9 research outputs found
Search for Axionlike and Scalar Particles with the NA64 Experiment
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 shielding, and would be observed either
through their decay in the rest of the HCAL detector or
as events with large missing energy if the a(s) decays downstream of the HCAL.
This method allows for the probing 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
electrons on target allowing to set new limits on the
-coupling strength for a(s) masses below 55 MeV.Comment: This publication is dedicated to the memory of our colleague Danila
Tlisov. 7 pages, 5 figures, revised version accepted for publication in Phys.
Rev. Let
Improved exclusion limit for light dark matter from e+e- annihilation in NA64
The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson AâČ were set by the NA64 experiment for the mass region mAâČâČ250 MeV, by analyzing data from the interaction of 2.84Ă1011 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including AâČ production via secondary positron annihilation with atomic electrons, we extend these limits in the 200-300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated e+ beam efforts
Search for Axionlike and Scalar Particles with the NA64 Experiment
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)âγγ 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 2.84Ă10^{11} electrons on target, allowing us to set new limits on the a(s)γγ-coupling strength for a(s) masses below 55 MeV
Search for pseudoscalar bosons decaying into e+e- pairs in the NA64 experiment at the CERN SPS
We report the results of a search for a light pseudoscalar particle a that couples to electrons and decays to e+e- performed using the high-energy CERN SPS H4 electron beam. If such light pseudoscalar exists, it could explain the ATOMKI anomaly (an excess of e+e- pairs in the nuclear transitions of Be8 and He4 nuclei at the invariant mass â17 MeV observed by the experiment at the 5 MV Van de Graaff accelerator at ATOMKI, Hungary). We used the NA64 data collected in the "visible mode"configuration with a total statistics corresponding to 8.4Ă1010 electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter Ï” we also used the data collected in 2016-2018 in the "invisible mode"configuration of NA64 with a total statistics corresponding to 2.84Ă1011 EOT. The background and efficiency estimates for these two configurations were retained from our previous analyses searching for light vector bosons and axionlike particles (ALP) (the latter were assumed to couple predominantly to Îł). In this work we recalculate the signal yields, which are different due to different cross section and lifetime of a pseudoscalar particle a, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space ma-Ï” in the mass range from 1 to 17.1 MeV is excluded. At the mass of the central value of the ATOMKI anomaly (the first result obtained on the beryllium nucleus, 16.7 MeV) the values of Ï” in the range 2.1Ă10-4<Ï”<3.2Ă10-4 are excluded
Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN
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
â
pairs observed in the excited
8
Be
â
nucleus decays. Because of its coupling to electrons, the
X
could be produced in the bremsstrahlung reaction
e
â
Z
â
e
â
Z
X
by a 100 GeV
e
â
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
â
pair. With
5.4
Ă
10
10
electrons on target, no evidence for such decays was found, allowing us to set first limits on the
X
â
e
â
coupling in the range
1.3
Ă
10
â
4
âČ
Δ
e
âČ
4.2
Ă
10
â
4
excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (
A
âČ
) from nonobservation of the decay
A
âČ
â
e
+
e
â
of the bremsstrahlung
A
âČ
with a mass
âČ
23
â
â
Me
Improved limits on a hypothetical X (16.7) boson and a dark photon decaying into eâșeâ» pairs
The improved results on a direct search for a new
X
(
16.7
â
â
MeV
)
boson that could explain the anomalous excess of
e
+
e
â
pairs observed in the decays of the excited
8
Be
â
nuclei (âBerillium or X17 anomalyâ) 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\ud
â
Z
â
e
â
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
â
pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to
8.4
Ă
10
10
electrons on target collected in 2017 and 2018. This allows one to set new limits on the
X
â
e
â
coupling in the range
1.2
Ă
10
â
4
âČ
Δ
e
âČ
6.8
Ă
10
â
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
âČ
) with a mass
âČ
24
â
â
MeV
. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling
Δ
. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics
Search for a light Z ' in the L-mu - L-tau scenario with the NA64-e experiment at CERN
The extension of Standard Model made by inclusion of additional U(1) gauge LÎŒ-LÏ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in B meson decays. This model predicts the existence of a new, light ZâČ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the ZâČ parameter space, obtained from the analysis of the data collected by the NA64-e experiment at CERN SPS, that performed a search for light dark matter with 2.84Ă1011 electrons impinging with 100 GeV on an active thick target. The resulting limits touch the muon g-2 preferred band for values of the ZâČ mass of order of 1 MeV, while the sensitivity projections for the future high-statistics NA64-e runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario
Measurement of the spin structure of the deuteron in the DIS region
Ageev ES, Alexakhin VY, Alexandrov Y, et al. Measurement of the spin structure of the deuteron in the DIS region. Phys.Lett. B. 2005;612(3-4):154-164.We present a new measurement of the longitudinal spin asymmetry Ad and the spin-dependent structure function g(1)(d) of the deuteron in the range 1 < Q(2) < 100 GeV2 and 0.004 < x < 0.7. The data were obtained by the COMPASS experiment at CERN using a 160 GeV polarised muon beam and a large polarised (LiD)-Li-6 target. The results are in agreement with those from previous experiments and improve considerably the statistical accuracy in the region 0.004 < x < 0.03. (c) 2005 Elsevier B.V. All rights reserved