American experiment finds no signs of dark matter - veja.com

Physics

studies the LUX detector searched for evidence of the existence of dark matter. During the first three months of operation of the equipment, the scientists found nothing – which is not necessarily bad

U.S. researchers announced the end of the first phase of operation of the Large Underground Xenon Detector (LUX, its acronym in English), the largest experiment ever conducted to find dark matter, mysterious kind of matter whose composition is unknown by scientists . The news released by researchers on Wednesday is that nothing has been found so far – which is not necessarily a bad outcome.

Located underground in the state of South Dakota, in the Midwest of the United States, the LUX experiment was created by the U.S. government in partnership with seventeen universities only to check for this type of matter. During the first three months of operation, the researchers calibrated their instruments and looked for three types of particles that had been found in previous research and were regarded as possible candidates for dark matter components. With the current results, these three particles can be discarded, leaving scientists with a wide field of research open to be explored.

Thus, the fact that nothing has been found by researchers was celebrated – in science, often a loss can be as important as a positive. “We show that the LUX is already producing the best results in the world and decreasing the space to find a particle of dark matter,” said Matthew Szydagis, researcher at the University of California, and responsible for coordinating the analysis of data among team members LUX.

Matter invisible – Dark matter is an invisible type of matter, whose existence can only be inferred by the researchers, but not directly detected. Since it does not emit or reflect light, scientists have no way of observing it from Earth. She is known only by the gravitational effects it has on the movement of stars in galaxies and clusters of galaxies.

Despite its mysterious nature, it is much more abundant in the cosmos than normal matter, whose existence can be observed. Physicists believe that dark matter accounts for 26.8% of mass and energy density of the universe, compared to 4.9% of ordinary matter. All the rest – 68.3% – is composed of dark energy, a component even more mysterious to which is attributed the expansion of the Universe.

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DARK MATTER
When scientists observe the way stars and galaxies move, there is something unusual. According to the laws of physics, the stars and planets must move more slowly as we move away from the center of it. But it does not happen in practice. For the equations of physics make sense, there must be some force pushing the heap of dust, gas, stars and planets on the outskirts of galaxies at speeds similar to those bodies that are closest to the nucleus. This additional force is gravity a manifestation of nature that has mass but does not emit any light.

DARK ENERGY
Dark energy is a recently discovered component representing 70% of the content of the Universe. She is supposedly responsible for the accelerating expansion of the Universe.

Researchers do not know exactly what the composition of dark matter, but they have some suspicions. The main candidates are called Weak Interaction Massive Particles (WIMPs, its acronym in English), so named because they interact weakly with ordinary matter. The LUX is designed to be the instrument with greater sensitivity to detect WIMPs already done.

Probing the Invisible – Scientists theorize that collisions between WIMPs and normal matter are extremely rare and difficult to detect. “To get an idea of ??how small this probability, imagine shooting a dark matter particle toward a lead block. To get a 50% chance of the interaction between the particle and the lead case, the block would need about 200 light-years long. Therefore, an interaction is incredibly rare, “says Richard Gaitskell, a researcher at Brown University who participated in the study.

If

find WIMPs would already be difficult in ideal research conditions, the task becomes much more complicated in a “polluted” like Earth, which is the target of a steady rain of particles from space – cosmic rays. These particles can produce interactions similar to dark matter and confuse the results obtained by the researchers.

Therefore, the LUX is housed in an abandoned gold mine located about 1.5 km from the surface of the United States, where few foreign matter may enter. To further enhance the isolation, the detector is protected from radiation by immersion in a tank of water.

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Eliminating candidates – In the center of this whole mechanism, titanium is a tank containing about 300 pounds of liquid xenon, cooled to – 100 degrees Celsius. If a WIMP achieve a xenon atom, it will be folded down and emit electrons and photons. The electron will be attracted by the electric field at the top of the tank, where it will interact with another layer of xenon gas, releasing more photons.

The apparatus is equipped with light detectors at the top and bottom of the tank, which are capable of detecting the action of a single photon. So, if by chance one WIMP reach the equipment, will be detected immediately, with the precision of a few millimeters. During the first three months of operation of the equipment, however, nothing was found.

The researchers sought specifically for three types of low-mass WIMPs, which had been detected in previous experiments. If they exist, however, have produced more than 1 600 interactions during operation of the detector LUX – or every 80 minutes. The result indicates that the data obtained in other experiments were probably the result of background radiation, not dark matter.

initial experience LUX complete, the team now must make some adjustments to fine tune the sensitivity of the device and start a new experiment in early 2014, with an estimated duration of 300 days. “This is just the beginning for LUX”, said Dan McKinsey, Yale University physicist who worked on the project. “Now that we understand the instrument, we will continue to collect data, testing for other candidates to increasingly elusive dark matter.” If at the end of its operation, the machine has not helped scientists figure out what dark matter is, it will at least have shown that it is not. For physics this is also important.