Three Minute Thesis Thursday

This is Day 4 of my outreach week challenge : a note on the TMT initiative, and my attempt at a (very) simplified explanation of my thesis.

The idea

The project was launched by the University of Queensland back in 2008 :

The Three Minute Thesis competition or 3MT, is an annual competition held in over 200 universities worldwide. It is open to PhD students, and challenges participants to present their research in just 180 seconds, in an engaging form that can be understood by an intelligent audience with no background in the research area. This exercise develops presentation, research and academic communication skills and supports the development of research students’ capacity to explain their work effectively.

You can find some cool videos here and here.

Dark Matter at the LHC

How do you take a picture of something that is invisible?

That might sound like a trick question, but it’s a very real question in physics. In fact, what do we mean by “visible”? Usually, something that emits visible light; but light itself could fall in a range of the spectrum where it’s no longer visible to a human eye or a commercial camera - UV or infrared light, for instance. Particle physicists take that reasoning one step further and define a visible particle as one that interacts with ordinary matter.

There are two keywords here.

First, “interact” : that’s a direct reference to quantum theory, our best shot at understanding and explaining the infinitesimally small. Different elementary particles can exchange information, interact - via what we call forces - in order to change their properties or their kinematics : a bit like billiard balls bouncing off each other, but a lot weirder.

The second keyword I mentioned is “ordinary” : for the past thirty years or so, we’ve had convincing evidence that something not ordinary at all is lurking in outer space. Something physicists have dubbed Dark Matter : “dark” because it doesn’t seem to interact much, if at all, with ordinary matter, but also because it reflects our ignorance of its true nature. My PhD thesis tries to bring us one step closer to elucidating that mystery.

Using the Large Hadron Collider at CERN, in Switzerland, we hope to collide “ordinary” particles at high enough energies that some Dark Matter might be produced in the aftermath of the collision. With the ATLAS detector (think 20 meter high, 7’000 tonne quantum camera), we can observe the outgoing particles and reconstruct the event that took place at the collision point - much like detectives investigating a crime scene. The only thing missing, of course, is the murderer! if Dark Matter really is dark, it will be invisible to our detector, which is only made of ordinary matter!

How then can we hope to detect it?

Well, that’s where, after 5 years of higher education, I have to go back to GCSE level physics : energy conservation.

We know three things : how much energy we put in the collision, how much energy we collected out of it (in terms of visible particles), and we know for a fact that these two quantities must match up. If they don’t, some energy has been “lost” into invisible particles, and we’ll have our first evidence of man-made Dark Matter. At first glance, it might seem like a clear-cut experimental signal : either you get missing energy or you don’t. 0 or 1. But that’s precisely where complications start.

For one thing, our detector isn’t 100% accurate : it is possible that it can miss a few out of the possibly hundreds of debris particles that emerge out of every collision - and there are 600 million collisions per second! More insidious, perhaps, is the fact that Nature already provided us with a near-invisible particle that isn’t registered by the detector either : the neutrino, which also happens to be produced in abundance.

All these challenges make the ultimate possibility of a discovery even more rewarding : we might finally have a shot at exploring the nature of what makes up more than 20% of the Universe - and, with a bit of luck, it’ll happen before I finish my PhD!

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