probing higgs in type iii seesaw at the large hadron...

Plan Neutrino Mass Seesaw Mechanisms Triplet fermion Collider Phenomenology Inverse Seesaw

Probing Higgs in Type III Seesaw at the Large Hadron

Collider

Priyotosh Bandyopadhyay

University of Helsinki & Helsinki Institute of Physics, Helsinki2nd KIAS Phenomenological Workshop,

KIAS, Seoul

Work done with Prof. Eung Jin Chun, Prof. Suyong Choi, Kyungnam MinJong Chul Park, Hiroshi Okada

Phys.Rev. D85 (2012) 073013, arXiv:1209.XXXX[hep-ph]

September 10, 2012


Plan

1 Neutrino Mass

2 Seesaw Mechanism

3 Type III Seesaw

4 Triplet fermions and Decay modes

5 Phenomenology at the LHC

2b + 3l2b + SSD

6 Inverse Seesaw

7 Conclusion


Neutrino Mass Dirac & Majorana

If neutrinos are of only Dirac type

⇒ Possible mass term is yνLHν

How do you have small ν mass O(0.1) ev)

By having yν ∼ O(10−12) ⇒ unnatural

Is there other way to get small neutrino mass?

If neutrinos are Majorana !


Majorana Neutrino

1 νc = ν, self-conjugate under the charge conjugation.

2 We can have a mass term

−L =1

2mL(ψL)cψL +

1

2mR(ψR)cψR + h.c

3 Introduction of this high scale mass can naturally explainsmall neutrino mass, generated by Seesaw Mechanism.


Seesaw Mechanism

Seesaw mechanism is one where the smallness of neutrinomass is explained by a large scale.

There are different versions of this seesaw mechanism buthave a basic structure:

Mν ≃ < v >2

Mseesaw

≃ MeV2

TeV≈ eV

Introduces two scales: very high scale and a moderate scale toget the very small scale.


Type III Seesaw Mechanism

SU(2)L triplet fermions with Y = 0, Σ = (Σ+,Σ0,Σ−).The matrix form of the triplet;

Σ =

(

Σ0√

2Σ+√

2Σ− −Σ0

)

where Σ+ is the antiparticle state of Σ−: Σ+ ≡ (Σ−)c .

Then the gauge invariant Yukawa terms are

L =[

yiHεΣPLli + h.c .]

+1

4Tr

[

ΣΣ]

where li is the lepton doublet and H is the Higgs doublet:l = (νi , ei )L and H = (H+,H0).


Neutrino mass and mixing

The neutrinos get a seesaw massmν,ij ∼ yiyjv

2/Mwhich becomes O ∼ 0.1 eV for yi ∼ 10−6 and M ∼ 1 TeV.

The neutrino Dirac mass, yiv , induces mixing between l andΣ.

The mixing angles for the neutral and charged part are

θνi≈ yiv

Mand θli ≈

√2yiv

M

There are bounds from EWPD on these mixing angles,|θiθj | < 10−7 − 10−4 Abada et.al, Aguila et.al


Gauge interaction

Due to the l–Σ mixing, we get the gauge couplings to triplets:

LVΣf = −gθνiW +

µ

[

1√2Σ0γµPLei + νiγ

µRRΣ−

]

−gθνiW−

µ

[

1√2eiγ

µPLΣ0 + Σ−γµPRνi

]

+gθνi

2cWZµ

[√2Σ−γµPLei +

√2eiγ

µPLΣ− − Σ0γµγ5νi

]


Production

As production of Triplet happens via gauge interaction, soindependent of these mixing angle.

Thus, we have the electroweak production of the triplets atthe LHC,

pp → Σ±Σ0, Σ±Σ∓

As, Y=0, the T3 = 0 component Σ0, does not couple to thegauge bosons, leading to no production of Σ0Σ0


Cross-section of the triplet fermions @ 14 TeV LHC

Cross-section is very low for higher triplet mass1.

1Hambye et al. 08, Aguila et al. 08


Decay

The triplet decays as follows:

Σ± → l±h

→ l±Z 0

→ νW±

→ Σ0π±

Σ0 → νh

→ νZ 0

→ l±W∓

We can see the final states with multi-leptons could beinteresting.2

2Aguila08


Decay branching of the triplet fermions

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

100 200 300 400 500 600 700 800 900 1000

Bra

nchi

ng fr

actio

n

mΣ+/-

m~ ν=10 meV

fπ=130 MeV

Σ- -> H lΣ- -> Z l

Σ- -> W- νΣ- -> π- Σ0

,

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

100 200 300 400 500 600 700 800 900 1000

Bra

nchi

ng fr

actio

n

MΓ in GeV

m~ ν=10 meV

Σ0 -> H νΣ0 -> Z Σ

Σ0 -> W+ l


Decay length of the triplet fermions

100 150 200 250 300 350 400 450 500

M in GeV

0.001

0.01

0.1

1

10

mν

in m

eV

0

5

10

15

20

25

30

35

40

45

50


Benchmark Points for the collider simulation

For the collider simulation we took mΣ = 250, 400 GeV andm = 10 meV as benchmark points

Production cross-sections (fb)

mΣ 250 GeV 400 GeV

Σ+Σ0 439.1 73.8

Σ+Σ− 320.0 50.0

Σ−Σ0 221.8 32.3

Table: Production cross-sections for the benchmark points.


Decay branching fraction for the Benchmark Points

Decay modes Branching fractions

mΣ 250 GeV 400 GeV

Σ0 → hν 0.17 0.22

Σ0 → Zν 0.27 0.26

Σ0 → W±l∓ 0.56 0.52

Σ± → hl± 0.17 0.22

Σ± → Zl± 0.27 0.26

Σ± → W±ν 0.55 0.52

Σ± → Σ0π± 0.009 0.003

Table: Branching fractions for the triplets with mν = 10 meV.


Final state topologies for collider simulation

Dominant decay modes are final states with Higgs and/orGauge bosons associated with leptons.

Higgs searches with multi-lepton final state could beinteresting.

We analyse all plausible leptonic final states.

Here we discuss only 2b + 3l and 2b + SSD at the LHC at 14TeV.


Collider simulation

We generate the events with MadGraph

Generated events thus interfaced with PYTHIA via LHEF

Hadronization, ISR/FSR effects and Jet formation (PYCELL)are done inside PYTHIA

CTEQ6L is used parton distribution function (PDF)

The renormalization/factorization scale is set at√

s.

Higgs mass was chosen to be 120 GeV.


Status of 2b + 3l @14TeVLHC

Typically hlZl ,hlWl ,ZlZl ,ZlWl dominantly contribute to thefinal state.

In particular final state with Higgs, i.e., hlZl ,hlWl areinteresting.

b pair can come from h and Z

Strategically we try to construct 2b + 3l final state

Invariant mass of b-jet pair could give the Higgs peak we areinterested in

Invariant mass of b − b − l where b-jets are from the Z and h

window can give rise to Σ mass peak.


Status of 2b + 3l @14TeVLHC

With the ISR/FSR, jet formation and b mis-tagging otherdecay modes also could contribute.

We define the signal by following cuts:

pjetT ,min = 20 GeV and jets are ordered in pT

leptons (ℓ = e, µ) are selected with pT ≥ 20 GeV and|η| ≤ 2.5no jet should match with a hard lepton in the event(∆Rj,l ≥ 0.4, ∆Rl,l ≥ 0.2)

Considering the b and lepton final state the main SMbackgrounds are:tt, ttZ , ttW ,tth, ttbb, WW , ZZ , WZ


≥ 2b+ ≥ 3l

The numbers for the signal and the background for the finaltopology ≥ 2b+ ≥ 3l

2b − jet + 3l

Signal Backgrounds

BP1 BP2 tt t tbb t tZ tth VV ttW

116.89 40.32 5.0 1.77 31.53 9.86 0.0 8.67

The dominant background is ttZ as expected

For ≥ 2b+ ≥ 3l at 10 fb−1 integrated luminosity the reach forBP1 is ∼ 9σ, where as for BP2 ∼ 4σ


Higgs peak in 2b + 3l final state

0

2

4

6

8

10

12

100 150 200 250 300 350 400 450 500

Nu

mb

er

of

eve

nts

mb-b in GeV

BP1, mΣ=250 GeV

TotalBackground

Figure: Invariant mass distributions of b-jet pair from ≥ 2b+ ≥ 3l finalstates for BP1 and SM background.


Higgs peak in 2b + 3l final state

The number of events in the window95GeV ≤ mb−b ≤ 145GeV

95GeV ≤ mb−b ≤ 145GeV

Signal Backgrounds


28.44 4.46 1.0 0.50 8.3 2.2 0.0 2.5

Higgs peak reconstructed with mbb a 5σ signal significance forBP1 will require 13 fb−1 integrated luminosity and for BP2,couple of 100 fb−1.


Sigma peak in 2b + 3l final state

The b-jet pair within 60-150 GeV of the invariant mass, alongwith a charged lepton are taken for b − b − l invariant massdistribution.

0

2

4

6

8

10

12

14

100 150 200 250 300 350 400 450 500

Nu

mb

er

of

eve

nts

mb-b-l in GeV

BP1, mΣ=250 GeV

TotalBackground

Figure: Invariant mass distributions of b-jet pair plus a charge leptonfrom ≥ 2b + 3l final state for BP1 and SM background.


Sigma peak in 2b + 3l final state

The number of events in the window mb−b−l within250(400) ± 50 GeV.

mb−b−l

Signal Backgrounds

tt t tbb t tZ tth VV ttW

BP1 61.89 2.0 0.1 5.9 1.5 0.0 0.9

BP2 5.19 0.0 0.0 1.5 0.06 0.0 0.5

mbbl at 10 fb−1 integrated luminosity the reach for BP1 is∼ 7σ


Status of 2b+ Same-sign di-lepton

≥ 2b − jet + SSD

Signal Backgrounds


127.38 29.09 24.0 7.5 41.6 29.0 0.0 41.4

For ≥ 2b + SSD at 10 fb−1 integrated luminosity the reachfor BP1 is ∼ 8σ, where as for BP2 ∼ 2σ



0

5

10

15

20

25

100 150 200 250 300 350 400 450 500

Nu

mb

er

of

eve

nts

mb-b in GeV

BP1, mΣ=250 GeV

TotalBackground

Figure: Invariant mass for b-jet pair from ≥ 2b + SSD final state for BP1and SM backgrounds.


2b+ Same-sign di-lepton

95GeV ≤ mb−b ≤ 145GeV

Signal Backgrounds


60.61 10.39 8.0 3.0 10.6 6.5 0.0 9.6

Higgs peak reconstructed with mbb have a significance of 6σfor BP1 at 10 fb−1 integrated luminosity and for BP2 it is1.5σ.



We take again the b-jets within the mass window of 60-150GeV and plot the invariant mass distribution with the lepton

0

5

10

15

20

25

100 150 200 250 300 350 400 450 500

Nu

mb

er

of

eve

nts

mb-b-l in GeV

BP1, mΣ=250 GeV

TotalBackground

Figure: Invariant mass for b-jet pair plus a charged lepton from≥ 2b + SSD final state for BP1 and SM backgrounds.


2b+ Same-sign di-lepton

The number of events in the window mb−b−l within250(400) ± 50 GeV.

mb−b−l

Signal Backgrounds

tt t tbb t tZ tth VV ttW

BP1 117.37 4.0 0.35 7.00 2.67 0.0 2.50

BP2 11.74 0.0 0.0 1.71 0.12 0.0 1.05

mbbl at 10 fb−1 integrated luminosity the reach is ≃ 10σ forBP1, and 3σ for BP2.


Bounds from LHC

CMS searches at√

S = 7 TeV at 4.9 fb−1 for trilepton +missing energy signatures put bounds on the triplet mass,mΣ ≥ 140 GeV to 200 GeV depending on the mixing angle θ.

CMS PAS EXO-11-073

The mixing angle, θ ≥ 10−6 for the bounds.


Inverse Seesaw

Inverse Seesaw mechanism explains tiny neutrino mass withO(1) Yukawa coupling.

This leads observable signature at the LHC.

We consider a gauged B − L symmetry which is spontaneouslybroken at the TeV scale.

The B − L Higgs boson can have sizable mixing with SMHiggs boson which can change the signatures at the LHC.

Some phenomelogical studies can be found in the literature3.

3Mohapatra et al, Khalil et al, Dev et al, Das et al, Okada et al, Fischer et al


Inverse Seesaw

The minimal field content realizing the Inverse Seesawmechanism with B − L

Particle Q uc , dc L ec ,N S1 S2 Φ χYB−L 1/3 -1/3 -1 1 -1/2 1/2 0 -1/2

A pair of fermionic S1 and S2 is required to cancel theanomaly.

The SM and B − L Higgs bosons are denoted by Φ and χ,respectively.


Inverse Seesaw

The leptonic sector of the Lagrangian

−L = yℓLΦec+yνLΦcN+ySNχS1+λS1

Λχ†2S2

1+λS2

Λχ2S2

2+h.c .

where Φc ≡ ǫΦ∗ and Λ is a cut-off scale

The mass term S1S2 can be suppressed by introducing, adiscrete symmetry Z2 under which S2 is odd and the othersare even.

The symmetry breaking leads, χ = (χ0 + v ′)/√

2,Φ = (φ+, φ)T with φ = (φ0 + v)/

√2,


Inverse Seesaw

After symmetry breaking the neutrino sector is,

Lνm = mDν

′N + MNNS1 + µSS21 + h.c .

where mD = yνv/√

2, MN = ySv ′/√

2 and µS = λS1v ′2/2Λ

The 3 × 3 neutrino mass matrix of one generation takes theform:

0 mD 0mD 0 MN

0 MN µs

.

⇒ The neutrino mass is given by,

mν = µsm2D/M

2N =

µs

MN

m2D

MN


Inverse Seesaw

In the Higgs sector χ and φ mixes as follows:

(

φ0

χ0

)

=

(

cosα sinα− sinα cosα

)(

h

H

)

,

where h is the SM like Higgs and H is the heavy Higgs.

LEP II data requiring mZ ′/gB−L = |Y χB−L|v ′ > 6 TeV,

we set v ′ = 12 TeV.


Phenomenology

h, H

h, H

ν

Ψ

l

W

l ′

ν ′


Phenomenology

We can see that one lepton is coming from the Higgs decaywhile the other one is from W± with opposite sign.

We explored the hadronically quiet opposite sign di-leptonfinal state

Following benchmark points are chosen for collider study

Benchmark mh mH mΨ cosα

Points (GeV) (GeV) (GeV)

BP1 50 125 100 0.1

BP2 50 125 100 0.25

BP3 125 200 100 0.8

BP4 125 300 100 0.8


Phenomenology

ATLAS and CMS looked for SM like H → WW ∗ → 2ℓ+ 6pT

The event selection requires 25 GeV and 15 GeV for theleading and sub-leading leptons.

In principle a soft lepton coming from the decay ofright-handed neutrino could be missed.

To look for our final state we select leptons of pT ≥ 5 GeV.


Phenomenology

We used PYTHIA for simulation with the following cuts

pjetT ,min = 20 GeV

leptons (ℓ = e, µ) are selected with pℓ1T ≥ 20, pℓ2

T ≤ 30 GeVand |η| ≤ 2.5

no jet should match with a hard lepton in the event(∆Rj ,l ≥ 0.4, ∆Rl ,l ≥ 0.2)

6pT ≥ 30 GeV


Phenomenology

WW , WZ , ZZ , tt, Z/γ + jets are considered as SMbackgrounds.

A 5σ signal significance at LHC@8 TeV require around 20fb−1 of integrated luminosity.

When yνµ>> yνe , favoured by the 7 TeV LHC data, we have

lepton flavour violating signal.⇒ 2µ− 2e final state.


Phenomenology

0

100

200

300

400

500

600

700

0 50 100 150 200

Nu

mb

er

of

eve

nts

Mll GeV

BP4 H

With 14 TeV and higher luminosity we can explore thedi-leptonic edge⇒ which will unveil the mass hierarchy of the right-handedneutrino and the Higgs.


Conclusions

Higgs searches from triplet fermions decay could beinteresting at the LHC

For low seesaw scale the reach could be possible at early dataof LHC

For higher seesaw mass scale as the production cross-sectiondrops down the reach is possible only at higher luminosity.to kill the standard model backgrounds.

In particular multi-lepton scenarios are good for massmeasurements; Higgs and the triplet fermions.

Hadronically quiet di-leptonic final state can probe the inverseseesaw scenario.

Lepton flavour violating signature could be an interestingprobe for the extra right-handed neutrino decay.

Di-leptonic edge in higher luminosity can probe the masshierarchy between the Higgs(es) and the right-handedneutrino.


Thank you

probing higgs in type iii seesaw at the large hadron...

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