This page expands on some of the descriptions of particle physics and my research interests.
The Standard Model of Particle Physics
Our current theory of particle physics, the Standard Model, was first proposed approximately 50 years ago and predicted the existence of many as-yet undiscovered particles. Over the intervening years, each of its predictions has come true, and the final piece was put into place in 2012 with the discovery of the Higgs boson at the Large Hadron Collider. Consisting of a handful of elementary "matter" particles, and three forces that act between elementary particles, the Standard Model is spectacularly successful at explaining how the building blocks of matter assemble into nuclei, atoms, molecules, and everything on up.
The quarks (purple) and leptons (green) make up the "matter" particles in the standard model. The gauge bosons (Red) mediate the electromagnetic, strong, and weak forces between matter particles. Finally, the Higgs Boson (yellow) gives mass to all particles. Credit: "Standard Model" Wikipedia page.
The Standard Model consists of fundamental "matter" particles such as the familiar electron, as well as quarks - this latter type of particle are the building blocks making up the proton and the neutron. Each of these matter particles interact via a combination of three forces: the electromagnetic force, the strong force, and the weak force. The electromagnetic force exists between any electrically charged particles and is familiar to us from our day-to-day experiences with electricity and magnets. The strong and weak forces are more obscure: the strong force keeps the quarks tightly stuck together to form protons, neutrons, and other so-called hadrons, while the weak force is responsible for radioactive decay processes that change a neutron into a proton. There is a "force carrying" particle that mediates each interaction: the photon for electromagnetism, the gluon for the strong force, and the W and Z bosons for the weak force.
What is remarkable about the Standard Model is that, by assembling the few constituent pieces into more complicated nuclei, atoms, and molecules, we can explain the rich array of physical phenomena that we see in the world. In that sense, our current understanding of the Standard Model is much like the periodic table in the 19th Century: the if we dig deeper into the structure of matter, will we see that the particles and forces in the Standard Model are themselves made up of simpler and more elegant pieces?
There are some hints that this may be true. From the picture on the right, it is evident that the matter particles come in three "families". For example, the electron is accompanied by heavier cousins known as the muon and the tau, which are the same as electrons in all aspects except for their larger masses. Is there a reason for there to be three copies of each matter particle? Also, the masses span a huge range: the top quark is some five orders of magnitude heavier than the electron, and the neutrinos are predicted to be massless in the Standard Model. There is very little understanding of why the particles have the masses and properties that they do. Finally, a simple calculation shows that the Higgs boson is expected to be much, much heavier than it has been observed to be. Perhaps one (or all) of these questions about the Standard Model hint at the existence of new particles and interactions that are within reach of our current experiments!
Dark Matter
The Standard Model explains nearly all physical phenomena we see on Earth. When we look to the heavens, though, it is a different story. For example, we can use the gravitational pull between astrophysical objects to measure how much mass each object has, and compare it to the amount of matter that gives off light in the form of stars, gas, etc. We find that there is significantly more matter in galaxies (and pretty much everywhere else we look) than can be found in stars and other luminous objects. This is creatively called dark matter. It is exciting and troubling that we are ignorant about what makes up so much of the matter in the Universe.
A large part of my research aims to identify what kind of elementary particles comprise this dark matter and how we can interact with dark matter. This includes understanding how dark matter was made in the early universe, and whether we can make dark matter at particle colliders (or see it in other experiments).
Rotational speed of stars in a spiral galaxy. Using newton's law, it is possible to relate the rotational speed to the gravitational force between the galaxy and each star. The dashed line shows the prediction of rotational speeds from the luminous matter, and the data points show the actual rotational speeds. The inclusion of dark matter gives the solid line. Credit: Corbelli, Salucci, 2000.
Here is what we know now about dark matter: there is about six times more of it than there is of "normal", visible matter, we know it doesn't give off (much) light, and we know it cannot be made of any of the types of particles found in the Standard Model. We also know that it clumps together early in the history of the Universe, creating a backbone around which all of the galaxies we see form. Beyond this, there is little we can say definitively about dark matter and its properties.
As a particle physicist, I am interested in finding out what this dark matter is and how it fits in with the Standard Model. Are the dark matter particles heavy or light compared to the familiar electron and proton? Is there one type of new particle, or an entirely new "Periodic Table" with its own dark forces? Are there dark atoms? Unfortunately, because of the nature of the gravitational interactions, we only know how much dark matter there is but we cannot answer any of these other questions. To do so, we need to observe the particle-physics properties of dark matter directly in experiments.
I research possible theories of dark matter that can explain the amount and spatial distribution of dark matter we see in the Universe, and connect these to the particle properties of dark matter that can be tested in terrestrial experiments. I am especially excited about the possibility that we could be producing dark matter or its friends in particle colliders, and I work on ways for us to determine whether or not we have, indeed, made dark matter in an experiment such as the LHC or lower-energy particle colliders. If we discover dark matter in the near future, then we will be able to study in a concrete way the links between tiny subatomic particles and their effects on large-scale structures in the Universe!
Specific research interests of mine include connections between dark matter and neutrinos, finding ways of testing thermal dark matter scenarios with colliders, and coming to a better understanding the connections between cosmology and experimental signatures of low-mass (i.e., lighter than proton mass) dark matter and its related forces.
The Matter-Antimatter Asymmetry
For every particle in the Standard Model, there exists a corresponding antiparticle. Antiparticles have exactly the same properties as the original particle, but with opposite signs for all charges under various forces (for instance, an antiproton has the same mass and spin as a proton, but with a charge of -e instead of +e). Since the discovery of antimatter in 1932, scientists have faced an important question: why is everything in our world made up of matter (protons, electrons) and NOT antimatter (antiprotons, positrons). The Standard Model puts particles and antiparticles on largely the same footing, and so our existence in the form of matter and not antimatter requires the existence of new particles or forces to explain this discrepancy.
Explaining the matter-antimatter asymmetry requires very special kinds of particles and interactions. My research connects these hypothesized particles & forces to current and upcoming experiments to understand what kinds of signals we should be looking for and what connections exist with dark matter and Standard Model particles. I also think of new ways to generate a matter-antimatter asymmetry and how we can experimentally test these theories.
Particles and antiparticles annihilate when they meet. Can we imagine a world with people made out of antimatter? Credit: CERN.
The problem is actually a bit worse than what I described already. When particles and antiparticles meet, they can annihilate into energy (typically in the form of photons or gluons, the carriers of the electromagnetic and strong forces). Immediately after the Big Bang, the Standard Model predicts that the Universe was in a hot and dense state with equal numbers of particles and antiparticles flying around. As the Universe cooled, all of the matter would have annihilated with all of the antimatter to leave....essentially nothing! The world would basically be a bath of electromagnetic radiation with no visible matter left over, and certainly no galaxies, stars, planets or humans. We owe our very existence to some primordial excess of particles over antiparticles, and we would like to know how that came about.
The process by which the Universe evolved into a state of unequal amounts of matter and antimatter is called baryogenesis ("genesis" = origin, "baryon" = fancy word for proton or neutron). One way in which baryogenesis can occur is to have a new particle that decays preferentially into matter relative to antimatter. This can explain why we now see an excess of matter vs. antimatter, but is challenging to realize in nature: the reason is that if we have a process that preferentially produces matter over antimatter, then the reverse process will tend to preferentially destroy the excess of matter we have made. Only very special kinds of new particles and interactions can successfully realize baryogenesis, and my research uses this information to make concrete predictions for what experiments can see to test the physics of baryogenesis. For example, I have shown that particles responsible for baryogenesis can give rise to very specific signatures when produced in a high-energy particle collision, and have worked with experimentalists to expand their search strategies to look for these kinds of particles. I have also studied how our predictions for baryogenesis in particular theories is tied to various assumptions about how the Universe evolved after the Big Bang, and consequently affects the links between our understanding of the matter-antimatter asymmetry and what we can see in the lab today.
Different types of neutrinos can transform into one another. It is also possible that neutrino particles can transform into antiparticles.
What I think is perhaps the most exciting possibility for baryogenesis is the connection between the matter-antimatter asymmetry and a particle in the Standard Model known as the neutrino. The neutrino interacts only very feebly with every known form of matter, and so its properties are relatively poorly known. It is also the only known type of matter particle that could be its own antiparticle. Thus, transformations involving neutrinos could convert matter into antimatter and vice-versa, and so neutrinos are a key that could unlock part of this puzzle. I am interested in finding new ways of connecting neutrinos to the matter-antimatter asymmetry, as well as devising and better understanding experimental tests for the neutrinos involved in baryogenesis at particle colliders.
Hunting New Particles At Colliders
According to Einstein's famous mass-energy relation, new types of heavy particles can be produced with a sufficient concentration of energy. One way to do this is to take protons and electrons, accelerate them to extremely high speeds/energies, and then smash them together. Heavy particles in the Standard Model, as well as previously unknown particles (potentially related to dark matter or the matter-antimatter asymmetry), are produced in the collisions and can be studied. The highest-energy collider ever built, the Large Hadron Collider (LHC) at CERN on the Swiss-French border, is currently colliding protons and giving us new insight into the basic building blocks of matter. However, trying to find a new particle amidst the debris of these collisions is like trying to find a needle in a haystack: we need to know what we are looking for before we find it!
I am interested in identifying ways in which current experimental search strategies could be failing to find new particles that are produced in colliders and which are lurking, undetected, in existing data. I also put my ideas into practice, having joined a particle collider experiment to look for evidence for dark matter and hidden forces in old datasets.
Proton collision at the LHC as recorded by the CMS detector. The debris from the collision reconstructs a Higgs boson decaying into two electrons and two muons. Credit: CMS Collaboration
To give a sense of the challenge of looking for new particles at high-energy colliders, consider that bunches of protons are collided at the LHC at the astounding rate of 30 million per second. The data recorded from the collision can only be recorded 1000 times per second, meaning that 99.997% of the data collected at the LHC is immediately thrown away. In order to pick out this tiny fraction of events, the by-product of the collision is quickly examined to see if it "looks interesting", and kept if it does. This means we'd better be sure we have the right ideas about what "looks interesting"; otherwise, we may find that much of the relevant data has been discarded! Even for collisions that survive this initial selection process, we have to dig through a lot of "boring" strong-interaction collisions to pick out the ones that we are most interested in looking at. For example, a Higgs boson is produced only every few seconds at this high collision rate, and some new particles may be produced only once every month or year! We have to work very hard to make sure these new processes are discovered.
Long-lived particle produced at the proton collision point (green ball), traversing the detector, and decaying at another position (purple ball). Credit: ATLAS collaboration
Recently, my research has focused on new types of particles that leave very striking signals in the detectors measuring debris from the collisions, but that would not be detected using current search strategies. One such example is the case of "long-lived particles", which are new particles that are produced at the collision point but travel partway through the detector before decaying back into familiar Standard Model particles. The detectors at the LHC are specifically geared to look at debris emanating directly from the collision point, not from partway through the detector. As a result, these kinds of new particles could be slipping through the cracks of existing experimental efforts. I have worked to propose a few new search strategies that the experiments can use to cover some of the gaps in their existing searches, and am co-leading the LHC Long-Lived Particles Community efforts to devise a comprehensive and systematic way of looking for long-lived particles.
Apart from the LHC, there are many other particle-physics experiments that can be used to detect traces of new particles or forces. The LHC offers an unprecedented particle collision energy to look for new particles that are too heavy to be made in other colliders. However, it's possible that the new particles connected with dark matter or the matter-antimatter asymmetry require far less energy to produce than is found in the LHC. Instead, these particles may interact with us in a very feeble manner, meaning that we care much more about the rate of collisions (in order to look for tiny deviations in the behaviour of Standard Model particles and forces) rather than the energy of collisions. For example, I have proposed searches that can be done at lower-energy particle colliders to look for new particles and forces beyond the Standard Model, and I joined the BABAR Experiment in order to look for the existence of new "dark" forces in electron-positron collision data collected over a decade ago.