ALICE and LHCb

Time for some more detectors! These last few weeks have been extremely busy with lectures and workshops, plus coding and a lot of summer student events (which I will elaborate on later).

However, my quest to visit all of the four detectors at the LHC has progressed well with a visit to ALICE last week and LHCb yesterday. I only have one left now- CMS which I will visit tomorrow. I spoke a lot about ATLAS, which of course is the detector project I'm working with, but I thought it would be good to give a mention to the others as well.

Beginning with ALICE (A Large Ion Collider Experiment- another testament to the ability of physicist to create strange acronyms), it is located at Point 2 just outside St Genis-Pouilly in France.
As its name suggests, the experiment is focused on using heavy ions (nuclei with some electrons removed to make it charged) such as lead to explore questions such as "Can we create free quarks?" Theory says that at the energy levels explored thus far, quarks (the constituents of protons, neutrons, and all hadrons) can't exist by themselves, but in the first few instances of the universe, it should have been possible. This is what ALICE hopes to examine.

As I mentioned, ALICE uses heavy ions for this research, but you might wonder- I thought the LHC collided protons? In fact, they also collide these ions! Even more interestingly, they have also done proton-lead collisions, which is neat because a lead ion has 82 protons and ~125 neutrons, meaning that it is roughly 200 times more massive than a proton. This is like colliding a mosquito with a nickle (in terms of relative masses)!

For some more detailed information on what they do: http://aliceinfo.cern.ch/Public/Welcome.html

ALICE

The detector itself is considerably smaller than ATLAS at 16 meters in diameter and 26 meters in length, but it weighs 10 000 tonnes! The setup itself is not so different from ATLAS,  in that it also involves an inner tracking detector, calorimeters and a muon detector. However, lead-lead collisions are very messy compared to proton-proton collisions which means that ALICE has to be able to collect a lot of data very quickly. Each event is over 10^7 bytes and there are roughly 1000 events per second. Considering that a CD holds 700 million bytes, this data collection is the equivalent of ~14 CD's per second! In total, they store over a petabyte per year (10^15 bytes).

Well, enough about ALICE, what about LHCb (Large Hadron Collider beauty experiment)? On the map above, it is locate at Point 8, just outside Meyrin. It is actually the deepest of the four detectors, being 100 meters underground. As you may have guessed, LHCb studyies what are known as "beauty" quarks, more commonly known as "bottom" quarks which only live for a million millionth of a second. These quarks can help us learn about the reason why our universe is made up of matter rather than antimatter- where has all the antimatter gone? It may also offer some explanation of dark matter, which makes up roughly 26% of our universe. (The number varies- I don't think I've ever seen two presentations with the same percentages.)

LHCb is also quite unique in it's setup. Rather than being built around the beam line and having the collision in the center, the collision actually occurs at one end of the detector. This is because the beauty quarks tend to travel very close to the beam line, meaning that it's really only necessary to look in one direction. So the detector is very much like a stack of pancakes. It also has some special detectors, such as the Cherenkov detectors, which help measure the particle's speed. When light travels through a material, it slows down, which means that it is possible for a particle to travel faster than the speed of light (but only in that material- NOT in a vacuum!!) This creates something like an optical sonic boom, which is then measured. It also has the usual tracking detectors, calorimeters and muon detectors too.

All of these detectors are unique and beautiful. I'm really lucky to be able to see them- it's a true testament to amazing engineering and innovation!

LHCb