The CMS detector in the tunnel of the LHC particle accelerator was used to find, among other things, the Higgs particle.
Photo: AFP/Valentin Flauraud
Scientists at the European Nuclear Research Center Cern (Conseil Européen pour la Recherche Nucléaire) near Geneva have already made a number of groundbreaking discoveries. A little over ten years ago, the famous Higgs boson was found here – often called the “God particle” in the media because it gives other particles their mass. And the search for the basic building blocks of the matter in which Cern specializes has also driven technological innovations that benefit all of humanity.
Cern was born out of necessity after the Second World War, when some leading scientists such as Niels Bohr and Louis de Broglie thought about a common European nuclear research center. This was intended to overcome the horror and divisions of war and nationalism and at the same time pool Europeans’ research opportunities in fundamental physics – also in order to catch up with the USA.
CERN was finally officially founded in 1954, initially with twelve member states. There are now 23, ten other countries are associated and around 17,000 employees from 110 countries work at or for the research center. Since these beginnings, CERN has pushed itself step by step to the forefront of particle physics with persistent work and is now home to the most powerful particle accelerator in the world: the Large Hadron Collider, or LHC for short.
Confirmation of the standard model
Soon after Cern was founded, some important measurements were made there that confirmed and advanced the theory of the so-called Standard Model of particle physics. This theory summarizes three of the four fundamental forces of nature. The strong nuclear force describes the cohesion of the quarks and gluons in atomic nuclei. The weak nuclear force deals with transformation processes of quarks and light particles such as electrons and muons. The electromagnetic force, in turn, explains all electrical and magnetic phenomena.
Only the fourth basic physical force, gravity, is not covered by the Standard Model but is described by Einstein’s theory of relativity. It is a great hope of many scientists at CERN to one day be able to integrate gravity into an extended standard model – even if this has not yet proven to be possible.
In order to get to grips with all these natural forces, particle accelerators are needed, with which tiny particles are brought to enormous energies and then collided with other particles. At the end of the 1950s, CERN briefly had the world’s most powerful accelerator, the Proton Synchrotron. Overhauled several times, this facility is still in operation today and feeds high-energy protons into the now much larger successor accelerators. An antideuteron was produced for the first time at the Proton Synchrotron in the 1960s – i.e. the atomic nucleus of the “heavy antihydrogen”.
In the 1970s, the “Super Proton Synchrotron” went into operation and was able to detect several fundamental particles of the Standard Model: the so-called W and Z bosons. These are responsible for the weak nuclear power and are involved, among other things, in all radioactive processes in which beta radiation and neutrinos are released.
Cern then carried out the largest construction project in its history with the decision to dig a 27 kilometer long, circular tunnel around 100 meters underground in order to install an even more powerful particle accelerator: the Large Electron Positron Collider, or LEP for short. This facility ran from 1989 and was intended to examine the W and Z bosons in detail and thereby determine the further development of particle physics. Together with Fermilab near Chicago and the particle accelerator there called Tevatron, CERN was now the leading research center for particle physics in the world.
Until I have LEP
After a long technical break in 1996, the system suddenly stopped working. Thanks to the good beam diagnostics, it was possible to narrow down the section in which the problem occurred. The jet pipe was opened – all time-consuming procedures in such high-vacuum systems – and lo and behold, the culprit was found: a beer bottle was stuck in the jet pipe! The neighboring sections were also examined and another beer bottle was found. To this day it is not clear whether someone pulled a rude prank here, whether it was deliberate sabotage by a dissatisfied worker or whether someone simply forgot the dig there after a somewhat damp workday. One of the many unsolved mysteries of particle physics…of the
Precision measurements at the LEP revealed, among other things, that there can only be three light types of neutrinos – an important indication of the predictive power of the Standard Model, in which there are exactly three types of neutrinos.
The LEP ran until 2000 and then had to make way for its successor, the Large Hadron Collider (LHC). Both systems did not fit together in the tunnel tube. In contrast to the LEP, the LHC does not accelerate electrons and positrons, but rather protons and achieves significantly higher energies.
Great moment in particle physics
The LHC has been running since 2008 and has not only found numerous exotic particles such as pentaquarks and the like, but also discovered the mysterious Higgs boson. The announcement of this result in 2012 can be seen as a great moment in particle physics: a confirmation of work to which thousands of scientists from several generations around the world have dedicated their life’s work.
This particle, which gives other elementary particles their mass, had been searched for decades in vain – not only at Cern, but also at LEP, Fermilab and elsewhere. The two large general-purpose detectors at the LHC, Atlas and CMS, were both ultimately able to find the Higgs in their data.
The so-called quark-gluon plasma was also measured at Cern. This is an extremely hot and dense state of matter, 100,000 times hotter than the center of the Sun and twenty times denser than atomic nuclei. A fraction of a second after the Big Bang, the entire universe was filled with this state of matter before it expanded and cooled. The most powerful particle accelerators can now routinely generate this particle plasma by firing heavy atomic nuclei at each other at the highest levels of energy.
What does the future hold?
However, with the discovery of the Higgs over ten years ago, the search for the building blocks of the Standard Model was completed: all the elementary particles that the theorists had predicted had now also been proven experimentally. And now? Is this already the end of particle physics? Far from it: there are numerous open questions and unsolved mysteries. Among other things, no one knows what dark matter is all about, which makes up significantly more mass in the cosmos than our usual matter.
So far, in all possible experiments around the world, there has not been the slightest hint of what dark matter might consist of. Many particle physicists therefore have high hopes for the next expansion stage of the LHC, the so-called High-Luminosity LHC. This will be installed as an upgrade to the LHC during the next major renovation break and will begin operations in 2029. The system will then generate significantly more collisions with only a slightly increased maximum energy.
The researchers hope that this will provide significantly more collision data and thus better statistical evaluations, not least in order to put the Higgs’ properties through their paces. In particular, the question arises as to whether the Higgs really behaves exactly as predicted by the Standard Model of particle physics. Maybe there are anomalies here or there? Is there possibly more than one Higgs boson? An extension of the Standard Model, the so-called supersymmetry, attempts to explain dark matter using heavy, previously undiscovered sibling particles of normal matter. According to her, the Higgs boson could have four sibling particles, some of which have similar properties to the Higgs boson.
Currently the LHC produces about one Higgs per second. But only one in 1000 Higgs bosons can be easily detected by the detectors. That’s why particle physicists want more Higgs bosons. It will be interesting to see what results the High-Luminosity LHC will produce, which is scheduled to run until the 2040s. Will he at least solve one or two partial puzzles about dark matter?
The question also arises as to whether an even larger particle accelerator should be built at CERN as a successor to the LHC – and who should pay for it all. And even if such a giant facility is built elsewhere: the scientists at CERN will be involved and will continue to advance particle physics and the associated technologies.
It is a great hope of many scientists to one day be able to integrate gravity into an extended standard model.
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