Session S5: Nuclear Physics

Investigating the kinematics of neutron-induced nuclear fission

Nuclear fission is the process by which a radioactive heavy nucleus breaks apart into smaller fragments, releasing energy. When this process occurs spontaneously and two fragments emerge, conservation of momentum requires that they be emitted back-to-back. If the process is caused by a neutron colliding with the nucleus, the fragments will emerge with an opening angle less than 180 degrees. For this investigation we calculated the opening angle of two fragments from the fission of major actinides caused by collisions with neutrons of energies in the range of 0.1 to 100 MeV in both a semi-classical and fully relativistic framework. In the former case, the energy liberated by the fission process, the Q-value of the reaction, is obtained from the semi-empirical mass formula for the particles involved. We find that the opening angles decrease by a few degrees relative to the back-to-back baseline over the range of neutron energies considered in both cases. These results will help us improve the tracking and pointing resolution of the NIFFTE time projection chamber.   

Search for Ternary Fission Events and Analysis With the NIFFTE Time Projection Chamber

Ternary fission is a rare occurrence in which three particles are produced from a single fission event. In most cases the third particle is an alpha or light ion but in some cases the fission can produce three fragments of nearly equal masses. Using tracked fission event data recorded by the NIFFTE collaboration for neutron-induced fission of actinide targets, a series of refined cuts was made to isolate all possible ternary events. An interactive 3-dimensional rendering was constructed in order to visualize these events. The ternary fission candidates were analyzed to determine the average opening angles and fragment energies so that their production rates and properties may be investigated. Preliminary results from these studies will be presented.   

Elastic Lambda-proton Scattering in CLAS

$\Lambda $p elastic scattering can lead to a greater understanding of the strong nuclear force. Obtaining a beam of $\Lambda $s is difficult, however, because they do not exist in nature, and because they decay rapidly. The CLAS Detector at the Thomas Jefferson National Accelerator Facility in Newport News, VA creates many $\Lambda $s using the process $\gamma $p$\to $K$^{\mathrm{+}}\Lambda $. The produced $\Lambda $ can then lead to the elastic scattering process $\Lambda $p$\to \Lambda $p using a second proton in the target. A data-mining project was undertaken by the CSUDH Hadronic Structure Laboratory using data from the CLAS g12 dataset, in which a tagged photon beam with E$_{\mathrm{\gamma }}$ from 3.6 to 5.4 GeV was incident on a liquid hydrogen target. The final state of the process $\gamma $p$\to $K$^{\mathrm{+}}\Lambda $; $\Lambda $p$\to \Lambda $p; $\Lambda \to \pi^{\mathrm{-}}$p is K$^{\mathrm{+}}\pi^{\mathrm{-}}$pp, an apparent violation of baryon number conservation which provides a very stringent cut, reducing the data sample considerably. In the remaining data, we have observed a number of $\Lambda $p$\to \Lambda $p events that is roughly twice the world's data sample. The upgraded CLAS12 detector and improvements to target design may allow better detection rates and will allow the study of more complicated processes beyond simple elastic scattering. This talk will present the motivation for this work, the current status of the project, and future work.   

The Fast Interaction Trigger (FIT) Upgrade to the ALICE experiment at the CERN LHC

The purpose of the ALICE experiment at CERN is to investigate the properties of the strongly interacting quark-gluon plasma formed in the high-energy collisions of lead nuclei in the CERN Large Hadron Collider. ALICE has been collecting data since 2009. The upcoming upgrade of the CERN LHC injectors during 2019-20 will boost the luminosity and the collision rate beyond the design parameters for several of the key ALICE detectors including the forward trigger detectors. The new Fast Interaction Trigger (FIT) will enable ALICE to discriminate beam-beam interactions with a 99\% efficiency for the collisions generated by the LHC at a rate of 50 kHz for Pb-Pb collisions and a rate of 200 kHz for p-p and p-Pb collisions. The FIT detector will be the main forward trigger, luminometer, and collision time detector. It will also determine multiplicity, centrality, and reaction plane of heavy ion collisions. We are involved in the development of the Cherenkov array for FIT, which will employ quartz radiators coupled directly to micro channel plate based photo-multiplier (MCP-PMT) light sensors. This talk will present an overview of the FIT detector and the new research capabilities that it will enable ALICE to explore after the 2019-20 LHC upgrade.   

Upsilon Reconstruction Efficiencies in Heavy-Ion Collisions with the CMS Detector

High energy heavy-ion collisions create a phase of matter characterized by color deconfinement known as the {\it Quark Gluon Plasma} (QGP). Quarkonium states, which are affected by Debye screening in the deconfined medium, are an effective tool for studying properties of the QGP. Relative suppression of excited bottomonium states are studied in heavy-ion collisions at the LHC at $\sqrt{s_{NN}}=5.02$ TeV. Double ratios of the yields of excited states to the ground state are calculated to eliminate various uncertainties that affect all states similarly. Reconstruction efficiency double ratios are needed to quantify the amount of non-cancellation and the possible effects of such non-cancellation on the double ratios. We present a study of the Upsilon efficiency based on Monte Carlo simulations used to model detector response. We illustrate $p_T$, $\eta$, and centrality dependences of the efficiencies in pp and PbPb collisions and present efficiency double ratios for $\Upsilon$ (2S) and $\Upsilon$ (3S). The deviations of the efficiency double ratio from unity for $\Upsilon$ (2S) are estimated to be $1.4\%$ or less, and are considered as systematic uncertainties for the yield double ratios.   

Polarized Heavy Quarkonia Production using the Color Evaporation Model

Even more than 40 years after the discovery of $J/\psi$, the production mechanism of quarkonia is still not well understood. Non-Relativistic Quantum Chromodynamics (NRQCD), perhaps the best known approach for studying quarkonia production can reproduce the $J/\psi$ $p_T$ distribution. However, the long distance matrix elements (LDMEs) fitted from the $p_T$ distributions fail to correctly describe the polarization. In this talk, I will outline the recent challenges to NRQCD and present the first leading order prediction of the polarization using the Color Evaporation Model, which integrates over all color states.   

Determining the CP odd pion-nucleon coupling with spectroscopic lattice QCD calculations: Part I

The universe is observed to have a slight excess of matter over anti-matter, as measured by the primordial baryon-to-photon ratio, $\eta \simeq 6.2\times10^{-10}$, which implies CP violation from beyond the Standard Model (BSM). This has inspired searches for permanent electric dipole moments (EDMs) in nucleons and nuclei, as CP violation gives rise to T violation and permament EDMs. In large nuclei, the EDMs may be dominated by contributions from CP-odd pion-nucleon couplings, as the pion can propagate over the entire nucleus, enhancing this contribution. Some of the largest uncertainties in constraining sources of BSM CP violation is lack of knowledge of these CP-odd pion-nucleon couplings. Lattice QCD can be used to compute these couplings with simple spectroscopic techniques by exploiting symmetries. In this talk, I will describe the relationship between the CP-odd pion-nucleon couplings and related spectroscopic quantities. These couplings depend upon isospin breaking in the hadron spectrum due to $m_{u} \neq m_{d}$. These isovector quantities can only be determined with Lattice QCD. We describe briefly how we include this splitting in our Lattice calculations, and we determine the $2 \delta = (m_{d} - m_{u})$ parameter using the Kaon mass splitting.   

Determining the CP odd pion-nucleon coupling with spectroscopic lattice QCD calculations: Part II

The universe is observed to have a slight excess of matter over anti-matter, as measured by the primordial baryon-to-photon ratio, $\eta \simeq 6.2\times10^{-10}$, which implies CP violation from beyond the Standard Model (BSM). This has inspired searches for permanent electric dipole moments (EDMs) in nucleons and nuclei, as CP violation gives rise to T violation and permament EDMs. In large nuclei, the EDMs may be dominated by contributions from a resulting CP-odd pion-nucleon couplings, as the pion can propagate over the entire nucleus, enhancing this contribution. Some of the largest uncertainties in constraining sources of BSM CP violation is lack of knowledge of these CP odd pion nucleon couplings. Lattice QCD can be used to compute these couplings with simple spectroscopic techniques by exploiting symmetries. In this talk, I will describe the lattice QCD calculation of the neutron-proton mass splitting arising from $m_d - m_u$. Chiral Pertubation Theory, the low energy effective theory of QCD, predicts the presence of a chiral-logarithm term in this quantity. I will show the evidence of this chiral-logarithm that we have observed in the above calculation. This result improves our knowledge of the CP odd pion-nucleon coupling arising from the QCD $\theta$ term.   

The model of complex structure of atomic nucleus

Physicists believe that physical world consists of ordinary matter and dark matter, the ordinary matter is composed of quarks, electrons and neutrinos ultimately, and the dark matter is composed of dark matter particles that physicists are looking for presently. This lecture proposes that: matter exists not only in the form of particle but also in the form of volume field; particle is a form of material existence in point space, it takes displacement motion in the form of continuous motion, particles have interaction between them by exchanging intermediate particles; volume field is a form of material existence in plane space, it takes volume-changing motion (or volume motion) in the form of non-continuous motion (or pulsation), volume fields have interaction between them by overlapping their volume fields. According to ``the combination principal of the least intensity of fundamental body'', this lecture predicts the existence of dark matter particle; according to the absoluteness of volume motion of volume field, this lecture predicts the existence of volume-field-like quark and volume-field-like neutrino. Based on these concepts, I further propose the model of atom-like structure of hadron and the model of molecule-like structure of atomic nucleus. In the end of this lecture, I will discuss the essences of color field force, nuclear force, the collision of high-energy hadrons, black hole, the universe before big bang, and dark matter.