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Results from the AMANDA neutrino telescope
Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170 www.elsevierphysics.com
Results from the AMANDA neutrino telescope Peter Steffena , for the AMANDA Collaborationb. a
DESY-Zeuthen, D-15735, Zeuthen, Germany For the full author list, see J. Ahrens, et al. ApJ 583 (2003) 1040.
b
The AMANDA neutrino telescope at the South Pole has been taking data since 1996. It has been upgraded in steps and reached its final stage in January 2000. Results are presented from the search for extraterrestrial neutrinos and neutrinos from dark matter annihilation.
1. INTRODUCTION One of the main purposes of AMANDA (Antarctic Muon and Neutrino Detector Array) is the search for extraterrestrial sources of highenergy neutrinos. They would give incontrovertible evidence for hadron acceleration in the emitting source. Neutrinos with energies in the TeV region and higher are predicted to be emitted by a variety of galactic and extragalactic sources, or in exotic processes associated with cosmological relics [1]. Apart from the search for extraterrestrial neutrinos, devices like AMANDA also search for neutrinos generated in annihilation processes of dark matter particles (such as WIMPs) in the center of the Earth or the Sun. 2. THE AMANDA DETECTOR The AMANDA detector (Fig. 1) is located at the Geographic South Pole. It uses the polar ice cap (about 3 km thick) both as the interaction and detection medium. The instrument consists of an array of 677 optical modules (OMs) on 19 vertical strings, mostly situated between 1500 m and 2000 m depth. They detect the Cherenkov light emitted by charged particles in the ice. AMANDA was deployed over several years, starting in 1996 [2]. The final configuration was completed in January 2000. It is named AMANDA-II. An OM consists of an 8-inch photomultiplier tube (PMT) housed in a glass pressure sphere. 0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.039
Figure 1. Schematics of the AMANDA detector
168
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
Analog PMT signals are transmitted to the surface via electrical (strings 1-10) and optical (strings 11-19) cables. The arrival time of the Cherenkov light is recorded with a resolution of about 5 nsec. 3. SEARCH FOR NEUTRINOS
ASTROPHYSICAL
AMANDA-II is optimized to measure muon tracks at energies above 1 TeV. The measurement of the arrival time of the Cherenkov light allows the reconstruction of the muon path. The angular resolution of the direction is about 2◦ . Muon energies above a TeV can be estimated from the amount of light emitted along the track; it is mostly due to pair production and bremsstrahlung. Most muons in AMANDA arise from air showers in the Earth’s atmosphere. They are separated from neutrino-induced events by the muon direction: atmospheric muons do come from above, while upward-moving muons are only produced in a CC-reaction of a νµ . However, misreconstructed atmospheric muons might fake a neutrino-induced up-going muon. This dominant background is reduced by a series of quality cuts. An unavoidable background is neutrinos generated in atmospheric air showers. The search for astrophysical neutrinos is based on different methods: point sources: search for localized neutrino sources manifesting as an accumulation of neutrino events at fixed directions. diffuse flux: search for neutrino events with high energy. Atmospheric neutrinos have a rather soft energy spectrum (spectral index ∼ E−3.7 ), while neutrinos from astrophysical sources are expected to have a harder spectrum (∼ E−2 ). In this case, one searches for an excess of neutrino events with higher energy. cascades: search for neutrino events with localized energy. CC-reactions from νe and ντ , as well as all NC reaction, produce localized showers. Also for this method, an
excess of high-energy events is required in order to identify the astrophysical origin of the events. Down-going atmospheric muons and muons from atmospheric neutrinos are AMANDA’s test beams. They have been extensively studied to understand, improve and calibrate the detector, and to continuously improve the simulation. As a result, good agreement is found both with theoretical predictions and with other experiments [3,4]. 3.1. Search for νµ point sources The complete AMANDA-II detector has a larger effective area as compared to the partial one operated during 1996 - 1999 (AMANDAB10)[12]. This results in an improved angular resolution (2 − 2.5◦ ) and a 4-5 times better sensitivity to point sources. A sample of 699 upward-going muons has been extracted from the AMANDA-II 2000 data. They are dominated by those induced by atmospheric neutrinos. The contamination of misreconstructed downward-going muons is small at declinations larger than 5◦ [13]. A binned search in the northern hemisphere and a study of pre-selected source candidates show no significant excess above background. Upper limits have been derived. They are tabulated in Table 1 for a few selected sources [12]. We would like to point out that the limit for the microquasar SS-433 is close to a prediction made in [14]. 3.2. Search for a diffuse extraterrestrial flux The diffuse flux of extraterrestrial neutrinos from unresolved directions has been searched for in data taken in 1997 with the smaller 10-string version of the detector (AMANDA-B10) [5]. The number of observed PMT-signals (hits) in the detector are used as a measure of energy, so muons from extraterrestrial neutrinos would appear as an excess of neutrino events at higher hit multiplicities. The final event sample was selected with an energy-dependent cut optimizing the detector sensitivity. No excess was observed. Instead one obtains a 90% CL limit of
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
169
Table 1 Preliminary 90% CL upper limits for candidate sources. Dec. and R.A. are declination and right ascension, nobs and nb the number of events in the search bin and the expected background. Neu−8 trino flux limits (Φlim cm−2 s−1 ), ν , in units of 10 −2 are integrated over an E energy spectrum (E > 10GeV). Candidate Markarian 421 Markarian 501 SS433 Cygnus X3 Crab Nebula Cassiopeia A M 87
Dec. [o ] 38.2 39.8 5.0 41.0 22.0 58.8 12.4
R.A. [h] 11.07 16.90 19.20 20.54 5.58 23.39 12.51
nobs
nb
Φlim ν
3 1 0 3 2 0 0
1.50 1.57 2.38 1.69 1.76 1.01 0.95
3.5 1.8 0.7 3.5 2.4 1.2 1.0
dN/dE · E2 = 8.4 · 10−7 GeVcm −2 s−1 sr−1 in the sensitive energy range of 6 to 1000 TeV (containing 90% of the signal of a hypothetical E−2 source). At present, analysis is going on with all available data taken with the three times better AMANDA-II detector. The outcome is expected to be sensitive to fluxes smaller than 1 · 10−7 GeV cm−2 s−1 sr−1 . A similar analysis was performed using a sample of downward-going muon events at energies above a few PeV [6]. At these energies the Earth is essentially opaque to neutrinos and a search for extraterrestrial sources must focus on almost horizontal events, where the expected signal accumulates. The main source of background is large bundles of downward-going atmospheric muons They are separated from the expected signal on the basis of the spatial light distribution in the detector. The 90% CL upper limit from this analysis is at dN/dE · E2 = 7.2 · 10−7 GeVcm −2 s−1 sr−1 , within a sensitive energy range of 2.5 to 6300 PeV. 3.3. Search for cascades The search for cascades of astrophysical origin is important since the expected ratio of neutrino fluxes is Φνe : Φνµ : Φντ ≈ 1 : 1 : 1. Cascade
Figure 2. Limits on the diffuse flux of all types of neutrinos (cf. text).
detection suffers from the reduced effective volume as compared to muon detection. However, the analysis profits from the good energy resolution for contained events (∆Log10(E)=0.1÷0.2 in the energy range from 1 to 100 TeV). The relatively low intrinsic background is mostly due to downward-going atmospheric muons of high energy. They can radiate a large fraction of their energy producing an electromagnetic cascade. An analysis of AMANDA-II year-2000 data yields a preliminary 90% CL upper limit of dN/dE · E2 = 9 · 10−7 GeV cm−2 s−1 sr−1 for the sum of all flavors [7], an order of magnitude lower than the previously published limit obtained with AMANDA-B10 [8]. The sensitivity range of this search is 80 TeV-7 PeV. Fig. 2 shows the limits on the diffuse flux of all three neutrino flavors, as obtained from the analysis of upward- and downward-going muons (“AMANDA-B10 νµ ×3”) and from the cascade analysis (“AMANDA-II νe + νµ + ντ ”), together with the corresponding BAIKAL limit [9] and the SSDS [10] and MPR [11] predictions (lower curve and upper curve respectively). The falling lines left represent the atmospheric neutrino fluxes (dashed lines for νµ and continuous line for νe ).
170
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
4. PARTICLE PHYSICS
REFERENCES
If cold dark matter consists of neutralinos, these would have accumulated in the center of the Earth and the Sun. The annihilation process of particles and antiparticles could lead to an excess of neutrinos from those directions. In the search for the annihilation process in the center of the Earth, vertically upward-moving muons are investigated using data taken in 1997 and 1999 with the AMANDA-B10 detector. No significant excess to the level of atmospheric neutrinos has been found. Instead limits on the neutrino-induced muon flux from neutralino annihilation [15] are derived (see [16] for details). At present, these limits are inferior to the ones obtained in direct searches. However, it should be noted that uncertainties in the understanding of the galactic halo can change considerably both the theoretical flux predictions and the significance of results of direct searches. The situation is different for neutralino annihilation in the Sun. The improved reconstruction of nearly horizontal muons of the complete AMANDA-II detector makes the detection of neutrinos from the center of the Sun feasible (which never sinks below 24 degrees below the horizon at the South Pole). Preliminary results indicate a sensitivity which is competitive with the results of direct searches.
1. J.G. Learned and K.Mannheim, Ann. Rev. Nucl. Part. 50 (2000) 679. 2. E. Andr`es et al., Astropart. Phys. 13 (2000) 1. 3. D. Chirkin, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1211. 4. H. Geenen, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1313. 5. J. Ahrens et al., Phys. Rev. Lett. 90 (2003) 251101. 6. S. Hundertmark, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1309. 7. M.Kowalski, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1301. 8. J. Ahrens et al., Phys. Rev. D. 67 (2003) 012003. 9. G.V. Domogatskii et al., arXiv:astroph/0211571. 10. F.W. Stecker et al., Phys. Rev. Lett. 66 (1991) 2697, Errata, ibid 69 (1992) 2738. 11. K. Mannheim, R.J. Protheroe and J.P. Rachen, Phys. Rev. D. 63 (2001) 023003. 12. J. Ahrens et al., Astroph. J. 583 (2003) 1040. 13. T. Hauschildt and D. Steele, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1305. 14. C. Distefano et al., Astroph. J. 575 (2002) 378. 15. J. Ahrens et al., Phys. Rev. D 66 (2002) 021005. 16. P. Olbrechts et al., Proc. 28th ICRC, Tsukuba (Japan) (2003) 1677.
5. CONCLUSIONS The analysis of AMANDA data covers different fields and energy ranges, from a few 100 GeV up to EeV. All events observed up to now are in agreement with background expectations. Upper limits have been derived which are in most cases the best available. AMANDA is taking data continuously. In present analyses it has reached the sensitivity to observe astrophysical neutrinos predicted by the currently most optimistic models. It will be extended to larger data samples and combined. The feasibility of an under-ice neutrino telescope has been proven by more than five years of successful operation. This opens the way to the realization of cubic-kilometer detectors.
Results from the AMANDA neutrino telescope Peter Steffena , for the AMANDA Collaborationb. a
DESY-Zeuthen, D-15735, Zeuthen, Germany For the full author list, see J. Ahrens, et al. ApJ 583 (2003) 1040.
b
The AMANDA neutrino telescope at the South Pole has been taking data since 1996. It has been upgraded in steps and reached its final stage in January 2000. Results are presented from the search for extraterrestrial neutrinos and neutrinos from dark matter annihilation.
1. INTRODUCTION One of the main purposes of AMANDA (Antarctic Muon and Neutrino Detector Array) is the search for extraterrestrial sources of highenergy neutrinos. They would give incontrovertible evidence for hadron acceleration in the emitting source. Neutrinos with energies in the TeV region and higher are predicted to be emitted by a variety of galactic and extragalactic sources, or in exotic processes associated with cosmological relics [1]. Apart from the search for extraterrestrial neutrinos, devices like AMANDA also search for neutrinos generated in annihilation processes of dark matter particles (such as WIMPs) in the center of the Earth or the Sun. 2. THE AMANDA DETECTOR The AMANDA detector (Fig. 1) is located at the Geographic South Pole. It uses the polar ice cap (about 3 km thick) both as the interaction and detection medium. The instrument consists of an array of 677 optical modules (OMs) on 19 vertical strings, mostly situated between 1500 m and 2000 m depth. They detect the Cherenkov light emitted by charged particles in the ice. AMANDA was deployed over several years, starting in 1996 [2]. The final configuration was completed in January 2000. It is named AMANDA-II. An OM consists of an 8-inch photomultiplier tube (PMT) housed in a glass pressure sphere. 0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.039
Figure 1. Schematics of the AMANDA detector
168
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
Analog PMT signals are transmitted to the surface via electrical (strings 1-10) and optical (strings 11-19) cables. The arrival time of the Cherenkov light is recorded with a resolution of about 5 nsec. 3. SEARCH FOR NEUTRINOS
ASTROPHYSICAL
AMANDA-II is optimized to measure muon tracks at energies above 1 TeV. The measurement of the arrival time of the Cherenkov light allows the reconstruction of the muon path. The angular resolution of the direction is about 2◦ . Muon energies above a TeV can be estimated from the amount of light emitted along the track; it is mostly due to pair production and bremsstrahlung. Most muons in AMANDA arise from air showers in the Earth’s atmosphere. They are separated from neutrino-induced events by the muon direction: atmospheric muons do come from above, while upward-moving muons are only produced in a CC-reaction of a νµ . However, misreconstructed atmospheric muons might fake a neutrino-induced up-going muon. This dominant background is reduced by a series of quality cuts. An unavoidable background is neutrinos generated in atmospheric air showers. The search for astrophysical neutrinos is based on different methods: point sources: search for localized neutrino sources manifesting as an accumulation of neutrino events at fixed directions. diffuse flux: search for neutrino events with high energy. Atmospheric neutrinos have a rather soft energy spectrum (spectral index ∼ E−3.7 ), while neutrinos from astrophysical sources are expected to have a harder spectrum (∼ E−2 ). In this case, one searches for an excess of neutrino events with higher energy. cascades: search for neutrino events with localized energy. CC-reactions from νe and ντ , as well as all NC reaction, produce localized showers. Also for this method, an
excess of high-energy events is required in order to identify the astrophysical origin of the events. Down-going atmospheric muons and muons from atmospheric neutrinos are AMANDA’s test beams. They have been extensively studied to understand, improve and calibrate the detector, and to continuously improve the simulation. As a result, good agreement is found both with theoretical predictions and with other experiments [3,4]. 3.1. Search for νµ point sources The complete AMANDA-II detector has a larger effective area as compared to the partial one operated during 1996 - 1999 (AMANDAB10)[12]. This results in an improved angular resolution (2 − 2.5◦ ) and a 4-5 times better sensitivity to point sources. A sample of 699 upward-going muons has been extracted from the AMANDA-II 2000 data. They are dominated by those induced by atmospheric neutrinos. The contamination of misreconstructed downward-going muons is small at declinations larger than 5◦ [13]. A binned search in the northern hemisphere and a study of pre-selected source candidates show no significant excess above background. Upper limits have been derived. They are tabulated in Table 1 for a few selected sources [12]. We would like to point out that the limit for the microquasar SS-433 is close to a prediction made in [14]. 3.2. Search for a diffuse extraterrestrial flux The diffuse flux of extraterrestrial neutrinos from unresolved directions has been searched for in data taken in 1997 with the smaller 10-string version of the detector (AMANDA-B10) [5]. The number of observed PMT-signals (hits) in the detector are used as a measure of energy, so muons from extraterrestrial neutrinos would appear as an excess of neutrino events at higher hit multiplicities. The final event sample was selected with an energy-dependent cut optimizing the detector sensitivity. No excess was observed. Instead one obtains a 90% CL limit of
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
169
Table 1 Preliminary 90% CL upper limits for candidate sources. Dec. and R.A. are declination and right ascension, nobs and nb the number of events in the search bin and the expected background. Neu−8 trino flux limits (Φlim cm−2 s−1 ), ν , in units of 10 −2 are integrated over an E energy spectrum (E > 10GeV). Candidate Markarian 421 Markarian 501 SS433 Cygnus X3 Crab Nebula Cassiopeia A M 87
Dec. [o ] 38.2 39.8 5.0 41.0 22.0 58.8 12.4
R.A. [h] 11.07 16.90 19.20 20.54 5.58 23.39 12.51
nobs
nb
Φlim ν
3 1 0 3 2 0 0
1.50 1.57 2.38 1.69 1.76 1.01 0.95
3.5 1.8 0.7 3.5 2.4 1.2 1.0
dN/dE · E2 = 8.4 · 10−7 GeVcm −2 s−1 sr−1 in the sensitive energy range of 6 to 1000 TeV (containing 90% of the signal of a hypothetical E−2 source). At present, analysis is going on with all available data taken with the three times better AMANDA-II detector. The outcome is expected to be sensitive to fluxes smaller than 1 · 10−7 GeV cm−2 s−1 sr−1 . A similar analysis was performed using a sample of downward-going muon events at energies above a few PeV [6]. At these energies the Earth is essentially opaque to neutrinos and a search for extraterrestrial sources must focus on almost horizontal events, where the expected signal accumulates. The main source of background is large bundles of downward-going atmospheric muons They are separated from the expected signal on the basis of the spatial light distribution in the detector. The 90% CL upper limit from this analysis is at dN/dE · E2 = 7.2 · 10−7 GeVcm −2 s−1 sr−1 , within a sensitive energy range of 2.5 to 6300 PeV. 3.3. Search for cascades The search for cascades of astrophysical origin is important since the expected ratio of neutrino fluxes is Φνe : Φνµ : Φντ ≈ 1 : 1 : 1. Cascade
Figure 2. Limits on the diffuse flux of all types of neutrinos (cf. text).
detection suffers from the reduced effective volume as compared to muon detection. However, the analysis profits from the good energy resolution for contained events (∆Log10(E)=0.1÷0.2 in the energy range from 1 to 100 TeV). The relatively low intrinsic background is mostly due to downward-going atmospheric muons of high energy. They can radiate a large fraction of their energy producing an electromagnetic cascade. An analysis of AMANDA-II year-2000 data yields a preliminary 90% CL upper limit of dN/dE · E2 = 9 · 10−7 GeV cm−2 s−1 sr−1 for the sum of all flavors [7], an order of magnitude lower than the previously published limit obtained with AMANDA-B10 [8]. The sensitivity range of this search is 80 TeV-7 PeV. Fig. 2 shows the limits on the diffuse flux of all three neutrino flavors, as obtained from the analysis of upward- and downward-going muons (“AMANDA-B10 νµ ×3”) and from the cascade analysis (“AMANDA-II νe + νµ + ντ ”), together with the corresponding BAIKAL limit [9] and the SSDS [10] and MPR [11] predictions (lower curve and upper curve respectively). The falling lines left represent the atmospheric neutrino fluxes (dashed lines for νµ and continuous line for νe ).
170
P. Steffen / Nuclear Physics B (Proc. Suppl.) 138 (2005) 167–170
4. PARTICLE PHYSICS
REFERENCES
If cold dark matter consists of neutralinos, these would have accumulated in the center of the Earth and the Sun. The annihilation process of particles and antiparticles could lead to an excess of neutrinos from those directions. In the search for the annihilation process in the center of the Earth, vertically upward-moving muons are investigated using data taken in 1997 and 1999 with the AMANDA-B10 detector. No significant excess to the level of atmospheric neutrinos has been found. Instead limits on the neutrino-induced muon flux from neutralino annihilation [15] are derived (see [16] for details). At present, these limits are inferior to the ones obtained in direct searches. However, it should be noted that uncertainties in the understanding of the galactic halo can change considerably both the theoretical flux predictions and the significance of results of direct searches. The situation is different for neutralino annihilation in the Sun. The improved reconstruction of nearly horizontal muons of the complete AMANDA-II detector makes the detection of neutrinos from the center of the Sun feasible (which never sinks below 24 degrees below the horizon at the South Pole). Preliminary results indicate a sensitivity which is competitive with the results of direct searches.
1. J.G. Learned and K.Mannheim, Ann. Rev. Nucl. Part. 50 (2000) 679. 2. E. Andr`es et al., Astropart. Phys. 13 (2000) 1. 3. D. Chirkin, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1211. 4. H. Geenen, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1313. 5. J. Ahrens et al., Phys. Rev. Lett. 90 (2003) 251101. 6. S. Hundertmark, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1309. 7. M.Kowalski, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1301. 8. J. Ahrens et al., Phys. Rev. D. 67 (2003) 012003. 9. G.V. Domogatskii et al., arXiv:astroph/0211571. 10. F.W. Stecker et al., Phys. Rev. Lett. 66 (1991) 2697, Errata, ibid 69 (1992) 2738. 11. K. Mannheim, R.J. Protheroe and J.P. Rachen, Phys. Rev. D. 63 (2001) 023003. 12. J. Ahrens et al., Astroph. J. 583 (2003) 1040. 13. T. Hauschildt and D. Steele, Proc. 28th ICRC, Tsukuba (Japan) (2003) 1305. 14. C. Distefano et al., Astroph. J. 575 (2002) 378. 15. J. Ahrens et al., Phys. Rev. D 66 (2002) 021005. 16. P. Olbrechts et al., Proc. 28th ICRC, Tsukuba (Japan) (2003) 1677.
5. CONCLUSIONS The analysis of AMANDA data covers different fields and energy ranges, from a few 100 GeV up to EeV. All events observed up to now are in agreement with background expectations. Upper limits have been derived which are in most cases the best available. AMANDA is taking data continuously. In present analyses it has reached the sensitivity to observe astrophysical neutrinos predicted by the currently most optimistic models. It will be extended to larger data samples and combined. The feasibility of an under-ice neutrino telescope has been proven by more than five years of successful operation. This opens the way to the realization of cubic-kilometer detectors.