24,862 research outputs found
Leptonic dark matter annihilation in the evolving universe: constraints and implications
The cosmic electron and positron excesses have been explained as possible
dark matter (DM) annihilation products. In this work we investigate the
possible effects of such a DM annihilation scenario during the evolution
history of the Universe. We first calculate the extragalactic -ray
background (EGRB), which is produced through the final state radiation of DM
annihilation to charged leptons and the inverse Compton scattering between
electrons/positrons and the cosmic microwave background. The DM halo profile
and the minimal halo mass, which are not yet well determined from the current
N-body simulations, are constrained by the EGRB data from EGRET and Fermi
telescopes. Then we discuss the impact of such leptonic DM models on cosmic
evolution, such as the reionization and heating of intergalactic medium,
neutral Hydrogen 21 cm signal and suppression of structure formation. We show
that the impact on the Hydrogen 21 cm signal might show interesting signatures
of DM annihilation, but the influence on star formation is not remarkable.
Future observations of the 21 cm signals could be used to place new constraints
on the properties of DM.Comment: 24 pages, 6 figures and 2 tables. Improved treatment of the energy
deposition process, the suppression on structure formation is weaker.
Accepted for publication by JCA
Matrix Product Representation of Locality Preserving Unitaries
The matrix product representation provides a useful formalism to study not
only entangled states, but also entangled operators in one dimension. In this
paper, we focus on unitary transformations and show that matrix product
operators that are unitary provides a necessary and sufficient representation
of 1D unitaries that preserve locality. That is, we show that matrix product
operators that are unitary are guaranteed to preserve locality by mapping local
operators to local operators while at the same time all locality preserving
unitaries can be represented in a matrix product way. Moreover, we show that
the matrix product representation gives a straight-forward way to extract the
GNVW index defined in Ref.\cite{Gross2012} for classifying 1D locality
preserving unitaries. The key to our discussion is a set of `fixed point'
conditions which characterize the form of the matrix product unitary operators
after blocking sites. Finally, we show that if the unitary condition is relaxed
and only required for certain system sizes, the matrix product operator
formalism allows more possibilities than locality preserving unitaries. In
particular, we give an example of a simple matrix product operator which is
unitary only for odd system sizes, does not preserve locality and carries a
`fractional' index as compared to their locality preserving counterparts.Comment: 14 page
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