Small Relative Momenta Research Articles

First measurement of proton-induced low-momentum dielectron radiation off cold nuclear matter

We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with HADES installed at GSI. The data represent the first high statistics measurement of proton-induced dielectron radiation from cold nuclear matter in a kinematic regime, where strong medium effects are expected. Combined with the good mass resolution of 2%, it is the first measurement sensitive to changes of the spectral functions of vector mesons, as predicted by models for hadrons at rest or small relative momenta. Comparing the e+e− invariant mass spectra to elementary p+p data, we observe for e+e− momenta Pee<0.8 GeV/c a strong modification of the shape of the spectrum, which we attribute to an additional ρ-like contribution and a decrease of ω yield. These opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons.

Bose-Einstein correlations with CMS detector at the LHC at √s = 0.9 and 2.36 TeV

Bose-Einstein correlations between identical particles are measured using samples of protonproton collisions at 0.9 and 2.36 TeV centre-of-mass energy, recorded by the CMS experiment at the CERN Large Hadron Collider. The signal is observed in the form of an enhancement of pairs of same sign charged particles with small relative momentum. A significant increase of the size of the correlated particle emission region with the particle multiplicity in the event is observed.

Λp femtoscopy in collisions of Ar + KCl at 1.76 A GeV with HADES

Results on Λp femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76.A GeV, the reaction Ar + KCl was studied with HADES at SIS18/GSI. A high-statistics and high-purity Λ sample was collected, allowing for the investigation of Λp correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the Λp emission source. The Λp radius is found significantly smaller than that for Au + Au/Pb + Pb collisions in the AGS, SPS and RHIC energy domains, but larger than that for electroproduction from He. Taking into account all available data, we find the Λp source radius to increase almost linearly with the number of participants to the power of one-third.

Measurement of Bose–Einstein correlations in pp collisions at the LHC with CMS

Bose–Einstein correlations are measured in samples of charged particles produced in proton–proton collisions at 0.9 and 7 TeV center-of-mass energies, recorded by the CMS experiment at the LHC. The signal is observed in the form of an enhancement of number of pairs of the same-sign charged particles with small relative momentum. The dependence of this enhancement on kinematic and topological features of the event is studied.

Measurement of Bose-Einstein correlations in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV

Bose-Einstein correlations between identical particles are measured in samples of proton-proton collisions at 0.9 and 7 TeV centre-of-mass energies, recorded by the CMS experiment at the LHC. The signal is observed in the form of an enhancement of number of pairs of same-sign charged particles with small relative momentum. The dependence of this enhancement on kinematic and topological features of the event is studied.

Correlations of antiprotons with small relative momentumin the SELEX Experiment

The main results of the data processing and physical analysis of the SELEX experiment and the preliminary data on correlations of antiprotons with small relative momentum are presented. Correlation femtoscopy is a powerful tool for studying the spatial-temporal characteristics of the emission region of identical particles. The correlation functions obtained in the SELEX and STAR experiments are compared.

Estimating the production rate of loosely bound hadronic molecules using event generators

We examine the use of hadronic event generators, such as Pythia or Herwig, to estimate the production rate of loosely bound hadronic molecules, such as the deuteron and the $X(3872)$. In the case of the deuteron, we point out that there are large uncertainties in the normalization of the predictions using event generators, because baryon pair distributions are not among the inputs used to tune the event generators. Predictions using Pythia for antideuteron production in $\ensuremath{\Upsilon}$ decay are compared to measurements by the CLEO Collaboration. They suggest that Pythia overpredicts the probability of producing pairs of baryons, at least in $\ensuremath{\Upsilon}$ decay into three gluons, and that the standard value of the coalescence parameter underpredicts the probability for formation of a deuteron from a neutron and proton with small relative momentum. In the case of the $X(3872)$, we discuss a proposed upper bound on the prompt cross section at the Tevatron that has been used as an argument against the $X(3872)$ being a loosely bound charm-meson molecule. We demonstrate that this proposed upper bound is invalid by showing that the analogous upper bound for the antideuteron would be smaller than the observed antideuteron cross section.

Bose–Einstein Correlation with CMS Detector at the LHC at [formula omitted] and 2.36 TeV

Bose–Einstein correlations between identical particles are measured using samples of proton-proton collisions at 0.9 and 2.36 TeV centre-of-mass energy, recorded by the CMS experiment at the CERN Large Hadron Collider. The signal is observed in the form of an enhancement of pairs of same sign charged particles with small relative momentum. A significant increase of the size of the correlated particle emission region with the particle multiplicity in the event is observed.

Λ−pfemtoscopy in collisions ofAr+KClat1.76A GeV

Results on $\Lambda$p femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76A~GeV, the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\Lambda$ sample was collected, allowing for the investigation of $\Lambda$ p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $\Lambda$p emission source. The $\Lambda$p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present $\Lambda$p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the $\Lambda$p source radius to increase almost linearly with the number of participants to the power of one-third.

Stability of the Excitonic Phase in Bilayer Quantum Hall Systems at Total Filling One –Effects of Finite Well Width and Pseudopotentials–

The ground state of a bilayer quantum Hall system at $\nu_{\rm T}=1$ with model pseudopotential is investigated by the DMRG method. Firstly, pseudopotential parameters appropriate for the system with finite layer thickness are derived, and it is found that the finite thickness makes the excitonic phase more stable. Secondly, a model, where only a few pseudopotentials with small relative angular momentum have finite values, is studied, and it is clarified how the excitonic phase is destroyed as intra-layer pseudopotential becomes larger. The importance of the intra-layer repulsive interaction at distance twice of the magnetic length for the destruction of the excitonic phase is found.

10.1007/s11450-008-3007-2

The theoretical analysis of pair correlations of low-energy neutrons produced in nuclear fission is performed on the basis of the general method of pair correlations of particles at small relative momenta. The analytical approach to the description of the effect of final-state interaction on narrow pair correlations of neutrons is used. It is shown that, at small but nonzero values of relative momenta, the two-neutron correlation function is sensitive to the Fermi statistics and final-state interaction effects only for pairs of scission neutrons, whereas for ordinary fission neutrons (emitted from fission fragments) these correlation effects are absent. Thus, the correlation method makes it possible, in principle, to determine the fraction of pairs of scission neutrons in the total amount of neutron pairs emitted in some fission process.

An ultra cold photoelectron gun for the Heidelberg TSR target section

We present a cryogenic photoelectron gun, developed for the target section of the Test Storage Ring (TSR) of the Max Planck Institute of Nuclear Physics (MPIK). Cooled to cryogenic temperatures by liquid nitrogen, the photocathode source provides an electron gas with an initial thermal energy spread of around 10 meV. The beam optics of the target section reduce the electron temperatures to much lower values in the comoving frame of the beam. Recently the photocathode source has seen significant improvements regarding its reliability. By controlling several cathode degradation mechanisms, including cryosorption, vacuum degrading leak electron currents, and backstream of ionised restgas particles, cathode lifetime and currents have been subject to substantial improvements. Presently the photoelectron gun can deliver currents of up to 1 mA at lifetimes of about 24h. The ability of the photoelectron beam to cool slow, heavy molecular ion beams was demonstrated by cooling a 3 MeV CF+ beam in the TSR. At an electron cooling energy of only 53 eV and a perveance-limited current of 0.34 mA, a cooling time below 2 s has been achieved, with a very small transverse relative momentum spread of 2.5 · 10−5 and final ion beam cross-section of 0.5×0.1 mm2.

Coulomb effects in femtoscopy

The correlation function of two identical particles---pions or kaons---interacting via Coulomb potential is computed. The particles are emitted from an anisotropic particle's source of finite lifetime. In the case of pions, the effect of halo is taken into account as an additional particle's source of large spatial extension. The relativistic effects are discussed in detail. The Bowler-Sinyukov procedure to remove the Coulomb interaction is carefully tested. In the absence of halo the procedure is shown to work very well even for an extremely anisotropic source. When the halo is taken into account the free correlation function, which is extracted by means of the Bowler-Sinyukov procedure, is distorted at small relative momenta but the source parameters are still correctly reproduced.

Difficulties in probing density dependent symmetry potential with the HBT interferometry

Based on the updated UrQMD transport model, the effect of the symmetry potential energy on the two-nucleon HBT correlation is investigated with the help of the coalescence program for constructing clusters, and the CRAB analyzing program of the two-particle HBT correlation. An obvious nonlinear dependence of the neutron-proton (or neutron-neutron) HBT correlation function (C np,nn ) at small relative momenta on the stiffness factor γ of the symmetry potential energy is found: when γ ≲ 0.8, C np,nn increases rapidly with increasing γ, while it starts to saturate if γ ≳0.8. It is also found that both the symmetry potential energy at low densities and the conditions of constructing clusters at the late stage of the whole process influence the two-nucleon HBT correlation with the same power.

Optimal spin-quantization axes for quarkonium with large transverse momentum

The gluon collision process that creates a heavy-quark-antiquark pair with small relative momentum and large transverse momentum predicts at leading-order in the QCD coupling constant that the transverse polarization of the pair should increase with its transverse momentum. Measurements at the Fermilab Tevatron of the polarization of charmonium and bottomonium states with respect to a particular spin-quantization axis are inconsistent with this prediction. However the predicted rate of approach to complete transverse polarization depends on the choice of spin-quantization axis. We introduce axes that maximize and minimize the transverse polarization from the leading-order gluon collision process. They are determined by the direction of the jet that provides most of the balancing transverse momentum.

Correlations of Λ hyperons with a small relative momentum in the SELEX experiment

The information on the shape of the potential between two baryons at small distances can be obtained by studying the baryon correlations at a small relative momentum in the creation reactions of high-energy hadron collisions. We have physically analyzed the SELEX experimental data and identified more than 20 × 103 events with creation of two Λ hyperons, which substantially exceeds the world statistical data. The obtained results are preliminary analyzed.

Direct calculation of the probability of pionium ionization in the target

The goal of the DIRAC experiment at CERN is the lifetime measurement of pionium (π + π − atom). Its lifetime is mainly defined by the charge-exchange process π + π − → π 0 π 0. The value of the lifetime in the ground state is predicted in the framework of chiral perturbation theory with high precision: τ 1S = (2.9 ± 0.1) × 10−15 s. The method used by DIRAC is based on analysis of π + π −-pair spectra with small relative momenta in their center-of-mass system in order to find out the signal from pionium ionization (breakup) in the target. Pioniums are produced in proton-nucleus collisions and have relativistic velocities (γ > 10). For fixed values of the pionium momentum and the target thickness, the probability of pionium ionization in the target depends on its lifetime in a unique way; thus, the pionium lifetime can be deduced from the experimentally defined probability of pionium ionization. On the basis of ionization cross sections of pionium with target atoms, we perform the first direct calculation of the pionium ionization probability in the target.

Higher-order contributions observed in three-dimensional(e,2e)cross-section measurements at 1-keV impact energy

We present experimental and theoretical fully differential cross sections for single ionization by fast, 1-keV $(v=8.6\phantom{\rule{0.3em}{0ex}}\text{a.u.})$ electron impact. The cross sections were measured using a momentum imaging technique for electrons and ions (reaction microscope), which covers a large fraction of the emission angles for emitted low-energy electrons $(E&lt;15\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and a wide range of scattering angles. Therefore comprehensive data sets are obtained for ionizing collisions at small relative momentum and energy transfer from the projectile to the target system. The experimental data are compared with predictions from several state-of-the-art theoretical calculations. At this high impact energy the calculated cross section for electron emission out of the scattering plane appears to be particularly sensitive to the treatment of higher orders in the projectile-target interaction within perturbative models.

On the pair correlations of nuclear fission neutrons with small relative momenta

The theoretical analysis of pair correlations of low-energy neutrons produced in nuclear fission is performed on the basis of the general method of pair correlations of particles at small relative momenta. The analytical approach to the description of the effect of final-state interaction on narrow pair correlations of neutrons is used. It is shown that, at small but nonzero values of relative momenta, the two-neutron correlation function is sensitive to the Fermi statistics and final-state interaction effects only for pairs of scission neutrons, whereas for ordinary fission neutrons (emitted from fission fragments) these correlation effects are absent. Thus, the correlation method makes it possible, in principle, to determine the fraction of pairs of scission neutrons in the total amount of neutron pairs emitted in some fission process.

TOWARDS RELIABLE CALCULATIONS OF THE CORRELATION FUNCTION

The correlation function of two identical pions interacting via Coulomb potential is computed for a general case of anisotropic particle's source of finite life time. The effect of halo is taken into account as an additional particle's source of large spatial extension. Due to the Coulomb interaction, the effect of halo is not limited to very small relative momenta but it influences the correlation function in a relatively large domain. The relativistic effects are discussed in detail and it is argued that the calculations have to be performed in the center-of-mass frame of particle's pair where the (nonrelativistic) wave function of particle's relative motion is meaningful. The Bowler–Sinyukov procedure to remove the Coulomb interaction is tested and it is shown to significantly underestimate the source's life time.