ATLAS

Other than perhaps last summer, this is certainly the best time to be at CERN. Thanks to the shutdown for upgrades that will last until 2015, we get to go down and visit the detectors!

ATLAS (A Toroidal Lhc ApparatuS)

It's true that no matter how many times you read about the ATLAS detector, and are told how big it is, once you see it, it's still mindboggling. Some numbers:

• 25 meters in diameter  and 46 meters long = half the size of Notre Dame in Paris
• weighs 7000 tonnes = the Eiffel tower weighs 7300 tonnes!
• 3000 km of cables

Some interesting facts:

• The data that is outputted in one year from the detector is 3200 terabytes, or 7 km of CD's stacked on top of each other
• The cavern was built from the top down
• They had to first put down 5 meters of concrete to hold the weight of the detector
• The detector was built using an access shaft, similar to a ship in a bottle
• Every year, the detector rises ~0.5 mm because the cavern acts a bit like a bubble in honey! They can adjust for this though.

The access shaft

So how does the detector actually work? It's a lot like a gigantic camera, and each layer looks at a particular part of a particle collision. It's starts off with sending two proton beams (or whatever you may have on hand) through the beam line, and colliding them at the detector - sounds simple, but it's more like firing two needles at each other, and expecting them to hit head on. However, the protons come in what are called "bunches", and each "bunch crossing" will have something along the lines of 20 collisions. It's not a lot, but because they can send the protons so fast, they can actually get 40 million crossings per second! This is approximately 1 million collisions per second. Of course, this is too much information for computer systems to handle, so there are triggers that filter the incoming data to only 200 INTERESTING events per second. This means the systems need to be told what is interesting- something like the Higgs boson.

That's how the data arrives, but how can we determine which particles came out of the collisions? This is where all the individual parts come in. I will quickly discuss their purpose, but I won't go into the details, because otherwise this would turn into an essay. If anyone is interested though: 

http://www.atlas.ch/pdf/ATLAS_fact_sheets.pdf

The first section is the Inner Detector (ID). This section lies right next to the beam line and is the first thing to be hit after a collision. This detector records the particles as they pass through the pixels and through this, it is possible to reconstruct a "track". The ID is also surrounded by magnets. This magnetic field will cause a charged particle to curve, meaning that we can then figure out how fast it was going and what its charge was (positive or negative). 

The second part is the Calorimeter. There is an electromagnetic and hadronic calorimeter, which focus on mostly electrons/photons and protons/neutrons respectively, even though there is some overlap. The Calorimeters actually "destroy" the particles by having them collide with material, and then the amount of energy that is left behind can tell us the energy and momentum of the incoming particles.

The endcap muon detector

More of the muon detector. The orange-striped pipes are huge toroid magnets.

The last piece is the Muon Detector (this is what you can see in the picture). Muons are a bit like heavy electrons, and they can pass through most of the detector with almost no problems. That's why the muon detector is the furthest outside. As particles pass through, they ionize the material in the muon detector (strip the electrons), and this can tell us when a muon arrived, and if we have enough pieces being hit by the muon, we can also determine the path it took. This section is also surrounded by magnets, which you can see in the picture too (they have the orange bands).

Mont Blanc, Lake Geneva and the Higgs boson!

I think this weekend, I would have made my French teachers proud! I was lucky enough to have some family visit me, and had a wonderful (though perhaps slightly stressful at times)  few days with them. Having a car is very useful, since it makes travelling a lot easier, but it helps to have a good GPS. I can't count the number of times "she" tried to take us off the highway, though the signs all said Geneva -20 km, or the closed streets where we just had to guess the general direction we needed to go in. But in the end, we made it.

I think I'm going to go in reverse chronological order, because I want to leave the physics for last. Sunday was a breathtaking experience (almost literally). We visited Chamonix, which lies in France, at the foot of Mont Blanc.

The church clock tower in Chamonix

 From there, we took the gondola up 3842 meters to visit the mountain. It was absolutely unbelievable! You travel up the mountain, and much of the trip was through thick clouds. However, as you reach the top, you pass through the top of the cloud cover, and emerge in a stunning alpine vista.

Reaching the summit.

 The pictures just don't do it justice. And to see all the people that are climbing the mountain- it's fantastic.

Mountaineers.

Even though I knew it would be, I was still surprised at how much thinner the air would be up there. Going up just a few stairs made me feel completely out of breath.

Mont Blanc (slightly hidden)

At 3842 meters, with the TRU moose!

Looking back down at the main platform

We also had a beautiful, sunny Saturday driving around Lake Geneva, and visiting the beautiful cities along the way. After lunch at the Mont Blanc hotel in Morges...

Family at Morges

...we continued on to Montreux. Apart from all the expensive casinos, and amazing lake views, Montreux is also home to a Freddie Mercury memorial, unveiled in 1996, due to Mountain Studios where Queen recorded some of their songs, but also because of Mercury's love of the city. I can see why, too!

Vineyards on the way to Montreux.

Freddie Mercury- "Lover of Life, Singer of Songs"

The mountain views

The fantastic Lake Geneva.

From Montreux, we continued on along the lake until we reached the small medieval town of Yvoire. It is absolutely beautiful, the small arts shops, the restaurants, church and of course, the view.

The houses in Yvoire.

I also noticed another very interesting statue when leaving the town. Mostly, I noticed because what looked like graffiti on the base was actually a series of physics equations and notes. This statue, is supposed to represent the recent discovery of the Higgs boson! That actually brings me to a more work related topic.

The Higgs boson- personified.

Most people have by now probably heard of the Higgs boson, but what I always find so amazing, is the way they found it. In the 1960's, Dr. Peter Higgs proposed what is called the Higgs field and the overarching Higgs mechanism. I included some links in the previous post, and if anyone is interested in some details:
http://arxiv.org/pdf/1207.2146v2.pdf
Beware - there is math involved!

Now, according to the Standard Model (the model that we use to describe the electromagnetic, strong and weak forces as well as particle interactions through these forces) each of the forces is mediated by an associated particle (which are called bosons). For example, let's say we have two electrons come together. Everyone learns in high school that like charges repel, but this repulsion is not just "magic". The Standard Model says that the repulsion is actually caused by one electron emitting one of these mediating particles, a photon in this case, and the other absorbing it. For the strong and weak forces, they are mediated by the W and Z bosons. However, the photon is massless, the W and Z are very heavy (Z is around 90 GeV. **Just a note- GeV is one billion eV, the energy of one electron accelerated across a 1V potential (like a 1V battery). However, in particle physics, because of Einstein's E=mc^2, it is used as a measure of mass). A huge question was, why this difference? This is where the Higgs boson became important; it was introduced to explain why certain particles are very heavy while other are light. In the simplest terms, it has to do with how much the particles "like" the field- I always like the dinner party analogy
http://www.exploratorium.edu/origins/cern/ideas/cartoon.html

The big problem was, though this theory could predict how the field would cause particles to have mass as well as how the Higgs boson could possibly decay (break up into other particles), it said nothing about how heavy it was. So, for approximately the last 30 years, first with LEP (Large Electron -Positron collider) and SLAC (Stanford Linear Accelerator Center) and now with the LHC (Large Hadron Collider), experimentalists have been searching for evidence. Based on the models we have, physicists had to comb through a very wide range of masses, and using what we know about how the boson should decay, they could determine (statistically) whether or not the data showed any excess. I may go into what this "excess" is a bit later. For now, it is enough to say that a lot of processes can mimic the Higgs boson (known as background) and the real boson would look like some extra Higgs signal- a bump- exceeding what we expect from the background.

After after years of ruling out various mass regions, physicists have finally found that "bump". Now, it's just a matter of seeing whether or not the data agrees completely with the theory in terms of charge, something known as spin, and other properties we would expect the Higgs boson to have.

Getting started at CERN

Well, I suppose that since I've been here for a week, I should get started with blog posts as promised. I will be using this to document random interesting facts about my time at CERN as well as some pictures of my time here.

Some background I suppose...

I am a fourth year (going into fifth year) student at Thompson Rivers University studying Physics and Honors Mathematics. I began this summer working with Dr. Dugan O'Neil at SFU in May as part of the SFU ATLAS group. ATLAS is one of the major detector experiments at CERN and is the project that Canada is primarily involved in. To learn more about ATLAS or ATLAS Canada, the following are great sites:
http://atlas.ch/,
http://www.atlas-canada.ca/

In particular, I have been working in the Higgs --> tau tau group. Now, the Higgs boson, which many of you have probably heard about since the announcement on July 4th 2012 of its discovery (and if not CERN has some fantastic resources: http://home.web.cern.ch/about/physics/search-higgs-boson), only lasts for a very short time before decaying into other particles. Using detectors like ATLAS, we can "see" these decays particles and trace them back to the Higgs itself. One of the main ways the Higgs can decay is to two tau particles (which then decay even further), and this is the area I'm working in.

Now this work is done all through programming. For this project, I've been learning Python, which if anyone is interested in learning, is actually a really nice and quite straightforward language to use. I can certainly recommend the following book for anyone starting out:
http://briggs.net.nz/snake-wrangling-for-kids.html

And now for CERN....

Since having arrived on Saturday, I've been familiarizing myself with the CERN "campus". I can't begin to say how convenient it is being able to walk only 100 meters and reach my office, the cafeteria and most meeting rooms, not to mention that I have a view of the Alps!

That's building 40 to the right, home to the ATLAS and CMS secretariats.

Inside building 40

Speaking of the cafeteria, the food here is amazing (of course), but rather expensive, as with most of the Geneva area.

At the moment, it is pouring buckets outside my office window, but for the last few days, it's been unbelievably humid. Luckily there is a pool not too far away, to provide some relief from the heat. Nonetheless, in the evenings, when it cools down a bit, it is nice to sit outside with other summer students and discuss life, the universe and everything. It's not often that you can have drinks with someone from the UK, USA, Greece, Norway and Finland all at once!

In my opinion, other than of course the unbelievable science and fundamental discoveries being made here, the people you meet and interact with here are the most amazing part about CERN. Though nearly everyone here speaks French or English in a group of 10 people, there are likely to be 6 countries represented. Everything from Madagascar and China to Costa Rica and Iceland.

Working in the office- a bit empty at the moment...