Search in leptonic channels for heavy resonances decaying to long-lived neutral particles

Abstract

A search is performed for heavy resonances decaying to two long-lived massive neutral particles, each decaying to leptons. The experimental signature is a distinctive topology consisting of a pair of oppositely charged leptons originating at a separated secondary vertex. Events were collected by the CMS detector at the LHC during pp collisions at √ s = 7 TeV, and selected from data samples corresponding to 4.1 (5.1) fb −1 of integrated luminosity in the electron (muon) channel. No significant excess is observed above standard model expectations, and an upper limit is set with 95% confidence level on the production cross section times the branching fraction to leptons, as a function of the long-lived massive neutral particle lifetime. . This Letter presents the first search using data from the Compact Muon Solenoid (CMS) for massive, long-lived exotic particles X that decay to a pair of oppositely charged leptons. We search for events containing a pair of oppositely charged electrons or muons (dileptons) originating from a common secondary vertex within the volume of the CMS tracker, that is significantly transversely displaced from the event primary vertex. These leptons are assumed to originate from a 2-body decay of a long-lived particle, and so are required to form a narrow resonance in the dilepton mass spectrum. This topological -1 - Keywords: Hadron-Hadron Scattering JHEP02(2013)085signature has the potential to provide clear evidence for physics beyond the standard model (SM). It is also very powerful in suppressing backgrounds from standard model processes.This signature is sensitive to a wide class of models. However, for the purpose of establishing a signal benchmark, a specific model of a long-lived, spinless, exotic particle X which has a non-zero branching fraction to dileptons is used. In this particular model, the X is pair-produced in the decay of a Higgs boson, i.e. H 0 → 2X, X → + − [5], where the Higgs boson is produced through gluon-gluon fusion. This model predicts up to two displaced dilepton vertices in the tracking volume per event.The D0 Collaboration has performed searches for leptons from delayed decays in its tracker volume [6, 7], but these searches are sensitive to a much smaller kinematic phase space region than CMS. The ATLAS Collaboration has performed searches that are sensitive to decay lengths up to about 20 m by exploiting the ATLAS muon spectrometer [8,9], using different decay channels from those considered in this Letter. The CMS detectorThe central feature of the CMS apparatus [10] is a superconducting solenoid of 6 m internal diameter providing an axial field of 3.8 T. Within the field volume are the silicon pixel and strip tracker, the lead-tungstate crystal electromagnetic calorimeter (ECAL), and the brass/scintillator hadron calorimeter. Muons are identified in gas-ionisation detectors embedded in the steel magnetic-flux return yoke of the solenoid.The silicon tracker is composed of pixel detectors (three barrel layers and two forward disks on either end of ...

Displaced vertices and long-lived charged particles in the NMSSM with right-handed sneutrinos

“…The resolution of the vertices in the pixel tracker for both ATLAS and CMS detectors is of the order of hundred µm, however as the displaced vertices must be distinguished from primary ones, usually |d 0 | 2 − 4 mm and L xy 4 − 8 mm is imposed [68][69][70][71]. These cuts also remove completely the SM background, as it was shown in refs.…”

“…These cuts also remove completely the SM background, as it was shown in refs. [69][70][71]73]. In fact, in ref.…”

“…[71] it was shown through MC simulation studies that the selection on |d 0 | > 2mm can remove 98% of all tracks from the primary pp vertices. Although in their searches for displaced vertices ATLAS and CMS use the whole detector volume, the identification of a displaced vertex decreases when d 0 grows [68,69,71]. In our analysis we will therefore restrict our searches to the inner detector 4 and for each simulated event we impose impose a cut on the decay length of the RH neutrino, 10 mm< cτ N <100 cm.…”

“…As it is shown in refs. [69][70][71]73] the main SM background is due to γ * /Z * → ℓ + ℓ − , Z * Z * . The cut imposed in the decay length is very effective and it can be seen that when it is combined with the condition that the invariant mass of two leptons are greater than 5 GeV, the SM background can be totally removed.…”

“…[68][69][70][71] are the present constraints from the LHC on displaced vertices. Some of these searches share the same signatures with this model.…”