Emission patterns of neutral pions in 40 A MeV Ta+Au reactions
K. Piasecki, T. Matulewicz, N. Yahlali, H. Delagrange, J. Díaz, D. G. d'Enterria, F. Fernández, A. Kugler, H. Löhner, G. Martínez-García, R. W. Ostendorf, Y. Sc… 展开作者
K. Piasecki, T. Matulewicz, N. Yahlali, H. Delagrange, J. Díaz, D. G. d'Enterria, F. Fernández, A. Kugler, H. Löhner, G. Martínez-García, R. W. Ostendorf, Y. Schutz, P. Tlustý, R. Turrisi, V. Wagner, H. W. Wilschut
备注：
16 pages (double-spaced), 5 figures; corrections after referee's comments and suggestions
Differential cross sections of neutral pions emitted in 181Ta + 197Au collisions at a beam energy of 39.5A MeV have been measured with the photon spectrometer TAPS. The kinetic energy and transverse momentum spectra of neutral pions cannot be properly described in the framework of the thermal model, nor when the reabsorption of pions is accounted for in a phenomenological model. However, high energy and high momentum tails of the pion spectra can be well fitted through thermal distributions with unexpectedly soft temperature parameters below 10 MeV.
We present transverse momentum and rapidity spectra of charged pions in central Ru + Ru collisions at 400$A$ and 1528$A$ MeV. The data exhibit enhanced production at low transverse momenta compared to the expectations from the thermal model that includes the decay of $Δ(1232)$-resonances and thermal pions. Modification of the $Δ$-spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The IQMD model reproduces the sha…
Transverse momentum spectra for neutral pions have been measured by the WA98 experiment in sqrt{s_{nn}}=17.3 GeV Pb+Pb collisions at the CERN SPS and by the PHENIX experiment in sqrt{s_{nn}}=130 GeV Au+Au collisions at RHIC. The neutral pion yields in central collisions for both reaction systems are compared to scaled transverse momentum spectra for nucleon-nucleon reactions at the respective energy. At SPS energies neutral pion production at high $p_T$ is enhanced compared to the n+n reference while at RHIC a significant suppression of high-pT neutral pions is observed.
A method is suggested to deduce the anisotropy in neutral pions by measuring the azimuthal anisotropy of photons in ultra-relativistic nuclear collisions. The ratio of the estimated anisotropy in photons to the anisotropy in neutral pions is seen to scale with a parameter which depends on photon multiplicity and anisotropy. This parameter can be determined from experimental data.
Measured values of the differential cross section for pion-nucleon charge exchange are presented at momenta 148, 174, 188, 212, 238, 271, 298, and 323 MeV/c, a region dominated by the Delta resonance. Complete angular distributions were obtained using the Crystal Ball detector at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL). Statistical uncertainties of the differential cross sections are typically 2-6%, exceptions being the results at the lowest momentum and at the most forward measurements of the five lowest momenta. We estimate the systematic uncertaint…
Measurements of direct photon and neutral pion production in heavy-ion collisions provide a comprehensive set of observables characterizing properties of the hot QCD medium. Direct photons provide means to test the initial stage of an AA collision and carry information about the temperature and space-time evolution of the hot medium. Neutral pion suppression probes the parton energy loss in the hot medium. Measurements of neutral meson spectra in pp collisions at LHC energies $\sqrt{s}=$0.9, 2.76, 7 TeV serve as a reference for heavy-ion collisions and also provide valuable input data for para…
The inclusive cross section for neutral pion production in the range 1<p_T<13GeV/c in |eta|<0.35 has been measured by the PHENIX experiment in proton-proton collisions at sqrt{s}=200GeV. An NLO pQCD calculation is, within the experimental and theoretical uncertainties, consistent with the measurement.
Recently, we have performed the experiment on studying the pion electroproduction at photon point on the internal polarized deuterium target at the VEPP-3 storage ring. For the more complete treatment of the reaction d(e,pp)X we investigate the reaction channel with an additional neutral pion production as background to the leading reaction of the single negative pion production. To study this background process we considered the nucleon pole mechanism and the two-body mechanism. In the latter mechanism a Delta(1232) and the pion are produced in one nucleon then the pion is absorbed by other n…
The cross section of the ${}^{16}{\rm O}(p,p'){}^{16}{\rm O}(0^-,T=1)$ scattering was measured at a bombarding energy of 295 MeV in the momentum transfer range of $1.0 \mathrm{fm^{-1}}$ $\le$ $q_{\rm c.m.}$ $\le$ $2.1 \mathrm{fm^{-1}}$. The isovector $0^-$ state at $E_x$ = 12.8 MeV is clearly separated from its neighboring states owing to the high energy resolution of about 30 keV. The cross section data were compared with distorted wave impulse approximation (DWIA) calculations employing shell-model wave functions. The observed cross sections around $q_{\rm c.m.}$ $\simeq$ $1.7 {\rm fm^{-1}}$…
New results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected 200 AGeV S + S and S + Au collisions. The spectra from all systems show a clear power-law like shape with similar curvature. Collisions of S + Au exhibit a larger mean transverse momentum than pp increasing with centrality. Predictions of string models and by hydrodynamic approaches including collective expansion and decays of short lived resonances are compared to the data and the implications are discussed.
Preliminary results are presented on high-pT inclusive neutral pion production in d+Au collisions at sqrt(sNN) = 200 GeV in the pseudo-rapidity range 0 < eta < 1. Photons from the decay pi0->gamma,gamma are detected in the Barrel electromagnetic calorimeter of the STAR experiment at RHIC. The analysis procedure is described in detail and the results are found to be in good agreement with earlier STAR findings and with next-to-leading order perturbative QCD calculations.
The production of neutral pions and eta mesons in collisions of light, intermediate-mass, and heavy nuclei at beam energies between 0.8 A GeV and 2.0 A GeV is discussed in the framework of a model assuming equilibrium at hadrochemical and thermal freeze-out, respectively. From the relative particle yields temperatures of 56-90 MeV and baryon chemical potentials of 780-675 MeV are deduced at chemical freeze-out. Midrapidity spectra of neutral pions and eta mesons are consistent with the obtained temperatures if, in addition, the measured transverse expansion of the collision zone is taken into …
Inclusive transverse momentum spectra of photons and neutral pions about mid-rapidity are presented for Au+Au collisions at sqrt(s_NN} = 130 GeV. Photon pair conversions have been reconstructed from charged tracks measured with the central Time Projection Chamber (TPC) of STAR. Contributions of the pi^0 -> γγdecay in the inclusive photon spectra have been studied. It was found that this contribution begins to decrease at a transverse momentum of 1.65 GeV/c in the top 11% most central centrality class.
Pion-nucleus elastic scattering at energies above the Delta(1232) resonance is studied using both pi+ and pi- beams on 12C, 40Ca, 90Zr, and 208Pb. The present data provide an opportunity to study the interaction of pions with nuclei at energies where second-order corrections to impulse approximation calculations should be small. The results are compared with other data sets at similar energies, and with four different first-order impulse approximation calculations. Significant disagreement exists between the calculations and the data from this experiment.
Invariant yields of neutral pions at midrapidity in the transverse momentum range $0.6 < p_{T} < 12 GeV/c$ measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV are presented for six centrality classes. The pp reference spectrum was measured in the range $0.4 < p_{T} < 10 GeV/c$ at the same center-of-mass energy. The nuclear modification factor, $R_{\rm AA}$, shows a suppression of neutral pions in central Pb-Pb collisions by a factor of up to about $8-10$ for $5 \lesssim p_{T} \lesssim 7 GeV/c$. The presented measurements are compared with results at lower center-of-mass energies and…
The production of neutral pions in 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied for the range 0.3 GeV/c < mT-m0 < 4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons which is most naturally explained by assuming an equ…
Neutral pions ($π^0$) in $\sqrt{s_\mathrm{NN}}$ = 200 GeV and 62.4 GeV Au+Au collisions were measured at RHIC-PHENIX Year-4 Run. In $\sqrt{s_\mathrm{NN}}$ = 200 GeV, $π^0$ specta are measured up to p$_\mathrm{T}$ = 20 GeV/$c$. %PHENIX extended the p$_\mathrm{T}$ region of A strong suppression by a factor of $\sim$ 5 is observed and stays almost constant up to 20 GeV/$c$.
In this paper, preliminary results are presented on high-$p_T$ inclusive neutral pion measurements in d-Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV in the pseudo-rapidity range $0<η<1$. The photons of the decay $pi^0\toγγ$ are detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at RHIC. The analysis of this first BEMC hadron measurement is described in detail. The results are compared to earlier RHIC findings. Furthermore, the obtained $π^{0}$ invariant differential cross sections show good agreement with next-to-leading order (NLO) perturbative QCD calculations.
An experimental technique for the light lambda-hypernuclei structure studies by using (negative kaon, neutral pion) reaction and Neutral Meson Spectrometer (NMS) developed at the BNL has been described. Position dependence calibration of the BGO conversion planes of the NMS was invented as the solution to the crucial constraint of the high resolution in single-lambda spectroscopy. Position parameters of the BGO crystal rods were fitted out from the out-of-kaon beam measured data as obtained by the original small highly collimated Co-60 source method, based on the coincidences between top and b…
Measurements of transverse-momentum p_T spectra of neutral pions in Au+Au and d+Au collisions at sqrt{s_NN}=200 GeV and 62.4 GeV by the PHENIX experiment at RHIC in comparison to p+p reference spectra at the same sqrt{s_NN} are presented. In central Au+Au collisions at sqrt{s_NN}=200 GeV a factor 4-5 suppression for neutral pions and charged hadrons with p_T > 5 GeV/c is found relative to the p+p reference scaled by the nuclear overlap function < T_AB >. In contrast, such a suppression of high-p_T particles is absent in d+Au collisions independent of the centrality of the collision. To study t…
Results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected 158$\cdot A$ GeV Pb + Pb, and Pb + Nb collisions. The distributions cover the range $0.5 \mathrm{GeV}/c^{2} \le m_{T} - m_{0} \le 4 \mathrm{GeV}/c^{2}$. The change of the spectral shape with centrality is studied in detail. In going from peripheral to medium central collisions there is a nuclear enhancement increasing with transverse mass similar to the well known Cronin effect, while for very central collisions this enhancement appears to be weaker than expected.
The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon-nucleon …
Transverse momentum spectra for identified $π^0$'s in the range 1 GeV/c $< p_T <$ 4 GeV/c have been measured by the PHENIX experiment in Au-Au collisions at $\sqrt{s}=130$ GeV. The spectra from peripheral nuclear collisions are consistent with the simple expectation of scaling the spectra from p+p collisions by the average number of nucleon-nucleon binary collisions. The spectra from central collisions and the ratio of central/peripheral spectra are significantly suppressed when compared to point-like scaling.
The (pi+,pd), and (pi+,dd) reactions were investigated with pions of 100 and 165 MeV kinetic energy on 6Li and 12C targets. In comparison with previously published (pi+,pp) data on the same targets and at the same beam energies, kinematic regions were identified in which the neutron pickup process n+p->d dominated the observed deuteron yield. The importance of this mechanism increases with energy, contributing half of the observed cross section at 165 MeV. The contribution of direct quasi-triton absorption is significant only at 100 MeV.
The question whether there is a pion transparency in 500 MeV $C(π, π^{'})$ scatterings is studied using a semiclassical, hadronic transport model. The double differential cross sections of this reaction measured at LAMPF can be largely accounted for, if one uses energy-dependent, anisotropic angular distributions which are fitted to pion-nucleon scattering data for the decay of $Δ(1232)$ and $N^*(1440)$ resonances. The remaining discrepancy between the data and the calculation sets a limit on effects of more exotic processes.
It has been observed that the yield of neutral pions at high transverse momentum (pT $>$ 5 GeV/c) region is strongly suppressed in central Au+Au collisions at Relativistic Heavy Ion Collider (RHIC), compared to the one expected in p+p collisions. This suppression may be due to an energy loss of hard scattered partons in the medium (jet quenching), that results in a decrease of the yield at a given pT. The magnitude of the suppression would depend on the path length of scattering partons in the medium, and therefore is associated with azimuthal angle from reaction plane in non-central collision…
We present the results for direct photon and neutral pion production obtained from proton-proton and lead-lead collisions in the ALICE experiment. Measurement of neutral pions in pp collisions allows to test the validity of QCD-inspired models. The study of neutral pion yields in Pb-Pb collisions permits to study the medium-induced suppression relative to the pp case. The yield of direct photons is compared to perturbative QCD calculations. In the case of central Pb-Pb collisions the observed excess of photons below 4 GeV/c is used to find the effective temperature of the matter.
The SciBooNE Collaboration reports a measurement of neutral current coherent neutral pion production on carbon by a muon neutrino beam with average energy 0.8 GeV. The separation of coherent from inclusive neutral pion production has been improved by detecting recoil protons from resonant neutral pion production. We measure the ratio of the neutral current coherent neutral pion production to total charged current cross sections to be (1.16 +/- 0.24) x 10-2. The ratio of charged current coherent pion to neutral current coherent pion production is calculated to be 0.14+0.30 -0.28, using our publ…
First measurements of the azimuthal anisotropy of neutral pions produced in PbPb collisions at a center-of-mass energy of sqrt(s(NN)) = 2.76 TeV are presented. The amplitudes of the second Fourier component (v2) of the neutral pion azimuthal distributions are extracted using an event-plane technique. The values of v2 are studied as a function of the neutral pion transverse momentum (pt) for different classes of collision centrality in the kinematic range 1.6 < pt < 8.0 GeV, within the pseudorapidity interval abs(eta) < 0.8. The CMS measurements of v2(pt) are similar to previously reported neut…
A description of low-energy scattering of pions and nuclei within a BUU transport model is presented. Implementing different scenarios of medium modifications, the mean free path of pions in nuclear matter at low momenta and pion absorption reactions on nuclei have been studied and compared to data and to results obtained via quantum mechanical scattering theory. We show that even in a regime of a long pionic wave length the semi-classical transport model is still a reliable framework for pion kinetic energies greater than ~20-30 MeV. Results are presented on pion-absorption cross sections in …
Within a transport model, we study effects of the momentum dependence of nuclear symmetry potential on pion observables in central Sn + Sn collisions at 270 MeV/nucleon. To this end, a quantity $U_{sym}^{\infty}(ρ_{0})$, i.e., the value of nuclear symmetry potential at the saturation density $ρ_{0}$ and infinitely large nucleon momentum, is used to characterise the momentum dependence of nuclear symmetry potential. It is shown that with a certain $L$ (i.e., slope of nuclear symmetry energy at $ρ_{0}$) the characteristic parameter $U_{sym}^{\infty}(ρ_{0})$ of symmetry potential affects signific…
We have measured pion single charge exchange differential cross sections on the proton at 27.5 MeV incident $π^-$ kinetic energy in the center of momentum angular range between $0^\circ$ and $55^\circ$. The extracted cross sections are compared with predictions of the standard pion-nucleon partial wave analysis and found to be in excellent agreement.
Invariant cross sections for neutral pions and eta mesons in pp collisions at sqrt(s) = 0.9, 2.76, and 7 TeV were measured by the ALICE detector at the Large Hadron Collider. Next-to-leading order (NLO) perturbative QCD calculations describe the pi0 and eta spectra at 0.9 TeV, but overestimate the measured cross sections at 2.76 TeV and 7 TeV. The measured eta/pi0 ratio is consistent with mT scaling at 2.76 TeV. At 7 TeV indications for a violation of mT scaling were found.
We investigate decay and production of neutral pions in strong magnetic fields. In the presence of strong magnetic fields, transition between a neutral pion and a virtual photon becomes possible through the triangle diagram relevant for the chiral anomaly. We find that the decay mode of a neutral pion into two photons cannot persist in the dominant mode in strong magnetic fields, and that decay into a dilepton instead dominates over the other modes. We also investigate the effects of magnetic fields on prompt virtual photons created in ultrarelativistic heavy-ion collisions. There is no anisot…
The invariant differential cross section for inclusive neutral pion production in p+p collisions at sqrt(s_NN) = 200 GeV has been measured at mid-rapidity |eta| < 0.35 over the range 1 < p_T <~ 14 GeV/c by the PHENIX experiment at RHIC. Predictions of next-to-leading order perturbative QCD calculations are consistent with these measurements. The precision of our result is sufficient to differentiate between prevailing gluon-to-pion fragmentation functions.
The STAR endcap electromagnetic calorimeter (EEMC) was designed to allow measurement of cross sections and spin observables in the forward direction, $1 < η< 2$ and with the full azimuth. Using the EEMC to measure double longitudinal spin asymmetries in photonic channels---such as inclusive neutral pions, prompt photon, and prompt photon + jet---allows access to $ΔG$ within a low Bjorken-$x$ region ($0.01 < x < 0.33$ at $\sqrt{s}=200$) where $ΔG$ is poorly constrained. Additionally, transverse spin asymmetries, shown to be zero at $η$ near zero and as large as 10\% at $η$ near 4, occupy a prev…
New results from polarized p-p collisions at sqrt(s)=200 GeV of double longitudinal spin asymmetry in pi0 production using the PHENIX detector in the 2005 RHIC run are presented. Both positive and negative maximal gluon polarization scenarios are inconsistent with the data. The data is consistent with small |Δg|, including Δg=0.
Laser-induced processes in ultra-dense deuterium D(0) layers eject multi-MeV u-1 particles using ns laser pulse energies of <200 mJ. Such particles have been observed previously as mA currents to time-of-flight (TOF) collectors at up to 1 m distance. The signal current is mainly due to the ejection of secondary electrons by impinging MeV particles on the collectors. Improved two-collector time-of-flight measurements now show that the energy of the particles is in the range 1-50 MeV u-1. Their distributions are almost thermal at up to 13 MeV u-1 or are sharper than thermal. The fastest sharp pe…
Transverse single-spin asymmetries of very forward neutral pions generated in polarized $p + p$ collisions allow us to understand the production mechanism in terms of perturbative and non-perturbative strong interactions. During 2017 the RHICf Collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral pions produced at pseudorapidity larger than 6 in polarized $p$+$p$ collisions at $\sqrt{s}$ = 510 GeV. The large non-zero asymmetries increasing both in longitudinal momentum fraction $x…
With the availability of the new neutral pion photoproduction from the proton data from the A2 and CB-TAPS Collaborations at Mainz it is mandatory to revisit Heavy Baryon Chiral Perturbation Theory (HBChPT) and address the extraction of the partial waves as well as other issues such as the value of the low-energy constants, the energy range where the calculation provides a good agreement with the data and the impact of unitarity. We find that, within the current experimental status, HBChPT with the fitted LECs gives a good agreement with the existing neutral pion photoproduction data up to $\s…
The contribution of D waves to physical observables for neutral pion photoproduction in the near threshold region is studied. Heavy Baryon Chiral Perturbation Theory to one loop, and up to ${\cal O}(q^4)$, is used to account for the S and P waves, while D waves are added in an almost model-independent way using standard Born terms and vector mesons. It is found that the inclusion of D waves is necessary to extract the $E_{0+}$ multipole reliably from present and forthcoming data and to assess the low-energy constants of Chiral Perturbation Theory. Arguments are presented demonstrating that F-w…
We investigate the neutrino induced coherent pion production reaction at low and intermediate energies. The model includes pion, nucleon and Delta(1232) resonance as the relevant hadronic degrees of freedom. Nuclear medium effects on the production mechanisms and pion distortion are taken into account. We obtain that the dominance of the Delta excitation holds due to large cancellations among the background contributions. We consider two sets of vector and axial-vector N-Delta transition form-factors, evidencing the strong sensitivity of the results to the axial coupling C5A(0). The difference…
Within the impulse approximation, the modeling of the energy of final state leptons in electron and neutrino quasielastic and pion production processes on nuclear targets in the region of the $Δ$ resonance depends on several parameters. These parameters include the removal energy of the initial state nucleon from the nucleus $ε^{P,N}$, the potentials of electrons, nucleons and pions in the Coulomb field of the nucleus $|V_{eff}|$, and the kinetic energy dependent nuclear potential for final state nucleons ($U^{QE}_{opt}$) and "nucleon plus pion" final states in the region of the $Δ$ resonance …
Two dedicated experiments on internal pair conversion (IPC) of isoscalar M1 transitions were carried out in order to test a 9 MeV/c2 X-boson scenario. In the 7Li(p,e+e-)8Be reaction at 1.1 MeV proton energy to the predominantly T=0 level at 18.15 MeV, a significant deviation from IPC was observed at large pair correlation angles. In the 11B(d,n e+e-)12C reaction at 1.6 MeV, leading to the 12.71 MeV 1+ level with pure T=0 character, an anomaly was observed at 9 MeV/c2. The compatibility of the results with the scenario is discussed.
