It won't end with LHC, whose results "will guide us into the new energy frontier," says CERN director-general

Declaring emphatically that the idea of particle colliders would not “wither” away after the Large Hadron Collider (LHC), Rolf Heuer, director-general of CERN (European Organization for Nuclear Research) in Geneva, said here on Thursday that there were exciting physics prospects with colliders in the energy frontier beyond the LHC's reach. The question, he said, was not ‘Wither Colliders?' but rather ‘Whiter Colliders?' meaning which type of colliders?

Dr. Heuer was speaking on ‘Whither Colliders After LHC?' at the ongoing XXV International Symposium on Lepton-Photon Interactions at the Tata Institute of Fundamental Research (TIFR). “We are just beginning to explore 95 per cent of the universe, and the LHC had delivered exciting results. But it is the results from the LHC that will guide us into the new energy frontier.”

Today, he said, the LHC, operating at a combined energy of 7 teraelectronvolts (TeV) delivered by two colliding beams of protons at 3.5 TeV each, had brought the world into unexplored energy territory and made excellent progress in the accelerator performance, in its various experiments and in the worldwide computational grid established to analyse the data coming out from it. “In the last eight months, the researchers had demonstrated an unprecedented efficiency in data taking of over 90 per cent.”

Talking specifically of the endgame that one now witnesses in the ongoing search for the Higgs particle (The Hindu, August 23), the crucial missing piece of the Standard Model (SM), the highly successful theory of fundamental particles and forces of nature, he said: “Finding Higgs is certainly a discovery, but excluding Standard Model's Higgs is an equally significant discovery.”

This was one of the five key messages he gave in his talk.

But either way, Dr. Heuer said, the LHC was poised to clarify the mechanism of the origin of the masses of fundamental particles, which Higgs was hypothesised to do. Answers to most outstanding questions of particle physics today would be through the new particles on the TeV energy scale, which lay in the LHC territory.

The new energy frontier that colliders of future would explore was at least two decades away till when the LHC and its upgrades were expected to continue generating new data, leading perhaps to new physics. The CERN had drawn up a 20-year programme for the LHC, the immediate target being to reach its design luminosity (a measure of proton-proton collisions in the machine) at the designed combined peak energy of 14 TeV by the end of 2012.

The next step is to increase its luminosity, but at 14 TeV, of which configuration he called the High-Luminosity LHC (HL-LHC).

The goal, according to Frederick Bordry, Head of Technology at CERN, is to attain 20-30 quadrillion proton-proton collisions per year from the current 100-200 trillion collisions per year, for which advanced magnet designs (with 13-15 tesla magnetic field from the current 9 tesla) are under way.

The energy frontier beyond the LHC scale, Dr. Heuer said, would have to be reached through a synergy of hadron-hadron colliders like a high-energy LHC (HE-LHC), lepton-hadron colliders such as an electron-proton collider, which he called LHeC, and lepton-lepton colliders such as the proposed International Linear Collider (ILC), which is already at an advanced stage of research and development, and the Compact Linear Collider (CLIC) at the CERN, which is in its early stage.

(Unlike circulating beams in LHC-like machines, linear colliders have beams travelling straight and colliding head-on. The earlier electron-positron machine at the CERN, called LEP, was a circulating one, in whose 27-km tunnel the LHC is now located.)

The preliminary specs for the HE-LHC were proposed in October 2010. Planned for launch during 2030-33, the machine is proposed to be a 33 TeV proton-proton collider with peak luminosity being twice what was planned for HL-LHC. This, of course, would require aggressive R&D on high field (20 tesla) magnet design, including high-temperature superconducting (HTS) magnets, Dr. Heuer said.

As for LHeC, Dr. Heuer said that since the LHC tunnel already existed, there were two options available: a ring-ring option that involved overlaying an electron ring in the same tunnel over the proton ring, and a linac-ring option involving an electron linear accelerator (linac) and a proton ring.

As regards ‘Multi-TeV electron-positron colliders' a la ILC and CLIC, he said that given the close R&D collaboration between the ILC and CLIC projects, it would be prudent to merge the two into a single Linear Collider (LC) project, and made a pitch for locating the LC at the CERN so as to minimise cost, power consumption and enhanced “value engineering.” The location for the ILC is yet to be decided.

The fourth option, he said, was the muon-muon collider, where muon is a heavier cousin of the electron. Though muons coupled with the Higgs particle more strongly, the technological challenges for muon-muon colliders were far greater.

Reiterating that the results from the LHC would determine the energies of these machines, Dr. Heuer said all projects needed continuing accelerator and detector R&D and close collaboration between theory and experiments, and it was important to make a convincing case for the future accelerator options so that the right decision could be made at the appropriate time.

“The earliest window of opportunity for enabling a decision on the next energy frontier target will be 2012,” he said. Calling for increased global collaboration, he said: “All these have to be built as global machines.”

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