Friday, April 22, 2011

Has the LHC found a hint of the Higgs boson?

David Shiga, reporter
(Image: ATLAS Experiment © 2011 CERN)
Physics blogs are alive with chatter about a possible sign of the Higgs boson – or perhaps an entirely unexpected particle – in data from the Large Hadron Collider near Geneva, Switzerland. But the claim has not gone through the experiment's vetting process and could easily turn out to be wrong, physicists say.
The LHC, which smashes beams of protons together, was built largely in the hope of netting the first observational evidence of the Higgs, which is thought to endow other particles with mass. The Higgs is the last undiscovered particle in the standard model of particle physics, which for three decades has reigned supreme in explaining how particles and forces interact.

The latest rumours of its possible sighting come from an abstract that was posted by an anonymous commenter on mathematician Peter Woit's blog on Thursday.
The abstract appears to be part of a longer paper written by four physicists involved with the LHC's ATLAS detector, though the full paper has not been posted publicly yet.

The authors of the abstract say ATLAS data shows more photon pairs than expected with an energy of 115 GeV.

That number is interesting because many physicists think the Higgs boson is likely to have a mass of around 115 GeV – at least if supersymmetry, a popular theory that wraps up some of the standard model's loose ends, is correct. (Physicists often use energy units when describing particle masses, since they are related according to Einstein's formula E=mc2.)

The Higgs should occasionally decay into a pair of photons, which would produce a bump in the photon-pair energy distribution. But if the Higgs has the properties predicted by the standard model, that bump should be much too small to see. The bump claimed in the abstract is 30 times bigger than the expected value.
The Résonaances blog by physicist Adam Falkowski has a good rundown of the possible explanations for the signal. Physicist bloggers Tommaso Dorigo and Lubos Motl have interesting discussions too.

The consensus seems to be that the paper itself is real rather than some sort of hoax, but the result may well turn out to be wrong.

However, it could be that the Higgs just behaves differently than expected. Physicists have dreamed up many ways to extend the standard model that would modify the properties of the Higgs. Some of these would boost the size of the photon-pair bump, though making it big enough still seems to be a stretch.
Or the bump could be from some unexpected new particle rather than the Higgs.
But perhaps the most likely explanation is that the bump is a mistake. Particle collisions are messy and it takes a lot of careful analysis to separate anomalies from mundane background events. An error somewhere along the way could make a bump appear that isn't really there.

It is worth noting that the claim is at a very early stage. Apparently the paper has not been reviewed or endorsed yet by the ATLAS collaboration, an organisation of hundreds of physicists that runs the detector.

By comparison, the 145 GeV bump seen recently in a different kind of measurement at Fermilab's Tevatron collider in Batavia, Illinois, had the backing of the collaboration that runs the CDF experiment the result was based on.

Even if the 115 GeV bump goes away, chances are good that the LHC will have more interesting results soon. CERN reports today that the LHC has broken the record for the world's most intense beams of colliding particles, snatching the title from the Tevatron. The LHC had already broken the record for collision energy, but now it has the highest rate of particle collisions per second as well, which should speed up new discoveries.

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