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Recent Noteworthy Physics Papers

  1. Aug 18, 2015 #181
    And another paper published yesterday by the ALICE (A Large Ion Collider Experiment) team, concerning antimatter and CPT invariance...

    ALICE Collaboration, Precision measurement of the mass difference between light nuclei and anti-nuclei, Nature Physics (2015), doi:10.1038/nphys3432
    Paper available for free here: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3432.html

    Abstract:
    The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories, but cannot yet be directly derived from quantum chromodynamics. Here we report a measurement of the difference between the ratios of the mass and charge of deuterons (d) and anti-deuterons (http://www.nature.com/nphys/journal/vaop/ncurrent/images/nphys3432-m1.gif), and 3He and http://www.nature.com/nphys/journal/vaop/ncurrent/images/nphys3432-m2.gif nuclei carried out with the ALICE (A Large Ion Collider Experiment) detector in Pb–Pb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. Our direct measurement of the mass-over-charge differences confirms CPT invariance to an unprecedented precision in the sector of light nuclei. This fundamental symmetry of nature, which exchanges particles with anti-particles, implies that all physics laws are the same under the simultaneous reversal of charge(s) (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).
     
    Last edited by a moderator: May 7, 2017
  2. Dec 12, 2015 #182
    More life for the electrons...

    M. Agostini et al. (Borexino Collaboration), Test of Electric Charge Conservation with Borexino
    Phys. Rev. Lett. 115, 231802 – Published 3 December 2015
    On arxiv: http://arxiv.org/abs/1509.01223

    Abstract:
    Borexino is a liquid scintillation detector located deep underground at the Laboratori Nazionali del Gran Sasso (LNGS, Italy). Thanks to the unmatched radio purity of the scintillator, and to the well understood detector response at low energy, a new limit on the stability of the electron for decay into a neutrino and a single monoenergetic photon was obtained. This new bound, τ ≥ 6.6 × 1028 yr at 90% C.L., is 2 orders of magnitude better than the previous limit.

    An APS synopsis is here: Synopsis: Still Waiting For Electron Decay
     
  3. Feb 11, 2016 #183
    B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
    Observation of Gravitational Waves from a Binary Black Hole Merger
    Phys. Rev. Lett. 116, 061102 – Published 11 February 2016

    Abstract:
    On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10−21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160−180Mpc corresponding to a redshift z=0.09+0.03−0.04. In the source frame, the initial black hole masses are 36+5−4M⊙ and 29+4−4M⊙, and the final black hole mass is 62+4−4M⊙, with 3.0+0.5−0.5M⊙c2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

    Article on APS: Viewpoint: The First Sounds of Merging Black Holes
     
  4. Aug 12, 2016 #184

    ZapperZ

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    R. Pohl et al., "Laser spectroscopy of muonic deuterium" Science v.353, p.669 (2016).

    Abstract: The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ–. We measured three 2S-2P transitions in μd and obtain rd = mml-math-1.gif fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value rd = mml-math-2.gif fm. The μd value is also 3.5σ smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a “small” proton radius rp, similar to the one from muonic hydrogen, amplifying the proton radius puzzle.

    There is an article on Ars Technica that describes this proton radius problem in a greater detail.

    Zz.
     
    Last edited: Aug 13, 2016
  5. Sep 16, 2016 #185

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    A. Hamo et al., "Electron attraction mediated by Coulomb repulsion" Nature v.535, p.395 (2016).

    Abstract: One of the defining properties of electrons is their mutual Coulomb repulsion. However, in solids this basic property may change; for example, in superconductors, the coupling of electrons to lattice vibrations makes the electrons attract one another, leading to the formation of bound pairs. Fifty years ago it was proposedhttp://www.nature.com/nature/journal/v535/n7612/full/nature18639.html#ref1 that electrons can be made attractive even when all of the degrees of freedom in the solid are electronic, by exploiting their repulsion from other electrons. This attraction mechanism, termed ‘excitonic’, promised to achieve stronger and more exotic superconductivity. Yet, despite an extensive search, experimental evidence for excitonic attraction has yet to be found. Here we demonstrate this attraction by constructing, from the bottom up, the fundamental building blockhttp://www.nature.com/nature/journal/v535/n7612/full/nature18639.html#ref8 of the excitonic mechanism. Our experiments are based on quantum devices made from pristine carbon nanotubes, combined with cryogenic precision manipulation. Using this platform, we demonstrate that two electrons can be made to attract each other using an independent electronic system as the ‘glue’ that mediates attraction. Owing to its tunability, our system offers insights into the underlying physics, such as the dependence of the emergent attraction on the underlying repulsion, and the origin of the pairing energy. We also demonstrate transport signatures of excitonic pairing. This experimental demonstration of excitonic pairing paves the way for the design of exotic states of matter.

    Also see the News and Views article on this paper in the same issue of Nature.

    So now we have two types of "glue" that can cause electrons to attraction each other in a solid. The phononic source that forms Cooper pairs in conventional superconductors, and now the excitonic glue that essentially is due to other electrons and charges in the material. This channel has been searched for for a long time, since William Little first proposed it in 1964.

    Zz.
     
    Last edited by a moderator: May 8, 2017
  6. Jan 13, 2017 #186

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    L. F. Pašteka et al., "Relativistic Coupled Cluster Calculations with Variational Quantum Electrodynamics Resolve the Discrepancy between Experiment and Theory Concerning the Electron Affinity and Ionization Potential of Gold", Phys. Rev. Lett. 118, 023002 (2017).

    Abstract: The first ionization potential (IP) and electron affinity (EA) of the gold atom have been determined to an unprecedented accuracy using relativistic coupled cluster calculations up to the pentuple excitation level including the Breit and QED contributions. We reach meV accuracy (with respect to the experimental values) by carefully accounting for all individual contributions beyond the standard relativistic coupled cluster approach. Thus, we are able to resolve the long-standing discrepancy between experimental and theoretical IP and EA of gold.

    A review of this work can be found at the APS Physics page.

    Zz.
     
  7. Feb 2, 2017 #187

    Andy Resnick

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    Precise measurements of antimatter systems might cast light on why the Universe is dominated by matter. The observation of a transition in an antihydrogen atom heralds the next wave of high-precision antimatter studies.

    Observation of the 1S–2S transition in trapped antihydrogen M. Ahmadi,et. al. Nature 541, 506–510 (26 January 2017) doi:10.1038/nature21040


    Abstract: The spectrum of the hydrogen atom has played a central part in fundamental physics over the past 200 years. Historical examples of its importance include the wavelength measurements of absorption lines in the solar spectrum by Fraunhofer, the identification of transition lines by Balmer, Lyman and others, the empirical description of allowed wavelengths by Rydberg, the quantum model of Bohr, the capability of quantum electrodynamics to precisely predict transition frequencies, and modern measurements of the 1S–2S transition by Hänsch1 to a precision of a few parts in 1015. Recent technological advances have allowed us to focus on antihydrogen—the antimatter equivalent of hydrogen. The Standard Model predicts that there should have been equal amounts of matter and antimatter in the primordial Universe after the Big Bang, but today’s Universe is observed to consist almost entirely of ordinary matter. This motivates the study of antimatter, to see if there is a small asymmetry in the laws of physics that govern the two types of matter. In particular, the CPT (charge conjugation, parity reversal and time reversal) theorem, a cornerstone of the Standard Model, requires that hydrogen and antihydrogen have the same spectrum. Here we report the observation of the 1S–2S transition in magnetically trapped atoms of antihydrogen. We determine that the frequency of the transition, which is driven by two photons from a laser at 243 nanometres, is consistent with that expected for hydrogen in the same environment. This laser excitation of a quantum state of an atom of antimatter represents the most precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of about 2 × 10−10.
     
  8. Feb 13, 2017 #188

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    J. Handsteiner et al., "Cosmic Bell Test: Measurement Settings from Milky Way Stars", Phys. Rev. Lett. v.118, p.060401 (2017).

    Abstract: Bell’s theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bell’s inequality, which is usually derived under the assumption that there are no statistical correlations between the choices of measurement settings and anything else that can causally affect the measurement outcomes. In previous experiments, this “freedom of choice” was addressed by ensuring that selection of measurement settings via conventional “quantum random number generators” was spacelike separated from the entangled particle creation. This, however, left open the possibility that an unknown cause affected both the setting choices and measurement outcomes as recently as mere microseconds before each experimental trial. Here we report on a new experimental test of Bell’s inequality that, for the first time, uses distant astronomical sources as “cosmic setting generators.” In our tests with polarization-entangled photons, measurement settings were chosen using real-time observations of Milky Way stars while simultaneously ensuring locality. Assuming fair sampling for all detected photons, and that each stellar photon’s color was set at emission, we observe statistically significant ≳7.31σ and ≳11.93σ violations of Bell’s inequality with estimated p values of ≲1.8×10−13 and ≲4.0×10−33, respectively, thereby pushing back by ∼600  years the most recent time by which any local-realist influences could have engineered the observed Bell violation.

    Read the synopsis of this paper here. This paper can also be obtained for free under the Creative Commons License.

    Zz.
     
  9. Mar 20, 2017 #189
    Mikhail Lemeshko, "Quasiparticle Approach to Molecules Interacting with Quantum Solvents", Phys. Rev. Lett. v.118, p.095301, (2017).

    arXiv pre-print here.

    Abstract: Understanding the behavior of molecules interacting with superfluid helium represents a formidable challenge and, in general, requires approaches relying on large-scale numerical simulations. Here, we demonstrate that experimental data collected over the last 20 years provide evidence that molecules immersed in superfluid helium form recently predicted angulon quasiparticles [Phys. Rev. Lett. 114, 203001 (2015)]. Most important, casting the many-body problem in terms of angulons amounts to a drastic simplification and yields effective molecular moments of inertia as straightforward analytic solutions of a simple microscopic Hamiltonian. The outcome of the angulon theory is in good agreement with experiment for a broad range of molecular impurities, from heavy to medium-mass to light species. These results pave the way to understanding molecular rotation in liquid and crystalline phases in terms of the angulon quasiparticle.

    Also featured on phys.org.
     
  10. Mar 25, 2017 #190

    ZapperZ

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    G. Rubino et al., "Experimental verification of an indefinite causal order", Sci. Adv., 3, e1602589 (2017).

    Abstract: Investigating the role of causal order in quantum mechanics has recently revealed that the causal relations of events may not be a priori well defined in quantum theory. Although this has triggered a growing interest on the theoretical side, creating processes without a causal order is an experimental task. We report the first decisive demonstration of a process with an indefinite causal order. To do this, we quantify how incompatible our setup is with a definite causal order by measuring a “causal witness.” This mathematical object incorporates a series of measurements that are designed to yield a certain outcome only if the process under examination is not consistent with any well-defined causal order. In our experiment, we perform a measurement in a superposition of causal orders—without destroying the coherence—to acquire information both inside and outside of a “causally nonordered process.” Using this information, we experimentally determine a causal witness, demonstrating by almost 7 SDs that the experimentally implemented process does not have a definite causal order.

    You may get the full paper here while it is available for free on Sci. Adv. Press release can be found here.

    Zz.
     
  11. Aug 3, 2017 #191

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    D. Akimov et al., "Observation of coherent elastic neutrino-nucleus scattering" Science 10.1126/science.aao0990 (2017).

    Abstract: The coherent elastic scattering of neutrinos off nuclei has eluded detection for four decades, even though its predicted cross-section is the largest by far of all low-energy neutrino couplings. This mode of interaction provides new opportunities to study neutrino properties, and leads to a miniaturization of detector size, with potential technological applications. We observe this process at a 6.7-sigma confidence level, using a low-background, 14.6-kg CsI[Na] scintillator exposed to the neutrino emissions from the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. Characteristic signatures in energy and time, predicted by the Standard Model for this process, are observed in high signal-to-background conditions. Improved constraints on non-standard neutrino interactions with quarks are derived from this initial dataset.

    As of now, you may get the full copy of the paper at the Science website.

    Zz.
     
  12. Aug 8, 2017 #192

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    M. Ahmadi, et. al., "Observation of the hyperfine spectrum of antihydrogen" Nature v.548, p.66 (2017).

    Abstract: The observation of hyperfine structure in atomic hydrogen by Rabi and co-workers1, 2, 3 and the measurement4 of the zero-field ground-state splitting at the level of seven parts in 1013 are important achievements of mid-twentieth-century physics. The work that led to these achievements also provided the first evidence for the anomalous magnetic moment of the electron5, 6, 7, 8, inspired Schwinger’s relativistic theory of quantum electrodynamics9, 10 and gave rise to the hydrogen maser11, which is a critical component of modern navigation, geo-positioning and very-long-baseline interferometry systems. Research at the Antiproton Decelerator at CERN by the ALPHA collaboration extends these enquiries into the antimatter sector. Recently, tools have been developed that enable studies of the hyperfine structure of antihydrogen12—the antimatter counterpart of hydrogen. The goal of such studies is to search for any differences that might exist between this archetypal pair of atoms, and thereby to test the fundamental principles on which quantum field theory is constructed. Magnetic trapping of antihydrogen atoms13, 14 provides a means of studying them by combining electromagnetic interaction with detection techniques that are unique to antimatter12, 15. Here we report the results of a microwave spectroscopy experiment in which we probe the response of antihydrogen over a controlled range of frequencies. The data reveal clear and distinct signatures of two allowed transitions, from which we obtain a direct, magnetic-field-independent measurement of the hyperfine splitting. From a set of trials involving 194 detected atoms, we determine a splitting of 1,420.4 ± 0.5 megahertz, consistent with expectations for atomic hydrogen at the level of four parts in 104. This observation of the detailed behaviour of a quantum transition in an atom of antihydrogen exemplifies tests of fundamental symmetries such as charge–parity–time in antimatter, and the techniques developed here will enable more-precise such tests.

    The paper is open access. You may read the entire paper at the Nature website. An overview of the result can be found here.

    Zz.
     
  13. Sep 2, 2017 #193
    I agree. This thread would really make a great category.
     
  14. Sep 2, 2017 #194

    mfb

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    What is a category?

    Individual papers can always get discussed in separate threads in their corresponding subforums.
     
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