Pentaquarks: A New State of Matter

Well actually they have always been here, We just didn't know it.

In 1964 it was proposed that protons and neutrons, called baryons, the main constituents of atoms, along with electrons, were composed of much smaller objects called quarks and that it took three of them to form a baryon. (Name taken from Joyce's Finnegans Wake: "Three quarks for Muster Mark.") The possibility was also raised that there could be some baryons that were formed of four quarks and one anti-quark, known as pentaquarks.

In the intervening half century, hundreds of so called "elementary particles," baryons and mesons, formed of quark-antiquark pairs were discovered, but no pentaquarks were found. There was an interlude around 2004 when some physicists thought they found them, but subsequent higher statistics measurements thoroughly debunked their claims. These searches only looked at the decay products of the proposed pentaquark and reconstructed a hypothetical mass that the particle had before it decayed.

Scientists know that, if a particle such as a pentaquark is present, examples of its decay (called events) accumulate at its mass after multiple decays. We call this "bump hunting," and it was used to find particles in the past, many real, but in the case of the 2004 project, not so much. These ephemeral results caused the word "pentaquark" to have serious negative connotations around physicists, especially if one claimed to have found them.

Our experiment, LHCb, is situated at CERN's Large Hadron Collider (LHC). It is underground in a tunnel that crosses the French-Swiss border several times with a circumference of 17 miles. It collides 4 trillion-electron-volt protons head on (now 6.5 trillion). Collisions occur 30 million times a second. They produce a plethora of particles of various masses. For example, the Higgs boson is a rarely produced particle that was found in 2012 in the ATLAS and CMS experiments that are designed to detect particles of masses of upwards of 100 billion-electron-volts (about the same mass as a silver nucleus). Researchers hope to discover even more massive particles that would be carriers of new, as yet undiscovered, forces.

LHCb has a different task. When particles containing the b-quark decay, they do it relatively slowly so that existence of all force carriers allowed in nature has an effect. These forces can be the known ones or unknown ones. Thus, by viewing the pattern of decays we can ascertain if there are heretofore unseen forces present. In our quest of measuring such decays we stumbled on the pentaquarks.

In order to explain this, let me first tell you what a J/ψ meson is. It has a dual-name because the two co-discoverer's of the particle could not agree on a single name when it was found in 1974. It is composed of charm quark and an anticharm quark. These are much heaver than the light quarks that make up the proton. We were looking for decays of a particle called a B meson into a J/ψ and two other particles.

One of our colleagues suggested that a particular background was possible that could fake our signal from the decay of a baryon called the Λb which contains a b-quark, a u-quark and a d-quark, compared with the proton which contains two u-quarks and a d-quark. The background process had the Λb decaying into a J/ψ plus a proton and a negatively charged kaon. This decay had never been seen before. The graduate student who was working on the B meson decay for his thesis was asked to look for the background and he begrudgingly went off and did it. He returned with a smile. He had found a very large signal. It was immediately used to measure the Λb lifetime which had be an outstanding issue for twenty years (unstable particles have lifetimes, they decay after a certain mean time just like radioactive nuclei). Previous measurements had disagreed with the theoretical prediction, but out data showed that the prediction was correct. There was, however, one feature of the data that surprised us; while most of the decay looked like what we expected, about 10% showed a bump in the J/ψ p mass. If real this would have to be the decay of a pentaquark.

We expected that others in at LHCb would explore this decay mode to explain why the bump existed not evoking pentaquarks, but it didn't happen. So eventually we decided that it was necessary to do a full analysis of how the Λb decayed by using a quantum mechanical description of the decay that used all the information in the decay besides the J/ψ p mass. The results of this analysis were startling. There were two pentaquark states in the data with different masses and different properties. They represented two new states of matter that had never been seen before. Our LHCb colleagues asked for many internal consistency checks that were all passed with flying colors.

Our work is far from finished. We need to compare the pentaquark properties with expectations from the theory of strong interactions called QCD. Such expectations need to be worked on by theoretical physicists. This will allow us to see the internal structure of these states: are the quarks separated or are they bound in pairs, etc..? We also need to look for more states and find out other ways they decay. This will take more data that we are acquiring. These studies will allow us to understand QCD better one of the pillars of modern science. It has wide applications and we do not yet fully understand the consequences of this discovery.