# Standard Model of Particle Physics: An Introduction

"What are we? "-This is the most relevant question that has always intrigued the generations of humankind. What if I say that today's science has an answer to this? Yes, science does have. Apparently, the science that we know so far says that we are made by some building blocks of matter called fundamental particles. Today, I am giving you an introduction to the standard model in Physics that incorporates the fundamental particles of nature.

The Standard Model in particle physics is a theory developed around the 1970s. This theory integrates all different types of fundamental particles in nature and the particles that govern their interaction with each other. The standard model comprises 17 building blocks of nature. They are six quarks, six leptons, four force-carrying particles, and one Higgs Boson. The 6 Leptons and 6 quarks are the building blocks of matter called Fermions and the remaining five are called Bosons. Let us talk about these, one by one

## 1) Fermions In Standard Model

The Fermions obey the famous Pauli's exclusion principle of Quantum mechanics. In simple words, this principle states that no two fermions can be at the same place at the same time. Let's look at the macroscopic picture. Electrons, protons, neutrons, and quarks are fermions. Similarly, these fermions combine to form matter or things that are also fermions such as molecules, people, walls, etc. In the macroscopic world, a person cannot pass through a wall unless the wall is removed. Hence, two fermions- people ad wall cannot be at an exact position at a particular time.

## Quarks

There are six different types of quarks in the standard model. They are: up, down, strange, charm, top, and bottom. Quarks bind into triplets or doublets giving rise to new particles. The three different quarks combine together to form Baryons. The doublets (two quarks) combine to form Mesons. Together, the Baryons and Mesons club in the category of Hadrons.

Baryons further classify into Nucleons and Hyperons. The Nucleons are protons and neutrons that sit within the nucleus of an atom. Hyperons are strange particles. They are the Baryons containing at least one strange quark.

## Leptons

The other six fermions are called leptons. The Greek word "leptos" means "thin", "delicate" or "small". Since the leptons don't bind with each other, they are apparently "thin" particles. Leptons comprise the electrons, muons, tau particles along with three neutrinos. Discovered in 1975, the tau-particle is the heaviest lepton. It has a mass around twice of the proton mass.

The neutrinos are an important subgroup within the leptons. They come in three flavors named for their partner leptons. The electron, muon, and tau match with the electron neutrino, muon neutrino, and tau neutrino. Neutrinos have very little mass (even for leptons) and interact so weakly with the rest of the particles that they are exceptionally difficult to detect.

## 2)Bosons In Standard Model

There are four fundamental forces of nature. Three of them are included in the standard model of particle physics — the electromagnetic force, strong force, and the weak force. The standard model does not include Gravity. Each force acts between particles because of some property of that particle — charge for electromagnetism, color for the strong force, and flavor for the weak force. The bosons associated with each force are called gauge bosons — the photon for electromagnetism, gluons for the strong force, and the W and Z bosons for the weak force.

## Photons

Photons are the fundamental carrier of the electromagnetic force. This means that whenever an electron repels an electron or an electron attracts a proton, a photon is exchanged between the two. Photons are massless and chargeless particles. They have an unlimited range.

## Gluons

Another force of nature that binds the particles in the nucleus together is the strong force. Gluon is the basic carrier of this force. Gluons glue the quarks together and they also stick to themselves. This is the reason why the range of the strong force isn't beyond the nucleus. However, the nucleus would blow up without the gluons.

## W & Z Boson

W & Z bosons are the carriers of weak forces in nature. These forces govern the interactions between flavored particles. According to a quantum mechanical idea, a neutrino at times is found to be as a combination of two pseudo-particles -the electron and W+ boson. The neutrinos acquire a weak charge in the plasma due to the non-uniform shielding of these pseudo particles by plasma. Such an interaction is termed as Electroweak interaction.

## Higgs Boson

The most recent discovered particle in the Standard Model of Physics is Higgs Boson. The existence of Higgs boson was confirmed in 2012 by the experiments based on collisions in the LHC at CERN. It is theorized that Higgs boson is produced by the quantum excitation of the Higgs field and is suggested to be responsible for the mass of those particles which should be massless as per the symmetries that control their interactions. It also resolves several other long-standing puzzles. One of these puzzles is the reason for the extremely short range of the weak force.

The standard model is indeed a thing of beauty. But it is still is not able to answer various mysteries so far such as- Why neutrinos have mass? What are dark matter and dark energy? Is there a fundamental particle associated with the force of gravity?. These are the questions that are still need to be answered. When answered, they will change the existing form of the standard model of Physics. Despite all these lacunas, the standard model has for the past 50 years helped the scientists to explain most of the visible world around us. I hope that now we have got some insight into the much-hyped question of "what we are? ".

Also Read: The story of Higgs Boson

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