Dielectron Measurements Research Articles

Thermal dielectron measurements in Au+Au collisions at √SNN = 7.7, 14.6, and 19.6 GeV with the STAR experiment

Dielectrons emitted during the evolution of the hot and dense QCD medium created in relativistic heavy-ion collisions offer an effective way to probe the medium properties, as they do not interact via the strong force. The rate of the dielectron emission is proportional to the medium’s electromagnetic spectral function. In the dielectron invariant mass range from 400 MeV/c2 to 800 MeV/c2, the spectral function probes the in-medium ρ meson propagator which is sensitive to the medium’s properties including the total baryon density and the temperature. Meanwhile, the low energy range of the spectral function provides information about the medium’s electrical conductivity. Therefore, by measuring thermal dielectron production, we can study the microscopic interactions between the electromagnetic current and the medium. The STAR experiment has recorded large datasets of Au+Au collisions during the Beam Energy Scan Phase-II (BES-II) program, spanning center-of-mass energies between √SNN = 3.0 and 19.6 GeV with detector upgrades that benefit the dielectron measurement via extended transverse momentum and rapidity coverages as well as enhanced particle identification capability. In these proceedings, we will report on the measurements of thermal dielectrons produced in Au+Au collisions at √SNN = 7.7, 14.6, and 19.6 GeV using the STAR experiment.

Commissioning of a hadron blind detector for dielectron measurement in pA reactions at J-PARC

The J-PARC E16 experiment measures mass spectra of low-mass vector mesons, ρ, ω, and ϕ, in nuclei via e + e - decays. Vector mesons are produced in pA reactions. A spectrometer with a large acceptance and a high-intensity beam up to 1 × 1010 protons per ∼2-sec duration pulse are utilized to collect a sufficient number of vector mesons. We have developed a spectrometer, which has two types of electron identification detectors: a hadron blind detector (HBD) and a lead-glass calorimeter. Commissioning runs have been performed at the J-PARC high-momentum beam line. The HBD successfully observed on average 11 ± 1 photoelectrons for an incident electron. We achieved the pion rejection of 0.9 ± 0.2% with the electron detection efficiency of 61 ± 4% for the HBD, which was consistent of the expected performance.

ALICE upgrades for Run 4 and Run 5

In view of Run 4 at the LHC, presently scheduled from 2029 onwards, ALICE is pursuing several upgrades to further extend its physics reach. In order to improve heavy-flavor hadron and dielectron measurements which rely on secondary vertexing, a reduction of the material budget of the innermost layers of the Inner Tracking System is needed. This can be achieved with bent pixel sensors, arranged in half-cylinder shapes with integrated power lines and data buses, allowing to get rid of most of the supporting structure and of the water cooling. Moreover, a new Forward Calorimeter (FoCal), covering pseudorapidities of 3.2<η<5.8, has been proposed to measure small-x (down to 10−6) gluon distributions via prompt photon production. The FoCal will be composed of a highly granular Si+W electromagnetic calorimeter combined with a conventional sampling hadronic calorimeter, and will significantly enhance the scope of ALICE for inclusive and correlation measurements with mesons, photons, and jets. For Run 5 and beyond, the ALICE 3 project has been proposed. It consists of a novel compact detector based on monolithic silicon sensors, with ultra-thin layers near the vertex, high readout rate capabilities, superb pointing resolution, excellent tracking and particle identification over a large acceptance. Such a detector enables a rich physics program, ranging from electromagnetic probes at ultra-low transverse momenta to precision physics in the charm and beauty sector. For particle identification, a sequence of detector systems is foreseen: a combination of a Time-Of-Flight system, a Ring-Imaging Cherenkov detector, an electromagnetic calorimeter, a muon identifier, and a dedicated forward detector for ultra-soft photons. In these proceedings, the upgrade plans will be shown together with the status of R&D on ITS3 and FoCal and with the concepts and requirements of ALICE 3.

Dilepton Program with Time-of-Flight Detector at the STAR Experiment

Pairs of lepton and antilepton (dilepton) in a continuous mass range are one of the most experimentally challenging and golden probes of the quark-gluon plasma (QGP) produced in heavy ion collisions because they do not strongly interact with the hot and dense medium, and reflect the properties of the medium at the time the dilepton is generated. The measurements of dileptons require lepton identification with high purity and high efficiency at large detector acceptance. STAR is one of two large experiments at the relativistic heavy ion collider with a primary goal of searching for the QGP and studying its properties. The STAR experiment launched a comprehensive dielectron (e+e−) program enabled by the time-of-flight (TOF) detector that had been fully installed in 2010. In this article, we review the decade-long R&amp;D, the construction and performance of the STAR TOF detector, and dielectron measurements, including thermal dielectron production and dielectron production from the Breit–Wheeler process. Future perspectives are also discussed.

Thermal radiation and direct photon production measurements with dielectrons in Pb–Pb and pp collisions

The latest ALICE results on dielectron measurements in Pb–Pb collisions at √SNN = 5.02 TeV, and in minimum-bias (MB) and high-multiplicity (HM) pp collisions at √S = 13 TeV with LHC Run 2 data are reported. The results are compared to the predictions of dielectron yields from known hadronic sources and thermal radiation from the medium. The contribution of virtual direct photons is extracted from the data by fitting the measured invariant mass distributions. We present the direct to inclusive photon ratio, as well as the direct photon yield in both pp and central Pb–Pb collisions.

Dielectron measurements with the HADES at GSI

In this work we present the dielectron analysis of 4.5 billion Ag+Ag collisions ((0−40)% centrality) at a center-of-mass energy of √sNN = 2.55 GeV measured with HADES. The obtained dielectron signal spectrum is compared to simulated hadronic cocktail and nucleon-nucleon reference spectra revealing a strong contribution from the hot and dense phases. The average temperature of the collision system can be extracted from the slope of the in-medium contribution. In a momentum differential analysis we observe modifications of the dielectron signal shape in the ω−ρ invariant mass region. Furthermore we compare the results to previous measurements in Au+Au collisions at slightly lower energy. The suggested modifications of the ρ spectral function is further studied within the HADES pion-beam program. Here, we present recently published results on the electromagnetic and hadronic coupling of baryonic resonances to the ρ − N final state.

Low-mass dielectron measurements with ALICE at the LHC

Recent measurements of e+e− pair production in pp and p–Pb collisions at the center-of-mass energy √SNN = 5.02 TeV are reported. Cold nuclear matter effects such as shadowing, as well as the possible presence of thermal radiation, are investigated in p–Pb collisions with the dielectron nuclear modification factor RpPb. Furthermore, results on dielectrons at low pT,ee in peripheral Pb–Pb collisions at √SNN = 5.02 TeV and in pp collisions at √S = 13 TeV are presented and compared to calculations.

Low-mass dielectron measurements in pp, p–Pb, and Pb–Pb collisions with ALICE at the LHC

Dielectrons are an excellent probe for the QCD matter created in created in ultra-relativistic heavy-ion collisions, since they are emitted during the whole evolution of the collision and do not interact strongly with the medium. To isolate the QGP signals, measurement of the dielectron production in vacuum and its modifications due to the presence of cold nuclear matter is necessary. We present and discuss results from a low magnetic field detector setup in proton-proton collisions at s=13TeV, as well as the measurement of dielectron production in pp, p–Pb, and Pb–Pb collisions at sNN=5TeV.

Dielectron production in proton-proton and proton-lead collisions at sNN=5.02TeV

The first measurements of dielectron production at midrapidity ($|\eta_{c}|<0.8$) in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass $m_{\rm{ee}}$ and the pair transverse momentum $p_{\rm{T,ee}}$ in the ranges $m_{\rm{ee}}$ < 3.5 GeV/$c^{2}$ and $p_{\rm{T,ee}}$ < 8.0 GeV/$c^{2}$, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at $\sqrt{s}$ = 7 and 13 TeV. The slope of the $\sqrt{s}$ dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for $\rm{e}^{+}\rm{e}^{-}$ pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same $\sqrt{s_{\rm{NN}}}$ via the dielectron nuclear modification factor $R_{\rm{pPb}}$. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.

Electromagnetic Radiation from Au + Au Collisions at √SNN = 2.4 GeV Measured with HADES

We present results of low-mass dielectron measurements from Au+Au collisions at √SNN = 2.4 GeV with HADES. The focus lies on the extraction of the effective temperature from the differential dilepton spectra and the analysis of the azimuthal anisotropy of virtual photons.

Measurement of dielectron production in central Pb-Pb collisions at sNN=2.76TeV

The first measurement of dielectron ($\mathrm{e}^{+}\mathrm{e}^{-}$) production in central (0$-$10$\%$) Pb-Pb collisions at $\mathbf{\sqrt{{\textit{s}}_{\mathrm{NN}}}}$ = 2.76 TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range $0 < m_{\mathrm{ee}}<3.5\ \mathrm{GeV}/\textit{c}^{2}$. The ratio of data and the cocktail of hadronic contributions without vacuum $\rho^{0}$ is measured in the invariant-mass range $0.15 < m_{\mathrm{ee}}<0.7\ \mathrm{GeV}/\textit{c}^{2}$, where an excess of dielectrons is observed in other experiments, and its value is $1.40 \pm 0.28\ (\mathrm{stat.}) \pm 0.08\ (\mathrm{syst.}) \pm 0.27\ (\mathrm{cocktail})$. The dielectron spectrum measured in the invariant mass range $0 < m_{\mathrm{ee}}<1\ \mathrm{GeV}/\textit{c}^{2}$ is consistent with the predictions from two theoretical model calculations that include thermal dielectron production from both partonic and hadronic phases with in-medium broadened $\rho^{0}$ meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs with invariant mass $0.1 < m_{\mathrm{ee}}<0.3\ \mathrm{GeV}/\textit{c}^{2}$, and in the transverse-momentum intervals $1<p_{\mathrm{T}, \mathrm{ee}}<2\ \mathrm{GeV}/\textit{c}$ and $2<p_{\mathrm{T}, \mathrm{ee}}<4\ \mathrm{GeV}/\textit{c}$. The measured fraction of virtual direct photons is consistent with the measurement of real direct photons by ALICE and with the expectations from previous dielectron measurements at RHIC within the experimental uncertainties.

Dielectron measurements in pp and Pb–Pb colllisions with ALICE at the LHC

The production of low-mass dielectrons is one of the most promising tools for the investigation of chiral symmetry restoration and thermal radiation from the QGP created in heavy-ion collisions. To single out the signal characteristics of the QGP, it is crucial to understand the primordial e+e− pair production in vacuum, i.e. in inelastic proton-proton (pp) collisions. Low-mass dielectrons have been measured with ALICE at the LHC in pp collisions at s=7and13TeV, and in Pb–Pb collisions at sNN=2.76TeV. An overview of the results on dielectron production is presented, together with their implications for the direct-photon and heavy-quark production.

Low mass dielectron measurements in pp, p–Pb, and Pb–Pb collisions with ALICE at the LHC

Low mass dielectrons are important probes of the hot and dense QCD medium created in heavy-ion collisions, since the production of low mass dielectrons is sensitive to the thermodynamical properties of the medium and chiral symmetry restoration in the medium.The ALICE experiment at the LHC has measured low mass dielectrons in pp collisions at s=7 TeV and p–Pb collisions at sNN=5.02 TeV to study and disentangle effects due to the cold nuclear matter. Recently, ALICE measured low mass dielectron mass spectra for different pair pT ranges in Pb–Pb collisions at sNN=2.76 TeV for the study of the properties of the hot and dense medium. In this paper, the status of the low-mass dielectron measurements in all collision systems produced at the LHC is summarized and the production of low mass dielectrons including virtual photons is discussed. Future prospects of the low-mass dielectron measurements in ALICE at the LHC Run3 and Run4 will be discussed.

Measurement of dielectrons in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Dielectrons are penetrating probes which carry unaffected information on the hot and dense medium created in ultra-relativistic heavy-ion collisions. The low-mass region of the dielectron spectrum (mπ0 < mee < mρ) is sensitive to virtual photon production and in-medium modifications of low-mass vector meson spectral functions. These modifications are connected to the restoration of chiral symmetry, which is expected at high temperature. Dielectrons are also sensitive to heavy-flavor production.In these proceedings, we present the preliminary results of dielectron measurements in pp collisions at = 7 TeV, in p–Pb collisions at = 5.02 TeV and in central (0-10%) Pb–Pb collisions at = 2.76 TeV with the ALICE experiment at the LHC. The expected scenario and perspectives for the dielectron measurement after the ALICE upgrade are also discussed.

Exploring neutron detection with HADES

The HADES experiment at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt (Germany) is fixed target experiment using SIS-18 accelerator to study collisions of protons, heavy-ions or secondary pions with target nuclei. HADES was designed and provides very accurate measurement of di-electrons and charged hadrons. The pion induced reactions provide unique opportunity to study exclusive reactions with neutrons in the final state. Using the inclusive channel π- + p → π- + π+ n we can optimize the selection criteria for neutron hits in TOF/RPC. Dedicated simulations are compared with preliminary results of real data analysis for reaction channels with two neutral particles in the final state.

Dielectron measurements in pp, p-Pb, and Pb-Pb collisions with the ALICE detector

Dielectrons are excellent tools to investigate the properties of the medium created in heavy-ion collisions. Since dielectrons generated in the medium are not affected by final state interactions, they carry information about the space-time evolution of the collision. The dielectron invariant mass shape is expected to be sensitive to the in-medium modification of vector mesons and thermal radiation from the hot medium created in heavy-ion collisions. In order to understand the nuclear effects, dielectron measurements in pp collisions serve as reference, while p-A collisions are important for the evaluation of the cold nuclear matter effects on the modification of the invariant mass shape. In this proceedings, the dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass spectra in pp at s=7 TeV and p-Pb collisions at sNN=5.02 TeV are extracted and in agreement with the estimated hadronic cocktail calculation. The cross section of the direct photons measured via virtual photons is consistent with NLO pQCD calculations within uncertainties. The uncorrected dielectron yields are also extracted in central and semi-central Pb-Pb collisions at sNN=2.76 TeV. The performance study of the dielectron excess measurement in Pb-Pb collisions by the ALICE detector upgrade is also discussed.

Beam energy dependence of dielectron production in Au+Au collisions from STAR at RHIC

We present the energy-dependent study of dielectron production in 0-80% minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ energies of 19.6, 27, 39, and 62.4 GeV in STAR. Invariant mass ($M_{ee}$) and transverse momentum ($p_T$) differential measurements of dielectron yields are compared to cocktail simulations of known hadronic sources and semi-leptonic charmed decays. The enhancement (excess yield) prominent in the Low-Mass Region (LMR) over the cocktail at all energies, is further compared to calculations of $\rho$ in-medium modifications. Within statistical and systematic uncertainties, we find that the model consistently describes this enhancement from SPS up to top RHIC energies in its $M_{ee}$- as well as $p_T$-dependence. Dielectron measurements drive the statistics for the future BES Phase-II program, which promises to improve our understanding of the LMR enhancement's trend with total baryon densities.

Dielectron measurements in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Electromagnetic probes are excellent messengers from the hot and dense medium created in high-energy heavy-ion collisions. Since leptons do not interact strongly, their spectra reflect the entire space-time evolution of the collision. The surrounding medium can lead to modifications of the dielectron production with respect to the vacuum rate. To quantify modifications in heavy-ion collisions, measurements in pp collisions serve as a reference, while the analysis of p-A collisions allows for the disentanglement of cold nuclear matter effects from those of the hot and dense medium. In this proceedings, dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass distributions in the range $ 0<m_{ee}<3 $ GeV/$ c^{2} $ are compared to the expected yields from hadronic sources for pp collisions at $ \sqrt{s}=7 $ TeV, and for p-Pb collisions at $ \sqrt{s_{\rm NN}}=5.02 $ TeV. The cross section of direct photons measured via virtual photons in pp collisions is compared to predictions from NLO pQCD calculations as a function of the transverse momentum. The status of the analysis of Pb-Pb collisions at $ \sqrt{s_{\rm NN}}=2.76 $ TeV is presented.

Investigation of surface homogeneity of mirrors for the CBM-RICH detector and low-mass di-electron feasibility studies

The Compressed Baryonic Matter (CBM) experiment at the future FAIR facility will investigate high net-baryon density matter at moderate temperatures in A+A collisions from 4-45 AGeV. One of the key observables of the CBM physics program is electromagnetic radiation as a probe of strongly interacting matter in heavy-ion collisions, carrying undistorted information on its conditions to the detector. This includes detailed investigations of low-mass vector mesons in their di-electron channel. A clean and efficient identification of electrons is required for such measurements. In CBM the electron identification will be performed by a Ring Imaging Cherenkov detector and several layers of Transition Radiation Detectors. The RICH detector will be operated with CO2 radiator gas, MAPMTs as photodetector and spherical glass mirrors as focusing elements. A high quality of the mirrors in terms of reflectivity and surface homogeneity is required. In the first part of the contribution results on measurements of the mirror surface homogeneity are presented. Results on the feasibility studies of low-mass di-electron measurements with realistic detector response are discussed in the second part of the contribution.

Measurability of , , and mesons via di-electron decays in high-temperature states produced in heavy-ion collisions

We discuss measurability of phi, omega and rho mesons via di-electron decays in high-temperature states produced in heavy-ion collisions, equivalently at different pion multiplicities per heavy-ion collision dN_{pi^{0} + pi^{+-}}/dy = 1000 and 2700 intended for the most central Au+Au collisions at sqrt(s_{NN}) = 200 GeV (RHIC) and the most central Pb+Pb collisions at sqrt(s_{NN}) = 5.5 TeV (LHC), by evaluating the signal-to-background ratios and the statistical significance for the idealized detection system in the numerical simulation. The simulation study provides a guideline to be applicable to a concrete detector design by focusing on only the key experimental issues relevant to the measurement of di-electrons. The results suggest that there are realizable parameter ranges to measure light vector mesons via di-electrons with the reasonable significance level, even in the highest multiplicity case.