New effects observed in the BaBar silicon vertex tracker: interpretation and estimate of their impact on the future performance of the detector

The silicon vertex tracker (SVT) of the BaBar experiment at PEP II is briefly described. It consists of five layers of double-sided AC-coupled silicon strip detectors, constituting the core of the BaBar tracking system. After six years of operation, some unexpected effects have appeared. In particular, a shift in the pedestal for the channels of the AToM readout chips that are most exposed to radiation has been observed. The behavior has been understood and reproduced in AToM chip irradiations with 1-GeV electrons at Elettra (Trieste) and the results of the studies are presented here. A second unexpected behavior has been detected, consisting of an anomalous increase in the bias leakage current for the modules in the outer layers. The effect is beam-related but not directly linked to radiation damage, as suggested by the fact that it is not present in the inner layers. The cause has been understood and the conclusions are presented here. The effect has been reproduced in a qualitative way in the laboratory. Over the next three years the luminosity is predicted to increase by a factor of three, leading to radiation and occupancy levels significantly exceeding the detector design. Estimates of future radiation doses and occupancies are shown together with the extrapolated detector performance and lifetime, in light of the new observations. Upgrade scenarios to deal with the increased luminosity and backgrounds are discussed.