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Now Physicists Need a Larger Hadron Collider


Physicists are proposing to build a new accelerator ring that is 100 kilometers around and run at seven times the energy of the Large Hadron Collider (LHC).

When Europe’s LHC started up back in 2008, particle physicists wouldn’t have dreamt of asking for something bigger until they got this $5-billion giant to work. However, with the 2012 discovery of the Higgs boson, the LHC has fulfilled its initial promise – and scientists began to get excited about designing and building a machine that may one day succeed it: the Very Large Hadron Collider (VLHC).

 “It is only prudent to attempt to sketch a vision decades into the future,” notes Michael Peskin, a theoretical physicist at SLAC National Accelerator Laboratory in Menlo Park, California, who presented the VLHC concept to a US government advisory penal on November 2nd.

The new giant machine will dwarf all its predecessors. It would collide protons at energies about 100 teraelectronvolts (TeV), when compared with the planned 14 TeV of the LHC at CERN, Europe’s particle-physics lab near Geneva in Switzerland. In addition, it will require a tunnel 80-100 kilometers around, in a contrary to the LHC’s 27-km circumference. For the past decade or so, there has been little study fund available worldwide to develop such concept, However, in this summer, at the Snowmass meeting in Minneapolis, Minnesota—where hundreds of particle physicists gathered together to dream up machines for their field’s future in long run—the VLHC concept stood out as a favorite.

Some physicists caution that the VLHC will be only a small part of the global particle-physics agenda. Some other priorities include: upgrading the LHC, which shut down in February for two years to boost its energies from 7 TeV to 14 TeV; plans to construct an International Linear Collider in Japan, to collide beams of electrons and positrons as a complement to the LHC’s proton discoveries; and a major US project to exploit high-intensity neutrino beams produced at the Fermi National Accelerator Laboratory in Batavia, Illinois. “These forthcoming projects need to be the focus and it is premature to highlight the VLHC.” Said Jonathan Rosner, a particle physicist at the University of Chicago, Illinois, who convened Snowmass.

To some extent, particle physicists’ interest in the VLHC is a sign that they are returning to their roots—pushing to higher energies to seek the fundamental building blocks of nature.

They will also need to prove that everything is worthwhile. The discovery of the Higgs particle lends support to the idea that some particles have mass since they interact with a pervasive, treacle-like Higgs field. However, a lot of aspects of the discovery are not totally understood, including why the mass of the Higgs particle is so large. A possible way to explain its heaviness is through supersymmetry theory, in which known particles are coupled with heavier ones that may be observed in bigger particle colliders. The LHC hasn’t detected any signs of supersymmetry, though, Peskin wishes that a hint might come before the end of the decade, which will help to inform the design of a larger collider.

Nima Arkani-Hamed is an advocate of a bigger collider, who is a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey. In December this year, he will assist to launch an institute in Beijing called the Center for Future High Energy Physics. A part of its explicit mission is to explore the physics that a future proton collider may investigate. This work is crucial to identify a machine size that can maximize the science per dollar, said William Barletta, an accelerator physicist at the Massachusetts Institute of Technology in Cambridge. “We will not just give hand-waving arguments,” he noted.

In order to build a 100-TeV collider, physicists need to develop superconducting magnets that could operate at higher fields than the current generation, maybe 20 tesla rather than 14 tesla, adds Barletta. One leading candidate material for this kind of magnets is niobium tin, which could withstand higher fields but it is costly and must be cooled below 18 kelvin.

CERN also has a similar plan for a collider to the VLHC. CERN accelerator physicist Michael Benedikt is directing a research of a “very high energy large hadron collider” that would pass under Lake Geneva. It will have the same key parameters as the suggested VLHC: an 80-100 km circumference and a 100 TeV collision energy. Benedikt recommended that construction may start in the 2020s so that the machine can be completed soon after the LHC shuts down for good around 2035. “One might not want to end up with a huge gap for high-energy physics,” he explains. He adds that it is still early to provide a price tag. But other physicists speculate that a next-gen collider may have to cost less than $10 billion for the project to be politically plausible.

Source: Nature

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