Individual and Combined Measurements of the Spin and Parity Proper9es of the Higgs boson using the ATLAS Detector

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Individual and Combined Measurements of the Spin and Parity Proper9es of the Higgs

boson using the ATLAS Detector

Kirill Prokoﬁev

Laboratori Nazionali di Frasca7 -‐ INFN

On behalf of the ATLAS Collabora9on

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Outline

•  Introduc9on

•  Op9ons for spin and parity of the new boson

•  Spin and parity measurements in di-‐boson channels

•  Summary

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ATLAS Spin and parity publica9ons used in this presenta9on:

Phys. Le). B 726 (2013), pp. 120-‐144 ATLAS-‐CONF-‐2013-‐029 ATLAS-‐CONF-‐2013-‐031 ATLAS-‐CONF-‐2013-‐013

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Introduc9on

Signiﬁcance

observed Analysis Reference Hàγγ 7.4 σ CONF-‐2013-‐ ^ATLAS-‐

012

HàZZ^(*)à4l 6.6 σ CONF-‐2013-‐ ^ATLAS-‐

013

HàWW^(*)àlνlν 3.8 σ CONF-‐2013-‐ ^ATLAS-‐

030

Hàττ 4.2 σ CONF-‐2013-‐ ^ATLAS-‐

108

•  The ini9al observa9on was reported on July 4 2012.

–  Published paper: a 5.9σ excess driven by ZZ^*, WW^*, γγ decays. About

11 `^-‐1 at 7 and 8 TeV.

•  Updates to full LHC run-‐I sample (~25`^-‐1)

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Op9ons for spin and parity of the new boson

•  The Standard Model Higgs boson: neutral scalar, J^CP=0⁺⁺_.

–  Theories with extended Higgs sector ofen allow for CP-‐mixed Higgses.

•  The new resonance is a neutral boson: it decays to pairs of gauge bosons with total charge 0.

–  Integer spin: currently considering 0, 1 and 2.

•  Spin-‐1 is strongly disfavored due to the observa9on of the γγ decay (Landau-‐

Yang theorem: Dokl. Akad. Nauk Ser. Fiz. 60 (1948) 207, Phys. Rev. 77 (1950) 242.)

–  The observa9on of di-‐photon decay also ﬁxes C=+1. Parity has to be determined.

•  To associate this par9cle to a par9cular model, one needs to measure the spin and parity without theore9cal prejudice.

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Measurements of Spin and Parity in ATLAS

•  Exclusion of alterna9ve hypotheses in favor of the Standard Model J^P=0⁺: –  J^P=0^-‐: gluon-‐fusion produc9on.

–  J^P=1⁺, 1^-‐: qqbar produc9on.

–  J^P=2⁺_m(Graviton-‐like tensor with minimal couplings):

gluon-‐fusion and qqbar produc9on.

•  Measurement of proper9es: deduce spin and parity from measured distribu9ons of kinema9c observables.

–  Angular distribu9ons of decay products in the rest frame of the resonance, invariant masses of intermediate gauge bosons.

arXiv:1001.3396 arXiv:1208.4018

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For all measurements the Monte Carlo samples for non-‐Standard Model

hypotheses are produced using JHU Leading Order generator.

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Cos θ

₁

m

_Z2

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Spin and parity in H→ZZ*→4l decay

Event selec9on follows the main ZZ* analysis (Phys. Leq. B 726 (2013), pp. 88-‐119) .

Signal Region: 115 GeV < m_4l < 130 GeV 43 candidate events.

Expected background: 16 events.

Expected signal (μ =σBR/σBR^SM=1): 18 events.

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hypo2 hypo1

Signal Type2 Signal

Type 1

Background

Low High Low m 4l

Spin and parity in H→ZZ*→4l decay

J

^P

=0

⁺

à

ß J

^P

=1

⁺

J

^P

=0

⁺

à

ß J

^P

=0

^-‐

BDT analysis in two bins: low S/B (115–121 and 127–130 GeV) and high S/B (121–127 GeV).

Reducible ZZ background from Monte Carlo.

Irreducible q, Zbb and Z+jets from control regions in data.

BDT training: m_Z1,m_Z2,Φ,Φ₁,cosθ_1,2,cosθ*

BDT (0⁺0^-‐) BDT (1⁺0⁺)

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p

₀

for 0

⁺

p

₀

for 0

^-‐

Sta9s9cal procedure

CL_S: the exclusion of the

alterna9ve J^P_alt hypothesis in favor of the Standard Model J^P= 0⁺ hypothesis:

Test sta9s9c: Ra9o of proﬁled likelihoods. (Independent signal strength per channel and per spin and parity hypothesis.)

In HàZZ*à4l data allow to exclude:

J^P=0^-‐ at 97.8% CL J^P=1⁺, 1^-‐ at >94% CL J^P=2⁺_m at >83% CL

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Data

CL

_S

= p

₀

( J

_alt^P

)

1 − p

₀

( J

^P

= 0

⁺

)

€

q = log L(0⁺, ˆ

µ

ˆ ₀+, ˆ ˆ

θ

₀+ ) L(J_alt^P , ˆ

µ

ˆ _J

altP , ˆ ˆ

θ

_J

altP )

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Spin measurement in H→γγ decay

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Study based on the single photon produc9on angle |cos θ*|.

Considered models: 0⁺ and 2⁺_m.Both qqbar and ggF produc9on mechanisms.

Signal region: 122 GeV <m_γγ< 130 GeV.

Fit to the product of |cos θ*| and m_γγ PDFs in the signal region.

20.7 `^-‐1 at 8 TeV

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Spin measurement in H→γγ decay

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For large values of the f_qq, the

|cos θ*|distribu9ons for J^P=0⁺ and 2⁺ become similar.

The data favor J^P=0⁺ hypotheses for all frac9ons where sensi9vity is present.

f_qq = σ(qq → H)

σ(qq→ H)+σ(gg→ H)

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Spin measurement in H→WW*→lνlν decay

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Analysis method: two dimensional ﬁts. Two BDT scores are calculated for each event: J^P=0⁺ or alterna9ve hypothesis against background.

Analysis uses the 0-‐jet e/μ channel only. Most sensi9ve to the spin due to the rela9vely low backgrounds.

Hypotheses: 0⁺, 1^-‐, 1⁺ and 2⁺. Observables: m^ll, p^ll_T, ∆φ^ll and m_T. Exclusion of the J ^P = 2⁺ up to 99% CL depending on the value of the f_qq.

The separa9on between hypotheses grows with f_qq. Complementary to γγ and ZZ*.

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Combina9on results

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The sta9s9cal procedure for the combina9on is iden9cal to the one of individual analyses. Exclusions:

J^P=2⁺ at >99.9% CL independently of f_qq (ZZ+WW+γγ);

J^P=0^-‐ at 97.8% CL (ZZ);

J^P=1^-‐at 99.73% CL; J^P=1⁺: 99.97% CL. (ZZ+WW).

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Summary

•  The spin and parity measurements to-‐date are based on exclusion of alterna9ve J^P hypotheses in favor of the Standard Model J^P=0⁺.

–  Results are consistent with the Standard Model predic9ons.

•  Tests are performed in di-‐boson channels where the new resonance is observed with high signiﬁcance.

–  Despite low signal sta9s9cs, most popular J^P=0^-‐,1⁺,1^-‐,2⁺_m models are excluded at >95% CL.

•  The evidence of spin-‐0 nature if the Higgs boson opens way for CP-‐mixing measurements in HZZ vertex.

–  Prospec9ve studies for 300 `^-‐1 and 3000 `^-‐1will be presented by Dr. Reina Camacho on Saturday.

•  Signiﬁcant observa9on of VBF Higgs produc9on and fermionic Higgs decays will increase the precision of ﬁxed hypotheses exclusion and CP-‐

mixing measurements.

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Backup

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Prospec9ve CP-‐mixing studies

•  HZZ vertex tensor structure:

•  f_g4 < 0.15 (0.04) at 95% CL f_g2 < 0.29 (0.12) at 95% CL for 300 fb^-1(3000 fb^-1)

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Individual and Combined Measurements of the Spin and Parity Proper9es of the Higgs boson using the ATLAS Detector