TeV Seesaws and

Signatures at LHC

Yong-Yeon KeumKorea U. and CTP-BUE

APCTP-2009 LHC-physics Workshop 25-27 Aug. 2009, Konkuk University

Contents:

• Origin of -mass: Seesaws

• Why TeV Seesaws?

• Type-I Seesaw and LHC-signatures

• Type-II Seesaw and LHC-signatures

• Type-(I+II) Seesaw and LHC-signatures

• Type-III Seesaw and LHC-signatures

Origin of -mass: Seesaws

A natural theoretical way to understand why 3 -masses are very small.

Type-III: SM + 3 triplet fermions (Foot, Lew, He, Joshi 89)

Type-I: SM + 3 right-handed Majorana ’s (Minkowski 77; Yanagida 79; Glashow 79; Gell-Mann, Ramond, Slanski 79; Mohapatra, Senjanovic 79)

Type-II: SM + 1 Higgs triplet (Magg, Wetterich 80; Schechter, Valle 80; Lazarides et al 80; Mohapatra, Senjanovic 80; Gelmini, Roncadelli 80)

Other variations or combinations (e.g., type-I + type-II in SO(10) GUT)

Some Recent Works★ Han, Zhang, PRL (06)★ Buckley, Murayama, PLB (06)★ del Aguila et al, JPCS (06)★ Bar-Shalon et al, PLB (06)

★ de Gouvea et al, PRD (07)

★ Atwood et al, PRD (07)

★ del Aguila et al, JHEP (07)

★ de Almeidaet al, PRD (07)★ Chen, Mahanthappa, PRD (07)★ Bajc et al, PRD (07)★ Graesser, PRD (07)★ Kersten, Smirnov, PRD (07) ★ Xing, PLB (08)★ de Gouvea, Jenkins, PRD (08)★ Chen et al, arXiv:0801.2011★ Bar-Shalon et al, arXiv:0803.2835★ Hirsch et al, arXiv:0804.4072★ del Aguila et al, arXiv:0806.0876★ Cogollo et al, arXiv:0806.3087★ Murayama, arXiv:0807.3775★ Perez et al., arXiv:0907.4186………

★ Hektor et al, NPB (07)★ Han et al, PRD (07)★ Dorsner, Mocioiu, NPB (08)★ Goravoa, Schwetz, JHEP (08)★ Chao et al, PRD (08)★ Akeroyd et al, PRD (08)★ McDonald et al, JCAP (08)★ Xing, PRD (08)★ Ren, Xing, PLB (08)★ Gogoladze et al, arXiv:0802.3257

★ Chao et al, arXiv:0804.1265

★ Fileviez Perez et al, arXiv:0805.3536★ Hirsch et al, arXiv:0806.3361★ ……

★ Barr, Dorsner, PLB (06)★ Bajc, Senjanovic, JHEP (07)★ Fileviez Perez, PLB (07)★ Dorsner, Fileviez Perez, JHEP (07)★ Abada et al, JHEP (07)★ Abada et al, arXiv:0803.0481

★ Franceschini et al, arXiv:0805.1613

★ Gogoladze et al, arXiv:0805.2129★ Mohapatra et al, arXiv:0807.4524★ Tong Li, X.G. He, arXiv:0907.4193……..

Type-I

Type-II

Type-III

How to experimentally distinguish one type from another?

What is the first step to test seesaws at the LHC?

ANSWER:

to discover the Higgs boson(s)

and

to verify the Yukawa interactions

Why TeV Seesaws?

Is the seesaw mechanism of -mass generation testable or not?

Planck

Fermi

GUT to unify strong, weak & electromagnetic forces?

TeV to solve the unnatural gauge hierarchy problem?

Is the “seesaw scale” close to a fundamental physics scale?

Conventional (Type-one) Seesaw Picture: close to the GUT scale

TeV Seesaw Idea: driven by testability at LHC

Naturalness? Testability?

RD M/M~S~RSeesaw:

Strength of Unitarity Violation

Hence V is not unitary

Diagonalization (flavor basis mass basis):

Type-I Seesaw: add 3 right-handed Majorana neutrinos into the SM.

or

Type-I Seesaw

Natural or Unnatural?

TeV-scale (right-handed) Majorana neutrinos: small masses of light Majorana neutrinos come from sub-leading perturbations.

Unnatural case: large cancellation in the leading seesaw term.

TMMMM D

1

RD

-

0.01 eV 100 GeVTeV 1

2

1

RD

-

10

10 ~Violation Unitarity

~M/M~S~R

Natural case: no large cancellation in the leading seesaw term.

TMMMM D

1

RD

-

0.01 eV 100 GeV

GeV 1015

26

13

RD

-

10

10 ~Violation Unitarity

~M/M~S~R

Structural Cancellation

Given diagonal M_R with 3 eigenvalues M_1, M_2 and M_3, the leading (i.e., type-I seesaw) term of the light neutrino mass matrix vanishes, if and only if M_D has rank 1, and if

(Buchmueller, Greub 91; Ingelman, Rathsman 93; Heusch, Minkowski 94; ……; Kersten, Smirnov 07).

0D

1

RD T

ν M-

MMM

DM

Tiny -masses can be generated from tiny corrections to this complete “structural cancellation”, by deforming M_D or M_R .

Simple example:

Lessons

Lesson 1: two necessary conditions to test a seesaw model with heavy right-handed Majorana neutrinos at the LHC:

(A) Masses of heavy Majorana neutrinos must be of O (1) TeV or below; (B) Light-heavy neutrino mixing (i.e., M_D/M_R) must be large enough.

Lesson 2: LHC-collider signatures of heavy Majorana ’s are essentially decoupled from masses and mixing parameters of light Majorana ’s.

Lesson 3: non-unitarity of the light neutrino flavor mixing matrix might lead to observable effects in neutrino oscillations and rare processes.

Lesson 4: nontrivial limits on heavy Majorana neutrinos can be derived at the LHC, if the SM backgrounds are small for a specific final state.

L = 2 like-sign dilepton events

Collider Signature

Lepton number violation: like-sign dilepton events at hadron colliders, such as Tevatron (~2 TeV) and LHC (~14 TeV).

collider analogue to 0 decay

N can be produced on resonance

dominant channel

Numerical Illustration

Han, Zhang (hep-ph/0604064, PRL): cross sections are generally smaller for larger masses of heavy Majorana neutrinos.

Tevatron LHC

Del Aguila et al (hep-ph/0606198): signal & background cross sections (in fb) as a function of the heavy Majorana neutrino mass (in GeV).

A single heavy N(minimal Type-II)

Type-II Seesaw

Type-II (Triplet) Seesaw: add 1 SU(2)_L Higgs triplet into the SM.

or

Potential:

L and B–L violationNaturalness? (t’ Hooft 79, …, Giudice 08)(1) M_ is O(1) TeV or close to the scale of gauge symmetry breaking. (2) _ must be tiny, and _ =0 enhances the symmetry of the model.

M

YM2

L

v

0.01 eV TeV 1

......

10

10 1,

106

12

12 ~Y~

~~Y

~Y

Collider Signature

From the viewpoint of direct tests, the triplet seesaw has an advantage: The SU(2)_L Higgs triplet contains a doubly-charged scalar that can be produced at colliders, depending only on its mass and independently of the Yukawa coupling.

Signatures: Rough number of events for pair (N_4l) and single (N_2l) production of doubly-charged Higgs at the LHC (See, e.g., Han et al07; Akeroyd et al 08; Perez et al 08; ……)

Type-(I+II) Seesaw

Incomplete cancellation between two leading terms of the light neutrino mass matrix in type-II seesaw scenarios. The residue of this incomplete cancellation generates the neutrino masses:

not small

not small

collider signature

tiny mass generation

Collider signatures: both heavy Majorana neutrinos and doubly-charged scalars are possible to be produced at the LHC (e.g., Azuleos et al 06; del Aguila et al 07; Han et al 07; ….). But decoupling between collider physics & the mechanism of neutrino mass generation is very possible.

Discrete flavor symmetries may be used to arrange the textures of two mass terms, but fine-tuning seems unavoidable in the (Big – Big) case.

Type-III Seesaw

• Type-III Seesaw: add left-handed triplet leptons into the SM.

• Unlike type I seesaw model, the doublet charged leptons mix with the triplet charged leptons to tree level flavor changing neutral current involving changed leptons

• LHC-signatures:

(1) Heavy triplet lepton production:

C

0 0

L 0 0

L Y(1,3,0) under SU(3) x SU(2) x U(1)

, Charge Conj. form

Define: ; and

/ 2 / 2

/ 2 / 2

, )

0(

c c

L

c c

L L L L R

L

l

m R R

L

L L Lc

Lc c

L L L L

mL l

Y M

0

0

+ h.c.

0 / 2 2 ( , ) + h.c.

/ 2 2 / 2

L

L

Lc c

L L

L

l

Y v

Y M

* * +

*

Z E E

W E N

pp

pp

Collider Signature

E ,N 0

0

Singnal channels for E and N:

1) 1-lepton channel

E Z, W ,

s:

2) 2-lepton channels:

H

N W , ,

H

l jjjj

l l jjjj

l l

l Z

,

3) 3-lepton channels:

4) 4-lepton channels:

5) 5-lepton channels:

6) 6-lepton channels:

l l

l l l

l l l l

l l l l l

l l l l

l l

Some Recent Works★ Han, Zhang, PRL (06)★ Buckley, Murayama, PLB (06)★ del Aguila et al, JPCS (06)★ Bar-Shalon et al, PLB (06)

★ de Gouvea et al, PRD (07)

★ Atwood et al, PRD (07)

★ del Aguila et al, JHEP (07)

★ de Almeidaet al, PRD (07)★ Chen, Mahanthappa, PRD (07)★ Bajc et al, PRD (07)★ Graesser, PRD (07)★ Kersten, Smirnov, PRD (07) ★ Xing, PLB (08)★ de Gouvea, Jenkins, PRD (08)★ Chen et al, arXiv:0801.2011★ Bar-Shalon et al, arXiv:0803.2835★ Hirsch et al, arXiv:0804.4072★ del Aguila et al, arXiv:0806.0876★ Cogollo et al, arXiv:0806.3087★ Murayama, arXiv:0807.3775★ Perez et al., arXiv:0907.4186………

★ Hektor et al, NPB (07)★ Han et al, PRD (07)★ Dorsner, Mocioiu, NPB (08)★ Goravoa, Schwetz, JHEP (08)★ Chao et al, PRD (08)★ Akeroyd et al, PRD (08)★ McDonald et al, JCAP (08)★ Xing, PRD (08)★ Ren, Xing, PLB (08)★ Gogoladze et al, arXiv:0802.3257

★ Chao et al, arXiv:0804.1265

★ Fileviez Perez et al, arXiv:0805.3536★ Hirsch et al, arXiv:0806.3361★ ……

★ Barr, Dorsner, PLB (06)★ Bajc, Senjanovic, JHEP (07)★ Fileviez Perez, PLB (07)★ Dorsner, Fileviez Perez, JHEP (07)★ Abada et al, JHEP (07)★ Abada et al, arXiv:0803.0481

★ Franceschini et al, arXiv:0805.1613

★ Gogoladze et al, arXiv:0805.2129★ Mohapatra et al, arXiv:0807.4524★ Tong Li, X.G. He, arXiv:0907.4193……..

Type-I

Type-II

Type-III

How to experimentally distinguish one type from another?

Remarks

Naturalness of the SM implies that there should exist a kind of new physics at the TeV scale. We wonder whether it is also responsible for the neutrino mass generation ---- TeV seesaws.

An uneasy feeling ---- the generation of tiny neutrino masses seems always to be decoupled from appreciable collider signals of TeV Majorana neutrinos. Unnatural? Unnatural? Unnatural?

Non-unitary CP Violation is a straightforward consequence of TeV seesaws---- it might manifest itself in both the oscillations of light neutrinos and the decays of heavy neutrinos.

It seems that people are struggling for a convincing reason to consider TeV seesaws ---- a balance between TH naturalness and EX testability as the guiding principle?