What Can We Learn from (Pseudo)Rapidity Distribution in High Energy Collisions?

Based on the (pseudo)rapidity distribution of final-state particles produced in proton-proton (pp) collisions at high energy, the probability distributions of momenta, longitudinal momenta, transverse momenta (transverse masses), energies, velocities, longitudinal velocities, transverse velocities, and emission angles of the considered particles are obtained in the framework of a multisource thermal model. The number density distributions of particles in coordinate and momentum spaces and related transverse planes, the particle dispersion plots in longitudinal and transverse coordinate spaces, and the particle dispersion plots in transverse momentum plane at the stage of freeze out in high energy pp collisions are also obtained.


Introduction
Due to the complexity of high energy collisions, it is impossible to measure all quantities and characteristics. Instead, some quantities and characteristics can be measured by experiment [1], while others can be obtained from the theoretical explanations based on the experimental results.
A lot of models were introduced in the past several decades. Different phenomenological mechanisms including initial interactions, intermediate processes, and final-state statistical laws have been proposed [14][15][16][17][18][19]. In a workshop held a few years ago at the CERN Theory Institute [20], many models reported their predictions for the heavy ion program at the LHC based on the explanations of the experimental results at the RHIC. Recently, some of the models are tested by , , and collisions at the LHC. Meanwhile, these models are further tested in , , , and collisions at the RHIC and SPS.
In the past years, we suggested a multisource thermal model [21][22][23] to describe the (pseudo)rapidity distributions, multiplicity distributions, transverse momentum distributions, azimuthal distributions, flow effects, and so forth. Some interesting quantities such as the temperature, speed of sound, and number of sources are obtained. It is noticed that some quantities and characteristics are related to others. Usually, based on a few distributions, other distributions can be obtained due to some modelling assumptions.
Very recently, the NA61/SHINE Collaboration reported the negatively charged pion productions in inelastic collisions at 20-158 GeV/c at the SPS [1]. The rapidity spectrums are parameterized by the sum of two Gaussian functions symmetrically displaced with respect to midrapidity. It is interesting for us to give a further test for the multisource thermal model by using the rapidity spectrums in collisions at SPS momenta (energies). According to the rapidity spectrums, we hope some other distributions can be obtained by the model. In this paper, in the framework of the multisource thermal model [21][22][23], we analyze the rapidity spectrums in collisions measured by the NA61/SHINE Collaboration at the SPS [1]. Then, a series of other distributions are obtained due to the description of rapidity spectrums.

The Model and Calculation Method
In the multisource thermal model [21][22][23], we assume that many emission sources are formed in high energy collisions. In rapidity space in the laboratory or centerof-mass reference frame, these sources distribute at different rapidity and form a target cylinder in rapidity interval [ min , max ] and a projectile cylinder in rapidity interval [ min , max ], respectively. Because of the symmetry in collision, we have [ min = − max ] and [ max = − min ].
In the rest frame of a given source, we can use different formalizations to describe the production of particles. For example, we can use the relativistic ideal gas model to give where = (1/ 2 0 )(1/ 2 ( 0 / )) is the normalization constant, 2 ( 0 / ) is the modified Bessel function of order 2, 0 is the rest mass of the considered particle, is the particle number, and is the temperature parameter of the source.
The particles are assumed to emit isotropically in the rest frame of the source. Then, the distributions of emission angle and the azimuth can be given by ( ) = (1/2) sin and ( ) = 1/(2 ), respectively.

Comparison and Extraction
The NA61/SHINE Collaboration has measured the rapidity distributions and other features of negatively charged pions produced in 20-158 GeV/c collisions at the SPS [1]. It is shown that the rapidity spectrums are parameterized by the sum of two Gaussian functions symmetrically displaced with respect to midrapidity: where , 0 , and 0 are the normalization constant, distribution width for one Gaussian, and peak position parameter, respectively. The values with errors of the three parameters at different momenta are obtained by the NA61/SHINE Collaboration [1]. We can compare directly our modelling results with the experimental distribution function of the NA61/SHINE Collaboration [1]. In Figure 1 Table 1. One can see that the model describes the experimental rapidity distributions of negatively charged pions produced in collisions at the SPS. The pseudorapidity distribution has a lower peak and wider range than those of the rapidity distribution. The temperature increases and the rapidity shift max increases with increase of the incident momentum, and the rapidity shift min does not change with the momentum. By using the above parameter values, we can obtain some interesting features for other quantities. Figure 2 Figures  6, 7, and 8, respectively. The circles and squares correspond, respectively, to the contributions of the target and projectile cylinders in 500 events. We can see that the density in large | | and small | ( )| region is greater than those in other regions. The density in small | ( )| and | | region is greater than those in other regions. The contribution points of the two cylinders are mixed in small longitudinal momentum region. The particle number densities in coordinate space and momentum space increase obviously with increase of the incident momentum.

Conclusions
From the above discussions, we obtain the following conclusions.
Advances in High Energy Physics (a) The multisource thermal model is used to describe the rapidity distributions of negatively charged pions produced in collisions at SPS energies (momenta). A target cylinder and a projectile cylinder are assumed to form in the collisions. For each source, a relativistic ideal gas model is used. The calculated results are in agreement with the rapidity distributions measured by the NA61/SHINE Collaboration [1]. In our calculation, the main parameters are the temperature, the maximum rapidity shift, and the minimum rapidity shift. The temperature and the maximum rapidity shift increase with increase of the incident momentum, and the minimum rapidity shift does not change with the momentum.
(b) According to the parameter values obtained from the rapidity distributions, the other distributions are obtained in the framework of the multisource thermal model. The other distributions include the probability distributions of momenta, longitudinal momenta, transverse momenta (transverse masses), energies, velocities, longitudinal velocities, transverse velocities, space angles, and pseudorapidities, as well as the number density distributions of particles in coordinate and momentum spaces. The final-state particles are far from the isotropic emission. The maximum ( ) appears at the maximum and the minimum ( ) appears in the region close to 0. The maximum values of ( ), ( ), and ( ) appear at 0 and the minimum values appear at the maximum coordinate or momentum (transverse momentum) values.
(c) According to the parameter values obtained from the rapidity distributions, the particle dispersion plots in ( )-, -, and ( )planes are also obtained in the framework of the model. The density in large | | and small | ( )| region is greater than those in other regions, and the density in small | ( )| and | | region is greater than those in other regions. The contributions of the target cylinder and the projectile cylinder are mixed in small longitudinal momentum region.