The Standard Model of particle physics represents our best understanding of the fundamental particles and forces that make up the Universe. There are observational, theoretical, and aesthetic reasons to believe that the Standard Model is an incomplete theory. Numerous astrophysical and cosmological observations point to the existence of some "dark matter" that outweighs Standard Model matter approximately five-to-one and "dark energy" that outweighs it approximately fourteen-to-one. There are many theoretically motivated candidates to explain the missing matter in the Universe (see below), however so far none of these candidates have been detected either in the laboratory or in astrophysical settings. My research involves identifying novel laboratories to look for physics that lies beyond the SM. My work focuses mostly on ultralight bosons: integer spin particles. Such particles are ubiquitous in theoretical models beyond the Standard Model (e.g., string theory). Let us take a look at some theoretically predicted ultralight bosons
Fundamental scales in physics and proposed dark matter/beyond Standard Model candidates.
Axions were first predicted to explain to explain why the neutron's electric dipole moment is least ten orders of magnitude smaller than theoretically expected (the so-called Strong CP problem). An elegant solution proposed that the parameter controlling the neutron's electric dipole moment was not a fixed value, but rather a field that can evolve in time. Non-perturbative effects in quantum chromodynamics (QCD) gives this field a potential whose minimum occurs at 0, meaning a small neutron electric dipole moment is energetically favorable. This field that controls the neutron's electric dipole moment is called the QCD axion. In addition to resolving the Strong CP problem, QCD axions are among the leading dark matter candidates. [FIGURE: Cartoon depiction of a neutron which consists of an up quark and two down quarks].
Attempts at theories of quantum gravity, such as various string theories and M-theory, predict the existence of several extra spatial dimensions. To describe the world around us, these extra dimensions need to be compactified on topologically non-trivial manifolds. The compactification of gauge fields leads to particles that share symmetry properties with the QCD axion, giving them the name axion-like particles. Depending on the geometry of the compactified extra dimenions, there may be many axion-like particles with masses spanning a large range. Discovering axion-like particles would provide strong evidence for theories like string theory that contain compactified extra dimensions.
Given the zoo of new particles predicted theoretically, the next question one can ask is how to go about looking for them. For the last several decade, the primary method by which physicists searched for new particles involved accelerating beams of particles to enormous energies, colliding them, and seeing what came out. This collider program has been remarkably successful in discovering particles within the Standard Model, including the Higgs boson in 2012. With future colliders poised to push to higher energies and luminosities, the prospect to discover new physics beyond the electroweak scale (~250 GeV) is promising. However, in the landscape of new particles, there are vast regions of parameter space that are not accessible to even future colliders. To look for new particles in these regions requires totally different experimental concepts. In the last few decades, there have been a host of proposed experiments to detect well-motivated dark matter candidates such as WIMPs and axions, with no conclusive signals seen so far. There are many reasons why dark matter detection is so difficult:
Astrophysical systems are very promising laboratories to look for new physics. The extreme conditions present in astrophysics are not achievable in any terrestrial experiment. As an example, the Sun is powered by nuclear fusion, but so far we have had limited success in reproducing the same processes in the laboratory. High-energy astrophysical systems such as neutron stars and black holes are also natural particle accelerators capable of achieving energies much larger than can be reached in colliders. My research shows that these settings, particularly pulsars, also provide ideal conditions to produce ultralight bosons, such as axions. What are pulsars and what makes them such good laboratories for new physics?
Pulsars are rapidly-rotating neutron stars that emit bright beams of electromagnetic emission from their poles (see NASA animation). We observe pulsations as the beams sweep across our line-of-sight. Electromagnetic radiation from pulsars spans a huge range of frequencies from radio to gamma ray. Of particular interest is the fact that much of the radiation is not thermal, which suggests there is considerable particle acceleration. Understanding the mechanisms of this non-thermal emission is an active area of research. A particularly curious example of non-thermal emission is fast radio bursts.
Fast radio bursts (FRBs) are bright, millisecond-duration radio transients of unknown origin. Due to their distribution on the sky and large estimated distances, we believe most FRBs are coming from well outside the galaxy. Recently, an FRB in our galaxy was associated with a highly magnetic neutron star, called a magnetar, supporting the hypothesis that at least some FRBs come from magnetars. But where and how are they produced? A pre-requisite to answering this question is understanding the basic physics of their progenitors, magnetars. For a summary of some of my research on pulsars and magnetars and how they can be used to search for axions, see my Research.
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