Atomic Experiments for Dark Matter and Gravity Exploration
A workshop on “Atomic Experiments for Dark Matter and Gravity Exploration” was held at CERN on 22nd and 23rd of July, More than 130 participants met at CERN to review the landscape of cold atom experiments for exploring dark sector physics, gravitational effects and new fundamental interactions.
The workshop offered a space to the cold atom community to meet with particle physicists and the gravitational-wave community, and review ground and space applications. Over two-days, participants discussed the physics opportunities and connections between the fundamental research areas of particle physics and gravitational physics using cold atoms as ultra-precise clocks and used as interferometers.
Cold atoms can be used as very precise clocks for searching ultra-light dark matter particles, particularly scalar particles but also pseudo-scalar axions, or more generally Axion Like Particles (ALPs), and vector dark matter, thus complementing other searches at collider and fixed-target experiments. Moreover, cold atom interferometry allows to search for mid-frequency gravitational waves thus bridging the range covered by LIGO, Virgo and the one of the future LISA mission. Searching for gravitational waves at this intermediate-frequency band should reveal signals from merging of massive black holes (between 100 and 100.000 solar masses) that power active galactic nuclei, and could further shed light on a first-order phase transition or cosmic strings that may have happened in the early universe.
The workshop shared information about long-baseline terrestrial cold-atom experiments that are already funded and under construction, such as MAGIS in the US, MIGA in France and ZAIGA in China, as well as ideas for future terrestrial experiments such as MAGIA-advanced in Italy, AION in the UK and ELGAR in France. Dark Matter particles or the presence of new interactions/particles could change the number of clock ticks and modify the clock-ticking rate.
As an example, the UK Atom Interferometry Observatory and Network (AION) project in the UK proposes a staged series of atom interferometers with baselines of 10m, 100m, and 1km, similar to MAGIS. AION has been proposed by a consortium of particle physicists, cold atom experts and gravitational wave enthusiasts, with a strong leading role by the particle physics community. It will provide a pathway for detecting gravitational waves from the very early universe in the, as yet mostly unexplored, mid-frequency band, ranging from several milliHertz to a few Hertz. The AION Project is foreseen as a 4-stage programme: The first stage (year 1-3) the plan is to develop existing technology (Laser systems, vacuum, magnetic shielding etc.) for a 10m demonstrator and prepare for Stage 2. The Stage 1 device is proposed to be located in Oxford, with the sites for the subsequent steps awaiting more study. For more details see http://www.hep.ph.ic.ac.uk/AION-Project/. Stage 2 requires a vertical shaft of order 100m. Obviously CERN has several such shafts on site which could make it an attractive host lab option. Furthermore, CERN has a good cryogenic infrastructure, expert know-how on vacuum technology, laser experience and potential experimental groups that may get interested, eg. from the AD facility.
The second stage (year 3-6) builds, commissions and exploits the 100m detector and also prepares design studies for the km-scale one. The third and fourth stage (year >6) prepare the groundwork for the continuing programme with a terrestrial km-scale detector and space based detector of order 104 km in size.
Space facilities like CACES (China) and CAL (NASA) and sounding-rocket experiments - MAIUS (Germany) - that also use cold atoms in space and microgravity were presented during the workshop as well.
One of the goals of the meeting was the preparation of a White Paper responding to ESA’s Voyage 2050 call. The White Paper focusses on the physics case for future space-based cold atom experiments names AEDGE. AEDGE is proposed as a multi-purpose facility/experiment that will enable a variety of experiments in atom optics and atom interferometry to cover a broad spectrum ranging gravitational waves astronomy to particle physics and dark matter searches. Figures 1 and 2 give the sensitivity that can be reached for the detection of light dark matter and gravitational waves as studied in the AEDGE project (arXiv:1908.00802).
Figure 1: Comparison of the strain measurements possible with AEDGE and other experiments, showing their sensitivities to black hole mergers of differing total masses at various redshifts. An indicating also given of the time remaining before the merger. Also shown is the possible gravitational gradient noise (GGN) level for a km-scale terrestrial detector.
Figure 2: The sensitivities of AEDGE in broadband (purple lines) and resonant mode (orange lines) to linear scalar DM interactions with electrons (top) and photons (bottom), compared to those of a km-scale terrestrial experiment (green lines). The grey regions show parameter spaces that have been excluded by the MICROSCOPE experiment (blue lines), searches for violations of the equivalence principle with torsion balances (red lines), or by atomic clocks (brown lines).
“This effort is an excellent opportunity to bring together the different communities in Europe and to work towards a strong science case that will build the foundation for future space-based, possibly also terrestrial, projects.” said Oliver Buchmueller (Imperial College London) one of the co-organizers of the workshop.
Today Dark Matter remains one of the open and perhaps most pressing question in particle physics. AEDGE could explore different scenarios for ultra-light dark matter in the regime of mDM down to 10-18 eV via searches for coherent effects of the oscillating DM field in the atomic cold. It could probe signals from scalar dark matter improving sensitivity of current searches while it is also promising for searches of other types of dark matter candidates.
“The use of this technology is fascinating both for the detection of gravitational waves and exploring new avenues of dark matter searches” said Albert De Roeck (CERN). “When progressing towards 100m and km-size detectors, CERN has a certainly a lot of experience to offer”.
Moreover, AEDGE can boost searches for gravitational waves. Astronomical observations in a range of different frequencies of electromagnetic waves allowed observing different structures and understanding cosmological processes in the evolution of the Universe. The same is expected to hold for gravitational waves. Probing the mid frequency band is optimal for probing collisions of black holes with masses between those detected by LIGO and super massive black holes that may be observed by LISA, as well as earlier evolution of LIGO binaries paving the way for multi-messenger astronomy.
As John Ellis (King’s College London, CERN) noted in his closing remarks: “AEDGE would be a uniquely interdisciplinary space mission, harnessing cold atom technologies to address key issues in fundamental physics, astrophysics and cosmology”.
More details and references can be found on https://indico.cern.ch/event/830432/ and arXiv:1908.00802
Image notice: The image used in the front page depicts gravitational waves as emitted during a black hole merger. (Image credit: S. Ossokine, A. Buonanno, Max Planck Institute for Gravitational Physics, Simulating eXtreme Spacetimes project, D. Steinhauser, Airborne Hydro Mapping GmbH)