Observation of the Higgs particle in events and …5.1 Higgs-boson production in association with...

Observation of the Higgs particle in γγ events andsearch for the Higgs particle in Zγ events at ATLAS

LIU Kun

1Laboratoire de Physique Nucleaire et de Hautes Energies (LPNHE)2University of Science and Technology of China (USTC)

June 24th, 2014

LIU Kun (LPNHE&USTC) Thesis defense June 24th , 2014 1 / 72

Outline

Outline

The Standard Model Higgs Boson

Thesis motivation

Experimental Setup

Photon Performance

Search for a Higgs Boson in H → Zγ → ``γ (` = µ, e)

Observation of the Higgs Boson in γγ Events

Summary


The Standard Model Higgs Boson Success of the Standard Model (SM)

Success of the Standard Model (SM), but...

Spin- 12 fermions, grains of matter.

Spin-1 bosons for interactions.Most of the SM predictions havebeen confirmed in experiments.Spontaneous Symmetry Breakingthrough Higgs mechanism:origin of particle masses.The spin-0 Higgs boson is pre-dicted by the Higgs mechanism.The Higgs boson had not beendetected before July 2011.The Higgs boson mass is a freeparameter.


The Standard Model Higgs Boson Constraints on the SM Higgs Boson Mass

Constraints on the SM Higgs boson mass in July 2011

Theoretical bounds: the existence of the Higgs boson leads to thecancellation of the ultraviolet divergence in the scattering amplitudeof W +

L W−L →W +

L W−L , provided the Higgs boson mass is not too

heavy:

mH <

√4π√

2

3GF∼ 700 GeV

Experimental direct limits (95 % CL),in July 2011, from LEP, Tevatron.Exclusion region:mH < 114.4 GeVmH within [156, 177] GeV.


The Standard Model Higgs Boson Constraints on the SM Higgs Boson Mass

Constraints on the SM Higgs boson mass in July 2011

A global fit to electroweak data (from Hfitter group):

Input parameters: W , Z , top masses and widths, cross sections

Best fitted mH = 91+30−23 GeV (without direct limits on mH as inputs)

This constraint favours a low mass value for the SM Higgs boson


The Standard Model Higgs Boson The Higgs Boson Production and Decay

The Higgs boson production and decay in the SM

Five main production processes (ggH,VBF ,WH,ZH, ttH):

q

q

q

q

H

V

V

q

q

q

q

H

V

V

q

q

q

q

H

V V

Fig. 4: Topologies of t-, u-, and s-channel contributions for electroweak Higgs-boson production, qq ! qqH atLO, where q denotes any quark or antiquark and V stands forW and Z boson.

!. The preferred choice, which should be most robust with respect to higher-order corrections, is theso-called GF scheme, where ! is derived from Fermi’s constant GF . The impact of EW and QCDcorrections in the favoured Higgs-mass range between 100 and 200 GeV are of order 5% and negative,and thus as important as the QCD corrections. Photon-induced processes lead to corrections at thepercent level.

Approximate next-to-next-to-leading order (NNLO) QCD corrections to the total inclusive crosssection for VBF have been presented in Ref. [75]. The theoretical predictions are obtained using thestructure-function approach [65]. Upon including the NNLO corrections in QCD for the VBF productionmechanism via the structure-function approach the theoretical uncertainty for this channel, i.e. the scaledependence, reduces from the 5"10% of the NLOQCD and electroweak combined computations [65,70]down to 1"2%. The uncertainties due to parton distributions are estimated to be at the same level.

3.2 Higher-order calculationsIn order to study the NLO corrections to Higgs-boson production in VBF, we have used two existing par-tonic Monte Carlo programs: HAWK and VBFNLO, which we now present. Furthermore we also giveresults of the NNLO QCD calculation based on VBF@NNLO and combine them with the electroweakcorrections obtained from HAWK.

3.2.1 HAWK – NLO QCD and EW correctionsHAWK [69–71] is a Monte Carlo event generator for pp ! H + 2 jets. It includes the completeNLO QCD and electroweak corrections and all weak-boson fusion and quark–antiquark annihilationdiagrams, i.e. t-channel and u-channel diagrams with VBF-like vector-boson exchange and s-channelHiggs-strahlung diagrams with hadronic weak-boson decay. Also, all interferences at LO and NLOare included. If it is supported by the PDF set, contributions from incoming photons, which are atthe level of 1"2%, can be taken into account. Leading heavy-Higgs-boson effects at two-loop orderproportional to G2

F M4H are included according to Refs. [76,77]. While these contributions are negligible

for small Higgs-boson masses, they become important for Higgs-boson masses above 400 GeV. ForMH = 700 GeV they yield +4%, i.e. about half of the total EW corrections. This signals a breakdownof the perturbative expansion, and these contributions can be viewed as an estimate of the theoreticaluncertainty. Contributions of b-quark PDFs and final-state b quarks can be taken into account at LO.While the effect of only initial b quarks is negligible, final-state b quarks can increase the cross sectionby up to 4%. While s-channel diagrams can contribute up to 25% for small Higgs-boson masses in thetotal cross section without cuts, their contribution is below 1% once VBF cuts are applied. Since thes-channel diagrams are actually a contribution to WH and ZH production, they are switched off in thefollowing.

18

2 Gluon-Fusion process2

2.1 Higgs-boson production in gluon–gluon fusionGluon fusion through a heavy-quark loop [6] (see Fig. 1) is the main production mechanism of theStandard Model Higgs boson at hadron colliders. When combined with the decay channels H ! γγ ,H ! WW, and H ! ZZ, this production mechanism is one of the most important for Higgs-bosonsearches and studies over the entire mass range, 100 GeV <" MH

<" 1 TeV, to be investigated at theLHC.

Ht,b

g

g

Fig. 1: Feynman diagram contributing to gg ! H at lowest order.

The dynamics of the gluon-fusion mechanism is controlled by strong interactions. Detailed studiesof the effect of QCD radiative corrections are thus necessary to obtain accurate theoretical predictions.In QCD perturbation theory, the leading order (LO) contribution [6] to the gluon-fusion cross sectionis proportional to !2

s , where !s is the QCD coupling constant. The main contribution arises from thetop quark, due to its large Yukawa coupling to the Higgs boson. The QCD radiative corrections to thisprocess at next-to-leading order (NLO) have been known for some time, both in the large-mt limit [7,8]and maintaining the full top- and bottom-quark mass dependence [9, 10]. They increase the LO crosssection by about 80#100% at the LHC. The exact calculation is very well approximated by the large-mt

limit. When the exact Born cross section with the full dependence on the mass of the top quark is used tonormalize the result, the difference between the exact and the approximated NLO cross sections is onlya few percent. The next-to-next-to-leading order (NNLO) corrections have been computed only in thislimit [11–17], leading to an additional increase of the cross section of about 25%. The NNLO calculationhas been consistently improved by resumming the soft-gluon contributions up to NNLL [18]. The resultleads to an additional increase of the cross section of about 7#9% (6#7%) at

$s = 7 (14) TeV. The

NNLL result is nicely confirmed by the evaluation of the leading soft contributions at N3LO [19–23].Recent years have seen further progress in the computation of radiative corrections and in the

assessment of their uncertainties. The accuracy of the large-mt approximation at NNLO has been stud-ied in Refs. [24–29]. These papers have definitely shown that if the Higgs boson is relatively light(MH

<" 300 GeV), the large-mt approximation works extremely well, to better than 1%. As discussedbelow, these results allow us to formulate accurate theoretical predictions where the top and bottom loopsare treated exactly up to NLO, and the higher-order corrections to the top contribution are treated in thelarge-mt approximation [30].

Considerable work has also been done in the evaluation of electroweak (EW) corrections. Two-loop EW effects are now known [31–35]. They increase the cross section by a factor that stronglydepends on the Higgs-boson mass, changing from +5% for MH = 120 GeV to about #2% for MH =300 GeV [35]. The main uncertainty in the EW analysis comes from the fact that it is not obvious how tocombine them with the large QCD corrections. In the partial factorization scheme of Ref. [35] the EWcorrection applies only to the LO result. In the complete factorization scheme, the EW correction insteadmultiplies the full QCD-corrected cross section. Since QCD corrections are sizeable, this choice has anon-negligible effect on the actual impact of EW corrections in the computation. The computation of thedominant mixed QCD–EW effects due to light quarks [30], performed using an effective-Lagrangian

2M. Grazzini, F. Petriello, J. Qian, F. Stoeckli (eds.); J. Baglio, R. Boughezal and D. de Florian.

4

u/d

d/u

H

WW±

q

q

H

ZZ

t

g

g H

Z

(a) (b) (c)

Fig. 7: (a), (b) LO diagrams for the partonic processes pp ! V H (V = W, Z); (c) diagram contributing to thegg ! HZ channel.

"s = 7 TeV ATLAS expects to exclude a Higgs boson at 95% CL with a cross section equivalent to

about 6 times the SM one [101], while with 5 fb!1 of data and"

s = 8 TeV CMS expects to excludea Higgs boson at 95% CL with a cross section equivalent to about 2 times the SM one [102]. Theseresults are very preliminary and partially rely on analyses which have not been re-optimized for thelower center-of-mass energy.

One of the main challenges of these searches is to control the backgrounds down to a precision ofabout 10% or better in the very specific kinematic region where the signal is expected. Precise differentialpredictions for these backgrounds as provided by theoretical perturbative calculations and parton-showerMonte Carlo generators are therefore crucial. Further studies (e.g. in Ref. [103]) suggest that with datacorresponding to an integrated luminosity of the order of 30 fb!1 the tt background might be extractedfrom data in a signal-free control region, while this seems to be significantly harder for theWbb or Zbbirreducible backgrounds, even in the presence of such a large amount of data.

For all search channels previously mentioned, a precise prediction of the signal cross section andof the kinematic properties of the produced final-state particles is of utmost importance, together witha possibly accurate estimation of the connected systematic uncertainties. The scope of this section is topresent the state-of-the-art inclusive cross sections for the WH and ZH Higgs-boson production modesat different LHC center-of-mass energies and for different possible values of the Higgs-boson mass andtheir connected uncertainties.

4.2 Theoretical frameworkThe inclusive partonic cross section for associated production of a Higgs boson (H) and a weak gaugeboson (V ) can be written as

!(s) =

! s

0dk2 !(V "(k))

d!

dk2(V "(k) ! HV ) + "! , (2)

where"

s is the partonic center-of-mass energy. The first term on the r.h.s. arises from terms where avirtual gauge boson V " with momentum k is produced in a Drell–Yan-like process, which then radiatesa Higgs boson. The factor !(V ") is the total cross section for producing the intermediate vector bosonand is fully analogous to the Drell–Yan expression. The second term on the r.h.s., "!, comprises allremaining contributions. The hadronic cross section is obtained from the partonic expression of Eq. (2)by convoluting it with the parton densities in the usual way.

The LO prediction for pp ! V H (V = W,Z) is based on the Feynman diagrams shown inFig. 7 (a),(b) and leads to a LO cross section of O(G2

F ). Through NLO, the QCD corrections are fullygiven by the NLO QCD corrections to the Drell–Yan cross section !(V ") [104–106]. For V = W, this

29

5 ttH process8

5.1 Higgs-boson production in association with tt pairsHiggs radiation off top quarks qq/gg ! Htt (see Fig. 12) plays a role for light Higgs masses below" 150 GeV at the LHC. The measurement of the ttH production rate can provide relevant informationon the top–Higgs Yukawa coupling. The leading-order (LO) cross section was computed a long timeago [113–117]. These LO results are plagued by large theoretical uncertainties due to the strong de-pendence on the renormalization scale of the strong coupling constant and on the factorization scales ofthe parton density functions inside the proton, respectively. For the LO cross section there are severalpublic codes available, as e.g. HQQ [64, 118], MADGRAPH/MADEVENT [119, 120], MCFM [112], orPYTHIA [121]. The dominant background processes for this signal process are ttbb, ttjj, ttγγ , ttZ,and ttW+W! production depending on the final-state Higgs-boson decay.

q

q

H

t

t

H

g

g

t

t

Fig. 12: Examples of LO Feynman diagrams for the partonic processes qq, gg ! ttH.

The full next-to-leading-order (NLO) QCD corrections to ttH production have been calculated[122–125] resulting in a moderate increase of the total cross section at the LHC by at most " 20%,depending on the value ofMH and on the PDF set used. Indeed, when using CTEQ6.6 the NLO correc-tions are always positive and the K-factor varies between 1.14 and 1.22 for MH = 90, . . . , 300 GeV,while when using MSTW2008 the impact of NLO corrections is much less uniform: NLO correctionscan either increase or decrease the LO cross section by a few percents and result in K-factors between1.05 and 0.98 forMH = 90, . . . , 300 GeV.

The residual scale dependence has decreased from O(50%) to a level of O(10%) at NLO, ifthe renormalization and factorization scales are varied by a factor 2 up- and downwards around thecentral scale choice, thus signalling a significant improvement of the theoretical prediction at NLO.The full NLO results confirm former estimates based on an effective-Higgs approximation [126] whichapproximates Higgs radiation as a fragmentation process in the high-energy limit. The NLO effects onthe relevant parts of final-state particle distribution shapes are of moderate size, i.e. O(10%), so thatformer experimental analyses are not expected to change much due to these results. There is no publicNLO code for the signal process available yet.

5.2 Background processesRecently the NLO QCD corrections to the ttbb production background have been calculated [127–131].By choosing µ2

R = µ2F = mt

!pTbpTb as the central renormalization and factorization scales the NLO

corrections increase the background cross section within the signal region by about 20–30%. The scaledependence is significantly reduced to a level significantly below 30%. The new predictions for the NLOQCD cross sections with the new scale choice µ2

R = µ2F = mt

!pTbpTb are larger than the old LO

predictions with the old scale choice µR = µF = mt + mbb/2 by more than 100% within the typical

8C. Collins-Tooth, C. Neu, L. Reina, M. Spira (eds.); S. Dawson, S. Dean, S. Dittmaier, M. Krämer, C.T. Potter andD. Wackeroth.

36

(a) (b)

(c) (e)(d) [GeV] Hm80 100 200 300 400 1000

H+

X)

[pb

]

→

(pp

σ

210

110

1

10

210

= 8 TeVs

LH

C H

IGG

S X

S W

G 2

012

H (NNLO+NNLL QCD + NLO EW)

→pp

qqH (NNLO QCD + NLO EW)

→pp

WH (NNLO QCD + NLO EW

)

→

pp ZH (NNLO QCD +NLO EW

)

→

pp

ttH (NLO QCD)

→pp

For a light Higgs boson in the SM

H → γγ and H → ZZ → 4` (` = µ, e)clean final states and large sensitivityH → Zγ → ``γ (` = µ, e) and H → µµ:clean final states but small BR

[GeV]Hm80 100 120 140 160 180 200

Hig

gs B

R +

Tota

l U

ncert

410

310

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110

1

LH

C H

IGG

S X

S W

G 2

013

bb

ττ

µµ

cc

gg

γγ γZ

WW

ZZ


Motivation The H → γγ and the H → Zγ → ``γ Decays

The H → γγ and the H → Zγ → ``γ decays in the SM

They are loop-induced decays, dominated by W loop.

Two kinds of backgrounds: irreducible and reducible background

γγ (75%), γ+jet(jet+γ), jet+jet (for H → γγ)``+ γ (82%), ``+jet (for H → Zγ → ``γ)


Motivation The H → γγ and the H → Zγ → ``γ Decays

The H → γγ and the H → Zγ → ``γ decays in the SM

At mH = 125 GeV, σH × BRH→γγ = 40 (50) fb at√

s = 7 (8) TeV,while σH × BRH→Zγ→``γ (`=µ,e) = 1.8 (2.3) fb.

H → γγ and H → Zγ → ``γ decays

clean final statesexcellent photon and lepton energy response resolutionloop-induced decays: sensitive probe for new physics

Reminders on the photon performance

high photon selection efficiency and jet rejection needed to improveS/√

Baccurate measurements of the photon trigger and identificationefficiencies are needed for precise measurements of cross sections andHiggs boson properties.


Experimental Setup The Large Hadron Collider (LHC)

The Large Hadron Collider (LHC)


Experimental Setup The ATLAS Detector

Running parameters in 2011 (2012)

ATLAS recorded 5.1 (21.3) fb−1 of pp collisions at√

s = 7 (8) TeV(left plot)

Mean number of interactions per crossing was 9.1 (20.7) in 2011(2012) (right plot)

Month in YearJan Apr Jul

Oct Jan Apr JulOct

1fb

Tota

l In

teg

rate

d L

um

inosity

0

5

10

15

20

25

30

ATLAS

Preliminary

= 7 TeVs2011,

= 8 TeVs2012,

LHC Delivered

ATLAS Recorded

1 fbDelivered: 5.461 fbRecorded: 5.08

1 fbDelivered: 22.81 fbRecorded: 21.3

Mean Number of Interactions per Crossing

0 5 10 15 20 25 30 35 40 45

/0.1

]1

Re

co

rde

d L

um

ino

sity [

pb

0

20

40

60

80

100

120

140

160

180 Online LuminosityATLAS

> = 20.7µ, <1Ldt = 21.7 fb∫ = 8 TeV, s

> = 9.1µ, <1Ldt = 5.2 fb∫ = 7 TeV, s



The ATLAS detector components



Excellent performance of the ATLAS tracker system

The precision tracking detectorsthe Inner detector (ID) : vertex and track reconstruction, precisiontrack momentum measurementthe Muon Spectrometer (MS) : muon track reconstruction andprecision momentum measurement

Very high vertex and track reconstruction efficiencies

µ

0 5 10 15 20 25 30 35 40 45

Ve

rte

x R

eco

nstr

uctio

n E

ffic

ien

cy

0.98

0.985

0.99

0.995

1

Simulation

tt

µµ→Z

ee→ZATLAS Preliminary



The Electromagnetic Calorimeter (EMC)

A Pb-LAr sampling dete-ctor with accordion-shapedkapton electrodes.3 layers to reconstructγ direction.1st layer segmentationto separate prompt γfrom photon pairs fromπ0, η...A thin LAr presampler(|η| <1.8) to estimatethe energy lost beforethe accordion calorimeter.


Photon performance Photon Reconstruction and Identification

Photon reconstruction

Unconverted photons: clusters not matched to tracks.Converted photons: clusters matched to at least one track(and ambiguity resolution w.r.t. electrons).About half of the photons convert before the EMC.Photon reconstruction efficiencies are very high

[GeV]T

p

20 40 60 80 100 120

Reconstr

uction e

ffic

iency

0.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

ATLAS PreliminarySimulation

All photons

Unconverted photons

Converted photons



Photon/electron isolation

Discriminating γ/e from fake and non-direct γ/e coming from jets.

Calorimeter isolation: collecting energy deposited in a cone ∆R = 0.4around the γ/e object

ET cone40: from all cellsE Topo

T cone40 : from cells belonging to topological clustersTrack isolation: in a cone of ∆R = 0.2 around the γ/e object

pT cone20: scalar sum of pT of tracksnucone20: number of tracks



Photon performance[ATLAS-CONF-2012-123]

Photon identification efficiency measurements, Photon trigger



Photon identification

9 discriminating variables

As example Eratio , ∆E

ratioE

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Entr

ies/0

.02

1

10

210

310

410

510

dataγll→Z

corrected MCγll→Z

ll)+jet corrected MC→Z(

1Ldt=20.3 fb∫=8 TeV, s

γConverted

ATLAS Preliminary

E [MeV]∆

0 1000 2000 3000 4000 5000 6000 7000 8000

En

trie

s/1

60

Me

V

1

10

210

310

410

dataγll→Z

corrected MCγll→Z

ll)+jet corrected MC→Z(

1Ldt=20.3 fb∫=8 TeV, s

γUnconverted

ATLAS Preliminary


Photon performance Photon Identification Efficiency Measurements

Photon identification efficiency measurements

Two identification algorithmscut-based: loose, tight IDneural-network based ID (only used in H → γγ at

√s =7 TeV)

Three data-driven methods:using Final State Radiative (FSR) photons from Z → ``γ decaysmatrix methodelectron extrapolation

[GeV]TE

10 20 30 100 200 1000 20000

0.2

0.4

0.6

0.8

1

datadriven

MCbased DataMC correction on signal MC

Matrix method

Extrapolation

ee→from Z

Z radiative decays



Photons from radiative Z decays

FSR photons from Z → ``γ (` = µ, e)decays, in low ET [10, 100] GeV.Event selection

requirements on lepton qualitym`` within [40, 83] GeVm``γ within [80, 96] GeV

120k photon probes in√

s =8 TeV dataPhoton purity from m``γ fit

∼90% for EγT in [10,15] GeV≥ 98% for EγT > 15 GeV

Residual backgroundssubtracted in 10 < EγT < 15 GeVas systematic uncertainty for higherEγT photons(< 2%).

[GeV]γllm

0 20 40 60 80 100 120 140 160 180 200

[G

eV

]ll

m

0

20

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100

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Eve

nts

0

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300

400

500

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900

1 L dt = 20.3 fb∫ = 8 TeVs

, e)µ (l=γll→Z

ATLAS work in progress

[GeV]γµµm

60 65 70 75 80 85 90 95 100 105 110

]1

[G

eV

γµ

µd

N/d

m

10

210

310

410datafitsignalbackgroundsignal region


1 L dt = 20.3 fb∫Unconverted photon

< 15 GeVT

γ10 < E

0.2)%±P = (93.4



The matrix method

Photon yield (and purity) before/after photon ID can be estimatedusing track isolation to discriminate photons from fakes.

NTpass = NS

pass + NBpass N Iso

pass = εsp × NS

pass + εbp × NB

pass

NTfail = NS

fail + NBfail =⇒ N Iso

fail = εsf × NS

fail + εbf × NB

fail

once track isolation efficiencies εsp, εs

f , εbp and εb

f are known.

Distributions of εsp and εb

p (left), εsf and εb

f (right):Passing tight-ID criteria Failing tight-ID criteria



The matrix method

Photon track isolation efficiencies (εsp, εs

f )

from simulated prompt photon eventsdata/MC difference in Z→ee (1%) included in systematic uncertainty

Fake photon track isolation efficiencies (εbp, εb

f )

measured in a fake photon enriched data sample selected by reversingpart of ID requirements (Fside, w3, ∆E , Eratio)signal leakage in the fake photon control region is subtractednon-closure in di-jet simulation included in systematic uncertainties

Sources of systematic uncertainties:

on the photon track isolation efficiencies (< 1%)on the fake track isolation efficiencies (1%–10%)



Efficiency comparison and combination

Three data-driven results are in good agreement within uncertainties.

Relative systematic uncertainty of combined efficiencyin 2011: 4% to < 1% from 20 to 200 GeVin 2012: 4% to < 1% from 10 to 500 GeV

Pile-up dependence is well modeled in MC.


Photon performance Photon Trigger

Photon medium trigger in 2012 data-taking

Reduce rate (to half for γγ trigger) and pile-up dependenceoptimizing criteria on variables used in loose triggeradding medium cut on extra variable Eratio

Trigger efficiency vs number of primary vertex:loose trigger and medium trigger

Nvtx

0 5 10 15 20 25 30 35 40 45 50

s∈

0.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

EF_g20_loose

Weta2(tighten) + Rhad(loosen) + Reta(loosen)

Weta2(tighten) + Rhad(loosen) + Reta(loosen) + Eratio(loose)

Weta2(tighten) + Rhad(loosen) + Reta(loosen) + Eratio(medium)

Weta2(tighten) + Rhad(loosen) + Reta(loosen) + Eratio(medium++)

Weta2(tighten) + Rhad(loosen) + Reta(loosen) + Eratio(tight)

The default trigger for H → γγ analysis in Run-II


Photon performance Photon Trigger

Photon trigger efficiency measurement

Using photons from radiative Z decays.W.R.T. off-line photon selection criteria.Di-photon trigger efficiencies are the products of the efficiencies forthe two single trigger objects.Trigger efficiency uncertainty is negligible (< 0.5%) compared toothers in the H → γγ analysis.

[GeV]T

γE

0 10 20 30 40 50 60 70 80 90 100

Effic

iency/2

GeV

0

0.2

0.4

0.6

0.8

1

EF_g20_medium

EF_g20_loose



Physics The H → γ and H → Zγ → ``γ Decays

We were ready to search for the Higgs boson


Search for a Higgs Boson in H → Zγ → ``γ decays

Search for a Higgs Boson in

H → Zγ → ``γ (` = µ, e) decays[ATLAS-CONF-2013-009], [Phys. Lett. B 732(2014)8-27]

Using 4.5 (20.3) fb−1 of data at√

s = 7 (8) TeV


Search for a Higgs Boson in H → Zγ → ``γ decays Event Selection

Event selection (I)

Using lepton triggersAt least one primary vertexMuon selection:

pT > 10 GeV, |η| < 2.7muon track qualityimpact parameter:|d0| <1 mm, |z0| < 10 mm

Electron selectionET > 10 GeV, |η| < 2.47identification based on shower shape and ID information|z0| <10 mmoverlap removal: remove the electron if its track is within a ∆R < 0.2around a muon track

[GeV]T

p

0 20 40 60 80 100 120 140

[1

/Ge

V]

Td

N/d

p

0

200

400

600

800

1000

1200

1400

1600

γ

1l

2l

µµ →, Z γ Z→H

= 8 TeVs = 125 GeV, H

m

process = gg fusion


Search for a Higgs Boson in H → Zγ → ``γ decays Event Selection

Event selection (II)

Photon selection

ET > 15 GeV, |η| <2.37∆R`γ > 0.3 and passing tight ID criteria

E TopoTcone40 < 4 GeV

Z → `` reconstruction and selection

two same flavor and opposite sign leptonsthe lepton pair whose m`` is closest to mZ

the reconstructed leptons are matched to trigger objectsm`` > 81.18 GeV

H → Zγ reconstruction and selection

lepton isolation requirementsmuon: pTcone20/pT < 0.15, ETcone20/ET < 0.3electron: pTcone20/ET < 0.15, ETcone20/ET < 0.3|d0|/σd0 < 3.5 (6.5) for muons (electrons)


Search for a Higgs Boson in H → Zγ → ``γ decays Event Categorization

Event categorization

Classify events exploiting the different kinematic properties of signaland background to improve analysis sensitivityTwo variables are used:

|∆ηZγ |, the |η| difference between the photon and Z bosonpTt , the component of the pH

T that is orthogonal to ~pγT − ~pZT

Optimized values of 30 GeV on pTt and of 2.0 on |∆ηZγ | are chosen

[GeV]Tt

p

0 20 40 60 80 100 120 140

/ 5

GeV

Tt

1/N

dN

/dp

410

310

210

110

gg fusion

VBF

WH/ZH

ttH

Data

MCγZ

ee→, Z γ Z→H

= 8 TeVs = 125 GeV, H

m

ATLAS

|γZ

η∆|

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

|γ

Zη

∆1/N

dN

/d|

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

gg fusion

VBF

WH/ZH

ttH

Data

MCγZ

ee→, Z γ Z→H

= 8 TeVs = 125 GeV, H

m

ATLAS

10 categories based on lepton flavour, center-of-mass energy, andprevious variables (in table in later slide).


Search for a Higgs Boson in H → Zγ → ``γ decays Background Composition

Background composition

Expecting 15 signal events with huge backgrounds after selection.

The m``γ distributions: ∼82% (∼ 17%) of Z + γ (Z +jets) events

[GeV]γllm

100 150 200 250 300

Eve

nts

/4 G

eV

0

200

400

600

800

1000

1200

γZ

Z+jets

+ WZttbkg uncertainty

Data

ATLAS

µµ →, Z γ Z→H

1Ldt = 20.3 fb∫ = 8 TeV, s


Search for a Higgs Boson in H → Zγ → ``γ decays Exclusion Limits and p−values

95% C.L. limit (CLs) and p-value

Likelihood-based statistical test :

95% C.L. limit on cross section × BRH→Zγ

p0-values : compatibility of data with background only hypothesis

Unbinned likelihood :

discriminating variable x = m``γ

profile likelihood ratio is used (µ =Nsignal

NSMsignal

): λ(µ) = L(µ,ˆθ(µ))

L(µ,θ)

in each category: Lc (µ,θc |xc ) = e−N′c N ′cNc∏Nc

k=1 Lc (xk |µ,θc )for each event Lc (x |µ,θc ) =Nsignal,c (µ,θnorm

c )Nsignal,c +Nbkg,c

fsignal,c (x |θshapec ) +

Nbkg,c

Nsignal,c +Nbkg,cfbkg,c (x |θbkg

c )

The nuisance parameters (θ) are constrained either with a Log-normaldistribution (affecting signal yields) or a Gaussian distribution.


Search for a Higgs Boson in H → Zγ → ``γ decays Signal Parameterization

Signal parameterization

Expected signal yield from: theoretical production cross section andbranching ratio, selection efficiency from simulation, and measuredluminositySignal model: Crystal Ball + Gaussian function fit from simulation

[GeV]γllm

105 110 115 120 125 130 135 140 145

]1

[G

eV

γlld

N/d

m

0

100

200

300

400

500

600µµ →, Z γ Z→H

= 125 GeVH

m

process = gg fusion

FWHM = 3.77 GeV

= 1.56 GeVCB

σ = 1.331

CBα

= 394 MeVCB

µ∆

= 90.46 %CBf = 3.69 GeV

Gσ



Search for a Higgs Boson in H → Zγ → ``γ decays Background Properties

Background properties

Choice of fit functions and fit range so that significance is maximumwhile potential bias is < 20 % of fitted signal uncertainty

Fit m``γ distribution in [115, 170] GeV on data

[GeV]γllm

120 130 140 150 160 170

Eve

nts

/Ge

V

0

100

200

300

400

500

600

Data

50)×SM

σ=125 GeV, H

(mγZ→H

=7 TeVs, 1Ldt = 4.5 fb∫=8 TeVs, 1Ldt = 20.3 fb∫

ATLAS


Search for a Higgs Boson in H → Zγ → ``γ decays Parametrization

Expected signal and background yields

In [-5, +5] GeV mass window around mH = 125 GeV: backgroundnumber (NB) is extracted in background-only data fit.

√s ` Category Model NS NB ND

NS√NB

FWHM

[TeV] [GeV]8 µ high pTt Exp. of 2nd order pol. 2.3 310 324 0.13 3.88 µ low pTt, low |∆η| 5th order Chebyshev pol. 3.7 1600 1587 0.09 3.88 µ low pTt, high |∆η| 4th order Chebyshev pol. 0.8 600 602 0.03 4.18 e high pTt Exp. of 2nd order pol. 1.9 260 270 0.12 3.98 e low pTt, low |∆η| 5th order Chebyshev pol. 2.9 1300 1304 0.08 4.28 e low pTt, high |∆η| 4th order Chebyshev pol. 0.6 430 421 0.03 4.57 µ high pTt Exponential 0.4 40 40 0.06 3.97 µ low pTt 4th order Chebyshev pol. 0.6 340 335 0.03 3.97 e high pTt Exponential 0.3 25 21 0.06 3.97 e low pTt 4th order Chebyshev pol. 0.5 240 234 0.03 4.0Inclusive 14 5145 5138


Search for a Higgs Boson in H → Zγ → ``γ decays Systematic Uncertainties

Systematic uncertainties at mH =125 GeV

Theory uncertaintiescross section: scale uncertainty(∼8% for ggF ) and PDFs+αs

uncertainty (gg − (qq−)initiated:8(4)%).branching ratio: 9.4%. An extra 5% uncertainty for the missingcontributions from H → ``∗ → ``γ and for the interference withH → Zγ → ``γ.

Main experimental uncertainties in 2012 (2011) analysisluminosity: 2.8 (1.8) %γ ID efficiency : ∼ 3 %e reconstruction+ID efficiency: ∼ 1.5⊕2.5 (2.5⊕1) %e/γ energy resolution: 10.6 (10.0) % for electron channel

2.7 (3.3) % for muon channelµ momentum resolution: 0.5 (1.5) %signal migration in categories: ∼ 3 %

All systematic uncertainties are taken correlated between 7 TeV and 8TeV categories, except the one of luminosity.


Search for a Higgs Boson in H → Zγ → ``γ decays Results

p0

The compatibility between data and the background-only hypothesis

[GeV]Hm

120 125 130 135 140 145 150

0Local p

210

110

1

10

0 observed pγ Z→H

0 expected pγ Z→SM H

= 8 TeVsData 2012, 1

Ldt = 20.3 fb∫ = 7 TeVsData 2011,

1Ldt = 4.5 fb∫

ATLASσ = 1.6 maxZ

= 142.0 GeVHm

σ1

σ2

No significant excess with respect to the background is observed.


Search for a Higgs Boson in H → Zγ → ``γ decays Results

Exclusion limit (CLs)

95% limit on σ(H → Zγ)/σSM(H → Zγ) at mH = 125.5 GeVexpected w/o Higgs boson: 9expected w/ Higgs boson: 10observed : 11

[GeV]Hm

120 125 130 135 140 145 150

)γZ

→(H

SM

σ)/γ

Z→

(Hσ

95

% C

L lim

it o

n

0

5

10

15

20

25

30ObservedExpected

σ 1±σ 2±

= 7 TeVs, 1

Ldt = 4.5 fb∫ = 8 TeVs ,

1 Ldt = 20.3 fb∫

ATLAS

Expected significance with 300 (3000) fb−1 of data at√

s = 14 TeV:2.3 (3.9) standard deviations


Observation of the Higgs Boson in γγ Events

Observation of the Higgs boson in γγ events[Phys. Lett. B716(2012)1-29], [ATLAS-CONF-2013-012]

Using 4.8 (20.7) fb−1 of data at√

s = 7 (8) TeVResults shown at Moriond EW2013


Observation of the Higgs Boson in γγ Events Event Selection and Categorization

Event selection and categorization at√s = 7 TeV

Di-photon selectiondiphoton triggerat least one primary vertex

two photons E(sub)leadingT > (30)40 GeV within |η| < 1.37 or

1.56 < |η| < 2.37tight ID criteriaisolated : pTcone20 < 2.6 GeV and E Topo

Tcone40 < 6 GeVVBF category: two forward jets

|ηjet | < 4.5, pjetT > 25 GeV

jet-vertex-fraction (JVF) > 0.75∆ηjj > 2.8mjj > 400 GeV∆φjj,γγ > 2.6

The remaining events are classified into 9 categories depending onphoton conversion status, pseudorapidity and the di-photon pTt .


Observation of the Higgs Boson in γγ Events Event Selection and Categorization

Event categorization at√s = 8 TeV

One-lepton category-at least one good lepton-veto events with 84< mγe <94 GeV

EmissT category-Emiss

T /σE missT

> 5

-veto events if γ passes e selection

Low-mass two-jet category-60< mjj <110 GeV-|ηjj | < 3.5-|ηjj,γγ | < 1-pTt > 70 GeV

High-mass two-jet categories (VBF)-multivariate technique (BDT) is used-8 discriminating variables:mjj , ηj1, ηj1, ∆ηjj , pTt ,∆φγγ,jj , (ηγγ −

ηj1+ηj2

2), ∆Rγj

min.-tight category: output of BDT ≥ 0.74-loose category: 0.44< BDT <0.74

diphoton selection

Onelepton

ll)H→)H, Z(ν l→W(

significancemiss

TE

)Hνν →)H, Z(ν l→W(

Lowmass twojet

jj)H→ jj)H, Z(→W(

Highmass twojet

VBF

tight

loose

conversionηTt

9 p

ggFggF enriched

VBF enriched

VH enriched

ATLAS Preliminary

γγ →H


Observation of the Higgs Boson in γγ Events Background Composition Studies

Two-dimensional template fit method

The data isolation distribution is fitted with the sum of 2D templates ofbackgrounds (γγ, γj , jγ, jj).

The 2D template of γγ, γj and jγ are 1×1 templates extracted from data

jet template: in jet control sample by reversing tight criteriaγ template: events passing tight criteria after subtracting jets

The jet-jet 2D template is extracted in data by reversing tight criteria on bothleading and subleading objects.

1D projection on leading object isolation of the 2d fit (left) and the estimatedbackground components mγγ distribution (right).

[GeV]isoT,1E

4 2 0 2 4 6 8

Events

/ (

0.2

5 G

eV

)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

γγ

jγ

+jjγj

+jjγj+jγ+γγ

Data

1 Ldt = 20.7 fb∫ = 8 TeV, sData 2012,

> 30 GeV,2γ

T > 40 GeV, E

,1γ

TE

[GeV]γγm

100 110 120 130 140 150 160

]1

[G

eV

γγdN

/dm

0

500

1000

1500

2000

2500

3000

3500

4000γ γ

γ j + j γ

j j

Fit statistical error

Total Error

Data

Data 2012

1 Ldt = 20.7 fb∫ = 8 TeV, s


Observation of the Higgs Boson in γγ Events Signal and Background Modeling

Signal and background modeling

Studies are similar to those of the H → Zγ analysis.In a mass window around mH = 126.5 GeV containing 90% of theexpected signal for

√s = 8 TeV categories.

Category σCB (GeV ) Observed NS NB NS/√

NB Background ModelUnconv. central, low pTt 1.50 911 46.6 881 1.56 Expo-pol 2

Unconv. central, high pTt 1.40 49 7.1 44 1.07 ExponentialUnconv. rest, low pTt 1.74 4611 97.1 4347 1.47 4th order pol.

Unconv. rest, high pTt 1.69 292 14.4 247 0.92 ExponentialConv. central, low pTt 1.68 722 29.8 687 1.14 Expo-pol 2

Conv. central, high pTt 1.54 39 4.6 31 0.83 ExponentialConv. rest, low pTt 2.01 4865 88.0 4657 1.29 4th order pol.

Conv. rest, high pTt 1.87 276 12.9 266 0.79 ExponentialConv. transition 2.52 2554 36.1 2499 0.72 Expo-pol 2

Loose High-mass two-jet 1.71 40 4.8 28 0.91 ExponentialTight High-mass two-jet 1.64 24 7.3 13 2.02 Exponential

Low-mass two-jet 1.62 21 3.0 21 0.65 ExponentialE miss

T significance 1.74 8 1.1 4 0.55 ExponentialOne-lepton 1.75 19 2.6 12 0.75 Exponential

Inclusive 1.77 14025 355.5 13280 3.08 4th order pol.


Observation of the Higgs Boson in γγ Events Signal and Background Modeling

Inclusive diphoton invariant mass mγγ distribution

Inclusive invariant mass distributions of di-photon candidates for thecombined

√s = 7 TeV and

√s = 8 TeV data sample

100 110 120 130 140 150 160

Events

/ 2

GeV

2000

4000

6000

8000

10000

ATLAS Preliminary

γγ→H

1Ldt = 4.8 fb∫ = 7 TeV, s

1Ldt = 20.7 fb∫ = 8 TeV, s

Selected diphoton sample

Data 2011+2012=126.8 GeV)

HSig+Bkg Fit (m

Bkg (4th order polynomial)

[GeV]γγm100 110 120 130 140 150 160E

ve

nts

F

itte

d b

kg

200

100

0

100

200

300

400

500


Observation of the Higgs Boson in γγ Events Systematic Uncertainties in 8 (7) TeV categories

Systematic uncertainties in 8 (7) TeV

Uncertainties on the signal yield (for mH = 126.5 GeV)

theory uncertainties on cross section (∼ 8⊕ 8%) and BR (4.8%)luminosity : 3.6 (1.8) %γ ID⊕isolation : 2.4⊕1 (8.4⊕1) %γ trigger : 0.5 %

Uncertainties on the mass resolution

EMC energy resolution ⊕ [e → γ extrapolation response]: 14→ 23%pile-up effect: 1.5%

Event migration between categories

material mis-modeling : ∼4%Higgs pT mis-modeling: 1→ 10%jet energy scale and resolutionunderlying event modeling: w/o multi-parton interactionmodeling of ∆φγγ,jj and η∗ variablesE miss

T uncertainty


Observation of the Higgs Boson in γγ Events Results in H → γγ Channel

Results in H → γγ channel

Expected and observed p0 at mH =126.5 GeV:

expected : 4.1 σobserved : 7.4 σ

Best-fit mass: mH = 126.6± 0.2(stat)± 0.7(syst) GeV

Inclusive signal strength: µ = 1.65+0.24−0.24(stat)+0.25

−0.18(syst). The excessw.r.t. µ = 1 is at level of 2.3 σ.

[GeV]Hm

110 115 120 125 130 135 140 145 150

0Local p

1410

1210

1010

810

610

410

210

1

210

410

510

σ1σ2σ3

σ4

σ5

σ6

σ7

(category)0

Observed p (category)

0Expected p

(inclusive)0

Observed p (inclusive)

0Expected p

= 7 TeVsData 2011, 1

Ldt = 4.8 fb∫ = 8 TeVsData 2012,

1Ldt = 20.7 fb∫

ATLAS Preliminary

γγ→H

Signal strength0 1 2 3 4 5 6

PreliminaryATLAS

20112012

= 126.8 GeVHm

γγ→H

= 7 TeVs, 1Ldt = 4.8 fb∫

= 8 TeVs, 1Ldt = 20.7 fb∫

Total

Stat.

Syst.

µ

ggH+ttHµ

VBFµ

VHµ


Observation of the Higgs Boson in γγ Events Measurements in Combined Channels

Higgs mass measurement

Channels: H → γγ and H → ZZ ∗ → 4`

mγγH = 125.98± 0.42(stat)± 0.28(syst) GeV

mZZ→4`H = 124.51± 0.52± 0.04(stat)(syst) GeV

Compatibility between the two meaurements:∆mH = 1.47± 0.67(stat)± 0.28(syst) GeV,corresponding to 2 standard deviations with ∆mH = 0.

Combined Higgs boson mass :mH = 125.36± 0.37(stat)± 0.18(syst) GeV


Observation of the Higgs Boson in γγ Events Measurements in Combined Channels

Signal strength (µ) and spin measurements

At mH = 125.5 GeV, thecombined signal strength isµ = 1.30± 0.12(stat)+0.14

−0.11(sys).The compatibility betweenthis measurement and theSM prediction is about 7%.The data favours the SMHiggs boson JP = 0+

w.r.t. 0−, 1±, 2+m.

) µSignal strength (

0.5 0 0.5 1 1.5 2

ATLAS Prelim.

1Ldt = 4.64.8 fb∫ = 7 TeV s

1Ldt = 20.3 fb∫ = 8 TeV s

= 125.5 GeVHm

0.28

0.33+

= 1.57µ

γγ →H

0.12

0.17+

0.18

0.24+

0.22

0.23+

0.35

0.40+

= 1.44µ 4l→ ZZ* →H

0.10

0.17+

0.13

0.20+

0.32

0.35+

0.29

0.32+

= 1.00µνlν l→ WW* →H

0.08

0.16+

0.19

0.24+

0.21

0.21+

0.20

0.21+

= 1.35µ, ZZ*, WW*γγ→H

Combined

0.11

0.13+

0.14

0.16+

0.14

0.14+

0.6

0.7+

= 0.2µb b→W,Z H

<0.1

0.4±

0.5±

0.4

0.5+

= 1.4µ

(8 TeV data only) ττ →H

0.1

0.2+

0.3

0.4+

0.3

0.3+

0.32

0.36+

= 1.09µττ, bb→H

Combined

0.04

0.08+

0.21

0.27+

0.24

0.24+

0.17

0.18+

= 1.30µCombined

0.08

0.10+

0.11

0.14+

0.12

0.12+

Total uncertainty

µ on σ 1±

(stat.)σ

)theorysys inc.(σ

(theory)σ


Summary Summary

Summary

Photon trigger optimization and efficiency measurementcriteria optimization to reduce trigger rate and pile-up dependencethe trigger efficiency uncertainty in the H → γγ analysis is reduced tobe less than 0.5%.

Photon identification efficiency measurementpure photon probes from radiative Z decaysmatrix method: measure efficiency for photons in wide ET rangeprovides accurate efficiency value, reducing uncertainty from 10%→ 1%

Search for a Higgs boson in H → Zγ → ``γno significant excess with respect to the background is observed95% C.L. limit on the σH→Zγ is 11 times the SM expectation

Observation of the Higgs Boson in γγ Events7.4 σ observation of the Higgs particlemH = 126.6± 0.2(stat)± 0.7(syst) GeVinclusive signal strength: µ = 1.65+0.24

−0.24(stat)+0.25−0.18(syst) (corresponding

to 2.3 standard deviations from 1)


Summary Summary

Publications

Measurements of the photon identification efficiency with the ATLASdetector using 4.9 fb−1 of pp collision data collected in 2011.ATLAS-CONF-2012-123.

Observation of a new particle in the search for the Standard Model Higgsboson with the ATLAS detector at the LHC. Phys. Lett. B 716 (2012) 1-29.

Measurements of the properties of the Higgs-like boson in the two photondecay channel with the ATLAS detector using 25 fb−1 of proton-protoncollision data. ATLAS-CONF-2013-012.

Measurement of isolated-photon pair production in pp collisions at√s = 7TeV with the ATLAS detector. JHEP01(2013)086.

Search for the Standard Model Higgs boson in the H → Zγ decay mode withpp collisions

√s = 7 and 8 TeV. ATLAS-CONF-2013-009.

Search for Higgs boson decays to a photon and a Z boson in pp collisions at√s= 7 and 8 TeV with the ATLAS detector. Phys. Lett. B 732 (2014) 8-27.


http://cds.cern.ch/record/1473426

http://dx.doi.org/10.1016/j.physletb.2012.08.020

http://cds.cern.ch/record/1523698

http://dx.doi.org/10.1007/JHEP01(2013)086

https://cds.cern.ch/record/1523683

http://dx.doi.org/10.1016/j.physletb.2014.03.015

Backup

Backup


Backup Spontaneous Symmetry Breaking

Spontaneous symmetry breaking of a global symmetry

The Lagrangian obeys the global symmetry (φ = (φ1 + iφ2)/√

2):

L = ∂µφ∗∂µφ− V (φ)

V (φ) = µ2φ∗φ+ λ(φ∗φ)2

Ground state: minimizing the potentialµ2 > 0, λ > 0 : φ = φ∗ = 0,

µ2 < 0, λ > 0: |φ|2 = −µ2

2λ≡ υ

The Lagrangian under small oscillationaround a chose vacuum state(φ1 = υ, φ2 = 0):

η = φ1 − υ, ξ = φ2

L =1

2(∂µη)2 − (λυ2)η2︸︷︷︸

massive scalar particle η

+1

2(∂µξ)2 + 0 ∗ ξ2︸︷︷︸

massless scalar particle ξ

+ · · ·︸︷︷︸higher order terms

One massive particle (η) and one massless particle (ξ) are generated.LIU Kun (LPNHE&USTC) Thesis defense June 24th , 2014 51 / 72

Backup Spontaneous Symmetry Breaking

Spontaneous symmetry breaking of a local symmetry

To keep the Lagrangian invariant under local U(1) phase transformation:

∂µ → Dµ = ∂µ + iqAµ

Aµ(x) → A′µ(x) = Aµ(x)− 1

q∂µθ(x)

The Lagrangian near the vacuum state (h = φ1 − υ, φ2 = 0):

φ =1√2

(υ + h)

L =1

2(∂µh)2 − λυ2h2︸︷︷︸

massive scalar particle h

+1

2e2υ2A2

µ︸︷︷︸gauge field with mass

+ e2υA2µh +

1

2e2A2

µh2︸︷︷︸Higgs and gauge fields interaction

− λυh3 − 1

4λh4︸︷︷︸

Higgs self-interactions

This mechanism gives rise to a mass for the gauge boson Aµ andintroduces a new real scalar field (h) with mass, interaction terms betweenthe gauge boson and h, and the self-interaction terms of the h field.


Backup The Higgs Mechanism

The Higgs mechanism - gauge bosons masses

A doublet of complex scalar fields: φ =(

φ†

φ0

)= 1√

2

(φ1 + iφ2

φ3 + iφ4

)

Symmetry group SU(2)L ⊗ U(1)Y and the transformation laws:

U(1)Y : φ→ φ′ = e−iI .θ(x)φ, SU(2)L : φ→ φ′ = e−i τ .θ(x)/2φ

∂µ → Dµ = ∂µ +ig

2τWµ +

ig ′

2Bµ

The kinetic density (Dµφ)†Dµφ =υ2

8[g2(W +)2 + g2(W−)2 + (g2 + g ′2)Z 2

µ + 0 ∗ A2µ]

leadsW +µ =

1√2

(W 1µ − iW 2

µ)

W−µ =1√2

(W 1µ + iW 2

µ)

Zµ =1√

g2 + g ′2(gW 3

µ − g ′Bµ)

Aµ =1√

g2 + g ′2(g ′W 3

µ + gBµ)

MW + = MW− =1

2υg

MZ =υ

2

√g2 + g ′2

Mγ = 0

MW

MZ=

g ′√g2 + g ′2

≡ cos(θw )

mh =√

2λυ2


Backup The Higgs Mechanism

The Higgs mechanism - fermions masses

Similarly, after spontaneous SU(2)L ⊗ U(1)Y symmetry breaking, theLagrangian of fermion mass term (taken electron e as an example):

L = −λe1√2

[(ψνe ,L, ψe,L

)L

(0

υ + h

)ψe,R + ψe,R

(0, υ + h

)( ψνe ,L

ψe,L

)]

=−λe (υ + h)√

2(ψe,Lψe,R + ψe,Rψe,L)

= −λeυ√2ψeψe︸︷︷︸

electron mass term

− λe√2

hψeψe︸︷︷︸electron-higgs coupling term

The electron mass : me = λeυ√2

The electron coupling to the Higgs boson, λe√2

= meυ , is proportional

to its mass.


Backup Constraints on the SM Higgs Boson Mass

Constraints on the SM Higgs boson mass

Theoretical bounds: the existence of the Higgs boson leads to thecancellation of the ultraviolet divergence in the scattering amplitudeof W +

L W−L →W +

L W−L , provided the Higgs boson mass is not too

heavy:

mH <

√4π√

2

3GF∼ 700 GeV

Experimental direct limits (95 % CL),til June of 2012, experiments fromLEP, Tevatron and LHC.Exclusion region:mH < 117.5 GeV,mH > 128 GeV,mH within [118.5, 122.5] GeV.

1

10

100 110 120 130 140 150 160 170 180 190 200

1

10

mH (GeV/c2)

95

% C

L L

imit

/SM

Tevatron Run II Preliminary, L ≤ 10.0 fb-1

Observed

Expected w/o Higgs

±1 s.d. Expected

±2 s.d. Expected

LE

P E

xclu

sio

n

Tevatron

+ATLAS+CMS

Exclusion

SM=1

Te

va

tro

n +

LE

P E

xc

lus

ion

CM

S E

xc

lus

ion

AT

LA

S E

xc

lus

ion

AT

LA

S E

xc

lus

ion

LE

P+

AT

LA

S E

xc

lus

ion

ATLAS+CMS

Exclusion

ATLAS+CMS

Exclusion

June 2012


Backup Constraints on the SM Higgs Boson Mass

Constraints on the SM Higgs boson mass

Besides the direct limits, Hfitter group provides constraints from a globalfit on the electroweak parameters.

Input parameters : W , Z , top masses and widths, cross sections

Best fitted mH = 94+25−22 GeV [1.3 σ deviation from the ATLAS and

CMS measurements].

Using the measured mH as input, the fitted MW and mt are in goodagreements with their experimental measurements.


Backup Photon Performance

Photon identification (9 discriminating variables)



FSR photons from radiative Z decays

ET spectra of FSR photon candidates selected from Z → ``γ decaysin data at

√s = 8 TeV.

[GeV]γTE

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

[1/G

eV

]T

/dE

γdN

110

1

10

210

310

410

γData 2012 Unconverted

= 8 TeVs , 1

Ldt = 20.7 fb∫

γ ll→Z

γµµ →Z

γ ee→Z

ATLAS Preliminary

[GeV]γTE

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

[1/G

eV

]T

/dE

γdN

110

1

10

210

310

410

γData 2012 Converted

= 8 TeVs , 1

Ldt = 20.7 fb∫

γ ll→Z

γµµ →Z

γ ee→Z

ATLAS Preliminary



Electron extrapolation

Exploiting the similarity of EM showers of electrons and photons.Mapping electron shower shapesto photon ones using Smirnovtransformation from MC samples.Electrons from Z → ee (9000k).Systematic uncertainties:

difference of e/γ EM showercorrelations and kinematics:< 2%,distorted material: < 4%,residual background inelectron sample < 1 %.

φR

0.4 0.5 0.6 0.7 0.8 0.9 1

pd

f

410

310

210

110 0.6≤| η 30 GeV and 0.1 < |≤

T25 GeV < E

electrons

photons

ATLAS Preliminary

Simulation

φR

0.4 0.5 0.6 0.7 0.8 0.9 1

CD

F

0

0.2

0.4

0.6

0.8

1

ATLAS Preliminary

Simulation

electrons

photons

electrons

0.4 0.5 0.6 0.7 0.8 0.9 1

ph

oto

ns

0.4

0.5

0.6

0.7

0.8

0.9

1

Smirnov Transform

ATLAS Preliminary

Simulation

φR

0.4 0.5 0.6 0.7 0.8 0.9 1

pd

f

410

310

210

110 Transformed electrons

photons

ATLAS Preliminary

Simulation



Pile up dependence of photon identification efficiency

Using photons from radiaitive Z decays

Photon tight-ID efficiency vs Number of primary vertex

0 5 10 15 20 25

IDε

0.5

0.55

0.6

0.65

0.7

0.75

0.8

γUnconverted

= 8 TeVs , -1

Ldt = 20.7 fb∫ data-drivenγZ->ll

simulationγZ->ll

ATLAS Internal

PVN0 2 4 6 8 10 12 14 16 18 20 22 24

Dat

aε

/ M

Cε 0.96

0.981

1.021.04 0 5 10 15 20 25

IDε

0.5

0.55

0.6

0.65

0.7

0.75

0.8 = 8 TeVs ,

-1 Ldt = 20.7 fb∫

γConverted

data-drivenγZ->ll

simulationγZ->ll

ATLAS Internal

PVN0 2 4 6 8 10 12 14 16 18 20 22 24

Dat

aε

/ M

Cε 0.96

0.981

1.021.04


Backup Search for a Higgs Boson in H → Zγ → ``γ (` = µ, e)

Improvements from corrections

Improvements on signal resolution: together with the identifiedprimary vertex, collinear FSR photons correction and Z massconstraints

[GeV]γµµm

105 110 115 120 125 130 135 140 145

]1

[G

eV

γµ

µ1/N

dN

/dm

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

ATLAS Simulation

µµ→, ZγZ→H→gg

=8 TeVs=125 GeV, Hm

γµµraw m

FWHM = 4.82 GeV

γµµcorrected m

FWHM = 3.80 GeV

[GeV]γeem

105 110 115 120 125 130 135 140 145]

1 [G

eV

γe

e1/N

dN

/dm

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

ATLAS Simulation

ee→, ZγZ→H→gg

=8 TeVs=125 GeV, Hm

γeeraw m

FWHM = 5.47 GeV

γeecorrected m

FWHM = 4.10 GeV



Decomposition of the expected signal in H → Zγ → ``γ

Decomposition of the expected signal from various productionprocesses at mH = 125.5 GeV

signal composition (%)

0 10 20 30 40 50 60 70 80 90 100

γZη∆ high T

tLow p

γZη∆ low T

tLow p

T

tHigh p

Inclusive

ggF VBF WH ZH ttHATLAS Simulation µµ→, ZγZ→H

= 8 TeVs = 125 GeVHm


0 10 20 30 40 50 60 70 80 90 100

γZη∆ high T

tLow p

γZη∆ low T

tLow p

T

tHigh p

Inclusive

ggF VBF WH ZH ttHATLAS Simulation ee→, ZγZ→H



0 10 20 30 40 50 60 70 80 90 100

T

tLow p

T

tHigh p

Inclusive

ggF VBF WH ZH ttHATLAS Simulation µµ→, ZγZ→H



0 10 20 30 40 50 60 70 80 90 100

T

tLow p

T

tHigh p

Inclusive

ggF VBF WH ZH ttHATLAS Simulation ee→, ZγZ→H




Exclusion limit (CLs)

95% CL limits on the production cross section of a Higgs bosondecaying to Zγ

[GeV]Hm

120 125 130 135 140 145 150

) [p

b]

γZ

→ x

BR

(Hσ

95

% C

L lim

it o

n

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2ObservedExpected

σ 1±σ 2±

SM cross section

25 x SM cross section

= 7 TeVs, 1

Ldt = 4.5 fb∫

ATLAS

[GeV]Hm

120 125 130 135 140 145 150

) [p

b]

γZ

→ x

BR

(Hσ

95

% C

L lim

it o

n

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1ObservedExpected

σ 1±σ 2±

SM cross section

10 x SM cross section

= 8 TeVs , 1

Ldt = 20.3 fb∫

ATLAS



CMS H → Zγ → ``γ (` = µ, e) analysis

5.0 (19.6) fb−1 data at√

s = 7 (8) TeV.

Events are classified into 11 categories according to the η`1, η`2, ηγ ,photon conversions and requirements for VBF events.

The observed and expected limits for m``γ = 125 GeV are within oneorder of magnitude of the SM prediction.



Interplay between H → Zγ and H → γγ channels

In the inert Higgs Doublet Model (IDM), decay rates of H → γγ(Rγγ) and H → Zγ (RZγ) normalized by their respective SMexpectations:

The continuous line represents the rates are enhanced or suppressed bythe same parameters associated with charged scalarThe straight line below (1,1) is for the case that rates are damped by abig common constant from invisible decay channels where the Higgsboson decays to inert scalars (dark scalars or D-scalars).


Backup Observation of the Higgs Boson in γγ Events

Decomposition of the expected signal in H → γγ

Decomposition of the expected signal from various productionprocesses at mH = 126.5 GeV for

√s = 8 TeV

signal composition (%)0 10 20 30 40 50 60 70 80 90 100

Onelepton

significancemiss

TE

Lowmass twojet

Tight highmass twojet

Loose highmass twojetConv. transition

TtConv. rest high p

TtConv. rest low p

TtConv. central high p

TtConv. central low p

TtUnconv. rest high p

TtUnconv. rest low p

TtUnconv. central high p

TtUnconv. central low p

Inclusive

ggF VBF WH ZH ttHATLAS γγ →Preliminary (simulation) H



Systematic uncertainties in 8 (7) TeV categories

Uncertainties on the mass measurement (only for mass measurement)

electron energy scale: 0.3%material effect (e → γ): 0.3%presampler energy scale: 0.1%non-linearities of the EM calorimeter electronics: 0.15%corrections for later energy leakage: 0.1%fraction of photon conversion: 0.13%relative calibration of the first and second sampling of the EMC: 0.2%photon direction: 0.03%background modeling: 0.1%

In total: 0.45%⊕0.32%, corresponding to 0.6 GeV⊕0.4 GeV.



Changes in mass measurement

Previous combined results (July 2013): mH = 125.5± 0.2+0.5−0.6 GeV

(2.4 σ tension: ∆m = 2.3+0.6−0.7 ± 0.6 GeV)

Changes in new measurementsnew MVA-based e/γ calibration

10% improvement on H → γγ mass resolutionmass shift expectation: −0.45± 0.35 GeV

improved e, γ and µ energy scale uncertainties

larger clean control samples: 5.5M Z → ee, 4.3M Z → µµ, 34MW → eν, 0.2M Jψ → ee and 0.2M Z → ``γ17% to 55% (depending on category) improvement for H → γγ channel10% improvement for H → ZZ channel

H → γγ: optimized event categorization for mass measurement (20%improvement on σmH

w.r.t inclusive analysis)H → ZZ : using 2D likelihood fit instead of using 1D fit(8%improvement on σmH

)



Limit on the Higgs width in ATLAS

Observed (expected) limits from H → γγ and H → ZZ ∗ analysis are< 2.5 (6.2) and 5.0 (6.2) GeV.

Higgs width from off-shell Higgs decay (ΓH/ΓSMH ) using H → 4` and

H → ``νν

combined observed (expected) limit: µoff−shell < 6.7 (7.3) at 95 % C.L.using the H → 4` µon−shell : ΓH/ΓSM

H < 4.1 at 95% C.L.



The spin and parity quantum numbers

The JP = 0+ hypothesis of theHiggs boson has been comparedto various hypotheses in threedi-boson decay channels

H → γγ: JP = 0+, 2+

H → ZZ → 4`: JP = 0±, 1±, 2+

H →WW → eνµν/µνeν:JP = 0±, 2+

The data favours the JP = 0+

JP = 0− is rejected at 97.8% C.L.JP = 1± is rejected at 99.7% C.L.JP = 2+ is rejected at 99.9% C.L.

= 0 PJ + = 1 PJ = 1 PJ m + = 2 PJ

ATLAS

4l→ ZZ* →H 1Ldt = 4.6 fb∫ = 7 TeV s

1Ldt = 20.7 fb∫ = 8 TeV s

γγ →H 1Ldt = 20.7 fb∫ = 8 TeV s

νeνµ/νµν e→ WW* →H 1Ldt = 20.7 fb∫ = 8 TeV s

σ1

σ2

σ3

σ4

Data

expectedsCL + = 0 Passuming J

σ 1 ±

) a

lt

P (

Js

CL

610

510

410

310

210

110

1



Latest results from other channels at mH = 125 GeV

H → τ+τ−(3 channels: lep-lep, lep-had, had-had): the observed(expected) significance corresponds to 4.1 (3.2) σ, µ = 1.4+0.5

−0.4.

H → µµ(only 8 TeV data): the observed (expected) limit is 9.8 (8.2).

ttH(→ bb): the observed (expected) limit is 4.1 (2.6).

VH → bb: the observed (expected in the absence of signal) is 1.4(1.3).



Expectation in Run-II

Relative uncertainty on the total signal strength µ for mH = 125 GeV

µ/µ∆0 0.2 0.4

(comb.)

(incl.)

(comb.)

(comb.)

(VBFlike)

(comb.)

ATLAS Simulation Preliminary

= 14 TeV:s 1Ldt=300 fb∫ ; 1Ldt=3000 fb∫

µµ→H

ττ→H

ZZ→H

WW→H

γ Z→H

γγ→H

µµ→H

ττ→H

ZZ→H

WW→H

γ Z→H

γγ→H

1.5→



Crystal Ball function (from WIKIPEDIA)


Observation of the Higgs particle in events and …5.1 Higgs-boson production in association with...

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