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Matthew Tamsett | USLHC | USA

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The perils of finding things out.

Friday, May 20th, 2011

– by Matthew Tamsett, US LHC

As of last week I’ve moved to CERN for the summer. It’s great to be back, it’s so vibrant and full of life here. However, it’s also a very busy place, and a place where, if you’re not careful, you can lose yourself in an infinite sea of meetings.

When I first came to CERN, several years ago, I found myself falling into this trap. I tried to attend every meeting that was relevant to me, and simply found that I wasn’t spending the time I needed on my research. I then recognised that a balance needed to be struck somewhere.

As the CERN folk-law tells it; once upon a time a committee was formed at CERN to look into the optimisation of meetings (so we didn’t all get lost at sea). Several ideas were circulated, including a rather shocking proposal to ban all laptops from meetings. I don’t think this idea would have gone down very well.

As the rumour goes, the committee was eventually disbanded because they couldn’t find the time to meet.

This tenuously links to what I wanted to talk about in this post. That is the 1981 Horizon documentary entitled “The pleasure of finding things out“, about Richard Feynman.

Feynman was an excellent speaker and this well-made programme allows the great man to wax freely in his inimitable style. I very much encourage everyone to watch it.

Among the many thoughts that stand out to me from this (and there are lots), one is that an idea in formation is like a house of cards. The idea itself is made up of a precarious stack of individual points (or cards) and requires a long period of uninterrupted thought to complete. Just like the house of cards, a nascent idea can easily fall apart if you’re distracted.

His solution is to cultivate the “myth of irresponsibility”, that is the idea that he can’t be trusted to take on extra responsibilities and that he doesn’t care about these responsibilities or the students. Of course this a is complete fabrication, but as he points out, it does enable him to free up the time he needs to work.

This is not necessarily something I’d try personally, but maybe it’s a more realistic idea for the CERN committee to propose than the removal of laptops from auditoriums.

As well as being one of the most important physicists of modern times, Feynman is also widely acknowledged as being one of the great educators and by watching this documentary you really get a sense for why this was.

His passion for science, and for life, shines through. As well as healthy doses of self knowledge, disrespect for authority and doubt. Doubt, he says, is in incredibly important part of science. He points out that one must be very careful in checking ones experimental results and data, before rushing to any conclusions.

The documentary ends with Feynman saying “I think it’s much more interesting to live with not knowing, than to have answers that might be wrong”, and I completely agree.

CERN is an excellent forum for checking results. The very state of belonging to a collaboration of several thousand physicists means that there exists the capability to cross check each others results very thoroughly. It also gives us the opportunity to collectively achieve things which we couldn’t individually, thus enabling us to get the very best out of our detector and the LHC machine.

And inevitably this is where the meetings come in. In order to work well as a collaboration meetings are a necessity, and unfortunately bigger the collaboration becomes the more meetings seem to proliferate. As a result each of us ends up with less time to build our respective houses of cards. Although in the end, the process of collaboration means anything we do come up with, should be very well thought through and checked.

In conclusion, it’s good to be at CERN, despite all the meetings.


Why Frank loves SUSY

Tuesday, May 3rd, 2011

This week I’ve been in Arlington Texas, attending the excellent south western ATLAS analysis jamboree. As a special treat the jamboree dinner was held in conjunction with an event at Southern Methodist University just to the north of Dallas.

The key speaker at this event was Frank Wilczek, the 2004 winner of the Nobel prize in physics. Frank won this prize for work he began during his Ph.D. studies (take note all you students) concerning the nature of the strong force. Tonight though, he did not talk about this, instead he focused on the LHC and on its ability to discover Supersymmetry (SUSY).


Me and Frank Wilczek

Me and Frank Wilczek


I’ve name dropped SUSY before, and once again explaining SUSY is way beyond the scope of what I intend to say today. In brief, SUSY solves a number of problems present in the Standard Model by introducing a new symmetry to the theory which allows the transformation of force particle (bosons) into matter particles (fermions). Essentially presenting these as two facets of the same thing.

SUSY has a lot of interesting and beautiful implications. It brings a greater level of symmetry to the Standard Model and by doing so explains all of the known particles and forces in a concise and elegant way.

Frank’s favourite property of SUSY is its ability to explain the strong, weak and electromagnetic forces each as manifestations of a single “grand-unified” force. These forces then only appear to be different to us as we’re forced to study them at the exceptionally low energies available in everyday life. However, if we were to look at these forces more closely, that is to say at much much higher energy, then SUSY predicts that we’d see that they are all one and the same thing.

The motivation for this grand-unification claim comes from, among other things, studying the how the strengths of these forces change with increasing energy. The idea being that if they are all the same force, then at some energy their strengths should all be the same.

If the Standard Model is the final word then this doesn’t happen. But, if we throw SUSY into the equation then, miraculously, it does. Moreover it happens at an energy that fits nicely(-ish) into our understanding of the universe.


The evolution of the strengths of the forces with energy in the Standard Model (1).



The evolution of the strengths of the forces with energy in the Minimal Supersymmetric Standard Model (1). Gravity is also shown in red.


Unfortunately even with the LHC studying the unification energy is way way out of reach. But, if SUSY is able to provide grand unification, then we’ll certainly be able to see it at the LHC.

Whether you buy this as a suitable motivation for SUSY or not is a matter of taste. Not everyone is convinced, one of the reason being that to get to the unification scale you have to extrapolate the strengths of the various forces over thirteen orders of magnitude. Yet, to date, we’ve only measured them over the first three.

Frank, however, doesn’t seem to feel this is an issue and as he’s the one with the Nobel prize maybe you should listen to him.


[1] Anticipating a New Golden Age, Frank Wilczek, arXiv:0708.4236v3.


Jet spotting

Saturday, April 30th, 2011

Every week I try to take a few hours to study something different. The idea being that this will give me a broader sense of what’s going on within the ATLAS collaboration and the world of particle physics at large. Last week I was mostly watching Gavin Salam’s superb lectures on jets. They’re available as videos from here.

So what is a jet? It’s certainly nothing to do with aeroplanes. Jets are what we observe at ATLAS when a highly energetic quark or a gluon (collectively referred to here as partons) is produced in a collision.

I won’t take the time to explain the physics behind a jet and how they come into being. Those interested can see Flip’s excellent post. In essence, instead of the individual partons, what we see in the detector is a spray of collimated particles. This is what we refer to as a “jet”.

At hadron colliders, such as the LHC, jets are everywhere. In fact the vast majority of interactions at a hadron collider will result in the creation of multiple jets. They are our window on partons and on to the strong force itself.

Being so ubiquitous it’s important that we’re able to reliably identify these within our detector. Unfortunately this isn’t always such an easy task. The event display below illustrates a typical jet event. How many jets do you see?



Here is ATLAS’s answer.



In this case the jets have helpfully been colour coded. In real life, this doesn’t happen.

As you can tell the definition of a jet can be somewhat ambiguous. At ATLAS the trigger system has to quickly identify thousands of jets a second in order to pick out the interesting events to record. Identifying such a large number of jets is no easy feat.

To solve this problem we use jet algorithms. These are pieces of software which define jets based on what we see in the detector. They come in all sorts of shapes and sizes, from “simple” versions where a jet is defined as all the particles inside a cone, to more advanced versions which sequentially combine together individual particles based on their separation and energy.

Different algorithms have different strengths and weaknesses. Cone based jets are relatively simple and provide nice, round jets. Unfortunately though, the jets they identify can easily be altered by changes within the jet itself, or by small amounts of energy coming from unrelated collisions. This makes them very hard to compare to the predictions from theory. More complicated algorithms such as the “kT” algorithm remove these ambiguities, but often result in “ugly” irregularly shaped jets.

The current vogue algorithm both at CMS and ATLAS is the so called “anti-kT” algorithm. This starts from the most energetic single particles and sequentially combines them with everything nearby, stopping at some pre-defined distance. This algorithm results in the identification of nice, round jets, and does this consistently regardless of the small amounts of additional energy or the structure of the jets themselves.


Delays and Space Shuttles

Thursday, April 7th, 2011

A canister, containing AMS being loaded onto the space shuttle Endeavour. Photo from NASA.

I had a plan for this first post, but unfortunately, like so much in life, it didn’t work out quite as I hoped. The plan went like this: On April 19th the space shuttle Endeavour was set to launch from the Kennedy Space Centre in Florida. Having recently relocated to Louisiana from the UK, I found myself within “easy” travelling distance of a launch for the first time ever. So like any good, space-obsessed 26-year-old, I immediately set plans in action to journey across the US and set up camp on the beach to watch this once-in–a-lifetime event.

Endeavour was set to transport the Alpha Magnetic Spectrometer 2 (AMS-02) to the International Space Station. AMS is a nifty little particle physics detector (little in the context of LHC-style detectors) designed to study the content of cosmic rays for evidence of dark matter and anti-helium. This provided the perfect tie–in between a trip to see the big rockets and my day-to-day work on ATLAS.

Even more so as I spent my PhD years helping design search strategies for neutralinos. Which are the hypothesised partner particles to the Standard Model gauge bosons, the force-carrying particles, proposed by a particularly popular extension to the Standard Model, Supersymmetry. And they represent a prime dark matter candidate.

Unfortunately though, this plan came crashing down when NASA postponed the launch date of the shuttle by 10 days, putting it right in the middle of a conference I’m scheduled to attend and hence scuppering both my travel and blog plans.

This set me thinking about delays and how delays to big experiments have a defining role in so many careers. The biggest and most public delay of recent years was the postponing of the LHC physics programme following the quenching incident of ’08. I was present at CERN at the time and remember well the sense of disappointment that surrounded this. Of course, it was quickly replaced with a cheery optimism as everyone settled in for another year with our treasured simulations and the always-relished chance to polish our respective code bases. I was lucky enough to be studying at a UK university at the time, so I was able to continue my studies — and complete the vast bulk of my thesis — using only these simulations and very little real data.

A lot of my US contemporaries were not so lucky. They needed data in order to be able to graduate. This led to a lost generation of students who had to either switch experiments or ride out the new LHC schedule and wait for the data. A lot of these students are still around today, all putting the finishing touches to their theses, and generally being massively over qualified for the post-doctoral positions they are only now applying for.

All of which goes to show that planning your life around science doesn’t always turn out the way you expected.