This material (for experts) is from an appendix to a document laying out the physics case for improved triggering on Higgs decays to two unusual jets — i.e., to two exotic jet-like objects.
Various Unusual Jets and Corresponding Trigger Objects
It is useful to consider what objects might arise in Higgs two-object decays. Each of these objects appears in existing theoretical models, which we will not review here.
First, consider the case of collimated particles. Obviously if the set of
collimated particles looks too similar to an ordinary QCD jet, triggering beyond the baseline V h events is impossible, even if an analysis were possible. At Level 1 neither ATLAS nor CMS is often likely to distinguish a QCD jet from other clusters of particles, except in the cases where
• multiple muons appear in the cluster or
• the cluster contains only electrons and/or photons, in which case it may be one hard or multiple soft electromagnetic objects
• the cluster is especially narrow, in which case it may be a tau candidate
The first case requires a special trigger pathway, which we assume (from lepton-jet studies if nothing else) already exists, with a very low threshold. The second case require additional disucssion if the cluster is sufficiently wide so as not to appear as a single electromagnetic object; it may appear both as a jet and as multiple soft electromagnetic objects.
Beyond Level 1, various objects might be identifiable at Level 2
• If the Level 1 object is a cluster of electromagnetic objects, it may later
be characterized as
– multiple photon objects, or
– a mostly-electromagnetic jet (i.e., a “low HCAL/ECAL jet.”)
• If the Level 1 object is a tau candidate, but is not a tau, then it may fail
tight tau requirements at higher trigger levels, but pass a looser one.
• If the Level 1 object is a jet, it may contain obvious and highly-delineated substructure — for instance, two tau-like objects, either of which may be a tau, or neither. (For example, consider a jet containing two charged pion pairs, each pair highly collimated, but the two pairs separated by R = 0.4.)
Second, consider the case of long-lived particles that typically produce a
jet-like object at Level 1. Beyond Level 1 we may consider the following cases as the decay location moves outward (defining d to be the distance from the decay location to the center of the beampipe)
• If d is no more than 10-20 cm, some of the reconstructed tracks may have non-zero impact parameter, and the object may be identifiable as a b jet at trigger level.
• If d is greater than 20-40 cm, the jet will have no reconstructed stiff tracks at trigger level; we will refer to such objects as “trackless jets”.
• If d is much larger than 50 cm, then the fact that the long-lived particle
decays close to or within the calorimeter will make it appear as a
narrow jet, even if its decay products are not intrinsically collimated by
• If d is large enough that the decay occurs within the hadronic calorimeter, the decay will result in a (probably narrow) jet whose energy is mostly hadronic (a “high HCAL/ECAL jet.”)
The objects which are produced in these two classes of decays are all relatively familiar. They include loose b tagged jets, loose taus, and loose photons. Trackless jets (typically with a muon required) and high HCAL/ECAL jets are already objects available for triggering at ATLAS; the situation at CMS has not been made public, but there should be no obstruction (other than time) to developing these objects if they are not yet available.
Possible Triggering Strategy
Any possible triggering strategies available are heavily dependent on the options available at Level 1, which we cannot evaluate reliably. Let us examine a possible strategy available at the higher level, imagining it is seeded either by a wide-open QCD three- or four-jet trigger, or even by an HT-based trigger; either way this is a high-bandwidth trigger so substantial reduction is required. The higher-level strategy for VBF production could potentially involve something like the following:
• Select four-jet events that have two VBF jet candidates (high jet-pair
invariant mass, at least one at high ||, possibly requiring one central
(with tracks) to reduce various backgrounds.
• Of the two remaining jets, require both of them be unusual:
– containing displaced tracks, or
– narrow, or
– trackless, or
– with exceptionally large or small HCAL/ECAL
Ideally one would not need to require that both jets be unusual in the same way, to allow for a wider variety of final states, but at the very least one should include cases where the strange property of one jet is shared by a second one. A supplementary approach which might work if the jets are sufficiently unusual might be to combine two unusual jets and two standard jets that are consistent with a hadronically-decaying W or Z.
For gg production without extra jets, one could proceed only if the unusual jets are recorded at Level 1 as taus or as electromagnetic objects, or contain muons. One would like to check that the resulting tau candidates and/or photon (or multi-photon) candidates are not commonly discarded at higher levels of the trigger as a result of failing the usual tight trigger requirements for taus and for photons. Only knowing the resulting thresholds could one estimate whether gg production has comparable sensitivity to the VBF production mode.