Why do we care about di-Higgs

Production?Tyler James Burch

US LHC Users AssociationFermilab - Batavia, Illinois

October 24-26, 2018

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From Double Higgs at Colliders Workshop

The Higgs Boson

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July 4, 2012 - Standard Model-like Higgs particle discovered by ATLAS and CMS!- Mass reconstructed at ~125 GeV- Observed in multiple channels with rates consistent with the Standard Model (SM)

But, plenty of outstanding questions! Among these… - Does the discovered Higgs boson couple as predicted by the SM?- Are there any additional Higgs bosons?- Do particles beyond those predicted by the SM exist? If so, how do they couple to the

Higgs boson?Searching for di-Higgs production gives insight into these questions

NYTimes

The Higgs Mechanism

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arXiv:1312.5672

V(ϕ*ϕ) = μ2(ϕ*ϕ) + λ(ϕ*ϕ)2

minimum corresponds to:μ

λ≡ v ~246 GeV

V = V0 +m2

h

2h2 +

m2h

2v2vh3 +

m2h

8v2h4

} }

λSMHHH λSM

HHHH

Higgs Vacuum expectation value (vev)

Does the discovered Higgs boson couple as predicted by the SM? Di-Higgs searches look to better understand this by probing the trilinear coupling, providing another check that this is, in fact, the SM Higgs bosonMeasuring λHHH checks that Electroweak Symmetry Breaking follows from a 𝝓4 potential

Higgs Potential is given by:

For fluctuations about the minima, ϕ → v + h

h h

h

Results in di-Higgs production!

di-Higgs Production

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Occurs through two diagrams which interfere destructively, which causes a very small cross section

σ(gg → HH )SM ≈ 33 fb @ 13 TeV~5000 HH events produced in the projected 150 fb-1 in Run-2

Gluon-gluon fusion (ggF) is the dominant di-Higgs production mode, accounting for ~87% of di-Higgs events at 13 TeV, current analyses target this production mode

di-Higgs production is a rare process, which will test the limits of the LHC

H

H

HH

SM non-resonant HH Production via gluon-gluon fusion

Higgs Self-Coupling, Κλ

H

Not sensitive to Κλ

Probing BSM

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Are there any additional Higgs bosons? Do particles beyond those predicted by the SM exist? di-Higgs searches look to investigate these questions by looking for Beyond the Standard Model (BSM) enhancements to di-Higgs production

X

BSM Resonant HH Production

Resonant enhancements: Enhancements can occur through a resonance that decays to to two Higgs bosons- Two-Higgs-doublet model (THDM) - Resonance is a

heavy scalar- Randall-Sundrum Model - Resonance is a Spin-2 Kaluza-

Klein gravitonNon-Resonant enhancements: Can also look toward enhanced production caused by couplings that differ from the SM- Higgs Self-Coupling (Κλ) - strength of coupling deviating from SM value can lead to

enhanced production- Additional couplings not predicted by the SM, e.g. direct ttHH vertex

Higgs Self-Coupling, Κλ

h Direct ttHH vertex

Models with enhanced production make di-Higgs already interesting to study with Run-2 data

di-Higgs Decays

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Morse Strong motivation to look at certain channels based on properties of decay products

Strongest contribution to current limits:- : fully takes advantage of high bb branching

ratio- : Excellent trigger and mass resolution for

photons, high bb branching ratio- : Taus are relatively clean while still having a

large branching ratio, high bb branching ratio

bb̄bb̄

γγbb̄

bb̄ττ

SM Landscape

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0 10 20 30 40 50 60 70 80ggFSMσ HH) normalized to → (pp ggFσ95% CL upper limit on

Combined

γγb b→HH

-τ+τb b→HH

bbb b→HH 12.9 20.7 18.5

12.6 14.6 11.9

20.4 26.3 25.1

6.7 10.4 9.2

Obs. Exp. Exp. stat.

ObservedExpected

σ 1±Expected σ 2±Expected

ATLAS Preliminary-1 = 13 TeV, 27.5 - 36.1 fbs

HH) = 33.4 fb→ (pp ggFSMσ

ATLAS-CONF-2018-043

Combination limits: SM di-Higgs production constrained to Expected: 0.35 pb (10.4*σSM)Observed: 0.22 pb (6.7*σSM)

Prospect studies for HL-LHC (3 ab-1):

Channel Exclusion (95% CL)

Discovey Significance

𝛾𝛾bb N/A 1.5σ

bbττ 4.3*σSM 0.6σ

bbbb 1.5*σSM N/A

*Based on extrapolation of current analyses, likely will improve with more advanced analysis techniques

BSM Landscape - Resonances

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300 400 500 600 700 800 900 1000 [GeV]Sm

3−10

2−10

1−10

1

10

210

310

HH

) [pb

]→

S

→(p

p σ

95%

CL

uppe

r lim

it on

SM

(exp.)bbbb (obs.)bbbb (exp.)-τ+τbb (obs.)-τ+τbb

(exp.)γγbb (obs.)γγbb

Combined (exp.)Combined (obs.)

(Combined)σ1±Expected (Combined)σ2±Expected

= 2)βhMSSM (tan

ATLAS Preliminary-1 = 13 TeV, 27.5 - 36.1 fbs spin-0

300 400 500 600 700 800 900 1000 [GeV]

KK*

Gm

3−10

2−10

1−10

1

10

210

HH

) [pb

]→

KK* G

→(p

p σ

95%

CL

uppe

r lim

it on

SM

(exp.)bbbb (obs.)bbbb (exp.)-τ+τbb (obs.)-τ+τbb

Combined (exp.)Combined (obs.)

(Combined)σ1±Expected (Combined)σ2±Expected = 2.0PlMBulk RS, k/

ATLAS Preliminary-1 = 13 TeV, 27.5 - 36.1 fbs = 2.0PlMspin-2, k/

Heavy Scalar

k /MPl = 2.0300 400 500 600 700 800 900 1000

[GeV]KK*

Gm

2−10

1−10

1

10

210

HH

) [pb

]→

KK* G

→(p

p σ

95%

CL

uppe

r lim

it on

SM

(exp.)bbbb (obs.)bbbb (exp.)-τ+τbb (obs.)-τ+τbb

Combined (exp.)Combined (obs.)

(Combined)σ1±Expected (Combined)σ2±Expected = 1.0PlMBulk RS, k/

ATLAS Preliminary-1 = 13 TeV, 27.5 - 36.1 fbs = 1.0PlMspin-2, k/

k /MPl = 1.0

KK-graviton ATLAS-CONF-2018-043

No significant excesses found, limits set

tanβ = 2: ratio of the vacuum expectation values of the two Higgs doublets

mX < 462 GeV @ 95% CL in hMSSM

k: curvature of the warped extra dimensionM¯ Pl: the effective four-dimensional Planck scale

k/MPl =1.0 constraints: 307 < mG < 1362 GeVk/MPl =2.0 constraints: mG < 1744 GeV

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BSM Landscape - λHHH

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20− 15− 10− 5− 0 5 10 15 20SMλ / HHHλ = λκ

2−10

1−10

1

10

210

HH

) [pb

]→

(pp

ggF

σ95

% C

L up

per l

imit

on

SM

(exp.)bbbb (obs.)bbbb (exp.)-τ+τbb (obs.)-τ+τbb

(exp.)γγbb (obs.)γγbb

Combined (exp.)Combined (obs.)

(Combined)σ1±Expected (Combined)σ2±Expected

NLO theory pred.

ATLAS Preliminary

-127.5 - 36.1 fb = 13 TeV,s

bb̄bb̄ bb̄τ+τ− γγbb̄

ATLAS-CONF-2018-043

Combination limits: Expected: -5.8 < Kλ < 12.0Observed: -5.0 < Kλ < 12.1

Prospect studies for HL-LHC (3 ab-1):

Channel Exclusion @ 95% CL

HH→bbbb -4.1 < κλ < 8.7

HH→𝛾𝛾bb 0.2 < κλ < 6.9

HH→bbττ -4.0 < κλ < 12.0Stefania Gori

So, Why Do We Care?

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However, SM di-Higgs production is a rare process that will challenge the limits of the LHC - With more data and improved analysis techniques, evidence may be possible with full LHC dataset Significant current work towards di-Higgs in ATLAS - Combination result recently published for 36.1 fb-1 (2015+2016 data), set best limits on

Higgs trilinear coupling to date- Work toward full Run-2 analyses ramping up now!

Searches for di-Higgs production seek to answer important outstanding questions still surrounding the discovered Higgs bosonStudying the Higgs Trilinear coupling investigates if the discovered Higgs couples as predicted by the SM- Provides insight into Electroweak Symmetry Breaking, a long-term goal of the LHC

h h

h

Searching for enhanced di-Higgs production targets BSM physics - Opportunity to discover additional Higgs bosons or new BSM particles

- Due to small SM hh cross section, BSM enhancements may be noticeable even with the full Run-2 dataset

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Thank you!

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Backup

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CMS Results

Luca Cadamuro

The Higgs Boson

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[GeV] HM120 122 124 126 128 130 132

H+X

) [pb

]

→(p

p σ

-110

1

10

= 8 TeVs

LHC

HIG

GS

XS W

G 2

014

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

bbH (NNLO QCD in 5FS, NLO QCD in 4FS)→pp

LHC Higgs XS WG

ggF = 87.3 %

VBF = 6.8%}VH = 4.1%

(for 13 TeV)

} ttH + bbH = 1.8%