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In the last 40 years (approximately) people have been "discovering", "rediscovering" and "studying" SUSY as a powerful tool and "symmetry principle".

Question:

What if SUSY is not realized in Nature at the end? Is SUSY the only path to "relate" fermions and bosons or what else? Remark: SUSY has not been discovered yet, so keep you totally conservative. What if there is no SUSY?

Bonus:

What are the merits of SUSY? What are its main issues? I do know some answers to this, but I think it could very enlightening if we "listed" pros and contras of current supersymmetric theories to see where we are NOW.

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There is a standard paradigm for thinking about the new physics that lies beyond the standard model, at higher and higher energies: weak-scale supersymmetry, grand unification, string theory. The purpose of weak-scale supersymmetry is to stabilize weak-scale physics (i.e. everything we know about) against quantum corrections. Grand unification can explain various features of the standard model, such as the assortment of representations and hypercharges. String theory offers a unique and subtle model of quantum gravity, and it contains supersymmetry and grand unification for free.

So it's a very compelling paradigm, but also a very broad one. It has been the work of decades to explore all the variations on this theme - there might be tens of thousands of models in the literature which fit this paradigm - and people continue to discover new theoretical possibilities every month. Conceptual and mathematical progress occurs too, alongside the exploration of concrete models. For example, the idea of a string-theory landscape, and the way it dovetails with eternal inflation, is a new perspective on the meaning of this "theory space" that people have been exploring, mostly at the level of field theory.

But of course theoretical work isn't confined to this paradigm. Numerous deviations from it are also being explored. We also continue to get new data about the world, both positive (new phenomena) and negative (confirmation of standard-model predictions), which has its impact on the viability and plausibility of all those thousands of specific models. And finally, there is that constant conceptual progress which incrementally clarifies our theoretical options, and which sometimes transforms our idea of what a particular theory means.

One feature of the present day is that the idea of weak-scale supersymmetry is under pressure, because only the Higgs has turned up, and no undeniable deviations from the standard model are being seen, and many possible deviations are not being seen. However, there are many possibilities, and the majority of the LHC's work still lies ahead. So I think the most we can say is that there is a shift in the theoretical centre. In the absence of a theoretical or experimental revolution, models of weak-scale supersymmetry will continue to be considered for years to come, but there will also be more attention to alternatives, such as high-scale supersymmetry and no supersymmetry (including string theory without supersymmetry). If we continue to see nothing but standard model at the LHC, there may be a shift in theoretical focus, from predicting masses of superpartners, to explaining the detailed features of the standard model (such as fermion mass ratios). Under such circumstances, the chief appeal of a model may become how many such features it can explain, and how well.

It seems clear that physics will continue to advance, even if not in ways that any one person managed to foresee. Even if we were to see nothing but standard model on Earth, there would still be the universe to explain. It would be a new era for fundamental physics if its primary empirical guidance were to become astrophysics, but if it happens, it happens, and methods and models will adjust accordingly. And meanwhile, that steady conceptual and mathematical progress always holds out the prospect of transforming how everything looks, even without further empirical input. Perhaps the string landscape will be completely mapped thanks to a generation-long effort; perhaps someone from one of the "deviant" streams of research will find a starting point which magically explains all the features of the standard model that are presently just assumptions.

Supersymmetry is there, or it isn't. Either way, I expect the human race to find out eventually, through a combination of empirical and theoretical advances, and as just one aspect of finding out the general truth about how physics works. It may show up undeniably at the LHC when it powers up to higher energies. Or we may already be seeing it as dark matter, in which case proving that dark matter is superparticles rather than something else may be a much more demanding task, than just turning the LHC up to maximum. Or supersymmetry may be confined to ultra-high energies, or not there at all, in which case there will have to be profound theoretical advances before we can understand the situation.

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