Nuclear Experiment

A summary of the QM19 conference is given by highlighting a few selected results. These are discussed as examples to illustrate the exciting future of heavy-ion collisions and the need for further instrumentation. (The arXiv version is significantly longer than the printed proceedings, with more figures.)

The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at s √ =13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity ( d N ch /dη∼26 ) as measured in p-Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p-Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and Statistical Hadronisation Models (SHM).

In this article, we present a measurement of flow rate, yield and effusion time of a 23 Ne production and transport system. We used an accelerator-driven Li(d,n) neutron source to produce neutrons up to 20 MeV. The radioactive atoms were produced by a 23 Na(n,p) reaction at a NaCl target. Later, the atoms were diffused out from the NaCl crystals and effused from the production chamber via a 10 m hose to a measurement cell and their decay products were detected using high purity germanium (HPGe) and plastic scintillator detectors. The resulting flow rate was $6.9\pm0.5\cdot 10^4\sfrac{atoms}{sec}$ and the total yield was $3.2\pm0.4\cdot10^{-9}\sfrac{atoms}{deuteron}$. We summarize our methods and estimates of efficiencies, rates of production and effusion.

β -decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. To search for new β -delayed protons and γ rays of 25 Si to investigate the properties of 25 Al excited states. 25 Si β decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and γ rays emitted in the decay were detected simultaneously. A Monte Carlo method was used to model the Doppler broadening of 24 Mg γ -ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal \textit{sd}-shell Hamiltonians, USDC and USDI, were performed. The most precise 25 Si half-life to date has been determined. A new proton branch at 724(4)~keV and new proton- γ -ray coincidences have been identified. Three 24 Mg γ -ray lines and eight 25 Al γ -ray lines are observed for the first time in 25 Si decay. The first measurement of the 25 Si β -delayed γ ray intensities through the 25 Al unbound states is reported. All the bound states of 25 Al are observed to be populated in the β decay of 25 Si. Several inconsistencies between the previous measurements have been resolved, and new information on the 25 Al level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of 25 Na and the shell-model calculations. The measured excitation energies, γ -ray and proton branchings, log~ ft values, and Gamow-Teller transition strengths for the states of 25 Al populated in the β decay of 25 Si are in good agreement with the shell-model calculations, offering gratifyingly consistent insights into the fine nuclear structure of 25 Al.

We comment on a recent manuscript by A. P. Serebrov, et al. regarding residual gas charge exchange in the beam neutron lifetime experiment

The proposed high-energy and high-luminosity Electron-Ion Collider (EIC) will provide one of the cleanest environments to precisely determine the nuclear parton distribution functions (nPDFs) in a wide x - Q 2 range. Heavy flavor production at the EIC provides access to nPDFs in the poorly constrained high Bjorken- x region, allows us to study the quark and gluon fragmentation processes, and constrains parton energy loss in cold nuclear matter. Scientists at the Los Alamos National Laboratory are developing a new physics program to study heavy flavor production, flavor tagged jets, and heavy flavor hadron-jet correlations in the nucleon/nucleus going direction at the future EIC. The proposed measurements will provide a unique way to explore the flavor dependent fragmentation functions and energy loss in a heavy nucleus. They will constrain the initial-state effects that are critical for the interpretation of previous and ongoing heavy ion measurements at the Relativistic Heavy Ion Collider and the Large Hadron Collider. We show an initial conceptual design of the proposed Forward Silicon Tracking (FST) detector at the EIC, which is essential to carry out the heavy flavor measurements. We further present initial feasibility studies/simulations of heavy flavor hadron reconstruction using the proposed FST.

The puzzle remains in the large discrepancy between neutron lifetime measured by the two distinct experimental approaches -- counts of beta decays in a neutron beam and storage of ultracold neutrons in a potential trap, namely, the beam method versus the bottle method. In this paper, we propose a new experiment to measure the neutron lifetime in a cold neutron beam with a sensitivity goal of 0.1% or sub-1 second. The neutron beta decays will be counted in a superfluid helium-4 scintillation detector at 0.5 K, and the neutron flux will be simultaneously monitored by the helium-3 captures in the same volume. The cold neutron beam must be of wavelength λ>16.5 A to eliminate scattering with superfluid helium. A new precise measurement of neutron lifetime with the beam method of unique inherent systematic effects will greatly advance in resolving the puzzle.

The future Electron-Ion Collider (EIC) will explore several fundamental questions in a broad Bjorken-x ( x BJ ) and Q 2 phase space. Heavy flavor and jet products are ideal probes to precisely study the tomography of nucleon/nuclei structure, help solve the proton spin puzzle and understand the hadronizaton processes in vacuum or in the QCD medium. Due to the asymmetric collisions at the EIC, most of the final state hadrons are produced in the nucleon/nuclei beam going (forward) direction. A silicon vertex/tracking is critical to precisely measure these forward hadrons at the EIC. Details of different conceptual designs of the proposed Forward Silicon Tracker (FST) and the relevant detector performance are presented in this technical note. The associated heavy flavor and jet studies with the evaluated FST performance are discussed as well.

Charged particle multiplicity fluctuations in pp collisions at s √ = 13 TeV have been studied by the method of scaled factorial moment for the minimum bias events generated by Ultra Relativistic Quantum Molecular Dynamics (UrQMD) model in the pseudo-rapidity ({\eta}), azimuthal angle ( ϕ ) and two dimensional anisotropic ( η - ϕ ) phase space. Strong intermittent type of fluctuations have been observed for the UrQMD simulated data. Systematic studies of intermittent fluctuations in terms of scaled factorial moments was utilized to extract the anomalous fractal dimension in the pseudo-rapidity ({\eta}), azimuthal angle ( ϕ ) and two dimensional anisotropic ( η - ϕ ) phase space. Search for the quark-hadron phase transition in the framework of Ginzburg-Landau theory of second-order phase transition by the analysis with the factorial moment method has also been performed.

With the aim of understanding the phase structure of nuclear matter created in high-energy nuclear collisions at finite baryon density, a beam energy scan program has been carried out at Relativistic Heavy Ion Collider (RHIC). In this mini-review, most recent experimental results on collectivity, criticality and heavy flavor productions will be discussed. The goal here is to establish the connection between current available data and future heavy-ion collision experiments in a high baryon density region.