Department of Radiation Sciences, Uppsala University, Sweden The Svedberg Laboratory, Uppsala, Sweden. 1 Introduction

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1 submittd to acta physica slovaca 1 6 η dcay into masurd in pd 3 Hη raction. M. Jacwicz 1, A. Kupść 2 for CELSIUS/WASA Collaboration Dpartmnt of Radiation Scincs, Uppsala Univrsity, Swdn Th Svdbrg Laboratory, Uppsala, Swdn Submittd Octobr 3, 25 Th raction pd 3 Hη has bn studid with WASA stup at CELSIUS. W prsnt prliminary rsults on th obsrvation of th η dcay. Th η msons wr producd from protondutron intractions at 893 MV incidnt proton nrgy with 3 H masurd in zrodgr spctromtr. 1 Introduction Exprimntal informations on th η mson dcays with lpton pair(s) ar scarc. Th η channl is of spcial intrst bcaus it can b usd to put nw limits on unconvntional mchanisms of th CP violation in flavour non consrving sctor which would show up as an asymmtry in th distribution of angl btwn th and dcay plans [1]. In addition th sam dcay channl, as wll as th η γ with ral photon in th final stat, can prob th intrplay btwn diagrams involving intrmdiat ρ and th contact trm. Th currnt PDG valu of th branching ratio: ( ) 4 has bn basd on th obsrvation of fiv vnts only: on vnt by Grossman t al. [2] and th four by Akhmshin t al [3]. 2 Th xprimntal mthod Th raction pd 3 Hη clos to thrshold as a sourc of ηs was first mployd for ta dcay xprimnts at Saturn II synchrotron at Saclay [4]. Th production of η was taggd in a vry clan way by masuring 3 H in a spctromtr at. Th η production cross sction in th raction raiss vry quickly from thrshold to a platau valu of.4 µb at 2 MV xcss nrgy, whil background from prompt pd 3 H raction rmains at th prcnt lvl [5]. This tagging mthod nabls to collct simultanously vry clan data sampl of all ta dcays. Th main problm with th raction is th rathr low cross sction. Th studis of η dcays using th pd 3 Hη raction clos to thrshold rquir dtction of outgoing 3 H ions at angl. That should provid a signal for th triggr and a masurmnt of th 3 H nrgy which givs a prcis dtrmination of th η momntum. 1 addrss: jacwicz@tsl.uu.s 2 addrss: kupsc@tsl.uu.s Institut of Physics, SAS, Bratislava, Slovakia 1

2 2 M. Jacwicz, A. Kupść For CELSIUS/WASA w usd acclrator dipol magnts as a filtr to dflct 3 H rcoils and masurd thir kintic nrgis in a tagging tlscops, an intrnal smiconductor HPG [6] dtctor, placd in th bam pip vacuum. Th spctromtr has an nrgy rsolution bttr than 1 MV for 3 H kintic nrgis lss than 42 MV and th accptanc at 1 MV abov thrshold is 5%. Th tagging dtctor provids triggr and vry clan signal of 3 H ions with kintic nrgy consistnt with th kinmatics of pd 3 Hη raction (Fig. 1). Th corrsponding missing mass rsolution is 15 kv MM 3 H [MV/c ] Fig. 1. Missing mass to 3 H with rsolution of 15 kv. Th products of th η dcays wr masurd with th WASA dtctor at th CELSIUS acclrator. Th main componnts ar shown in Fig. 2 and dscribd in dtail in [7]. For th studis of η dcay mostly th Cntral Dtctor was usd, whr chargd particls momnta wr masurd by th Mini Drift Chambr (MDC) in th prsnc of th axial magntic fild of 1T supplid by a suprconducting solnoid (SCS). Th nrgis of th particls wr masurd in th Plastic Scintillator Barrl (PSB) and th Scintillator Elctromagntic Calorimtr (SEC). Fig. 2. Th cross sction of th WASA apparatus with Cntral Dtctor built around intraction point, Forward Dtctor and zrodgr spctromtr. (Th scal of th spctromtr s plot was intnsionally incrasd and adjustd to fit th figur.) Th intrnal pllt targt systm with dutrium was availabl on rgular basis from Spring

3 η dcay into masurd in pd 3 Hη raction Th triggr rat of th tagging systm during th opration with th dutrium pllts and th proton bam with luminosity 5 3 cm 2 s 1 was fw Hz, yilding on avrag 1η vnt pr scond in th data acquisition. Th quality of th tagging in pd 3 Hη raction can b tstd by th dcay η γγ. Th procss has ovrconstraind and vry simpl kinmatics sinc th η is narly at rst in th cntr of mass systm. Th sum of nrgis of th photons is constant and qual to 64 MV and momnta of th photons ar coplanar with th bam. Th vlocity (β) of th η can b rconstructd from th photon scattring angls alon. Th β is givn by a simpl formula: β = sin(θ 1 θ 2 ) sin θ 1 sin θ 2 =.42 whr θ 1,2 ar scattring angls of th photons in th xprimnt dtrmind from th impact point in th calorimtr undr assumption of bam targt intractions (z =). Th xprimntal β distribution is show in (Fig. 3(lft))for vnts with corrct photon nrgis. Th flat background is du to bamrst gas intractions as illustratd in Fig. 3(right) whr rlation btwn vrtx position along th bam axis and th rconstructd β is plottd for MC simulation. Tagging by masurmnt of 3 H at tnds to nhanc bam rst gas contribution sinc it is not snsitiv to th position of th intraction point in principl all vnts along CELSIUS straight sction ar accptd. Th background can b rmovd by a constrain on th vrtx position for th dcays with chargd particls. Sinc th triggr is not biasd by a particular ta dcay channl it is nough to dtrmin th vrtx point distribution for on dcay channl to b abl to tak it into account in th MC for all rmaining channls. An unbiasd triggr allows also to dtrmin total numbr of ta dcay vnts collctd in th xprimnt from th numbr of obsrvd η γγ vnts. Using that mthod it was found that data usd in th analysis blow corrsponds to 25 of ta vnts. Evnts 3 z [cm] β β Fig. 3. (Lft) Rconstructd β distribution from η γγ. Thick solid lin xprimntal data, thin lins Mont Carlo simulations of bamrst gas and vrtx intractions. (Right) Corrlation of rconstructd β vs vrtx position along th bam Mont Carlo studis. 3 Analysis and rsults Th mthod of tagging 3 H rcoils mittd in a narrow forward con clos to o with a magntic spctromtr provids an almost background fr η production. Th only background for th

4 4 M. Jacwicz, A. Kupść dcay ar othr dcays with chargd particls. Th main background was idntifid with Mont Carlo studis as η with subsqunt Dalitz dcay of, ( γ), or with a photon convrsion in th bam pip. 3 Th vnt candidats for th η dcay into wr slctd by rquiring four chargd tracks in th cntral drift chambr, with two tracks carrying positiv and two ngativ charg. Small fraction of vnts with additional tracks in th Forward Dtctor was discardd to diminish th possibility of ovrlapping of chanccoincidnc vnts. Th unassociatd clustrs in th calorimtr (SEC) wr howvr allowd in ordr to kp high accptanc, sinc th prsnc of th nutral clustr can b du to splitoffs. In ordr to distinguish btwn pions and lctrons, th valu of th opning angl btwn two particls with opposit charg was calculatd. With two positivly and two ngativly chargd particl, thr ar four combinations to b tstd. Basd on th Mont Carlo simulations, th pair with th smallst opning angl is most likly a positronlctron pair. Th ffctivnss of th particl sparation basd on that mthod is dmonstratd in Fig. 4. In mor than 85% cass th sparation was corrct for a Mont Carlo data sampl of η. In addition a EP mthod of particl idntification was applid for particls scaping th MDC using th nrgy dposits in th PSB and/or th SEC. Fig. 4. Particl sparation basd on th opning angl for Mont Carlo data sampl of η. (First row) EP distributions for all rconstructd chargd tracks with nrgy dposition in th PSB (lft) and in th SEC (right). On th X axis is plottd signd particl momntum obtaind from th MDC. Pionand lctronbands ar wll sparatd. (Two lowr rows) Th particls wr sparatd basd on opning angl condition ( / middl row, / lowst row) and th sparation is bttr than 85%. 3 Bam pip is mad of bryllium with th wall thicknss of 1.2 mm (.34 X ).

5 η dcay into masurd in pd 3 Hη raction. 5 In ordr to supprss th background, th following slction critria wr applid to th data sampl: cosin of th opning angl btwn pair gratr than.5 total missing mass within th rang from.14 GV to.4 GV. In cas of vnts with an unassociatd clustr in th calorimtr, th invariant mass of γ, M ( γ), was calculatd and rquird to b within th rang from.11 GV to.16 GV. Th ncssity of this constrain is illustratd in Fig. 5, whr on s a clar pak in th invariant mass spctrum for th xprimntal data as xpctd from η dcay. 16 a) b) c) IM [GV] γ IM [GV] γ IM [GV] γ Fig. 5. Invariant mass of γ, M ( γ), calculatd for η dcay into (a), (b), and data (c). Th significant admixtur of th background vnts in th data manifsts itslf by a pronouncd pak in th mass spctra. Fig. 6 shows th distribution of th invariant mass M ( ) for th data aftr applying ths slction critria. Th spctrum is wll undrstood by suprposition of th Mont Carlo simulations of th signal (η ) and th background (η ) dcays. W idntifid 22±7 candidats for η dcay. Th dtction fficincy was stimatd to 2% η ( γ ). 12 η IM [GV] Fig. 6. Invariant mass M ( ) for pd 3 Hη, η aftr all constrains (top). Th spctrum can b wll undrstood by suprposition of Mont Carlo simulations of signal (η ) and background (η ) dcays (bottom).

6 6 M. Jacwicz, A. Kupść 4 Conclusion Th mthod of tagging 3 H rcoils using zro dgr magntic spctromtr provids an almost background fr η production. W prsntd th prliminary analysis of th η dcay using th total data sampl masurd in th WASA dtctor, whr 22 vnts wr obsrvd. Acknowldgmnt: This work was supportd by EtaMson Ntwork within Sixth Framwork Programm. Rfrncs [1] DaoNng Gao, Mod.Phys.Ltt., A 17, 1583 (22). [2] R. A. Grossman t al., Phys. Rv. 146, 993 (1966). [3] R. R. Akhmshin t al., Phys. Ltt. B 51, 191 (21). [4] J. Brgr t al., Phys. Rv. Ltt (1988). [5] B. Mayr t al., Phys. Rv. C (1996). [6] Chr. Bargholtz t al., Nucl.Instrum.Mth. A (1997). [7] J.Zabirowski t al.,celsius/wasa Collaboration, Phys. Scripta T 99, 159 (22).