Investigation of the φ-meson resonance by electron-positron colliding beams

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  • Volume 34B, number 4 PHYS ICS LETTERS 1March 197:

    INVEST IGAT ION OF THE (b -MESON RESONANCE BY

    ELECTRON-POSITRON COLL ID ING BEAMS

    V. E. BALAKIN, G. I . BUDKER, E .V . PAKHTUSOVA, V .A .S IDOROV A. N. SKRINSKY, G .M. TUMAIKIN and A. G. KHABAKHPASHEV '

    Institute of Nuclear Physics of the Siberian Division of the USSR Academy of Sciences, Novosibirsk, USSR

    Received 20 January 1971

    Using the colliding beam machine, VEPP-2, the excitation curve of the (b-meson resonance was invest i - gated in the three main decay modes. The resonance parameters obtained are the width, the total c ross - section and the branching ratios. Namely:

    F - (4.67 0.42)MeV

    (~o - (3.96 0.35) ~B

    B(e+e -) = (2,81 0.25) x 10 -4

    B(K*K-) = (54.0 3,4) %

    o O B(KsK~) - (25.7 3,0) c/r:

    B@r+T;~ ) = (20.3 4,2) %

    In addition, the pion form factor at 2E :: 1020 MeV is obtained together with an upper l imit for branching ratio of the (b-meson decay into a pion pair:

    i FTr 12 2.9 1,2 B(TT+Tr -) < 0.8 ~ .

    Us ing two co l l id ing -beam ins ta l la t ions (Novo- s ib r i sk and Orsay) a number of neat ra l vector meson invest igat ions have been per fo rmed [1-8]. The present work on the (/)-meson resonance is an extens ion of these researches . I ts d i f fe rence f rom the s imi la r work [6, 7] by the Orsay group cons is ts in the s imul taneous detect ion of the three main ~-meson decay modes : K+K ", K~K~ (through K~ ~ ~+~-) and 7r+~-n , as wel l as the :r+~ - pa i rs . The pre l iminary resu l t s of the work are a l ready pub l i shed [8].

    The exper iments have been made us ing the e lec t ron-pos i t ron s torage r ing VEPP-2 . The work ing condi t ions of the s torage r ing dur ing the exper iments (in the summer of 1969) a re spec i f ied by the fo l lowing parameters , namely : the average luminos i ty is 1028 cm -2 sec -1, the in i t ia l cur rents a re 60 mA and 40 mA, for e lec - t rons and pos i t rons , respect ive ly , and the ef - fec t ive l i fe t ime of the beams is 5 hours .

    The spark chamber sys tem used in the ex- per iment cons is ts of two ident ia l par ts (upper and lower) cover ing the so l id ang le 2 0.9 s terad ians near the ver t i ca l ax is . The layout of the spark chamber sys tem is shown in f ig. 1. Th inp la te chambers a re used to determine the ang les of outcoming par t i c les as wel l as the co -

    o rd inates of the in teract ion point. The par t i c le ident i f i ca t ion is ach ieved by observ ing the i r in - te ract ion with the mater ia l p la tes in the "shower" - and the " range" -spark chambers . The "shower" chamber cons is ts of 11 lead p la tes of 2 mm th ick each. The " range" cham- ber cons is ts of 21 p la tes each of 8 mm th ick s ta in less s tee l . An e laborate sys tem of mi r ro rs a l ]owed the use of a s ing le camera for record ing

    The spark chamber sys tem is t r iggered by four sc in t i l l a t ion counters in co inc idence with the reso lv ing t ime of 20 nsec . The four - fo ld co - inc idence detected the min imum ion iz ing par t i c - les with an ef f ic iency of 97%. The two- fo ld co - inc idence between the inner sc in t i l l a t ion coun- te rs , c lose to the vacuum chamber , detected charged K -mesons with an ef f ic iency of 100%. The k inet i c energy of K -mesons at the (b- reso- nance max imum is only 16 MeV, and hence the l ight y ie ld in the sc in t i l l a to r is cons iderab ly greater than that of re la t iv i s t i c par t i c les . To fac i l i ta te the la t te r ana lys i s the magn i tude of these pu lses for each event was recorded on the f i lm of the spark chamber p ic tures .

    An ant ico inc idence counter of d imens ions 160 160 5 cm 3 was v iewed by a s ing le photo - mul t ip l ie r FEU-65 and is used for veto ing cos -

    328

  • Volume 34B, number 4 PHYS ICS LETTERS 1March [971

    II \ "",,,\ . . ,, i

    p

    i l l i , I L - ~- -

    _'2 ! ~'~ ---

    - i e / e - - : -

    .- j .~ ~ -~_A

    I

    . - . , _ _ ~ : : ==

    i l~ , i I I

    ~ ' ,

    i '

    Fig. 1. Spark Chamber system, 1) anticoincidence scinti l lation counter, 2) lead absorber, 200 mm thick, 3} range spark chamber, 4) shower spark chamber, 5), 7) scinti l lation counters, 6) duraluminium absorber, 20mm thick, 8), t0) thin-plate spark chamber, 9) s tor - age ring magnet, 11) interaction region, 12), 13) inner

    and outer vacuum chambers.

    mic rays . Between the veto -counter and the chambers a 20 cm th ick lead absorber is p laced to prevent the par t i c les under invest igat ion f rom reach ing th is counter . The ant ico inc idence coun- te r reduced the cosmic - ray t r igger ing ra te by more than a fac tor of 50. An add i t iona l reduct ion of a fac tor 4 was obta ined when the counters were gated by the r f s igna l . Under these cond i - t ions the t r igger ing ra te due to cosmic - rays is 40 counts per hour whi le the t r igger ing ra te due to the par t i c les f rom the co l l id ing beams was much h igher .

    The accuracy of the abso lu te energy deter - minat ion by means of magnet ic f ie ld measure - ments is about 1%. The s tab i l i ty in energy and i ts reproduc ib i l i ty over var ious runs was checked by the nuc lear resonance method wi th an accuracy of 0.01%. The abso lu te energy ca l ib ra - t ion was made by us ing the tabu la ted energy va lue , 1019.5 MeV of the (~-meson resonance [9].

    Check ing the cond i t ions of the beam co l l i s ion and the luminos i ty mon i to r ing ( in tegrat ion) were

    Table 1 Summarizes the results of measurements.

    effect background cosmic

    Measurement time (hour) 222 67 68

    2y-moni tor (103) 1292 2.3

    Number of pictures (103 ) 86.1 25.6 10.2

    Luminosity integral (1033 crn_2) 8.5

    car r ied out by record ing the double bremsst rah- lung events [16]. Th is p rocess was recorded by the two NaI sc in t i l l a t ion counters in co inc idence (2 v -mon i to r ) . The energy thresho ld of these counters was set at 5 MeV.

    The main bulk of record ing took 1.5 months of 24 hours per day operat ion of the s torage r ing . The exper iments were per fo rmed at n ine energy va lues of the co l l id ing beams, f rom 508 MeV up to 514 MeV. One- four th of the whole per iod was spent on "background" measurements with the beams d isp laced ver t i ca l ly f rom each o ther at a d i s tance of 2 mm. In addi t ion the background of cosmic - rays was measured .

    Through the ana lys i s of the photographed events the fo l lowing s ix types of react ion were se lec ted :

    1) e+e - -* e+e -. The co l l inear i ty and the pre - sence of character i s t i c showers in both upper and lower chamber sys tems a l low us to se lec t th i s type of p rocess c lear ly wi thout background contaminat ion . The dev iat ion , /x~o, f rom co l - l inear i ty was requ i red to sat i s fy AW < 10 o.

    2) e+e - -* K+K - . The main c r i te r ia for se lec - t ion were co l l inear i ty (A~o < 10 ) and pu lses of h igh ampl i tude in a pa i r of t r igger ing sc in t i l l a - t ion counters . In "shower" and " range" cham- bers about one- th i rd of the reg is tered events of th i s p rocess showed a character i s t i c p ic ture of the decay of a K -meson s topped in the f i r s t sc in - t i l l a t ion counter . The la t te r events have no background.

    o o 3) e+e - ~ KsK L. The decay K~ ~ ~+~- was

    recorded . The branch ing ra t io of th i s mode is 68.7 + 0.6% [9]. The ang le of non-co l l inear i ty , Aw, was requ i red to be f rom 10 to 34 . The upper l im i t i s set by the K~-meson momentum. The range of the ~-mesons f rom th is p rocess does not a l low them to penet ra te beyond the four th gap of the " range" chamber . Th is cond ic t ion was used as an add i t iona l c r i te r ion for se lec t ion . The

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  • Volume 34B. number 4 PHYS ICS LETTERS

    Tab le2

    1March 1971

    Channel

    e+e-

    K+K-

    O O KsK L

    77-+ T/~-T/-O

    ~+~-

    Number of events detected

    back- effect cosmics ground

    530 0 0

    633 20 2

    95 0 0

    21 0 0

    40 3 15

    Normalization factor 1 0.298 1.32

    Pure effect Efficiency of (background detection substracted)

    530 23 (0.0625gb)

    565 29 7.1 :~ 0.1

    95 10 3.4 + 0.2

    21 4.6 0.71 :e 0.08

    28.6 7 6.0 0.7

    events with az imutha l component , A(p, of A0 smal le r than 4 were not accepted , s ince they are cont imated by the process e+e - ~ ~+Tr-).

    4) e+e - ~ ~+~r-ro. To th i s p rocess were as - s igned those events in which the ang le of non- co l l inear i ty or the range of one of the p ions ex - ceeded the va lues expected for the preced ing process . The c r i te r ion A~ < 4 is a l so re ta ined here . An add i t iona l condi t ion of A~ < 65 i s fu r ther imposed in o rder to decrease the back- g round contaminat ion .

    5) e+e - -* ~r+Tr -. To th is p rocess were as - s igned those events in which both ends of the co l - l i near t rack , (Aw < 2.5 o) were e i ther in the "shower" - or in the " range" chamber .

    6) e+e - ~ ~+ g- . To th i s p rocess were as - s igned those events in which the co l l inear t rack (Aw < 2.5 ) extended in both d i rec t ions beyond the " range" chambers . The resu l t s of th is p ro - cess a re g iven e l sewhere [10] where the domain of app l i cab i l i ty of QED is invest igated .

    The resu l t s of the exper iment a re summa- r i zed in tab le 2.

    The number of background events among the above accepted events was normal i zed to that in the "background" run by us ing the tota l numbers of p ic tures taken in the respect ive runs . In es t i - mat ing the background for the process ~+~r- we used the "cosmic" run ins tead of "background" run because of the bet ter s ta t i s t i ca l accuracy .

    Dur ing the course of the exper imenta l runs the re la t ive in tegra l luminos i ty was checked by the 2 ~-moni tor . The abso lu te va lue of the lumi - nos i ty was determined f rom the la rge ang le e lec - t ron -pos i t ron e las t i c scat ter ing .

    The geometr i ca l detect ion ef f ic iency of the spark chamber sys tem was ca lcu la ted by a

    330

    Monte Car lo method. The detect ion probab i l i t ies g iven in tab le 2 inc lude cor rect ions for the e f - f i c iency of the co inc idence c i rcu i t , and for the probab i l i ty of ~-mesons sat i s fy ing the range re - qu i rement , etc. The e las t i c scat ter ing c ross - sect ion g iven in tab le 2 fo r the present detector geometry a l ready inc ludes the rad ia t ive cor rec - t ion [11]. In ca lcu la t ing the I~K 0 mode cross - sect ion it was taken into account~hat a par t of the events 7r+Tr-Tr o would sat i s fy the se lec t ion re - qu i rements for K~K~. The probab i l i ty fo r an 1r+~-~ -event to be mis taken for a K~K~-event i s (0.14 + 0.02) %.

    The rad ia t ive cor rect ions are ra ther impor - tant for the ana lys i s of the resonance process under invest igat ion [12]. The measured c ross - sect ion , am, is re la ted by the fo l lowing fo rmula to the " idea l " c ross -sect ion , ~, which we are looking for , a m = a(1 + 5R). At the peak of the ~- resonance , 6 R =-0 .24 .

    The durat ion of the runs is f rom 2 to 5 hours . S ince the t ime of rad ia t ive po la r i za t ion of the beams at the energy 510 MeV in s torage r ing VEPP-2 is th ree hours [13], in the absence of depo lar i za t ion e f fects the po la r i za t ion of the beam wil l in f luence the resu l t s of c ross -sect ion measurements in the K+K - channe l . In th is case the cor rect ion for the c ross -sect ion wi l l be 5 R = 0.06. In o ther words , if there are no depo lar i za - t ion ef fects , our resu l t s for the tota l c ross -sec - t ion of th is mode should be reduced by 6%. It i s poss ib le to es t imate th is cor rect ion f rom the ob- servat ion of the ra te of occurence of K+K - -events as a funct ion of t ime a f te r the beams were in jec ted into the s torage r ing . Th is method gave 5p = 0.02 + 0.03 and we took the va lue 5p = 0 in the present ana lys i s .

  • Volume 34B, number 4 PHYS ICS

    i

    I E, AC KGR OL~t~ ! 'J 0 ~ I0~'6 ~0'20 1024 2( , NeV

    Fig. 2. e+e ~ -~

  • Volume 34B. number 4 PHYS ICS LETTERS 1March 1971

    decay mode of the S -meson. This is

    B(~+n -) < 0,8 %

    with the conf idence level of 95%. Including the poss ib le in ter fe rence ef fect the max imum l ike ly- hood es t imate g ives

    iF~ i 2 = 2,9 1,2.

    The authors a re gratefu l to V. N. Bayer , A . I . Va inshte in and I. B. Khr ip lov ich for usefu l d i s - cuss ions and to the la rge group of co -workers taking par t in data process ing .

    RefeT"ences [1] V. L. Auslender et al.. Phys. Letters 25B (1967)

    433. [2] V. L. Auslender et al., Yadern. Fiz. 9 (1969}

    [3] I. E. Augustin et al.. Phys. Rev. Letters 20 (1968) 126.

    [4] I. E. Augustin et al., Phys. Letters 28B (1969) 508. [5] I. E. Augustin et al., Phys. Letters 28B (1969) 513. [6] I. E. Augustin et al., Phys. Letters 28B ~l .q69) 517. [71 I. Perez-y-Yorba. Intern. Symposium (Daresbury.

    (1969). [8] V. A. Sidorov, Intern. Symposium (Daresbury,

    1969). [9] A. N. Rosenfeld, Tables (1970).

    [10] V. E. Balakin et al., Submitted to the Intern. Con- ference (Kiev, 1970).

    [11] S. M. Sukhanov, V. S. Fadin and V. A. Khoze, Sov. Phys. Dokl., 178 (1968) 822.

    [12] V. N. Bayer and V. S. Fadin, Phys. Letters 27B (1968) 223.

    [13] V. N. Bayer. International School of Physics "Enrico Fermi" XLVI (Academic Press , 1970).

    [14] F.Renard. Nucl. Phys. B15 (1970) 267. [15] L. D. Landau and E. M. Lifshits., Phys. 6 (1950)

    615. [16] P. I. Golubnichiy et al., Proc. Int. Symp. on elec-

    tron and positron storage rings (Saclay, 1966).

    332