Azimuthal anisotropy and multiplicities of hard photons and free nucleons in intermediate-energy heavy-ion collisions
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Regular Article - Theoretical Physics
Azimuthal anisotropy and multiplicities of hard photons and free nucleons in intermediate-energy heavy-ion collisions S. S. Wang1,2,3 , Y. G. Ma1,a , X. G. Cao4 , D. Q. Fang1, C. W. Ma5 1
Key Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Fudan University, Shanghai 200433, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 3 University of Chinese Academy of Sciences, Beijing 100049, China 4 Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China 5 Institute of Particle and Nuclear Physics, Henan Normal University, Xinxiang 453007, China
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Received: 10 June 2020 / Accepted: 27 September 2020 / Published online: 6 October 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Communicated by Ralf Rapp
Abstract Anisotropic flow can offer significant information of evolution dynamics in heavy-ion collisions. A systematic study of the directed flow v1 and elliptic flow v2 of hard photons and free nucleons is performed for 40 Ca + 40 Ca collisions in a framework of isospin-dependent quantum molecular dynamics (IQMD) model. The study firstly reveals that thermal photons emitted in intermediate-energy heavy-ion collisions have the behaviors of directed and elliptic flows. The interesting phenomena of incident energy dependence of v1 and v2 for thermal photons in central collisions also confirmed that it can be regarded as a good probe of evolution dynamics. Moreover, the multiplicities of hard photons and free nucleons and their correlation are also investigated. We find that direct photon emission is positively related to free nucleons emission, however, there exists an anti-correlation for thermal photons with free nucleons.
1 Introduction Heavy-ion collisions make it possible to investigate on the properties of nuclear matter, especially the phase diagram of nuclear matter where a transition from the Fermi liquid ground state to the nucleon gas phase was predicted [1–10]. Due to the considerable advantage of hard photons not being disturbed by the final-state interactions except for weakly interacting with the surrounding nuclear medium through the electromagnetic interaction, they are very clean probes of the reaction dynamics and can deliver an undisturbed picture of the emitting source [11–18]. Therefore, particular attentions are paid to hard photons emitted from intermediate-energy heavy-ion collisions since hard photons were observed in a e-mail:
experiments [19]. So far experimental [20–25] and theoretical works [26–39] demonstrated that hard photons mainly originate from incoherent bremsstrahlung from individual neutron–proton collisions, which own two distinct sources (i.e. direct photon and thermal photon sources) in space and time according to the experimental evidence as well as the Boltzmann–Uehling–Uhlenbeck (BUU) model calculations [36,37]. Although there are some hot fragments formed in heavy-ion
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