The great disillusionment – So far nothing at the LHC except the standard model

The joy among scientists was immense when the nuclear research center CERN announced on July 4, 2012 that its physicists had discovered the signature of a new particle: the long-awaited Higgs boson. For once, the international newspapers were full of reports on basic research in elementary particle physics. Finally, her so-called „standard model“ had a solid foundation and was complete. At the same time, this discovery marked a great moment in theoretical physics. Because more than 50 years ago, the existence of this particle had been predicted as a consequence of an abstract theoretical framework that physicists call „spontaneous symmetry breaking“. Only this process should give the elementary particles their mass in an initially mass-free theory. „Without the Higgs boson, there is no mass in the universe“ is how the theoretical findings of the particle’s namesake, Peter Higgs, and two colleagues from the early 1960s can be summarized.

Now the physicists have learned a new unit of measure in their search for the fundamental structures over the years. It is arguably the only entity they share with Wall Street investment bankers: the US dollar. It gives a direct measure of the probability that a particular experiment can be carried out at all. The final evidence that there is mass in the universe seemed to have been worth more than $ 6 billion to the physicists (you can tell they are people who want to know exactly: everyday experience of gravity is enough not out of them). This was the cost of the LHC (Large Hadrom Collider) particle accelerator needed to discover the Higgs particle. A lot of money for a single particle, one would think, especially since its existence was actually to be assumed a priori (its discovery was the least surprising scientific sensation of the past decades).

In fact, the physicists‘ expectations of the LHC were a lot higher than (only) confirming the standard model. One could even say that the discovery of the Higgs particle should only be a first resting place on the way to a completely new basic theory of physics. In fact, the physicists‘ hopes for the LHC are enormous: a completely new symmetry (the name of which tells us a lot about the psychology of physicists): „supersymmetry“; the explanation of the ominous „dark matter“ and the even more ominous „dark energy“; solving the matter-antimatter puzzle (why is there so much of the former and so little of the latter in the universe?); the description of such complex structures as quark-gluon plasmas, in which conditions like the big bang should prevail for a short time; or even the evidence of such exotic things as microscopic (actually far beyond „femto-scopic“) rolled up additional spatial dimensions. The list of promises was long. Because it really isn’t that easy to get politicians to spit out so much money. Something similar must spring out like the explanation for the origin of the universe.

And this is exactly where the particle physicists‘ current problem lies: Up to now, the LHC has given them nothing, absolutely nothing, apart from the Higgs particle and thus the final confirmation of the standard theory. Now the almost obsessive longing of particle physicists for signs of failure of their best theory is not so easy to understand for non-physicists. Shouldn’t the scientists be happy that their mathematically so complex models seem to give such accurate images of nature? Her attitude is something like Newton hoping his theory was wrong. In order to understand this setting in more detail, one has to know a special property of the standard model that fundamentally differentiates it from Newton’s theory. Because in addition to its own correctness and enormous predictive power for all experiments carried out so far, this theory predicts by itself that it only has to be an approximation of an even more comprehensive theory. In other words, it tells us that it needs to collapse from certain areas of energy (whereas Newton believed that his theory should explain all structures in the universe once and for all). The standard model of elementary particle physics thus intrinsically contains the limitation of its own correctness. (Strictly speaking, Einstein’s general theory of relativity also describes its own limits by allowing the possibility of black holes). Above a certain energy, new physics must come into play purely for reasons inherent in theory.

In a broader sense, these theoretical reasons are what the theoretical physicists call the „hierarchy problem“: The mass of the Higgs particle – and thus that of all particles – should actually grow immeasurably through necessary (quantum field theoretical) interactions with other particles. Only a super-precise fine-tuning of all terms contributing to the Higgs mass can prevent this. However, this appears to the theoretical physicists as unnatural and downright ugly. It violates their basic aesthetic needs. The standard model also has other serious deficits. So it cannot tell us what dark matter and dark energy are made of, the existence of which comes from cosmological measurements that attribute up to 95 percent of the mass of our universe to these two unknown entities.

Unfortunately, the physicists do not know at which energy exactly the standard model loses its validity. This is exactly what the LHC was built for (and not primarily to find the Higgs particle). And now it could look as if the scope of the standard model extends much further than the physicists had hoped for.

There is no shortage of speculations about what the new physics could look like beyond the standard model. The most important and mathematically „most beautiful“ and „most elegant“ variant suggests to send each known particle a („supersymmetric“) partner particle with opposite spin. It thus predicts a whole series of new so-called “SUSY particles”, the energy scales and masses of which physicists are not yet familiar. These particles provide an elegant solution to the hierarchy problem: Assuming that there is a super symmetric partner for each particle of the standard model, their contributions to the mass of the Higgs particle are mutually exactly balanced, which is how this is done in a very natural way the measured value is brought.

Therefore, last year (end of 2015) the excitement among particle physicists was again very high when the LHC data concentrated indications of a new particle with a mass of 750 gigaelectronvolts (approximately six times the Higgs particle mass). Unfortunately, until August 2016 it turned out that these signals represented statistical fluctuations. Almost simultaneously (mid-July 2016), another working group announced that an almost two-year search for possible components of the dark matter had not brought the hoped-for success. The reports from the ice cube experiment at the South Pole were equally sobering: there doesn’t seem to be a fourth besides the three known neutrino varieties. So far, 2016 has not been a good year for particle physics (as of September).

The latest results at CERN raise doubts as to whether supersymmetry can still naturally balance the mass contributions to the Higgs particle and thus solve the hierarchy problem. Because the masses of the SUSY particles should meanwhile be quite large so that their previous non-observation could be explained in particle accelerators. But if the mass is too large, the contributions of normal and supersymmetric particles to the Higgs mass can no longer be completely equalized. In addition, if the supersymmetric particles are significantly heavier than they can be measured in the LHC, they are no longer suitable as candidates for dark matter. Because the resulting density of dark matter would simply be too great. So if no supersymmetric particles can be measured at CERN in the coming years, the particle physicists have a real problem. And you should know: The probability of building even larger particle accelerators is in inverse proportion to a new unit of measurement in physics: the US dollar.

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