Sunday, August 2, 2015

The Pentaquark

Recently the discovery of a “pentaquark” has been in the news.

To explain what a pentaquark is, I first have to explain the standard model. The standard model is a theory of sub-atomic particles. It states that all the matter in the universe is composed of three types of particles: bosons, leptons and quarks. Bosons and leptons will not be discussed further in this article.

Quarks come in six different “flavors:” up, down, strange, charm, bottom and top. In turn each flavor has two versions: quark and anti-quark. This makes 12 different kinds of quarks. up, anti-up, down, anti-down, strange, anti-strange and so on.

Quarks group together to form larger particles; take a quark and an anti-quark (not necessary of the same flavor) and you get a meson. An up and anti-down OR a down and anti-up form the first mesons to be discovered: the positive pi meson and the negative pi meson respectively. Other combinations lead to other mesons. All mesons are unstable, and rapidly decay into other particles.

If you take three quarks or three anti-quarks, you get a baryon. The two lightest baryons, the first to be discovered, and the best known are the proton and the neutron. There are many other baryons. Except for the proton, all decay into other particles. The neutron takes about 10 minutes to decay, heavier particles decay much more quickly. (Note the electro-weak theory predicts that the proton should be unstable, but with a very long half-life. The decay of the proton has never been experimentally detected, in spite of four decades of attempts).

In turn you can combine protons and neutrons to form a larger structure: an atomic nucleus.

There are rules for how quarks combine. These rules come from quantum mechanics and the concept of asymptotic freedom. The later is a feature of  Quantum Chromodynamics (QCD). One of the rules can be summarized as follows: if you count each quark as +1 and each anti-quark as -1, and you add up all the quarks/anti-quarks in a quark structure, the sum must be zero, a positive multiple of 3 (such as 3, 6, 9 etc), or a negative multiple of 3 (such as -3, -6, -9 etc).

The sum will be 3 for all baryons, -3 for all anti-baryons and 0 for all mesons and all anti-mesons. Note that this rule prohibits a single quark or anti-quark from existing in isolation, but allows any combination of two more provided there is an appropriate mixture of quarks and anti-quarks.

In case you are wondering, forget about breaking a meson or baryon apart to obtain individual quarks. This is forbidden by asymptotic freedom and QCD.

Ignoring occasional claims for the discovery of tetraquarks, or pentaquarks (which I'll explain momentarily), these are the only quark structures known from experiments. Are other structures possible? There doesn't seem to be any theoretical reason why not.

The first possibilities to look for are the “tetraquark,” a combination of two quarks and two anti-quarks. Or a “pentaquark,” a combination of four quarks and one anti-quark. As I said a moment ago, there have been occasional claims for tetraquarks and/or pentaquarks, but these claims have typically been found be false.

Fast forward to July of this year. Another claim of a pentaquark. This was not the result of a deliberate search, but rather of experiments conducted at the Large Hadron Collider (LHC) in Geneva. These experiments were designed to probe the properties of a particle known as Lambda (1405). This particle has generally been considered to be a baryon (with three quarks); however there was an idea (not universally accepted) that Lambda (1405) might be a pentaquark (with 5 quarks).

The end result of the LHC experiments strongly suggest that Lambda (1405) is in fact a pentaquark. These results await peer review and could turn out to be false, though these results are much stronger than earlier claims.

An unresolved question: are tetraquarks, pentaquarks and larger structures (assuming they really exist) just formless bags of quarks, or are they built up from combinations of mesons and/or baryons? There is strong reason to believe the latter is true. Computer simulations suggest not only that the Lambda (1405) is a pentaquark, but a pentaquark formed by combining one meson with one baryon. Also we know that the atomic nucleus is a combination of baryons, so there are already examples of quark structures composed of baryons. It is not a great leap to consider other quark structures.

This story is not finished; there will no doubt be additional developments over time.

For more information read the following…