The e−e+ decay of the candidate new particle X(17) has already been confirmed by previous experiments. However, theoretical models give different predictions for the spin and the parity of this particle. The double gamma-decay process could be an appropriate probe to shed light on such properties. Thus, for the first time, we searched for the gamma-gamma decay of X(17) created in nuclear transitions. In this paper, we report preliminary results of two experiments on the Jπ= 0− -> 0+ transition in 4He

Ciemala, M

Csatlós, Margit

Csige, Lóránt

Gácsi, Zoltán

Hunyadi, Mátyás

Klaus, Tobias

Kmiecik, M

Maj, Adam

Nagy, Ádám

Pietralla, Norbert

Révay, Zsolt

Sas, Nándor

Stieghorst, Christian

Tornyi, Tamás Gábor

Tímár, János

Wasilewska, B

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DOI 10.1393/ncc/i2019-19124-8Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 124Searching for the double γ-decay of the X(17) particleA. Nagy(1)(2), A. J. Krasznahorkay(2), M. Ciemala(3) M. Csatlo´s(2),L. Csige(2), Z. Ga´csi(2), M. Hunyadi(2), T. Klaus(4), M. Kmiecik(3),A. Maj(3), N. Pietralla(4), Zs. Re´vay(5), N. Sas(1), C. Stieghorst(5),J. T´ıma´r(2), T. Tornyi(2) and B. Wasilewska(3)(1) University of Debrecen - University square 1, 4032, Debrecen, Hungary(2) Institute for Nuclear Research, Hungarian Academy of Sciences - Bem ter 18/c, H-4026Debrecen, Hungary(3) Institute of Nuclear Physics, Polish Academy of Sciences - ul. Radzikowskiego 152, 31-342Krakow, Poland(4) Institut fu¨r Kernphysik, Technische Universita¨t Darmstadt - Schlossgartenstraße 9, 64289Darmstadt, Germany(5) Technical University Munich, Heinz Maier-Leibnitz Centrum (FRM II) - Lichtenberg str.1, D-85748, Garching, Germanyreceived 5 February 2019Summary. — The e−e+ decay of the candidate new particle X(17) has alreadybeen conﬁrmed by previous experiments. However, theoretical models give diﬀerentpredictions for the spin and the parity of this particle. The double γ-decay processcould be an appropriate probe to shed light on such properties. Thus, for the ﬁrsttime, we searched for the γγ decay of X(17) created in nuclear transitions. In thispaper, we report preliminary results of two experiments on the Jπ = 0− → 0+transition in 4He.1. – IntroductionPhenomenological models predict light neutral bosons (dark photons, axion-like par-ticles, etc.) in the 10 MeV−1 GeV range [1]. According to these models, the e−e+ decayof these particles is allowed. The ﬁrst positive indications for such a light boson was pub-lished recently by Krasznahorkay et al. [2]. The EX = 17.6 MeV and EX = 18.15 MeVstates of 8Be were excited by using proton resonant capture on 7Li. The de-excitation ofthese states (Jπ = 1+, T = 1→ 0+, T = 0; Jπ = 1+, T = 0→ 0+, T = 0) is allowed byinternal pair creation. The angular correlation (σ(Θ)) between the e− and e+ emitted inthe process was measured. The experimental result for isovector transition (EX = 17.6MeV) is indeed in a good agreement with the theoretical expectations. However, in thecase of the isoscalar transition, a signiﬁcant enhancement has been observed at Θ  145◦,which was interpreted as a possible signature of a new particle, called X(17), with a massof m0c2 = 16.70± 0.35(stat)±0.6(sys) MeV.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. NAGY et al.Zhang and Miller have discussed in detail a possible nuclear physics origin of theobserved anomaly [3]. They performed calculations considering interferences between E1,E2, and M1 multipoles and introducing important constraints from photon productionmeasurements. The anomaly could not be explained by their modiﬁed model.Feng and his co-workers analysed our results along with other previous measurementsand gave a new interpretation of our experimental observations by proposing a newtheory with a ﬁfth, fundamental force [4]. According to their explanation, a vector-boson is created in the nuclear transition with a mass of m0c2 = 16.7 MeV and Jπ = 1+.Very recently, Ellwanger and Moretti published another theoretical interpretation [5].They assumed a hypothetical particle emitted with L = 1 orbital momentum. Thus, itimplies that we observed a particle with Jπ = 0−.We have performed two experiments to proof which is the right scenario. Accordingto the Landau-Yang theorem, the γγ-decay of a m > 0, J = 1+ or J = 1− particle isstrictly forbidden. Therefore, we searched for possible signatures of the X(17) → γγdecay in the Jπ = 0− → 0+ transition of 4He.2. – Experimental results on 3H(p, γγ)4HeWe have investigated the 3H(p, γγ)4He reaction at the new, 2-MV Tandetron Accel-erator of MTA Atomki by impinging E=1 MeV protons on a 3 mg/cm2 thick tritiated(3H) Ti target layer evaporated on a 0.4 mm thick Mo disk and cooled by liquid N2.The second excited state of 4He with spin-parity, Jπ = 0− and energy EX = 21.01 MeVwas populated by resonant proton capture. The beam energy has been chosen to belower than the threshold energy of the (n,p) channel (Ethr = 1.018 MeV) to avoid anyneutron-induced background. However, due to the large overlap of the ﬁrst and secondexcited states and by applying Ep = 1 MeV, the ﬁrst excited state was also populatedwith spin-parity, Jπ = 0+ and energy EX = 20.21 MeV.Our γ-ray coincidence spectrometer consisted of 14 LaBr3 scintillators. Twelve detec-tors had a volume of 3”× 3” and two detectors had a volume of 6.5”× 3.5”. The smallerdetectors were placed on a surface of a sphere with a radius of 30 cm. The two largerdetectors were located behind the target. The energy resolution of the LaBr3 detectorswas 0.5% at E = 17 MeV.The γ spectrometer was calibrated by the γ cascade de-exciting the second excitedstate of 12C (E∗ = 16.6 MeV, Jπ = 2−) which was populated in the 11B(p,γ)12C reactionby a proton beam at Ep = 0.685 MeV. The 2− state can decay to the ﬁrst excited state atE = 4.43 MeV, thus the two gamma photons of the cascade with Eγ1 = 12.17 MeV andEγ2 = 4.43 MeV could be detected in coincidence. The single gamma energy spectrum ofthe calibration measurement is shown in Fig. 1a. Single escape (SE) and double escape(DE) peaks are indicated by arrows. In Fig. 1b, the E = 16.6 MeV energy sum of thetwo cascade gammas can be clearly seen as well as the corresponding single and doubleescape peaks and their combinations.In the experiment on 3H(p, γγ)4He, we measured the energy sum of the detected γphotons in coincidence. To reduce the eﬀect of the scattering between the detectors,we set a strict condition on the energy symmetry parameter y = (Eγi − Eγj)/(Eγi +Eγj), where Eγi and Eγj denote the detected γ-energies. The energy sum spectrum isshown in Fig. 2a. Random events and cosmic background events were subtracted. Thecontribution of the background to the γ spectrum is relatively high since the cross sectionfor the direct capture γ rays is large. However, a peak clearly shows up at E = 20.6 MeV.A large Compton edge is also visible at the low–energy side of the energy spectrum.SEARCHING FOR X → γγ 34 5 6 7 8 9 10 11 12 13 (MeV)E310410CountsSEDE4.43 MeV12.17 MeVa) 6 8 10 12 14 16 18 (MeV)E01020304050607080CountsDE, DE SE, SE16.6 MeVb) Fig. 1. – a) γ energy spectrum of 12C showing the 12.17 and 4.43 MeV cascade transitions,which are excepted in coincidence and b) the sum energy spectrum of detectors ﬁred in coin-cidence. The response of the spectrometer shows the energy summing of the 4.443 and 12.17MeV transitions. The escape peaks are indicated.3. – Experimental results on 3He(n, γγ)4HeIn the second experiment, we investigated the 3He(n, γγ)4He reaction at the coldneutron beamline of the high-ﬂux reactor Research Neutron Source Heinz Maier-Leibnitz(FRM II) of the Technical University Munich (Munich, Germany) in order to search forany signature of the X(17) → γγ decay channel. The Q-value of the reaction is verylarge (Q=20.6 MeV), so the neutron resonance absorption populates the EX = 20.21MeV (Γ = 0.50 MeV) as well as the EX = 21.01 MeV (Γ = 0.84 MeV) state of 4Hedue to their signiﬁcant overlap. These states can decay by the emission of one or two γphotons with a branching ratio of σ(1γ)/σ(2γ) = 0.28 [6].The incident neutron ﬂux was collimated down to a slit of 8x18 mm2. The totalﬂux on the target was around 1010/s (thermal equivalent). We used a 3He gas targetat a pressure of p=2 bar contained in a 90 cm long cylinder and closed by 50 μm thickaluminized Mylar foil at both ends. By adopting such a geometry, 90% of the incidentneutron ﬂux was absorbed via the 3He(n,p) reaction, which has a very large cross section(σ=5400 barn). The photons were detected in coincidence by an array of twelve 3”× 3”LaBr3 scintillators placed perpendicularly to the beam direction at 30 degree betweeneach other. In addition, we used a plastic scintillator array above our spectrometer toreject the cosmic-ray coincidences in the region of interest.Fig. 2b shows the energy sum spectrum of the γγ coincidences. The γ energy dis-tribution is very similar to the one measured at Atomki, but the background is muchsmaller since the direct capture of the cold neutrons has much smaller cross section.Although, a peak can be clearly seen at E ≈ 20 MeV, no conclusive angular correlationcould be extracted for the γγ pairs. Unfortunately, the experiment was limited in termsof counting rate due to the scattering of the neutron beam halo on the Al wall of the 3Hegas container. Thus, only 25% of the available neutron ﬂux could be used in the experi-ment. According to our data, the plastic veto detector array could reduce the cosmic raybackground only by a factor of 0.5 resulting in a signiﬁcant cosmic ray contribution tothe coincidence events. We plan to revisit the experiment soon with an improved setup.4 A. NAGY et al.16 18 20 22 24 26 (MeV)E01002003004005006007008009001000Countsa)20.6 MeV 16 18 20 22 24 26 (MeV)E0510152025Countsb)20.6 MeV Fig. 2. – a) Energy sum spectrum of the detected γ photons in 3H(p, γγ)4He and b) in3He(n, γγ)4He. Both histogram were random event and cosmic background subtracted.4. – ConclusionWe studied the γγ decay of the 17-MeV particle candidate in order to distinguishbetween the vector or the pseudoscalar boson scenario, both of which have been sug-gested recently by diﬀerent theoretical groups interpreting our previous experimentalobservations. Employing the 3H(p, γγ)4He and the 3He(n, γγ)4He reactions in diﬀerentexperiments, we measured the sum energy and the angular correlation of the γ pairs byusing a LaBr3 spectrometer array. We recorded true γγ coincidences in both experi-ments, however, no clear indication of a boson decay was observed. It may be due to thelarge cosmic ray background and the limited statistics, hence, we plan to repeat bothexperiments with an improved setup.∗ ∗ ∗This work is based on upon experiments performed at the PGAA instruments oper-ated by FRM II at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany andsupported by the Hungarian NKFI Foundation No. K124810, by GINOP-2.3.3-15-2016-0034 and the Janos Bolyai Research Fellowship of the Hungarian Academy of Sciences(L. Csige). The contributions of T. Klaus and N. Pietralla were supported by the DFGunder grant No. SFB 1245.REFERENCES[1] Feng J. L. and et al., , Phys. Rev., D98 (2018) 055021, and references therein.[2] Krasznahorkay A. J. and et al., Phys. Rev. Lett., 116 (2016) 042501.[3] Zhang X., Miller G. A. , Phys. Lett., B773 (2017) 159.[4] Feng J. L., and et al., Phys. Rev. Lett., 117 (2016) 071803.[5] Ellwanger U., Moretti S., arXiv:1609.01669v2.[6] Suffert M., Bertholett R., Nucl. Phys., A318 (1979) 54.

Searching for the double gamma-decay of the X particle

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Searching for the double gamma-decay of the X particle

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