Long Live LLPs!

Particle physics news today...

I’ve been spending my mornings this week at the 11th Long-Lived Particle Workshop, a Zoom-based gathering of experts on the subject.  A “long-lived particle” (LLP), in this context, is either

  • a detectable particle that might exist forever, or
  • a particle that, after traveling a macroscopic, measurable distance — something between 0.1 millimeters and 100 meters — decays to detectable particles

Many Standard Model particles are in these classes (e.g. electrons and protons in the first category, charged pions and bottom quarks in the second).

Typical distances traveled by some of the elementary particles and some of the hadrons in the Standard Model; any above 10-4 on the vertical axis count as long-lived particles. Credit: Prof. Brian Shuve

But the focus of the workshop, naturally, is on looking for new ones… especially ones that can be created at current and future particle accelerators like the Large Hadron Collider (LHC).

Back in the late 1990s, when many theorists were thinking about these issues carefully, the designs of the LHC’s detectors — specifically ATLAS, CMS and LHCb — were already mostly set. These detectors can certainly observe LLPs, but many design choices in both hardware and software initially made searching for signs of LLPs very challenging. In particular, the trigger systems and the techniques used to interpret and store the data were significant obstructions, and those of us interested in the subject had to constantly deal with awkward work-arounds. (Here’s an example of one of the challenges... an older article, so it leaves out many recent developments, but the ideas are still relevant.)

Additionally, this type of physics was widely seen as exotic and unmotivated at the beginning of the LHC run, so only a small handful of specialists focused on these phenomena in the first few years (2010-2014ish).  As a result, searches for LLPs were woefully limited at first, and the possibility of missing a new phenomenon remained high.

More recently, though, this has changed. Perhaps this is because of an increased appreciation that LLPs are a common prediction in theories of dark matter (as well as other contexts).  The number of new searches, new techniques, and entirely new proposed experiments has ballooned, as has the number of people participating. Many of the LLP-related problems with the LHC detectors have been solved or mitigated. This makes this year’s workshop, in my opinion, the most exciting one so far.  All sorts of possibilities that aficionados could only dream of fifteen years ago are becoming a reality. I’ll try to find time to explore just a few of them in future posts.

  But before we get to that, there’s an interesting excess in one of the latest measurements… more on that next time.

Just a few of the unusual signatures that can arise from long-lived particles; (Credit: Prof. Heather Russell)

2 responses to “Long Live LLPs!

  1. Andrew Oh-Willeke

    It used to be the case that an LLP was presumed when you didn’t detect anything but had missing momentum on the theory that LLPs decayed beyond the point where the direct decay products could be observed. It seems like there are additional tools being brought to bear, but it is a little hard to tell which ones.

    • Fair enough. Yes, the idea here is to actually observe, if possible, the location where the particle decays, and what it decays to. Or, alternatively, the particle might actually be electrically charged, and then it would be observed through its interaction with ordinary matter.

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