The experimental detection of dark matter is one of the great challenges of current fundamental research in physics. This year’s prize of the Swiss Institute of Particle Physics (CHIPP) is awarded to the physicist Dr. Johanna Gramling for her latest contributions to the search for this mysterious component of matter.
"I am very pleased to get this award and I see it as an appreciation of my work performed at CERN in recent years," says Johanna Gramling. The 29-year-old physicist got the CHIPP prize on the occasion of today's in Geneva. The prize is awarded every year to a young researcher for an outstanding scientific contribution. It is the most prestigious award in the field of particle physics in Switzerland.
Johanna Gramling is awarded the CHIPP Prize for her doctorate, which she has completed end of 2016 at the University of Geneva. She was supervised by Prof. Xin Wu, a specialist in dark matter. The scientific work is the outcome of Gramling's multi-year research performed with the ATLAS detector. ATLAS is one of the four large experiments operated at the CERN particle accelerator LHC. The jury of the CHIPP Prize honors the scientist "for her work on dark matter searches with the ATLAS detector and her role in establishing the use of simplified models for their theoretical interpretation," as the laudatio states.
The great questions of mankind
Johanna Gramling was born in 1988 and grew up as the daughter of a teacher and an electrical engineer near Darmstadt (Germany). During high school she discovered her enthusiasm for physics and philosophy: "I found these subjects very exciting, especially in this combination, since physics and philosophy are both dealing with the great questions of mankind, much more than other scientific disciplines," Gramling says. As a high school student, the German was not afraid of complex questions. In a two-week summer course, she worked on interpretations of quantum mechanics, a physical field that challenges our intuitive understanding of the world.
Physics and philosophy were also Gramling’s fields of study at the University of Heidelberg. After five years she completed her studies in 2011 with the diploma. At that time, she had already discovered CERN as the world's leading laboratory for particle physics research while doing a research internship during her studies. Gramling did her diploma thesis with ALICE, also one of the large experiments carried out by scientists at the CERN particle accelerator LHC.
Search for an invisible kind of matter
From spring 2012, Johanna Gramling devoted herself to her doctoral thesis. For this she moved from Heidelberg to Geneva and joined the ATLAS experiment at CERN. In the 27 km long, circular tunnel of the Large Hadron Collider (LHC), protons are accelerated to very high energies and then brought to collision. The physicists of the ATLAS experiment investigate which particles are produced during these proton collisions. By analyzing these particles, they get valuable information on the structure of matter. In this way, the Higgs particle was discovered five years ago.
Johanna Gramling would also like to find new particles together with the ATLAS team: those that make up the so-called dark matter. These particles are named WIMPs - for: Weakly Interacting Massive Particles. Although no one has ever observed dark matter, there is a consensus among physicists that our universe is full of it. Dark matter is supposed to be five times more abundant than the matter we know – observed by eye or with microscopes or telescopes. Many astrophysical phenomena can only be explained by assuming the existence of dark matter. However, the existence of this 'invisible' matter still lacks unambiguous evidence.
There have been a lot of attempts to establish this proof since the Swiss astronomer Fritz Zwicky has postulated the existence of dark matter in the early 1930s. Over the last years, large underground detectors have been built in the hope of directly observing collisions of WIMPs with the known matter in order to prove the existence of dark matter. Physicists also attempted to measure dark matter ‚indirectly’: they directed telescopes to areas of presumably high density of dark matter, like the center of our galaxy or our sun. This approach is based on the assumption that the collision of two WIMPs would result in the production of known matter which could then be detected. However, the detection of dark matter has not yet been achieved either directly or indirectly.
Two Signatures of Dark Matter
Johanna Gramling and many research colleagues take a third way to track down dark matter: they want to use the particle accelerator LHC to produce WIMPs. Under the assumption that WIMPs could be produced during the collision of two particles of known matter (concretely: protons), dark matter can possibly be detected at the LHC. "We are looking for traces of particles produced in the collision of two protons, consistent with the production of particles known to us as well as dark matter," says Johanna Gramling. Theoretical considerations suggest that dark matter can for example be observed together with a particle jet, or – more difficult to prove – together with a pair of top quarks. The top quark is the heaviest of the six known quarks.
If scientists measure events with one of these two signatures in detail in the traces of proton-proton collisions, the detection of dark matter could succeed. This is exactly what Johanna Gramling aimed for in her doctoral thesis. For this task intellectual brilliancy was required: The particle physicist had to select the candidates for one of the two signatures mentioned from the 700 million proton-proton collisions per second occuring in the LHC. In the second step, she had to check whether the selected events were so frequent that they could be interpreted as statistically relevant signals - and thus hint at the existence of dark matter.
For her work the scientist used the data of the proton-proton collisions the LHC had produced in 2012 and later in 2015/16. To give an impression of the complexity of this task: in the search for candidates for the top quark signature, Johanna Gramling has examined a million times a billion collision events and identified 35 candidates. For a while, it looked as if she and her colleagues had found an experimental evidence for the existence of dark matter. As further data were included and the statistical significance of the analysis increased, the disillusionment followed: the supposed discovery of dark matter turned out to be a fallacy. Scientists speak of a 'statistical fluctuation'.
More data, complex signatures
In the past years Johanna Gramling has shown: dark matter can be found neither in combination with a jet nor with top quarks in the LHC data of 2012 and 2015/16. Does that mean that dark matter will never be seen via these two signatures? "Not at all," says Johanna Gramling, "we can only say that we cannot observe dark matter with the analysed data. Perhaps the signals hoped for are very, very rare. If this is the case, we can still find them via one of these signatures if we can use a larger number of LHC collision data for our analysis in the future. "
Anyone who thinks the negative intermediate result would discourage Johanna Gramling will be wrong. "My work is extraordinarily exciting, I want continue," says the physicist, who plans to include new concepts of dark matter in her future work. "It is also possible that dark matter does not just consist of WIMPs, but of several dark matter particles leading to more complex signatures than the ones we have studied so far. We have a lot of fascinating work ahead of us. "
The hunt for the matter, which no one has seen so far, continues. Johanna Gramling is currently working as a postdoctoral researcher at the University of California in Irvine. She belongs to the subgroup, which works directly at CERN. "This position allows me to pursue those questions that interest me most," says Gramling. She and her colleagues can rely on further results, which have also emerged from Gramling's dissertation, in the future search for the dark matter. There, Johanna Gramling has studied simplified theory models that improve the chances of success in the search for dark matter. These are the 'simplified models' the jury of the CHIPP Prize has mentioned in its laudatio as an outstanding scientific achievement.
Author: Benedikt Vogel
 Joint Annual Meeting of the Swiss Physical Society and Austrian Physical Society: https://indico.cern.ch/event/611331/overview
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Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Michele Weber
University of Bern
Laboratory for High Energy Physics LHEP