Naturalness problem

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In theoretical physics and philosophy of science, naturalness is understood to be a specific, aesthetically motivated construction principle for physical theories . In this sense, “natural” means a theoretical explanation of a phenomenon that does not require fine-tuning of parameters that appear to be very improbable , which are located on quantitatively far apart scales (orders of magnitude). In other words, naturalness requires that the dimensionless ratios of these parameters do not differ too much from 1 and that no poorly justified fine-tuning is necessary in order to reduce interfering parameters.

The standard model of elementary particle physics , which is able to explain the experimental findings with great accuracy, has a problem of naturalness , since certain of its parameters have to be fine-tuned.

Closely related is the hierarchy problem of theoretical physics and the question of fine-tuning the natural constants .

Mass of the Higgs particle

The measured mass of the Higgs boson (125  GeV ) is in the order of magnitude on the energy scale of the electroweak symmetry breaking (246 GeV, see Higgs mechanism ) and the mass of the electroweak gauge bosons .

The mass of the Higgs boson can also be calculated using a quantum field theory . The calculated mass is influenced by radiation and loop corrections ( Feynman graphs of higher order). The result is the effective mass of the Higgs boson, which greatly exceeds its bare mass . The masses of gauge bosons are not influenced in this way because of local gauge symmetries ; Fermions are not because of chiral symmetries . This kind of protection is  not given with the scalar Higgs boson, which has spin 0: the mass of scalars diverges quadratically with the highest energy scale Λ of the theory.

One therefore expects a natural mass of the order of magnitude of the Planck scale for scalars . The Planck scale is 16 orders of magnitude higher than the electroweak scale, which corresponds to the measured mass of 125 GeV. The experimentally accessible mass of the Higgs boson is therefore not at its natural value close to the Planck mass , but significantly below it. This is the naturalness problem.

It is possible to bring the Higgs mass by renormalization to a value close to the actual measured value. However, this renormalization is technically demanding and requires unnatural fine-tuning. The naturalness problem cannot therefore be solved in this way.

The remaining fine-tuning is interpreted as a gap in the understanding of the Standard Model.

meaning

The importance of naturalness lies in the multiple, successful application as a construction principle of elementary particle physics and especially renormalizable , effective field theories . This makes important contributions to the determination of the energetic validity range Λ (lambda) of quantum electrodynamics  (QED), to hadronic resonances in the GeV range and to the charm quark .

Many of the possibilities or expectations proposed in the past decades to expand the standard model of elementary particle physics are motivated by naturalness as a construction principle (e.g. technicolor and supersymmetry ). In these cases, new elementary particles and fields are proposed which could solve the naturalness problem of the Higgs particle or the Higgs mechanism . The existence of certain WIMPs , which would also be able to illuminate the character of dark matter , also appeared to be highly attractive .

Crisis of naturalness

Due to arguments about naturalness, the research community expected that the Large Hadron Collider (LHC) at CERN would not only detect the Higgs particle, but also provide clear indications of elementary particles beyond the Standard Model. Since 2017, however, the LHC measurements have increasingly contradicted central elements of the Technicolor and supersymmetry hypotheses, which were intended to solve the naturalness problem of the Higgs particle. Theorists like GF Giudice from CERN have therefore proclaimed a post-naturalness era . Others ( S. Hossenfelder ) consider naturalness to be a "fashion" based on logically flawed foundations.

literature

  • R. Gast, At the End of Naturalness, Spectrum - Die Woche, 14/2018, ( online , accessed February 13, 2019).
  • GF Giudice, Odyssey in the Zeptoraum: A Journey into the Physics of the LHC, Springer-Verlag, Berlin Heidelberg 2012, ISBN 978-3-642-22394-5 .
  • GF Giudice, The Dawn of the Post-Naturalness Era, CERN-TH-2017-205, submitted on 18 Oct 2017, ( online , accessed 13 February 2019).
  • S. Hossenfelder. Lost in Math: How Beauty Leads Physics Astray, 2018, Basic Books.