Life at the interface of particle physics and string theory

If the results of the first LHC run are not betraying us, many decades of particle physics are culminating in a complete and consistent theory for all nongravitational physics: the standard model. But despite this monumental achievement there is a clear sense of disappointment: many questions remain unanswered. Remarkably, most unanswered questions could just be environmental, and disturbingly to some the existence of life may depend on that environment. Meanwhile there has been increasing evidence that the seemingly ideal candidate for answering these questions, string theory, gives an answer few people initially expected: a large “landscape” of possibilities that can be realized in a multiverse and populated by eternal inflation. At the interface of “bottom-up” and “top-down” physics, a discussion of anthropic arguments becomes unavoidable. Developments in this area are reviewed, focusing especially on the last decade.

Figure 1

Habitable regions in QFT space. The gray circle represents the experimental bounds on the standard model. The dots show the distribution of QFT points in a hypothetical landscape.

Figure 2

Brane configurations: (a) the Madrid model, (b) SU(5) GUTs, and (c) trinification.

Figure 3

Distribution of standard model couplings in a class of intersecting brane models.

Figure 4

Distribution of CKM angles at small and large angles for a scale-invariant distribution. The black line is for ${\theta }_{12}$ and ${\theta }_{23}$, the gray line is for ${\theta }_{13}$.

Figure 5

Distribution of up-type ($u,c,t$) and down-type ($d,s,b$) masses. On the horizontal axis powers of ten are indicated.