Skip to main content
Accelerators and detectors

Accelerators and detectors

CERN split over collider closure

01 Dec 2000

The decision by CERN to shut down its Large Electron Positron (LEP) collider last month after it had caught a glimpse of the Higgs boson, the “holy grail” of particle physics, has incensed and saddened physicists. Many researchers at the particle-physics laboratory are unhappy that the management has not given LEP an extension to operate in 2001, with evidence for the elusive particle having already increased by running the collider for a month longer than originally intended. The LEP researchers believe that the lab’s director general, Luciano Maiani, has not provided convincing scientific reasons to support the closure.

LEP

“We felt there was evidence to justify running LEP again next year,” says Tiziano Camporesi, spokesman for LEP’s DELPHI experiment. The CERN staff association, which represents more than 70% of the employees at the lab, has denounced the “lack of clarity in the procedure that led the director general to take such a decision”. It said: “Such a decision cannot be taken on the sly by a director general who no longer knows how to listen to the scientific community as a whole.”

But there are many researchers who think that it is important to dismantle the accelerator in order to make way for CERN’s next major facility, the Large Hadron Collider (LHC), which will be built in the tunnel that currently houses LEP. “I think overall it is the right decision since it maximizes the future of the lab,” says Peter Jenni, spokesman for the LHC’s ATLAS experiment. “I don’t think it would be right to put at risk the other science at the LHC.”

Lyn Evans, LHC project leader, believes that physicists must accept the management’s decision. “If they don’t put all of this behind them there is going to be a serious chance that the particle-physics community splits,” he says. “In the end I hope common sense prevails, but it is absent at the moment.”

Supporters of LEP are dismayed that the opportunity to discover the Higgs will now pass to the Tevatron collider at Fermilab in the US, which is likely to gather convincing evidence for the particle over the next five years. “Not running in 2001 has clear consequences,” says Patrick Janot, LEP physics coordinator. “Fermilab will put all its efforts and resources to solve the question before the LHC. Then CERN will look ridiculous at having missed this opportunity, and the future of CERN will be very dark.”

Fermilab director Michael Witherell believes that Maiani’s was “as hard a decision as it gets” and says that he has mixed feelings about the closure. “I relish the prospect of going after the Higgs,” he says, “but the definite observation of the Higgs that could be produced by an extension of the LEP run would emphasize the importance of pushing our accelerator performance to study it at Fermilab.”

A rollercoaster ride

Emotions have been running high at CERN since the summer when LEP first detected signs of the Higgs, the particle that is thought to provide all other particles with mass and forms the missing link in the Standard Model of particle physics (Physics World October p5). Keen to squeeze every last drop out of the 11-year-old machine before it was due to shut down at the end of September, physicists hiked its centre-of-mass energy up to just over 206 GeV. They saw five events in two of LEP’s four detectors, which they interpreted as the tell-tale signs of a Higgs particle with a mass of about 115 GeV c-2. This led CERN to extend the running of the machine by a further four weeks in the hope that more data would strengthen its claim on the Higgs.

On Friday 3 November hundreds of scientists packed into the CERN auditorium to hear the results from LEP’s extended run. Researchers reported evidence for a sequence of particle decays in the L3 detector that were different from the events seen thus far. This reduced the chances that the combined measurements were due to statistical fluctuations from about 2.2 sigma (2.2 standard deviations) to 2.8, equivalent to a probability of 2 in 1000.

The results were greeted with shouting and screaming, and the researchers presenting the new data received a standing ovation, says Steve Myers, head of machine physics at LEP. “The only other time I can remember a reception like that was when Carlo Rubbia presented his results on the discovery of the Z boson, for which he received the Nobel prize,” adds Myers.

Scientists at CERN thought that these results would be good enough to convince the management to run LEP for another year in the hope that it could gather enough data to reach the magical 5 sigma, which means that the uncertainty in the measurements would be reduced to three parts in 10 million. But within days their euphoria had changed to agony. Technical staff were dumbfounded the following Wednesday when they found out through a press release that LEP had been switched off for the last time. “The new data were not sufficiently conclusive to justify running LEP in 2001, which would have had an inevitable impact on LHC construction and CERN’s scientific programme,” said the press release. “The CERN management decided that the best policy for the laboratory is to proceed full-speed ahead with the LHC.”

To reach his decision Maiani took advice from three committees. The LEP science committee could not decide whether or not to shut down the ageing accelerator. Then CERN’s high-level research board spent hours debating the problem, but it too could not agree an outcome. And CERN’s scientific policy committee was split down the middle. Since none of the committees could reach a consensus, Maiani decided to shut LEP down. “If LEP had seen a 4.5 sigma effect then we probably would have said go for it,” says Roger Cashmore, CERN’s director of research. “But LEP has seen less than 3 sigma. Experience tells you that need around 5 sigma to be sure.”

The science committee put the chances of LEP producing 5 sigma evidence by the end of next year at “50:50”. If the Higgs mass is 115 GeV c-2, as thought, then the committee estimated that LEP would have produced a 5.3 sigma event by the end of 2001, with an uncertainty of ±0.5 sigma. However, if the mass is in fact 116 GeV c-2 then this figure falls to 4.3 sigma.

But the science was not the only the factor that the management had to consider, says Cashmore. He maintains that in addition to delaying the LHC by a year, running LEP in 2001 would have cost around SFr 100m (about £40m), partly due to the default on engineering contracts. “We could have ended up with the situation where we had delayed the LHC, spent all that money and still only have had a 3.5 sigma effect,” he says.

For just over a week after Maiani reached his decision, physicists hoping to revive LEP clung to the faint hope that a committee representing CERN’s member states would reverse the closure. But when this committee met on 17 November it too could not reach a consensus, so Maiani’s decision stood.

Many physicists, however, remain unconvinced by the management’s verdict. Rumours spread through the corridors of CERN that an extension to LEP would not significantly affect the LHC because the LHC’s schedule was reckoned to be set back anyway by delays to engineering work. Lyn Evans, LHC project leader, says this analysis is not correct. He calculates that if LEP were to have run in 2001 then there would have been a “serious chance” that the LHC’s first physics run would have been put back from 2006 to 2007.

But many researchers believe that such considerations are peripheral. Belen Gavela, a physicist at the Universidad Autonoma de Madrid and an independent member of the CERN research board, says that she has not heard “any serious scientific argument” in favour of closing LEP. “I understand that there may be political and financial issues,” she says. “But in my opinion, it would be a very great service to science if you can overcome them. [A delay to the LHC is a] minor consequence, as there is no scientific competitor for that physics at the time that the LHC will open. Besides, if the signal disappears during 2001, it would still imply a relatively quick answer to a timely and fundamental question.”

“We should have carried on with LEP while we had the chance,” says Sam Ting of the Massachusetts Institute of Technology, who shared the 1976 Nobel Prize for Physics for the discovery of the J/* particle. “Its full potential for discovering the Higgs was not explored,” he says. Ting, who worked on the L3 experiment, believes it is difficult to object to an extension of LEP on economic grounds when billions of pounds have already been spent on it. “And who knows how long it will be before the LHC detects the Higgs,” he says. “The complexity of that machine and the detectors makes it difficult to estimate when it will produce useful results.”

Supporters of the LHC point out that, unlike LEP, the LHC is tailor-made to study the Higgs over a wide range of masses. The LHC is due to come on line in 2005 and should get its first sight of the Higgs about two years later. It will operate at a higher energy, a superior luminosity and will produce Higgs via several decay channels.

But this machine is likely to be beaten into second place by Fermilab’s Tevatron. Fermilab director Michael Witherell is confident that the Tevatron will get the upgrades it needs over the next few years to produce 5 sigma evidence for the Higgs in 2005 or 2006. “The visibility of what has happened at CERN should put us in a very good position,” he says.

Related events

Copyright © 2024 by IOP Publishing Ltd and individual contributors