ATLAS And CMS Publish 2011 Higgs Results
February 7th 2012
You have seen it already two months ago, but those were
"preliminary" results. Now both CMS and ATLAS have produced
full-fledged documents (CMS
describing their respective combinations of different Higgs
boson searches, using data collected in 2011 by the two
experimental apparata at the CERN Large Hadron Collider.
So what - you might retort - these are still "unpublished", in
the sense that they have not seen the light of printed matter on
scientific publication; they still only exist in the form of
preprints. Let me clear the ground from misunderstandings here:
between December 13th and today the two collaborations have
worked hard at polishing off their results and documenting them
as well as possible. The results have not changed
significantly, but this was not granted: it is what was meant by
the word "preliminary" attached to every plot. Now, however,
they are guaranteed to not change anymore.
Today is therefore a good time to give a look at the results
once again. I will not discuss the results, but maybe just paste
below the p-value distributions produced by the experiments.
These, I believe, are the most informative graphs for those who
believe that the Higgs boson does exist. So here we go, Atlas
A few remarks on these pics.
1) the expected p-value in case of a Higgs boson in the data, in
the two graphs, is shown by black hatched lines. Please observe
that at 125 GeV ATLAS expected to see a p-value of the
background-only hypothesis equal to 0.005, or a bit less than
three sigma; CMS expected to see a p-value of exactly three
sigma, so CMS had in principle more power (due to using more
final states in their searches, and slightly more statistics in
some cases IIRC). The data, however, chose to fluctuate the
other way: ATLAS has an observed p-value of about 0.0002, CMS
"only" 0.001, for the (a posteriori) most probable Higgs mass
2) The various p-values for the individual searches are shown by
different colored lines. The ATLAS plot has the expected p-value
for each, while the CMS plot doesn't - a understandable choice
given that CMS is displaying six independent analysis results
against the three of ATLAS.
3) Don't be deceived by observing a "disagreement" between
expected and observed p-values in the ATLAS plot (for the 125
GeV hypothesis). What is plot is the "median" expected p-value,
but the statistical fluctuations intrinsic of the searches
naturally allow that quantity to be observed at very different
values. Plus, it's a semi-logarithmic plot!
4) I find the agreement of the two experimental searches
impressive, and as I have already said here, I believe this is
really the effect of Higgs boson decays in the ATLAS and CMS
data we are looking at.
To summarize, the two figures do talk of something going on in
the 120 GeV region. For a Higgs believer, in fact, one might
argue that the Higgs boson mass has already been measured: a
mass of about 121+-5 GeV results from an eyeballing of the 95%
CL limits. But that is too large a leap of faith. Let us then
just say that there is a fat chance that the Higgs boson has
indeed a mass of about 124-126 GeV. That being true, the LHC
this year will definitely allow CMS and ATLAS to conclusively
observe it - 8 TeV running conditions have been confirmed last
week at the Chamonix workshop, and 20 inverse femtobarns of
proton-proton collisions are a fair estimate for the amount of
data that will be delivered next year.
One word about the two publications: the documents are rather
technical, so I do not advise newcomers to download them and try
making sense of them. However, graduate students working in LHC
experiments should, in my humble opinion, spend the better part
of a weekend over them. I guarantee there is something to learn,
and it is stuff that will be needed in order to really make
sense of the next round of results! And don't be scared by the
length of the papers upon downloading them: most of the pages
just contains the authors names !!
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