Signals of a Light Dark Force in the Galactic Center

Recent evidence for an excess of gamma rays in the GeV energy range about the Galactic Center have refocused attention on models of dark matter in the low mass regime ($m_\chi \lesssim m_Z/2$). Because this is an experimentally well-trod energy range, it can be a challenge to develop simple models that explain this excess, consistent with other experimental constraints. We reconsider models where the dark matter couples to dark photon, which has a weak kinetic mixing to the Standard Model photon, or scalars with a weak mixing with the Higgs boson. We focus on the light ($\lesssim 1.5 GeV$) dark mediator mass regime. Annihilations into the dark mediators can produce observable gamma rays through decays to $\pi^0$, through radiative processes when decaying to charged particles ($e^+e^-, \mu^+\mu^-,...$), and subsequent interactions of high energy $e^+e^-$ with gas and light. However, these models have no signals of $\bar p$ production, which is kinematically forbidden. We find that in these models, the shape of resulting gamma-ray spectrum can provide a good fit to the excess at Galactic Center. We discuss further constraints from AMS-02, and find regions of compatibility.Comment: 39 pages, 14 figures, references updated and discussion of CMB constraints include

The new interaction suggested by the anomalous 8^8Be transition sets a rigorous constraint on the mass range of dark matter

The WIMPs are considered one of the favorable dark matter (DM) candidates, but as the upper bounds on the interactions between DM and standard model (SM) particles obtained by the upgraded facilities of DM direct detections get lower and lower. Researchers turn their attentions to search for less massive DM candidates, i.e. light dark matter of MeV scale. The recently measured anomalous transition in $^8$Be suggests that there exists a vectorial boson which may mediate the interaction between DM and SM particles. Based on this scenario, we combine the relevant cosmological data to constrain the mass range of DM, and have found that there exists a model parameter space where the requirements are satisfied, a range of $10.4 \lesssim m_{\phi} \lesssim$ 16.7 MeV for scalar DM, and $13.6 \lesssim m_{V} \lesssim$ 16.7 MeV for vectorial DM is demanded. Then a possibility of directly detecting such light DM particles via the DM-electron scattering is briefly studied in this framework.Comment: 13 Pages, 7 figures, references added, version accepted by journa

Note on Thermodynamics Method of Black Hole/CFT Correspondence

In the paper we further refine the thermodynamics method of black hole/CFT correspondence. We show that one can derive the central charges of different holographic pictures directly from the entropy product $S_+S_-$ if it is mass-independent, for a black hole in the Einstein gravity or the gravity without diffeomorphism anomaly. For a general black hole in the Einstein gravity that admits holographic descriptions, we show that the thermodynamics method and asymptotic symmetry group (ASG) analysis can always give consistent results in the extreme limit. Furthermore, we discuss the relation between black hole thermodynamics and the hidden conformal symmetry. We show that the condition $T_+A_+=T_-A_-$, with $A_\pm$ being the outer and inner horizon areas, is the necessary, but not sufficient, condition for a black hole to have the hidden conformal symmetry. In particular, for the Einstein(-Maxwell) gravity $T_+A_+=T_-A_-$ is just the condition $T_+S_+=T_-S_-$, with $S_\pm$ being the outer and inner horizon entropies, which is the condition for the entropy product $S_+S_-$ being mass-dependent. When there exists the hidden conformal symmetry in the low-frequency scattering off the generic non-extremal black hole, it always leads to the same temperatures of dual CFT as the ones got from the thermodynamics method.Comment: 31 pages, references added, published versio

On the role of information disclosures in capital markets

This dissertation uses economic modelling to explore the value of information disclosures to the firm and investors in capital markets. Specifically, two chapters investigate the interaction between manager’s decisions and the value of information to investors, while another two chapters examine how disclosures affect the risk sharing among heterogeneous investors. The first chapter studies how manager’s information acquisition strategy affects the information content of management forecast and shows that the manager’s private forward looking information would lead to informative management forecasts and forecast errors only when there is a positive relation between firm performance and manager’s forecasting ability. The second chapter investigates the impact of corporate social responsibility (CSR) reporting on the firm's CSR investment decision and demonstrates that disclosing either the firm’s contribution to the externalities from CSR investment or the cost of CSR investment can discipline the manager’s CSR investment policy, but disclosing two pieces of information together cannot further improve the investment efficiency. The third chapter analyses how disclosure format affects the risk sharing among informed and uninformed investors and proves that separating multiple pieces of information and disclosing each piece of information at a different time can achieve efficient risk allocation among investors and thus lower the firm’s cost of capital. The fourth chapter examines the heterogeneous investment horizons of investors and suggests that long-horizon investors perform short-term trading to share the first period stock price risk with short-horizon investors. This lowers the risk premium in the stock price. Moreover, such short-term trading increases with the firm’s guidance on the short-term cash flow while decreases with the guidance on the long-term cash flow