High Momentum Region Research Articles

Role of two-nucleon mechanisms in pion photoproduction on nuclei in the region of high momentum transfers

The role of two-nucleon mechanisms in pion photoproduction on nuclei was studied in the region of high momentum transfers to the residual nucleus. The process in which the photoproduction of negative pions on a 12C nucleus is accompanied by proton emission was considered by way of example. The results of the calculations were compared with available experimental data.

Emission of η meson with high transverse momentum in Au–Au, d–Au and p–p collisions at $$\sqrt{\mathrm{\mit s_{NN}}}=200$$ s NN = 200 GeV

Inclusive transverse momentum spectra of η meson in nucleus–nucleus, deuton–nucleus and proton–proton collisions are studied in the framework of a thermalized cylinder model. A single component distribution can only describe a narrow transverse momentum range. In the region of high transverse momentum, the considered distributions of η meson have a tail part at the maximum energy of relativistic heavy ion collider. To explain the wider transverse momentum spectra, we consider the relative importance of hard versus soft processes in the particle production mechanisms. The improved cylinder model utilizing two-component spectra is used to describe the experimental data of the PHENIX Collaboration. It is found that the two-component distribution can describe a broadened range of the transverse momentum. It can offer information about soft and hard interactions in the collisions.

Meson Production in High Energy p+p Collisions at the RHIC Energies

Transverse momentum spectra of mesons produced in p+p collisions are studied in the framework of a thermalized cylinder model. In the region of high transverse momentum, the considered distributions have a tail part at the maximum energy of RHIC. A two-component distribution based upon the improved cylinder model is used to fit the experimental data of the PHENIX Collaboration. It is found that the improved approach can describe the meson production in the wider range of transverse momenta.

WWZ/γproduction in the large extra dimensions model at the LHC and ILC

We investigate the effect induced by the Kaluza-Klein (KK) graviton in the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production in the framework of the large extra dimensions (LED) model at both the CERN Large Hadron Collider (LHC) and the International Linear Collider (ILC). The integrated cross sections and various kinematic distributions in the LED model are presented and compared with those in the standard model. The results show that the contributions from KK-graviton exchange remarkably affect the observables of the triple gauge boson (${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$) production processes at both the ILC and LHC, particularly either in the high transverse momentum region or in the central rapidity region. We also find that the relative LED discrepancy for the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production at the LHC is generally larger than that at the ILC due to the additional LED contribution via $gg$ fusion subprocess and the KK-graviton exchanging resonant effect induced by the continuous large colliding energy in $pp$ collision. We conclude that the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}$ and ${W}^{+}{W}^{\ensuremath{-}}Z$ productions at the LHC could have the distinct advantage over at the ILC from the aspect of effectively exploring the LED signal in measuring ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production.

Identification of Vacancy Formation Sites in LaNi5Cu During Hydrogenation Using in Situ Coincidence Doppler Broadening Technique

To study the formation mechanism of lattice defects introduced during hydrogenation, we developed equipment that can measure in situ positron lifetime and coincidence Doppler broadening (CDB) simultaneously with pressure–composition (P–C) isotherms at room temperature. The equipment, which consisted of a high-pressure sample holder, reduced the intensity of the background signals in the relatively high momentum region of the Doppler broadening spectrum by more than 2 orders of magnitude in comparison to the background shown by conventional Doppler broadening equipment. Further, the quality of data obtained by our instrument was similar to that obtained from ex situ CDB equipment without a high-pressure sample holder. The substitution effect on the positron annihilation behavior was observed from the ratio curves of CDB spectra for LaNi5Cu and LaNi5-xAlx. In addition, using the equipment we developed, we attempted to identify the vacancy formation sites introduced in LaNi5Cu during hydrogenation. Changes i...

Quark degrees of freedom in the deuteron and their testing in nucleon-deuteron scattering

The discrepancies between theoretical predictions and experimental data for nucleon-deuteron scattering at high momentum transfers are usually attributed to the three-nucleon force effects but inclusion of 3N forces (of conventional type) in exact Faddeev calculations helps to improve the agreement with the data only partially. Alternatively, the short-range part of 2N interaction (manifesting itself in processes with large transferred momenta) may be modified by incorporating quark degrees of freedom. Such a microscopic NN-force model - the dressed bag model - was proposed by the Moscow-Tuebingen group and is based on the production of an intermediate 6q bag dressed by a strong σ field. It is demonstrated that taking into account the formation of an intermediate 6q bag leads to a noticeable enhancement of the deuteron wave function in the high-momentum region. Our preliminary calculations performed within the dressed bag model show that the suggested mechanism of the short-range NN interaction gives significant contribution to the nucleon-deuteron scattering at high momentum transfers both through 2N and 3N forces.

Nuclear fragmentation study at ITEP heavy ion facility

In an experiment performed at ITEP TWA heavy ion accelerator, the yields of hydrogen (p,d,t) and helium (from 3He to 8He) isotopes at 3.5° from fragmentation of 12C at T0 = 0.2 − 3.2 GeV/nucleon on a Be target have been measured. Momentum spectra of the fragments in the projectile rest frame have been obtained in larger momentum intervals than in the previous experiments with heavy ion beams. The main attention was given to the region of high momentum where fragment velocity exceeds the velocity of the projectile nucleus. The obtained data cover about 6 orders of the differential cross section magnitude. It made possible the observation of a transition from the Gaussian shape of the longitudinal momentum spectra in projectile rest frame, expected for the evaporation mechanism, to the exponential shape, typical for the cumulative (pre-equilibrium) processes. The Feynman x distributions for protons are analyzed in the framework of quark-gluon string model. The probabilities of existence of six-and nine-quark clusters are estimated and compared with the results on two- (three-) nucleon short range correlations in nuclei measured at Jefferson Laboratory.

Nucleon momentum distributions and elastic electron scattering form factors for some 1p-shell nuclei

The nucleon momentum distributions (NMD) and elastic electron scattering form factors of the ground state for 1p-shell nuclei with Z = N (such as 6Li, 10B, 12C and 14N nuclei) have been calculated in the framework of the coherent density fluctuation model (CDFM) and expressed in terms of the weight function \(\left| {f( x )} \right|^2\). The weight function has been expressed in terms of nucleon density distribution (NDD) of the nuclei and determined from the theory and the experiment. The feature of the long-tail behaviour at high-momentum region of the NMDs has been obtained by both the theoretical and experimental weight functions. The experimental form factors F(q) of all the considered nuclei are very well reproduced by the present calculations for all values of momentum transfer q. It is found that the contributions of the quadrupole form factors FC2(q) in 10B and 14N nuclei, which are described by the undeformed p-shell model, are essential for obtaining a remarkable agreement between the theoretical and experimental form factors.

Shear layers in the turbulent pipe flow of drag reducing polymer solutions

A range of high molecular weight polymers (polyethylene oxide) was dissolved at very low concentrations – in the order of few wppm – in a solvent (water). The Newtonian character of the polymer solutions was confirmed by rheological measurements. The polymer solutions were then pumped through a long horizontal pipe section in fully developed turbulent conditions. The flow experienced a reduction in frictional drag when compared to the drag experienced by the equivalent flow of the pure solvent. Specifically, drag reduction was measured at Reynolds numbers ranging from 3.5×104 to 2.1×105 in a pressure driven flow facility with a circular tube section of internal diameter 25.3mm. The turbulent flow was visualized by Particle Image Velocimetry and the resulting data were used to investigate the effect of the drag reducing additives on the turbulent pipe flow. Close attention was paid to the mean and instantaneous velocity fields, as well as the two-dimensional vorticity and streamwise shear strain rate. The results indicate that drag reduction is accompanied by the appearance of “shear layers” (i.e. thin filament-like regions of high spatial velocity gradients) that act as interfaces separating low-momentum flow regions near the pipe wall and high-momentum flow regions closer to the centerline. The shear layers are not stationary. They are continuously formed close to the wall at a random frequency and move towards the pipe centerline until they eventually disappear, thus occupying or existing within a “shear layer region”. It is found that the mean thickness of the shear layer region is correlated with the measured level of drag reduction. The shear layer region thickness is increased by the presence of polymer additives when compared to the pure solvent, in a similar way to the thickening of the buffer layer. The results provide valuable insights into the characteristics of the turbulent pipe flow of a solvent containing drag reducing polymers that can be used to further our understanding of the role of polymers on the mechanism of drag reduction and to develop advanced drag reduction models.

Dispersions of image potential states on surfaces of clean graphite and lead phthalocyanine film

Dispersions of image potential states on a graphite surface (denoted IPS1) and on 1 monolayer (ML) film (denoted IPS2) of lead phthalocyanine (PbPc) are investigated by the micro-spot angle-resolved two-photon photoemission (micro-AR-2PPE) spectroscopy. On the graphite surface, whole dispersions of the two members of IPS1 (n = 1 and 2) are observed. The n = 1 IPS1 peak is weakly visible at energy higher than the vacuum level. The effective mass of an electron in the n = 1 IPS1 becomes slightly light at the high momentum region, suggesting the interaction between the IPS1 and the unoccupied σ-band of graphite. On the PbPc film, the IPS2 band forms a band gap and back-folds at the boundary of the Brillouin zone. A 1-dimensional Kronig-Penny model is used to reproduce the effective mass and the shift of binding energy.

Very-large-scale motions in a turbulent boundary layer

Direct numerical simulation of a turbulent boundary layer was performed to investigate the spatially coherent structures associated with very-large-scale motions (VLSMs). The Reynolds number was varied in the range Reθ = 570–2560. The main simulation was conducted by using a computational box greater than 50δo in the streamwise domain, where δo is the boundary layer thickness at the inlet, and inflow data was obtained from a separate inflow simulation based on Lund's method. Inspection of the three-dimensional instantaneous fields showed that groups of hairpin vortices are coherently arranged in the streamwise direction and that these groups create significantly elongated low- and high-momentum regions with large amounts of Reynolds shear stress. Adjacent packet-type structures combine to form the VLSMs; this formation process is attributed to continuous stretching of the hairpins coupled with lifting-up and backward curling of the vortices. The growth of the spanwise scale of the hairpin packets occurs continuously, so it increases rapidly to double that of the original width of the packets. We employed the modified feature extraction algorithm developed by Ganapathisubramani, Longmire & Marusic (J. Fluid Mech., vol. 478, 2003, p. 35) to identify the properties of the VLSMs of hairpin vortices. In the log layer, patches with the length greater than 3δ–4δ account for more than 40% of all the patches and these VLSMs contribute approximately 45% of the total Reynolds shear stress included in all the patches. The VLSMs have a statistical streamwise coherence of the order of ~6δ; the spatial organization and coherence decrease away from the wall, but the spanwise width increases monotonically with the wall-normal distance. Finally, the application of linear stochastic estimation demonstrated the presence of packet organization in the form of a train of packets in the log layer.

Neutral pion production with respect to reaction plane in Au+Au collisions at RHIC-PHENIX

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 collisions. Studying the path length dependence of energy loss would give additional information on understanding the energy loss mechanism. We discuss the parton energy loss mechanism using the nuclear modification factor ($R_{AA}$) of neutral pion with respect to reaction plane. A new reaction plane detector was installed in the PHENIX detector in RHIC Year-7 run, and improved the reaction plane resolution. More precise measurement of the hadron suppression with respect to path length is expected using the detector. I will report about analysis status of neutral pion production in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Calculation of the pressure of a hot scalar theory within the Non-Perturbative Renormalization Group

We apply to the calculation of the pressure of a hot scalar field theory a method that has been recently developed to solve the Non-Perturbative Renormalization Group. This method yields an accurate determination of the momentum dependence of n-point functions over the entire momentum range, from the low momentum, possibly critical, region up to the perturbative, high momentum region. It has therefore the potential to account well for the contributions of modes of all wavelengths to the thermodynamical functions, as well as for the effects of the mixing of quasiparticles with multi-particle states. We compare the thermodynamical functions obtained with this method to those of the so-called Local Potential Approximation, and we find extremely small corrections. This result points to the robustness of the quasiparticle picture in this system. It also demonstrates the stability of the overall approximation scheme, and this up to the largest values of the coupling constant that can be used in a scalar theory in 3 + 1 dimensions. This is in sharp contrast to perturbation theory which shows no sign of convergence, up to the highest orders that have been recently calculated.

CALCULATION OF THEORETICAL ISOTROPIC COMPTON PROFILE FOR MANY PARTICLE SYSTEMS

Theoretical isotropic (spherically symmetric) Compton profiles (ICP) have been calculated for many particle systems' He , Li , Be and B atoms in their ground states. Our calculations were performed using Roothan–Hartree–Fock (RHF) wave function, HF wave function of Thakkar and re-optimized HF wave function of Clementi–Roetti, taking into account the impulse approximation. The theoretical analysis included a decomposition of the various intra and inter shells and their contributions in the total ICP. A high momentum region of up to 4 a.u. was investigated and a non-negligible tail was observed in all ICP curves. The existence of a high momentum tail was mainly due to the electron–electron interaction. The ICP for the He atom has been compared with the available experimental data and it is found that the ICP values agree very well with them. A few low order radial momentum expectation values 〈pn〉 and the total energy for these atomic systems have also been calculated and compared with their counterparts' wave functions.

Emission of Neutral Pions with High Transverse Momenta in A-A and p-p Collisions at Energies Available at the BNL Relativistic Heavy Ion Collider

Invariant neutral pion (π0) yields in Au-Au and Cu-Cu collisions and invariant π0 cross sections in p-p collisions are studied in the framework of a two-cylinder model. The considered distributions of neutral pions at the maximum energy of the relativistic heavy ion collider (RHIC) have a tail part in the region of high transverse momentum. A two-component distribution based upon the two-cylinder model is used to describe the experimental data of the PHENIX collaboration.

Positron lifetime and Doppler broadening study of defects in hydrogen charged Al–Mg alloys

The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime τav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion.

A new aerogel Cherenkov detector for DIRAC-II

A new threshold Cherenkov detector using silica aerogel radiators is in operation in the DIRAC-II experiment at CERN. A duplex design was chosen to discriminate between kaons and protons in the momentum range 4–8 GeV/ c. The counter consists of overlapping modules with two different refractive indices, 1.015 and 1.008, covering the low and high momentum regions, respectively. We developed a new design of alternating layers of aerogel tiles and diffusive reflector foils coated with wavelength shifter to improve light collection in the aerogel. A pyramidal shape of the aerogel radiator was also adopted for best signal uniformity over a large sensitive area. We report on the detector status and its performance in the DIRAC 2007 run.

Top production from black holes at the LHC

In theories with large extra dimension and with low quantum gravity scale near a TeV, it is expected that TeV-scale black holes to be produced in proton-proton collisions at the LHC with the center of mass energy of 14 TeV. Since the black holes temperature can be around 1 TeV, top quark production is expected from them via Hawking radiation. Within the Standard Model of particle physics top quarks are produced via strong interaction in $t\bar{t}$ pairs or via electroweak interaction singly. Therefore, black holes can be the new source of top quark production. In this article we present the total cross sections and transverse momentum distributions of top quark production from black holes at the LHC. We find that the top quarks from black holes tend to reside at very high transverse momentum region so it can be a very useful signature for the black holes at the LHC.

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence. By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. 68Ge, instead of the widely used positron emitter 22Na. The 68Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low. When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10 −3 m 0 c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.

Applying Bayesian neural networks to identify pion, kaon and proton in BESII**Supported by National Natural Science Foundation of China (10605014)

The Monte-Carlo samples of pion, kaon and proton generated from 0.3 GeV/c to 1.2 GeV/c by the `tester' generator from SIMBES which are used to simulate the detector of BESII are identified with the Bayesian neural networks (BNN). The pion identification and misidentification efficiencies are obviously better at high momentum region using BNN than the methods of χ2 analysis of dE/dX and TOF information. The kaon identification and misidentification efficiencies are obviously better from 0.3 GeV/c to 1.2 GeV/c using BNN than the methods of χ2 analysis. The proton identification and misidentification efficiencies using BNN are basically consistent with the ones of χ2 analysis. The anti-proton identification and misidentification efficiencies are better below 0.6 GeV/c using BNN than the methods of χ2 analysis.