Particle physics

Delay in LHC restart

Print edition : April 17, 2015

A SNAG may delay the start of Run 2 of the large Hadron Collider (LHC), the particle accelerator at CERN, which made the historic discovery of the long-sought-after particle, the Higgs boson.

Though seven of the machine’s eight sectors have successfully been commissioned to the new operating energy of 6.5 tera, or trillion electronvolts (TeV), per beam, and the eighth is not far behind, the delay could be anywhere between a few days to several weeks. According to a CERN press release, an intermittent short circuit to ground in one of the machine’s magnet circuits was identified on March 21. The LHC was shut down in February 2013 to upgrade the system and increase the beam energy. The commissioning was expected to begin in April 2015.

“It is a well-understood issue, but one that could take time to resolve since it is in a cold section of the machine and repair may therefore require warming up and recooling after repair,” the release said.

“Any cryogenic machine,” said Frédérick Bordry, CERN’s Director for Accelerators, “is a time amplifier. So what would have taken hours in a warm machine could end up taking us weeks.”

A full assessment was going on, and a revised schedule would be announced as soon as it was known, the release said. The impact on LHC operation would, however, be minimal, it added. In any case, the rest of the current year would be spent studying the performance of the upgraded machine with a view to full-scale physics running in 2016-18. “All the signs are good for a great Run 2,” said CERN Director General Rolf Heuer.

It would be recalled that the LHC operated at 3.5 TeV per beam in 2010 and 2011 and at 4 TeV in 2012. It was during this Run 1 phase in 2012 that the Higgs boson was discovered. Using the data of about 4,000 trillion proton-proton collisions collected at the LHC in 2011 and 2012 by two collaborations ATLAS and CMS at centre-of-mass energies of 7 and 8 TeV, the mass of the Higgs particle was determined to be 125-126 G(iga)eV. The Higgs boson decays into various different particles. For this measurement, data on the two decay channels that best reveal the mass of the Higgs boson were used: decay into two photons and into four leptons.

According to the Standard Model of particle physics, the theory that describes all known elementary particles and their interactions, the Higgs boson is responsible for bestowing mass to all matter in the universe. The Model does not predict the mass of the Higgs boson itself. It must, therefore, be measured experimentally. However, once supplied with a Higgs mass, the Standard Model does make predictions for all the other properties of the Higgs boson, which can then be tested by the experiments.

Up to now, increasingly precise measurements from the two experiments have established that all observed properties of the Higgs boson, including its spin, parity and interactions with other particles, are consistent with the Standard Model. By combining other Run 1 Higgs results and more collisions with higher energy to come during LHC Run 2, physicists expect to increase even more the precision of the Higgs boson mass and explore in more detail the particle’s properties.

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