Hint Of New Physics At LHC – Explaining the LHCb Results

4. Decay Processes and how the D0 and D0 bar particles decay

We have seen, in the previous section, how the heavy charm quark can decay into lighter particles. Quarks cannot be seen in isolation (due to an effect known as Quark Confinement). The lighter quarks immediately form stable bound states, which can be detected either directly or through their own decay processes.

D0 decay process

The D0 particle is a bound state of a charm quark and an anti-up quark, while the D0bar contains an anti-charm and an up quark. We know how the charm decays (previous section). We are now ready to construct the decay products. The up and anti-strange quarks, from the charm decay, can form a positive Kaon, while the other strange and the up (which did not decay) can now form a negative Kaon. Diagrammatically, it looks like the following. (The diagrams below are known as Feynman Diagrams. Ignore the W-boson in between, if you want.)

Similarly, the decay of the charm to give down quarks, instead of strange quarks, give pion-pion end products.

The D0 bar decay process

The D0bar also gives the same end-products, but through different intermediate channels. Here’s the illustration.

For the more technically oriented, it is worthy to note that the diagrams are just tree-level diagrams. There are higher level diagrams, involving one loop (penguin diagrams) and so on.

5. Standard Model and what it says:

The Standard Model is the backbone of particle physics. This describes the interaction of all known forces, except gravity, and all interactions of all particles with these forces. The Standard Model is flexible enough to allow mixing in the quark sector. The quarks can thus mix’ with each other. This amount of mixing is given as an angle, called the Cabibbo angle. The sine of this angle gives the quantitative measure of the mixing. The angle being zero shows no mixing at all.

The quantity we are interested in measuring is this:

This gives the relative rates of the decay into the lighter particles (pions or kaons). If the Cabibbo angle was zero, this would’ve been zero. The fact that the Cabibbo angle is small, is indicated by the fact that the quantity shown above (ACP) is very small.

6. The LHCb Result!

Finally, we want to pull all the strings together and see what the LHCb results indicate. The LHCb results show that the mixing is much greater than anything expected from the Standard Model. The observed value of ACP is 0.82% +/- 0.22%, which is way greater than the very small, nearly zero value. The results are at 3.5 sigma confidence level. This is an important result, not only because this is the first observed CP violation in the charm sector, but also because of the magnitude.

We don’t know what to make of this result at this moment. Further analysis might actually make this result go away. Even though this is not as exciting as the faster-than-light neutrino results to the general public, this might be far more significant.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.