3.5 TeV : a good start!

To the pessimists out there, the 3.5 TeV starting energy of the LHC will be like a half-empty glass. However, the thousands of physicists working at the experiments certainly do not share these feelings. On the contrary, they are as excited as ever since they will be the first to observe what happens to matter in these (still) unprecedented conditions.

Although one might think that 3.5 TeV for a machine designed to operate beams at 7 TeV is as frustrating as driving a Ferrari when the speed limit is 60 km/h, physicists working at the LHC experiments see the glass half full: they are now focusing on how to make the best use of this intermediate energy. For them, having the opportunity to test their detectors at non-extreme conditions is rather a reassuring feeling. "So far, the CMS detector has been commissioned using cosmic rays. After start-up, the first thing CMS will do is to check its performance again, this time with collision data - where the particles originate from the centre of the detector rather than passing from the top to the bottom as is the case with cosmic rays", explains Jim Virdee, CMS Spokesperson.

The start-up energy is still 3.5 times higher than the world’s current most powerful accelerator, the Tevatron (Fermilab, US). "This energy is large enough for the LHC to produce interesting samples of top-quarks, the heaviest quark and the only one that has not yet been observed in Europe", says Fabiola Gianotti, ATLAS Spokesperson. "As events due to top-quark production contain a large number of physics "signatures" (electrons, muons, jets, jets from b-quarks, missing energy), observation of top-quark production will demonstrate that the detectors, the calibration procedures and the reconstruction tools are in good shape. At that point, we will be ready to embark on the discovery phase, which, at this energy, might even include supersymmetry", she adds.

The ALICE detector is optimized for lead-ion collisions, the second part of the LHC scientific programme that will start towards the end of the initial run, in 2010. "As it turns out, the proton-proton collision energy equivalent to full energy ion beams is about 5.5 TeV, and therefore the initial 7 TeV (that is, 3.5 TeV per beam) proton run is actually better suited for comparing proton-proton collisions to lead collisions than the full 14 TeV! For both our proton and heavy ion physics programs the lower start-up energy will therefore be extremely useful", explains Jurgen Schukraft, the experiment’s Spokesman.

The most specialized of the four large detectors is LHCb. It will search for new physics by observing the interactions between new particles and the beauty quarks. "The kind of indirect search for new particles that we perform makes our experiment less sensitive to the LHC collision energy", explains Andrei Golutvin, LHCb Spokesperson. What really matters to us is to have a stable run at a reasonable luminosity. The 3.5 TeV is enough for us to perform new measurements of some rare decays of the beauty quarks with unprecedented accuracy. If we discover any discrepancy with the expected values, we will be able to point to some new physics".

The current plan for the LHC is to safely move up in energy to around 5 TeV per beam in 2010 and to gradually push up the luminosity by increasing the number of protons per bunch and the number of bunches. "We fully support such a plan in which steps will be taken in the light of experience", says Virdee. "Although significant exploration of the Higgs ‘territory’ will take longer than expected, we could be fortunate enough to see signs of new phenomena such as supersymmetry or extra-dimensions if they exist in Nature at these energies".

Did you know?

What is a TeV?

1 TeV corresponds to 10¹² electronvolts. The electronvolt is an energy unit particularly convenient in particle physics because, in absolute terms, the energies that particle physicists deal with are very small. If we take the LHC as an example, the total collision energy is 14 TeV, making it the most powerful particle accelerator in the world. Still, if we convert this into joules – the energy unit accepted by the International System – we obtain only 22.4 x 10^–7 joules.

This is a very small amount of energy if compared, for example, to the energy of an object weighing 1kg and falling from a height of 1m, that is, 9.8 joules.

What does "luminosity" mean?

Luminosity is a measure of how efficiently a particle accelerator produces collision events. It determines the rate at which these collisions take place. In a collider, particles are stored in a string of bunches to make a beam. Each bunch is about the size of a grain of rice and contains a few billion particles. Pushing the limits of technology, accelerator physicists increase luminosity by putting more particles in each bunch, colliding more bunches per second, and squeezing the bunches to the smallest possible size at the collision point (extract from Symmetry Magazine).