Particle physics has been historically the ground of long-standing scientific challenges between the US and Europe, especially since the birth of the CERN laboratories in 1954. And in parallel, another challenge has kept the field alive and thriving for over half a century: the one between theoretical and experimental physics. 
The fifties were glorious years for experimental research in "high-energy physics" (HEP) - where high energy meant to signify the use of very energetic reactions to study new states of matter. A plethora of new hadrons (subatomic particles made of the same ingredients that form nuclear matter, quarks and gluons) were discovered as the energy of particle beams was gradually increased in various experimental facilities. The US led the pace, and most discoveries happened there. 

In those years experiments were generally ahead of theory, as the picture appeared in many ways confusing: what all those unstable particles meant was initially very unclear. However, soon theorists managed to make sense of experimental inputs in a spectacular way. First with the hypothesis of parity nonconservation in weak interactions (Lee and Yang, 1956), a prediction that was very soon confirmed in a brilliant experiment by Wu and collaborators at the National Bureau of Standards; then with Gell-Mann, Neeman and Zweig's successful description of hadrons as objects composed of quarks; and finally, with the formulation by a handful of theorists of the Higgs mechanism, the Standard Model, and the proof of the renormalizability of the theoretical construct. The challenge and then the confirmation of those theoretical ideas would keep experimentalists busy for four more decades.

Geographically speaking, until the sixties the most important discoveries in HEP arguably took place in the US, a country which had invested a lot in that direction, mostly a lasting effect of the white-hot message brought about by the invention of nuclear weapons during world war two. In the seventies, however, the pendulous finally started to swing toward Europe: the discovery of neutral currents, of the gluon (a spin one mediator of the strong force), and then the observation of the W and Z bosons (again, spin one particles) finally challenged the US leadership, despite the extraordinarily important discovery of the J/Psi meson in 1974 at Brookhaven and SLAC, and the Upsilon discovery at Fermilab in 1977.

The more recent history has seen a balance between the successes on both sides of the Atlantic ocean: although the top quark discovery still happened in the US (again at Fermilab, in 1995), that was counter-balanced by the exquisite precision of electroweak measurements by the LEP collider at CERN; and more recently, the Higgs boson discovery (2012) has confirmed that Europe is the place to be, especially if you want to discover particles of integer spin.

The above summary is a very quick-and-dirty one, as it does not pay justice to parallel advancements in the field of neutrino physics, B physics, and other important topics in fundamental physics, which saw a growing contribution from Asian countries (suffices to mention the groundbreaking 1998 discovery of neutrino oscillations by the Kamiokande experiment in Japan). Yet I think it is useful, as it allows to form a picture of how HEP has evolved, and where we are now. 

On one side, with the exhaustion of the mandate of testing the standard model and finding the Higgs boson a good fraction of all HEP experimentalists, branding the powerful LHC hammer, are looking around for nails. Many of their colleagues are scattered around in smaller endeavours (although in some case still quite large on an absolute scale, such as e.g. the Belle II experiment in Japan). Theory is indicating too many possible avenues for making progress, and none of them seems altogether very convincing at this point. For a decade or two, it looked like the tandem of theoretical ideas and experimental verifications could continue by using Supersymmetry as the driver of the next big project, but recently it has become clear that we cannot consider SUSY a promising way to justify our existence as HEP physicists.

On the other hand, governments have caught the message - funding fundamental physics research will not buy new powerful weapons. The justification for building new large facilities has to come from other arguments today. Arguably, the US had already given up its leadership at the forefront of HEP in 1993, when its Congress decided to cancel the plan of building a Superconducting Super Collider in Texas; the following strategic decisions and the funding profile of HEP in recent years have only strenghtened that impression. A possible exception is China, which has shown to be much more ready to invest big money in large new projects. However, it is not clear what those projects will be.  

If you consider the above statements a fair summary of the situation, it is clear that HEP is is a deep crisis. What can be the strategy to move forward in a principled way ? Where should we put our money, at a time when funds are thinning and theory provides no more clear guidance ?

These questions are at the center of the agenda of a process called "Update of the European Strategy for Particle Physics", which has been involving physicists throughout the old continent in the task of finding a consensus of what experiments should be planned, what lines of research have to be privileged, and what synergies can be constructed with the other main players - the US, China and Japan above all - in the furthering of fundamental research. 

It is clear that one of the biggest questions is, what big toy should we build next ? I have sat through many heated discussions in the past few years, with colleagues at various degrees in the decision-making hierarchy, and I have heard a wide range of different opinions. Funnily, it looks as if when left out in the woods, the average physicist will find refuge in the physics ideas they have had most satisfaction playing with during their past careers. It must for sure be the highest-energy hadron collider we can afford to build, for there await us them big discoveries. No, it has to be a precision electron-positron machine, where we can deepen our understanding of the Higgs boson. On the contrary, it has to be a muon collider, where for the first time we can open our eyes to unexpected new physics, thanks to those second-generation projectiles. Arguments will be scientifically motivated, as everybody is honestly squeezing their brains to answer the question in a principled way, and yet convergence is nowhere near - reasonable people can disagree, even if they are physicists.

And there are many other "minor" questions to address. The synergy between different projects, the possibility of "killing more birds with one collider", must be considered. Secondary beams may allow us to discover dark matter, or increase our understanding of the neutrino sector; hadron beams can also bring us forward in the understanding of quark-gluon plasma with heavy ions as projectiles. In addition, a whole set of ideas of "physics beyond colliders" are being developed to complement the plan. 

According to a statement by the Chair of the European Strategy Group Halina Abramowicz,

The Strategy process is about reviewing the state of particle physics by bringing together the whole community to discuss what Europe’s long-term vision should be. It is about shaping the field for the next decade and beyond. We have to start discussing what we would like the landscape of particle physics research to look like in the post-LHC era,”.

All in all, I am sure that those colleagues who have taken on the challenge to design the future path of the European Strategy are having a lot of fun and interesting discussions, while learning new things in the future development of cutting-edge technologies and how these can be used for fundamental research. And you can contribute, too: until December 18, 2018 you have a chance to submit your ideas to the group. There will follow a scientific open symposium (Granada, 13-16 May 2019), and other steps. The full process will take a long time to be completed - it will be concluded by a CERN Council meeting in May 2020.

I am happy to see that we are writing a plan for HEP research in Europe, but on the other hand let me be a bit aggravating now, at the end of this post, where nobody will read my text anyway: although the update of European strategy for HEP is a good thing, maybe (just maybe, huh) we would be okay even without one. First, I note that the high-luminosity program of the LHC is approved, so it will happen. That means the LHC will keep a generation of physicists busy for at least some 15 more years (for what purpose, that's a wholly different question). Second, it looks clear to me that the bang-for-the-buck is much better in astro-particle physics research, and we all have an opportunity to take a break at smashing things against each other to see what's inside, and rather give a refreshing look at the stars above. And third, the centrality of CERN and Europe in general in HEP is probably temporary, in the long run: I see the capital of fundamental research moving eastward with time, as opposed to the capital of economic power, which has showed an oppositely directed drift in the past few centuries. Time will tell.


Tommaso Dorigo is an experimental particle physicist who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can get a copy of the book on Amazon.