1. The Hunt for Heavy Neutrinos
at the FCC
Alain Blondel Right handed Neutrinos at the FCC Clermont 2015
Alain Blondel University of Geneva
with many thanks to
S. Ganjour, M. Mitra, S. Pascoli, E. Graverini, P. Mermod, N. Serra, M. Shaposhnikov
courtesy J. Wenninger

2. Future Circular Collider Study - SCOPE
CDR and cost review for the next ESU (2018)
International collaboration to study:
100 TeV pp-collider (FCC-hh) Ultimate goal
 defining infrastructure requirements
e+e- collider (FCC-ee) as potential intermediate step
p-e (FCC-he) option
km infrastructure in Geneva area
Alain Blondel Right handed Neutrinos at the FCC Clermont 2015

3. possible long-term strategy
FCC-ee ( km,
e+e-, GeV
Interm. step
LEP
PSB
PS (0.6 km)
LHC (26.7 km)
HL-LHC
SPS (6.9 km)
FCC-hh
(pp, up to
100 TeV c.m.)
Ultimate goal
& e± (120 GeV)–p (7, 16 & 50 TeV) collisions FCC-eh)
≥60 years of e+e-, pp, ep/A physics at highest energies

4. 1000 50 5 4 at ZH threshold in LEP/LHC tunnel X 4 in FCC tunnel
Original motivation (end 2011): now that m_H and m_top are known,
explore EW region with a high precision, affordable, high luminosity machine
 Discovery of New Physics in rare phenomena or precision measurements
ILC studies  need increase over LEP 2 (average) luminosity by a factor 1000
How can one do that without exploding the power bill?
Answer is in the B-factory design:
a low vertical emittance ring with small *y (1mm vs 5cm at LEP)
 much shorter lifetime (few minutes)
 top up continuously with booster ==> increase operation efficiency
Increase SR beam power to 50MW/beam 50 5 4
1000
at ZH threshold
in LEP/LHC tunnel
X 4 in FCC tunnel
X 4 interaction points
EXCITING!

5. beam commissioning
will start in 2016
K. Oide

6. 93 km option – current baseline
LHC P1/P8 extraction (avoids Jura limestone)
Deepest shaft close to 400 m
(not optimized)
10,500m

7. Goal performance of e+ e- colliders
WOW!
complementarity with ILC/CLIC
complementarity
Luminosity : Crossing point between circular and linear colliders ~ GeV
As pointed out by H. Shopper in ‘The Lord of the Rings’ (Thanks to Superconducting RF…)
Combined know-how {LEP, LEP2 and b-factories} applied for large e+e- ring collider
High Luminosity + Energy resolution and Calibration  precision on Z, W, H, t
CAN WE DO IT? Many accelerator and experimental challenges!

8. Beam polarization and E-calibration @ FCC-ee
Precise meast of Ebeam by resonant depolarization
~100 keV each time the meast is made
At LEP transverse polarization was achieved routinely at Z peak.
instrumental in 10-3 measurement of the Z width in 1993
led to prediction of top quark mass ( GeV) in March 1994
Polarization in collisions was observed (40% at BBTS = 0.04)
At LEP beam energy spread destroyed polarization above 60 GeV
E  E2/  At FCC-ee transverse polarization up to at least 80 GeV
to go to much higher energies requires spin rotators and siberian snake
FCC-ee: use ‘single’ bunches to measure the beam energy continuously
no interpolation errors due to tides, ground motion or trains etc…
but saw-toothing must be well understood! require Wigglers to speed up pol. time
<< 100 keV beam energy calibration around Z peak and W pair threshold.
mZ ~0.1 MeV, Z ~0.1 MeV, mW ~ 0.5 MeV

9. PUBLISHED

10. TLEP: PARAMETERS & STATISTICS (e+e- -> ZH, e+e- → W+W-, e+e- → Z,[e+e-→ t 𝑡 ] )
TLEP-4 IP, per IP
statistics
circumference
80 km
max beam energy
175 GeV
no. of IPs 4
Luminosity/IP at 350 GeV c.m.
1.3x1034 cm-2s-1
106tt pairs
Luminosity/IP at 240 GeV c.m.
6.0x1034 cm-2s-1
2 106 ZH evts
Luminosity/IP at 160 GeV c.m.
1.6x1035 cm-2s-1
108 WW pairs
Luminosity/IP at 90 GeV c.m.
/36 cm-2s-1
1012/13 Z decays
at the Z pole repeat the LEP physics programme in a few minutes…

11. -- Model independent Higgs couplings and invisible width
First look at the physics case of TLEP, arXiv: v3 scoped the precision measurements:
-- Model independent Higgs couplings and invisible width
-- Z mass (0.1 MeV), W mass (0.5 MeV) top mass (~10 MeV), sin2Weff , Rb , N etc...
 powerful exploration of new physics with EW couplings up to very high masses
 importance of luminosity and Ebeam calibration by beam depolarization up to W pair
So far: simulations with CMS detector (Higgs) -- or «just» paper studies.
Snapshot of novelties appeared in recent workshops
Higher luminosity prospects at W, Z with crab-waist
 sensitivity to right handed (sterile) neutrinos
 s-channel e+e-  H(125.2) production almost possible (  monochromators)
 rare Higgs Z W and top decays, FCNCs etc...
 discovery potential for very small couplings
 precision event generators (Jadach et al)

12. Englert and Higgs get Nobel Prize
Higgs boson mass cornered (LEP H, MZ etc +Tevatron mt , MW)
Higgs Boson discovered (LHC)
Englert and Higgs get Nobel Prize
(c) Sfyrla
9/19/2018

13. Is it the end?
9/19/2018

14. -- Baryon Asymmetry in Universe -- Neutrino masses
Is it the end?
-- Dark matter
-- Baryon Asymmetry in Universe
-- Neutrino masses
-- and... why are the charges of e and p identical
to 21 significant digits?
are experimental proofs that there is more
to understand.
We must continue our quest
9/19/2018

15. Nima

16. At higher masses -- or at smaller couplings?
Nima
At higher masses -- or at smaller couplings?

17. But at least 3 pieces are still missing
THE STANDARD MODEL IS COMPLETE .....
But at least 3 pieces are still missing
neutrinos have mass...
and this very probably implies new degrees of freedom
 Right-Handed, Almost «Sterile» (very small couplings) Neutrinos
completely unknown masses (meV to ZeV), nearly impossile to find.
.... but could perhaps explain all: DM, BAU,-masses

18. FCC design study and FCC-ee http://cern.ch/fcc-ee
some REFERENCES
arxiv:
arxiv:
FCC design study and FCC-ee
and presentations at FCC-ee physics workshops
Phys.Lett.B631: ,2005
arXiv:hep-ph/

19. Electroweak eigenstates
𝒆 𝒗 𝒆
 𝒗 
 𝒗  𝒆  
Q= -1 R R R L L L
 𝒆
 
 
Q= 0 R R R
I = 1/2
I = 0
Right handed neutrinos
are singlets
no weak interaction
no EM interaction
no strong interaction
can’t produce them
can’t detect them
-- so why bother? --

20. Adding masses to the Standard model neutrino 'simply' by adding a Dirac mass term (Yukawa coupling)
implies adding a right-handed neutrino (new particle)
No SM symmetry prevents adding then a term like
and this simply means that a neutrino turns into a antineutrino
(the charge conjugate of a right handed antineutrino is a left handed neutrino!)
It is perfectly conceivable (‘natural’?) that both terms are present  ‘see-saw’
B. Kayser, the physics of massive neutrinos (1989)

21. See-saw in a general way : MR  0 mD  0 Dirac + Majorana mass terms
Mass eigenstates
See-saw in a general way :
MR  0
mD  0
Dirac + Majorana
mass terms
MR > mD  0
Dirac + Majorana
L NR R NL
½ ½
4 states , 2 mass levels
see-saw
MR = 0
mD  0
Dirac only, (like e- vs e+):
L R R L
½ ½
4 states of equal masses
MR  0
mD = 0
Majorana only
L R
½ ½
2 states of equal masses m m m
dominantly:
Iweak=
Iweak=
Iweak=
Some have I=1/2 (active)
Some have I=0 (sterile)
All have I=1/2 (active)
m1 have ~I=1/2 (~active)
m2 have ~I=0 (~sterile)

22. There even exists a scenario that claims to explain everything: the MSM
Shaposhnikov et al
TeV
N2, N3
can generate Baryon Asymmetry of Universe
if mN2,N3 > 140 MeV
GeV
MeV
constrained:
mass: 1-50 keV
mixing :
10-7 to 10-13 N1
keV
decay time:
N1 > Universe eV 3 2
N1 v 
meV 1
soon excluded?

23. Manifestations of right handed neutrinos
𝒗 = light mass eigenstate
N = heavy mass eigenstate
 𝒗𝑳 , active neutrino
which couples to weak inter.
and  NR, which does’nt.
𝒗= 𝒗𝑳 cos - 𝑵𝒄 𝑹 𝐬𝐢𝐧
one family see-saw :
  (mD/M)
𝒎𝒗  𝒎𝑫𝟐 𝑴
mN  M
|U|2  2  𝒎𝒗 / mN
can be larger with 3 families
𝑵= 𝑵 𝑹 cos+ 𝒗𝑳 c sin
what is produced in W, Z decays is:
𝒗𝑳= 𝒗 cos + 𝑵 𝐬𝐢𝐧
-- mixing with active neutrinos leads to various observable consequences
-- if very light (eV) , possible effect on neutrino oscillations (short baseline)
-- if in keV region (dark matter), monochromatic photons from galaxies with E=mN/2
-- possibly measurable effects at High Energy
If N is heavy it will decay in the detector (not invisible)
 PMNS matrix unitarity violation and deficit in Z «invisible» width
 Higgs, W, Z exotic decays H ii and Z ii , W-> li i
 also in charm and b decays via W*-> li i
 violation of unitarity and lepton universality in Z, W or  decays
-- etc... etc...
-- Couplings are small (𝒎𝒗 / mN) (but who knows?) and generally out of reach of hadron colliders (but this deserves to be revisited for detached HL-LHC, FCC-hh)

24. by virtual neutrino exchange and to flavour violation
Indirect effects
-- neutrino Majorana mass term can lead to lepton number violating processes
by virtual neutrino exchange and to flavour violation
-- neutrinoless double beta decay (the most powerful one)
-- FCNC (e) etc...
-- at a Z factory : Z  Z e Z->  ,      e etc...

25. Indirect constraints from lepton flavour violation are weak
arxiv:
LEP2 limits (DELPHI)
LEP1 limits (DELPHI)
(projected)

26. Goal performance of e+ e- colliders
FCC-ee as Z factory: 1012 Z
(possibly even 1013 with crab-waist) (few years)
possible
upgrade
complementarity ww
NB: ideas for lumi upgrades:
ILC arxiv: (not in TDR. Upgrade at 250GeV by reconfiguration after 500 GeV running; under discussion)
FCC-ee (crab waist)

27. -- direct search of heavy neutrino decays
-- invisible widths
-- FCNC
-- direct search of heavy neutrino decays

28. At the end of LEP: N = 2.984 0.008 Phys.Rept.427:257-454,2006
- 2  :^) !!
This is determined from the Z line shape scan
and dominated by the measurement of the
hadronic cross-section at the Z peak maximum 
The dominant systematic error is the theoretical
uncertainty on the Bhabha cross-section (0.06%)
which represents an error of  on N
Improving on N by more than a factor 2 would require a large effort
to improve on the Bhabha cross-section calculation!

30. Neutrino counting at TLEP
given the very high luminosity, the following measurement can be performed

31. incl. invisible = (dark matter?)
Higgs factory
(constrained fit
including ‘exotic’)
4 IPs
(2 IPs)
ZH events in 5 years
«A tagged Higgs beam».
sensitive to new physics in loops
incl. invisible = (dark matter?)
NB will improve with inclusion of
ZH-> qq H tagging
 total width
HHH (best at FCC-hh)
Htt (best at FCC-hh)
<1%
28%
13%
from HZ thresh
from tt thresh

32. Lepton flavour violating Z decays with 1013 Z decays
A. Abada et al, arXiv: Indirect searches for sterile neutrinos at a high-luminosity Z-factory A. Abada, V. De Romeri, S. Monteil, J. Orloff, A. M. Teixeira

33. RHASnu’s production in Z decays
multiply by 2 for anti neutrino and add contributions of 3 neutrino species (with different |U|2 )
Decay
Decay length: cm
NB CC decay always leads to
 2 charged tracks
Backgrounds : four fermion: e+e-  W*+ W*- e+e-  Z*(vv) + (Z/)*

34. Order-of-magnitude extrapolation of existing limits
4 106 Z decays
maybe achievable with
Z decays?
BAU
see-saw

35. search e+ e- v N N v(/Z)* monojet Find: one event in 4x106Z: e+e-
Search for heavy neutral leptons
search e+ e- v N
N v(/Z)* monojet
Find: one event
in 4x106Z:
e+e- e+ * Z* e-

36. Decay length ~1 evt with 1013Zs
Interesting region
|U|2 ~ 10-9 to 50 GeV
Decay length
L=1mm
L=1m
L=10m
~1 evt with 1013Zs
heavy neutrino mass ~ M  N +
W-qq
a large part of the interesting region will lead to detached vertices
... very strong reduction of background!
Exact reach domain will depend on detector size
and details of displaced vertex efficiency & background

38. A.B, Elena Graverini, Nicola Serra, Misha Shaposhnikov
NZ = mm<L<1m
region of interest
FCC-ee sensitivity
A.B, Elena Graverini, Nicola Serra, Misha Shaposhnikov

39. NZ = 1012 1mm<L<1m NZ = 1013 100𝒎 <L<5m
region of interest
FCC-ee sensitivity

40. NZ = 1012 1mm<L<1m NZ = 1013 100𝒎 <L<5m
region of interest
FCC-ee sensitivity

41. SHIP NZ = 1012 1mm<L<1m NZ = 1013 100𝒎 <L<5m
region of interest
FCC-ee sensitivity
NB very large detector caverns for FCC-hh may allow very large FCC-ee detector (R=15m?)
leading to improved reach at lower masses.

42. -- also search for same sign muons or electrons at the LHC
(e.g. CMS: )
ATLAS arXiv:
CMS arXiv:
arXiv:
7 GeV run
papers on 8 GeV run
in preparation
limits at |U|2 ~ level

43. Preliminary projection for LHC (P. Mermod, very preliminary)

44. Comment/Outlook for FCC-hh
We have seen that the Z factory offers a clean method
for detection of Heavy Right-Handed neutrinos
Ws are less abundant at the lepton colliders
At the 100 TeV hadron machine the W is the dominant particle.
There is a lot of /pile-up/backgrounds/lifetime/trigger issues which need to be investigated.
BUT.... in the regime of long lived HNLs the simultaneous presence of
-- the initial lepton from W decays
-- the detached vertex with kinematically constrained decay
allows for a significant background reduction.
But it allows also a characterization both in flavour and charge of the produced neutrino, thus
information of the flavour sensitive mixing angles and a test of the fermion violating nature
of the intermediate (Majorana) particle.
VERY interesting...

45. Conclusions
1. H, Z factory will investigate invisible widths with exp precision
of < 0.15% (Higgs) and < 
2. The direct quest for the
«Right-Handed-Almost-Sterile-See-saw-partners neutrinos»
(dextrinos? RHASnus? Heavy Neutral Leptons? Shaposhninos? heavinos?)
is not desperate at all
for Majorana mass at weak scale. In particular it may lead to spectacular
‘detached vertex’ signatures in a beam dump experiment (SHIP)
or in Z-> neutrino decays at a Tera-Z factory like FCC-ee,
Observation in W decays at hadron colliders with enough energy and luminosity
(100 TeV FCC-hh), would allow charge and flavour identification (mixing angles!) ...
be very interesting!
But here a detached vertex trigger and some way to reduce the hadron interaction background
will be needed.

46. SPARES

47. Higgs Decay into nu + N
Chen, He, Tandean, Tsai (2011)

48. H-> vN or Z-> vN arxiv:1208.3654 LEP2 limits (DELPHI)
(projected)
? TLEP-Z ?

49. determine the number of neutrinos from the radiative returns
Another solution:
determine the number of neutrinos from the radiative returns
e+e-   Z ( vv )
in its original form (Karlen) the method only counts the ‘single photon’ events
and is actually less sensitive than claimed. It has poorer statistics and requires running
~10 GeV above the Z pole. Systematics on photon selection are not small.
present result: Nv= 2.920.05