Table of contents

Hacienda Cocoyoc, Morelos, Mexico January 7-10, 2019

Editors:

LUIS ACOSTA

Instituto de Física

Universidad Nacional Autónoma de México

PAULINA AMADOR-VALENZUELA

Instituto Nacional de Investigaciones Nucleares

DANEIL JOSÉ MARÍN-LÁMBARRI

Instituto de Física

Universidad Nacional Autónoma de México

INTERNATIONAL ADVISORY COMMITTEE

Daniel Bardayan University of Notre Dame, USA

Francesco Cappuzzello INFN-Laboratori Nazionali del Sud, Italia

Osvaldo Civitarese Universidad Nacional de La Plata, Argentina

Jerry Draayer Louisiana State University, USA

Alfredo Galindo-Uribarri Oak Ridge National Laboratory, USA

María José García Borge ISOLDE-CERN, Switzerland

Reiner Krücken TRIUMF, Canada

Jorge López University of Texas at El Paso, USA

Takashi Nakamura Tokyo Institute of Technology, Japan

Thomas Papenbrock University of Tennessee, USA

Jorge Piekarewicz Florida State University, USA

Stuart Pittel University of Delaware, USA

Elena Santopinto Università degli Studi di Genova, Italia

W. Michael Snow Indiana University, USA

Adam Szczepaniak Indiana University, USA

Piet Van Isacker GANIL, France

Michael Wiescher University of Notre Dame, USA

ORGANIZING COMMMITTEE

Luis Acosta IF-UNAM

Paulina Amador-Valenzuela ININ

Daniel José Marín-Lámbarri IF-UNAM

SPONSORS

Instituto de Ciencias Nucleares, UNAM

Instituto de Física, UNAM

Instituto Nacional de Investigaciones Nucleares

División de Física Nuclear de la SMF

PREFACE

As every beginning of the year, the 42nd edition of the Symposium on Nuclear Physics was celebrated at the Hacienda Cocoyoc, most precisely, from January 7th to 10th, 2019. The Symposium was early thought as a small meeting designed to bring together some of the leading nuclear scientists around the world. Its most distinctive feature is to provide a forum for specialists in different areas of nuclear physics: both theorists and experimentalists, students, postdocs, senior and junior scientists, in a relaxed and informal environment, providing them with a unique opportunity to exchange ideas and thus, maybe establish new scientific collaborations as well as to upgrade the already existing. After the first meeting in Oaxtepec in 1978, the Symposium was organized every year without interruption, arriving now to its 42nd edition.

The scientific program consisted of 36 invited talks and 15 posters on a wide variety of hot topics in contemporary nuclear physics, ranging from the traditional fields of nuclear structure and reactions to nuclear astrophysics, hadronic physics, relativistic heavy ions, nuclear instrumentation and applied nuclear physics.

At the meeting there were representatives of many large international collaborations to report on the most recent research progress and the most recent developments in laboratories like Notre Dame, Cyclotron Institute Texas A&M and NSCL/MSU (USA); LUNA-LNGS, LNS and LNL from INFN (Italy); the facilities of TRIUMF (Canada), TU-Dresden (Germany) and iThemba Labs (South Africa), as well as recent results obtained in local Mexican facilities like ININ and LEMA.

On the theoretical side, interesting new developments were presented in nuclear dynamics, nuclear structure, Equation of State, neutrino physics, beyond-the-Standard-Model physics, weakly bound nuclei, nuclear clusters, shell model, hadronic physics and QCD.

The high quality of the talks and posters, the expertise of the speakers and the broad spectrum of subjects covered in the meeting, shows that nuclear physics continues being a very active area at the frontier of scientific research which establishes bridges between many different disciplines, putting together theoretical and experimental researchers and having an extension to the instrumentation developments and applications. Is important to mention the large amount of young researchers and students that participated in this Mexican meeting, giving a nice spirit to the Symposium.

This special volume of Journal of Physics: Conference Series contains 23 contributions that were presented on this Symposium edition. We want to thank the support received from the Nuclear Physics Division of the Mexican Physics Society, the IF-UNAM and the ICN-UNAM.

Finally, we would like to thank all our colleagues for their enthusiastic participation in this 42nd edition of the meeting. In particular, we are grateful to the members of the International Advisory Committee for their invaluable suggestions and to the anonymous referees for their time and dedication in helping us to maintain the high standard of the contributions published in the proceedings of the Symposium.

We want as well to thank to Nicoll De Los Ríos who repeated as logo-designer for the 42nd edition of this international Symposium.

Luis Acosta

Paulina Amador-Valenzuela Daniel José Marín-Lámbarri

Conference Photograph available in this pdf.

Conference Logo available in this pdf.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The distribution of the heavy metals ₈₀Hg, ₈₂Pb, ₉₀Th and ₉₂U in the biogeochemical microcosm of three geo and hydro thermal sites of the Los Azufres volcanic complex have been determined by the nuclear analytical techniques of Polarised Energy Dispersive X-Ray Fluorescence spectrometry and Instrumental Neutron Activation Analysis. We have shown that the sites chosen can be considered to be 'real-time' natural laboratories to simulate the Ordovician-Devonian periods. We observe that (1) the primitive, extremophilic plants of the fumaroles – the mosses and ferns – are efficient agents of pedogenesis; (2) that their rhizospheric soil retains more heavy metal than the hydrothermal sediments. The former implies that large-scale photosynthesis and the consequent burial of soil organic carbon could have substantially contributed to the late-Ordovician climate cooling by CO₂ draw-down, and the latter, that dissolved heavy metals in the hydrothermal seas could have been one of the reasons for the marine extinction of that period. This 'holistic' approach at simulating the Paleozoic environment differs from extant purely laboratory-based studies. However, it corroborates the findings of some earlier works.

An updated overview of recent results on Heavy-Ion induced reactions of interest for neutrinoless double beta decay is reported in the framework of the NUMEN project. The NUMEN idea is to study heavy-ion induced Double Charge Exchange (DCE) reactions with the aim to get information on the nuclear matrix elements for neutrinoless double beta (0νββ) decay. Moreover, to infer the neutrino average masses from the possible measurement of the half- life of 0νββ decay, the knowledge of the nuclear matrix elements is a crucial aspect.

Beta - neutrino correlation measurements are key in the research of physics beyond the Standard Model. In pure Fermi beta transitions, the beta-neutrino correlation coefficient, a_βν, is sensitive to the presence of scalar currents. The present limits were established by experimental studies of various nuclear systems with allowed Fermi transitions. A new experiment to improve the constraints on scalar currents is being developed, by the WISArD collaboration at ISOLDE/CERN, where the aim is to measure the energy shift of the β-delayed protons emitted from the isobaric analogue state of the ³²Ar ground state. To enhance the sensitivity, protons and positrons are guided by a strong magnetic field and measured in coincidence between the two detection configurations located on both sides of a catcher foil in which the radioactive samples are implanted. Kinematic energy shifts of the protons in coincidence with positrons, in the same or opposite hemisphere of the catcher foil, will be more or less pronounced as a function of the possible scalar current component of the weak interaction. Details of the apparatus and preliminary results of the experiment are presented.

The ¹⁵O(α,γ)¹⁹Ne reaction is an important trigger reaction leading to the rapid proton (rp) capture process in X-ray bursts. The primary uncertainty in determining its astrophysical rate is the uncertain α branching ratios of levels near E_x = 4.1 MeV in ¹⁹Ne. These states have been populated in a study of the ²⁰Ne(p, d)¹⁹Ne reaction, and α branching ratios are reported in this manuscript.

In this contribution, we present the cluster shell model which is analogous to the Nilsson model, but for cluster potentials. Special attention is paid to the consequences of the discrete symmetries of three α-particles in an equilateral triangle configuration. This configuration is characterized by a special structure of the rotational bands which can be used as a fingerprint of the underlying geometric configuration. The cluster shell model is applied to the nucleus ¹³C.

In the past several years, there has been a large interest of Time Projection Chambers (TPCs) for use in experimental nuclear physics. This has continued in tandem with the requirement for high efficiency detectors with low intensity radioactive ion beams. TexAT is a Active Target TPC (AT-TPC) built at Texas A&M University utilizing MICRO MEsh GASeous (MicroMegas) pads and GET electronics developed specifically for TPCs. This design combines good TPC pixelation with a surrounding shell of Si/CsI telescopes to make an extremely versatile detector capable of a wide range of different experimental techniques with only minor modifications to the electronics setup. Two recent experiments performed at the Cyclotron Institute, Texas A&M University, are detailed here demonstrating versatility beyond the usual Thick Target Inverse Kinematics (TTIK) or transfer reactions that these TPCs are more typically used for. The first, a measurement of the ¹²N → ¹²C^* → 3α decay demonstrates the capabilities of TexAT as a low-energy detector operating at low pressure (20 Torr) to measure β-delayed particle decay. The second, a direct measurement of the ⁸B+⁴⁰Ar fusion cross section shows the advantages of operating in active target mode where the target also functions as the detector gas.

The ³²S(³He,d)³³Cl one-proton transfer reaction is a powerful tool to investigate the spectroscopy of low-lying states in the proton-rich ³³Cl nucleus. However, the extraction of firm differential cross-section data at various angles, against which benchmarking theoretical models to correctly constrain the spectroscopy of ³³Cl, is made challenging by the presence of competitive reaction products contaminating the detected energy spectra. We have recently measured the ³²S(³He,d)³³Cl reaction at 9.8 MeV incident energy by using a new generation hodoscope of silicon detectors, capable to detect and identify emitted deuterons down to energies of the order of 2 MeV. The high angular segmentation of our hodoscope allowed to unambiguously disentangle the contribution of one-proton transfer reactions in the ground state of ³³Cl and in its 0.810 MeV, 2.352 MeV, 2.685 MeV, 2.846 MeV excited states from contaminant deuteron-emitting reactions. These data will be crucial to help to constrain J^π and spectroscopic factor C²S_p values of low-lying ³³Cl states, still ambiguous in the literature. The present status of the analysis is discussed in the paper.

We present an analysis of unpolarized Drell-Yan pair production in pion-nucleus scattering with a particular focus into the pion dynamics. The study consists in analyzing the effect of the partonic longitudinal and, especially, transverse distributions of the pion in a Nambu–Jona-Lasinio (NJL) framework, with Pauli-Villars regularization. In order to consistently take into account the QCD evolution effects, we have estimated the hadronic scale corresponding to the NJL model's degrees of freedom through a minimization procedure at NLO: The NLO evolved pion distributions have been compared to rapidity differential DrellYan cross sections data. That hadronic scale so determined represents the only free parameter in our approach.

The NJL transverse momentum PDF, evolved up to next-to-leading logarithmic accuracy, is then tested against the transverse momentum spectrum of dilepton pairs up to a transverse momentum of 2 GeV. We found a fair agreement with available pion-nucleus data. We find sizable evolution effects on the shape of the distributions and on the generated average transverse momentum of the dilepton pair.

The cross sections of nuclear reactions relevant for astrophysics are crucial ingredients to understand the energy generation inside stars and the synthesis of the elements. In stars, nuclear reactions take place at energies well below the Coulomb barrier. As a result, their cross sections are often too small to be measured in laboratories on the Earth's surface, where the signal would be overwhelmed by the cosmic-ray induced background. An effective way to suppress the cosmic-ray induced background is to perform experiments in underground laboratories. LUNA is a unique facility located at Gran Sasso National Laboratories (Italy) and devoted to Nuclear Astrophysics. The extremely low background achieved at LUNA allows to measure nuclear cross sections directly at the energies of astrophysical interest. Over the years, many crucial reactions involved in stellar hydrogen burning as well as Big Bang Nucleosynthesis have been measured at LUNA. This paper provides a short overview on underground Nuclear Astrophysics and discusses the latest results and future perspectives of the LUNA experiment.

There is a long-standing discrepancy between the neutron lifetime measured in beam and bottle experiments. We propose to explain this anomaly by a dark decay channel for the neutron, involving a dark sector particle in the final state. If this particle is stable, it can be the dark matter. Its mass is close to the neutron mass, suggesting a connection between dark and baryonic matter. In the most interesting scenario a monochromatic photon with energy in the range 0.782 MeV – 1.664 MeV and branching fraction 1% is expected in the final state. We construct representative particle physics models consistent with all experimental constraints.

We investigate the role of the Pauli Exclusion Principle (PEP) for light nuclei, at the examples of ¹²C and ¹⁶O. We show that ignoring the PEP does lead not only to a too dense spectrum at low energy but also to a wrong grouping into bands. Using a geometrical mapping, a triangular structure for ¹²C and a tetrahedral structure in ¹⁶O in the ground state is obtained by using the indistinguishably of the α-particles.

The Trojan Horse Method (THM) makes use of quasi-free reactions to deduce the cross section of nuclear reactions relevant for astrophysics at the energies of interest. Thanks to the suppression of the Coulomb barrier, the THM cross section does not exponentially vanishes at astrophysical energies. Here we will briefly summarise the fundamentals of the method, then we will discuss two applications of the method to reactions that have a pivotal role in the latest stages of stellar evolution, leading to explosive scenarios. In particular, we will focus on the indirect investigation of the ¹⁸F(p,α)¹⁵O reaction, which is the most important ¹⁸F destruction channel in novae, and the ¹²C + ¹²C reaction, which plays a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments, including supernovae.

Accurate studies on ¹³C spectroscopy have great impact in the present understanding of the role played by extra-neutrons in stabilizing α-cluster structures formed in light nuclei. Carbon-13 is in fact the simplest systems that can be formed by adding a neutron to make a triple-α molecular-like structure. The accurate spectroscopy of excited states at energies above the α emission threshold is fundamental to benchmark the calculations of theoretical models aiming to describe clustering in light nuclei.

To improve our knowledge of ¹³C structure, we performed a comprehensive R-matrix fit of α+⁹Be elastic and inelastic scattering data in the energy region E_α ≈ 3:510 MeV at several backward polar angles. To carefully determine the partial decay widths of states above the α-decay threshold we included in the fit also ⁹Be(α n)¹²C cross section data taken from the literature. This analysis allows to improve the (poorly known) spectroscopy of excited states in ¹³C in the E^x ≈ 12-17 MeV region, and tentatively suggests the presence of a large-deformation negative-parity molecular band.

While ab initio many-body techniques have been able to successfully describe the properties of light and intermediate mass nuclei based on chiral effective field theory interactions, neutron-rich nuclei still remain out of reach for these methods. Conversely, energy density functional approaches can be used to calculate properties of heavy nuclei but rely mostly on phenomenological interactions. A usable form of the nuclear energy density functional that is rooted in the modern theory of nuclear forces was presented recently. The first component of this new set of functionals corresponds to the direct part (Hartree term) of the expectation value of local chiral potentials on a Slater determinant. The exchange term, which is a functional of the non-local density, is transformed into a local functional by applying the density matrix expansion. In order to reduce the computational cost due to the direct implementation of non-separable, local interactions in the Hartree term, we use an approximation to represent the regularized Yukawa functions in terms of a sum of (separable) Gaussian functions. These proceedings analyze the accuracy of such an approximation in terms of the number of Gaussian functions and look for an optimal value that gives an acceptable level of accuracy while maintaining the computational memory requirements in a many-body calculation as low as possible.

In this contribution, we discuss the electromagnetic couplings of pentaquark states with hidden charm. This work is motivated by recent experiments at CERN by the LHCb Collaboraton and current experiments at JLab to confirm the existence of hidden-charm pentaquarks in photoproduction experiments.

Asymptotic Giant Branch (AGB) stars have been proven to be sites of F production through spectroscopy observations by several authors, but it is not clear whether these stars might account for the total fluorine abundance of the Galaxy. Recently the two main channels for ¹⁹F destruction in AGB stars, namely the ¹⁹F(α,p)²²Ne and ¹⁹F(p,α)¹⁶O reactions, have been studied via the Trojan Horse Method in the energy range of interest for astrophysics. In both cases experimental results have shown the presence of resonant structures below 500 keV, hinting to an enhancements in efficiency of fluorine destruction by stellar H- and He- burning. In particular the ¹⁹F(p, α)¹⁶O reaction rate at T₉≤ 0.2K turns out to be increased up to a factor of 1.7 while the ¹⁹F(p, α)¹⁶O is enhanced more than a factor of 4 at 0.1 ≤ T₉≤ 0.25. We present here a re-analysis of the role of AGB stars as fluorine galactic source by comparing stellar observations with predictions of AGB nucleosynthesis (for stellar masses from 1.5 to 5M_⊙) computed by employing in state-of-the-art models the THM reaction rates for ¹⁹F destruction.

The PANDA experiment will be one of the pillars of the new Facility for Antiproton and Ion Research (FAIR), which is currently under construction in Darmstadt, Germany. PANDA will be a fixed-target experiment which will allow the study of non-perturbative phenomena of the strong interaction. These will be probed in antiproton-proton collisions in the momentum range of 1.5 - 15 GeV/c. Within the PANDA physics program, strangeness production will be addressed through $\bar{p}p\to \bar{Y}Y$ processes. Measurements of the $\bar{p}p\to {\bar{\sum }}^{0}\Lambda$ channel for its comparison with the existing data of the $\bar{\Lambda }\Lambda$ channel are highly encouraged to study the role of isospin symmetry in hadron production dynamics. This work consists on a simulation study focusing on the feasibility of measuring the $\bar{p}p\to {\bar{\sum }}^{0}\Lambda$ channel at PANDA.

The beta decay of the Tz=-2 nucleus 64Se has been studied in a fragmentation reaction at RIKEN-Nishina Center. 64Se is the heavies Tz=-2 nucleus that decays to bound states in the daughter nucleus and the heaviest case where the mirror reaction 64Zn(3He,t)64Ga on the Tz=+2 64Zn stable target exists and can be compared. Beta-delayed gamma and proton radiation is reported for the 64Se and 64As cases. New levels have been observed in 64As, 64Ge (N=Z), 63Ge and 63Ga. The associated T1/2 values have been obtained.

Present paper reports the mass distribution of fission events in the ¹⁴N+¹⁸¹Ta reaction at four different projectile energies viz., 82.2 ± 0.8, 79.18 ± 0.82, 76.8 ± 1.2 and 72.9 ± 0.91 MeV using recoil catcher technique followed by off-line γ-ray spectrometry. A single peaked broad Gaussian mass curve has confirmed the nonappearance of any non-compound nuclear fission. Further, the variance of the mass distribution is compared with the existing literature data. It has been observed that the lower mass asymmetric system results in a lower variance of the mass distribution.

The first measurements of the capture reactions for the astrophysical γ-process using HECTOR, a total absorption spectrometer are presented. Two reactions: ¹⁰²Pd(p,γ)¹⁰³Ag and ⁹⁰Zr(α,γ)⁹⁴Mo were measured using the γ-summing technique. The results are compared with the previous measurements found in the literature and with the NON-SMOKER predictions. The results from the current work are in a good agreement with the data found in the literature and serve as a test of the newly developed detector.

Two recent experiments have indicated that the break-up of the ¹²C Hoyle state is dominated by the sequential ⁸Be(g.s.) + α decay channel. The rare direct 3α decay was found to contribute with a branching ratio of less than 0.047% (95% C.L.). However, the ability of experimentalists to successfully disentangle these two competing decay channels relies on accurate theoretical predictions of how they each manifest in phase space distribution of the three break-up α-particles. The following paper reviews the current theoretical approaches to calculating the break-up of the Hoyle state and introduces a semi-classical WKB approach, which adequately reproduces the results of more sophisticated calculations. It is proposed that a more accurate upper limit on this branching ratio may be obtained if these new theoretical results are taken into account when analysing experimental data.

The Trojan Horse Method has been applied to many neutron-induced reactions using the deuteron as a virtual source of neutrons, to explore wide energy regions of interest for astrophysics and applied physics and to investigate the suppression of the centrifugal barrier, that is one of the key advantages of this Method. The neutron-induced experimental campaign has already concerned ¹⁷O(n, α)¹⁴C, ⁶Li(n,α)³H, ⁷Be(n,p)⁷Li, ⁷Be(n,α)⁴He, ¹⁸F(n,α)¹⁵O and ¹⁴N(n,p)¹⁴C, and very recently ²⁵Mg(n,α)²²Ne and ²⁷Al(n,p)²⁷Mg, while others are planned to be measured soon, thus influencing different astrophysical scenarios. Particular attention is dedicated to a new measurement regarding the ¹⁰B(n,α)⁷Li reaction, aimed to disentangle the ⁷Li ground state contribution from its first excited state to the cross section.

Various kinds of rare events can be studied but only a small fraction of these are mentioned in these proceedings. The search for rare events is quite some fun but also very hard work as typically very high backgrounds must be tackled for a very small signal. The first process discussed is double beta decay, which is an extremely rare process and requires half-live measurements around 10²⁰ years for the neutrino accompanied mode, while for the neutrinoless mode much longer half-lives are already explored. Nuclear physics input is needed for the matrix elements and a quenching measurement of g_A will be presented. In this case the focus is on the highly forbidden Cd-113 beta decay. New half-lives for the forbidden EC/beta decay measurement of V-50 will be presented. In addition new precision half-lives of long-living alpha decays will be presented as well.