1. Discovery of Sterile Neutrinos at the LHC
Intensity Frontier (Sep 26, 2016)
UW-Madison (Aug 17, 2016)
Planck-2016 (May 25, 2016)
Univ. of Vienna (May 10, 2016)
TUM (May 3, 2016)
MPI-Heidelberg (Apr 25, 2016)
SNU (Dec 10, 2015)
IHEP (Nov 24, 2015)
C. S. Kim
(Yonsei University)
Sterile N at LHC C S Kim

2. Discovery of Light Sterile Neutrino (M_N<M_W) at the LHC
Talk based on
arXiv: (PRD 92 (2015) )
C Dib, C S Kim
arXiv: (PRD 94 (2016) )
C Dib, C S Kim, K Wang, J Zhang
Sterile N at LHC C S Kim

3. “Physics – Spotlighting Exceptional Research” http://physics.aps.org/
APS News (Nov 18, 2015) for
“Physics – Spotlighting Exceptional Research”
Particles & Fields
SYNOPSIS: LHC Data Might Reveal Nature of Neutrinos
A long-standing question over whether the neutrino is its own antiparticle might be answered by looking at decays of W bosons.
Sterile N at LHC C S Kim

4.
Sterile N at LHC C S Kim

5. Outline 1. Introduction Probing Majorana Neutrinos via (a) 0nuBB
(b) at the LHC
2. Discovery of Light Sterile Neutrino at LHC
2.-1 Counting out Dirac Sterile Neutrinos
3. Summary & Conclusions
**4. K, D, Ds, B, Bc meson RARE decays
**5. A few Issues in probing experimentally
Sterile N at LHC C S Kim

6. 1. Introduction Neutrinos are massless in the SM
No right-handed ’s  Dirac mass term is not allowed.
Conserves the SU(2)_L gauge symmetry, and only contains
the Higgs doublet (the SM accidently possesses (B - L)
symmetry);  Majorana mass term is forbidden.
BUT, Neutrinos are massive.
Sterile N at LHC C S Kim

7. Why are physicists interested in neutrino mass ?
 Window to high energy physics beyond the SM!
How exactly do we extend it?
 Without knowing if neutrinos are Dirac or Majorana,
any attempts to extend the Standard Model are
not successful.
Effective Observability of Difference between
Dirac and Majorana Nu is proportional to
Sterile N at LHC C S Kim

8. Possible range of (Sterile) Nu mass
From neutrino oscillation and WMAP:
from neutrino oscillation
from WMAP and Astrophysics
from nuMSM (a model)
(b) From dark matter searches:
from nuMSM, warm DM, …
from DAMA, CDMS, XENON, …
from SUSY, EDM, …
(C) From BAU and Inflation
(D) From usual see-saw
(E) We can assume any value of , which will be determined
by experiments.
Sterile N at LHC C S Kim

9. Possible bound of (Sterile) N mixing:
 PMNS Mixing Sterile N Mixing
In nuMSM, see-saw with (light RH sterile) N gives:
We can assume any value of , which will be determined
by experiments.
Present bounds for heavy N [Nardi etal, PLB327,319]
Sterile N at LHC C S Kim

10. A. Atre et al, JHEP05(2009)030
Sterile N at LHC C S Kim

11. * Probing Majorana neutrinos
Lepton number violation by 2 units plays a crucial role
to probe the Majorana nature of ’s,
(a) The observation of 0
Black Box 0 ,
Provides a promising lab. method for determining the
absolute neutrino mass scale that is complementary to
other measurement techniques
Sterile N at LHC C S Kim

12.
Sterile N at LHC C S Kim

13. the half-life time, ,of the 0nbb decay can be factorized as :
In the limit of small neutrino masses :
the half-life time, ,of the 0nbb decay can be factorized as :
: Nuclear matrix element
: phase space factor
: effective neutrino mass (model independent)
 depends on neutrino mass hierarchy

14. Large uncertianties in NME
Uncertainties
(O.Cremonesi, 05)
Large uncertianties in NME
About factor of 100 in NME 
affect order 2-3 in |< mn>|
Sterile N at LHC C S Kim

15. (b) Probing Majorana Neutrinos at LHC
In accelerator-based experiments, neutrinos in the final
state are undetectable by the detectors, leading to the
“missing energy”.
So it is desirable to look for charged leptons in the final
state.
It is hard to avoid the TeV-scale physics to contribute
to flavor-changing effects in general whatever it is,
SUSY, extra dimensions, TeV seesaw, technicolor, Higgsless, little Higgs
Sterile N at LHC C S Kim

16. transition rates are proportional to
Basic process we consider
transition rates are proportional to
Sterile N at LHC C S Kim

17. Testability at the LHC Two necessary conditions to test at the LHC:
-- Masses of heavy Majorana n’s must be less than TeV
-- Light-heavy neutrino mixing (i.e., MD/MR) must be large
enough.
LHC signatures of heavy Majorana ’s are essentially
decoupled from masses and mixing parameters of
light Majorana ’s.
Non-unitarity of the light neutrino flavor mixing matrix might lead to observable effects.
Sterile N at LHC C S Kim

18. 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
j j =2 well isolated energetic jets; N=Majorana, M(N)>100 GeV
If N=Dirac and/or M(N)<100 GeV  Large Background
Sterile N at LHC C S Kim

19. 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
Sterile N at LHC C S Kim

20. Some Results
Cross sections are generally smaller for larger masses of heavy
Majorana neutrinos. [ Han, Zhang (hep-ph/ ) ]
Signal & background cross sections (in fb) as a function of the
heavy Majorana neutrino mass (in GeV) :
[ Del Aguila et al (hep-ph/ ) ]
*Background could be much larger by soft-piling up !!
Tevatron
LHC
Sterile N at LHC C S Kim

21. 2. Discovery of light sterile N at LHC
C Dib, CS Kim, arXiv: (PRD92(2015)093009)
In previous works for LHC, covered.
Is a way to cover ?
Cosmology & astrophysics motivate strongly GeV.
How about on-shell W leptonic decays?
Possible problems
Large radiative decays,
we choose from W+ decay (no radiative bg)
Final neutrino flavor not observed or
LNV (Majorana N) or LNC-LFV (Maj./Dirac N)
Sterile N at LHC C S Kim

22. vs two competing processes (for ) : LNV (only Majorana neutrino N)
LNC but LFV (Majorana or Dirac neutrino N)
** We cannot indentify the final neutrino flavor, or
Sterile N at LHC C S Kim

23. Comments: Same processes
and their C.C. (as long as no + - for the same flavor)
Comparison of vs.
need well isolated energetic 2 jets (same for )
(otherwise large background from WW, instead of WWW)
 m(N) > ~ m(W) for W -> l l j j
 m(N) < m(W) for W -> l l l nu
3. arXiv: considered only
not (flavor of final nu unidentifiable)
4. Reconstruction of on-shell W at LHC (hadronic colliders)
or for m(nu)~0
reconstructable event-by-event by using p(l) and m_T.
Sterile N at LHC C S Kim

24. Reconstruction event-by-event from on-shell W
Even with missing neutrino,
assuming p_T(W) = 0:
Sterile N at LHC C S Kim

25. LNV, Pure Majorana, Process:
Sterile N at LHC C S Kim

26. LNC but LFV, Pure Dirac, Process:
Sterile N at LHC C S Kim

27. Numerical studies and discussions
(A) branching ratios:
reduced BR for
Sterile N at LHC C S Kim

28. (B) Spectrum analysis to separate Majorana from Dirac neutrino:
Sterile N at LHC C S Kim

29.
Sterile N at LHC C S Kim

30. 2.-1 Discovery of light Sterile Neutrino at the LHC when Hierarchical Mixing
When |U_N e| <<, >> |U_N mu|
arXiv: (PRD94 (2016) )
(C Dib, C S Kim, K Wang, J Zhang)
Sterile N at LHC C S Kim

31. LNC but LFV, Pure Dirac, Process:
ALWAYS !!
Sterile N at LHC C S Kim

32. LNV, Pure Majorana, Process:
Only if
Sterile N at LHC C S Kim

33. Counting out Dirac Sterile Neutrinos
If N=Majorana, N decays as Dirac or as Majorana,
THEREFORE:
If N=Dirac,
If N=Majorana,
Sterile N at LHC C S Kim

34. C.L. of excluding Dirac N
Sterile N at LHC C S Kim

35. 3. Summary and Conclusions
Knowing that neutrinos are Dirac or Majorana is THE MOST
important to go beyond the SM.
We have discussed a new way to probe Majorana/Dirac
neutrinos with much less uncertainty for mass ranges
of , from the rare leptonic decay of on-shell
and
We investigated Br(W^+  e^+ e^+ mu^- nu)
as well as the energy spectrum of the decays to
separate the Majorana neutrino from Dirac one.
Sterile N at LHC C S Kim

36. Extra Slides
Sterile N at LHC C S Kim

37. **4. Probe of Majorana neutrinos via
rare decays of mesons
(G.Cvetic, C. Dib, S.Kang, C.S.Kim, arXiv: (PRD82,053010,2010))
Taking mesons in the initial and final state to be
pseudoscalar (M : K, D, Ds, B, Bc / M’=pi, K, D,…)
Not involve the uncertainties from nuclear matrix
elements in 0bnn
Sterile N at LHC C S Kim

38. Effective Hamiltonian:
Decay Amplitude:
Sterile N at LHC C S Kim

39. transition rates are proportional to
Sterile N at LHC C S Kim

40. For example, leptonic current :
Sterile N at LHC C S Kim

41. Model Independence of Effective Theory approach
This could be any gauge boson,
e.g ,…
This could be any Majorana particle,
e.g neutralino, heavy N,…
Propagator changed
Sterile N at LHC C S Kim

42. Intermediate mass scale neutrino case
dominant contribution to the process is from the “s-type” diagram because the neutrino propagator is kinematically entirely on-shell
Sterile N at LHC C S Kim

43. Effective amplitude at meson level:
If we neglect charged lepton masses;
Sterile N at LHC C S Kim

44. Br for as function of mN, with lepton mixings divided out
Sterile N at LHC C S Kim

45.
Sterile N at LHC C S Kim

46. **5-1. Issues on Displacement of Vertices
CSK et al
Gronau et al, PRD29(1984)2539
When in
a) N is on mass-shell
b) Two charged leptons at displaced vertices
c) Possibly 2ndary vertex fall outside of detector =
12/6/2018
IHEP Seminar C S Kim

47. Life time of N, Present bound For , Therefore, for 
Atre et al, JHEP05,035
Life time of N,
CSK et al
Present
bound
For ,
Therefore, for 
 Need consider vertex displacement
12/6/2018
IHEP Seminar C S Kim

48. Correct Branching Ratios
1 Theoretical BR w/ unlimited detectability
2 However, experimentally observed BR
where probability for N to decay inside detector (size )
12/6/2018
IHEP Seminar C S Kim

49.  Huge implication to the present bounds on
3 Therefore, the correct (theoretical) BR is
Ex) KEKB (3.5 on 8 GeV),
Ex) CERN LHC, m(N)~ GeV, P_N ~1
 Huge implication to the present bounds on
12/6/2018
IHEP Seminar C S Kim

50. **5-2. Issues on vs. What Belle/BaBar had done: Need recalculate this
G Cvetic, CS Kim, YJ Kwon, PRD 93(2015)
What Belle/BaBar had done:
Need recalculate this
12/6/2018
IHEP Seminar C S Kim

51. [ gone] Need to consider: Due to long lifetime of N, (and small )
When you see ,
is the event or ?
(where massive sterile (Dirac or Majorana) particle)
tau decay involves neutrinos, so m(nu) cannot be constrained.
12/6/2018
IHEP Seminar C S Kim

52. 12/6/2018
IHEP Seminar C S Kim

53. Need new numerical analysis (possibly 4 cases)
1 Within SM, 
2 With new physics (eg. 2HDM) with 
3 Within SM with sterile N, 
4 With new physics (eg. 2HDM) with 
Br(B^+  tau^+ + missing)_exp= (PDG average)= ( ) X 10^-4
Br(…)_SM= (CKMfitter)= ( ) X 10^-4
All parameter values from CKMfitter
12/6/2018
IHEP Seminar C S Kim

54. |B_tauN| from DELPHI, ZPC74(1997)57
Recent LHC (ATLAS, CMS) Results
for charged Higgs boson
NPB(Proc.Suppl.) 253(2014)171
12/6/2018
IHEP Seminar C S Kim

55. Numerical analysis
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IHEP Seminar C S Kim

56. 12/6/2018
IHEP Seminar C S Kim

57. 12/6/2018
IHEP Seminar C S Kim

58. 12/6/2018
IHEP Seminar C S Kim

59. 12/6/2018
IHEP Seminar C S Kim

60. **5-3. Issues of CP Violation in decays
CSK et al JHEP02(2015)108
CSK et al
**5-3. Issues of CP Violation in decays
CERN-SPSC (arXiv: [hep-ex]) :
CERN SPS Proposal to search Heavy Neutral Leptons
through CPV in at a new fixed target exp.
Theoretical background: nuMSM model
[T. Asala etal, PLB631,151; M. Shaposhnikov, PLB639,414, …] 
Experimental bounds for HNLs
12/6/2018
IHEP Seminar C S Kim

61. CSK et al
CP Violation in decays
CPV in
12/6/2018
IHEP Seminar C S Kim

62. 12/6/2018
IHEP Seminar C S Kim