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Neutrino Mass Origin of Matter: Probing at LHC . R. N. Mohapatra MPI-Heidelberg Seminar,2009. Universe is full of matter and “no” anti-matter. How do we know ? (i) Solar probes have not exploded- (ii) Sun sends us billions of particles and no

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neutrino mass origin of matter probing at lhc

Neutrino Mass Origin of Matter: Probing at LHC

R. N. Mohapatra

MPI-Heidelberg Seminar,2009

universe is full of matter and no anti matter
Universe is full of matter and “no” anti-matter
  • How do we know ?

(i) Solar probes have not exploded-

(ii) Sun sends us billions of particles and no

antiparticles since there are no natural fireworks

in the sky-

(iii) Anti-matter fraction in cosmic rays: 1 in 10,000

(completely understandable in terms of known

particle physics.)

big bang nucleosynthesis
Big Bang Nucleosynthesis
  • In the beginning, when the Universe was only a second old, there were only protons and neutrons- so how did all the elements we are made of formed ?
matter amount
Matter Amount
  • Detailed analysis
was it put in by hand at the beginning
Was it put in by hand at the beginning ?

Not tenable since inflation empties the universe—

  • It must be created by microphysics during evolution
baryon asymmetry from microphysics
Baryon asymmetry from Microphysics
  • Sakharov Proposed 3 conditions for generating baryon asymmetry using microphysics (1967)
  • Baryon number violation;
  • CP violation;
  • Out of Thermal Equilibrium.
  • Standard model cannot do-although it has both CP and B-violation (too small CPV and not light enough Higgs).
premise of the talk
Premise of the Talk:
  • Discovery of neutrino mass requires new physics beyond SM which has provided a promising possibility for explaining the matter-antimatter asym.
  • Can we test this physics at LHC ?
  • TeV scale Z’ related to -mass:

(Blanchet, Chacko, Granor, RNM: arXiv:0904.2974)

seesaw paradigm for neutrino mass
Seesaw Paradigm for neutrino mass
  • Why ?
  • Add right handed neutrinos to SM with Majorana mass:
  • Breaks B-L

sym. of SM:

  • After electroweak

symmetry breaking

  • Note: if MR=0, (too small)

whereas even with TeV MR, (more reasonable)

B-L breaking crucial to seesaw:

Minkowski; Gell-Mann, Ramond Slansky,Yanagida, R.N.M.,Senjanovic,Glashow

seesaw and origin of matter
Seesaw and Origin of matter
  • Proposal:(Fukugita and Yanagida ,1986)
  • Generates lepton asymmetry:
  • Gets converted to baryons via sphaleron interactions

of SM (‘t Hooft) (Kuzmin,Rubakov,Shaposnikov)

  • No new interactions needed other than those already used for generating neutrino masses !!
  • Seesaw provides a common understanding of both neutrino masses and origin of matter in the Universe.
  • What is the seesaw scale ?
  • Is scale right for baryogenesis ?
  • Important because scale determines whether the idea is testable !!
seesaw scale
Seesaw scale
  • Neutrino masses do not determine the seesaw scale- we do not know in seesaw formula
  • Type I seesaw +

GUT - GeV- Small neutrino mass could be indication for SUSYGUT;

Many interesting SO(10) GUT models.

  • No collider signals ! Possible tests in nu-osc.
  • With SUSY, in (dependent on SUSY scale)
  • Seesaw scale is around TeV

(corresponding Yukawa~ ) ;

  • Natural -protected by chiral sym.
  • Many collider signals, possibly ,
leptogenesis scale
Leptogenesis Scale
  • Diagrams:
  • Two classes of models depending on RH mass pattern
  • High Scale leptogenesis: Expected in GUT theories:Adequate asymmetry for lightest RH (for hierarchical masses)(Buchmuller, Plumacher,di Bari; Davidson, Ibarra)
  • Resonant leptogenesis: degenerateN’s, self energy diagram dominates:~ ; Resonance when ;works for all B-L scales.

(Liu and Segre’94; Covi et al’95 ; Flanz et al.’95 Pilaftsis’97)

an issue with high scale susy leptogenesis
An Issue with High scale SUSY Leptogenesis
  • Recall the lower bound on the lightest RH neutrino mass for enough baryons in GUTs
  • Problem for supersymmetric models:

they have gravitinos with TeV mass that are produced during inflation reheat along with all SM particles-

  • Will overclose the universe if stable for TR>10^9 GeV.
  • If unstable, Once produced they live too long -affect BBN. . (Kohri et al.)
  • Possible tension between LFV and leptogen.
tension between gravitino and high scale leptogenesis
Tension between gravitino and high scale leptogenesis
  • Overclosure for stable and BBN constraint for unstable ones: (Kawasaki, Kohri, Moroi,Yatsuyanagi,2008)
leptogenesis and
Leptogenesis and
  • Both depend on RH neutrino mass hierarchy !!
  • (Chun,Evans,Morrissey,Wells’08; Petcov,Rodejohann,Shindou,Takanishi’05)

No such conflict for TeV scale leptogenesis !! Goes well with collider friendly TeV seesaw !

minimal tev scale seesaw
Minimal TeV scale seesaw
  • No new interactions: N production at LHC can happen only through mixing ;

cross section observable

only if mixing is >

  • (del Aguila,Aguilar-Savedra, Pittau)
  • However observed neutrino masses via seesaw for

~100 GeV implies Not observable at LHC.

  • Exceptions possible with specific extra global symmetries-
tev seesaw with b l forces z
TeV Seesaw with B-L forces (Z’)
  • Seesaw effect observable at LHC even with tiny

mixings as in generic neutrino models.

  • pp Z’+X; Z’NN followed by N-decay;
  • Like sign dileptons is the tell-tale seesaw signal.
how plausible is local b l
How plausible is Local B-L ?
  • Neutrino masses  seesaw scale much lower than Planck scale New symmetry(B-L).
  • Is B-L global or local ?
  • SM  only Tr (B-L)[SU(2)]^2=0 but
  • But SM + 
  • B-L is a potential gauge symmetry-
  • Gauged B-L eliminates R-parity problem of MSSM and ensures proton stability and dark matter: Another advantage of B-L(RNM’86; Martin’92)
  • Extend SM gauge symmetry to include B-L- many ways-
two faces of b l
Two Faces of B-L
  • Separate B-L vs SO(10) inspired B-L:
  • For low B-L scale(TeV range), need B-L=2 Higgs

to break symmetry to implement seesaw, if no new physics upto Planck scale.

tev z cross section at lhc
TeV Z’ cross section at LHC
  • LHC Z’ reach - 4 TeV
  • Cross section for ppZ’NN (Z’NN branching ratio ~20%)

2.5 TeV Z’


5 TeV

testing seesaw with z decay
Testing seesaw with Z’ decay
  • PPZ’+X; xsection for a 3 TeV Z’ ~fb
  • Seesaw signal: N=Majorana
  • N l W , Wjj ,
  • Di and Multi-lepton events: (X=jjjj)
  • Important for signal to bg: very high pT leptons coming from N-decay; inv mass reconstruction:

(Del Aguila, Aguilar-Saavedra; Aguilar-Saavedra )

tev scale resonant leptogenesis with z
TeV scale Resonant leptogenesis with Z’
  • Conditions:

(i)RH neutrinos must be degenerate in mass;

since h >10^-5 degeneracy ok anywhere from ;technically natural and enough for baryogenesis!

(ii) Since there are fast processes at that temperature, the net lepton asymmetry and primordial lepton asym are related by

where <1 (efficiency factor); depends on rates for Z’ med. scatt. ;inverse decay

  • Finding :

(Buchmuller,di Bari Plumacher)

  • Note: very small, when S >> D- i.e. lighter Z’;
  • As MZ’ increases, S ~ D, gets bigger and there is a large range where adequate leptogen is possible.
  • Adequate leptogenesis implies a lower limit on MZ’
  • Is the lower limit in the LHC accessible range ?

Yes; MZ’ > 2.5 TeV for MZ’ > 2MN

  • Can LHC directly probe the primordial lepton asymmetry ?
can lhc directly probe the primordial lepton asym
Can LHC Directly probe the primordial lepton asym. ?
  • Since , small efficiency means large ; Search for where is tiny so is order 1.
  • Detectable at LHC by searching for like sign leptons
  • (Blanchet, Chacko, Granor, RNM: arXiv:0904.2974)
  • Basic idea:
  • At LHC, PPZ’+X
  • 12.5% of time NNl Xl X
  • Look for a CP violating observable !
direct probe of resonant leptogenesis contd
Direct probe of resonant leptogenesis, contd.
  • Direct link between primordial lepton asymmetry and CP violating LHC observable:
  • For a ranges of Z’-N mass,

very small so that ~0.1-1;

visible at LHC:

  • Similarly for tri-lepton events.
  • Lower bound on MZ’ >2.5 TeV.
  • 300 fb^-1, expect 255 dilepton events (85% det eff.)
  • 90% of events with jets or one missing E.
  • With no CP violation: 16 ++ and - - events;
  • Should rule out at 2 sigma level.
  • An observation will directly probe leptogenesis, if RH mass deg. is inferred from inv mass study.
  • How to tell how many N’s ?
  • For one N, there are 5 observables, but only two inputs; we have three relations of type:
  • For 2 N’s, 4 inputs and 5 observables; only one relation. none for three !
how natural is degenerate rh spectrum
How natural is degenerate RH spectrum ?
  • Degenerate RH neutrino specctrum might look odd since quark and charged lepton masses are very hierarchical:
  • RH vs Q,L masses:

(i) RH nu’s are Majorana masses whereas q, l masses

are Dirac;

(ii) RH masses arise at different scale and from a

different mechanism (B-L breaking) as against the

Q, L masses which arise from SM symmetry br.

(iii) Already large neutrino mixings are an indication

that in the seesaw formula RH neutrinos must

have some peculiarity.

a model
A model
  • Gauge group xO(3)H

with RH nu’s triplet under O(3)H – all other fermion fields singlet.

  • Higgs: 1,2 + SM like Higgs.
  • Seesaw arises from following Yukawa Lagrangian:
  • Choose will give desired parameters.
  • Since Dirac Yukawas are ~10^-5, RH neutrino mass splitting is radiatively stable.
left right embedding
Left-right embedding
  • Left-right Model:
  • Solves SUSY and Strong CP in addition to automatic RP
  • UnlessMWR > 18 TeV,
  • L-violating scatterings e.g.

willerase lepton asymmetry.

(Frere, Hambye and Vertongen)

- Sym br. to U(1)I3RxU(1)B-L SM at TeV-

to do resonant leptogenesis.

avoiding the wr bound
Avoiding the WR bound:
  • If there are heavy vector like D-quarks mixing with d in such a way that the doublet coupling to WR becomes: for D-mass in the 10 TeV range,

the dominant process does not occur. We need

to avoid the WR bound.

  • WR can be in the LHC range but the decay modes purely leptonic.
resonant leptogenesis in generic lr model
Resonant leptogenesis in generic LR model
  • Key question is whether degenerate RH neutrino spectrum is radiatively stable to have leptogeneesis possible generic LR models !!
  • Yes- since largest rad correction to RH masses


  • Whereas CP asymmetry is:
  • Which gives for h~10^-5.5,
  • Not visible from Z’ decay but nonetheless a viable low scale model for leptogenesis and dark matter !!
a specific lr model
A specific LR model:
  • LR+extra symmetries: xU(1)xxU(1)Z
  • Leads to RH mass matrix of form:
  • Leads to two deg RH nu’s;
  • Dirac mass matrix:
  • Leads to realistic nu masses and mixings as well as resonant leptogenesis with tiny sym br. Effects.
new collider signals for lr case
New collider signals for LR case
  • Even if WR may be out of reach due to baryogenesis constraints, other exotic Higgs bosons in LHC reach
  • gets embedded into
  • Predicts doubly charged Higgs bosons in the sub-TeV mass range coupling to like sign dileptons:
  • Resonant leptogenesis  dominant modes;
  • No but allowed.
unification prospects an so 10 possibility
Unification Prospects: An SO(10) possibility
  • Triplets with B-L=2 hard to unify to SUSY SO(10).
  • Both for TeV Z’ and WR, unification possible with B-L =1 doublets breaking U(1)B-L; (Deshpande, Keith and Rizzo; 93; Malinsky, Romao, Valle’05);
neutrino masses
Neutrino masses
  • Requires double seesaw for neutrino masses: Add an extra singlet field S in addition to left and RH neutrinos which are part of {16};
  • Double seesaw:N S)
  • (RNM’86; RNM,Valle’86)
  • Important: Unlike type I seesaw, Majorana character of RH N depends on how large is.
  • Suppresses like sign dileptons at LHC unless ~1.

Leptogenesis possible but visible only for ~1.

low scale susy lr an alternative to mssm
Low scale SUSY LR-an Alternative to MSSM
  • MSSM: SO(10) Unified SUSYLR

1. Rapid p-decay due to

RP breaking

2. Neutrino mass not easy

3. EW baryo in a corner

of parameter space.

4. Light Higgs and stop

5. DM gravitino/Neutra


  • 1.No dim4 p-decay due to B-L
  • 2.Double seesaw for nu mass
  • Explains Origin of matter
  • Z’ and like sign dileptons
  • at LHC
  • 5. DM gravitino/Neutralino
  • LHC can directly probe the seesaw mechanism for neutrino masses if the seesaw scale is in the TeV range and there is a TeV scale Z’ regardless of neutrino mass pattern.
  • For certain ranges of the Z’-N mass, LHC can probe resonant leptogenesis mechanism for the origin of matter directly -find Z’-N in the allowed range simultaneously with large like sign dilepton CP asymmetry.
  • Use of inv mass peak and large PT leptons to reduce background.
  • There are left-right and SO(10) SUSY GUT models where such scenarios can be embedded, providing theoretical motivation for low scale Z’ as well as TeV scale leptogenesis .
extra slides
Extra slides
  • Post-sphaleron baryogenesis and color sextet scalars at LHC.
what if rh neutrinos are tev scale but non degenerate
What if RH neutrinos are TeV scale but non-degenerate ?
  • Can one have seesaw scale around a TeV so LHC can see it and still understand the origin of matter related to seesaw physics ?
  • Yes- baryogenesis can arise from seesaw related physics below 100 GeV (but not from RH N decay) (post-sphaleron baryogenesis) (Babu, RNM, Nasri’06)
  • Predicts light color sextet Higgs (< TeV) that can be observed at LHC via decay to two tops.
q l unify tev seesaw
Q-L unify TeV seesaw
  • SU(2)LxU(1)RxU(1)B-L SU(2)LxU(1)RxSU(4)PS.
  • Recall Origin of RH nu mass for seesaw is from
  • Q-L unif. implies quark partners for i.e. - color sextet scalars coupling to up quarks ; similar for dd- only right handed quarks couple. Come from (1, 1, 10)
  • SU(4)PS breaks toU(1)B-L above 100 TeV
baryon violation graph
Baryon violation graph
  • +

+ h. c.

  • B=2 but no B=1; hence proton is stable but neutron can convert to anti-neutron!
  • N-N-bar diagram
  • coupling crucial to get baryogenesis (see later)
origin of matter
Origin of matter
  • (Babu, Nasri, RNM, 2006)
  • Call Re = Sr ; TeV mass : S-vev generates seesaw and

leading to B-violating decays

  • Baryogenesis: Due to high dimension of operator, B-violating process goes out of eq. below 100 GeV.
upper limits on s r and color sextet masses
Upper limits on Sr and color sextet masses:
  • Two key constraints:

 MS < 500-700 GeV to get right amount of baryons.

  • Decay before QCD phase transition temp:
  • Implies MS< MX < 2 MS.
two experimental implications
Two experimental implications:
  • oscillation:successful baryogenesis implies that color sextets are light (< TeV) (Babu, RNM, Nasri,06; Babu, Dev, RNM’08);

arises via the diagram:

  • Present limit: ILL >10^8 sec. similar bounds from Soudan,S-K etc.
  • 10^11 sec. reachable with available facilities !!
  • A collaboration for NNbar search with about 40 members exists-Exploration of various reactor sites under way for a second round search.
color sextet scalars at lhc
Color sextet scalars at LHC
  • Low seesaw scale + baryogenesis requires that sextet scalars must be around or below a TeV:
  • Two production modes at LHC:

(I) Single production:

xsection calculated in (RNM, Okada, Yu’07;) resonance peaks above

SM background- decay to tt or tj depending on RH nu

Majorana coupling; directly measures seesaw parameters.

(II) Drell-Yan pair production:

( Chen, Klem, Rentala, Wang, 08)

  • Leads to final states: LHC reach < TeV

Single Sextet production at LHC:

Diquark has a baryon number & LHC is ``pp’’ machine

Depends on Yukawa coupling

pair production of deltas
Pair Production of Deltas
  • Due to color sextet nature, Drell-Yan production reasonable- independent of Yukawa coupling
  • Leads to final states:
  • Can be probed upto a TeV

using like sign dilepton mode.


Phenomenological Aspects

Constraints by rare processes


Similarly B-B-bar etc. Can generate neutrino masses - satisfying FCNC

examples of color sextet couplings that work
Examples of color sextet couplings that work.
  • Down sector:
  • Fits neutrino mass via type I seesaw.
collider signal with wr
Collider signal with WR
  • Depends on mass of WR; for WR in the few TeV range, N-decay profile changes:
  • No WR case:
  • With WR (TeV)
  • No missing E in second case;
  • Trilepton signal very sub-dominant.