HADES Measures Proton-Proton Elastic Scattering at 4.53

The HADES collaboration has released the first physics results from its recently upgraded detector system. The work, detailed in a paper submitted to arXiv in September 2026, measures elastic proton-proton scattering using beams with kinetic energies of 4.53 GeV and 1.60 GeV striking a liquid hydrogen target.

This investigation serves a dual purpose. It provides new high-precision data on a fundamental strong interaction process. It also validates the performance of the upgraded HADES experimental setup.

The Upgraded HADES Detector

The measurements mark the debut of HADES's enhanced capabilities. A key new component is a Forward Detector featuring straw-tube tracking planes. This hardware was originally designed for the future PANDA experiment at FAIR. Its integration into HADES allows for precise tracking of particles scattered at very forward angles.

Researchers exploited the clean, two-body kinematics of elastic scattering. By analyzing the correlated angles and momenta of the scattered proton and recoil proton, the team could perform detailed calibrations. This process refined the detector alignment, verified the beam alignment on the target, and yielded a final, precise measurement of the incident beam energy itself.

Determining Integrated Luminosity

A important output of the study is the determination of the experiment's integrated luminosity. This figure represents the total number of collisions delivered and is essential for calculating absolute cross sections. The HADES team normalized their data in a specific range of the squared four-momentum transfer, denoted as t.

This range was chosen to overlap with established measurements from previous experiments conducted worldwide. By matching their event rates to these well-known reference cross sections, the researchers could accurately calculate the luminosity recorded during their data-taking runs. The paper states this method provides a reference for luminosity determinations by future experiments operating in the same energy regime.

Differential Cross Sections and Slope Parameters

The core physics result is the measurement of the differential cross section, dσ/dt, across a large range of t. For elastic scattering at these energies, the cross section typically follows an exponential decrease with |t|. The rate of this decrease is characterized by the slope parameter, B.

The analysis extracted the following slope parameters from the data:

The parameter B is larger at the higher beam energy, indicating a slower fall-off of the cross section with increasing momentum transfer. These precise values contribute to the global database on proton-proton elastic scattering. They provide critical input for refining theoretical models of the strong force at intermediate energies. The paper concludes that these results offer a valuable benchmark for upcoming experiments.