Weird-looking distribution of Muon::isolationR03().emEt vs muon ϕ in Phase 2 samples #43858
Description
While studying the Phase2 TkAl ALCARECO events produced in the Phase2Fall22DRMiniAOD campaign, a peculiar feature was noticed in the ϕ distribution of the tracks from Z → µµ decays selected from alignment (see here for more details).
Upon dedicated check at the level of ALCARECO sample, it was observed that the feature was already present in the tracks persisted in the input sample, but also in the muon tracks from W → μν decays.
Checking on the input muons, the reason of the ϕ-dependent inefficiency was spotted in an abnormal distribution of the muon electromagnetic relative isolation distribution Muon::isolationR03().emEt as a function of the muon azimuth. This quantity is used together with the tracker and hadronic relative isolation to select the muons entering the alignment samples.
For reference it is how the isolationR03().emEt looks like in a NO PU SingleMuon gun [*] (hence I would naively expect it to be close to zero):
And here the full relative combined isolation vs muon 𝜂-ϕ.
[*]
/RelValSingleMuFlatPt2To100/CMSSW_14_0_0_pre2-133X_mcRun4_realistic_v1_STD_2026D98_noPU_RV229-v1/GEN-SIM-RECO