined using the CC reaction νe + 12 C → 12 N + e− after statistical subtraction of the ν¯ e -induced charged current reaction on 12 C with very similar signature. Neutral current processes, sensitive to all neutrino flavors, would give information on the total flux: νX + 12 C → 12 C∗ + νX is a pure flux measurement without spectral information, while the recoil spectra of elastic electron scattering and proton scattering reflect the incoming SN neutrino spectrum. Due to the strong dependence of the measured event rate on the mean neutrino energy, proton scattering is very sensitive to the temperature of the SN neutrinosphere. 3.3. Diffuse supernova neutrino background In the absence of a galactic SN as a spectacular neutrino source within the operational time of LENA, the cumulative neutrino flux from all past SNe in the Universe, the so-called diffuse supernova neutrino background (DSNB) can be used to investigate the core-collapse explosions mechanism. The DSNB signal is sensitive to the evolution of the SN rate, which is closely related to the star formation rate. In LENA, the electron antineutrino part of this signal can be measured using the IBD reaction on protons with the benefit of the delayed coincidence signal from the captured neutron. If LENA was installed at Pyhäsalmi (Finland), there would be an observational energy window from 9.7 to 25 MeV that is given by the remaining background of atmospheric and reactor neutrinos that are indistinguishable from the actual DSNB signal. For the most likely value of the SN rate, 6 to 13 events per year will be contained in this window which is almost free of background [7]. This would be the first detection of the DSNB. Within ten years of exposure, it will be possible to derive constraints on both core-collapse supernova models and the supernova rate in the near Universe up to redshifts of z < 2. 3.4. Solar and geo neutrinos in LENA If a level of radioimpurities similar to that in the Borexino experiment is reached, solar neutrinos will be detected via neutrino electron scattering with event rates exceeding the Borexino rates by at least two orders of magnitude even if the fiducial volume is reduced to increase the shielding against external gamma background. A rate of about 10,000 7 Be events per day is expected. It might be of interest to search for temporal variations in the 7 Be flux. The statistics of one year exposure is sufficient to identify a modulation of 1.5%. The detection of CNO and pep neutrinos delicately depends on the background level induced by the decays of cosmogenic 11 C. At Pyhäsalmi, the ration of the combined CNO/pep signal to 11 C background would be a factor five better than in Borexino. 3.5. Indirect WIMP detection The observational window for neutrinos from dark matter annihilation is very similar to the one for the observation of the DSNB: it is mainly determined by the remaining fluxes of reactor, diffuse SN and atmospheric neutrinos. It has been demonstrated in [8] that LENA would be very sensitive to νe originating from the annihilation of light dark matter particles. The discovery potential will be very large as the antineutrino flux from annihilation is an excess line in the energy spectrum whose position corresponds to the mass of the dark matter particle. On the other hand, the low background level in this energy regime will also allow to put a very stringent limit on the existence of dark matter of MeV mass scale if no signal is detected. 4. Conclusions and outlook LENA is a proposed multi-purpose detector based on liquid-scintillator technology with high discovery potential in different fields. The physics program covers particle physics, astrophysics and geophysics. Rare-event searches in a large,
T. Lachenmaier et al. / Progress in Particle and Nuclear Physics 64 (2010) 381–383
383
low-background fiducial volume would be possible as well as high-statistics measurements of astrophysical neutrino sources. Furthermore, the possibility to use LENA as the far detector of a long-baseline neutrino beam experiment is being investigated with Monto Carlo studies of the detector response for more complex event topologies of events at GeV energy, with promising first results [9]. This detector would be largely complementary to a water Cherenkov detector of several 100 kt being discussed for different locations in Europe, USA and Japan. Due to the competence and expertise present in Europe, a LENA-type detector may be of particular interest to be investigated in Europe. Common technical requirements and options for possible underground locations are discussed within a European context in the LAGUNA initiative [10]. Acknowledgements This work has been supported by funds of the DFG (SFB/TR 27 ‘‘Neutrinos and Beyond’’), the Cluster of Excellence ‘‘Origin and Structure of the Universe’’, and the Maier-Leibnitz-Laboratorium in Garching. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
L. Oberauer, F. von Feilitzsch, W. Potzel, Nucl. Phys. B (Proc. Suppl.) 138 (2005) 108. T. Marrodán Undagoitia, et al., Rev. Sci. Instrum. 80 (2009) 043301. M. Wurm, et al. (in preparation). T. Marrodán Undagoitia, et al., Phys. Rev. D 72 (2005) 075014. K. Kobayashi, et al., Phys. Rev. D 72 (2005) 052007. J.M.A. Winter, Diploma Thesis, Technische Universität München, 2007. M. Wurm, et al., Phys. Rev. D 75 (2007) 023007. S. Palomares-Ruiz, S. Pascoli, PRD 77 (2008) 025025. J. Peltoniemi, arXiv:0909.4574 and arXiv:0911.4876. D. Autiero, et al., JCAP 0711 (2007) 011.
Contents lists available at ScienceDirect
Progress in Particle and Nuclear Physics journal homepage: www.elsevier.com/locate/ppnp
Review
Physics with the large liquid-scintillator detector LENA T. Lachenmaier a,b,∗ , F. von Feilitzsch a , M. Göger-Neff a , T. Lewke a , T. Marrodán Undagoitia a,c , Q. Meindl a , R. Möllenberg a , L. Oberauer a , J. Peltoniemi b , W. Potzel a , M. Tippmann a , J. Winter a , M. Wurm a a
Physik Department E15, Technische Universität München, Garching, Germany
b
Excellence Cluster Universe, Technische Universität München, Garching, Germany
c
Physik-Institut, Universität Zürich, Zürich, Switzerland
article
info
Keywords: Neutrino astronomy Liquid-scintillator detector Proton decay LENA
abstract A large liquid-scintillator detector with 50 kt target mass in an underground location of at least 4000 m.w.e. is considered as a unique tool for low-energy neutrino detection and the search for rare events from astrophysical sources and up to now unobserved processes beyond the Standard Model. In this contribution, the physics potential of LENA for selected topics is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A 50 kt detector for low-energy neutrino astronomy (LENA) has been proposed to address fundamental questions in particle astrophysics, elementary particle physics, and geophysics [1]. The physics goals of LENA include the search for proton decay with unprecedented sensitivity, detection of the diffuse supernova neutrino background, time-resolved flavorspecific detection of galactic supernova neutrinos, indirect WIMP detection, high-statistics solar neutrino spectroscopy, and detection of geo neutrinos. 2. Detector design The current detector design is based on a large volume of liquid scintillator with vertical cylindrical shape, with a diameter of 30 m and a height of approximately 100 m. A photo coverage of 30% is achieved if the inner volume of the detector is instrumented with e.g. 13,000 photomultiplier tubes of 20 inch diameter. Using 50 kt of a PXE or LAB (linear alkyl benzene) based scintillator with high light yield, this coverage corresponds to a photoelectron yield of at least 150 p.e./MeV. Both options have been characterized in terms of fluorescence decay-time constants, attenuation and scattering lengths as the exact light yield is dependent on the latter two [2,3]. The outer part of the detector is filled with water as shielding and is a Cherenkov detector for cosmic muons. To benefit from the full physics potential, the detector should be placed underground at a depth of 4000 m.w.e. or deeper. 3. Physics potential 3.1. Proton decay Among the primary goals of all large, liquid based, underground detectors is the search for proton decay. Liquidscintillator detectors offer one particular advantage over water Cerenkov detectors: the energy deposit of the charged
∗
Corresponding author at: Physik Department E15, Technische Universität München, Garching, Germany. E-mail address: [email protected] (T. Lachenmaier).
0146-6410/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2009.12.054
382
T. Lachenmaier et al. / Progress in Particle and Nuclear Physics 64 (2010) 381–383
kaon created in the decay channel p → K + ν¯ , which is favored by some supersymmetric theories, can be detected directly. The subsequent kaon decay provides a very fast coincidence signal which allows for a efficient suppression of the atmospheric neutrino background in the relevant energy range. Monte Carlo simulations have been performed and it has been estimated that with this suppression, less than one count in 10 years due to atmospheric neutrinos is expected while still maintaining 65% efficiency for proton decay events [4]. In the absence of a signal in 10 years the 90% C.L. lifetime limit could be increased to τ > 4 · 1034 y for this decay mode. This is an order of magnitude improvement over the current best limit by the Super-Kamiokande experiment [5]. 3.2. Neutrino signal from a galactic supernova LENA will allow for time-resolved and flavor-specific detection of supernova (SN) neutrinos in the case of a galactic core-collapse SN explosion. A total of 10,000–15,000 events is expected for an SN explosion of an 8 M progenitor star at a distance of 10 kpc, depending on the initial neutrino spectra and matter effects [6]. The dominant detection channel is the inverse beta decay (IBD) ν¯ e p → ne+ , but a variety of other possible reactions can be exploited with useful signal rates. These will provide invaluable information on the astrophysics of the core-collapse explosion and on the neutrino mixing parameters. The νe flux can be determined using the CC reaction νe + 12 C → 12 N + e− after statistical subtraction of the ν¯ e -induced charged current reaction on 12 C with very similar signature. Neutral current processes, sensitive to all neutrino flavors, would give information on the total flux: νX + 12 C → 12 C∗ + νX is a pure flux measurement without spectral information, while the recoil spectra of elastic electron scattering and proton scattering reflect the incoming SN neutrino spectrum. Due to the strong dependence of the measured event rate on the mean neutrino energy, proton scattering is very sensitive to the temperature of the SN neutrinosphere. 3.3. Diffuse supernova neutrino background In the absence of a galactic SN as a spectacular neutrino source within the operational time of LENA, the cumulative neutrino flux from all past SNe in the Universe, the so-called diffuse supernova neutrino background (DSNB) can be used to investigate the core-collapse explosions mechanism. The DSNB signal is sensitive to the evolution of the SN rate, which is closely related to the star formation rate. In LENA, the electron antineutrino part of this signal can be measured using the IBD reaction on protons with the benefit of the delayed coincidence signal from the captured neutron. If LENA was installed at Pyhäsalmi (Finland), there would be an observational energy window from 9.7 to 25 MeV that is given by the remaining background of atmospheric and reactor neutrinos that are indistinguishable from the actual DSNB signal. For the most likely value of the SN rate, 6 to 13 events per year will be contained in this window which is almost free of background [7]. This would be the first detection of the DSNB. Within ten years of exposure, it will be possible to derive constraints on both core-collapse supernova models and the supernova rate in the near Universe up to redshifts of z < 2. 3.4. Solar and geo neutrinos in LENA If a level of radioimpurities similar to that in the Borexino experiment is reached, solar neutrinos will be detected via neutrino electron scattering with event rates exceeding the Borexino rates by at least two orders of magnitude even if the fiducial volume is reduced to increase the shielding against external gamma background. A rate of about 10,000 7 Be events per day is expected. It might be of interest to search for temporal variations in the 7 Be flux. The statistics of one year exposure is sufficient to identify a modulation of 1.5%. The detection of CNO and pep neutrinos delicately depends on the background level induced by the decays of cosmogenic 11 C. At Pyhäsalmi, the ration of the combined CNO/pep signal to 11 C background would be a factor five better than in Borexino. 3.5. Indirect WIMP detection The observational window for neutrinos from dark matter annihilation is very similar to the one for the observation of the DSNB: it is mainly determined by the remaining fluxes of reactor, diffuse SN and atmospheric neutrinos. It has been demonstrated in [8] that LENA would be very sensitive to νe originating from the annihilation of light dark matter particles. The discovery potential will be very large as the antineutrino flux from annihilation is an excess line in the energy spectrum whose position corresponds to the mass of the dark matter particle. On the other hand, the low background level in this energy regime will also allow to put a very stringent limit on the existence of dark matter of MeV mass scale if no signal is detected. 4. Conclusions and outlook LENA is a proposed multi-purpose detector based on liquid-scintillator technology with high discovery potential in different fields. The physics program covers particle physics, astrophysics and geophysics. Rare-event searches in a large,
T. Lachenmaier et al. / Progress in Particle and Nuclear Physics 64 (2010) 381–383
383
low-background fiducial volume would be possible as well as high-statistics measurements of astrophysical neutrino sources. Furthermore, the possibility to use LENA as the far detector of a long-baseline neutrino beam experiment is being investigated with Monto Carlo studies of the detector response for more complex event topologies of events at GeV energy, with promising first results [9]. This detector would be largely complementary to a water Cherenkov detector of several 100 kt being discussed for different locations in Europe, USA and Japan. Due to the competence and expertise present in Europe, a LENA-type detector may be of particular interest to be investigated in Europe. Common technical requirements and options for possible underground locations are discussed within a European context in the LAGUNA initiative [10]. Acknowledgements This work has been supported by funds of the DFG (SFB/TR 27 ‘‘Neutrinos and Beyond’’), the Cluster of Excellence ‘‘Origin and Structure of the Universe’’, and the Maier-Leibnitz-Laboratorium in Garching. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
L. Oberauer, F. von Feilitzsch, W. Potzel, Nucl. Phys. B (Proc. Suppl.) 138 (2005) 108. T. Marrodán Undagoitia, et al., Rev. Sci. Instrum. 80 (2009) 043301. M. Wurm, et al. (in preparation). T. Marrodán Undagoitia, et al., Phys. Rev. D 72 (2005) 075014. K. Kobayashi, et al., Phys. Rev. D 72 (2005) 052007. J.M.A. Winter, Diploma Thesis, Technische Universität München, 2007. M. Wurm, et al., Phys. Rev. D 75 (2007) 023007. S. Palomares-Ruiz, S. Pascoli, PRD 77 (2008) 025025. J. Peltoniemi, arXiv:0909.4574 and arXiv:0911.4876. D. Autiero, et al., JCAP 0711 (2007) 011.