Search for supersymmetry with displaced dileptons at the ATLAS - - PowerPoint PPT Presentation

Search for supersymmetry with displaced dileptons at the ATLAS experiment

Dominik Krauss

Max-Planck-Institut f¨ ur Physik

March 30, 2017

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 1 / 11

Search for displaced dileptons

Search for massive long-lived particles decaying to two charged leptons (e or µ) Experimental signature: Displaced vertices with two lepton tracks Sensitive to lifetimes of about 1 ps to 1 ns Model independent search interpreted in supersymmetric models

˜ q ˜ q ˜ χ0

1

˜ χ0

1

p p q λ ℓ ℓ ν q λ ℓ ℓ ν

Example of a R-parity violating model

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 2 / 11

Reconstruction of displaced vertices

Standard tracking reconstructs tracks up to |d0| = 10 mm Additional tracking optimised for tracks up to |d0| = 300 mm Secondary vertices reconstructed by standard ATLAS vertexer Tracking and vertexing very resource-intensive → Event preselection based on photon and muon spectrometer triggers

|d0| z0 q/p,θ,φ Reference Point Particle Track Local z axis y x z

Transverse radius [mm] 20 40 60 80 100 120 140 Vertex reconstruction efficiency 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ATLAS Work in progress = 100 mm τ ) = 500 GeV, c

1

χ ∼ ) = 700 GeV, m( q ~ m( ν µ µ →

1

χ ∼ Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 3 / 11

Displaced vertex selection

Displaced vertex with at least two oppositely charged leptons

Lepton tracks: pT > 10 GeV and |d0| > 2 mm Displacement: 4 mm in transverse plane to all PVs Fiducial volume: Vertices inside detector material are vetoed mDV > 10 GeV Vertex has to pass at least one criterion used to preselect data events

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 4 / 11

Background sources of displaced vertices

Vertex invariant mass [GeV] 5 10 15 20 25 30 35 40 45 50 Number of vertices 1 10

2

10

3

10

4

10 Work in progress ATLAS MC t = 13 TeV, t s

Conversions Hadrons Crossings (PV+PV) Crossings (PV+PU) Crossings (PU+PU)

Plot shows origin of displaced vertices with two tracks in a t¯ t Monte Carlo sample No leptons required and pT cut on tracks lowered to 1 GeV Random crossing of tracks dominant background for mDV > 10 GeV

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 5 / 11

Hadron decays

Vertex invariant mass [GeV] 1 2 3 4 5 6 7 8 9 10 Number of ee vertices / 0.2 GeV 1 2 3 4 5 6 7 ATLAS Work in progress

• 1

= 13 TeV, 32.7 fb s

ee vertices

Vertex invariant mass [GeV] 1 2 3 4 5 6 7 8 9 10 vertices / 0.2 GeV µ µ Number of 1 10

2

10

3

10 ATLAS Work in progress

• 1

= 13 TeV, 32.7 fb s

µµ vertices Validation region on data with inverted mass cut and loosened vertex selection Most vertices originate from displaced J/ψ particles of B-hadron decays No dilepton vertex with mDV > 5.5 GeV observed → Background from hadron decays negligible

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 6 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria Randomly select a given number of lepton pairs (“seed pairs”)

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria Randomly select a given number of lepton pairs (“seed pairs”) Run vertex algorithm on each pair

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria Randomly select a given number of lepton pairs (“seed pairs”) Run vertex algorithm on each pair Count number of vertices passing vertex selection

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria Randomly select a given number of lepton pairs (“seed pairs”) Run vertex algorithm on each pair Count number of vertices passing vertex selection Calculate crossing probability pxing = Number of vertices found in this procedure

Number of seed pairs

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Random crossings

Unrelated lepton tracks can randomly cross and form a vertex Dominant background of this search Estimation procedure for signal regions:

Collect all electrons and muons in data passing our track selection criteria Randomly select a given number of lepton pairs (“seed pairs”) Run vertex algorithm on each pair Count number of vertices passing vertex selection Calculate crossing probability pxing = Number of vertices found in this procedure

Number of seed pairs

Estimate: Number of lepton pairs in data × pxing

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 7 / 11

Validation of random crossing estimation

Validation region: Vertices with two tracks that fail lepton identification Enlarge statistics: No trigger and opposite charge requirements All other vertex selection criteria applied VR Number of pairs 1.1 × 108

• Avg. crossing prob.

1.6 × 10−4 Predicted vertices 17947 Observed vertices 14775

Vertices / 10 GeV 500 1000 1500 2000 2500 3000 3500 4000 4500 Observation Prediction Work in progress ATLAS

• 1

= 13 TeV, 32.7 fb s Vertex invariant mass [GeV] 20 40 60 80 100 120 140 160 180 200 Data/Estimate 0.5 1 1.5

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 8 / 11

Background estimate for signal regions

SR Nℓℓ pxing/10−5 Nest

vx /10−4

ee 22 +0.6

−8.9 (syst.)

0.52 ± 0.05 (stat.) ± 0.13 (syst.) 1.2 ± 0.1 (stat.) +0.3

−0.6 (syst.)

eµ 11 +0

−2.7 (syst.)

6.2 ± 0.2 (stat.) ± 1.4 (syst.) 6.9 ± 0.2 (stat.) +1.6

−2.3 (syst.)

µµ 5 +0

−2.6 (syst.)

9.7 ± 0.3 (stat.) ± 2.2 (syst.) 4.9 ± 0.1 (stat.) +1.1

−2.8 (syst.)

Random crossing background is of the order 10−4 for all SRs pxing larger in VR due to missing trigger requirement pxing smaller for ee than for µµ (also observed on MC) Total uncertainties on the estimates not larger than 60%

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 9 / 11

Cosmic muons

Cosmic muons sometimes reconstructed as a back-to-back muon pair Back-to-backness: ∆Rcosmic =

• (η1 + η2)2 + (|∆φ| − π)2

Veto cosmic muons in signal regions by requiring: ∆Rcosmic > 0.04 Invert cosmic veto to study back-to-backness of cosmic muons:

cosmic

R ∆ 0.01 0.02 0.03 0.04 0.05 0.06 Dimuon pairs

1 −

10 1 10

2

10 ATLAS Work in progress

• 1

= 13 TeV, 32.7 fb s Cosmic rays control region

All dimuon pairs (scaled) DV matched dimuon pairs Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 10 / 11

Summary

Search for displaced vertices with at least two lepton tracks Interpreted in supersymmetric models Dominant background from random crossings of leptons Data-driven estimate of random crossings Background is of the order 10−4 for all SRs Potential signal can be identified very clearly in data

Dominik Krauss (MPI Physik) Displaced dileptons March 30, 2017 11 / 11