Higgs Boson Definitely ‘Observed’, But Not ‘Discovered’ As Yet: Official Word From CERN
By on December 13th, 2011

The Higgs search is not yet over and is all set to go on at LHC, CERN. This is the natural consequence of the CERN official seminar.

The Higgs has been definitely observed at the energy 126 GeV at a 3.6  2.3-sigma confidence level at ATLAS, combining all decay channels!

The data presented at ATLAS, by ATLAS boss Fabiola Gianotti, is more-or-less in line with Standard Model expectations.

Result from ATLAS:

The Higgs officially lies between 114 GeV to 141 GeV. The rest of the mass range has been eliminated with 95% confidence level.Several channels like the Higgs-> WW* has been excluded.

The mass range between 113 t0 115.5 GeV has been excluded, as has been the range from 131-453 GeV, with the exception of a window from 237-251 GeV at 95% confidence.

The Higgs-> gamma-gamma is a very promising channel and this suggests the 126 GeV figure for the mass of the Higgs. This suggests the presence of a ‘low-mass’ Higgs, which is quite in line with the Standard Model.  More data in 2012 will help CERN make a more definitive statement.

  • Bottom Line: Local Significance – 3.6-sigma; Global Significance – 2.3-sigma  at 126 GeV

Result from CMS:

The CMS results ruled out a high mass Higgs, much like the ATLAS results. 270-440 GeV was excluded and the Higgs->gamma-gamma channel gave very clear results. This low mass Higgs is very consistent with the previously announced ATLAS results, which is extremely good news.  There were excess events noticed between 110-130 GeV, in the tau-tau and bottom-bottom decay channels; this eliminates 134-158 GeV mass range.

A curious 4-lepton excess was noticed at 125 GeV, which is bang on target, if you take the ATLAS results (above) at face-value. This is again, very good news. The Higgs-> WW and Higgs-> ZZ excludes 129-270 GeV mass range. Multiple channel “modest excess” was noticed just below 129 GeV!

  • Bottom Line: Local Significance – 2.6-sigma; Global Significance – 1.9-sigma  at 124 GeV  

One of the key slides from today's seminar. Look at the excess in each of the channels at 126 GeV! (Courtesy: CERN live webcast)

The global results take into account the so-called ‘look elsewhere’ effect, which means that it factors in the chances of observing this same local excess at any point within a certain range and also in all channels.

The CERN announcement

CERN announced today that the Higgs has been observed’, but not detected’. The subtle difference between these two words lies in mathematics. When CERN says that they have observed the Higgs, it means that they are 99.73% sure that the Higgs is there. This is, however, not enough to guarantee the tag of a discovery. For that, the confidence level has to go up to 5-sigma, which gives a 99.99994% surety. This is very important, since 3-sigma effects have been known to go away in the past.

The non-discovery of the Higgs, as yet

The only reasonable explanation for the less-than-discovery tag at the moment is because LHC still doesn’t have enough data or rather, not enough data has been crunched.

This is surely great news for the particle physics community. The Higgs may be there in this and there are strong indications from both ATLAS and CMS that it is there and this means that the Standard Model has passed its stringent test yet! However, the mass is still to be ascertained exactly. The error bars haven’t been fully established.

So, the wait continues.

The Super Symmetric Models

This mass of the Higgs Boson, if actually true, is extremely exciting. It lends credibility to the cMSSM models, which is one of the basic Super Symmetric Models. There were widespread news reports that LHC has ruled out super-symmetric models or at least the simplest ones. Not quite! The cMSSM can accommodate a Higgs of 121 GeV mass and no higher. However, a small tweaking of the parameters yield a different version of the theory, which can very well accommodate a 125 GeV Higgs.

Another revolution may be just around the corner! Watch out!

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Author: Debjyoti Bardhan Google Profile for Debjyoti Bardhan
Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.

Debjyoti Bardhan has written and can be contacted at debjyoti@techie-buzz.com.
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    thanks

 
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