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Strange, unexplained signals spotted in world's most sensitive dark matter experiment

Scientists from the international XENON collaboration said data from their XENON1T, the world’s most sensitive dark matter experiment, show a surprising excess of events.

June 17, 2020 / 10:48 PM IST
The XENON1T detector. Visible is the bottom array of photomultiplier tubes, and the copper structure that creates the electric drift field.

The XENON1T detector. Visible is the bottom array of photomultiplier tubes, and the copper structure that creates the electric drift field.

Data from one of the world's most sensitive dark matter experiments, the XENON1T, shows a surprising excess of events. While scientists do not claim to have found dark matter, there seem to be some other unexplained signals, reasons behind which could be many.

When a certain type of event is observed more than expected in a data plot, scientists term it an excess. The statistical significance of such excesses is then studied by scientists to determine how certain they are that these excesses result from new physics, and are not random fluctuations.

An unexpected rate of events has been observed by scientists, who are part of the XENON1T experiment, the source of which is not yet fully understood. But, they say, there is a possibility that this could be a sign of the existence of a new particle called the "solar axion".

"The signature of the excess is similar to what might result from a tiny residual amount of tritium (a hydrogen atom with one proton and two neutrons), but could also be a sign of something more exciting—such as the existence of a new particle known as the solar axion, or the indication of previously unknown properties of neutrinos," as per a release.

The XENON experiment, a collaboration of over 150 scientists, is a dark matter research project operated at the INFN Laboratori Nazionali del Gran Sasso in Italy, a deep underground research facility.

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The series of experiments under the XENON collaboration were originally designed to seek weakly interacting massive particles, or WIMPs. Any particle traversing the detector should would collide with a xenon nucleus, generating tiny signals of light and would free electrons from a xenon atom.

The XENON1T detector works on the same principle, and was preceded by the XENON100 and XENON 10 detectors in the series. In this experiment, the XENON1T was filled with 3.2 tonnes of ultra-pure liquefied xenon, 2 tonnes of which serve as the target for particle interactions. Most of these interactions occur from particles that are known to exist.

According to the release, "Scientists therefore carefully estimated the number of background events in XENON1T. When data of XENON1T were compared to known backgrounds, a surprising excess of 53 events over the expected 232 events was observed."

The excesses, that were recorded, raised a question as to their source. Among the few explanations given for the observation, scientists say one could be the existence of a new particle. They noted that the excess observed during this experiment has an energy spectrum similar to that expected from axions produced in the Sun. The observed excess seems to be most consistent with the solar axion signal hypothesis.

"In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2/10,000 chance that the observed excess is due to a random fluctuation rather than a signal," the release said.

Axions are hypothetical particles that were proposed to preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them.

"While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but never observed class of new particles, with a large impact on our understanding of fundamental physics, but also on astrophysical phenomena. Moreover, axions produced in the early universe could also be the source of dark matter," they said.

One of the other two possible explanations cited is a new, previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the XENON1T detector. The other one could be due to neutrinos, trillions of which pass through your body, unhindered, every second.

"The magnetic moment (a property of all particles) of neutrinos is larger than its value in the Standard Model of elementary particles. This would be a strong hint to some other new physics needed to explain it," it said.
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first published: Jun 17, 2020 10:48 pm

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