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UHE neutrino detection by a satellite-based air shower experiment
Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282 www.elsevier.com/locate/npbps
UHE neutrino detection by a satellite-based air shower experiment K. Miyazawaa, N. Inouea , K. Higashidea a
The Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
Ultra-High-Energy(UHE) neutrinos have not yet been detected because of the small interaction cross-section with the target matter. UHE neutrinos could be produced in astrophysical objects where UHE cosmic rays are accelerated and could also be expected in the top-down process such as the decay of SHR/topological defects. Extensive Air Showers (EASs) induced by neutrinos have a uniform Xmax distribution over atmospheric slant depth though that of hadronic EASs is limited between 800 g/cm2 and 1200 g/cm2 . Assuming UHE neutrino fluxes, the detection efficiencies by satellite-based observation with a target mass of 1015 kg are estimated.
1. Introduction
2. Simulation procedure
The chemical composition of UHE cosmic rays is one of the unsolved questions, and its study will provide information on the origin and the acceleration mechanism of UHE cosmic rays. In fact, the detection of UHE neutrinos with energies > 1019 eV will be a key to solve these questions. Cosmogenic neutrinos have been expected to be steadily produced in the GZK process [1,2]. The possibility of neutrino production during the acceleration in AGNs or GRBs has also been pointed out. UHE neutrinos have such a small interaction cross-section with matter, and will be the best probe to study UHE cosmic ray accelerators at cosmological distances. UHE neutrinos rarely interact with atmospheric nuclei and the produced UHE electrons generate EAS in the atmosphere. Neutrino induced EASs are clearly distinguished from hadronic EASs in terms of the Xmax , as they will be typically largely inclined EASs or upwardgoing EASs. From the experimental point of view, a huge amount of target mass is required to ensure interactions in the atmosphere. To overcome this problem, JEM-EUSO[3], a mission installed on the International Space Station to study chargedparticle astronomy in the energy region above 3×1019 eV, is a suitable observation platform because of its large Field of View (FoV), 250 km×170 km, and the atmospheric matter in the acceptance is larger than 1 tera-ton.
UHE neutrino simulation codes consist of neutrino interactions, EAS generation and the production of air fluorescence/Cherenkov UV photons along the EAS development. The interaction cross-section with the atmospheric nucleus (σcc (νN )) is assumed to be a function of equation (1)[4] in this calculation. The interaction length (Lint ) related to the cross-section in the atmosphere is expressed by (2).
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.09.053
σcc (νN ) = 5.53 × 10−36 (Eν /1GeV )0.363 [cm2 ] (1) Lint = 1/(σcc (νN ) · NA ) [g/cm2 ]
(2)
where NA and Eν are the Avogadro constant and the neutrino energy. The US standard atmospheric model [5] is used in the calculation to estimate the air density, ρ(r)[g/cm3 ], as a function of the vertical height, (r). A neutrino EAS is induced by secondary electrons produced in the neutrino interaction and the ratio between the EAS energy (Ee ) and the initial energy (Eν )[6] is given by Ee /Eν = (0.6724 + 0.0058 log Eν )
(3)
Electro-magnetic EASs have been generated by the AIRES code (Ver.2.6.0) [7]. The individual longitudinal structure is fitted by the GaisserHillas formula with 3 parameters1 , and are ac x a2 ×a3 1 N (x) = a1
a2
× exp [(a2 − x)a3 ]
280
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
Interaction probability [%]
Interaction Probability in F.O.V. 0.02
0.01
0 19
Figure 1. The distributions of interaction depths in the atmosphere. Neutrinos with zenith angle of 90◦ and energies of 1019.0 eV, 1019.5 eV, 1020 eV are initiated.
20
21 Log10(E) [eV]
Figure 2. The interaction probability in a FoV of JEM-EUSO. 106 neutrinos for each energy between 1019 eV and 1021 eV are injected.
3. Results
cumulated in the database of EAS longitudinal structures. Emissions of air fluorescence/Cherenkov photons from the secondary charged particles in EAS are calculated assuming the photon yield and their attenuation in the atmosphere due to Rayleigh and the Mie scattering during propagation in atmosphere. The number of photons at the JEM-EUSO telescope is estimated with the following known features: (i) the optical lens size of 2.5 m in diameter, (ii) the observation from a height of 430 km with a FoV of ±30◦ , (iii) a time resolution of 2.5 μs (GTU) and the size of the ground segment of 0.8 km. The simulation is executed with the input parameters of the initial energy, the arrival direction and the position of incidence. The characteristics of arrival photons at the telescope; the arrival time, the wave length and the emission point (x,y,z), are recorded as the output data.
Figure 1 shows the distributions of interaction depths (in g/cm2 ) of UHE neutrinos in the atmosphere. Neutrinos with a zenith angle of 90◦ and energies of 1019 eV, 1019.5 eV and 1020 eV are injected into the atmosphere with statistics of 106 for each energy. The interaction mean free path in the atmosphere for these energies are 7.0 × 107 , 4.6 × 107 and 3.0 × 107 g/cm2 . Therefore, the distributions in 0 - 72,000 g/cm2 (the maximum slant depth for the horizontal) are almost uniform though the distribution of hadronic cosmic rays does not exceed 1,400 g/cm2 because of the shorter interaction mean free path < 100 g/cm2 . The interaction probability of UHE neutrinos as a function of energy is shown in figure 2. The target atmospheric matter is assumed to be the acceptance of JEM-EUSO with a FoV of ±30◦ from observations at a height of 430 km. 106 neutrinos for each energy between 1019 eV and 1021 eV are injected into the FoV with zenith angles up to 95◦ sampled from the zenith angle distribution following a sine function. The interaction proba-
281
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
Number of photons/(2.5 μ sec bin)
Histgram of photon distribution
arrival time (μ sec)
Figure 3. The relation between the first interaction depth and the incident zenith angle for neutrinos with the fixed energy of 1020 eV. 107 neutrinos are injected to a FoV of JEM-EUSO.
bility, which indicates the ratio of the number of interacted neutrinos to all incident ones, is nearly 0.01% at 1020 eV and increases with energy. Figure 3 shows the relation between the first interaction depths in a FoV and incident zenith angles. 107 neutrinos with a fixed energy of 1020 eV are injected with zenith angles sampled from the same distribution. EASs initiated by neutrinos in the larger zenith angle region, typically > 70◦ , are observed dominantly because the larger slant depth of atmosphere has a larger probability for neutrinos to interact. Several events that appeared in 2 × 106 g/cm2 ∼ 5 × 107 g/cm2 are ground-skimming events initiated by neutrinos with zenith angles of 90◦ ∼ 95◦ . After determining the interaction point in a FoV, a neutrino EAS is generated. The emission of air fluorescence/Cherenkov photons from the EAS particles and their propagations in atmosphere are calculated. The time profiles of arrival photons at the telescope with a diameter of 2.5 m are shown in figure 4 for EASs with an energy of
Number of photons/(2.5 μ sec bin)
Histgram of photon distribution
arrival time (μ sec)
Figure 4. The time profiles of arrival photons of neutrino EASs with an energy of 1020 eV at a zenith angle of 90◦ . EASs passing through at different altitudes of 2.0 km(top) and 9.6 km(bottom).
1020 eV at a zenith angle of 91◦ . The numbers of photons binned in every 2.5 μs are plotted for events passing through at different heights of 2.0 km (top) and 9.6 km (bottom). An EAS developed in the higher atmosphere, produces a larger number of photons in its development than that
282
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
of an EAS developed in the lower atmosphere, because the fluorescence photon yield[8] does not depend so much on the height between 0 km and 20 km. In order to estimate the detection possibility of neutrino EASs by JEM-EUSO, the following procedures have been taken into account, (i) 3 different neutrino energy spectra (a), (b) and (c) are assumed for the examination as shown in figure 5, and 5 ×106 neutrinos are injected with energies sampled from each energy spectrum, (ii) after confirming the interaction in a FoV, a neutrino induced EAS is generated in the atmosphere and the time profile of arrival photons at the optical lens is calculated, (iii) a simple judgement on the event trigger is done by satisfying the condition of 5 successive GTUs with > 35 photons in each GTU. This required photon number in a GTU corresponds to a level of 3σ higher than the average background photons (500 photons/(m2 · sr · ns)). The expected numbers of neutrino EASs with > 1019.7 eV are 1.3, 0.3 and 0.3 events/year for the energy spectra of (a),(b) and (c), respectively. neutrino induced EASs with > 1020.1 eV are also expected as 0.65, 0.18 and 0.11 events/year, respectively. The statistics are still small from this estimation, however 1-5 neutrino EASs will be expected in 5 years observation and candidates of neutrino induced EAS could be well distinguished from the background EASs by Xmax with high reliability. JEM-EUSO plans a tilted mode observation to increase its acceptance by 3 times larger than the nadir mode observation in the energy range > 1020 eV. The expected numbers over 3 years in the tilted mode observation will be 2.7 and 1.9 events for energies of > 1019.7 eV and > 1020.5 eV for case (b).
Figure 5. The 3 different energy spectra used in the simulation are plotted by the lines of (a), (b) and (c). The intensity of the Waxman-Bahcall limit [9] is also shown as a black line in the figure.
REFERENCES 1. K. Greisen, Phys. Rev. Lett. 16, (1966) 748. 2. G.T. Zatsepin and V.A. Kuzumin, Pis’ma Zh. Eksp. Theor. Fiz, 4 (1966) 144. 3. T. Ebisuzaki et al., Nuclear Physics B (Proc. Suppl.) 175-176 (2008) 237. 4. R. Gandihi, C. Quigg, M. H. Reno and I. Sarcevic, Phys. Rev. D58 (1998) 093009 5. U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976 6. R. Gandihi, C. Quigg, M. H. Reno and I. Sarcevic, Astropart. Phys. 5 (1996) 81. 7. S.J. Sciutto,astro-ph/9911311. 8. M. Nagano, K. Kobayakawa, N. Sakaki and K. Ando, Astropart. Phys. 22 (2004) 235. 9. E. Waxman and J. Bahcall, Phys. Rev. D, 73 (2006) 083003.
UHE neutrino detection by a satellite-based air shower experiment K. Miyazawaa, N. Inouea , K. Higashidea a
The Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
Ultra-High-Energy(UHE) neutrinos have not yet been detected because of the small interaction cross-section with the target matter. UHE neutrinos could be produced in astrophysical objects where UHE cosmic rays are accelerated and could also be expected in the top-down process such as the decay of SHR/topological defects. Extensive Air Showers (EASs) induced by neutrinos have a uniform Xmax distribution over atmospheric slant depth though that of hadronic EASs is limited between 800 g/cm2 and 1200 g/cm2 . Assuming UHE neutrino fluxes, the detection efficiencies by satellite-based observation with a target mass of 1015 kg are estimated.
1. Introduction
2. Simulation procedure
The chemical composition of UHE cosmic rays is one of the unsolved questions, and its study will provide information on the origin and the acceleration mechanism of UHE cosmic rays. In fact, the detection of UHE neutrinos with energies > 1019 eV will be a key to solve these questions. Cosmogenic neutrinos have been expected to be steadily produced in the GZK process [1,2]. The possibility of neutrino production during the acceleration in AGNs or GRBs has also been pointed out. UHE neutrinos have such a small interaction cross-section with matter, and will be the best probe to study UHE cosmic ray accelerators at cosmological distances. UHE neutrinos rarely interact with atmospheric nuclei and the produced UHE electrons generate EAS in the atmosphere. Neutrino induced EASs are clearly distinguished from hadronic EASs in terms of the Xmax , as they will be typically largely inclined EASs or upwardgoing EASs. From the experimental point of view, a huge amount of target mass is required to ensure interactions in the atmosphere. To overcome this problem, JEM-EUSO[3], a mission installed on the International Space Station to study chargedparticle astronomy in the energy region above 3×1019 eV, is a suitable observation platform because of its large Field of View (FoV), 250 km×170 km, and the atmospheric matter in the acceptance is larger than 1 tera-ton.
UHE neutrino simulation codes consist of neutrino interactions, EAS generation and the production of air fluorescence/Cherenkov UV photons along the EAS development. The interaction cross-section with the atmospheric nucleus (σcc (νN )) is assumed to be a function of equation (1)[4] in this calculation. The interaction length (Lint ) related to the cross-section in the atmosphere is expressed by (2).
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.09.053
σcc (νN ) = 5.53 × 10−36 (Eν /1GeV )0.363 [cm2 ] (1) Lint = 1/(σcc (νN ) · NA ) [g/cm2 ]
(2)
where NA and Eν are the Avogadro constant and the neutrino energy. The US standard atmospheric model [5] is used in the calculation to estimate the air density, ρ(r)[g/cm3 ], as a function of the vertical height, (r). A neutrino EAS is induced by secondary electrons produced in the neutrino interaction and the ratio between the EAS energy (Ee ) and the initial energy (Eν )[6] is given by Ee /Eν = (0.6724 + 0.0058 log Eν )
(3)
Electro-magnetic EASs have been generated by the AIRES code (Ver.2.6.0) [7]. The individual longitudinal structure is fitted by the GaisserHillas formula with 3 parameters1 , and are ac x a2 ×a3 1 N (x) = a1
a2
× exp [(a2 − x)a3 ]
280
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
Interaction probability [%]
Interaction Probability in F.O.V. 0.02
0.01
0 19
Figure 1. The distributions of interaction depths in the atmosphere. Neutrinos with zenith angle of 90◦ and energies of 1019.0 eV, 1019.5 eV, 1020 eV are initiated.
20
21 Log10(E) [eV]
Figure 2. The interaction probability in a FoV of JEM-EUSO. 106 neutrinos for each energy between 1019 eV and 1021 eV are injected.
3. Results
cumulated in the database of EAS longitudinal structures. Emissions of air fluorescence/Cherenkov photons from the secondary charged particles in EAS are calculated assuming the photon yield and their attenuation in the atmosphere due to Rayleigh and the Mie scattering during propagation in atmosphere. The number of photons at the JEM-EUSO telescope is estimated with the following known features: (i) the optical lens size of 2.5 m in diameter, (ii) the observation from a height of 430 km with a FoV of ±30◦ , (iii) a time resolution of 2.5 μs (GTU) and the size of the ground segment of 0.8 km. The simulation is executed with the input parameters of the initial energy, the arrival direction and the position of incidence. The characteristics of arrival photons at the telescope; the arrival time, the wave length and the emission point (x,y,z), are recorded as the output data.
Figure 1 shows the distributions of interaction depths (in g/cm2 ) of UHE neutrinos in the atmosphere. Neutrinos with a zenith angle of 90◦ and energies of 1019 eV, 1019.5 eV and 1020 eV are injected into the atmosphere with statistics of 106 for each energy. The interaction mean free path in the atmosphere for these energies are 7.0 × 107 , 4.6 × 107 and 3.0 × 107 g/cm2 . Therefore, the distributions in 0 - 72,000 g/cm2 (the maximum slant depth for the horizontal) are almost uniform though the distribution of hadronic cosmic rays does not exceed 1,400 g/cm2 because of the shorter interaction mean free path < 100 g/cm2 . The interaction probability of UHE neutrinos as a function of energy is shown in figure 2. The target atmospheric matter is assumed to be the acceptance of JEM-EUSO with a FoV of ±30◦ from observations at a height of 430 km. 106 neutrinos for each energy between 1019 eV and 1021 eV are injected into the FoV with zenith angles up to 95◦ sampled from the zenith angle distribution following a sine function. The interaction proba-
281
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
Number of photons/(2.5 μ sec bin)
Histgram of photon distribution
arrival time (μ sec)
Figure 3. The relation between the first interaction depth and the incident zenith angle for neutrinos with the fixed energy of 1020 eV. 107 neutrinos are injected to a FoV of JEM-EUSO.
bility, which indicates the ratio of the number of interacted neutrinos to all incident ones, is nearly 0.01% at 1020 eV and increases with energy. Figure 3 shows the relation between the first interaction depths in a FoV and incident zenith angles. 107 neutrinos with a fixed energy of 1020 eV are injected with zenith angles sampled from the same distribution. EASs initiated by neutrinos in the larger zenith angle region, typically > 70◦ , are observed dominantly because the larger slant depth of atmosphere has a larger probability for neutrinos to interact. Several events that appeared in 2 × 106 g/cm2 ∼ 5 × 107 g/cm2 are ground-skimming events initiated by neutrinos with zenith angles of 90◦ ∼ 95◦ . After determining the interaction point in a FoV, a neutrino EAS is generated. The emission of air fluorescence/Cherenkov photons from the EAS particles and their propagations in atmosphere are calculated. The time profiles of arrival photons at the telescope with a diameter of 2.5 m are shown in figure 4 for EASs with an energy of
Number of photons/(2.5 μ sec bin)
Histgram of photon distribution
arrival time (μ sec)
Figure 4. The time profiles of arrival photons of neutrino EASs with an energy of 1020 eV at a zenith angle of 90◦ . EASs passing through at different altitudes of 2.0 km(top) and 9.6 km(bottom).
1020 eV at a zenith angle of 91◦ . The numbers of photons binned in every 2.5 μs are plotted for events passing through at different heights of 2.0 km (top) and 9.6 km (bottom). An EAS developed in the higher atmosphere, produces a larger number of photons in its development than that
282
K. Miyazawa et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 279–282
of an EAS developed in the lower atmosphere, because the fluorescence photon yield[8] does not depend so much on the height between 0 km and 20 km. In order to estimate the detection possibility of neutrino EASs by JEM-EUSO, the following procedures have been taken into account, (i) 3 different neutrino energy spectra (a), (b) and (c) are assumed for the examination as shown in figure 5, and 5 ×106 neutrinos are injected with energies sampled from each energy spectrum, (ii) after confirming the interaction in a FoV, a neutrino induced EAS is generated in the atmosphere and the time profile of arrival photons at the optical lens is calculated, (iii) a simple judgement on the event trigger is done by satisfying the condition of 5 successive GTUs with > 35 photons in each GTU. This required photon number in a GTU corresponds to a level of 3σ higher than the average background photons (500 photons/(m2 · sr · ns)). The expected numbers of neutrino EASs with > 1019.7 eV are 1.3, 0.3 and 0.3 events/year for the energy spectra of (a),(b) and (c), respectively. neutrino induced EASs with > 1020.1 eV are also expected as 0.65, 0.18 and 0.11 events/year, respectively. The statistics are still small from this estimation, however 1-5 neutrino EASs will be expected in 5 years observation and candidates of neutrino induced EAS could be well distinguished from the background EASs by Xmax with high reliability. JEM-EUSO plans a tilted mode observation to increase its acceptance by 3 times larger than the nadir mode observation in the energy range > 1020 eV. The expected numbers over 3 years in the tilted mode observation will be 2.7 and 1.9 events for energies of > 1019.7 eV and > 1020.5 eV for case (b).
Figure 5. The 3 different energy spectra used in the simulation are plotted by the lines of (a), (b) and (c). The intensity of the Waxman-Bahcall limit [9] is also shown as a black line in the figure.
REFERENCES 1. K. Greisen, Phys. Rev. Lett. 16, (1966) 748. 2. G.T. Zatsepin and V.A. Kuzumin, Pis’ma Zh. Eksp. Theor. Fiz, 4 (1966) 144. 3. T. Ebisuzaki et al., Nuclear Physics B (Proc. Suppl.) 175-176 (2008) 237. 4. R. Gandihi, C. Quigg, M. H. Reno and I. Sarcevic, Phys. Rev. D58 (1998) 093009 5. U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976 6. R. Gandihi, C. Quigg, M. H. Reno and I. Sarcevic, Astropart. Phys. 5 (1996) 81. 7. S.J. Sciutto,astro-ph/9911311. 8. M. Nagano, K. Kobayakawa, N. Sakaki and K. Ando, Astropart. Phys. 22 (2004) 235. 9. E. Waxman and J. Bahcall, Phys. Rev. D, 73 (2006) 083003.