Speaker
Description
Within the last 30 years, the sub-field of ultra-high energy cosmic ray (UHECR) astronomy has emerged as a vibrant experimental and theoretical sub-field within the larger field of particle astrophysics, comprising studies of both charged and neutral particles at energies greater than 1 EeV. The physics interest in UHECR lies in understanding the nature of the cosmic accelerators capable of producing such enormously energetic particles at energies millions to billions of times higher than what we are capable of producing in terrestrial accelerators, the details of the interaction of UHECR with the cosmic ray background, correlations in the arrival directions of UHECR with exotic objects such as neutron stars, gamma-ray bursts (GRB), and active galactic nuclei (AGN), and testing the Standard Model of particle physics at the ultra high energy frontier. The NASA-sponsored Antarctic Impulsive Transient Antenna (ANITA) project has the goal of detecting ultra high energy neutrinos of energies above 1 EeV. Ultra high energy neutrinos provide a new window to the Universe since they can propagate in intergalactic medium without attenuation, i.e. they do not suffer from the GZK cutoff.
ANITA is a balloon-borne suite of 48 high precision radio receivers designed to register radio frequency (50-1000 MHz) signals produced by UHECR, synoptically observing an area of ice of order 15,00000 km^2. Although originally purposed for detection of neutrinos, the ANITA-1 mission (2006) unexpectedly observed 14 extremely high-energy radio-frequency signals with a nonneutrino-like radio wave signal polarization (horizontal [HPol] vs. vertical [VPol]) which traced back to the Antarctic surface beneath the balloon. After considerable work, it was demonstrated that these events were the result of collisions of down-coming protons corresponding to energies 10,000,000 times greater than the energy of particles accelerated in the Large Hadron Collider in Geneva, Switzerland. The radio frequency signals produced in the collision of UHECR particles with atmospheric molecules are explained as the combined result of the Askaryan effect resulting from the net charge excess acquired by the shower as it descends through the atmosphere, plus the geomagnetic signal resulting from separation of different charged species due to the Earth’s magnetic field. These signals subsequently get reflected off the Antarctic surface and back up to the ANITA detector at an altitude of about 38 Km. An upcoming ultra high energy neutrino produces coherent radio emission after collision with ice nuclei and the refracted radio wave from the ice-air interface is also detected by ANITA. We developed a reliable theoretical framework based on the Weyl formalism to model the reflection and refraction properties of these radio pulses on Antarctic ice surface, incorporating curvature of Earth as well as the surface roughness. The incident pulses are first decomposed into their Fourier components, each of which are monochromatic spherical waves. The propagation distance turns out to be very large in comparison to the wavelength. However, despite the large propagation distance, it is not a good approximation to assume these reflected waves to be plane waves. The reflection of each of these spherical waves can be handled by decomposing them into plane waves and then using the standard Fresnel formalism for each plane wave. This is called the Weyl formalism in literature. The resulting integral over all the plane waves shows very rapid fluctuations and hence is computationally intensive. These results are described in our paper Antarctic Surface Reflectivity Calculations and Measurement from the ANITA-4 and HiCal-2 Experiments, (arXiv:1801.08909,(2018)). We have worked on calibration of the detector by using HiCal (High-altitude Calibration) I and II. HiCal consist of a calibrated radio-frequency transmitter source flying on a balloon, several hundred km away from the ANITA payload. The main goal of HiCal experiment is to directly measure the local surface reflectivity by sending radio pulses towards ANITA both directly, and also via surface reflection. Taking the ratio of direct and reflected field, one can directly measure the local surface albedo. The earlier treatment of the reflected pulses used the standard Kirchoff integral formalism which is reasonable but not rigorous. Our first application of the Weyl formalism was based on the assumption that a plane wave after getting reflected at the spherical ice surface remains a plane wave. This assumption is reasonable since the radius of Earth is much larger than the wavelength of the wave. However direct comparison with experimental data revealed that it fails for small elevation angles (i.e. angle with respect to ground). We have now solved this problem by assuming that the reflected wave is a plane wave only over a small neighbourhood of the wave vector pointing towards the detector. We term this procedure as local plane wave approximation. This provides a theoretically rigorous treatment of this problem without relying on any uncontrolled approximations. We also studied more realistic surface roughness models to incorporate the surface roughness properties of the ice-air interface into our formalism. We found that our results are in very good agreement with the ANITA-HiCal data. Our model is the first complete theoretical model that can be applied rigorously to radio pulses generated by UHECR particles including neutrinos. We have calculated the H-Pol and V-Pol field strengths assuming that the pulses are generated by a horizontal dipole radiator, applicable to the HiCal experimental case. ANITA -1 has detected two strange events, dominantly H-Pol, with polarity consistent with direct cosmic ray event. However their arrival directions are such that they must either originate from inside the ice or the radio pulse be reflected. Both of these possibilities are inconsistent with standard expectations. We seek to understand these mystery events using our formalism based on the local plane wave approximation. We start with the incident wave as dominantly H-Pol and expect that the reflected wave will be 180 degrees out of phase with the incident wave. Any deviation from this behaviour might provide an explanation for the observed mystery events. This has the potential to lead to a breakthrough discovery of new physics.
