Experimental High Energy Physics for dummiesPart II ::: The Large Hadron Collider

We continue our journey to the world of Particle Physics and after the first general introduction let’s go and look the beast named Large Hadron Collider at more detail. The LHC will be accelerating protons at energies of 7 TeV. The protons will need 89 microseconds to make one turn in the accelerator’s 27 km (17 mi) circular tunnel and their speeds will be amazingly close to the speed of light. How close? Well at full speed they’ll have 0.999999991 times the speed of light!

That beam of protons will carry a massive 362 MJ of energy. That’s almost 77.5 kg of TNT or a car traveling at 1700 km/hr!!! Another common example that is being used is that 7 TeV is approximately the energy of a flying mosquito. Mosquitoes weigh about 2 to 2.5 mg (thousands of a gram). Now imagine the same amount of energy being compressed on a single proton (they weigh 1.673E-24 grams!). This is more than 20 orders of magnitude difference!

The LHC will be built at the same old ring of the previous accelerator LEP. Have a look:

LHC Ring aerial view

To build this huge ring, they’ll use 14.3 meters long magnets. There will be dipole magnets to deflect the beam and quadrupole magnets in the arcs and in the insertions to focus the beam. During operation, they’ll be at 1.9 K (-271.25 degrees Celsius or -456.25 degrees Fahrenheit !!!) and their magnetic field will be 8.33 T. This is approximately 150000 times stronger than the magnetic field of Earth.This is the interior of the tunnel during magnet assembly:

Ring assembly

In order to operate at such temperatures approximately 1500 magnets, a state-of-the-art cooling system had to be developed. LHC will use superfluid helium due to its unusually efficient heat transfer properties; it allows kilowatts of refrigeration to be transported over more than a kilometer with a temperature drop of less than 0.1 K. In all, LHC cryogenics will need 40,000 leak-tight pipe junctions, 12 million liters of liquid nitrogen will be vaporised during the initial cooldown phase of 31,000 tons of material. The total inventory of liquid helium will be 700,000 liters. Due to the huge power consumpion of LHC, it will be operationg only ~140 days per year.

Although LHC by itself is really powerful, it wouldn’t be able to accelerate particles at these amazing speeds just by itself. A series of pre-accelerators will be used to “feed” the main LHC ring with protons. In the picture below you can see PS (Proton Synchrotron) CERN’s first major accelerator. It has a circumeference of ~600m and it’ll give an initial boost of 26GeV to the particles. Next, the SPS (Super PS) has a circumeferece of 6km and it’ll be accelerating the protons from PS energy to 450 GeV.

LHC Accelerators and Detectors

The primary “mission” of LHC is to search for the existence and estimate the mass of the Higgs boson which has become something like the “holy grail” of modern particle physics. The Higgs plays a key role in explaining the origins of the mass of other elementary particles, in particular the difference between the massless photon and the very heavy W and Z bosons [wikipedia]. It is the last piece of the puzzle called “Standard Model”. Other physics may emerge as well such as proof for the existence of supersymmetry (SUSY for short) [wikipedia].

Collisions will happen as fast as 800 million per second. This means that the particles from one collision will still be travelling through the detectors when the next collision happens. This calls for very fast and efficient detectors. LHC will have five detectors: ATLAS and CMS are large, “general purpose” particle detectors. The other three (LHCb, ALICE, and TOTEM) are smaller and more specialized. We’ll have to opportunity to talk about one of them (the Compact Muon Solenoid) at our next meeting.

The next part will appear probably after a month or so because I have to study for my qualifiers…

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