Professor Laura Baudis (The University of Zurich) is a leading figure in the science of dark matter detection and a JPhysG Editorial Board member. She is a key member of the XENON collaboration; an experiment designed to search for the elusive particle.
Her latest work in the journal titled ‘Dark matter detection‘ looks at her perspectives on the current status of the search and an outlook for the future. Its accessible nature makes it recommended reading for anyone with an interest in the topic, and its even free to read.
We asked Laura a few questions to give you more of a background about dark matter, the development of the XENON experiment and what happens if a signal is detected.
What is dark matter and why is it important?
Dark matter is an invisible form of matter that accounts for 27 percent of all matter and energy in our Universe. Due to its gravitational attraction, it leads to the formation of our Cosmos’ largest structures, and to the formation of galaxy clusters and galaxies, including our own Milky Way. A major challenge is the fact that the nature of dark matter is unknown. We believe that it could be made of weakly interacting, elementary particles emerging from an early phase in our Universe, with negligible thermal velocities. Such particles are key to explain the observed structure formation, and to extend the Standard Model of particle physics. I find it essential that this hypothesis is testable by experiment. Dark matter particles are searched for at the LHC, by direct production, via indirect detection experiments through particle-antiparticle annihilation products, and in direct detection experiments. These look for a faint and rare scattering signal between such an invisible particle and an atomic nucleus.
Tell us about XENON. What is it, and how will it help us understand dark matter?
XENON is a programme to observe dark matter particles colliding with xenon nuclei in ultra-low background time projection chambers filled with liquid xenon. The current stage in this programme, XENON1T, operates a total of 3.5 tonnes of the heavy liquid in the Gran Sasso Underground Laboratory (LNGS) in Italy. It is contained in a double-walled cryostat, itself surrounded by a large, 10 m in diameter and 10 m high, instrumented water shield to protect the experiment from external radiation and detect cosmic muons that penetrate the 1400 m of rock shield. Such interactions would cause a too high background for the actual signal we are trying to observe: a minuscule amount of light and of charge liberated in the rare collision. XENON’s goal is to measure the rate and the energy spectrum of dark matter particle collisions and ultimately to uncover the new particle’s properties, such as its mass and its interaction strengths with normal matter.
What’s your role in the team and what are you currently working on?
XENON1T is operated by a team of about 130 physicists from twenty institutions based in ten countries. Each institution is responsible or co-responsible for one or several crucial sub-systems of the experiment. At the University of Zurich, we are mostly involved with the inner detector, the time projection chamber. We have participated in its design, in the construction and assembly of several parts, including the so-called field cage, both in our laboratory, and underground at LNGS. We have screened for radioactivity and tested a significant part of the 3-inch photosensors in liquid xenon, developed and built their electronic readout, as well as their calibration system based on LEDs and optical fibres. These photosensors, 248 in total, and arranged in two arrays, will detect the small number of photons that will be produced in the rare collisions of particles with xenon atomic nuclei. We are currently working on the first in-situ calibration of the photosensors in the detector during its on-going commissioning phase. We are also working on Monte Carlo simulations, and on the 3-D vertex reconstruction software, and are of course preparing for the imminent data analysis.
When will the experiment go ‘live’, and how long until we start to see results?
At the moment, the water shield is being filled with ultra-pure water, we have already reached the top of the cryostat. Next, we will start filling the cryostat with xenon. The xenon must be continuously purified against electronegative impurities, to reach an optimum light and charge signal. It must also be purified for the radioactive 85Kr, of anthropogenic origin and present in small traces in the xenon gas. Finally the detector must be well characterised with various internal and external calibration sources, before the actual science run can start. I thus hope that we can go ‘live’ this spring, and initial results would soon follow. The design sensitivity, a factor of hundred improvement compared to the previous phase, XENON100, will be reached after two years of dark matter data.
What would it mean for physics if XENON discovered dark matter?
First, if XENON1T saw a signal that is compatible with dark matter, and incompatible with background noise, such a signal would require confirmation by an independent search. In the best case, another direct detection experiment, and in addition an unexpected signal for new physics at the LHC. In the best of all worlds, it would be accompanied by a dark matter annihilation signal that may for instance reveal itself in gamma rays observed by the Fermi satellite, or in a high-energy neutrino signal from the Sun seen by IceCube at the South Pole. In any event, considering what we already know about the strength of dark matter interactions from its non-observation so far, XENON1T would only see a handful of events. We would then urgently require confirmation with higher statistics, and we are already working on its upgrade, XENONnT. It will use about 7 tonnes of liquid xenon in a larger inner detector, while reusing most of the infrastructure that we have installed in the underground lab. Finally we would need at least a few hundred of events to also learn about the properties of the dark matter particles. A few years ago we started the R&D for a 50 tonne liquid xenon experiment, that we call DARWIN. While DARWIN would also be operated at the Gran Sasso Laboratory, it will most likely require a larger water shield.
And finally, what’s the best thing about being a physicist?
The fact that one is constantly faced with one’s own ignorance? Being a physicist allows me to combine my genuine passion for science with my every day work. Of course, passion and scientific curiosity is not sufficient. As in other sciences, pursuing a career in physics also requires hard work, creativity, flexibility and the ability to learn from one’s failures. Working in experimental physics also calls for the ability to function in a large, international team, to compromise. One of the largest rewards is that political or ideological barriers hardly play any role, we are all united in our quest to uncover the dark matter particle, to be there first.
Find out more about dark matter
Dark matter detection Laura Baudis 2016 J. Phys. G: Nucl. Part. Phys. 43 044001
XENON1T Collaboration website
Dark matter direct-detection experiments by Teresa Marrodán Undagoitia and Ludwig Rauch
On the hunt for WIMPs: a JPhys+ image of the week
Is there dark matter in our neighbourhood?
More on the search for dark matter over at Ars Technica, including discussion on XENON.
This work is licensed under a Creative Commons Attribution 3.0 Unported License
Images supplied by Laura Baudis and used with permission.
This work was commissioned by JPhysG as part of its 40th anniversary celebrations. Read more interviews and perspectives over at IOPscience.