Synopsis

Higgs-like Particle in a Mirror

The particle thought to be the Higgs boson seems likely to pass an important identity check.

Physicists have been searching for the Higgs boson for nearly 50 years. In July 2012, two collaborations at the Large Hadron Collider at CERN announced the discovery of a new particle that meets the general expectations for a Higgs boson. But is this particle the final piece of the standard model, or something more exotic?

One important test is the parity of the particle: how its mirror image behaves. In a mirror, even-parity particles look the same, whereas odd-parity particles appear reversed. The standard model Higgs boson is a scalar, a spin- 0 even-parity particle. But there are many models that include a spin- 0 odd-parity particle known as a pseudoscalar.

For the first time, the CMS Collaboration at the LHC has placed constraints on this possibility. They study decays of the new particle to a pair of Z bosons, each of which, in turn, decays to a pair of leptons. They analyze the angular distribution of the leptons under the assumption that the new particle is spin 0 , and find that the odd-parity pseudoscalar scenario is disfavored, having an effective statistical p value of only 2 . 4 % . So all the evidence thus far is consistent with the new particle being the standard model Higgs boson. – Robert Garisto