Performance of the trigger system for displaced muons in CMS

Performance of the trigger system for displaced muons in CMS

ALEJANDRO SOTO RODRÍGUEZ (ON BEHALF OF THE CMS COLLABORATION)

FPU20/02225 funded by Grant PID2020-113341RB-100 funded by

Trigger and readout at the LHC experiments for Run 3 and beyond in Spain – COMCHA

20-22 March 2023

Introduction

• What are displaced muons?

• Muons that are not produced in the primary vertex (PV).

• They can be produced through the decay of a long- lived particle (LLP).

• Therefore, they do not have to point to the PV.

• Why are they interesting:

• Signature of new physics.

• Many exotic models can produce displaced muons:

• RPV, Exotic Higgs models, dark photon, …

• Challenges:

• They need a special reconstruction which doesn’t take into account the PV constrain.

• From the trigger perspective, this can explode the rate with fake tracks from pileup vertices.

• Highly displaced muons can be misidentified with low 𝑝_𝑇 non-displaced muons.

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The CMS Muon Detector

Barrel Overlap

En dc ap

• Regarding L1 trigger, in this presentation I will focus on the overlap muon track finder (OMTF).

• The OMTF performs muon reconstruction in the overlap region.

• Challenges in the overlap region:

• Combines information from 3

subdetectors: DTs, RPCs and CSCs.

• Complicated geometry and difficult magnetic field.

• In each overlap area, the OMTF receives information from 15 layers.

Level 1 trigger OMTF

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OMTF: Naïve-Bayes Classifier

• Reconstruction based on a Naïve-Bayes classifier:

• It performs muon identification and 𝑝_𝑇 measurement in one step.

• The foundation of the algorithm is the Bayes theorem.

• In this case:

𝑃 𝑝_𝑇 𝑥₁, … , 𝑥_𝐿 = 𝑃 𝑥₁, … , 𝑥_𝐿 𝑝_𝑇 𝑃(𝑝_𝑇) 𝑃(𝑥₁, … , 𝑥_𝐿)

• It is assumed that the log-likelihood that a muon has a given 𝑝_𝑇 𝑃(𝑝_𝑇|hits) is just a sum of the log-likelihoods of the muon hit 𝜙 positions in each detector layer 𝑃_{𝑙𝑎𝑦𝑒𝑟}(𝜙_𝑖^{𝑑𝑖𝑠𝑡}|𝑝_𝑇).

• 𝜙_𝑖^{𝑑𝑖𝑠𝑡} = Δ𝜙_𝑖 − Δ𝜙_𝑖^{𝑚𝑒𝑎𝑛}(𝑝_𝑇)

• Compare with precomputed patterns → pattern with the biggest likelihood gives the 𝑝_𝑇 estimation.

• The values of the log 𝑃_{𝑙𝑎𝑦𝑒𝑟}(𝜙_𝑖^{𝑑𝑖𝑠𝑡}|𝑝_𝑇) are stored in LUTs implemented in the FPGA BRAM modules.

OMTF: Run3 performance

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• Efficiency evaluated with Run3 data of era E.

• Efficiency of the OMTF over 95% for 𝑝_𝑇 > 15 GeV.

• Good turn-on of the 𝑝_𝑇 curve.

OMTF: Phase-2 upgrade

• For the Phase-2 update of the CMS detector the DT trigger primitives are going to be updated.

• See Jaime’s presentation: link.

• A new set of patterns is trained with this new input.

• A slight gain in efficiency can be achieved for 𝜂 𝜖 (0.7, 0.9) which is the region where DTs are located.

Displaced OMTF

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• A new approach is proposed to reconstruct displaced muons.

• The algorithm is modified to give two values for the muon 𝑝_𝑇: constrained and unconstrained to the interaction point.

• Same patterns are used for prompt and displaced muons.

• An extrapolation is performed using 𝜙_𝑟𝑒𝑓^𝑏 .

• 𝜙_𝑖^{𝑑𝑖𝑠𝑡} = 𝜙_𝑖^𝑏 − 𝜙_𝑟𝑒𝑓^𝑏 − Δ𝜙_𝑖 − Δ𝜙_𝑖^{𝑚𝑒𝑎𝑛} 𝑝_𝑇 ; if layer 𝑖 has bending measurement.

• 𝜙_𝑖^{𝑑𝑖𝑠𝑡} = Δ𝜙_𝑖 − Δ𝜙_𝑖^extrpol − Δ𝜙_𝑖^{𝑚𝑒𝑎𝑛}(𝑝_𝑇); if layer 𝑖 does not have bending measurement.

• Only MB1 and MB2 are used in the extrapolation.

Displaced OMTF: performance

• Highly displaced muons are reconstructed as low 𝑝_𝑇 muons.

• Using 𝑢𝑝_𝑇 allow us to trigger on highly displaced muons.

• Efficiency of ~50 % for 𝑑_𝑥𝑦 𝜖[0, 100] cm.

Displaced OMTF: performance

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• Rate of ~2kHz for a 𝑝_𝑇 threshold of 20 GeV.

• Additional rate of +1kHz if we allow displaced muons with a 𝑢𝑝_𝑇 threshold of 20 GeV.

Additional rates

High Level

Trigger

Displaced global reconstruction at HLT

• CMS has three types of muons:

• Standalone muons: reconstructed using only information from the muon chambers.

• Global muons: reconstructed using information from the tracker and segments from the muon chambers.

• Tracker muons: tracker tracks that are matched to muon segments.

• Global muons have better resolution than muons reconstructed only in the chambers.

• We have developed a new displaced global reconstruction (dGbl+dTks) that incorporates:

• Improved seeding that targets displaced signatures.

• Final estimate of the muon momentum based on the global track fit.

• Displaced tracker muons (dTks).

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Tracker track

Muon segments

Efficiencies for displaced muons

𝐿_𝑥𝑦 = 𝑣_𝑥 − 𝑏_𝑥 ² + 𝑣_𝑦 − 𝑏_𝑦 ² ቊ𝑣: muon vertex 𝑏: beam spot

• Here we compare the efficiency of our proposal dGbl+dTks and the up to now reconstructions used in CMS:

• Standard iterative reconstruction.

• Cascade reconstruction.

• Very good improvement in efficiency and momentum resolution 𝜎_𝑝.

Rate of the displaced paths

• Two double muon paths are built using the cascade and dGbl+dTks reconstructions.

• To reduce the rate, muons with low displacement are vetoed (𝑑_𝑥𝑦 < 0.01 cm).

• Less rate of the new proposal thanks to the better 𝑑_𝑥𝑦 resolution.

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Trigger path Total rate (Hz) Total timing (ms)

HLT_DoubleL3Mu16_10NoVtx_DxyMin0p01cm_v1 (cascade reco) 16.7 5.9 HLT_DoubleL3dTkMu16_10NoVtx_DxyMin0p01cm_v1 (dGbl+dTks) 14.7 4.9

HLT_DoubleL3dTkMu16_10NoVtx_DxyMin0p01cm_v1 HLT_DoubleL3Mu16_10NoVtx_DxyMin0p01cm_v1

Rate vs PU

Rate measured with real data.

Displaced standalone reconstruction at HLT

• The performance of the standalone reconstruction is evaluated using cosmics recorded during Run3.

• Two paths are studied:

• Standalone collision seeded: HLT_L2Mu10NoVtx_v1.

• Standalone cosmic seeded: HLT_L2Mu10NoVtx_CosmicSeed_v1.

• Good performance of both paths is observed.

Summary

• The performance and capabilities of the CMS trigger system for reconstructing displaced muons has been presented.

• The overlap muon track finder has been adapted to trigger on displaced muons with an efficiency of ~50% for medium displaced muons.

• This comes with a moderate increase in rate.

• Currently working on deploying online the displaced OMTF for the 2023 data taking.

• From the HLT side, a new displaced global muon reconstruction has been developed.

• It offers a moderate gain in efficiency but a good reduction in rate and timing.

• An HLT path using the new displaced reconstruction has been used during 2022 data taking with excellent performance.

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Thanks for your attention!

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Backup

The CMS L1 Trigger

Muon Track Finders:

Combines local info from different stations to find the trajectory and

estimate momentum.

Local reconstruction:

Local information, reconstructs segment

direction in each station →

(TPs).

Global Trigger:

Combine Muon and Calo trigger into a single

decision.