Large Acceptance Spectrometer Research Articles

Quark core-gluon model for heavy hybrid baryons

Besides the ordinary hadrons, quantum chromodynamics (QCD) allows the existence of states in which excitations of the gluonic field can play the role of valence particles, either alone in a glueball, or coupled to quarks in a hybrid. So, hybrid baryons, made of three quarks and a gluon, can exist. Till now, there is no experimental evidence for such exotic hadrons but experimental efforts are being made to search for them at CEBAF Large Acceptance Spectrometer. In this work, a hybrid baryon is considered as a two-body system composed of a color octet three-quark core and a gluon, interacting via a QCD-inspired interaction. A semirelativistic potential model is built in which the dominant interaction is a potential simulating the flux tube confinement, and the Casimir scaling is assumed to link interactions between triplet and octet color sources. This picture is similar to the quark-diquark description for baryons. It is chosen in order to take properly into account the helicity of the gluon. Only cccg and bbbg states are considered because the strong mass asymmetry between the quark core and the gluon is expected to favor the formation of the core. As the results for heavy hybrid baryons seem relevant, we consider this paper as a proof of concept which can be extended for the study of light hybrid baryons. Published by the American Physical Society 2024

Beam spin asymmetry measurements of deeply virtual π0 production with CLAS12

The new experimental measurements of beam spin asymmetry were performed for the deeply virtual exclusive π0 production in a wide kinematic region with the photon virtualities Q2 up to 6.6 GeV2 and the Bjorken scaling variable xB in the valence regime. The data were collected by the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab with longitudinally polarized 10.6 GeV electrons scattered on an unpolarized liquid-hydrogen target. Sizable asymmetry values indicate a substantial contribution from transverse virtual photon amplitudes to the polarized structure functions. The interpretation of these measurements in terms of the Generalized Parton Distributions (GPDs) demonstrates their sensitivity to the chiral-odd GPD E¯T, which contains information on quark transverse spin densities in unpolarized and polarized nucleons and provides access to the nucleon's transverse anomalous magnetic moment. Additionally, the data were compared to a theoretical model based on a Regge formalism that was extended to the high photon virtualities.

Measurement of the helicity asymmetry {mathbb {E}} for the vec {gamma }vec {p} rightarrow p pi ^0 reaction in the resonance region

The double-spin-polarization observable {mathbb {E}} for vec {gamma }vec {p}rightarrow ppi ^0 has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies E_gamma from 0.367 to 2.173~textrm{GeV} (corresponding to center-of-mass energies from 1.240 to 2.200~textrm{GeV}) for pion center-of-mass angles, cos theta _{pi ^0}^{c.m.}, between - 0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will allow us to refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties, which will be presented in a future publication.

Deep learning level-3 electron trigger for CLAS12

Fast, efficient and accurate triggers are a critical requirement for modern high energy physics experiments given the increasingly large quantities of data that they produce. The CEBAF Large Acceptance Spectrometer (CLAS12) employs a highly efficient electron trigger to filter the amount of recorded data by requiring at least one electron in each event, at the cost of a low purity in electron identification. Machine learning algorithms are increasingly employed for classification tasks such as particle identification due to their high accuracy and fast processing times. In this article we show how a convolutional neural network could be deployed as a Level 3 electron trigger at CLAS12. We demonstrate that the Artificial Intelligence (AI) trigger would achieve a significant data reduction compared to the traditional trigger, whilst preserving a 99.5% electron identification efficiency. The AI trigger purity as a function of increased luminosity is improved relative to the traditional trigger. As a consequence, this AI trigger can achieve a data recording reduction improvement of about 65% at standard CLAS12 luminosities when compared to the traditional trigger. A reduction in data output also reduces storage costs and post-processing times, which in turn reduces time to publication of new physics measurements.

CATLIFE (Complementary Arm for Target LIke FragmEnts): Spectrometer for Target like fragments at VAMOS++

The multi-nucleon transfer reaction between 136Xe beam and 198Pt target at the beam energy 7 MeV/u was studied using the large acceptance spectrometer VAMOS++ coupled with the newly installed second arm time-of-flight and delayed γ-ray spectrometer CATLIFE (Complementary Arm for Target LIke FragmEnts). The CATLIFE detector is composed of a large area multi-wire proportional chamber and the EXOGAM HPGe clover detectors with an ion flight length of 1230 mm. Direct measurement of the target-like fragments (TLF) and the delayed γ-rays from the isomeric state helps to improve TLF identification. The use of the velocity of TLFs and the delayed γ-ray demonstrate the proof of principle and effectiveness of the new setup.

J/ψ Near-Threshold Photoproduction off the Proton and Neutron with CLAS12

Near-threshold J/ψ photoproduction is a key aspect of the physics program at the Thomas Jefferson National Accelerator Facility (JLab) 12 GeV upgrade due to the wealth of information it has to offer. J/ψ photoproduction proceeds through the exchange of gluons in the t-channel and is expected to provide unique insight about the nucleon gravitational form factors and the nucleon mass radius. The JLab based CLAS Collaboration, which uses the CEBAF Large Acceptance Spectrometer (CLAS12), aims to measure the J/ψ near threshold photoproduction cross section using both a proton and a deuteron target, from threshold up to 10.6 GeV. The deuteron target further offers the possibility of comparing the proton and neutron gluonic form factors and mass radii in a first measurement of the cross sections off a proton or neutron within the deuteron target. The analysis towards these measurements is ongoing and well advanced, with machine learning based techniques for particle identification already designed and validated with CLAS12 data.

Identification of medium mass (A=60–80) ejectiles from 15 MeV/nucleon peripheral heavy-ion collisions with the MAGNEX large-acceptance spectrometer

An approach to identify medium-mass ejectiles from peripheral heavy-ion reactions in the energy region of 15 MeV/nucleon is developed for data obtained with a large acceptance magnetic spectrometer. This spectrometer is equipped with a focal plane multidetector, providing position, angle, energy loss and residual energy of the ions along with measurement of the time-of-flight. Ion trajectory reconstruction is performed at high order and ion mass is obtained with a resolution of better than 1/150. For the unambiguous particle identification however, the reconstruction of both the atomic number Z and the ionic charge q of the ions is critical and it is suggested, within this work, to be performed prior to mass identification. The new proposed method was successfully applied to MAGNEX spectrometer data, for identifying neutron-rich ejectiles related to multinucleon transfer generated in the 70Zn+64Ni collision at 15 MeV/nucleon. This approach opens up the possibility of employing heavy-ion reactions with medium-mass beams below the Fermi energy (i.e., in the region 15–25 MeV/nucleon) in conjunction with large acceptance ray tracing spectrometers, first, to study the mechanism(s) of nucleon transfer in these reactions and, second, to produce and study very neutron-rich or even new nuclides in previously unexplored regions of the nuclear landscape.

Beam-spin asymmetry Σ for Σ− hyperon photoproduction off the neutron

We report a new measurement of the beam-spin asymmetry, Σ, for the γ→n→K+Σ− reaction using quasi-free neutrons in a liquid-deuterium target. The new dataset includes data at previously unmeasured photon energy and angular ranges, thereby providing new constraints on partial wave analyses used to extract properties of the excited nucleon states. The experimental data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS), housed in Hall B of the Thomas Jefferson National Accelerator Facility (JLab). The CLAS detector measured reaction products from a liquid-deuterium target produced by an energy-tagged, linearly polarised photon beam with energies in the range 1.1 to 2.3 GeV. Predictions from an isobar model indicate strong sensitivity to N(1720)3/2+, Δ(1900)1/2−, and N(1895)1/2−, which corroborates results from a recent combined analysis of all KΣ channels. When our data are incorporated in the fits of partial-wave analyses, one observes significant changes in γ-n couplings of resonances which have small branching ratios to the πN channel.

Improved Λp Elastic Scattering Cross Sections between 0.9 and 2.0 GeV/c as a Main Ingredient of the Neutron Star Equation of State.

Strange matter is believed to exist in the cores of neutron stars based on simple kinematics. If this is true, then hyperon-nucleon interactions will play a significant part in the neutron star equation of state. Yet, compared to other elastic scattering processes, there is very little data on Λ-N scattering. This experiment utilized the CEBAF Large Acceptance Spectrometer (CLAS) detector to study the Λp→Λp elastic scattering cross section in the incident Λ momentum range 0.9-2.0 GeV/c. These are the first data on this reaction since the 1970s. The new cross sections have significantly better accuracy and precision than the existing world data, and the techniques developed here can also be used in future experiments.

Measurement of deeply virtual Compton scattering off He4 with the CEBAF Large Acceptance Spectrometer at Jefferson Lab

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off $^4$He using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized $^4$He gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling $^4$He nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent ($e^4$He$~\to~e'$$^4$He$'\gamma'$) beam spin asymmetries compared to those observed on the free proton, while the incoherent ($e^4$He$~\to~e'$p$'\gamma'$X$'$) asymmetries exhibit a 30$\%$ suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only $^4$He Compton form factor, $\cal H_A$, leading the way toward 3D imaging of the partonic structure of nuclei.

Double polarisation observable [formula omitted] for single pion photoproduction from the proton

We report measurements of π+ and π0 meson photoproduction from longitudinally spin-polarised protons by an energy tagged (0.73-2.3 GeV) and linearly polarised photon beam. A close to complete solid angle coverage for the reaction products was provided by the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The double-polarisation observable G is extracted from Maximum Likelihood fits to the data, enabling the first accurate determination for the reaction γ→p→→π+n, while also significantly extending the kinematic coverage for γ→p→→π0p. This large data set provides an important constraint on the properties and spectrum of excited nucleon states decaying to Nπ in the mass range from 1.4 to 2.2 GeV, as well as for background (non-resonant) photoproduction processes. The considerable improvement achieved in the description of the observable G within the SAID and Bonn-Gatchina approaches after implementation of our data, illustrates that the partial-wave analyses now significantly extend the knowledge on Nπ photoproduction amplitudes at W>1.8 GeV. A partial-wave analysis using the new high-precision data set has a large impact on the extracted properties of high-spin nucleon excited states.

Exploring dense baryonic matter and multi-strangeness at J-PARC

J-PARC is one of the world's highest-intensity proton synchrotron facilities for material and life sciences, neutrino physics, hadron and nuclear physics. By acceleration of heavy-ion beams, J-PARC could also become a high-intensity frontier heavy-ion beam facility (J-PARC-HI). For heavy-ion acceleration, we will build a heavy-ion linac and a booster ring as a compact heavy-ion injector, while we utilize the existing RCS and MR synchrotrons to accelerate 1011 Hz heavy-ion beams at 1–12 AGeV/c. We aim at exploring phase structures of the QCD phase diagram in a high baryon-density regime such as the first-order phase boundary and the QCD critical point, and also aim to search for multi-strangeness systems. We are developing the experimental design for a large acceptance spectrometer. In this work, physics feasibility of J-PARC-HI such as flow and hypernucleus measurements will be shown. Also, a plan to transport heavy-ion beams in the primary proton beamline at Hadron Experimental Facility and an upgrade plan to study heavy-ion collisions at the p+A experiment J-PARC E16 will be presented.

The MUGAST campaign at GANIL

The paper describes the MUGAST campaign at GANIL designed for direct reaction measurement. The physics cases addressed in this campaign deal with shell model evolution, nuclear astrophysics and unbound states. The set-up consist of state-of-art stripped silicon array, MLTGAST coupled with the large acceptance spectrometer, VAMOS and with the state-of-the-art gamma array AGATA. It also accommodates cryogenic targets and specific targets. The challenges of the measurements performed are described.

Extraction of moments from the hard exclusive π+ channel in a wide range of kinematics with CLAS

The beam-spin asymmetry (BSA) has been measured for the hard exclusive e p → e n π+ reaction over a wide range of kinematics in the deep inelastic regime. The measurements were performed with the CEBAF Large Acceptance Spectrometer (CLAS) using a 5.5 GeV polarized electron beam at Jefferson Lab (JLAB). The ϕ dependence of the BSA as well as the −t, Q 2 and xB dependence of the extracted ALU sin(ϕ) moment will be presented. For ALU sin(ϕ) a clear sign change can be observed between pions emitted in forward and backward direction with a smooth transition around 90° in CM. The results will be discussed in the context of formalisms depending on generalized parton distributions (GPDs) and transition distribution amplitudes (TDAs), which can be used to describe complementary kinematic regimes.

Calibration of the electromagnetic calorimeter ECal of the HADES experiment

HADES is a large acceptance spectrometer operating at SIS18, GSI, Germany. It is aimed at exploration of QCD phase diagram at the ion beam energies of 1-2 AGeV in the region of high baryonic densities. The new segmented electromagnetic calorimeter (ECal) was built to extend experimental opportunities of the HADES detector. The electromagnetic calorimeter will allow to study new reaction channels involving the production of neutral mesons and neu-tral resonances in elementary and heavy-ion reactions via detection of their two photon decay. An additional advantage of such a device is the resulting improvement of the electron-to-pion separation at large momenta.The detector is based on 978 Cherenkov lead glass modules divided into 6 sectors, and it covers forward angles of 12° < θ < 45° and almost full azimuthal angle. Currently four out of six sectors planned are assembled in the experimental area. The first raw beam data obtained with the ECal detector in Ag+Ag reactions at 1.65 AGeV beam are presented.

Simulation study of effects induced by final granularity of detector in particle flow

HADES is a large acceptance spectrometer operating at SIS18, GSI, in Darmstadt, Germany [1]. It is aimed at exploration of the QCD phase diagram at the ion beam energies of 1-2 AGeV in the region of high density baryonic matter. An unphysical behavior of particle directed flow deduced from raw experimental data obtained during experimental study of Au+Au collisions at 1.23 AGeV is observed. It is found that dependence of the directed flow on rapidity does not show the antisymmetry expected in the symmetric projectile-target system, and there is significant non-zero directed flow at midrapidity where this component has to be zero, again from the system symmetry reasons. Possible explanation of the effect is due to the final granularity of corresponding detector and hence limited resolution of close particle tracks.

First measurement of direct photoproduction of the a2(1320)0 meson on the proton

We present the first measurement of the exclusive reaction $\gamma p \rightarrow a_2(1320)^0 \, p$ in the photon energy range $3.5$-$5.5$ GeV and four-momentum transfer squared $0.2<-t<2.0$ GeV$^2$. Data were collected with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The neutral $a_2$ resonance was detected by measuring the reaction $\gamma p \rightarrow \pi^0 \eta p$ and reconstructing the $\pi^0 \eta$ invariant mass. The differential cross section $d\sigma/dt$ was extracted at different beam energies in each $-t$ bin. The most prominent feature of the differential cross section is a dip at $-t\simeq 0.55$ GeV$^2$. This can be well described in the framework of Regge phenomenology, where the exchange degeneracy hypothesis predicts a zero in the reaction amplitude for this value of the four-momentum transfer.

Prospects for Studying Hyperons and Hypernuclei on the NICA Collider

NICA is a new flagship project in Dubna aimed at constructing a new accelerator complex for heavy ions. The main goal of the project is experimental exploration of the still poorly known region of the nuclear matter phase diagram of the highest net-baryon density. The MPD detector is a multi-purpose large acceptance spectrometer for studying heavy-ion collisions. An overview is presented of the NICA physics program for studying strangeness production and present MPD performance for reconstructing hyperons and hypernuclei.

The CLAS12 drift chamber system

The CEBAF Large Acceptance Spectrometer at 12 GeV (CLAS12) is located in Hall B, one of the experimental halls at Jefferson Lab. The forward part of CLAS12 is built around a superconducting toroidal magnet. The six coils of the toroid divide the detector azimuthally into six sectors. Each sector contains three multi-layer drift chambers for reconstructing the trajectories of charged particles originating from a fixed target.Each of the 18 planar chambers has two “superlayers” of six layers each, with the wires in the two adjacent superlayers oriented at ±6° stereo angles. Each layer has 112 hexagonal cells spanning a range from about 5° to 40° in polar angle. The six-layer structure provides redundancy in track segment finding and good tracking efficiency even in the presence of some individual wire inefficiency. The design, construction, operation, and calibration methods are described, and estimates of the efficiency and resolution are presented from in-beam measurements.

The CLAS12 software framework and event reconstruction

We describe offline event reconstruction for the CEBAF Large Acceptance Spectrometer at 12 GeV (CLAS12), including an overview of the offline reconstruction framework and software tools, a description of the algorithms developed for the individual detector subsystems, and the overall approach for charged and neutral particle identification. We also present the scheme for data processing and the code management procedures.