The pentaquark

This has been a big week for news of all kinds, not least of which is the arrival of the New Horizons spacecraft at Pluto after its decade-long, three-billion-mile journey through the solar system. But forget all that. The really big news — what all the cool kids are talking about — is the pentaquark.

Ordinary matter, the stuff you and I, and everything else we can see and touch, is made of particles called hadrons. The two best-known examples of hadrons are the proton and the neutron. These particles are each made up of three quarks1 and thus could be called triquarks. (As far as we can tell, quarks represent the fundamental bits of matter, which means, unlike molecules, atoms, and protons, you can’t break them up into smaller bits.) Another type of hadron is the meson, which is made up of a quark and an anti-quark2 (a biquark?).

A new class of hadron was proposed by physicist Murray Gell-Mann in 1964, and, like the Higgs boson, seemed quite clever in eluding particle physicists for many years. However, that has apparently changed with what scientists at the Large Hadron Collider are pretty sure is the pentaquark. Unlike a proton or a neutron, the pentaquark, as the name suggests, is made of five quarks2. A boring old proton is made of 2 up quarks and 1 down quark1. A neutron is made of 2 down quarks and 1 up quark1. In principle, you could have a lot of different kinds of pentaquarks, all made of different combinations of quarks. The one discovered by LHC is made of 2 up quarks, 1 down quark, 1 charm quark, and 1 anti-charm quark2,3.

What’s important about this discovery is that it: a) further validates what is called the Standard Model, the prevailing theory governing particle physics; and b) raises new questions, which is music to a physicist’s ears. There is nothing better in science than a new question. One of the new questions is, what holds a pentaquark together? The quarks in a proton are bound tightly together by gluons, however it’s not clear what holds a pentaquark together. Is it tightly bound by its gluons like a proton or is it made up of a proton and a meson that are, themselves, somehow bound together? This’ll keep particle physicists busy for a while.


1. Plus zillions of gluons and zillions of quark-antiquark pairs. (Yes, gluons are called gluons, because they glue quarks together.)

2. Plus, presumably, zillions of gluons and zillions of quark-antiquark pairs.

3. The six known ‘flavors’ of quark are called: up, down, top, bottom, charm, and strange. As counterintuitive as it may seem, the top quark is the strangest quark of them all.

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