Dileptons as a penetrating probe of high-$$\mu _\textrm{B}$$ matter

Abstract In heavy-ion collision experiments, it is particularly interesting to probe the stage of the QCD medium’s evolution at the highest temperatures and baryon densities. It cannot be done directly with hadronic observables, as their yields and kinematics are fixed at the late, dilute stage, when the probability for strong interactions diminishes. In contrast, dileptons interact only electromagnetically and thus, effectively decouple from the system immediately after they are produced. The High-Acceptance DiElectron Spectrometer (HADES), while being a multipurpose detector system, has been specifically designed to measure $$e^+e^-$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>-</mml:mo> </mml:msup> </mml:mrow> </mml:math> pairs in hadronic and heavy-ion collisions at SIS18 energies. A comparison of measurements in C+C, Ar+KCl, Ag+Ag, and Au+Au collisions at $$\sqrt{s_\textrm{NN}}\sim 2\div 2.5~\textrm{GeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mtext>NN</mml:mtext> </mml:msub> </mml:msqrt> <mml:mo>∼</mml:mo> <mml:mn>2</mml:mn> <mml:mo>÷</mml:mo> <mml:mn>2.5</mml:mn> <mml:mspace/> <mml:mtext>GeV</mml:mtext> </mml:mrow> </mml:math> allowed HADES to establish the excess dilepton yield over a properly normalized NN (nucleon-nucleon) reference measurement, attributed to the emission from a hot and dense medium. The shape of the invariant mass spectrum is dominated by an exponential fall-off, indicating the thermal character of the medium’s radiation. Dedicated $$p+p$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>p</mml:mi> <mml:mo>+</mml:mo> <mml:mi>p</mml:mi> </mml:mrow> </mml:math> , $$d+p$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>d</mml:mi> <mml:mo>+</mml:mo> <mml:mi>p</mml:mi> </mml:mrow> </mml:math> , and $$\pi ^-+p$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>-</mml:mo> </mml:msup> <mml:mo>+</mml:mo> <mml:mi>p</mml:mi> </mml:mrow> </mml:math> runs allowed to experimentally establish the NN reference for extracting medium physics in heavy-ion collisions, as well as to better understand the microscopic processes, including the Vector Meson Dominance (VMD). More details can be obtained from further observables whose analysis is currently ongoing. They encompass dilepton azimuthal anisotropy (flow), the polarization of virtual photons, and the spectra of extremely soft dileptons. The latter can be related to electrical conductivity – an important transport coefficient of QCD matter – or to a transition to a color-superconducting phase. As a result, HADES dilepton measurements will mark the high $$\mu _\textrm{B}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>μ</mml:mi> <mml:mtext>B</mml:mtext> </mml:msub> </mml:math> end of various excitation functions that cover a wide region of the QCD phase diagram, mapped with several other (future) experiments.