Occasional Musings of a Particle Physicist

Searching for New Physics

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Last week I attended the annual LBNL ATLAS physics group meeting. On the final day some interesting comments were made about the strategy for searching for new physics at ATLAS and with the LHC in general. Here’s a brief summary to the best of my memory…

1. Measure the Standard Model

To find new physics you have to know what you are seeing in your detector. To this end the particles from the Standard Model (W and Z bosons, photons, electrons, etc.) must be well measured. That is to say when you see a photon in the calorimeter you must be confident it is indeed a photon! The best way to do this is measure the Standard Model cross-sections – and who knows while doing so you might see something unexpected! Of course you probably won’t but this relates to the next point…

2. Don’t just set limits

The rate at which the LHC is collecting data means a limit on a particular model can be superceded pretty quickly. In fact the winter conference results on the full 2010 dataset will quickly be overtaken by the initial summer results on 2011 data; however as the LHC energy is extremely likely to be changing (caveat: this decision hasn’t been made yet) a Standard Model cross-section measurement on 2010 data will stand the test of time. Setting limits in a model independent way which also makes a useful Standard Model measurement is preferential. The exception to this rule is the Standard Model Higgs on which limits are well defined and motivated.

3. What to do when new physics is found

The first thing to do when new physics is found is to characterise it. This is not a trivial task and will represent significant work at the LHC if new effects are observed. Another key thing on the discovery of new physics is not to continue looking for alternative exotic models. The new physics is only likely to take one form!


Written by Mark

10/01/2011 at 7:21 pm

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Quantum Mechanics meet Special Relativity

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Here as promised is my attempt to explain antiparticles.

First there’s quantum mechanics, the theory of the very small where determinism in the classical sense breaks down; to get anywhere with an experiment you have to predict in terms of probabilities. Probabilities of specific observations must always be between 0 and 1 and the overall probability of observing something must be 1. Then there’s special relativity in which Galilean relativity breaks down as the speed of light turns out to be the same in all reference frames, you may know it predicts E2=(mc2)2+(pc)2 (that’s right I’m leaving the momentum in because it’s correct to; FYI wikipedia rest mass is Lorentz invariant and therefore not the same as energy which isn’t, are you suggesting photons, which have no mass, have no energy?!…)

Now, showing wikipedia is unreliable aside, these theories describe different things: quantum mechanics deals with things which are very small but not moving very fast and special relativity deals with things which are not very small but which are moving very fast (near light speed: relativistic). But what happens if you want to measure something very small but also relativistic as you might in a particle collider like the LHC?

Well you clearly have to find some way of combining quantum mechanics and special relativity. In fact you don’t because British physicist Paul Dirac did exactly that in 1928, you just have to be able to understand how to solve this equation:

Unless you’ve studied quantum mechanics and special relativity the symbols in Dirac’s equation won’t mean much. However, mathematically what it does is satisfy the key requirement of combining the two scenarios: all the probabilities of observing a particle in a given state will be between 0 and 1 and the total probability of observing a particle at all will be 1. Critically this is now true no matter how fast the particle is moving!

Dirac’s equation tells you more than that. If you solve it for a particle like an electron you find there are four solutions. From quantum mechanics we expect two types of electron with the distinction being the quantum mechanical property of spin. Spin is related to the angular momentum of a particle and the two distinct spin states of electrons can be observed by passing them through magnetic fields. The other two solutions are a complete surprise, they predict so called ‘negative-energy solutions’ which can be interpreted as a particle with identical mass and spin states as the electron but opposite charge. This particle, the antiparticle electron or positron, was first seen experimentally in 1933.

Antiparticles represent a profound prediction of theoretical physics later being confirmed by experiment. All of the fundamental particles listed in the table in the previous post have corresponding antiparticles. Also, as mentioned last time, a class of subatomic particle known as mesons, are formed from quark-antiquark pairs. In fact all of the particles formed from quarks have antiparticle partners by exchanging quark for antiquark and vice versa. For example the antiproton is formed of two antiup quarks and one antidown quark.

The Dirac equation as discussed above concerns particles which exist for all time somewhere in space; however, in experiments particles come into and decay out of existence in fractions of a second. The description of such processes requires a more general form of relativistic quantum mechanics: quantum field theory.

Written by Mark

17/10/2010 at 6:21 pm

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The Generation Game

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One of the (many) strange and elegant things about the standard model is its generational structure. The matter that makes up everything around us, as any school student will tell you, is formed of protons (p), neutrons (n) and electrons (e). Electrons, as far as we can tell, are fundamental particles; they are not themselves made up of yet smaller particles. The protons and neutrons on the other hand are composite particles each containing quarks, which again we think are fundamental. They contain two flavours types of quark: the up quark (u) and the down quark (d). Protons are made of two ups and one down (p=uud) and neutrons of two downs and one up (n=udd). Interestingly, unlike the electron, you can never see a quark on it’s own due to the nature of the force which attracts one quark to another. They only come in threes like the proton and neutron (you can also get pairs of matter-antimatter quarks together, I’ll explain antimatter in a future post). The only stable combination of these quarks is the proton, all other combinations decay pretty quickly.

So far all well and good, we have three fundamental particles which explain all the visible matter in the universe, except for some unknown reason it doesn’t stop there. Measurements of cosmic rays (the stream of particles which cascade to earth as extra-terrestrial particles collide with particles in the upper atmosphere) and of controlled collisions in laboratories show that there are six more fundamental particles and this is where things get weird. Two of those particles are exactly like the electron only heavier; another two are exactly like the down quark only heavier; the final two are exactly like the up quark only heavier! In order of increasing mass, the electron type particles are the muon (μ) and the tau (τ); the down type particles are the strange quark (s) and the bottom quark (b); the up type particles are the charm quark (c) and the top quark (t). All of these heavier particles decay pretty quickly to combinations of lighter ones as I’ll explain later, but they do exist if only briefly. It makes sense to group the particles in generations as the table shows. You can see that there are three other particles, the neutrinos (ν), which are related to the electron type particles by the weak force, again to be explained later I promise!

But why are there three generations and why is the only difference between them their respective masses? Well that I don’t know, I wish I did!

Written by Mark

13/10/2010 at 8:57 am

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Good Things Come to Those Who Wait?

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The reason I might now have interesting things to blog about is because I have just started a new job. As of last Monday I am a physicist postdoctoral fellow at Lawrence Berkeley National Laboratory. My position is based away from Berkeley at CERN near Geneva. I am working on the ATLAS Experiment.

ATLAS is one of the so called general purpose detectors measuring the resulting debris from protons collided by the Large Hadron Collider. These collisions are at record energies under laboratory conditions and if you don’t find that exciting then this blog is not for you! The reason we investigate these data is to push the boundaries of our current understanding of the fundamental building blocks of matter and their interactions. At the moment our current theory, the standard model, describes all the data from numerous previous experiments extremely well; however, for interesting technical reasons we know the standard model is not a complete theory: some new physics processes which it can’t explain will more than likely be visible in the LHC collision data.

My intention with this blog is to give an inside view into what working on such an experiment actually involves and occasionally (try to) give my views on the results from CERN as and when they appear. The results are not instant as we have to play a statistical game with the data, distinguishing signatures of what really is new and unknown from substantial backgrounds. These backgrounds we understand in principle from the standard model but their phenomenology (characteristic signature in the detector) is currently at best sophisticated guess work from computer models and must also be verified with the data. So when looking for the new physics signatures patience is required!

Written by Mark

10/10/2010 at 4:22 pm

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Ok so I had a blog once and rarely updated it. It started reasonably well and then kind of petered out, as these things tend to do if you’re busy with multiple things in the real world. I was unemployed. I think the trouble was, like all workshy layabouts, all I had to blog about were the times when politics made me angry. Thus I achieved the effect of coming across like some rabid opinionated loon, although perhaps not on the BBC Have Your Say level. Arguing political points was never a strong area of mine. I tend to get bored of such arguments pretty quickly; most of them never seem to end, other than when you agree to disagree. That’s probably another reason my previous blogging attempt died out: my short attention span. I’d often start writing a post and then abandon it (less than) half way through; no commitment to the cause.

So why start again? Well I think I might occasionally have interesting things to write about. I’ll explain why in my next post…

For this to be more successful than last time I’m going to set myself two rules:

1) Posts will be apolitical
2) Posts will be under 500 words

Written by Mark

10/10/2010 at 3:23 pm

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