Dark Matter Transducer

Scientists are on a quest to solve one of physics’ biggest mysteries: What exactly is dark matter – the invisible substance that accounts for 85 percent of all the matter in the universe but can’t be seen even with our most advanced scientific instruments? Most scientists believe it’s made of ghostly particles that rarely bump into their surroundings; that’s why billions of dark matter particles might zip right through our bodies every second without us even noticing. Leading candidates for dark matter particles are WIMPs, or weakly interacting massive particles. SLAC (Stanford Linear Accelerator Center) National Accelerator Laboratory is helping to build and test the LUX-ZEPLIN or LZ detector, one of the biggest and most sensitive detectors ever designed to catch hypothetical dark matter particles known as WIMPs.

Identifying the building blocks of dark matter is one of the highest priorities of modern physics. Most scientists believe it’s made of ghostly particles that rarely bump into their surroundings; that’s why billions of dark matter particles might zip right through our bodies every second without us even noticing. Leading candidates for dark matter particles are WIMPs, or weakly interacting massive particles. Now SLAC is helping to build and test one of the biggest and most sensitive detectors ever designed to catch a WIMP – the LUX-ZEPLIN or LZ detector. LZ’s heart is a tank filled with 10 tons of liquid xenon. If a particle streaks through the tank and strikes a xenon nucleus, two things will happen: The xenon will emit a flash of light, and it will also release electrons, which drift in an electric field to the top of the tank where they produce a second flash of light. A WIMP will produce a characteristic combination of flashes, which will be detected with nearly 500 light-sensitive tubes at the top and bottom of the tank. But to have any hope of seeing a potential WIMP signal, LZ scientists will have to deal with a lot of background noise, such as unwanted signals from cosmic rays from space and gamma rays and neutrons released in natural radioactive decays in the environment and detector materials. Therefore, the detector core will be surrounded by several layers that eliminate this noise as much as possible. First, the researchers will purify the xenon to an incredible degree, to get rid of traces of radioactive krypton. The goal is less than one krypton atom per 100 trillion xenon atoms. Next, they’ll use only the innermost 80 percent of the purified xenon in the tank to detect dark matter. The outer 20 percent will serve as a radiation shield. Another thin layer of xenon, called the xenon 'skin', will reject signals from gamma rays and neutrons. The tank that holds the xenon will be made of ultraclean, medical-grade titanium that generates very few background signals. The titanium tank will sit inside a bigger tank filled with a liquid that like the xenon skin detects gamma rays and neutrons. And both of those tanks will be inside a third tank filled with 70,000 gallons of water as the outer radiation shield. All of this will be located nearly a mile underground at the Sanford Underground Research Facility in South Dakota, where it’s protected from cosmic rays.

The nervous system is composed of billions of communicating cells called neurons. Neurons conduct electrical signals or action potentials along the cell's membrane, along the axon to the terminus, where synapses are formed. The synapse, also called neuronal junction, consists of the membrane of the presynaptic cell, a synaptic cleft and the postsynaptic cell membrane. Since an action potential can only propagate along the neurons membrane, a chemical known as a neurotransmitter must bridge the gap between the adjacent cells. The presynaptic cell releases the neurotransmitters into the synaptic cleft where they will move to bind the receptor proteins in the postsynaptic cell membrane. When the action potential arrives at the synapse it opens voltage-gated calcium channels resulting in an inward flow of calcium. The calcium binds to the vesicular membrane. This binding activates the vesicles to fuse with the presynaptic membrane. As fusion occurs, neurotransmitter molecules are released into the synaptic cleft. Once in the cleft, the neurotransmitter molecules diffuse throughout the interstitial fluid in the cleft. Some of the neurotransmitters will cross the synaptic cleft and bind with receptors in a postsynaptic membrane. This binding produces an effect in the postsynaptic cell by opening the protein channels and allowing the flow of ions into and out of the cell. If the effect of a neurotransmitter excites the postsynaptic membrane, an excitatory postsynaptic membrane potential, also known as EPSP, is generated. This localized depolarizing signal generally causes long-distance signals, action potentials, in the dendrite of the postsynaptic cell. If the effect of the neurotransmitter inhibits the postsynaptic membrane, an inhibitory postsynaptic membrane potential, also known as an IPSP, is generated. This localized hyperpolarizing signal generally prevents the formation of long-distance signals in the postsynaptic cell.

An example of a specific neurotransmitter that functions in these ways is serotonin. Some serotonin will cross the synaptic cleft and bind with the specific serotonin receptors within the membrane of the postsynaptic cell producing an effect in the target cell. Seratonin will eventually unbind from the receptors and either diffuse out of the synaptic cleft and be lost or be captured by the serotonin reuptake transporter in the presynaptic membrane. This protein complex will transport the biologically active serotonin back into the presynaptic neuron for repackaging and re-release.

The brain acts like a dark matter transducer. Just like the LZ detector's heart is a tank filled with 10 tons of liquid xenon, the neuron synapse consists of the membrane of the presynaptic cell, a synaptic cleft (filled with neurotransmitters) and the postsynaptic cell membrane. If a dark matter particle streaks through the tank and strikes a xenon nucleus or if a dark matter particle passes through a neuron synapse and hits a neurotransmitter, electrons are released. A dark matter particle will produce a characteristic combination of flashes, which will be detected with nearly 500 light-sensitive tubes at the top and bottom of the tank or in the neuron's case, excite the postsynaptic membrane.

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