The LHCb Level 1 trigger LHC Symposium, October 27, 2001 Frederic Teubert, CERN, EP Division, on behalf of the LHCb collaboration The LHCb VELO Detector The L1 Algorithm and its Performance New Developments Outlook 27-10-2001 Frederic Teubert

VELO Overview Vacuum Vessel Precise vertexing requires, to be: as close as possible to the decay vertices with a minimum amount of material between the first measured point and the vertex  silicon sensors are placed in vacuum ~1 m Number of silicon sensors: 100 Area of silicon: 0.32 m2 Number of channels: 204,800 27-10-2001 Frederic Teubert

VELO Setup One detector half: p p forward Positioning and number of stations is defined by the LHCb forward angular coverage of 15mrad <  < 390mrad (4.9>>1.6) together with the inner/outer sensor dimensions Also need to account for spread of interaction region: s = 5.3 cm partial backward coverage for improved primary vertex measurement One detector half: Interaction region p p Final configuration: 25 stations 1 station = 2 modules (left and right) 1 module = 2 sensors (R and ) forward 27-10-2001 Frederic Teubert

Sensor Design Symmetry of the event production suggests sensors which measure  and R coordinates. Advantages of R geometry: - Resolution: smallest strip pitch where it is needed, optimal cost/resolution. - Radiation: short strips, low noise, (40mm x 6283mm) close to beam. - L1:  segmentation of R-sensors (45o) is enough for primary vertex reconstruction. Seeding region Inner radius is defined by the closest possible approach of any material to the beam: 8 mm (sensitive area)  LHC machine Outer radius is constrained by the practical wafer size: 42 mm 27-10-2001 Frederic Teubert

LHCb Trigger System p 40 MHz  1 MHz Fixed latency: 4.0 ms Data is kept in analog/digital pipelines L0: look for high pt particles hadron (HCAL), muon (MUON) or electron (ECAL) pile up VETO 40 MHz  1 MHz Fixed latency: 4.0 ms p L0 YES: Data is digitized, but stays in memory of individual detector electronics L1: vertex trigger 1 MHz  40 kHz L1 buffer = 1927 events L1 max. latency: 1.7 ms L1 YES: Digitized data is read-out, event building  CPU farm L2: refined vertex trigger 40 kHz  5 kHz 5 kHz  O(200 Hz) L3: online event selection 27-10-2001 Frederic Teubert

L1 (Vertex) Trigger Input rate: 1 MHz Output rate: 40 kHz Maximum latency: 1.7 ms Purpose: Select events with detached secondary vertices Needs: Standalone tracking and vertex reconstruction Occupancy < 1% per channel ~ 1.5 tracks in the seeding region (innermost 450 sectors of R-sensors)  track reconstruction is straightforward 27-10-2001 Frederic Teubert

Algorithm 2d track reconstruction using only R-hits Efficiency: 98% z triplets 2d track reconstruction using only R-hits Primary vertex: Histogram of two 2d-tracks crossings  z -segmentation  x,y sz ~ 70 mm sx,y ~ 20 mm 27-10-2001 Frederic Teubert

Algorithm 2d tracks Impact Parameter Efficiency: 95% 3d tracks Minimum bias Efficiency: 95% Tracks from PV 3d tracks (only 2d tracks with large IP) Search for two 3d tracks close in space (rough secondary vertex) Minimum bias, fraction of tracks with IP/ > 2 Calculate probability to be a non b-event based on impact parameter and geometry 27-10-2001 Frederic Teubert

Physics Performance (test beam) Reconstruction of Primary Vertex using 2d tracks (test beam data) sz ~ 79 mm 27-10-2001 Frederic Teubert

Execution Time of Algorithm 800 MHz Pentium III Expect CPU’s to be 10 times faster in 2005 27-10-2001 Frederic Teubert

Implementation Basic Idea: 150-250 processors SWITCH CPU farm x20 SWITCH x100 1 MHz CPU farm Readout Unit Digitizer Board: zero-suppression, common mode correction, cluster finding Challenge: 4 Gb/s and small event fragments of ~170bytes 2d torus topology based on SCI shared memory 27-10-2001 Frederic Teubert

Physics Performance (MC studies) Main limitations: No momentum information No information on Pt Significance of impact parameter No particle ID Working point 40kHz 27-10-2001 Frederic Teubert

Can we do better? The L1 topological trigger is mainly limited by the lack of Pt information, so: Link L0 objects: large Pt (e,,h) with VELO tracks. Essentially comes “for free” Get a rough estimation of the momentum for large IP tracks. Needs a bit of work 27-10-2001 Frederic Teubert

Level 1 with L0 information MUON ECAL HCAL B dl = 4 Tm, pt kick = 1.2 GeV/c matching efficiency B+- : 78% for at least one  BJ/()Ks: 96% for at least one  27-10-2001 Frederic Teubert

Level 1 with L0 information BJ/()Ks is saturated using L0(). B+- is improved by a factor >2 due to the Pt information. 27-10-2001 Frederic Teubert

Level 1 with Pt Maximum Pt Adding Pt information, Events with at least 2 tracks with IP/ > 3 after L0   B0s  Ds-(K+K--) K+   B0d  +- Minimum bias B0d  +- Maximum Pt Adding Pt information, allows to work with low L1 output rates (5-40) KHz and high signal efficiencies. 27-10-2001 Frederic Teubert

Momentum resolution using VELO tracks + TT1: Level 1 with Pt Silicon tracking station (TT1), with ~10% of By VELO Momentum resolution using VELO tracks + TT1: s(1/p) ~ 0.2/p  0.01/GeV Increase of L1 event size by ~30% TT1 27-10-2001 Frederic Teubert

Outlook The feasibility of fast standalone tracking and primary vertex reconstruction has been checked both with MC and test-beam data. The TP L1 algorithm is limited by the lack of Pt information and lepton ID. The use of L0 information (High Pt: e,,h) improves the performance. If a rough estimation of the Pt of the high d0 tracks is available at L1, one can work with much lower output rates (~10 KHz) while keeping similar signal efficiencies  Work in progress. Final answer: Trigger TDR, end 2002 27-10-2001 Frederic Teubert