Opportunities in a new year of [natural, unnatural]ness

 Breakfast in Naperville

In October 2003, I went to Fermilab for a Workshop on Future Hadron Colliders. One morning Guido [Altarelli], Fabiola Gianotti, and I went for breakfast in a coffee shop in Naperville. While the blueberry muffins were markedly forgettable, our conversation was memorable. Although at the time ATLAS was only an empty cavern, the LHC was already very present in everyone’s mind and so Fabiola started to ask what should we expect the LHC to find: supersymmetry, technicolor, extra dimensions, little Higgs? Nothing, said Guido dispassionately, nothing other than the Higgs. Guido had a natural skepticism towards complicated model building or elaborate constructions whose only rationale seemed to be their ability of ignoring what LEP was telling us. But never did his skepticism turn into a bleak view of the future. He kept a sober attitude towards the latest fashions in new-physics theories, but had a sincere interest in them. Prejudices were not hampering his scientific curiosity. His natural optimism was not incompatible with his rational skepticism. This wise balance is a useful lesson to learn for any scientist who experiences the rollercoaster of excitements and disappointments that characterise research at the frontier of knowledge. I will not forget our breakfast in Naperville.  

The Dawn of the Post-Naturalness Era

Gian Francesco Giudice

(Submitted on 18 Oct 2017)

 Natural optimism is not incompatible with a rational skepticism

While discussing the role of naturalness in the post-naturalness era, I focused on the Higgs. However, the cosmological constant poses an even bigger problem. The dominant attitude during the naturalness era was to believe that the two problems could be addressed separately. This was probably not dictated by a deep theoretical conviction, but mostly due to pragmatic reasons. While progress with the cosmological constant was difficult, Higgs naturalness seemed an endless source of revolutionary ideas with promises of direct experimental verification. From a post-natural perspective, it seems that the problem of the cosmological constant can no longer be ignored. By making a field expansion of the SM effective potential, it appears that the problem with the cosmological constant and the Higgs mass have a common origin. The only difference is that the vacuum energy, unlike the Higgs mass, becomes an observable only through its coupling to gravity. But it seems likely that progress during the post-naturalness era will only come by addressing the two problems simultaneously. The cosmological constant corresponds to a scale of about 2×10−3 eV and affects physics in the deep IR, at astronomical and cosmological distances. Modifications of gravity at large distances are attempts to tackle the problem from an IR perspective [37]. However, the conceptual problem with the cosmological constant comes from quantum effects in the deep UV. Attempts to tackle the UV problem with the traditional methods used for the Higgs seem hopeless because the corresponding UV cutoff is way lower than any energy scale entering those theories. This confusion among scales is at the basis of the problem. It is a big source of confusion because the systematic approach of effective field theories has taught us how to separate energy scales in successive shells and make sense of the theory at each shell separately. The cosmological constant seems to resist this approach. Naturalness is an offspring of effective field theory and so it is not surprising that the difficulty we are encountering with the effective theory description leads to a problem with naturalness. A theory that manifests an active interplay between the IR and the UV would not be simply describable by an effective field theory, as it would violate its inner logic. It is not impossible that quantum gravity will exhibit some kind of IR/UV interplay. An indication could come from the classical behaviour of gravity. Consider the head-on collision of two particles at ever increasing energies. Once you pass the threshold for forming a black hole, the more energy you feed in the system the larger the Schwarzschild radius becomes. In other words, higher energy collisions are less sensitive to short distances, in contrast with our effective-theory intuition for a separation between IR and UV. A radical conclusion that could be derived from these considerations is that we are facing the end of validity of field theory and the solution of the cosmological constant lies beyond our familiar theories. The new framework should incorporate an interplay of physical effects occurring at all scales. Although it is difficult to tell how such a framework would look, one can easily expect that the cosmological constant problem will play a key role in the post-naturalness era.

...now I realise that my chance is today. As a scientist, I have the privilege to live in a new era of krisis. Ideas thrive in the periods of krisis dominated by uncertainty and confusion, when physicists are in search of a paradigm change able to deal with the puzzles they are confronted with. There is no lack of open fundamental questions we must tackle today: the nature of the Higgs boson, the structure of quarks and leptons, inflation, the cosmic baryon asymmetry, dark matter, dark energy, quantum gravity, and more. But there is also a widespread feeling that our theoretical tools – which have been so successful in bringing particle physics to its present stage of maturity – are becoming inadequate to address the next layer of open questions. A new paradigm change seems to be necessary. Experimental physics is reacting to the present status of krisis with a broad and ambitious program that will enable humanity to cross the borders of knowledge on many fronts. Theoretical physics is exploring new directions and looking beyond the boundaries of traditional particle physics, across different disciplines. Revolutions in science don’t happen overnight: it took thirty years for quantum mechanics to develop from Planck’s black-body radiation to Dirac’s equation; twenty-five years for the Standard Model to go from QED to the asymptotic freedom of QCD. We can’t expect to find all answers today. But we are experiencing all the right symptoms – unresolved fundamental questions, an old paradigm that seems to run out of mileage, bold experimental projects, revived theoretical curiosity – that indicate we are living in the dawn of a new era. This is the post-naturalness era, which may soon become a new chapter in the continuing story of human exploration of knowledge.

Id.