Yes ! Discovery of a Higgs boson !!

Finally, after monumental work, we have discovered a new particle !!!

This is an historic day in particle physics, fireworks are very well justified. CMS and ATLAS presented status reports showing that the 2012 data confirm the hint of a Higgs boson seen in the 2011 data. The strongest evidence comes from the H→γγ mode and from the H→ZZ→4L mode, mainly because these channels give precise mass information.

Here are the γγ mass distributions from ATLAS and CMS:

ATLAS gammagamma mass distribution

CMS gammagamma mass distribution

These results are absolutely beautiful. No one would doubt that “there is something there.”

Here are the distributions for the H→ZZ→4L channel:

ATLAS 4-lepton mass distribution. The blue signal histogram is the right one.

CMS 4-lepton mass distribution

The significance from each experiment is 5σ so both CMS and ATLAS have independent grounds for discovery. This is what experimenters want – this level of confidence sweeps aside any nagging doubts that a new state has been observed. We have a new particle, no doubt, and it is a boson and seems to resemble the Higgs boson.

Here are the local p-values:

ATLAS p-value.

CMS p-value

There were some interesting differences in the work done by CMS and ATLAS. For example, CMS shows results from several channels, γγ, ZZ, WW, Vbb and ττ, while ATLAS restricted new results to the γγ and ZZ channels (the most important ones in this mass range) only. The mass values are not precisely the same, but they are probably compatible. The signal strengths are quite similar, and both experiments see a γγ signal that is larger than expected.

CMS presented an actual mass measurement: M_H=125.3±0.6 GeV.

Higgs mass measurement from CMS

CMS presented a check of custodial symmetry, reporting the ratio of WW and ZZ signal strengths: H→WW/H→ZZ=0.9+1.1-0.6, consistent with unity. Finally, CMS also presented a study of the couplings, introducing a factor for vectors and another factor for fermions, and find C_V=1 and C_F=0.5.

analysis of couplings from CMS

I will post again later (it is the middle of the night here in Illinois). Let me share a few quotes from Joe Incandela, Fabiola Gianotti and Rolf Heuer:

Joe thanked the theorists, accelerator scientists and even the six directors general who guided CERN through the LHC project. Then he made a beautiful statement: Thanks to CERN for opening up the lab to the world. These results are for all of mankind.

Fabiola pointed out that we are now entering the era of Higgs measurements. Then she added: The LHC and the experiments have been doing miracles. People have worked very very hard – so theorists, please be patient! I hope these results will open the door to a very bright future.

Rolf made the official and happy pronouncement: We have a discovery!

Joe Incandela, Fabiola Gianotti and Rolf Heuer

There was a standing ovation at the end, as was very appropriate.

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