Celia Fernández Madrazo
Instituto de Física de Cantabria (UC-CSIC)
CMS searches for long-lived particles in Run 2 and Run 3
November 2022
Introduction: Long-lived particles
C. F. Madrazo | CPAN 2022
New physics could be hiding in BSM models which predict the existence of particles with high lifetimes: Long-lived particles (LLP).
LLPs may decay away from the interaction point and produce atypical topologies that offer appealing and challenging searches where we have to opportunity to improve:
- The estimation of non-standard backgrounds - Loss of efficiency in reconstruction
- Reduced trigger performance
They arise in a wide variety of BSM theories and covering different regions of the phase space due to
- Small couplings
- Very compressed spectra - Heavy off-shell mediators
P(decay)
Travelled distance
The long-lived signature and CMS analyses
EXO-21-006
EXO-20-014
EXO-18-003
EXO-19-010
EXO-20-003
EXO-19-021 EXO-20-015
EXO-19-013
(This talk only covers the most recent ones,
which are some of
(from J. Antonelli)
Analyses with displaced leptons
LLPs decaying into leptons manifest as leptons displaced from the interaction point that could be produced in the same vertex or not.
Main discriminant: Lepton impact parameter d0 or the vertex distance Lxy (if there is a vertex)
Major backgrounds: Prompt misreconstruction, QCD, low-mass resonances, cosmic muons, leptonic tau
decays, B hadrons… That are often estimated from data Triggers: Generally not optimized. Analyses rely on muon chambers for muons and on ECAL for electrons.
→ Limiting factor for the sensitivity
C. F. Madrazo | CPAN 2022
(from EXO-21-006)
→ Run 2 analyses were optimized to provide sensitivity on different lepton flavors, displacement or mass range.
Displaced vertices with leptons (IFCA analysis)
Run 2 IFCA analysis is targetting LLPs decaying in vertices with electrons and muons displaced within the tracker volume.
- Target: Middle 2023
BSM Higgs RPV SUSY
→ Two channels: dielectron and dimuon
→ Use of non-standard reconstruction algorithms to recover efficiency.
→ LLP candidates are identified with dilepton vertices fitted from lepton tracks
→ Dedicated calibration of the objects by using cosmic muons.
→ Background estimated from data in control regions.
Work in progress
Signal region Control region
Recent CMS analyses with displaced leptons (Run 2)
C. F. Madrazo | CPAN 2022
EXO-18-003
EXO-21-006
EXO-20-014
Search for displaced muons and electrons, without a common vertex requirement.
→ Signal region defined with both lepton d0. → Background estimated with ABCD method.
Search for OS charged displaced muon vertices.
→ Splitted in 3 analyses depending on where the muons were reconstructed.
→ Dedicated data-driven background estimation
Search for OS charged displaced muon vertices with scouting data.
→ Loose requirements (pT and mass) but limited information in the events → Signal + background fits in mass windows of the spectra
Recent CMS analyses with displaced leptons (Run 2)
EXO-18-003 sets limits on RPV SUSY with LL
stops and LL sleptons production EXO-18-003 and EXO-21-006 constrains BSM Higgs scenarios with LL scalars:
C. F. Madrazo | CPAN 2022
Limits set on a Hidden Abelian Higgs model (HAHM) with dark photons.
EXO-21-006 and EXO-20-014 complement
each other at high lifetimes and low masses.
(Bands are masked SM low-mass resonances)
Recent CMS analyses with displaced leptons (Run 2)
Hadronically decaying LLPs
LLPs decaying hadronically appear as jets whose origin is displaced from the interaction point.
They can be studied in a wide variety of topologies:
- Displaced jets that can be tagged with track, vertex or angular properties - Displaced tracks clustered into displaced vertices
- MET if LLPs decay after inner subdetectors
- Unusual energy clusters e.g. in muon chambers
Discriminants depend on the topology:
- Jet tracks d0 distribution
- Vertex position (if reconstructed) - MET
Major background: QCD and multijet events
Triggers:
- Dedicated displaced jet triggers - MET triggers
- If there are other particles e.g. leptons, trigger on them
(from EXO-20-003)
Run 2 analyses deployed a wide variety of strategies to target hadronically decaying LLPS:
C. F. Madrazo | CPAN 2022
Topology: Displaced secondary vertex (SV) associated with a dijet system.
Gradient Boosted Decision Tree (GBDT) with SV information to remove QCD background.
EXO-19-021 EXO-20-003
Search for H decays into hadronically decaying LLPs in associated Z boson production.
Prompt leptons are used to trigger the events
Displaced jets are tagged with track level information variables.
→ Number of displaced jets used as discriminant.
Recent CMS analyses with hadronic LLPs (Run 2)
Run 2 analyses deployed a wide variety of strategies to target hadronically decaying LLPS:
EXO-19-013 EXO-20-015
Topology: Two vertices reconstructed from the intersection of charged particles trajectories.
Distance dvv between vertices is the main
discriminant.
Background estimated from data using a dvv template
Search for LLPs decaying in muon system endcaps.
→ Objects are clusters of hits in CSC chambers.
→ Use MET and Nhits as main discriminants.
(also MET triggers)
→ Background estimated from data with ABCD method.
Cluster in EMS (LLP decay)
MET
Recent CMS analyses with hadronic LLPs (Run 2)
Recent CMS analyses with hadronic LLPs (Run 2)
Hadronic LLP analyses set limits in H SS models →
EXO-19-021 (left) and EXO-20-015 (right) set limits on S dd (bb):→
EXO-20-015 studies S→ττ decays
EXO-20-003 set limits on B(H SS) in ZH →
production
C. F. Madrazo | CPAN 2022
Recent CMS analyses with hadronic LLPs (Run 2)
Hadronic LLP analyses set limits in SUSY models with LL particles:
EXO-19-021
sets upper limits on GMSB models with LL gluino
EXO-19-013 and EXO-19-021 provide sensitivity in different lifetime ranges in RPV SUSY models:
Direct detection: Disappearing tracks
Charged LLPs that decay within the silicon tracker into undetected decay products may appear as tracks that “disappear” → Disappearing tracks
C. F. Madrazo | CPAN 2022
EXO-19-010
→ Signature:
- Isolated tracks with missed hits in outer tracker layers, calorimeters and muon system.
- IRS recoil jet
IRS recoil jet
(undetected)
→ Backgrounds:
- Spurious reconstructed tracks
- Leptons
→ Interpreted in the context of Anomaly- mediated-supersymmetry-breaking (AMSB) models:
Wrapping up
Run 2 analyses have provided sensitivity in many models and different regions of the
phase space:
Long-lived analyses facing Run 3
Long-lived analysis exploited all the tools available in Run 2 but there is room for improvement towards Run 3 and a huge effor ongoing:
→ Standard triggers were a critical limiting factor in Run 2 as they were optimized for prompt signals (vertex contraints, worse resolusion… etc). Both L1 and HLT performance were
improved
→ Lessons learned from Run 2 analyses allowed to improve offline displaced particle
reconstruction with the objective of increasing the efficiency, resolution and introducing the posibility to deploy new analysis techniques.
→ There are some interesting regions that are of common interest for the analyses:
- Low mass and lower pT - Higher displacement
- Improve some channels e.g. displaced electrons or displaced leptonic taus.
IFCA and Universidad de Oviedo are participating in a search for displaced muons that will make use of new triggers and new object reconstructions.
C. F. Madrazo | CPAN 2022
