Members of the CMS (Compact Muon Solenoid) and the ATLAS detectors working with the Large Hadron Collider in Switzerland have independently identified signals that could lead to the discovery of a new fundamental particle of nature. Both experiments have observed an excess of pairs of photons which could arise from the decay of heavy particles created during the collision.
While it is not exactly clear what this particle could be, if the existence of the new particle turns out to be verified when more data come in next year, physics would have a new elementary particle about six times as massive as the Higgs boson which explains why other particles have mass.
The data was collected after the LHC was fired up again in June to continue smashing particles together at record-breaking energy levels of 13 Teraelectronvolts (TeV). Both the teams presented their data on December 15 in a meeting held at CERN, Geneva.
With the discovery of the Higgs boson, all the particles in the Standard Model of particle physics have been seen. So if a new particle is discovered, it would mean evidence for physics beyond the established mode of thinking about elementary particles from the mid-1970s onwards, namely the Standard Model of particle physics.
CERN’s media spokesperson, Arnaud Marsollier, cautiously reiterates that it is far too early to say anything about a discovery. “ATLAS and CMS presented many results on Tuesday, and among these, there is a small intriguing signal that might be something new. But we will definitely need more data as such signal can also easily be due to statistical fluctuations…”
But the caution cannot detract from the excitement that the little statistical bump has created, as the signals leading up to the discovery of the Higgs boson were also similarly tantalising.
Summing up the excitement of the community, Arnaud Marsollier says, “Such a signal if confirmed would be unexpected and physicists will study this in more detail with more data to come in 2016. As the LHC restarted in 2015 at a new unexplored energy, there is a very strong interest with new data collected, but the scientific process needs time and we have learnt to be patient!”
In the experiments, two proton beams of extremely high energy are made to collide with each other. The protons shatter and in the combined effect of energy and mass fragmentation, various elementary particles are created, many of which decay into states like photons or Z bosons, to name a few.
By looking at the fragments that emerge from beyond the zone of these cataclysmic collisions, the scientists backtrack and estimate what particles were created in the collision. Protons and other elementary particles are essentially quantum in nature and governed by laws of statistics and probability, so the scientists have to use statistical analysis to interpret the data.
In this case, both CMS and ATLAS experiments have independently observed an excess of pairs of photons which could have resulted from the breaking up of a particle that decayed into two photons. Since the energies of these photon pairs add up to 750 gigaelectronvolts (GeV) or roughly 750 billion electron volts, this could be a measure of the mass of the particle that decayed. The mass of a proton is close to one GeV.
These results would constitute a discovery if they had a statistical significance of at least 5 sigma, and in this case, CMS reports a significance of 2.6 sigma and ATLAS a significance of 3.6 sigma. The sigma value indicates the confidence level of the measurement; for instance, 5 sigma will mean that the probability of the bump being due to a chance background event would be one in 3.5 million. Also, this was the result obtained when the scientists were scanning the entire energy spectrum and not just 750 GeV. This fact causes the statistical significance to drop a little more. But there more data to come in next year, and if this signal should improve to required levels, it would actually yield evidence for a new particle.
In 2016, the LHC will continue experiments that probe not only the two-photon decay channel, but also others which will strengthen these searches. Immaterial of whether the existence of this new particle turns out to be proved or not, 2016 is for sure going to be an exciting year for particle physics.
As of now, apart from interesting possibility at 750 GeV, the scientists have seen no signs of physics beyond the Standard Model, such as supersymmetry.
