Particle physics in Europe: plans for the future

Particle physics has historically been the site of long-standing scientific challenges between the United States and Europe, particularly since the birth of the CERN laboratories in 1954. And in parallel, another challenge has kept the field alive and flourished for more than half a century: the bridge between theoretical physics and experimental physics.
The 1950s were golden years for experimental research in “high energy physics” (HEP) – where high energy meant the use of very energetic reactions to study new states of matter. A plethora of new hadrons (subatomic particles made up of the same ingredients that form nuclear matter, quarks and gluons) have been discovered as the energy of particle beams has gradually increased in various experimental facilities. The United States led the way and most of the discoveries took place 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 hazy. However, theorists quickly succeeded in making sense of the experimental contributions in spectacular ways. First with the hypothesis of non-conservation of parity in weak interactions (Lee and Yang, 1956), a prediction which was very quickly confirmed by a brilliant experiment by Wu and collaborators of the National Bureau of Standards; then with the successful description by Gell-Mann, Neeman and Zweig 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 these theoretical ideas would occupy experimenters for another four decades.

Geographically speaking, until the 1960s, arguably the most important discoveries in HEP ​​took place in the United States, a country that had invested heavily in this direction, mainly as a lasting effect of the white-hot message brought by the invention of nuclear weapons in the world. war two. In the 1970s, however, the pendulum finally began to swing towards Europe: the discovery of neutral currents, of the gluon (mediator of spin one of the strong force), then the observation of the W and Z bosons (still of spin one particles) eventually challenged American leadership, despite the extraordinarily important discovery of the J/Psi meson in 1974 at Brookhaven and SLAC, and the discovery of Upsilon at Fermilab in 1977.

The most recent history has seen a balance between the successes on both sides of the Atlantic Ocean: although the discovery of the top quark has always occurred in the United States (still at Fermilab, in 1995), it has counterbalanced by the exquisite precision of electroweak measurements by the LEP collider at CERN; and more recently the discovery of the Higgs boson (2012) confirmed that Europe is the place to be, especially if you want to discover particles of integer spin.

The above summary is very quick and crude, as it does not do justice to parallel advances in the field of neutrino physics, B-physics and other important topics of fundamental physics, which have seen a contribution growth in Asian countries (enough to mention the groundbreaking discovery in 1998 of neutrino oscillations by the Kamiokande experiment in Japan). Still, I think it’s useful, because it gives an idea of ​​how HEP has evolved, and where we are now.

On the one hand, with the mandate of testing the Standard Model and finding the Higgs boson exhausted, a good portion of all HEP experimenters, wielding the mighty LHC hammer, are looking for nails. Many of their colleagues are dispersed in smaller companies (although in some cases still quite large on an absolute scale, as for example the Belle II experiment in Japan). The theory points to too many possible paths to progress, and none of them seem quite convincing at this point. For a decade or two it seemed that the tandem of theoretical ideas and experimental verifications could continue using supersymmetry as the engine of the next big thing, but recently it has become clear that we cannot consider SUSY as a promising way to justify our existence as HEP Physicists.

On the other hand, governments have gotten the message – funding basic physics research will not buy powerful new weapons. The justification for the construction of new large facilities must come from other arguments today. It can be said that the United States had already given up its leadership at the forefront of the HEP in 1993, when its Congress decided to cancel the project to build a superconducting supercollider in Texas; subsequent strategic decisions and HEP’s funding profile in recent years have only reinforced this impression. A possible exception is China, which has shown itself to be much more willing to invest a lot of money in new large-scale projects. However, it is unclear what those plans will be.

If you consider the statements above as a good summary of the situation, it is clear that HEP is a deep crisis. What can be the strategy to move forward in a reasoned way? Where should we put our money, at a time when funds are dwindling and theory no longer gives clear indications?

These questions are at the center of the agenda of a process called “Update of the European Strategy for Particle Physics”, which involves physicists from all over the old continent to find a consensus on the experiments to be planned, the axes of research should be favored and what synergies can be built with the other main players – the United States, China and Japan in the first place – in the advancement of basic research.

Clearly one of the biggest questions is which big toy should we build next? I have witnessed many heated discussions over the past few years, with colleagues at various levels in the decision-making hierarchy, and heard a wide range of different opinions. Oddly enough, it seems that when left in the woods, the average physicist will find refuge in the ideas of physics they’ve had the most fun playing with in their past careers. It must certainly be the highest energy hadron collider we can afford to build, because great discoveries lie ahead. No, it must be a precision electron-positron machine, where we can deepen our understanding of the Higgs boson. Rather, it must be a muon collider, where, for the first time, we can open our eyes to unexpected new physics, thanks to these second-generation projectiles. Arguments will be scientifically motivated, because everyone is honestly trying to answer the question in a principled way, and yet the convergence is far from close – reasonable people can disagree, even if they are physicists.

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

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

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

All in all, I’m sure that the colleagues who have taken up the challenge of designing the future path of the European strategy are having lots of fun and interesting discussions, while learning new things about the future development of cutting-edge technologies and the way these can be used for basic research. And you too can contribute: until December 18, 2018, you have the possibility to submit your ideas to the group. This will be followed by an open scientific symposium (Granada, May 13-16, 2019), and other steps. The full process will take a long time to complete – it will be concluded with a CERN Council meeting in May 2020.

I’m glad to see that we are writing a plan for HEP research in Europe, but on the other hand, let me be a little annoying now, at the end of this post, where no one will read my text at all way: although the strategy update for HEP is a good thing, maybe (just maybe, huh) we’d be fine even without it. First of all, I note that the LHC high luminosity program is approved, it will be occur. This means that the LHC will keep a generation of physicists busy for at least another 15 years (to what end is another question). Secondly, it seems clear to me that the value for money is much better in astroparticle physics research, and we all have the opportunity to pause and smash things against each other to see what’s has inside, and rather give a refreshing look at the stars above. And thirdly, the centrality of CERN and Europe in general in HEP ​​is probably temporary, long-term: I see the capital of fundamental research moving eastward over time, as opposed to the capital of power economic, which has shown a drift in the opposite direction in recent centuries. Time will tell us.

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

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