Seminario finale di dottorato - uniroma1.it · Seminario finale di dottorato Giuseppe Fasanella...

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17/05/2017

Seminario finale di dottorato

Giuseppe Fasanella

“La Sapienza” University of Rome & INFN Roma 1Co-supervision with

ULB Université Libre de Bruxelles

Search for new physics in dielectron and diphoton final states at CMS

Direttori di tesi: Dr Paolo Meridiani Prof. Barbara Clerbaux

Giuseppe Fasanella, ULB and INFN Roma I

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Outline

● Introduction

● Motivation

● LHC and the CMS detector

● Heavy (neutral) resonance searches- dielectron channel- diphoton channel

● Conclusions


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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?”


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MotivationThe 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


M [GeV]

# Ev

en

ts

# Ev

en

tsM [GeV]

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The LHC

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


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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


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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.

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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

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Search for high mass resonances (Z ') decaying in dielectron final state


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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


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Z' to dielectron: backgroundsThree 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 processeswith 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)


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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

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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

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Dielectron mass spectra

● Observed mass spectra are compatible with the SM-only hypothesis


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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)


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Search for high mass resonances decaying in diphoton final state


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Event selection● Many similarities with the dielectron case:

– Cut based selection targeting high-pT 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

p1T

75 GeV

p2T

75 GeV

||max

<2.5

||min

<1.45

categorization EB-EBEB-EE


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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.

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

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Diphoton invariant mass spectra(latest 2016 results)

EBEEEBEE


● Background fitted directly from data● Observed mass spectra are compatible with the SM-only hypothesis

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Observed limits 2016

Resonance widthResonance width


● 95% confidence level limit are set on the production cross-section for a heavy resonance decaying in diphoton final state

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Results: p-values 2015+2016

SPIN0

SPIN2

WIDTHWIDTH

Spin 0

Spin 2

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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 data-taking

● 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


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Back-up


Energy corrections vs ET


● Residual scale corrections vs ET 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 ET

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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

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General calibration strategy:

● Identification and reconstruction of leptons/photons with very high transverse momentum (pT)

● 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 (pT~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


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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 pT >~ 300 GeV ● with the level of statistics cumulated in 2015, the region up-to pT >~150

GeV was inspected

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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

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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

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Event selection● Simple set of requirements

– Fixed pT cuts, at least one photon in the barrel region (EB: ||<1.45).

– Events categorized in barrel-barrel (EBEB) and barrel-endcap (EBEE) configurations.

– 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

p1T

75 GeV

p2T

75 GeV

||max

<2.5

||min

<1.45

categorization EB-EBEB-EE


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Energy corrections

● Quite similar technique to the dielectron one (electrons and photons, expecially at high-pT , are quite similar objects)

● Data/simulation comparison shows very good agreement


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