Update: On 26 July, the Tevatron’s CDF and DZero experiments announced they had further narrowed down the window of possible masses where the Higgs boson might be found. In 2009, combined data from the two experiments excluded the possibility that the Higgs lies between a mass of 162 and 166 gigaelectronvolts with 95% certainty. Now that excluded region has been expanded so that it spans from 158 and 175 gigaelectronvolts, nearly a quarter of the allowed range.
IT COULD do with a lick of paint and may not break any records any time soon. But the Tevatron collider in Batavia, Illinois, which has been slamming protons and antiprotons together for the last 27 years, is poised to beat Europe’s much-vaunted Large Hadron Collider (LHC) in the race to find the first hints of a Higgs boson. How has an ageing workhorse come to have the edge on its successor?
A new batch of data collected at the Tevatron will be presented next week at the International Conference on High Energy Physics in Paris, France. The results are likely to tighten the constraints on the possible mass of the Higgs boson, the particle thought to be responsible for giving other particles their mass.
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The Tevatron (pictured) is set to shut up shop by the end of September 2011, but the progress revealed in Paris could bolster the case to let it operate for another three years. ” Soon after the meeting, the laboratory management has to come to a decision about what to do,” says Kurt Riesselmann, a spokesman for the Fermi National Accelerator Laboratory, which manages the Tevatron.
If it is allowed to stay open – at a cost of around $50 million a year – the steady collision rate the Tevatron can achieve, combined with improvements in its data analysis and the closure of the LHC throughout 2012 for repairs, will favour the Tevatron in the hunt for the first signs of the Higgs, say researchers at the collider.
There was a swirl of speculation this month that the collider had already found a signal produced by the Higgs. The rumour, which by Tommaso Dorigo of the University of Padua, Italy, has been denied by Tevatron physicists, but the resultant media attention did no harm to the case for extending the collider’s life.
The race between the rival colliders is tight: the Tevatron has analysed many more collisions than the LHC, but the LHC can reach greater energies. By the end of 2011, each of the two Higgs-seeking experiments at the Tevatron, named CDF and DZero, will have collected data from a few hundred trillion collisions. By this measure, the Tevatron will be a factor of 10 ahead of the LHC. But the likelihood of creating a Higgs boson increases with collision energy, so by the end of next year the LHC should have produced roughly the same number of Higgs particles as the Tevatron.
Producing the Higgs is one thing; detecting it reliably is another. At first, the hunt involves a process of elimination, whittling down the mass range where the Higgs most likely lies. Here, a decade’s worth of data collected by the older collider could give it the lead, at least for a few years, while the LHC ramps up to full power. By 2011, the Tevatron will have finished this task, says CDF spokesman Robert Roser, and will be ready to move onto the next stage: discerning a positive signal. This will show up as a slight excess in certain particles that reach the Tevatron’s detectors. Establishing that the excess is a signal of a Higgs may take a few more years, because it requires high statistical significance.
It is more likely that the Higgs – if it exists – is at the lighter end of the possible mass range, which is well within the Tevatron’s reach (see diagram). The Tevatron’s measurements, coupled with increasingly precise data on other particles, suggest the Higgs probably has a mass below 140 gigaelectronvolts. “All indications are that the Higgs is more likely light than heavy,” says , who works on the LHC’s CMS experiment, “and at low masses the Tevatron actually has some advantage.”
“The indications are that the Higgs is more likely light, and at low masses the Tevatron has an edge”
That’s because if the Higgs is light, it will likely decay into a pair of bottom quarks, which combine to form jets of heavier particles that can be picked up by the Tevatron’s detectors. To confirm that these jets are produced by a Higgs, the experiments will look for other particles such as W bosons, which are also likely to be produced.
If the Higgs is light, there is also a small probability that it will decay not into bottom quarks but into photons. This decay, which the LHC has been set up to look for, should stick out like a sore thumb, Korytov says, but only a tiny fraction of Higgs particles will decay into photons, so it may take the LHC a while to see a signal.
By the end of 2014, says Roser, the Tevatron could have turned up a detection of the Higgs with at least a 3-sigma significance – meaning there is a 99.7 per cent likelihood that the signal is real. Projections at the LHC are similar: “By the end of 2014 certainly, we hope to have mopped it up, whatever it is,” says LHC operations group leader Mike Lamont. A 3-sigma hint, however, is not an official discovery. This requires a 5-sigma level of certainty, corresponding to a chance of less than 1 in a million that the signature is due to a random fluctuation. The LHC will be needed to do this.
Will extending the Tevatron’s life be worth it? Korytov, who has worked at both colliders, suggests it could be counterproductive. “Running the two in parallel is a dilution of resources, particularly human resources,” he says.
Others point out that recent Tevatron results, such as hints of a new generation of fundamental particles, show that the collider is far from over the hill. They also insist two colliders are better than one, especially if there are more teething problems at the LHC. “The LHC is still an untested machine,” says Roser. “We can expect where we’re going to be by 2014. They can only make projections.”
