Epistemic Status and Research-Strategic Role of Theories beyond the Standard Model after the Large Hadron Collider

After a discovery claim is established, the world is reported to be different; by contrast, when something is lost which was never had, seemingly nothing has changed. Drawing on interviews with experimental physicists involved in the search for, and (thus far) non-discovery of, super symmetry (SUSY) at the Large Hadron Collider (LHC), this paper explores transformations in epistemic strategies. In particular, the paper explores how the absence of direction in research strategies following the negative results at the LHC is being accommodated by experimental particle physicists in the ATLAS and CMS experiments at the LHC. 
Prior to the start of data collection, many in the high energy physics community strongly expected that evidence for SUSY would be observed at the LHC. To date, these expectations have not been realised. Van Lente (2012) has argued that "expectations are statements that do something, rather than being descriptive statements that may be true or false … expectations are performative". Drawing on van Lente's insights, I will show the whilst the negative results have not transformed the ontology of particle physics, the violated expectations have resulted in transformations in epistemic strategies away from targeted searches for evidence of SUSY (and other beyond standard model physics) to attempts to find evidence for 'unconceived alternatives' (Stanford, 2006). Epistemic strategies that aim at 'unconceived alternatives' are apparent in the shift toward 'model independent searches' and standard model measurements. This paper will focus on one such example: the attempts to measure the self-coupling of the Higgs boson. These attempts are in part motivated by the possibility that the measured result will disconfirm the value predicted by the standard model, thereby providing a path to unconceived and alternative new physics. 
Stanford (2019) has recently argued that, due to conservative attitudes, today's scientific communities are "less effective" than their predecessors "in developing fundamentally novel theoretical conceptions of nature in the first place" (p.3931). Whilst Godfrey-Smith (2008) and others have questioned this global claim, the current situation in particle physics makes for an opportunity to examine, at the local level, a case where the transformation in epistemic strategies within the experimental particle physics community indicates direct attempts to find evidence for 'unconceived alternatives', or disconfirming and disruptive experimental results that could provide fundamentally novel conceptions. 

One of the key philosophical debates in contemporary particle physics is how much to pay attention to the many ``beyond the Standard Model" (BSM) theories discussed in the literature when designing and operating new experiments. Should we only seek to "confirm" or "rule out" BSM theories, or should we conduct experiment unencumbered by pre-conceived theoretical notions for what "new physics" nature might have in store for us?


The debate has intensified in recent years given the lack of discovery of any new physics at the LHC beyond the Higgs boson, which itself was not new physics but rather a confirmation of the naïve Standard Model choice for electroweak symmetry breaking and mass generation of elementary particles. None of the hundreds of new BSM ideas (e.g., supersymmetry, extra dimensions, etc.) that were thought to have had good chances of discovery at the LHC have materialized yet. This has led a large group of physicists to resurrect a form of "signalism," which explicitly states that theorists "should not be listened to" and experimentalists should only design and operate experiments with an eye toward searching for signals that differ from Standard Model expectations, independent of how motivated one might think they are from a theoretical perspective.


The signalism perspective is reviewed with some examples of its advocacy and how it is implemented at the LHC. It is then contrasted with the opposing BSM perspective, where the differences in perspective are detailed. Signalism vs. the BSM perspective is not just a sterile debate on perspective, but also a debate that has profound implications on how sciences is conducted and what high-investment projects are funded. It is argued that physicists must confront these two perspectives in open debate in order to determine even the practical steps of how the field moves forward. It is argued here that careful scrutiny of signalism shows that it is self-contradictory despite the possibility that one could still accidentally make discoveries adhering to it. The BSM-oriented perspective, in contrast, is self-consistent and is argued to be the optimal perspective to make decisions about analysis of current experiment and justifications for future experiments.

LHC experiments led to the discovery of a standard model-like Higgs particle but have not generated any empirical evidence for physics beyond the standard model. In particular, low energy supersymmetry (SUSY), which is considered a promising hypothesis for a number of reasons, has not found empirical support up to this point. How should the LHC results reasonably affect trust in low energy SUSY? The present paper aims to analyze this question by developing a Bayesian model of belief updating under the most relevant observations. The goal is not to come up with a specific quantification of the currently justified degree of trust in low energy SUSY. Rather, the paper aims to identify structures of reasoning and salient connections between prior assumptions and resulting degrees of trust. The analysis can play a significant role in assessing the pros and cons of future research strategies in experimental high energy physics. Philosophically, it aims to identify the epistemic element in deliberations on research strategies in an important case of contemporary scientific reasoning. 
It is obvious from a subjective Bayesian perspective that posterior credences depend on subjectively chosen priors. Given that the data collected at CERN is far from having conclusive implications regarding low energy SUSY, convergence effects are weak and posteriors of individual agents differ considerably in dependence on the chosen priors. What may be more surprising, however, is the fact that physicists differ substantially in their assessment of the impact the LHC data should have on belief updating regarding SUSY. A careful Bayesian analysis can be helpful for providing a more cogent understanding of the way the data's impact should be understood. 
Using the discovery of the top quark in 1995 as a starting point, two important experimental steps can be distinguished. LEP II experiments collected data between 1997 and 2000. LHC experiments collected data reaching substantially higher energy scales from 2009 onwards. At LEP II, no Higgs particle was discovered and, despite expectations to the contrary, no SUSY candidates were found. Both results lowered credences in low energy SUSY. At the LHC, the Higgs particle was found with a mass favored by SUSY models. However, no indications for SUSY have been found up to this point, which substantially constrains the parameter space of low energy SUSY model and enforces some degree of finetuning if SUSY exists at low energies. Therefore, The LHC data increases credences in low energy SUSY based on the Higgs discovery and reduces credences due to ruling out parts of SUSY parameter space. It is nontrivial to assess the net effect of all these considerations.
Several issues need to be addressed in a Bayesian reconstruction of the described assessment: a probabilistic treatment of reducing the allowed parameter space; striking a balance between accounting for a given degree of finetuning and accounting for unconceived ways of settling those finetuning issues; and factoring in conceptual reasoning. The analysis will indicate in which ways specific choices regarding those issues lead to specific assessments of the current state of low energy SUSY. 

The concept of theory space plays a crucial role in empirical and non-empirical methods of theory assessment as well as any discussion about scientific underdetermination more generally (see Stanford, 2006; Dawid, 2013, 2018; Dardashti et al., 2019). But what are we referring to when we speak of "theory space" and what kind of epistemic access do we have to it? While these questions may not need to be addressed explicitly in most philosophical discussions – as the arguments remain valid independent from a precise explication of that concept – its implications for scientific practice and their reliable normative consequences will, as I will argue, strongly depend on the chosen explication and corresponding ontological commitment. The focus of this talk will be the relation between the argumentative role theory space plays in the philosophical analysis of theory assessment and the epistemic access practicing scientists actual have to it. This analysis will lay bare the implicit assumptions that need to be made explicit for reliably linking the formal analysis with the claimed normative consequences for the work of the practicing scientist. 
The divide between our practice-oriented knowledge about theory space and the formal role it purportedly plays in its application in empirical and meta-empirical theory assessment will be the starting point. After reviewing the formal role theory space plays in empirical and meta-empirical theory assessment, it is compared with the role it can play in scientific practice and the epistemic limitations to access it. I will argue that Dawid's account of meta-empirical theory assessment can be understood as providing the regulative framework within which the practicing scientist will more reliably apply their assessment of the theory in the context of theory development. This provides a strong dependence between the context of discovery and the context of justification. This claim can, as I will argue, similarly be applied to empirical theory assessment with a less strong dependence between the two contexts.
To concretize the above analysis, I will apply it to the context of beyond the standard model particle physics and develop the normative consequences of meta-empirical theory assessment. The analysis will provide a framework within which one may understand how the trust and reliance on certain models of particle physics comes about even before any experiments have started (Lykken & Spiropolou, 2014) and how the non-observation and therefore disconfirmation of a large class of models beyond the standard model at the LHC has impacted the scientists trust in the theories. This is then complemented with the normative analysis which may allow to critically assess the rationale of the scientist's assessment and provide tools for a more reliable assessment. The aim of this work is to both allow us to better understand the historical development as well as providing the tools to normatively evaluate the work of the practicing scientist.