Updating the particles parade: Exotic hadrons!

What better way to ringing in the new year than finding out that another particle has gate-crashed the 43-year old Standard Model party! Last year, superluminal neutrinos sent Nobel Prize winners and other like-minded geniuses scurrying back to their whiteboards in an effort to tweak Einstein's theories of relativity. Today, physicists at Japan's High Energy Accelerator Research Organization (KEK) found conclusive evidence of a strange hadron on the site of the B Factory (KEKB), a high-energy electron-positron collider. The so-called Belle Experiment found conclusive evidence of a hadron that had four quarks instead of the mesonic two or the baryonic three.

Hadrons are the heavier of the fundamental particles, with their lighter counterparts being called the leptons. Together with bosons, quarks and gluons, they comprise the Standard Model. The Model was cemented as a "precise" framework in 1967 when Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into it as a way to explain the origin of mass in the universe. Thereafter, it has been uncannily precise in its prediction of the masses and other properties of various particles in its umbra. However, with the construction of high-energy and high-luminosity colliders such as the LHC, the Tevatron and the KEKB, unanticipated insight has been obtained into the workings of nature with the discovery of particles much outside the ambit of the Standard Model.

Hadrons, by definition, were thought to contain two quarks (the building blocks of matter, as it were) of one colour and its anti-colour or three quarks of the same colour. Essentially, they had to be "white" because of a principle called colour confinement (these aren't actual colours but markers used to identify certain properties of the particles' quantum states). When a hadron contained two quarks, it was a meson; when it contained three quarks, it was a baryon. The hadron discovered today has four quarks.

[caption id="attachment_21244" align="aligncenter" width="253" caption="Image from Wikipedia"][/caption]

Since the colour confinement principle cannot break down under any circumstances, the hadron, despite being "exotic", has to be white. This can be achieved by multiple combinations of the twelve different types of quarks and anti-quarks. In order to further quantify its innards, therefore, physicists studied its decay pattern. They found that the new hadron decayed into a bottomonium and a charged pi meson, π±. A bottomonium is a quark-anti-quark pair and is therefore charge-neutral (the bottom quark is the second-heaviest amongst all the quarks; the heaviest is the top quark). The π meson, on the other hand, was found to be charged, indicating that the hadron, too, must be charged because of charge conservation.

The decay birthed a bottomonium weighing 10,610 MeV/c² and a π± weighing 10,650 Mev/c², placing their individual masses at close to 11 times that of a proton. Because the bottomonium is colour-confined and charge-neutral, the meson too must be colour-confined but could consist of an up and anti-down pair, giving the exotic hadron a charge of +¹/₃. Even though the measurements made by the KEKB are precise enough to merit confirmations, a Belle II Experiment is in the works that will record up to 50 times more data starting early 2015 with an astounding luminosity of 50 attobarn.

Such increasingly frequent additions to the Standard Model's particulate load has sparked concerns that there will come a point where just tweaking its principles will become insufficient. For this reason, research interest in such previously-fringe phenomena is growing because they shed important light on things we never thought could exist. At first, there were the protons, neutrons and electrons, but today, there are more than 50 different particles that make up our universe, defining everything from the strong nuclear force between baryons to the rise and fall of galaxies. It seems such is the material of impossibility: the more we try to look for what may not be there, the more we understand what the hell is going on.