398 research outputs found
Effective quantum kinetic theory for spin transport of fermions with collsional effects
We systematically derive the collision term for the axial kinetic theory, a
quantum kinetic theory delineating the coupled dynamics of the vector/axial
charges and spin transport carried by the massive spin-1/2 fermions traversing
a medium. We employ the Wigner-function approach and propose a consistent
power-counting scheme where the axial-charge distribution function, a
non-conserved quantity for massive particles, is accounted as the first-order
quantity in the expansion, while the vector-charge distribution
function the zeroth-order quantity. This specific power-counting scheme allows
us to organize a reduced expansion for the collision term and to
formally identity the spin-diffusion effect and the spin-polarization effect at
the same order. We confirm that the obtained collisional axial kinetic theory
smoothly reduces to the chiral kinetic theory in the massless limit, serving as
a consistency check. In the absence of electromagnetic fields, we further
present the simplified axial kinetic equations suitable for tracking dynamical
spin polarization of heavy and light fermions, respectively. As an application
to the weakly coupled quark-gluon plasma at high temperature, we compute the
spin-diffusion term for massive quarks within the leading-log approximation.
The formal expression for the first-order terms provides a path toward
evaluation of the spin polarization effect in quantum chromodynamics.Comment: 50 pages, no figures, typos in Eqs. (34), (37), (42) correcte
On the Role of Charmed Meson Loops in Charmonium Decays
We investigate the effect of intermediate charmed meson loops on the M1
radiative decays and
as well as the isospin violating
hadronic decays using heavy hadron
chiral perturbation theory (HHPT). The calculations include tree level as
well as one loop diagrams and are compared to the latest data from CLEO and
BES-III. Our fit constrains the couplings of 1S and 2S charmonium multiplets to
charmed mesons, denoted and , respectively. We find that
there are two sets of solutions for and . One set, which
agrees with previous values of the product extracted from
analyses that consider only loop contributions to , can only fit data on radiative decays with fine-tuned
cancellations between tree level diagrams and loops in that process. The other
solution for and leads to couplings that are smaller by a
factor of 2.3. In this case tree level and loop contributions are of comparable
size and the numerical values of the tree level contributions to radiative
decays are consistent with estimates based on the quark model as well as
non-relativistic QCD (NRQCD). This result shows that tree level HHPT
couplings are as important as the one loop graphs with charmed mesons in these
charmonium decays. The couplings and are also important for
the calculations of the decays of charmed meson bound states, such as the
X(3872), to conventional charmonia.Comment: 16 pages, 3 figures, minor modifications, more references adde
Jet Quenching and Holographic Thermalization with a Chemical Potential
We investigate jet quenching of virtual gluons and thermalization of a
strongly-coupled plasma with a non-zero chemical potential via the
gauge/gravity duality. By tracking a charged shell falling in an asymptotic
AdS background for and , which is characterized by the
AdS-Reissner-Nordstr\"om-Vaidya (AdS-RN-Vaidya) geometry, we extract a
thermalization time of the medium with a non-zero chemical potential. In
addition, we study the falling string as the holographic dual of a virtual
gluon in the AdS-RN-Vaidya spacetime. The stopping distance of the massless
particle representing the tip of the falling string in such a spacetime could
reveal the jet quenching of an energetic light probe traversing the medium in
the presence of a chemical potential. We find that the stopping distance
decreases when the chemical potential is increased in both AdS-RN and
AdS-RN-Vaidya spacetimes, which correspond to the thermalized and thermalizing
media respectively. Moreover, we find that the soft gluon with an energy
comparable to the thermalization temperature and chemical potential in the
medium travels further in the non-equilibrium plasma. The thermalization time
obtained here by tracking a falling charged shell does not exhibit,
generically, the same qualitative features as the one obtained studying
non-local observables. This indicates that --holographically-- the definition
of thermalization time is observer dependent and there is no unambiguos
definition.Comment: 25 pages, 15 figures, minor modification, references adde
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