In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged pion transverse-mass and rapidity spectra, as ...well as on pion-pion momentum correlations is investigated in Au+Au collisions at \(\sqrt{s_{NN}}\) = 2.4 GeV. We show that the low-mt part of the data (\(m_t < 0.2\) GeV/c\(^2\)) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential deviates strongly from a \(A_{part}^{2/3}\) scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.
First information on the timelike electromagnetic structure of baryons in the second resonance region has been obtained from measurements of invariant mass and angular distributions in the quasi-free ...reaction \(\pi^- p \to nee\) at \(\sqrt{s_{\pi^- p}}\) = 1.49 GeV with the High Acceptance Di-Electron Spectrometer (HADES) detector at GSI using the pion beam impinging on a CH\(_2\) target. We find a total cross section \(\sigma (\pi^- p \to nee) = 2.97 \pm 0.07^{data} \pm 0.21^{acc} \pm 0.31^{\rm{Z}_{\rm{eff}}} \mu\)b. Combined with the Partial Wave Analysis of the concurrently measured two-pion channel, these data sets provide a crucial test of Vector Meson Dominance (VMD) inspired models. The commonly used "strict VMD" approach strongly overestimates the \(e^+e^-\) yield. Instead, approaches based on a VMD amplitude vanishing at small \(e^+e^-\) invariant masses supplemented coherently by a direct photon amplitude provide a better agreement. A good description of the data is also obtained using a calculation of electromagnetic timelike baryon transition form factors in a covariant spectator-quark model, demonstrating the dominance of meson cloud effects. The angular distributions of \(e^+e^-\) pairs demonstrate the contributions of virtual photons with longitudinal polarization, in contrast to real photons. The virtual photon angular dependence supports the dominance of J=3/2, I=1/2 contributions observed in both the \(\gamma^{\star} n\) and the \(\pi \pi n\) channels.
In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at \sqrt{s_{NN}} = 2.42 GeV collected with HADES. In this energy ...regime the population of \Delta(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dM{\pi}pdpTdy distributions of correlated {\pi}p pairs have been determined by subtracting the {\pi}p combinatorial background using an iterative method. The invariant-mass distributions in the \Delta(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1 < Minv (GeV/c2) < 1.4, the yield is found to be similar for {\pi}+p and {\pi}-p pairs, and to follow a power law ^{\alpha}, where is the mean number of participating nucleons. The exponent {\alpha} depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is {\alpha} = 1.5 \pm 0.08stat \pm 0.2sys.
High precision measurements of flow coefficients \(v_{n}\) (\(n = 1 - 4\)) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at ...\(\sqrt{s_{NN}} = 2.4\) GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentum \(p_{t}\) and rapidity \(y_{cm}\) over a large region of phase space and for several classes of collision centrality. A clear mass hierarchy is found for the slope of \(v_{1}\), \(d v_{1}/d y^{\prime}|_{y^{\prime} = 0}\) where \(y^{\prime}\) is the scaled rapidity, and for \(v_{2}\) at mid-rapidity. Scaling with the number of nucleons is observed for the \(p_{t}\) dependence of \(v_{2}\) and \(v_{4}\) at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation. \(v_{2}\) is found to scale with the initial eccentricity \(\langle \epsilon_{2} \rangle\), while \(v_{4}\) scales with \(\langle \epsilon_{2} \rangle^{2}\) and \(\langle \epsilon_{4} \rangle\). The multi-differential high-precision data on \(v_{1}\), \(v_{2}\), \(v_{3}\), and \(v_{4}\) provides important constraints on the equation-of-state of compressed baryonic matter.
High-statistics \(\pi^-\pi^-\) and \(\pi^+\pi^+\) femtoscopy data are presented for Au+Au collisions at \(\sqrt{s_\mathrm{NN}}=2.4\) GeV, measured with HADES at SIS18/GSI. The experimental ...correlation functions allow the determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the number of participants, irrespective of transverse momentum. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.
Phys. Rev. Lett. 125, 262301 (2020) Flow coefficients $v_{n}$ of the orders $n = 1 - 6$ are measured with the
High-Acceptance DiElectron Spectrometer (HADES) at GSI for protons, deuterons
and tritons ...as a function of centrality, transverse momentum and rapidity in
Au+Au collisions at $\sqrt{s_{NN}} = 2.4$ GeV. Combining the information from
the flow coefficients of all orders allows to construct for the first time, at
collision energies of a few GeV, a multi-differential picture of the angular
emission pattern of these particles. It reflects the complicated interplay
between the effect of the central fireball pressure on the emission of
particles and their subsequent interaction with spectator matter. The high
precision information on higher order flow coefficients is a major step forward
in constraining the equation-of-state of dense baryonic matter.
We present high-statistic data on charged pion emission from Au+Au collisions
at $\sqrt{s_{\rm{NN}}}$ = 2.4 GeV (corresponding to $E_{beam}$ = 1.23 A GeV) in
four centrality classes in the range 0 - ...40$\%$ of the most central collisions.
The data are analyzed as a function of transverse momentum, transverse mass,
rapidity, and polar angle. Pion multiplicity per participating nucleon
decreases moderately with increasing centrality. The polar angular
distributions are found to be non-isotropic even for the most central event
class. Our results on pion multiplicity fit well into the general trend of the
world data, but undershoot by $2.5 \sigma$ data from the FOPI experiment
measured at slightly lower beam energy. We compare our data to state-of-the-art
transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find
substantial differences between the measurement and the results of these
calculations.
Phys. Rev. C 102, 024001 (2020) Pion induced reactions provide unique opportunities for an unambiguous
description of baryonic resonances and their coupling channels by means of a
partial wave ...analysis. Using the secondary pion beam at SIS18, the two pion
production in the second resonance region has been investigated to unravel the
role of the $N(1520) \frac{3}{2}^-$ resonance in the intermediate $\rho$
production. Results on exclusive channels with one pion ($\pi^{-}p$) and two
pions ($\pi^{+}\pi^{-}n$, $\pi^{0}\pi^{-}p$) in the final state measured in the
$\pi^{-}-p$ reaction at four different pion beam momenta (0.650, 0.685, 0.733,
and 0.786 GeV/c) are presented. The excitation function of the different
partial waves and $\Delta\pi$, $N\sigma$ and $N\rho$ isobar configurations is
obtained, using the Bonn-Gatchina partial wave analysis. The $N(1520)
\frac{3}{2}^-$ resonance is found to dominate the $N\rho$ final state with the
branching ratio $BR=12.2 \pm 1.9\%$.
Phys. Rev. C 102, 024914 (2020) We present an analysis of proton number fluctuations in $\sqrt{s_{NN}}$ = 2.4
GeV Au+Au collisions measured with the High-Acceptance DiElectron Spectrometer
(HADES) at ...GSI. With the help of extensive detector simulations done with IQMD
transport model events including nuclear clusters, various nuisance effects
influencing the observed proton cumulants have been investigated. Acceptance
and efficiency corrections have been applied as a function of fine grained
rapidity and transverse momentum bins, as well as considering local track
density dependencies. Next, the effects of volume changes within particular
centrality selections have been considered and beyond-leading-order corrections
have been applied to the data. The efficiency and volume corrected proton
number moments and cumulants Kn of orders n = 1, . . . , 4 have been obtained
as a function of centrality and phase-space bin, as well as the corresponding
correlators C_n . We find that the observed correlators show a power-law
scaling with the mean number of protons, i.e. $C_n \propto ^n$, indicative
of mostly long-range multi-particle correlations in momentum space. We also
present a comparison of our results with Au+Au collision data obtained at RHIC
at similar centralities, but higher $\sqrt{s_{NN}}$.
We investigate identical pion HBT intensity interferometry for central Au+Au collisions at 1.23A GeV. High-statistics \(\pi^-\pi^-\) and \(\pi^+\pi^+\) data are measured with HADES at SIS18/GSI. The ...radius parameters, derived from the correlation function depending on relative momenta in the longitudinal-comoving system and parametrized as three-dimensional Gaussian distribution, are studied as function of transverse momentum. A substantial charge-sign difference of the source radii is found, particularly pronounced at low transverse momentum. The extracted Coulomb-corrected source parameters agree well with a smooth extrapolation of the center-of-mass energy dependence established at higher energies, extending the corresponding excitation functions down towards a very low energy. Our data would thus rather disfavour any strong energy dependence of the radius parameters in the low energy region.
