Cutting edge particle physics today:
I’ve been spending the week at an inspiring and thought-provoking scientific workshop. (Well, “at” means “via Zoom”, which has been fun since I’m in the US and the workshop is in Zurich; I’ve been up every morning this week before the birds.) The workshop brings together a terrific array of particle theorists and Large Hadron Collider [LHC] experimenters from the ATLAS and CMS experiments, and is aimed at “Semi-Visible Jets”, a phenomenon that could reveal so-far-undiscovered types of particles in a context where they could easily be hiding. [Earlier this week I described why its so easy for new particles to hide from us; the Higgs boson itself hid for almost 25 years.]
After a great set of kick-off talks, including a brand new result on the subject from ATLAS (here’s an earlier one from CMS) we moved into the presentation and discussion stage, and I’ve been learning a lot. The challenges of the subject are truly daunting, not only because the range of possible semi-visible jets is huge, but also because the scientific expertise that has to be gathered in order to design searches for semi-visible jets is exceptionally wide, and often lies at or beyond the cutting edge of research.
In particle physics, an ordinary run-of-the-mill “jet” is a spray of ordinary “hadrons“, which are particles made from ordinary quarks, anti-quarks and gluons. These jets are themselves the result of a quark or anti-quark or gluon being produced with lots of motion-energy — typically moving at or near the speed of light — in a particle collision, such as those that are happening at the LHC (which just started producing collisions at a record energy-per-collision.) Below is an event from ATLAS showing an event with two back-to-back jets.
A “semi-visible” jet is the name given to something more complex. We don’t know if such jets occur in nature, but experimentalists at the LHC are now trying to look for them. These jets are sprays of currently-unknown, novel particles from a “hidden valley” or “dark sector” (almost equivalent names for more or less the same thing, and the words “hidden” and “dark” will be used almost interchangeably in this post.) These particles are typically themselves “dark hadrons”, made analogously to ordinary hadrons from “dark quarks”, “dark anti-quarks” and “dark gluons”. Why call these particles “dark?”
- They are completely unaffected by the forces, other than gravity, that dominate daily life: the electromagnetic, weak-nuclear and strong-nuclear forces. That makes them undetectable; such particles would almost always pass through ordinary matter leaving no trace, even more “ghostly” than neutrinos.
- Some of these types of undetectable dark hadrons might be the universe’s dark matter (or might decay to dark matter), though that’s a speculative issue that the LHC experiments won’t be able to determine by themselves.
If that were all there was to the story, these exotic jets would be completely invisible. What makes them visible, or “semi-visible”, is that while some of these dark hadrons may depart the scene leaving no trace, a certain fraction of these dark hadrons may decay, within a tiny fraction of a second, to ordinary particles that we know and love — quarks and anti-quarks, electrons and positrons, or others of the known elementary particles. If they decay to quarks and anti-quarks, each one of these makes its own ordinary jet. In that case, a semi-visible jet becomes, in a way, a jet of ordinary jets… along with some signs that something’s gone missing.
To give you a sense for this, here (from work I did in 2008) are a couple of simulated proton-proton collision events of these jets of hidden hadrons, both entirely visible (left) and semi-visible (right); on each side of each event is a jet of jets (and I promise you there are undetected particles, not shown, in the right-hand event.) Compare these with the jets in the real ATLAS event pictured at the top of the post.
But this picture shows just one of many examples of what semi-visible jets can look like. There are tremendous variations from one semi-visible jet to another even within a single choice of hidden valley [you can actually see this in the figure above], and if you change to another hidden valley with, say, a different set of dark quarks or a different choice for their masses, you may find your semi-visible jets are qualitatively different. This is what makes looking for semi-visible jets so difficult; the space of possibilities is so large that we have to find strategies that are able to cover many possibilities at once, but we don’t yet have all the basic tools and techniques that we need in order to design those strategies.
That’s why we hold workshops: to bring the experts together to brainstorm and share ideas and knowledge. Hopefully we’ll solve some of these problems in the coming six months to a year, and perhaps in two to four years you’ll see the fruits of this labor — a range of new and more powerful searches for this phenomenon, and a chance for a discovery.