Generally when used, scientific observation refers to the empirical apprehension of things in the world classically through sense data, though there are objections to objective notions of sense data today by conceptualists and less extreme views in the post-positivist philosophical ecology. It is a classic tenet of empiricism that phenomena are said to exist scientifically if and only if there is strong empirical evidence for them. In contemporary strong forms of ontological commitment (SEP), one can posit the existence of truthmakers. From WP:
Truthmaker theory is "the branch of metaphysics that explores the relationships between what is true and what exists".1 The basic intuition behind truthmaker theory is that truth depends on being. For example, a perceptual experience of a green tree may be said to be true because there actually is a green tree. But if there were no tree there, it would be false. So the experience by itself does not ensure its truth or falsehood, it depends on something else.
In physics, where the ontological status of subatomic particles stretches conventional notions of empiricism given the heavy application of mathematics and the exotic laboratory procedures that govern the metaphysics, it is of course natural that operationalization was first developed and applied. Hempel in his monograph on definitions, for instance, considers Bridgman's articulation of the theory in his survey of definitions. Thus, the correspondence theory of truth remains a central pillar of modern physics despite the challenges that the uncertainty principle and the wave-particle duality present. Russell's teapot is still a good way to understand existence in the philosophy of physics.
Yet, there seems to be to me an extension of philosophical principles to make claims about the existence of things in modern physics that goes beyond this classic and conservative view about objects, let us call it admitting the Meinogian jungle to the classically conservative physical ontology. It is simply admitting that abstract objects should be counted as physical objects if the abstract objects (SEP) exist in physical theory. Let us call this the "liberal interpretation of physical existence".
Physics, like all sciences, requires metaphysical speculation. For instance, phlogiston was speculated to exist prior to it being ruled out through empirical activity. Luminiferous aether is another example. Both were posited to exist, but subsequently discarded. That is, sometimes we have to posit a teapot in space before we can design tests and use them to determine whether or not the teapots exist. There are such teapots that are well-funded and well-received in today's mathematical physics: dark matter, strings, branes, and modal realist universes distinct from ours. None of these things have a lick of empirical support, and yet many accomplished physicists commit to them ontologically. What's going on?
In this case, while immediate apprehension and operationalization are still considered valid truthmakers for things that are directly and indirectly observable, physicists in question seem to be implicitly blurring the distinction between real objects and abstract objects. If, for instance, we accept the wavefunction as a real object, and rely on its predictive capability as a truthmaker by ignoring the distinction between model of reality, and physical reality itself, then we are now admitting abstract objects as real objects. Thus, the wavefunction IS the particle. This is consistent with Wheeler's It From Bit and his Participatory Anthropic Principle.
Under Wheeler and other physicists who are pushing against a radical physicalist interpretation such eliminative materialism, idealism, which 150 years ago was the default metaphysical presupposition, is attempting to reassert itself. Thus John Archibald Wheeler, Seth Loyd, Max Tegmark and other mathematical physicists are pushing against the strict dichotomy between real and unreal that the logical positivists attempted to prove was beyond question. Now, we can answer your question
You asked:
Is there a flavour of philosophy where the particle would be considered to exist before it is observed?
Yes. It's philosophical realism in the broadest sense: from vanilla, naive realism, Searle's direct realism, and scientific realism. In realism, we may not commit ontologically to an object until it's verified physically, be we do consider things to exist physically before they are observed.
Now consider Wheeler's anti-realist position, something that goes beyond using mathematics to merely predict existence. If one admits as a truthmaker mathematical physical theory and the existence of abstract objects and then ignores the Korzybski's maxim that the map is not the territory, we find ourselves at Wheeler's position where we show what is real through math first, and then bring about their physical existence by observing them. Note that this is not phenomenalism because the criterion of existence establish the Ding-an-sich-ness through the proof of the abstract object. In this way of thinking, objects are therefore mathematically real and then are created physically after the fact.
So, hidden variables, model-independent existence, and using mathematical physics to predict physical observation is actually nothing other than plain old scientific realism pushing back against the ontological ambiguity of instrumentalism and radical forms of phenomenalism. Kant introduced ontological ambiguity with his phenomenon/noumenon dichotomy, and how a thinker responds to his argument is key in understanding their respective position. Today, there are flavors of philosophy of science where the particle would be considered proven to exist before observation, and these are the anti-realist positions. (And somewhat unscientific in my estimation.)
Is there a flavour of philosophy where the particle would be considered to exist before it is observed?- pretty sure that's the norm, not the exception. If we discover a particle on Tuesday, most physicists and philosophers would happily agree that the particle existed on Monday, and they were just incorrect on Monday. – TKoL Jan 29 '24 at 16:43