What Do the Quark Oddities at the Large Hadron Collider Mean?

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What Do the Quark Oddities at the Large Hadron Collider Mean?

Amid the chaotic chains of events that ensue when protons smash together at the Large Hadron Collider in Europe, one particle has popped up that appears to go to pieces in a peculiar way.

All eyes are on the B meson, a yoked pair of quark particles. Having caught whiffs of unexpected B meson behavior before, researchers with the Large Hadron Collider beauty experiment (LHCb) have spent years documenting rare collision events featuring the particles, in hopes of conclusively proving that some novel fundamental particle or effect is meddling with them.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

In their latest analysis, first presented at a seminar in March, the LHCb physicists found that several measurements involving the decay of B mesons conflict slightly with the predictions of the standard model of particle physics—the reigning set of equations describing the subatomic world. Taken alone, each oddity looks like a statistical fluctuation, and they may all evaporate with additional data, as has happened before. But their collective drift suggests that the aberrations may be breadcrumbs leading beyond the standard model to a more complete theory.

“For the first time in certainly my working life, there are a confluence of different decays that are showing anomalies that match up,” said Mitesh Patel, a particle physicist at Imperial College London who is part of LHCb.

The B meson is so named because it contains a bottom quark, one of six fundamental quark particles that account for most of the universe’s visible matter. For unknown reasons, the quarks break down into three generations: heavy, medium, and light, each with quarks of opposite electric charge. Heavier quarks decay into their lighter variations, almost always switching their charge, too. For instance, when the negatively charged heavy bottom quark in a B meson drops a generation, it usually becomes a middleweight, positively charged “charm” quark.

The LHCb collaboration scours the wreckage of particle pileups for exceptions to this rule. For every million B meson decays they see, one fringe event showcases a rebellious bottom quark metamorphosing into a “strange” quark instead, dropping a generation but keeping its negative charge. The standard model predicts the exceedingly low rate of these events and how they will play out. But because they are so rare, any tweaks coming from undiscovered particles or effects should be obvious.

LHCb’s new analysis covered about 4,500 rare B meson decays, roughly doubling the data from their previous study in 2015. Each transformation ends with four outbound particles hitting a ring-shaped detector. When experimentalists compared the various angles between the particles with the angles predicted by the standard model, they found a deviation from the expected pattern. The collective significance of the anomalous angles grew slightly since the last analysis, and researchers say the new measurements also tell a more unified story. “Suddenly the consistency between the different angular observables got much better,” said Felix Kress, an LHCb researcher who helped crunch the numbers.

Statistically, the deviation in the angular pattern is equivalent to flipping a coin 100 times and getting 66 heads, rather than the usual 50 or so. For a fair coin, the odds of such a deviation are about 1 in 1,000.

But amid oodles of particle collisions, statistical fluctuations are bound to arise, so a 1-in-1,000 deviation doesn’t count as hard proof of a break with the standard model. For that, the physicists will need to accumulate enough B meson decays to demonstrate a deviation of 1 in 1.7 million, akin to flipping 75 heads. “If this is new physics,” Jure Zupan, a theoretical physicist at the University of Cincinnati, said of the current update, “it’s not significant enough.”

Still, the observed pattern hints that something is off with B meson decay products in the lepton family, the other category of matter particles aside from quarks. Like quarks, leptons come in heavy, medium, and light generations (called tau particles, muons, and electrons, respectively); the standard model says they’re all identical except for their mass. Each B meson decay ends by shooting off a twin pair of any of the three types of leptons. LHCb’s latest update focused on the anomalous angular pattern produced by muon events, which are easiest to detect.

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