Seminario finale di dottorato Search for new physics in dielectron and diphoton final states at CMS 17/05/2017 Giuseppe Fasanella La Sapienza University of Rome & INFN Roma 1 Co-supervision with ULB Université Libre de Bruxelles Direttori di tesi: Dr Paolo Meridiani Prof. Barbara Clerbaux 1
Outline Introduction Motivation LHC and the CMS detector Heavy (neutral) resonance searches - dielectron channel - diphoton channel Conclusions 2
The present picture + Higgs boson (2012) The standard model (SM) of particle physics includes: all known leptons and quarks the force carriers of 3 of the 4 fundamental forces of Nature The Higgs boson (which accounts for the particles masses) The question is: Is there something else? 3
Motivation The searches for new physics in the dielectron/diphoton final state are Theoretically motivated: new heavy neutral resonances predicted in several BSM scenarios (GUT, extra-dimensions, theories with non-minimal Higgs sectors) Experimentally motivated: very clean channels, hence high discovery potential Signal Signature: Excess of events (peak) or a distortion in the invariant dielectron/diphoton invariant mass distribution compared to the SM processes # Events # Events M [GeV] M [GeV] 4
The LHC Giuseppe Fasanella, ULB and INFN Roma I The LHC (Large Hadron Collider), located under the Geneva area, is: Hadron accelerator and collider (only proton-proton collisions considered here) Design center-of-mass energy of 14 TeV 27 km of circumference It hosts 4 main experiments (ALICE, ATLAS, CMS, LHCb) After the Higgs discovery, the goal is the search for new physics beyond the SM In my thesis: The searches for new physics in the dielectron and diphoton are presented Different datasets are analyzed colled by the CMS experiment during 2015-2016 5
The CMS detector The main detector for the final states targeted in these analyses is the e.m. calorimeter (ECAL): made-up of ~76k scintillating crystals 6
Particles in CMS Each particle has a specific signature in the CMS detector. Both electrons/photons release most of their energy in the ECAL. Electrons also leave hits in the silicon Tracker. Quarks appear in the detector as jets (spray of collimated particles) with high hadronic activity. 7
Electrons and photons in CMS Electrons (photons) identification based on isolation: search for isolated ECAL deposit with (or without) associated track. on shape: the energy of the impacting particle is shared by a cluster of crystals, with a peculiar form and a certain spread in due to the magnetic field 8
Search for high mass resonances (Z ' ) decaying in dielectron final state 9
Event Selection High energy electron pairs (HEEP) selection is used Cut-based selection designed to be highly efficient at high ET Events categories: Barrel-Barrel (BB) or Barrel-Endcap (BE) The pair with the highest invariant mass is selected The selection efficiency is studied with data-driven techniques 10
Z' to dielectron: backgrounds Three main types of SM backgrounds (BG) in the di-electron channel All 3 of them resulting in a falling and continuous BG vs mass The most significant one is the irreducible SM Drell-Yan (DY) process Background predicted using simulations The second most important BG comes from real electrons in processes with W and Z bosons involved (WW, tt, tw, ZZ,...) These processes are flavor-symmetric: Verify the simulations by looking at the e-mu spectrum The third type of background is the jet background, where one or more jet is misidentified as an electron (di-jets events, W + jets ) Estimated directly with data (Fake Rate method) 11
Energy scale & resolution corrections For narrow resonances, experimental peak width dominated by calorimeter energy resolution need to control scale & resolution of the ECAL Derived in 2 steps: Data/simulation comparisons at the Z-peak region (pt ~45 GeV) ~15 M Zee events: it is possible to define several kinematical regions the scale in data is matched to the one in simulation the resolution in simulation is broaden to match the one in data EBEB EBEE 12
Energy scale & resolution corrections For narrow resonances, experimental peak width dominated by calorimeter energy resolution need to control scale & resolution of the ECAL Derived in 2 steps: Data/simulation comparisons at the Z-peak region (p T ~45 GeV) On top of that, need to control the pt spectrum going from Z-peak region to unknown territory (several effects could lead to non-linearity: electronics, shower containements, energy losses,.) A clear trend was exposed at high-pt due to events acquired with different gains solved 13
Dielectron mass spectra Giuseppe Fasanella, ULB and INFN Roma I Observed mass spectra are compatible with the SM-only hypothesis 14
Observed limits 95% confidence level limit are set on the (normalized) production cross-section for a heavy resonance decaying in dielectron final state Two models considered: a Z' particle with couplings identical to the SM ones (Z'SSM ) and a Z' particle coming from GUT theory (Z' ψ ) 15
Search for high mass resonances decaying in diphoton final state 16
Event selection Many similarities with the dielectron case: Cut based selection targeting high-p T photons Main difference is the requirement that the cluster in the calorimeter should not be associated to a track in tracker Analysis results are interpreted in terms of a spin-0 and spin-2 resonance hypothesis p 1 T p 2 T 75 GeV 75 GeV max <2.5 min <1.45 categorization EB-EB EB-EE 17
Backgrounds Irreducible backgrounds from processes with real photons: Reducible backgrounds: from +jets and di-jet processes: Differently w.r.t the dielectron case, the background is fitted direcly from data, by modeling the mass distribution with an analytical function. 18
A bit of history At mid-dicember 2015, some excess of events (~3 standard deviations in the local p-value) have been reported by both ATLAS and CMS around the mass region ~750 GeV 2016
Diphoton invariant mass spectra (latest 2016 results) EBEE Background fitted directly from data Observed mass spectra are compatible with the SM-only hypothesis 20
Observed limits 2016 95% confidence level limit are set on the production cross-section for a heavy resonance decaying in diphoton final state Resonance width 21
Results: p-values 2015+2016 Spin 0 SPIN0 WIDTH Spin 2 SPIN2 22
Conclusions Searches for new resonances both in dielectron and diphoton final states have been presented The analyses, thanks to the increase in the center-of-mass energy from 8 TeV to 13 TeV, explored a way wider region w.r.t Run1 datataking No significant excesses seen in data over the SM-only expectations Dielectron resonance search: Mass below ~ 4 TeV are excluded at 95% CL Diphoton resonance search: The 2015 dataset showed a mild excess of events (~2.9 standard deviations in the local p-value) in the mass region ~750 GeV It turned out to be simply a statistical fluctations, which disappeared with more statistics 23
Back-up 24
Energy corrections vs E T Residual scale corrections vs E T are inside the 1% syst. Uncertainty band Note: scale corrections at the level of 0.5 % (for EB 3.8 T) vs 1.5 % (for EB 0T) extra 1% syst. uncertainty on scale is added for 0T dataset to take into account possible mismodelling of the difference in energy scale between the two dataset Smearing corrections are roughly constant vs E T
Interpretation of results Three datasets considered: 19.7/fb at s=8tev 3.3/fb at s=13tev (2015) 12.9/fb at s=13tev (2016) Combinations: 2015 + 2016 8 + 13 TeV 26
General calibration strategy: Identification and reconstruction of leptons/photons with very high transverse momentum (p T ) Since we are searching for a peak at high mass, it is important to control the electron energy resolution in this regime The strategy is divided in 2 steps: 1) Calibrate the detector response at the Z peak (p T ~45 GeV): Comparison of the data-mc Z peak (scale and broadness) 2) After applying 1), we inspect the high mass region: Either via MC-only or looking at boosted Z events 27
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Energy scale & resolution corrections Important input for both analyses: the signal shape is convoluted with the detector resolution Derived in 2 steps: Data/simulation comparisons at the Z-peak region (p T ~45 GeV) On top of that, inspect the high-pt region the target is to control the energy scale at the level of ~1% for p T >~ 300 GeV with the level of statistics cumulated in 2015, the region up-to pt >~150 GeV was inspected 29
Energy scale & resolution corrections For narrow resonances, experimental peak width dominated by calorimeter energy resolution need to control scale & resolution of the ECAL Derived in 2 steps: Data/simulation comparisons at the Z-peak region (p T ~45 GeV) On top of that, need to control the pt spectrum going from Z-peak region to unkown territory (several effects could lead to non-linearity: electronics, shower containements, energy losses,.) A clear trend was exposed at high-pt 30
Observed limits 95% confidence level limit are set on the (normalized) production cross-section for a heavy resonance decaying in dielectron final state (left) Two models considered: a Z' particle with couplings identical to the SM ones (Z'SSM ) and a Z' particle coming from GUT theory (Z' ψ ) Combined with dimuon channel 31
Event selection Simple set of requirements Fixed p T cuts, at least one photon in the barrel region (EB: <1.45). Events categorized in barrel-barrel (EBEB) and barrel-endcap (EBEE) configurations. p 1 T p 2 T 75 GeV 75 GeV max <2.5 min <1.45 categorization EB-EB EB-EE Efficient cut-based photon identification criteria. Per-photon efficiency in the barrel: 90%. Per-photon efficiency in the endcaps: 85%. Analysis results interpreted in terms of a spin-0 and spin-2 resonance hypothesis 32
Energy corrections Quite similar technique to the dielectron one (electrons and photons, expecially at high-p T, are quite similar objects) Data/simulation comparison shows very good agreement 33