First Λ Baryons for CBM

The Compressed Baryonic Matter (CBM) experiment at the Facility for Antiproton and Ion Research (FAIR) presently under construction in Darmstadt, Germany, is part of a worldwide research program devoted to study quantum chromodynamics (QCD) matter in the laboratory under extreme conditions. CBM will contribute to the understanding of QCD matter properties and phases at large baryon densities similar to those expected inside the core of heavy neutron stars with unprecedented precision measurements of rare probes; among others, hadrons containing several strange quarks, di-leptons, and hypernuclei [1]. To obtain statistically significant results, CBM is designed to be capable of fully reconstructing up to 107 heavy-ion reactions per second. Hence, preparing for the particle and data rate challenges the demonstrator setup mCBM was installed and is operated at the Society for Heavy Ion Research (GSI) Darmstadt, as shown in Figure 1. The mCBM experiment employs preseries detectors from all CBM subsystems, read out by (close to) final data acquisition components of CBM [2]. The particle trajectories are measured in two stations of the Silicon Tracking System based on double-sided silicon micro-strip sensors, three layers of Transition Radiation Detector (TRD1D, TRD2D) modules and a time-of-flight wall composed of 30 Multi-Gap Resistive Plate Chambers with low resistivity glass electrodes providing a time resolution of 60 ps. The test setup implements the final free-streaming data processing chain of CBM and transports all timestamped raw signal messages via optical links into the compute farm located in the Green IT Cube of GSI. Here, data reformatting, event building, reconstruction, data selection, and archiving are done in a scalable FairMQbased framework. Due to the limited space inside the experimental area, mCBM does not have a magnetic field, which limits the possibilities to define rare probes. At energies presently available to mCBM, Λ – baryon production is a rare process due to its strangeness content. Thus, the weak decay Λ → p + πwith a lifetime of 263 ps represents a suitable test case for the CBM data acquisition and reconstruction concepts. Making use of high-precision tracking and timing devices of CBM, the trajectories and velocities of the daughter particles can be determined accurately enough to identify the secondary vertex and assign momenta to the daughters, allowing the reconstruction of the invariant mass of the mother. Accordingly, preliminary results from a 2-hour-long data-taking period of the reaction Ni + Ni at beam kinetic energy of 1.9 AGeV in May 2022 are depicted in Figure 2 [3]. Within the limited geometrical acceptance of