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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%