Catching long-lived particles from Hidden Valley models:

new challenges for the ATLAS trigger

9 February 2009

A number of extensions of the Standard Model (SM) result in particles that are neutral, weakly-coupled and have macroscopic decay lengths that can be comparable with LHC detector dimensions. These long lived particles occur in many models: gauge-mediated SUSY extensions of the MSSM (addition of one singlet field), MSSM with R-parity violation, split SUSY, inelastic dark matter and Hidden Valley Scenarios. A common feature of many of these beyond the Standard Model extensions is that they result in non standard Higgs decays to final states that have displaced vertices. Here we consider a Hidden Valley Scenario and explore the challenges that long lived particles present to the ATLAS trigger.

What is a Hidden Valley? To the Standard Model is appended a hidden sector ("v-sector"), and a communicator (or communicators) which interacts with both sectors. A barrier (perhaps the communicator's high mass, weak couplings, or small mixing angles) weakens the interactions between the two sectors, making production even of light v-sector particles (''v-particles'') rare at low energy. At the LHC, by contrast, production of v-particles, through various possible channels, may be observable. The communicator can be any neutral particle or combination of particles, including the Higgs boson, the Z boson, Z' boson, neutralinos, neutrinos, or loops of particles charged under both Standard Model and v-sector gauge groups. The Hidden Valley Scenario includes a broad range of models in which neutral long lived particles may appear, with a wide range of final states including multi-lepton and multi-jet states.

The performance of ATLAS for triggering on and detecting  a low mass Higgs (140 GeV) that decays to long-lived Hidden Valley particles (πv,''v-pions'') has been studied. The πv is neutral and has a displaced decay mainly to bottom quarks. These displaced decays lead to distinctive event signatures as can be seen in the event displays below. The first event (left picture) shows a πv decaying in the Muon Spectrometer resulting in a large number of charged hadrons traversing the Spectrometer. A second event (right picture) shows a πv decaying in the Hadronic Calorimeter, producing a jet with no energy deposited in the EM Calorimeter and no associated tracks in the Inner Detector.

Events showing on the left side a πv decaying in the Muon Spectrometer and on the right side, decaying in the Hadronic Calorimeter.

Due to the low Higgs mass, the jets produced from the πv decay will have relatively low energy and because of the πv's displaced decay vertex, any track segments produced will not point back to the primary vertex. Given these particular event signatures, the standard ATLAS triggers only select a small fraction of these unique events. However, we are currently developing trigger algorithms that could use the displaced vertex signatures as trigger objects in order to increase the fraction of events accepted. We have developed signature-driven triggers that can be used to preferentially select events containing Hidden Valley long-lived particles when these particles either decay in the Muon Spectrometer from the end of the Hadronic Calorimeter to the first muon trigger plane; or decay in the Calorimeters from the end of the EM Calorimeter to the end of the Hadronic Calorimeter.

Long-lived particles predicted by a number of Standard Model extensions are challenging to the ATLAS Detector, in particular for the online trigger selection. However, by implementing new signature-based triggers, it will be possible to increase the selection efficiency, thus allowing for a potential early discovery of this new physics.

For more information about Hidden Valleys see a few articles by M. J. Strassler and K. M. Zurek:
Echoes of a Hidden Valley at Hadron Colliders
Discovering the Higgs through highly-displaced vertices
Possible Effects of a Hidden Valley on Supersymmetric Phenomenology

or by N. Arkani-Hamed and N. Weiner,
LHC Signals for a SuperUnified Theory of Dark Matter




Daniel Ventura

University of Washington