observation of h→ɣɣ at cms using 2016 data at s=13 tev...observation of h→ɣɣ at cms using...

Observation of H→ɣɣ at CMS using 2016 data at √s=13 TeV

Louie Corpe (Imperial College) on behalf of the CMS Collaboration.

Louie Corpe, Imperial College 1

31.08.16 Higgs Hunting 2016, Paris.

Young Scientists Forum - Part 1

Public analysis summary: CMS-PAS-HIG-16-020

http://cds.cern.ch/record/2205275

Introduction


• H→ɣɣ: key role in discovery of the Higgs.

• Clean final state: two highly energetic photons. However, low branching fraction (~0.2%) and large irreducible background.

• In a nutshell: Look for ‘bump’ on diphoton invariant mass (mɣɣ) spectrum.

• General strategy: categorise events by resolution and production topology (using additional objects in the event).

• This talk presents first results with 12.9 fb-1

of prompt data collected by CMS in 2016 at √s=13 TeV.

• Increased Higgs cross-section (XS): similar statistical sensitivity to Run 1 analysis.

Run 1 result:Sig+Bkg fit, all

categories summed

BDT score of the photon ID1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1

Even

ts210

310

410

510

610

710

810DataSimulation:

4 = 125 GeV)x10H

(mγγ→Htotal background+stat.uncert.γ-γ

-jetγjet-jet

DataSimulation:

4 = 125 GeV)x10H


-jetγjet-jet

DataSimulation:

4 = 125 GeV)x10H


-jetγjet-jet

DataSimulation:

4 = 125 GeV)x10H


-jetγjet-jet

DataSimulation:

4 = 125 GeV)x10H


-jetγjet-jet

DataSimulation:

4 = 125 GeV)x10H


-jetγjet-jet

(13 TeV)-112.9 fbCMS Preliminary

Vertex ID and photon/diphoton ID


Vertex ID

Photon ID and diphoton pairs• Photon ID selects prompt photons against π0/

η→ɣɣ and electrons. • Multivariate approach combining shower shape

and isolation variables. • A further BDT is used to identify signal-like

diphoton pairs: kinematics, high photon ID scores, correct vertex probability and good mass resolution.

• Vertex assignment important for mɣɣ resolution. |zchosen- ztrue|<1 cm ⇒ angular contribution negligible wrt energy resolution.

• Vertex ID uses Multivariate approach (BDT):exploits tracks recoiling from ɣɣ system and conversion tracks. Estimate of vertex probability extracted for use in diphoton classification.

α

Eɣ1

Eɣ2

γ γTransformed BDT0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Even

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.02

1

10

210

310

410

510

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710 Datajet jet jetγγ γ

MC stat. uncert.

ggHVBFVHttH

Preliminary CMS (13TeV)-112.9 fb

Simulation background =125 GeVH

, mγγ→SM H

Event categorisation


• Events with additional objects characteristic of specific production modes are tagged, and remaining events are categorised using BDTɣɣ.

• TTH: look for hadronic or leptonic decays of tops quarks.

• VBF: Distinctive 2-jet + 2-photon signature, use BDTs to identify VBF jets and split VBF Tags by mass resolution.

• Inclusive categories split by mass resolution using BDTɣɣ output.

TTH Leptonic

TTH Hadronic

VBF Tag 0

Untagged 0

VBF Tag 1

Untagged 1

Untagged 2

Untagged 3

}}

}Target ttH (~0.5 pb)

Events categorised by top decay

Target VBF (~3.8 pb). Events categorised by sensitivity

Inclusive categories, mostly populated by ggH (~49 pb)

and VH (~2 pb). Events categorised by sensitivity using BDT.

(GeV)Hm121 122 123 124 125 126 127 128 129

Acc

epta

nce

(%)

×Ef

ficie

ncy

36

38

40

42

44

Simulation Preliminary CMS TeV 13

γγ→H

A× εSignal model

syst. uncertaintyσ 1 ±

• Signal Model: For each category/process, fit sum of Gaussians to mɣɣ distribution, separately for:

• Vertex correctly identified: mass resolution dominated by energy resolution.

• Vertex incorrectly identified: mass resolution dominated by uncert. on vertex position.

• Smooth parametric model in each process/category. Interpolated between 7 mass points in the range 120-130 GeV.

• Background model: data-driven method. Sidebands used to determine background shape under signal peak in mɣɣ distribution.

• Treat choice of background function as discrete parameter in final minimisation.

Signal and Background Modelling


(GeV)γγm105 110 115 120 125 130 135 140

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FWHM = 3.58 GeV

All Categoriesγγ→H

Simulation

modelParametric

= 1.88 GeVeffσ

13 TeVCMS Simulation Preliminary

Diphoton mass spectrumAll categories summed


• Invariant mass spectrum. Background-only and signal-plus-background fits to the data are shown.

• Clear Higgs signal, visible by eye when background subtracted. Applying the weighting by sensitivity makes the peak even more obvious.

Even

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DataS+B fitB component

σ1 ±σ2 ±

All categories=0.95µ=126.0 GeV, Hm --

Preliminary CMS TeV) (13-1 12.9 fbγγ→H

(GeV)γγm100 110 120 130 140 150 160 170 180

100−

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100

200 B component subtracted

S/(S

+B) W

eigh

ted

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σ1 ±σ2 ±

S/(S+B) weightedAll categories=0.95µ=126.0 GeV, Hm

--


(GeV)γγm100 110 120 130 140 150 160 170 180

100−

0

100


Unweighted Weighted

P-value and signal strength


• We report an observation with significance 5.6σ where 6.2σ was expected for the SM Higgs boson at mH=125.09 GeV. The maximum significance of 6.1σ is observed at 126 GeV.

• Best fit signal strength (XS*BR) relative to SM, for profiled MH, is found to be:

(GeV)Hm116 118 120 122 124 126 128 130 132 134

Loca

l p-v

alue

10−10

9−10

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1σ1 σ2

σ3

σ4

σ5

σ6

Observed

Expected

= 125.09 GeVHmExpected

Preliminary CMSγγ→H

(13TeV)-112.9 fb

µ0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

ln L

∆-2

0

1

2

3

4

5

6

0.18−0.21+ = 0.95 µ


TeV) (13-112.9 fb

ProfiledHm

Fiducial Cross-section measurement


(fb)fidσ30 40 50 60 70 80 90 100 110

ln L

∆-2

0

1

2

3

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fb 22− 18+ = 69 fidσ

HXSWG YR4+ aMC@NLO

=125.09 GeVHm

Preliminary CMS

H, profiling mγγ→H

(13 TeV)-112.9 fb

(TeV)s7 8 9 10 11 12 13 14

(fb)

fid.

σ20

30

40

50

60

70

80

90

100Preliminary CMS

(13 TeV)-1 (8 TeV) + 12.9 fb-119.7 fb

γγ→H)Hbest-fit mData (

syst. uncertainty)=125.09 GeVHmSM (

- norm. LHC Higgs XSWG YR4MC@NLOA- acc.

• Measurement of fiducial cross section using different categorisation scheme.

• Consistency with SM observed.

µ2− 0 2 4 6 8

-1.2+1.5 1.91

ttHµ

-0.8+0.9 1.61

VBFµ

-0.23+0.25 0.77

ggHµ

0.18−0.21+ = 0.95

combinedµ

ProfiledHm

σ 1±Combined

σ 1±Per category

SMµ=µ

Preliminary CMS

γγ→H

TeV) (13-1 12.9 fb

= 1VHµ

Production rate modifier


• Best fit signal strength split into bosonic and fermionic production modes:

• Also split the signal strength by individual production mode: consistent with SM.

ggH,ttHµ

0.5− 0 0.5 1 1.5 2 2.5 3

VBF,

VHµ

0

0.5

1

1.5

2

2.5

3

3.5

4Best Fit

SM

σ1

σ2


TeV) (13-112.9 fb

ProfiledHm

Vκ0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

fκ

1−

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0

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1

1.5

2

2.5

3 ) fκ, Vκq(

0

2

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Best Fitσ1σ2

SM

Preliminary CMS TeV) (13-112.9 fb

γγ→H ProfiledHm

γκ0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

gκ0

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2 ) gκ, γκq(

0

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Best Fitσ1σ2

SM


γγ→H ProfiledHm

Coupling strength modifiers


• 𝞳ɣ, 𝞳g, 𝞳V, 𝞳f measure strength of Higgs coupling to particles relative to SM. For Higgs couplings via loops, an effective coupling is defined.

• Results consistent with SM expectation.

Summary and Conclusions


• CMS has prepared its first results using the 2016 13 TeV dataset with 12.9 fb-1. Roughly equivalent to Run 1 dataset in terms of sensitivity.

• We report an observation of the Higgs boson with over 5σ significance in the diphoton decay channel.

• The measured signal strength is consistent with the SM expectation, with overall uncertainty of ~20%:

• Fiducial XS measurement performed - in line with SM expectation. • Consistency with SM when splitting signal strength into production modes. • Best-fit values of Higgs coupling strength modifiers for gluons, photons, vector

bosons and tops are found to be consistent with SM expectation. • Higgs physics: begin transitioning from discovery to precision

measurement era.

BACKUP


BACKUP

The CMS detector

HF Calorimeter

Hadron Calorimeter

Silicon Tracker

Preshower

Muon Detectors

EMCalorimeter


Solenoid Magnet 3.8T

Vertex ID


• Inputs to the vertex ID BDT.

• ptasym

• ptbal

• sumpt2

• pull

• correct vertex (dZ<1cm)

• incorrect vertex (dZ >1cm)

Photon ID


• Inp

• Distribution of BDTɣ ID score for signal and background. • Comparison of Distribution of BDTɣ ID score in data and MC using Z→ee events

where electrons are reconstructed as photons.

Energy Reconstruction


• data vs MC comparison for invariant mass of Z→ee electrons reconstructed as photons.

• Events split into |η| and R9 categories. (R9 is the ratio of the amount of energy in a 3x3 array of ECAL crystals around the seed divided by the total energy in the supercluster. High R9 (>0.94) photons are likely to be unconverted, while Low R9 photons are likely to have undergone ɣ→e+e-).

Diphoton MVA


• Distribution of BDTɣɣ score for signal and background. • Comparison of Distribution of BDTɣɣ score in data and MC using Z→ee events

where electrons are reconstructed as photons.

Diphoton Selection andEvent Classification


• Diphotons split into categories, exploiting different S/B ratios and mass resolution => maximum sensitivity:

• TTH categories: make cuts on photon quality and requirements on bTags, Jets and absence/presence of leptons .

• VBF categories: use an MVA to identify VBF-like events with dijets. A further MVA using the diphoton and dijet MVAs as inputs is used to classify VBF events by sensitivity into VBFTags 0 and 1.

• Untagged categories: mostly populated by ggH, bring the largest contribution to analysis’ sensitivity. Category boundaries defined by Diphoton MVA.

• Event tagging sequence is defined as follows:

ttHLeptonic ttHHadronic VBF0-1 Untagged0-3

Background Model Plots Unblinded


• Data and Signal + Background Fits shown, in the scenario where mH is profiled.

• Figures also show the Background-subtracted distributions.

• Uncertainty bands achieved by throwing toys from the post-Fit distributions and finding locations of relevant quantiles.

Weighted S/S+B and P-Value


• Shown here is the total invariant mass distribution where we reweight using the factor S/(S+B) in a 1GeV window around the point.

Unweighted Weighted

Signal Model


Signal Model


(GeV)γγm105 110 115 120 125 130 135 140

Even

ts /

( 0.5

GeV

)

02468

1012141618202224

FWHM = 5.11 GeV

Untagged 3γγ→H

Simulation

modelParametric

= 2.44 GeVeffσ


(GeV)γγm105 110 115 120 125 130 135 140

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GeV

)

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FWHM = 3.45 GeV

Untagged 2γγ→H

Simulation

modelParametric

= 1.70 GeVeffσ


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Untagged 1γγ→H

Simulation

modelParametric

= 1.35 GeVeffσ


(GeV)γγm105 110 115 120 125 130 135 140

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1.4

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FWHM = 2.42 GeV

Untagged 0γγ→H

Simulation

modelParametric

= 1.18 GeVeffσ


Signal Model


(GeV)γγm105 110 115 120 125 130 135 140

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FWHM = 2.84 GeV

TTH Hadronic Tagγγ→H

Simulation

modelParametric

= 1.39 GeVeffσ


(GeV)γγm105 110 115 120 125 130 135 140

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0.08

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FWHM = 3.17 GeV

TTH Leptonic Tagγγ→H

Simulation

modelParametric

= 1.61 GeVeffσ


(GeV)γγm105 110 115 120 125 130 135 140

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FWHM = 3.12 GeV

VBF Tag 0γγ→H

Simulation

modelParametric

= 1.60 GeVeffσ


(GeV)γγm105 110 115 120 125 130 135 140

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FWHM = 3.28 GeV

VBF Tag 1γγ→H

Simulation

modelParametric

= 1.71 GeVeffσ


Fiducial XS measurement


(fb)fidσ30 40 50 60 70 80 90 100 110

ln L

∆-2

0

1

2

3

4

5

6

fb 22− 18+ = 69 fidσ

HXSWG YR4+ aMC@NLO

=125.09 GeVHm

Preliminary CMS

H, profiling mγγ→H

(13 TeV)-112.9 fb

(TeV)s7 8 9 10 11 12 13 14

(fb)

fid.

σ

20

30

40

50

60

70

80

90

100Preliminary CMS

(13 TeV)-1 (8 TeV) + 12.9 fb-119.7 fb

γγ→H)Hbest-fit mData (

syst. uncertainty)=125.09 GeVHmSM (

- norm. LHC Higgs XSWG YR4MC@NLOA- acc.

μggH,ttH vs μVBF,VH Run1 vs Run2


ggH,ttHµ

0.5− 0 0.5 1 1.5 2 2.5 3VB

F,VH

µ0

0.5

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2

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3Best Fit

SM

σ1

σ2


TeV) (13-112.9 fb

ProfiledHm

Run 1

Figures shows with same axis ranges

Run 2

𝞳g vs 𝞳ɣRun1 vs Run2


γκ0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2gκ

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Best Fitσ1σ2

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γγ→H ProfiledHm

Run 1 Run 2

𝞳g vs 𝞳ɣRun1 vs Run2


Run 1 Run 2

Vκ0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

fκ

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3 ) fκ, Vκq(

0

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Best Fitσ1σ2

SM


γγ→H ProfiledHm

Analysis Flowchart


Mass Measurement


• Observed best fit is at mH=126.0 GeV.• Obtained from prompt-reconstructed data. • Statistical uncertainty ~0.3 GeV. • Systematic uncertainty ~0.2-0.4 GeV. • Details of the systematic uncertainties on

the mass require further refinement and are still under study.

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All categories=0.95µ=126.0 GeV, Hm --


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observation of h→ɣɣ at cms using 2016 data at s=13 tev...observation of h→ɣɣ at cms using...

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