It is known for 60 years now that particles like the proton and neutron are not elementary particles but are made up of more fundamental building blocks called quarks—and we now know that there are six kinds of quarks. 

The interaction between particles at high energies is described in terms of the interactions between quarks and related particles called gluons within a theoretical framework called Quantum Chromodynamics (QCD) but the understanding of how quarks bind together to form the observed particles is still lacking. 

This is one of the big open problems in particle physics but some progress has been made in understanding the bound states of heavier quarks (called charm and bottom quarks) using a theory derived from QCD called NRQCD (Non-Relativistic Quantum Chromodynamics).

Within NRQCD, it is possible to rigorously analyse the production of the bound states of charm quarks and while this theory explains several features of experimental data, it fails some crucial tests. One such test is the production of a charm quark bound-state called the eta_c. 

In a paper written by K Sridhar and his collaborator way back in 2010, they predicted, using NRQCD, the number of eta_​c events that should be seen at the Large Hadron Collider in CERN. The experimental results were published four years later and were seen to be much smaller than the theoretical predictions — almost by a factor of hundred! 

There has been no successful explanation of this discrepancy but more recently with his collaborators, Biswal and Mishra, Sridhar formulated a modified theoretical framework called Modified NRQCD which predicts exactly the number of events that the LHC experiment has measured. 

Their theoretical results are genuine predictions with no adjustable parameters and the remarkable agreement with the data suggests that their modified theory is coming closer to apprehending the true dynamics of the bound-state formation of quarks.

About the Researchers

Sridhar K is a faculty member at Azim Premji University. He may be contacted at sridhar.​k@​apu.​edu.​in

Sudhansu S Biswal is part of the Department of Physics, Ravenshaw University, Cuttack. He may be contacted at sudhansu.​biswal@​gmail.​com.

Sushree S Mishra is part of the Department of Physics, Ravenshaw University, Cuttack. She may be contacted at sushreesimran.​mishra97@​gmail.​com.

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Image by Gerd Altmann from Pixabay