What pattern would emerge in a double slit experiment in which you tried to measure the particle before the slits? Asked this question many times before, on different sites as well, but i still haven't gotten a good answer for some reason. Some will say one thing, others another, some won't really give me a direct answer at all. Probably going to get downvoted but what the heck.
We all know that if you shoot a single particle at the slits, the probability distribution that you will find on the screen after multiple trials will be that associated with a wave-like entity (interference pattern). However, if you decide to measure which slit the particle goes through, the probability distribution pattern that emerges will change, it will be a pattern consistent with a particle-like entity (the "particle" is most likely to be detected directly behind the slits and the probability will taper off everywhere else). My question is this, what pattern, "wave" or "particle", will emerge if we decide to take a measurement before it goes through the slits?
 A: We all know that if you shoot a single particle at the slits, the probability distribution that you will find on the screen after multiple trials will be that associated with a wave-like entity (interference pattern).
Yes, you see a parttern like this one, courtesy of Brown University:
 
Note though that each photon has energy E=hf or E=hc/λ. It has a frequency and a wavelength. It's a wave, it goes through both slits. See weak measurement work by Jeff Lundeen et al. Don't think the photon is some point particle because detection leaves a dot on the screen. For all you know that detection could involve some kind of interaction that's akin to the optical Fourier transform, see Steven Lehar's web page:
 
However, if you decide to measure which slit the particle goes through, the probability distribution pattern that emerges will change, it will be a pattern consistent with a particle-like entity
Detection at one slit would presumably involve something akin to a Fourier transform  that converts the wave into a dot that goes through that slit only. So there's no interference pattern any more.   
My question is this, what pattern, "wave" or "particle", will emerge if we decide to take a measurement before it goes through the slits?
I imagine it would be a wave pattern of dots, similar to what you'd get if the photon was emitted from your measurement point. 
A: The pattern on the screen depends on where before the slit you measure the particles.


*

*If you measure the particle's position in a way, that it will propagate on in a way
that will cover both slits later on, then you will just see the interference pattern as before.

*If you measure directly before the slits, the results will be (almost) the same as when measuring after the slits.

*If you measure somewhere such that the particle flux on the both slits becomes uneven you get results, where you have an interference pattern and "ballistic" background.


These results are easily obtained by considering, that position measurement
puts the particle in the measured position eigenstate (this obviously only holds approximately and, depending on the measurement device, not at all for other observables).
You can even construct measurement devices that measure only with a certain probability, then you will get "intermediate" results (material on this can easily be found).
In other words the important thing about measuring the particle at a position, is whether the measurement predicts the taken slit, then the quantum coherence
is obviously no longer given and you measure the "ballistic" results.
The correct mathematical formalism to handle this is a density matrix approach, (and then you could easily describe the measurement by taking the partial trace over the environment and getting the results from decoherence theory, by-passing the above assumption about the measurement process).
You can use this logic to answer your own question: The particles you measure before the screen form a simple ballistic pattern that is determined by the source! The measured particles are not coherently related with the interference. In otherwords, the measurement before the slits is completely unaffected by the presence of the slits (but may affect the pattern observed after the slits).
