Conceptually, particle physics experiments are surprisingly simple. Smash a shitload of particles together, and look at what comes out. The results will either confirm whatever the business-as-usual theory is, or, if there’s a really crystal clear deviation from that theory, they might prove some new hypothesis about some new particles. But the middle ground, where the difference between what we know and what we see is still fuzzy, is where lots and lots of results live.
New results from LHCb, one of the experiments observing particle collisions at the Large Hadron Collider in Geneva, Switzerland, are showing one such fuzzy discrepancy. Physicists are cautiously excited, because if these results hold up, they would imply the existence of some brand-new particles. Unlike last year, when a small signal that seemed like it could have been something new turned out to be just a statistical fluke, these new results are popping up in the wake of another hint observed in a different way a few years ago that hasn’t gone away. So, this time, there may really be something there.
If these signals do turn out to indicate real discoveries, it might “imply the existence of some new kind of particles or other physics that’s still unknown,” LHCb physics coordinator Vincenzo Vagnoni told Gizmodo. “This is a way to unveil the existence of a new family of particles.”
LHC beauty, or LHCb, is a building-sized detector placed around one of the points of the intersecting Large Hadron Collider tubes where particles slam together. Protons and neutrons are combinations of three particles called “up” and “down” quarks, but there are four other heavier quarks you don’t usually hear about, called the charm, strange, top and bottom quarks. LHCb focuses on measuring the properties and decays of B mesons, a special combination of two quarks containing antimatter version of the bottom quark and the regular matter version of other kinds of quarks.
These B mesons quickly decay into a collection of other particles, and for the newest research, LHCb scientists focused on a specific B meson, the B0, decaying into a particle called the kaon, and either a pair of electrons or a pair of muons.
Muons and electrons are the same kind of particle (called leptons), but muons outweigh their electron cousins by a factor of about 200. Their similarity implies that B0s should decay into equal amounts of the two. But yesterday, at a seminar, scientists at LHCb reported that B0s decayed into muons less often than the theory predicted. This violation of “lepton universality” would require the presence of never-before-observed particles.
The results weren’t huge—the difference between the observation and what’s expected according to the usual theory of particle physics, the Standard Model, was only significant to 2.2 to 2.5 standard deviations, or “sigma.” This means the scientists don’t have enough data to claim a discovery—they’ve got some weird statistics that only have a 1 percent chance of happening by accident. Physicists usually require 5 sigma, or a 1 in 3.7 million chance of a difference happening by chance. The only way to increase your sigma value is to collect more data points, which would decrease the chance of the result being caused by random chance.
Everyone is being cautious. Last year, a 3.6 sigma fluctuation washed away with more data, much to the disappointment of many physicists hoping for some never-before-seen particles to look at.
An interesting read via io9