Brane-Antibrane Kinetic Mixing, Millicharged Particles and SUSY Breaking

TL;DR

This work demonstrates that brane-antibrane configurations generically generate kinetic mixing between visible and hidden U(1) factors via a non-planar annulus diagram. The authors compute the mixing amplitude in compact extra dimensions, separating a tadpole piece (volume-divergent) from a finite threshold, and derive an explicit dependence of the mixing parameter on the interbrane separation and internal volumes. They show that kinetic mixing can induce millicharged particles and can dominantly mediate SUSY breaking to the visible sector, which imposes a stringent upper bound on the string scale, typically $M_s \lesssim 10^8$ GeV for many setups. These results strongly constrain intermediate-scale brane models and suggest that avoiding unbroken U(1)s on antibranes or ensuring vanishing trace factors is necessary for phenomenological viability.

Abstract

It is known that hidden U(1) gauge factors can couple to visible U(1)'s through Kinetic Mixing. This phenomenon is shown generically to occur in nonsupersymmetric string set-ups, between branes and antibranes. Kinetic Mixing acts either to give millicharges (of e.g. hypercharge) to would-be hidden sector fermions, or to generate an enhanced communication of supersymmetry breaking that dominates over the usual gravitational suppression. In either case, the conclusion is that the string scale in nonsupersymmetric brane configurations has a generic upper bound of M_s <~ 10^8 GeV.

Figures (5)

• Figure 1: Kinetic Mixing in field theory. $f$ is a fermion carrying charge under both $U(1)$'s.
• Figure 2: Kinetic Mixing in string theory: Annulus diagram with two open-string vertex operator insertions.
• Figure 3: Relative contribution of different winding modes to $Z_{p\overline{p}}$, for $p=3$ and $p=5$. Even if $Y = 2\pi R/2$, we see that the zero-winding contribution is strongly dominant.
• Figure 4: When to resum winding modes; a dimension that has $R_i \ll |Y|$ ceases to contribute to the exchange of closed string modes between the branes as lines of flux are confined. Winding modes in that dimension are not resummed.
• Figure 5: The annulus as obtained from the torus by setting $\tau_1=1$ and identifying under $\sigma^1=-\sigma^1$ (i.e. $z=-\bar{z}$). A fundamental region is shown shaded. The propagator is obtained by supplementing each transition (marked A,D) by an image charge piece (marked B,C).