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Progress in Very High Energy Neutrino Astronomic Experiments
Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382 www.elsevierphysics.com
Progress in Very High Energy Neutrino Astronomic Experiments Zhen Caoa∗ a
19B Yuquan St., Shijingshan Beijing P.O. Box 918, Beijing 100049, P.R. China In this review, I will report progress in UHE neutrino search using existing experiments and status of constructing detectors. Physics relevant to UHE neutrinos are updated as well.
1. Introduction From April 23rd to 26th, 2006, the International Ultra High Energy τ Neutrino Workshop was held in IHEP, Beijing, China. About 50 people from US, France, Italy, Span, Taiwan and Mainland China attended the workshop. Among them, there are about 40 experimentalists and 10 theoreticians. In the workshop, attendances reported the progresses in both experimental and theoretic researches. Upon a request of the international advisory committee of this symposium, I provide a summary report about the workshop. I will report 1. Ultra high energy neutrino search with currently operational experiments; 2. status of new experiments; and 3. Progresses in ultra high energy neutrino sources, related physics etc. In the last part, I will introduce some new ideas that may be useful for the next generation of detectors to explore cosmogenic neutrinos.
2. Ultra High Energy Neutrino Search with Operational Experiments Existing ultra high energy cosmic ray detector are HiRes at Utah, USA and Auger at Melargue, Argentina. They are searching for neutrinos above 1EeV (1018 eV). At lower energies, the AMANDA II experiment is exploring the region above 1013 eV. ∗ This
work is supported by the Innovation fund (U526) of IHEP and Hundred Talents & Out- standing Young Scientists Abroad Program (U- 610/112901560333) IHEP/CAS, China.
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.035
2.1. Auger The Auger Experiment is the largest cosmic ray detector array in the world and monitors the south hemisphere for air showers above 1EeV. By March 2006, there are about 1000 water Cerenkov detectors are in operation as a surface array (SD). With a spacing of 1.5km between water tanks, the array has covered about 2000km2. There are also three fluorescence buildings completed each with 6 telescopes (FD) over watching the surface array with an elevation coverage up to 30◦ . Since the surface water tank Cerenkov detector has a height about 1m, the array is sensitive to near horizontal showers. Putting the FD and SD together, the whole detector is sensitive to 3 types of neutrino events. Class A events are earth skimming neutrinos, those neutrinos, mainly ντ , skim in the earth shell, interact with rock, generate τ s escaping from the shell and produces up-going showers in the air near the horizon. Class B events are called “Andes Skimming” events. They are also mainly from ντ s that interact in the Mountain Andes, which is about 60 km away from the detector array, produce τ leptons escaping from the mountain and produce air showers near the horizon. Class C events are due to neutrinos that interact in the air and generate showers directly. All three types of neutrino events are illustrated in Figure.1. Using Waxman-Bahcall flux, the number of events is estimated 0.1 νe and ντ per year for E>1EeV using FD only. Using surface detector array, horizontal events have been found as candidates. One of them is shown in Figure.1 with zenith angle of 87.6◦ . However, those candidates are not neutrino showers because if they are, one of characteristics should be that the fall-time
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Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
Figure 1. Category of neutrino events that Auger experiment can see with FD. An example SD horizontal event.
to rise-time of SD pulses must both be greater than 100ns and well correlated. By March 2006, no neutrino event has been found in the existing data set. 2.2. HiRes The HiRes Experiment is composed of two sites of fluorescence light telescope arrays 12.6 km apart. At one site, 22 telescopes (HR1) cover an elevation angle range from 3◦ to 17◦ and start operation since 1997 and 42 telescopes (HR2) cover elevation angles from 3◦ to 31◦ and start operation since the end of 1999 at another site. Both telescope arrays have full azimuthal coverage. The aperture of each HiRes telescope array is hundreds of km2 sr above their threshold ( 0.6EeV) and more than 10,000km2sr at 1020 eV. Working in 300-400nm UV band, the detector can only be operated in moon-less clear night without clouds. Until its termination of operation in April, 2006, the HiRes Experiment has live time 20,132,360 sec for HR1 and 13,096,693 sec for HR2, respectively. Skimming τ neutrinos through the earth shell and nearby mountains are signals. Both νe s and ντ s can also interact in the air and generate showers directly. Figure 2 shows a simulated distribution of all shower start point surrounding the HiRes detectors and sensitivity of the HiRes detector as a function of neutrino energy. There are expectation for several events above 10EeV according to estimated cosmogenic neutrino flux due to interaction of protons or nuclei with CMB photons above GZK cut-off and
Figure 2. Simulated Neutrino interaction location distribution surround HiRes detectors and corresponding sensitivity of GZK-neutrino detection using HiRes detectors.
decay of pions in products. Preliminary analysis shows no up-going or horizontal event in the HiRes data set. Matching events observed by HR1 and HR2 simultaneously, the HiRes detectors can be used in stereoscopic mode. An event at zenith angle=76.2◦ is found at 40EeV, Figure3. According to the shower maximum location of 800g/cm2 as measured, the shower does favor a proton origin but not a neutrino origin. Further search for horizontal events is still in progress.
Figure 3. A near-horizontal HiRes event. Upper panels show shower images in both HR1 and HR2 detectors and lower panels show shower development measured by both detectors, vertical axes are the number of measured signals and the horizontal axes are slant depth of the atmosphere.
2.3. Amanda The AMANDA Experiment starts taking data since 1997, with 10 strings of optical modules (OM) buried in ice from 1500m to 2000m underneath the South Pole. The experiment was
Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
enlarged to diameter of 200m from 120m and to 19 strings of OMs in 2000. Combining the data from 2000 to 2003, AMANDA experiment searches possible point sources in the north sky. In total, 3329 upward going events measured with the highest excess of 3.4σ as shown in sky map Figure 4. For diffuse neutrino search, no signal is observed therefore upper limits for flux are set as shown in Figure 4. The AGN source model by Stecker et al.[1] is clearly ruled out. The new limit set by using 2000-2003 combined data is approaching the model based on AGN jet effect. Further analysis result is available now [2].
379
all type of sources of neutrinos, including point source, diffuse neutrinos originated from AGNs, GRBs and cosmogenic processes. Building larger detectors to boost sensitivity up is the way to approach. Great efforts have been making in the society throughout the whole world. In this section, status of all under construction experiments are reported. 3.1. IceCube To enlarge the AMANDA detector by a factor of 100, the IceCube Experiment has been started since 2004. The total detector volume of the IceCube will be 0.9km3 and 80 kilometerlong strings each with 60 OMs will be deployed from depth 1450m to 2450m in ice. Figure 5 illustrates the Whole IceCube detector and status of deployment. By March 2006, 9 strings have been deployed. 12 more strings are going to be deployed in 2006-2007 season. The capability of the
Figure 4. Point source search with AMANDAII data from 2000 to 2003 (left panel). In right panel, limits for diffuse neutrino background with AMANDAII data. 2: B10νμ (1997); 3: B10 UHE (1997); 4: AMANDAII Cascades (2000); 6: AMANDAII νμ (2000); 7: AMANDAII UHE; 8: AMANDAII νμ (2000-2003).
3. Status of Constructing Neutrino Experiments Existing experiments have been gradually opening a new window for people to explore UHE astronomy using neutrinos. Point source search and diffuse neutrino detection cover all energy range from tens of TeV to 1020 eV. UHE neutrino provides direct evidence for proton source in high energy astronomic processes. There is no difficulty for exploring remote sources using neutrinos because they do not interact with CMB photons. However, the results from existing experiments show that we are strongly limited by the sensitivity of the detectors at all energies for
Figure 5. Illustration of IceCube detector and deployment status by March, 2006.
IceCube detector allows it to detect cosmogenic neutrinos at about 4 events per year. The effective area reaches 1.4 km2 at 100PeV, the angular resolution to 0.7◦ for neutrinos between 1PeV and 100PeV.
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Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
3.2. ANITA Another Antarctic experiment is the balloonborne radio Cerenkov detector, ANITA. Illustrated in Figure 6, the ANITA Experiment uses 32 quad-ridged horn antennas to catch UHF radio Cerenkov radiation signals from showers induced by ice-skimming neutrinos. Neutrinos with energy greater than 0.1EeV and interact in Antarctic ice sheet, induce showers in ice that produce Cerenkov radio in cones. Radio signals continuously propagate after being deflected into the air and are picked up by the ANITA detector. Since the attenuation is negligible, the ANITA detector is surveying an area of 1 million km2 on the surface from an altitude of 37km. Prototype detector ANITA-lite had a test flight during 2003-2004 with two antennas. In 18 days of flight, the prototype detector working properly but did not see signals from skimming neutrinos. This results in a limit for high energy neutrino flux, as shown in Figure 6. It clearly rules out the zburst model. In June 2006, ANITA detector was calibrated using an electron beam at SLAC. The ANITA project is scheduled for its first full scale flight by the end of this year.
3.3. Antares The Antares Experiment is an underwater neutrino telescope in Mediterranean near La Seyne, France. The Antares telescope deploys strings of OMs in sea water from depth of 2000m to 2500m. In total, 900 OMs in 12 strings will be deployed by the end of 2007. All OMs in a string are distributed in 25 storeys and 3 OMs at each storey. The first string was deployed in Feb. 2006. The first muon track has been measured as shown in Figure 7. The complete Antares telescope will see
Figure 7. Deployment of the first string of OMs of the Antares Experiment. The first muon track measured by the neutrino telescope.
the Galactic center in 2/3 of observational time, and have a 0.5 πsr instantaneous common view of Galactic center and 1.5 πsr common view per day with South Pole detector, i.e. IceCube. The angular resolution is 0.2◦ above 100TeV. It is very useful for background suppression in point source search. It will be a complementary measurement to the HESS gamma ray experiment.
Figure 6. Illustration of ANITA Experiment and results from a flight with prototype detector.
3.4. Nemos Another under water neutrino telescope is Nemo near Sicily. A tower structure made of horizontal bars and strings forms a semi-3D structure under water. OMs are attached to the ends of bars which is 15m long. Vertical spacing between bars is about 40m. An OM is made with downwards and sidewards watching PMTs. The total height of the tower is 750m. Depth of the site near Capo Passero is about 3350m. The Nemo Experiment has tested a tower with 4 bars in shallow water and deployed a cable junction box and
Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
finished the test with it. All electronics are under test including power supply, cable junction box, online calibration system, data transition system etc. The Phase 1 deployment-connection and DAQ start-up was tested in June-July 2006. 3.5. KM3 Another European under water neutrino project is Nestor located at Pylos, Greece. No speaker from the Nestor Experiment attends the workshop. The three Mediterranean water Cerenkov neutrino telescopes are planning to integrate their efforts together and building a larger telescope at a scale of km3 . This will result in a complementary detector at north hemisphere to the South Pole detector IceCube, called KM3 Experiment, illustrated in Figure 8. 81 towers
Figure 8. the KM3 Experiment is planning to build a under water neutrino telescope at a scale of km3 .
with 5832 PMTs are arranged in a 9x9 array with a spacing of 140m between towers and 40m between floors. Angular resolution is about 0.19◦ . The physics expectations of the experiment is to measure the diffuse neutrino fluxes at event rates of 0.5/yr from GZK processes, 50/yr from GRBs and few/yr or more than 100/yr from AGNs depending on model. For point source search, the experiment expects to see 1-10 GRBs per year, few AGNs like 3c279 per year, few galactic SNR per year and 1 to 100 micro-quasars per year. 3.6. CRTNT Because of neutrino oscillation, τ neutrinos are expected at the earth even though there is no ντ
381
at the remote sources. To detect ντ , air shower Cerenkov/fluorescence light telescope can be useful if there is a mountain in front of the telescope array to play the role of a target. Generated τ leptons have a chance to escape from rock and decay in the air. About 80% of decay products (hadrons and electrons) will initiate showers in the air. Showers with energy greater than 10TeV could be detectable by the telescope array. This is the idea behind the CRTNT Experiment. A mountain at Balikun, China has been selected as an ideal site for the CRTNT Experiment that has 16 C/F light telescopes configured in an array as Figure 9 shows. 1/3 IceCube sensitivity for diffuse AGN flux is expected. Two prototype tele-
Figure 9. Left: The CRTNT Experiment consists 16 portable fluorescence/Cerenkov light telescopes watching into mountains for air showers induced by ντ interacting inside. Right: Each telescope consists of 5m2 alumina spherical mirror, imaging camera made of 256 PMT, equivalent 12 bit FADC read-out and online UV LED calibration. Attached hydraulic lift and trailer system make the telescope portable.
scopes have been completed and passed final test in lab, as shown in Figure 9. Currently, we are installing them near by the ARGO Experiment at Tibet, China to coincidentally measure cosmic rays above 100TeV as an in-situ calibration of the telescopes. 3.7. Nutel The Nutel project is a similar ντ detection experiment initiated by National Taiwan Univ. and
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Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
used to collaborate with ASHRA experiment at Hawaii, USA. By April 2006, a whole electronics readout system for a prototype telescope was completed. Production of mirrors and telescope housing are in progress. The Nutel group developed a complete simulation software for ντ interaction, air shower development. More specific detector simulation can be easily fit in by setting proper geometry for detectors. The package is named SHENIE.
4. Updates on Physics Successful observations of galactic VEH γsources by HESS and MAGIC experiments enlarged a galactic neutrino source candidate group. Sources near galactic center, such as SS433,GX3394, are most favorable neutrino sources. Micro-quasars, LSi+61303[3] and HESS J1826-148[4], newly discovered by MAGIC and HESS open another category of candidates for galactic UHE neutrinos. Nearby GBSs (Z < 0.03) can be 500 times[5] more than expected as hinted by Swift observation, this results in a possible shift upwards for GRB neutrino energies. Cosmogenic neutrinos produced in proton scattering on IR background photons [6] shift the traditional GZK neutrinos downwards in energy. Both effects make the neutrino detection more effective for most of neutrino telescopes. Neutrino detection may open a channel to go beyond the standard model. Super-symmetry particles, such as neutrilino and sleptons, are expected to play a role to transfer UHE neutrino energy to other side of the earth [7]. They will induce exotic showers in the field of view of most large scale cosmic ray detectors, e.g. HiRes and Auger etc. Sterile neutrinos play similar roles for producing exotic events [8]. All of them are candidates for dark matter. Due to effects associated with extra dimension, neutrino interaction cross section could be greatly enhanced above 1PeV[9]. This changes predictions about event rates and can be useful for exploring those extra dimensions if a higher event rate are observed at energies well above 1PeV.
5. Outlook and Summary As a “guaranteed” source for UHE neutrinos, cosmogenic neutrinos generated in “GZK process”, namely cosmic ray protons scatter on CMB photons and produce pions and eventually decay into neutrinos, are the goal for projects post IceCube. IceCube is still too small. A larger effective area, cheaper detector is needed. Both radio technique and acoustic technique are under development at the South Pole. IceCube has a plan to hybrid those two techniques together and build an array with a size 75 times larger than IceCube detector in the shallower ice above the IceCube strings. This will enable a statistic study of GZK neutrinos. The Salsa Project is another expedition of detection of cosmogenic neutrinos by using natural salt-dumb underground. The idea is to drill holes in salt-dumb to 3000m deep and bury strings of radio antennas to form an array with Vef f about 100 to 200 km3 . With such a detector, hundreds of GZK neutrinos are expected per year. As a fast growing field, UHE neutrino detection has been gradually opening a new astronomic window for us to explore the universe. This allows us to directly locate sources where UHE processes occur. This also allows us to explore remote sources beyond the distance that 1PeV (and higher) photons can pass through, to discover completely different and unknown aspects of the universe. REFERENCES 1. F.W. Stecker et al., Phy.Rev. Lett. 66, 2697, 1991. 2. A. Achterberg et al., Astropart.Phys.26,2006. 3. J. Albert et al., Science 312,1771,2006. 4. F. Aharonian et al., Science 309, 746, 2005. 5. N. Gupta and B. Zhang, astro-ph/0610770. 6. T.Stanev, astro-ph/0607515. 7. M.H. Reno, I. Sarcevic, S. Su, Astropart.Phys.24,107,2005. 8. A.Kusenko, hep-ph/0609158. 9. T.Weiler, Int.J.Mod.Phys.A20,1168,2005.
Progress in Very High Energy Neutrino Astronomic Experiments Zhen Caoa∗ a
19B Yuquan St., Shijingshan Beijing P.O. Box 918, Beijing 100049, P.R. China In this review, I will report progress in UHE neutrino search using existing experiments and status of constructing detectors. Physics relevant to UHE neutrinos are updated as well.
1. Introduction From April 23rd to 26th, 2006, the International Ultra High Energy τ Neutrino Workshop was held in IHEP, Beijing, China. About 50 people from US, France, Italy, Span, Taiwan and Mainland China attended the workshop. Among them, there are about 40 experimentalists and 10 theoreticians. In the workshop, attendances reported the progresses in both experimental and theoretic researches. Upon a request of the international advisory committee of this symposium, I provide a summary report about the workshop. I will report 1. Ultra high energy neutrino search with currently operational experiments; 2. status of new experiments; and 3. Progresses in ultra high energy neutrino sources, related physics etc. In the last part, I will introduce some new ideas that may be useful for the next generation of detectors to explore cosmogenic neutrinos.
2. Ultra High Energy Neutrino Search with Operational Experiments Existing ultra high energy cosmic ray detector are HiRes at Utah, USA and Auger at Melargue, Argentina. They are searching for neutrinos above 1EeV (1018 eV). At lower energies, the AMANDA II experiment is exploring the region above 1013 eV. ∗ This
work is supported by the Innovation fund (U526) of IHEP and Hundred Talents & Out- standing Young Scientists Abroad Program (U- 610/112901560333) IHEP/CAS, China.
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.035
2.1. Auger The Auger Experiment is the largest cosmic ray detector array in the world and monitors the south hemisphere for air showers above 1EeV. By March 2006, there are about 1000 water Cerenkov detectors are in operation as a surface array (SD). With a spacing of 1.5km between water tanks, the array has covered about 2000km2. There are also three fluorescence buildings completed each with 6 telescopes (FD) over watching the surface array with an elevation coverage up to 30◦ . Since the surface water tank Cerenkov detector has a height about 1m, the array is sensitive to near horizontal showers. Putting the FD and SD together, the whole detector is sensitive to 3 types of neutrino events. Class A events are earth skimming neutrinos, those neutrinos, mainly ντ , skim in the earth shell, interact with rock, generate τ s escaping from the shell and produces up-going showers in the air near the horizon. Class B events are called “Andes Skimming” events. They are also mainly from ντ s that interact in the Mountain Andes, which is about 60 km away from the detector array, produce τ leptons escaping from the mountain and produce air showers near the horizon. Class C events are due to neutrinos that interact in the air and generate showers directly. All three types of neutrino events are illustrated in Figure.1. Using Waxman-Bahcall flux, the number of events is estimated 0.1 νe and ντ per year for E>1EeV using FD only. Using surface detector array, horizontal events have been found as candidates. One of them is shown in Figure.1 with zenith angle of 87.6◦ . However, those candidates are not neutrino showers because if they are, one of characteristics should be that the fall-time
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Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
Figure 1. Category of neutrino events that Auger experiment can see with FD. An example SD horizontal event.
to rise-time of SD pulses must both be greater than 100ns and well correlated. By March 2006, no neutrino event has been found in the existing data set. 2.2. HiRes The HiRes Experiment is composed of two sites of fluorescence light telescope arrays 12.6 km apart. At one site, 22 telescopes (HR1) cover an elevation angle range from 3◦ to 17◦ and start operation since 1997 and 42 telescopes (HR2) cover elevation angles from 3◦ to 31◦ and start operation since the end of 1999 at another site. Both telescope arrays have full azimuthal coverage. The aperture of each HiRes telescope array is hundreds of km2 sr above their threshold ( 0.6EeV) and more than 10,000km2sr at 1020 eV. Working in 300-400nm UV band, the detector can only be operated in moon-less clear night without clouds. Until its termination of operation in April, 2006, the HiRes Experiment has live time 20,132,360 sec for HR1 and 13,096,693 sec for HR2, respectively. Skimming τ neutrinos through the earth shell and nearby mountains are signals. Both νe s and ντ s can also interact in the air and generate showers directly. Figure 2 shows a simulated distribution of all shower start point surrounding the HiRes detectors and sensitivity of the HiRes detector as a function of neutrino energy. There are expectation for several events above 10EeV according to estimated cosmogenic neutrino flux due to interaction of protons or nuclei with CMB photons above GZK cut-off and
Figure 2. Simulated Neutrino interaction location distribution surround HiRes detectors and corresponding sensitivity of GZK-neutrino detection using HiRes detectors.
decay of pions in products. Preliminary analysis shows no up-going or horizontal event in the HiRes data set. Matching events observed by HR1 and HR2 simultaneously, the HiRes detectors can be used in stereoscopic mode. An event at zenith angle=76.2◦ is found at 40EeV, Figure3. According to the shower maximum location of 800g/cm2 as measured, the shower does favor a proton origin but not a neutrino origin. Further search for horizontal events is still in progress.
Figure 3. A near-horizontal HiRes event. Upper panels show shower images in both HR1 and HR2 detectors and lower panels show shower development measured by both detectors, vertical axes are the number of measured signals and the horizontal axes are slant depth of the atmosphere.
2.3. Amanda The AMANDA Experiment starts taking data since 1997, with 10 strings of optical modules (OM) buried in ice from 1500m to 2000m underneath the South Pole. The experiment was
Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
enlarged to diameter of 200m from 120m and to 19 strings of OMs in 2000. Combining the data from 2000 to 2003, AMANDA experiment searches possible point sources in the north sky. In total, 3329 upward going events measured with the highest excess of 3.4σ as shown in sky map Figure 4. For diffuse neutrino search, no signal is observed therefore upper limits for flux are set as shown in Figure 4. The AGN source model by Stecker et al.[1] is clearly ruled out. The new limit set by using 2000-2003 combined data is approaching the model based on AGN jet effect. Further analysis result is available now [2].
379
all type of sources of neutrinos, including point source, diffuse neutrinos originated from AGNs, GRBs and cosmogenic processes. Building larger detectors to boost sensitivity up is the way to approach. Great efforts have been making in the society throughout the whole world. In this section, status of all under construction experiments are reported. 3.1. IceCube To enlarge the AMANDA detector by a factor of 100, the IceCube Experiment has been started since 2004. The total detector volume of the IceCube will be 0.9km3 and 80 kilometerlong strings each with 60 OMs will be deployed from depth 1450m to 2450m in ice. Figure 5 illustrates the Whole IceCube detector and status of deployment. By March 2006, 9 strings have been deployed. 12 more strings are going to be deployed in 2006-2007 season. The capability of the
Figure 4. Point source search with AMANDAII data from 2000 to 2003 (left panel). In right panel, limits for diffuse neutrino background with AMANDAII data. 2: B10νμ (1997); 3: B10 UHE (1997); 4: AMANDAII Cascades (2000); 6: AMANDAII νμ (2000); 7: AMANDAII UHE; 8: AMANDAII νμ (2000-2003).
3. Status of Constructing Neutrino Experiments Existing experiments have been gradually opening a new window for people to explore UHE astronomy using neutrinos. Point source search and diffuse neutrino detection cover all energy range from tens of TeV to 1020 eV. UHE neutrino provides direct evidence for proton source in high energy astronomic processes. There is no difficulty for exploring remote sources using neutrinos because they do not interact with CMB photons. However, the results from existing experiments show that we are strongly limited by the sensitivity of the detectors at all energies for
Figure 5. Illustration of IceCube detector and deployment status by March, 2006.
IceCube detector allows it to detect cosmogenic neutrinos at about 4 events per year. The effective area reaches 1.4 km2 at 100PeV, the angular resolution to 0.7◦ for neutrinos between 1PeV and 100PeV.
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Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
3.2. ANITA Another Antarctic experiment is the balloonborne radio Cerenkov detector, ANITA. Illustrated in Figure 6, the ANITA Experiment uses 32 quad-ridged horn antennas to catch UHF radio Cerenkov radiation signals from showers induced by ice-skimming neutrinos. Neutrinos with energy greater than 0.1EeV and interact in Antarctic ice sheet, induce showers in ice that produce Cerenkov radio in cones. Radio signals continuously propagate after being deflected into the air and are picked up by the ANITA detector. Since the attenuation is negligible, the ANITA detector is surveying an area of 1 million km2 on the surface from an altitude of 37km. Prototype detector ANITA-lite had a test flight during 2003-2004 with two antennas. In 18 days of flight, the prototype detector working properly but did not see signals from skimming neutrinos. This results in a limit for high energy neutrino flux, as shown in Figure 6. It clearly rules out the zburst model. In June 2006, ANITA detector was calibrated using an electron beam at SLAC. The ANITA project is scheduled for its first full scale flight by the end of this year.
3.3. Antares The Antares Experiment is an underwater neutrino telescope in Mediterranean near La Seyne, France. The Antares telescope deploys strings of OMs in sea water from depth of 2000m to 2500m. In total, 900 OMs in 12 strings will be deployed by the end of 2007. All OMs in a string are distributed in 25 storeys and 3 OMs at each storey. The first string was deployed in Feb. 2006. The first muon track has been measured as shown in Figure 7. The complete Antares telescope will see
Figure 7. Deployment of the first string of OMs of the Antares Experiment. The first muon track measured by the neutrino telescope.
the Galactic center in 2/3 of observational time, and have a 0.5 πsr instantaneous common view of Galactic center and 1.5 πsr common view per day with South Pole detector, i.e. IceCube. The angular resolution is 0.2◦ above 100TeV. It is very useful for background suppression in point source search. It will be a complementary measurement to the HESS gamma ray experiment.
Figure 6. Illustration of ANITA Experiment and results from a flight with prototype detector.
3.4. Nemos Another under water neutrino telescope is Nemo near Sicily. A tower structure made of horizontal bars and strings forms a semi-3D structure under water. OMs are attached to the ends of bars which is 15m long. Vertical spacing between bars is about 40m. An OM is made with downwards and sidewards watching PMTs. The total height of the tower is 750m. Depth of the site near Capo Passero is about 3350m. The Nemo Experiment has tested a tower with 4 bars in shallow water and deployed a cable junction box and
Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
finished the test with it. All electronics are under test including power supply, cable junction box, online calibration system, data transition system etc. The Phase 1 deployment-connection and DAQ start-up was tested in June-July 2006. 3.5. KM3 Another European under water neutrino project is Nestor located at Pylos, Greece. No speaker from the Nestor Experiment attends the workshop. The three Mediterranean water Cerenkov neutrino telescopes are planning to integrate their efforts together and building a larger telescope at a scale of km3 . This will result in a complementary detector at north hemisphere to the South Pole detector IceCube, called KM3 Experiment, illustrated in Figure 8. 81 towers
Figure 8. the KM3 Experiment is planning to build a under water neutrino telescope at a scale of km3 .
with 5832 PMTs are arranged in a 9x9 array with a spacing of 140m between towers and 40m between floors. Angular resolution is about 0.19◦ . The physics expectations of the experiment is to measure the diffuse neutrino fluxes at event rates of 0.5/yr from GZK processes, 50/yr from GRBs and few/yr or more than 100/yr from AGNs depending on model. For point source search, the experiment expects to see 1-10 GRBs per year, few AGNs like 3c279 per year, few galactic SNR per year and 1 to 100 micro-quasars per year. 3.6. CRTNT Because of neutrino oscillation, τ neutrinos are expected at the earth even though there is no ντ
381
at the remote sources. To detect ντ , air shower Cerenkov/fluorescence light telescope can be useful if there is a mountain in front of the telescope array to play the role of a target. Generated τ leptons have a chance to escape from rock and decay in the air. About 80% of decay products (hadrons and electrons) will initiate showers in the air. Showers with energy greater than 10TeV could be detectable by the telescope array. This is the idea behind the CRTNT Experiment. A mountain at Balikun, China has been selected as an ideal site for the CRTNT Experiment that has 16 C/F light telescopes configured in an array as Figure 9 shows. 1/3 IceCube sensitivity for diffuse AGN flux is expected. Two prototype tele-
Figure 9. Left: The CRTNT Experiment consists 16 portable fluorescence/Cerenkov light telescopes watching into mountains for air showers induced by ντ interacting inside. Right: Each telescope consists of 5m2 alumina spherical mirror, imaging camera made of 256 PMT, equivalent 12 bit FADC read-out and online UV LED calibration. Attached hydraulic lift and trailer system make the telescope portable.
scopes have been completed and passed final test in lab, as shown in Figure 9. Currently, we are installing them near by the ARGO Experiment at Tibet, China to coincidentally measure cosmic rays above 100TeV as an in-situ calibration of the telescopes. 3.7. Nutel The Nutel project is a similar ντ detection experiment initiated by National Taiwan Univ. and
382
Z. Cao / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 377–382
used to collaborate with ASHRA experiment at Hawaii, USA. By April 2006, a whole electronics readout system for a prototype telescope was completed. Production of mirrors and telescope housing are in progress. The Nutel group developed a complete simulation software for ντ interaction, air shower development. More specific detector simulation can be easily fit in by setting proper geometry for detectors. The package is named SHENIE.
4. Updates on Physics Successful observations of galactic VEH γsources by HESS and MAGIC experiments enlarged a galactic neutrino source candidate group. Sources near galactic center, such as SS433,GX3394, are most favorable neutrino sources. Micro-quasars, LSi+61303[3] and HESS J1826-148[4], newly discovered by MAGIC and HESS open another category of candidates for galactic UHE neutrinos. Nearby GBSs (Z < 0.03) can be 500 times[5] more than expected as hinted by Swift observation, this results in a possible shift upwards for GRB neutrino energies. Cosmogenic neutrinos produced in proton scattering on IR background photons [6] shift the traditional GZK neutrinos downwards in energy. Both effects make the neutrino detection more effective for most of neutrino telescopes. Neutrino detection may open a channel to go beyond the standard model. Super-symmetry particles, such as neutrilino and sleptons, are expected to play a role to transfer UHE neutrino energy to other side of the earth [7]. They will induce exotic showers in the field of view of most large scale cosmic ray detectors, e.g. HiRes and Auger etc. Sterile neutrinos play similar roles for producing exotic events [8]. All of them are candidates for dark matter. Due to effects associated with extra dimension, neutrino interaction cross section could be greatly enhanced above 1PeV[9]. This changes predictions about event rates and can be useful for exploring those extra dimensions if a higher event rate are observed at energies well above 1PeV.
5. Outlook and Summary As a “guaranteed” source for UHE neutrinos, cosmogenic neutrinos generated in “GZK process”, namely cosmic ray protons scatter on CMB photons and produce pions and eventually decay into neutrinos, are the goal for projects post IceCube. IceCube is still too small. A larger effective area, cheaper detector is needed. Both radio technique and acoustic technique are under development at the South Pole. IceCube has a plan to hybrid those two techniques together and build an array with a size 75 times larger than IceCube detector in the shallower ice above the IceCube strings. This will enable a statistic study of GZK neutrinos. The Salsa Project is another expedition of detection of cosmogenic neutrinos by using natural salt-dumb underground. The idea is to drill holes in salt-dumb to 3000m deep and bury strings of radio antennas to form an array with Vef f about 100 to 200 km3 . With such a detector, hundreds of GZK neutrinos are expected per year. As a fast growing field, UHE neutrino detection has been gradually opening a new astronomic window for us to explore the universe. This allows us to directly locate sources where UHE processes occur. This also allows us to explore remote sources beyond the distance that 1PeV (and higher) photons can pass through, to discover completely different and unknown aspects of the universe. REFERENCES 1. F.W. Stecker et al., Phy.Rev. Lett. 66, 2697, 1991. 2. A. Achterberg et al., Astropart.Phys.26,2006. 3. J. Albert et al., Science 312,1771,2006. 4. F. Aharonian et al., Science 309, 746, 2005. 5. N. Gupta and B. Zhang, astro-ph/0610770. 6. T.Stanev, astro-ph/0607515. 7. M.H. Reno, I. Sarcevic, S. Su, Astropart.Phys.24,107,2005. 8. A.Kusenko, hep-ph/0609158. 9. T.Weiler, Int.J.Mod.Phys.A20,1168,2005.