On the same day that the New Horizon probe makes history as the first man-made object to do a fly-by of the planet Pluto, the LHCb experiment is claiming a fascinating new discovery on the subatomic scales. According to results announced today, they seem to have produced and detected a pentaquark system.

Allow me to begin by (briefly) reviewing the theory of quarks. According to the Standard Model of particle physics, there are six flavours of quark as well as a matching set of six anti-quark flavours. Each quark can be in one of three states, usually labelled as red, green, or blue. (Although it must be said that they do not actually have a visible colour, but these names have stuck for the three possible states). The antiquarks can be in the states antired (which is blue-green), antigreen (which is red-blue) or antiblue (which is red-green). 

However the forces between quarks are so strong that we can never isolate a single colour or anti-colour. All physical particles that can be detected must be effectively white. That gives us effectively three possible states:
  • Baryon: Three quarks, with one red quark, one green quark, and one blue quark.

  • Antibaryon: Three antiquarks, with one anti-red,one anti-green, and one anti-blue. Or alternatively, one green-blue, one red-green, and one blue-green so that we have a total of two reds, two blues, and two greens to create a total of a white state.

  • Mesons: One quark and one antiquark, with the quark being either red, green or blue, and the antiquark carrying the other two possible colours to make a total of white.
Those are the three simplest combinations of quarks that can be found in nature. Baryons include all of the protons and neutrons in the Universe. Antibaryons are not common in nature, but are generated in high energy events in the form of anti-protons and anti-neutrons. And mesons are very short-lived, but are still produced in large quantities in every particle accelerator and in every cosmic ray. These three states have been studied extensively for the past century, and are well understood.

However the LHCb is claiming now that they have found a fourth state. Theorists have long suspected that there could be a state with four quarks and one anti-quark, called a pentaquark. However there has always been debate about its stability, since the antiquark could easily pair with one of the quarks to become a separate meson, rather than forming a pentaquark. 

If the LHCb result is confirmed and proven correct, then it would indicate that this really is a fourth state of quark particle. It could still be a baryon and a meson close together but not actually a single particle. Only time and more data can prove their results.

For now, the official announcements can be found here and here.