Al Collisions Research Articles

Evidence for Nonlinear Gluon Effects in QCD and Their Mass Number Dependence at STAR.

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π^{0}s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass energy of 200GeV. We observe a clear suppression of the correlated yields of back-to-back π^{0} pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale Q_{s}^{2} on the mass number A. A linear scaling of the suppression with A^{1/3} is observed with a slope of -0.09±0.01.

ϕ meson production in p+Al, p+Au, d+Au, and He3+Au collisions at sNN=200GeV

Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects; however, the collective behavior observed in these collisions shows a hint of hot-nuclear-matter effects. The identified-particle spectra, especially the $\phi$ mesons which contain strange and antistrange quarks and have a relatively small hadronic-interaction cross section, are a good tool to study these effects. The PHENIX experiment has measured $\phi$ mesons in a specific set of small collision systems $p$$+$Al, $p$$+$Au, and $^3$He$+$Au, as well as $d$$+$Au [Phys. Rev. C {\bf 83}, 024909 (2011)], at $\sqrt{s_{_{NN}}}=200$ GeV. The transverse-momentum spectra and nuclear-modification factors are presented and compared to theoretical-model predictions. The comparisons with different calculations suggest that quark-gluon plasma may be formed in these small collision systems at $\sqrt{s_{_{NN}}}=200$ GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. Comparison with calculations suggests that the main production mechanisms of $\phi$ mesons at midrapidity may be different in $p$$+$Al versus $p/d/$$^3$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. While thermal quark recombination seems to dominate in $p/d/$$^3$He$+$Au collisions, fragmentation seems to be the main production mechanism in $p$$+$Al collisions.

Transverse single spin asymmetries of forward neutrons in p+p , p+Al , and p+Au collisions at sNN=200 GeV as a function of transverse and longitudinal momenta

In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded $p$$+$$p$, $p$$+$Al, and $p$$+$Au collision data at center of mass energies of $\sqrt{s_{_{NN}}}=200$ GeV with the proton beam(s) transversely polarized. At very forward rapidities $\eta>6.8$ relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction $x_F$. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultra-peripheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on $x_F$. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for $p$$+$$p$ collisions, moderate for $p$$+$Al collisions, and large for $p$$+$Au collisions.

The ϕ meson production in small collision systems observed by PHENIX

The measurements of light hadron production in small collision systems (such as p+Al, p+Au, d+Au, 3He+Au) may allow to explore the quarkgluon plasma formation and to determine the main hadronization mechanism in the considered collisions. Such research has become particularly crucial with the observation of the light hadrons collective behavior in p/d/3He+Au collisions at √SNN = 200 GeV and in p+Al collisions at the same energy at forward and backward rapidities. Among the large variety of light hadrons, ϕ meson is of particular interest since its production is sensitive to the presence of the quark-gluon plasma. The paper presents the comparison of the obtained experimental results on ϕ meson production to different light hadron production in p+Al and 3He+Au at √SNN = 200 GeV at midrapidity. The comparisons of ϕ meson production in p+Al, p+Au, d+Au, and 3He+Au collisions at √SNN = 200 GeV at midrapidity to theoretical models predictions (PYTHIA model and default and string melting versions of the AMPT model) are also provided. The results suggest that the QGP can be formed in p/d/3He+Au collisions, but the volume and lifetime of the produced medium might be insufficient for observation of strangeness enhancement effect. Conceivably, the main hadronization mechanism of ϕ meson production in p+Al collisions is fragmentation, while in p/d/3He+Au collisions this process occurs via coalescence.

Nuclear-modification factor of charged hadrons at forward and backward rapidity in p + Al and p + Au collisions at sNN=200GeV

The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<\eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<\eta<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.

Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+A Collisions at sqrt[s_{NN}]=200 GeV.

During 2015, the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized p+p collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in p+p collisions predicts only a moderate atomic-mass-number (A) dependence. In contrast, the asymmetries observed at RHIC in p+A collisions showed a surprisingly strong A dependence in inclusive forward neutron production. The observed asymmetry in p+Al collisions is much smaller, while the asymmetry in p+Au collisions is a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed A dependence.

The STAR Heavy Flavor Tracker and Upgrade Plan

The Heavy Flavor Tracker (HFT) of the STAR experiment at RHIC is the first application of the state-of-the-art thin Monolithic Active Pixel Sensors (MAPS) technology in a collider environment. The HFT is composed of two silicon PiXeL detector (PXL) layers, an Intermediate Silicon Tracker (IST) and a Silicon Strip Detector (SSD). It greatly improves the impact parameter resolution of STAR tracking and enables reconstruction of secondary decay vertices of open heavy hadrons in heavy ion collisions, providing unique probes for studying the Quark-Gluon Plasma. In these proceedings we discuss the HFT hardware design, and current detector status and performance. The HFT was successfully commissioned during the 2014 RHIC run, taking data in Au+Au collisions at 200 GeV. The HFT performance during this run matches the expected performance, most significantly for track pointing resolution. Preliminary results have been obtained from 2014 Au+Au data analyses, demonstrating the capabilities of open charm hadron reconstruction with the HFT. Modifications to HFT subsystems have been made to improve its performance in the 2015 run in p+p, p+Au and p+Al collisions at sNN=200 GeV. In order to further improve such capabilities to measure bottom quark hadrons at RHIC energies, a faster heavy flavor tracker (HFT+) is needed to collect data at higher luminosity with good efficiency. The proposed HFT+ will be equipped with new generation of MAPS sensors with a much shorter integration time (≤40μs) and possibly extend the current PXL detector acceptance with minimal modification to the original mechanical and air cooling infrastructure. Requirements for the upgraded HFT+ detector and expected performance are also presented in these proceedings.

PHENIX results on collectivity tests in high-multiplicity p + p and p + Au collisions at [formula omitted

The observation of possible collective effects in high-multiplicity p+p and p+Pb collisions at the LHC and in d+Au and 3He+Au collisions at RHIC challenge our understanding of the ingredients necessary for quark-gluon plasma formation. For further investigation of these effects, the PHENIX collaboration has taken high statistics data in p+p and p+Au and p+Al collisions in 2015. For these data sets, high-multiplicity triggers were implemented using the forward silicon detector (FVTX) and the beam-beam counter (BBC) covering pseudo-rapidity 1.0<|η|<3.0 and 3.1<|η|<3.9, respectively. The multi-hundred million high-multiplicity event samples recorded enable highly differential analysis to look for collective effects. We report results on large pseudo-rapidity separation correlations to elucidate if the so-called ridge phenomena exists in certain p+p event classes at RHIC. The flow coefficients from azimuthal anisotropies in p+Au are extracted and compared with theoretical expectations in various models, including viscous hydrodynamics where the elliptic flow strength is expected to be substantially smaller than in d+Au and 3He+Au at the same energy due to the smaller initial spatial eccentricity.

Strange meson production in Al+Al collisions at 1.9 A GeV

The production of K$^+$, K$^-$ and $\varphi$(1020) mesons is studied in Al+Al collisions at a beam energy of 1.9A GeV which is close or below the production threshold in NN reactions. Inverse slopes, anisotropy parameters, and total emission yields of K$^{\pm}$ mesons are obtained. A comparison of the ratio of kinetic energy distributions of K$^-$ and K$^+$ mesons to the HSD transport model calculations suggests that the inclusion of the in-medium modifications of kaon properties is necessary to reproduce the ratio. The inverse slope and total yield of $\phi$ mesons are deduced. The contribution to K$^-$ production from $\phi$ meson decays is found to be [17 $\pm$ 3 (stat) $^{+2}_{-7}$ (syst)] %. The results are in line with previous K$^{\pm}$ and $\phi$ data obtained for different colliding systems at similar incident beam energies.

Evidence for charge exchange effects in electronic excitations in Al by slow singly charged He ions

We report on experiments of secondary electron emission in the interaction of helium ions with aluminum surfaces. Comparison between the electron emission induced by the impact of 3He+ and 4He+ on Al illustrates similarities and differences between the two projectiles. The intensity of emission shows the same dependence on velocity for the two isotopes, showing that KEE yields for helium ions impact on Al are dominated by direct excitation of valence electrons and not by electron promotion. Electron promotion and charge transfer processes are unambiguously identified by the observation of Auger electron emission from Al, at energies below the excitation threshold of Al–Al collisions, indicating energy losses for the projectiles higher than those commonly considered.

Azimuthal anisotropy relative to the participant plane from a multiphase transport model in centralp+Au,d+Au, andHe3+Aucollisions atsNN=200GeV

Recent data from p+p and p+Pb collisions at the Large Hadron Collider (LHC), and d+Au and $^3$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) reveal patterns that---when observed in the collision of heavy nuclei---are commonly interpreted as indicators of a locally equilibrated system in collective motion. The comparison of these data sets, including the forthcoming results from p+Au and p+Al collisions at RHIC, will help to elucidate the geometric dependence of such patterns. It has recently been shown that A-Multi-Phase-Transport-Model (AMPT) can describe some of these features in LHC data with a parton-parton scattering cross section comparable to that required to describe A+A data. In this paper, we extend these studies by incorporating a full wave function description of the $^3$He nucleus to calculate elliptical and triangular anisotropy moments $v_2$ and $v_3$ for p+Au, d+Au and $^3$He+Au collisions at the RHIC top energy of 200 GeV. We find reasonable agreement with the measured $v_2$ in d+Au and $^3$He+Au and $v_3$ in $^3$He+Au for transverse momentum ($p_{T}$) $\lesssim$ 1 GeV/c, but underestimate these measurements for higher values of \pt. We predict a pattern of coefficients ($v_{2}$, $v_{3}$) for \pau, dominated by differences in the number of induced local hot spots (i.e. one, two, or three) arising from intrinsic geometry. Additionally, we examine how this substantial azimuthal anisotropy accrues during each individual evolutionary phase of the collision in the AMPT model. The possibility of a simultaneous description of RHIC- and LHC-energy data, the suite of different geometries, and high multiplicity p+p data is an exciting possibility for understanding the underlying physics in these systems.

Emission of forward neutrons by 158A GeV indium nuclei in collisions with Al, Cu, Sn and Pb

The cross sections of forward emission of one, two and three neutrons by 158A GeV 115In nuclei in collisions with Al, Cu, Sn and Pb targets are reported. The measurements were performed in the framework of the ALICE–LUMI experiment at the SPS facility at CERN. Various corrections accounting for the absorption of beam nuclei and produced neutrons in target material and surrounding air were introduced. The corrected cross section data are compared with the predictions of the RELDIS model for electromagnetic fragmentation of 115In in ultraperipheral collisions, as well as with the results of the abrasion–ablation model for neutron emission in hadronic interactions. The measured neutron emission cross sections well agree with the RELDIS results, with the exception of In–Al collisions where the measured cross sections are larger compared to RELDIS. This is attributed to a relatively large contribution of hadronic fragmentation of In on Al target with respect to electromagnetic fragmentation, in contrast to similar measurements performed earlier with 30A GeV 208Pb colliding with Al.

What can we learn from the subthreshold ϕ(1020) production?

Strange particles are sensitive probes of hot and dense nuclear matter formed in the relativistic nucleus - nucleus collisions. In our experimental studies performed at SIS18 (GSI Darmstadt) with the FOPI spectrometer, we have measured the ϕ(1020) meson production in Al+Al collisions at 2A GeV. The production of ϕ(1020) mesons, almost half of them decaying via the K+K−channel, obviously affects the rate of kaon production. In order to quantify the contribution of ϕ mesons decays to the observed K−mesons yield, the ϕ meson production probability was estimated.

Measurement ofK*(892)0andK0mesons inAl+Alcollisions at 1.9AGeV

A new measurement of subthreshold K*(892)0 and K0 production is presented. The experimental data complete the measurement of strange particles produced in Al+Al collisions at 1.9A GeV measured with the FOPI detector at SIS at GSI (Darmstadt). The K*(892)0/K0 yield ratio is found to be 0.0315±0.006(stat.)±0.012(syst.) and is in good agreement with the transport model prediction. These measurements provide information on the in-medium cross section of K+-π− fusion, which is the dominant process in subthreshold K*(892)0 production.Received 21 April 2010DOI:https://doi.org/10.1103/PhysRevC.81.061902©2010 American Physical Society

Neutral fractions calculations for grazing incidence H+/Al(111) collisions

We have calculated outgoing H(1s) fractions in a large range of impact velocities for protons impinging on Al(111) with a grazing incidence of 0.5°. Capture and loss for Resonant and Auger processes are taken into account and the role of the surface-plasmon-assisted electron capture mechanism is also investigated. Consideration of all these mechanisms allow to reach a good agreement with experimental measurements. A detailed analysis of our results show that Resonant processes dominate for v||≤1.5 although the surface-plasmon-assisted capture mechanism has a significant contribution around the Fermi velocity (vF≈0.9). For higher velocities Auger capture and loss mechanisms (and also Resonant loss) mainly explain the experimental findings.

He(1s2) angular distributions simulations for He+(1s) on Al(1 1 1) collisions in the keV energy range at grazing incidences

We have calculated charge fractions and angular distributions of scattered He atoms resulting from the interaction of keV He+ beams at grazing incidences with an Al(111) surface. Several improvements over our earlier approaches have been incorporated. These are a more sophisticated RPA image potential for the ion–metal interaction as well as kinematic factors affecting the static rates for the Auger transfer mechanisms. For interaction cases in which both perpendicular and parallel velocities are low, we obtain angular distributions which have a better agreement with the experimental data than our earlier version of the theory, although still the theoretical results do not reach total neutralization of the beam as the experimental results show.

Strange resonances measured in Al+Al collisions at GeV with the FOPI detector

The FOPI Collaboration performed a high statistic experiment to study the strangeness production in Al+Al collisions at a beam kinetic energy of 1.9A GeV. The collected huge data sample allows for the first time the reconstruction of sub-threshold resonances production such as the Σ(1385) and the K(892). These measurements constrain the parameters of statistical model and provide information on in-medium cross sections for the different processes involved in the strangeness production close to or below their thresholds.

Ionization in symmetric and nearly symmetric low energy ion–surface collisions

Multiply charged ions are emitted following bombardment of Al(100) and Si(111) by low energy Si+ and P+ ions. The ion formation is attributed to inner-shell electron promotion during a hard collision between symmetric or nearly symmetric atomic species, followed by Auger decay outside the surface. The relative yield of triply charged Si ions for Si+→Si(111) is much smaller than that of triply charged Al ions in direct recoil Si+→Al(100) experiments. This difference can be explained by assuming that only one 2p hole is produced in a Si atom during the symmetric collision, whereas a double 2p hole is also produced in the Al atom following the nearly symmetric Si–Al collision. Further evidence is provided by the complimentary experiment P+→Si(111), where Si3+ regains its intensity and Si4+ emerges as a result of a double 2p hole decay with shake-off.

Multiply charged Al recoils with impact of 2.0keV Si+ ions

Scattered Si and fast recoiled Al are both present in time-of-flight spectra collected when 2.0keV Si+ ions are incident on an Al(100) surface. Monte Carlo simulations were used to determine the origin of the spectral features, and electrostatic analysis was used to measure the ion yields and charge states as a function of the recoil and incident angles. The analysis indicates that all of the scattered Si projectiles are neutralized, while the recoiled Al is produced as neutrals and in singly, doubly, and triply charged states. The formation of multiply charged ions at this low energy is attributed to inner-shell promotion during the nearly symmetric Si–Al collision. Further experiments used Cl+ and SiCl+ projectiles for comparison. Multiply charged recoils were not observed with Cl+, while the behavior of SiCl+ could be interpreted as a linear combination of Si+ and Cl+ projectiles.

Angular distributions of atoms sputtered from NiAl{1 1 0} and Ni{1 0 0}

Using the laser post-ionization surface analysis technique, I have for the first time studied angular distributions of Ni and Al atoms sputtered from NiAl{1 1 0}. Emission angular distributions from Ni{1 0 0} have also been measured. I have observed preferential emissions of Ni and Al atoms along 〈1 1 1〉 and 〈1 0 0〉crystallographic directions for NiAl{1 1 0} and of Ni atoms along 〈1 1 0〉 and 〈1 0 0〉 directions for Ni{1 0 0}. The observed preferential ejections can be explained in terms of the theory of focusing-collision sequences. Because of the difference in surface binding energy between Al and Ni atoms, preferential ejection angles of Ni atoms are slightly different from those of Al atoms along the 〈1 1 1〉 ejections. For NiAl, the 〈1 1 1〉 preferential ejections were less prominent than the 〈1 0 0〉 preferential ejections and this can be related to the low efficiency of momentum transfer in Ni–Al collision sequences along 〈1 1 1〉 lattice directions. The low efficiency of momentum transfer due to the mass mismatch can also be responsible for the experimental observation that the preferential ejections in the alloy were less prominent than those in the Ni metal.