A study of the transverse polarization of Λ0 and Λ0 hyperons produced in π−Cu interactions at 230 GeV/c

ELSEVIER

21 April 1994

Physics Leaers B 325 (1994) 531-535

PHYSICS LETTERS B

A study of the transverse polarization of A ° and )-o hyperons produced in 7r--Cu interactions at 230 GeV / c

ACCMOR Collaboration Amsterdam-Bristol-CERN-Cracow-Munich-Rutherford-Valencia

S. Barlag a'l, H. Becker e'2, A. Bo2ek d, T. Btihringer c,3, M. Bosman e,4, V. Castillo c'5, V. Chabaud c, C. Damerell f, C. Daum a, H. Dietl e, A. Gillman f, R. Gilmore h, T. Gooch b,

P. Grasg, Z. Hajduk d, E. Higong, D.P. Kelsey c,6, R. Klanner e,7, S. Kwan c'8, B. Lticking e, G. Ltitjens e, G. Lutz e, J. Malos b, W. M~Lnner e, E. Neugebauer e,9, H. Patka d, M. Pep6 f,l°, J. Richardson f'll, K. Rybicki d, R. Ry~ko d,12, H.J. Seebrunner c, U. Stierlin e, H.G. Tiecke a,

G. Waltermann e, S. Watts h, p. Weilhammer c, F. Wickens f, L.W. Wiggers a, M. Witek d a NIKHEF-H, NL-IO09 DB Amsterdam, The Netherlands

b University of Bristol, Bristol BS8 1TL, UK c CERN, CH-1211 Geneva 23, Switzerland

d Institute of Nuclear Physics, PL-30055 Cracow, Poland e Max-Planck-lnstitutffir Physik, D-80805 Munich, FRG

f Rutherford Appleton Laboratory, Chilton, Didcot OXll OQX, UK g IFIC, CS1C and University of Valencia, Valencia, Spain

h Brunel University, Uxbridge, UB8 3PH, UK

Received 25 February 1994 Editor: L. Montanet

Abstract

We study the polarization with respect to the normal to the production plane for a very clean sample of 27217 A ° / - ~

hyperons produced in 230 G e V / c ~ r - -Cu interactions. In general we find P ( A °) ~ P ( - ~ ) ~ 0 except for XF > 0, p r > 1 G e V / c where P ( A °) = - 0 . 2 8 4- 0.09(stat.) 4- 0.02(syst.).

I Present address: KNMI, De Bilt, The Netherlands. 2 Present address: Gesamthochschule, Saarbriicken, FRG. 3 Present address: University of Lausanne, CH-1015 Lausanne, Switzerland. 4 Present address: Univ. Autonoma de Barcelona, E-08193 Bel- laterra, Spain 5 Present address:IFIC,CSIC and University of Valencia, E-46100 Valencia, Spain. 6 Present address: Rutherford Appleton Laboratory, Chilton, Did-

Elsevier Science B.V. SSDI 0370-269 3 ( 94 ) 003 3 9-9

cot OXll 0QX, UK. 7 Present address: DESY, D-22603 Hamburg, FRG. 8 Present address: FNAL, Batavia, IL60510, USA. 9 Present address: Universi~t-GH, D-57608 Siegen, FRG. lo Present address: CERN,CH-1211 Geneva 23, Switzerland. tl Present address: University of Geneva, CH-1211 Geneva 4, Switzerland. 12 Present address: Queen Mary and Westfield College, London E1 4NS, UK.

532

1. Introduct ion

ACCMOR Collaboration/Physics

Since the discovery of the transverse hyperon polarization in high energy, collisions by Bunce et al. [ 1 ] there have been many studies of this interesting phenomenon (see Ref. [2] and references therein), mainly using proton beams. In proton-induced interactions P ( A ° ) is negative (using the Basel convention - see below) and increases with the A ° transverse momentum PT while p(~O) ~ 0 as it was shown e.g. in a high-statistics experiment of Lundberg et al. [ 3 ] for 400 GeV/c protons• There exist but a few results for 7r-p interactions, namely at 3.95 GeV/c [4], at 15 GeV/c [5], at 16 GeV/c [6] and at 18.5 GeV/c [7] as well as a combined result at 100, 200 and 360 GeV/c [ 8 ]. Except for a high-statistics result from the Multi-Particle Spectrometer [6] they come from hydrogen bubble chambers or from a hybrid system [ 8]. In addition to the above-mentioned measurements on a hydrogen target there is a propane-bubble-chamber result [9] at 40 GeV/c, in which the majority of 1219 hy.y__~erons come from ~ ' - - C interactions• Most of A°/A v hyperons observed in bubble chambers are emitted in the target fragmentation region (Feynman

• 0 - - r0 . XF < 0) with A /A tranverse momenta PT hardly exceeding 1 GeV/c. A strong A°/A°polarization is generally claimed in low-statistics experiments• This is particularly true for the propane experiment [9], in which the polarization amounts to -0 .22 4- 0.07 when averaged over the entire xp and Pr range. This disagrees with high-statistics experiments [4,6] but the difference could still be attributed to the nuclear target used in Ref. [9].

The ACCMOR Collaboration performed a ded- icated charm experiment NA32 at the CERN SPS using a 230 GeV/c ~-- beam and a copper target. The experiment yielded many results on hadronic production and decay properties of charmed particles. Some studies of spin effects of Ac + [ 10,11 ] and D*+(2010) [12] were also based on the ACCMOR results• Simultaneously, the experiment delivered nearly 29000 A°/ -~ hyperons decaying in the silicon vertex detector. This allowed a study of the A°/-~ polarization for both positive and negative XF values with a reasonable statistics•

In this study we use the Basel convention, i.e. the direction of Pbea m ×PA for the normal to the production plane. We call 0t~v the angle between the normal and

Letters B 325 (1994) 531-535

the proton (antiproton) momentum in the A ° (~-0) rest frame. The distribution of x = cos 0t~v can be written in the form

l ( x ) = ½(1 + otPx) (1)

where the A ° decay asymmetry ot = 0.642 [13] and P stands for the polarization under study.

The letter is organized as follows: In Section 2 and Section 3 we briefly review the ACCMOR experiment, data processing and acceptance corrections. The results are shown in Section 4 and discussed in Section 5.

2. The exper iment and data analysis

The data used in this study come from the second phase of the NA32 experiment which was performed at the CERN-SPS using a negative 230 GeV/c beam (96% pions and 4% kaons) and a 2.5 mm Cu target. Charm decays were reconstructed with an im- proved silicon vertex detector and a large-acceptance spectrometer. The latter consisted of two magnets, 48 planes of drift chambers and three multicellular Cherenkov counters allowing pion, kaon and proton identification in the momentum range (4-80) GeV/c. The vertex detector consisted of a beam telescope with seven microstrip planes and a vertex telescope with two charge-coupled devices (CCDs) and eight microstrip planes. The overall precision of our vertex detector allowed a purely topological charm search, which was restricted neither to a limited number of decay modes nor to any mass window•

Event reconstruction is done in several steps (see Ref. [ 14] 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 only accept events with the primary vertex inside the target and at least two tracks not originating from the vertex. These tracks are then used as a seed for the search for one or more secondary vertices. The vertices should be between the target and the first microstrip plane (65 mm from the target). In addition we require the vector sum of the momenta

A CCMOR Collaboration/Physics Letters B 325 (1994) 531-535 533

of all particles originating from the secondary vertex to pass through the primary one. This procedure has yielded about 1200 fully reconstructed decays of charmed particles (see e.g. Ref. [15]) and tens of thousands of decays of neutral strange particles.

3. Signal, background and acceptance corrections

As a byproduct of our charm search we have found nearly 29000 A ° / ' ~ decays in the silicon detector. There are more A°/ -~ decaying downstream in the remaining part of the apparatus but our V ° reconstruction program was found to have a rather large (,-~25%) and poorly measured inefficiency. Consequently, we restrict our study to A°/"~ decaying in the vertex detector (this restriction limits the available region to XF < 0.35), which was thoroughly tested in the charm search and for which the reconstruction efficiency is well under control. Removing weak reflection from the K~s ~ ~r+Tr - decays we are left with 28567 events for which the effective mass of decay products meff is in the interval (1111-1120) MeV. About 4% of them are produced in K--induced interactions and they are ig- nored since the statistics of 1350 events is too low for a meaningful polarization study. We fit the meff distribution of the remaining 27217 events (see Fig. 1 ) with a Gaussian signal and a polynomial background. The fit yields m ( A ° / ' ~ ) = ( 1115.77=/=0.01 ) MeV, close to the RPP value [ 13] of ( 1115.63 4- 0.05) MeV. From the same fit we obtain 0.7% as an upper limit of the background under the A°/-~ peak.

Acceptance corrections are calculated using a Monte Carlo program which generates the A°/'-~ hyperon in the reaction ~ - C u ~ A°/-~X, where X stands for other particles produced in the interaction. The hyperons are generated from the distribution

d2o"

dxedpr oc ( 1 - XF) n e -bp2 (2)

where the values of n = 3 and b = 3.0 ( G e V / c ) - 2 were iteratively found to describe the experimental distribution. The parameters of the particles X are read a sample of interaction-trigger events recorded during the experiment. Then the tracks of the A°/-A ° decay products are merged with the real tracks and the re- suiting event is submitted to our selection criteria. The

3000

2500

2000

1500

1000

500

1.105 1.11 1.115 1.12

A m o s s ( G e V / c 2)

Fig. I. Experimental distribution of the A°/A ° invariant mass.

acceptance is small (not exceeding 0.3%) and weakly depends on cos 0try (changes of the order of 15%).

4. Results

We study the PT dependence of the polarization P separately for positive and negative xF values. It should be stressed that we have very few A°/-A ° for xr < -0 .15 thus our XF < 0 results refer mainly to the central region. On the other hand in the forward hemisphere our spectrum extends to xe ,~ 0.35 as it has been mentioned in Section 3. These limits must be taken into account when comparing our results to those of other experiments. In particular there is no overlap between our results and those of the Multi- Particle Spectrometer [6], which cover only an xe > 0.5 region.

In each region of xF and Pr we fit Eq. (1) to the acceptance-corrected distribution (20 bins) of cos 0t~ and the resulting P values are listed in Table 1 and Table 2. The X 2 values always refer to 18 degrees of freedom. The systematic errors reflect the uncertain- ties of acceptance corrections.

534

Table 1 A ° and ~-o polarization for XF > 0

ACCMOR Collaboration/Physics Letters B 325 (1994) 531-535

P'r range [GeV/c] No. of A ° P(A °) X 2 No. of ~-o p(~0) X2

0.00-0.25 1968 -0.04 4- 0.06 4- 0.01 21.1 1578 -0.10 4- 0.07 4- 0.01 14.4 0.25-0.50 4030 --0.06 4- 0.04 4- 0.01 25.3 3129 0.03 4- 0.05 4- 0.01 15.0 0.50-0.75 2730 --0.01 4- 0.05 4- 0.01 28.4 2275 0.16 4- 0.06 4- 0.01 25.3 0.75-1.00 1306 0.02 4- 0.08 4- 0.02 11.1 1022 -0.14 4- 0.09 4- 0.02 11.9 > 1.00 839 --0.28 4- 0.09 4- 0.02 23.8 563 -0.01 4- 0.12 4- 0.02 42.6

full 10873 -0.04 4- 0.03 4- 0.01 41.1 8567 0.02 4- 0.03 4- 0.01 29.6

Table 2 A ° and ~o polarization for xe < 0

PT range [GeV/c] No of A ° P(A °) X 2 No of ~o p(~O) X2

0.0-0.5 3164 -0.01 4- 0.05 4- 0.01 16.5 2078 -0.08 4- 0.06 4- 0.01 37.4 > 0.5 1636 0.03 4- 0.07 4- 0.02 11.6 899 0.05 4- 0.09 4- 0.02 17.3

full 4800 0.01 4- 0.04 4- 0.01 11.5 2977 -0.03 4- 0.05 4- 0.01 33.6

The results for XF < 0 are well consistent with p ( A o ) = p ( ~ 0 ) = 0 for any Pr . For xF > 0, we ob-

serve a non-van i sh ing P (A °) in the high p r region at the level o f three standard deviat ion (see Fig. 2) , 50 while p(~-0) fluctuates around zero with a possible excursion between PT = 0.5 G e V / c and Pr = 0.75

G e V / c . 40

5. Discussion of results

Since we are us ing a copper target it seems natural to compare our results first to those of the propane bubble chamber at 40 G e V / c [9 ] . With a superior statistics we do not support their c la im about a significant overall polar izat ion even though we have relatively more x r > 0 hyperons which, according to them, should be more strongly polarized. The 3.95 G e V / c [4] experi- men t with still h igher statistics has also measured P 0 for - 0 . 2 < xF < 0.35 and p r < 1 G e V / c . Other results [5,7,8] at low p r are consis tent with ours.

There have been many models a iming at the de- script ion o f the transverse hyperon polarizat ion (see Ref. [ 17] ) with p - or K- induced reactions. The parameters of these models are general ly valid on ly for a part icular process. Recent ly proposed dynamical models (see Refs. [ 18,19] ) may improve the situation. This group of models gives predict ions for the differ-

30

20

10

1 f L

-0 .5 0 0.5

t

COS6)t., Fig. 2. Acceptance-corrected distribution of cos 0try for A ° at XF > 0 and Pr > 1 GeV/c. Straight line shows the result of the fit described in Section 4.

ential cross section and takes various hadronic intermediate states into account [ 20] when calculat ing the

ACCMOR Collaboration/Physics Letters B 325 (1994) 531-535 535

polarization. It seems interesting to compare our P ( A °) at high

pr with the only result on P ( A +) in hadronic production, which also comes from our experiment [ 11 ]. For XF > 0 and Pr > 1.1 GeV/c the product of the kaon asymmetry 13 t~r in the A + --~ pK-Tr + decay and the polarization P ( A +) was measured as aKP = _n ~+0.22 using an opposite sign convention v.v- '_0.18 for the normal to the production plane. Assuming that the kaon asymmetry is close to dr = --1 as it may be expected from heavy-quark phenomenology (see Ref. [21] for description of the model and Ref. [22] for some experimental results; see also Bjorken [23] for a comment on A+c -+ pK-Tr + decay) we obtain P ( A +) ~ -0 .6 , the sign corresponding to the Basel convention. This value combined with the present result on P ( A °) in the same kinematic region is well consistent with the predictions [ 24,25 ] that the scale of the polarization is given by the mass of the polarized baryon.

6. Conc lus ions

We have studied the transverse polarization for 27217 A° / '~ hyperons inclusively produced in 230 GeV/c ~r - -Cu interactions. At XF > 0 and Pr > 1.0 GeV/c we measure P ( A °) = -0 .28 + 0.09 + 0.02. For the remaining xF and pr r_~. ions P ( A °) ,,~ p ( ~ 0 ) ,.~ 0, except maybe for P ( A ) at positive XF and intermediate/77" range.

7. A c k n o w l e d g e m e n t s

One of us (K.R.) is very grateful to the Werner Heisenberg Institute for support, and another (R.R.) acknowledges partial support by the KBN Grant No. 20-38-09101.

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Documents

A study of the transverse polarization of Λ0 and Λ0 hyperons produced in π−Cu interactions at 230 GeV/c