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54
result(s) for
"Lysan, V."
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Probing the explanation of the muon (g-2) anomaly and thermal light dark matter with the semi-visible dark photon channel
We report the results of a search for a new vector boson (A′) decaying into two dark matter particles χ1χ2 of different mass. The heavier χ2 particle subsequently decays to χ1 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′→χχ¯ and axion-like or pseudo-scalar particles a→γγ. 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 2me up to 390 MeV and mixing parameter ε between 3×10-5 and 2×10-2.
Journal Article
First constraints on the Lμ− Lτ explanation of the muon g-2 anomaly from NA64-e at CERN
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.
Journal Article
Proof of principle for a light dark matter search with low-energy positron beams at NA64
A
bstract
Thermal light dark matter (LDM) with particle masses in the 1 MeV–1 GeV range could successfully explain the observed dark matter abundance as a relic from the primordial Universe. In this picture, a new feeble interaction acts as a “portal” between the Standard Model and LDM particles, allowing for the exploration of this paradigm at accelerator experiments. In the last years, the “missing energy” experiment NA64
e
at CERN SPS (Super Proton Synchrotron) has set world-leading constraints in the vector-mediated LDM parameter space, by exploiting a 100 GeV electron beam impinging on an electromagnetic calorimeter, acting as an active target. In this paper, we report a detailed description of the analysis of a preliminary measurement with a 70 GeV/c positron beam at NA64
e
, performed during summer 2023 with an accumulated statistics of 1
.
596 × 10
10
positrons on target (hereafter referred to as
e
+
OT). This data set was analyzed with the primary aim of evaluating the performance of the NA64
e
detector with a lower energy positron beam, towards the realization of the post-LS3 program. The analysis results, other than additionally probing unexplored regions in the LDM parameter space, provide valuable information towards the future NA64
e
positron campaign.
Journal Article
Hunting down the X17 boson at the CERN SPS
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.
Journal Article
Searches for the light invisible axion-like particle in K+→π+π0a decay
A high-statistics data sample of the
K
+
decays is recorded by the OKA collaboration. A missing mass analysis is performed to search for a light invisible pseudoscalar axion-like particle (ALP)
a
in the decay
K
+
→
π
+
π
0
a
. No signal is observed, and the upper limits for the branching ratio of the decay are calculated. The
90
%
confidence level upper limit changes from
2.5
·
10
-
6
to
2
·
10
-
7
for the ALP mass from 0 to 200 MeV/
c
2
, except for the region of
π
0
mass, where the upper limit is
4.4
·
10
-
6
.
Journal Article
Observation of K+→π+π0π0γ decay
The
K
+
→
π
+
π
0
π
0
γ
decay is observed by the OKA collaboration. About 60 events of the decay observed with signal:noise
≈
1
. The branching ratio obtained by normalization to
K
+
→
π
+
π
0
π
0
is measured to be
(
3.7
±
0.9
(
s
t
a
t
)
±
0.3
(
s
y
s
t
)
)
×
10
-
6
for
E
γ
∗
>
10
MeV
. The branching ratio,
γ
energy spectrum and angular distribution are consistent with ChPT prediction.
Journal Article
Study of K+→π0e+νγ decay with OKA setup
Results of a study of the K+→π0e+νγ decay at OKA setup are presented. More than 32,000 events of this decay are observed. The differential spectra over the photon energy and the photon–electron opening angle in kaon rest frame are presented. The branching ratios, normalized to that of Ke3 decay are calculated for different cuts on Eγ∗ and cosΘeγ∗. In particular, the branching ratio for Eγ∗>30 MeV and Θeγ∗>20∘ is measured R = Br(K+→π0e+νeγ)Br(K+→π0e+νe) = = (0.587±0.010(stat.)±0.015(syst.))×10-2, which is in a good agreement with ChPT O(p4) calculations.
Journal Article
Search for heavy neutrino in K + → μ + ν H decay
A high statistics data sample of the K+→μ+νμ decay was accumulated by the OKA experiment in 2012. The missing mass analysis was performed to search for the decay channel K+→μ+νH with a hypothetic stable heavy neutrino in the final state. The obtained missing mass spectrum does not show peaks that could be attributed to existence of stable heavy neutrinos in the mass range (270
Journal Article
Investigation of the compressed baryonic matter at the GSI accelerator complex
The Compressed Baryonic Matter (CBM) experiment at FAIR will play a unique role in the exploration of the QCD phase diagram in the region of high net-baryon densities, because it is designed to run at unprecedented interaction rates. High-rate operation is the key prerequisite for high-precision measurements of multi-differential observables and of rare diagnostic probes which are sensitive to the dense phase of the nuclear fireball. The goal of the CBM experiment at SIS100 (√sNN = 2-4.9 GeV) is to discover fundamental properties of QCD matter, namely, the equation-of-state at high density as it is expected to occur in the core of neutron stars, effects of chiral symmetry, and the phase structure at large baryon-chemical potentials (μB ≥ 500 MeV). We are focusing here on the contribution of JINR to the CBM experiment: design of the superconducting dipole magnet; manufacture of the straw and micro-strip silicon detectors, participation in the data taking and analysis algorithms and physics program.
Journal Article
Observation of$$K^{+} \\rightarrow \\pi ^{+}\\pi ^{0}\\pi ^{0}\\gamma $$decay
The$$K^{+} \\rightarrow \\pi ^{+}\\pi ^{0}\\pi ^{0}\\gamma $$K + → π + π 0 π 0 γ decay is observed by the OKA collaboration. About 60 events of the decay observed with signal:noise$$\\approx 1$$≈ 1 . The branching ratio obtained by normalization to$$K^{+} \\rightarrow \\pi ^{+}\\pi ^{0}\\pi ^{0}$$K + → π + π 0 π 0 is measured to be$$(3.7 \\pm 0.9(stat) \\pm 0.3(syst))\\times 10^{-6}$$( 3.7 ± 0.9 ( s t a t ) ± 0.3 ( s y s t ) ) × 10 - 6 for$$E_{\\gamma }^*>10\\,\\textrm{MeV}$$E γ ∗ > 10 MeV . The branching ratio,$$\\gamma $$γ energy spectrum and angular distribution are consistent with ChPT prediction.
Journal Article