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  • richardmitnick 12:35 pm on June 17, 2017 Permalink | Reply
    Tags: , , , CP-Symmetry, , Helen Quinn and Roberto Peccei, Peccei-Quinn symmetry, , ,   

    From Quanta: “Roberto Peccei and Helen Quinn, Driving Around Stanford in a Clunky Jeep” 

    Quanta Magazine
    Quanta Magazine

    June 15, 2017
    Thomas Lin
    Olena Shmahalo, Art Director
    Lucy Reading-Ikkanda, graphics

    Ryan Schude for Quanta Magazine
    Helen Quinn and Roberto Peccei walking toward Stanford University’s new science and engineering quad. Behind them is the main quad, the oldest part of the campus. “If you look at a campus map,” said Quinn, who along with Peccei proposed Peccei-Quinn symmetry, “you will see the axis that goes through the middle of both quadrangle areas. We are on that line between the two.”

    Four decades ago, Helen Quinn and Roberto Peccei took on one of the great problems in theoretical particle physics: the strong charge-parity (CP) problem. Why does the symmetry between matter and antimatter break in weak interactions, which are responsible for nuclear decay, but not in strong interactions, which hold matter together?

    “The academic year 1976-77 was particularly exciting for me because Helen Quinn and Steven Weinberg were visiting the Stanford department of physics,” Peccei told Quanta in an email. “Helen and I had similar interests and we soon started working together.”

    Encouraged by Weinberg, who would go on to win a Nobel Prize in physics in 1979 for his work on the unification of electroweak interactions, Quinn and Peccei zeroed in on a CP-violating interaction whose strength can be characterized by an angular variable, theta. They knew theta had to be small, but no one had an elegant mechanism for explaining its smallness.

    “Steve liked to discuss physics over lunch, and Helen and I often joined him,” Peccei said. “Steve invariably brought up the theta problem in our lunch discussions, urging us to find a natural solution for why it was so small.”

    Quinn said by email that she and Peccei knew two things: The problem goes away if any quarks have zero mass (which seems to make theta irrelevant), and “in the very early hot universe all the quarks have zero mass.” They wondered how it could be that “theta is irrelevant in the early universe but matters once it cools enough that the quarks get their masses?”

    They proceeded to draft a “completely wrong paper based on conclusions we drew from this set of facts,” Quinn said. They went to Weinberg, whose comments helped clarify their thinking and, she said, “put us on the right track.”

    They realized they could naturally arrive at a zero value for theta by requiring a new symmetry, now known as the Peccei-Quinn mechanism. Besides being one of the popular proposed solutions to the strong CP problem, Peccei-Quinn symmetry also predicts the existence of a hypothetical “axion” particle, which has become a mainstay in theories of supersymmetry and cosmic inflation and has been proposed as a candidate for dark matter.

    Peccei and Quinn discussing their proposed symmetry with the aid of a sombrero. Ryan Schude for Quanta Magazine

    That year at Stanford, Quinn and Peccei regularly interacted with the theory group at the Stanford Linear Accelerator Center (SLAC) as well as with another group from the University of California, Santa Cruz.

    “We formed a large and active group of theorists, which created a wonderful atmosphere of open discussion and collaboration,” Quinn said, adding that she recalls “riding with Roberto back and forth from Stanford to SLAC in his yellow and clunky Jeep, talking physics ideas as we went.”

    See the full article here .

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

  • richardmitnick 9:28 am on July 17, 2016 Permalink | Reply
    Tags: , CP-Symmetry, ,   

    From Science Alert: “Physicists just confirmed a pear-shaped nucleus, and it could ruin time travel forever” 


    Science Alert

    27 JUN 2016 [Just today in social media]


    Physicists have confirmed the existence of a new form of atomic nuclei, and the fact that it’s not symmetrical challenges the fundamental theories of physics that explain our Universe.

    But that’s not as bad as it sounds, because the discovery could help scientists solve one of the biggest mysteries in theoretical physics – where is all the dark matter? – and could also explain why travelling backwards in time might actually be impossible.

    “We’ve found these nuclei literally point towards a direction in space. This relates to a direction in time, proving there’s a well-defined direction in time and we will always travel from past to present,” Marcus Scheck from the University of the West of Scotland told Kenneth MacDonald at BBC News.

    So let’s back up here, because to understand this new form of atomic nuclei, you have to get to know the old ones first. Until recently, it was established that the nuclei of atoms could be one of just three shapes – spherical, discus, or rugby ball.

    These shapes are formed by the distribution of electrical charge within a nucleus, and are dictated by the specific combinations of protons and neutrons in a certain type of atom, whether it’s a hydrogen atom, a zinc atom, or a complex isotope created in a lab.

    The common factor across all three shapes is their symmetry, and this marries nicely with a theory in particle physics known as CP-Symmetry. CP-symmetry is the combination of two symmetries that are thought to exist in the Universe: C-Symmtery and P-Symmetry.

    C-Symmetry, also known as charge symmetry, states that if you flip an atomic charge to its opposite, the physics of that atom should still be the same. So if we take a hydrogen atom and an anti-hydrogen atom and mess with them, both should respond in identical ways, even though they have opposite charges.

    P-Symmetry, also known as Parity, states that the the spatial coordinates describing a system can be inverted through the point at the origin, so that x, y, and z are replaced with −x, −y, and −z.

    “Your left hand and your right hand exhibit P-Symmetry from one another: if you point your thumb up and curl your fingers, your left and right hands mirror one another,” Ethan Siegel from It Starts With a Bang explains.

    CP-Symmetry is a combination of both of these assumptions. “In particle physics, if you have a particle spinning clockwise and decaying upwards, its antiparticle should spin counterclockwise and decay upwards 100 percent of the time if CP is conserved,” says Siegel. “If not, CP is violated.”

    The possibility that the Universe could actually violate both C-Symmetry and CP-Symmetry is one of the conditions that have been proposed to explain the mystery of antimatter in the Universe. But proving that would mean the Standard Model of Physics needs a serious rethink.

    According to the laws of physics, at the time of the Big Bang, equal amounts of matter and antimatter had to have been created, but now, billions of years later, we’re surrounded by heaps of matter (solid, liquid, gas, and plasma), but there appears to be almost no naturally occurring antimatter.

    Okay, so back to our atomic nuclei shapes. Most of our fundamental theories of physics are based on symmetry, so when physicists at CERN discovered an asymmetrical pear-shaped nucleus in the isotope Radium-224 back in 2013, it was a bit of a shock, because it showed that nuclei could have more mass at one end than the other.

    Now, three years later, the find has been confirmed by a second study, which has shown that the nucleus of the isotope Barium-144 is also asymmetrical and pear-shaped.

    “[T]he protons enrich in the bump of the pear and create a specific charge distribution in the nucleus,” Scheck told the BBC. “This violates the theory of mirror symmetry and relates to the violation shown in the distribution of matter and antimatter in our Universe.”

    While physicists have suspected that Barium-144 has a pear-shaped nucleus for some time now, Scheck and his team finally figured out how to directly observe that, and it turns out its distortion is even more pronounced than predicted.

    So what does all of this have to do with time travel? It’s a pretty out-there hypothesis, but Scheck says that this uneven distribition of mass and charge causes Barium-144’s nucleus to ‘point’ in a certain direction in spacetime, and this bias could explain why time seems to only want to go from past to present, and not backwards, even if the laws of physics don’t care which way it goes.

    Of course, there’s no way of proving that without further evidence, but the discovery is yet another indication that the Universe might not be as symmetrical as the Standard Model of Physics needs it to be, and proving that could usher us into a whole new era of theoretical physics.

    The study has been published in Phyiscal Review Letters, and can be accessed for free at arXiv.org.

    See the full article here .

    Please help promote STEM in your local schools.

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