Wednesday, November 19, 2014

Two New Baryons Predicted By The Standard Model Discovered At LHC

The large hadron collider (LHC) has discovered two never before detected baryons (three quark composite structures in the same category as protons and neutrons), the Xi_b'- and Xi_b* 

The discovery is reported in a preprint here. The baryons have a bottom quark, a down quark and a strange quark each.  One has spin 1/2 and the other has spin 3/2.  The masses are essentially at the Standard Model predicted values.

A summary of the possible mesons and baryons in the Standard Model can be found at a previous post at this blog.  In the pertinent part that post noted that:
3. A chi baryon (isospin 1/2) (three horizontal lines) (J=1/2 or 3/2) have[sic] one light quark and two heavy quarks.

All four chi baryons without charm and bottom quarks, and all four chi baryons with one strange quark and one charm quark have been observed experimentally. Both of the J=1/2 bottom chi baryons and one of the two J=3/2 bottom chi baryons have been observed experimentally. One of the two J=1/2 double charmed chi baryon has also been observed.

Scientists have not yet observed one of the J=3/2 bottom chi baryons, one of the J=1/2 double charmed chi baryons, both of the J=3/2 double charmed baryons, any of the four of the charmed bottom chi baryons, or any of the four of the double bottom chi baryons.

Thus, exactly half of the twenty-four kinds of chi baryon ground states have been observed so far. In general, the baryons not yet observed are the heaviest ones.
This discovery involves the J=3/2 bottom chi baryon, and a first excited state of the J=1/2 bottom chi baryon above the ground state (the ground state of which has been observed already).  The double charmed chi baryons and the double bottom chi baryons remain to be observed, as do a variety of excited states.

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