We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e^{+}e^{-} pairs observed in the excited ^{8}Be^{*} nucleus decays. Because of ...its coupling to electrons, the X could be produced in the bremsstrahlung reaction e^{-}Z→e^{-}ZX by a 100 GeV e^{-} beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a e^{+}e^{-} pair. With 5.4×10^{10} electrons on target, no evidence for such decays was found, allowing us to set first limits on the X-e^{-} coupling in the range 1.3×10^{-4}≲ε_{e}≲4.2×10^{-4} excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A^{'}) from nonobservation of the decay A^{'}→e^{+}e^{-} of the bremsstrahlung A^{'} with a mass ≲23 MeV.
We carried out a model-independent search for light scalar (s) and pseudoscalar axionlike (a) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new ...particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The a (s) would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their a (s) → γγ decay in the rest of the HCAL detector, or as events with a large missing energy if the a (s) decays downstream of the HCAL. This method allows for the probing of the a (s) parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to 2.84 × 10 11 electrons on target, allowing us to set new limits on the a(s)γγ -coupling strength for a (s) masses below 55 MeV.
A search for sub-GeV dark matter production mediated by a new vector boson A′, called a dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in ...an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with 2.84×1011 electrons on target no evidence of such a process has been found. The most stringent constraints on the A′ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range ≲0.2 GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.
We performed a search for a new generic X boson, which could be a scalar (S), pseudoscalar (P), vector (V), or an axial vector (A) particle produced in the 100 GeV electron scattering off nuclei, e− ...Z → e− ZX, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such a process was found in the full NA64 dataset of 2.84 × 1011 electrons on target. We place new bounds on the S, P, V, A coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment ae,|ΔaX|≲ 10−15 –10−13 for the X mass region 1 MeV ≲ mX ≲ 1 GeV. These results are an order of magnitude more sensitive compared to the current accuracy on a e from the electron g − 2 experiments and recent high-precision determination of the fine structure constant.
We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active ...target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100 MeV as an explanation of the muon g_{μ}-2 anomaly.
The improved results on a direct search for a new X ( 16.7 MeV ) boson that could explain the anomalous excess of e+e− pairs observed in the decays of the excited 8Be ∗ nuclei ("Berillium or X17 ...anomaly") are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, 4 He , seems to support this anomaly spurring the need for an independent measurement. If the X boson exists, it could be produced in the bremsstrahlung reaction e − Z → e − Z X by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into e+e− pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4 × 1010 electrons on target collected in 2017 and 2018. This allows one to set new limits on the X−e− coupling in the range 1.2 × 10−4 ≲ ε e ≲ 6.8 × 10−4 , excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons ( A ′ ) with a mass ≲ 24 MeV . For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling ε . This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.
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
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hunting down the X17 boson at the CERN SPS Depero, E.; Andreev, Yu. M.; Banerjee, D. ...
The 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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
