616 research outputs found
Two-mediator dark matter models and cosmic electron excess
The cosmic electron energy spectrum recently observed by the DAMPE experiment
exhibits two interesting features, including a break around 0.9 TeV and a sharp
resonance near 1.4 TeV. In this analysis, we propose a dark matter explanation
to both exotic features seen by DAMPE. In our model, dark matter annihilates in
the galaxy via two different channels that lead to both a narrow resonance
spectrum near 1.4 TeV and electron excess events over an extended energy range
thus generating the break structure around TeV. The two annihilation channels
are mediated by two gauge bosons that interact both with dark matter and with
the standard model fermions. Dark matter annihilations through the s-channel
process mediated by the heavier boson produce monoenergetic electron-positron
pairs leading to the resonance excess. The lighter boson has a mass smaller
than the dark matter such that they can be on-shell produced in dark matter
annihilations in the galaxy; the lighter bosons in the final state subsequently
decay to generate the extended excess events due to the smeared electron energy
spectrum in this process. We further analyze constraints from various
experiments, including HESS, Fermi, AMS, and LHC, to the parameter space of the
model where both excess events can be accounted for. In order to interpret the
two new features in the DAMPE data, dark matter annihilation cross sections in
the current galaxy are typically much larger than the canonical thermal cross
section needed for the correct dark matter relic abundance. This discrepancy,
however, is remedied by the nonperturbative Sommerfeld enhancement because of
the existence of a lighter mediator in the model.Comment: 23 pages, 21 figure
Harish-Chandra modules over the \Q Heisenberg-Virasoro Algebra
In this paper, it is proved that all irreducible Harish-Chandra modules over
the \Q Heisenberg-Virasoro algebra are of intermediate series (all weight
spaces are 1-dimensional)
Wavefront Prediction Using Artificial Neural Networks for Adaptive Optics
Latency in the control loop of Adaptive Optics (AO) systems can severely limit its performance. Theories describing the temporal evolution of the atmospheric turbulence, such as the frozen flow hypothesis, justify the feasibility of predicting the turbulence (or equivalently its measurements) to compensate for the resultant temporal error in the system. This will mostly benefit AO assisted High Contrast Imaging (HCI) instruments for enhanced contrast, or wide-field AO systems for improved sky coverage.
In this thesis, we explore the potential of an Artificial Neural Network (ANN) as a nonlinear tool for open-loop wavefront prediction. The ANN predictor composes mainly Long Short-Term Memory (LSTM) cells, an ANN type specialised in sequence modelling and prediction. We demonstrate the efficiency and robustness of an ANN predictor both with simulated and on-sky 7 × 7 Shack-Hartmann Wavefront Sensor (SHWFS) CANARY data measured at 150 Hz, an AO demonstrator on the 4.2 m William Herschel Telescope (WHT), La Palma. We provide evidence that in addition to accurately predicting the wavefronts, an ANN predictor is also filtering high temporal frequencies such as Wavefront Sensor (WFS) noise. We show that an ANN predictor is adaptive to time-variant turbulence on sub-second level without user tuning. Specifically, we show that an ANN predictor is capable of predicting both frozen flow and non-frozen flow such as dome seeing, and that the ANN prediction can be based on a per-subaperture basis. As a pioneer, this thesis examines in great detail the characteristics of an ANN wavefront predictor and provides implications towards an on-sky implementation
Exploring open-charm decay mode of charmonium-like state
The newly observed exotic states are definitely not in the standard
structures, thus their existence composes a challenge to our
understanding on the fundamental principles of hadron physics. Therefore the
studies on their decay patterns which are determined by the non-perturbative
QCD will definitely shed light on the concerned physics. Generally the
four-quark states might be in a molecular state or tetraquark or their mixture.
In this work, we adopt the suggestion that is a charmonium-like
tetraquark made of a diquark and an anti-diquark. If it is true, its favorable
decay mode should be decaying into an open-charm baryon pair, since
such a transition occurs via strong interaction and is super-OZI-allowed. In
this work, we calculate the decay width of
in the framework of the quark pair creation (QPC) model. Our numerical results
on the partial width computed in the tetraquark configuration coincide with the
Belle data within a certain error tolerance.Comment: 8 pages, 4 figures, 1 table. Accepted by Eur. Phys. J.
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