Higgs cornered in Grenoble
Jul 22, 2011 27 comments
The latest data from experiments at the Large Hadron Collider (LHC) leave significantly less room for the Higgs boson to hide – that is the take-home message from the Europhysics Conference of High-Energy Physics in Grenoble, France.
"It's getting real!" remarked one particle physicist excitedly as delegates emerged from Friday afternoon's session of the conference, which runs until 27 July. In the space of a few presentations, the possible hiding places for the Higgs boson have been shrunk dramatically thanks to data collected this year at the LHC at CERN.
The Higgs boson is a hypothetical particle the existence of which would provide the last missing piece in the nearly 40-year-old Standard Model of particle physics. It is the simplest explanation for how the electroweak symmetry was broken in the very early universe, giving mass to elementary particles.
The new results even hinted that the infamous boson may already be rearing its head in particle collisions taking place at the LHC deep beneath the Franco–Swiss border near Geneva.
Nowhere to hide
At the Tevatron collider at Fermilab in the US, the CDF and D0 experiments have been excluding possible Higgs masses for the past several years. On Friday morning CDF and D0 physicists presented their latest results in Grenoble, ruling out mass regions of 157–174 GeV and 162–170 GeV, respectively. Direct searches at CERN's previous collider, LEP, ruled out a Higgs lighter than about 115 GeV, while indirect constraints from precision measurements of other Standard Model parameters disfavour a Higgs heavier than about 180 GeV.
The LHC has been performing well in recent months, with its four giant particle detectors taking more data per day than were collected during the whole of last year. Although the LHC has not yet collected as much data as the Tevatron, its higher-energy collisions (7 TeV compared with 2 TeV) are much more likely to create Higgs bosons and so the LHC has greater power to exclude certain mass intervals.
The LHC has finally entered the Higgs game, giving the first direct constraints in the high-mass region Dave Charlton, Atlas
Indeed, the LHC's ATLAS experiment has now excluded the regions 155–190 GeV and 295–450 GeV, while its sister experiment CMS rules out a Higgs in the ranges 149–206 GeV and 300–440 GeV. "The LHC has finally entered the Higgs game, giving the first direct constraints in the high-mass region ever reached by experiment," ATLAS deputy spokesperson Dave Charlton told physicsworld.com. "The data were only collected up until three weeks ago so there's much we still have to look into; but if the Higgs exists, then we've now got hints that we should be looking more carefully at the 130–150 GeV region. "
Higgs peeking out?
Intriguingly, both experiments saw slightly more events over background in this low-mass region than would be expected if the Higgs does not exist. The statistical significance of the excess is about 2.7 sigma, which means a roughly 8% chance that the excess could have been produced by chance fluctuations of the data. That is nowhere near the 5 sigma "gold standard" for a discovery, but the fact that both ATLAS and CMS see the same pattern generated a distinct buzz at the Grenoble conference.
It is precisely what you would expect to see if a low-mass Higgs was starting to show itself Gigi Rolandi, CMS
"It could be an unlucky fluctuation of the background or it could be something common in the way the two experiments model the background, but it is also precisely what you would expect to see if a low-mass Higgs was starting to show itself," CMS physics coordinator Gigi Rolandi told physicsworld.com. No excess events were seen at higher masses by either ATLAS or CMS.
With Fermilab's Tevatron collider set to close down in the autumn, the transatlantic race for the Higgs prize is all but over, unless a Higgs with a very low mass exists. ATLAS and CMS will perform the all-important combination of their results in time for the Lepton–Photon conference in Mumbai, India, next month, by which time they we will have doubled the amount of data. "Assuming the LHC keeps running as it is, we will know by the end of October whether the Higgs exists or not," says Rolandi. Researchers from the Tevatron experiments will present their combined exclusion limits in Grenoble on Wednesday.
Some physicists, however, were expressing caution. "It's too early to get carried away," said theorist Matt Strassler of Rutgers University in the US. "It's vital that the background, especially events where two W bosons are produced, is correctly understood." A signature of the Higgs is that it decays into two W bosons – but such pairs can also be produced by decaying quarks. Distinguishing between the two processes is difficult and getting it wrong could make it seem like there is an excess of Higgs-like events.
Theories ruled out
The newHiggs limits rule out theories that predict a fourth generation of quarks, and followed a day of presentations in Grenoble on Thursday during which ATLAS and CMS teams cut swathes through models that attempt to describe nature at scales beyond the Standard Model.
A blizzard of plots displaying the latest LHC data showed no deviation from the Standard Model of particle physics. Black holes and other exotic states that could arise if there are extra dimensions of space now have much less room to hide, as do new force-carrying bosons and other heavy particles that would indicate the existence of physics beyond the Standard Model. Quarks also remain point-like at the energies probed so far.
Perhaps the most troubling finding of the LHC so far is that supersymmetric particles – heavy copies of the Standard Model particles that arise from new quantum dimensions of space–time – have not been seen. ATLAS and CMS independently exclude such "sparticles" with masses less than roughly 900 GeV.
Grappling with the unexpected
Meanwhile, Tevatron researchers are grappling with a number of anomalous features in their data that do not fit within the Standard Model. An unexpected bump at a mass of around 150 GeV that turned up at the CDF detector earlier this year, for example, is still present in a slightly larger data sample, yet it is not seen by its sister experiment D0 or by new analyses performed by the LHC experiments. ATLAS and CMS are also closing in on other anomalous Tevatron results.
"We are in discovery mode," CMS spokesperson Guido Tonelli says. "This is just the beginning, but within the next year we will have, one way or another, a completely different view of nature which will have major consequences for the field."
About the author
Matthew Chalmers is a science writer based in the UK