How CERN's experiment could shake up the Standard Model of particle physics

The European organisation for nuclear research (CERN) on Tuesday came out with a news that has more than raised an eyebrow among particle physicists. The LHCb experiment in CERN has shown a feeble but persistent sign of physics that contradicts a basic assumption of the Standard Model, indicating that this theory which has ruled the roost may not be complete in itself. Here is what you need to know:

What is the Standard Model of particle physics?

Nature as we know it is governed by four fundamental forces – electromagnetic, strong, weak and gravitational. One of the major programmes in physics is to unify these four forces and have one equation to describe everything – the theory of everything! However, so far scientists have been able to devise a theory that only gives a unified description of the first three forces. This theory is called the Standard Model (SM).

What are the particles in particle physics?

It is known that all subatomic particles are composed of quarks. These come in six flavours or types: Up, Down, Truth, Beauty, Charm and Strange. The quarks do not occur as singles, they can come in pairs – forming the mesons (e.g. pions and kaons), or triplets, to form the Baryons (e.g., protons and neutrons).

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CERN sees “indications” of new physics

Leaving out gravity, which all particles are subject to, let us look at the list of elementary particles: One set of elementary particles are low-mass leptons (electron, muon, tau) which are not made up of quarks and interact only through electromagnetic and weak interactions.

Then come the heavy-mass baryons (protons neutrons etc) which undergo all types of interactions. These are all fermions, or spin ½ particles.

There are the mesons (pions, kaons etc). These are bosons and they are highly unstable and hence shortlived. These interact through strong interactions.

Gauge bosons are particles that mediate the various forces – gluons mediate the strong interactions, the W and Z bosons mediate the weak interaction and the photons, the electromagnetic interactions.

All these particles have been observed and the last particle to be seen experimentally was the Higgs particle. This is a boson and is involved in the mechanism by which the baryons get their mass.

What are the gaps in the Standard Model?

The SM does not include anything like a description of the dark matter particles. So an experimental discovery of a dark matter particle such as a WIMP (weakly interacting massive particle) would be seeing physics beyond the standard model.

As mentioned earlier, quarks come in flavours. The standard model does not allow these to be changed in processes that are strongly observed. So experimental evidence of flavour-changing neutral currents would also go beyond the Standard Model.

What is the “indication” that LHCb experiment found?

Today, LHCb has described an “indication” (which is a weaker statement than saying “discovery”) that they have observed a difference in the way electrons and muons behave. They have observed two types of reactions: in one, B meson decays to an excited K Meson and a muon-plus and muon-minus pair. Now the standard model predicts that these two reactions should have the same rate, however, the experimentalists find a significant difference in the rates. This indicates that there is something different from what the Standard Model predicts.

This is a massive announcement. They are cautious to say that the statistical significance is not sufficient for it to be termed a discovery. With the next runs bringing in some five times more data it is very possible that they would get a stronger indication of this.