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Archive for August, 2010

CERN at night

Wednesday, August 25th, 2010

It’s 5:38 AM.  Do you know where we physicists are?

Right now I’m on a test beam shift for the ALICE electromagnetic calorimeter (EMCal).  The test beam delivers particles at a fixed momentum – right now a mixture of 60% electrons and 40% pions at 10 GeV/c.  We have a miniature version of our EMCal, 64 towers (8×8) complete with read out electronics.  It’s positioned in front of the beam line so that we can measure the response of the EMCal to these particles.  We move the beam around on the detector so that we can see the response of each tower to the beam.  We also try different momenta.

We have about a week to use the test beam and we want to make the most of our time, so we take shifts around the clock.  This is where I am right now:

The building to the right – the barracks – is where we sit when we take data.  Our little detector is to the left, behind the large cement blocks.  The cement blocks are there to shield people in the hall from radiation from the beam.  The beam comes from the far end of the hall.  The cables take data from our detector to the barracks.

And we are not alone – there are several other groups using data from the test beam and doing other experiments right now.  The lab that never sleeps.  Our test beam comes from the Super Proton Synchrotron – once the highest energy accelerator in the world and now both the injection source for the LHC and the beam source for multiple ongoing experiments.


Sorry, can you repeat that?

Monday, August 23rd, 2010

This post is meant to have a positive tone. Really.

The LHC experiments all rely heavily on some form of teleconferencing to get their work done. As experimental collaborators number in the thousands, we can’t get by without conversing with each other. And with collaborators all over the world, we can’t expect people to physically appear at every single meeting. This could work fifteen or twenty years ago, when people typically participated in experiments on the regional or national scale. I know a whole fleet of professors who used to drive a car or take a plane to Fermilab once every two weeks, or even every week, so that they could be in the room for some particular meeting. Now that we are spread over so many miles, it seems too much to ask. But teleconferencing has allowed us to move past that era. It is absolutely not as good as being there in person, but given the monetary costs of moving people around, and the amount of people’s time that can be wasted in transit, not to mention the wear and tear on all of us when we are away from home, it makes sense to take advantage of teleconferencing technology.

The good news in all this is that we have reached a point in teleconferencing technology where anyone who has a computer with a microphone, speaker and network connection can take part, from any office that they might be sitting in, making teleconferences much more convenient than ever before. The bad news, of course, is that we have reached a point in teleconferencing technology where anyone who has a computer with a microphone, speaker and network connection can take part, from any office that they might be sitting in. Not all microphones are of such high quality. Some microphones tend to be rather close to computer speakers. Some connections are unreliable and have limited bandwidth.

So today I found myself on yet another conference in which we had to remind people to mute because we were hearing other speakers echo through their sound pickup, and had to work our way through some parties becoming inaudible or distorted at times, and had to listen to the occasional background conversation, and had to ask people to repeat themselves, a little louder please. It is, honestly a bit of a drag. I’ll admit that I pine for the days when you really just could sit around the table with a couple of co-workers and point at the plots in your notebook and be done with it.

But this post has a positive tone, really. I just try to keep in mind that yes, we are able to work with people who are scattered all around the globe, and actually get things done, thanks to this technology, even though it gives me fits.


Off to Geneva

Friday, August 20th, 2010

I’m off to Geneva for a couple of weeks. While I’m there I’ll work on the test beam for the ALICE electromagnetic calorimeter. I’ll tell you more about that in the next posts. But I thought I’d share with you the contents of my long trip survival kit:

A travel pillow, a bandana (which serves both as an eye mask and a lazy hair style), an outlet adapter, a netbook and mini-optical mouse, ear plugs, an mp3 player with a 30 hour battery, a hair brush and extra hair bands, two change purses (one for Euros, the other for Swiss Francs) and little mini-toothbrushes with toothpaste already on them.  I don’t deal with sleep deprivation very well so these flights are never very fun – but they’re easier to take than flights between the US and Asia.


Workers in France are guaranteed at least 5 weeks paid vacation time each year.[1,2] Many people take that time off in the month of August.  I don’t know how August became the vacation month, but that’s the way it is.  Hours for many stores become even more limited or simply close – for the month!

Even in my hometown of Madison, WI there is a French bakery owned by a french family and they close up shop for most of August.

The disappearing of French workers also happens at CERN – professors, scientists, etc, many of them are gone.  That leaves the rest of us with the chance to either get ahead in our work, or relax and take it easy as well.

(Oh, and did I mention that the French also have a 35-hour work week?[3])

Don’t worry though, the LHC is still on and they’re trying to reach higher beam luminosities.  At the moment they’re working on some cryo problems:


Hamburg or Munich?

Thursday, August 19th, 2010
Lighthouse near Munich... what's wrong with that picture?

Lighthouse near Munich... what's wrong with that picture?

Did I get on the wrong flight? Not really possible nowadays, I know… That’s why they don’t do the “destination check” announcement anymore. But, if this is Munich, what is a Lighthouse doing next to my place?

Apparently, a Hamburg Fishmarket will be here for the next couple of days… Lets see, it might be more what a Bavarian would expect a fishmarket to be like instead of authentic Hamburg style… That reminds me, this year I have not been to Hamburg as often as in previous years… It seems to be more of a Geneva year.


A Physicist’s Dinner in Paris

Thursday, August 19th, 2010

One of the nights at ICHEP, I ended up by myself and wandered around the middle of Paris a bit. At last I was hungry, and decided I wanted something easy and affordable if at all possible. The best solution for this, in Paris, is one of the touristy restaurants. So what I ate is below. Some aspects of it are typically French, but there is nothing unfamiliar to an American about it except the concept of an omelet at dinner.

Dinner in Paris

You can also see what I was reading: a book of papers on the “multiverse hypothesis” adapted from some conference lectures. Among some theoretical physicists trying to build a fundamental theory of life, the universe, and everything, there is serious research and debate on this subject — but to me as an experimentalist, it’s crazy far-out philosophy. But it’s also amusing dinner reading, and the university publishers who had booths set up at ICHEP were the only source of English-language books I knew of in Paris.



The fifth annual CERN / Fermilab Hadron Physics summer school is currently going on right now at Fermilab in the Wilson Hall High Rise.

This annual summer school is a great opportunity for young physicists to listen to lectures about the field of high energy physics both experimental and theory. Furthermore, these  lectures are designed to give someone with limited experimental knowledge some real understanding of how these accelerators, detectors, and analysis computing take place in the real world of particle physics

This summer series is going on August 17th – August 20th and is available streaming online:

As well as the slides from the various speakers are publicly available here:

So even if you aren’t able to be at Fermilab for these lectures you can still enjoy them. Much thanks to the Fermilab visual media services!


I usually don’t just post links, but this one really deserves it: check out Matt Might‘s take on what is exactly a doctorate?
It definitely rings true to me.


We toss around the term “TeV” – a teraelectron volt, 1012 electron volts (eV). But how much energy is it really?

An electron volt is the energy an electron gains when it is accelerated through a potential difference of one volt. An electron volt is defined as a unit of energy. (Various prefixes are defined here.)

Let’s put this in terms we can all understand. A Lindt 70% cocoa chocolate bar has 194 Calories. To convert this to electron volts:

194 Calories × 1000 calories/Calorie ×4.2J/calorie / 1.6 × 10-19 J/eV = 5 × 1024eV

(Note that the dietary unit, a Calorie, is 1000 calories, the amount of energy needed to raise the temperature of one mL of water by one degree Celsius.) So it would take a hundred billion (1011) proton-proton collisions at top energy (14 TeV in the center of mass) to get the same amount of energy as in a chocolate bar.

The difference is how much space we pack that energy into. A proton has a volume of roughly 1 fm3, or about 10-39 cm3. A Lindt chocolate bar is about 10 cm x 1/2 cm x 20 cm = 100 cm3. A chocolate bar then has an energy density of about 194 Cal/100 cm3, or around 2 Cal/cm3. A proton-proton collision at 14 TeV has an energy density around 14 x 1012 eV/10-39 cm3 x 1.6 × 10-19 J/eV *1000 Calorie/4.2J = 5 x 1035 Cal/cm3. So our proton-proton collisions have an energy density about 1035 times a chocolate bar.

We also use an electron volt as a unit of temperature. An atom in a monatomic (helium, argon, etc.) ideal gas has a kinetic energy of 3/2kBT where kB is the Boltzmann constant. The factor in front (3/2) is different for different systems. For instance, it’s 5/2 for a diatomic gas, such as hydrogen (H2), oxygen (O2), or nitrogen (N2). But the energy is usually kBT times some factor between 1-10. So to convert an electron volt into a unit of temperature, we use eV=kBT and T=eV/kB=11604 Kelvin.

So how hot is a cup of coffee in electron volts? When I worked at a coffee shop in high school, we made our cappuccinos and lattes at 160°F (71°C). This works out to be 344K, or 0.03 eV. So a proton moving at 7 TeV is about 100,000,000,000,000 (1014) times more energetic than the average molecule in a cup of coffee.

The Quark Gluon Plasma created at the Relativistic Heavy Ion Collider is at a temperature of about 170 MeV. (Note this is the temperature of the medium produced, not the energy of the incoming beam.) The fluid we’ll create at the LHC will be hotter – over ten billion (1010) times hotter than a cup of coffee.

These collisions are hot stuff!

[Note these are all what we call “back-of-the-envelope” calculations. The goal is to figure out the right order of magnitude for various quantities, not to do a detailed, precise calculation.]


Over the weekend the LHC was able to deliver our first pb^-1 of data! Milestones keep rolling on by and the data keeps rolling in. This is a big first step in getting what will hopefully be lots and lots of data. I’ve included a link to the ATLAS luminosity plot for your viewing pleasure. (CMS has one too… but I’m on ATLAS :))

To anyone who isn’t a particle physicist an inverse picobarn (pb^-1) is a pretty bizarre unit. I’ll start out with the base unit: the barn (b). It’s a measurement of area, proportional to m^2 or cm^2. The barn unit comes from when nuclear physics was in its infancy and refers to a uranium nucleus which is as big as a barn (1 barn = 10^-24 cm^2). (I still think physicists should hire writers to come up with this stuff… anywho, back to the post).

An inverse barn (or b^-1) in the particle physics world is a measure of collision events in an area of a barn. Throw in a metric prefix (pico which is 10^-12*base unit) and now you’re all caught up to speed. But what does that mean really? Fermilab has over an inverse femtobarn (fb^-1, which means 1000x an inverse picobarn) of data but of course they’ve been running their collider for over a decade. We’ll still need much more data to do searches for things like the Higgs, but very early searches are definitely underway – not to mention all the Standard Model physics and calibration that’s going on too.

So cheers to the first pb-1 of data… I can’t wait to start analyzing.