Title: BTeV Trigger and Data Acquisition System
1 BTeV Trigger andData Acquisition System
Presentation to the DOE/Fermi Group, January 11,
2001
2Overview
- The challenge for the BTeV trigger and data
acquisition system is to analyzeparticle tracks
and interaction vertices for every interaction
that occurs in the BTeV detector (15 million per
second), and to select interactions with Bs. - The trigger performs this task using 3 stages,
referred to as Levels 1, 2 and 3L1 looks at
every interaction, and rejects 99 of
uninteresting dataL2 uses L1 results and
performs a more refined analysis for data
selectionL3 performs a complete analysis
that uses all of the data for an interaction - Reject 99.8 of background. Keep gt 50 of
signal. - The data acquisition system saves all of the data
in memory for as long as isnecessary to analyze
each interaction ( 1 millisecond on average),
and movesdata to processing units and data
storage for selected interactions. - The key ingredients that make it possible to meet
this challenge - BTeV pixel detector with its exceptional pattern
recognition capabilities - rapid development in technology FPGAs, DSPs,
microprocessors - good ideas
FPGAs field programmable gate arrays DSPs
digital signal processors
Erik Gottschalk f
3Trigger/DAQ
Erik Gottschalk f
4Vertex Trigger
- The vertex trigger is the primary physics trigger
for BTeV. The trigger looks forparticles that do
not come from the main interaction, but appear to
come from aB-decay vertex close to (of order 1
10 mm) the main interaction vertex. - The first stage of the trigger, L1, reconstructs
tracks and vertices for every beamcrossing (7.6
million beam crossings per second), and every
interaction (averageof 2 interactions per beam
crossing). L1 handles 15 million
interactions/second! - With new data arriving at a rate of 7.6 MHz, the
L1 trigger must produce atrigger decision
(accept or reject) every 132 ns. - To accomplish this the L1 trigger uses
parallelism pipelining - data for each beam crossing is subdivided and
processed in parallel - processor farms use parallelism to handle
multiple crossings simultaneously - data can remain in the processing pipeline for
as long as a millisecond, but trigger decisions
occur at most every 132 ns, on average
Erik Gottschalk f
5Simulated Pixel Data (compressed along beam
direction by 10)
- pixel data for a beam crossing (an interaction
with B-decay vertices) input to L1 - data are subdivided and processed in parallel
Erik Gottschalk f
6Track segments found by the L1 vertex trigger
A key feature of the L1 vertex trigger is that
partial track reconstruction issufficient at
this stage of the trigger. Therefore, we find
tracks as the enterthe pixel detector (close to
vertex), and as they exit (momentum measurement).
entering track segment exiting track segment
Erik Gottschalk f
7L2 and L3 Vertex Trigger
- Unlike the L1 trigger, which is built with field
programmable gate arrays (FPGAs)and digital
signal processors (DSPs), the L2 and L3 triggers
are implemented as afarm of commercial
processors (e.g. INTEL processors running LINUX). - L2 trigger (data for one beam crossing are sent
to a single processor) - starts with tracks and vertices found by the L1
trigger - requests data from other detectors (such as the
forward tracking system) - improves the track and vertex reconstruction
- imposes more selection criteria to reject 80
90 of crossings that pass L1 - L3 trigger (L3 begins when all data for a beam
crossing are sent to a processor) - further refinement of selection criteria
- reject 50 75 of crossings that pass L2
- reduce the amount of data that will be recorded
- Output 4000 crossings per second, with an
average size of 50 Kbytes 200 Mb/sec
Erik Gottschalk f
8Final Remarks
- The BTeV trigger and data acquisition system is
an ambitious project. - We benefit from new technologies that help us
meet the challenge - pixel detectors
- rapid development of FPGAs, DSPs,
microprocessors - BTeV trigger rejects 99.8 of background, keeps gt
50 of signal.
Erik Gottschalk f