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Particle astrophysics is the probing of events or objects in the sky with the help of particles other than light such as cosmic rays and neutrinos. When it comes to traveling the largest cosmic distances at the highest energies, light is not the most reliable messenger of astrophysics.
We live in a boring part of the Universe.
Thankfully for us, we live in a boring part of the Universe. This allows life and the life sciences to thrive here. However, everything that is interesting (to me) in astrophysics takes place far, far away. For example, most Gamma Ray Bursts take place about 1 Gigaparsec away from us. That is over three billion light-years away!
To give you an idea of the scale, our galaxy is about 100,000 light-years across. That is, it takes light a hundred thousand years to get from one end of our galaxy to the other.
Why do I care about Gamma Ray Bursts?
Well, in short, they are Nature’s most powerful accelerators and they outshine an entire galaxy when they occur! They are super duper explosions. What is more, the physics behind these exotic events continues to remain mysterious for over 50 years.
(Gamma Ray Bursts were discovered during the Cold War by the U.S. military)
Traditional astrophysical messengers are not able to completely probe physics that takes place at the farthest distances and at the highest energies. This is why we need new messengers or previously unused particles. Since the beginning of astronomy, we have relied on optical light to study objects in the sky. This is the light that our eyes can see.
In the last few decades, we have started utilizing light of other wavelengths such as X-rays and gamma rays. X-rays and gamma rays have shorter wavelengths than optical light and therefore higher frequencies. Higher frequency light is more energetic than lower frequency light.
High energy light can directly convert to matter and get absorbed by matter. Light of energy 13.6 eV gets absorbed by Hydrogen atoms, the most abundant element in the Universe. So, there is an inevitable need for complementary messengers.
Fortunately, in the last century, we have opened up multiple new windows to peer into the Universe. About 100 years ago, cosmic rays were discovered by Victor Hess in a balloon-based experiment. These are charged particles hitting the earth all the time.
In the last several years, the IceCube neutrino observatory has discovered the first astrophysical neutrinos up to energies of a few PeV.
1 PeV = 10^15 eV
These neutrinos are from other galaxies where more exciting things are happening.
Moreover, gravitational waves were discovered by the LIGO collaboration in the last few years, confirming, for the first time, the association of short Gamma Ray Bursts with neutron star-neutron star mergers. (That is so beautiful it makes me cry.)
Neutron stars are very compact objects (10 km radius!) that form when a star dies.
Neutrinos: my favorite space particles
Neutrinos are so light that for a long time they were thought to be massless. They are potentially perfect candidates for carrying information about distant particle accelerators all the way to us. This is because these elementary particles are neutral and weakly interacting! Unlike cosmic rays, they can’t get bent around by magnetic fields because they are neutrally charged!
So neutrinos remain unattenuated and point straight back to their source.
Neutrinos are the side product of almost every nuclear reaction and can carry versatile information about particle physics taking place at cosmic distances. Our sun makes neutrinos! But, they are low-energy and not very interesting (to me). The neutrinos I am interested in are ultra-high-energy neutrinos.
Ultra-high-energy means > 10^18 eV!
Such neutrinos may be associated with powerful phenomena like Gamma Ray Bursts and help us to understand the physics driving these super-duper astrophysical phenomena!
So, that’s particle astrophysics. It is the study of astrophysics that happens so far away that we need the least extroverted particles to carry information about them to us. Particles that don’t stop to talk to other particles. Because if they stop to talk to other particles then the information they carry changes, and we don’t want that.
Particles like neutrinos, my favorite.
Summary of my thesis:
I am happy to report that I finished my Ph.D. a few days ago! My defense is done and the thesis submitted.
So now I have a final draft of the thesis and share the short version/conclusions as well as a link to the full text in this post.
I climbed Ob Hill (Antarctica) a few times during my Ph.D. so here is a glimpse of that
Abbreviations you see a lot
ANITA: ANtarctic Impulsive Transient Antenna
EAS: Extensive air shower
UHE: ultra-high-energy
GRB: Gamma Ray Burst
Short version/conclusions
It is an exciting time for particle astrophysics (says me). There have been major developments in the radio detection of UHE neutrinos and extensive air showers.
ANITA has made two observations of potential UHE tau neutrino candidates for the first time.
It has been found that ANITA also has the potential to be sensitive to exotic physics involving supersymmetric particles (cute theory paper below).
The search for a diffuse flux of UHE neutrinos in data from the third flight of ANITA has been completed.
A new limit has been placed on the diffuse flux of UHE neutrinos (analysis paper below).
Around 25 EAS candidates were also discovered in the ANITA-3 data, two of which were only discovered in the new binned analysis presented in this thesis.
This thesis is also the first document to describe the (gory) details of the binned analysis results published for the first time in the analysis paper below.
The plot in my thesis that has gained emoji status in the collaboration 😉
The binned analysis has also been successfully extended to perform a search for neutrinos from GRBs with progress made in the development of the first search constrained in time as well as direction.
Lastly, the total instrument livetime of ANITA has been tripled in ANITA-4 by the TUFF notch filters. Details on these filters and associated results along with the first descriptions of the ANITA-3 and -4 instruments are also part of this thesis and an associated publication (instrument paper below).
This has paved the way for a much more (about 4 times more) sensitive instrument and the potential for further confirmation of ground-breaking observations as well as for new discoveries in particle astrophysics.
This work is organized into seven chapters in the thesis.
The main publications associated with this thesis are:
Instrument paper
There are four appendices that I wrote especially keeping in mind the new graduate students who will continue this work.
I hope you enjoy and please leave any comments or questions that you might have about particle astrophysics below!
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