Explanation of Higgs mass exclusion graphs?

[Eonic]

This is a graph from an article from last November on the search for the Higgs with the Atlas detector.

It shows the cross section for a Higgs process relative to the standard model prediction.
And then the upper limit of this, at the 95% confidence level. And this for different Higgs masses.

For the upcoming new results I would like to understand these kind of graphs, can someone explain what it shows?
The standard model prediction, is that with or without the Higgs particle?
Why is the expected ratio not 1, the standard model is expected?
How do you read from this graph the mass range that is excluded?
How does a positive identification of the Higgs at a certain mass look like?

 

[mfb]

The standard model prediction, is that with or without the Higgs particle?

With. It is "the SM prediction for the Higgs boson" in particular. "1" means as many Higgs bosons as predicted by the SM.

Why is the expected ratio not 1, the standard model is expected?

These are exclusion limits. An exclusion limit of 10 means they can exclude that 10 times as many Higgs bosons as predicted by the SM are produced. That doesn't help much, of course - people are interested in the region where the exclusion limit is better than 1: "We can exclude the production of Higgs bosons with a mass of X as often as the SM predicts." The range where the exclusion limit is below 1 is excluded (at 95% confidence level - you can never be sure, of course).

The expected exclusion limits are just what you expect to get on average if there is no Higgs boson. If you are not very sensitive to the Higgs at some mass then your expected exclusion limit will be high: You expect that you cannot make a strong statement about the existence or non-existence of the particle at this mass.

How does a positive identification of the Higgs at a certain mass look like?

The exclusion plots have a mass range where the expected exclusion limit is way below 1 ("if there is no Higgs as often as the SM predicts, we expect that we can rule out the presence of it there") but the observed exclusion limit is above 1 ("we cannot exclude the SM Higgs there").

Figure 7 from ATLAS shows this feature (left side):

You can't properly judge the significance of the identification from such a plot, so what the experiments show in addition is a bit different: How consistent is the data with the hypothesis "no Higgs boson here" and how consistent is it with "SM Higgs boson here"?
Here is figure 13 from CMS:

Without a Higgs boson at 125 GeV CMS expected that they can rule out the SM Higgs boson at ~99.999% CL, but what they actually saw was very consistent with the SM Higgs at 125 GeV.