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Hidden Valley Higgs Decays in the ATLAS detector The ATLAS Collaboration

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The Hidden Valley. Hidden Valley: parameters of the model. Is the ATLAS detector able to cope with “unexpected” long-lived neutral particles?. Model parameter h v production and decay: M( h v ) =140 GeV M( p v ) =40 GeV c t =1500mm.

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The Hidden Valley

Hidden Valley: parameters of the model

Is the ATLAS detector able to cope with “unexpected” long-lived neutral particles?

Model parameter

hv production and decay:

M(hv)=140 GeV

M( pv )=40 GeV


“Hidden Valley” models predict a new dynamic accessible (may be) at LHC energies

Hidden Valley and SM communicate through a mediator communicator(Higgs, Z’,LSP)

All v-particles are neutral under the SM

The lightest v-particles (pv) are stable in the v-sector and decay (weakly) only in the SM

pv decay in heavy quarks (heavy leptons) pairs (bb,tt)

Hidden Valley models are a general class of models that predict neutral, long-lived particles

Hidden Valley Monte Carlo simulation based on Pythia

Simulated process:

Higgs → pvpv

Decay length choosen to provide pvdecays throughout

the ATLAS detector

pv decay probability

M. Strassler and K. Zureck ,

Phys.Lett.B 661:263(2008)

Phys.Lett.B 651:374(2007)

pv proper c (mm)

“Pythia” event display (no detector simulation)

“Atlantis” ATLAS event display

pv decaying in the calorimeter

  • Small energy deposit in Electromagnetic calorimeters
  • Large deposit in Hadronic calorimeters
  • No tracks associated in ID

pv decaying in the Muon Spectrometer

  • Large multiplicity
  • Tracks not pointing to nominal interaction vertex
  • No tracks in ID
  • No energy in calorimeters

HV Calorimetric trigger

HV muon trigger

Ratio of energy deposit in calorimeters

Log(EHAD/EEM) vs pv decay distance

Di-jet sample

70 < ET< 140 Gev

140 < ET< 280 Gev

Mean number of Level-1 Muon Candidates vspv decay length

Large Level-1 multiplicity

<NCandidates> ~ 3

pv decaying in Hadronic


Number of LVL-1 Muon Candidates in cluster with D R=0.4 around pv line of flight

pv decaying in Inner Detector

or in the EM calorimeter

appears as “usual” jets

Hidden Valley Higgs Decays in the ATLAS detectorThe ATLAS Collaboration

L.Bodine, G.Ciapetti, A.Coccaro, D.Depedis, C.Dionisi, S.Giagu, H.Lubatti, F.Parodi, M.Rescigno, G.Salamanna, C.Schiavi, A.Sidoti, M.Strassler, D.Ventura, L.Zanello


Displaced high multiplicity neutral vertices  Unique topological signature

No SM process can mimick those signatures, HV processes almost background free

Displaced high multiplicity neutral vertices  Non-pointing tracks and no charged track in the Inner Detector

Low efficiencies for “conventional” trigger selections (jet trigger, muon triggers, tracking algorithms in Inner Detector) and reconstruction algorithms


ATLAS “Hidden Valleys” TriggersImplemented Level-2 triggers specific for Hidden Valley decays selection

Jet Log(EHAD/EEM):

  • Jet ET>35 GeV in | h |<2.5
  • log(EHAD/EEM)>1
  • no reconstructed tracks pT>1GeV/c

Muon Cluster:

  • at least 3 Level-1 muon candidates pT>6GeV/c
  • contained in a cone of D R=0.4
  • isolated (D R=0.7) from jets

Trackless Jet with muon:

  • Jet ET>35 GeV in | h |<2.5
  • 1 Level-1 muon candidate with pT>6GeV/c
  • No tracks with pT>1GeV/c contained within the jet cone (D R=0.4)

Expected Level1&Level2 background trigger rate from HV triggers:

~3 Hz at L=1033 cm-2s-1

evaluated with minimum bias and dijet samples



* Overlap removed