- Email: [email protected]
Study of the sensitivity to point-like neutrino sources for a flexible-tower geometry for KM3NeT
Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S200–S202
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the sensitivity to point-like neutrino sources for a flexible-tower geometry for KM3NeT Rosa Coniglione n, Carla Distefano, Piera Sapienza Laboratori Nazionali del Sud INFN, Via S. Sofia 62, Caatnia 95123, Italy
For the KM3NeT Consortium a r t i c l e in f o
a b s t r a c t
Available online 12 June 2010
The KM3NeT consortium has investigated the performance of a high energy neutrino detector by means of Monte Carlo simulations. In this work the study of the sensitivity to point-like neutrino sources as a function of detector size and of source declination is presented. It is shown that the detector sensitivity improves less than linearly with increasing homogeneously instrumented volume and that the detector sky view is very large (about 3.5p). & 2010 Elsevier B.V. All rights reserved.
Keywords: Neutrino telescope Sensitivity
1. Introduction
2. Sensitivity to point-like sources: simulations and results
The detection of high energy neutrino fluxes from astrophysical sources is the main objective of an under-water or under-ice cubic-kilometre detector. Since the origin of high energy charged cosmic rays [1] is still unknown, the discovery of neutrino fluxes from galactic or extragalactic sources will shed light on the acceleration mechanisms responsible for the emission of both neutrinos and charged particles at high energies. Since neutrinos are electrically neutral and interact weakly, they are not sensitive to magnetic fields and can traverse very dense layers of matter. These properties make them ideal messengers for discovering new sources and exploring their interior. Estimates of the neutrino fluxes from known sources indicate that a detector larger than a cubic kilometre is required. The aim of the KM3NeT consortium [2] is the construction of a research infrastructure including a cubic-kilometre-scale detector in the Mediterranean Sea. An optimisation [3] to find the best detector geometry was carried out with the main focus on pointlike sources producing neutrinos in the energy range between a few TeV and about 1 PeV. The concepts behind the design of the detector, the technical requirements for the assembly, deployment and operation of a deep underwater detector will be reported in a technical design report. In this work sensitivity studies on point-like sources for a detector based on flexible towers will be presented.
The search for a point-like neutrino source is based on statistical techniques to detect a weak neutrino signal from a cosmic source amongst a large diffuse background from atmospheric muons and neutrinos. Both are produced by the interactions of primary charged cosmic rays with atmosphere. The atmospheric muon background, which represents a reducible background for a high energy neutrino telescope, is attenuated in intensity and energy at locations below the sea surface—it falls to zero for angles near and below the horizon, where the large path in water and in the Earth shield all the muons. For this reason the search for cosmic neutrinos is limited mainly to the study of upgoing muons. The diffuse atmospheric neutrino background is irreducible for the detection of cosmic neutrinos. Owing to the excellent angular resolution of the detector ( 0.21), this background can be reduced to a manageable level by performing the search in a restricted cone around the source direction. The two main statistical approaches developed to look for steady point-like sources are binned and unbinned methods [4]. The sensitivity reported in this document has been estimated by applying a binned method. Following the approach of Feldman and Cousins [5], the sensitivity is the limit that can be placed on neutrino flux model from a source if no signal is detected. The average flux limit is defined as
m Nbkg F90 ¼ FS MRF ¼ FS 90 hNS i
n
Corresponding author. Tel.: +39 095 542288; fax: + 39 095 7141815. E-mail address: [email protected] (R. Coniglione).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.053
where Fs is the normalization factor of the flux model. The sensitivity values are obtained by minimising the model rejection factor (MRF) [5]. The MRF is defined as the ratio of the average upper limit m90 to the average number Ns of events from the
R. Coniglione et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S200–S202
source; m90 is the average upper limit of background fluctuations at 90% confidence level that would be observed in hypothetical repetitions of an experiment with a given background expectation and no true signal. By means of Monte Carlo simulations, the average number of events Ns inside a cone around a point-like source at fixed declination and the average number of background events, Nbkg, are estimated. ANTARES codes [6] modified for a cubic-kilometre detector [7], including the detector geometry, the neutrino interaction, the light generation, the light propagation in water and the detector response, have been used for the simulations. A track reconstruction algorithm [8], based on a maximum likelihood estimator, developed inside the ANTARES collaboration and adapted to the cubic kilometre geometry, has been employed to reconstruct the muon and consequently, the neutrino direction. During the KM3NeT design study, three combinations of technical options have been investigated and optimized in simulations studies. In this work the sensitivity to point-like sources for the geometry based on bar-structured, three-dimensional detection units (DUs) will be presented. The DUs are arranged in a hexagonal lay-out with a spacing of 180 m. The DU consists of a sequence of 20 rigid bars, 6 m long, placed orthogonally to each other with a vertical spacing of 40 m. Each bar holds three couples of optical modules each one containing 800 PMTs (35% peak quantum efficiency): two couples at each bar edge with one PMT horizontal-looking and the other downlooking and a third one, located in the bar center, with the PMTs down-looking at an angle of 451 with respect to the vertical direction. In the simulation the Capo Passero optical water parameters [9], latitude and depth were used. The assumed random optical background due to the decay of 40K and bioluminescence was 47 kHz per PMT. The sensitivity to a point-like source with E 2 muon neutrino spectrum in the energy range 102–107 GeV was evaluated for several declinations. Atmospheric neutrinos have been isotropically generated assuming the Bartol flux [10] (approximately an E 3.7 spectrum). A prompt contribution from charm decays is also included. The parameters that are optimized in order to get the minimum value of MRF are the size of the search cone, the cut on the reconstruction quality parameter and a cut on the number of recorded PMT hits that is related to the neutrino energy. Only neutrinos up to 61 above the horizon were considered. Fig. 1 shows the sensitivity to a point-like source at 601 of declination (full visibility) with an E 2 spectrum for 1 year of observation time reported as a function of number of DUs arranged in a homogeneous setup (solid line). Sensitivities estimated for detectors made of two building blocks are also shown (dashed line). The improvement in sensitivity is not proportional to the size of the detector. For a detector of about 130 DUs a 20% increase in sensitivity requires about 30% more DUs. Due to its location the KM3NeT neutrino telescope will observe a large fraction of the Galactic Plane, where many candidate sources are situated. Fig. 2 shows (solid line) the sensitivity to a E 2 point-like source for 1 year of observation time as a function of source declination for a detector of 310 DUs. The result shows that the KM3NeT field of view allows for observing a large fraction of the sky (about 3.5p) including the Galactic Centre (sin(d)¼ 0.48) and most of the Galactic Plane. The shape of the sensitivity curve is due to the combination of three effects: the source visibility, which is determined by the geographic location, the neutrino absorption in the Earth and the detector response as a function of zenith angle. The KM3NeT sensitivity compared to that of IceCube [11] (dashed line) extends substantially the visible region of the sky
S201
Fig. 1. Sensitivity to an E 2 point-like source at a declination of 601 for one year of observation time as a function of the number of DUs for detector arranged in a homogenous set up (solid line) and in two building blocks (dashed line).
Fig. 2. Solid line—sensitivity for a E 2 point-like sources for one year of observation time as a function of the source declination. Dashed line is the IceCube sensitivities [11] estimated with the unbinned method.
and even improves the discovery potential in the same field of view. This gain is due to the far larger instrumented volume possible within the expected project budget, together with the significantly better angular resolution achievable in water compared with ice. Moreover, as shown by the IceCube collaboration [4], using the unbinned method, where in the probability density function a neutrino energy related component is included, a further improvement in sensitivity over the binned method is obtained. Thus a similar improvement on the KM3MeT sensitivity can be obtained once an ubinned method is implemented. References [1] J. Abraham for the AUGER collaboration, Phys. Rev. Lett. 101 (2008) 06110. [2] KM3NeT web page /http://www.km3net.orgS and the KM3NeT Conceptual Design Report for a Deep-Sea Research Infrastructure in the Mediterranean Sea Incorporating a Very Large Volume Neutrino Telescope. on /http://www. km3net.org/CDR/CDR-KM3NeT.pdfS. April 2008 KM3NeT collaboration ISBN 978-90-6488-031-5. [3] R. Coniglione, et al.for the KM3NeT collaboration, Nucl. Instr. and Meth. A 602 (2009) 98; P. Sapienza, et al.for the KM3NeT collaboration, Nucl. Instr. Meth. A 602 (2009) 101;
S202
R. Coniglione et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S200–S202
P. Sapienza et al. for KM3NeT collaboration, ICRC 2009 Proceedings; P. Sapienza et al. for KM3NeT consortium, Nucl. Instr. Meth. A, these proceedings; G. Vannoni for KM3NeT consortium, Nucl. Instr. Meth. A, these proceedings. [4] J. Braun, et al., Astropart. Phys. 29 (2008) 299–305. [5] G.J. Feldman, R. Cousins, Phys. Rev. D 57 (1998) 3873–3889. [6] D. Bailey Ph.D. Thesis in the ANTARES web page, 2004, /http://antares.in2p3. fr/PublicationsS.
[7] P. Sapienza For the NEMO collaboration, in: Proceedings of the VLVnT 2003, 5–8 October 2003, Amsterdam, in /http://www.vlvnt.nl/proceedingsS. [8] A. Heijboer Ph.D. thesis in the ANTARES web page, 2004, /http://antares. in2p3.fr/PublicationsS. [9] G. Riccobene For the NEMO collaboration, in: Proceedings of the VLVnT 2003, 5–8 October 2003, Amsterdam, in /http://www.vlvnt.nl/proceedings.pdfS. [10] V. Agrawal, et al., Phys. Rev. D 53 (1996) 1314. [11] R. Abbasi, et al., IceCube collaboration, Astrophys. J. 701 (2009) L47.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the sensitivity to point-like neutrino sources for a flexible-tower geometry for KM3NeT Rosa Coniglione n, Carla Distefano, Piera Sapienza Laboratori Nazionali del Sud INFN, Via S. Sofia 62, Caatnia 95123, Italy
For the KM3NeT Consortium a r t i c l e in f o
a b s t r a c t
Available online 12 June 2010
The KM3NeT consortium has investigated the performance of a high energy neutrino detector by means of Monte Carlo simulations. In this work the study of the sensitivity to point-like neutrino sources as a function of detector size and of source declination is presented. It is shown that the detector sensitivity improves less than linearly with increasing homogeneously instrumented volume and that the detector sky view is very large (about 3.5p). & 2010 Elsevier B.V. All rights reserved.
Keywords: Neutrino telescope Sensitivity
1. Introduction
2. Sensitivity to point-like sources: simulations and results
The detection of high energy neutrino fluxes from astrophysical sources is the main objective of an under-water or under-ice cubic-kilometre detector. Since the origin of high energy charged cosmic rays [1] is still unknown, the discovery of neutrino fluxes from galactic or extragalactic sources will shed light on the acceleration mechanisms responsible for the emission of both neutrinos and charged particles at high energies. Since neutrinos are electrically neutral and interact weakly, they are not sensitive to magnetic fields and can traverse very dense layers of matter. These properties make them ideal messengers for discovering new sources and exploring their interior. Estimates of the neutrino fluxes from known sources indicate that a detector larger than a cubic kilometre is required. The aim of the KM3NeT consortium [2] is the construction of a research infrastructure including a cubic-kilometre-scale detector in the Mediterranean Sea. An optimisation [3] to find the best detector geometry was carried out with the main focus on pointlike sources producing neutrinos in the energy range between a few TeV and about 1 PeV. The concepts behind the design of the detector, the technical requirements for the assembly, deployment and operation of a deep underwater detector will be reported in a technical design report. In this work sensitivity studies on point-like sources for a detector based on flexible towers will be presented.
The search for a point-like neutrino source is based on statistical techniques to detect a weak neutrino signal from a cosmic source amongst a large diffuse background from atmospheric muons and neutrinos. Both are produced by the interactions of primary charged cosmic rays with atmosphere. The atmospheric muon background, which represents a reducible background for a high energy neutrino telescope, is attenuated in intensity and energy at locations below the sea surface—it falls to zero for angles near and below the horizon, where the large path in water and in the Earth shield all the muons. For this reason the search for cosmic neutrinos is limited mainly to the study of upgoing muons. The diffuse atmospheric neutrino background is irreducible for the detection of cosmic neutrinos. Owing to the excellent angular resolution of the detector ( 0.21), this background can be reduced to a manageable level by performing the search in a restricted cone around the source direction. The two main statistical approaches developed to look for steady point-like sources are binned and unbinned methods [4]. The sensitivity reported in this document has been estimated by applying a binned method. Following the approach of Feldman and Cousins [5], the sensitivity is the limit that can be placed on neutrino flux model from a source if no signal is detected. The average flux limit is defined as
m Nbkg F90 ¼ FS MRF ¼ FS 90 hNS i
n
Corresponding author. Tel.: +39 095 542288; fax: + 39 095 7141815. E-mail address: [email protected] (R. Coniglione).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.053
where Fs is the normalization factor of the flux model. The sensitivity values are obtained by minimising the model rejection factor (MRF) [5]. The MRF is defined as the ratio of the average upper limit m90 to the average number Ns of events from the
R. Coniglione et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S200–S202
source; m90 is the average upper limit of background fluctuations at 90% confidence level that would be observed in hypothetical repetitions of an experiment with a given background expectation and no true signal. By means of Monte Carlo simulations, the average number of events Ns inside a cone around a point-like source at fixed declination and the average number of background events, Nbkg, are estimated. ANTARES codes [6] modified for a cubic-kilometre detector [7], including the detector geometry, the neutrino interaction, the light generation, the light propagation in water and the detector response, have been used for the simulations. A track reconstruction algorithm [8], based on a maximum likelihood estimator, developed inside the ANTARES collaboration and adapted to the cubic kilometre geometry, has been employed to reconstruct the muon and consequently, the neutrino direction. During the KM3NeT design study, three combinations of technical options have been investigated and optimized in simulations studies. In this work the sensitivity to point-like sources for the geometry based on bar-structured, three-dimensional detection units (DUs) will be presented. The DUs are arranged in a hexagonal lay-out with a spacing of 180 m. The DU consists of a sequence of 20 rigid bars, 6 m long, placed orthogonally to each other with a vertical spacing of 40 m. Each bar holds three couples of optical modules each one containing 800 PMTs (35% peak quantum efficiency): two couples at each bar edge with one PMT horizontal-looking and the other downlooking and a third one, located in the bar center, with the PMTs down-looking at an angle of 451 with respect to the vertical direction. In the simulation the Capo Passero optical water parameters [9], latitude and depth were used. The assumed random optical background due to the decay of 40K and bioluminescence was 47 kHz per PMT. The sensitivity to a point-like source with E 2 muon neutrino spectrum in the energy range 102–107 GeV was evaluated for several declinations. Atmospheric neutrinos have been isotropically generated assuming the Bartol flux [10] (approximately an E 3.7 spectrum). A prompt contribution from charm decays is also included. The parameters that are optimized in order to get the minimum value of MRF are the size of the search cone, the cut on the reconstruction quality parameter and a cut on the number of recorded PMT hits that is related to the neutrino energy. Only neutrinos up to 61 above the horizon were considered. Fig. 1 shows the sensitivity to a point-like source at 601 of declination (full visibility) with an E 2 spectrum for 1 year of observation time reported as a function of number of DUs arranged in a homogeneous setup (solid line). Sensitivities estimated for detectors made of two building blocks are also shown (dashed line). The improvement in sensitivity is not proportional to the size of the detector. For a detector of about 130 DUs a 20% increase in sensitivity requires about 30% more DUs. Due to its location the KM3NeT neutrino telescope will observe a large fraction of the Galactic Plane, where many candidate sources are situated. Fig. 2 shows (solid line) the sensitivity to a E 2 point-like source for 1 year of observation time as a function of source declination for a detector of 310 DUs. The result shows that the KM3NeT field of view allows for observing a large fraction of the sky (about 3.5p) including the Galactic Centre (sin(d)¼ 0.48) and most of the Galactic Plane. The shape of the sensitivity curve is due to the combination of three effects: the source visibility, which is determined by the geographic location, the neutrino absorption in the Earth and the detector response as a function of zenith angle. The KM3NeT sensitivity compared to that of IceCube [11] (dashed line) extends substantially the visible region of the sky
S201
Fig. 1. Sensitivity to an E 2 point-like source at a declination of 601 for one year of observation time as a function of the number of DUs for detector arranged in a homogenous set up (solid line) and in two building blocks (dashed line).
Fig. 2. Solid line—sensitivity for a E 2 point-like sources for one year of observation time as a function of the source declination. Dashed line is the IceCube sensitivities [11] estimated with the unbinned method.
and even improves the discovery potential in the same field of view. This gain is due to the far larger instrumented volume possible within the expected project budget, together with the significantly better angular resolution achievable in water compared with ice. Moreover, as shown by the IceCube collaboration [4], using the unbinned method, where in the probability density function a neutrino energy related component is included, a further improvement in sensitivity over the binned method is obtained. Thus a similar improvement on the KM3MeT sensitivity can be obtained once an ubinned method is implemented. References [1] J. Abraham for the AUGER collaboration, Phys. Rev. Lett. 101 (2008) 06110. [2] KM3NeT web page /http://www.km3net.orgS and the KM3NeT Conceptual Design Report for a Deep-Sea Research Infrastructure in the Mediterranean Sea Incorporating a Very Large Volume Neutrino Telescope. on /http://www. km3net.org/CDR/CDR-KM3NeT.pdfS. April 2008 KM3NeT collaboration ISBN 978-90-6488-031-5. [3] R. Coniglione, et al.for the KM3NeT collaboration, Nucl. Instr. and Meth. A 602 (2009) 98; P. Sapienza, et al.for the KM3NeT collaboration, Nucl. Instr. Meth. A 602 (2009) 101;
S202
R. Coniglione et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S200–S202
P. Sapienza et al. for KM3NeT collaboration, ICRC 2009 Proceedings; P. Sapienza et al. for KM3NeT consortium, Nucl. Instr. Meth. A, these proceedings; G. Vannoni for KM3NeT consortium, Nucl. Instr. Meth. A, these proceedings. [4] J. Braun, et al., Astropart. Phys. 29 (2008) 299–305. [5] G.J. Feldman, R. Cousins, Phys. Rev. D 57 (1998) 3873–3889. [6] D. Bailey Ph.D. Thesis in the ANTARES web page, 2004, /http://antares.in2p3. fr/PublicationsS.
[7] P. Sapienza For the NEMO collaboration, in: Proceedings of the VLVnT 2003, 5–8 October 2003, Amsterdam, in /http://www.vlvnt.nl/proceedingsS. [8] A. Heijboer Ph.D. thesis in the ANTARES web page, 2004, /http://antares. in2p3.fr/PublicationsS. [9] G. Riccobene For the NEMO collaboration, in: Proceedings of the VLVnT 2003, 5–8 October 2003, Amsterdam, in /http://www.vlvnt.nl/proceedings.pdfS. [10] V. Agrawal, et al., Phys. Rev. D 53 (1996) 1314. [11] R. Abbasi, et al., IceCube collaboration, Astrophys. J. 701 (2009) L47.