Is there a new heavy fundamental particle - Palahalli Vishwanath

This appeared in Deccan Herald Spectrum 26/1/2016

Is there a new heavy fundamental particle ?

In most High Energy Physics experiments two beams of protons are accelerated to a very high energy and then made to collide which results in the creation of myriads of particles, mostly known but some hitherto unknown. With each new accelerator, the energies are higher and the possibility of detection of heavier particles is also higher . With their first run 3 years ago with proton beam energies of 4000 GeV , both experiments with the Large Hadron Collider (LHC) - ALICE and CMS- detected the famous Higgs Boson with mass of 125 GeV ( Proton, has a mass of about 1 GeV) . They also gave copious information about the Top Quark which has a mass of 175 GeV and had been detected earlier.

After a break of about two years , the LHC experiments restarted in June 2015 with beam protons each having a much higher energy viz 6500 GeV. New results presented in mid December have proved to be very interesting. Both of them see the possibility of a very high mass particle with energy of about 750 GeV ! This would be the heaviest particle ! The particle is detected by its decay into two photons . The signal levels of the possible discovery are not very high but it is interesting that both the experiences see the signal at the same energy. The experiments are continuing and if the signal is real, it will be known in the next few months

At present, the fundamental particles known are quarks, leptons and the force carrier bosons. Atoms have a nucleus at the centre with electrons (a lepton) revolving round the centre at various distances . Nucleus is made up of protons and neutrons which themselves are made up of quarks. The number of quarks and leptons is six; however since all these particles have anti particles and quarks have colour, the total number of quarks and leptons is 48 . This makes up the Standard Model of Particle Physics. The crowning glory of the model was the discovery of Higgs Boson

which was the last missing puzzle. While the neutrino oscillation discovered recently has dented the model some what, at the moment this hallowed model neither expects nor wants a new particle.

However if the particle is real it shows the lack of completeness of the standard model. New physics will have to be invoked to explain the particle. Therefore, the very possibility of a new particle has resulted in a frenzy in the particle physics community. The number of papers about what the particle could be has reached more than hundred. . The possibilities are endless. It could be a partner of Higgs or a Graviton , the particle responsible for gravitational interactions or a Dark Matter particle or a SUSY particle proposed by theories which put fermions an bosons on the same footing. It should be noted that several cosmic ray experiments in the past have detected heavy mass particles but because of lack of control in those experiments there is no proper information about the cross section or the mass of those particles. Therefore it is not unlikely that heavy particles exist. As a researcher puts it " If the 750 GeV bump is real, then the LHC has just made the most profound discovery in physics since the development of the Standard Model in the early 1970s "

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Figure shows the results from the ATLAS experiment. The red line shows the number of

events expected as a function of energy. It can be seen that there is an excess of events around 750 GeV(the energy scale not shown in figure )