Standard Model, with its present precision, predicts the neutron
β
-decay time
τ
SM
=
878.7
±
0.6
s which is perfectly compatible with the neutron lifetime measured in the trap experiments
τ
trap
=
...879.4
±
0.6
s. However, the lifetime measured in the beam experiments via counting the protons produced by
β
-decay
n
→
p
e
ν
¯
e
,
τ
beam
=
888
±
2.0
s, is deviated from
τ
SM
by 9 seconds (
4.4
σ
). This discrepancy can be explained via the neutron
n
conversion into mirror neutron
n
′
, its dark partner from parallel mirror sector. Provided that
n
and
n
′
have a tiny mass splitting
∼
10
-
7
eV, in magnetic fields of few Tesla used in beam experiments,
n
-
n
′
transition is resonantly enhanced converting a 1
%
fraction of neutrons into mirror neutrons which decay in invisible mode
n
′
→
p
′
e
′
ν
¯
e
′
. Thus less protons are produced and the measured value
τ
beam
appears larger than the true decay time
τ
SM
=
τ
trap
.
We discuss the possibility that baryon number
B
is spontaneously broken at low scales, of the order of MeV or even smaller, inducing the neutron–antineutron oscillation at the experimentally ...accessible level. An associated Goldstone particle–baryonic majoron can have observable effects in neutron to antineutron transitions in nuclei or dense nuclear matter. By extending baryon number to an anomaly-free
B
-
L
symmetry, the baryo-majoron can be identified with the ordinary majoron associated with the spontaneous breaking of lepton number, and it can have interesting implications for neutrinoless
2
β
decay with the majoron emission. We also discuss the hypothesis that baryon number can be spontaneously broken by QCD itself via the six-quark condensates.
Existing bounds on the neutron-antineutron mass mixing,
ϵ
n
n
¯
<
few
×
10
-
24
eV, impose a severe upper limit on
n
-
n
¯
transition probability,
P
n
n
¯
(
t
)
<
(
t
/
0.1
s
)
2
×
10
-
18
or so, ...where
t
is the neutron flight time. Here we propose a new mechanism of
n
-
n
¯
transition which is not induced by direct mass mixing
ϵ
n
n
¯
but is mediated instead by the neutron mass mixings with the hypothetical states of mirror neutron
n
′
and mirror antineutron
n
¯
′
. The latter can be as large as
ϵ
n
n
′
,
ϵ
n
n
¯
′
∼
10
-
15
eV or so, without contradicting present experimental limits and nuclear stability bounds. The probabilities of
n
-
n
′
and
n
-
n
¯
′
transitions,
P
n
n
′
and
P
n
n
¯
′
, depend on environmental conditions in mirror sector, and they can be resonantly amplified by applying the magnetic field of the proper value. This opens up a possibility of
n
-
n
¯
transition with the probability
P
n
n
¯
≃
P
n
n
′
P
n
n
¯
′
which can reach the values
∼
10
-
8
or even larger. For finding this effect in real experiments, the magnetic field should not be suppressed but properly varied. These mixings can be induced by new physics at the scale of few TeV which may also originate a new low scale co-baryogenesis mechanism between ordinary and mirror sectors.
After the recent high precision determinations of
V
us
and
V
ud
, the first row of the CKM matrix shows more than
4
σ
deviation from unitarity. Two possible scenarios beyond the Standard Model can be ...investigated in order to fill the gap. If a 4th non-sequential quark
b
′
(a vector-like weak isosinglet) participates in the mixing, with
|
V
u
b
′
|
∼
0.04
, then its mass should be no more than 6 TeV or so. A different solution can come from the introduction of the gauge horizontal family symmetry
S
U
(
3
)
ℓ
acting between the lepton families and spontaneously broken at the scale of about 6 TeV. Since the gauge bosons of this symmetry contribute to muon decay in interference with Standard Model, the Fermi constant is slightly smaller than the muon decay constant so that unitarity is recovered. Also the neutron lifetime problem, that is about
4
σ
discrepancy between the neutron lifetimes measured in beam and trap experiments, is discussed in the light of the these determinations of the CKM matrix elements.
Neutron-mirror neutron mixing and neutron stars Berezhiani, Zurab; Biondi, Riccardo; Mannarelli, Massimo ...
The European physical journal. C, Particles and fields,
11/2021, Letnik:
81, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The oscillation of neutron
n
into mirror neutron
n
′
, its mass degenerate partner from dark mirror sector, can gradually transform the neutron stars into the mixed stars consisting in part of mirror ...dark matter. In quark stars
n
-
n
′
transitions are suppressed. We study the structure of mixed stars and derive the mass-radius scaling relations between the configurations of purely neutron star and maximally mixed star (MMS) containing equal amounts of ordinary and mirror components. In particular, we show that the MMS masses can be at most
M
NS
max
/
2
, where
M
NS
max
is a maximum mass of a pure neutron star allowed by a given equation of state. We evaluate
n
-
n
′
transition rate in neutron stars, and show that various astrophysical limits on pulsar properties exclude the transition times in a wide range
10
5
year
<
τ
ε
<
10
15
year
. For short transition times,
τ
ε
<
10
5
year, the different mixed stars of the same mass can have different radii, depending on their age, which possibility can be tested by the NICER measurements. We also discuss subtleties related with the possible existence of mixed quark stars, and possible implications for the gravitational waves from the neutron star mergers and associated electromagnetic signals.
The oscillation of the neutron n into mirror neutron n′, its partner from the dark mirror sector, can gradually transform an ordinary neutron star into a mixed star consisting in part of mirror dark ...matter. The implications of the reverse process taking place in the mirror neutron stars depend on the sign of baryon asymmetry in the mirror sector. Namely, if it is negative, as predicted by certain baryogenesis scenarios, then n′¯−n¯ transitions create a core of our antimatter gravitationally trapped in the mirror star interior. The annihilation of accreted gas on such antimatter cores could explain the origin of γ-source candidates with an unusual spectrum compatible with baryon–antibaryon annihilation, recently identified in the Fermi LAT catalog. In addition, some part of this antimatter escaping after the mergers of mirror neutron stars can produce the flux of cosmic antihelium and also heavier antinuclei which are hunted in the AMS-02 experiment.
A
bstract
The Standard Model does not constrain the form of the Yukawa matrices and thus the origin of fermion mass hierarchies and mixing pattern remains puzzling. On the other hand, there are ...intriguing relations between fermion masses and mixing angles which may point towards specific textures of Yukawa matrices. One of the classic hypothesis is the zero texture proposed by Fritzsch which is, however, excluded by present precision tests since it predicts a too large value of |
V
cb
| as well as a too small value of the ratio |
V
ub
/
V
cb
|. In this paper we discuss a minimal modification which still maintains the six zero entries as in the original Fritzsch ansatz. This modification consists in introducing an asymmetry between the 23 and 32 entries in the down-quark Yukawa matrix. We show that this flavour structure can naturally emerge in the context of models with inter-family SU(3)
H
symmetry. We present a detailed analysis of this Fritzsch-like texture by testing its predictions and showing that it is perfectly compatible with the present precision data on quark masses and CKM mixing matrix.
We analyze status of C, P and T discrete symmetries in application to neutron–antineutron transitions breaking conservation of baryon charge B by two units. At the level of free particles all these ...symmetries are preserved. This includes P reflection in spite of the opposite internal parities usually ascribed to neutron and antineutron. Explanation, which goes back to the 1937 papers by E. Majorana and by G. Racah, is based on a definition of parity satisfying P2=−1, instead of P2=1, and ascribing P=i to both, neutron and antineutron. We apply this to C, P and T classification of six-quark operators with |ΔB|=2. It allows to specify operators contributing to neutron–antineutron oscillations. Remaining operators contribute to other |ΔB|=2 processes and, in particular, to nuclei instability. We also show that presence of external magnetic field does not induce any new operator mixing the neutron and antineutron provided that rotational invariance is not broken.
The tension between the neutron lifetimes measured in the beam and trap experiments suggests that the neutron $n$ might have a new invisible decay channel $n\to n' X$ into mirror neutron, its dark ...partner from parallel hidden sector and nearly degenerate in mass with the neutron, with $m_n-m_{n'} \simeq$ 1 MeV or so, and $X$ being ordinary and mirror photons, as well as more exotic massless bosons. I discuss some phenomenological and astrophysical consequences of this scenario, which depends on the mass range of mirror neutron $n'$. Namely, the case $m_{n'} < m_p + m_e$ leads to a striking possibility is that the hydrogen atom $^1$H (protium), constituting 75 per cent of the baryon mass in the Universe, could in fact be unstable: it can decay via the electron capture into $n'$ and $\nu_e$, with relatively short lifetime. If instead $m_{n'} > m_p + m_e$, then the decay $n'\to pe\bar \nu_e$ is allowed and $n'$ can represent unstable but very long living dark matter component. Nevertheless, this decay would produce substantial diffuse gamma background. This explanation, however, is in tension with the latest results of the experiments measuring $\beta$-asymmetry in the neutron decay.
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