The bible of particle physics is dying for an upgrade. And physicists may have just the thing: Some particles and forces might look in the mirror and not recognize themselves. That, in itself, would send the so-called Standard Model into a tailspin.
Just about all fundamental reactions between the universe’s subatomic particles look the same when they are flipped around in a mirror. The mirror-image, called parity, is then said to be symmetrical, or to have parity symmetry, in physics speak.
Of course, not everyone follows the rules. We know that, for instance, reactions involving the weak nuclear force, which is also weird for a whole bunch of other reasons, violates parity symmetry. So it stands to reason other forces and particles in the quantum world are also rule-breakers in this area.
Physicists have some ideas about these other hypothetical reactions that wouldn’t look the same in the mirror and hence would violate parity symmetry. These strange reactions could point us toward new physics that could help us move past the Standard Model of particle physics, our current summary of all things subatomic.
Unfortunately, we will never see most of these strange reactions in our atom smashers and laboratories. The interactions are just too rare and weak to detect with our instruments, which are tuned to other kinds of interactions. But there might be some rare exceptions. Researchers at the world’s largest atom smasher, the Large Hadron Collider (LHC), located near Geneva, have been hunting for these rare interactions. So far, they’ve come up empty-handed, but even that result is illuminating. Those negative results help weed out fruitless hypotheses from consideration, allowing physicists to focus on more-promising avenues in the hunt for new physics. [18 Times Quantum Particles Blew Our Minds]
Mirror, mirror on the wall
One of the most important concepts in all of physics is that of symmetry. You could even reasonably argue that physicists are just symmetry hunters. Symmetries reveal the fundamental laws of nature that govern the innermost workings of reality. Symmetry is a big deal.
So what is it? A symmetry means that if you change one element in a process or interaction, the process stays the same. Physicists then say that the process is symmetric with respect to that change. I’m being deliberately vague here because there are so many different kinds of symmetry. For example, sometimes you can change the sign of the charges on particles, sometimes you can run processes forward or backward in time, and sometimes you can run a mirror-image version of the process.
This last one, looking at a process in the mirror, is called the symmetry of parity. Most subatomic interactions in physics give you the exact same result whether they’re done right in front of you or in the mirror. But some interactions violate this symmetry, like the weak nuclear force, especially when neutrinos are produced in interactions involving that force.
Neutrinos always spin “backward” (in other words, the axis of their spin points away from their direction of motion), while antineutrinos spin “forward” (their axis of spin points straight ahead as they fly around). That means there are very subtle differences in the numbers of neutrinos and antineutrinos produced when you run a regular, versus a mirror-flipped experiment that relies on the weak nuclear force. [Strange Quarks and Muons, Oh My! Nature’s Tiniest Particles Dissected]
As far as we know, the weak nuclear force and the weak nuclear force alone violates the symmetry of parity. But maybe it’s not alone.
We know that physics beyond what we currently understand must exist. And some of those hypothetical ideas and concepts also violate the symmetry of parity. For example, some of these theories predict subtle asymmetries in otherwise-normal interactions that involve the kinds of particles the LHC typically examines.
Of course, these hypothetical ideas are exotic, complex and very hard to test. And in many cases, we’re not exactly sure what we’re looking for.
The problem is that while we know that our current conception of the particle world, called the Standard Model, is incomplete, we don’t know where to look for its replacement. Many physicists hoped that the LHC would reveal something — a new particle, a new interaction, anything at all — that would point us toward something new and exciting, but so far all those searches have failed.
Many of the former front-runner theories for what’s beyond the Standard Model (like supersymmetry) are slowly being ruled out. This is where parity-symmetry violation might come in handy.
Almost all common hypothetical extensions to the Standard Model include the limitation that only the weak nuclear force violates parity symmetry. (This is baked into the fundamental mathematics of the models, in case you were wondering how this works.) That means concepts like supersymmetry, axions and leptoquarks all keep this symmetry breaking exactly where it is, and nowhere else.
But look, folks, if these common extensions aren’t panning out, maybe it’s time to broaden our horizons.
Peeling back parity
For that reason, a team of researchers searched for parity violations in a cache of data released by the Compact Muon Solenoid (CMS) experiment at the LHC; they detailed their results in a study published April 29 to the preprint server arXiv. This was a pretty tricky search, since the LHC isn’t really set up to look for parity violations. But the researchers cleverly figured out a way to do it by examining the leftovers in interactions between other particles.
The result: No hints of parity violation were found. Hooray for the Standard Model (again). Though it’s a tad disappointing that this research didn’t open up a new frontier of physics, it will help clarify future searches. If we keep searching and still turn up no evidence for parity violation outside of the weak nuclear force, then we know that whatever lies beyond the Standard Model must have some of the same mathematical structures as that mainstay theory and allow only the weak nuclear force to look different in the mirror.