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Elementary (particle physics), my dear Watson

Ground Control to Major Tom: Shruti Patel checking in. Since my dear friend and fellow-physicist Rahul Poruri is homeward bound, and shall remain there for a week, I'm stepping in to continue his tradition of daily posts to Astronut. Even as I write, I'm not quite sure what this post is going to be about, so please forgive me for occasional rants and digressions. 

By the way Rahul, I totally resent you for the name of this blog- I'm never going to start my own blog because there's no way I'll come up with a name as cool as Astronut! For now, I shall try and be happy with being a guest writer!

Before I begin, a little bit on who I am and what I do; so that you can trust that my general views on science- particle physics in particular- are not entirely baseless. I graduated with a masters in Physics from IIT Madras in July this year, and I'm currently working towards (an eventual) PhD with the Theory Group at the Deutsches Electronen-Synchrotron (DESY) in Hamburg, Germany. 

My work is more phenomenology than theory- I work on Supersymmetry- SUSY for short- or more specifically the Minimal Supersymmetric Extension of the Standard Model (MSSM) with complex parameters. In this post, it would be a good idea I think, to give a Level-0 crash course in the Standard Model (SM), while subsequent posts will talk about why the Standard Model is not the end all and be all of Particle Physics and why there most definitely is physics Beyond the Standard Model (BSM). 

Developed in the latter half of the 20th century, the Standard Model of particle physics is in my humble opinion a remarkable example of what collaborative science- both theoretical and experimental- can achieve. This theory, as it stands today, was driven by both experimentalists and theorists alike. The theoretical predictions have been verified by experiments to a fantastic degree of precision, and the theory has been able to accommodate all the particles discovered as a result of experiments in the most satisfying way. The SM explains how the particles that make up the observable universe interact with each other through three of the four fundamental forces of nature- the Strong, Weak and Electromagnetic forces. In all its beauty and elegance though, the Gravitational force still remains that one that got away. 

The basic structure of the theory itself is quite simple. There are two classes of particles: Fermions and Bosons. Let's look at them one at a time. 

Fermions (named after Italian physicist Enrico Fermi) are the building blocks of matter. They are characterized by a half-integer spin and they follow Pauli's Exclusion Principle (for the non-physics junta, that quite simply means that two fermions cannot exist at the same place at the same time). Also known as "matter particles" for obvious reasons, Fermions are further divided into two more families- Quarks and Leptons. Each of these families contain six elementary particles (along with their anti-particles) that are organised into three generations.

  • Quarks, as I've already mentioned, come in six flavours; namely Up (u), Down (d), Charm (c), Strange (s), Top/Truth (t) and Bottom/Beauty (b) (physicists are cool like that). The quarks u, c and s are called "up type" and d, s and b are called "down type". The first, second and third generations constitute of up-down, charm-strange and top-bottom respectively, with the first generation being the lightest and so on. All stable matter in the universe is made up of the first generation of particles, with the heavier particles decaying into the next-lightest particles. All the up-type quarks carry a charge of +2e/3, where e is the charge of the electron/proton. The down-type quarks carry a charge of -e/3. Each of these quarks, in addition to the electric charge, also carries a colour charge. The colour charge is similar to the electric charge, except that it corresponds to the Strong force rather than the Electromagnetic force, and comes in three types, not two. The three types of colour charges are named- no surprises here- red, blue and green. The principle of colour confinement dictates that coloured particles cannot exist independently: all stable matter should be colourless. This means that quarks cannot exist in isolation. They combine to form colourless-composites called Hadrons. Quarks can combine in groups of two or three. The former is in the form of quark-anti quark pairs, called Mesons. Groups of three quarks called similarly called Baryons. The most famous of these are the proton (uud) and the neutron (udd). 
  • Leptons are come in six flavours as well- the electron, muon and tauon accompanied by their corresponding neutrinos. The electron and the electron neutrino make up the first generation of leptons; the muon and its neutrino the second, and the tauon and the tau neutrino the third generation. Tauons are the heaviest of the leptons- they decay into muons, which decay into electrons. All stable matter, analogous to the quarks, is made up of first generation leptons. The electron, muon and tauon carry an electric charge of -e, while the neutrinos are electrically neutral. The leptons carry no colour charge, which means that they do not interact via the strong force. 
Bosons (named after the Indian scientist Satyendra Nath Bose) are the "force carriers". Unlike fermions, boson carry integer spin, and do not follow the Exclusion Principle. The interactions between fermions through the three forces encapsulated in the Standard Model are mediated by the bosons. Each boson is a quanta of the force field. Two or more fermions interact by the exchanging these bosons. The photons carry the electromagnetic field, the W+, W- and Z bosons carry the weak force, and gluons (g) carry the strong force. The sixth of these bosons is the (now legendary) Higgs Boson (AKA the God Particle, but don't get me started on that). The Higgs doesn't correspond to a force, but in the Standard Model it is responsible for imparting mass to the W and Z bosons and the fermions. A seventh boson- the graviton- has been postulated for a long time, but it doesn't fit into the Standard Model yet. When we try to reconcile General Relativity (the theory of gravity) and Quantum Mechanics, all kinds of weird things start to happen, as anyone who has suffered through Nolan's Interstellar would attest for, but this is a story for another day.

I think this is a good time to stop. I believe (and hope) that this lays down a fairly comprehensive outline of what the Standard Model is, and should prove to be sufficient to explain why it is not the complete picture.

I cannot end the post without talking about the merits of this theory though, and how it is a standing proof of what humanity is capable of achieving. In the last 100 years, we have come a long way: from the Plum Pudding model of the atom, to smashing protons together at terascale in what can only be called a true engineering marvel, the crowning jewel of the international scientific community. Regardless of the fact that the theory itself is incomplete, it is a testimony to our abilities to ask questions to the universe we live in and successfully decipher the answers it throws back at us. The study of elementary particles takes us to the very core of what it means to be human- to wonder, to question, to discover. 

We live in exciting times, and it feels like the field of High Energy Physics is on the brink of a paradigm shift. The second run of the LHC (Spring 2015) may yet throw some surprises- it's time to buckle in and hold on tight! 

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