Tuesday, December 1, 2015

Chasing Fundamental Physics

Several recent preprints take a stab at considering deeper order that could give rise to the constants of the Standard Model.

One considers a non-supersymmetric alternative to a key assumption of quantum chromodynamics (QCD) in the Standard Model that governs the strong force. Francesco Sannio's unusually well written article, "Challenging Asymptotic Freedom" (submitted November 29, 2015) considers a change to how the strong force coupling constant runs at high energies, which is consistent with experimental data from particle physics experiments, but would make the low energy QCD behavior known as "asymptotic freedom" merely an approximate effective description of how the strong force acts, rather than an assumption that is rigorously true at all energy scales, with deep implications for cosmology and very high energy physics. The abstract of that article reads as follows:
Several extensions of the standard model feature new colored states that besides modifying the running of the QCD coupling could even lead to the loss of asymptotic freedom. Such a loss would potentially diminish the Wilsonian fundamental value of the theory. However, the recent discovery of complete asymptotically safe vector-like theories, i.e. featuring an interacting UV fixed point in all couplings, elevates these theories to a fundamental status and opens the door to alternative UV completions of (parts of) the standard model. If, for example, QCD rather than being asymptotically free becomes asymptotically safe there would be consequences on the early time evolution of the Universe (the QCD plasma would not be free). It is therefore important to test, both directly and indirectly, the strong coupling running at the highest possible energies. I will review here the attempts made in to use pure QCD observables at the Large Hadron Collider (LHC) to place bounds on new colored states. Such bounds do not depend on the detailed properties of the new hypothetical states but on their effective number and mass. We will see that these direct constraints cannot exclude a potentially safe, rather than free, QCD asymptotic nature. A safe QCD scenario would imply that quarks and gluons are only approximately free at some intermediate energies, otherwise they are always in chains.
An article by Yithsbey Giraldo, "Five-Zero Texture non-Fritzsch like Quark Mass Matrices in the Standard Model" (submitted November 28, 2015) explores a rather unambitious theoretical approach that, if true, would remove one of the ten experimentally measured parameters (the six quark masses and four CKM matrix parameters) that are found in the Standard Model. Its abstract says:
We will consider a five-zero texture non-Fritzsch like quark mass matrices that is completely valid and generates all the physical quantities involved, including the quark masses, the Jarlskog invariant quantity and the inner angles of the Cabibbo-Kobayashi-Maskawa unitarity triangle, and explaining the charge parity violation phenomenon at 1σ confidence level. To achieve this, non-physical phases must be included in the unitary matrices used to diagonalize the quark mass matrices, in order to put the Cabibbo-Kobayashi-Maskawa matrix in standard form. Besides, these phases can be rotated away so they do not have any physical meaning. Thus, the model has a total of nine parameters to reproduce ten physical quantities, which implies physical relationships between the quark masses and/or mixings.
Finally, an article by Venkitesh Ayyar and Shailesh Chandrasekharan, "Origin of fermion masses without spontaneous symmetry breaking"(submitted on November 29, 2015) explores an unconventional mechanism that could impart mass to fundamental fermions in the Standard Model. It's abstract is as follows:
Using a simple three dimensional lattice four-fermion model we argue that massless fermions can become massive due to interactions without the need for any spontaneous symmetry breaking. Using large scale Monte Carlo calculations within our model, we show that this non-traditional mass generation mechanism occurs at a second order quantum critical point that separates phases with the same symmetries. Universality then suggests that the new origin for the fermion mass should be of wide interest.
All three preprints are basically speculative theoretical proposals with almost no impact on any of the experimental data points in particle physics to date. The odds of any particular one of them being right are modest.

But, these articles do prove, by example, that there are viable and fruitful paths for theoretical physicists to explore that do not involve supersymmetry and that could provide a path forward to a deeper understanding of the deeper fundamental physics that give rise to the Standard Model after half a century of work with supersymmetry theories and string theory seem to have done nothing but lead us into a theoretical rabbit hole.

Last week, Jester at the Resonaances blog explored the extent to which experimental evidence is consistent with Leptoquarks, another non-SUSY beyond the Standard Model theory which isn't a bad fit to several current moderately statistically significant anomalies at the LHC, but which few people, myself included, are very excited about as a theory.  Lubos explores the angle with decidedly more enthusiasm, although even he, in the end, admits he doesn't really think that this will prove to be the correct interpretation of the data.

Finally, I will note an interesting more rigorous treatment of what Hawking Radiation really is at the Backreaction blog the explains in which respects the heuristic explanation given by Stephen Hawking in his book, "A Brief History of Time" actually oversimplifies the reality as it has been understood by physicists since the 1970s and how it is related to the Unruh effect which is another truly weird consequence of the intersection of relativity and quantum physics.

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