The Physics Case for Axions, WIMPs, WISPs and Other Weird Stuff

Abstract

Over the years both theoretical as well as experimental evidence has accumulated that strongly suggests the existence of physics beyond the current standard model of particle physics (SM). The Large Hadron Collider currently starting up at CERN will test many of the ideas for such physics beyond the standard model (BSM) and hopefully will provide us with a wealth of new information. In this note we argue that there is also a very good motivation to search for new physics in low energy experiments that can provide us with powerful complementary information on currently open questions and in particular on how the standard model is embedded into a more fundamental theory. Let us begin by briefly repeating some of the main reasons why we believe that there must be physics beyond the standard model. On the theoretical side there are a number of deficiencies in the SM. Some of them could be just aesthetic defects but some may go deeper. First of all the SM has a relatively large number O(30) free parameters that cannot be determined from theory alone but must be measured experimentally. Although this does not indicate an inconsistency of the theory it certainly is not in line with the hope that a fundamental theory of everything should have very few, possibly only 1 or even 0, free parameters. Moreover, some of the parameters seemingly need to require an enormous degree of finetuning or appear unnaturally small. Well known examples are the Higgs mass but also the θ parameter of QCD (which must be extremely small in order not to be in conflict with the observed smallness of strong CP violation). Another dissatisfying feature is that gravity is not incorporated into the SM but rather treated as a separate part. This is not just an aesthetic defect but also an expression of the fact that the quantization of gravity is still not (fully) understood. Finally, strictly speaking the SM will most likely not be valid up to arbitrary high energy scales. On the one hand this is due to our current inability to properly quantize gravity. But even the non-gravity parts are probably encountering problems in the form of Landau poles (places where the coupling becomes infinite) in the QED sector (at a very high scale much beyond the Planck scale) but probably also in the Higgs sector (where the problem is much more immediate and will occur at scales much below the Planck scale -