A search for rare and forbidden decays of η-meson with GAMS-4π

Nuclear Physics B (Proc. Suppl.) 162 (2006) 155–160 www.elsevierphysics.com

A search for rare and forbidden decays of η-meson with GAMS-4π F.Binona , A.Blikb , A.Gorinb , S.Donskovb , S.Inabac , V.Kolosovb, M.Ladyginb , A.Lednevb , V.Lishinb , I.Manuilovb, Yu.Mikhailovb , J.P.Pegneuxd, V.Polyakovb, V.Samoylenkob, A.Sobolb , J.P.Stroota , V.Sugonyaevb, K. Takamatsuc, T.Tsuru c and G. Khaustovb a

Universite Libre de Bruxelles, CP 229, B-1050 Bruxelles, Belgium

b

Institute for High Energy Physics, 142281, Protvino, Russia

c

National Laboratory for High Energy Physics – KEK, Tsukuba, Ibaraki 305-0801, Japan

d

Laboratoire d’Annecy de Physique des Particules, F-74019 Annecy-le-Vieux, France

A search for the rare and forbidden neutral decays of η-meson with the GAMS-4π setup has been performed. The charge-exchange reaction at 32.5 GeV/c was used as a source of 3.7·106 η-mesons. At the 90% confidence level the following upper limits were obtained: BR(η → 3γ) < 1.6 · 10−4 , BR(η → 4γ) < 2.8 · 10−4 , BR(η → π o π o ) < 3.5 · 10−4 , BR(η → π o π o γ) < 1.7 · 10−3 , BR(η → π o π o γγ) < 4.0 · 10−3 , BR(η → 3π o γ) < 2.4 · 10−4 , BR(η → 4π o ) < 2.0 · 10−5 .

Rare and forbidden decays of η mesons are considered as a possible source of C and CP violating processes and other interested phenomena. The modern situation and physical motivation are discussed in detail in [1]. Before the GAMS Collaboration performed the research of rare decays of η, ω and η  mesons [2–4]. In this report we present our results obtained with the upgraded version of the detector.

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1. Introduction

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2. Experimental Setup The current experiment was carried out with the upgraded setup GAMS-4π which significantly differ from the previous version GAMS-2000 [5]. The beam particle track is measured by scintillator hodoscopes [6] with high spatial (0.3 mm) and timing (1 ns) resolutions. The last property is particularly important for the performance in a high-intensity beam for reducing the pile-up effect. The kind of the beam particle is defined by two Cherenkov threshold counters. The interaction point in the 40 cm liquid target is measured by a special Cherenkov counter. The lead-glass guard system has high aperture and increased up 0920-5632/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2006.09.080

Figure 1. Mass spectrum: a: 2γ (1C fit). b: 3π o (4C fit).

to 5Xo radiation lengths. The old veto counter [7] was replaced by a wide-angle detector (WAD) which can measure the energy and position of gammas. WAD is a sandwich type electromagnetic calorimeter with planar structure and has 12.5Xo radiation lengths. The threshold energy of photon detected is enhanced from 200 to 50

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MeV compared with veto counters used before. The central part of the GAMS electromagnetic calorimeter was replaced by a PWO crystal detector [8] with cell size 20 × 20 mm2 . As a result, the reconstruction of the overlapping showers in the most loaded central part of the detector was improved. Despite this upgrade, detector calibration and event reconstruction procedures, practically did not change and are described elsewhere [9–12]. Data quality is shown in Fig. 1.

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3. Decay Search The charge-exchange reaction (1) at 32.5 GeV/c π − momentum π − p → ηn

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is used as a source of η-mesons. All neutral decay modes of η meson were detected under the same conditions to reduce systematic errors. A set of the cuts applied to events in order to suppress physical and instrumental backgrounds without substantial loss in detection efficiency was carefully described in [3]. Our results are based on 3.73 · 106 η-mesons produced in the Run. 3.1. η → 3γ A search of C violating decay η → 3γ

(2)

requires detailed analysis of the physical background which consists of other decay modes of η mesons such as η → 3π o ,

(3)

η → π o γγ

(4)

and the π o π o system with soft non-detected photons. 3γ events were subjected to 1C kinematical fit (fix the mass of recoiled neutron) and only events with Confidence Level CL(1C, n) > 0.1 are used for analysis. The physical background from reactions (3), (4) and π o π o is suppressed by 2C kinematical fit (masses of π o and neutron were fixed). Events with CL(2C, n, π o ) > 1·10−4 are excluded from further analysis. Some events can occur

Figure 2. a: Solid histogram — experimental mass spectrum of γ pair in η meson region (450 < M (3γ) < 650 MeV), dashed —in the decay (2) (Monte-Carlo events). b: γ pair mass after cuts applied, 3 ent./event.

from the decay η → 2γ with one false γ. To reject these events, 1C fit with only 2γ in the event was performed, and if CL2γ (1C, n) > CL3γ (1C, n), the event was rejected. The results of these cuts are presented in Fig. 2. We applied the method [13] to determine the upper limit of the decay (2). This method1 allows to determine the weight and the corresponding error for each source of the events in the experimental mass spectrum, Fig. 3. In our case the reactions (3), (4), and the π o π o system are considered as sources of 3γ events. It is expected that the desired decay (2) is also the source of events. The detection efficiency for each system was determined from Monte Carlo taking into account the production t-dependence and mass spectrum. For the decay (2) the efficiency equals 23%. The upper limit is calculated as BR(η → 3γ) <

Nexp · (W (η → 3γ) + dW (η → 3γ)) ,(5 ) (η → 3γ) · Nη

where W (η → 3γ) is the weight of the decay in the spectrum, dW (η → 3γ) — the error corresponding to 90% confidence level, (η → 3γ) — the detection efficiency, Nexp — the event num1 Subroutine

HMCMLL in [24].

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ber in the experimental spectrum and Nη is the number of η-mesons. As a result, the upper limit2 of the decay (2) is (6)

3.2. η → 4γ Rare radiative decay η → 4γ

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Figure 3. a: 3γ experimental mass spectrum after cuts applied. b: Solid histogram — η → 3γ, dashed — η → 3π o → 3γ, dot — π o π o → 3γ, dashed-dot — η → π o γγ → 3γ (Monte-Carlo events).

BR(η → 3γ) < 1.6 · 10−4 .

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was not studied experimentally. A simple theoretical estimation gives the branching at a level ≈ 10−6 [1]. 4γ events with CL(1C, n) > 0.1 and momentum transfer t < −0.1 (GeV/c)2 are used to determine the branching of this process. Events originating from 3π o and π o π o systems are suppressed by 2C kinematical fit (fixed neutron and π o masses) CL(2C, n, π o ) < 1 · 10−2 . Simultaneously, this cut makes the contribution of the decay (4) negligible. The background from the decay Kso → π o π o concentrated in the 400 MeV region and is of no importance. As a result of the cuts for pure 4γ events selecting the detection efficiency of the decay (7) is reduced to 3.7% 2 Recently KLOE Collaboration set a more stronger limit of 1.6 · 10−5 [14].

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Figure 4. a: Solid histogram —experimental γpair mass spectrum after cuts applied, dashed — γ-pair mass spectrum of the decay (7) (MonteCarlo events), 6 ent./event. b: Solid histogram — experimental 4γ mass spectrum after cuts applied, dashed — η → 4γ (Monte-Carlo events).

In the experimental 4γ mass spectrum the 3π o system and decay (7) are considered as event sources. Upper limit of the decay (7) at 90% confidence level equal BR(η → 4γ) < 2.8 · 10−4 .

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3.3. η → π o π o This decay forbidden by P and CP parity was studied in the experiments SND [15] and CMD-2 [16]. A modern upper limit is 4.3 · 10−4 . To search the decay η → πo πo

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4γ events were selected. For further analysis the events were fitted under the π o π o hypothesis and only events with CL(3C, n, 2π o ) > 0.1 and high momentum transfer t < −0.20 GeV/c2 were used. The data quality of the π o π o system is shown in Fig. 5. With cuts used the detection efficiency of the π o π o system equals 24%. No signal of the decay (9) was observed. An upper limit at 90% confidence level is BR(η → π o π o ) < 3.5 · 10−4 .

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Figure 5. a: Mass spectrum of non-fitted γ-pair when M (4γ) is in the η region after 2C fit. b: Solid histogram — experimental mass spectrum of the π o π o system after cuts applied, dashed — the decay η → π o π o (Monte-Carlo events).

3.4. η → π o π o γ The intensive decay in 3π o with one nondetected gamma makes a search of C violating decay η → πo πo γ

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very problematic. To set the upper limit we used 5γ events with CL(3C, n, 2π o ) > 0.1 and momentum transfer t < −0.10 (GeV/c)2 , Fig. 6. A possible contribution from the π o π o system with an additional gamma which can arise from the fluctuation of a photon cluster was removed by the cut Eγ5 > 1 GeV/c. The final detection efficiency of the process (11) is 15%. The 3π o system and desired decay (11) are assumed as the sources of events in the π o π o γ mass spectrum. The upper limit at 90% confidence level is o o

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3.5. η → π o π o γγ This decay mode is discussed in connection with chiral perturbative models [17–19] which predict the branching at the level ≈ 10−7 . The selection of the decay η → π o π o γγ

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Figure 6. a: Experimental mass spectrum π o π o γ system after cuts applied. b: Solid histogram — η → π o π o γ, dashed — η → 3π o → π o π o γ. (Monte-Carlo events).

is quite difficult because the mass of the nonpaired gammas overlaps with the π o mass, Fig. 7. To separate decay (13) from the 3π o system two contradicted cuts were used — CL(3C, n, 2π o ) > 0.1 and CL(4C, n, 3π o ) < 1 · 10−2 . The last cut suppresses the 3π o system (Fig. 7) and reduces the final detection efficiency to 2.2%. It should be pointed out that finite energetic and spatial resolutions of the calorimeter did not allowed fully exclude 3π o . The experimental mass spectrum of π o π o γγ is shown in Fig. 8. The upper limit at 90% confidence level equals BR(η → π o π o γγ) < 4.0 · 10−3 .

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3.6. η → 3π o γ The decay η → 3π o γ

(15)

is strictly forbidden by C parity. The background sources are the 3π o system with fake gamma and the ηπ o system in the 8γ decay mode. The first case is difficult to simulate by the Monte Carlo method, but indirect evidence for the 3π o origin of the event is a small (≈ 30 MeV) mass of γ-pair or low energy (≈ 500 MeV) of nonpair gamma. The contribution of the ”direct”

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Figure 7. a: Solid histogram —mass spectrum of γ-pair, non-linked in π o in the decay 13, dashed —γ-pair mass spectrum from the decay η → 3π o after 3C fit (Monte-Carlo events). b: Experimental mass spectrum of non-linked γ-pair after cuts applied.

4π o system is negligible. To suppress the background from 3π o events with mγγ < 30 MeV and Eγ7 < 1 GeV were rejected from further analysis. The additional requirement implies that events, which have the combination 6γ satisfying the hypothesis η → 3π o with CL6γ (5C, n, 3π o , η) > 0.1 have been discarded. The events with CL(4C, n, 3π o ) > 0.1 were used to determine the upper limit of the decay. The mass spectrum is shown in Fig. 9. After all cuts the detection efficiency of the mode (15) equals 4.2%. The upper limit at 90% confidence level is BR(η → 3π o γ) < 2.4 · 10−4 .

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3.7. η → 4π o This decay is suppressed by small phase space and a high power of a π o momentum in matrix element. Previously Crystal Ball [20] has been obtained an upper limit which is now in [21]. At first to determine the background level in the η mass region 8γ events were fitted under the hypothesis (n, 3π o ) and mass spectrum of non-fitted gamma pair is presented in Fig. 10a. From this Figure we notice that the background is less than 10%. In the experimental mass spectrum of 4π o

Figure 8. Solid histogram — experimental mass spectrum of π o π o γγ system, dashed — the decay (13), dot — 3π o system suppressed by cuts (Monte-Carlo events).

with CL(5C, n, 4π o ) > 0.1 (Fig. 10b) no events were observed in the region of η meson. This corresponds to the upper limit of 2.3 events [22,23] at 90% confedence level. Taking into account the efficiency 3%, the upper limit equals BR(η → 4π o ) < 2.0 · 10−5 .

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4. Conclusion The upper limits for the rare and forbidden neutral decays of η meson were established in our report. The summary results are presented in Tab. 1. We would like to thank S.Bityukov and Yu.Kharlov (IHEP) for useful discussions. REFERENCES 1. B.M.K.Nefkens, J.W.Price; e-Print Archive: nucl-ex/0202008. 2. D.Alde et al., Z.Phys.C 25 (1984) 225. 3. D.Alde et al., Z.Phys.C 61 (1994) 35. 4. D.Alde et al., Z.Phys.C 36 (1987) 603. 5. F. Binon et al., Nucl. Instr. and Meth. A248(1986) 86. 6. A.M.Gorin et al., Proc. of SCIFI 97, Edited by Alan D. Bross et al. (1998) 627. 7. Preprint IHEP 85-35, Protvino, 1985, in Russian.

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Figure 9. a: Experimental mass spectrum of the 3π o γ system after cuts applied. b: Solid histogram —η → 3π o γ, dashed — ηπ o → 4π o → 3π o γ (Monte-Carlo events).

Figure 10. a: Experimental mass spectrum of non-fitted γ-pair after 4C fit, M (8γ) < 1500 MeV. b: Solid histogram —- experimental mass spectrum of 4π o system, dashed — η → 4π o (Monte-Carlo events).

Table 1 Summary of η Branchings. Decay Mode PDG 2002 η → 3γ 5 · 10−4 η → 4γ η → πo πo 4.3 · 10−4 o o η→π π γ η → π o π o γγ η → 3π o γ η → 4π o 6.9 · 10−7

ex/0307042. 15. M.N.Achasov et al., Phys.Lett. B425 (1998) 388; e-Print Archive: hep-ex/9803008. 16. R.R.Akhmetshin et al., Phys.Lett. B462 (1999) 380; e-Print Archive: hep-ex/9907006. 17. M. Kolesar, J. Novotny, e-Print Archive: hepph/0301005. 18. G. Knochlein, S. Scherer, D. Drechsel, Phys.Rev.D 53 (1996) 3634; e-Print Archive: hep-ph/9601252. 19. G. Knochlein, S. Scherer, D. Drechsel, Prog.Part.Nucl.Phys.36 (1996) 137; e-Print Archive: hep-ph/9510374. 20. S.Prakhov et al., Phys. Rev. Lett., V.84 (2000) 4802. 21. Particle Data Group, Review of Particle Physics, Phys. Rev. D. V. 66(2002). 22. G.Zech, Eur.Phys.J direct C4 (2002) 12. 23. S.I.Bityukov, N.V.Krasnikov, V.A.Tapereshkina, Preprint IHEP 2000-61, Protvino, 2000; e-Print Archive: hep-ex/0108020. 24. CERN Program Library, Geneva, 1996.

GAMS-4π 1.6 · 10−4 2.8 · 10−4 3.5 · 10−4 1.7 · 10−3 4.0 · 10−3 2.4 · 10−4 2.0 · 10−5

8. O.V.Buyanov et al., Nucl.Inst. and Meth. A349(1994) 64. 9. D.Alde et al., Phys.Lett.B 216 (1989) 451. 10. Cherenkov Detectors and their applying in Physics and Techniques, Moscow, 1990, P.149, in Russian. 11. Preprint IHEP 93-152, Protvino, 1993, in Russian. 12. Preprint IHEP 85-17, Protvino, 1985, in Russian. 13. R.Barlow, C.Beeston. Comp.Phys.Comm. V.77 (1993) 219. 14. A.Aloisio et al., e-Print Archive: hep-