Quantum Diaries

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.

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 3/2k_BT where k_B 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 k_BT times some factor between 1-10. So to convert an electron volt into a unit of temperature, we use eV=k_BT and T=eV/k_B=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.

-Regina