How many fundamental particles are there in the universe? Answer to this question was different in past and can also be different in future. Because particles were discovered gradually. Many-a-times the theory predicted a new particle and other times the results from particle detectors required a new particle.
What is interesting is that each particle has its own story. And I want to share some of them here including a brief about the heroes of those stories – the physicists and experimentalists.
Born -15 September 1929 (age 88)
Gell-Mann arranged hadrons in beautifully simple geometric patterns. Hadrons include mesons and baryons. He arranged these particles in octets and decuplets. And he called this arrangement “Eightfold way” in a joking manner. It was independently discovered by Yuval Ne’eman (14 May1925 -26 April 2006) also.
Here Gell-Mann is holding in his hands the bubble chamber photograph which proved the existence of the Omega minus baryon. It was predicted by his “Eightfold way” of arranging hadrons. And with him is the physicist Yuval Ne’eman.
In this picture Ω¯ (omega minus) baryon is produced when K⁻ meson interacts with proton. And it also shows the decay of particles. Ω¯ decays into ≡° (Xi not) and a π¯ (pi minus) meson. The dashed lines are the path of neutral particles.
But why Hadrons can be arranged in such a way? And the answer came again from Gell-Mann and independently from Geroge Zweig.
Born: 20 May 1937 (age 80)
He discovered that the reason that hadrons can be arranged in such a way is because of a deeper symmetry. That is the hadrons are made of fundamental particles and he named them “aces”. Gell-Mann called them “quarks”. Even though Gell-Mann didn’t believe that they were real particles but George did.
At that time only three quarks were predicted: up, down, strange.
But the real problem was the quarks themselves could not be detected. A hypothesis called “quark confinement” was proposed which simply says that quarks are confined inside hadrons thus cannot be detected separately. But it was still a hypothesis and why it should be true was not known.
Later a new heavy meson was discovered in the year 1974 by two different detectors and it was found to be bound state of fourth quark named “charm”.
3.Samuel Chao Chung Ting
Born: 27 January 1936 (age 81)
At Brookhaven laboratory with his team he discovered the new heavy meson particle and named it (psi) ψ.
Born: 22 March 1931 (age 86)
At SLAC with his team he discovered the new heavy meson and named it “J”.
We now call the particle J/Ψ (because the two experimentalists discovered it independently and gave it different names). It’s bound state of charm and anti-charm quarks.
5.Sheldon Lee Glashow
Born: 5 December 1932 (age 84)
He already predicted the existence of the new quark and named it “charm” quark using the symmetry of leptons and quarks. Only 4 leptons were known at that time so he thought there should be 4 quarks also, not 3.
According to their spin, particles are described as Fermions (spin 1/2) and Bosons (integer spin). So leptons and quarks are Fermions.
So the question that I asked in the beginning of this post can now be answered.
There are 61 fundamental particles. Leptons, quarks, force carriers and Higgs Boson. The total number comes as follows,
6 leptons + 6 anti-leptons = 12
6 quarks (each with 3 different colours)+6 anti-quarks (with 3 different anti-colours) = 36
8 gluons + W(±) + Z° + Υ = 12
Graviton, the hypothetical particle for gravitational interaction is not included in the standard model. A theory that unifies all 4 interactions is yet to be discovered.
Griffiths, David J. (1987). Historical Introduction to the Elementary Particles. Introduction to elementary particles (pp. 11-48). Retrieved from Internet Archive website: Introduction to the Elementary Particles