Recent Noteworthy Physics Papers

[mfb]

A measurement of the atomic hydrogen Lamb shift and the proton charge radius
N. Bezginov et al, Science 06 Sep 2019: Vol. 365, Issue 6457, pp. 1007-101

Thread about it

The surprising discrepancy between results from different methods for measuring the proton charge radius is referred to as the proton radius puzzle. In particular, measurements using electrons seem to lead to a different radius compared with those using muons. Here, a direct measurement of the n = 2 Lamb shift of atomic hydrogen is presented. Our measurement determines the proton radius to be r_p = 0.833 femtometers, with an uncertainty of ±0.010 femtometers. This electron-based measurement of r_p agrees with that obtained from the analogous muon-based Lamb shift measurement but is not consistent with the larger radius that was obtained from the averaging of previous electron-based measurements.

 

[mfb]

Probing gravity by holding atoms for 20 seconds
Science 08 Nov 2019: Vol. 366, Issue 6466, pp. 745-749 DOI: 10.1126/science.aay6428

Atom interferometers are powerful tools for both measurements in fundamental physics and inertial sensing applications. Their performance, however, has been limited by the available interrogation time of freely falling atoms in a gravitational field. By suspending the spatially separated atomic wave packets in a lattice formed by the mode of an optical cavity, we realize an interrogation time of 20 seconds. Our approach allows gravitational potentials to be measured by holding, rather than dropping, atoms. After seconds of hold time, gravitational potential energy differences from as little as micrometers of vertical separation generate megaradians of interferometer phase. This trapped geometry suppresses the phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters.

The sensitivity seems to be low - measuring g at 10^-7 or 10^-8 or so, 4-5 orders of magnitude worse than the best gravimeters - but the measurement is done within a few millimeters, with the main measurement happening just a few micrometers apart, and it is a new method so we can expect improvements in the future. This might become interesting to measure gravity over short length scales.

 

[Wrichik Basu]

Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN

M. Aker et al. (KATRIN Collaboration)

Phys. Rev. Lett. 123, 221802 – Published 25 November 2019

We report on the neutrino mass measurement result from the first four-week science run of the Karlsruhe Tritium Neutrino experiment KATRIN in spring 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are energy analyzed by a high-resolution MAC-E filter. A fit of the integrated electron spectrum over a narrow interval around the kinematic end point at 18.57 keV gives an effective neutrino mass square value of (−1.0^+0.9_−1.1)  eV². From this, we derive an upper limit of 1.1 eV (90% confidence level) on the absolute mass scale of neutrinos. This value coincides with the KATRIN sensitivity. It improves upon previous mass limits from kinematic measurements by almost a factor of 2 and provides model-independent input to cosmological studies of structure formation.

https://doi.org/10.1103/PhysRevLett.123.221802

https://physics.aps.org/articles/v12/129

 

[Wrichik Basu]

Light Dark Matter Search with Ionization Signals in XENON1T

E. Aprile et al. (XENON Collaboration)

Phys. Rev. Lett. 123, 251801 – Published 17 December 2019

We report constraints on light dark matter (DM) models using ionization signals in the XENON1T experiment. We mitigate backgrounds with strong event selections, rather than requiring a scintillation signal, leaving an effective exposure of (22±3) tonne day. Above ∼0.4  keV_ee, we observe <1  event/(tonne day keV_ee), which is more than 1000 times lower than in similar searches with other detectors. Despite observing a higher rate at lower energies, no DM or CEvNS detection may be claimed because we cannot model all of our backgrounds. We thus exclude new regions in the parameter spaces for DM-nucleus scattering for DM masses mχ within 3–6  GeV/c², DM-electron scattering for m_χ>30  MeV/c², and absorption of dark photons and axion-like particles for m_χ within 0.186–1  keV/c².

Link to the paper: https://doi.org/10.1103/PhysRevLett.123.251801

Related article: https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.123.251801

 

[Wrichik Basu]

Absence of evidence for chiral Majorana modes in quantum anomalous Hall-superconductor devices

Morteza Kayyalha et al.

Science 03 Jan 2020:
Vol. 367, Issue 6473, pp. 64-67

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.

DOI: 10.1126/science.aax6361

Related article in ScienceDaily:
The case of the elusive Majorana: The so-called 'angel particle' is still a mystery

 

[ZapperZ]

L. Prochaska et al., "Singular charge fluctuations at a magnetic quantum critical point." Science v.367, p.285 (2020).

Abstract: Strange metal behavior is ubiquitous to correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in quantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh₂Si₂, a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter.

ArXiv version of this paper may be found here.

This is a very tedious and difficult experiment, especially in the growth of a pristine sample to actually be able to detect this effect. This is another example where the principle of quantum criticality is often most apparent in a condensed matter system.

And oh, note that this material is antiferromagnetic, which is similar to the "parent" or undoped state of the cuprate high-Tc superconductors, that are also believed to have quantum criticality underlying the various properties.

Zz.

 

[Wrichik Basu]

Testing the Seesaw Mechanism and Leptogenesis with Gravitational Waves

Jeff A. Dror et al.

Phys. Rev. Lett. 124, 041804 – Published 28 January 2020

DOI: 10.1103/PhysRevLett.124.041804

We present the possibility that the seesaw mechanism with thermal leptogenesis can be tested using the stochastic gravitational background. Achieving neutrino masses consistent with atmospheric and solar neutrino data, while avoiding nonperturbative couplings, requires right handed neutrinos lighter than the typical scale of grand unification. This scale separation suggests a symmetry protecting the right-handed neutrinos from getting a mass. Thermal leptogenesis would then require that such a symmetry be broken below the reheating temperature. We enumerate all such possible symmetries consistent with these minimal assumptions and their corresponding defects, finding that in many cases, gravitational waves from the network of cosmic strings should be detectable. Estimating the predicted gravitational wave background, we find that future space-borne missions could probe the entire range relevant for thermal leptogenesis.

 

[anorlunda]

Transition from Static to Dynamic Friction in an Array of Frictional Disks
Harish Charan, Joyjit Chattoraj, Massimo Pica Ciamarra, and Itamar Procaccia
Phys. Rev. Lett. 124, 030602 – Published 24 January 2020

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.030602

The nature of an instability that controls the transition from static to dynamical friction is studied in the context of an array of frictional disks that are pressed from above on a substrate. In this case the forces are all explicit and Newtonian dynamics can be employed without any phenomenological assumptions. We show that an oscillatory instability that had been discovered recently is responsible for the transition, allowing individual disks to spontaneously reach the Coulomb limit and slide with dynamic friction. The transparency of the model allows a full understanding of the phenomenon, including the speeds of the waves that travel from the trailing to the leading edge and vice versa.

Popular report here
https://arstechnica.com/science/2020/02/flat-surfaces-surf-past-each-other-on-the-peak-of-a-wave/

I find it interesting that

1. friction can be studied by statistical mechanics,
2. that dynamic friction could be an emergent property
3. that a simulation based on a hypothetical array of disks could relate to real physics.

 

[Wrichik Basu]

Direct Measurements of Collisional Dynamics in Cold Atom Triads

L. A. Reynolds, E. Schwartz, U. Ebling, M. Weyland, J. Brand, and M. F. Andersen
Phys. Rev. Lett. 124, 073401 – Published 18 February 2020

https://doi.org/10.1103/PhysRevLett.124.073401

The introduction of optical tweezers for trapping atoms has opened remarkable opportunities for manipulating few-body systems. Here, we present the first bottom-up assembly of atom triads. We directly observe atom loss through inelastic collisions at the single event level, overcoming the substantial challenge in many-atom experiments of distinguishing one-, two-, and three-particle processes. We measure a strong suppression of three-body loss, which is not fully explained by the presently availably theory for three-body processes. The suppression of losses could indicate the presence of local anticorrelations due to the interplay of attractive short range interactions and low dimensional confinement. Our methodology opens a promising pathway in experimental few-body dynamics.

An article on this paper at phys.org:
https://phys.org/news/2020-02-physicists-individual-atoms-groundbreaking.html

 

[TeethWhitener]

Mesoscale All-Atom Influenza Virus Simulations Suggest New Substrate Binding Mechanism

Jacob D. Durrant, Sarah E. Kochanek, Lorenzo Casalino, Pek U Ieong, Abigail C. Dommer, Rommie E. Amaro
ACS Cent. Sci. 2020, 6, 2, 189-196

https://doi.org/10.1021/acscentsci.9b01071

Influenza virus circulates in human, avian, and swine hosts, causing seasonal epidemic and occasional pandemic outbreaks. Influenza neuraminidase, a viral surface glycoprotein, has two sialic acid binding sites. The catalytic (primary) site, which also binds inhibitors such as oseltamivir carboxylate, is responsible for cleaving the sialic acid linkages that bind viral progeny to the host cell. In contrast, the functional annotation of the secondary site remains unclear. Here, we better characterize these two sites through the development of an all-atom, explicitly solvated, and experimentally based integrative model of the pandemic influenza A H1N1 2009 viral envelope, containing ∼160 million atoms and spanning ∼115 nm in diameter. Molecular dynamics simulations of this crowded subcellular environment, coupled with Markov state model theory, provide a novel framework for studying realistic molecular systems at the mesoscale and allow us to quantify the kinetics of the neuraminidase 150-loop transition between the open and closed states. An analysis of chloride ion occupancy along the neuraminidase surface implies a potential new role for the neuraminidase secondary site, wherein the terminal sialic acid residues of the linkages may bind before transfer to the primary site where enzymatic cleavage occurs. Altogether, our work breaks new ground for molecular simulation in terms of size, complexity, and methodological analyses of the components. It also provides fundamental insights into the understanding of substrate recognition processes for this vital influenza drug target, suggesting a new strategy for the development of anti-influenza therapeutics.

I know it's on the biochem side, but the size of this MD simulation blew my mind: all-atom (160 million atoms) totally solvated 121-ns simulation of an influenza virus in 1-fs timesteps. I remember how big a deal it was about a decade ago when Klaus Schulten's group did the first 1-million atom simulation (of satellite tobacco mosaic virus), but this makes that work look like small potatoes.

 

[TeethWhitener]

Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations

• The T2K Collaboration

Nature volume 580, pages339–344(2020)

https://www.nature.com/articles/s41586-020-2177-0

The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 19641, and CP violation in the weak interactions of quarks was soon established2. Sakharov proposed3 that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis4. Leptonic mixing, which appears in the standard model’s charged current interactions5,6, provides a potential source of CP violation through a complex phase δCP, which is required by some theoretical models of leptogenesis7,8,9. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments10,11. Until now, the value of δCP has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δCP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δCP, which is cyclic and repeats every 2π, is [−3.41, −0.03] for the so-called normal mass ordering and [−2.54, −0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.

Evidence of CP violation in leptons. Seems pretty important.

 

[George Jones]

Evidence of CP violation in leptons. Seems pretty important.

This has been discussed in a thread in the High Energy, Nuclear, Particle Physics forum,

https://www.physicsforums.com/threads/t2k-sees-3-sigma-evidence-for-neutrino-cp-violation.987402/

 

[Wrichik Basu]

GW190521: A Binary Black Hole Merger with a Total Mass of $150\ M_\odot$

R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)

Phys. Rev. Lett. 125, 101102 – Published 2 September 2020

https://doi.org/10.1103/PhysRevLett.125.101102

On May 21, 2019 at 03:02:29 UTC Advanced LIGO and Advanced Virgo observed a short duration gravitational-wave signal, GW190521, with a three-detector network signal-to-noise ratio of 14.7, and an estimated false-alarm rate of 1 in 4900 yr using a search sensitive to generic transients. If GW190521 is from a quasicircular binary inspiral, then the detected signal is consistent with the merger of two black holes with masses of $85^{+21}_{−14}  M_\odot$ and $66^{+17}_{−18} \ M_\odot$ (90% credible intervals). We infer that the primary black hole mass lies within the gap produced by (pulsational) pair-instability supernova processes, with only a 0.32% probability of being below $65 M_\odot$. We calculate the mass of the remnant to be $142^{+28}_{−16}  M_\odot$, which can be considered an intermediate mass black hole (IMBH). The luminosity distance of the source is $5.3^{+2.4}_{−2.6} \ \text{Gpc}$, corresponding to a redshift of $0.82^{+0.28}_{−0.34}$. The inferred rate of mergers similar to GW190521 is $0.13^{+0.30}_{−0.11} \ \text{Gpc}^{−3} \ \text{yr}^{−1}$.