**It does not answer the most fundamental mystery: what constitutes the dark energy and dark matter that make up the majority of our universe**

The Standard Model evolved as the fundamental model of elementary particle physics in the 2nd half of the 20th century. It is considered today as the best description of the building blocks of universe. It explains how quarks (which form protons and neutrons) and leptons (electrons etc) make up all the known matter. It is also an explanation of how quarks and leptons are influenced via the exchange of intermediating force carriers. It describes three of the four fundamental interactions that sum up the structure of matter down to the measure of 10 raised to -18 metre. It is in short a quantum theory of three basic theories of electromagnetic interaction, strong interaction and weak interaction. The development of the Standard Model was driven both by huge number of experimental and theoretical physicists alike. The mathematical structure or framework for Standard Model is provided by Quantum Field theory

According to the Standard Model, all matter is made of three kinds of elementary particles: leptons, quarks, and their mediators. There are six leptons which fall into three generations. There are six anti-leptons as well, so the total number of leptons is 12. Similarly, the number of quarks is six, with each coming in three colours (this colour has no resemblance with the concept of colour in our everyday life), which accounts for 36 quarks in total including anti quarks. Quarks like leptons have three generations. Finally, for every interaction we have a mediator. The carrier of the electromagnetic interaction is a massless photon while carriers of the weak interaction are called intermediate vector bosons, which are two charged W’s and a neutral heavy Z. Finally, for exchange of strong interaction we have 8 gluons.

The missing link in the Standard Model, the Higgs Boson theoretically predicted by Peter Higgs in early 1960’s which accounts for the mass of elementary particles via Higgs mechanism, involving chiral symmetry breaking, was found in 2012 at the Large Hadron Collider in Geneva Switzerland. The marvellous achievement of the Standard Model can be gauged by the simple fact that it has led to over 50 Nobel Prizes in Physics so far.

Loopholes:

Even though the Standard Model is currently the best description we have of the sub-atomic world but despite its robust predictions, there is a consensus among physicists that the Standard Model is neither complete nor is it the final theory. “There is a degree of ugliness in Standard Model,” says Steve Weinberg, one of its prime architects. First, the Standard Model is totally silent on dark energy and dark matter. It does not answer the question of what constitutes the dark energy and dark matter that make up the majority of matter in our universe. Secondly, it does not explain neutrino oscillations and most importantly, it does not incorporate one of the most fundamental interactions, gravity, that accounts for the large-scale structure of the universe. On a more basic level it fails to explain why there are precisely three generations of quarks and leptons. Similarly, the difference in masses of the elementary particles which they gain as a result of their interaction with the Higgs Field via Higgs Boson remains a mystery.

Possible way out:

In order to account for many of the shortcomings of the Standard Model as listed above, physicists over the passage of time have come up with different theories and approaches. All these theories and approaches fall in the category of ‘Physics Beyond Standard Model’. Theories that lie Beyond Standard Model include the various extensions of supersymmetry and entirely novel explanations and theories such as string theory, Loop Quantam Gravity, and extra dimensions. But the theory that has gained most prominence among them is string theory. String theory has captured the imagination of an entire generation of particle physicists in the last 40 years. String theory not only promises the reconciliation of quantum mechanics with Einstein’s General Relativity and eliminates the infinities that plague Quantam Field Theory, it also provides a unified theory of everything from which all elementary particle physics, including gravity, would emerge as an inescapable consequence. But the bottleneck of particle physics as string theorist and Nobel laureate David Gross says, is experimental and not theoretical, so in absence of experimental evidence to back up its predictions, string theory’s future seems bleak, or at least one has to keep fingers crossed. If string theory meets expectations, which seems unlikely, it will be the ultimate triumph of the human mind.

—The writer is a student of Physics

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