The Readout of the sPHENIX Tracking System

Martin L. Purschke, for the sPHENIX Collaboration

The sPHENIX Collaboration at RHIC is upgrading the PHENIX detector in
a way that will enable a comprehensive measurement of jets in
relativistic heavy ion collisions. The upgrade will give the
experiment full azimuthal coverage within a pseudorapidity range of
$-1.1 < \eta < 1.1$. In addition to measuring heavy-ion collisions,
the new apparatus will provide enhanced physics capabilities for
studying nucleon-nucleus and polarized proton collisions, and
eventually allow a detailed study of electron-nucleus collisions at an
envisioned Electron Ion Collider at Brookhaven.

We have presented the status of the sPHENIX project at the last
conference. Since then, we have selected the technologies for the
tracking system, which will consist of a silicon detector based on
MAPS (Monolithic Active Pixel Sensors), followed by an Intermediate
Tracker (INTT), and then by a TPC. With the exception of the INTT,
which is based on the former PHENIX Forward Vertex Detector and also
re-uses the readout technology, both the MAPS detector and the TPC
introduce new readout hardware and strategies.

In order to achieve high event rates, the TPC needs to be read out in
continuous, or streaming, mode, without the use of a gating grid. The
resulting continuously sampled waveforms of the TPC sensors must then
be processed and correlated with the actually triggered events of the
full detector.

Since the TPC is using the ALICE Sampa ASIC, one of the possible
readout cards is the ALICE "Common Readout Unit" (CRU). We are also
looking into a card in use in the ATLAS experiment, called
"FELIX". Both cards are PCIe-based and have a large number (up to 48)
of fiber inputs to connect to the front-end cards, and a powerful FPGA
for waveform processing.

The MAPS detector will use the eventually chosen technology for its
readout as well.

We will present the envisioned design of the streaming readout, and
explain the challenges with the high data rates generated by this
readout method, which could reach as much as 80 GBit/s. We will show
the design of our data acquisition to cope with those data rates, and
present the status of the ongoing R&D. By the time of the conference,
we will likely have a number of prototype setups, and some actual
performance data.