High Energy for Dummies: A Brief Glossary of Technical Jargon Used in Particle Physics
By on August 23rd, 2011

If a high energy collision means fast billiard balls colliding or a train wreck, this article is tailor made for you. As the world, or at least the 0.042% of it who are interested in particle physics, gets their opinions ready about the yet unsuccessful Higgs search at the Large Hadron Collider (LHC), CERN, we thought it might be a good idea to explain what physicists mean by what they say. However, we don’t want to explain why they speak that way.

Tunnel of the LHC

Here we will list out a few things you’ve heard and are likely to hear in the coming days, along with what they might possibly mean.

Units and Particles:

Energy units

In particle physics, people prefer to use units of energy, instead of that of mass to measure mass. The idea is a good one, since we can use Einstein’s relation E=mc2 directly. The unit of energy is an electron volt or eV. If you force an electron to go through one volt, then it has the energy of 1 eV. It’s too small to use, so people use 1Mega eV (MeV) 1 million eV, or 1Giga eV (GeV) 1 billion eV. If that is not large enough, the LHC demands that we use 1 Tera eV (TeV) or 1 million million eV. The LHC is currently operating at 7 TeV.

The more important point to note about energy is what is considered big! How much really is 1 TeV of energy in human scales? It’s much less than the kinetic energy of a house-fly! So why is it called high-energy physics? It’s because this energy is carried by particles that are really really small! If a proton having 3 TeV of energy is scaled up to our scales, then we would each have energy exceeding that of a Supersonic jet plane. There’s a large amount of energy packed into a small volume.

Particles types

About particles, there are just two types Fermions and Bosons. Know that fermions don’t like hanging out together, while bosons have no such ego issues. You can stuff a lot of bosons at one place, while that is impossible in case of fermions. Leptons are one kind of fermions, which are electron-like. Photons are bosons. (As to why this happens, it’s related to the Pauli Exclusion Principle, a fundamental result from quantum mechanics, responsible for single-handedly creating all of chemistry! Fermions follow Pauli Principle, bosons do not).

Hullabaloo about Higgs:

This is the biggest name in particle physics you’ve ever heard. The electron is passé, it’s time for the Higgs. Named after Peter Higgs, this particle is supposed to endow all other particles in the Universe with mass. It is a so-called boson, unlike the electron. It is itself massive (meaning that it has mass), theoretically predicted to be about 140 GeV. Through a mechanism, known as the Higgs mechanism, an example of spontaneous symmetry breaking (explanation below) of a field theory, mass is generated.

Spontaneous Symmetry breaking:

This is probably the most important phenomenon in this story, so pay attention. This is the deal. Every modern physical theory has some symmetry associated with it. For example, if you made all the positive charges in the Universe negative and the negative charges positive, the Universe will still look exactly like it does. Here’s the crucial part with every (continuous) symmetry, is associated a quantity that is conserved, or doesn’t change value. Example: Take time translation. If time were to flow backwards, at the microscopic scale, we would notice nothing. This symmetry leads to conservation of energy. Symmetry breaking refers to the fact that under certain circumstances, a physical theory loses the symmetry it started out with. We can then distinguish between different states. We can choose.

A Useful Analogy

The best analogy I’ve heard asks you to picture a round table with 10 people sitting equally spaced from each other. There is one glass of water kept identically placed between each two adjacent persons, making it 10 glasses in total. Assuming that the glasses are all identical, one can go and reach out for either one there is no preferred choice as yet. (Situation 1 in the graphic below.) But, say someone does pick up a glass say the glass placed to his/her right.(Situation 2 in the graphic below.) Then, everybody will HAVE to pick the glass to his/her right (assuming that no one wants chaos!). Now, you can differentiate between this system and another such system in which someone went for the glass on the left. Symmetry breaking has allowed us to identify a parameter, which had earlier left the system invariant. The word spontaneous’ is clear from this context. The perturbation has to come from within the system and everyone will choose a glass. The symmetry has been spontaneously broken!

In physics too, there arises situations in which the ground state (or state of lowest energy) of a theory is invariant under certain symmetries. This symmetry can, however, be broken and spontaneously, too if the system interacts with a field. It can be shown that this leads to two distinguishable states one having mass. Physicist believe that this is how mass is generated. The interaction field is named Higgs field’ and the force carrying boson associated with the field (as with any force field) is the Higgs Boson.

Final Word

We could like to conclude this brief glossary of explanations here. Know that these explanations are highly simplified and the whole picture is much richer in beauty and technical details than this. Unfortunately, I won’t be able to communicate that beauty to a person, not having enough background in physics (being a major in physics is a must) and maths. If you already have a background, you’ve probably already seen a bit of the beauty. Go in search of more.

The world is not only queerer than we suppose, remember; it is much queerer than we can possibly suppose. In the words of the immortal Richard Feynman, I think Nature’s imagination is so much greater than Man’s, She’s never going to let us relax.

Links to related articles:

The Higgs search being unsuccessful so far, ATLAS and CMS collaborations of CERN jointly announce. Initial report here.

The figures and numbers (statistics) from the CERN announcement here.

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Author: Debjyoti Bardhan Google Profile for Debjyoti Bardhan
Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.

Debjyoti Bardhan has written and can be contacted at debjyoti@techie-buzz.com.
 
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