search of higgs to invisible decays with the atlas detector · in atlas, the search is performed in...

Search of Higgs to invisible decayswith the ATLAS detector

Stefano Rosati

INFN Roma

5 July 2014

On behalf of the ATLAS Collaboration

Stefano Rosati (INFN Roma) ICHEP 2014 - Valencia 5 July 2014 1 / 15

Motivation

Various extensions of the Standard Model (SM) allow Higgs bosondecays to a pair of stable or long-lived particles

Observation of a large Branching Ratio to invisible decays can meanevidence of New Physics BSM

For example, particles with very low interaction with SM particles, e.g.Dark Matter through the so-called Higgs-portal model

LHC data collected by the ATLAS experiment can be used to constrainthe BR of the Higgs boson @125.5 GeV to invisible particles

Limits can also be set on σ·BR to invisible particles of any additionalHiggs boson over a wide mass range


Introduction and outlineIn ATLAS, the search is performed in Higgsassociated production with a Z decaying toleptons, Z→ll (l=e,µ).Signature is a pair of isolated leptons withmass close to the Z, plus missingtransverse energyResults based on the full LHC Run-1dataset of 4.5 fb−1 at 7 TeV and 20.3 fb−1

at 8 TeVPhys. Rev. Lett. 112, 201802 (2014)

q

q

ZH χ

χ

Z

ℓ−

ℓ+

Outline of the talkEvent selectionBackgrounds and main systematic errorsLimits on the cross section and on the invisible decayDark Matter interpretation in the Higgs-portal modelIndirect limits


Event Selection - 1Di-lepton invariant mass consistent with a Z decay

I 76 < mll < 106 GeV

Missing ET >90 GeV to remove most of the Z+jets backgroundCorrelation between azimuthal angles of Emiss

T and track-based missingmomentum Pmiss

T :I Avoids Emiss

T from misreconstructed calo energy, pileupI ∆φ(Emiss

T ,PmissT ) <0.2

Eve

nts

1

10

210

310

410

510

610

710

810

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

0 50 100 150 200 250 300 350 400 450 500Da

ta / M

C

0.5

1

1.5

/8E

ve

nts

/

1

10

210

310

410

510

610

710

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

ϕ

0 0.5 1 1.5 2 2.5 3Da

ta / M

C

0

1

2


Event Selection - 2Require the Z momentum to balance the Emiss

TI ∆φ(pll

T,EmissT ) >2.6

Cut on the fractional pT differenceI |Emiss

T − pllT|/pll

T <0.2Exploit the boost of the Z

I ∆φ(l , l) <1.7Veto jets with pT>25 GeV and |η| <2.5

I Mostly against t t̄ and Z+jets

/8E

ve

nts

/

1

10

210

310

410

510

610

710

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

ϕ ℓℓ

0 0.5 1 1.5 2 2.5 3Da

ta / M

C

0.5

1

1.5

/16

Eve

nts

/

10

210

310

410

510

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

ϕ ℓ ℓ

0 0.5 1 1.5 2 2.5 3Da

ta / M

C

0.5

1

1.5


BackgroundsDi-boson processes (ZZ, WZ) dominate the backgrounds after theselection

I Determined from NLO MC, validated in control regionsInclusive Z→ ee, Z→ µµ estimated in three sideband regions withreverted ∆φ(Emiss

T ,PmissT ) and fractional pT difference cuts

Other backgrounds with an isolated same-flavour lepton pair(WW,tt,Wt,Z→ ττ )

I Estimated in a control region of events with an eµ pair

0 50 100 150 200 250 300 350

Eve

nts

/ 20

GeV

1

210

410

610

810

1010

1210 ATLAS-1 L dt = 20.3 fb∫=8 TeV, s

ZH → ℓℓ + inv.

Data

µµee, → Z

Other BG

Top quark

WW

WZ → ℓνℓℓ (incl.τ)

ZZ → ℓℓνν (incl.τ)

ZH → ℓℓ + inv., BR(H → inv.) = 1

Signal Region∆ϕ(E

missT , p

missT ) < 0.2, |E

missT − p

ℓℓT | /p

ℓℓT < 0.2

[GeV]missTE

0 50 100 150 200 250 300 350

Dat

a / M

C

0.5

1

1.50 50 100 150 200 250

Eve

nts

/ 20

GeV

1

210

410

610

810

1010

1210 ATLAS-1 L dt = 20.3 fb∫=8 TeV, s


Data

µµee, → Z

Other BG

Top quark

WW

WZ → ℓνℓℓ (incl.τ)

ZZ → ℓℓνν (incl.τ)


Sideband Region∆ϕ(E

missT , p

missT ) > 0.2, |E

missT − p

ℓℓT | /p

ℓℓT > 0.2

[GeV]missTE

0 50 100 150 200 250

Dat

a / M

C

2

4


Selection results and systematicsNumber of expected and observed events

For the signal, the momentum of the reconstructed Zboson is expected to be balanced by the momentum of theinvisibly decaying Higgs boson. Therefore the azimuthalseparation between the dilepton system, where the magni-tude of its transverse momentum is defined as pll

T , and theEmissT , !!!pll

T ; EmissT ", is required to be greater than 2.6. The

boost of the Z boson causes the decay leptons to be producedwith a small opening angle. The azimuthal opening angle ofthe two leptons, !!!l;l", is thus required to be less than1.7. Furthermore pll

T and EmissT are expected to be similar.

Therefore the fractional pT difference, defined asjEmiss

T ! pllT j=pll

T , is required to be less than 0.2. Finally,for the majority of the signal no additional high-pT jets areexpected to be observed in the events, while for the back-ground from boosted Z bosons and from tt̄ pairs one or morejets are expected. Thus, events are required to have noreconstructed jets with pT > 25 GeV and j"j < 2.5.After the selection requirements, the dominant back-

ground is SM ZZ production followed by SM WZproduction, as shown in Table I. These backgrounds aresimulated using MC samples normalized to NLO crosssections. The simulation of WZ events is validated bycomparing them to data events in which the third-leptonveto is replaced by an explicit third-lepton requirement.The theoretical prediction of the ZZ production is inagreement with the ATLAS cross-section measurementat

!!!s

p# 7 TeV [50].

Background contributions from events with a genuineisolated lepton pair, not originating from a Z ! ee or Z !## decay (WW, tt̄, Wt, and Z ! $$), are estimated byexploiting the flavor symmetry in the dilepton final state ofthese processes. Distributions for events with an e# pair,appropriately scaled to account for differences in electronand muon reconstruction efficiencies, can be used toestimate this background in the electron and muon chan-nels. The difference between the efficiencies for electronsand muons is estimated using the square root of the ratio ofthe numbers of dimuon and dielectron events in data withinthe mll window. Events in the e# control region notoriginating from WW, tt̄, Wt, or Z ! $$ backgrounds aresubtracted using simulated samples. Important sources of

systematic uncertainty are variations in the correction factorfor the efficiencies for electrons and muons and uncertain-ties in the simulated samples used for the subtraction. Thecombined systematic uncertainty is 23% for both the 7 and8 TeV data. The estimated background from these sourcesis consistent with the expectation from the simulation.The background from inclusive Z ! ee and Z ! ##

production in the signal region is estimated from the back-ground in three sideband regions [51].These sideband regionsare formed by considering events failing one or both of thenominal selection requirements applied to !!!Emiss

T ; pmissT "

and the fractional pT difference. Contributions from non-Zbackgrounds in the sideband regions are subtracted. Theimpact from a correlation between the above two variables isdetermined from the simulation and a correction, of at most7%, is applied to account for it. Themain uncertainties are dueto variations in this correction and differences in the shape ofthe Emiss

T distribution in the control regions. The overallsystematic uncertainty is 52% in the 7 TeV data and 59% inthe 8 TeV data.The small background from events with only one genuine

isolated lepton (inclusive W, single-lepton top pairs andsingle top production) or from multijet events is estimatedfrom data using control samples, selected by requiring twolepton candidates of which at least one fails the full leptonselection criteria. These samples are scaled with a measuredpT-dependent factor, determined from data as described inRef. [52]. Systematic uncertainties are determined followingthe procedures used in Ref. [52], yielding an uncertainty of40% in the 7 TeV data and 21% in the 8 TeV data.Systematic uncertainties on the signal and the SM ZZ

and WZ backgrounds are derived from the luminosityuncertainty, the propagation of reconstructed object uncer-tainties, and from theoretical uncertainties on the produc-tion cross sections. The luminosity uncertainty is 1.8% forthe 7 TeV data-taking period and 2.8% for the 8 TeV data-taking period [53].Lepton trigger and identification efficiencies as well as

the energy scale and resolution are determined from datausing large samples of Z events. After appropriate correc-tions to the simulation, uncertainties are propagated to the

TABLE I. Number of events observed in data and expected from the signal and from each background source forthe 7 and 8 TeV data-taking periods. Uncertainties on the signal and background expectations are presented withstatistical uncertainties first and systematic uncertainties second.

Data period 2011 (7 TeV) 2012 (8 TeV)

ZZ ! ll%% 20.0$ 0.7$ 1.6 91$ 1$ 7WZ ! l%ll 4.8$ 0.3$ 0.5 26$ 1$ 3Dileptonic tt̄, Wt, WW, Z ! $$ 0.5$ 0.4$ 0.1 20$ 3$ 5Z ! ee, Z ! ## 0.13$ 0.12$ 0.07 0.9$ 0.3$ 0.5W % jets, multijet, semileptonic top 0.020$ 0.005$ 0.008 0.29$ 0.02$ 0.06Total background 25.4$ 0.8$ 1.7 138$ 4$ 9Signal (mH # 125.5 GeV, &ZH;SM, BR!H ! inv:" # 1) 8.9$ 0.1$ 0.5 44$ 1$ 3Observed 28 152

PRL 112, 201802 (2014) P HY S I CA L R EV I EW LE T T ER Sweek ending23 MAY 2014

201802-3Main systematics uncertainties:I Luminosity uncertainty (1.8-2.8%)I Lepton trigger and reconstruction uncertainties (1.0-1.5%)I Pileup uncertainty on Emiss

T (1-2%)I Jet energy scale and resolution (3-6%)I Total uncertainties on ZZ/WZ backgrounds (8-13%)I Theoretical uncertainties on ZH cross section (3.6-5.7%) and Higgs pT boost

(1.9%)


Limits settingsThe Emiss

T distribution after the full selection is used to extract the limits onthe cross section times Branching Ratio to invisible

I Use a Maximum Likelihood fit following the CLS formalism with profilelikelihood

I Consider MH in the range 110-400 GeV

Systematics as nuisance parameters constrained by Gaussiandistributions

Data vs MC @ 7 TeV Data vs MC @ 8 TeV

100 150 200 250 300 350 400 450

Eve

nts

/ 30

GeV

-210

-110

1

10

210

310 ATLAS-1 L dt = 4.5 fb∫ = 7 TeV, s


Data

ZZ → ℓℓνν (i�cl. τ)

WZ → ℓνℓℓ ( i�cl. τ)

WW, dilep. ��̄, W�, Z → ττ

µµee, → Z

W + jets, multijet, semilep. top


[GeV]missTE

100 150 200 250 300 350 400 450Dat

a / E

xpec

ted

1

2

3 100 150 200 250 300 350 400 450

Eve

nts

/ 30

GeV

1

10

210

310 ATLAS-1 L dt = 20.3 fb∫ = 8 TeV, s


Data

ZZ → ℓℓνν (incl. τ)

WZ → ℓνℓℓ (incl. τ)

WW, dilep. ��̄, W�, Z → ττ

µµee, → Z

W + jets, multijet, semilep. top


[GeV]missTE

100 150 200 250 300 350 400 450Dat

a / E

xpec

ted

0.51

1.52


ResultsLimits on the σ·BR to invisible particles are given for MH in the range110-400 GeV

I In this case, no assumption on the production cross sectionI No significant deviation from the SM expected limit is observed

Assuming the predicted SM ZH production cross section, with MH=125.5GeV, an upper limit can be set on the BR

I Observed limit at 95% C.L. is 75%I Expected in absence of invisible decays is 62%

[GeV]Hm

150 200 250 300 350 400

inv.

) [fb

]→

H B

R(

× Z

Hσ

0

100

200

300

400

500

600

,SMZHσ

Observed 95% CL limit

Expected 95% CL limit

σ1±

σ2±

ATLAS


-1 L dt = 4.5 fb∫ = 7 TeV, s-1 L dt = 20.3 fb∫ = 8 TeV, s

inv.) →H BR(

0 0.2 0.4 0.6 0.8 1

1-C

L

-210

-110

1ObservedExpected

68% CL

95% CL

ATLAS


-1 L dt = 4.5 fb∫ = 7 TeV, s-1 L dt = 20.3 fb∫ = 8 TeV, s


Dark Matter in the Higgs portalThe results can be interpreted in the context of a Higgs-portal DMscenario

I Higgs as mediator between DM and SM particlesForm factor associated to Higgs nucleon coupling set to 0.33+0.3

−0.07 (Phys.Lett. B 709, 65 (2012))Assume that DM decays account for the full invisible BRVery stringent limits at low mass, degrading as the DM massapproaches MH/2

DM Mass [GeV]1 10 210 310

]2N

ucle

on c

ross

sec

tion

[cm

−D

M

-5110

-5010

-4910

-4810

-4710

-4610

-4510

-4410

-4310

-4210

-4110

-4010

-3910

-3810

-3710

σDAMA/LIBRA 3 σCRESST 2CDMS 95% CL CoGeNTXENON10 XENON100LUX ATLAS, scalar DMATLAS, vector DM ATLAS, fermion DM

ATLAS = 7 TeV,s ∫ -1Ldt=4.5 fb = 8 TeV,s ∫ -1Ldt=20.3 fbZH → ℓℓ + inv.

Higgs-portal Model


Indirect limitsIndirect limits on the BR to invisiblecan be derived from the combinationof rates in γγ,ZZ ∗ → 4l ,WW → lνlν,ττ,bb̄ channels

I Fix H couplings to SM values

Add contribution to the total widthfrom invisible, undetected decays

I ΓH = k2HΣi

ki(1−BRi,u)

ΓSMH

Introduce effective couplings to γ andgluons (possible new particles inloops)

I Fit with κγ , κg and BRi,u free

Observed limit BRi,u<0.41 at 95%C.L. (expected from SM BRi,u <0.55)If ZH→ ll + Emiss

T is included in the fitthe limit becomes BRi,u <0.37(expected BRi,u <0.39)

iBR-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

) i(B

RΛ

-2 ln

0

2

4

6

8

10

12

14 PreliminaryATLAS

-1 Ldt = 4.6-4.8 fb∫= 7 TeV, s

-1 Ldt = 20.3 fb∫= 8 TeV, s

]i

, BRgκ, γκ[ : b, bττ, ZZ*, WW*, γγ →h

,b, bττ, ZZ*, WW*, γγ →h

:Tmiss ll + E→Zh

obs. exp.

obs. exp.

i,uBR

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7)

i,u(B

RΛ

-2 ln

0

2

4

6

8

10 ATLAS Preliminary-1Ldt = 4.6-4.8 fb∫ = 7 TeV, s

-1Ldt = 20.3 fb∫ = 8 TeV, s

b,bττ,ZZ*,WW*,γγ →Combined H

> 0i,u

Limiting to physical range BR

]i,u

,BRgκ,γκ[

Observed

SM expected

ATLAS-CONF-2014-09ATLAS-CONF-2014-10


Summary and conclusions

A direct search for invisible decay modes of the Higgs boson with theATLAS experiment has been presented

I Based on the full Run 1 LHC dataset of 4.5 fb−1 at 7 TeV and 20.3 fb−1 at 8TeV collected with the ATLAS experiment

No deviation from the SM expectation is observed in the σ·BR measuredover the mass range 110-400 GeV

Assuming the SM ZH production cross section @ MH=125.5 GeV, thedirect search allows to set an upper limit of 75% at 95% C.L. on the BR toinvisible particles

I Limit at 37% from the combination of observed rates in all SM channels andupper limits from the direct search

In the Higgs-portal Dark Matter scenario, the results have beeninterpreted interpreted as limits on low-mass Dark Mattercandidates-nucleon cross sections


BACKUP SLIDES


Other selection variables

Fractional pT differenceI |Emiss

T − pllT|/pll

T <0.2

Number of jets

Eve

nts

1

10

210

310

410

510

610

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

ℓℓ ℓℓ

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Da

ta / M

C

0.5

1

1.5

Eve

nts

1

10

210

310

410

510

610

710

→ ℓ ℓℓ

→ ℓℓ ℓ

→ ℓℓ →

-1 L dt = 20.3 fb = 8 TeV, s

ATLAS

→ ℓℓ

0 1 2 3 4 5Da

ta / M

C

0

1

2


Dark Matter limits from the indirect search

95% confidence level limits from the combined fit of SM channels ratesand upper limits from the direct search of invisible decays

[GeV]χm1 10 210 310

]2 [c

m-Nχσ

-5710

-5510

-5310

-5110

-4910

-4710

-4510

-4310

-4110

-3910

DAMA/LIBRA (99.7% CL)CRESST (95% CL)CDMS (95% CL)CoGeNT (90% CL)XENON10 (90% CL)XENON100 (90% CL)LUX (95% CL)

Scalar WIMPMajorana WIMPVector WIMP

ATLAS Preliminary

Higgs portal model:ATLAS (95% CL) in

-1dt = 4.6-4.8 fbL∫ = 7 TeV, s-1dt = 20.3 fbL∫ = 8 TeV, s

,νlνl→WW*→4l, h→ZZ*→, hγγ→hmiss

Tll+E→bb, Zh→, hττ→h


search of higgs to invisible decays with the atlas detector · in atlas, the search is performed in...

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