Collider data hint at unexpected new subatomic particles

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A handful of measurements of decaying particles has appeared barely off-kilter for years, intriguing physicists. Now a new decay measurement at the Large Hadron Collider in Geneva has amplified that interest into tentative enthusiasm, with theoretical physicists proposing that bizarre new particles may clarify the outcomes. Scientists with the LHCb experiment reported the new end result on April 18 in a seminar at the European particle physics lab CERN, which hosts the LHC.

“It’s incredibly exciting,” says theoretical physicist Benjamin Grinstein of the University of California, San Diego. The new measurement is “a further hint that there’s something new and unexpected happening in very fundamental interactions.”

Other physicists, nevertheless, are extra cautious, betting that the collection of hints won’t result in a new discovery. “One should always remain suspicious of an effect that does not show up in a clear way” in any particular person measurement, Carlos Wagner of the University of Chicago wrote in an e-mail.

Taken in isolation, not one of the measurements rise past the extent that may be defined by a statistical fluctuation, that means that the discrepancies may simply disappear with extra data. But, says theoretical physicist David London of the University of Montreal, there are a number of impartial hints, “and they all seem to be pointing at something.”

The measurements all contain a category of particle referred to as a B meson, which may be produced when protons are smashed collectively within the LHC. When a B meson decays, it might produce a sort of particle referred to as a kaon that’s accompanied both by an electron and a positron (an antimatter model of an electron) or by a muon — the electron’s heavier cousin — and an antimuon.

According to physicists’ accepted theories, muons and electrons ought to behave basically identically apart from the results of their differing lots. That means the 2 sorts of particles ought to have a fair probability of being produced in such B meson decays. But within the new end result, the scientists discovered solely about seven decays with muons for each 10 with electrons.

There are a number of sorts of B mesons. All are made up of 1 quark — a sort of elementary particle that additionally makes up protons and neutrons — and one antiquark. One of the 2 particles is a sort referred to as a “bottom” quark (or antiquark), therefore the B meson’s identify.

Earlier measurements of one other number of B meson decay additionally discovered a muon scarcity. What’s extra, measurements of the angles at which particles are emitted in some varieties of B meson decay additionally seem barely out of whack, including to the sense that one thing humorous could also be happening in such decays.

“We are excited by how [the measurements] all seem to fit together,” says LHCb spokesperson Guy Wilkinson, an experimental physicist at the University of Oxford in England. If extra data affirm that B mesons misbehave, a probable rationalization could be a new particle that interacts in another way with muons than it does with electrons. One such particle might be a leptoquark — a particle that acts as a bridge between quarks and leptons, the category of particle that features electrons and muons. Or it might be a heavy, electrically impartial particle referred to as a Z-prime boson.

Physicists spawned an identical hubbub in 2016, when the ATLAS and CMS experiments at the LHC noticed hints of a possible new particle that decayed to 2 photons (SN: 5/28/16, p. 11). Those hints evaporated with extra data, and the present anomalies may do likewise. Although the 2 units of measurements are very totally different, says Wolfgang Altmannshofer of the University of Cincinnati, “from the point of the overall excitement, I would say the two things are roughly comparable.”

Luckily, LHCb scientists nonetheless have much more data to dig into. The researchers used particle collisions solely from earlier than 2013, when the LHC was working at decrease power than it’s now. “We have to get back to the grindstone and try and analyze more of the data we have,” says Wilkinson. Updated outcomes might be prepared in about half a yr, he says, and will permit for a definitive conclusion.

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