New Higgs ATLAS Results on the Two Photon and Four Lepton Decays

This is an update on my lifelong learning lectures (OLLI) on the Higgs.  For real experts discussion I refer you to my sources of the lecture by Tom LeCompte at the KITP workshop, and the discussion on Resonaances on Dec. 13, called “Twin Peaks in ATLAS”.

The LHC has completed its 2012 data run, and will resume in 2015.  It has delivered 23 /fb of collisions at 8 TeV, of which 20 /fb will be useful.

ATLAS has now presented its preliminary analysis of its Higgs to two photons (H → γγ), and Higgs to four leptons (4l) via a real Z boson and a virtual Z boson.  These complete the update on the 13 /fb of data at 8 TeV collision energy.  The following graph shows the measured ratios of production in the various channels to those predicted by the standard model Higgs.  The number 1 on the x-axis, or the dashed line, shows agreement with the standard model.

Higgs to 4l or four leptons occurs by the Higgs decaying to a real Z boson while emitting a virtual Z boson (or Z*) which then emerges as two leptons.  The real Z also decays to two leptons, which can be two muons or two electrons.  This process is centered at 1, so it agrees with the standard model prediction.  The new Higgs to two photons (H → γγ) value is high by about two error bar lengths or two ‘sigmas’.  This means that the odds of it agreeing with the standard model is only about five percent.  However, two sigmas is a common discrepancy in particle physics, and is considered a hint, but not a discovery.

There is a discrepancy between the masses of the Higgs measured in the two types of events by about 3 GeV, which are inconsistent by 2.7 sigmas.  However, Tom LeCompte says these are due to four muon final states in the Z-Z* process, where there are 8 events of which 4 could be the expected background.  The different mass results in the two types of decays caused the delay in the ATLAS announcement while they tried to resolve it.

In the standard model, the Higgs is a spin zero particle with positive parity.  In other new ATLAS results, they can eliminate the negative parity spin zero possibility at 99% confidence level, which means that there is less than one percent probability that it does have the negative parity spin zero values.  They can also eliminate the possibility that the new particle has a spin of 2 and a positive parity at the 85% confidence level.

Finally, the combined ATLAS certainty that they have discovered the particle is now at 7 sigma, or only a one in a trillion possibility that they have a statistical fluctuation in the background rather than a real particle.

I include here a data plot of 4 lepton production showing the Higgs peak in blue.  The red and purple events are from other standard model processes without a Higgs.

 

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For two decades I have been active in learning about energy and the environment, and in reporting on those topics for a decade. For the last four years I have added science policy. Lately, I have been reporting on the Covid-19 pandemic of our times.
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