Random Thoughts on the Universe

Wednesday, April 06, 2011

A New Discovery

There is a rumor circulating through the particle physics community that this afternoon the Fermilab particle accelerator team is going to announce preliminary results that indicate they may have discovered a new fundamental particle.

The rumors are claiming that the data shows a massive particle which is decaying into light quarks (which are the ones that form all of the protons and neutrons in the Universe), which adds to the excitement. The only particle that theorists have strong motivation to expect to find is the Higgs particle, which I have discussed in previous articles, and which gives mass to all the particles in the Universe. The problem is that the Higgs we expect to find will decay to very heavy short-lived quarks.

And that means that this new discovery is not the Higgs, and it is not any other particle in the Standard Model. If the data is real, it indicates an unexpected discovery of the first new particles or forces in several decades (the W,Z bosons and the top quark were both predicted before they were discovered).

But as always the theorists have plenty of candidates, even if they are not as well defined:

  • Dark Matter: The first candidate that physicists will discuss is dark matter. We know from astrophysics experiments that 95% of the energy in the Universe is not included in the Standard model, and roughly a third of that is a form of electrically neutral matter dubbed dark matter. But aside from its existence, none of its properties are known so this new discovery could indicate any of a hundred dark matter candidates. Some of the experiments that have searched for dark matter in the galaxy have indicated a candidate which is too light to decay into the heavier quarks, which agrees with this new data.

  • Supersymmetry: There are two classes of particles, fermions and bosons, and to make the Standard Model work at higher energy levels requires a symmetry between them that has never been observed. As with dark matter, there are hundreds of models for this supersymmetry and several of them could be adjusted to explain this new data.

  • Extra Dimensions: For almost a century physicists have considered the possibility that the Universe contains higher dimensions, with various properties. Several of these models contain a second complete copy of the Standard Model with masses above what previous experiments have been able to achieve. It is not difficult to model a higher mass copy of the u and d quarks that decay to their lighter counterparts in the Standard Model, in agreement with the observations. Because of the limited energy range of the Fermilab experiments, only the heavier u and d quarks can be produced and not the b and t quarks which would decay to heavier quarks, which makes this another prime candidate.
  • Fourth Generation: This one is a little harder to justify, but is still interesting. The Standard Model is neatly organized with 24 fermionic particles divided into three generations, each with two classes (quarks and leptons), each class divided into particles and antiparticles. There is no reason to exclude a fourth generation of particles, which would be very heavy and would quickly decay into lighter quarks. The downside is that they should prefer to decay into the third generation quarks rather than first generation, but that could be explained with a suitable model.
Of course theorists are notorious for generating thousands of models to explain a single anomaly, so this list is far from exhaustive. It must also be remembered that these large experiments often announce data that ends of being just an error or a statistical anomaly, and they end up meaning nothing.

For now all we can do is watch and wait...

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