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Indirect Search for Dark Matter Using the VERITAS Telescope Array

Final Report Summary - UCDVERITAS (Indirect Search for Dark Matter Using the VERITAS Telescope Array)

Very-high-energy (VHE) astronomy provides the means to probe the most extreme processes in the universe, as well as the nature and evolution of the universe. VHE astronomy is performed using multiple large optical reflectors that image the Cherenkov light from showers of particles that occur in the atmosphere when energetic gamma rays and cosmic ray particles collide with the atmosphere. Offline image analysis techniques are used to extract the gamma-ray signal from a huge background of cosmic ray events. This project utilized the VERITAS ( ) array of imaging Cherenkov telescopes, located in southern Arizona, which is one of the three state-of the-art VHE observatories currently in operation. The project had two main components: the first focused on increasing the sensitivity of the VERITAS gamma-ray telescope array through the use of machine learning algorithms (specifically boosted decision trees), and the second was the application of the these algorithms to observational cosmology.

By better distinguishing gamma rays from the isotropic background of cosmic rays, we can, for example, better search for faint signatures from dark matter annihilation and decay, better probe the acceleration and accretion processes in supermassive black holes, and better constrain the properties of the intergalactic medium. Following the development of a boosted decision tree analysis for categorizing gamma and cosmic rays, the analysis was applied to VERITAS data. In particular, a study probing the strength of the weak intergalactic magnetic field that resides in the voids between galaxy clusters was made which benefited from the sensitivity improvement. Other applications included the search for new extragalactic sources of VHE gamma rays, and the study of the spectra several distant sources to constrain the cosmic extragalactic background light intensity, which absorbs VHE gamma rays travelling cosmological distances.

The boosted decision tree analysis developed as part of this project uses properties of particle showers in Earth’s atmosphere to distinguish between gamma-ray-induced showers and cosmic-ray-induced showers. While traditional analysis uses the same properties to categorize showers, the boosted decision trees offer several advantages: they account for correlations between different shower properties, and they combine the properties into a single variable, which allows for better categorization of showers. A shower that would fail traditional gamma-ray selection due to a single poorly reconstructed shower property can be retained in the boosted decision tree analysis.

A main goal of this project was to develop a new analysis that could be used broadly for all analysis of VERITAS data, rather than a small tool optimized for a particular type of study. This was accomplished: the boosted decision tree analysis is now used as the default for one of the two VERITAS analysis pipelines. Sensitivity improvements of 10-40% from the traditional analysis have been demonstrated, depending on the properties (e.g. strength, spectral characteristics) of the gamma-ray emitter, with the largest improvement at low energies.

Another main goal of this project was to study fundamental physics using the VERITAS telescope array. One potential target was dark matter, which is believed to account for more than 80% of the matter in the universe, but does not emit or absorb light. The boosted decision tree analysis will be used in future studies, with substantial improvement expected. Theoretical predictions favor a potential signal at the low end of the VERITAS energy range, where the boosted decision tree analysis performs best.

Two other fundamental physics topics were explored during this project, both using the boosted decision tree analysis: the strength of the intergalactic magnetic field, and the power spectrum of the extragalactic background light. The former is a weak field that may have been generated in the early universe. Measuring its strength and power spectrum would provide information about early-universe conditions, as well as giving a clue to the riddle of observed strong magnetic fields in galaxies and galaxy clusters. In the course if this project, new constraints were set on the strength of the intergalactic magnetic field, using measurements of the spatial properties of an ensemble of distant galaxies with supermassive black holes at their centers. The measurement relied on a careful characterization of the angular resolution of the VERITAS telescope array. A window of magnetic fields strengths was ruled out, narrowing the range of experimentally allowed values of the field strength. The results of this study have been published in the Astrophysical Journal.

Like the intergalactic magnetic field, the extragalactic background light is an important probe of the evolution of the universe. It comprises the sum total of light produced in star and galaxies from early in the universe’s history, as well as lower energy photons from light that is absorbed by dust and re-radiated. Interaction of very-high-energy gamma rays from distant galaxies and extragalactic background light attenuates the VHE emission and leads to an imprint on the observed spectral properties of the gamma-ray-emitting galaxies that varies as a function of distant. In the context of this project, constraints on the extragalactic background light intensity were set using a similar sample of distant galaxies to the intergalactic magnetic field study. This is a work that is about 75% complete and the intention is to publish the results later this year.