2016, a breakthrough anomaly was reported in the isoscalar magnetic dipole transition in8Be via the Internal Pair Creation process. An unexpected angular distribution of the relative angle of the e+e− was measured at the Atomki Laboratory. This phenomenon was explained considering the assumption of an emission of a neutral boson, named X17, with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c2 and Jπ =1+. This finding triggered a global campaign to search for the new boson claimed. In Italy, at the Legnaro National Laboratories, a novel scintillator detector array has been designed and built. The present work reports the status of the first in-beam experiments performed in 2023 and 2024

Aguilera P.

Angelini F.

Azaiez F.

Baldesi L.

Balogh M.

Barlini S.

Benito-Garcia J.

Bolzonella R.

Bottoni S.

Brugnara D.

Caletano A.

Camaiani A.

Carollo S.

Casini G.

Cobari G.

Dell'Aquila D.

Ercolano F.

Ertroprak A.

Fabris D.

Galtarossa F.

Goasduff A.

Gongora Servin B.

Gottardo A.

Gozzelino A.

Gramegna F.

Lazzaretto D.

Lombardo I.

Marchi T.

Mengoni D.

Nannini A.

Palombini L.

Pellumaj J.

Perez-Vidal R. M.

Piantelli S.

Pigliapoco S.

Pilotto E.

Polettini M.

Recchia F.

Redigolo L.

Rezynkina K.

Rigon L.

Rocchini M.

Rossi M.

Sedlak M.

Sigmund M.

Simpsi F.

Spina G.

Stramaccioni D.

Tagnani D.

Valdre S.

Valiente-Dobon J. J.

Vigilante M.

Zago L.

Zanon I.

Zappacosta L.

English

AIR Universita degli studi di Milano

DOI 10.1393/ncc/i2025-25023-4Colloquia: WPCF-Resonance 2023IL NUOVO CIMENTO 48 C (2025) 23Searching for internal pair creation anomalies and the X17 bosonat LNLB. Gongora Servin(1)(2)(∗), T. Marchi(1), D. Tagnani(3), A. Caletano(4),A. Goasduff(1), J. J. Valiente-Dobon(1), P. Aguilera(5)(6), F. Angelini(1)(5),F. Azaiez(1), L. Baldesi(8), M. Balogh(1), S. Barlini(8), J. Benito-Garcia(17),R. Bolzonella(2), S. Bottoni(11), D. Brugnara(1), A. Camaiani(8),S. Carollo(5)(6), G. Casini(8), G. Cobari(11), D. Dell’Aquila(9)(10),F. Ercolano(8), A. Ertroprak(15), D. Fabris(5), F. Galtarossa(6),A. Gottardo(1), A. Gozzelino(1), F. Gramegna(1), D. Lazzaretto(5),I. Lombardo(7), D. Mengoni(5)(6), A. Nannini(8), L. Palombini(5),J. Pellumaj(1)(11), R. M. Perez-Vidal(1)(14), S. Piantelli(8),S. Pigliapoco(5)(6), E. Pilotto(5)(6), M. Polettini(5)(6), M. Sigmund(13),G. Spina(11), D. Stramaccioni(1)(5), F. Recchia(1)(5), L. Redigolo(7),K. Rezynkina(6), L. Rigon(5), M. Rocchini(8), M. Rossi(5), M. Sedlak(16),F. Simpsi(5), S. Valdré(8), M. Vigilante(9), L. Zago(1)(5), I. Zanon(12)and L. Zappacosta(5)(1) INFN Laboratori Nazionali di Legnaro - Padova, Italy(2) Università degli Studi di Ferrara - Ferrara, Italy(3) INFN Sezione di Roma Tre - Roma, Italy(4) INFN Sezione di Genova - Genova, Italy(5) Università degli Studi di Padova - Padova, Italy(6) INFN Sezione di Padova - Padova, Italy(7) INFN Sezione di Catania - Catania, Italy(8) INFN Sezione di Firenze - Firenze, Italy(9) INFN Università di Napoli Federico II - Napoli, Italy(10) INFN Sezione di Napoli - Napoli, Italy(11) INFN Sezione di Milano - Milano, Italy(12) Stockholm University - Stockholm, Sweden(13) RBI Zagreb - Zagreb, Croatia(14) IFIC Valencia - Valencia, Spain(15) ANL Argonne - Lemont, IL USA(16) Bratislava University - Bratislava, Slovakia(17) Universidad Complutense de Madrid - Madrid, Spainreceived 23 July 2024(∗) On behalf of the NUCLEX Collaboration.Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0) 12 B. GONGORA SERVIN et al.Summary.— 2016, a breakthrough anomaly was reported in the isoscalar magneticdipole transition in 8Be via the Internal Pair Creation process. An unexpectedangular distribution of the relative angle of the e+e− was measured at the AtomkiLaboratory. This phenomenon was explained considering the assumption of anemission of a neutral boson, named X17, with a mass of 16.70±0.35(stat)±0.5(syst)MeV/c2 and Jπ =1+. This finding triggered a global campaign to search for the newboson claimed. In Italy, at the Legnaro National Laboratories, a novel scintillatordetector array has been designed and built. The present work reports the status ofthe first in-beam experiments performed in 2023 and 2024.1. – IntroductionIn nature, physical systems look for lower energy configurations. The case of theatomic nucleus is of particular interest because the release of the energy contains in-formation about its structure, spin, angular momentum, and internal dynamics. In theelectromagnetic transitions, the emission of intermediate particles is observed. The mostcommon case is the gamma-ray emission. Other processes become essential under cer-tain conditions. For example, the internal conversion (IC) and the Internal Pair Creation(IPC) become relevant in 0+j → 0+i transitions (i < j). The angular distribution of theemission of those particles contains information about the type of electromagnetic tran-sition, spin, and parity. In the case of the IPC, a complete theoretical description waspublished by Rose in 1949 [1, 2]. The model had demonstrated consistency with theexperiments performed in light nuclei since 1950 [3, 4]. But in 2016, Krasznahorkay etal. published the breakthrough of an anomaly in the IPC of 8Be [5]. An unexpectedbehavior in the Internal Pair Creation Coefficient was found in the isoscalar magneticdipole transition (18.15 MeV state (Jπ = 1+, T=0+) → ground state (Jπ =0+, T=0+)).The Rose model predicts a quick drop with the relative emission angle of the leptons. Incontrast, the Hungarian group found a peak-like behavior at large angles [5]. This resulthas been interpreted as the signature of the emission of a previously unknown neutralisoscalar particle, named X17, with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c2 andJπ =1+.The present work reports the status of the first experiments performed at the LegnaroNational Laboratories to study this anomaly. The project aims to measure the angulardistribution of the relative angle of the emission of the e+e− in the IPC process in theisoscalar magnetic dipole transition in 8Be.2. – MethodThe detector array was designed to be placed in the same reaction chamber as thetarget. This fact reduces the uncertainty of the incidence position determination of theleptons in the detector because of the absence of scattering material between the targetand the detector. The material used to manufacture the detectors is the commercial plas-tic scintillator EJ-200 (polyvinyl toluene). The telescope configuration has been proposedsince the coincidence among the different layers can reduce the gamma-ray backgroundSEARCHING FOR INTERNAL PAIR CREATION ANOMALIES AND THE X17 BOSON AT LNL 3Fig. 1. – Picture of four clover detectors.by up to 4 orders of magnitude, according to Geant4 simulations [6]. Moreover, since theΔE layer cannot be used for particle identification, like in the case of other ions, it hasbeen designed to be sensible to the incidence position of the leptons. Two sublayers of10 bars of 5x50x2 mm3 in a perpendicular configuration constitute the front part of thetelescope. Every bar has been drilled through the longest part to create a 1 mm diametercanal. An optical fiber has been introduced inside this canal for better light collection.The light is read out by a 10x1 of 2x2 mm2 SiPM array produced by the FondazioneBruno Kessler (FBK). This system is cooling down at 0◦ for a better performance of theSiPM. On the other hand, a scintillator block of 50x50x100mm3 is placed in the back ofthe telescope. This layer aims to measure energy in calorimeter mode. Two HamamatsuSiPM of 6x6 mm2 read out the scintillation light.In the project’s first stage, 2 cluster detectors (called “clover”) of 4 telescopes wereinstalled in the reaction chamber. They were placed perpendicular to the beam directionat 30◦ and 315◦, at a 12.5 cm distance from the target. LiF targets from 60 to 900μm/cm2 on Cu/C backing (20-50 μm/cm2 thickness) were irradiated with a proton beamof 450 keV energy. The latest configuration of the setup consists of 4 clover detectors.A two-part experiment has been carried out in the last six months. Two configurationswere used to cover the angular range of the anomaly. In the first one, the clovers wereplaced at 35◦, 165◦, 240◦, and 315◦. While in the second at 30◦, 150◦, 240◦, and 300◦(see fig. 1). A 1.036 MeV proton beam impinging on LiF targets from 30-100 μm/cm2on 20-50 μm/cm2 thickness C backing was the object of study in this stage.4 B. GONGORA SERVIN et al.Fig. 2. – Position spectrum of the bar front layer.3. – Results.The position determination of the scintillator bar system showed a well-separate peakspectrum in correspondence to one bar fired in one of the sublayers (see fig. 2). The po-sition in the 3D space was accomplished by considering the coincide of the two sublayers.The primary validation statement of the events is the coincidence in the three layers oftwo telescopes inside a 10 ns time window. The interaction of leptons with matter causesa tortuous path. To avoid those events that produce a wrong position determination, ahigh energy limit of 1 MeV has been set in every sublayer of the ΔE. This restrictionallows us to select the straightest paths.In the 451 MeV proton beam irradiation, the population of the 0+2 in16O via thenuclear reaction 19F(p,α)16O and the 1+ in 8Be via 7Li(p,γ)8Be were observed. Thereconstruction of the transition energy of the populated states was calculated via thesum of the energy of the pair detected in coincidence. The energy spectrum as a functionof the asymmetry of the deposited energy (see fig. 3) shows three structures along theFig. 3. – Lepton pair sum energy spectrum at 451 MeV proton beam irradiation as a functionof the energy asymmetry of the pair detected.SEARCHING FOR INTERNAL PAIR CREATION ANOMALIES AND THE X17 BOSON AT LNL 5Y-axis. The bottom corresponds to the transition to the ground state in 16O. The secondone looks less defined due to the wall effect in the detectors and the intrinsic width of the2+1 state in8Be. The transition to the ground state in 8Be is at the top of the matrix.These results indicate that the setup and the analysis method can be used to study theanomaly reported in the isoscalar magnetic dipole transition [5].4. – Conclusions.A new lepton spectrometer has been commissioned at the Legnaro National Labo-ratories, and its performance indicates that it can be used to study the anomaly foundat the Atomki laboratories in 2016. The setup can determine the incidence position ofthe leptons in coincidence and measure their energy with a limit close to 20 MeV perlepton. The analysis of the former experiments is in progress, and it will be of relevantimportance to understand the anomaly reported.REFERENCES[1] Rose E., Phys. Rev., 76 (1949) 678.[2] Rose E., Phys. Rev., 78 (1950) 184.[3] Devons S. et al., Proc. Phys. Soc. A, 67 (1954) 134.[4] Devons S. et al., Proc. Phys. Soc. A, 67 (1954) 413.[5] Krasznahorkay J. et al., Proc. Phys. Soc. A, 116 (2016) 2016.[6] Bolzonella R., Master Thesis, UniPD (2021).

Searching for internal pair creation anomalies and the X17 boson at LNL

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Searching for internal pair creation anomalies and the X17 boson at LNL

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