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CERN | Geneva | Switzerland

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The mystery remains on the Higgs boson

Ever since the discovery of what might be the Higgs boson last July, physicists from the CMS and ATLAS experiments have been trying to pinpoint its true identity. Is this the Higgs boson expected by the Standard Model of particle physics or some “Higgs-like boson” befitting a different theoretical model?

To tell the difference, we must check all its properties, like how often this boson decays into different types of particles, and determine its spin and parity, two properties of fundamental particles.

Since the new boson has a short lifetime, it breaks apart immediately after being created. There are five ways a Standard Model Higgs boson should decay that we can study at the Large Hadron Collider (LHC): breaking into two photons, two W or two Z bosons, two b quarks or two tau leptons in well defined proportions.  We must check both the presence of and the rate at which each decay mode occurs.

Last summer, just after the discovery of the new boson, both experiments reported unambiguous observations in only three channels. Unfortunately, the data sample was still too small to really be able to check if the new boson could decay into a pair of b quarks or tau leptons.

With more data available, the two experiments have just shown results for all channels today at a conference held in Kyoto as shown on the two figures below.









The left figure is for CMS and the right one for ATLAS. The values “σ/σSM” and “μ” are equivalent and represent the ratio of what is seen to what is expected from the Standard Model. So if μ is exactly one for a given channel, it means that channel decays at the rate expected from the theory. A value of zero would imply this particular decay channel is not seen at all, contrary to expectation. If μ has any other value, it implies the new boson does not behave quite as predicted. But one must take into account the error margin (the horizontal bar) before drawing any conclusion.

Both experiments now measured decays into two b quarks and two tau leptons and the errors have gone down for several channels. For now, CMS obtains a combined value of 0.88 ± 0.21whereas ATLAS mesures 1.3 ± 0.3. Both are compatible with 1.

The confirmed presence of all five modes would be compatible with a spin-zero particle. Having in addition all the correct decay rates would make the new boson look much more like a Higgs boson but it would still not be quite sufficient. The new boson must also have positive parity as the Standard Model predicts.

The spin of a fundamental particle refers to its rotation on itself, as the name suggests. Parity has to do with flipping direction in space, exactly like what happens when we watch an event directly or through a mirror where the left and right directions are inverted. Particles with a positive parity look the same when you observe them directly or through a mirror.

The parity can be determined by looking at the direction taken by all fragments after the boson decays. Depending on its parity, its debris will fly in a preferred direction. For example, CMS measured all angles between the four electrons or muons when the “Higgs-like” bosons decay into two Z bosons, each one ending in a pair of electrons or muons. Then they compared the distributions with two standards: one for positive, one for negative parity as shown on the figure below.

The left curve in blue shows the probability one would measure for a particular point on the horizontal axis if the new boson had a negative parity. The right curve in pink shows the same for a particle with positive parity. The value measured by CMS (green arrow) indicates the new boson most likely has a positive parity as expected by the Standard Model.

CMS also started looking for other bosons with masses beyond 600 GeV, the current excluded limit. If new bosons turn up, it could mean we have found one of the five Higgs bosons predicted by supersymmetry, a new theoretical model, and not the single Higgs boson predicted by the Standard Model.

So where do we stand? With more than twice as much data as shown in July, scientists have moved from searching for this elusive particle to starting to measure its properties. Once the decay channels, decay rates, spin and parity are clearly established, we will be able to determine its identity.

It is still too early to tell but the new boson looks like, sings like and dances more and more like a Higgs boson. More certainty will come out next March at a winter conference with still more data and improved analyses. But it will take a long time to figure out beyond any doubt if the discovered boson was really the Standard Model Higgs boson.

Pauline Gagnon

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