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Direct dark matter searches with LUX ZEPLIN

LUX ZEPLIN

How to detect Dark Matter?

The challenge for dark matter detectors is that dark matter usually passes straight through, with any hint of its passage being incredibly rare. However, with a device that’s big enough and sensitive enough such that a few interactions are registered, we could confirm its existence and explore its properties. In the case of LZ, our device is made of about 7 tons of liquid xenon. Xenon has the useful property that should a particle of dark matter collide with one of the xenon atoms, small bursts of light will be emitted. Very sensitive light detectors are constantly watching the xenon, recording any signals that might occur.

What is a TPC?

The TPC is the Time Projection Chamber, which is the most important central section of the instrument. In the case of LZ, the xenon inside the TPC is mostly liquid, although a thin gaseous xenon layer is maintained at the surface. As stated above, when particles interact with the xenon, small flashes of light are emitted. In addition, one or more of the electrons of the xenon atom that was hit can be released, and by application of a strong electric field, these electrons are drifted to the liquid surface, where a second flash of light is produced. By measuring the time between the two flashes of light, we can ‘project’ the position at which the interaction occurred. Being able to see where the events occurred within the detector allows us to remove many sources of background that might otherwise bemisinterpreted. That LZ uses both liquid and gas means it’s often referred to as a dual phase TPC.

Why do we use xenon?

Xenon has numerous properties that make it ideal as a dark matter detector. It scintillates, which is the technical name for its property to give out light when struck by other particles. Moreover, it is then transparent to its own scintillation light, which means that we can get the signal out of all parts of a large detector. It is a noble element, which means we can easily purify large quantities to a very high degree. And xenon has a very high atomic mass which makes it an ideal target for dark matter interactions as each atom has many neutrons and protons to act as targets for the dark matter to scatter from. Finally, xenon naturally has several stable isotopes, that is, atoms of xenon which all have 54 protons naturally occur with various different numbers of neutrons, ranging from 70 to 82. There are different ways dark matter could interact with these different isotopes, and having a detector made from all of them makes the experiment sensitive to a wider range of types of dark matter. Some of the isotopes are candidates for an exciting process known as neutrinoless double beta decay, which is a whole different area of physics, but which LZ is also sensitive to - a bonus for free!

Why do we go underground to detect it?

Working underground presents a number of challenges, not the least the difficulty of moving our detector, equipment and scientist a mile underground safely - so why would we do this? The reason is that even given the size of the LZ detector, we still only expect to see very few events per year as a result of dark matter interactions. If our detector was above ground, it would be completely swamped with millions of background noise events from the cosmic radiation that is constantyl bombarding the Earth’s surface. We go deep underground as a natural shield against these sources of background.

It’s worth pointing out that this is just the start of trying to escape sources of background events. It’s a truism that everything is radioactive - even your average human naturally emits around 1000 gamma rays every second. We need to control the radiation of the laboratory within which the detector is sited, and of the detector itself as well - these all emit some small amount of radiation, completely harmless to humans, but our detectors are now so sensitive even these small amounts would contribute significantly to our backgrounds. Our method is to build increasingly radiation free shells of shielding, like russian dolls, to reduce the radioactive background until we have the least radioactive cubic metre in the known universe at the heart of our detector. Dark matter, by its very nature, will be unaffected by our efforts to block it, so this central volume is where we search.

The LUX ZEPLIN Collaboration

The experiment is a collaboration of some 250 scientists, engineers, and technicians from 35 institutions across the US, UK, Portugal and South Korea.

To learn more about Dark Matter and LZ:

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