Fig 1: The Standard Model of particle
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1. Introduction
Since the birth of humanity, we humans have been asking some questions like, “What is the ultimate composition of the Universe?”, “Where did we come from?”, “Why are we here?”, “Why do the laws of physics function the way it is?”, well it seems a very reasonable question that we need to answer and thought to be able to answer but till now we haven’t been able to unlock the secrets of the universe. It has taken a whole era of scientific revolution but these questions are left unanswered.
The question, “What is the ultimate composition of the universe?” led to the discovery of known subatomic particles and many unknown kinds of stuff. The closest but not the ultimate composition of the universe has been achieved in what we now call the “Standard Model Of Particles.”
2. History
Ancient Greeks later supported by Aristotle believed that all the matters in the universe are all made up of combinations of four elements, “Earth, Air, water and fire.” It is been used as the basis of all branches of knowledge, medicine, philosophy, Astronomy etc... Perhaps these are the four states that matter exists, solid(Earth), liquid(water) and gases(air) and Plasma(fire). It is one of the first answers that attempted to answer the question.
Later, it has been realized that all four elements are matter which is composed of “Atoms,” and it is thought to be the ultimate building block universe. When atoms are combined formed a molecule, compound and element. John Dalton in his atomic theory described the building block of matter as “atoms,” and some analysis of their interactions. Later eminent chemists like Dimitre Mandeelyve and Mosley classified the elements as what we call now, the “Periodic table of elements,” which is one of the finest classifications ever made.
Since then, the realization was not the end of the story. Chemists like Rutherford gave more hints that atoms are composed of further small particles like electrons and nucleus(through his famous gold foil experiment). J.J Thompson discovered the electron in 1897, proton by Goldstein in 1886 and neutron by Chadwick in 1932. Since then, the discovery of three particles of the matter was sufficient to describe almost the interactions that we observe in the physical world around us. It was such cool milestones that did so much to answer the question. The simplified model of an atom is visualized as electrons revolving around the nucleus composed of protons and neutrons like our solar system. Though it is more complex than we had that simple picture(lower or middle schools).
When further zoomed through the three subatomic particles, the current advanced technology of science allowed us to see more in detail. The story continued when protons and neutrons are composed of small particles now called “Quarks and Leptons.” And this is where the “Standard Model,” comes into the picture. It is the collection of elementary particles that we know so far. It is the modern periodic table of particles.
3. The Standard Model
The standard Model is not as simple as some letters written on the tables and piled up one over another. Perhaps, it is one of the masterpieces developed from discoveries of particles that involved various scientific efforts and financial grants. It is a collection of the extreme limit of the composition of the universe that we know so far and the three fundamental forces. Sadly, gravity is not in this picture.
The standard Model has three stacks of particles plunged into the Higgs Field. The first and second fall under one category called “Fermions,” and the other is called “Bosons.” The fermions are the elementary particles that make the visible matter and bosons are the “force carriers.” Well, another way to distinguish is with regards to the spin of the particle.
SPIN
Well, spin is another exciting and confusing concept in quantum mechanics. It should be started with something called the wave function of Particles. A wave function is a mathematical way of expressing information that we know about the quantum states of particles. Well, the information can be position, momentum, energy, spin etc… The wave function is hard to figure out as it is different for different quantum states but can be visualized as something like sine waves, (or sinusoidal). It can be symmetric or asymmetric waves). The fermions have the asymmetric wave function and bosons have the symmetric wave function.
Therefore, the fermions have the odd integer half spin, it can be 1/2, 3/2, 5/2, etc.. and other non-integer spins like 1/3, 4/5 are not observed. We don’t know whether they exist or we fail to detect them. And only all fermions have +1/2 spin.
And the bosons have integer spin 0,1,2,3 etc…
The fermions are further classified into 2 types: 1) Quarks and 2) Leptons.
There are six quarks: Up, Down, charm, strange, and top and bottom. UP and DOWN ARE FOUND IN NUCLEUS OF AN ATOM AND others required advanced particle accelerators to detect them(anti-particles are counterparts and also included in the precise standard model).
Leptons don’t interact with strong nuclear force. There are six leptons: 3 being charged and 3 being neutral. They are electron, muon and tau(and their anti-particles: positron, anti-muon and anti-tau).
The neutral leptons are neutrinos which come from the three above sources namely: Electron neutrino, muon neutrino and tau neutrino(and their antiparticles: Electron anti-neutrino, muon anti-neutrino, tau anti-neutrino). All of this makes up the 12 elements of ordinary matter(24 in all anti-fermion particles are included).
To understand Bosons we need to have some basics to understand: there are four known forces of the universe; gravity, electromagnetic force, and strong and weak nuclear forces. In the subatomic realm, the strong nuclear force is the strongest.
Gravity is the force at a large scale which governs the motion of the heavens, makes an apple fall, makes the ball slow down and makes the planets revolve around the sun. The electromagnetic force makes the intuition of sight that is light, lasers and photo electricity, and magnets that attract things. The strong forces bind neutrons and protons in the nucleus which explains why stars shine. Why do protons exist? The weak nuclear forces help to explain how some nuclei decay giving rise to nuclear reactors that give enormous energy. When talked about force, in plain language, it is rather regarded as pull or push. When force is applied there is an exchange of some particles that we don’t see but gets exchanged resulting in it. So these are three elementary force carriers.
Photons(neutral) for the electromagnetic force, Gluons(neutral) for the strong nuclear force, W(- & +) and Z boson(neutral) for the weak force have the spin of 1 and gravitons are suspected to be the carrier of gravitational force with the spin of 2. The Higgs Boson has the spin of 0.
The standard model predicted that all these elementary particles must be massless of course it is not true. Even neutrinos that are thought to be massless have non-zero mass. And bosons like photons have no rest mass. To address this problem to explain where some particles get mass from and some do not, Higgs's theory was developed. Peter Higgs along with other 5 scientists predicted the existence of another field called Higgs field when excited gives particles Higgs Boson. It is when some particles like quarks interact with the Higgs field that gets their mass and some don’t interact such as photons that have no mass. The particles which interact more are heavier and less are lighter. Higgs boson was discovered in 2012 at LHC, CERN which earned Nobel Prize for Peter Higgs and Englert in 2013. The discovery of the Higgs field completed the Jigsaw of the standard model.
4. Conclusion
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Although the standard model gave us a refined picture of the composition of the universe it only constitutes roughly 5% of the total matter in the universe. And even it is not the ultimate building block that we think of. So, there comes the picture of superstring theory. Others are thought to be dark matter and dark energy that constitute 95% of the universe. But all particles discovered constitutes 5% of the universe and the rest particles or energy carriers not known so far account for dark matter. It means that we don’t know still 95% of what the universe is made up of. Moreover, mysteries have been still not solved: why does the universe have to be made out of quarks and leptons? Why there should be four forces or even more undiscovered? Is dark energy even real or are we skipping some important considerations that can explain the motions of the cosmos? For almost the scientific era, the questions beginning with “HOW,” has been somewhat answered but “WHY,” turned out to be still difficult questions to answer. Maybe PHYSICS enthusiasts will tell us some days. Let’s wait for it...
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