Dielectron Invariant Mass Spectra Research Articles

Pair invariant mass spectrum and polarization asymmetry in the event generation of gamma-ray conversions

We examine the dielectron invariant mass spectrum and the variation of the polarization asymmetry as a function of that mass in the event generator of the five-dimensional Bethe–Heitler differential cross section of the conversion of linearly polarized photons that we have implemented recently as a Geant4 Physics Model. We compare the results obtained with simulated samples to analytical expressions.Studies of b→sγ decays at LHC and Belle II experiments could provide hints of physics beyond the standard model in flavor-changing neutral current processes; issues have been reported in the simulation of the properties of the background induced by genuine γ-ray conversions in their detectors. Our results demonstrate that using the five-dimensional model would help solve these issues.

Dielectrons and Direct Virutal Photons at RHIC-STAR

Electromagnetic probes are unique probes of the hot and dense medium cre- ated in relativistic heavy-ion collisions due to their minimal interactions with the partonic and hadronic medium. They can be produced at all stages of a collision, thus they provide unique ways to study the medium properties of the whole collision evolution. We present the dielectron invariant mass spectrum at √sNN = 27, 39, 39, and 62.4 GeV. Comparing to hadronic cocktail simulation, significant excesses of dielectron invariant mass spectrum at low mass are observed. The direct virtual photon invariant yields derived from the low-mass e+e− continuum in Au+Au collisions at √sNN = 200 GeV is presented and the excesses from thermal photons in low transverse momentum (pT) have been observed. Model calculations can simultaneously describe both dielectron low mass excesses and direct virtual photon low pT excesses.

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.

Investigating hadronic resonances in pp interactions with HADES

In this paper we report on the investigation of baryonic resonance production in proton-proton collisions at the kinetic energies of 1.25 GeV and 3.5 GeV, based on data measured with HADES. Exclusive channels npπ+ and ppπ0 as well as ppe+e- were studied simultaneously in the framework of a one-boson exchange model. The resonance cross sections were determined from the one-pion channels for Δ(1232) and N(1440) (1.25 GeV) as well as further Δ and N∗ resonances up to 2 GeV/c2 for the 3.5 GeV data. The data at 1.25 GeV energy were also analysed within the framework of the partial wave analysis together with the set of several other measurements at lower energies. The obtained solutions provided the evolution of resonance production with the beam energy, showing a sizeable non-resonant contribution but with still dominating contribution of Δ(1232)P33. In the case of 3.5 GeV data, the study of the ppe+e- channel gave the insight on the Dalitz decays of the baryon resonances and, in particular, on the electromagnetic transition form-factors in the time-like region. We show that the assumption of a constant electromagnetic transition form-factors leads to underestimation of the yield in the dielectron invariant mass spectrum below the vector mesons pole. On the other hand, a comparison with various transport models shows the important role of intermediate ρ production, though with a large model dependency. The exclusive channels analysis done by the HADES collaboration provides new stringent restrictions on the parameterizations used in the models.

Charm quarks in medium and their contribution to di-electron spectra in relativistic heavy ion collisions

We study the dynamics of charm quarks in the partonic medium and its implication for the di-electron spectra in high-energy heavy ion collisions. The charm quarks traversing a thermal medium are simulated by the relativistic Langevin equation for elastic scattering of charm quarks by thermal partons in an expanding fireball. The transport coefficients of the charm quarks are calculated by the in-medium $T$-matrix method, where a static heavy quark potential is used with parameters fitted by the lattice QCD results. The di-electron invariant mass spectra are computed in the most central collisions and are compared to the STAR data. The angular correlations of di-electrons are almost the same in $p+p$ and $\text{Au}+\text{Au}$ collisions in the mass range $1.1&lt;M&lt;2.5\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}/{c}^{2}$ with a back-to-back feature. This means that the angular correlation is intact even with medium interaction at the energy of the Relativistic Heavy Ion Collider.

Dielectron differential cross section in Au + Au collisions at different beam energies at STAR

Abstract Dielectron invariant mass spectra have been measured at mid-rapidity by the STAR experiment at RHIC in Au + Au collisions at s N N = 19.6 , 39, 62.4, and 200 GeV. The spectra exhibit excess yields for M e e 1.0 GeV / c 2 , when compared to the expected contributions from known hadronic sources. The enhanced production rates can be consistently described by a broadened ρ spectral function. In addition, the elliptic flow, ν 2 , has been measured for dielectron invariant mass M e e 1.1 GeV / c 2 in Au + Au collisions at s N N = 200 GeV .

Propagation of spectral functions and dilepton production at SIS energies

The time evolution of vector meson spectral functions is studied within a BUU-type transport model. Applications focus on ρ and ω mesons being important pieces for the interpretation of the dielectron invariant mass spectrum. Since the evolution of the spectral functions is driven by the local density, the inmedium modifications turn out to compete, in this approach, with the known vacuum contributions.

Measurements of low mass [formula omitted] pairs in p+p and Au+Au collisions with the HBD upgrade of the PHENIX detector

The Hadron Blind Detector (HBD) was developed, installed and successfully operated in the PHENIX detector during RHIC Runs in 2009 (p+p) and 2010 (Au+Au). The HBD is a windowless Cherenkov detector, operated with CF 4 gas in proximity focus configuration. It uses triple GEM elements for signal amplification with CsI photocathode evaporated on top of the first GEM. The purpose of the HBD is to reduce the combinatorial background from the dielectron invariant mass spectrum by recognizing and rejecting e + or e − originating from π 0 Dalitz decays and conversion. This article reports on the in-beam performance of the HBD.

Off-shell transport of broad resonances and dilepton production at SIS energies

The time evolution of vector meson spectral functions is studied within a Boltzmann-Uehling-Uhlenbeck (BUU) type transport model. Applications focus on ρ and ω mesons being important pieces for the interpretation of the dielectron invariant mass spectrum. Since the evolution of the spectral functions is driven by the local density, the in-medium modifications turn out to compete, in this approach, with the known vacuum contributions.

ψ′ to J/ψ ratio measurements in PHENIX at RHIC

The ratio of the $\psi'$ over the $J/\psi$ production cross section in the dielectron channel has been measured in $\sqrt{s}=$ 200 GeV $p+p$ collisions with the PHENIX detector at RHIC. The analysis is based on fitting of the dielectron invariant mass spectra in the area around the $J/\psi$ and $\psi'$ signals in order to extract a ratio $\psi'$ over $J/\psi$ of 0.019$\pm 0.005($stat$)\pm 0.002($sys$)$ and a fractional feed-down contribution to $J/\psi$ from $\psi^\prime$ of $8.6 \pm 2.5 %$.

Search for a heavy neutral gauge boson in the dielectron channel with 5.4 fb−1 of [formula omitted] collisions at [formula omitted

We report the results of a search for a heavy neutral gauge boson Z′ decaying into the dielectron final state using data corresponding to an integrated luminosity of 5.4 fb−1 collected by the D0 experiment at the Fermilab Tevatron Collider. No significant excess above the standard model prediction is observed in the dielectron invariant-mass spectrum. We set 95σ(pp¯→Z′)×BR(Z′→ee) depending on the dielectron invariant mass. These cross section limits are used to determine lower mass limits for Z′ bosons in a variety of models. For the sequential standard model Z′ boson a lower mass limit of 1023 GeV is obtained.

In-Medium Modifications of Low-Mass Vector Mesons in PHENIX at RHIC

Measurements at RHIC have established the creation of a Quark Gluon Plasma (QGP) in most central heavy-ion collisions. An important tool to understand properties of the QGP is study of the spectral shapes of low-mass vector mesons (LVM's), ρ, ω and ϕ, which can be modified in the medium by partial restoration of chiral symmetry. This modification may be accessed directly by measuring low-momentum LVM's via their decays into lepton pairs inside the hot matter. Since leptons are not subject to the strong interaction, they do not rescatter on their way out of the medium. The PHENIX experiment at RHIC has measured LVM production at mid-rapidity in p+ p, d+Au and Au+Au collisions at s N N = 200 GeV . Mass peaks for the LVM's have been observed in the di-electron invariant mass spectra with a resolution of 10 MeV/ c 2 in all of the three collision systems. The extracted spectra, mass and width of ω and ϕ in p+ p, d+Au and Au+Au, in the leptonic and hadronic decay channels are reviewed. As the widths of the mesons may be affected in the medium, the branching ratios of various decay modes may also be modified from the values in vacuum. The relative branching ratio is compared between ϕ → e + e − and ϕ → K + K − , which may be sensitive to the mass modification due to the small Q-value of ϕ → K + K − .

Lepton pairs from thermal mesons

We study the net dielectron production rates from an ensemble of thermal mesons, using an effective Lagrangian to model their interaction. The coupling between the electromagnetic and the hadronic sectors is done through the vector meson dominance approach. For the first time, a complete set of light mesons is considered. We include contributions from decays of the type $V(P)\ensuremath{\rightarrow}P(V)+{e}^{+}{e}^{\ensuremath{-}}$, where $V$ is a vector meson and $P$ is a pseudoscalar, as well as those from binary reactions $P+P$, $V+V$, and $V+P\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$. Direct decays of the type $V\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ are included and shown to be important. We find that the dielectron invariant mass spectrum naturally divides into distinct regions: in the low mass domain the decays from vector and pseudoscalar mesons form the dominant contribution. The pion-pion annihilation and direct decays then pick up and form the leading signal in an invariant mass region that includes the $\ensuremath{\rho}\ensuremath{-}\ensuremath{\omega}$ complex and extends up to the $\ensuremath{\varphi}$. Above the invariant mass $M\ensuremath{\approx}1$ GeV other two-body reactions take over as the prominent mechanisms for lepton pair generation. These facts will have a quantitative bearing on the eventual identification of the quark-gluon plasma.

Dielectron production in proton-nucleus reactions.

Dielectron production in proton-nucleus reactions is studied in the cascade model. In addition to production from the proton-neutron bremsstrahlung and the decay of delta, we have also taken into account the contribution from the pion-nucleon interaction. The calculated dielectron invariant mass spectra are compared to the data obtained recently for proton-Be reactions at incident energies of 1.05, 2.1, and 4.9 GeV.