Free Nucleon-nucleon Collisions Research Articles

Effects of shadowing in Pb + Pb collisions at energies available at the CERN Large Hadron Collider within the HIJING code

The charged particle central rapidity density per participant nucleon pair ( $(\mathrm{d} N_{ch}/\mathrm{d} \eta)/(0.5\langle N_{part} \rangle)$ ) as a function of the average number of participants ( $ N_{part}$ ) for $ \mathrm{Au}+\mathrm{Au}$ ( $ \mathrm{Cu}+\mathrm{Cu}$ ) collisions at $ \sqrt{s_{\mathrm{NN}}}=62.4$ and 200 GeV, and $ \mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ and 5.02 TeV is investigated within an improved HIJING (ImHIJING/Cas) model. Compared to the standard HIJING version, it incorporates: i) more consistent parton densities, ii) a QCD running coupling, defined by the exact solution of the renormalization group equation, and iii) a spatial dependent nucleon shadowing, triggered by collective nucleon interactions with an incoming nucleon, and simulated by the collective cascade recipe. Predictions are compared using four different settings of nuclear shadowing of partons, based on the standard HIJING1.0(2.0) parameterizations. It is shown that ImHIJING/Cas/Set2 (that with both quark and soft gluon shadowing of strength $ s_{q(g)}$ can nicely reproduce the measured centrality dependence of $ (\mathrm{d} N_{ch}/\mathrm{d} \eta)/(0.5\langle N_{part} \rangle)$ at RHIC energy. As for LHC, a reduction of $ s_{g}$ by $ \sim 40$ % at very peripheral (90-100%) $ \mathrm{Pb}+\mathrm{Pb}$ collisions, as compared to free nucleon-nucleon collisions, is found to reproduce the measured data. The values of $s_{q(g)}$ used in ImHIJING/Cas/Set2 calculations for most central 0-3(5)% $ \mathrm{Au}+\mathrm{Au} (\mathrm{Pb}+\mathrm{Pb})$ collisions at RHIC(LHC) energy, are found to be consistent with those employed in Set2 that reproduce the measured structure function ratios ( $ 12F^{Pb}_{2}/207F^{C}_{2}$ ) as a function of x.

Dielectron production in Ar + KCl collisions at 1.76AGeV

We present results on dielectron production in Ar+KCl collisions at 1.76A GeV. For the first time $\omega$ mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The omega multiplicity has been extracted and compared to the yields of other particles, in particular of the phi meson. At intermediate e+e- invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by delta Dalitz decays.

Probing the deuteron structure at small N-N distances by cumulative pion production

The fragmentation of deuterons into pions emitted forward in the kinematic region forbidden for free nucleon-nucleon collisions is analyzed. It is shown that the inclusion of the non-nucleonic degrees of freedom in a deuteron results in a satisfactory description of the data for the inclusive pion spectrum and improves the description of the data about $T_{20}$. According to the data, $T_{20}$ has very small positive values, less than 0.2, which contradicts the theoretical calculations ignoring these degrees of freedom.

Theoretical investigation of tensor analyzing powers in deuteron fragmentation into pions

A detailed analysis of the fragmentation of tensorially polarized deuterons into pions in the kinematical region forbidden for free nucleon-nucleon collisions is performed. Within the relativistic impulse approximation, the inclusive spectrum of product pions and the tensor analyzing power T20 are explored by using various forms of the deuteron wave function. The effect of the P-wave component of the deuteron wave function on these observables is also investigated. It is shown that the invariant spectrum is more sensitive to the form of the NN → πX invariant amplitude than the tensor analyzing power T20. The inclusion of nonnucleon degrees of freedom in the deuteron makes it possible to reproduce the pion spectrum over the entire kinematical region and improves the description of experimental data on T20. According to present-day experimental data, T20 is positive and very small in magnitude, less than 0.2, which is at odds with the results of theoretical calculations disregarding nonnucleon degrees of freedom.

Polarization phenomena by deuteron fragmentation into pions

The fragmentation of deuterons into pions emitted forward in the kinematic region forbidden for free nucleon-nucleon collisions is analyzed. The inclusive relativistic invariant spectrum of pions and the tensor analyzing power T20 are investigated within the framework of an impulse approximation using various deuteron wave functions. The influence of -wave inclusion in the deuteron wave function is also studied. The invariant spectrum is shown to be more sensitive to the amplitude of the NN↦πX process than the tensor analyzing power T20. It is shown that the inclusion of the non-nucleonic degrees of freedom in a deuteron results in a satisfactory description of the data for the inclusive pion spectrum and improves the description of the data about T20. According to the data, T20 has very small positive values, less than 0.2, which contradicts the theoretical calculations ignoring these degrees of freedom.

Angle dependence of polarization observables by fragmentation of deuterons to pions

The fragmentation of deuterons into pions with nonzero angle emitted in the kinematical region forbidden for free nucleon-nucleon collisions is analyzed. The inclusive relativistic invariant spectrum of pions and the tensor analyzing power A YY are investigated within the framework of an impulse approximation using different kinds of the deuteron wave function. The influence of P-wave contribution to the deuteron wave function is studied, too. Our results are compared with the experimental data and other calculations performed within both the non-relativistic and relativistic approaches. It was found that theoretical calculations based on IA do not provide a consistent understanding of the new data in a whole cumulative region that can be caused by a influence of non-nucleon degrees of freedom.

Production and flow of K 3 0 -mesons and a-hyperons in58Ni+58Ni collisions at 1.93 a gev

We present preliminary results of an analysis of the neutral strange particlesK and A produced in58Ni+58Ni collisions at 1.93 A GeV. The experiment was performed at GSI in Darmstadt with the detector system FOPI. The incident energy is close to the threshold of strange particle production in free nucleon-nucleon collisions. Therefore the production of kaons and A-particles is sensitive to multi particle effects and multi step processes. The particle yields and their phase-space distributions can be used as probes of the hot and high-density phase of nuclear matter. Neutral strange particles are of special interest since they are not influenced by the Coulomb interaction and are less affected by detection thresholds. The reconstruction ofK meson and A hyperon events and the determination of preliminary physical results is presented.

Tensor analyzing power T20 for cumulative pion production from deuterons in the GeV energy region

The tensor analyzing power T 20 for the reaction d ↑ + 12 C → π ± (0°) + X has been measured with a polarized deuteron beam from 6.2 to 9.0 GeV/ c at a pion momentum3.0 GeV/ c. This experiment is focused on “cumulatively produced pions”, which are produced beyond the kinematically allowed limit for free nucleon-nucleon collisions. The measured values of T 20 turn out to be close to zero. They are in disagreement with the results of our impulse approximation calculation which is based on a single NN → πNN interaction and takes into account the internal motion of nucleons in the deuteron. Possible explanations of the result are discussed.

Subthreshold antiproton production in heavy ion collisions

We measured cross sections for production of antiprotons, K- and π- mesons at θlab = 0° in nucleus-nucleus collisions for the systems Ne + NaF, Ne + Cu, Ne + Sn, Ne + Bi and for Ni + Ni, using 20Ne and 58Ni beams for the heavy ion synchrotron (SIS) at GSI with incident energies between 1.6 and 2.0 GeV/u. This energy range is far below the threshold for the production of antiprotons in free nucleon-nucleon collisions of 5.6 GeV/u. The dependency on the incident energy, the size of the system and the momentum of the detected particle are discussed.

Direct-photon production in heavy-ion collisions.

To study plasma photons in relativistic heavy-ion collisions one must account for direct photons from all possible sources. The aim of this work is the examination of the effect that initial-state parton distributions in nuclei, distorted to accommodate the European Muon Collaboration (EMC) effect, will have on the direct-photon spectrum produced relative to the basic QCD subprocesses in free nucleon-nucleon collisions. Since this parton distortion has been quite satisfactorily described by an expansion of the nuclear state in multiquark color-singlet (cluster) basis states, this «quark cluster model» should give useful predictions for the reflection of the EMC effect in the nucleus-nucleus direct photons

Statistical model of subthreshold pion production.

A statistical model has been developed to compute total pion production cross sections at energies below the energy threshold for pion production in free nucleon-nucleon collisions. The model assumes that during a collision between two nuclei a fraction of their energy is released within an interaction volume and is rapidly distributed among the different degrees of freedom. The calculation of the probability of creating a pion is based on the statistical weights of the various possibilities. The model has been applied successfully to reactions between /sup 12/C, /sup 14/N, and /sup 16/O projectiles and /sup 12/C, /sup 27/Al, /sup 58/Ni, W, and U targets for energies ranging from 25 to 84 MeV/nucleon.

Inclusive particle production at forward angles from collisions of light relativistic nuclei: Negative pions

We have measured single particle inclusive spectra of negative pions produced at angles from 0\ifmmode^\circ\else\textdegree\fi{} to 12\ifmmode^\circ\else\textdegree\fi{} (lab) in collisions of 1.05 and 2.1 GeV/nucleon protons, deuterons, alpha particles, and carbon nuclei with targets of C, Cu, Pb, and H (from a C${\mathrm{H}}_{2}$---C subtraction). Most of the pions are produced in the kinematical domains allowed in free nucleon-nucleon collisions, but for alpha and carbon projectiles we have also observed pions whose energies range up to nearly twice the kinetic energy of a nucleon in the projectile. Our results suggest that processes involving more than two colliding nucleons and/or high internal momentum components are involved in the production of these high energy pions. Comparison is made with several hypotheses of scaling including specific dynamical models, and some disagreement is observed. We present fits to the kinetic energy dependence of the data, and the target and projectile mass dependence. We also show transverse momentum distributions.NUCLEAR REACTIONS p + H, p + C, p + Cu, p + Pb, d + H, d + C, d + Cu, d + Pb, $\ensuremath{\alpha}$ + H, $\ensuremath{\alpha}$ + C, $\ensuremath{\alpha}$ + Cu, $\ensuremath{\alpha}$ + Pb, C + H, C + C, C + Cu, C + Pb, $\frac{E}{A}=1050$ MeV/nucleon; p + H, p + C, p + Cu, p + Pb, d + H, d + C, d + Cu, d + Pb, $\ensuremath{\alpha}$ + h, $\ensuremath{\alpha}$ + C, $\ensuremath{\alpha}$ + Cu, $\ensuremath{\alpha}$ + Pb, C + H, C + C, C + Cu, C + Pb, $\frac{E}{A}=2100$ MeV/nucleon; measured $\ensuremath{\sigma}(p, \ensuremath{\vartheta})$ for ${\ensuremath{\pi}}^{\ensuremath{-}}$.

Polarization of Vector Mesons Produced in Nucleon-Nucleon Collisions

The polarization of vector mesons produced in free nucleon-nucleon collisions near the threshold is studied. The calculation is made under the Born approximation, assuming a phenomenological nuclear potential obtained from $n\ensuremath{-}p$ scattering data for the interaction between nucleons. The vector meson produced is shown to be completely polarized (to the order of $\frac{m}{M}$) parallel to the direction of motion of the incoming nucleon for vector coupling between meson and nucleon, and perpendicular to this direction for tensor coupling; similar to the polarization produced by the interaction between a nucleon and a nuclear force field, as studied by Wentzel. In particular, the same effect is shown to exist for the reactions $p+p\ensuremath{\rightarrow}p+n+{\ensuremath{\pi}}^{+}$ and $n+n\ensuremath{\rightarrow}p+n+{\ensuremath{\pi}}^{\ensuremath{-}}$ when the final proton and neutron fall into $^{3}s$ or $^{1}s$ deuteron states. The angular distribution of the out-going nucleons in the baricentric system for the inverse reactions can be employed as a means for detecting the polarization of the mesons. Cross sections for these reactions are also estimated.