The rate of collisions at the Tevatron or the LHC is many orders of magnitude too large for a record of every collision to be stored. The detectors are so enormous, with so many data channels, that to store the record of a single collision requires a stunning amount of memory. Moreover, recording the events takes time. Roughly 100,000,000 events per second occur, but only about 100 of these can be recorded.

So how do experimentalists decide how to select 10² out of 10⁸ events each second? The detector must contain an elaborate “trigger” as part of its hardware and software which does a partial analysis of each collision to decide whether it is sufficiently interesting to be worth recording.

For instance, if an event has a muon in it, it has a good chance of being interesting. If there is a large amount of missing transverse momentum, it has a good chance of being interesting. If there are several jets with a TeV of transverse momentum, it’s interesting, and so on. So the trigger consists of a set of conditions: if an event appears to satisfy one or more of these conditions, the detector software will trigger a full readout of the detector data. Otherwise, the event is dumped into the void, and lost forever.

Triggering is all about compromise. We can’t record all events with candidate electrons in them without having to throw away some events that have large missing transverse momentum — there are just too many. So we require any interesting electrons to have some minimum amount of transverse momentum... unless, say there are two leptons in the event, which is more rare, in which case we can lower that minimum, or, say, the event has both an electron and a substantial amount of missing tranverse momentum, which is also rare, in which case again the minimum could be lowered. QCD produces huge numbers of events with high-p_T jets, so we can’t record all events with high-p_T jets without having no storage space for events with muons. So we might demand that an event, in order to be stored, have at least three jets that satisfy a condition: one jet must have at least 650 GeV of p_T, the next must have at least 300 GeV, then next at least 150 GeV, etc.

Now here’s the problem: this means that the detector records events that pass a rather complicated set of conditions. Even ignoring the fact that the detector trigger decisions are imperfect, this makes for a very complex situation. For instance, we cannot easily ask how many events in a new signal have a muon compared to how many events have no leptons. Or rather, we can ask it, but it doesn’t tell us anything, because the trigger conditions for events with muons and for events with no leptons are completely different, and the effect of this difference is very hard to estimate unless, in addition to understanding your detector very well, you have a precise and detailed model of the new physics.... which was precisely what we were trying to construct in the first place! So the problem is circular, and very difficult to solve.

Triggering involves a decision that must be made very rapidly. Real detectors have to therefore make these decisions based on partial and incomplete and often erroneous information. This means that interesting events are sometimes missed by the trigger, and conversely, events which seem interesting a first glance may turn out to be less so after being more carefully analyzed. For instance, an initial look at the calorimeter may reveal a narrow isolated cluster of energy in the hadronic calorimeter that hints at being a tau. To check if it is likely to be a tau, the triggering system looks to see if there are a small number of tracks in the vicinity (one or three would be expected.) But because of the time available, the detector will reconstruct tracks quickly, using only two projected dimensions (radius and azimuthal angle phi) of the three-dimensional tracking information. This allows two types of errors, with either sign: (a) the detector may fail to reconstruct a track which is actually present, for example because of tracks which are superposed and crossing when projected onto radius and phi, or (b) the detector will see a track that points at the tau-candidate cluster, but later, with more time for track reconstruction, this turns out to be a coincidence: although the track has the same phi as the cluster, its eta (psuedorapidity) is completely different. Because of these errors, or more precisely, inefficiencies and fakes, an event with a tau may be thrown away, or an event without a tau may be kept, on the basis of the triggering process.

Thus it is essential to distinguish between trigger objects (the imperfectly reconstructed electrons, muons, jets, etc. on the basis of which a trigger decision is made) and reconstructed objects (which are the objects that, having been carefully reconstructed, are used for data analysis.) However, the objects in the triggers at the LHC experiments (in contrast to those at the Tevatron) are sufficiently close to reconstructed objects that the decision has been made, following consultations with experimentalists, to run the trigger off the reconstructed objects in PGS4. The advantage of a simpler data structure for the data challenges appears to outweigh the small price in realism.

The trigger table used for this rounds of LHCO black boxes is listed below. For each line in the table, there are two sets of thresholds.

• L1 : The lower values are close to the actual values used by the CMS experiment, and can be accessed by running the blackbox software with the L1 flag. These low settings are used for the signal-only black boxes.
• L2 : The second set are the high thresholds used for generating the black boxes with SM background, and can be acessed by running the blackbox software with the L2 flag.

The L1 and L2 flags are set by editing the last line of ./olympics.sh. The energy thresholds below refer to the pT of the object. In the case of the dilepton (ditau, diphoton, or dijet) triggers, the trigger requires that both leptons (taus, photons, or jets) are above threshold.

    Trigger Name                                    |  L1               |  L2
----------------------------------------------------+-------------------+---------------- 
1)  Inclusive isolated lepton (muon/electron)       |  30 GeV           |  180 GeV
2)  Lepton (muon/electron) plus jet                 | (20 GeV, 100 GeV) | (130 GeV, 200 GeV)  (lepton, jet)
3)  Isolated dileptons (mumu/ee)                    |  15 GeV           |  60 GeV
4)  Dileptons (mumu/ee) plus jet                    | (10 GeV, 100 GeV) | (45 GeV, 150 GeV)   (ll, jet)
5)  Isolated dileptons (emu)                        |  10 GeV           |  30 GeV
----------------------------------------------------+-------------------+----------------
6)  Isolated lepton (mu/ e) plus isolated tau       | (15 GeV, 45 GeV)  | (45 GeV, 60 GeV)    (lepton, tau)
7)  Isolated ditau                                  |  60 GeV           |  60 GeV
8)  Inclusive isolated photon                       |  80 GeV           |  80 GeV
9)  Isolated diphoton                               |  25 GeV           |  40 GeV
----------------------------------------------------+-------------------+----------------
10) Inclusive MET                                   |  90 GeV           |  200 GeV
11) Inclusive single-jet                            |  400 GeV          | 1000 GeV
12) Jet plus MET                                    | (180 GeV, 80 GeV) | (300 GeV, 125 GeV)  (jet, MET)
13) Acoplanar jet and MET    (1 < Dphi < 2)         | (100 GeV, 80 GeV) | (150 GeV, 80 GeV)   (jet, MET)
14) Acoplanar dijets         (Dphi < 2)             |  200 GeV          |  400 GeV

With the L2 settings, a table with approximate values for accepted cross sections (in pb) of the dominant SM processes is given below.

| trigger: ||  1  |  2  |  3  |  4  |  5  ||  6  |  7  |  8   |  9  || 10  | 11  | 12  | 13  | 14  || total  |
--------------------------------------------------------------------------------------------------------------
| dijets:  ||  1  |  4  |  0  |  0  |  0  ||  0  |  0  |  6   |  0  ||  4  | 60  | 18  |  7  | 23  ||  118   |
| singlew: || 10  | 17  |  0  |  0  |  1  ||  0  |  0  |  3   |  0  || 22  |  1  | 15  |  5  |  0  ||   54   |
| singlez: ||  4  |  7  |  8  | 10  |  0  ||  0  |  0  |  1   |  0  ||  1  |  0  |  0  |  0  |  0  ||   18   |
| drellyan:||  2  |  3  | 11  |  7  |  0  ||  0  |  0  |  2   |  0  ||  0  |  2  |  1  |  0  |  0  ||   22   |          
| ttbar:   ||  2  |  5  |  0  |  1  |  6  ||  0  |  0  |  1   |  0  ||  6  |  2  |  9  |  7  |  0  ||   30   | 
--------------------------------------------------------------------------------------------------------------        
| total:   || 20  | 36  | 19  | 18  |  8  ||  1  |  0  | 13   |  0  || 33  | 65  | 45  | 19  | 25  ||  236   |

An event may pass more than one trigger at once. Its “trigger word” is the decimal value of the corresponding binary string. For example, the event below

   #   typ     eta    phi       pt  jmass  ntrk  btag   had/em  dummy dummy  
   0           103   2563                                                    this is event number 103, and its trigger word value is 2563 
   1    2   -1.219  4.739   449.95   0.11   1.0   0.0     0.00   0.0   0.0   a muon with a pT of 450 GeV 
   2    4   -1.729  1.557   687.76 592.46  37.0   0.0     4.41   0.0   0.0   a jet with a pT of 688 GeV, invariant mass of 592 GeV, and 37 charged tracks 
   3    4   -0.829  2.540    67.26  20.33   5.0   0.0     3.55   0.0   0.0   a jet with a pT of 67 GeV, invariant mass of 20 GeV, and 5 charged tracks 
   4    6    0.000  4.857   275.16   0.00   0.0   0.0     0.00   0.0   0.0   the "missing transverse energy" in the event is 275 GeV 

has trigword value 2563 = 2^11 + 2^9 + 2^1 + 2^0 = 00101000000011 in binary, and thus has passed the 1st, 2nd, 10th and 12th trigger. So the term 2^n refers to the n+1-st trigger.