blind analysis and the combination of results

Human beings, even careful scientists, see what they hope to see. If you keep tweaking your selection cuts while watching whether a bump grows, you will, sooner or later, sculpt a fluctuation into a 'signal' without ever cheating consciously. Blind analysis is the discipline invented to defend against exactly this temptation: you design and fix your entire analysis — every cut, every method, every uncertainty — while deliberately hiding the answer from yourself, and only 'unblind' to look at the result once everything is locked.

In practice, blinding can take several forms. You might hide the data in the signal region (the narrow mass window where a new particle would appear) and develop the whole method using only sideband data and simulation; or you might shift all the numbers by a secret offset that is revealed only at the end. Either way, the rule is that once you unblind, you are not allowed to keep changing the analysis to chase a nicer result. This removes the unconscious bias that would otherwise creep in from optimizing against the very data you are trying to judge.

Combination is the complementary idea: pooling results from different decay channels, different experiments, or different years into a single, stronger statement. A signal seen weakly in several independent channels can together cross the discovery threshold, and crucially, an independent confirmation by a separate experiment guards against a mistake unique to one detector — which is why the Higgs was announced by ATLAS and CMS together. The honest caveat is that combination must be done carefully: results often share common systematic uncertainties (the same luminosity measurement, the same PDFs), and naively treating them as fully independent would overstate the combined significance.