Experiment –I. Ultra High Energy Neutrinos and Cosmic Rays
Our primary goal is to detect UHE cosmogenic neutrinos in the energy higher than E>1018eV.
Since LeCosPA group joined in the ANITA experiment in 2006, our laboratory has been utilized for ultra-high energy particle detection with the radio technique. Upon the great successes of ANITA flights, we have preceded alternative experiments, ARA for improved sensitivity and TAROGE as an application for cosmic rays detection.
Why neutrinos?
Classical astronomies use electromagnetic (EM) waves in various bands such as radio, infrared, optical, X-ray, and gamma-ray. However, they have a disadvantage that the EM waves are absorbed in dense environments, so that people can see only surface images of objects. The high energy gamma-ray has a further limitation in observation of distant objects due to gamma-gamma interaction with the cosmic micro background (CMB) photon. Astronomy using charged particles has a weak point which is an uncertainty of directional information caused by deflection when particles travel in the galactic magnetic fields. This effect can be reduced for UHE (E>1019eV) particles. However, propagation distances of these particles are limited by proton-gamma interaction with the CMB photons. On the other hand, neutrinos which only weekly interact having no electric charge, are not affected by dense environmental matters, CMB photons, and magnetic fields. The neutrinos are highly attractive astrophysical messengers to deliver secrets of their origins without distortions.
Why high energies?
Neutrinos have various origins depending on their energies. For example, the Sun or supernovae produce neutrinos in MeV range and cosmic ray showers in GeV range. UHE neutrinos would be originated from extremely energetic objects such as Active galactic nucleus, Gamma ray bursts, or unknown cosmic accelerators. Exploiting the UHE would answer to one of the most important questions in the astrophysics; the origin of UHE cosmic rays which has been selected as one of the “Eleven Science Questions for the 21st Century” by the influential US NRC Turner Committee Report (2003). UHE neutrinos can play an important role in cosmology study because they can travel cosmological distances. The UHE neutrinos are interesting in particle physics as well, since their energies are beyond that human being can create. The UHE neutrinos can excellent test beams to probe the standard model investigating elementary subjects neutrino flavors, mixing, life time, cross-section, etc beyond the LHC.
Why Antarctica?
The fact that neutrinos interact only weekly, now becomes a critical disadvantage in observations because they are penetrating not only environmental media but also detectors on Earth. Furthermore, neutrino detection in very high energy is awfully challenging due to extremely low fluxes of neutrinos. Thus, neutrino experiments require large and massive target to collect enough number of events. The target medium must be transparent to measure signals emitted from neutrino interaction. The huge and clear Antarctic ice sheet with which is wonderful target medium for neutrino experiment. The biggest neutrino telescope; IceCube installed in the South Pole uses a cubic kilometer of Ice as a target. Optical modules embedded into the clear South Pole ice detect the Cherenkov lights emitted during the interaction of neutrino in the detector volume.
Why radio detector?
In 1962, Gurgen Askaryan postulated possible strong radio-frequency (RF) emissions from high energy showers in dielectric media. This, so called Askaryan effect, is due to the charge asymmetry produced during developments of the high energy showers. Since the secondary charged particles in the shower move faster than light, the signals are coherently enhanced as similar as the Cherenkov radiation. With the progress of RF radio technology in recent decades, the Askaryan effect has risen up as a feasible method for the UHE neutrino detection. The effect has been observed experimentally in various media such as silica sand, rock salt, and ice. The ANITA collaboration even used the real Askaryan signal to calibrate instruments. There is an outstanding advantage using RF over lights, which is the fact that radio signals can propagate distances in media. For instance, the attenuation length of radio in the South Pole ice in is about 1km which is an order of magnitude longer than lights. The observational coverage of unit detector is consequently enlarged, which coverage allows a highly effective way to obtain a huge the detection volume for UHE neutrino detection.
The ANITA Experiment
ANITA is utilized to enlarge the detection acceptance for Askaryan signals by collecting data at observation points on high altitudes. Using the NASA long-duration balloons, ANITA can fly about 35km altitude above the Antarctic ice shelf so that the entire ice within ANITA’s horizon can be a vast target for neutrino detection. ANITA consists of 48 high-gain quad-ridge horn antennas (32 antennas for ANITA-1, 40 for ANITA-2) receiving signals from all azimuth direction in a frequency range 200MHz – 1,000MHz, which provides >1 million km2 of instant field of view. The payload launched at the Williams field around the McMurdo station is circling around the continent. Duration of experimental is determined by the balloon and weather. Average flight time is about 1 month using NASA Long-duration balloon, but can be extended up-to 3 months with Ultra-long-duration balloon in the future.
ANITA has completed two successful flights; ANITA-1 during 2006-2007 Austral summer and ANITA-2 during 2008-2009.
ANITA has been reporting many of important results including
-UHE Neutrino flux limits from ANITA-1 and ANITA-2 which include observation of one neutrino candidate,
-Observation of UHE cosmic rays,
-Flux limit of GRB neutrinos,
-Monopole Search,
-Observation of the Askaryan effect in Ice,
-Measurement of Radio Propagation in Ice, and
-Measurement of Radio Albedo on Antarctic surface.
Two reports, the Askaryan effect in Ice and Observation of UHE cosmic rays were spotlighted by covering Physics Review Letters. Talking about the later report, it was turned out that ANITA had strong detection capabilities not only for UHE neutrinos but also UHE cosmic rays. Radio emissions, generated in deflections of charged particles of air showers in the Earth magnetic field, can be detected after reflection off the Antarctic ice surface. This was highly suggestive result opening new promising method of UHE cosmic rays.
ANITA-3 is in preparation of the launch in December 2014 while ANITA-4 has been funded for the next flight in a couple of years.
LeCosPA activities on the ANITA Experiment
Our team in LeCosPA is active in data analysis and instrumentations. For data analysis, we accomplished the initial discovery of UHE cosmic rays in ANITA-1 data. We also pointed out a critical mistake in the ANITA-2 data analysis published in PRL, and the correction was reported in the following erratum. For instrumentations, our group has been responsible for in-situ data storage using solid-state drives (SSD). After a great success of the first 1 TB SSD array for ANITA-2, we delivered a standalone storage server with 6TB SSD array for ANITA-3. One of the most significant hardware contributions to ANITA was low-noise amplifiers (LNAs). The LeCosPA team, led by Jerry Shihao, has successfully developed LNAs with world best quality lowering noise level. Total 32 units of LNAs were deployed for drop-ring antennas in ANITA-3, and are going to be used for future flights.
The ARA Experiment
Drawbacks in the ANITA experiment for neutrino detection is the limited observation time due to balloon duration and signal weakening during RF propagation. The signal weakening is caused by power loss in refraction on the ice-air boundary and considerable decrement of power density by 1/R2 for long distance to the payload, which makes ANITA to be insensitive in energy below 1019eV where highest GZK-neutrino fluxes are expected.
A solution came up was the ARA experiment which is an in-ice antenna array in a wide area. This is somewhat similar to the IceCube, but use Askaryan radio signals instead of the Cherenkov lights as a cost-effective way to increase target volume. Total 37 antenna stations form a hexagonal array covering about 100km2 of area near the IceCube detector at the South Pole. Each station consists of 12 antennas buried in the ice, about 200 m depth.
Starting from a prototype; ARA-testbed in 2010-2011, installation of three stations ARA-1,2,3 have been successfully completed in 2011-2013. We plan to complete the following ARA-4,5,6,7,8 stations in a couple of years.
LeCosPA activities on the ARA Experiment
LeCosPA supported by the Vanguard program of MOST in Taiwan is responsible for Initial hardware of the ARA experiment; ARA-1 to ARA-8. Our team’s activies spread in simulation, data analysis, and instrumentations. For the instrumentations, we are responsible for antennas, LNAs, DAQ boxes, and cables. These are produced or assembled in the LeCosPA laboratory. The final full-system integration and calibration for ARA-2 and ARA-3 were carried out in the anechoic chamber and the extreme-low temperature freezers in the laboratory.
The TAROGE Experiment
The TAROGE experiment is an antenna array observatory on the high mountains of Taiwan’s east coast for the detection of UHE cosmic rays. The antennas point toward the ocean to detect RF signals reflected off the ocean surface. Looking down from the high mountain, it can cover a vast area in field of view. Its detection efficiency is further enhanced because it can collect both the direct-emission as well as the ocean-reflected signals. In addition to the detection of UHE cosmic rays, this instrument also provides the capability of detecting Earth-skimming tau-neutrinos. A great advantage using radio over the fluorescence detectors used in the Pierre Auger Observatory and the Telescope Array is its year-round operation with 100% of duty cycle. ANITA has the same scheme of detection covering much wider area, but TAROGE has merits in the long operation time, enhanced signal strength due to shorter distance to the interaction point, and easiness of extension.
As a prototype station, TAROGE-1 which consists of 12 Log-period dipole array (LPDA) antennas for 110MHz – 300MHz, has been successfully built on the mountain top of YoungShiShan (1km altitude) near the Heping Township in July 2014. The TAROGE has been funded by the pioneer program of MOST for initial two stations for two years.
