46,919 research outputs found
Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))
The authors claim to have found a "proper", "gauge-invariant" definition of a
charged-particle's momentum in gauge theory, which is more "superior" than the
textbook version. I show that their result arises from a misunderstanding of
gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is
not physical
Effect of nickel on the microstructure and mechanical property of die-cast Al–Mg–Si–Mn alloy
The effect of nickel on the microstructure and mechanical properties of a die-cast Al–Mg–Si–Mn alloy has been investigated. The results show that the presence of Ni in the alloy promotes the formation of Ni-rich intermetallics. These occur consistently during solidification in the die-cast Al–Mg–Si–Mn alloy across different levels of Ni content. The Ni-rich intermetallics exhibit dendritic morphology during the primary solidification and lamellar morphology during the eutectic solidification stage. Ni was found to be always associated with iron forming AlFeMnSiNi intermetallics, and no Al3Ni intermetallic was observed when Ni concentrations were up to 2.06 wt% in the alloy. Although with different morphologies, the Ni-rich intermetallics were identified as the same AlFeMnSiNi phase bearing a typical composition of Al[100–140](Fe,Mn)[2–7]SiNi[4–9]. With increasing Ni content, the spacing of the α-Al–Mg2Si eutectic phase was enlarged in the Al–Mg–Si–Mn alloy. The addition of Ni to the alloy resulted in a slight increase in the yield strength, but a significant decrease in the elongation. The ultimate tensile strength (UTS) increased slightly from 300 to 320 MPa when a small amount (e.g. 0.16 wt%) of Ni was added to the alloy, but further increase of the Ni content resulted in a decrease of the UTS.The Engineering and Physical Sciences Research Council (EPSRC), Technology Strategy Board (TSB) and Jaguar Land Rover (JLR) in the United Kingdom
The Rotation Average in Lightcone Time-Ordered Perturbation Theory
We present a rotation average of the two-body scattering amplitude in the
lightcone time()-ordered perturbation theory. Using a rotation average
procedure, we show that the contribution of individual time-ordered diagram can
be quantified in a Lorentz invariant way. The number of time-ordered diagrams
can also be reduced by half if the masses of two bodies are same. In the
numerical example of theory, we find that the higher Fock-state
contribution is quite small in the lightcone quantization.Comment: 25 pages, REVTeX, epsf.sty, 69 eps file
A topological realization of the congruence subgroup Kernel A
A number of years ago, Kumar Murty pointed out to me that the computation of
the fundamental group of a Hilbert modular surface ([7],IV,6), and the
computation of the congruence subgroup kernel of SL(2) ([6]) were surprisingly
similar. We puzzled over this, in particular over the role of elementary
matrices in both computations. We formulated a very general result on the
fundamental group of a Satake compactification of a locally symmetric space.
This lead to our joint paper [1] with Lizhen Ji and Les Saper on these
fundamental groups. Although the results in it were intriguingly similar to the
corresponding calculations of the congruence subgroup kernel of the underlying
algebraic group in [5], we were not able to demonstrate a direct connection
(cf. [1], 7). The purpose of this note is to explain such a connection. A
covering space is constructed from inverse limits of reductive Borel-Serre
compactifications. The congruence subgroup kernel then appears as the group of
deck transformations of this covering. The key to this is the computation of
the fundamental group in [1]
Implications of Color Gauge Symmetry For Nucleon Spin Structure
We study the chromodynamical gauge symmetry in relation to the internal spin
structure of the nucleon. We show that 1) even in the helicity eigenstates the
gauge-dependent spin and orbital angular momentum operators do not have
gauge-independent matrix element; 2) the evolution equations for the gluon spin
take very different forms in the Feynman and axial gauges, but yield the same
leading behavior in the asymptotic limit; 3) the complete evolution of the
gauge-dependent orbital angular momenta appears intractable in the light-cone
gauge. We define a new gluon orbital angular momentum distribution
which {\it is} an experimental observable and has a simple scale evolution.
However, its physical interpretation makes sense only in the light-cone gauge
just like the gluon helicity distribution y.Comment: Minor corrections are made in the tex
Electromagnetic fields in a 3D cavity and in a waveguide with oscillating walls
We consider classical and quantum electromagnetic fields in a
three-dimensional (3D) cavity and in a waveguide with oscillating boundaries of
the frequency . The photons created by the parametric resonance are
distributed in the wave number space around along the axis of the
oscillation. When classical waves propagate along the waveguide in the one
direction, we observe the amplification of the original waves and another wave
generation in the opposite direction by the oscillation of side walls. This can
be understood as the classical counterpart of the photon production. In the
case of two opposite walls oscillating with the same frequency but with a phase
difference, the interferences are shown to occur due to the phase difference in
the photon numbers and in the intensity of the generated waves.Comment: 8 pages revTeX including 1 eps fi
Nucleon-Quarkonium Elastic Scattering and the Gluon Contribution to Nucleon Spin
It is shown that the amplitude for the scattering of a heavy quarkonium
system from a nucleon near threshold is completely determined by the fraction
of angular momentum, as well as linear momentum, carried by gluons in the
nucleon. A form for the quarkonium-nucleon non-relativistic potential is
derived.Comment: 4 pages, no figures. Author's e-mail: [email protected]
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