Do theoretical particle physicists really believe the particles they work on do not exist? Inspired by this article in The Guardian.

In private, many physicists admit they do not believe the particles they are paid to search for exist – they do it because their colleagues are doing it

This makes no sense to me - there are surely better things to work on than stuff one believes to be wrong.
Is it really the case that many theoretical particle physicists don't believe the particles they work on exist?
 A: This is probably not aimed at the particles in the standard model. Probably also not the ones who study gravitons and quantized collective modes/quasi-particles like phonons, plasmons, polarons, etc. I'd bet even people who study dark matter are confident that it is real. This is probably more about the hypothetical particles that have been postulated for so long: axions, the supersymmetric pairs (the zoo of -inos), and strings. There is, as far as I'm aware, no direct evidence that those exist, just arguments that they should exist because they make the math prettier. A lot of people thought the LHC would find at least the lightest of the supersymmetry particles, and it came up empty on that account.
Much of it I would honestly characterize as being akin to looking for possible patterns of yet-to-be-observed epicycles in a Ptolemaic model of the solar system. I don't know what insight will lead to the sort of simplification that was achieved by Kepler, or even if that's possible, but we'll see.
As such, I think it's probably better to think of this as a Bayesian. The particle physicists hypothesizing these particles have a low confidence that their specific particle will pan out, I mean the track record for finding hypothetical particles has been mostly downhill since they failed to find proton decay generally predicted by grand unified theories (GUTs), but they're willing to place a bet in the hopes that it might. Before then there were some spectacular successes in predicting particles based on symmetry concerns (isospin, the eightfold way, etc), so it's not like they're unjustified in thinking that this sort of strategy could work again. It's just that the theory has gotten so far ahead of observed phenomena that it seems unlikely that these bets will pay off in our lifetimes, if ever.
To give another example, the Higgs was postulated based on mathematical concerns, and we found it. But saying "the" Higgs is a little misleading. The truth is that there was a job that needed to be done in the standard model, giving the weak gauge bosons mass without violating the gauge symmetry, so we could be confident that there was something that was doing that job, we just weren't sure exactly what. There were a whole family of possible Higgs particles that could do the job. So, the odds that any one arrangement of particles would turn out to be right was quite low, but the odds that one of them existed was quite high. Last I heard, the particle physicists were a bit disappointed because the evidence was pointing to the simplest Higgs model that doesn't point to new physics at even higher energy levels.
My own money is on the idea that we need to try something else. The "postulate symmetry, derive particles needed for the symmetry, and predict you'll find them at the next accelerator" is probably played out for a while.
A: 
Is it really the case that many theoretical particle physicists don't believe the particles they work on exist?

I hope not.
To be an adequate researcher, in physics particularly, you have to focus on what you are researching, the way an artist focuses on his/her work.
I think this is a biased article by a person who does not understand that theorists in basic theories of physics explore many roads, hoping to find the one that models what nature is doing.
Some acquaintances may have expressed the belief that the complex particles predicted by the theory they are working on are tentative and may not be measurable, which is fair enough, and the statement is misinterpreted.
If a researcher is not behind his/her research, he/she should get out of
physics research, particularly theoretical physics.
A: She is referring to particles in speculative theories like Garrett Lisi's E8, various string theories, GUT, and SUSY, etc. The experimental physicists and engineers sometimes have theories but mostly they are testing others weird ideas. Sabine makes good arguments that it's probably a waste of money and resources. It's unfortunate, but there's no particular reason to assume that the grand unified theory is something we can confirm or even could confirm. So, experimental physicists are like drunks looking for their keys around the lamp post because that is where the light is shining brightest.
