The multiplicities of positive and negative pions, kaons and unidentified hadrons produced in deep-inelastic scattering are measured in bins of the Bjorken scaling variable x, the relative virtual-photon energy y and the fraction of the virtual-photon energy transferred to the final-state hadron z. Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam of both electric charges and a liquid hydrogen target. These measurements cover the kinematic domain with photon virtuality Q^2 > 1 (GeV/c)^2, 0.004 < x < 0.4, 0.1 < y < 0.7 and 0.2 < z < 0.85, in accordance with the kinematic domain used in earlier published COMPASS multiplicity measurements with an isoscalar target. The calculation of radiative corrections was improved by using the Monte Carlo generator DJANGOH, which results in up to 12% larger corrections in the low-x region.

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar η and η' mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV π^- on nuclei of an active target as the source of neutral mesons. The η, η' → invisible events would exhibit themselves via a striking signature - the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with 2.9×10^9 pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio Br(η' → invisible) < 2.1 × 10^-4 improving the current bound by a factor of ≃3. We also set a limit on Br(η→ invisible) < 1.1 × 10^-4 comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

We present the results of a missing-energy search for Light Dark Matter which has a new interaction with ordinary matter transmitted by a vector boson, called dark photon $A^\prime$. For the first time, this search is performed with a positron beam by using the significantly enhanced production of $A^\prime$ in the resonant annihilation of positrons with atomic electrons of the target nuclei, followed by the invisible decay of $A^\prime$ into dark matter. No events were found in the signal region with $(10.1 \pm 0.1)~\times~10^{9}$ positrons on target with 100 GeV energy. This allowed us to set new exclusion limits that, relative to the collected statistics, prove the power of this experimental technique. This measurement is a crucial first step toward a future exploration program with positron beams, whose estimated sensitivity is here presented.

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 $K^{+} \to \pi^{+} \pi^{0} a$. No signal is observed, the upper limits for the branching ratio of the decay are calculated. The $90\%$ confidence level upper limit is changing from $2.5\cdot10^{-6}$ to $2\cdot10^{-7}$ for the ALP mass from 0 to 200 MeV/$c^{2}$, except for the region of $\pi^{0}$ mass, where the upper limit is $4.4\cdot10^{-6}$.

We report the first search for dark sectors performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of 160 GeV/c are directed to an active target. The signal signature consists of a single scattered muon with momentum <80 GeV/c in the final state, accompanied by missing energy, i.e., no detectable activity in the downstream calorimeters. For a total dataset of (1.98±0.02)×1010 muons on target, no event is observed in the expected signal region. This allows us to set new limits on the remaining (mZ′,gZ′) parameter space of a new Z′ (Lμ−Lτ) vector boson which could explain the muon (g−2)μ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal dark matter relic abundance. Our results pave the way to explore dark sectors and light dark matter with muon beams in a unique and complementary way to other experiments. Published by the American Physical Society 2024

A search for the K+ → π0π0π0e+ν decay is performed by the OKA collaboration. The search is based on 3.65 × 109 K+ decays. No signal is observed. The upper limit set is BR(K+ → π0π0π0e+ν) < 5.4 × 10–8 90

Abstract A high-statistics data sample of the $$K^{+}$$ 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 $$K^{+} \rightarrow \pi ^{+} \pi ^{0} a$$ K + → π + π 0 a . No signal is observed, and the upper limits for the branching ratio of the decay are calculated. The $$90\%$$ 90 % confidence level upper limit changes from $$2.5\cdot 10^{-6}$$ 2.5 · 10 - 6 to $$2\cdot 10^{-7}$$ 2 · 10 - 7 for the ALP mass from 0 to 200 MeV/ $$c^{2}$$ c 2 , except for the region of $$\pi ^{0}$$ π 0 mass, where the upper limit is $$4.4\cdot 10^{-6}$$ 4.4 · 10 - 6 .

Abstract 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.

Abstract The inclusion of an additional U(1) gauge Lμ − Lτ 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 Lμ − Lτ 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 × 1011 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 <$$ {m}_{Z^{\prime }} $$ m Z ′ ≲ 2 MeV range, in complementarity with the limits recently obtained by the NA64-μ experiment with a muon beam.

A search for dark sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles (X) could be produced in the bremsstrahlung-like reaction of high-energy 160 GeV positively charged muons impinging on an active target, mu N -> mu NX, followed by their decays, X -> invisible. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the subdetectors located downstream from the interaction point. The full sample of the 2022 run is analyzed through the missing-energy/momentum channel, with total statistics of (1.98 +/- 0.02) x 10(10) muons on target. We demonstrate that various muonphilic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector case, besides a L-mu - L-tau Z' vector boson, we also consider an invisibly decaying dark photon (A' -> invisible). This search is complementary to NA64 running with electrons and positrons, thus opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams.

Owing to a change in the scope of physics research, the AMBER spectrometer at CERN is undergoing an update on its instrumentation and trigger strategy for the data acquisition system. One of the key updates is the adoption of a free-running and trigger-less operation, which differs from the predecessor of AMBER, COMPASS, by working in a continuous mode but in an event-based triggered way. In this article, we present a multichannel data-acquisition platform developed in line with the next generation of trigger-less and free-running data acquisition systems. The platform is based on a pre-existing Mezzanine Sampling ADC board for the analog-to-digital conversion and a Xilinx Zynq Ultrascale+ System on Module for online real-time data processing. We also present the evaluation of the system operating in continuous mode, taking data from a 25-element electromagnetic calorimeter prototype with a muon beam, during the first AMBER pilot run. The acquired data were used for noise and pulse shape studies required for the design of the algorithms for lossless compression and data feature extraction needed for trigger-less operation.

The K (+) -> pi (+) pi( 0) pi (0) gamma decay is observed by the OKA collaboration. About 60 events of the decay observed with signal:noise approximate to 1. The branching ratio obtained by nor-malization to K (+) -> pi (+) pi( 0) pi (0) gamma is measured to be (3.7 +/- 0.9(stat)+/- 0.3(syst))x10(-6)for E-gamma(& lowast;)>10 MeV. The branching ratio,gamma energy spectrum and angular distribution are consistent with ChPT prediction

The K^+→π ^+π ^0π ^0γ 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 ^0π ^0 is measured to be (3.7 ± 0.9(stat) ± 0.3(syst))× 10^-6 for E_γ^*>10 MeV . The branching ratio, γ energy spectrum and angular distribution are consistent with ChPT prediction.

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar eta and eta ' mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV pi- on nuclei of an active target as the source of neutral mesons. The eta,eta'-> invisible events would exhibit themselves via a striking signature-the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with 2.9x109 pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio Br(eta'-> invisible) < 2.1 x 10(-4) improving the current bound by a factor of similar or equal to 3. We also set a limit on Br(eta -> invisible) < 1.1 x 10(-4) comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarised $^6$LiD target. The data were taken in 2022 with the COMPASS spectrometer using the 160 \gevv\ muon beam at CERN, balancing the existing data on transversely polarised proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the $u$ and $d$ quark, as well as the tensor charge in the measured $x$-range are extracted. In particular, the accuracy of the $d$ quark results is significantly improved.

We report on a search for a new $Z'$ ($L_\mu-L_\tau$) vector boson performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of 160 GeV/c are directed to an active target. A signal event is a single scattered muon with momentum $<$ 80 GeV/c in the final state, accompanied by missing energy, i.e. no detectable activity in the downstream calorimeters. For a total statistic of $(1.98\pm0.02)\times10^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on part of the remaining $(m_{Z'},\ g_{Z'})$ parameter space which could provide an explanation for the muon $(g-2)_\mu$ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal Dark Matter relic abundance. Our results pave the way to explore Dark Sectors and light Dark Matter with muon beams in a unique and complementary way to other experiments.

The $K^{+} \to \pi^{+}\pi^{0}\pi^{0}\gamma$ decay is observed by the OKA collaboration. The branching ratio is measured to be $(4.1 \pm 0.9(stat) \pm 0.4(syst))\times 10^{-6}$. The branching ratio and $\gamma$ energy spectrum are consistent with ChPT prediction.

A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2003–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160GeV/c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e− annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of π+π− pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x>0.032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of π+π− measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions.

The K^+→π^+π^0π^0γ decay is observed by the OKA collaboration. The branching ratio is measured to be (4.1 ± 0.9(stat) ± 0.4(syst))× 10^-6. The branching ratio and γ energy spectrum are consistent with ChPT prediction.