Quote of the Day

[Professor Steven] Weinberg raises an eyebrow and points to his office. 

His office, it turns out, is half the size of mine, an observation that vaporizes what little ambition I ever had to win the Nobel Prize.

- Sabine Hossenfelder, "Lost in Math" (2018) at page 96.

For what it is worth, "Lost in Math" is a treasure trove of dry wit for those with some familiarity with modern physics, and this is merely one of many gems that her book contains.

An abstract from today that sums up the attitudes in the field about high energy physics that she is critiquing is this one:

The standard model of particle physics is an extremely successful theory of fundamental interactions, but it has many known limitations. It is therefore widely believed to be an effective field theory that describes interactions near the TeV scale. A plethora of strategies exist to extend the standard model, many of which contain predictions of new particles or dynamics that could manifest in proton-proton collisions at the Large Hadron Collider (LHC). As of now, none have been observed, and much of the available phase space for natural solutions to outstanding problems is excluded. If new physics exists, it is therefore either heavy (i.e. slightly above the reach of current searches) or hidden (i.e. currently indistinguishable from standard model backgrounds). We summarize the existing searches, and discuss future directions at the LHC.

Salvatore Rappoccio, "The experimental status of direct searches for exotic physics beyond the standard model at the Large Hadron Collider" (October 24, 2018).

Thirty three pages of null results follow. The review begins with the following introduction:

Particle physics is at a crossroads. The standard model (SM) explains a wide range of phenomena spanning interactions over many orders of magnitude, yet no demonstrated explanation exists for a variety of fundamental questions. Most recently, the discovery of the Higgs boson [1, 2, 3, 4, 5, 6, 7, 8, 9] at the ATLAS [10] and CMS [11] detectors has elucidated the mechanism of electroweak symmetry breaking, but there is no explanation for why the scale of its mass is so much different from naive quantum-mechanical expectations (the “hierarchy problem”) [12, 13, 14, 15, 16, 17, 18, 19, 20]. Dark matter (DM) remains an enigma, despite extensive astronomical confirmation of its existence [21, 22, 23]. Neutrino masses are observed to be nonzero [24, 25, 26, 27], and elements of the Pontecorvo-Maki-Nakagawa-Sakata matrix [28, 29] have been measured, but these masses are not easily accounted for in the SM [30]. Unification of the strong and electroweak forces is expected, but not yet observed nor understood [31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44]; such models often predict the existence of yet-to-be-observed leptoquarks (LQs) or proton decay [45]. Furthermore, there are unexpected observations that are not explained in the SM, such as the baryon asymmetry [46], anomalies in the decays of bottom-quark hadrons [47], a discrepancy in the anomalous magnetic moment of the muon (g-2) [48], and the strong CP problem [49, 50, 51]. Even further, there are open questions about long-standing observations, such as whether or not there is an extended Higgs sector [52], why there are multiple generations of fermions with a large mass hierarchy [32, 53, 54, 55], and why no magnetic monopoles are observed to exist [56]. For these reasons, the SM is considered to be an effective field theory, and that physics beyond the SM (BSM) should exist.  

In this Review, we will (non-exhaustively) discuss a subset of these questions that have been investigated recently at the LHC with 13 TeV proton-proton collisions by the ATLAS, CMS, and LHCb [57] experiments. From a collider standpoint, we will discuss the solution to the hierarchy problem, dark matter, the origins of neutrino masses, unification, and compositeness. We will also discuss the possibilities for improvements of these searches at the High-Luminosity LHC (HL-LHC) or other future colliders. One very popular group of theories to explain several of these phenomena involve supersymmetric (SUSY) extensions to the SM [12, 13]. With a few exceptions, this Review will focus on answers to the above questions that do not involve SUSY, although it remains a theoretically attractive solution. This Review will also primarily not focus on solutions that involve an extended Higgs sector, nor open anomalies in hadron spectroscopy.  

Many models of BSM physics that can be tested at the LHC involve spectacular signatures that distinguish them from SM backgrounds. It is therefore worthwhile to discuss the searches for new physics with their unique signatures in mind. As such, we will first broadly discuss the signatures used for LHC BSM searches, and then discuss the implications on various scenarios.

I have highlighted the problems identified and underlined those that are legitimate problems as opposed to mere quibbled with what the laws of Nature actually happen to be. The decision to omit SUSY limitations and extended Higgs sectors is a telling sign of the decreasing popularity of these theories.

The decision to ignore "open anomalies in hadron spectroscopy" is a reflection of the extent to which QCD is so inexact, compared to other aspects of high energy physics, that anomalies often don't mean very much.