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The Royal Society of Edinburgh
Knowledge Exchange event organised by the ERC project
Perspectival Realism.
Science, Knowledge and Truth from a Human Vantage Point
(ERC Consolidator Grant 647272,
Horizon2020 Research and Innovation Programme)
Modelling Beyond Standard Model Physics
School of Philosophy, Psychology and Language Sciences
Eidyn Research Centre, University of Edinburgh
Thursday 13 April 2017
Report by Michela Massimi
The goal of this half-day workshop was to bring to the attention of the general public
some cutting-edge methodological and philosophical issues arising from contemporary
physics at the Large Hadron Collider, CERN, Geneva. The event was part of a project in
the philosophy of science entitled Perspectival Realism, which has received funding from
the European Research Council (ERC) under the Horizon2020 Research and Innovation
Programme and is hosted by the Eidyn Research Centre in Philosophy at the University
of Edinburgh (Principal Investigator: Professor Michela Massimi).
What is next in physics after the discovery of the Higgs boson? Is there any evidence for
physics Beyond the Standard Model (BSM)? And what are the challenges facing the
search for BSM? The workshop, chaired by Sir David Wallace, RSE Vice-President for
the Physical Sciences, opened with a talk by Tiziano Camporesi, former Run and
Commissioning coordinator responsible for the first run to collect data at the Large
Hadron Collider in 2010, and spokesperson until August 2016 of the CMS Experiment at
CERN. Tiziano introduced the audience to the physics of LHC at CERN, where a second
run of proton–proton collisions at higher energy (ca. 13 TeV) started in 2015 with the
hope of unveiling some of the outstanding mysteries surrounding the existence of
possible physics Beyond the Standard Model. Tiziano pointed out the success of the socalled Standard Model to date, which the discovery of the Higgs boson in 2012 seemed
to have sealed. Are high-precision measurements within the Standard Model all that is
left to do for physics? Some ‘wrinkles’ in the Standard Model suggest a negative answer
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to this question. How can we reconcile gravity with quantum mechanics? What is dark
matter? How do we solve the so-called ‘naturalness problem’, concerning the
explanation of the low-value measured mass for the Higgs boson? What is causing the
asymmetry between matter and anti-matter, which allows our Universe to exist? The
future answers to these still-open questions will depend to a large degree on the
outcomes of current CMS and ATLAS experiments at CERN.
The second speaker was Monica D’Onofrio, Reader in Physics at the University of
Liverpool and former ATLAS SUSY Working Group convener (2012–14), where she
spearheaded searches for third-generation squarks and coordinated the top squark
search group. Monica introduced Supersymmetric (SUSY) theory as a possible way of
explaining the outstanding problems with the Standard Model. According to SUSY, in
addition to the existing particles – quarks and leptons – of the Standard Model,
additional particles (called ‘sparticles’) must be introduced (called ‘squarks’ and
‘sleptons’); as well as supersymmetric counterparts for the Standard Model force carriers
(i.e., photinos for photons, gluinos for gluons and Zino and Bino for the Z and W
bosons); plus Higgsino, gravitino, chargino and neutralino (the latter being regarded as a
possible candidate for dark matter). While no supersymmetric particle has yet been
found, Monica pointed out the reasons why theoretical physicists have found SUSY such
an attractive theory to supplement the Standard Model. From an experimental point of
view, the main challenge concerns the low production rates for SUSY, the bewildering
number of production modes and decay channels, and a very complex SUSY phasespace. Monica explained how experimental searches have concentrated on events with
missing transverse energy, as a possible signature for dark matter candidates arising
when SUSY particles produced at the LHC promptly disintegrate, and why the top
quarks are special in the hunt for SUSY. Top quarks, as heavy as a gold atom, are
copiously pair-produced, their decay mimics the signature of SUSY particles and
physicists have developed very sophisticated strategies to recognise and reject these
events. The search is far from being concluded.
The third speaker was Jon Butterworth, Professor of Physics at UCL and member of
the ATLAS Collaboration at CERN. Jon pointed out the importance of the high-precision
measurements routinely done at CERN, not only in confirming the validity of the
Standard Model to date, but also in excluding possible extensions of it. By measuring the
final states of proton-proton collisions, the hope is to identify suitable events of interest
based on how experimental data compare with Standard Model expectations (given by
simulation). A distinctive feature of the modelling practices currently adopted in these
searches is their model-independence. Looking for possible new physics Beyond the
Standard Model (BSM) requires being able to produce models that do not necessarily
involve hefty assumptions from the Standard Model itself. A widely used strategy resorts
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to so-called ‘simplified models’. Simplified models are models that have been – so to
speak – stripped of any hefty theoretical assumptions (concerning SUSY or other BSM
candidates) and that usually feature only a couple of possible candidate particles and
their possible range of mass values and couplings. In the specific example that Jon
illustrated (taken from a recent paper co-authored with Krämer et al.)1, simplified models
for a dark matter particle candidate and a hypothetical boson Z’ were considered, with
different possible mass values for the Z’ particle. For some mass values, the models
were found to be incompatible with current measurements made by both CMS and
ATLAS at CERN. This strategy allows physicists to create what are called ‘heat maps’
where exclusion regions for the hypothetical production of the dark matter particle from
the Z’ boson can be identified for specific ranges of the mass values and with specific
confidence levels, mapping therefore not just the space of what we know already exists,
but also – more importantly – the space of what we can reasonably exclude as existing
on experimental grounds.
Alan Barr, Professor of Physics, University of Oxford, ATLAS SUSY Group, further
explored the methodological implications concerning the degree of model-dependence
of LHC searches. Among the approaches currently adopted in the hunt for SUSY, Alan
has been working on the so-called phenomenological Minimal SuperSymmetric Model
(pMSSM). This is a ‘phenomenological’ model that reduces the high number of SUSY
parameters down to 19 and considers a range of possible values for these 19
parameters, with an eye to surveying possible (randomly selected) SUSY scenarios. The
hope is that by sampling a sufficiently high number of such hypothetical scenarios, the
types of model which might represent a supersymmetric extension to the Standard
Model realised in nature could be identified. As in Butterworth’s example, these pMSSM
models are themselves to some extent model-independent, because they make
relatively few theoretical assumptions about SUSY, while considering a manageable
number of parameters and some of their possible values. As more data become
available, some of the parameter values for the hypothetical sparticle candidates get
excluded. Thus, pMSSM provides an effective strategy for narrowing down possible
future research directions in BSM.
The last talk was by Michela Massimi, Professor of Philosophy of Science, University of
Edinburgh, who, as PI on the ERC project Perspectival Realism, has been conducting
fieldwork at CERN with a view to getting acquainted with the modelling practices for
BSM searches both at CMS and ATLAS. Michela explained the various reasons why
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Jon Butterworth, D Grellscheid (IPPP Durham), M Krämer, B Sarrazin (Aachen), D Yallup (UCL), JHEP
1703 (2017) 078, arXiv:1606.05296
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philosophers of science should pay more attention to LHC physics (from Big Data, to the
increasing use of simulation) and she focused her attention on models. Against a deeply
entrenched philosophical view of scientific models that sees them as accomplishing (first
and foremost) a representational task, Michela stressed the heuristic function of models,
which is exemplarily displayed by LHC modelling practices. The search for BSM physics
takes place via a variety of modelling techniques (e.g., from the heat maps, to the
pMSSM-19 models, and the simplified models of the CMS Collaboration2, to mention just
three examples), whose main role is to explore what might be the case. Exploring the
space of what is possible raises important philosophical questions: how can possibilities
act as a guide to reality? And how do we re-think the role of models in allowing scientists
to make how-possible inferences about what is real?
The talks were followed by Q&A from the public, with questions concerning the
predictive power of models, and the complementary searches for dark matter coming
from cosmology, among others.
The Vote of Thanks was offered by Sir David Wallace, RSE Vice-President for the
Physical Sciences.
Opinions expressed here do not necessarily represent the views of the RSE, nor of its Fellows.
The Royal Society of Edinburgh, Scotland’s National Academy, is Scottish Charity No. SC000470
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The CMS Collaboration [2016]: ‘Search for new physics with the MT2 variable in all-jets final states
produced in pp collisions at √s = 13 TeV’, Journal of High Energy Physics, doi: 10.1007/JHEP10(2016)006
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