Papers by D. Protopopescu
Measurement of the $K^+\to\pi^+\gamma\gamma$ decay
The first measurement of the branching ratio of the ultra-rare 𝐾 + → 𝜋 + 𝜈 ν decay and the search... more The first measurement of the branching ratio of the ultra-rare 𝐾 + → 𝜋 + 𝜈 ν decay and the searches for production of feebly interacting particles predicted by dark scalar models from the NA62 experiment at CERN are presented. The results of a search for in-flight decays of dark photon to ℓ + ℓ -pairs with data collected in beam-dump mode in 2021 are also reported.
Journal of High Energy Physics, Nov 20, 2023
Measurement of the ultra-rare K + → π + ν ν decay at the NA62 experiment at CERN requires high-pe... more Measurement of the ultra-rare K + → π + ν ν decay at the NA62 experiment at CERN requires high-performance particle identification to distinguish muons from pions. Calorimetric identification currently in use, based on a boosted decision tree algorithm, achieves a muon misidentification probability of 1.2 × 10 -5 for a pion identification efficiency of 75% in the momentum range of 15-40 GeV/c. In this work, calorimetric identification performance is improved by developing an algorithm based on a convolutional neural network classifier augmented by a filter. Muon misidentification probability is reduced by a factor of six with respect to the current value for a fixed pion-identification efficiency of 75%. Alternatively, pion identification efficiency is improved from 72% to 91% for a fixed muon misidentification probability of 10 -5 .
High Intensity Kaon Experiments (HIKE) at the CERN SPS Proposal for Phases 1 and 2
Search for K+ decays into the π+e+e−e+e− final state
Physics Letters B, Oct 31, 2023
The first search for ultra-rare K+ decays into the π+e+e−e+e− final state is reported, using a da... more The first search for ultra-rare K+ decays into the π+e+e−e+e− final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017–2018. An upper limit of 1.4×10−8 at 90% CL is obtained for the branching ratio of the K+→π+e+e−e+e− decay, predicted in the Standard Model to be (7.2±0.7)×10−11. Upper limits at 90% CL are obtained at the level of 10−9 for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: K+→π+aa, a→e+e− and K+→π+S, S→A′A′, A′→e+e−.
A search for the $K^+\to\mu^-\nu e^+e^+$ decay
HAL (Le Centre pour la Communication Scientifique Directe), Dec 31, 2022
Journal of High Energy Physics, Sep 6, 2023
A bstract A sample of 1 . 3 × 10 5 K + → π 0 e + νγ candidates with less than 1% background was c... more A bstract A sample of 1 . 3 × 10 5 K + → π 0 e + νγ candidates with less than 1% background was collected by the NA62 experiment at the CERN SPS in 2017–2018. Branching fraction measurements are obtained at percent relative precision in three restricted kinematic regions, improving on existing results by a factor larger than two. An asymmetry, possibly related to T-violation, is investigated with no evidence observed within the achieved precision.
HAL (Le Centre pour la Communication Scientifique Directe), Dec 31, 2022
The NA62 experiment at CERN, designed to study the ultra-rare decay K + → π + ν ν, has also colle... more The NA62 experiment at CERN, designed to study the ultra-rare decay K + → π + ν ν, has also collected data in beam-dump mode. In this configuration, dark photons may be produced by protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to µ + µ -pairs is reported, based on a sample of 1.4 × 10 17 protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV/c 2 .
Journal of High Energy Physics, May 1, 2019
The results of a search for π 0 decays to a photon and an invisible massive dark photon at the NA... more The results of a search for π 0 decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of 4.12 × 10 8 tagged π 0 mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60-110 MeV/c 2 . The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay π 0 → γν ν, improving the current limit by more than three orders of magnitude.
Physics Letters B, Aug 1, 2020
A search for heavy neutral lepton (N) production in K + → e + N decays using the data sample coll... more A search for heavy neutral lepton (N) production in K + → e + N decays using the data sample collected by the NA62 experiment at CERN in 2017-2018 is reported. Upper limits of the extended neutrino mixing matrix element |U e4 | 2 are established at the level of 10 -9 over most of the accessible heavy neutral lepton mass range 144-462 MeV/c 2 , with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The |U e4 | 2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c 2 .
Physics Letters B, Sep 1, 2006
Journal of High Energy Physics, Jun 1, 2021
The NA62 experiment reports the branching ratio measurement BR(K + → π + ν ν) = (10.6 +4.0 -3.4 |... more The NA62 experiment reports the branching ratio measurement BR(K + → π + ν ν) = (10.6 +4.0 -3.4 | stat ± 0.9 syst ) × 10 -11 at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016-2018. This provides evidence for the very rare K + → π + ν ν decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10 -11 , corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10 -11 . This measurement is also used to set limits on BR(K + → π + X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.
Physical Review Letters, Jan 22, 2015
A measurement of the electroproduction of photons off protons in the deeply inelastic regime was ... more A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6-GeV electron beam, a longitudinally polarized proton target and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep → e p γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q 2 , xB, t and φ, for 166 fourdimensional bins. In the framework of Generalized Parton Distributions (GPDs), at leading twist the t dependence of these asymmetries provides insight on the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center. These results also bring important and necessary constraints for the existing parametrizations of chiral-even GPDs.
Physical Review Letters, Dec 30, 2010
The electric form factor of the neutron was determined from studies of the reaction 3 -→ He( e, e... more The electric form factor of the neutron was determined from studies of the reaction 3 -→ He( e, e n)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling
Search for $K^+$ decays into the $\pi^+e^+e^-e^+e^-$ final state
HAL (Le Centre pour la Communication Scientifique Directe), Jul 13, 2023
Physics Letters B, Feb 1, 2009
The spin structure functions g1 for the proton and the deuteron have been measured over a wide ki... more The spin structure functions g1 for the proton and the deuteron have been measured over a wide kinematic range in x and Q 2 using 1.6 and 5.7 GeV longitudinally polarized electrons incident upon polarized NH3 and ND3 targets at Jefferson Lab. Scattered electrons were detected in the CEBAF Large Acceptance Spectrometer, for 0.05 < Q 2 < 5 GeV 2 and W < 3 GeV. The first moments of g1 for the proton and deuteron are presented -both have a negative slope at low Q 2 , as predicted by the extended Gerasimov-Drell-Hearn sum rule. The first extraction of the generalized forward spin polarizability of the proton γ p 0 is also reported. This quantity shows strong Q 2 dependence at low Q 2 . Our analysis of the Q 2 evolution of the first moment of g1 shows agreement in leading order with Heavy Baryon Chiral Perturbation Theory. However, a significant discrepancy is observed between the γ p 0 data and Chiral Perturbation calculations for γ p 0 , even at the lowest Q 2 .
Physical review, Mar 7, 2007
New measurements of the spin structure functions of the proton and deuteron g p 1 (x, Q 2 ) and g... more New measurements of the spin structure functions of the proton and deuteron g p 1 (x, Q 2 ) and g d 1 (x, Q 2 ) in the nucleon resonance region are compared with extrapolations of target-masscorrected next-to-leading-order (NLO) QCD fits to higher energy data. Averaged over the entire resonance region (W < 2 GeV), the data and QCD fits are in good agreement in both magnitude and Q 2 dependence for Q 2 > 1.7 GeV 2 /c 2 . This "global" duality appears to result from cancellations among the prominent "local" resonance regions: in particular strong σ 3/2 contributions in the ∆(1232) region appear to be compensated by strong σ 1/2 contributions in the resonance region centered on 1.5 GeV. These results are encouraging for the extension of NLO QCD fits to lower W and Q 2 than have been used previously.
Physics Letters B, Mar 1, 2018
A search for heavy neutral lepton production in K + decays using a data sample collected with a m... more A search for heavy neutral lepton production in K + decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10 -7 to 10 -6 level are established on the elements of the extended neutrino mixing matrix |U e4 | 2 and |U μ4 | 2 for heavy neutral lepton mass in the ranges 170-448 MeV/c 2 and 250-373 MeV/c 2 , respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |U e4 | 2 , and above 300 MeV/c 2 for |U μ4 | 2 .
Physics Letters B, Apr 1, 2019
The NA62 experiment at the CERN SPS reports the first search for K + → π + ν ν using the decay-in... more The NA62 experiment at the CERN SPS reports the first search for K + → π + ν ν using the decay-in-flight technique, based on a sample of 1.21 × 10 11 K + decays collected in 2016. The single event sensitivity is 3.15 × 10 -10 , corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of 14 × 10 -10 on the K + → π + ν ν branching ratio at 95% CL.
Physical review, Jul 8, 2008
The light vector mesons (ρ, ω, and φ) were produced in deuterium, carbon, titanium, and iron targ... more The light vector mesons (ρ, ω, and φ) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the ρ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to e + e -. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The ρ meson mass spectrum was extracted after the ω and φ signals were removed in a nearly model-independent way. Comparisons were made between the ρ mass spectra from the heavy targets (A > 2) with the mass spectrum extracted from the deuterium target. With respect to the ρ-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.
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Papers by D. Protopopescu