The Large Hadron Collider LHC, CERN´s largest particle accelerator, restarted operating in early June after an almost two year shutdown. It is delivering new data that will enable physicists around the world to gain completely new insights into the composition of matter. Tobias Golling (39) is one of the participating scientists. His academic career took him to Freiburg, Heidelberg, Bonn and the Chicago Fermilab. Since fall 2014 he has been Associate Professor at the University of Geneva as well as researcher at the LHC´s ATLAS experiment. According to Golling the data that the LHC will deliver in the coming months and years could take our understanding of modern physics a big step forward.
Tobias Golling is only one of many thousand physicists who played a part in finding the Higgs boson in July 2012. However he did make a particularly visible contribution by working on the pixel-detector of the ATLAS-experiment. This pixel-detector constitutes the inner core of a huge measuring instrument that helps to capture particle traces. These traces emerge whenever two protons collide inside the LHC with high energy and decay into numerous subatomic particles, out of which one can reconstruct the Higgs boson.
First class flight for the pixel detector
Golling developed an important component of the pixel detector at the Berkeley Laboratory in California, where he had worked as a Postdoc since 2005. “The pixel detector was part of my luggage when I moved from Berkeley to Geneva to work at CERN”, Tobias Golling says. “Literally. We had built the detector in Berkeley, wrapped and packed it with great care for its trip to Geneva. It was attached to springs to prevent any damage during the trip. I flew with the detector from San Francisco to New York and then on to Geneva. The detector was small enough to fit onto the seat next to me on the plane”, the physicist remembers. “It even had its own boarding pass in the name of Bob. Like every passenger it had to go through security before boarding the plane”, Golling recounts with a smile.
After its arrival in Geneva the detector component was assembled with two other parts to the complete pixel detector. Since then it has formed the core piece of the ATLAS experiment at the LHC. ATLAS and CMS are two of the four major LHC-experiments that have been analyzing data from particle collisions since the LHC started operating in 2010. On the basis of this data the Higgs boson was discovered in 2010.
Without any doubt the discovery of the Higgs boson was the most spectacular scientific success during the LHC’s first term of operation between 2010 and 2012. In the beginning of 2013 the particle accelerator’s operation was put on hold for a thorough maintenance and upgrade. In a press release CERN called the revision a “Hercules task”. Several ten thousand electrical connections between magnets had to be tested and replaced, protection systems for magnets were installed. Engineers improved the cooling system for the magnets as well as the vacuum for the acceleration system and its electronics. Now the proton beams are travelling with a new distance of only 25 nanoseconds instead of 50. Thereby the number of proton-proton collision per seconds is twice as high as before,
Experts did not only maintain the LHC´s 27-kilometer underground ring, they also put in place many small and major improvements at the four LHC experiments (ATLAS, CMS, LHCb, ALICE). The pixel detector of the ATLAS experiment received an additional layer, as Tobias Golling recounts. A part of this crucial process took place in a cleanroom at the University of Geneva. “The new layer is only three centimeters away from the point of collision instead of five centimeters like before. As a result we can measure with even higher precision the particles that emerge from the proton collision, Golling explains. He taught at the prestigious Yale University in Connecticut (USA) before he became professor in Geneva in September. The University of Geneva also helped with the upgrade of the trigger: This filter has the task to filter relevant information from the mass of data that the LHC produces and thereby cuts the mass of information down to an amount that can be handled by the available computing capacities.
On the way to new physics
After being on hold for two years and a test run that lasted several months the LHC took up operation again on June 3rd 2015 and is back to delivering data. As soon as it reaches its maximum capacity the LHC will provide one billion proton-proton collisions – per second. Not only the number of collisions (the so called luminosity) has increased significantly during season 2 at the LHC, so has the level of energy. The collision energy has been almost doubled to 13 teraelectronvolts /TeV compared to the first phase of operation. In the microscopic world this amounts to a gigantic level of energy. “Einstein´s equation e=mc2 explains how energy and mass are connected”, says Tobias Golling: “The higher the energy of the protons inside the accelerator, the more massive new particles we can produce in our collisions. 13 TeV should help us to produce particles that we need in order to solve the still unanswered questions. In other words: A higher energy level enables us to detect smaller structures of the matter.”
With the increase of energy and luminosity the LHC is taking a big leap forward, that probably cannot be repeated in later years, says Tobias Golling. “We have three very exciting years ahead of us”, the particle physicist declares adding a metaphor: “We open a window and now are able to see a country that we never had access to.” Golling mentions the discovery of the Top-Quark and the Higgs boson (2012). “The discovery of these particles was in a certain way easy – acknowledging all scientific excellence. Scientists looked for a new particle whose mass essentially had already been known to some extend from experimental measurements and theoretical predictions. At this point we have found all particles that we expected to be out there. If we find something new in the future, it will open up a completely new dimension.”
Dark matter and supersymmetric particles
„It is time for new physics!” CERN´s director general Rolf Heuer said in early June, summarizing the high expectations towards LHC´s second run from 2015 to 2018. What does this new physics imply, which is already coming up on the horizon behind the standard model. The first question concerns dark matter. This still unknown form of matter is believed to fill the universe. It helps scientists to align the perceived movements of the stars with the laws of gravity. “If dark matter does indeed exist we should be able to create it”, Golling says. Since October 2013 he is in lead of the search for exotic new physics inside the ATLAS team, including in particular the search for dark matter and extra dimensions (see below). “The chances are high that we find such dark matter particles or at least hints of such particles during the LHC´s second run.”
Furthermore, the concept of supersymmetry reaches beyond the standard model. It states that each known elementary particle has a supersymmetric partner. According to this concept three of the four known forces, electromagnetism, strong and weak interaction, could be explained with a primary interaction (Urwechselwirkung). This force also lies at the heart of the “theory of everything” which would again include the hitherto unexplained force of gravity. The concept of supersymmetry could also help to understand dark matter. Furthermore it could be relevant in the search for an answer to what physicists call the “hierarchy problem”. Supersymmetry could provide the long sought explanation for this problem. “Supersymmetric particles should become measurable at the LHC with energy levels of 13, 14 TeV”, says Tobias Golling. “It is possible that they remain invisible even in such high energies, but this would be unnatural.” In his view it would be quite a surprise if physicists were unable to find an explanation for the “hierarchy problem” in the course of the LHC´s second run.
Hidden forces in unknown dimensions
Apart from dark matter and the concept of supersymmetry exists a third field in which CERN physicists hope to advance towards a new physics. It is known as “extra dimensions”. It may sound like magic to hear physicists discuss the existence of additional dimensions to our known three dimensions. However this concept could explain physics phenomena in an elegant way. Like: Why is the force of gravity so much weaker in the microcosm than the other three known fundamental forces? Can this be explained by the fact that part of the gravitational force escapes into the fourth dimension and as a result is not conceivable for us humans – who can only experience a three dimensional space?
The existence of extra dimensions could therefore be a key to our understanding of matter. The according theory would also solve the earlier mentioned hierarchy problem, Golling underlines. In order to find tell-tale signs of extra dimensions particle physicists have to prove the existence of the graviton – this still undetected elementary particle that mediates the force of gravitation (as electromagnetism is mediated by the photon, weak interaction by the W- and Z-bosons and strong interaction by the gluons). „A possible sign of extra dimensions would be a collision in which a particle disappeared, perhaps indicating a graviton leaving our visible universe and entering extra dimensions“, says Tobias Golling. „It is very well possible that we see evidence of the graviton when we use an energy of 13 or 14 TeV.“
The Higgs boson stays on the agenda
Dark matter, supersymmetry, extra dimensions. “Anything we find in this process would outshine the discovery of the Higgs boson”, says Tobias Golling. But in any case the Higgs boson will keep its place on the agenda of the CERN physicists. The continuing operation of the LHC will provide an immense amount of new data allowing more precise measurements of the Higgs boson and its properties. Compared to what is waiting to be discovered in new dimension this task may appear to be a trivial. However, as long as new physics is still a dream of the future the Higgs boson will keep its fascination for scientists.
Benedikt Vogel (published July 13th 2015)