Research

First inferred to explain the mass of the Coma cluster by Fritz Zwicky in 1933, Dark Matter is a non-luminous type of matter whose existence is inferred from its gravitational interactions and its effect on the formation of structure in the Universe. With the absence of consistent hints of its detection, dark matter continues to elude us despite eight decades of research.

If we hope to discover dark matter some day, there are three ways we could do it in principle: direct detection, production or indirect detection. Direct detection method is where we set up a low-background particle detector underground, let it collect data over some volume and time and later analyze the candidate events to see if any non-standard model particle has interacted with and left a signal in the active volume of the detector. As we push our sensitivity up, our upper bounds on the probability of interaction of dark matter particles with the visible matter naturally goes down.

If we are bored waiting for a visible-but-not-so-visible particle to be registered in our detector, we could alternatively build a particle accelerator, smash high energy particles together and hope that some of the products of the collisions are invisible, and hence, escape the detector. This is what is currently being done at the LHC in Geneva, Switzerland. We are looking forward to the new data set to be collected starting in Spring 2015 following the long shutdown.

Thirdly we could look up at the sky, measure fluxes of particles like photons, electrons, positrons, anti-protons or neutrinos, account for known astrophysical backgrounds and look to see if there is any residual that can be explained by dark matter decay or self-annihilation. The latter is relatively easy, i.e., the dark matter literature is rich enough that exotic signals can explain almost everything. The hard part is the accurate and self-consistent exclusion of the astrophysical emitters as the source of the excess.


Here are some projects I have lately been thinking about.

The gamma-ray excess in the inner Milky Way

As a graduate student my first project was on the data analysis of the gamma-ray excess in the center of the Milky Way. By doing a maximum Poisson likelihood analysis on the Fermi-LAT data, we characterized the features of the signal. The morphology, normalization and spectrum of the signal is consistent with a dark matter annihilation scenario with the thermal cross-section. However other interpretations also exist such as a dim and unresolved millisecond pulsar population in the galactic center or a recent cosmic ray burst from the supermassive black hole in the galactic center.


A Bayesian approach to Point source inference

See PCAT!


Potential contribution of Dark Matter annihilation to the ionization state of the IGM

Regarding the potential impact of the ~ GeV scale dark matter annihilation interpretation of the gamma-ray excess in the galactic center of the Milky Way, I am also currently working on a follow-up project. In particular I am studying a possible connection between the energy injection due to the associated dark matter annihilations and the apparent mismatch between the predicted metagalactic ionization rate and that required by simulations tied to the Lyman-alpha observations.


Scrutinizing the CMB spectral distortions for new physics

In the last few decades the Cosmic Microwave Background (CMB) has taught us a lot about the energy budget and its time evolution and revolutionized our understanding of cosmology. However there is yet to learn more from the CMB than has been possible with the current technology. Even though the Universe was in an almost thermal state in its infancy, there must be some relic non-thermal component of the CMB, which potentially carries information about energy injection into the primordial plasma at early times, i.e., even before recombination.


Mapping the ISRF as a background for the gamma-ray sky

All of the star light emitted by the stars in our galaxy is reprocessed by dust in the interstellar medium, i.e., dust particles in the galaxy selectively absorb and scatter star light, reddening and re-emitting it thermally at lower energies. Therefore the radiative transfer of dust is a nonlinear and nonlocal problem, which makes the calculation of the radiation field everywhere in our galaxy, in every direction and at every wavelength computationally demanding. Nevertheless the interstellar radiation field is an important unknown if one wants to predict the inverse Compton scattering of high energy electrons on star light in our galaxy. The latter is the major background in our gamma-ray searches for dark matter annihilation and decay. Therefore I have recently been interested in the radiative transfer of dust in a long-term effort to eventually produce a better dataset on the inverse Compton scattering of high energy electrons in our galaxy.


High energy photon event reconstruction using AMS-02

As an undergraduate I was involved in the AMS-02 (Alpha Magnetic Spectrometer 02) experiment hosted by CERN. AMS-02 is a particle detector with a silicon tracker to track, and a permanent magnet to bend charged particles. It was launched into space in 2011 and is currently located on the International Space Station. It has several sub-detectors that allow particle identification as well as determination of charge, energy and momentum. In AMS-02 I worked on the reconstruction of high energy photons, i.e., selection of genuine photon events from raw data and determination of their arrival direction and energy. In an environment dominated by a huge background flux of charged particles, this requires an efficient selection and accurate reconstruction algorithm that combines data recorded in different sub-detectors.