On Dark Matter

Some quotes from "New Directions in the Search for Dark Matter",(https://arxiv.org/abs/2204.03085 by Surjeet Rajendran, John Hopkins University.  

The paper is a good backgrounder on how we might find out what dark matter is comprised of; but there is also a philosophy of physics that has largely been forgotten in all the stringy revolutions.

The existence of dark matter proves that there is physics beyond the standard model. But, other than its existence, observational limits on its properties are extremely weak.

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Given the vastness of this parameter space, how can we hope to make progress? When confronted with this vastness, there is a human tendency to artificially restrict it by focusing on “theoretically well motivated” dark matter - in this context, “theoretically well motivated” means particles that theorists have already written down for some other reason. While it is certainly possible that the existence of dark matter may be tied to the solution to some other problem in particle physics, such a connection is not a logical requirement. It is a fantasy to think that the particle spectrum of the world can be figured out entirely from first principles. I have not come across a physicist who has convinced me that their refined sense of theoretical insight would have allowed them to figure out (without experimental input) that the Standard Model is a SU (3) × SU (2) × U (1) gauge theory with the SU (3) confined at low energies, the SU (2) × U (1) broken in a weird way leaving an unbroken U (1), with three generations of quarks and leptons that have hierarchial yukawa couplings with only the top quark possessing a naturally large yukawa coupling while also containing nearly massless neutrinos and a highly fine tuned Higgs boson. Our job as physicists is to discover what nature actually is rather than attempt to constrain it from the armchair.

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A skeptical reader may ask if we should actually care about technical naturalness. After all, we now have very solid evidence of at least two fine tuned quantities in our universe - the cosmological constant and the higgs boson itself. Neither of these terms are protected by symmetry and the absence of symmetry did not prevent their existence, creating confounding theoretical problems. Our job as physicists is to figure out what is out there in the world instead of imposing philosophies on it - especially philosophies that are already empirically known to be violated.

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The identification of the nature of dark matter is pretty clearly one of the major problems confronting particle physics. It is exceedingly unlikely that humanity will solve this problem from the armchair by guessing a sufficiently pretty theory. Physics is an experimental field - the belief that we can figure out what is out there in the world without experimental input has always just been a silly fantasy. Given the vastness of the parameter space of dark matter, there is a tremendous need to dramatically widen the experimental program that has been pursued to detect its properties. Now, it could have been the case that this dramatic widening could only come at great cost - if every probe of a part of dark matter parameter space required billions of dollars and thousands of working hours, we will not be able to appreciably probe the dark matter parameter space in our lifetimes. Luckily, this is not the case - the methods and experiments described in these lectures are experiments that can be pursued by a small number of investigators at the cost of several million dollars per experiment. It is thus possible to sustain a robust ecosystem of dark matter experiments which will cover a significant range of parameter space. While the creation of such a program is not up to me, I certainly hope that this broad ranged program will come to be realized.