We report the results of a search for a new vector boson (
A
′
) decaying into two dark matter particles
χ
1
χ
2
of different mass. The heavier
χ
2
particle subsequently decays to
χ
1
and an ...off-shell Dark Photon
A
′
∗
→
e
+
e
-
. For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay
A
′
→
χ
χ
¯
and axion-like or pseudo-scalar particles
a
→
γ
γ
. With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for
A
′
masses from 2
m
e
up to 390 MeV and mixing parameter
ε
between
3
×
10
-
5
and
2
×
10
-
2
.
Hunting down the X17 boson at the CERN SPS Depero, E.; Andreev, Yu. M.; Banerjee, D. ...
European physical journal. C, Particles and fields,
12/2020, Letnik:
80, Številka:
12
Journal Article
Recenzirano
Odprti dostop
Recently, the ATOMKI experiment has reported new evidence for the excess of
e
+
e
-
events with a mass
∼
17 MeV in the nuclear transitions of
4
He, that they previously observed in measurements with
...8
Be. These observations could be explained by the existence of a new vector
X
17
boson. So far, the search for the decay
X
17
→
e
+
e
-
with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining
X
17
parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the
X
17
decay with the proposed method. To reach this goal an optimization of the
X
17
production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results
1
, validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.
A
bstract
The inclusion of an additional U(1) gauge
L
μ
−
L
τ
symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm ...assumes the existence of a new, light
Z
′ vector boson, with dominant coupling to
μ
and
τ
leptons and interacting with electrons via a loop mechanism. The
L
μ
−
L
τ
model can also explain the Dark Matter relic abundance, by assuming that the
Z
′ boson acts as a “portal” to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the
Z
′ search performed by the NA64-
e
experiment at CERN SPS, that collected ~ 9 × 10
11
100 GeV electrons impinging on an active thick target. Despite the suppressed
Z
′ production yield with an electron beam, NA64-
e
provides the first accelerator-based results excluding the
g −
2 preferred band of the
Z
′ parameter space in the 1 keV
<
m
Z
′
≲ 2 MeV range, in complementarity with the limits recently obtained by the NA64-
μ
experiment with a muon beam.
This work reports an original approach to the synthesis of composite “core-shell” nanoparticles of o-EuFeO3@am-EuFeO3 via solution combustion synthesis (SCS) followed by heat treatment in the air. ...PXRD, 57Fe and 151Eu Mössbauer spectroscopy, FTIR, DSC-TGA, SEM-EDX, TEM-SAED and vibrating-sample magnetometry were used to analyze the as-prepared and heat-treated samples. It was shown that the formation of amorphous am-EuFeO3 “shell” on the surface of crystalline o-EuFeO3 “cores” arises from the partial carbonatization of europium oxide during the solution combustion. It was found that the average crystallite size of o-EuFeO3 “cores” (37–92 nm) and thickness of am-EuFeO3 “shell” (1–10 nm) can be easily varied through the simple temperature change of heat treatment in the range of 500–900 °C. Acquired composite nanoparticles o-EuFeO3@am-EuFeO3 demonstrate uniform isometric morphology with a well distinct core and shell. Magnetometry results indicate the paramagnetic behavior of composite nanoparticles in a wide range of temperatures and applied magnetic fields that contradict the Mössbauer spectroscopy results showing the magnetic ordering of Fe-sublattice of o-EuFeO3 “cores”. Thus, the results of this work confirm the possibility of obtaining a new class of rare earth orthoferrite nanomaterials with the core-shell structure that can have promising functional applications.
Display omitted
•New approach is proposed to the synthesis of EuFeO3-based core-shell nanoparticles.•Procedure is based on a solution combustion synthesis followed by heat treatment.•Core-shell structure forms due to partial carbonatization of nanoparticle surface.•Core and shell sizes can be controlled by simply varying the treatment temperature.•synthesized composite nanoparticles exhibit unusual magnetic behavior.
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