Soft and hard processes in QCD

QCD equations for the generating functions are applied to separate soft and hard jets in e⁺e^? processes of multiparticle production. The dependence of average multiplicities and higher moments of multiplicity distributions of particles created in “newly born” soft subjets on the share of energy devoted to them is calculated in fixed coupling gluodynamics. This dependence is the same as for the total multiplicity up to a constant factor if soft jets are defined as those carrying out a fixed share of initial energy at all energies. The constant factor depends on this share in a nontrivial way. Other definitions are also proposed. The relation between these quantities for soft and hard processes is discussed.     

Soft and hard QCD dynamics in hadroproduction of charmonium

Both hard and soft QCD dynamics are important in charmonium production, as presented here through a next-to-leading order QCD matrix element calculation combined with the color evaporation model. The observed and distributions of in hadroproduction at fixed target and collider energies are reproduced. Quite similar results can also be obtained in a more phenomenologically useful Monte Carlo event generator where the perturbative production of pairs is instead obtained through leading order matrix elements and the parton shower approximation of the higher order processes. The soft dynamics may alternatively be described by the soft color interaction model, originally introduced in connection with rapidity gaps. We also discuss the relative rates of different charmonium states and introduce an improved model for mapping the continuous mass spectrum on the physical charmonium resonances Received: 29 November 2001 / Published online: 1 March 2002     

Soft X-ray resonant magnetic scattering studies on Fe/CoO exchange bias system

We have used soft X-ray resonant magnetic scattering (XRMS) to search for the presence of an effective ferromagnetic moment belonging to the antiferromagnetic (AF) layer which is in close contact with a ferromagnetic (F) layer. Taking advantage of the element specificity of the XRMS technique, we have measured hysteresis loops of both Fe and CoO layers of a CoO(40 Å)/Fe (150 Å) exchange bias bilayer. From these measurements we have concluded that the proximity of the F layer induces a magnetic moment in the AF layer. The F moment of the AF layer has two components: one is frozen and does not follow the applied magnetic field and the other one follows in phase the ferromagnetic magnetization of the F layer. The temperature dependence of the F components belonging to the AF layer is shown and discussed.     

Soft gluon emission and the QCD Λ mass parameter

We use a simple and transparent method for the resolution of the modified Altarelli-Parisi equation (including soft gluon emission) in the non-singlet case. We point out some of the phenomenological consequences of this modification. In particular, we show that the QCD Λ mass scale has to be lowered by a factor of two.     

Pomeron structure function and diffraction dissociation of virtual photons in perturbative QCD

We study scaling properties of the diffraction dissociation of virtual photons in a deep inelastic scattering. We concentrate on the total diffraction dissociation rate, diffraction excitation mass spectrum and the pomeron structure function to the lowest order in perturbative QCD. We calculate the valence structure function and the strangeness and charm content of the pomeron and estimate the ocean structure function using the pomeron factorization property. We find that quarks carry ≈ 10% of pomeron's momentum. Differential cross section of the (virtual) photon-pomeron scattering is found to exhibit features typical of the hadronic two-body reactions, supporting a treatment of the Pomeron as a particle, whereas the flavor dependence of structure functions does not support the particle treatment of the pomeron. Diffraction dissociation of photons is predicted to make ≈ 15% of the total deep inelastic scattering rate at smallx and largeQ ². Detailed predictions for the mass spectrum and angular distribution of jets produced on the valence component of the pomeron are presented.     

Soft diffraction at the LHC: a partonic interpretation

We present a ‘new generation’ model for high energy proton–proton ‘soft’ interactions. It allows for a full set of multipomeron vertices as well as for including multichannel eikonal scattering. It describes the behaviour of the proton–proton total, σ_tot, and elastic, dσ_el/dt, cross sections together with those for low- and high-mass proton dissociation. Although the model contains a comprehensive set of multipomeron diagrams, it has a simple partonic interpretation. Including the more complicated multipomeron vertices reduces the absorptive effects as compared to the predictions in which only the triple-pomeron vertex is considered. Tuning the model to describe the available ‘soft’ data in the CERN ISR–tevatron energy range, we predict the total, elastic, single- and double-diffractive dissociation cross sections at the LHC energy. An inescapable consequence of including multichannel eikonal and multipomeron effects is that the total cross section is expected to be lower than before: indeed, we find σ_tot≃ 90 mb at the LHC energy. We also present differential forms of the cross sections. In addition, we calculate soft diffractive central production.     

The shrinkage of the diffraction peak of the bare pomeron in QCD

The new parameterQ ₀ is introduced in our integration over transverse momentaq _t in perturbative QCD to avoid infrared divergencies. Consideringq _t >Q ₀, the distribution of wee partons in impact parameter (b _t ) and the mean radius of interaction are calculated in the framework of the leading logarithmic approximation (LLA) of perturbative QCD. It is shown that the slope of the elastic amplitude increases as \(B \propto {{\sqrt {\alpha _s In s} } \mathord{\left/ {\vphantom {{\sqrt {\alpha _s In s} } {Q_0^2 }}} \right. \kern-0em} {Q_0^2 }}\) .     

Soft photon production rate in resummed perturbation theory of high temperature QCD

We calculate the production rate of soft real photons from a hot quark-gluon plasma using Braaten-Pisarski's perturbative resummation method. To leading order in the QCD coupling constantg we find a logarithmically divergent result for photon energies of ordergT, whereT is the plasma temperature. This divergent behaviour is due to unscreened mass singularities in the effective hard thermal loop vertices in the case of a massless external photon.Laboratoire associé du Centre National de la Recherche Scientifique     

Magnetization and neutron diffraction studies on FeCrP

Crystal structure and magnetic behaviour of FeCrP have been investigated using magnetization and neutron diffraction measurements. FeCrP crystallizes in orthorhombic FeZrP type structure (P_nma space group,Z = 4) in which Cr atoms occupy the pyramidal site and Fe atoms occupy the tetrahedral site with total preference. Structural parameters including positional parameters have been refined. The refined values of positional parameters for Fe and Cr are quite different from those in FeZrP. The nature of magnetization-temperature curve is suggestive of antiferromagnetic nature withT _N = 280 (±10) K. Preliminary analysis of neutron diffraction pattern at 13 K is indicative of a rather complicated magnetic structure.     

DSC and X-ray diffraction studies of HxBPA

A detailed DSC study has been done for the substance HxBPA in the temperature range 170 to 380 K and the scheme of transitions in this compound has been deduced. From X-ray work, values for the smectic layer thickness and inplane intermolecular distance have been found in two of the liquid crystalline phases, P₂ and S_H.     

Soft diffraction and the elastic slope at Tevatron and LHC energies: a multi-Pomeron approach

We present a formalism for high energy soft processes, mediated by Pomerons, which embodies pion-loop insertions in the Pomeron trajectory, rescattering effects via a two-channel eikonal and high-mass diffractive dissociation. It describes all the main features of the data throughout the ISR to Tevatron energy interval. We give predictions for soft diffractive phenomena at the LHC energy, and we calculate the different survival probabilities of rapidity gaps which occur in various diffractive processes. Received: 1 August 2000 / Published online: 27 October 2000     

Phase transitions in BaCeO3: neutron diffraction and Raman studies

Polycrystalline samples and small single crystals of the perovskite BaCeO₃ were studied by neutron diffraction and Raman spectrometry between 300 and 1200 K. The controversy about the phase transitions originally deduced from our previous Raman study and those observed since by neutron diffraction by Knight has stimulated this work. Pretransitional effects which are detected by Raman much before long-range ordering takes place can partly explain the above disagreement. A continuous monitoring of the structural changes by neutron diffraction and by Raman spectroscopy including polarization analysis has allowed discussion of the transition mechanisms: The first transition Pnma–Imma takes place at 573 K and is of second order. Although some modes soften when the temperature is raised as in many of these perovskite compounds the transition is likely partly displacive partly order–disorder. The Raman modes which disappear transform in modes at the X point of the Brillouin zone of the Imma phase. The second transition Imma–R c takes place at 673 K and is first order. The last transition R c–Pm3m occurs above 1200 K and the transition temperature which can be deduced by extrapolation to zero Raman intensity is in good agreement with neutron results. This second order transition is progressive and begins at about 400 K, the intermediate R c structure appearing as an attempt for slowing down the structural evolution toward the cubic perovskite form.     

Nonlinear Dynamics of Soft Boson Excitations in Hot QCD Plasma. I. Plasmon-Plasmon Scattering

On the basis of the pure gauge sector of the Blaizot-Iancu equation, we derive a kinetic equation of Boltzmann type, taking into account 2n+2-colorless plasmon decay processes, n=1, 2,…. Using the so-called Tsytovich correspondence principle, a direct connection between matrix elements of the plasmon decay processes and a certain effective current, generating these processes, is established. The procedure of calculating a matrix element for the simplest four-plasmon decay is considered comprehensively. The limiting value of the plasmon occupation number (∼1/g², where g is a strong coupling) wherein all plasmon decays with n≥1 contribute to the right-hand side of the Boltzmann equation, is defined. The iterative method of calculation of matrix elements for higher decay processes (n>1) is proposed, and a problem of their gauge-invariance is discussed. We proceed from the general reasons the problem of extending the suggested approach to the case of color plasmons is considered. The explicit form of a linearized Boltzmann equation for color plasmons is written out, and it is shown that this equation covariantly conserves the color current, resulting from color-plasmon number density.     

Kapitza-Dirac效应的最新进展

自由电子束经光的驻波散射服从布拉格(Bragg)定律的设想由Kapitza和Dirac(KD)于1933年提出．这一效应在近70年以后的2001年经Batelaan等人用实验证实．近日，电子束的KD衍射的理论以中国物理学家为主用非微扰量子电动力学做出．新理论不但解释了Batelaan的实验，还预言了若干新现象．由于玻色一爱因斯坦凝聚的发展及高科技领域的需要，原子和电子形成的物质波须由KD效应进行控制．巨大的应用前景正在使KD效应的研究由冷门变为热门．     

High-pressure X-Ray diffraction and optical absorption studies of CsI

Static-compression data and absorption spectra for CsI have been collected to 61 GPa (610 kbar) at room temperature. The band gap closes with increasing pressure and CsI is expected to metallize at 105 (± 15) GPa. A second order phase transition to the CuAu I structure is observed at 39 (± 1) GPa. The elastic constants measured at low pressures do not predict that an elastic instability, and hence a structural distortion, would occur at elevated pressures. Similarly, an ionic pair-potential model which reproduces the properties of CsI at low pressures does not show the distortion to be stabilized at high pressures.     

High-pressure behavior of red mercuric iodide: in situ X-ray diffraction and optical absorption studies

We have investigated the behavior of red mercuric iodide (α-HgI₂) under high pressures using in situ X-ray diffraction and optical absorption techniques. Our experimental results indicate that the tetragonal→orthorhombic phase transformation, observed at 1.4 GPa, is accompanied by an abrupt increase in the band gap while the nature of the gap does not change. However, across the orthorhombic→hexagonal phase transformation, observed at ∼7.2 GPa, the gap decreases discontinuously and changes from direct to indirect type. These studies suggest that HgI₂ may metallize at ∼40 GPa, if not prevented by any other structural change.