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PYTHIA [27] is an event generator that is extensively and successfully used for the study of proton-proton and proton-lepton collisions. In pp collisions, a multi-parton interaction (MPI) is generated under the assumption that every partonic interaction is almost independent. PYTHIA8 does not natively support heavy-ion systems. Recently, the PYTHIA8 Angantyr model [28] extrapolated pp dynamics into heavy-ion collisions using the PYTHIA8 event generator, enabling the study of heavy nuclei collisions, namely, proton-nuclei (pA) and nuclei-nuclei (AA) collisions. The Angantyr model combines several nucleon-nucleon collisions into one heavy-ion collision. It is a combination of many-body physics (theoretical) models suitable for producing hard and soft interactions, initial and final-state parton showers, particle fragmentations, multi-partonic interactions, color reconnection (CR) mechanisms, and decay topologies. However, it does not include any mechanism of the QGP medium believed to be produced in AA collisions.
In the current version of the PYTHIA8 Angantyr model [29], in a heavy-ion collision, each projectile nucleon can interact with several target nucleons, and the number of participant nucleons is determined by the Glauber model. This model incorporates several algorithms to distinguish different types of nucleon-nucleon interactions, such as elastic, diffractive, and absorptive interactions, and its purpose is to effectively describe final-state properties, such as multiplicity and transverse momentum distributions in AA collisions [30, 31].
We use the 8.308 version of PYTHIA8 in this study. A simulation is considered with different PYTHIA tunes using MPI and CR configurations. Approximately one million events are generated for each collision energy in the Au + Au collisions. In the PYTHIA8 Angantyr model, both the density and density fluctuation are dependent on system volume. To remove the system volume effect, we use two dimensionless statistical quantities,
$ \langle (\delta p)\rangle /\langle p\rangle $ and$ \langle (\delta n)\rangle /\langle n\rangle $ (Sec. II.B). The nucleons are extracted for different rapidity ranges and centralities. We define the centrality intervals based on the summed transverse energy ($ \sum E_T $ ) in the pseudorapidity interval [–0.5, 0.5]. -
Based on Ref. [24, 32, 33], it has been proposed that the creation of light nuclei occurs through the process of nucleon coalescence. Via this mechanism, protons and neutrons come together to form deuterons and tritons. If we neglect the binding energy, the number of these particles can be expressed as
$ \begin{equation} N_d \approx \frac{3}{2^{1/2}}\left(\frac{2\pi}{mT}\right)^{3/2}N_p\langle n \rangle(1+\alpha), \end{equation} $
(1) $ \begin{equation} N_t \approx \frac{3^{3/2}}{4}\left(\frac{2\pi}{mT}\right)^{3}N_p\langle n \rangle^2(1+\Delta n+2\alpha), \end{equation} $
(2) and then, to eliminate the energy dependence on the local effective temperature (T) at coalescence, we have the light nuclei ratio
$ \begin{equation} \frac{N_tN_p}{N_d^2}=g\frac{1+\Delta n+2\alpha}{(1+\alpha)^2}, \end{equation} $
(3) where
$ N_p $ ,$ N_d $ , and$ N_t $ represent the number of protons, neutrons, and tritons, respectively, m denotes the mass of a nucleon, and$ \Delta n = \langle (\delta n)^2\rangle /\langle n\rangle^2 $ is the dimensionless relative neutron density fluctuation. The standard deviation and mean value of neutron production are represented by$ \langle (\delta n) \rangle $ and$ \langle n\rangle $ , respectively, in our paper.$ \alpha= \langle\delta n \delta p\rangle/ (\langle n\rangle\langle p\rangle) $ is the neutron and proton number density correlation coefficient, and g =$\dfrac{3^{3/2}}{4} \Big/\left(\dfrac{3}{2^{1/2}}\right)^{2}$ =$ \dfrac{1}{2\sqrt{3}} $ .If we assume that the correlation coefficient between the neutron and proton density is small, α can be approximated as zero (
$ \alpha\approx0 $ ), and Eq. (3) can be rewritten as$ \begin{equation} \frac{N_tN_p}{N_d^2}=\frac{1+\Delta n}{2\sqrt{3}}. \end{equation} $
(4)
Study of neutron density fluctuation and neutron-proton correlation in Au+Au collisions using PYTHIA8/Angantyr
- Received Date: 2023-05-04
- Available Online: 2023-11-15
Abstract: Utilizing the PYTHIA8 Angantyr model, which incorporates the multiple-parton interaction (MPI) based color reconnection (CR) mechanism, we study the relative neutron density fluctuation and neutron-proton correlation in Au+Au collisions at