The excitement among physicists was enormous, when on July 4th 2012 the CERN announced that a new particle had been discovered: the Higgs boson. For once the international newspapers were full of reports on fundamental research in particle physics. Finally the so called “standard model” had received a solid foundation and was thus complete. At […]
The excitement among physicists was enormous, when on July 4th 2012 the CERN announced that a new particle had been discovered: the Higgs boson. For once the international newspapers were full of reports on fundamental research in particle physics. Finally the so called “standard model” had received a solid foundation and was thus complete. At the same time the discovery represented a triumph for theoretical physics, as the existence of the Higgs particle had been predicted more than 50 years earlier as a consequence of a highly abstract theoretical framework, which the physicists call “spontaneous symmetry breaking”. This process was suggested to give masses to the elementary particles within a theory that was inherently massless at first. “Without Higgs boson no matter in the Universe”, one can summarize the theoretical findings of Peter Higgs, who gave the particle its name, and his two colleagues from the early 1960s.
In their search for the fundamental structures physicists have over the years stumbled across a new measurement unit. It is probably the only unit, they share with Wall Street investment bankers: the US dollar. It provides a quantification for the probability that a particular experiment may be carried out at all. The final proof that there is mass in the universe was apparently worth more than six billion dollars to the physicists (you can tell that they are people who want to know things very precisely: the everyday experience of gravity does not suffice for them). So much did the particle accelerator LHC (Large Hadrom Collider), which was necessary for the discovery of the Higgs boson, cost. A lot of money for a single particle, one would think, especially as its existence had a priori already been out of question (its discovery was then probably the least surprising scientific sensation of recent decades).
In fact the expectations of physicists from the LHC were a quite bit higher than to (only) confirm the standard model. One could even say that the discovery of the Higgs particle should have merely been a first rest stop on the way to a completely new fundamental theory in physics. Physicists had hoped for exciting stuff to be discovered within the LHC: an entirely new symmetry (whose name tells us quite a bit about the psychology of physicists): the “supersymmetry”; an explanation for the ominous “dark matter” and the even more ominous “dark energy”; the elucidation of the matter-antimatter mystery (why is there apparently so much of the former and so relatively little of the latter in the universe?); the description of such complex structures as quark-gluon plasmas, in which for a short time we find conditions as prevalent during the Big Bang; or even the evidence of such exotic things as microscopically (in fact far beyond “femto-scopically) coiled additional spatial dimensions. The list of promises was long. It is really not that simple go get politicians to spit out such an amount of money. This requires something as big as the explanation for the origin of the universe.
And here lies the current problem of particle physicists: Except for the Higgs particle and thus the final confirmation of the standard theory the LHC has so far given them … nothing, absolutely nothing. Now, for non-physicists the almost obsessive desire of particle physicists for signs of their best theory to fail is not quite that easy to understand. Should they not be pleased that their mathematically so highly complex models seems to provide such a precise map to the natural world? Their attitude is as if Newton had hoped for his theory to be wrong. In order to understand this enigma one should know about a very special property of the standard model, which is fundamentally different from Newton’s theory. In addition to its accuracy and enormous predictive power for all experiments performed so far this theory inherently predict, that itself can only be an approximation of a more comprehensive theory. The theory itself thus tells us that it must fall apart at a certain energy (whereas Newton believed that his theory should explain – once and for all – all structures in the universe). The standard model of elementary particle physics thus entails intrinsically the limitations of its accuracy (strictly speaking, also Einstein’s General Theory of Relativity describes its own limits by allowing the possibility for black holes). As of a certain energy, on pure theoretical grounds new physics must come into play.
These theoretical reasons are most broadly about what theoretical physicists call the “hierarchy problem”: Due to necessary (quantum field theoretical) interactions with other particles the mass of the Higgs particle – and thus the one of all the particles – should grow beyond any measurable limits. Only a super-accurate fine-tuning of all contributing to the Higgs mass can prevent this from happening. To theoretical physicists this appears unnatural (and downright ugly). It violates their fundamental belief in aesthetics, one can say. In addition, the standard model has further major shortcomings. For example it cannot tell us anything about dark matter and dark energy, the existence of which physicists must conclude from cosmological measurements which assign up to 95 percent of the mass of our universe to those two unknown entities.
Unfortunately the physicists do not know, at which exact energy level the standard model begins to fail. Just to figure this out the LHC was built (and, as mentioned, not primarily to find the Higgs particle). And now it looks as if the scope of the standard model extends much further than the physicists had hoped.
There is no shortage on speculations about how the new physics beyond the standard model might look like. The most important and mathematically most “beautiful” and “elegant” version proposes that any known particles comes with a (“supersymmetric”) partner particles with opposite spin. It thus predicts a whole set of new, so called “SUSY-particles” whose energy scales or masses are not yet know to the physicist. These particles provide an elegant solution to the hierarchy problem: Assuming that for every particle of the Standard Model there exists a supersymmetric partner, their overall contributions to the Higgs particle’s mass cancel out precisely, and it thus quite naturally assumes its measured value.
Therefore particle physicists got once again very excited last year (late 2015), as evidence of a new particle with a mass of 750 GeVs (about six times the Higgs particle’s mass) appeared in the LHC data. Unfortunately in August 2016 it had become clear that these signals constitute statistical fluctuations. Almost simultaneously (in mid-July 2016) another research group announced that a nearly two-year search for possible constituents of dark matter had not produced the success scientists had hoped-for. Equally sobering were the messages from Ice Cube experiment at the South Pole: There seems to be no fourth addition to the three known types of neutrinos. So far 2016 (as of September) is not a good year for particle physics.
The most recent results at CERN raise doubts whether supersymmetry can still create the natural balance of mass contributions to the Higgs particle and thus solve the hierarchy problem. The mass of the SUSY-particles must meanwhile be quite large in order to explain that they have not been observed in particle accelerators so far. And for larger masses the contributions of ordinary and supersymmetric particles to the Higgs mass no longer completely balance out. In addition, if the supersymmetric particles are significantly heavier than what can be measured in the LHC, they can no longer be candidates for the dark matter. The resulting density of dark matter would simply be too great. Therefore, if no supersymmetric particles are detected at CERN in the coming years, particle physicists face a real problem. And they should be reminded: The likelihood of building even larger particle accelerators is inversely proportional to another fundamental measure in physics: the US Dollar.
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