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Tevatron refines Higgs boson picture

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“Unfortunately, this hint is not significant enough to conclude that the Higgs boson exists”, says Fermilab physicist Rob Roser – but the difficult last particle is getting closer to revealing its secrets.

Today’s particle physics excitement comes from Fermilab results presented at a conference in La Thuille in Italy, where researchers think they’ve spotted the Higgs boson in an analysis of 500 trillion collisions. Fermilab says two independent experiments, from the CDF and DZero collaborations, have both spotted hints of the particle that’s thought to have created mass at the beginning of the universe.

The new results have a greater than 2 sigma confidence – a huge improvement over last summers 0.5 sigma results.

In the typically-cautious phraseology of high-energy particle physics, Fermilab says the two teams “found excesses in their data that might be interpreted as coming from a Higgs boson with a mass in the region of 115 to 135 GeV (giga-electron volts).

“In this range, the new result has a probability of being due to a statistical fluctuation at a level of significance known among scientists as 2.2 sigma. This new result also excludes the possibility of the Higgs having a mass in the range from 147 to 179 GeV.”

The 2.2 sigma result means the chance that these experiments have produced a statistical fluke is 1 in 250; at three sigmas (1 in 740), the physicists will upgrade the observation to providing evidence of a new particle, and at five sigmas (1 in 3.5 million), they’ll declare the particle “discovered”.

Even so, the latest announcement is important. Coming from two independent experiments is enough to spark excitement, but the mass range turned up in the DZero and CDF experiments also agrees with last year’s announcements from the Large Hadron Collider at CERN. Agreement between Fermilab and LHC results matters because the two accelerators used different types of beams: Tevatron, until its last run in September 2011, fired protons at antiprotons, while the LHC creates collisions between two beams of protons.

This, as explained by Nature, means that the two colliders produced different kinds of decays. The Tevatron was most sensitive to a Higgs emerging as it decays into a bottom quark/antiquark pair, while the LHC spots decays such as the production of photon pairs.

One of the challenges over the years has been to find result sets in which the Higgs boson could be identified in different decay modes.

Fermilab’s analysts have now used most of the 10 inverse femtobarns of data produced by its now-retired Tevatron beam, with only enough data to support experiment until June.

As CERN scales up the energy of the LHC to 8 TeV this year, its physicists hope to end up with a five sigma result for the Higgs boson – sufficient to declare its discovery.

Differentiating the Higgs

The problem in identifying the Higgs signal is that the trillions of events produced by the particle accelerators are detected by the energy the collisions produce – and particularly by the decay of particles produced in the collisions – and the Higgs isn’t the only particle that produce similar particles.

“The Higgs signal is tiny. It’s buried under an avalanche of backgrounds”, a Fermilab presenter* said in a presentation streamed online. Those backgrounds, she explained, come from collisions that have “the same final state”.

Fermilab’s “needle in a haystack” problem is to find particular signatures predicted for Higgs events, and there aren’t many. In approaching their 10 inverse femtobarns of data, the researchers expected to find just 167 Higgs events, and those still be buried under 200,000 background events. ®

*In trying to get the stream to work on a connection badly affected by a Sydney, Australia, thunderstorm, I missed the early slides in the Fermilab presentation. I believe the researcher I was listening to was Michelle Stancari, but am not certain. ®

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