# A Signal strength parameter (interpretation)

1. May 13, 2016

### ChrisVer

How can in general the signal strength parameter $\mu$ be interpreted?

I am talking for the parameter defined in Eq.1 here and plots like the Fig.1 here:
http://arxiv.org/abs/1507.04548

It says that it's the ratio of the i->H->f of the observed over what's expected by the SM... is the last the cross section prediction of the Higgs or for any other background?
Then what would the <1 or >1 indicate? I think the >1 indicate a signal excess, while the <1 indicate a signal underestimation(????)

2. May 13, 2016

### Staff: Mentor

Only Higgs. The background in data is subtracted before μ is calculated.
A deviation from the standard model. If μ=1 gets ruled out in some channel, things get interesting.

3. May 15, 2016

### ChrisVer

Is that the case even if you have background overestimation compared to data?

4. May 15, 2016

### Staff: Mentor

I'm not sure if I understand your question. If you overestimate something, you are doing something wrong and should fix it, or not use what you cannot get right.

5. May 15, 2016

### ChrisVer

Well it's not unusual to get overestimates like these [at <120 or 140GeV]:
http://cms.web.cern.ch/sites/cms.we...ZMass_7Plus8TeV_70-180_3GeV.png?itok=k2MlxuLT
In which case subtracting the estimates from data will result to negative values.

6. May 15, 2016

### Staff: Mentor

That is (hopefully) not an overestimate, just a statistical fluctuation. Yes, estimated signal strengths can be negative. As a random example, this ATLAS note has -0.4 +- 1.1 for VH -> Vbb in table 2. Note that it is consistent with 1, the uncertainties were just very large in 2012.