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 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 —> Read More