Fri·Apr 24, 2020Friday·April 24, 2020

1. 01*

   The n${}^3$He Experiment: Parity Violation in Polarized Neutron Capture on ${}^3$He

   n3He Collaboration: M. McCrea (1)🇺🇸 · M.L. Kabir (2)🇺🇸 · N. Birge (3)🇺🇸 · C.E. Coppola (3)🇺🇸 · C. Hayes (3)🇺🇸 · E. Plemons (3)🇺🇸 · A. Ramírez-Morales (4)🇲🇽 · E. M. Scott (3)🇺🇸 · J. Watts (5) · S. Baessler (6 and 7)🇺🇸 · L. Barrón-Palos (4)🇲🇽 · J.D. Bowman (7)🇺🇸 and 16 other authors

   Significant progress has been made to experimentally determine a complete set of the parity-violating (PV) weak-interaction amplitudes between nucleons. In this paper we describe the design, construction and operation of the n${}^3$He experiment that was used to measure the PV asymmetry $A_{\mathrm{PV}}$ in the direction of proton emission in the reaction $\vec{\mathrm{n}}+{}^3\mathrm{He}\to {}^3\mathrm{H}+\mathrm{p}$, using the capture of polarized cold neutrons in an unpolarized gaseous ${}^3\mathrm{He}$ target. This asymmetry has was recently calculated \cite{Viviani,Viviani2}, both in the traditional style meson exchange picture, and in effective field theory (EFT), including two-pion exchange. The high precision result (published separately) obtained with the experiment described herein forms an important benchmark for hadronic PV (HPV) theory in few-body systems, where precise calculations are possible. To this day, HPV is still one of the most poorly understood aspects of the electro-weak theory. The calculations estimate the size of the asymmetry to be in the range of $(-9.4\to 3.5)\times 10^{-8}$, depending on the framework or model. The small size of the asymmetry and the small overall goal uncertainty of the experiment of $\delta A_{\mathrm{PV}}\simeq 1\times 10^{-8}$ places strict requirements on the experiment, especially on the design of the target-detector chamber. In this paper we describe the experimental setup and the measurement methodology as well as the detailed design of the chamber, including results of Garfield++ and Geant4 simulations that form the basis of the chamber design and analysis. We also show data from commissioning and production and define the systematic errors that the chamber contributes to the measured $A_{\mathrm{PV}}$. We give the final uncertainty on the measurement.

   nucl-exphysics.ins-det2 citations
2. 02*

   Positive-parity linear-chain molecular band in ${}^{16}$C

   Y. Liu🇺🇸 · Y. L. Ye🇨🇳 · J. L. Lou🇨🇳 · X. F. Yang🇨🇳 · T. Baba🇯🇵 · M. Kimura🇯🇵 · B. Yang · Z. H. Li🇨🇳 · Q. T. Li🇨🇳 · J. Y. Xu🇨🇳 · Y. C. Ge🇨🇳 · H. Hua🇨🇳 and 27 other authors

   An inelastic excitation and cluster-decay experiment ${}^2\mathrm{H}{(}^{16}\mathrm{C},{}^4\mathrm{He}+{}^{12}\mathrm{Be}\mathrm{or}{}^6\mathrm{He}+{}^{10}\mathrm{Be}){}^2\mathrm{H}$ was carried out to investigate the linear-chain clustering structure in neutron-rich ${}^{16}\mathrm{C}$. For the first time, decay-paths from the ${}^{16}\mathrm{C}$ resonances to various states of the final nuclei were determined, thanks to the well-resolved $Q$-value spectra obtained from the three-fold coincident measurement. The close-threshold resonance at 16.5 MeV is assigned as the $J^{\pi }=0^{+}$ band head of the predicted positive-parity linear-chain molecular band with $(3/2_{\pi }^{-}{)}^2(1/2_{\sigma }^{-}{)}^2$ configuration, according to the associated angular correlation and decay analysis. Other members of this band were found at 17.3, 19.4, and 21.6 MeV based on their selective decay properties, being consistent with the theoretical predictions. Another intriguing high-lying state was observed at 27.2 MeV which decays almost exclusively to ${}^6\mathrm{He}+{}^{10}\mathrm{Be}(\sim 6\mathrm{MeV})$ final channel, corresponding well to another predicted linear-chain structure with the pure $\sigma$-bond configuration.

   nucl-exPRL(2020)·43 citations
3. 03*

   Astrophysics in the Laboratory: The CBM Experiment at FAIR

   P. Senger · for the CBM collaboration

   The future Facility for Antiproton and Ion Research (FAIR) is an accelerator-based international center for fundamental and applied research, which presently is under construction in Darmstadt, Germany. An important part of the program is devoted to questions related to astrophysics, including the origin of elements in the universe and the properties of strongly interacting matter under extreme conditions, which are relevant for our understanding of the structure of neutron stars and the dynamics of supernova explosions and neutron star mergers. The Compressed Baryonic Matter (CBM) experiment at FAIR is designed to measure promising observables in high-energy heavy-ion collisions, which are expected to be sensitive to the high-density equation-of-state (EOS) of nuclear matter and to new phases of QCD matter at high densities. The CBM physics program, the relevant observables and the experimental setup will be discussed.

   nucl-exastro-ph.IMhep-exhep-ph+1Particles(2020)·15 citations
4. 04*

   Probing the core of the strong nuclear interaction

   A. Schmidt🇺🇸 · J. R. Pybus🇺🇸 · R. Weiss🇮🇱 · E. P. Segarra🇺🇸 · A. Hrnjic🇺🇸 · A. Denniston🇺🇸 · O. Hen🇺🇸 · E. Piasetzky🇮🇱 · L. B. Weinstein🇺🇸 · N. Barnea🇮🇱 · M. Strikman🇺🇸 · A. Larionov🇩🇪 and 153 other authors

   The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of Quantum Chromodynamics (QCD). However, as these equations cannot be solved directly, physicists resort to describing nuclear interactions using effective models that are well constrained at typical inter-nucleon distances in nuclei but not at shorter distances. This limits our ability to describe high-density nuclear matter such as in the cores of neutron stars. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations thereby accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta above 400 MeV/c. As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor-force to a predominantly spin-independent scalar-force. These results demonstrate the power of using such measurements to study the nuclear interaction at short-distances and also support the use of point-like nucleons with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of atomic nuclei.

   nucl-exnucl-thNature(2020)·100 citations
5. 05*

   Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at $\sqrt{s}=200$ GeV with the STAR detector at RHIC

   STAR Collaboration: J. Adam · L. Adamczyk · J. R. Adams · J. K. Adkins · G. Agakishiev · M. M. Aggarwal · Z. Ahammed · I. Alekseev · D. M. Anderson · A. Aparin · E. C. Aschenauer · M. U. Ashraf and 352 other authors

   We report on the measurement of the Central Exclusive Production of charged particle pairs $h^{+}{h}^{-}$ ($h=\pi ,K,p$) with the STAR detector at RHIC in proton-proton collisions at $\sqrt{s}=200$ GeV. The charged particle pairs produced in the reaction $pp\to p^{\prime }+h^{+}{h}^{-}+p^{\prime }$ are reconstructed from the tracks in the central detector, while the forward-scattered protons are measured in the Roman Pot system. Differential cross sections are measured in the fiducial region, which roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range $0.04\text{mbox}{GeV}^2<-t_1,-t_2<0.2\text{mbox}{GeV}^2$, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range $|\eta |<0.7$. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of ${\pi }^{+}{\pi }^{-}$ and $K^{+}{K}^{-}$ pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to ${\pi }^{+}{\pi }^{-}$ production. The fiducial ${\pi }^{+}{\pi }^{-}$ cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the $f_0(980)$, $f_2(1270)$ and $f_0(1500)$, with a possible small contribution from the $f_0(1370)$. Fits to the extrapolated differential cross section as a function of $t_1$ and $t_2$ enable extraction of the exponential slope parameters in several bins of the invariant mass of ${\pi }^{+}{\pi }^{-}$ pairs. These parameters are sensitive to the size of the interaction region.

   ↳ hep-exnucl-exJHEP(2020)·29 citations
6. 06*

   Designing Optimal Experiments: An Application to Proton Compton Scattering

   J. A. Melendez🇺🇸 · R. J. Furnstahl🇺🇸 · H. W. Griesshammer🇺🇸 · J. A. McGovern🇬🇧 · D. R. Phillips🇺🇸 · M. T. Pratola🇺🇸

   Interpreting measurements requires a physical theory, but the theory's accuracy may vary across the experimental domain. To optimize experimental design, and so to ensure that the substantial resources necessary for modern experiments are focused on acquiring the most valuable data, both the theory uncertainty and the expected pattern of experimental errors must be considered. We develop a Bayesian approach to this problem, and apply it to the example of proton Compton scattering. Chiral Effective Field Theory ($\chi$EFT) predicts the functional form of the scattering amplitude for this reaction, so that the electromagnetic polarizabilities of the nucleon can be inferred from data. With increasing photon energy, both experimental rates and sensitivities to polarizabilities increase, but the accuracy of $\chi$EFT decreases. Our physics-based model of $\chi$EFT truncation errors is combined with present knowledge of the polarizabilities and reasonable assumptions about experimental capabilities at HI$\gamma$S and MAMI to assess the information gain from measuring specific observables at specific kinematics, \emph{i.e.}, to determine the relative amount by which new data are apt to shrink uncertainties. The strongest gains would likely come from new data on the spin observables ${\Sigma }_{2x}$ and ${\Sigma }_{2x^{\prime }}$ at $\omega \simeq 140$ to $200$ MeV and $40^{\circ }$ to $120^{\circ }$. These would tightly constrain ${\gamma }_{E1E1}-{\gamma }_{E1M2}$. New data on the differential cross section between $100$ and $200$\,MeV and over a wide angle range will substantially improve constraints on ${\alpha }_{E1}-{\beta }_{M1}$, ${\gamma }_{\pi }$ and ${\gamma }_{M1M1}-{\gamma }_{M1E2}$. Good signals also exist around $160$ MeV for ${\Sigma }_3$ and ${\Sigma }_{2z^{\prime }}$. Such data will be pivotal in the continuing quest to pin down the scalar polarizabilities and refine understanding of the spin polarizabilities.

   ↳ nucl-thnucl-exphysics.data-anEPJA(2021)·29 citations
7. 07*

   Evidence for Nonlinear Isotope Shift in Yb${}^{+}$ Search for New Boson

   Ian Counts🇺🇸 · Joonseok Hur🇺🇸 · Diana P. L. Aude Craik🇺🇸 · Honggi Jeon🇰🇷 · Calvin Leung🇺🇸 · Julian Berengut🇦🇺 · Amy Geddes🇦🇺 · Akio Kawasaki🇺🇸 · Wonho Jhe🇰🇷 · Vladan Vuletić🇺🇸

   We measure isotope shifts for five Yb${}^{+}$ isotopes with zero nuclear spin on two narrow optical quadrupole transitions ${}^2{S}_{1/2}\to {}^2{D}_{3/2}$, ${}^2{S}_{1/2}\to {}^2{D}_{5/2}$ with an accuracy of $\sim 300$ Hz. The corresponding King plot shows a $3\times 10^{-7}$ deviation from linearity at the 3 $\sigma$ uncertainty level. Such a nonlinearity can indicate physics beyond the Standard Model (SM) in the form of a new bosonic force carrier, or arise from higher-order nuclear effects within the SM. We identify the quadratic field shift as a possible contributor to the nonlinearity at the observed scale, and show how the nonlinearity pattern can be used in future, more accurate measurements to separate a new-boson signal from nuclear effects.

   ↳ physics.atom-phhep-exnucl-exPRL(2020)·117 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.