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CERN spots Higgs boson decay breaking the rules

So much for the Standard Model of particle physics?

Evidence discovered at CERN of a rare form of Higgs boson decay may be just what scientists need to prove the existence of particles beyond those predicted by the Standard Model of particle physics – indirectly, at least.

Speaking at the Large Hadron Collider Physics Conference last week, researchers working on a pair of CERN experiments – ATLAS and CMSsaid their combined datasets offer the first evidence of a Higgs boson decaying into a Z boson (an electrically neutral carrier of the weak force) and a photon.

Higgs bosons decay in various ways. They can split into a pair of leptons – an electron and positron – for example, or a pair of the electron's heavier cousin, muons. It's also possible for a Higgs boson to decay into two photons, but here's where things start to get tricky and weird: a Higgs boson doesn't get there directly.

Instead of going from Higgs directly to photons, "the decays proceed via an intermediate 'loop' of 'virtual' particles that pop in and out of existence and cannot be directly detected. These virtual particles could include new, as yet undiscovered particles that interact with the Higgs boson," CERN said. 

As with decay into two photons, a Higgs boson that decays into a Z boson and photon goes through the same loops of virtual, and potentially undiscovered, particles. That's not all, either: the ATLAS/CMS findings also suggest the Standard Model of particle physics, which the Higgs boson should have completed, is actually pointing toward theories that extend the Standard Model.

According to said Standard Model and CERN, around 0.15 percent of Higgs bosons should decay into a Z boson and photon, but the data indicates it's actually happening in around 6.6 percent of decays picked up by the Large Hadron Collider. In theoretical models that extend the Standard Model to include other particles, the Higgs' Z boson/photon decay rate varies from the 0.15 percent predicted by the Standard Model. In other words, something interesting and potentially undiscovered is going on.

"Through a meticulous combination of the individual results of ATLAS and CMS, we have made a step forward towards unraveling yet another riddle of the Higgs boson," said ATLAS physics coordinator Pamela Ferrari. 

Of course, there's also the certainty of this discovery to assess, and it's not as sure a thing as the discovery of the Higgs boson itself by CERN scientists in 2012. While the Higgs boson's evidence was given a statistical significance of 5-Sigma (roughly equivalent to a one in 3.5 million chance that its discovery was an error), the Z boson/photon decay discovery only rates 3.4-Sigma – still a pretty low chance of being a mistaken observation, but greater than the discovery of the Higgs boson itself. 

In other words, the science continues with hopes more Higgs observations will help clear things up. "This study is a powerful test of the Standard Model. With the ongoing third run of the LHC and the future High-Luminosity LHC, we will be able to improve the precision of this test and probe ever rarer Higgs decays," said CMS physics coordinator Florencia Canelli. ®

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