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New physics at colliders and beam dumps

Is there a “fifth force” beyond the four we currently know of? How do we make dark matter in the laboratory? How can we use muon beams to achieve the best sensitivity to new physics?

The world as we know it can be described by four forces: gravity, electromagnetism, and the strong and weak nuclear forces. In the language of particle physics, each of those forces is “mediated” by an associated particle: the graviton for gravity, the photon for electromagnetism, the gluon for the strong force, and the W/Z bosons for the weak force. But the existence of dark matter suggests that there may be additional forces responsible for mediating interactions between dark matter particles. If we’re lucky, these same mediators may interact (very weakly!) with visible particles, making it possible to detect dark matter.

One possibility for this “fifth force” is a particle we nickname the dark photon. “Dark,” because it interacts mostly with dark matter, and “photon” because its interactions with charged particles are just like the interactions of the ordinary photon, but much weaker. Unlike the massless photon, the dark photon can be massive, which leads to distinctive experimental signatures: for example, a dark photon radiated from a high-speed electron wants to steal all of the electron’s energy, while an ordinary photon would prefer to sneak away with as little energy as possible. My research is focused on designing novel experiments to detect such dark photons and their interactions with dark and visible matter, and on using a dark photon to explain anomalies in precision low-energy physics.

New forces can also have effects which are purely quantum in nature. A new force which couples only to muons could affect the value of the muon magnetic moment (known as “g-2” for historical reasons) and may explain why the measured and predicted values for this quantity have been discrepant by more than 3 standard deviations for several decades. This anomaly was recently confirmed by the g-2 experiment at Fermilab; my collaborators (including David Curtin) and I showed that a muon collider with sufficient energy is guaranteed to discover the new particles or forces responsible for this anomaly. As the particle physics community gears up for the next generation of high-energy colliders, this provides one piece of motivation to consider a muon collider as a possible way forward. Muon colliders are also an opportunity for theorists to think creatively about accelerator design and collider observables: muons are point particles, but at the highest energies, they are "made" of photons and weak gauge bosons, and so one can talk about "parton distribution functions" of the muon which are nonetheless perturbative and therefore calculable.

New forces might also let us make our own dark matter in the laboratory! Everything we know about dark matter — so far — comes from its gravitational influence on other things: stars, gas, photons in the early universe, and so on. We’d like to detect dark matter on Earth, but to do so, we’re stuck with the properties of dark matter in our galaxy that Nature has given us. Specifically, dark matter is slow (with typical speeds about a thousandth of the speed of light), and its mass density is fixed (about one proton’s worth of mass per 3 cubic centimeters). Already we can see that there are a couple failure modes for dark matter detection: if its mass is too light, its slow speed might not give it enough kinetic energy to make anything exciting happen in a detector, and if there isn’t enough of it around, the event rate might be too slow.

The solution to both these problems is for us to make our own dark matter! This strategy has been around for a while, and is usually known as “collider detection,” where the signature at a high-energy collider like the LHC is a large amount of missing energy which would otherwise seem to violate conservation of momentum. But strictly speaking, this is detecting the absence of something, rather than the something (dark matter) itself. Recently, there has been a revival of interest in using a type of collider experiment more common in the 1960’s to detect lighter-mass dark matter. Here, a beam of electrons or protons collides with a fixed target (of graphite, beryllium, lead, or something else), rather than having two beams of electrons or protons collide with each other. The price we pay is that the total center-of-mass energy of the collision is much lower than the beam energy, but the reward is that the luminosity of the beam can be much higher, leading to much larger event rates. Most of the beam particles just stop inside the target without doing anything interesting, so these experiments are also known as beam dumps. But even if only a tiny fraction of these collisions produce dark matter, we can dial both the speed (by changing the beam energy) and the density (by changing the flux of beam particles) of the dark matter, and set up a detector downstream which is tailored to the properties we now have experimental control over. Instead of just seeing missing energy as at the LHC, we would see the telltale signatures of dark matter scattering at the detector.

There is a nice synergy with neutrino physics, since neutrino “beams” are made by using precisely this setup, and one can piggyback a dark matter experiment on top of a neutrino experiment which is already running. I’m interested in ways we can leverage the amazing infrastructure which has developed around neutrino physics to look for dark matter, and also in designing new experiments using beam dumps which are specifically tailored to look for models of dark matter which escape other experimental probes.

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