Well, quite.
But that's not the same thing as "what is beauty", is it? Or even "Why are things beautiful?". They're different types of questions, though you seem to want to conflate them.
Roger says "I'm defining beauty as "what somebody finds beautiful"" but that doesn't solve the question. Such a definition would, indeed, be useful for the types of experiments you describe. But we still need philosophy should we want to break the recursion in that sentence and actually try and define "beautiful".
A philosophical conception of beauty comes prior - even linguistically prior, let alone epistemologically prior - to any experiments that seek to understand how people process beauty.
There's no need for philosophy in any of this.
To define "beauty" you need rhetoric and linguistics, not philosophy.
And you've drawn a false dichotomy between the questions "Why did our sense of beauty evolve?" and "Why do we each find different things to be beautiful?"
Both can be approached scientifically, without philosophy -- as was nicely demonstrated by the "Your Brain on Art" PDF that another poster linked to above.
For some reason, when scientific questions are applied to humans, you want to leap into philosophy. But it's an unjustified and unnecessary leap.
As an example, suppose we observe that about 60% of dogs love green beans. They salivate when they smell them, gobble them up when given a chance, and prefer them over most other foods when given a choice.
About 30% of dogs hate green beans. They turn away from them and won't eat them even when hungry.
About 10% of dogs don't seem to much care. They'll eat green beans, but don't especially prefer them.
We can use the scientific method to answer the question "Why do some dogs like green beans while others dislike them?" No one would suggest that this is somehow a philosophical issue. Nor do we need philosophy in order to define "dog", "like", "dislike", or "green bean".
Nothing changes when we shift our focus to humans, or when we change our topic to why different people find different things beautiful or ugly.