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Theory and experiment – closer together by being apart


Eckhard Elsen is Director for Research and Computing

How the Theory Department contributes to the evolving research of particle physics and the CERN research programme

One year ago, when the new directorate started to consider the idea of separating theory and experiment into two departments, the decision to do so was quickly made. While the new Theory Department may be small in terms of numbers, it is host to a large number of eminent scientists over the year. It seems inconceivable to me for a world-leading centre for fundamental physics not to have a vibrant, independent activity in theoretical physics. Creating a Theory Department has given us that. Perhaps paradoxically, it is also helping to bring theory and experiment closer together.

One essential purpose of theory at CERN is to provide the bridge that makes theory testable by our experiments. Today that means enabling precision physics at a hadron collider: a task made all the more pressing by the superb performance of the LHC in 2016.

One example, illustrating the vibrancy of the interplay between theory and experiment, came with the infamous 750 GeV bump. Much as we would all have liked it to develop into a discovery, rather than a statistical fluctuation, the tantalising excess seen by ATLAS and CMS nevertheless catalysed the theory community to examine just how flexible the margins of the Standard Model might be. Their numerous and imaginative attempts to accommodate the observation in extensions of the Standard Model has value for future experimental studies.

CERN’s theorists work on many fronts. The LHC Physics Centre at CERN, LPCC, has for many years been our local primary interface between physics and experiment. As well as providing the day-to-day conduit between theory and experiment, it also allows new avenues to be thoroughly explored before detectors are put in place. A case in point is the expansion of forward physics at the LHC, which has seen major developments recently with the LPCC paving the way. The Physics Institutes that the Department regularly hosts provide another important contribution to the scientific life of CERN. They allow the intellectual firepower of the Department to be increased by addressing topical issues with eminent invited scientists from around the world. The upcoming meeting on neutron stars is just one example.

Recent developments in the Theory Department reflect the evolving nature of particle physics and the CERN research programme. One such initiative explores ways of exploiting the diversity of CERN’s on-site facilities, looking at the potential for physics beyond colliders.

Another has its sights set further afield: a neutrino activity, recently established with leading neutrino theorists spending extended periods at CERN. This complements the new neutrino group in the Experimental Physics Department: together their role is to survey the global neutrino physics landscape and act as a resource for CERN users wishing to engage with neutrino programmes wherever they may be. This is a clear example of the realignment taking place in our field to better coordinate and better make use of globally distributed facilities by a global physics community.

Our theory Department concerns itself not only with the very small, but also with the very large. While CERN makes resources available to collaborations in neighbouring fields through the recognised experiment programme, our theorists also engage with those researching fields such as cosmology. This makes CERN an attractive place for recognised experiments to hold their collaboration meetings, and it is good for CERN in exchange.

All this activity makes CERN’s Theory Department an invaluable resource for an experimental laboratory and the community at large. It is by working hand in hand that theory and experiment advance human knowledge, as equal partners in a shared endeavour.