THE STANDARD MODEL OF PARTICLE PHYSICS

From the tiniest speck to the massive galaxies, something is there from which they all are composed of and something which binds the small fundamental particles together. The standard model is the name given in 1970s to a theory of fundamental particles and how they interact. I'm going to start with the very beginning, from the earliest ideas of building blocks of matter.

How can it be possible that steam, water and ice are actually the same thing? Though they seem to have totally different properties. Essentially the question can be boiled down to what are the ultimate building blocks of reality and what are the rules that govern them. Questions like these have perplexed humanity for so long. And of course, these questions have come up with answers, with varying degrees of sensibility.

In the study for fundamental particle from the four elements- fire, water, air and earth to the modern ideas of chemistry (periodic table). However for the last 100 years or so we have come up with very rapid progress in the study of fundamental particles. Indeed, our modern understanding of the universe can explain phenomena from the behaviour of atoms to how stars burn. We have name for such understanding as STANDARD MODEL OF PARTICLE PHYSICS.

To understand it we need to recall our Chemistry class where we have learnt that all the matters is made up of about 114 elements of periodic table or simply atoms. However near a century ago physicist realised atom isn't the final word (atom is not the smallest particle for building matter which was discovered by ancient Greek philosophers). Physicist discovered nucleus of atom is made up of proton and neutron. And electron revolve around the nucleus. The discovery of electron was a pretty impressive achievement because even now electron is one of the fundamental particle in standard model. In early 1950 Isochronous cyclotron was built in University of California, Berkeley which was used for experiments in nuclear and particle physics. Physicist discovered around 80 more subatomic particles in experiment using particle accelerator. In later 1960s physicist realised that the familiar proton and neutron are made of smaller objects still. These smaller objects are called quarks.

There are 6 kinds of quarks- up, down, Charm, strange, top and bottom. Up and Down quarks are present inside proton and neutron while others are necessary to explain vast number of discoveries made in particle accelerator. In addition to quarks there is another class of subatomic particles called leptons. The most familiar lepton is electron, although it turns out that where are 6 leptons as well. Three of these leptons have electrical charge (electron, muon and tau).  The other three are neutrinos which are electrically neutral. These quarks and leptons include every particle that we know of. The up and down quarks and electron are building blocks of Cosmos. The other 9 particles are observed in our particle accelerator.

DISCOVERY OF QUARKS

In the 1960s, when scientists shooting electrons at matter saw them veer off in different directions, seemingly for no reason. Looking at how and when the electrons changed direction, scientists concluded that the nucleus had to be made up of smaller parts, some of which the electrons were "running into." These parts were smaller than the protons that the scientists knew were in atomic nuclei. The parts, they realized, had to be inside the protons themselves. This was good news for scientists who had been trying to simplify what had come to be known as a ''zoo'' of particles. In the early part of the decade, two different physicists, George Zweig and Murray Gell-Mann, first speculated that the particles weren't elementary, but were made up of different particles that carried either one third or two thirds the charge of either particle. Both came up with the idea of three very basic elementary particles that would make up many of the particles that so profoundly proliferated in physics. Zweig named the particles ''aces.'' Gell-Mann called them ''quarks,'' after a read through of James Joyce's Finnegans Wake and the poem, ''Three quarks for Muster Mark.''

POETRY OF QUARKS

This new theory worked very well in explaining charge, spin, and mass. Two different combinations of quarks could make up a proton or a neutron just the way two different combinations of hydrogen and oxygen atoms could make up a water or an acid.  The force that pulls them together in pairs or threes grows stronger as they move farther away from each other, like an elastic band. Only incredibly high energy events can separate them for even a short time. Quarks can also change "flavor." While no one to this day has ever "seen" a quark on its own, experimental results and observed properties of particles match up so perfectly with the theory of their existence, and don't match up as well to any other theory, that scientists are satisfied that they exist. They explain too many things too well not to be in there somewhere.

Quarks

Name	Symbol	Antiparticle	Charge (e)	Mass (MeV/c2)
up	u	u	+ 2⁄3	1.5–3.3
down	d	d	− 1⁄3	3.5–6.0
charm	c	c	+ 2⁄3	1,160–1,340
strange	s	s	− 1⁄3	70–130
top	t	t	+ 2⁄3	169,100–173,300
bottom	b	b	− 1⁄3	4,130–4,370

As said before, quarks aren't found on their own. They roam in pairs, and certain pairs always team up. The pairs are as follows, up and down, charm and strange, top and bottom. The first quark mentioned in each of these pairs had a charge of two-thirds of a proton unit of charge. The second quark in each pair has a charge of negative one third. In the original theory, two up quarks and a down quark add up to make a charge of positive one - or a proton. Two downs and an up have charges that add up to zero, and make neutrons. But if three quarks have positive two-thirds charges and three quarks have negative one-third charges, then why aren't there just two quarks total? What's the difference? Each of the quarks have just slightly different masses. This is why protons and neutrons, when studied, were found to have slightly different masses. The different combination of quarks gave them a different mass. This combination of charge and mass, as well as a few more esoteric qualities, make up the ''flavor'' of each quark. As to why they can't just be called ''types'' — perhaps we should ask James Joyce.

LEPTONS

A lepton is an elementary, half-integer spin (spin 1/2) particle that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.

There are six types of leptons, known as flavours, forming three generations. The first generation is the electronic leptons, comprising the electron(e) and electron neutrino (ν­­_e); the second is the muonic leptons, comprising the muon(μ) and muon neutrino (ν_μ); and the third is the tauonic leptons, comprising the tau(τ) and the tau neutrino (ν_τ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction.

For every lepton flavor there is a corresponding type of antiparticle, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.

The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, which was classified as a meson at the time. After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particle to be proposed. The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay. It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956. The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory. The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.

Name	Symbol	Antiparticle	Charge (e)	Mass (MeV/c2)
Tau	τ−	τ+	−1	1,777
Tau neutrino	ν τ	ν τ	0	< 15.5
Muon neutrino	ν μ	ν μ	0	< 0.170
Electron neutrino	ν e	ν e	0	< 0.0000022
Muon	μ−	μ+	−1	105.7
Electron	e−	e+	−1	0.511

However, while the building blocks of nature are important, we have forgotten an important point. the important point is force, without force these particles however around the Cosmos without interacting with each other if something does not stick quarks and leptons then there would be no atoms and consequently no us, fortunately which doesn't happen. We should be thankful for that.

We know of four different forces. First is Gravity which is weakest force among all. And we don’t understand how it works in quantum level. However, other three forces are very well understood. Electromagnetism which is responsible for electricity and magnetism and in terms of building matter, this force binds electron to the atomic nuclei and makes atoms. The other two forces are less familiar. First is strong nuclear force, it is this force that ties quarks inside protons and neutrons. And the weak force is responsible for some types of radioactivities.

Gravity and Electromagnetism have a very long range whereas weak and strong nuclear force sparingly affect at long distances, at distances bigger than an atom these forces don’t exist.

FORCE	STRENGTH
Strong Force	1
Electromagnetic Force	10-2
Weak Force	10-5
Gravitational Force	10-40

This weakness of Gravity is basic reason that we can’t study it at particle accelerator and is a huge mystery. We don’t understand why gravity is so much weaker than other forces. Though currently gravity is not a part of standard model.

Now the question is how do these forces interact at quantum level. At quantum level forces are caused by exchange in particle. In a nutshell, all forces work by exchanging different kind of particle at some atomic level. Particles are gluon (G) for strong nuclear force, phorton (𝛄) for electromagnetic force and bosons (W,Z) for weak nuclear force. So this is the standard model of 12 particles of matter governed by 3 forces that are caused by exchange of 4 different particles. From all these building blocks, we can make recipe for anything in this universe.

It is not just a theory but the experiments of particle accelerator has completed our understanding with standard model with amazing precision. Though standard model is the most successful theory ever devised till now but still it is not the end to the questions that we still have to explore. Standard model also include a particle called higgs boson which is thought to give mass to all the particles. We still need to learn the origins of mass which I will explore in my next post...