Introduction

This chapter presents a brief overview of the original physics goal of HERA-$B$ - measuring CP violation in the decays of neutral $B$ mesons. Finally the modified physics program is outlined.

The violation of CP symmetry was first observed in the system of neutral K mesons ( $K^0 - \bar K^0$). The $K_{L}^{0}$ has a CP eigenstate of -1 but it was shown that it can also decay into two charged pions with a CP eigenstate of +1. The Standard Model offers a mechanism which can explain this violation. The system of neutral $B$ mesons should also show CP violation mediated by the same mechanism and in addition the CP violation in the B-system should be large relative to the Kaon system.

HERA-B was approved in 1994 [#!proposal!#] but the start of the HERA-B experiment was delayed by several years due to problems during detector development and commissioning. The $e^+e^-$ B factory approach was chosen by two experiments: BaBar [#!BaBar!#] collaboration at SLAC and Belle [#!Belle!#] collaboration at KEK, they were able to present their first measurements of CP violation already in 2000. Since that moment the measurements have reached an accuracy level that can not be reached by HERA-B. HERA-B was commissioned in 1999 with a short data taking run. In 2000/01, during the HERA luminosity upgrade, HERA-B detector was partially upgraded and finalized. Since it became obvious that HERA-B is no longer competitive in B-physics a new physical program was proposed for the data taking in 2002/2003.