Dark matter contribution to b→sμ+μ−b\to s \mu^+ \mu^- anomaly in local U(1)Lμ−LτU(1)_{L_\mu-L_\tau} model

We propose a local $U(1)_{L_\mu-L_\tau}$ model to explain $b \to s \mu^+ \mu^-$ anomaly observed at the LHCb and Belle experiments. The model also has a natural dark matter candidate $N$. We introduce $SU(2)_L$-doublet colored scalar $\widetilde{q}$ to mediate $b \to s$ transition at one-loop level. The $U(1)_{L_\mu-L_\tau}$ gauge symmetry is broken spontaneously by the scalar $S$. All the new particles are charged under $U(1)_{L_\mu-L_\tau}$. We can obtain $C_9^{\mu,{\rm NP}} \sim -1$ to solve the $b \to s\mu^+\mu^-$ anomaly and can explain the correct dark matter relic density of the universe, $\Omega_{\rm DM} h^2 \approx 0.12$, simultaneously, while evading constraints from electroweak precision tests, neutrino trident experiments and other quark flavor-changing loop processes such as $b \to s \gamma$ and $B_s-\overline{B}_s$ mixing. Our model can be tested by searching for $Z'$ and new colored scalar at the LHC and $B \to K^* \nu \overline{\nu}$ process at Belle-II.Comment: 15 pages, 6 figure

Which part of a chain breaks

This work investigates the dynamics of a one-dimensional homogeneous harmonic chain on a horizontal table. One end is anchored to a wall, the other (free) end is pulled by external force. A Green's function is derived to calculate the response to a generic pulling force. As an example, I assume that the magnitude of the pulling force increases with time at a uniform rate $\beta$. If the number of beads and springs used to model the chain is large, the extension of each spring takes a simple closed form, which is a piecewise-linear function of time. Under an additional assumption that a spring breaks when its extension exceeds a certain threshold, results show that for large $\beta$ the spring breaks near the pulling end, whereas the breaking point can be located close to the wall by choosing small $\beta$. More precisely, the breaking point moves back and forth along the chain as $\beta$ decreases, which has been called "anomalous" breaking in the context of the pull-or-jerk experiment. Although the experiment has been explained in terms of inertia, its meaning can be fully captured by discussing the competition between intrinsic and extrinsic time scales of forced oscillation.Comment: 18 pages, 11 figure

Fermions in an anisotropic random magnetic field

We study the localization of fermions in an anisotropic random magnetic field in two dimensions. It is assumed that the randomness in a particular direction is stronger than those in the other directions. We consider a network model of zero field contours, where there are two types of randomness - the random tunneling matrix element at the saddle points and unidirectional random variation of the number of fermionic states following zero field contours. After averaging over the random complex tunneling amplitude, the problem is mapped to an SU(2N) random exchange quantum spin chain in the $N \to 0$ limit. We suggest that the fermionic state becomes critical in an anisotropic fashion.Comment: 5 pages, replaced by revised version, accepted for publication in Europhysics Letter