Searches for Invisible Decay Modes of the Higgs Boson with the CMS Detector

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Searches for Invisible Decay Modes of the Higgs Boson with

the CMS Detector

Daniele Trocino – Northeastern University on behalf of the CMS Collaboration

XXXVII International Conference on High Energy Physics Valencia (Spain) – July 4, 2014

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Why Invisible Higgs?

●

Recent measurements of the branching fractions of the 125 GeV Higgs boson are compatible with the Standard Model expectations, but with large uncertainties

● can still accommodate significant non-SM decays

● moreover, additional Higgs bosons with exotic decays are still possible

●

Many models predict Higgs boson(s) decaying to heavy, undetectable particles

● fourth-generation neutrinos, neutralinos, graviscalars in extra-dimensions, etc

● some of these constitute viable dark matter (DM) candidates

● in “Higgs-portal” models, the DM does not couple to any SM particles but the Higgs

– Higgs as a mediator between the SM and DM sectors

CMS-PAS-HIG-13-005

updated results in M. Chen's talk

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Indirect and Direct Searches

Γ

_tot

= Γ

_tot^SM

⋅ ∑

^obs_X

κ

²^X

⋅ B

^SM^X

1−B

_BSM

,

^κX

2 = Γ^obs_X Γ^SM_X

●

Non-SM decays modify the total Higgs width

● indirect constraints on B_BSM can be inferred by fitting the observed decay modes X

●

Direct searches are conducted in production channels where the Higgs boson recoils

against a visible system

● vector boson fusion (VBF):

qq → qq VV → qq H → 2 jets + MET

● associated production with a Z boson, decaying to a pair of leptons or bb

qq → Z* → ZH → ℓ

⁺

ℓ

^–

+ MET qq → Z* → ZH → bb + MET

CMS-PAS-HIG-13-005

68% CL 95% CL B

_BSM

< 0.52

updated results in M. Chen's talk

inv. H Z

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VBF: Signal and Selection

Signature and selection

➢ Two forward jets, separated by a large rapidity gap, with high invariant mass

– 2 jets with p_T > 50 GeV/c, |η| < 4.6 – η₁·η₂ < 0, Δη > 4.2, M(jj) > 1100 GeV/c² – central-jet veto (CJV): no additional jets

with η₁ < η < η₂ (p_T > 30 GeV/c)

➢ Large missing transverse energy (MET)

– MET > 130 GeV

Backgrounds and rejection

➢ Z(νν) + jets

➢ W(ℓν) + jets, DY(ℓℓ) + jets

– lepton veto: no ℓ with p_T > 10 GeV/c

➢ Single top, tt, diboson

➢ QCD multijets

jet

jet MET

arXiv:1404.1344

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VBF: Backgrounds

Z(νν) + jets estimated from Z(μμ) + jets events

● same selection as for the signal +

● exactly 2 muons with M(μμ) = 60-120 GeV/c²

● control→signal extrapolation factor from MC

N_{Z( ν ν)} =

99

±

29 (stat)

±

25 (syst)

W(ℓν) + jets estimated from single e / μ / τ

_had

events

N_W_(e_{ν )} =

63

±

9 (stat)

±

18 (syst)

N_W_{(μ ν)} =

67

±

5 (stat)

±

16 (syst)

N_W_{(τ ν)} =

53

±

18 (stat)

±

18 (syst)

QCD estimated from sidebands of two uncorrelated variables: MET and CJV (“ABCD” method)

MET 130 GeV

CJV

pass fail

signal

B A

D C

control

N_QCD =

30.9

±

1.6 (stat)

±

23.0 (syst)

CMS-PAS-HIG-13-013

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VBF: Results

● Mass independent selection, cut-and-count approach

● Using a CL_S method, we set limits on σ × B(H→invisible) and σ × B(H→invisible) / σ_VBF

● Assuming SM VBF production cross section and acceptance:

observed 95% CL limit on B(H→inv) for m_H = 125 GeV/c²: 0.65 (expected: 0.49)

Final yields

125 GeV/c

arXiv:1404.1344

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Z(ℓℓ) + H(inv): Signal and Selection

➢ Two isolated leptons from Z decay

– 2 leptons (e, μ) with p_T > 20 GeV/c, v|η| < 2.5 – M(ℓℓ) = M_Z ± 15 GeV/c²

➢ Large missing transverse energy (MET)

– MET > 120 GeV

➢ ZZ(ℓℓνν) + jets, WW(ℓνℓν) + jets

➢ WZ(ℓνℓℓ) + jets

– 3^rd lepton veto: no e / μ with p_T > 10 GeV/c

➢ DY(ℓℓ) + jets

– MET cut

– requirements on MET-ℓℓ balance

➢ tt, single top, W(ℓν) + jets, QCD

– ≤ 1 jet (p_T > 30 GeV/c)

– no b-tagged jets (p_T > 20 GeV/c)

MET

arXiv:1404.1344

MET > 120 GeV DY

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Z(ℓℓ) + H(inv): Backgrounds

DY(ℓℓ) + jets estimated from photon + jets events

● resembles high-p_T Z production in all relevant aspects – production mechanism, hadronic recoil, pile-up,

underlying event conditions

● re-weighting of p_T(γ) to match Z spectrum in data

WW, single top, tt, Z(ττ) estimated cumulatively from e μ events

● eμ → ee and eμ → μμ extrapolation factors are computed from the side-bands (SB) of the Z peak (m(ℓℓ) = 40-70 + 110-200 GeV/c²)

α_ee = N_ee^SB/N_e^SB_μ, N_ee^sign = α_ee⋅N_e^sign_μ α_{μ μ} = N_{μ μ}^SB/N_e^SB_μ, N_{μ μ}^sign = α_{μ μ}⋅N_e^sign_μ

ZZ(ℓℓνν) + jets and WZ(ℓνℓℓ) + jets backgrounds are estimated from MC predictions

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Z(ℓℓ) + H(inv): Results

Limits computed from a fit to 2D distributions m_T vs Δφ(ℓℓ)

(only m_T in 7 TeV dataset) Azimuthal angle Δφ(ℓ⁺ℓ^–)

m_T=

√

²^p^T^E^T^miss ^[^1−cos^{Δ ϕ (Z}^{, E}^T^miss^)]

Assuming SM ZH cross section, the 95% CL observed limit on B(H→inv) for m_H = 125 GeV/c²

is 0.83 (expected: 0.86)

iv:1404.1344

0-jet and 1-jet events analyzed separately (different S/B)

Final yields

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Z(bb) + H(inv): Signal and Backgrounds

➢ Two b-jets from Z decay

– 2 b-tagged jets with

p_T > 30 / 60 GeV/c, |η| < 2.5 – p_T (jj) > 100-130 GeV/c

➢ Large MET

– three boost categories:

100-130, 130-170, >170 GeV

➢ Z(νν) + jets, W(ℓν) + jets

➢ tt, single top

➢ ZZ (ννbb), WZ (ℓνbb)

– no isolated leptons with p_T>15 GeV/c

➢ QCD multijets

– requirements on MET quality

MET

b-jet

● Background modeled with MC, normalized from data

● Seven control regions defined for the main backgrounds – Z + jets (0, 1, 2 b-jets), W + jets (0, 1, 2 b-jets), tt

The Combined Secondary Vertex (CSV) algorithm tags b-jets combining information from secondary vertices and

track impact parameters

Z + bb enriched region arXiv:1404.1344

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Z(bb) + H(inv): Results

High-p_T region

●

Final discrimination is obtained with a multivariate analysis (BDT), separately for each Higgs mass hypothesis and in each boost category

●

The BDT output distributions are used to set limits on σ × B(H→inv) / σ

_ZH

●

95% CL observed limit on σ × B(H→inv) / σ

_ZH

for m

_H

= 125 GeV/c

²

: 1.82 (expected: 1.99)

arXiv:1404.1344

Final yields

different boost categories analyzed separately

(different S/B)

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VBF + ZH Combination

●

Individual limits on σ × B(H→invisible) / σ

_SM

from VBF and ZH channels are combined

●

Assuming SM VBF and ZH cross sections, they are interpreted as limits on B(H→invisible)

For m

_H

= 125 GeV/c

²

at 95% CL

VBF: 0.65 (expected 0.49) ZH (ℓℓ + bb): 0.81 (expected 0.83) VBF + ZH: 0.58 (expected 0.44)

Comparable with recent indirect limits from measurement of visible decays

arXiv:1404.1344

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Higgs-Portal Dark Matter

●

Limits on the Higgs invisible branching fraction can be interpreted in the context of Higgs-portal DM models

● direct-detection experiments measure the DM-nucleon elastic interaction, mediated by a Higgs

● if the DM has mass below m_H/2, the decay width Γ_inv can be translated to spin-independent DM-nucleon elastic cross section and compared to results from direct detection

λ

_hχχ

λ

_hχχ

f

_N

direct detection

production at colliders

from effective field theory (EFT)

scalar DM

vector DM

fermion DM

parametrizes the DM-Higgs interaction

f

_N

=0.33

_−0.07^+0.30

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Limits on DM-Nucleon Cross Section

Using the combined limits from CMS VBF, Z(ℓℓ)H, and Z(bb)H searches for m

_H

= 125 GeV/c

²

B(H→inv) < 0.51 at 90% CL

Results are competitive with current results from direct-detection experiments in the low DM mass region

arXiv:1404.1344

we set limits on the

spin-independent DM-nucleon cross section for 3 scenarios:

scalar, fermion, vector DM

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Conclusions

●

Direct searches for invisible decay modes of the Higgs boson(s) are performed using CMS 2011 and 2012 data sets, in a Higgs mass range of 105-400 GeV/c

²

● VBF, Z(ℓℓ)H, and Z(b

b

)H channels were considered

● no significant excess observed over the expected background

● combined limits from the three searches are comparable with the indirect constraints from visible decay channels for the 125 GeV Higgs boson, assuming SM production cross sections

– 95% CL observed limits: B(H→inv) < 0.52 (indirect search), < 0.58 (direct search)

●

The limits on invisible branching fraction of the 125 GeV Higgs boson are interpreted as upper bounds on the DM-nucleon cross section in a Higgs-portal DM scenario

● results competitive with DM direct-detection experiments for DM masses < m_H/2

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References

●

CMS-PAS-HIG- 13‐005

http://cds.cern.ch/record/1542387?ln=en

●

arXiv:1404.1344, CMS-HIG-13-030, CERN-PH-EP-2014-051

http://arxiv.org/abs/1404.1344

submitted to European Physical Journal C

●

CMS-PAS-HIG-13-013

http://cds.cern.ch/record/1596283?ln=en

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Backup

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VBF: Signal and Selection

➢ Two forward jets, separated by a large rapidity gap, with high invariant mass

– 2 jets with p_T > 50 GeV/c, |η| < 4.6 – η₁·η₂ < 0, Δη > 4.2, M(jj) > 1100 GeV/c²

– MET > 130 GeV

➢ Z(νν) + jets

➢ W(ℓν) + jets

– veto leptons with p_T > 10 GeV/c

➢ QCD multijets

– no additional jets with p_T > 30 GeV/c

and η₁ < η < η₂ (central-jet veto, or CJV) – Δφ_{j j} < 1.0 rad

➢ Single top, tt, diboson, DY + jets

jet

jet MET

CMS-PAS-HIG-13-030

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VBF: Backgrounds

Z(νν) + jets estimated from Z(μμ) + jets events

● exactly 2 muons with M(μμ) = 60-120 GeV/c²

● MET > 130 GeV (removing muons)

● other backgrounds subtracted using MC

N_{Z( ν ν)} =

99

±

29 (stat)

±

25 (syst)

W(ℓν) + jets estimated from single e / μ / τ

_had

events

N_W_(e_{ν )} =

63

±

9 (stat)

±

18 (syst)

N_W_{(μ ν)} =

67

±

5 (stat)

±

16 (syst)

N_W_{(τ ν)} =

53

±

18 (stat)

±

18 (syst)

QCD estimated from sidebands of two uncorrelated variables: MET and CJV (“ABCD” method)

● electroweak backgrounds subtracted using MC

● possible correlation between the two variables is accounted for in the systematic uncertainty

MET 130 GeV

CJV

pass fail

signal

B A

D C

control

N_QCD =

30.9

±

1.6 (stat)

±

23.0 (syst)

CMS-PAS-HIG-13-013

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VBF: Results

● Mass independent selection, cut-and-count approach

● Using a CL_S method, we set limits on σ × B(H→invisible) and σ × B(H→invisible) / σ_VBF

● Assuming SM VBF production cross section and acceptance:

observed 95% CL limit on B(H→inv) for m_H = 125 GeV/c²: 0.65 (expected: 0.49)

Final yields Main systematics

CMS-PAS-HIG-13-030

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Z(ℓℓ) + H(inv): Signal and Selection

Signature and selection

➢ Two isolated leptons from Z decay

– 2 leptons (e, μ) with p_T > 20 GeV/c, |η| < 2.5 – M(ℓℓ) = M_Z ± 15 GeV/c²

– MET > 120 GeV

➢ ZZ(ℓℓνν) + jets, WW(ℓνℓν) + jets

➢ WZ(ℓνℓℓ) + jets

– no 3^rd e/μ with p_T > 10 GeV/c

➢ DY(ℓℓ) + jets

– Δφ(MET, ℓℓ) > 2.7 rad – |MET – p_T(ℓℓ)|/p_T(ℓℓ) < 0.25

➢ tt, single top, W(ℓν) + jets, QCD

– at most 1 jet with p_T > 30 GeV/c – no b-tagged jets with p_T > 20 GeV/c

MET

CMS-PAS-HIG-13-030

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Z(ℓℓ) + H(inv): Backgrounds

DY(ℓℓ) + jets estimated from photon + jets events

● resembles high-p_T Z production in all relevant aspects

● production mechanism, hadronic recoil, pile-up, underlying event conditions

● re-weighting of p_T(γ) to match Z spectrum in data

WW, single top, tt, Z(ττ) estimated cumulatively from e μ events

● no Z peak in dilepton invariant mass ⇒ scale factors from eμ to ee/μμ final states are computed from the side-bands (SB) of the Z peak: m(ℓℓ) = 40-70 GeV/c² and 110-200 GeV/c²

α_ee = N_ee^SB/N_e^SB_μ, N_ee^sign = α_ee⋅N_e^sign_μ α_{μ μ} = N_{μ μ}^SB/N_e^SB_μ, N_{μ μ}^sign = α_{μ μ}⋅N_e^sign_μ

ZZ(ℓℓνν) + jets and WZ(ℓνℓℓ) + jets backgrounds are estimated from MC predictions

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Z(ℓℓ) + H(inv): Results

● Events with 0 jets and 1 jet analyzed separately, due to the different S/B

● Main systematic uncertainties

● theory (ZZ / ZH)

– PDFs: 5.0 / 5.7%

– QCD scales: 6.4 / 7.0%

● DY normalization from data: 5.4%

Limits computed from a fit to 2D distributions m_T vs Δφ(ℓℓ)

(only m_T in 7 TeV dataset) Azimuthal angle Δφ(ℓ⁺ℓ^–)

m_T=

√

²^p^T^E^T^miss ^[^1−cos^{Δ ϕ (Z}^{, E}^T^miss^)]

Assuming SM ZH cross section, the 95% CL observed limit on B(H→inv) for m_H = 125 GeV/c²

is 0.83 (expected: 0.86)

iv:1404.1344

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Z(bb) + H(inv): Results

High-p_T region

● Final discrimination is obtained with a multivariate analysis (BDT) using a set of kinematic variables after full selection and background normalization, separately for each Higgs mass hypothesis and in each boost region

● Expected and observed events in the three boost regions

● Main systematics

● signal cross section: PDF and QCD scales (6%) and NNLO QCD + NLO EWK corrections (7%)

● background normalization from data: 8%

● b-tagging efficiency effect on background shape: 7%

● The BDT output distributions are used to set limits on σ × B(H→inv) and σ × B(H→inv) / σ_ZH

● Assuming SM ZH cross section, the 95% CL observed limit on B(H→inv) for m_H = 125 GeV/c²is 1.82 (expected: 1.99)

arXiv:1404.1344

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Higgs-Portal Dark Matter

●

Limits on the Higgs invisible branching fraction can be interpreted in the context of Higgs-portal DM models

● hidden sector with stable neutral particle χ coupling directly to the Higgs boson

● direct-detection experiments measure the nuclear recoil from DM-nucleon elastic interaction, mediated by the Higgs boson

● if χ has mass below m_H/2, H→χχ decay width Γinv can be translated to spin-independent DM-nucleon elastic cross section and compared to results from direct detection

λ

_hχχ

λ

_hχχ

f

_N

direct detection

production at colliders

Three scenarios for χ

scalar

vector

fermion

parametrizes the

DM-nucleon interaction

f

_N

=0.33

_−0.07^+0.30