Azimuthal correlations between charmed particles produced in 230 GeV/c π−Cu interactions

Volume 257, number 3,4 PHYSICS LETTERS B 28 March 1991

Azimuthal correlations between charmed particles produced in 230 GeV/c rt-Cu interactions

A C C M O R Collaboration

Amsterdam-Br i s to l -CERN-Cracow-Munich-Ruther ford-Valenc ia

S. Barlag a,l H. Becker a,2, T. B6hringer b.3, M. Bosman a V. Castillo b,4, V. Chabaud b C. Damerell c, C. D a u m d, H. Dietl a, p. Dziembaj e, A. Gil lman c, R. Gilmore f, T. Gooch f, L. G~Srlich e, p. Gras g, Z. Hajduk e, E. Higon g, D.P. Kelsey b,5, R. Klanner a.6 S. Kwan b, B. Lficking a, G. Liitjens a, G. Lutz a, j. Malos f, W. Manner a, E. Neugebauer a,7, H. Patka e, M. Pep6 ~,s, j. Richardson c,9, K. Rybicki e H.J. Seebrunner b, U. Stierlin a, H.G. Tiecke f, G. Waltermann a, S. Watts h, p. Weilhammer b, F. Wickens ~, L.W. Wiggers d M. Witek e and T. 7.etudziewicz e.10 " Max-Planck-InstitutJ~r Physik, W-8000 Munich, FRG b CERN, CII-1211 Geneva 23, Switzerland c Rutherford-Appleton Laboratory Chilton, Didcot OXI 10QX, UK d NIKHEF-H, NL- I009 DB Amsterdam, The Netherlands e Institute QfNuclear Physics, PL-30-055 Cracow, Poland f University o f Bristol, Bristol BS8 ITL, UK g IFIC, CS1Cand University o f Valencia, Valencia, Spain h Brunel University ~rbridge UB8 3PH, UK

Received 9 November 1990

In the CERN NA32 experiment a high-resolution silicon vertex detector and a purely topological approach were used to collect 642 events with two or more secondary vertices. They are consistent with double production of charm. Azimuthal angle correlations are studied for D °, D +, D + and A~+; we also investigate these correlations as a function of the number of leading D mesons.

1. Introductions

Although many properties of the hadronic production of charm are known (see refs. [1,2], and references therein) there have been only a few studies of charmed-particle correlations. The LEBC-EHS Col-

laboration [3] collected 53 double-charm events in 360 GeV/c rt-p interactions and demonstrated an azimuthal angle anisotropy with charmed particles emitted preferentially into opposite hemispheres. This anisotropy was later observed with much higher statistics (233 events) in 400 GeV/c p -p interactions

1 Present address: KNMI, De Bilt, The Netherlands. 2 Present address: Gesamthochschule, W-6600 Saarbrficken,

FRG. 3 Present address: University of Lausanne, CH-1015 Lausanne,

Switzerland. 4 Present address: University of Valencia, Valencia, Spain. 5 Present address: Rutherford-Appleton Laboratory, Chilton,

Didcot OX11 0QX, UK.

6 Present address: DESY, W-2000 Hamburg, FRG. 7 Present address: Universit/it-GH Siegen, W-5900 Siegen,

FRG. 8 Present address: CERN, CH-1211 Geneva 23, Switzerland. 9 Present address: University of Geneva, CH-1211 Geneva 04,

Switzerland. l0 Present address: University of Melbourne, Melbourne, Vic-

toria, Australia.

0370-2693/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland ) 5 19


[4]. Using 102 charm pairs produced in 350 GeV/c r t - -emuls ion interactions the WA75 Collaboration [5] showed that this effect appears for all charge combinations o f the D mesons. In this letter we de- scribe a study of azimuthal angle correlations between various charmed particles in a much larger sample of events with two charm decay vertices seen in our apparatus. The details of selection of these events are described in our earlier paper [ 6 ] devoted to branching fractions of charmed particles; here we recall only the essential features of the experiment and the data analysis.

2. The experiment

The second phase of the NA32 experiment was performed at the CERN SPS using a negative 230 G e V / c beam (96% pions and 4% kaons) and a 2.5 mm Cu target. Charm decays were reconstructed with an improved silicon vertex detector and a large- acceptance spectrometer. The latter consisted of two magnets, 48 planes of drift chambers and three mul- ticellular Cerenkov counters, allowing ~, K, p identification in a wide momentum range. A two-level trigger, described in ref. [ 7 ], selected events with a p K - or a K + K - pair, since the experiment was originally intended to be a study of A + -~pK- r t + and D + -+ K + K - = + decays ( throughout this letter a particle symbol stands for particle and antiparticle). Never- theless we have also observed charm signals in many other channels (cf. refs. [6 ,8]) . This is due to the associated production of charm often yielding two kaons of opposite charge, to pions misidentified as kaons at the trigger level and finally to kaons or pro- tons from the primary vertex.

The vertex detector consisted of a beam telescope (seven microstrip planes) and a vertex telescope with two charge-coupled devices (CCDs) and eight microstrip planes. The precise space-point information from the CCDs was essential for our study of events with two secondary vertices. The overall precision of our vertex detector allowed a purely topological charm search which is restricted neither to a limited number of decay modes nor to any mass window.

The results described in this letter are based on the full sample of 17 million triggers.

3. Data analysis

Event reconstruction is done in several steps (see refs. [6,9] for more details). First, all tracks are reconstructed in the drift chambers and particle identification is performed. Independently the beam track and the secondary tracks are reconstructed in the beam and vertex telescopes, respectively. Then, tracks found in the drift chambers and in the vertex telescope are matched. Finally, the reconstruction of the primary vertex is performed. We accept only events with the primary vertex inside the copper target and at least two tracks not originating from the vertex. These tracks are then used to fit one or more secondary vertices. The vertices should be between the target and the first microstrip plane (65 mm from the target). In addition to exclude secondary interactions, we demand a separation of the secondary vertex of at least two standard deviations from the target edge and from the edges of both CCDs. Secondary vertices containing a track seen in the vertex telescope but not in the drift chambers and thus of un- known momentum are not considered. Then we se- lect events with two secondary vertices. These events are further cleaned by the following cuts:

(i) we reject all decays of strange particles e.g. by demanding that the effective mass mvis of the charged decay products for ~+~- vertices must be signifi- cantly larger than the kaon mass.

(ii) the visible transverse momentum p~S of the charged decay products with respect to the direction of the parent charmed particle P cannot exceed the maximum transverse momentum kinematically allowed for any particle in a decay channel under consideration.

(iii) decays attributed to short-lived D °, D + and Ac + must occur upstream of the second CCD.

After this selection we are left with 673 events hav- ing two secondary vertices. In ref. [6] we have as- signed these vertices to various decay modes of D °, D +, D + and A + . This was done with the help ofm<~ and of the neutral mass defined as

v "~ 9 2 " m6 =mr, q-mg, is-- 2mex/m,,is q- (p,},s)2 .

For the purpose of this letter the correct assignment is not fully needed since we only want to know the quark content of the parent charmed particle P. Tile D O is recognized by two or four charged decay prod-

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Volume 257, n u m b e r 3,4 PHYSICS LETTERS B 28 March 1991

ucts, so that the only problem relevant for this analysis is to distinguish between D o and 0% This is easy for vertices involving a single charged kaon (except for weak channels like I ) ° ~ K - K ° r c +, the K - always comes from D°). Vertices with none or two charged kaons can correspond to a neutral D with either sign o f the charmed quark. In such a case we assume the sign opposite to that in the other secondary vertex. In ref. [6] we estimated the error of this assumption to be about 5%, i.e. up to 5% of our D o charm-ant i - charm sample could correspond to charm-charm events.

For the charged charmed particles there is no am- biguity in the sign of the charmed quark. The A f (apart from (~erenkov kaon/pro ton ambiguities) is uniquely identified by the presence of a proton among its decay products so that the only remaining ambi- guity is that between the Cabibbo-suppressed D + decays and those of D~ for rnv~s < roD. We estimate that ~ 8 events can be misidentified which represents 15% of all D~ + events and much less of events containing D + mesons.

After assigning all vertices to various charm decay modes we are left with 584 events consistent with charm-ant icharm ( "cC ' ) production. We have com- pared the number of DI) events with those calculated from charm production cross section, branching fractions and our acceptance. In particular we expect (231_+35) D°I) °, (166_+23) D + D ° or D - D ° and (63 _+ 9) D + D - pairs. These predictions are reason- ably consistent with the values quoted in the first three rows of table 1. For D + and Ac + there is a similar consistency within much larger errors which are

mainly due to a poor knowledge of branching fractions.

In addition we observe 58 events in which both de- caying particles seem to contain quarks of the same sign ("cc") . The latter number is surprisingly large and could at first sight be taken as evidence of a large amount of double associated charm production. There are, however, several possible ways in which real ( "cC ' ) events can contaminate the ( "cc" ) sample. Unfortunately it is very difficult to calculate this contamination in a reliable way. Taking into account track inefficiencies and particle identification ambiguities, it is possible that our data is fully consistent with the measurement by the WA75 Collaboration [10] of double associated charm production at the 1% level. The important issue for this letter is how much contamination there is of the ( "cc" ) sample by real ( "cc" ) events even if they are more abundant than 1% of the total. For this reason the results will also be shown for the 58 ( "cc" ) events in section 5 where it will be seen that the effect on the ( "ce" ) results is very small.

4. Acceptance corrections

Simulation o f the geometrical acceptance of our apparatus, of the trigger and of the selection criteria requires a complex Monte Carlo program. This program generates a pair o f charmed particles Pi and P, The particles decay into observed channels, and we calculate the acceptance Aij for each combinat ion of decays: each decay being generated from phase-space

Table l

Statist ics, average az imutha l angle <0> ( in degrees) and the a s y m m e t r y A for var ious categories of events.

C o m b i n a t i o n of cha rmed p No. o <0> ( ° ) A

D°D ° 195 107.6 + 3.9 D°D and D + I ) ° 209 110.0_+3.6

D + D - 56 111.4_+6.1 all D D 460 109.2 _+ 2.4 D~ + I) and D/- D 63 95.4 _+ 6.8 A + 17) and Ag- D 49 104.5_+7.6

( " c c " ) 58 99.0_+6.9 - ~ DIT); both leading 115 112.4 _+ 4.8 - ~ DO; one leading 241 108.4 _+ 3,4

; t - -~ DO; none leading 90 105.7 -+ 5,4

-0 .27_+0 .07 -0 .35_+0 .07 -0 .46_+0 .12

-0 .33_+0 .05

-0 .08_+0 .13 -0 .18_+0 .15 - 0 . 2 4 + 0 . 1 3 - 0.44 + 0.09

- 0 . 3 1 -+0.07 -0 .22 -+0 .11

521


distribution. The distributions of the Feynman vari- able xv and of the transverse momenta of P, or Pj [ 1,2 ] as well as the lifetime o f A + [ 9 ] are taken from our experiment; the lifetimes of other charmed particles are taken from ref. [ 11 ]. The generated pair of charmed particles is mixed with tracks from an event randomly chosen from an interaction-trigger sample recorded with our apparatus, thus faking a real event. All tracks from such an event are subsequently traced through our apparatus and subjected to the same cuts as those made in our analysis (for more details see

refs. [ 1,2] ). The acceptance A o is in the range of ( 1- 5 )% depending on the decay channel. It depends very weakly on the relative azimuthal angle 4, the angle between the flight directions of charmed particles P, and Pj in the plane transverse to the beam direction. A slight increase ( ~ 10%) of Aij with increasing ~ is due to our trigger. This was preferring ("cc") pairs yielding pK- or K+K - where one K or p goes to the left and the other to the right part of our apparatus. This change of A,j was included in our results.

(a)

0~2100 I I ! 40 60 80 I0o 120 1 4 0 1 6 0 150

'(b)

60 80 I00 120 1 4 0 1 6 0 (49180

2O

15

60 eO 1oo 120 140 1 6 0 1 2 O

200 o~

_ FUSION ' ~ i FUSION CLUSTER I/ I ZO CLUSTER

]

'el,' 1 /s 1

30

>

zO I P

~ o ~ . . . . . . ~ 0 20 4(3 60 80 100 120 140 160 16¢J 4(3 60 80 100 120 140 160 180 0

~o

1 2 ~

ifil FUSION CLUSTER

,4

29 40 60 80 100 120 140 160 ]80

G Zo 40 6O 20 I00 120 140 160(,0180

Fig. I. Distribution of relative azimuthal angle ¢~ (in degrees) for various pairs of charmed particles: (a) D°[3 °, (b) D ° D - and D+[7) °, (c) D + D - , (d) all D[3, (e) D + [3 and D;- D, ( f ) A + 13, (g) "'cc". Dashed and dotted lines are the predictions of ref. [ 12 ] and ref. [ 13 ] respectively.

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5. R e s u l t s

Since most of our decays involve missing neutrals and we do not know the momen ta of the charmed part icles we study only the correlat ions in the relat ive az imuthal angle 0- The ~ d is t r ibut ions for various classes of events are shown in fig. 1. They have been corrected for acceptance and normal ized to the total number of events. The number o f events and average values o f ( 0 ) for these classes are l isted in table 1. The ( 0 ) values were used as a measure of the asymmetry in refs. [3,5,14]. We also quote the value of the asymmet ry A def ined as

A = N ( ~ < 9 0 ° ) - N ( O > 9 0 ° ) N ( O < 9 0 O ) + N ( O > 9 0 o) •

Since, within the l imi ted statist ics of events produced by an incoming K - , the asymmet ry does not seem to depend on the type of beam particle, the first six rows of table 1 and all parts of fig. 1 refer to the mixture of n - - and K - - i n d u c e d events. The ~ distr i- but ions for 2 Dsl)s, 6D~Ac and 1 AcAc pairs are not shown because of the very low statistics.

It is immedia te ly seen that the ~ dis t r ibut ions are very s imilar and strongly asymmetr ic for all combinat ions of D mesons. This has been observed previ- ously in ref. [5]; we confi rm this fact with much higher statistics. We show for the first t ime the ~ dis- t r ibut ions for D + , A + and ( " c c " ) events. It seems that the dis t r ibut ion for the A + is less asymmetr ic than that for the D mesons. The ~ d is t r ibut ion for the D , I ) pairs is pract ical ly isotropic. This might indi- cate a difference in product ion mechanism between

D + and D mesons. It is interest ing to note that in the same exper iment [1,2] we have found that the x v

spectra are the same for all charmed particles, however the PT spectrum for the D + seems to be wider than those for the other particles. Fur ther studies are needed to show whether D + is p roduced differently from the other charmed particles. The results for the ( " cc" ) sample are inconclusive as they are halfway between isotropy and the asymmet ry measured for DO pairs, being consistent with both. For genuine ( " cc" ) events one would not expect a strong corre- lation but, if our sample is heavily con tamina ted by the ( " ce" ) events, the d is t r ibut ions will tend to be similar. It should be stressed that any wrong assignment (e.g. admixture of ( " c c " ) events in ( " c e " ) sample or contaminat ion of D + events by D + events) would d iminish the differences between various distr ibutions; thus the real differences are larger than those seen in fig. 1.

In fig. 2 we show the ~ distr ibutions for three classes of DO combina t ions depending on the number of leading D mesons i.e. those containing a c o m m o n quark with the beam (here only a p ion) particle. It is interesting to note that this is the first a t tempt to study leading pairs of charmed particles. An inspect ion of fig. 2 and table 1 shows that the asymmet ry seems to increase with the number of leading D mesons. How- ever, we see no dependence of the asymmet ry on the total m o m e n t u m of the charm pair.

Concluding this chapter we would like to comment on a compar ison with phenomenological predictions. There are two of them, nei ther based on very solid grounds (see ref. [ 14] for detai led discussion) .

I (a) ~ ~ I 20 4O

I r J ,

5 ° 2'o ," 6" o' . . . . . . . . . . . . rp

i b ) . . . . . . i(c)

1

60 80 0 20 40 60 80 100 120 140 160olf lO 0 20 40 100 120 140 1 6 0 1 8 0

Fig. 2. Distribution of relative azimuthal angle q~ (in degrees) for various classes of DI) pairs: (a) both leading, (b) one leading, (c) none leading.

523


The fusion model [ 12 ], independently of uncertain- ties connected with the fragmentation of a charm quark into a charmed hadron, yields predictions for the 0 distribution only for the second-order processes q + q - , c + e and g + g ~ c + e where q and g stand for a light quark and a gluon, respectively. This prediction even in the version with transverse momentum smearing in the fragmentation process yields an asymmetry which is too large (see fig. 1 ). However, it was shown by Nason et al. [ 15 ] that the third-order processes g + g- , c + e + g and g + q--, c + c + q play an essential role in describing the experimental cross section for the hadronic production of charm using a reasonable mass of the charm quark. The higher-order processes are expected to reduce the asymmetry thus bringing it closer to the experimental value.

In an alternative model [ 13 ] a hadronic cluster is produced via quark-quark scattering. This cluster of a fixed mass of ~ 5 GeV/c 2 decays with a ~1% probability into a DI) pair. The fragmentation function is assumed to be of Boltzmann type in the rest frame of the cluster. The prediction of this model (also shown in fig. 1 ) roughly agrees with the experimental results.

6. Conclusions

We have studied azimuthal angle correlations in a sample of 642 double-charm events. There is a clear peaking at 180 ° for DI) pairs, less pronounced for A~?-IS) combinations and probably even weaker for D~IS) events. This peaking is slightly stronger for leading D mesons.

References

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[2 ] ACCMOR Collab., S. Barlag et al., Production properties of D o , D +, D *+ and D~ + in 230 GeV/c rt - and K - - C u interactions, submitted to Z. Phys. C.

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C.E. Carlson and R. Suaya, Phys. Lett. B 81 (1979) 329; R. Winder and C. Michael, Nucl. Phys. B 173 (1980) 59; V. Barger et al., Phys. Rev. D 25 ( 1982 ) 112; R. Odorico, Nucl. Phys. B 209 (1982) 77.

[13] R.M. Godbole and D.P. Roy, Phys. Rev. Lett. 48 (1982) 1711; D.P. Roy and B.R. Desai, Z. Phys. C 22 (1984) 149.

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Azimuthal correlations between charmed particles produced in 230 GeV/c π−Cu interactions