Physicists review three experiments that hint at a phenomenon beyond the Standard Model of particle physics

June 8, 2017, University of California - Santa Barbara
Event display recorded by the BaBaR detector showing the decays of two B mesons into various subatomic particles, including a muon and a neutrino. Credit: SLACNATIONAL ACCELERATOR LABORATORY

To anyone but a physicist, it sounds like something out of "Star Trek." But lepton universality is a real thing.

It has to do with the Standard Model of particle physics, which describes and predicts the behavior of all known particles and forces, except gravity. Among them are charged leptons: electrons, muons and taus.

A fundamental assumption of the Standard Model is that the interactions of these elementary particles are the same despite their different masses and lifetimes. That's lepton universality. Precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, but recent studies of the higher-mass tau lepton have produced observations that challenge the theory.

A new review of results from three experiments points to the strong possibility that lepton universality—and perhaps ultimately the Standard Model itself—may have to be revised. The findings by a team of international physicists, including UC Santa Barbara postdoctoral scholar Manuel Franco Sevilla, appear in the journal Nature.

"As part of my doctoral thesis at Stanford, which was based on earlier work carried out at UCSB by professors Jeff Richman and Michael Mazur, we saw the first significant observation of something beyond the Standard Model at the BaBaR experiment conducted at the SLAC National Accelerator Laboratory," Franco Sevilla said. This was significant but not definitive, he added, noting that similar results were seen in more recent experiments conducted in Japan (Belle) and in Switzerland (LHCb). According to Franco Sevilla, the three experiments, taken together, demonstrate a stronger result that challenges lepton universality at the level of four standard deviations, which indicates a 99.95 percent certainty.

BaBaR, which stands for B-Bbar (anti-B) detector, and Belle were carried out in B factories. These particle colliders are designed to produce and detect B mesons—unstable particles that result when powerful particle beams collide—so their properties and behavior can be measured with high precision in a clean environment. The LHCb (Large Hadron Collider b) provided a higher-energy environment that more readily produced B mesons and hundreds of other particles, making identification more difficult.

Nonetheless, the three experiments, which measured the relative ratios of B meson decays, posted remarkably similar results. The rates for some decays involving the heavy lepton tau, relative to those involving the light leptons—electrons or muons—were higher than the Standard Model predictions.

"The tau lepton is key because the electron and the muon have been well measured," Franco Sevilla explained. "Taus are much harder because they decay very quickly. Now that physicists are able to better study taus, we're seeing that perhaps lepton universality is not satisfied as the Standard Model claims."

While intriguing, the results are not considered sufficient to establish a violation of universality. To overturn this long-held physics precept would require a significance of at least five standard deviations. However, Franco Sevilla noted, the fact that all three experiments observed a higher-than-expected tau decay rate while operating in different environments is noteworthy.

A confirmation of these results would point to new or interactions and could have profound implications for the understanding of . "We're not sure what confirmation of these results will mean in the long term," Franco Sevilla said. "First, we need to make sure that they're true and then we'll need ancillary experiments to determine the meaning."

Explore further: SLAC particle physicist discusses the search for new physics

More information: Gregory Ciezarek et al. A challenge to lepton universality in B-meson decays, Nature (2017). DOI: 10.1038/nature22346

F. Archilli et al. Flavour-changing neutral currents making and breaking the standard model, Nature (2017). DOI: 10.1038/nature21721

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Osiris1
1 / 5 (4) Jun 09, 2017
Graviton will probably turn out to be a preon or a quark.
jjesterj
3 / 5 (8) Jun 09, 2017
Wait. The scientific community didn't scream "Standard Model Deniers!" at them? WTF kinda science is this?
sedumjoy
1.4 / 5 (5) Jun 09, 2017
If you can't get the numbers to match ...mother nature is playing some real nice tricks on you. On the other hand there have been false alarms at CERN so it's hard to tell. They are really struggling at CERN. so far one field has been found . The Higgs, which begs the question for it's razor edge mass being on the cuspid of instability and none of the symmetric predictions of particles have come about. That's an awful lot of physics geeks knocking their heads together and not really coming up with much, which is very remarkable considering Fessenden invented and built an AM radio for a few hundred bucks back in the day.
Hyperfuzzy
1 / 5 (5) Jun 09, 2017
There are no particles.
sevensixtwo
4 / 5 (2) Jun 09, 2017
My theory predicts three generations of non-universal leptons.
Da Schneib
5 / 5 (2) Jun 09, 2017
Experiments in the lepton sector have the most potential for new physics. This has also been hinted at by neutrino physics.
Seeker2
5 / 5 (1) Jun 10, 2017
Experiments in the lepton sector have the most potential for new physics. This has also been hinted at by neutrino physics.
Could be but they're working awfully hard on the hidden sector at http://physicstod....3.3594.
Nik_2213
5 / 5 (1) Jun 10, 2017
#762: Link to arxiv or reviewed papers, please ??
eachus
5 / 5 (2) Jun 11, 2017
I suspect that general relativity will be the explanation. But this would be very "new physics" in that those GR corrections will have to be applied to a QM model. No way to really do that today...

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