Higgs hunters face long haul
Feb 4, 2010 10 comments
Apart from being so huge, complex and expensive, CERN's Large Hadron Collider (LHC) is perhaps most famous for having broken down just nine days after it switched on in September 2008. Fourteen months and some CHF40m of repairs later, the LHC came spectacularly back to life late last year as jubilant physicists collided particles at record-breaking energies.
But to reduce the chances of the LHC being derailed again by a similar accident, physicists at the Geneva lab have decided to run the collider at just half its design energy for the next 18-24 months. The decision will potentially increase the time it will take the CHF6bn machine to unearth new fundamental particles, particularly the Higgs boson.
Under the latest schedule announced this week, the 27 km circumference collider will begin to smash beams of protons into one another at an energy of 7 TeV (3.5 TeV per beam) in early March. Experiments will then continue until its detectors have accumulated one "inverse femtobarn" of data – roughly 10 trillion proton–proton collisions – with the run ending after two years at the latest. By the time it was shut down on 16 December last year after just four weeks of operation, the LHC had delivered more than 50,000 collision "events" at a record energy of 2.36 TeV to its two largest particle detectors, ATLAS and CMS.
5 TeV per beam now looks very risky Roger Bailey, CERN
The previous plan to step the collision energy to 10 TeV this year was shelved following lab tests carried out late last year that simulated the accident of 19 September 2008. It occurred when a connection between two of the LHC's superconducting magnets evaporated while carrying a current of 8.7 kA, puncturing the machine's liquid-helium cooling system and causing significant collateral damage.
By opting to run at just 7 TeV, CERN is playing it safe. "5 TeV per beam now looks very risky," LHC operations leader Roger Bailey told physicsworld.com.
Once the 7 TeV run is over, CERN will shut the LHC down in 2012 for a year or more to prepare it to go straight to maximum-energy 14 TeV collisions in 2013. This will be a complex job that will involve replacing some 10,000 superconducting magnet connections with more robust ones.
However, Chiara Mariotti, who co-convenes the Higgs working group on the CMS experiment, says that choosing to stay at lower energies is a big price to pay in terms of the Higgs search. "We will need more than twice the data at 7 TeV compared to that needed at 10 TeV to reach the same discovery potential," she says. "At this energy we can at best expect to exclude a Higgs with a mass between 155 and 175 GeV."
Her CMS colleague Tommaso Dorigo, who has written extensively about the Higgs search on his blog, reckons the hope of discovering the Higgs boson at the LHC before 2012 is "faint".
However, the decision to run at lower energies still offers plenty of opportunity for CERN researchers, who could make major discoveries such as supersymmetric particles – or even something totally unexpected – relatively early. Indeed, the run energy of 7 TeV is still 3.5 times greater than at the Tevatron collider at Fermilab in the US, which until December was the world's most powerful collider. What will be discovered – if at all – depends largely on how heavy such new particles are and on how easy they are to spot among "background" processes taking place in the proton–proton collisions.
The Higgs boson is the last missing piece of the Standard Model of particle physics, and its discovery would confirm the most compelling explanation physicists have for how elementary particles acquire mass. Although the theory does not predict the mass of the Higgs boson, precision measurements of known Standard Model particles mean that its mass is unlikely to be more than 186 GeV. Meanwhile, direct searches made at CERN's Large Electron–Positron collider – the forerunner to the LHC – rule out a Higgs that is lighter than 114 GeV.
There is less and less room for the Higgs to hide Stefan Söldner-Rembold, D0 experiment
Efforts are therefore being focused on the region inbetween, and not only by physicists who work at the LHC. Keen to spot evidence for the Higgs first, researchers at the Tevatron's two experiments – CDF and D0 – have spent the past few years feverishly gathering data from proton–antiproton collisions at an energy of 1.96 TeV. These experiments suggest that physicists could be in for a long slog: a joint paper accepted for publication this week in Physical Review Letters rules out a Higgs with a mass of around 165 GeV, while disfavouring (at lower statistical significance) the region 160–180 GeV. "There is less and less room for the Higgs to hide," says D0 co-spokesperson Stefan Söldner-Rembold.
A lighter Higgs?
Although such exclusion limits allow physicists on both sides of the Atlantic to focus more sharply on the region where the Higgs might exist, the data – when taken with indirect limits from measurements at previous colliders – tentatively point to a light Higgs, which would be harder to discover. For example, a Higgs weighing less than about 140 GeV would be less likely to decay into pairs of W or Z bosons, which would leave clear, quick-to-find signatures in the LHC's detectors, and more likely to decay into pairs of b-quarks, which are much harder to distinguish from background. The LHC experiments would therefore need to collect more data to build a strong enough statistical case to identify a Higgs "signal".
Although the Tevatron does not have the capability to discover the Higgs outright – that task will only be possible with the LHC – it could produce the first strong hints of the particle's existence if the Higgs is lighter than about 160 GeV. "There is very high level of excitement at Fermilab and in other places including the US Department of Energy [which funds the laboratory]," say D0 co-spokesperson Dmitri Denisov. "But in order to claim evidence for Higgs we need to see the signal, not just exclude other areas. And keep in mind that the Higgs might not exist at all."
The Tevatron result certainly is adding more pressure for the LHC to join this race without delay Pedro Teixeira-Dias, ATLAS experiment
The Tevatron is now expected to run in tandem with the LHC's 7 TeV run until the end of 2011 following President Obama's budget request, which was made earlier this week. "Anything beyond that is a guess," says CDF co-spokesperson Jacobo Konigsberg.
As the high-energy baton passes from Fermilab to CERN, the race for the Higgs and perhaps other ground-breaking discoveries is on. "The Tevatron result certainly is adding more pressure for the LHC to join this race without delay," says ATLAS physicist Pedro Teixeira-Dias. "Compared with the Tevatron the LHC will have a much higher Higgs cross-section and a better signal-to-background ratio, even at 'just' 7 TeV. But the Tevatron is now at the top of its game and is clearly not to be discounted. We live in exciting times."
About the author
Matthew Chalmers is a science writer based in Bristol, UK