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1.
《The European Physical Journal C - Particles and Fields》2007,52(2):255-265
The longitudinal double spin asymmetry A1
ρ for exclusive leptoproduction of ρ0 mesons, μ+N→μ+N+ρ, is studied using the COMPASS 2002 and 2003 data. The measured reaction is incoherent exclusive ρ0 production on polarised deuterons. The Q2 and x dependence of A1
ρ is presented in a wide kinematical range, 3×10-3<Q2< 7 (GeV/c)2 and 5×10-5<x<0.05. The results presented are the first measurements of A1
ρ at small Q2 (Q2< 0.1 (GeV/c)2) and small x (x<3×10-3). The asymmetry is in general compatible with zero in the whole kinematical range.
PACS 13.60.Le; 13.88.+e 相似文献
2.
3.
We introduce a new class of two-dimensional cellular automata with a bootstrap percolation-like dynamics. Each site can be
either empty or occupied by a single particle and the dynamics follows a deterministic updating rule at discrete times which
allows only emptying sites. We prove that the threshold density ρ
c
for convergence to a completely empty configuration is non trivial, 0<ρ
c
<1, contrary to standard bootstrap percolation. Furthermore we prove that in the subcritical regime, ρ<ρ
c
, emptying always occurs exponentially fast and that ρ
c
coincides with the critical density for two-dimensional oriented site percolation on ℤ2. This is known to occur also for some cellular automata with oriented rules for which the transition is continuous in the
value of the asymptotic density and the crossover length determining finite size effects diverges as a power law when the
critical density is approached from below. Instead for our model we prove that the transition is discontinuous and at the same time the crossover length diverges faster than any power law. The proofs of the discontinuity and the lower bound on the crossover length use a conjecture on the critical behaviour for
oriented percolation. The latter is supported by several numerical simulations and by analytical (though non rigorous) works
through renormalization techniques. Finally, we will discuss why, due to the peculiar mixed critical/first order character of this transition, the model is particularly relevant to study glassy and jamming transitions. Indeed, we will show that
it leads to a dynamical glass transition for a Kinetically Constrained Spin Model. Most of the results that we present are
the rigorous proofs of physical arguments developed in a joint work with D.S. Fisher. 相似文献
4.
The analytic solution of EIP model in the MSA via the OCP criterion yields temperature dependent results in RPA quite satisfactorily
at high temperatures. The same set of parameters produces low and high angle structural properties. The model can work almost
near to the critical point if the ion-core radius is allowed to vary systematically in the regionρ<2ρ
c,ρ
c being the critical density. The model can also accommodate the general scaling behaviour observed for the structure factors
of liquid alkali metals. 相似文献
5.
We found an approximate simple solution of sea-quark evolution equation in terms ofρ(=√ln(x
0/x)/ln[ln(Q
2/Λ2)/(Q
0
2
/Λ2)]) andζ(≡ln[ln(Q
2Λ2)/ln(Q
0
2
/Λ2)]) in the small-x region whenρ is fixed and compared with HERA data. Agreement with data is found for largeQ
2 and smallρ. Comparison with double asymptotic scaling prediction is made. We found a critical value ofρ. More data are needed to confirm this point. 相似文献
6.
E.G. Drukarev M.G. Ryskin V.A. Sadovnikova 《The European Physical Journal A - Hadrons and Nuclei》1999,4(2):171-186
We calculate the contribution of pions to the $\bar qq$-expectation value κ(ρ) =<M|ˉq
q|M> in symmetric nuclear matter. We employ exact pion propagator renormalized by nucleon-hole and isobar-hole excitations. Conventional
straightforward calculation leads to the “pion condensation” at unrealistically small values of densities, causing even earlier
restoration of chiral symmetry. This requires a self-consistent approach, consisting in using the models, which include direct
dependence of in-medium mass values on κ(ρ), e.g. the Nambu–Jona-Lasinio–model. We show, that in the self-consistent approach
the ρ-dependence of the condensate is described by a smooth curve. The “pion condensate” point is removed to much higher values
of density. The chiral restoration does not take place at least while ρ < 2.8ρ0 with ρ0 being the saturation value. Validity of our approach is limited by possible accumulation of heavier baryons (delta isobars)
in the ground state of nuclear matter. For the value of effective nucleon mass at the saturation density we found m
*(ρ0) = 0.6m, consistent with nowadays results of other authors.
Received: 8 October 1998 相似文献
7.
We have obtained a generalization of the hydrodynamic theory of vacuum in the context of general relativity. While retaining
the Lagrangian character of general relativity, the new theory provides a natural alternative to the view that the singularity
is inevitable in general relativity and the theory of a hot Universe. We show that the macroscopic source-sink motion as a
whole of ordinary (dark) matter that emerges during the production of particles out of the vacuum can be a new source of gravitational
vacuum polarization (determining the variability of the cosmological term in general relativity). We have removed the well-known
problems of the cosmological constant by refining the physical nature of dark energy associated precisely with this hydrodynamically
initiated variability of the vacuum energy density. A new exact solution of the modified general relativity equations that
contains no free (fitting) parameter additional to those available in general relativity has been obtained. It corresponds
to the continuous and metric-affecting production of ultralight dark matter particles (with mass m
0 = (ħ/c
2) $
\sqrt {12\rho _0 k}
$
\sqrt {12\rho _0 k}
≈ 3 × 10−66 g, k is the gravitational constant) out of the vacuum, with its density ρ0, constant during the exponential expansion of a spatially flat Universe, being retained. This solution is shown to be stable
in the regime of cosmological expansion in the time interval −∞ < t < t
max, when t = 0 corresponds to the present epoch and t
max= 2/3H
0
cΩ0m
≈ 38 × 109 yr at Ω0m
= ρ0/ρc ≈ 0.28 (H
0 is the Hubble constant, ρc is the critical density). For t > t
max, the solution becomes exponentially unstable and characterizes the inverse process of dark matter particle absorption by
the vacuum in the regime of contraction of the Universe. We consider the admissibility of the fact that scalar massive photon
pairs can be these dark matter particles. Good quantitative agreement of this exact solution with the cosmological observations
of SnIa, SDSS-BAO, and the decrease in the acceleration of the expansion of the Universe has been obtained. 相似文献
8.
Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T
c
superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one
approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T
−1/3, changes to ln ρ ∝ T
−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed
in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator
state. The behavior of the width of the superconducting transition and of the temperature of its onset, T
con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity
of the IM transition the superconducting transition becomes broader, and the onset of the transition T
con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in
the crystal of superconducting droplets with different values of T
c
, which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions,
superconductivity has a percolation character.
Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998) 相似文献
9.
A. A. Blinkin V. V. Derevyanko T. V. Sukhareva V. L. Uvarov V. A. Finkel Yu. N. Shakhov I. N. Shlyakhov 《Physics of the Solid State》2011,53(2):245-255
This paper reports on the results of investigations of the influence of irradiation of the two-band BCS superconductor MgB2 by electrons with an average energy $
\bar {\rm E}
$
\bar {\rm E}
∼ 10 MeV at high doses (0 ≤ ϕt ≤ ∼2.5 × 1018 cm−2) on the temperature and width of the transition to the superconducting state, the temperature dependence of the electrical
resistivity in the normal state, the crystal lattice parameters, and the diffraction line intensity. An increase in the electron
irradiation dose ϕt leads to the following effects: a decrease in the critical temperature T
c
; an increase in the width of the superconducting transition ΔT
c
; and a decrease in the “residual electrical resistivity” ρ273 K/ρ40 K, in the parameters a and c of the hexagonal crystal lattice, and in the ratio between the diffraction line intensities I
110/I
100. From analyzing the results obtained, it has been established that the main type of radiation damages under irradiation of
the BCS superconductor MgB2 by high-energy electrons is the formation of vacancies in the B sublattice, which leads to a narrowing of the large band
gap Δσ on the Fermi surface. 相似文献
10.
The broadening and shift of the Q bands of the 1388/1285-cm−1 Fermi doublet of carbon dioxide have been measured by means of the spectroscopy of coherent anti-Stokes Raman scattering
in a wide density range realized at compression in dense gaseous and liquid states. The spectrum of the low-frequency Q band exhibits an essential narrowing upon the compression of the liquid in the density range of 320–400 amagat from a maximum
width of about 2.2 cm−1 to about 1.7 cm−1 determined by elastic dephasing. The observed dependence is connected with the progressive narrowing of the spectral contribution
attributed to the collapsed rotational structure. 相似文献
11.
L. S. Li W. Chen W. Dong X. S. Chen 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,80(2):189-193
Using Monte Carlo simulations and finite-size scaling, we
investigate the critical behavior of two-dimensional magnetic
lattice gas at densities ρ = 0.90, 0.95, 1.0. There is a
ferromagnetic phase transition at each density. As expected, the
critical temperature T
c
depends on system density ρ.
Unexpectedly, there is a density dependence of the critical
exponent of correlation length ν. For densities
ρ = 0.90,0.95,1.0, we obtain the inverse of critical exponent
1/ν = 0.835(5), 0.905(5), 1.00(1) respectively. It is found that
the ratios of critical exponent β/ν and γ/ν of
magnetization and susceptibility are independent of density. 相似文献
12.
A scenario that removes the contradiction between the suppression of the η(1475) → γγ decay width and the strong coupling
of η(1475) to the ρρ, ωω, and γρ0 channels and which leads to a nontrivial prediction for the manifestation of η(1475) in γγ*(Q
2) collisions is considered. Data on the dependence of the cross section for the reaction γγ*(Q
2) → K[`(K)]pK\bar K\pi on the photon virtuality in the energy range 1.35–1.55 GeV are explained here by the production of an η(1475) resonance in
contrast to their standard interpretation in terms of the f
1(1420) resonance. Experimental verification of the present explanation requires determining the spin-parity of resonance contributions,
R, in the reactions γγ*(Q
2) R → R → K[`(K)]pK\bar K\pi and J/ψ → γR → γ(γρ0, γϕ). 相似文献
13.
The two-dimensional dilute granular flow on a smooth incline bounded by rough sidewalls is investigated experimentally, and
the transverse density profiles of granular flux (ζ = ρ v) across the channel are measured. The results show that the transverse density profiles of granular flux are symmetric about
the channel center and that the density of granular flux near the boundary is clearly lower than that of the center. There
is a critical width of channel W
c for the transition of the density of granular flux. The density of granular flux near the boundary decays with the increasing
of inclination (sinθ) of the channel.
Supported by the National Natural Science Foundation of China (Grant No. 10274071) and the Department of Education of Zhejiang
Province (Grant Nos. 20050916 and y200804093) 相似文献
14.
S. S. Murzin 《JETP Letters》2010,91(3):155-157
It has been shown that the observation of the transitions between the dielectric phase and the integer-quantum-Hall-effect
phases with the quantized Hall conductivity σ
xy
q ≥ 3e
2/h announced in a number of works is unjustified. In these works, the crossing points of the magnetic-field dependence of the
diagonal resistivity ρ
xx
at different temperatures T and ωcτ = 1 have been misidentified as the critical points of the phase transitions. In fact, these crossing points are due to the
sign change of the derivative dρ
xx
/dT owing to the quantum corrections to the conductivity. Here, ωc = eB/m is the cyclotron frequency, τ is the transport relaxation time, and m is the effective electron mass. 相似文献
15.
M. A. Yurishchev 《Journal of Experimental and Theoretical Physics》2010,111(4):525-533
The behavior of Ising chains with the spin-spin interaction value λ in a transverse magnetic field of constant intensity (h = 1) is considered. For a chain of infinite length, exact analytical formulas are obtained for the second central moment
(dispersion) of the entropy operator Ŝ = -lnρ with reduced density matrix ρ, which corresponds to a semi-infinite part of the model chain occurring in the ground
state. In the vicinity of a critical point λc = 1, the entanglement entropy fluctuation ΔS (defined as the square root of dispersion) diverges as ΔS ∼ [ln(1/|1 − λ|)]1/2. For the known behavior of the entanglement entropy S, this divergence results in that the relative fluctuation δS = ΔS/S vanishes at the critical point, that is, a state with almost nonfluctuating entanglement is attained. 相似文献
16.
The quantum corrections to the law of corresponding states are studied by calculating the critical pressure, temperature, and density to first order in Planck's constanth on an exactly soluble model. The ratio of the critical parameters to the corresponding classical values are found to be (p
c/p
c
0)1/2=c/c
0 = Tc/Tc
0 = 1–0.67, with=h
c
1/3(mkT
c)–1/2. The critical ratio is independent ofh to first order. The results are compared with critical data for noble gases and hydrogen isotopes. 相似文献
17.
M. A. Zelikman 《Physics of the Solid State》1997,39(11):1751-1755
A system of pancake vortices formed near the boundary of a sample in a monotonically increasing external magnetic field is
calculated with allowance for pinning due to the cellular structure of the medium for various values of the pinning parameter
I, which is proportional to the critical current of the junction and the cell diameter. The shortest distance from the outermost
vortex to the nearest neighbor is proportional to I
−11. It is shown that the pinning parameter has a critical value I
c separating two regimes with different types of critical states. For I<I
c the external magnetic field has a threshold value H
t(I), above which the field immediately penetrates the interior of the junction to an infinite distance. For I>I
c the magnetic field decays linearly from the boundary into the interior of the junction. The value obtained in the study,
I
c=3.369, differs from the value of 0.9716 postulated by other authors. The dependence of the slope of the magnetic field profile
near the boundary on I is determined. It is shown that the slope is independent of I in intervals 2πk<I<2πk+π.
Fiz. Tverd. Tela (St. Petersburg) 39, 1958–1963 (November 1997) 相似文献
18.
L. L. Sviridova V. G. Neudatchin I. T. Obukhovsky D. K. Fedorov 《Physics of Atomic Nuclei》2010,73(7):1203-1206
The electroproduction of charged ρ mesons on a nucleon under conditions of quasielastic kinematics (Q
2 = 2−3 (GeV/c)2, W = 2 GeV) is considered. Dominant diagrams are taken into account. It is shown that the transverse cross section for ρ-meson knockout, in just the same was as the respective longitudinal cross section, is dominated by the t-pole diagram involving a virtual ρ meson. This means that the leading contribution to the process comes from direct ρ-meson knockout. 相似文献
19.
G.W. Canters J. van Egmond T.J. Schaafsma J.H. van der Waals 《Molecular physics》2013,111(6):1203-1215
The absorption and fluorescence spectra of Zn porphin in an n-octane single crystal at 4·2 K are reported in the region between 17 400 and 18 500 cm-1. A strong peak appears in both spectra at 17 961 cm-1 and is assigned to the origin of one component (|x, 0>) of the nearly degenerate Q-band. In absorption a second strong line occurs at a frequency δ = 109 cm-1 above the first; a corresponding line is almost totally absent in the emission spectrum at 4·2 K, but it appears as a hot band of appreciable intensity when the temperature is raised to 80 K. This feature is assigned to the origin of the other component (|y, 0>) of the Q-band. The lifting of the degeneracy of the Q-band is interpreted as a crystal field splitting of the Jahn-Teller unstable 1 Eu state. The Zeeman effect is investigated for the 0–0 transition of the phosphorescence spectrum and the |x, 0> and |y, 0> components of the Q-band absorption spectrum. From the phosphorescence experiment it is concluded that the great majority of the ZnP guests are oriented in the host with an angle of about 25° between the out-of-plane molecular axes and the crystal a-axis. The analysis of the Zeeman effect in absorption (H//crystal a-axis) is complicated by the Jahn-Teller instability which causes two additional unknowns to appear in the problem: the frequency ν and the nuclear displacement parameter α of the active mode. When not making an assumption about these parameters one can only derive a lower limit for the matrix element of the orbital angular momentum between the two electronic components: Λ > 4·6. If is identified with the low-frequency mode of 180 cm-1 appearing in the absorption spectrum, then it follows that Λ = 6·1 ± 0·6 with α = 1·4 ± 0·1. 相似文献
20.
A simple method has been applied to solve the approximate gluon evolution equation for small-x at fixedρ(≡√ln(x
0/x)/ln[ln(Q
2/Λ2)/ln(Q
0
2
/Λ2)]. Numerical comparison is made with the predictions from ‘double asymptotic scaling’ and fit. Better agreement is found
between our solution and fit nearρ=1. The solution gives approximate double scaling in this region having ‘hard’ pomeron with small contamination. 相似文献