c proton-emulsion interactions

Volume 263, number 3,4

PHYSICS LETTERSB

18 July 1991

Charm meson production in 800 GeV/c proton-emulsion interactions Fermilab E653 Collaboration K. K o d a m a a, N. Ushida a A. Mokhtarani b,1, V.S. Paolone b, J.T. Volk b,t, J.O. Wilcox b, P.M. Yager b, R.M. Edelstein c, A.P. Freyberger c,2, D.B. Gibaut c, R.J. Lipton c,~, W.R. Nichols ~, D.M. Potter c, J.S. Russ c, y . Zhang c,3, H.I. Jang d, j . y . Kim d, T.I. Kim a, I.T. Lim d, M.Y. Pac d, B.R. Bailer e, R.J. Stefanski e,4, K. Nakazawa f, S. Tasaka f, K.S. Chung g, S.H. Chung g, D.C. Kim g, I.G. Park g, M.S. Park g, J.S. Song g, C.S. Yoon 8, M. Chikawa h, T. Abe i, T. Fujii i, G. Fujioka i, K. Fujiwara i, H. F u k u s h i m a i, T. Hara i, y . Takahashi i, K. T a r u m a i, y . Tsuzuki i, C. Y o k o y a m a ~, S.D. Chang J, B.G. Cheon J, J.H. Cho J, J.S. Kang J, C.O. K i m J, K.Y. Kim J, T.Y. Kim J, J.C. Lee J, S.B. Lee J, G.Y. Lira J, S.W. N a m J, T.S. Shin J, K.S. Sim J, J.K. Woo J, Y. Isokane k, y . Tsuneoka k, S. Aoki ~, A. Gauthier ~,5, K. Hoshino ~, H. Kitamura Q, M. Kobayashi 2, M. Miyanishi ~, K. N a k a m u r a 2, M. N a k a m u r a 2, Y. N a k a m u r a 2, S. Nakanishi ~, K. Niu 2, K. Niwa 2, H. Tajima 2, J.M. Dunlea m,6, S.G. Frederiksen m,4, S. K u r a m a t a m,7, B.G. Lundberg m,~, G.A. Oleynik re, l, N.W. Reay m, K. Reibel m, R.A. Sidwell m, N.R. Stanton m, K. Moriyama ", H. Shibata n, G.R. Kalbfleisch o, P. Skubic o, J.M. Snow o, S.E. Willis o,s, W.Y. Yuan o, O. K u s u m o t o p,9, T. Okusawa P, M. Teranaka P, T. Tominaga P, T. Watanabe P, J. Yamato P, H. Okabe q, J. Yokota q, M. Adachi r, M. Kazuno r, F. Minakawa r, E. Niu r, H. Shibuya r, S. Watanabe r, O. Fukuda s, Y. Sato s, I. Tezuka ~, S.Y. Bahk t and S.K. Kim t a b c a e f g

Aichi University of Education, Kariya 448, Japan University of California at Davis, Davis, CA 95616, USA Carnegie-Mellon University, Pittsburgh, PA 15213, USA Chonnam National University, Kwangju 500-757, Korea Fermi NationalAcceleratorLaboratory, Batavia, IL 60510, USA Gifu University, Gifu 501-11, Japan GyeongsangNational University, Jinju 660-300, Korea Kinki University, Higashi-Osaka 577, Japan i Kobe University, Kobe 657, Japan J Korea University, Seoul 136-701, Korea k Nagoya Institute of Technology, Nagoya 466, Japan Nagoya University, Nagoya 464, Japan m Ohio State University, Columbus, OH 43210, USA n Okayama University, Okayama 700, Japan o University of Oklahoma, Norman, OK 73019, USA P Osaka City University, Osaka 558, Japan q Science Education Institute of Osaka Prefecture, Osaka 558, Japan r Toho University, Funabashi 274, Japan Utsunomiya University, Utsunomiya 350, Japan t Wonkwang University, Iri 570-749, Korea

Received 10 May 1991 0370-2693/91/$ 03.50 © 1991 - ElsevierSciencePublishers B.V. ( North-Holland )

573

Volume 263, number 3,4

PHYSICS LETTERS B

18 July 1991

We report results on DOand D ÷ production in proton-emulsion interactions at x/s=38.7 GeV. A fit to the form ( 1- IxrJ )n exp( -bp 2) yields n=6.9+_~:9 and b=0.84_+o°:~° (GeV/c) -2. The total inclusive cross section, assuming linear A dependence, is measured to be 38 + 3 (stat.) + 13(sys.) ~tb for the DOand 38 + 9 + 14 Ixbfor the D +. A comparison of these results with previous measurements indicates that nuclear effects do not strongly influence charm production. The predictions of QCD are in good agreement with our data.

An important requirement of perturbative Q C D calculations for hadronic charm production is that the cross section per nucleon be i n d e p e n d e n t of the atomic weight of the target. Recent results [ 1 ] indicate that this is nearly true of the total cross section, but more subtle effects [ 2 ] could still appear in kinematic distributions. These are best studied at the highest possible energy to distinguish between threshold effects [ 3 ] and non-perturbative effects related to quark mass. In this paper we report results on charm production in p r o t o n - e m u l s i o n interactions at x / ~ = 3 8 . 7 GeV a n d compare them to previous measurements and to Q C D predictions. Data for this analysis were recorded during the first run of Fermilab experiment E653. Detailed descriptions of the apparatus and technique appear elsewhere [4,5], and are summarized here. An 800 G e V / c proton beam struck a 1.47 cm long nuclear emulsion target in which the primary vertex and at least one decay vertex were measured. The trigger, which was optimized for s e m i m u o n i c decays of charm particles, required a beam particle to interact in the target and a m u o n candidate track to penetrate 5 m of steel. The apparatus was triggered by 5% of all interactions, and 5.4 X 106 events were recorded. Tracks were reconstructed in a silicon microstrip vertex detector and m o m e n t u m analyzed in a large aperture

Present address: Fermilab, Batavia, IL 60510, USA. 2 Presentaddress: Universityof Florida, Gainesville,FL 32611, USA. 3 Present address: Pennsylvania State University, University Park, PA 16802, USA. 4 Present address: SSC Laboratory, Dallas, TX 75237, USA. 5 Present address: University of Illinois, Urbana-Champaign, IL 61801, USA. 6 Present address: University of Rochester, Rochester, NY 14627, USA. 7 Presentaddress: HirosakiUniversity,Hirosaki-city036, Japan. 8 Present address: Northern Illinois University, DeKalb, IL 60115, USA. 9 Presentaddress: Soai University, Osaka 559, Japan.

574

magnetic spectrometer. Initial event selection required the m u o n m o m e n t u m to be greater than 8 G e V / c and its transverse m o m e n t u m to be greater than 0.2 GeV/c. In 56 000 events the m u o n track appeared to be unassociated with the reconstructed primary vertex. For each of these events in the fiducial volume of the emulsion, the primary vertex was located visually, a n d the m u o n trajectory in the spectrometer was compared to the tracks in the emulsion in order to find a match. This procedure identified 162 2-prong a n d 65 3-prong s e m i m u o n i c charm decay candidates for which all emulsion tracks could be matched, and typically rejected an event as having been triggered by the muonic decay o f a pion or a kaon associated with the primary vertex or with a secondary interaction vertex. In order to find the second charm decay, which was unassociated with the m u o n track, a "partner" vertex search was carried out. Emulsion tracks from the primary vertex were followed to the end of the emulsion if they were not matched to spectrometer tracks; decays of charged charm particles were found in this way. To find neutral charm particle decays, spectrometer tracks that were u n m a t c h e d to emulsion tracks were projected into the emulsion and followed upstream as far as the primary. Table 1 lists the various types of decay vertices, all of which were inside the emulsion. Since this analysis used no hadron identification, the 3-prong vertices were an u n k n o w n mixture of D +, D + , and A + de-

Table 1 Types of decay vertices and average RMS momentum resolution Decay vertex type

Number of events

a(p)/p

2-prong semimuonic 3-prong semimuonic 2-prongpartner 4-prong partner

108 18 13 7

0.30 0.16 0.28 0.19

Volume 263, number 3,4

PHYSICS LETTERS B

cays ~1 To select D + decays, only s e m i m u o n i c 3prongs with Kn invariant mass within 50 M e V / c 2 o f the K * ( 8 9 2 ) ° were used. This criterion preserved a mass peak containing 50% o f the s e m i m u o n i c 3prongs (see fig. 1 ), a n d yielded a sample d o m i n a t e d [ 6 ] by the decay m o d e D + --.K* (892)°lx+v. Most o f the c h a r m decays in our d a t a c o n t a i n e d missing neutral daughters and could not be fully constrained. Consequently, it was necessary to estimate the c h a r m m o m e n t u m . In the c h a r m center o f mass the most probable polar angle for the visible m o m e n t u m vector o f the decay products is at 90 ° to the charm direction. The m o m e n t u m e s t i m a t o r we used is the product o f the D mass and the boost from this frame to the laboratory. To calculate the visible mom e n t u m vector in any other than the l a b o r a t o r y frame, particle masses must be known. Because only the muon track was identified, the mass o f every other track was assigned its most probable value. Each vertex was assumed to have a charged kaon o f sign consistent with that o f the muon; all other tracks were assumed to be pions. The e s t i m a t e d m o m e n t u m was then scaled so that its m e a n was correct when averaged over the branching m o d e s [ 7 ] for each type o f decay vertex. Table 1 lists the average R M S resolu~ Throughout this paper references to particles also include antiparticles

10,0i[ I t,

,,,i,,,,i ~, i r

5.0

18 July 1991

tion o f the e s t i m a t o r for the events in our sample. Backgrounds to the charm vertices fell into four categories: ( 1 ) Strange particle decays were e l i m i n a t e d by requiring at least one daughter track to have a transverse m o m e n t u m greater than 250 M e V / c with respect to the parent direction. ( 2 ) Most secondary interactions were e l i m i n a t e d during emulsion scanning by rejecting vertices with heavily ionizing tracks from nuclear b r e a k u p or with a p p a r e n t lack o f charge conservation. The secondary interactions that survived were "white stars", which show no evidence o f nuclear b r e a k u p (e.g., diffractive interactions). Background from this source in the partner sample was negligible because each decay was p a i r e d with a s e m i m u o n i c vertex. To estimate the white star background in the semimuonic sample, the total n u m b e r (1010) o f secondary interactions to which the muon track was linked during scanning was scaled by the probability ~2 (0.028) per secondary interaction to produce a 3-prong white star. This number (29) is the background estimate for the semim u o n i c 3-prong sample before any cuts. To calculate the background in the final sample, the 29 events were scaled b o t h by the fraction (0.28) o f 3-prongs that survived cuts ( p r i o r to K * ( 8 9 2 ) ° selection) and by the fraction (0.11 ) o f the remaining background retained by K * ( 8 9 2 ) ° selection. F o r the 2-prong sample the 29 events were scaled by the fraction (0.52) that survived cuts and by the ratio #3 ( 0 . 1 3 ) o f neutral to charged long-lived h a d r o n s p r o d u c e d in the p r i m a r y interaction. These calculations for the white star b a c k g r o u n d yield 0.9 event in the 3-prongs and 2.0 events in the 2-prongs, and are an overestimate, since the background is less likely to survive cuts than charm. ( 3 ) Incorrect matches between spectrometer and emulsion tracks were suppressed by the following t12 We use the average of the values from ref. [8] and ref. [9],

,,,

-0.4 -0.2 0 0.2 0.4 MK.zr-MK*(e92) ° (GeV/c2) Fig. I. Invariant K~ mass distribution for semimuonic 3-prong decays. The zero of the horizontal axis corresponds to the

K*(892) ° mass.

which report on interactions of 22.5 GeV protons in emulsion. The procedure for identifying secondary interactions was the same in these two experiments as in this work. We assume that the probability for a neutral parent to produce a 2-prong white star is equal to the probability for a charged parent to produce a 3-prong white star. We use data from ref. [ 10] and ref. [ 11 ] for the average number of neutrons, lambdas, and neutral kaons per interaction at 300 GeV/c and from ref. [ 12 ] for average charged multiplicityto compute the neutral to charged ratio. 575

Volume 263, number 3,4

PHYSICS LETTERS B

three criteria: (a) track momenta had to be greater than 1.25 G e V / c , (b) impact parameters at the secondary vertex had to be less than 400 ~tm, and (c) no spectrometer track that would retain correct vertex charge and that had an acceptable probability to belong to the vertex could be within 4 mr of any track matched to the vertex. The background from this source remaining in the sample is estimated to be 4 vertices. This is the number of charge 2 vertices that survive all cuts except the requirement that the vertex charge be less than 2. (4) A background to the semimuonic 2-prong sample is hadronic charm decays in which a daughter pion or kaon decays in flight. The level of this hadronic feedthrough was estimated by Monte Carlo simulation to be 3%, and alters the results of the fits by less than 10% of their errors. It was also required that each decay be kinematically compatible with a charm hypothesis. After all selection criteria were applied, the total background, excluding hadronic feedthrough, was estimated to be fewer than 7 events, or less than 5% of the data ~4. It was checked that data samples rejected by cuts, and thus enriched in background, did not preferentially populate large XF or p zx; otherwise the background was ignored in subsequent analysis. Acceptance and reconstruction efficiency were determined with a GEANT-based Monte Carlo program that simulated the charge deposition in silicon detectors and the drift time and time-over-threshold in drift chambers. Uncorrelated charm pairs were superimposed on proton-emulsion interactions generated by the Fritiof program [ 14 ]. These simulated data were processed by the routines used for real data, and thus provided a test of the analysis procedure, as well as the determination of the efficiency. Since the initial selection of candidate semimuonic vertices depended on reconstructed tracks and vertices, efficiency varied rapidly at short decay lengths. To reduce the variation and thus improve the reliability of the simulation, the decay length measured in the emulsion was required to be greater than 2 mm. Similarly, a 0.5 m m cut was used for partner vertices, since scanning efficiency decreased for very short decays. Doubling the decay length cuts eliminated 45 #4 Our previous work [ 13 ] includes additional evidence that the background is small.

576

18 July 1991

events and changed the results of the fits by less than half of their statistical errors. Fig. 2 shows the raw XF distribution of the data and the experimental efficiency function. The maximum likelihood method was used to fit the data to the form ( 1 - IXF In. For each event the normalized probability distribution was computed as e(x'v;n) =Q(X'F;n)/N ,

where 1

Q(x'v;n) = ) R(XF, X'F)e(XF)(1 -- IXF I )"dXF, --I X~nax

N=

f

Q ( x F/ ,, n ) d x Fr ,

x~in

primed variables are measured, unprimed are true, and R and e are the resolution and efficiency functions, respectively. For each event X'ax was calculated from the measured lifetime and distance between the primary vertex and the end of the emulsion target. Similarly, Xm~, depended on the minimum decay length cut and lifetime. The resolution function was also evaluated for every event. The fit yields n = o+ 1,8, ~,9 where the error is statistical only, and larger "~-80

I[11

t II

~ll

I

Ifllrtttt

I

0.3 60

0.2 ~-

40

~

20

o

\'\

2

0.1

I I I I I I I I I I I t I I I I I I I I

-0.4

-0.2

0

0.2

0

0.4

XF

Fig. 2. The raw XF distribution of the data. Also shown are the efficiency function (dot-dashed) and histograms of the fit to the data (solid) and of the QCD prediction ofref. [ 15] (dashed).

Volume 263, number 3,4

PHYSICS LETTERS B

than that due to systematic effects. Histograms, which include experimental acceptance and efficiency, are displayed in fig. 2 for the fit and for the QCD prediction of ref. [ 15 ]. The QCD calculation, which was for charm quarks, and included next-to-leading order terms, is in good agreement with our data. Our value for n is also consistent with 8.6 -+2.0, the result of the E743 measurement [ 16 ] at the same energy in proton-proton interactions. The distribution is consistent with being symmetric about XF=0. For the 96 events with x ~ > 0, n = 6.8_+2;~; for the 50 events with X~<0,

n=8.3

+6"° --5,6.

Fig. 3 shows the raw p2 distribution of the data ~5. Efficiency in p2 was flat to 3%, but experimental momentum resolution affects the shape of the distribution. A maximum likelihood fit, which included resolution and efficiency, was made to the form exp( -bp 2) and yielded the result that ~,=u. ~" n 84+O.lO_oo8 ( G e V / c ) -2. The error on the result is statistical only and dominates errors due to systematic effects. Our result agrees well with the E743 result that b = 0.8+_0.2 ( G e V / c ) -2. Simulations, which included experimental resolution, were made for the fit and for the QCD prediction of ref. [ 17 ], and are dis~5 There are four events with p~ > 10(GeV/c)2; these events do not appear in fig. 3, and are excluded from the fit, since their errors exceed 8 ( G e V / c ) 2.

IIIIIIIIIIIIIIIIIIlllll

10 2

101 or) I,Z

t

"'

10o

1(~ 1

0

Illlllll

2

IIIllllllliill

4

6

8

10

p2 ((GeV/c)2)

Fig. 3. The raw p2 distribution of the data. Also shown are histograms of the fit to the data (solid) and of the Q C D prediction ofref. [ 17] (dashed).

! 8 July 1991

played as histograms in fig. 3. The QCD prediction, which is a next-to-leading order calculation for charm quarks, represents our data as well as the exponential fit. The cross sections for D Oand D ÷ production were determined from the two samples of semimuonic decays. Branching ratios were taken to be equal [ 18 ] to 96% of those for the corresponding semielectronic modes [ 12 ], and a linear dependence on the mean atomic weight, 26.6, of the emulsion target was assumed. Expressed as total inclusive cross sections per nucleon, the results are 38 + 3 (stat.) + 13 (sys.)~tb for the D °, and 38+9_+ 14 ~tb for the D +. Uncertainties in branching ratios, charm production dynamics, and experimental efficiencies contribute comparably to the systematic errors. Our results are somewhat higher than, but consistent with the E743 results: 22_+9 + 5 ~tb for the D Oand 26 + 4 -+ 6 ~tb for the D ÷. These resuits are in agreement with next-to-leading order QCD predictions [ 19 ]. In conclusion, we find the XF and p2 distributions to be the same within statistics as those measured at the same energy in p - p collisions [ 16 ], and the total cross sections for D Oand D ÷ production to be consistent with a linear dependence on atomic weight when compared to those measured in the hydrogen experiment. In addition, the XF distribution is consistent with being symmetric about XF=0. Thus, we see no evidence for any nuclear effects in our data, and conclude that they cannot strongly influence charm production at high energy. Our XF and p~ distributions are in agreement with next-to-leading order QCD predictions [ 15,17 ] for charm quarks. This observation suggests that hadronization plays a minor role in shaping the kinematic distributions of charm mesons. We gratefully acknowledge the efforts of the Fermi National Accelerator Laboratory staff in staging this experiment. This work was supported in part by the US Department of Energy; the US National Science Foundation; the Japan Society for the Promotion of Science; the Japan-US Cooperative Research Program for High Energy Physics; the Ministry of Education, Science and Culture of Japan; the Korea Science and Engineering Foundation; and the Basic Science Research Institute Program, Ministry of Education, Republic of Korea. 577

Volume 263, number 3,4

PHYSICS LETTERS B

References [ 1 ] L. Lueking, Hadroproduction of charm at Fermilab E769, in: Proc. XXVth Rencontre de Moriond (Les Arcs, March 1990), ed. J. Tran Tranh Van (Editions Fronti~res, Gif-surYvette, 1990) p. 202; M.I. Adamovich et al., CERN preprint CERN-EP/89-123 (1989). [2]S.J. Brodsky, Heavy quark production in quantum chromodynamics, in: Proc. 23th Intern. Conf. on High energy physics (Berkeley, CA, July 1986), Vol. 2, ed. S.C. Loken (World Scientific, Singapore, 1987 ) p. 1373; S.J. Brodsky, J.F. Gunion and D.E. Soper, Phys. Rev. D 36 (1987) 2710; S.J. Brodsky and P. Hoyer, Phys. Rev. Lett. 63 (1989) 1566; P. Hoyer, M. Vanttinen and U. Sukhatme, Phys. Lett. B 246 (1990) 217. [3] K. Abe et al., Phys. Rev. D 33 (1986) 1; M.I. Adamovich et ah, Phys. Lett. B 187 (1987) 437. [4] K. Kodama et al., Nuch Instrum. Methods A 289 (1990) 146. [5] J.M. Dunlea, Ph.D. Thesis, Ohio State University ( 1987); G.A. Oleynik, Ph.D. Thesis, Ohio State University ( 1987); K. Taruma, Ph.D. Thesis, Kobe University (1987) [in Japanese ]; T.S. Jaffery, M.S. Thesis, University of Oklahoma ( 1988); A. Mokhtarani, Ph.D. Thesis, University of California at Davis ( 1988);

578

18 July 1991

Y.L. Zhang, Ph.D. Thesis, Carnegie-Mellon University (1989); V.S. Paolone, Ph.D. Thesis, University of California at Davis (1990); A.P. Freyberger, Ph.D. Thesis, Carnegie-Mellon University (1990); T. Watanabe, Ph.D. Thesis, Osaka City University ( 1990); W.R. Nichols, Ph.D. Thesis, Carnegie-Mellon University (1991). [ 6 ] J.C. Anjos et al., Phys. Rev. Lett. 62 ( 1989 ) 722. [7] Particle Data Group, G.P. Yost et al., Review of particle properties, Phys. Lett. B 204 (1988) 1. [ 8 ] H. Winzeler, Nucl. Phys. 69 ( 1965 ) 661. [ 9 ] S. Hasegawa et al., Suppl. Prog. Theor. Phys. 47 ( 1971 ) 126. [ 10] F.T. Dao et ah, Phys. Rev. D 10 (1974) 3588. [ 11 ] A. Sheng et al., Phys. Rev. D 11 (1975) 1733. [ 12 ] Particle Data Group, J.J. Hern~ndez et al., Review of particle properties, Phys. Lett. B 239 (1990) 1. [ 13 ] K. Kodama et al., Phys. Rev. Lett. 66 ( 1991 ) 1819. [ 14] B. Nilsson-Almquist et al., Comput. Phys. Commun. 43 (1987) 387. [ 15 ] W. Beenakker et al., DESY preprint DESY 90-064 (1990). [16] R. Ammar et al., Phys. Rev. Lett. 61 (1988) 2185. [17] P. Nason, S. Dawson and R.K. Ellis, Nucl. Phys. B 327 (1989) 49. [18]D.M. Coffman, Ph.D. Thesis, California Institute of Technology ( 1986 ), unpublished. [ 19] G. Altarelli et al., Nucl. Phys. B 308 (1988) 724.