arXiv:hep-ex/9809010v1 12 Sep 1998 rnhn ai,bsdo supino pure of φ assumption on based ratio, branching ut o,o h re f5 of order the of low, quite eaiginto decaying inwt hoeia ecito fthe of description theoretical with tion ea,adeeti ioerdaietransition φ radiative dipole electric and decay, 5,and [5], rsn,isqaksrcuei o eles- including well exist of not modifications models is several and structure tablished quark its present, a eas h eut togydpn nthe the on of depend structure strongly quark results the because tutr of structure h -ur oe [][][],mkn the making ([1],[7],[2]), φ model 4-quark the a siain of estimations cal inlo h aerdaiedecay radiative rare the of Signal Abstract pnigt oa nertdlmnst of corre- luminosity data, integrated of total 4 analysis a the at to on sponding experiment based is SND re- sult The the collider. - in VEPP-2M observed was esrdbacigrtoof ratio branching measured aisvr rmaot10 about from vary ratios eo,wt subsequent a with , and a odce nteN xeiet[]at [8] experiment ND the in conducted was eea ehnssmycnrbt into contribute may φ mechanisms Several Introduction 1 π V seult (0 to equal is pb 0 (1020) → (980) hoeia rdcin fthe of predictions Theoretical h rtsac o the for search first The → → ∗ 0 − email:[email protected] 1 K π V yetastos where transitions, type a r8 or , a 0 0 K γ (980) (980) 0 K ..ohnk ..ui,DABkn ..udn T.V.Dimova, S.V.Burdin, D.A.Bukin, A.D.Bukin, A.V.Bozhenok, → ukrIsiueo ula hsc,Nvsbrk 300 Russia 630090, Novosibirsk, Physics, Nuclear of Institute Budker ea smc oecomplicated, more much is decay ea ehns [] 2,[],are [3]), [2], ([1], mechanism decay oeua oesu o10 to up models molecular K ..oisi ..rzii,MSDboi,I.A.Gaponenko, M.S.Dubrovin, V.P.Druzhinin, S.I.Dolinski, ηπ ..ohb,APLsno ..ahuoa A.A.Salnikov, E.V.Pakhtusova, A.P.Lysenko, S.V.Koshuba, a · ..eenao,VVSay uMSauo,V.A.Sidorov, Yu.M.Shatunov, V.V.Shary, S.I.Serednyakov, . γ γ ..cao,VMAlhno ..au A.V.Berdyugin, S.E.Baru, V.M.Aulchenko, M.N.Achasov, oeua oe 6.Theoreti- [6]. model molecular 0 83 10 ηπ 90 meson. (980) V.B.Golubev 0 ea nqepoeo the of probe unique a decay ihascalar a with γ 6 q ± 0 vdneo the of Evidence q ¯ . ea aeincluding rate decay φ 0 cee[] -ur model 4-quark [4], scheme φ msn rdcd The produced. - . 23) → ..iaaz,ANSrnk,Yu.V.Usov A.N.Skrinsky, Z.K.Silagadze, · a a 10 0 0 90 eo.At meson. (980) V (980) φ − − φ 5 4 → → · a ..vnhno ..vnv A.A.Korol, P.M.Ivanov, V.N.Ivanchenko, , ∗ . V → 0 10 ntwo-quark in 90 meson (980) γ ηγ ηπ ηπ − savector a is φ φ branching 6 0 radiative Situa- . 0 → → γ γ − decay decay ηπ ηπ φ φ 4 0 0 → → in γ γ 1 φ re,adtesldagecvrg s9%of de- 90% is 1.5 coverage to angle 4 close solid the is and calorimeter grees, the of lution VEPP-2M pe ii f2 of limit upper eouinsatdoeaina VEPP-2M. of at indications First operation better started spatial having and resolution detector SND granularity, hermeticity, new the 1995 enb N n19 9.Tersl was result The the of [9]. half of 1997 analysis in on based SND by seen ryrnefo 8 o14 e ihthe vicinity with close the the MeV in of collected 1040 data en- the to of the 980 most in from 1996-1997 range in ergy recorded was decay i ffl aasml a are u.The out. carried was analy- sample work φ data this full of In sis 1996–1997. in recorded ryrnewr efre ihattlinte- 4 total of a luminosity with grated performed were range ergy xeiet[0,[1 a are u n1996 in VEPP-2M out at carried was [11] experi- [10], Experiment and Detector 2 measured. was ratio branching its and time f3 mo 4rdainlnts h en- The described be 4 lengths. can as radiation for 14 resolution or ergy thickness cm total 35 a of with crystals 3- NaI(Tl) universal a 1630 a is of consisted part is calorimeter main electromagnetic [12] layer Its detector detector. SND nonmagnetic The tor. e of ber π → h aa sdfrtesuyof study the for used data, The → steradian. . 2% ment ηπ φ φ ηπ pa.Sxscesv cn fteen- the of scans successive Six -peak. / msn rdcdo 8 of produced -mesons 0 γ 4 p e 0 ea a bevdfrtefirst the for observed was decay + E γ e e ( + − GeV . e 5 decay − oldri 97 hr an where 1987, in collider · 10 oldrwt N detec- SND with collider 1] h nua reso- angular the [13], ) φ − . 0 3 → pb a salse.In established. was − ηπ 1 n oa num- total and 0 . γ 2 φ msndata -meson · ea were decay 10 φ 6 → . ηπ 0 γ 3 Data analysis e+e− → 5γ ones and corresponding param- 2 eter χe, describing the likelihood of this as- The main background sources for the process 2 sumption was calculated. Events with χe > under study 10 were also rejected. Study of simulated

− 5γ events of the processes (1), (4), and (5) e+e → φ → ηπ0γ → 5γ (1) 2 showed, that the χe and ζ cuts reject less are the following φ-meson decays: than 15% of true 5γ events, suppressing the process (2) by a factor of 3 and making the − e+e → φ → ηγ → 3π0γ (2) expected background due to the process (3) very small, of the order of 10 events. Al- + − e e → φ → KSKL → neutrals (3) though, in contrast with the other processes, − e+e → φ → π0π0γ (4) the Monte Carlo simulation of the process (3) is much less accurate due to nuclear inter- and nonresonant process action of KL, so the data analysis may not − completely rely on this estimation. e+e → ωπ0 → π0π0γ. (5) Characteristic feature of the process (1) The expected number of events of the pro- is that each event must contain two pho- cess (1) at a branching ratio of φ → ηπ0γ ton pairs with invariant masses of η and π0 − equal to 10 4 is about 300, while the number mesons. Simulation shows that if the photons of background events (2) is 3 · 104. Although in an event are enumerated in descending or- the process (2) does not produce 5γ events di- der in energy, the photons from η → γγ decay rectly, it can fake the topology of the process have the numbers of either 1 and 2 or 1 and 3. (1) due to either merging of close photons or Corresponding experimental and simulated loss of soft photons through openings in the distributions in m12 and m13 with an addi- calorimeter. The process (3) produces 8 · 105 tional requirement, that rest of photons con- 0 0 2 KSKL events with KS → π π decays. The tains a pair with |mij − mπ0 | < 20 MeV/c KL-s in φ-meson decays are slow and have and more stringent quality require- a decay length of 3 m, while the nuclear in- ment ζ < −5. are shown in Fig.1. Back- teraction length in NaI(Tl) is about 30 cm. ground estimations are based on PDG ta- Characteristic feature of spurious 5γ events ble value for φ → ηγ decay branching ratio + − 0 produced by KSKL decays due to either nu- [15], our measurements of e e → ωπ cross 0 0 clear interaction of KL-s or their decays in section [11] and φ → π π γ decay branch- flight is an energy-momentum imbalance and ing ratio [16]. Distribution of experimental poor quality of at least one photon in the events in m12 + m13 shows an enhancement event. centered at η-meson mass, while the sum of Primary event selection was based on sim- background processes is nearly flat in this re- ple criteria: the number of reconstructed gion. The sum of all simulated background photons is equal to five, there are no tracks processes, where each one was normalized to in the central drift chambers, the total en- the number of events expected at a given inte- ergy deposition ranges from 0.8 to 1.1 cen- grated luminosity and total number φ-meson ter of mass energy 2E0, and the total trans- decays, describes the spectrum outside the verse momentum is less than 0.15E0/c. Such enhancement quite well. If this excess of criteria greatly reduce background from the events is due to the decay (1), it corresponds processes (2) and (3), not affecting the de- to a φ → ηπ0γ branching ratio of the order of cays (1), (4), and (5). Also rejected were 7 · 10−5, but it is difficult to extract accurate the events containing close photon pairs with result from these inclusive spectra, because of spatial angle between photons less than 27 poor signal to background ratio and unknown degrees, where could be poorly re- decay dynamics. constructed. Next step in the event selection was based Detailed study of the process (1) requires on quality of photons and kinematic fit. Pa- substantial suppression of background. It could be done using kinematic fit with in- rameter ζ [14] was constructed describing the 0 likelihood of a hypothesis, that given trans- termediate η and π mesons taken into ac- count. For each event ηπ0γ and π0π0γ hy- verse energy deposition profile of a cluster 2 of hit crystals in the calorimeter can be at- potheses were tried and corresponding χηπ0γ, 2 tributed to a single photon. The require- and χπ0π0γ parameters calculated. To sup- ments were imposed that ζ < 0, for all pho- press the processes (4) and (5) the follow- 2 tons. Then kinematic fit was performed un- ing requirements were imposed: χηπ0γ < 20, 2 der the assumption, that selected events are χπ0π0γ > 20, and for additional suppression

2 the process (2): ζ < −4. With these require- ground was subtracted using events with Mγγ ments the contribution from the processes (4) outside η peak. The result is shown in Fig.4. and (5), which are relatively rare themselves, The invariant mass spectrum was fitted us- becomes negligible. ing formulae from Ref.[1] and assuming that

The resulting spectra of the Eγmax/E0, en- the decay is a pure φ → a0(980)γ transition. ergy of the most energetic photon, are shown The result is also shown in Fig.4. It is seen, in Fig.2. Since the recoil photon in the pro- that although the errors are very large, the cess (2) has a narrow spectrum peaked at 360 observed mass spectrum is consistent with MeV, Eγmax/E0 must be more than 0.7 in a0γ decay mechanism. Constraint fit with this process. The spectrum in Fig.2b, where gaηπ = 0.85gaK+K− [5] gives the following true 5γ events are suppressed by an order values of a0(980) meson parameters: +23 2 of magnitude, is well reproduced by simula- Ma0 = 986−10 MeV/c , 2 2 tion of the process (2) alone, while the spec- gaK+K− /4π = (1.5 ± 0.5) GeV . trum in Fig.2a shows excess of experimen- tal events with ζ < −4 over the simula- tion of (2). The background (2) still dom- 4 Conclusion inates at Eγmax/E0 > 0.7, so events with E /E0 > 0.7 were rejected. From Fig.2a In this work evidence of the new decay φ → γmax 0 one can see, that this roughly halves the de- ηπ γ with a branching ratio of (0.83 ± 0.23) · 10−4 is presented. The observed enhance- tection efficiency for the process (1). This 0 also enhances dependence of the detection ef- ment in the ηπ invariant mass spectrum at ficiency on the invariant mass of ηπ0 system, large mηπ0 is consistent with domination of 2 a0γ decay mechanism, while relatively high which varies from 1% at Mηπ0 = 970 MeV/c 2 decay rate agrees with the theoretical pre- up to 5% at Mηπ0 = 700 MeV/c . The distributions of the events over pho- dictions based on 4-quark model of a0(980)- meson [1], [2]. ton quality parameter ζ are shown in Fig.3. Selection criteria here were the same as de- scribed above, except loosened requirement on photon quality: ζ < 10. It can be 5 Acknowledgements seen, that simulation well describes the back- This work is supported in part by Russian ground from the process (2), while the excess Fund for Basic Researches, grants No. 96-02- of events in Fig.3a at low ζ is compatible 19192 and 96-15-96327, and STP “Integra- with existence of the decay (1) with BR of tion” No.274. the order of 10−4. Also was checked the dis- 2 tribution in χηπ0γ . While the distributions of experimental events with 0 <ζ < 10 is References in a good agreement with simulation of the process (2), the distribution of the events [1] N.N.Achasov, V.N.Ivanchenko, Nucl. with ζ < −4 agrees with the simulation of Phys., B 315 (1989) 465. the process (1) with a small contribution of the process (2). The total number of se- [2] N.N.Achasov, V.V.Gubin, Phys. Rev. D lected events satisfying the selection criteria 56 (1997) 4084. is 25, from which 5 were estimated to be a background from the process (2). The corre- [3] A.Bramon, A.Grau and G.Pancheri, sponding branching ratio was calculated us- Phys. Lett. B 283 (1992) 416. ing the ηπ0 invariant mass distribution of the selected events and detection efficiencies, ob- [4] N.T¨ornqvist, Phys. Rev. Lett. 49 (1982) tained from simulation. The resulting value 624. is (0.83 ± 0.23) · 10−4, where error is a statis- tical one. For such stringent selection crite- [5] R.L.Jaffe, Phys. Rev. D 15 (1977) 267, ria the systematic error in detection efficiency 281. could be as high as 15%, but at present level [6] J.Weinstein, N.Isgur, Phys. Rev. D 41 of experimental statistics the statistical error (1990) 2236. dominates. 0 In order to study the ηπ mass spectrum [7] F.E.Close, N.Isgur, S.Kumano, Nucl. the Eγmax cut was dropped. This increased Phys. B 389 (1993) 513. the background from the φ → ηγ decay, but made detection efficiency twice higher and [8] V.P.Druzhinin et al., Phys. Reports 202 less dependent on ηπ0 mass. The ηγ back- (1991) 99.

3 [9] M.N.Achasov et al., e-print archive: hep-ex/9711023; Proc. of HADRON97 Conference, Upton NY, August 24–30, 1997, p.26–35, p.783–786. To be pub- lished in Phys. Atom. Nucl.. [10] M.N.Achasov et al., Preprint Budker INP 96-47, Novosibirsk, 1996. [11] M.N.Achasov et al., Preprint Budker INP 97-78, Novosibirsk, 1997. [12] V.M.Aulchenko et al., Proc. Workshop on Physics and Detectors for DAΦNE, Frascati, April 9-12, (1991), p.605. [13] M.N.Achasov et al., Nucl. Instr. and Meth. A401(1997)179 20 [14] A.V.Bozhenok, V.N.Ivanchenko, a Z.K.Silagadze, Nucl. Instr. and Meth. A 15 Events 379 (1996) 507. 10 [15] Review of , Phys. Rev. D 54 (1996) 1. 5 [16] V.M.Aulchenko et al., e-print 0 hep-ex/9807016, Submitted to 0.5 0.6 0.7 0.8 0.9 1 Phys.Rev.Lett.. Eγ max /E0

60 b Events 40

20

0 0.5 0.6 0.7 0.8 0.9 1 300 E /E c γ max 0 240 Events 180 b Figure 2: Spectra of the most energetic pho- 120 ton in the event: a – photons quality ζ < −4, b–0 <ζ< 10. Circles with error bars de- 60 a pict experimental data, shaded histogram – simulation of the process (2) and clear his- 0 togram – simulated sum of (2) and (1) with 400 500 600 700 −4 2 a BR = 10 . mγγ , MeV/c

Figure 1: Invariant masses of pairs of most energetic photons: circles with error bars – experimental data, a – simulated signal from φ → ηπ0γ decay corresponding to a branch- ing ratio of 0.7 · 10−4, b – estimated back- ground from the e+e− → ωπ0 and φ → ηγ,f0(980)γ events, c – sum of a and b

4 10 a Events -1 14

5 MeV 12 7 10

× 10 0 -10 -5 0 5 10 )/dm 8 ζ 6 30 φ→ηπγ (

Β 4

b d Events 20 2 10 0 700 750 800 850 900 950 1000 m , MeV 0 ηπ -10 -5 0 5 10 ζ Figure 4: ηπ0 invariant mass spectrum: points – experimental data, curve – optimal fit according to Ref. [1]. Figure 3: Distribution of events over ζ: a– events with Eγmax/E0 < 0.7, b – events with 0.7 < Eγmax/E0 < 0.8 circles with error bars – experimental data, shaded histogram de- picts simulation of the process (2) and clear histogram – simulated sum of (2) and (1) with a BR = 10−4.

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