25,701 research outputs found
Adaptation to criticality through organizational invariance in embodied agents
Many biological and cognitive systems do not operate deep within one or other
regime of activity. Instead, they are poised at critical points located at
phase transitions in their parameter space. The pervasiveness of criticality
suggests that there may be general principles inducing this behaviour, yet
there is no well-founded theory for understanding how criticality is generated
at a wide span of levels and contexts. In order to explore how criticality
might emerge from general adaptive mechanisms, we propose a simple learning
rule that maintains an internal organizational structure from a specific family
of systems at criticality. We implement the mechanism in artificial embodied
agents controlled by a neural network maintaining a correlation structure
randomly sampled from an Ising model at critical temperature. Agents are
evaluated in two classical reinforcement learning scenarios: the Mountain Car
and the Acrobot double pendulum. In both cases the neural controller appears to
reach a point of criticality, which coincides with a transition point between
two regimes of the agent's behaviour. These results suggest that adaptation to
criticality could be used as a general adaptive mechanism in some
circumstances, providing an alternative explanation for the pervasive presence
of criticality in biological and cognitive systems.Comment: arXiv admin note: substantial text overlap with arXiv:1704.0525
QCDQED evolution of TMDs
We consider for the first time the QED corrections to the evolution of
(un)polarized quark and gluon transverse-momentum-dependent distribution and
fragmentation functions (TMDs in general). By extending their operator
definition to QCDQED, we provide the mixed new anomalous dimensions up
to and the pure QED ones up to .
These new corrections are universal for all TMDs up to the flavor of the
considered parton, i.e., the full flavor universality of TMD evolution found in
pure QCD is broken in QCDQED by the presence of the electric charge. In
addition, we provide the leading-order QED corrections to the matching
coefficients of the unpolarized quark TMD parton distribution function onto its
integrated counterparts at .Comment: 9 pages, 3 figures. v2: minor corrections, accepted for publication
in Phys. Lett.
Soft and Collinear Factorization and Transverse Momentum Dependent Parton Distribution Functions
In this work we consider how a parton distribution function, with an explicit
transverse momentum dependence can be properly defined in a
regularization-scheme independent manner. We argue that by considering a
factorized form of the transverse momentum dependent spectrum for the
production of a heavy lepton pair in Drell-Yan reaction, one should first split
the relevant soft function into two boost invariant contributions. When those
soft contributions are added to the pure collinear contributions, well-defined
hadronic matrix elements emerge, i.e., the transverse momentum dependent
distributions. We also perform a comparison with Collins' definition.Comment: 9 pages, 3 figures. We modified the discussion about the equivalence
between Collins (JCC) and Echevarria-Idilbi-Scimemi (EIS) definitions of
TMDPDF. The results remain intact. Version to appear in Phys. Lett.
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