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  • richardmitnick 3:10 pm on February 17, 2020 Permalink | Reply
    Tags: , Bosons and fermions, , , , , , , Shedding new light on the internal structure of atomic nuclei.   

    From KTH Royal Institute of Technology via phys.org: “Exotic atomic nuclei reveal traces of new form of superfluidity” 

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    From KTH Royal Institute of Technology

    via


    phys.org

    Published Feb 17, 2020
    David Callahan

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    The team behind the discovery of the new form of superfluidity: from left, Bo Cederwall, professor of physics at KTH Royal Institute of Technology, Xiaoyu Liu, Wei Zhang, Aysegül Ertoprak, Farnaz Ghazi Moradi and Özge Aktas.Published Feb 17, 2020

    Recent observations of the internal structure of the rare isotope ruthenium-88 shed new light on the internal structure of atomic nuclei, a breakthrough which could also lead to further insights into how some chemical elements in nature and their isotopes are formed.

    Led by Bo Cederwall, Professor of Experimental Nuclear Physics at KTH Royal Institute of Technology, an international research team identified new rotational states in the extremely neutron-deficient, deformed, atomic nucleus 88Ru. The results suggest that the structure of this exotic nuclear system is heavily influenced by the presence of strongly-coupled neutron-proton pairs.

    “Such a structure is fundamentally different from the normal conditions observed in atomic nuclei, where neutrons and protons interact in pairs in separate systems, forming a near-superfluid state,” Cederwall says.

    The results may also suggest alternative explanations for how the production of different chemical elements, and in particular their most neutron-poor isotopes, proceeds in the nucleosynthesis reactions in certain stellar environments such as neutron star-red giant binaries, he says.

    The discovery, which was published February 12 in the journal, Physical Review Letters, results from an experiment at the Grand Accélérateur National d’Ions Lourds (GANIL), France, using the Advanced Gamma Tracking Array (AGATA) [below].

    The researchers used nuclear collisions to create highly unstable atomic nuclei with equal numbers of neutrons and protons. Their structure was studied by using sensitive instruments, including AGATA, detecting the radiation they emit in the form of high-energy photons, neutrons, protons and other particles.

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    The Advanced Gamma Tracking Array (AGATA), which researchers from KTH used to study unstable atomic nuclei generated at the Grand Accélérateur National d’Ions Lourds.

    According to the Standard Model of particle physics describing the elementary particles and their interactions, there are two general types of particles in nature; bosons and fermions, which have integer and half-integer spins, respectively. Examples of fermions are fundamental particles like the electron and the electron neutrino but also composite particles like the proton and the neutron and their fundamental building blocks, the quarks. Examples of bosons are the fundamental force carriers; the photon, the intermediate vector bosons, the gluons and the graviton.

    The properties of a system of particles differ considerably depending on whether it is based on fermions or bosons. As a result of the Pauli principle of quantum mechanics, in a system of fermions (such as an atomic nucleus) only one particle can hold a certain quantum state at a certain point in space and time. For several fermions to appear together, at least one property of each fermion, such as its spin, must be different. At low temperature systems of many fermions can exhibit condensates of paired particles manifested as superfluidity for uncharged particles (for example, the superfluid 3He), and superconductivity for charged particles, such as electrons in a superconductor below the critical temperature. Bosons, on the other hand, can condense individually with an unlimited number of particles in the same state, so-called Bose-Einstein condensates.

    In most atomic nuclei that are close to the line of beta stability and in their ground state, or excited to an energy not too high above it, the basic structure appears to be based on pair-correlated condensates of particles with the same isospin quantum number but with opposite spins. This means that neutrons and protons are paired separately from each other. These isovector pair correlations give rise to properties similar to superfluidity and superconductivity. In deformed nuclei, this structure is for example revealed as discontinuities in the rotational frequency when the rotational excitation energy of the nucleus is increased.

    Such discontinuities, which were discovered already in the early 1970s by KTH Professor emeritus Arne Johnson, have been labeled “backbending”. The backbending frequency is a measure of the energy required to break a neutron or proton pair and therefore also reflects the energy released by the formation of a pair of nucleons in the nucleus. There are long-standing theoretical predictions that systems of neutron-proton pairs can be mixed with, or even replace, the standard isovector pair correlations in exotic atomic nuclei with equal numbers of protons and neutrons. The nuclear structure resulting from the isoscalar component of such pair correlations is different from that found in “ordinary” atomic nuclei close to stability. Among different possible experimental observables, the backbending frequency in deformed nuclei is predicted to increase significantly compared with nuclei with different numbers of neutrons and protons.

    The KTH research group has previously observed evidence of strong neutron-proton correlations in the spherical nuclear nucleus 92Pd, which was published in the journal Nature (B. Cederwall et al., Nature, volume 469, p 68-71 (2011)). The ruthenium isotope 88Ru, with 44 neutrons and 44 protons, is deformed and exhibits a rotation-like structure that has now been observed up to higher spin, or rotational frequency, than previously possible. The new measurement provides a different angle on nuclear pair correlations compared with the previous work. By confirming the theoretical predictions of a shift towards higher backbending frequency it provides complementary evidence for the occurrence of strong isoscalar pair correlations in the heaviest nuclear systems with equal numbers of neutrons and protons.

    See the full article here .

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    An innovative European technical university

    Since its founding in 1827, KTH Royal Institute of Technology in Stockholm has grown to become one of Europe’s leading technical and engineering universities, as well as a key centre of intellectual talent and innovation. We are Sweden’s largest technical research and learning institution and home to students, researchers and faculty from around the world dedicated to advancing knowledge.

     
  • richardmitnick 4:33 pm on August 20, 2018 Permalink | Reply
    Tags: , Anomalies, Bosons and fermions, Branes, , , , , , Parity violation, , , , , , , The second superstring revolution, Theorist John Schwarz   

    From Caltech: “Long and Winding Road: A Conversation with String Theory Pioneer” John Schwarz 

    Caltech Logo

    From Caltech

    08/20/2018

    Whitney Clavin
    (626) 395-1856
    wclavin@caltech.edu

    John Schwarz discusses the history and evolution of superstring theory.

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    John Schwarz. Credit: Seth Hansen for Caltech

    The decades-long quest for a theory that would unify all the known forces—from the microscopic quantum realm to the macroscopic world where gravity dominates—has had many twists and turns. The current leading theory, known as superstring theory and more informally as string theory, grew out of an approach to theoretical particle physics, called S-matrix theory, which was popular in the 1960s. Caltech’s John H. Schwarz, the Harold Brown Professor of Theoretical Physics, Emeritus, began working on the problem in 1971, while a junior faculty member at Princeton University. He moved to Caltech in 1972, where he continued his research with various collaborators from other universities. Their studies in the 1970s and 1980s would dramatically shift the evolution of the theory and, in 1984, usher in what’s known as the first superstring revolution.

    Essentially, string theory postulates that our universe is made up, at its most fundamental level, of infinitesimal tiny vibrating strings and contains 10 dimensions—three for space, one for time, and six other spatial dimensions curled up in such a way that we don’t perceive them in everyday life or even with the most sensitive experimental searches to date. One of the many states of a string is thought to correspond to the particle that carries the gravitational force, the graviton, thereby linking the two pillars of fundamental physics—quantum mechanics and the general theory of relativity, which includes gravity.

    We sat down with Schwarz to discuss the history and evolution of string theory and how the theory itself might have moved past strings.

    What are the earliest origins of string theory?

    The first study often regarded as the beginning of string theory came from an Italian physicist named Gabriele Veneziano in 1968. He discovered a mathematical formula that had many of the properties that people were trying to incorporate in a fundamental theory of the strong nuclear force [a fundamental force that holds nuclei together]. This formula was kind of pulled out of the blue, and ultimately Veneziano and others realized, within a couple years, that it was actually describing a quantum theory of a string—a one-dimensional extended object.

    How did the field grow after this paper?

    In the early ’70s, there were several hundred people worldwide working on string theory. But then everything changed when quantum chromodynamics, or QCD—which was developed by Caltech’s Murray Gell-Mann [Nobel Laureate, 1969] and others—became the favored theory of the strong nuclear force. Almost everyone was convinced QCD was the right way to go and stopped working on string theory. The field shrank down to just a handful of people in the course of a year or two. I was one of the ones who remained.

    How did Gell-Mann become interested in your work?

    Gell-Mann is the one who brought me to Caltech and was very supportive of my work. He took an interest in studies I had done with a French physicist, André Neveu, when we were at Princeton. Neveu and I introduced a second string theory. The initial Veneziano version had many problems. There are two kinds of fundamental particles called bosons and fermions, and the Veneziano theory only described bosons. The one I developed with Neveu included fermions. And not only did it include fermions but it led to the discovery of a new kind of symmetry that relates bosons and fermions, which is called supersymmetry. Because of that discovery, this version of string theory is called superstring theory.

    When did the field take off again?

    A pivotal change happened after work I did with another French physicist, Joël Scherk, whom Gell-Mann and I had brought to Caltech as a visitor in 1974. During that period, we realized that many of the problems we were having with string theory could be turned into advantages if we changed the purpose. Instead of insisting on constructing a theory of the strong nuclear force, we took this beautiful theory and asked what it was good for. And it turned out it was good for gravity. Neither of us had worked on gravity. It wasn’t something we were especially interested in but we realized that this theory, which was having trouble describing the strong nuclear force, gives rise to gravity. Once we realized this, I knew what I would be doing for the rest of my career. And I believe Joël felt the same way. Unfortunately, he died six years later. He made several important discoveries during those six years, including a supergravity theory in 11 dimensions.

    Surprisingly, the community didn’t respond very much to our papers and lectures. We were generally respected and never had a problem getting our papers published, but there wasn’t much interest in the idea. We were proposing a quantum theory of gravity, but in that era physicists who worked on quantum theory weren’t interested in gravity, and physicists who worked on gravity weren’t interested in quantum theory.

    That changed after I met Michael Green [a theoretical physicist then at the University of London and now at the University of Cambridge], at the CERN cafeteria in Switzerland in the summer of 1979. Our collaboration was very successful, and Michael visited Caltech for several extended visits over the next few years. We published a number of papers during that period, which are much cited, but our most famous work was something we did in 1984, which had to do with a problem known as anomalies.

    What are anomalies in string theory?

    One of the facts of nature is that there is what’s called parity violation, which means that the fundamental laws are not invariant under mirror reflection. For example, a neutrino always spins clockwise and not counterclockwise, so it would look wrong viewed in a mirror. When you try to write down a fundamental theory with parity violation, mathematical inconsistencies often arise when you take account of quantum effects. This is referred to as the anomaly problem. It appeared that one couldn’t make a theory based on strings without encountering these anomalies, which, if that were the case, would mean strings couldn’t give a realistic theory. Green and I discovered that these anomalies cancel one another in very special situations.

    When we released our results in 1984, the field exploded. That’s when Edward Witten [a theoretical physicist at the Institute for Advanced Study in Princeton], probably the most influential theoretical physicist in the world, got interested. Witten and three collaborators wrote a paper early in 1985 making a particular proposal for what to do with the six extra dimensions, the ones other than the four for space and time. That proposal looked, at the time, as if it could give a theory that is quite realistic. These developments, together with the discovery of another version of superstring theory, constituted the first superstring revolution.

    Richard Feynman was here at Caltech during that time, before he passed away in 1988. What did he think about string theory?

    After the 1984 to 1985 breakthroughs in our understanding of superstring theory, the subject no longer could be ignored. At that time it acquired some prominent critics, including Richard Feynman and Stephen Hawking. Feynman’s skepticism of superstring theory was based mostly on the concern that it could not be tested experimentally. This was a valid concern, which my collaborators and I shared. However, Feynman did want to learn more, so I spent several hours explaining the essential ideas to him. Thirty years later, it is still true that there is no smoking-gun experimental confirmation of superstring theory, though it has proved its value in other ways. The most likely possibility for experimental support in the foreseeable future would be the discovery of supersymmetry particles. So far, they have not shown up.

    What was the second superstring revolution about?

    The second superstring revolution occurred 10 years later in the mid ’90s. What happened then is that string theorists discovered what happens when particle interactions become strong. Before, we had been studying weakly interacting systems. But as you crank up the strength of the interaction, a 10th dimension of space can emerge. New objects called branes also emerge. Strings are one dimensional; branes have all sorts of dimensions ranging from zero to nine. An important class of these branes, called D-branes, was discovered by the late Joseph Polchinski [BS ’75]. Strings do have a special role, but when the system is strongly interacting, then the strings become less fundamental. It’s possible that in the future the subject will get a new name but until we understand better what the theory is, which we’re still struggling with, it’s premature to invent a new name.

    What can we say now about the future of string theory?

    It’s now over 30 years since a large community of scientists began pooling their talents, and there’s been enormous progress in those 30 years. But the more big problems we solve, the more new questions arise. So, you don’t even know the right questions to ask until you solve the previous questions. Interestingly, some of the biggest spin-offs of our efforts to find the most fundamental theory of nature are in pure mathematics.

    Do you think string theory will ultimately unify the forces of nature?

    Yes, but I don’t think we’ll have a final answer in my lifetime. The journey has been worth it, even if it did take some unusual twists and turns. I’m convinced that, in other intelligent civilizations throughout the galaxy, similar discoveries will occur, or already have occurred, in a different sequence than ours. We’ll find the same result and reach the same conclusions as other civilizations, but we’ll get there by a very different route.

    See the full article here .

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    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

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