# Difference between expected $p_0$-value and observed $p_0$-value？

In the ATLAS paper for the higgs discovery, they used 2 kinds of different $$p_0$$-values. One is the expected $$p_0$$-value, the other one is observed $$p_0$$-value.

What's the difference between the expected $$p_0$$-value and observed $$p_0$$-value?

How can one expect a $$p_0$$-value without real experiment? Seems like the expected p-$$p_0$$-value is from simulation by assuming the higgs as a given mass. If so, why does the observed $$p_0$$-value has a large discrepancy with the expected one at 125GeV？

On the right axis the $$σ$$ is the standard deviation from the null result for each set of data displayed. The little circles are the published $$σ$$ . One gives the mass fitted and the standard deviations from background in the publication for the PDG tables.