Spectator Nucleon Research Articles

Coherent Photoproduction of Neutral Pseudoscalar Mesons $$\boldsymbol{\pi^{0}\pi^{0}}$$ and $$\boldsymbol{\pi^{0}\eta}$$ on Light Nuclei

The coherent photoproduction of $$\pi^{0}\pi^{0}$$ and $$\pi^{0}\eta$$ on nuclei in the mass-number region of $$A\leqslant 7$$ is studied systematically over the range between the threshold and the laboratory photon energy of $$E_{\gamma}=2$$ GeV. The amplitude for photoproduction on nuclei is constructed on the basis of the impulse approximation with allowance for a number of processes responsible for meson–nucleus final-state interaction. The pion-rescattering processes $$\pi N\to\pi N$$ and $$\pi N\to\eta N$$ occurring on spectator nucleons, $$\pi^{+/-}\pi^{0}\pi^{0}$$ and $$\pi^{+/-}\pi^{0}\eta$$ production followed by $$\pi^{+/-}$$ absorption by spectator nucleons, and three-body $$\pi$$ –NN and $$\eta$$ –NN interactions are described for the first time within a unified microscopic approach.

Formation of Free and Bound Spectator Nucleons in Hadronic Interactions between Relativistic Nuclei

The abrasion–ablation model is implemented in the form of the computer code Abrasion–Ablation Monte Carlo for Colliders (AAMCC) and applied to collisions of relativistic nuclei at the collider NICA now under construction. The total number of free nucleons, the fraction of free nucleons in the total volume of spectator matter, and their n/p ratio are calculated as functions of the impact parameter in 197Au–197Au collisions at NICA. Correlations are calculated between the total volumes of spectator matter, as well as between the numbers of spectator nucleons from each of the colliding nuclei. The obtained results can be used to improve the method of centrality determination in nucleus–nucleus collisions by registering spectator nucleons and to estimate the impact of heavy spectator fragments on NICA components, including superconducting magnets.

Magnetic fields in heavy ion collisions: flow and charge transport

At the earliest times after a heavy-ion collision, the magnetic field created by the spectator nucleons will generate an extremely strong, albeit rapidly decreasing in time, magnetic field. The impact of this magnetic field may have detectable consequences, and is believed to drive anomalous transport effects like the Chiral Magnetic Effect (CME). We detail an exploratory study on the effects of a dynamical magnetic field on the hydrodynamic medium created in the collisions of two ultrarelativistic heavy-ions, using the framework of numerical ideal MagnetoHydroDynamics (MHD) with the ECHO-QGP code. In this study, we consider a magnetic field captured in a conducting medium, where the conductivity can receive contributions from the electromagnetic conductivity sigma and the chiral magnetic conductivity sigma _{chi }. We first study the elliptic flow of pions, which we show is relatively unchanged by the introduction of a magnetic field. However, by increasing the magnitude of the magnetic field, we find evidence for an enhancement of the elliptic flow in peripheral collisions. This effect is stronger at RHIC than the LHC, and it is evident already at intermediate collision centralities. Next, we explore the impact of the chiral magnetic conductivity on electric charges produced at the edges of the fireball. This initial sigma _chi can be understood as a long-wavelength effective description of chiral fermion production. We then demonstrate that this chiral charge, when transported by the MHD medium, produces a charge dipole perpendicular to the reaction plane which extends a few units in rapidity. Assuming charge conservation at the freeze-out surface, we show that the produced charge imbalance can have measurable effects on some experimental observables, like v_1 or langle sin phi rangle . This demonstrates the ability of a MHD fluid to transport the signature of the initial chiral magnetic fields to late times. We also comment on the limitations of the ideal MHD approximation and detail how further development of a dissipative-resistive model can provide a more realistic description of the QGP.

Color transparency in $${{\bar{p}}} d \rightarrow \pi ^- \pi ^0 p$$ reaction

We consider exclusive two-pion production in antiproton-deuteron interactions at the beam momenta around 10 GeV/c in the kinematics with large momentum transfer in the underlying hard process $\bar p n \to \pi^- \pi^0$. The calculations are performed taking into account the antiproton and pion soft rescattering on the spectator proton in the framework of the generalized eikonal approximation. We focus on the color transparency effect that is modeled by introducing the dependence of rescattering amplitudes on the relative position of the struck and spectator nucleons along the momentum of a fast particle. As a consequence of the interplay between the impulse approximation and rescattering amplitudes the nuclear transparency ratio reveals a pretty complicated behaviour as a function of the transverse momentum of the spectator proton and the relative azimuthal angle between the $\pi^-$-meson and the proton. Color transparency significantly suppresses rescattering amplitudes which leads to substantial modifications of the nuclear transparency ratio moving it closer to the value obtained in the impulse approximation. By performing the Monte-Carlo analysis we determine that this effect can be studied at PANDA with a reasonable statistics.

On the Detection of Multinucleon Events in Nucleus—Nucleus Collisions with Forward Calorimeters (ZDC)

The detection of spectator nucleons emitted forward in the beam direction in experiments on the study of nucleus—nucleus interactions using forward Zero Degree Calorimeters (ZDCs) allows the determination of the event centrality and the detection of the nucleons from electromagnetic dissociation (EMD). In this paper, we propose a simple combinatorial model for calculating the energy releases in the ZDC, taking into account the calorimeter acceptance and efficiency. Using this model, the effect of the limited acceptance and nucleon detection efficiency on the spectral shape in the ZDC and the possibility of measuring the yield of a certain number of nucleons from the EMD is studied at LHC energies.

Forward Proton Detectors in Heavy-ion Physics

The forward proton detectors, already existing at the LHC, are considered in the context of heavy ion collisions. It is shown that such detectors have the potential to measure nuclear debris originating from spectator nucleons. The geometric acceptance for different nuclei is studied, and how it is affected by the motion of the nucleons in the nucleus and by the experimental conditions. A possibility of reconstructing the impact parameter of the collision from the measurement of the nuclear fragments is discussed.

Performance of the ALICE Zero Degree Calorimeters and upgrade strategy

The Zero Degree Calorimeters (ZDCs) of the ALICE experiment were designed with the twofold purpose of both estimating the centrality in heavy ion collisions by measuring the energy carried away by the spectator nucleons and of measuring the luminosity delivered to the experiment exploiting the high cross sections for neutron emission from electromagnetic dissociation processes. The measurement of centrality has been successfully extended to p–A collisions with the detection of nucleons ejected from the nucleus by the collisions with the projectile proton (“gray” nucleons) and those resulting from de-excitation processes (“black” nucleons). The applications of the detector in triggering and analysis have expanded during the years of operation in the LHC RUN1 and RUN2. These now include both the reaction plane and the longitudinal asymmetry measurements in heavy ion collisions. Moreover the ZDCs are used to reject the parasitic interactions of main bunches with satellite bunches in A–A and p–A collisions and to tag diffractive events in pp collisions. The foreseen operation in RUN3 with the tenfold increase in the luminosity delivered by LHC in heavy ion collisions, together with the continuous acquisition strategy that is being adopted by ALICE, will be challenging for the ZDC readout system. The readout upgrade will be based on FMC digitizers with trigger, timing and charge integration functionality performed through FPGA.

Coherent J/ψ photoproduction in hadronic heavy-ion collisions

Significant excesses of $J/\ensuremath{\psi}$ yield at very low transverse momentum (${p}_{T}<0.3$ GeV/$c$) were observed by the ALICE and STAR collaborations in peripheral hadronic $A+A$ collisions. This is a sign of coherent photoproduction of $J/\ensuremath{\psi}$ in violent hadronic interactions. Theoretically, the photoproduction of $J/\ensuremath{\psi}$ in hadronic collisions raises questions about how spectator and nonspectator nucleons participate in the coherent reaction. We argue that the strong interactions in the overlapping region of incoming nuclei may disturb the coherent production, leaving room for different coupling assumptions. The destructive interference between photoproduction on ions moving in opposite directions also needs to be included. This paper presents calculations of $J/\ensuremath{\psi}$ production from coherent photon-nucleus ($\ensuremath{\gamma}+A\ensuremath{\rightarrow}J/\ensuremath{\psi}+A$) interactions in hadronic $A+A$ collisions at BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider energies with both nucleus and spectator coupling hypotheses. The integrated yield of coherent $J/\ensuremath{\psi}$ as a function of centrality is found to be significantly different, especially towards central collisions, for different coupling scenarios. Differential distributions as a function of transverse momentum, azimuthal angle, and rapidity in different centrality bins are also shown, and found to be more sensitive to the Pomeron coupling than to the photon coupling. These predictions call for future experimental measurements to help better understand the coherent interaction in hadronic heavy-ion collisions.

Effects of longitudinal asymmetry in heavy-ion collisions

In collisions of identical nuclei at a given impact parameter, the number of nucleons participating in the overlap region of each nucleus can be unequal due to nuclear density fluctuations. The asymmetry due to the unequal number of participating nucleons, referred to as longitudinal asymmetry, causes a shift in the center of mass rapidity of the participant zone. The information of the event asymmetry allows us to isolate and study the effect of longitudinal asymmetry on rapidity distribution of final state particles. In a Monte Carlo Glauber model the average rapidity-shift is found to be almost linearly related to the asymmetry. Using toy models, as well as Monte Carlo data for Pb-Pb collisions at 2.76 TeV generated with HIJING, two different versions of AMPT and DPMJET models, we demonstrate that the effect of asymmetry on final state rapidity distribution can be quantitatively related to the average rapidity shift via a third-order polynomial with a dominantly linear term. The coefficients of the polynomial are proportional to the rapidity shift with the dependence being sensitive to the details of the rapidity distribution.Experimental estimates of the spectator asymmetry through the measurement of spectator nucleons in a Zero Degree Calorimeter may hence be used to further constrain the initial conditions in ultra-relativistic heavy-ion collisions.

Studies of Final-State Interactions via Helicity Asymmetries in Exclusive Pseudoscalar Meson Photoproduction off Deuteron

Exclusive meson photoproduction off deuteron is a great tool to study final-state interactions (FSI) between the hadrons produced in the scattering of the incident photon off one of the target nucleons and the spectator nucleon. FSI in the reaction \(\gamma d\rightarrow K^+\varLambda n\) allow to access the dynamics of \(\varLambda n\) elastic scattering and provide a method of studying the hyperon-nucleon (YN) interaction. Also, as deuteron is often used as a neutron target, FSI contribute to the reaction dynamics in addition to the quasi-free mechanism and their understanding is critical for the estimate of observables off the free neutron. Here we show preliminary results for beam-helicity asymmetries for the reactions \(\mathbf {\gamma } d\rightarrow K^+\varLambda n\) and \(\mathbf {\gamma } d\rightarrow p\pi ^+\pi ^- n\) and their evolution over the spectator-nucleon momentum. The data were obtained with the CEBAF Large Acceptance Spectrometer at Thomas Jefferson National Accelerator Facility (JLab). We show how to kinematically select an event sample dominated by a specific FSI mechanism. We also discuss a method to extract polarization observables for photoproduction off the free nucleon using the helicity asymmetries for reactions off the deuteron.

Centrality and collision event-plane determination in ALICE at the LHC

Investigation of physical phenomena in heavy-ion collisions requires knowledge about collision geometry, which is characterized by the energy distribution in the overlap region of the colliding nuclei and of the collision spectators. Both the event-by-event distributions of produced particles and the spectator nucleons can be used to estimate the collision geometry. Due to the pressure gradients, the spatial anisotropy of initial state geometry is converted during the system evolution to an anisotropy in momentum space. The event-planes of this anisotropy can be estimated via measured azimuthal distributions of particles produced in the collision. We report on the performance of the ALICE experiment at the LHC for the centrality and the event-plane determination for different harmonics using measured distribution of produced particles at central and forward rapidity, and the energy distribution of the spectator neutrons at beam rapidity.

Participant and spectator scaling of spectator fragments in Au + Au and Cu + Cu collisions atsNN=19.6and 22.4 GeV

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to Nitrogen (Z=7), are measured in PHOBOS. These fragments are observed in Au+Au (sqrt(sNN)=19.6 GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity ($\eta$). The dominant multiply-charged fragment is the tightly bound Helium ($\alpha$), with Lithium, Beryllium, and Boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the $\alpha$ fragments to be released than Lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for $\alpha$ and Lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.

Nucleon-gold collisions at200AGeV using taggedd + Auinteractions in the PHOBOS detector

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.

Role of pion exchange inηmeson photoproduction on the deuteron

Incoherent $\eta$ photoproduction on the deuteron is studied with the main emphasis on the role of final state interactions. In addition to the previously studied mechanisms of $NN$ and $\eta N$ rescatterings, the role of an intermediate pion exchange is considered in detail, where first a pion is photoproduced on one nucleon and then rescatters into an $\eta$ meson on the other, the spectator nucleon. It is found, that the role of this pion mediated contribution is comparable in size to that of $\eta N$ rescattering. Results for total and semi-inclusive differential cross sections and associated polarization observables are presented. In particular polarization observables show a significant sensititvity to final state interactions.

Sivers Function in the Quasi-Classical Approximation

In this brief article we summarize the derivation of the Sivers function of a heavy nucleus, which we obtain by generalizing the quasi-classical McLerran-Venugopalan model to incorporate the role of spin and orbital angular momentum. In doing so we obtain a new channel which is capable of generating the Sivers function of the nucleus from the orbital motion of its nucleons. An essential role is played in this channel by the multiple rescatterings on spectator nucleons which screen the distribution function during the initial- or final-state interactions. The combination of orbital angular momentum together with multiple rescattering yields a new interpretation of the sign flip relation between the Sivers function measured in semi-inclusive deep inelastic scattering and the Drell-Yan process.

Magneto-sonoluminescence and its signatures in photon and dilepton production in relativistic heavy ion collisions

The matter produced in the early stages of heavy ion collisions consists mostly of gluons, and is penetrated by the coherent magnetic field produced by spectator nucleons. The fluctuations of gluonic matter in an external magnetic field couple to real and virtual photons through virtual quark loops. We study the resulting contributions to photon and dilepton production that stem from the fluctuations of the stress tensor ${T}_{\ensuremath{\mu}\ensuremath{\nu}}$ in the background of a coherent magnetic field $\stackrel{P\vec}{B}$. Our study extends significantly the earlier work [G. Basar, D. E. Kharzeev, and V. Skokov, Phys. Rev. Lett. 109, 202303 (2012)], in which only the fluctuations of the $trace$ of the stress tensor ${T}_{\ensuremath{\mu}\ensuremath{\mu}}$ were considered (the coupling of ${T}_{\ensuremath{\mu}\ensuremath{\mu}}$ to electromagnetic fields is governed by the scale anomaly). In the present paper we derive more general relations using the operator product expansion (OPE). We also extend the previous study to the case of dileptons, which offers the possibility to discriminate between various production mechanisms. Among the phenomena that we study are magneto-sonoluminescence [MSL, the interaction of magnetic field $\stackrel{P\vec}{B}(x,t)$ with the sound perturbations of the stress tensor $\ensuremath{\delta}{T}_{\ensuremath{\mu}\ensuremath{\nu}}(x,t)$] and magneto-thermoluminescence [MTL, the interaction of $\stackrel{P\vec}{B}(x,t)$ with smooth average $\ensuremath{\langle}{T}_{\ensuremath{\mu}\ensuremath{\nu}}\ensuremath{\rangle}$]. We calculate the rates of these process and find that they can dominate the photon and dilepton production at early stages of heavy ion collisions. We also point out the characteristic signatures of MSL and MTL that can be used to establish their presence and to diagnose the produced matter.

Neutron emission from electromagnetic dissociation of Pb nuclei at √sNN= 2.76 TeV measured with the ALICE ZDC

The ALICE Zero Degree Calorimeter system (ZDC) is composed of two identical sets of calorimeters, placed at opposite sides with respect to the interaction point, 114 meters away from it, complemented by two small forward electromagnetic calorimeters (ZEM). Each set of detectors consists of a neutron (ZN) and a proton (ZP) ZDC. They are placed at zero degrees with respect to the LHC axis and allow to detect particles emitted close to beam direction, in particular neutrons and protons emerging from hadronic heavy-ion collisions (spectator nucleons) and those emitted from electromagnetic processes. For neutrons emitted by these two processes, the ZN calorimeters have nearly 100% acceptance.

Incoherent photoproduction ofϕ-meson from deuteron at low energies

The LEPS and CLAS data of the incoherent photoproduction of $\phi$ meson from deuteron at low energies are studied with a model for $\phi$ meson photoproduction from nucleon consisting of Pomeron, $\pi$, and $\eta$ meson exchanges in the t-channel, and a postulated resonance, with parameters fitted to recent LEPS data on $\phi$ production from proton near threshold. The resonance was introduced to explain an observed bump in the forward differential cross section. Within impulse approximation, we find that the Fermi motion, final state interaction, and the resonance excitation all give important contributions to improve the agreement with data. However, discrepancies remain. Contributions from $\phi$ production via spectator nucleon by other mesons like $\pi, \rho,$ and $\phi$ produced from the first nucleon need to be calculated in order to gain insight on the medium effects as well as the existence of the postulated nucleon resonance.

Directed Flow of Charged Particles at Midrapidity Relative to the Spectator Plane in Pb-Pb Collisions atsNN=2.76 TeV

The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at √(s(NN))=2.76 TeV relative to the collision symmetry plane defined by the spectator nucleons. A negative slope of the rapidity-odd directed flow component with approximately 3 times smaller magnitude than found at the highest RHIC energy is observed. This suggests a smaller longitudinal tilt of the initial system and disfavors the strong fireball rotation predicted for the LHC energies. The rapidity-even directed flow component is measured for the first time with spectators and found to be independent of pseudorapidity with a sign change at transverse momenta p(T) between 1.2 and 1.7 GeV/c. Combined with the observation of a vanishing rapidity-even p(T) shift along the spectator deflection this is strong evidence for dipolelike initial density fluctuations in the overlap zone of the nuclei. Similar trends in the rapidity-even directed flow and the estimate from two-particle correlations at midrapidity, which is larger by about a factor of 40, indicate a weak correlation between fluctuating participant and spectator symmetry planes. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.

Probing vector mesons in deuteron breakup reactions

We study vector meson photoproduction from the deuteron at high momentum transfer, accompanied by break-up of the deuteron into a proton and neutron. The large $-t$ involved allows one of the nucleons to be identified as struck, and the other as a spectator to the $\gamma N\rightarrow VN$ subprocess. Corrections to the plane wave impulse approximation involve final state interactions (FSIs) between the struck nucleon or the vector meson, either of which is energetic, with the slow spectator nucleon. In this regime, the eikonal approximation is valid, so is employed to calculate the cross-section for the reaction. Due to the high-energy nature of the FSIs, the maxima of the rescatterings are located at nearly transverse directions of the fast hadrons. This results in two peaks in the angular distribution of the spectator nucleon, each corresponding to either the $V$-$N$ or the $p$-$n$ rescattering. The $V$-$N$ peak provides a new means of probing the $V$-$N$ interaction. This is checked for near-threshold $\phi$ and $J/\Psi$ photoproduction reactions which demonstrate that the $V$-$N$ peak can be used to extract the largely unknown amplitudes of $\phi$-$N$ and $J/\Psi$-$N$ interactions. Two additional phenomena are observed when extending the calculation of $J/\Psi$ photoproduction to the sub-threshold and high-energy domains. In the first case we observe overall suppression of FSI effects due to a restricted phase space for sub-threshold production in the rescattering amplitude. In the second, we observe cancellation of the $V$-$N$ rescattering amplitudes for vector mesons produced off of different nucleons in the deuteron.