Higgs Two Photons ATLAS - Yamamura 12

8/9/2019 Higgs Two Photons ATLAS - Yamamura 12

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Higgs Boson Searches in the two photon

Taiki Yamamura (Univ. of Tokyo)

On behalf of the ATLAS collaboration

High Energy Physics

in the LHC Era

4th International workshop

06/01/2012


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Outline

Higgs boson search in the two photon

(1) Search for the Standard Model(SM) H→-1

Main topic of this presentation

with 4.9fb (= full dataset of 2011)

H→ (2 photons)

decay channel (H→ ) at ATLAS

(2) Search for the Fermiophobic H→ with 1.08fb-

(3) Summary & future prospects

3High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura

(Univ. of Tokyo)

will be presented briefly.

(※ This is not a result with the full 4.9fb sample,-1

that has not been published yet. )

Search result with 1.08fb-1


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https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/

◆ SM H→ : ATLAS-CONF-2011-161

References

◆ Fermiophobic H→ : ATLAS-CONF-2011-149


(Univ. of Tokyo)


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earc or e →


(Univ. of Tokyo)


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Search for the SM H→

At LHC, H→ is the most sensitive

・Narrow si nal eak in M s ectrum

・Small branching ratio, but large event

Branching fraction

for SM Higgs boson

channel in the low mass range.

yield due to high selection efficiency.

(110<


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H→ analysis

◆ Backgrounds

・Irrreducible BG : Main contribution

・Reducible BG : +jets, di-jet

・Z→ee(DY) (・・・ very small contribution)

◆ M( ) reconstruction

・ M( ) = 2E1E2・(1ーcos α)2


(Univ. of Tokyo)

✓α ・・・ Opening angle of the two photons

・For the precise reconstruction, careful understandings

are needed for the followings :

✓Energy calibration & resolution

✓Primary vertex position (related to α)

(related to E1 and E2)

(→ For details, see the following slides.)

✓ E1 [E2] ・・・ Energy of 1st [2nd ] photon


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Energy calibration & resolution

＜Z→ee peak＞

Photon energy calibration

◆ MC-based calibration

(Tuned by beam-test result)

◆ After MC-based calibration,

electron energy scale corrections


(Univ. of Tokyo)

.(Scale factor is obtained from Z→ee.)

Energy resolution

Resolution correction is applied to MC.

(This correction factor is also determined by comparing

the Zee peak between data and MC.)


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Vertex reconstruction

Vertex position is measured by

(z-vertex)

・Unconverted photon :

“1st + 2nd layer of EM calorimeter”

“pointing method”.

・Converted photon :

“1st layer of EM calorimeter”

+ “conversion oint →ee ”

＜Calorimeter

(i) Measure photon-direction.

pointing＞

EM

calorimeter

Beam axis

Robust measurement against pile-up.

＜H→ peak with various conditions for pile-up＞

M( ) resolution also becomesstable against pile-up effect.

By using “pointing method”,

(ii) z of primary vertex

is deduced.

9


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Event selection

◆ 2-photon trigger

◆ Primary vertex selection

(for selecting collision event)

Event signature is very simple.

Event with two high-Et photons

(ET( 1, 2)>40, 25GeV )

Diphoton event candidate

(Obtained from data)

◆ Selection for di-photon event

Photon is required to satisfy :

・|η|


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(conv, η)

Event categorization

To improve sensitivity, data sample

different S/B and M( ) resolution.

is divided into 9 categories with

9 categories

(a) Conversion- η categories(5 categories)◆ Both photons unconverted

・Central ・Rest

◆ At least one photon converted

・Central ・Rest・Transition

Thrust axis

)()( 21 T T p pt rr

r

pT,t

・pT,t is the transverse component of

pT( γγ) with respect to the thrust axis.

・pT,t has more discriminative power

than pT( γγ).11

Definition of pT,t

(b) pT,t categories ( 5 → 9 categories )

◆ Low pT,t (40GeV)

(pT,t = “pT-thrust”)


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Analysis results


(Univ. of Tokyo)


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Background decomposition

◆ Decomposition for “ +DY”, “ +jets” and “di-jet” is performed

as a control sample.

(data-driven)

in a data-driven manner.Control sample is obtained from “anti-cut” region that is defined

with photon-ID and isolation variables for the two photons.

(※For details, see backup slide.)

◆ DY contribution is also estimated by using “e events”

※

Enriched with Z→ee where one electron

is faking as photon.

Result

The contribution from irreducible BG ( )

is dominant. (Fraction = 71% )

It could be also confirmed that the

(


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Background modeling

BG shape is defined by the fit with single-exponential

(100


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Signal modeling

◆ Signal MC

(100-150GeV with 5GeV step)

・Function : “Crystal-ball + Gaussian”

(ggF, VBF, WH/ZH, ttH)

Samples are available at 11 mass points.

◆ Peak shape modeling

Signal shape

inclusive

Peak resolution

( =120GeV)

( =120GeV)

σCB

(GeV)

・Global fit :

Simultaneous fit is performed for allHm

Hm

mH(GeV) 110 115 120 125 130 135 140 145 150

#evts 69.9 71.5 70.9 68.3 63.7 57.5 49.8 40.8 30.6

inclusive 1.7

Best category(unconv central)

1.4

Worst category(conv transition)

2.3

(for =110-125GeV)

◆ Expected # of signal events (4.9fb , inclusive)-1

mass points in each category.

The signal peak shape are parameterized

linearly as a

Nsig ～ 70evts

function of mH.

15Hm


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Main systematics

(a) Signal yield : ～20%

(b) H→

mass resolution : ～14%

(c) Migration of signal events

between categories

(Error on # of signals)

Summary of sys errors

(a)

(b)

. ,

⊿Nsig = ±8% (for high PT,t-bin)

(d) BG modeling

⊿Nsig = 0.1-5.6 events

The intrinsic difference between

・ Chosen background model (= exponential)

・ True background shape

(Depending on categories)

It is included into sys uncertainty on # of signal events.


(Univ. of Tokyo)

(c)


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Result for H→ search

Exclusion limit w.r.t

SM prediction

Limit setting

By using profile likelihood ratio

method, exclusion limit is

obtained with CLs.(95%C.L.)

◆ Expected limit(1.61-2.87)×SM @ = 110-150GeV

(1.61-1.78)×SM @ = 115-130GeV

◆ Observed exclusion=114-115GeV

=135-136GeV


(Univ. of Tokyo)

Hm

Hm

Hm

Hm


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Result for H→ search

p0-value w/o LEE

(Look-Elsewhere-Effect)

※p0 value :

Probability of seeing upward

fluctuation in the background-

only hypothesis as large as or

◆ Observed excess at mH=126GeV

・w/o LEE : 2.8σ

・w/ LEE : 1.5σ


(Univ. of Tokyo)

(p0=0.27%)

(p0=6.5%)

larger than the obtained excess.


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(2) Search for the Fermiophobic H→


(Univ. of Tokyo)


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◆ Fermiophobic benchmark scenario

・Higgs does not couple to fermions

but couples only to bosons.

Branching fraction

in Fermiophobic

・Production mechanism

✓ ggF is gone.

scenario

γγ

Search for the Fermiophobic H→


(Univ. of Tokyo)

BF and VH become dominant.

Br (H→ ) is strongly enhanced

in the low mass range.

(


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Analysis results◆ Using 1.08fb

-1

◆Analysis entirely based on the SM analysis (SM H→ search)・Two isolated photons with pT>40 and 25GeV

・M( ) reconstruction : Using calo pointing / conv vertex

◆ Some differences w.r.t. SM analysis

・Categorization : 3 categories with pT( )

・BG model : 2nd order polynomial (Bernstein basis)

Note

Results with the full 4.9fb sample

has not been released yet.

-1

← Func. is optimized based on the categorization strategy.

Exclusion limit w.r.t the prediction

◆ Expected exclusion

=110-116GeV

◆ Observed exclusion

= 110-111GeV,

113.5-117.5GeV

21

(95%C.L.)

Hm

Hm


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Summary

◆ Search for the Higgs boson decaying to two photons have been

made by using 2011 data with the ATLAS detector.

・ Upper limit on x-sec

✓ Expected limit

✓ Observed exclusion

◆ Search for the SM H→ (with 4.9fb )-1

H→ search at the ATLAS experiment

. - . or - e

(1.61-1.78)×SM (for 115-130GeV)

= - e

=135-136GeV

・ Observed excess at =126GeV

w/o LEE : 2.8σ

w/ LEE : 1.5σ (※LEE : look-elsewhere effect)


(Univ. of Tokyo)

◆ Search for the Fermiophobic H→ (with 1.08fb )

-1

・ Expected exclusion : = 110-116GeV

・ Observed exclusion : = 110-111GeV, 113.5-117.5GeV

Hm

Hm

Hm

Hm

Hm

(※Results with the full 4.9fb sample has not been released.)-1


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Future prospects for SM H→ search

◆ Improvement of analysis sensitivity

・Multivariate technique for photon-ID selection

・Exclusive analysis (H+0/1/2 jets) etc.

◆ 2012 run at LHC

・Expected to obtain ～15fb (or more?).

・

-1

The x-sec for Hi s roduction will increase

◆ Prospects for SM H→ search in 2012

・ Exclusion is possible in complete region for 110<


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(Univ. of Tokyo)


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ac up →


(Univ. of Tokyo)


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Backgrounds

◆ Irreducible background ( )

◆ Reducible back round + et di- et

Born Brems Box

◆ Drell-Yan (Z→ee) ・・・ Very small contribution


(Univ. of Tokyo)

di-jet+jet


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LAr electromagnetic calorimeter

◆ EM calorimeter consists of 3 layers.

・Strip (1st

layer)・Middle (2nd layer)

・Back (3rd

layer)

＜ATLAS detector ＞＜EM calorimeter ＞

EM calorimeter

High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura

(Univ. of Tokyo)27


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Photon reconstruction◆ Longitudinal segmentation

・ Strip (S1)

・ Middle (S2)

・ Back (S3)

・ Pre-sampler (in |η|


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E scale

◆ Understood from Z→ee.

◆ Energy scale at m(Z) known to ～0.5%.

◆ Linearity : better than 1%

◆ Uniformity

(Constant term of resolution) :

- .


(Univ. of Tokyo)


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(Univ. of Tokyo)

(eta, conv)


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9 categories based on conversion status, η and pT,t

(a) Based on η and conversion (5 categories)

◆ Both photons unconverted

・Central

・Rest

◆ At least one photon converted

・Central


※To improve sensitivity, data sample

different S/B and M( ) resolution.

is divided into 9 categories with

・Rest

・Transition

(b) Based on pT,t ( 5 → 9 categories )

◆ “Central” and “Rest” categories are divided into “Low-pT,t” and “High-pT,t”.

Thrust axis

)()( 21 T T p pt rr

r

・pT,t is the transverse component of

pT( γγ) with respect to the thrust axis.

・pT,t has more discriminative power

than pT( γγ).

(“Transition” category is not divided.)

◆ pT,t : “pT-thrust”

pT,t 31


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# of selected events in each category

(4.9fb , 100


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M( γγ) spectrum after the event selection

(data, 4.9fb , 100


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Background decomposition

◆ Data-driven BG decomposition

・Using “ABCD” method (i.e. A=B*C/D), # of

fake photons in “tight-isolated” region can be checked.

・This method is applied to 1st and 2nd photons iteratively.

◆ Z→ee

Control sample : “e ” events Z(ee) with one electron faking as photon

(for , +jets and di-jet)

Result

・Contribution from irreducible BG ( )

is dominant. (Fraction = 71% )

・Estimated result is consistent with

the predicted one from theory and MC.

・Very small contribution from Zee.

(


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Signal MC◆ Process

◆ Generator

・ggF, VBF : POWHEG

・WH/ZH, ttH : Pythia

◆ x-sec

・ggF, VBF, WH/ZH : NNLO

・ttH : NLO

ggF, VBF, WH/ZH, ttH

◆ Branching ratio : HDECAY

Correction to MC samples

・


(Univ. of Tokyo)

・Reweighting for longitudinal beam spot distribution

(σZ(data)=5.6cm)・Photon ID variables (shifted to match data)

・Energy smearing (Understood from Zee)

・Isolation variable (for lateral shower leakage, UE , pile-up)

・ggF : Reweighting for pT( ) distribution to matchthe one with HqT.

・# of ggF events : Corrected by considering the interference

with gg→ (BG).


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Signal modeling

・Function : “Crystal-ball + Gaussian”

・Global fit model :

◆ Peak shape modeling

Simultaneous fit is done for all of mass points in each category

to extract the best values of all parameters.

✓ Width and position of the peak depend on mH linearly.

✓ The other parameters : constant (not depending on mH)

Result of global fit

inclusive

Peak resolution

( =120GeV)

(for =120GeV)

σCB(GeV)

inclusive 1.7

Best category

(unconv central) 1.4

Worst category

(conv transition) 2.3

Hm

Hm

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# of signal events in each category (4.9fb )-1


(Univ. of Tokyo)


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Systematics

(i) # of signal events

・x-sec : +15%/-11%

・Iso-cut eff. : ±5%

・photon-ID eff. : ±11%(pdf and scale variations)(from comparison between data and MC)

(material effect)

(Understood from Zee)

・Pileup effect on photon-ID : ±4%

・Luminosity : ±3.9%

・Trigger : ±1%

・Higgs pT modeling : ±1%


(Univ. of Tokyo)

(Diff between HqT and Resbos)


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Systematics

(ii) Resolution of signal peak

・Cluster energy resolution : ±12%(Constant term)

・Energy calibration : ±6%

・Pileup : ±3%

(Extrapolation from electron energy

measurements) (Material effect)

(Pileup noise impact on cluster energy)

・Resolution of photon-angle measurement

(pointing)

: ±1%


(Univ. of Tokyo)

(Studied with ⊿z between two photons)


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Systematics

(iii) Uncertainty due to event migration among the categories

Uncertainty on # of signal events in the category is also defined.

・Between converted/unconverted

・Between hi h and low T,t cate ories : ±8% for hi h T,t-bin

categories : ±4.5% (for unconv-bin)

(Comparing #evts fraction with low and high pileup condition.)

(Material effect)


(Univ. of Tokyo)

(Scale variation in HqT)

(photon energy scale uncertainty)

(iv) Uncertainty due to BG modeling

See the following slides.


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ncer a n y ue o mo e ng


(Univ. of Tokyo)


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BG modeling

◆ In H→ analysis, the background shape is modeled by

single-exponential function.

◆ There can be intrinsic difference between the true background

shape and the chosen background model (= exponential).

(detail)

Reliability of our BG model needs to be checked.

(Check 1) Use of another function

Modeling is also cross-checked with another functions.

(2nd order Bernstein polynomial etc.)

No large difference found for sensitivity.


(Univ. of Tokyo)


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(Check2) Difference between the exponential model

・It is checked by using parton-level

and the true background shape

generator(RESBOS, process).

・Integrated the residual in a sliding

window (±2GeV) over the search

range of 110-150GeV.

(= “residual”)

Largest integrated residual is

regarded as sys error.

＜Uncertainty on # of signal events due to BG modeling＞

(in each of 9 categories)

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Limit calculation on H→

◆ Profile likelihood ratio method is used.

(Limit setting is done with CLS.)◆ Unbinned maximum likelihood fit is performed simultaneously

in 9 categories.

・ Signal parameters : fixed

・ BG parameters : free

・ → .

◆ PDF for (S+B)-fit

bb s spr spr s signal f n f f n pdf )(

Signal model(Defined by MC)

BG term

Signal term which is produced by

the bias due to BG modeling.


(Univ. of Tokyo)


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List of nuissance parameters


(Univ. of Tokyo)


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0


(Univ. of Tokyo)

BG model


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p0 calculation with alternative BG model

Two alternatives have been also tried for BG modeling.

(i) Hybrid model

・High pT,t category : Bernstein

・Others : Exponential

(ii) Bernstein-only model


(Univ. of Tokyo)

.

◆ Uncertainties due to BG modeling

The values when using single exponential are

set to be zero or doubled.

◆ Photon energy scale uncertainty of 0.5%

is introduced in the likelihood fits.


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Result for p0-value (with alternative BG models)

The obtained results are quite

to the nominal one.

The largest change in the

observed significance at

=126GeV isH

m


(Univ. of Tokyo)

p0-value for the 126GeV excess

nominal Hybrid Bernstein-only

Local p0 0.27%(2.8σ) 0.38%(2.7σ) 0.25%(2.8σ)

Global p0 6.5%(1.5σ) 8.9%(1.3σ) 6.0%(1.5σ)

0.16 standard deviations.


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Combined result for SM Higgs boson in ATLAS


(Univ. of Tokyo)


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Combined result for SM Higgs boson in ATLAS


(Univ. of Tokyo)

Excluded region (at 95% C.L.)

112.7<


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ac up erm op o c →


(Univ. of Tokyo)


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◆ Fermiophobic benchmark scenario

・Higgs does not couple to fermions

but couples only to bosons.

Branching fraction

in Fermiophobic

・Production mechanism

✓ ggF is gone.

scenario

γγ

Search for the Fermiophobic Higgs boson


(Univ. of Tokyo)

BF and VH become dominant.

Br (H→ ) is strongly enhanced

in the low mass range.

(


53/57

Search for the Fermiophobic Higgs boson at ATLAS

◆ Using 1.08fb-1

◆ Analysis entirely based on the SM analysis (SM H→ search)・Two isolated photons with pT>40 and 25GeV

・M( ) reconstruction : Using calo pointing / conv vertex

◆ Some differences w.r.t. SM analysis

・3 categories with pT( ) :

(H→ )

BG is modeled by 2nd order polynomial (Bernstein basis)

pT( )


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M( ) spectrum

◆ low pT( )

(100


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Systematics

Most of the uncertainties are the same as for SM analysis

・Theory : ±9%

±6.5evts (low pT( ))

・BG modeling :

except for :

±2.2evts (middle pT( ))

55

±0.65evts (high pT( ))

・Migration : ±2% between low-pT and high-pT categories

High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura

(Univ. of Tokyo)


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◆ Expected exclusion :

◆ Observed exclusion :

Exclusion limit

Exclusion limit w.r.t the prediction (95%C.L.)

=110-116GeVHm

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・CMS (H→ ) :


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https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults

H→ public results

(1) Search for the Standard Model(SM) H→


(Univ of Tokyo)

(2) Search for the Fermiophobic H→

Documents

Higgs Two Photons ATLAS - Yamamura 12