Searches for Long-Lived Particles Using Displaced Vertices Matthew Walker for the ATLAS and CMS Collaborations Rutgers University November 12, 2013
Outline Review of ATLAS and CMS Tracking Detectors Overview of primary vertexing techniques Use of vertexing in the ATLAS displaced vertex search Use of vertexing in the ATLAS displaced jet search Use of vertexing in the CMS displaced dijet search Summary November 12, 2013 Matthew Walker, Rutgers University 2
ATLAS Tracker Inside 2T solenoid Insertable B-Layer (IBL) Added during LS1 r = 33 mm Pixel detector 3 barrels (r = 51, 89, 123 mm) 2x3 endcaps (z = 495, 580, 650 mm) Silicon micro-strip tracker double sided modules 4 barrel layers (r = 300, 371, 443, 514 mm) Coverage up to |eta| < 2.5 2x9 forward disks (z in 835-2788 mm) Transition radiation tracker Designed so tracks with p T > 0.5 GeV will cross 30 straws November 12, 2013 Matthew Walker, Rutgers University 3
CMS Tracker Inside 4T solenoid Pixel detector -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -1.6 1.6 → η → r (cm) 3 barrels (r = 44, 73, 102 mm) 110 100 -1.8 1.8 TOB 90 2x2 endcaps (z = 345, 465 mm) 80 -2.0 2.0 70 Silicon Strip tracker -2.2 2.2 60 TIB TID TID+ − -2.4 2.4 50 -2.6 2.6 4 inner barrel layers ( 200 < r < 550 mm) 40 TEC TEC+ − -2.8 2.8 30 -3.0 3.0 6 outer barrel layers (550 < r < 1160 mm) 20 10 PIXEL 3 inner disks (580 < z < 1240 mm) 0 -300 -200 -100 0 100 200 300 z (cm) 9 outer disks (1240 < z < 2820 mm) Coverage up to |eta| < 2.5 November 12, 2013 Matthew Walker, Rutgers University 4
Outline Review of ATLAS and CMS Tracking Detectors Overview of primary vertexing techniques Use of vertexing in the ATLAS displaced vertex search Use of vertexing in the ATLAS displaced jet search Use of vertexing in the CMS displaced dijet search Summary November 12, 2013 Matthew Walker, Rutgers University 5
Primary Vertex Clustering (CMS) Select a list of tracks based on some quality cuts and compatibility with beam spot Clustering in CMS is done with Deterministic Annealing Minimizes a free-energy-like function to determine the best number of vertices and their locations Changing “temperature” allows relaxation of the system - if the system goes through a phase-change, the relevant vertex is split in two Once the system reaches a minimum temperature, splitting is stopped, and only track assignment is changed An additional outlier rejection term is added to down-weight tracks that are not near any vertex November 12, 2013 Matthew Walker, Rutgers University 6
Primary Vertex Clustering (CMS) Free Energy: # tracks # vertices ( z T i − z V k ) 2 � − 1 X X F = − T p i log ρ k exp 2 σ z T i i k Critical Temperature ◆ 2 ✓ z T i − z V p i p i k p i p ik X X T k k / c = 2 σ z 2 2 σ z σ z i i i i i November 12, 2013 Matthew Walker, Rutgers University 7
Primary Vertex Fitting (CMS) Adaptive Vertex Fitter applied to each vertex with at least two tracks that are incompatible with other vertices Least squares estimator that weights each track based on its compatibility with the vertex n dof = -3 + 2 * (sum of weights) Improves the robustness of the fit in the event of misassociated tracks or mismeasured track errors Iterative fitting stops when vertex position has not changed by more than 1 micron Vertex resolution tested by splitting tracks into two sets and refitting November 12, 2013 Matthew Walker, Rutgers University 8
Outline Review of ATLAS and CMS Tracking Detectors Overview of primary vertexing techniques Use of vertexing in the ATLAS displaced vertex search Use of vertexing in the ATLAS displaced jet search Use of vertexing in the CMS displaced dijet search Summary November 12, 2013 Matthew Walker, Rutgers University 9
ATLAS displaced vertices Phys. Rev. D 92, 072004 (2015) Searching for RPV, GGM, and split-SUSY scenarios Uses a retracking pass to find additional displaced tracks in the inner detector Searches for multitrack displaced vertices or dilepton displaced vertices November 12, 2013 Matthew Walker, Rutgers University 10
ATLAS displaced vertices Track requirements: p T > 1 GeV > 1 SCT hits, > 0 TRT hits, > 1 pixel hits d 0 > 2mm Vertexing Seed with vertices made between all pairs of tracks if the vertex has chi2 < 5 (for 1 d.o.f.) Tracks in these vertices can’t have any hits with radius less than the vertex Also requirements on having hits in the first or second layer outward from the vertex November 12, 2013 Matthew Walker, Rutgers University 11
ATLAS displaced vertices Additional vertexing procedure Define distance vector as the vector between the primary and ~ secondary vertices d = ~ r DV − ~ r P V ~ Require > -20 mm d · ˆ p Iterative track combination procedure Look for tracks in multiple vertices If the chi2 for the tracks and one of the vertices is greater than 6, remove it from the vertex If not, find the second vertex with the smallest distance significance If the significance of the distance < 3, merge the two vertices If not, remove the track from the vertex that it has the higher chi2 from November 12, 2013 Matthew Walker, Rutgers University 12
ATLAS displaced vertices Vertex selection chi2 / ndof < 5 r DV < 300 mm, |z DV | < 300 mm Transverse distance to any PV > 4 mm Veto vertices in material Require invariant mass of vertices > 50 MeV, also remove K 0s with mass cut Number of tracks > 4 Density of vertices with less than 5 Divide the vertices in high/low mass tracks that are excluded by material veto bins using a cut of 10 GeV November 12, 2013 Matthew Walker, Rutgers University 13
ATLAS displaced vertices Signal Regions DV + lepton Triggering muon with p T > 55 GeV, d 0 > 1.5 mm, |eta| < 1.07, cosmic-ray rejection Triggering electron with p T > 125 GeV, d 0 > 1.5 mm Lepton distance of closest approach to DV < 0.5 mm DV + jets or MET 4 Jets with p T > 90, 5 Jets with p T > 65 or 6 jets with p T > 55 GeV and all jets have to pass quality criteria MET > 180 GeV November 12, 2013 Matthew Walker, Rutgers University 14
ATLAS displaced vertices Background estimates Accidental crossing: a low m DV vertex is accidentally crossed by a high-p T track at large angle Low-m DV component modeled using the m DV distribution of vertices whose tracks are highly collimated then scaled High-m DV component modeled by mixing vertices with tracks from other events which have their momentum rotated so that azimuthal and polar angles with respect to the distance vector are the same Merged vertices: two low-m DV vertices that are less than 1 mm apart are reconstructed as a single vertex that passes selection criteria November 12, 2013 Matthew Walker, Rutgers University 15
ATLAS displaced vertices Results No events were seen in any of the signal regions Upper limits are set on signal yields and cross sections for a variety of models, taking into account vertex efficiency for given ctau values Limits are calculated in each model for a variety of values of model dependent parameters November 12, 2013 Matthew Walker, Rutgers University 16
Outline Review of ATLAS and CMS Tracking Detectors Overview of primary vertexing techniques Use of vertexing in the ATLAS displaced vertex search Use of vertexing in the ATLAS displaced jet search Use of vertexing in the CMS displaced dijet search Summary November 12, 2013 Matthew Walker, Rutgers University 17
ATLAS displaced hadronic jets Phys. Rev. D 92, 012010 (2015) Searching for decays of Higgs or other scalar bosons to long- lived particles, Hidden Valley Z’ and Stealth SUSY Requiring there to be 2 displaced vertices in the event Looking for displaced vertices in both the inner tracker and in the muon spectrometer November 12, 2013 Matthew Walker, Rutgers University 18
ATLAS displaced hadronic jets Inner tracker vertexing Use tracks with d 0 > 10 mm Remove d 0 and z 0 significance requirements Loosen requirements on numbers of pixels and strip hits Remove beamspot constraint Apply material cut (signficance > 6) Require number of tracks > 7 November 12, 2013 Matthew Walker, Rutgers University 19
ATLAS displaced hadronic jets Muon spectrometer vertexing Specialized tracking algorithm Match tracklets from the two layers in a given chamber Cluster tracklets from all chambers using a cone algorithm Calculate phi line-of-flight from cluster Barrel approach All tracklets projected to r-z plane Tracklets back-extrapolated to lines of constant radius in this plane Clustering performed using z intercepts on each line Select the radius and z position with the most tracklets associated November 12, 2013 Matthew Walker, Rutgers University 20
ATLAS displaced hadronic jets Endcap approach No momentum or charge measurement Tracklets back-extrapolated as straight lines Iterative vertex finding performed by least squares fit and dropping farthest tracklet until the distance to farthest tracklet is less than 30 cm Vertices required to match to at least 3 tracklets Because extrapolation starts outside magnetic field and goes into magnetic field region, radius and z positions are biased to larger values November 12, 2013 Matthew Walker, Rutgers University 21
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