Mr Dissertori, the LHC has been back in service since February 28 after an extended Christmas break. Tomorrow, on the Hönggerberg media representatives and interested parties will be able to follow how two proton beams aimed at each other collide at a center-of-mass energy of 7 TeV– a new world record. What’s been going on between February 28 and today?
The LHC project outstrips anything that has gone before it in terms of technology and size. Countless tests were thus carried out to make sure everything goes like clockwork – keeping the proton beam on its predefined path, for instance. At full intensity, a proton beam transports energies capable of melting a 400-kilogram block of copper. The protons in the beam basically move in tiny clouds; they can go astray and crash into the magnets, where they then give off heat. In order to prevent this and avoid compromising the superconductivity, the beam runs between metal blocks – so-called collimators – which are supposed to catch the “rogue” protons. We have to test that these blocks are working beforehand, for example. Over the past weeks, the LHC’s beam energy has gradually been increased to 3.5 TeV, which will ultimately allow a center-of-mass energy of 7 TeV.

How high was the energy when the LHC broke down shortly after its initial launch in September 2008?
Beforehand, proton beams were circulated at 0.45 TeV, but not made to collide. Back then, we were in the preparatory phase where the magnets were supposed to reach a certain power in order to increase the beam to a higher energy level. The aim was to reach today’s level, but there weren’t any proton beams in the machine when it broke down.

The glitch was put down to faulty electrical connections between the magnets. A recently published study by a Cern physicist reveals that, amongst other things, the joints were poorly soldered and with the wrong solder material. The flaws still haven’t been corrected. What is being done to prevent another breakdown?
Since the mishap, the resistances of nearly all the connections have been checked and a new so-called quench protection system developed, tested and installed in the LHC. ETH Zurich offered support in testing the associated electronics. The system monitors superconductivity. If the cable heats up, the superconductivity is suspended and the cable becomes normally conductive – referred to as a quench. In the event of imminent danger, the new system is supposed to trigger the machine’s switch-off mechanism automatically in order to prevent a breakdown on the same scale as the one back in the fall of 2008. Based on previous tests and analyses – equipped with the new system – we assume that no such accident will happen any more at 7 TeV.

The plan is to decommission the LHC after a period of about two years for at least a year in order to replace the thousands of defective connections between the 1,200 magnets. Only then – 2013 at the earliest – can the LHC be operated at full power: 14 TeV. Why not carry out the repairs now?
We discussed this question at length. On the one hand, of course, we’re in competition with Fermilab’s Tevatron collider in the USA. On the other hand, our decision to begin by operating the LHC at half power also had a psychological aspect. The convincing quality of the results of the recent publication (see ETH Life article from 18.02.2010) on the collision data collected a few weeks before Christmas 2009 came as a surprise to all the experts. After the long forced break, this was a huge motivation boost for everyone involved in the project and amongst other things encouraged us to at least get the LHC up and running at half power.

Can new findings be obtained or even the elusive Higgs boson discovered at this energy level?
We have calculated that the data gathered using the LHC at 7 TeV of center-of-mass energy over two years can match or even improve on most of the results of the Tevatron operated for ten years at about 2 TeV. Some models even suggest that new discoveries are possible. However, calculations indicate that it’s unlikely the Higgs boson will be discovered in the next two years.

Are the new connections vital if we are to operate the LHC at 14 TeV?
Yes, it’s the only way to reach the ultimate energy levels in a safe and secure manner. Only once they have been replaced can the proton beam circle the 27-kilometer collider at maximum intensity. Consequently, the engineers are already developing new connections. These will be tested before they replace the old ones.

Why is so much energy needed if we’ve already been successful at lower levels?
Higher energy levels enable us to examine physical models that predict new phenomena and particles with extremely high masses. Naturally, experiments at lower energies test a lower mass range. Furthermore, the intensity of the beams is also of great importance: if the beam circulates at the highest intensity, it generates an extremely high collision rate. We need this to see rare occurrences.

CMS center on the Hönggerberg

Tomorrow, two proton beams each carrying 3.5 TeV – i.e. 7 TeV of center-of-mass energy in total – are set to collide at Cern. There will be a live feed from the CMS control room on large screens in the HPK building on the Hönggerberg and physicists will be reporting on the events online. If everything goes smoothly, as many results as possible will be communicated at the first major summer conferences and Dissertori expects the first publications in the subsequent months. However, here it is less about discoveries than good measurements that one can show, says the scientist. Tomorrow, with a bit of luck the first images of the collision in the individual detectors will already be shown. ETH Zurich has an instrumental role in the CMS experiment with the CMS detector. The additional detectors are ATLAS, ALICE and LHCb.