Search for muoproduction of X(3872) at COMPASS and indication of a new state X˜(3872)

We have searched for exclusive production of exotic charmonia in the reaction μ+N→μ+(J/ψπ+π−)π±N′ using COMPASS data collected with incoming muons of 160 GeV/c and 200 GeV/c momentum. In the J/ψπ+π− mass distribution we observe a signal with a statistical significance of 4.1 σ. Its mass and width are consistent with those of the X(3872). The shape of the π+π− mass distribution from the observed decay into J/ψπ+π− shows disagreement with previous observations for X(3872). The observed signal may be interpreted as a possible evidence of a new charmonium state. It could be associated with a neutral partner of X(3872) with C=−1 predicted by a tetraquark model. The product of cross section and branching fraction of the decay of the observed state into J/ψπ+π− is determined to be 71±28(stat)±39(syst) pb

Search for invisible decays of sub-GeV dark photons in missing-energy events at the CERN SPS

We report on a direct search for sub-GeV dark photons (A') which might be produced in the reaction e^- Z \to e^- Z A' via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The A's would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75\cdot 10^{9} electrons on target. We set new limits on the \gamma-A' mixing strength and exclude the invisible A' with a mass < 100 MeV as an explanation of the muon g_\mu-2 anomaly.Comment: 6 pages, 3 figures; Typos corrected, references adde

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 $a(s)\to\gamma \gamma$ 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 $2.84\times10^{11}$ electrons on target allowing to set new limits on the $a(s)\gamma\gamma$-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

Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering

A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the range $0.003 < x < 0.4$, fraction of the virtual photon energy carried by the hadron in the range $0.2 < z < 0.8$, square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/$c)^2 < P_{\rm{hT}}^{2} < 3$ (GeV/$c$)$^2$. The multiplicities are presented as a function of $P_{\rm{hT}}^{2}$ in three-dimensional bins of $x$, $Q^2$, $z$ and compared to previous semi-inclusive measurements. We explore the small-$P_{\rm{hT}}^{2}$ region, i.e. $P_{\rm{hT}}^{2} < 1$ (GeV/$c$)$^2$, where hadron transverse momenta are expected to arise from non-perturbative effects, and also the domain of larger $P_{\rm{hT}}^{2}$, where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small $P_{\rm{hT}}^{2}$ to study the dependence of the average transverse momentum $\langle P_{\rm{hT}}^{2}\rangle$ on $x$, $Q^2$ and $z$. The power-law behaviour of the multiplicities at large $P_{\rm{hT}}^{2}$ is investigated using various functional forms. The fits describe the data reasonably well over the full measured range.Comment: 28 pages, 20 figure

Statistique mensuelle de la viande. 1968 N° 4 APRIL-AVRIL = Monthly statistiques of meat. 1968 No. 4 April

In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV pi- beam is used to test the method in the NA64 experiment resulting in a suppression factor of 10−5 while the efficiency for electron detection is 95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved.ISSN:0168-9002ISSN:1872-957

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 $\sim$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 $X17$ boson. So far, the search for the decay $X17 \rightarrow e^+ e^-$ 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 $X17$ parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the $X17$ decay with the proposed method. To reach this goal an optimization of the $X17$ 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 [Phys. Rev. D101, 071101 (2020)], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate shows that the goal of the proposed search is feasible

Constraints on New Physics in the Electron g-2 from a Search for Invisible Decays of a Scalar, Pseudoscalar, Vector, and Axial Vector

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, $e^- Z \to e^- Z X$, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such process was found in the full NA64 data set of $2.84\times 10^{11}$ 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 $a_e$, $|\Delta a_{X}| \lesssim 10^{-15} - 10^{-13}$ for the $X$ mass region $m_X\lesssim 1$ 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.Comment: 6 pages, 4 figure