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      New value for W boson mass dims 2022 hints of physics beyond Standard Model

      news.movim.eu / ArsTechnica · Friday, 24 March, 2023 - 18:06 · 1 minute

    ATLAS Event Displays: W boson production

    Enlarge / Event display of a W-boson candidate decaying into a muon and a muon neutrino inside the ATLAS experiment. The blue line shows the reconstructed track of the muon, and the red arrow denotes the energy of the undetected muon neutrino. (credit: ATLAS Collaboration/CERN)

    It's often said in science that extraordinary claims require extraordinary evidence. Recent measurements of the mass of the elementary particle known as the W boson provide a useful case study as to why. Last year , Fermilab physicists caused a stir when they reported a W boson mass measurement that deviated rather significantly from theoretical predictions of the so-called Standard Model of Particle Physics —a tantalizing hint of new physics. Others advised caution, since the measurement contradicted prior measurements.

    That caution appears to have been warranted. The ATLAS collaboration at CERN's Large Hadron Collider (LHC) has announced a new, improved analysis of their own W boson data and found the measured value for its mass was still consistent with Standard Model. Caveat: It's a preliminary result. But it lessens the likelihood of Fermilab's 2022 measurement being correct.

    "The W mass measurement is among the most challenging precision measurements performed at hadron colliders," said ATLAS spokesperson Andreas Hoecker . "It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modeling uncertainties. This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions.”

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      The quiet search for dark matter deep underground

      Matthew Francis · news.movim.eu / ArsTechnica · Monday, 6 September, 2021 - 12:39 · 1 minute

    A mile below ground, a sign hangs over the door to the LUX dark matter experiment telling visitors how far to Wall Drug—in both dimensions.

    A mile below ground, a sign hangs over the door to the LUX dark matter experiment telling visitors how far to Wall Drug—in both dimensions. (credit: Matthew R. Francis)

    Update, Sept. 6, 2021: It's Labor Day Weekend in the US, and even though most of us are continuing to call home " the office ," Ars staff is taking a long weekend to rest and relax. And given we can't travel like we could during Labor Day Weekends past, we thought we'd revisit one of our favorite trips from the archives. This story on our adventure to the Large Underground Xenon (LUX) dark matter experiment in South Dakota originally ran in July 2014, and it appears unchanged below.

    One of the quietest, darkest places in the cosmos isn’t out in the depths of space. It’s at the center of a tank of cold liquid xenon in a gold mine deep under the Black Hills of South Dakota. It needs to be that quiet: any stray particles could confuse the detectors lining the outside of the tank. Those detectors are looking for faint, rare signals, ones that could reveal the presence of dark matter.

    The whole assembly—the container of liquid and gaseous xenon, the water tank enveloping that, and all the detectors—is called the Large Underground Xenon (LUX) dark matter experiment. So far, LUX hasn’t found anything , but the days of its operation are just beginning: the detector was installed and started operations just last year.

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      Unstable helium adds a limit on the ongoing saga of the proton’s size

      John Timmer · news.movim.eu / ArsTechnica · Wednesday, 27 January, 2021 - 20:11 · 1 minute

    A huge pavement campus surrounded by green fields.

    Enlarge / The small particle accelerator in Switzerland where, surrounded by farms, the work took place. (credit: Paul Scherrer Institut )

    Physicists, who dedicate their lives to studying the topic, don't actually seem to like physics very much, since they're always hoping it's broken. But we'll have to forgive them; finding out that a bit of theory can't possibly explain experimental results is a sign that we probably need a new theory, which is something that would excite any physicist.

    In recent years, one of the things that's looked the most broken is a seemingly simple measurement: the charge radius of the proton, which is a measure of its physical size. Measurements made with hydrogen atoms, which have a single electron orbiting a proton, gave us one answer. Measurements where the electron was replaced by a heavier particle called a muon gave us a different answer‐and the two results were incompatible. Lots of effort has gone into eliminating this discrepancy, and it's gotten smaller —but it hasn't gone away.

    That's gotten theorists salivating. The Standard Model has no space for these kind of differences between electrons and muons, so could this be a sign that the Standard Model is wrong? The team behind some of the earlier measurements is now back with a new one, this one tracking the behavior a muon orbiting a helium nucleus. The results are consistent with other measurements of helium's charge radius, suggesting there's nothing funny about the muon. So the Standard Model can breathe a sign of relief.

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