Experimental High Energy Physics for dummiesPart I ::: Experimental High Energy… what?

The reason to write this is mainly to “force” myself to read about stuff I always wanted to but found excuses not to; plus, it is a nice way to explain to my friends what I’m trying to do as an (ahem) future experimental physicist. The full title should probably be: “Experimental High Energy Physics for dummies (from a complete Idiot)” but I preferred the short version :). That said, let’s begin:

As the title reveals, more posts will follow. This first article is supposed to be an introduction to the whole concept of High Energy Physics (HEP for short) and how experiments are done.I will focus only on colliders and (surprise!) the Large Hadron Collider (LHC) and the Compact Muon Solenoid (CMS) detector that are at the final stages of assembly at CERN in Geneva. This means that the next posts will have to do with these two. There is no actual schedule for the articles, but my goal right now is to try to post once every two-three weeks. If you want to be updated on when the next articles appear you can simply subscribe to my Physics RSS feed (avoiding the rest of the boring posts in my blog)

To begin with: Why do physicists need particle accelerators anyway? How do these devices help them do research? What is the history behind them? What is the present situation and their future?

Why particle accelerators?

A very brief (and completely irrelevant as you’ll soon realize) answer could be: Because microscopes have limits. Before I explain what I mean let’s talk about the past. A century or more ago our knowledge about the structure of matter could be described at best as simplistic. Scientists knew that Dmitri Mendeleev’s periodic table classified all known materials so everybody believed that matter was made of atoms. At the end of the 19th century, J.J. Thomson made one step further: He showed that atoms were made of positive and negative particles. And twenty years later Ernest Rutherford with his famous scattering experiments (along with Geiger and Marsden) showed that the positive charge was inside a very compact nucleus and the electrons were moving around it just like the planets of our solar system. The puzzle needed just one more piece which came in 1932: The neutron’s discovery by James Chadwick. And everyone was happy. Sort of.

Where do the microscopes fit in? Patience my young apprentice; patience… You see, first we have to talk about another important thing. As you know like charges repel each other and opposite charges attract. Right? Right. Well then why don’t the electrons crash on the nucleus and why doesn’t the nucleus tear apart? Well, the first issue was tackled by Neils Bohr who “simply” assumed that the electrons were allowed to move only in particular orbits without falling on the nucleus. This was later established on firmer grounds thanks to quantum mechanics. For the second issue, physicists assumed that there must be some new kind of mysterious force that keeps protons together (strong force they named it).

Let’s make one more small (I promise, it will be the last!) deviation: Let’s talk about photons. You see photons, besides being as we all know the carrier of light have one more important property. It is the particle that mediates when two (like or opposite) charges interact. This means that when two electrons repel each other this doesn’t happen instantaneously (btw this is forbidden by Einstein’s General Theory of Relativity) but what happens essentially is they “exchange” a photon which carries a message from one to the other at max speed the speed of light of course. So, in the case of two electrons you could say that the message the photon carries says “Go away”! Now because photons have no mass, they can take this message to eternity. This is why we say that electromagnetic interactions have infinite range. Now lets go back to our new kind of force. This “strong force” needed a “carrier” particle as well. And since this force has a very small range (only inside the nucleus obviously – otherwise we’d know about it earlier!) its carrier particle had to be heavy. Hideki Yukawa predicted theoretically its mass with great accuracy. The proof of existence of this particle (the pion) came when it was discovered in cosmic radiation (the torrent of particles coming in the Earth’s atmosphere from outer space).

By the 60’s the physicists had to cope with an enormous amount of particles which were discovered continuously. Classifying all these was a big pain. The solution came when the quark theory was proposed. But there was a very big catch: they couldn’t see individual quarks but only the particles they were supposed to be making! How can this be? Well, as Einstein has taught us, E=m*c^2 which means that mass is another form of energy. When you try to separate two quarks, you have to spend an enormous amount of energy – so big that it’s enough to create two more quarks that’ll pair with the ones you’re trying to separate! What we can do though it crush particles together and observe whatever comes out of that collision. If energies are big enough the quarks of different particles come really close and interact. From those observations – believe it or not! – physicists can make extraordinary discoveries. (Actually microscopes are completely useless for the dimensions we’re talking about here – I was just trying to make a point. At present, the most efficient microscopes – Scanning tunneling microscopes for example – are able to look at individual atoms)

Summing all the above, we can say that the main use of particle accelerators is to crush particles together. At the collision point, complex detectors are used to collect as much information as possible about the products. Physicists then use this information to test existing theories or look for new ones.

About accelerators

There are two major categories of accelerators:

  • Linear Accelerators (or linacs) and
  • Circular Accelerators (or cyclotrons)

Each has its own advantages and disadvantages. In linacs the particles are accelerated on a straight line as their name suggests. If you have a cathode-ray tube TV in your household you are the proud owner of a small linac! They accelerate particles using alternating currents: A very common design utilizes metal plates placed at a certain distance. As the particles are moving towards the plate, it is charged accordingly so that it attracts them, therefore accelerates them. When the particles pass the first plate and start moving towards the second, then the first changes its polarity to repel them this time, while the next plate starts attracting them and so on… Their major advantage over the circular accelerators is that they don’t suffer from the emission of synchrotron radiation. This type of radiation is emitted whenever a particle is not moving on a straight line and since it loses energy, it slows down.

Circular accelerators on the other hand are operating using electromagnets. The magnetic field is used in this case to accelerate the particles and to keep them on track. They are much more in use than linacs because the circular track allows the particles to make several turns to accelerate more before colliding. Particles can even be stored in the ring travelling indefinitely. The aforementioned synchrotron radiation plays a very important role in this case; it slows down the particles and careful studies have to be made before starting to build such an accelerator because different particles emit different amounts of radiation (it depends on the particle’s mass).

Present and future

The largest linac at present (3 km/2 mi) is the Stanford Linear Accelerator while the largest cyclotron ever built was the Large Electron Positron (LEP) at CERN with a diameter of 8.5 kilometers (which gives a circumference of 26.6 km/16.53 mi). At the same underground ring as the LEP, LHC is being built right now and at 2007 is expected to start operating.
End of part I. Hope you like it.
Part II: More on the Large Hadron Collider. ETA: Unknown.

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