Rice University Dark Matter Search | New Experimental Limits

A floating target waiting for a push

The POLONAISE experiment at Rice University uses a permanent magnet on the scale of a grain of sand, held in a superconducting tantalum trap. The levitated assembly weighs 0.356 milligrams and includes a small glass sphere attached to the magnetic material. With no ordinary mechanical support rubbing against the moving target, tiny forces can leave measurable traces in its motion. This setup allows researchers to monitor physical impulses that would remain hidden in conventional detectors.

The cold surroundings help make superconducting operation possible. A superconducting pick-up coil senses changes in magnetic flux as the magnet moves, and an exceptionally sensitive device called a SQUID reads the resulting signal. The instrument watches motion rather than waiting for a flash of light or an electrical recoil inside a conventional particle detector. The target was small, but the effort to keep it still was substantial.

According to the research team, the cryostat rested on a 25-tonne concrete block supported by pneumatic dampers. Inside, a suspended system further reduced vibrations. Pumps and cooling machinery were mechanically separated as far as possible, because ordinary laboratory disturbances could imitate the impulse being sought.

Why heavier dark matter needs a different search

The analysis considered dark matter that could interact with neutrons through a hypothetical new force carried by a very light mediator particle. If such a dark-matter object passed close to the sensor, the combined response of many constituents in the magnet could produce a detectable momentum kick. This is a particular interaction model, not a detector for every conceivable form of dark matter. The force’s strength and range affect whether a passing object would move the target enough to register.

Mass introduces another difficulty. For a fixed amount of dark matter in a region, making each object heavier means there are fewer objects passing through. A search for rare, massive visitors needs a suitable target and sufficient observation time. The paper reports sensitivity across roughly nine orders of magnitude in candidate mass. Its upper reach extends about seven orders of magnitude beyond earlier optical-levitation searches, whose suspended targets were much lighter. That seven-order-of-magnitude jump represents the ten-million-fold extension in reach.

Data selection and null results

Measurements ran from December 22, 2025, to January 21, 2026. The team selected overnight periods, between 7 p.m. and 7 a.m. local time, when external activity was expected to be lower. It also removed calibration periods and dates affected by apparatus work or a deliberate vibration test. That left 219.66 hours of usable exposure, equivalent to about 9.15 days.

Eight candidate impulses survived the main selection. They were consistent with instrumental disturbances, and their behaviour correlated with laboratory activity. The researchers had no reliable model that would let them subtract those disturbances as a known background. Instead, they treated every surviving candidate as potentially due to dark matter when calculating conservative upper limits. The outcome does not mean the magnet never moved. It means that the observations supplied no confirmed evidence for the proposed dark-matter interaction.

Next steps and industry implications

The collaboration reported exclusions at the 95 per cent confidence level for specified combinations of particle mass, mediator properties, and interaction strength. Those qualifications define the result. They cannot be discarded to claim that ultraheavy dark matter as an entire category has been ruled out. There are boundaries at both ends of the search, as a particle must deliver enough momentum to cross the detection threshold.

Rice researchers previously looked for a repeating force associated with a wave-like field in June 2025. That experiment also found no evidence for its target signal. Reusing sensitive levitation technology does not make those two hypotheses interchangeable, but it shows how one experimental platform can address widely separated possibilities. The team plans further cooling, longer measurements, and multiple levitated magnets. Several sensors could help distinguish a possible passing particle from a disturbance affecting the laboratory. For now, the achievement is a measured expansion of experimental reach: a small floating magnet has tested a region that earlier levitated targets could not cover.