A New Baryon on the Block
I just chanced upon the news that a new particle has been discovered at the Large Hadron Collider. This is probably old hat for people who work at CERN, but for those of us following along in their wake it definitely belongs to the category of things marked Quite Interesting.
The new particle is a baryon, which means that it consists of three quarks. These quarks are held together by the colour force (which I refuse to spell the American way); baryonic states exist by virtue of the colours of constituent quarks being a red-green-blue mixture that is colourless.
Quarks are fermions with spin 1/2. The new particle has spin 3/2 which contrasts with the most familiar baryons, the proton and the neutron, which also consist of three quarks but which have spin 1/2. The difference can be understood from basic quantum mechanics: spins have to be added like vectors, so the three individual quark spins can be added to produce total spin 3/2 or 1/2.
The most familiar spin 3/2 baryons are made from the lightest quarks (the up, down and strange) as shown in the diagram below:
The top row contains no strange quarks, only up and down. In fact the Δ0 and Δ+ contain exactly the same quark compositions as the proton and the neutron (udd and uud respectively), but differ in spin. The next row down contains one strange quark (e.g. uds) , the one below two (e.g uss), and the particle at the bottom is a very famous one called the Ω– which is entirely strange (sss). For reasons I’ve never really understood, a strange quark carries a strangeness quantum number S=-1 (why not +1?) and the electrical charge is labelled by q in the diagram.
There are six quark flavours altogether so one can construct further baryonic states by substituting various combinations of heavier quarks (c,b and t) in the basic configurations shown above. There are also excited states with greater orbital energy; all the particles shown above have quarks in the lowest state of orbital angular momentum (L=O). There is then a potential plethora of baryonic particles, but because all are unstable you need higher and higher energies to bring them into existence. Bring on the LHC.
The new particle is called the Ξb*, and it consists of a combination of up, strange and bottom quarks that required collision energies of 7 TeV to make it. The nomenclature reflects the fact that this chap looks a bit like the particles in the third row of the figure, but with one strange quark replaced by a much more massive bottom quark; this one has zero electrical charge because the charges on the u, s and b are +2/3, -1/3 and -1/3 respectively.
Anyway, here’s the graph that represents the detection of the new baryon on the block:
Only 21 events, mind you, but still pretty convincing. For technical details, see the arXiv preprint here.
Whether you really think of this as a new particle depends on how fundamental you think a particle should be. All six quark species have been experimentally detected and in a sense those are the real particles. Things like the Ξb* are merely combinations of these states. You probably wouldn’t say that an excited state of the hydrogen atom (say with the electron in the 2s energy level) is actually a different particle from the ground state so why do different permutations of the same quarks warrant distinct names?
The answer to this I guess is the fact that the mass of an excited hydrogen atom differs from the ground state by only a tiny amount; electronic energy levels correspond to electron-volt scales compared to the 1000 MeV or so that is the rest-mass energy of the nucleus. It’s all very different when you’re talking about energy levels of quarks in baryonic particles. In such situations the binding energies of the quarks are comparable to, or even larger than, their rest masses because the colour force is very strong and the quarks are whirling around inside baryons with correspondingly enormous energies. When two creatures have enormously different masses, it’s difficult to force yourself to think of them as different manifestations of the same beast!
Anyway, the naming of this particle isn’t really the important thing. A rose by any other name would smell as sweet. What matters is that existence of this new quark state provides another example of a test of our understanding of quark-quark interactions based on the theory of quantum chromodynamics. You might say that it passed with flying colours…Follow @telescoper