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

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




    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 -


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

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


    i l l i , I L - ~- -

    _'2 ! ~'~ ---

    - i e / e - - : -

    .- j .~ ~ -~_A


    . - . , _ _ ~ : : ==

    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


  • Volume 34B. number 4 PHYS ICS LETTERS

    Tab le2

    1March 1971




    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 ine