## Abstract

The optimal fluctuation method-essentially geometrical optics-gives a deep insight into large deviations of Brownian motion. Here we illustrate this point by telling three short stories about Brownian motions, 'pushed' into a large-deviation regime by constraints. In story 1 we compute the short-time large deviation function (LDF) of the winding angle of a Brownian particle wandering around a reflecting disk in the plane. Story 2 addresses a stretched Brownian motion above absorbing obstacles in the plane. We compute the short-time LDF of the position of the surviving Brownian particle at an intermediate point. Story 3 deals with survival of a Brownian particle in 1 + 1 dimension against absorption by a wall which advances according to a power law xw (t) ∼ t^{γ}, where γ > 1/2. We also calculate the LDF of the particle position at an earlier time, conditional on the survival by a later time. In all three stories we uncover singularities of the LDFs which have a simple geometric origin and can be interpreted as dynamical phase transitions. We also use the small-deviation limit of the geometrical optics to reconstruct the distribution of typical fluctuations. We argue that, in stories 2 and 3, this is the Ferrari-Spohn distribution.

Original language | American English |
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Article number | 415001 |

Journal | Journal of Physics A: Mathematical and Theoretical |

Volume | 52 |

Issue number | 41 |

DOIs | |

State | Published - 18 Sep 2019 |

### Bibliographical note

Publisher Copyright:© 2019 IOP Publishing Ltd.

## Keywords

- Brownian motion
- dynamical phase transitions
- large deviations
- optimal fluctuation method