arXiv:1102.2770v1 [hep-ph] 14 Feb 2011
Neutrino oscillations and uncertainty relations
S. M. Bilenky
Joint Institute for Nuclear Research, Dubna, R-141980, Russia
Physik-Department E15, Technische Universit¨ at M¨ unchen,
D-85748 Garching, Germany
F. von Feilitzsch and W. Potzel
Physik-Department E15, Technische Universit¨ at M¨ unchen,
D-85748 Garching, Germany
We show that coherent flavor neutrino states are produced (and
detected) due to the momentum-coordinate Heisenberg uncertainty
relation. The Mandelstam-Tamm time-energy uncertainty relation re-
quires non-stationary neutrino states for oscillations to happen and de-
termines the time interval (propagation length) which is necessary for
that. We compare different approaches to neutrino oscillations which
are based on different physical assumptions but lead to the same ex-
pression for the neutrino transition probability in standard neutrino
oscillation experiments. We show that a M¨ ossbauer neutrino experi-
ment could allow to distinguish different approaches and we present
arguments in favor of the163Ho -163Dy system for such an experiment.
The observation of neutrino oscillations in atmospheric , solar , reactor
 and accelerator experiments [4, 5] is one of the most important recent
discoveries in particle physics. Small neutrino masses can not be of Standard-
Model origin and are commonly considered as a signature of new physics
beyond the Standard Model.
All existing neutrino-oscillation data with the exception of the data of
the LSND  and MiniBooNE antineutrino experiments , which require
confirmation, are perfectly described under the assumption of three-neutrino
Here U is the PMNS [8, 9] 3 × 3 mixing matrix, which is characterized by
three mixing angles θ12,θ23,θ13 and the CP phase δ, νi(x) is the field of
neutrinos (Dirac or Majorana) with mass mi, and the ”mixed field” νlL(x) is
the SM field which enters into the standard charged current
Existing neutrino-oscillation data are analyzed under the assumption that
the transition probabilities between different flavor neutrinos are given by
the following standard expression (see, for example, )
P(νl→ νl′) = δl′l− 2 Re
l′kUlk(1 − e−i
Here, L is the distance between neutrino source and neutrino detector, E is
the neutrino energy, ∆m2
use for the transition probability another expression
k. Notice that it is also convenient to
P(νl→ νl′) = δl′l− 2
|Uli|2(δl′l− |Ul′i|2)(1 − cos∆m2
where the index j is fixed.
The character of neutrino oscillations is determined by the following two
observed features of the neutrino-oscillation parameters:
• The solar-KamLAND mass-squared difference ∆m2
than the atmospheric-accelerator mass-squared difference ∆m2
Sis much smaller
• The mixing angle θ13is small :
sin2θ13≤ 4 · 10−2.(6)
From (4), (5) and (6) follows (see, for example, ) that the leading
oscillations in the atmospheric and accelerator experiments are νµ⇄ ντand
¯ νµ⇄ ¯ ντ and in solar and KamLAND experiments the leading oscillations
are νe⇄ νµ,τand ¯ νe⇄ ¯ νµ,τ.
In the leading approximation it is impossible to distinguish two possible
neutrino mass spectra:
• Normal spectrum
m1< m2< m3,∆m2
• Inverted spectrum
m3< m1< m2,∆m2
In the case of the normal spectrum ∆m2
the case of the inverted spectrum ∆m2
From the recent three-neutrino analysis of the Super-Kamiokande data
 the following 90% CL limits were found for the normal (inverted) neutrino
1.9 (1.7)·10−3≤ ∆m2
For the parameter sin2θ13the following bounds were obtained
A≤ 2.6 (2.7)·10−3eV2,0.407 ≤ sin2θ23≤ 0.583. (7)
sin2θ13≤ 4 · 10−2(9 · 10−2).(8)
From the two-neutrino analysis of the MINOS data was found 
A= (2.43 ± 0.13) · 10−3eV2,sin22θ23> 0.90 (9)
From the three-neutrino global analysis of the solar and reactor KamLAND
data was obtained 
−0.20) · 10−5eV2,tan2θ12= 0.452+0.035
For the parameter sin2θ13was found
At present four neutrino-oscillation parameters (∆m2
tan2θ12) are known with accuracies within the (3-10)% range. In the accel-
erator neutrino oscillation experiment T2K  the parameter ∆m2
measured with an accuracy of δ∆m2
will be measured with an accuracy of δ(sin22θ23) ≃ 10−2. One of the major
aims of this experiment and the reactor experiments DOUBLE CHOOZ,
RENO , and Daya Bay  is to determine the value (or to improve the
upper bound by one order of magnitude or better) of the parameter sin2θ13.
In case that this parameter is relatively large, it is envisaged that in future
neutrino experiments the value of the CP phase δ will be determined and
the problem of the neutrino mass spectrum will be resolved (see ).
Thus, we are entering into the era of high precision neutrino oscillation
experiments. Despite that the neutrino oscillation formalism, on which the
analysis of experimental data is based, has been developed and debated in
many papers starting from the 1970s (see reviews [16, 17]), these debates
and discussions are continuing (see recent papers ). From our point of
view the importance of uncertainty relations was not sufficiently analyzed in
previous discussions. We will show here that the phenomenon of neutrino
oscillations is heavily based on the Heisenberg uncertainty relation and the
Mandelstam-Tamm time-energy uncertainty relation. We briefly consider dif-
ferent approaches to neutrino oscillations and discuss a M¨ ossbauer neutrino
experiment which could allow to distinguish them.
A< 10−4eV2and the parameter sin22θ23
2 Flavor neutrinos: production, evolution,
Which neutrino states are produced in CC weak processes together with
charged leptons in the case of neutrino mixing, eq.(1): Neutrino flavor states,
coherent superpositions of plane waves, or superpositions of wave packets?
Here we will present arguments based on the QFT, the Heisenberg uncer-
tainty relation and the knowledge of the neutrino mass-squared differences
that ”mixed” flavor states which describe the flavor neutrinos νe, νµand ντ
are physical states (fully analogous to the ”mixed” states which describe K0
and¯K0, B0and¯B0, etc.).
Let us consider (in the lab. system) the decay 
a → b + l++ νi,(i = 1,2,3)(12)
where a and b are some hadrons.
The state of the final particles is given by
where ?νil+b|S|a? is the matrix element of the transition a → b + l++ νi,
|νi? is the state of a neutrino with mass mi, momentum ? pi= pi?k (?k is the
unit vector) and helicity equal to -1. We assume, as usual, that initial and
final particles have definite momenta.
Because neutrino masses are small, we can use the expansion
i≃ E −m2
where E is the energy of neutrinos for m2
momenta we have
i→ 0. For the difference of neutrino
|pi− pk| ≃|∆m2
rc4/eV2)m,r = A,S(16)
is the oscillation length. For E ≃ 1 GeV and ∆m2
spheric and LBL accelerator neutrinos) we have LA
MeV and ∆m2
On the other side, from the Heisenberg uncertainty relation we have
A≃ 2.4 · 10−3eV2(atmo-
osc≃ 103km. For E ≃ 3
S≃ 7.5 · 10−5eV2(reactor antineutrinos) we have LS
Here d characterizes the quantum-mechanical size of the source. Taking into
|pi− pk| ≪ (∆p)QM.(19)
Thus, we conclude that due to the uncertainty relation it is impossible to
resolve the emission of neutrinos with different masses.1
determines the leptonic part of the matrix element of the process (12). We
li¯ uL(pi)γαuL(−pl) ≃ U∗
li¯ uL(p)γαuL(−pl), (21)
where plis the momentum of l+, and p = E is the momentum of the neutrino
i→ 0. For the total matrix element of the process (12) we have
?νil+b|S|a? ≃ U∗
where ?νll+b|S|a?SMis the Standard Model matrix element of the emission
of the flavor neutrino νlwith the momentum p in the process
a → b + l++ νl.(23)
From (13) and (22) we find
|f? = |νl?|l+?|b ??νll+b|S|a?SM, (24)
where the state of the flavor neutrino νlis given by the relation
li|νi?(l = e,µ,τ)(25)
and |νi? is the state of a neutrino with mass mi, negative helicity and mo-
Let us stress that
• Flavor neutrino states do not depend on the production process.
1For the energy of a neutrino with mass mi we have Ei ≃ E(1 +
oscillation experiments E ? 1 MeV and
different neutrino energies in production (and detection) processes.
2Let us notice that the theory of the evolution of neutrinos in matter and the MSW
effect [20, 21] are based on the assumption that a flavor neutrino state is a state with
2E2). In neutrino
2E2 ? 10−12. Thus, it is impossible to resolve
• Flavor states are characterized by the momentum (if there are no spe-
cial conditions of neutrino production).
• Flavor states are orthogonal and normalized
?νl′|νl? = δl′l.(26)
The evolution of states in QFT is given by the Schr¨ odinger equation
= H |Ψ(t)?, (27)
where H is the total Hamiltonian and time t is a parameter both of which
characterize the evolution of the system.
If at t = 0 in a CC weak process νlis produced, we have for the state of
the neutrino at the time t
|νl?t= e−iHt|νl? =
H|νi? = Ei|νi?,Ei≃ E +m2
Neutrinos are detected via the observation of weak CC and NC processes.
Let us consider the production of a lepton l′in the CC process
νi+ N → l′+ X.(30)
Taking into account that effects of neutrino masses can not be resolved in
neutrino processes we have
?l′X|S|νiN? ≃ ?l′X|S|νl′N?SMUl′i,(31)
where ?l′X|S|νl′ N?SMis the SM matrix element of the process
νl′ + N → l′+ X.
From (24), (28) and (31) follows that the chain of processes a → b + l++
νl,νl → νl′,
product of amplitudes
νl′ + N → l′+ X corresponds to the following factorized
?b l+νl|S|a?SM. (33)
Only the amplitude of the transition νl→ νl′
A(νl→ νl′) =
depends on the properties of massive neutrinos (mass-squared differences
and mixing angles). The matrix elements of the neutrino production and
detection are given by the Standard Model expressions in which effects of
neutrino masses can safely be neglected. Let us stress that the property of
the factorization (33) is based on the smallness of the neutrino masses and
on the Heisenberg uncertainty relation.
3 Mandelstam-Tamm uncertainty relation
and neutrino oscillations
All uncertainty relations in Quantum Theory are based on the inequality
∆A ∆B ≥1
which follows from the Cauchy inequality. In (35) A and B are hermitian
and A = ?a|A|a? is the average value of the operator A. For example, for
operators of momentum p and coordinate q which satisfy the commutation
relation [p,q] =1
The Mandelstam-Tamm time-energy uncertainty relation  is based on
the inequality (35) and the equation
operators, |a? is any state, ∆A =
?a|(A − A)2|a? is the standard deviation
iwe have the Heisenberg uncertainty relation ∆p ∆q ≥1
for any operator O(t) in the Heisenberg representation (H is the total Hamil-
From (35) and (36) we have
∆E ∆O(t) ≥1
This inequality gives nontrivial constraints only in the case of non-stationary
Taking into account that ∆E does not depend on t we find
∆E ∆t ≥1
|O(∆t) − O(0)|
For the time interval ∆t during which the state of the system is significantly
changed (O(t) is changed by the value which is characterized by the standard
deviation) the right-hand part of (38) is of the order of one. We obtain the
Mandelstam-Tamm time-energy uncertainty relation
∆E ∆t ? 1. (39)
From (34), for the normalized probability of the transition νl→ νl′ we
obtain the expression
P(νl→ νl′) = |
Ul′i(e−i(Ei−Ej)t− 1) U∗
which obviously gives the standard transition probability (3).
From (40) follows that neutrino oscillations can be observed if the condi-
|Ei− Ej| t ? 1
It is obvious that this inequality is the Mandelstam-Tamm
time-energy uncertainty relation. According to this relation a change of
the flavor neutrino state in time requires energy uncertainty (i.e., a non-
stationary state). The time interval required for a significant change of the
flavor neutrino state is given by t ≃
4On plane wave and wave packet approaches
to neutrino oscillations
We will now briefly discuss other approaches to neutrino oscillations. In the
approach based on the relativistic quantum mechanics, in CC processes to-
gether with charged leptons coherent superpositions of plane waves are
3This is a necessary condition for the observation of oscillations. It is also necessary
that mixing angles would be relatively large.
4Let us notice that the inequality (41) can be interpreted in another way: In order
to reveal a small energy difference |Ei− Ej| ≃
|Ei−Ej|. This corresponds to another interpretation of the time-energy uncertainty
relation (see ).
we need a large time interval
produced and absorbed. In this case, for the normalized νl→ νl′ transition
probability the following expression can be obtained (see, for example[24, 25])
P(νl→ νl′) = |
Here pi= (Ei,? pi) is the 4-momentum of a neutrino with mass miand x =
Let us assume that ? pi= pi?k, where?k is the unit vector. For the phase
difference which is gained by a plain wave at the distance x = (? x?k) = L after
the time interval t we have
(pi− pj) · x = (Ei− Ej)t − (pi− pj)L.
For ultrarelativistic neutrinos we have
t ≃ L.
2E, from (43) and (44) we come to the
Taking into account that Ei≃ pi+
standard oscillation phase
(pi− pj) · x =∆m2
and the standard expression (4) for the transition probability.
Let us stress that in the approach based on the QFT Schr¨ odinger equation
the small oscillation phase difference is the result of the cancellation of large
terms in the expressions for the neutrino energies. The cancellation takes
place because neutrino states are characterized by definite momentum. In
the QM plane wave approach, small oscillation phases are the result of the
cancellation of large terms in the time and space parts of the phase difference.
The cancellation is due to the relation (44).
A direct generalization of the QM plane wave approach is the wave
packet approach (see  and references therein) in which the plane wave
transition probability (42) is changed to
P(νl→ νl′) = |
ei(? pi′? x−E′
′− ? pi) d3p′U∗
at the point ? pi
′= ? pi.
iand the function f(? pi
′− ? pi) has a sharp maximum
iat the point ? pi
′= ? piwe find
ei(? pi′? x−E′
′− ? pi) d3p′= ei(? pi? x−Eit)g(? x −? vit), (47)
g(? x −? vit) =
ei? q (? x−? vit)f(? q) d3q (48)
? vi=? pi
If we make the standard assumption that the function f(? q) has the Gaussian
f(? q) = N e
(σpis the width of the wave packet in the momentum space) we find
g(? x −? vit) = N(π
−(? x−? vit)2
The probability of the transition νl→ νl′ in the wave packet approach
is determined as a quantity integrated over time. From (47) we find the
following expression for the integrated normalized transition probability
2σpcharacterizes the spacial width of the wave packet.
P(νl→ νl′) =
Here L is the distance between neutrino source and neutrino detector, Lik
is the oscillation length, ξ is a constant of the order of one and
is the coherence length.5Taking into account that (pi− pk) − (Ei− Ek) =
a distance between neutrino source and detector at which νiand νkare separated by an
interval comparable to the size of the wave packet.
we come to the conclusion that the νl→ νl′ transition probability in
coh∼ 2√2σx. Thus, the coherence length is such
5We have |vi− vk|Lik
the wave packet approach is given by the standard expression (3) which is
multiplied by the decoherence factor e
Thus, the wave packet approach (after integration over t) assures the
equality t = L and the standard oscillation phase in the transition probability.
For usual neutrino oscillation experiments with L being a few times LA,S
two additional exponential factors are practically equal to one.
In many papers (see ), neutrinos propagating about 100 km (reactor
¯ ν’s ) or about 1000 km (atmospheric and accelerator ν’s ), are considered
as virtual particles in a Feynman diagram-like picture with the neutrino
production process at one vertex and the neutrino absorption process in
another vertex. This approach gives the wave packet picture of neutrino
oscillations with a transition probability which (before integration over t)
depends on x and t.
The major difference between different approaches to neutrino oscillations
can be summarized as follows:
and the factor e
1. The QFT approach with the Schr¨ odinger evolution equation is based
on the assumption of the existence of ”mixed” flavor neutrinos νe,νµ,ντ
which are described by coherent states |νl? =?
energy uncertainty relation. Neutrino oscillations can take place only
in the case of non-stationary neutrino states with ∆E∆t ? 1, where
∆t is the time interval during which the oscillations happen. The QFT
approach is based on the same general principles as the approach to
K0⇄¯K0, B0⇄¯B0, etc. oscillations studied in detail at B-factories
and other facilities.
li|νi?. The important
characteristic feature of this approach is the Mandelstam-Tamm time-
2. Other approaches are based on the assumption that in weak processes,
mixed coherent superpositions of plane waves or wave packets describ-
ing neutrinos with different masses, are produced and detected. The
evolution of mixed neutrino wave functions in space and time is de-
termined by the Dirac equation. There is no notion of flavor neutrino
states in these approaches. Neutrino oscillations are possible also in
the case of monochromatic neutrinos.
Different approaches to neutrino oscillations lead to the same expression for
the neutrino transition probability P(νl → νl′) in the standard neutrino
oscillation experiments. In order to distinguish 1. and 2. special neutrino
oscillation experiments are necessary. Such experiments could be M¨ ossbauer
neutrino experiments which we will discuss in the next sections.
5 M¨ ossbauer ¯ νe: Basic considerations
The basic concept is to use electron antineutrinos (¯ νe) which are emitted
without recoil in a bound-state β-decay and are resonantly captured again
without recoil in the reverse bound-state process. As an example, let us
consider the3H -3He system  with the transitions
(source) and ¯ νe+3He →3H (target).
In the source, the electron (e−) is emitted directly into a bound-state
atomic orbit of3He. This decay is a two-body process, thus the emitted ¯ νe
has a fixed energy (18.6 keV). In the target the reverse process occurs, a
monochromatic ¯ νewith an energy of 18.6 keV and an e−in an atomic orbit
of3He are absorbed to form3H.
To suppress thermal motions of the3H and3He atoms, they have to be
imbedded in a solid-state lattice, e.g., in Nb metal . In addition, for a
M¨ ossbauer ¯ νe experiment it is mandatory that no phonons are excited in
the lattice when the ¯ νeis emitted or absorbed, because only then a highly
monochromatic ¯ νe radiation and the large cross section of the M¨ ossbauer
resonance of typically 10−19to 10−17cm2can be achieved. However, it be-
came apparent ,,, that there exist several basic difficulties to
observe M¨ ossbauer ¯ νe with the system3H -3He in Nb metal. The main
problem originates from lattice expansion and contraction processes. They
occur when the nuclear transformations (from3H to3He and from3He to
3H) take place during which the ¯ νe is emitted or absorbed and can cause
lattice excitations (phonons) which change the ¯ νeenergy and thus destroy
the M¨ ossbauer resonance. It has been estimated that due to these lattice ex-
citations the probability for phononless emission and consecutive phononless
capture of ¯ νeis ∼ 7 · 10−8which makes a real experiment with the3H -3He
system extremely difficult ,,,. Another basic problem is caused
by inhomogeneities in an imperfect lattice which directly influence the energy
of the ¯ νe.
A promising alternative is the rare-earth system163Ho -163Dy. It offers
several advantages: Due to the highly similar chemical behaviour of the
rare earths also the lattice deformation energies for163Ho and163Dy can be
expected to be similar, thus leaving the ¯ νe energy practically unchanged.
3H→3He +¯ νe
In addition, the ¯ νeenergy is very low (2.6 keV), i.e., the recoil originating
from the emitted (absorbed) ¯ νeis highly unlikely to generate phonons in the
lattice. Altogether, the probability of phononless emission and absorption
could be larger than for the3H -3He system by ∼ 7 orders of magnitude.
Furthermore, due to the similar chemical behaviour, the163Ho -163Dy system
can also be expected to be less sensitive to variations of the binding energies
in the lattice. For this reason, variations of the ¯ νeenergy will also be reduced
improving the monochromaticity (linewidth) of the ¯ νeM¨ ossbauer resonance.
On the negative side, the magnetic moments of the 4f electrons of the
rare-earth atoms are large and might cause broadening of the M¨ ossbauer ¯ νe
resonance ,. Fortunately, conventional M¨ ossbauer spectroscopy (with
photons) gathered a wealth of information on the behaviour of rare-earth
systems in the past. Of particular interest is the 25.65 keV M¨ ossbauer res-
onance in161Dy where an experimental linewidth of Γexp≈ 5 · 10−8eV has
been reached ,. We will show in the following section that the163Ho
-163Dy system might be suitable to investigate the question concerning the
different approaches to neutrino oscillations.
the evolution of the ¯ νestate in time
163Dy M¨ ossbauer system and
If the evolution of the ¯ νestate occurs in time only, M¨ ossbauer ¯ νeoscillations
with an oscillation length LA
the relative energy uncertainty fulfills the relation 
oscdetermined by ∆m2
Awill not be observed if
For the163Ho -163Dy system, eq. (54) requires?∆E
For the 25.65 keV γ-transition in
Γexp≈ 5·10−8eV has been observed , which is ∼ 5 times below the limit
∆E ? 2.4 · 10−7eV just mentioned. It might be expected that a similar
value for Γexpcan be reached for the163Ho -163Dy system. In particular,
using the usual M¨ ossbauer γ-transition in161Dy, relevant physical properties,
e.g., the experimental linewidth in the Ho - Dy system can be investigated
A≃ 2.4 · 10−3eV2is the atmospheric mass-squared difference.
Ho−Dy≪ 9.2 · 10−11or
∆E ≪ 2.4 · 10−7eV.
161Dy an experimental linewidth of
and improved if necessary.
M¨ ossbauer ¯ νeoscillate can be answered experimentally. For Γexp≈ 5 · 10−8
eV, according to the Mandelstam-Tamm time-energy uncertainty relation a
significant change of the ¯ νestate in time can occur only very slowly leading
to a long oscillation path-length Lchangesince the ¯ νeis ultrarelativistic:
Thus it looks promising that the question if
Lchange≃ c ·
· 2π. (55)
For the163Ho -163Dy system, Lchange≈ 25 m for the ¯ νestate.
In comparison, for an evolution of the ¯ νe state in space and time, the
oscillation length is given by eq. (16). With E = 2.6 keV for the163Ho -
163Dy system, and ∆m2
times shorter than Lchange. If the evolution occurs in time only, in such a
M¨ ossbauer-neutrino experiment with Γexp≈ 5 · 10−8eV, instead of LA
much longer Lchangewould be observed.
If M¨ ossbauer ¯ νeoscillate, an interesting application would be the search
for the conversion to sterile neutrinos ¯ νe→ ¯ νsterile involving additional
mass eigenstates. Since ¯ νsterile does not show the weak interaction of the
Standard Model of elementary particle interactions, such a conversion would
have to be tested by the disappearance of ¯ νe. The results of the LSND
(Liquid Scintillator Neutrino Detector) experiment , indicate a mass
splitting of ∆m2≈ 1 eV2. Unfortunately, several experiments performed
by the MiniBooNE collaboration to check the LSND results have not been
conclusive, although the MiniBooNE results are compatible with the LSND
observation . For M¨ ossbauer ¯ νeof the163Ho -163Dy system (E=2.6 keV)
the oscillation length LA
A≃ 2.4 · 10−3eV2, we obtain LA
osc≃ 2.6 m, about 10
oscwould be only ∼ 1 cm if ∆m2≈ 1 eV2.
After the golden years of the discovery of neutrino oscillations in atmospheric,
solar and reactor neutrino experiments we now enter into the era of detailed
studies of this phenomenon. Measurements of the small mixing angle θ13, of
the CP phase δ, and the establishment of the character of the neutrino-mass
spectrum will require high-precision neutrino-oscillation experiments which
are already ongoing now or are under preparation or in the R&D stage.
Is there a consensus in the treatment and understanding of the neutrino
oscillation phenomenon? Many recent papers on the theory of neutrino os-
cillations (see, for example, ) certify that such a consensus still does not
Is the notion of flavor neutrinos νe, νµand ντin the case of neutrino mix-
ing a convenient terminology coming from ”the times of massless neutrinos”
or are they real physical states? From the momentum-coordinate Heisen-
berg uncertainty relation follows that due to the small values of the neutrino
mass-squared differences in weak processes ”mixed” flavor neutrinos νl(sim-
ilar to the ”mixed” K0,¯K0; B0,¯B0; etc), which are described by coherent
superpositions of states of neutrinos with definite mass, are produced and
detected. We showed that in this approach for neutrino oscillations to be
observed the Mandelstam-Tamm time-energy uncertainty relation must be
satisfied. This means that neutrino oscillations can take place only in the
case of non-stationary neutrino states.
We compared different approaches to neutrino oscillations. In approaches
in which flavor neutrinos are described by coherent superpositions of plane
waves or wave packets and in the approach in which neutrinos are consid-
ered as virtual particles in a Feynman diagram with the neutrino production
process at one vertex and the neutrino absorption process in another vertex
neutrino oscillations are possible also in the case of monochromatic neutrinos
(M¨ ossbauer neutrinos).
Usual neutrino oscillation experiments do not allow to distinguish these
different approaches. The realization of an idea concerning the M¨ ossbauer
resonance neutrino experiment with practically monoenergetic ¯ νecould be
the way of probing the real nature of mixed flavor states, different con-
jectures on the evolution of such states and the universal applicability of
the time-energy uncertainty relation. Such an experiment was discussed for
the3H −3He source-detector pair . Recently, however, it was shown
that the performance of a M¨ ossbauer neutrino experiment in the case of the
3H −3He system is most probably not possible in practice ,,. We
present here arguments in favor of a M¨ ossbauer neutrino experiment with
the163Ho−163Dy source-detector system . The possibility to perform an
experiment in such a system looks promising but is still very challenging and
requires further investigations.
This work was supported by funds of the Deutsche Forschungsgemeinschaft
DFG (Transregio 27: Neutrinos and Beyond), the Munich Cluster of Ex-
cellence (Origin and Structure of the Universe), and the Maier-Leibnitz-
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