Fixed-target Mode Research Articles

Anisotropic Flow Measurements of Identified Hadrons with Fixed-Target Mode of MPD Detector at NICA

Anisotropic Flow Measurements of Identified Hadrons with Fixed-Target Mode of MPD Detector at NICA

First-Order Event Plane Correlated Directed and Triangular Flow from Fixed-Target Energies at RHIC-STAR

We report the measurement of first-order event plane-correlated directed flow (v1) and triangular flow (v3) for identified hadrons (π±, K±, and p), net particle (net-K, net-p), and light nuclei (d and t) in Au + Au collisions at sNN = 3.2, 3.5, and 3.9 GeV in the fixed-target mode from the second phase of the beam energy scan (BES-II) program at RHIC-STAR. The v1 slopes at mid-rapidity for identified hadrons and net particles except π+ are found to be positive, implying the effect of dominant repulsive baryonic interactions. The slope of v1 for net-kaon undergoes a sign change from negative to positive at a lower collision energy compared to net-proton. An approximate atomic mass number scaling is observed in the measured v1 slopes of light nuclei at mid-rapidity, which favors the nucleon coalescence mechanism for the production of light nuclei. The v3 slope for all particles decreases in magnitude with increasing collision energy, suggesting a notable integrated impact of the mean-field, baryon stopping, and collision geometry at lower collision energies.

Light- and hyper-nuclei collectivity in Au+Au collisions at RHIC-STAR

We report the first results on collision energy and particle mass dependence of directed flow v1 of light- and hyper-nuclei in mid-central Au+Au collisions at center of mass energies per nucleon pair of 3.0, 3.2, 3.5, and 3.9 GeV. All data have been collected by the STAR experiment in the fixed-target mode during the second phase of the RHIC beam energy scan. The mid-rapidity v1 slope, dv1/dy|y=0, of hyper-nuclei shows a similar energy and particle mass dependence to that of light-nuclei. The results suggest that the coalescence mechanism plays a dominant role in the formation of light- and hyper-nuclei.

R & D of prototype iTOF-MRPC at CEE

The cooling storage ring (CSR) external-target experiment (CEE) is a spectrometer running at the Heavy Ion Research Facility (HIRFL) at Lanzhou. The CEE is the first large-scale nuclear physics experimental device by China to operate in the fixed-target mode with an energy of ∼1 GeV. The purpose of the CEE is to study the properties of dense nuclear matter. CEE uses a multi-gap resistive plate chamber (MRPC) as its internal time-of-flight (iTOF) detector for the identification of final-state particles. An iTOF-MRPC prototype with 24 gaps was designed to meet the requirements of CEE, and the readout electronics of the prototype use the FPGA-based time digitization technology. Using cosmic ray tests, the time resolution of the iTOF prototype was found to be approximately 30 ps. In order to further understand how to improve the time resolution of MRPC, ANSYS HFSS was used to simulate the signal transmission process in MRPC. The main factors affecting the timing performance of the MRPC and, accordingly, the optimization scheme are presented.

An overview of new measurements of flow, chirality and vorticity from STAR experiment

In relativistic heavy-ion collisions, the properties of quark–gluon plasma (QGP) and complex dynamics of multi-scale processes in Quantum Chromodynamics (QCD) are studied by analyzing the final state produced particles in a variety of different ways. In these proceedings, we present an overview of new detailed measurements of flow, chirality and vorticity by the STAR experiment at Relativistic Heavy Ion Collider (RHIC). Furthermore, STAR’s future opportunities for the precision measurements on small systems, fixed-target (FXT) mode, and Beam Energy Scan (BES-II) program are discussed.

A Monte Carlo Study of Hyperon Production with the MPD and BM@N Experiments at NICA

Study of the strangeness production in heavy-ion collisions is one of the most important parts of the physics program of the BM@N and MPD experiments at the NICA accelerator complex. With collision energies sNN of 2.3–3.3 GeV in the fixed target mode at BM@N and 4–11 GeV in the collider mode at MPD, the experiments will cover the region of the maximum net baryon density and provide high-statistics complementary data on different physics probes. In this paper, some results of Monte Carlo studies of hyperon production with the BM@N and MPD experiments are presented, demonstrating their performance for investigation of the objects with strangeness.

Collective flow of light nuclei and hyper-nuclei in Au+Au collisions at √SNN = 3, 14.6, 19.6, 27, and 54.4 GeV using the STAR detector

Light nuclei and hyper-nuclei are produced in abundance in heavyion collisions. The production mechanism of these species in heavy-ion collisions still remains to be understood. In these proceedings, we report the transverse momentum and centrality dependence of elliptic flow (υ2) of d, t, and 3He in Au+Au collisions at √SNN = 14.6, 19.6, 27, and 54.4 GeV. Mass number scaling of υ2(pT) of light nuclei is shown. We also report the first observation of hyper-nuclei Λ3H and Λ4H directed flow (υ1) in √SNN = 3 GeV mid-central (5-40%) Au+Au collisions in the fixed target mode.

Recombination mechanism for D0-meson production and [formula omitted] production asymmetry in the LHCb p+20Ne fixed-target experiment

We discuss production of neutral D mesons in proton-proton collisions at the LHC (fixed target mode) in the framework of the BJM recombination model. We present rapidity and transverse momentum distributions of D mesons and compare the recombination contribution to the dominant gluon-gluon fusion mechanism. Both the direct production, as dictated by the matrix element, and fragmentation of the associated c or c¯ are included. The latter mechanism generates D mesons with smaller rapidities than those produced directly. We calculate the D0+D‾0 meson distributions relevant for fixed target p+4He collisions at s = 86.6 GeV as well as for p+20Ne collisions at s = 69 GeV. The recombination component is consistent with the LHCb data and in addition results in production asymmetry. The asymmetries in D0−D0‾ production as a function of rapidity and transverse momentum are shown and the cancellation of terms for direct production and associated c/c¯ fragmentation is discussed.

Colliding and Fixed Target Mode in a Single Experiment—A Novel Approach to Study the Matter under New Extreme Conditions

Here, we propose a novel approach to experimentally and theoretically study the properties of QCD matter under new extreme conditions, namely having an initial temperature over 300 MeV and baryonic charge density over three times the values of the normal nuclear density. According to contemporary theoretical knowledge, such conditions were not accessible during the early Universe evolution and are not accessible now in the known astrophysical phenomena. To achieve these new extreme conditions, we proposed performing high-luminosity experiments at LHC or other colliders by means of scattering the two colliding beams at the nuclei of a solid target that is fixed at their interaction region. Under plausible assumptions, we estimate the reaction rate for the p+C+p and Pb+Pb+Pb reactions and discuss the energy deposition into the target and possible types of fixed targets for such reactions. To simulate the triple nuclear collisions, we employed the well-known UrQMD 3.4 model for the beam center-of-mass collision energies sNN = 2.76 TeV. As a result of our modeling, we found that, in the most central and simultaneous triple nuclear collisions, the initial baryonic charge density is approximately three times higher than the one achieved in the ordinary binary nuclear collisions at this energy.

The LHCspin project

Broad and unexplored kinematic regions can be accessed at the LHC with fixed-target pppp, pApA and PbAPbA collisions at \sqrt{s_{NN}}sNN=72-115 GeV. The LHCb detector is a fully-instrumented forward spectrometer able to run in fixed-target mode, and currently hosts a target gas cell to take data in the upcoming Run 3. The LHCspin project aims at extending this physics program to Run 4 and to bring polarised physics at the LHC. An overview of the physics potential and a description of the LHCspin experimental setup are presented.

Fixed-target program upgrade and prospects at LHCb

LHCb has the unique capability to operate in fixed-target mode to study collisions of the LHC beams on fixed targets. In Run3, the internal gas target is going to be upgraded to allow for a wider selection of target gas species and a significant increase of the rates of fixed-target collisions by up to two orders of magnitude. Along with significant data acquisition and tracking upgrades, the SMOG2 system greatly enhances the reach of LHCb’s heavy ion program.

Global Λ -hyperon polarization in Au+Au collisions at sNN=3 GeV

Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While na\"{i}ve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.

Probing Axion‐Like‐Particles at the CERN Gamma Factory

AbstractThe aim of the proposed CERN Gamma Factory is to produce ≈1017 photons per second with energies up to 400 MeV. The photon beam intensity is expected to be a factor of larger than that of the presently available photon beams in the MeV energy range. In this work, its potential to probe physics beyond the Standard Model is explored. In particular, searches for axion like particles (ALPs) with dominant couplings to photons are discussed and various production scenarios—fixed target, photon–photon collision, and conversion by a magnetic field—and detection schemes considered—via decay to photons or back‐conversion. It is found that the Gamma Factory in a fixed target mode can probe ALPs with mass and decay constants larger than 107 GeV, improving by an order of magnitude the discovery potential of previous beam dump experiments.

Flow and interferometry results from Au+Au collisions at sNN=4.5 GeV

The beam energy scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC) was extended to energies below sNN=7.7 GeV in 2015 by successful implementation of the fixed-target mode of operation in the STAR (Solenoidal Tracker At RHIC) experiment. In this mode, ions circulate in one ring of the collider and interact with a stationary target at the entrance of the STAR time projection chamber. The first results for Au+Au collisions at sNN=4.5 GeV are presented, demonstrating good performance of all the relevant detector subsystems in fixed-target mode. Results presented here include directed and elliptic flow of identified hadrons, and radii from pion femtoscopy. The latter, together with recent HADES results, reveal a long-sought peak structure that may be caused by the system evolving through a first-order phase transition from quark-gluon plasma to the hadronic phase. Directed and elliptic flow for pions are presented for the first time at this beam energy. Pion and proton elliptic flow show behavior which hints at constituent quark scaling, and demonstrate that a definitive conclusion will be achievable using the full statistics of the ongoing second phase of BES (BES-II). In particular, BES-II to date has recorded fixed-target data sets with two orders of magnitude more events at each of nine energies between sNN=3.0 and 7.7 GeV.13 MoreReceived 29 July 2020Revised 12 December 2020Accepted 2 March 2021DOI:https://doi.org/10.1103/PhysRevC.103.034908©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective flowParticle correlations & fluctuationsRelativistic heavy-ion collisionsNuclear Physics

The STAR detector upgrades for the BES II and beyond physics program

The Beam Energy Scan program (BES) at the Relativistic Heavy Ion Collider (RHIC) is dedicated to explore the Quantum Chromodynamics (QCD) phase diagram and to search for the critical point of the QCD phase transition. The results from the BES phase I program show hints of a first-order phase transition in the QCD phase diagram and the turn-off of the characteristic signatures of the quark gluon plasma at low collision energies, sNN<20GeV. Three upgrades of the STAR detector, the inner Time Projection Chamber, the Event Plane Detector, and the endcap Time Of Flight, provide unique opportunities to further investigate the nature of the QCD phase diagram during the BES phase II program (BES II), which covers the sNN from 7.7 to 19.6 GeV in the collider mode and from 3 to 7.7 GeV in the fixed-target mode. Beyond the BES II, the STAR Collaboration currently designs, constructs, and installs a suite of new detectors in the forward rapidity region (2.5 <η < 4) over the next two years, enabling a program of novel measurements in p+p, p+A and A+A collisions. This extension of STAR's kinematic reach will allow a detailed study of cold QCD physics at both very high and very low partonic momentum fraction. The new subdetectors to be installed comprise a Forward Calorimeter System, with electromagnetic and hadronic calorimetry. As well as a Forward Tracking System, which consists of 3 layers of silicon mini-strip detectors and 4 layers of small-strip Thin Gap Chambers. In this presentation, the detailed description on the STAR detector upgrades for the BES II and beyond, their performance, as well as the future physics opportunities, will be given.

Single-spin asymmetries in p^uparrow p rightarrow J/psi +X within a TMD approach: role of the color octet mechanism

We calculate the transverse single-spin asymmetry (SSA) for J/psi production in proton-proton collisions, within non-relativistic QCD, employing the transverse momentum dependent, generalized parton model, which includes both spin and intrinsic motion effects. In particular, we study the role of the color octet mechanism in accessing the gluon Sivers function. In doing that, we also show, within this approach, how the singularities coming from color octet terms in the low-P_T region can be handled, leading to finite cross sections. Predictions for both unpolarized cross sections and SSAs are given and compared against PHENIX data. Estimates for LHCb in the fixed target mode are also presented.

Heavy-Ion and Fixed-Target Physics in LHCb

Selected results of the LHCb experiment on heavy ion collisions studied in the collider and fixed-target modes are presented. The clear evidence of the impact of the production mechanism (prompt/delayed, p-p or p-Pb systems) on the pT and rapidity distributions for J/ф, D0 and ϒ(ns) species is demonstrated. The interpretation of the observations in frames of theoretical models is briefly discussed. Some original results, as well as prospects of fixed-target mode studies, are presented.

Phenomenological NLO analysis of ηc production at the LHC in the collider and fixed-target modes

In view of the good agreement between the LHCb prompt-ηc data at s=7 and 8 TeV and the NLO colour-singlet model predictions –i.e. the leading v2 NRQCD contribution–, we provide predictions in the LHCb acceptance for the forthcoming 13 TeV analysis bearing on data taken during the LHC Run2. We also provide predictions for s=115 GeV for proton-hydrogen collisions in the fixed-target mode which could be studied during the LHC Run3. Our predictions are complemented by a full theoretical uncertainty analysis. In addition to cross section predictions, we elaborate on the uncertainties on the pp¯ branching ratio –necessary for data-theory comparison– and discuss other usable branching fractions for future studies.

Single-transverse-spin asymmetries in exclusive photo-production of J/ψ in ultra-peripheral collisions in the fixed-target mode at the LHC and in the collider mode at RHIC

We investigate the potentialities offered by the study of J/ψ exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected cross sections in both set-ups. Studying Single-Transverse-Spin Asymmetries (AN) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) Eg(x,ξ,t). We evaluate the expected precision on AN for realistic conditions with the LHCb detector in pH↑ and PbH↑ collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.

First Measurement of Charm Production in its Fixed-Target Configuration at the LHC.

The first measurement of heavy-flavor production by the LHCb experiment in its fixed-target mode is presented. The production of J/ψ and D^{0} mesons is studied with beams of protons of different energies colliding with gaseous targets of helium and argon with nucleon-nucleon center-of-mass energies of sqrt[s_{NN}]=86.6 and 110.4GeV, respectively. The J/ψ and D^{0} production cross sections in pHe collisions in the rapidity range [2, 4.6] are found to be σ_{J/ψ}=652±33(stat)±42(syst) nb/nucleon and σ_{D^{0}}=80.8±2.4(stat)±6.3(syst) μb/nucleon, where the first uncertainty is statistical and the second is systematic. No evidence for a substantial intrinsic charm content of the nucleon is observed in the large Bjorken-x region.