Posted on behalf of Alexandra Witze.
COPENHAGEN — Physicists working at CERN, the European particle accelerator near Geneva, Switzerland, have snared a new first for the Higgs boson. They have watched it decay directly to the particles that make up matter (called fermions) rather than just the particles that convey force (bosons), as they had before.
“It’s yet another important step in understanding the behaviour of the particle,” Fabiola Gianotti, a CERN physicist, said today at a meeting of the EuroScience Open Forum here.
A CERN spokesman called it the most important Higgs discovery since the particle itself was revealed in July 2012 (see ‘Higgs triumph opens up field of dreams‘).
The results appeared on 22 June in Nature Physics. They come from the Compact Muon Solenoid (CMS) experiment at CERN’s Large Hadron Collider (LHC), which smashed protons together at great energies to tease out the Higgs. Until now, the Higgs had been seen to decay directly only to other bosons — namely, the carriers of the electromagnetic force (photons) and those of the weak force (Z and W particles). There had been indirect hints that it might decay to fermions as well, but measuring the fermion link directly is more challenging, Gianotti said. It needed to be done, though, to confirm that the Higgs was behaving as predicted.
CMS scientists measured the Higgs decaying to two different kinds of fermions: bottom quarks and their antimatter counterpart, and tau leptons and their antimatter counterpart. “Now we have actually observed it with very high significance,” said Gianotti of the CMS findings. (Gianotti is a member of ATLAS, another giant experiment on the LHC ring.)
The LHC has been shut down since early 2013 for maintenance and upgrades to its instruments and to the superconducting magnets that guide particle beams through it at high energies. It had to be opened every 20 metres along its 27-kilometre length for the upgrades. The machine is expected to re-start around April 2015, CERN director Rolf-Dieter Heuer told the meeting. Plans for this second science run call for it to operate with an energy of 6.5 teraelectronvolts (TeV) in each beam, giving a total combined energy when they smash of 13 TeV.
Only after scientists get some initial data from the 13-TeV run, Heuer said, will CERN consider beefing up the accelerator even more to get to its final design energy, of 14 TeV.
Between 2009 and 2012, in its first science run of smashing protons, the collider operated at a maximum of 8 TeV. “Altogether we foresee another 20 years of additional running,” said Heuer. “Discovering the Higgs boson was easy — the work starts here.”