If new particles existed but had long lifetimes… would we have missed them?

The ATLAS and CMS detectors were built to search for new particles at the LHC. You can think of them as enormous 3D cameras. But after 10 years of LHC operation, no sign of new particles has been seen… and yet we have plenty of evidence from astrophysics that there must exist new physics beyond the Standard Model to explain Dark Matter.

The famous “bullet cluster”, one of the most striking and undeniable pieces of evidence we have for Dark Matter. The image shows two colliding clusters of galaxies. The colour scale shows the location of the visible matter, while the contour lines show the location of the overall mass as measured by gravitational lensing. The fact that these are so offset suggest form of matter, the Dark Matter, which we cannot see.

So where are the new particles? There are two broad possibilities:

either the new particles are much heavier than all the other particles we know about, and are inaccessible at the energy scale of LHC collisions.
… or we are looking in the wrong place. For example, the ATLAS and CMS detectors were built with the expectation that any new particles would decay immediately after being produced at the collision point at the centre of the detectors. What if this assumption was not justified?

Either of the possibilities above would be fascinating. Even if no new particles are discovered at the LHC, if we are able to say with confidence that the new-physics scale is decoupled from the so-called Electroweak Scale which the LHC can probe, then this would be a very valuable result, with important theoretical implications. For example it could mean that the masses of Dark Matter particles arise through a different mechanism than interactions with the Higgs fields. This would restrict which new-physics models are viable.

However, if we are to make such a statement, we need to make sure we don’t miss any new particles in our blind spots!

Checking our blind spots

There are plenty of particles in the Standard Model which have macroscopic lifetimes: fundamental particles like the b-quark, and composite particles like the kaon or the neutron. In general, long-lived particles can arise when decays prevented by weak couplings, when the decay products are close in mass to the original particle, or when a decay is mediated by a much heavier particle. Such situations occur frequently in both the SM and proposed extensions to the SM, so it’s very possible that there are exotic long-lived particles waiting to be discovered…

But these particles could have been missed entirely by the standard research program of ATLAS and CMS! That’s because the detectors and the algorithms we use to analyse the data we collect were designed assuming that any new particles would decay promptly. To be fair, it was a reasonable assumption: this was the case for the top quark, the Higgs boson, the W/Z bosons… But at this stage, we need to cover all possibilities to ensure we leave no stones left unturned.

What would long-lived particles look like in the ATLAS detector?

The answer to this question depends a lot on where in the detector the decay takes place, as well as the properties of the particle, like its mass and electric charge. Let’s focus on neutral long-lived particles which decay to quarks.

Decays in the tracker would lead to displaced vertices: tracks pointing back to a location away from the usual collision point.
Decays in the calorimeters would lead to narrow, trackless jets (sprays of collimated particles). If the decay happens in the hadronic part of the calorimeter, then you’d expect little or no energy in the electromagnetic part of the calorimeter. This is the type of decay I’ve been chiefly searching for!

Decays in the muon system would lead to vertices or jets in the outer layer of the detector.
… or the long-lived particle could be so long-lived that it does not decay within the detector at all! In that case, you will still expect some missing energy to be recorded to balance the energy of all the visible particles!

So what next?

Since October 2021, I’ve been the ATLAS convener for the “Unusual Signatures and Exotic Higgs” subgroup. In simple terms, that means that I’m responsible for coordinating the work of about 70 physicists in a dozen teams, as we search for these long-lived particles and other weird and wonderful realisations of theories which we might otherwise have missed. I’m also a co-organiser of the LHC Long-lived Particle Workshop, a twice-yearly meeting which hosts > 100 attendees from all the major LHC experiments as well as the theory community.

The long-lived particle community is small but dynamic, and we have a broad programme to search for these exotic particles. So far, nothing has turned up, but we are about to start Run 3 of the LHC, and in the coming years we will double the size of our dataset. The search goes on!