JOIN USToday, the subatomic world is very well explained by the Standard Model of particle physics. The basic tenets of this model are conservation laws that give rise to various types of symmetries and four types of interaction (gravitational, electromagnetic, strong nuclear and weak nuclear). Further, all particles are divided into two types, depending on the value of quantum mechanical spin. The particles with half integer values for spin are called fermions and the rest are bosons. The model requires several bosons (include photons) as carriers of different types of interaction whereas fermions (include electrons, protons, quarks) are considered as building blocks of matter. Leptons consist of familiar electron and unfamiliar particles like neutrinos and muons.
However, well-known particles like protons and neutrons are not in the list of elementary particles. This is because they are considered as mixtures of quarks. The beginning of the 20th century saw man take a deeper look into atoms. It was found that the atom consists of three particles — electron, proton and neutron — which were all considered elementary. However, from 1930 onwards, there were many more particles detected by cosmic ray experiments in the next few decades: the positron, the anti-particle of electron, which is also an example of symmetry in nature, medium mass (between that of electron and proton) particles called pi mesons (pion), which is the carrier of nuclear forces and a seemingly purposeless particle called the muon, which could penetrate a large amount of matter. Strange particles which were later categorised as K mesons (kaon), hyperons (also classified as baryons) etc were also discovered.
But with time, the accelerators came on the scene and a plethora of particles that consisted of hitherto unknown mesons and baryons were detected. These were not stable like the proton, and were shortlived. With so many new particles, it was natural to wonder whether they were all really fundamental. It was Murray Gell-Mann who brought order into this world of chaos in 1964. He reasoned that the basis of all the baryons and mesons was a triplet ,which he called quarks. The revolutionary aspect was that unlike particles like protons and electrons, these quarks have non-integer charges like 1/3 and 2/3. He proposed three quarks called up, down and strange.
With time, three more types of quarks were proposed — charm, bottom and top — and they have all been detected in the laboratory. Top, the heaviest quark with about 175 times the mass of the proton was discovered 20 years ago in Fermilab in Chicago. The standard model has also been deemed complete after the discovery of the famous Higgs Boson in 2013 in the Large Hadron Collider (LHC) in Geneva.
Quarks were considered as just mathematical entities for quite some time since
many experiments to detect quarks gave no results. However, scattering experiments at Stanford University showed that just like how an atom has nucleus as a hard constituent, a proton also has three hard constituents, which could be identified with quarks. Apart from electric charge, quarks also have another charge called the colour charge, which can have three values represented by red, blue and green. These three colours add up to colourless particles like protons and neutrons in the same way that red, green, and blue light combine to create a white glow. Because of colour, there are actually 36 quarks.
Pentaquarks
At the time of the original theory itself, possible mixtures of more than three quarks had been suggested. The strong interaction theory also does not forbid exotic type of particles like tetra, penta and hexa quarks. While no search had given conclusive results about such particles, last year, a group working on the CERN’s LHC claimed detection of pentaquarks with high significance. The data has been further analysed and two new studies show that the evidence for pentaquarks is robust. Since then, the same experiment has also detected few tetraquarks, a combination of four quarks.
In the LHC, two protons collide at very high energy to create various new particles and a lot of normal particle debris. The present experiment showed that the collision occasionally produces bottom quarks which travel a short distance and then decay into a pentaquark plus other particles all of which are recognised and registered in a series of detectors. The experiment has evidence for two new pentaquarks with masses of about 4.4 GeV, four times that of a proton.
There are two ways to envision the pentaquark: it could be thought of as a baryon and a meson ‘molecule’ bound together or as a mixture of four quarks and an antiquark (specifically 2 up, 1 down, 1 charm and 1 anticharm). It is considered
colourless.
Scientists believe that the discovery of the pentaquark is just the tip of the iceberg. According to them, it is not just another new particle and studying its properties allows one to understand better even ordinary matter apart from complex forces between quarks. One might stumble upon more and more combinations. It is also expected to throw light on interactions at the core of neutron stars and the possible existence of quark stars.