Heavy-Ion Collisions Reveal Photon Emission Sources at Fermi Energies.

Analyses of heavy-ion collisions at Fermi energies reveal that hard photons primarily originate from initial nucleon-nucleon collisions, exhibiting collective nuclear motion. Subsequent thermal emission contributes less significantly to the observed photon energy spectrum, as demonstrated by calculations mirroring Ca-Ca collisions at 35 A MeV.

The intense conditions created during collisions of heavy ions, such as those studied in nuclear physics, briefly recreate the extreme temperatures and densities thought to have existed shortly after the Big Bang. Understanding the electromagnetic radiation, specifically photons, emitted from these fleeting events provides valuable insight into the fundamental forces governing matter at its most basic level. Recent research, detailed in the article ‘Photon Emission from Nucleon-Nucleon Bremsstrahlung in Fermi-energy Heavy-Ion Collisions’, investigates the origin of these photons, focusing on those produced by the braking of nucleons – protons and neutrons – during collisions. Thomas Onyango, affiliated with both Texas A&M University and Lawrence Livermore National Laboratory, alongside Ralf Rapp of Texas A&M University, present a theoretical analysis utilising a field-theoretical model and nucleon distribution functions derived from transport simulations to quantify the contribution of initial, non-equilibrium collisions and the subsequent thermalisation process to the observed photon spectrum from calcium-calcium collisions. Their work aims to reconcile theoretical predictions with experimental measurements, refining our understanding of matter under extreme conditions.
Heavy-ion collisions at Fermi energies, involving the acceleration of ions to energies typically measured in millions of electron volts, provide valuable information about the behaviour of dense nuclear matter, the state of matter existing within the nuclei of atoms compressed to extremely high densities. Researchers actively investigate direct photon emission, the production of photons directly during the collision rather than as secondary products of particle decay, as a means of probing the dynamics of these collisions. They utilise a sophisticated model integrating field-theoretical calculations of photon emission rates with nucleon distribution functions, which describe the probability of finding nucleons (protons and neutrons) at specific locations and with specific momenta, derived from transport model simulations. This approach allows for a detailed understanding of the complex processes occurring during these collisions.

The model explicitly accounts for the non-equilibrium dynamics present in the initial stages of collisions, particularly along the beam direction, recognising that the system is not immediately in thermal equilibrium after impact. Simultaneously, it incorporates a thermal description of transverse momentum distributions, the motion of particles perpendicular to the beam direction, for a more realistic representation of the process. Calculations quantify the contributions to the photon energy spectrum from both first-chance nucleon-nucleon collisions, where nucleons collide for the first time, and the subsequent transition to a thermal source, where the system approaches a state of thermal equilibrium. These calculations are specifically performed for calcium-calcium (Ca-Ca) collisions at a bombarding energy of 35 A MeV, where ‘A’ represents the mass number of the nucleus.

Results demonstrate that the majority of hard photons, those with relatively high energies, originate during the initial stages of heavy-ion collisions, stemming from primordial collisions where nucleons retain their initial collective nuclear motion. Emission from later stages contributes a sub-dominant component to the observed spectrum, confirming the importance of initial collision dynamics in photon production. A key component of the work involves a detailed mathematical derivation presenting the calculation of the electromagnetic factor, a quantity influencing photon emission, crucial for determining the probability of photon production. This factor accounts for the interaction between charged particles and electromagnetic fields, governing the emission of photons.

Researchers derive this electromagnetic factor through integration over all possible photon directions, employing approximations including the soft-photon limit, which simplifies calculations by considering photons with relatively low energies, and non-relativistic approximations, valid when particle velocities are much lower than the speed of light, to simplify calculations. The resulting expression aligns with established theoretical models, as evidenced by comparison with results presented in references [41] and [42], validating the accuracy of the approach.

Quantitative comparison with experimental data from calcium-calcium collisions at 35 A MeV reveals strong agreement, validating the model’s predictive power and confirming the theoretical framework. The inclusion of detector acceptance cuts, which simulate the limitations of the experimental apparatus in detecting photons emitted in certain directions, in the calculations further enhances the fidelity of the comparison, demonstrating the model’s ability to accurately reproduce observed differential photon-energy spectra. This agreement supports the assertion that primordial nucleon-nucleon collisions dominate hard photon production, solidifying the understanding of the underlying mechanisms.

Future research will focus on extending this model to explore the effects of different nuclear systems and collision energies, providing a more comprehensive understanding of photon emission in heavy-ion collisions. Researchers also plan to investigate the sensitivity of the results to the underlying equation of state of nuclear matter, a relationship describing the pressure, temperature and density of nuclear matter, potentially providing new insights into the properties of dense nuclear matter. This work represents a significant step forward in our understanding of the complex dynamics of heavy-ion collisions and the role of direct photon emission as a probe of the collision process.