IL NUOVO CIMENTO VOL. 107A, N. 11 Novembre 1994

Antiproton-Proton Annihilation in Flight(*).

CRYSTAL BARREL COLLABORATION

C. AMSLER (13), D. S. ARMSTRONG(I), I. AUGUSTIN (7), C. A. BAKER(4) B. M. BARNETT(l°), C. J. BATTy(4), K. BEUCHERT(2), P. BIRIEN(1), J. BISTIRLICH(1) P. BLiJM(7), R. BOSSINGHAM(1), H. BOSSy(I), K. BRAUNE(I1), J. BROSE(1°) D. V. BUGG(S), M. BURCHELL(3), T. CASE (1), i . COOPER(S), K. M. CROWE (1) H. P. DIETZ (11), S. VON DOMBROWSKI (13), M. DOSER (3), W. DUNNWEBER (11) D. ENGELHARDT(7), M. ENGLERT(11), M. A. FAESSLER(ll), C. FELIX(ll), G. FOLGER(11) R. HACKMANN(lO), R. P. HADDOCK(9), F. H. HEINSIUS(6), N. P. HESSEY(5) P. HIDAS(3), P. ILLINGER(11), D. JAMNIK(ll), Z. JAVORFI(3), H. KALINOWSKY(10) B. K~MLE (6), T. KIEL(6), J. KISIEL(ll), E. KLEMPT(lO), M. KOBEL (5), H. KOCH(2) C. KOLO(11), K. K(~NIGSMANN(ll), M. KUNZE(2), R. LANDUA(5), J. LUDEMANN(2) H. MA2WHAEy(2), M. MERKEL(1O), J. P. MERLO(lO), C. A. MEYER(13) V. MEYER-BERKHOUT(ll), L. MONTANET(5), A. NOBLE (13), F. OULD-SAADA(13) K. PETERS (2), G. PINTER (3), S. RAVNDAL (2), j . SALK(2), A. H. SANJARI (8) E. SCH~ER(lO), B. SCHMID(13), P. SCHMIDT(6), S. SPANIER(10) C. STRA~BURGER(I°), V. STROHBUSCH (6), M. SUFFERT (12), D. URNER (13) C. VOLCKER (11), D. WALTHER(2), U. WIEDNER (6), N. WINTER (7) J. ZOLL(5) and C. ZUPANSI5 (11)

presented by S. RAVNDAL

(1) University of California - LBL, Berkeley, CA 94720, USA (e) Universit~it Bochum - D-4630 Bochum, Germany (3) Academy of Science - H-1525 Budapest, Hungary (4) Rutherford Appleton Laboratory - Chilton, Didcot, 0 X l l oQX, UK (5) CERN - CH-1211 Gen~ve, Switzerland (8) Universitdt Hamburg - D-2000 Hamburg, Germany (7) Universitdt Karlsrnhe - D-7500 Karlsrnhe, Germany (s) Queen Mary and Westfield College - London E1 4N5, UK (9) University of California - Los Angeles, CA 90024, USA (lo) Universitdt Mainz - D-6500 Mainz, Germany (11) Universitdt Mi~nchen - D-8000 Mi~nchen, Germany (12) Centre de Recherches Nuclgaires - F-67037 Strasbourg, France (13) Universitdt Zi~rich - CH-8001 Zi~rich, Switzerland

(ricevuto il 14 Aprile 1994)

(*) Paper presented at the conference HADRON '93, Como, June 21-25, 1993.

2305

2306 s. RAVNDAL

Summary. - - This is an overview of the preliminary results of ~p annihilation at incident ~ momenta of 600, 1200 and 1940 MeV/c (*). The data was taken at LEAR with the Crystal Barrel Detector (E. AKER et al.: NucL Instram. Methods A, 321, 108 (1992)). In the two pseudoscalar final states seven different channels are observable and their angular distributions are measured. They are compared to older data, if existent, and are found to be equal within the experimental errors. In the three-meson final states, an overview of the already examined final states is given. Striking signals in the Dalitz plots and invariant-mass projections for a set of final states are observed. In the final states ~7: ° and ~ a signal -~ with the invariant mass of 1480MeV/c 2 and a width of about r

70 MeV/c 2 is visible. In the final-state ~o~ ° a signal in the o~ invariant-mass spectrum at 1680 MeV/c 2 with a width/" = 150 MeV/c 2 is observed. Finally, in the final-state o~o)~ ° two visible structures in the Dalitz plot are detected at 1600 MeV/c 2 (F----90 MeV/c 2) and 1900 MeV/c 2 ( F ~ 150 MeV/c2 ).

PACS 13.75 - Hadron-induced low- and intermediate-energy reactions and scattering (energy ~< 10 GeV). PACS 14.40 - Mesons and meson resonances. PACS 01.30.Cc - Conference proceedings.

1. - D a t a s e t and d a t a s e l e c t i o n .

The data was taken using an on-line 0-prong trigger. The 0-prong t r igger is defined by an incoming antiproton, no hits in the inner proportional wire chambers (PWC) and no antiproton exceeding the liquid-hydrogen target . The t r iggered and calibrated data was reconstructed and preselected using an off-line cut on residual tracks in the je t drift chamber which is embedded in the electromagnetic calorimeter. A kinematic fit was applied to each event demanding total energy and momentum conservation and a free z-vertex for the annihilation point. The z-axis is defined by the incoming antiproton beam. To each event a set of appropriate hypotheses which match the multiplicity of reconstructed photons was tested. The data presented is based on 1.2.106 t r iggered 0-proton events at an antiproton momentum of 600 MeV/c, 1.6.106 t r iggered events at 1200 MeV/c, and a subset of 4.5- 106 of the 7.3.106 0-prong triggered events at an incoming antiproton momentum of 1940 MeV/c.

2. - T w o p s e u d o s c a l a r f i n a l s t a t e s .

Preselected events with a photon multiplicity of 47's were kinematically fitted requiring energy and momentum conservation in the 4~, phase space, without the constraint of a fixed z-vertex. The experimental V~, mass resolution am at the n ° mass and the ~ mass were determined for each initial antiproton momentum. In a two- dimensional Goldhaber plot the ~°'s and the ~'s were selected in a mass window of

(*) The presented data comprise a part of the PhD theses of K. Beuchert, J. Lfidemann, S. Ravndal and J. Salk.

A N T I P R O T O N - P R O T O N A N N I H I L A T I O N I N F L I G H T 2307

4or m . This yields in the case of the dominant ~0~0 final state 393 events in the 600 MeV/c momentum data sample, 24 721 events in the 1200 MeV/c momentum data sample and 72542 events at a momentum of 1940MeV/c. The number of reconstructed events in the less dominant final states is significantly lower, so in this presentation only the =0~0 final state is shown. In the rest system of the two pseudoscalars, the decay angle 0 is determined. This is the opening angle between the flight direction of the incoming antiproton and the flight direction of one produced pseudoscalar. The spectra shown can be compared with previous measure- ments [1,2]. In fig. 1 the angular distribution (cos O) for the three different incident momenta is shown. These angular distributions I(O) may be described using a set of Legendre polynomials Pl (cos O) [3]:

(1) I(0) = ~ atP1 cos O, l

25

20

15

10

0 1000

8O(

6O(

4OO

2OO

b)

! 11::: :::: i l l

0 d i:i:i: !i:!:!;i:ii:!:!?!:i ! !~i:i i:i !~!:: i iii i i:!:!i!:!:ii:ii iii ii:i i:;,

c)

1600 ft~ 1 ~

1200 ~I ..............

800 I::::: ~

i " : I k:::::::::::::: ~ tt 2 : : : 4:00 : : : : : ~ : : : : !

, :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ,

- .0 - 0 . 5 0 0.5 1.0 cos (0)

Fig. 1. - The angular distribution of two 7:°'s in the rest frame of the ~°'s at an incoming antiproton momentum of a) 600 MeV/c, b) 1200 MeV/c, and c) 1940 MeV/c is shown. The spectra are not corrected for the acceptance.

2308 s. RAVNDAL

where a~ is a real parameter to be determined. The a~'s at different momenta of the antiproton are determined by a fit to the angular distribution of O. The preliminary result supports the dominance of the Legendre polynomial with 1 = 4 at an antiproton momentum of 600 MeV/c e, a dominance of 1 = 6 at 1200MeV/c ~ and I = 8 at 1940 MeV/c z . This is in good agreement with the measurements of [1]. The aim of this still ongoing analysis is to determine the distribution of initial states in ~p annihilation at the three different incoming antiproton momenta. This is an important starting point for future partial-wave analyses in higher-multiplicity final states. If, however, the information in the two pseudoscalar sectors is not constrained enough in order to obtain reliable results, more constrained final states like o~= ° and ~o~ are promising to lead to stable solutions. They were measured as well, and are under study.

3. - T h r e e - m e s o n f i n a l s ta te s .

We present the preliminary results from the data analyses of ~p annihilation into three mesons at an incoming antiproton momentum of 1940MeV/c. The data is a reconstructed subset of the 0-prong triggered data taken. So far we only present the Dalitz plot and the invariant-mass spectra. This data is not corrected for acceptance and background. Monte Carlo studies show an acceptance variation over the phase space of the final states of the order of less than 5%. Background contributions from combinatorics or adjacent final states are of the same order of magnitude. In this respect the spectra shown have to be viewed as preliminary. The masses and widths determined in the invariant-mass spectra have been determined by a fit with a Breit-Wigner function folded with a Gaussian. The Gaussian in this fit represents the experimental mass resolution of the detector. In the mass region between 1400-1900 MeV/e z the width of the experimental resolution ~ has been determined by comparison between reconstructed Monte Carlo data and reconstructed experimental data. In this mass region the width ~ varies between ~ = 8-10MeV/c ~ which is negligible in comparison to the widths of the signals measured in the invariant-mass spectra. The Dalitz plots presented here are displayed as boxes illustrating the density in the Dalitz-plot cells.

iiiii!! iiiii iii %

a)

. . . . . . . . . . . . . . . . . . oo . . . . . . . . . . . . . .

i iiiiiiiiii iiii!i iii}iiiii , i!!ii!!!!!!!!!!i!iii!!i!!iii!!i!!!!!!iiii il

1 2 3 4 5 2 2 4

m~o~o (GeV /c )

6000

5000 > N 4000

3000

2000

1000

~:~ b)

¢:!:i:ii

ii!iiiiiiii~i %i!:!~!i!i), :iii!i!i!)!:~!i!i!!i!i!:i!!i~tO ~

~:iiiiiii:ili:i i:iii:i:!ii::)i:i:i:i:i:iN ~!iiii?!?ii!iii! iliiiii?i?!iii!i!i!iiiiiiiiiiiiiii~'~ ~:: : : ; : : : : : : :::::::::::::::::::::::::::::::::

f (127o)[iiiiii5 ...,-~iiiiiii!~iii!iii iiii~i~!~ii!ii-

/~;{i~iii~;~!~!~:~!!~i~i[~iiiii~!~i~!~i~i~i!i~i~[~!~i!i~!ii~i~i~i~i~i~!~iii~i!i~!iii~i[i~iiii~?.~ 500 1000 1500 2000

2 m~o~o (MeV/c)

Fig. 2. - a) ~o ~0 ~o Dalitz plot and b) ~o ~0 invariant-mass spectrum for the reaction ~p --* ~o ~o ~o at a ~ momentum of 1940MeV/c.

ANTIPROTON-PROTON ANNIHILATION IN FLIGHT 2309

3"1. 6 - p h o t o n states. - A data set of preselected events with 6 reconstructed photons was kinematically fitted with the different hypotheses. The hypotheses were tested simultaneously for each event. In the case of 6 photons the hypotheses were =o=o~o, .r~o~o, ~ o and ~-~. The fit used the z-vertex from the previous 3C fit. The kinematic fits were tried for each combinatorial case, where the demanded number of photon pairs had an invariant V~ mass of either (134 _+ 50)MeV/c e and (547 _+ _+ 100) MeV/c e , respectively. We define this fitted dataset as dataset A. For the ~o ~o ~o final state a confidence level cut at 10% for the best combinatorial case of the =o ~o ~o fit was applied to the fitted dataset A. This yields 70 968 fitted ~o o 7.o events. The dominant feature of the ~o ~o ~o Dalitz plot (fig. 2a)) is the signal in the region of the f2(1270) with an edge around 1000MeV/c e and a shoulder above 1500MeV/c ~ (fig. 2b)) The edge around 1000 MeV/c e might correspond to the fo(975). The shoulder above 1500 MeV/c 2 might be a reflection of the fe (1270), but an ongoing partial-wave analysis indicates additional intensity in this mass region.

3"2. ~ o ~o f i n a l state. - From data sample A, events with a confidence level above 10% from the ~ o o fit were selected. For these events a countercut of 1% to the confidence level of the fit to the ~o~o~o hypothesis was performed. 36177 fitted ~o~o events survived the countercut. In the ~=o =o Dalitz plot (fig. 3a)) the diagonal band of the f2 (1270) is visible in the =o~o invariant mass (see also fig. 3b)). In the ~ o invariant mass a band at the position of the ae (1320) is observed.

3"3. ~ o f i n a l state. - In the case of the ~-r~ °, final-state events from data sample A were selected with a confidence level cut of 10% from the ~ o hypothesis. A countercut of 1% on the confidence levels of the o ~ o o and the ~o~o hypotheses was applied. These countercuts left 3259 fitted ~ o remaining events. The Dalitz plot is shown in fig. 4a). Three arrows indicate the bands in the Dalitz plot, which are also visible in the ~ o and ~ invariant-mass spectra (fig. 4b)). The peaks in the ~ o spectrum were investigated using a line shape fit and were assigned to the resonances ao(980) and a2(1320). In the Dalitz plot a diagonal structure at the low-mass part of the ~ invariant-mass spectrum can be seen. The line shape fit to the ~ invariant-mass spectrum yields a mass of about 1480 MeV/c 2 with a width of F

70 MeV/c 2 . In the peak region approximately 100 events contribute to the signal, which has to be compared with a similar measurement [4]. A spin-parity assignment is not available at this stage of the analysis.

3"4. ~ f i n a l state. - In the case of the ~ final state, events from the data sample A were selected with a confidence level cut on the ~ hypothesis of 10%. An event with a confidence level above 1% for any other hypothesis in the 6~ final state is removed from the ~ data sample. This leads to 1532 kinematically fitted ~-~ events. An enhancement at the edges of the Dalitz plot is striking (fig. 5a)). A resonance behaviour is not favourable. In addition three crossing bands in the Dalitz plot are present. In the ~ invariant-mass spectrum (fig. 5b)) this band is identified with a low ~ peak at around 1480 MeV/c 2 with a width of F ~ 70 MeV/c 2 mass, compatible with the signal in the r~= ° =o final state.

3"5. 7-pho ton f i n a l s tates . - For the ~o~ ° final state, a data sample of events with 7 reconstructed photons was kinematically fitted with 7C and 8C hypotheses. The hypotheses tested were r~ ° no r)~,, ~ o ~o ~, ~ o no and o~n ° . Events with a confidence

2310 s. RAVNDAL

"C 4

> ~ 3 v %

i : : : : : : : : , a )

-....ooo . . . . . . . . . . . . . =======================

:ilE~!!i~fi~i~iiiiE!i!::

............................... ii.. ~- :J!}~}~?!EE!~!!!~!~i~iii!!2ii}}ii

. . . . . . . . . . . . . . . . . . ° , ° . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ° o . . ° . . . . . . . . . . .

: : : : : : : : : : : : : : : : : : : : : : : : : : : :

t l l t ] l , , , I . . . . I , , nn I . . . . I . . . . I n l l , $ . . . . . I . , - , ,., I ,

1 2 3 4 ~ 5 2 2 4

mn~o(GeV /c )

1600 ~-

¢~ 1400[ {~ b)

2oo I looo

s o o :

"E 600 • . . "~ : : : : : : : : : : : : : : : : : : : : : : :

400 : ~++'~ ::1~:ii ~,

200 : f261270)

500 1000 1500 2

m~o=o ( M e V / c )

Fig. 3. - a) .q~o =o Dalitz plot and b) ~]~:o invariant-mass spectrum for the reaction ~p ---) ~ o 7:o at a momentum of 1940 MeV/c.

3.5

2.5 £,q~"

>

% 1.5 ¢-q ~-

0.5

l l , . ; , _ , • • mmi

wmnn

• - ~ t l I , , , m l

• .mmo °

. : :_, : , : : : . , ,u

i . m I I

• • a) wmmmm.

m - a u i o m m . i , m a i m m m i , m m i , .miamumtmamm. ,mm,,mmm,mmm,. mmmm,,

" " I I : ~ ' " ' " ' " • mm - m a m , , m - . • I . l . . . . . . . . . ~ . . . . .

t l • t l l t l m r |

, n w ~ m n i m n o : m

• m l m i l ~ a m m w , - . . . . . .

' ..... "'"'1.5 ..... ' . . . . . . . . . . . . . 2.5 3.5 ~ 2 2 4

mnno(GeV /c )

200

N 15o

~ lOO r~

"~ 50

~ 0

@ ' t b)

1000 1500 2000 2

mn ~o ( M e V / c )

"-- I00 > N sO

~ 60 if/

1000 1500 2000 2

mnn (MeV/c)

Fig. 4. - a) ~-r~ ° Dalitz plot, b) ~ o invariant mass and c) ~r, invariant-mass spectrum for the reaction ~p___>~]~o at a ~ momentum of 1940MeV/c 2. The arrows indicate the masses determined by line shape fits.

ANTIPROTON-PROTON ANNIHILATION IN FLIGHT 2311

v

35i .:::::. 1 ~.-':::.:::!.. : i n n . • , , , .

3.0

2.5

2.0

1.5

a)

7 J l l l l l l l i l l n i i

i l U l | i . n l l i n l l i

I | : . . . . . . . . . • :~| o R • • m m m

I r a , • - ~ = • • l n i l l l i : i i ~ : : i : , : i .

• ~: . . . . 11;;:::.-1... • mm. ,in.

m I ....... ~ ...... :nun • • wm, .w.

. , i . • . . m i i l i • m e • o • o ,

--w..m.mm. m,m,, ..,....lmmmil m

. . .

. . . . . . . . . . . . • . . , , , , . : . i . : : . , m i m i n u m m , .

, l ~ l l l l l l l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5 2.5 3 . 5 ~ 2 2 4

m n n ( G e V /c )

100

~.. 80 >

60 CD c q

o~ 40

® 20

1200 1600 2

mnn ( M e V / c )

b)

2000

Fig. 5. - a) ~ Dalitz plot and b) ~ invariant-mass spectrum for the reaction ~p --> ~ , at a momentum of 1940MeV/c.

Cq "~-

>

v

%

2.5

2.0

1.5'

1.0

0.5

. . , . . . !

. . . . U . l . i l l . a) ' ' 1 , i ' ' i i , , I i

i l l i m . , i l i . | . . . ,m, n , , l U | , | , , i n , .

. m | n m u , U m i n n n . a , , • m|,|nUm|nin|Bm,ms:m...m. ~ • , m | i i l i i i i n u l l i n _ _ _ i , ~ "" . , l l i l l l l i l i l i l l l l l . ,

. , e e o l l i l i l n i i . , , I , . . . , n , i , u l i , i u i | w . | . , . , .

i m . l i J a l i l i , , , J i J ,

| t t , , | | , | | , . . | , . . . , * .

. . | e - I n l J , , u , . . , l , l l i n . J l l O l . . | , . , . . . I . I I . '

. . . | . l | = . . | , . . | , . * l . . . W . . . . . . , . , . .

2 3 4 5 2 2 4

m ~ (GeV / c )

"C 40

~ 0 ~ L ~ 750 1250

2 mn ~0 ( M e V / c )

c',l °

120 ~ i00 o 80 '~ 60

[ t[!I!~!t!~ t c)

40 I ~ 20 JI x(168o)I i!,+,, 1250 1750 2250

2 mm n (M e V /c )

Fig. 6. - a) co~ ° Dalitz plot, b) ~=o invariant mass and c) co-~ invariant-mass spectrum for the reaction ~p ~ co-rj~ ° at a ~ momentum of 1940 MeV/c.

2312 s. RAVNDAL

level above 10% for the ~ o ~ov were selected. F rom this subsample the events with a confidence level grea ter than 1% for ei ther the uo~o~oy or the ~o~:°~ ° hypothesis were removed. In order to finally accept an ( o ~ ° event, the energy of the single ~, of the ~o::o~, fit had to be above 160MeV/c e and the confidence level for the ~o~: ° hypothesis above 10%. This yields 2909 fitted ~o~ ° events in the Dalitz plot (fig. 6). A dominant production of ae (1320) is seen in the .q~o invariant-mass spectrum and the Dalitz plot. The ae band is crossed in the Dalitz plot by a band in the o)~ mass. This corresponds to an invariant o)~ mass of 1680 MeV/c e and a width of - 150 MeV/c e. A similar signal in o)~ has already been observed [5, 6].

3"6. 8-photon f i n a l states. - For the ¢o~o~ ° final state, a data sample of events with 8 reconstructed photons was kinematicaUy fitted with 7C and 9C hypotheses. The hypotheses tested were no~:o::o 77, ~:o~o ~/~,, ~oo)::0. Events with two different ~:07 pairs with an invariant mass of (782 + 60)MeV/c e are selected. Two side bins were defined and events with entries in one of the side bins are selected. The selected side bin events are subtracted from the Dalitz plot and the invariant-mass distributions. In fig. 7 the Dalitz plot of o)o)~ ° shows three different main features. A band in the o~n ° mass is identified with the b~ (1235). Two separated diagonal bands in the Dalitz plot are indicated in fig. 7a). In the o)o~ mass distribution these two bands can be identified

3.0

"~'~ 2.5

> ~5 2.0

% ~ 1.5

1.0

1 a ) . .

- i N i m i . - n i U m , m n , -

- n R l i i n u w l J I l i l U , - , n i m -

- I n J J i n i . * i l , • u , u l i l , . . , i .

- , n l l l i l , l l m w l l . I , l l l l l n l n , • • • , I m l ~ l l l l l n l l l l '

n n , l l _ i l l n l l l - I I ~ m l u - l U , i i m n i U i n

, , n m u m i i m m n l l m ,

1.0 1.5 2.0 2.5 2 2 4

m ~ o ( G e V / c )

3.0

~ 300

250

200

¢~ 150 cq

"~ lOOi {D

50 0

> 200

150

lOO

• ~ 50

09 ¢D 800 1200 1600

2 mcouo (MeV/c )

! !!!i!iiiii!iiii!iii!!!i!i!! i!~!il;ilililiiiiiiiiiiiiiiii!iliiiii~ii~ii~

~[X(1640)1 ~ . .

1600 2000 2400 2

r n ~ (MeV/c )

Fig. 7. - a) ~o7: ° 7: ° Dalitz plot, b) co~ ° invariant mass and c) coco invariant-mass spectrum for the reaction ~p--> coo)n ° at a ~ momentum of 1940 MeV/c 2.

ANTIPROTON-PROTON ANNIHILATION IN FLIGHT 2313

with two peaks (fig. 7b)). The low-mass peak is located at 1640 MeV/c 2 with a width of 90 MeV/c 2. The high-mass peak is positioned at around 1900MeV/c 2 and has a

width of - 1 5 0 MeV/c 2. Previous measurements of [7,8] have also claimed to see a resonance decay into ~oo~. Our measurement might preliminarily confirm those measure- ments.

4 . - C o n c l u s i o n s .

As is shown in detail and in many different final states, the Crystal Barrel detector is a suitable device for measurements of ~p annihilation in flight. The indication of four states, previously measured, but not established, shows the importance of further investigations, in particular at high initial antiproton momenta. The determination of spin and parity of the observed signal in the ~-q, ~o~ and ~co invariant-mass spectra is ongoing.

R E F E R E N C E S

[1] R. S. DELUDE et al.: Phys. Lett. B, 79, 325 (1978). [2] E. EISENHANDLER et al.: Nucl. Phys. B, 96, 109 (1975). [3] R. S. DELUDE et al.: Phys. Lett. B, 79, 335 (1978). [4] T. A. ARMSTRONG et al.: Phys. Lett. B, 309, 394 (1993). [5] G. L. LANDSBERG: Proceedings of HADRON '91, University of Maryland, College Park,

12-16 Augus~ 1991, edited by S. ONEDA and P. C. PEASLEE (World Scientific, Singapore, 1992), p. 12.

[6] M. ATKINSON et al.: Z. Phys. C, 34, 303 (1987). [7] D. ALDE et al.: Phys. Lett. B, 216, 451 (1989). [8] G. M. BELADIDZE et al.: Z. Phys. C, 54, 367 (1992).