Particle physicists are on a high-energy boost — two major experiments in CERN are looking like they are on the verge of a massive discovery. Both experiments have independently observed what seems like signs of a new particle at energy of 750 giga electronvolts. The experiments in question are the Compact Muon Solenoid experiment and the ATLAS experiment, of CERN, both of which smash together high energy proton beams and, by studying what comes out of these collisions, probe the deep secrets of particle physics. The experiments still have not announced the detection as a discovery, because the statistical significance of these events is a mite below the threshold level of 5 sigma, which is the minimum needed to constitute a discovery.
Yet, conferences of particle physicists are vibrant with talk of the 750 GeV di-photon excess, as it is known. Theorists are rolling up their sleeves to get to work, and experimentalists are expressing a cautious optimism. “If it is true, it has got to imply new physics,” says Rohini Godbole of Centre for High Energy Physics, Indian Institute of Science. A peek into this phenomenon is revealing. It is also interesting to wonder what makes this potential discovery so exciting for physicists.
Though at present it is only accorded the status of a statistical blip, the observed structure has been detected independently by the two experiments which are totally different in what they are geared to detect. And what is more, the signal has been picked up at the same energy of 750 giga electrovolts.
Since two different experiments have detected a blip at the same energy, given the wide range of energies scanned, it would be too much of a coincidence for it to be attributed to a systemic error. However, Prof. Godbole adds, “The two experiments would like the width of the resonance to be very different [in one experiment the significance is higher for a smaller width whereas in the other the significance is high for a larger width]. That is, the best value of the width for which they get the highest significance is different for the two experiments. That is the real stumbling block.” This is not to say an overlap optimizing the significance cannot be found.
To be sure, experimentalists are waiting for more data to verify this and obtain a result of higher sigma value.
It is interesting that with the 2012 discovery of the Higgs Boson at 125 GeV, all the particles predicted by the Standard Model of physics have been discovered. So this massive would-be particle would mean that there is some physics beyond the known. Physicists have hypothesized on this and theories are already flying around. Some 235 papers have already been written, expounding on possible explanations of this particle.
If this resonance (or blip) would gather statistical significance, with further analysis and data, and if it is established as a new particle, it could perhaps give evidence of supersymmetry (susy).
In particle physics, susy is a space-time symmetry relating particles having integer spin, known as bosons (e.g., the Higgs particle or photons), and those having half-integer spins, known as fermions (e.g. electrons, protons). This property has not been discovered in nature so far. But if it exists, and evidence for this property is found, it could open out a whole new world of particles and their physics. Susy is also used in String theories; hence, evidence of susy would add support to string theories, which aim to unify all the forces in the world.
“It does not fit straight into any known model… perhaps supersymmetry or a two-Higgs doublet model… but the values of the parameters needed for a supersymmetric model seem to present a challenge. With the given values, we should have seen supersymmetry earlier itself. [To explain] why we haven’t would be a challenge,” says Prof. Godbole.
So while things are still not quite clear, there is a nascent excitement brewing - of important discoveries to come. One thing, however, seems true: If this resonance were to be established as a particle, it would not come alone.
It would only herald the discovery of a host of other particles, not even hypothesized until now via the Standard Model. Also, as Prof. Godbole puts it, “It [the discovery] will help us set the energy goals and luminosity goals for future accelerators.”
