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
.
A
bstract
We address production of massive axion-like particles by two electromagnetic modes inside a superconducting radio-frequency (SRF) cylindrical cavity. We discuss in detail the choice of pump ...modes and cavity design. We numerically compute time-averaged energy density of produced axion field for various cavity modes and wide range of axion masses. This allows us to estimate optimal conditions for axion production within a cavity. In addition, we consider photon regeneration process initiated by produced axion field in a screened radio-frequency cavity and derive constraints in parameter space (
g
aγγ
,
m
a
) for different choice of pump modes.
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.
We discuss prospects of searching for a dark photon (A′) which serves as mediator between Standard model (SM) particles and light dark matter (LDM) by using the combined results from the NA64 ...experiment at the CERN SPS running in high-energy electron (NA64e) and muon (NA64μ) modes. We discuss the most natural values and upper bounds on the A′ coupling constant to LDM and show they are lying in the range accessible at NA64. While for the projected 5×1012 electrons on target (EOT) NA64e is able to probe the scalar and Majorana LDM scenarios, the combined NA64e and NA64μ results with ≃1013 EOT and a few 1013 MOT, respectively, will allow covering significant region in the parameter space of the most interesting LDM models. This makes NA64e and NA64μ extremely complementary to each other and increases significantly the discovery potential of sub-GeV DM.
Dark photon (A′) that couples to the standard model fermions via the kinetic mixing with photons and serves as a mediator of dark matter production could be observed in the high-energy electron ...scattering e−+Z→e−+Z+A′ off nuclei followed by the bremsstrahlung A′→invisible decay. We cross check the exact tree-level calculations of the A′ production cross sections by other results and implement them in the program for the full simulation of such events in the experiment NA64 at the CERN SPS . Using simulations results, we study the missing energy signature for the A′→invisible decay that allows to probe the γ−A′ mixing strength in a wide, from sub-MeV to sub-GeV, A′ mass range. We refine and expand our earlier studies of this signature by including corrections to the previously used calculations based on the improved Weizsaker–Williams (IWW) approximation, which turn out to be significant. We find that the commonly used IWW approach can lead to substantial overestimation of the sensitivity to A′ in fixed target experiments. The possibility of future searches with high-energy electron beams and their sensitivity to A′ are briefly discussed.
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•The JKR model was applied for analysis of the AFM force curves with adhesion.•The approach and retraction curves obtained on cells and PA gels were compared.•Loading rate, holding ...time and indentation depth affected the measured parameters.•Observed effects were caused by both bulk and interfacial viscoelasticity.•BSA addition decreased adhesion of the probe to gels but increased adhesion to cells.
The force-distance curves (FCs) obtained by the atomic force microscope (AFM) with colloid probes contain information about both the viscoelastic properties and adhesion of a sample. Here, we processed both the approach and retraction parts of FCs obtained on polyacrylamide gels (in water or PBS) and Vero cells (in a culture medium). The Johnson–Kendall–Roberts model was applied to the retraction curves to account for the adhesion. The effects of loading rate, holding time and indentation depth on adhesion force and Young’s modulus, calculated from approach and retraction curves, were studied. It was shown that both bulk and local interfacial viscoelasticity can affect the observed approach-retraction hysteresis and measured parameters. The addition of 1% bovine serum albumin (BSA) decreased adhesion of the probe to the PAA gel surface, so interfacial viscoelasticity effects were diminished. On the contrary, the adhesiveness of Vero cells increased after BSA addition, indicating the complex nature of the cell-probe interaction.
Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble ...interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A^{'}, in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37×10^{11} electrons on target collected during 2016-2022 runs NA64 probes for the first time the well-motivated region of parameter space of benchmark thermal scalar and fermionic dark matter models. No evidence for dark matter production has been found. This allows us to set the most sensitive limits on the A^{'} couplings to photons for masses m_{A^{'}}≲0.35 GeV, and to exclude scalar and Majorana dark matter with the χ-A^{'} coupling α_{D}≤0.1 for masses 0.001≲m_{χ}≲0.1 GeV and 3m_{χ}≤m_{A^{'}}.
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