[Table of Contents] [Previous Section] [Next Section]
[The Trigger Framework] [Level 1,2 and 3 Trigger Systems]
The present D0 triggering system includes two hardware triggers Level 0 (L0) and Level 1 (L1), and a Level 3 (L3) software trigger (we have replaced the existing D0 nomenclature of L0, L1, L1.5, and L2.) Interactions with coincident hits in the small angle counters on both sides of the interaction region give a L0 trigger. After a L0 accept, the L1 trigger requires either a minimum $E_{T}$ deposition in the calorimeter or primitive tracks of minimum $p_T$ in the muon chambers. Once an event passes the L1 trigger, the entire detector is read out and the event assembled in a farm of VAX computers. This farm, the L3 trigger, performs a nearly complete reconstruction of the event. If the event includes objects of sufficient interest, it is written to tape. Between L1 and L3, a third hardware trigger (L2) refines the calorimeter-based trigger for electron candidates by examining the shape of the energy deposition, and refines the muon trigger by using finer granularity hardware information. L2 presently interrogates only a subset of the L1 accepts and inhibits data taking while examining the event. Typically, this limits the current L1/L2 accept rate to 150 Hz. The maximum L3 accept rate is 4 Hz.
A typical Run I trigger menu includes high $p_T$ jet, electron, muon and large missing $E_T$ triggers. The L1/L2 and L3 cross-sections, at a luminosity of $2 \times 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$, are 10 and 0.05 $\mu$barns, respectively. These correspond to trigger rates of 2000 Hz for L1/L2 and 10 Hz for L3 at $2 \times 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$, which are beyond our present capabilities. In order to deal with these high rates, the D0 triggering system requires significant enhancement. The upgrade must include a new trigger framework to deal with the increased rates and several new triggering elements, including the fiber track trigger (CFT), the central preshower trigger (CPS), the forward preshower trigger (FPS), and muon trigger detectors, to provide the necessary rejection of background.
The present L0 provides a trigger and luminosity measurement. The magnetic field requires that the L0 system be replaced because of its conventional phototube readout. The luminosity monitor functions will be replaced by the new L0 system.
There are two distinguishing characteristics of the new framework. First, all events will be examined by L2 hardware engines -- not just a subset of events. Second, there will be event buffers between L1 and L2 and between L2 and L3. The addition of eight buffers between each trigger stage de-randomizes the Poisson distributed arrival times of the events, decreasing deadtime due to pileup. The buffers also eliminate the present L1 disable during L2 operation. These two improvements alone increase the L1 accept rate to 5-10 kHz and the L2 accept rate to 800 Hz. Since the Run II event sizes will be half that of Run I, an 800 Hz event transfer rate to L3 and a 10 Hz rate to tape are feasible. In summary, the expected Run II trigger accept rate limits are 10 kHz, 800 Hz, and 10 Hz at L1, L2, and L3, respectively.
The L1 high-transverse-momentum electron trigger will be upgraded from the simple threshold in $E_T$ to include a CFT track and CPS deposition for $|\eta| < $ 1.2 . The CFT momentum threshold gives good rejection against minimum bias (QCD) background. Requiring an energy deposition in the CPS which matches spatially with the track further improves rejection against isolated charged pions. Since the calorimeter L1 trigger does not contain spatial information, it cannot be used to further improve the background rejection. The forward electron trigger will use the calorimeter, as before, and the new forward preshower. The L1 high $p_T$ trigger rate will be $\sim$ 1000 Hz at $2\cdot 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$.
Both the forward and central L2 electron triggers will retain the present electron isolation and shape requirements. In addition, a rejection factor of approximately two should be possible by requiring a coincidence among the calorimeter, CFT, and CPS in the central region and between the calorimeter and the FPS in the forward region. With these new elements, the L2 electron trigger rate at $2\cdot 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$ will be $\sim$ 200 Hz for $E_T>15$ GeV. A L3 electron rejection factor of 100 can be achieved by importing current off-line shape cuts into the software farm; the high-momentum electron rate at $2\cdot 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$ will be $\sim$ 2 Hz for $E_T>20$ GeV.
The L1 high $p_T$ central muon triggers ($|\eta | <$ 1.6) will also incorporate the CFT. Coincidences between the CFT and the inner-layer muon scintillators, and/or the muon chambers themselves, will provide substantial background rejection. The forward (2 $< |\eta | <$ 3) muon triggers will rely on the A, B, and C pixel layers and SAMUS coincidences. Additional shielding, multiplicity cuts, multiple interaction cuts and pulse height discrimination will reduce the rates further. At the lowest $p_T$ (~1.5 GeV), a pair of CFT $\bullet$ A-layer $\phi$ coincidences will serve as a di-muon or J/$\psi$ trigger. A Level 2 di-muon mass trigger could provide rejection factors of five for a J/$\psi$ trigger. The high $p_T$ L1 muon rate at $2\cdot 10^{32} {\rm cm}^{-2} {\rm s}^{-1}$ will be $\sim$ 200 Hz. The goal is to trigger on one muon with $p_T^\mu> 8$ GeV unprescaled, and on two muons for $p_T^\mu > 2-3$ GeV. Measurements and extrapolations based on data and Monte Carlo calculations indicate that this is possible.
Changes in the trigger framework and the addition of triggering elements will meet the high rate demands of Run II. Table 2 is a summary of the L1, L2 and L3 rates for various generic triggers at $2\cdot 10^{32} cm^{-2} s^{-1}$. For completeness, high $p_T$ jet, photon, and missing $E_T$ triggers have been included. Note that there is sufficient bandwidth for more specific low rate top, di-lepton and tri-lepton search triggers.