We present a preliminary study from Fermilab experiment E791 of Lambda^0 /
Lambda^0 bar, Xi^- / Xi^+ and Omega^- /Omega^+ production asymmetries from pi^-
nucleus interactions at 500 Gev/c. The production asymmetries for these
particles are studied as a function of x_F and pt^2. We observed an asymmetry
in the target fragmentation region for Lambda^0's larger than that for Xi's,
suggesting diquark effects. The asymmetry for Omega's is significatively
smaller than for the other two hyperons consistent with the fact that Omega's
do not share valence quarks with either the pion or the target particle. In the
beam fragmentation region, the asymmetry tends to 0.1 for both Lambda^0's and
Xi's. The asymmetries vs pt^2 are approximately constant for the three strange
baryons under study.Comment: Typos correcte

collaboration, E791

Magnin, J.

Simao, F. R. A.

Solano, J.

English

arXiv

We present a preliminary study from Fermilab experiment E791 of {Lambda}{sup 0}/{anti {Lambda}}{sup 0}, {Xi}{sup -}/{Xi}{sup +} and {Omega}{sup -}/{Omega}{sup +} production asymmetries from {pi}{sup -} nucleus interactions at 500 Gev/c. The production asymmetries for these particles are studied as a function of x{sub F} and p{sup 2}{sub T}. We observed an asymmetry in the target fragmentation region for {Lambda}{sup 0}`s larger than that for {Xi}`s, suggesting diquark effects. The asymmetry for {Omega}`s is significatively smaller than for the other two hyperons consistent with the fact that {Omega}`s do not share valence quarks with either the pion or the target particle. In the beam fragmentation region, the asymmetry tends to 0.1 for both {Lambda}{sup 0}`s and {Xi}`s. The asymmetries vs p{sup 2}{sub T} are approximately constant for the three strange baryons under study

Simao, F.R.A.

Collaboration, E791

UNT Digital Library

Fermi National Accelerator Laboratory a w FERMILAB-Conf-97/368-E E791 Hyperon Production Asymmetries in 500 GeVk Pion Nucleus Interactions J. Solano Representing the E79 1 Collaboration Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510 J. Magnin and F.R.A. Simgo Centro Brasileiro de Pesquisas Fisicas CBPF Rio de Janeiro, Brazil October 1997 Published Proceedings of the Relativistic Aspects of Nuclear Physics '97, Rio de Janeiro, Brazil, August 26-29, 1997 Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of Energy Disclaimer This report was prepared as an  account of work sponsored by an  agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility f o r  the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that i ts  use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply i ts  endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Distribution Approved f o r  public release; further dissemination unlimited. HYPERON PRODUCTION ASYMMETRIES IN 500 GeV/c PION NUCLEUS INTERACTIONS J. SOLAN0 representing the FERMILAB E791 COLLABORATION and J. MAGNIN, F.R.A. SIMAO Centro Brasileiro de Pesquisas Fisicas - C B P F  Rio de Janeiro, Brazil. We present a preliminary study from Fermilab experiment E791 of A o / P ,  E-/E+ and 0-/0+ production asymmetries from T- nucleus interactions at 500 Gev/c .The production asymmetries for these particles are studied as a function of XF and p$. We observed an asymmetry in the target fragmentation region for Ao’s larger than that for E’s, suggesting diquark effects. The asymmetry for 0’s is significatively smaller than for the other two hyperons consistent with the fact that 0’s do not share valence quarks with either the pion or the target particle. In the beam fragmentation region, the asymmetry tends to 0.1 for both Ao’s and E’s. The asymmetries vs p$ are approximately constant for the three strange baryons under study. Leading particle effects in charm hadron production, which are manifest as an enhancement in the production rate of particles which share valence quarks over that  of particles with no valence quarks in common with the ini- tial hadrons, have been extensively studied in the recent years from both the experimental and theoretical point of view. The same type of leading effects are expected to  occur in light hadron production. Indeed, there is some evidence of asymmetries in ho/p production in sr-Cu interactions at 230 GeV/cl and in ho/h and E-/=+ production in 250 GeV/c sr-p interactions’. Some ad- ditional evidence for ho/h asymmetry can be found in Ref. 3, but in general light hadron production asymmetries in sr-p interactions are not systemati- cally studied. 4 4 As a byproduct of our charm program in Fermilab Experiment E791 we collected a large sample o f h o / p ,  E-/=+ and O-/O+ which was used to  mea- sure the particlelanti-particle production asymmetries reported in this paper. The ZF range covered by our experiment is -0.12 5 ZF 5 0.12 allowing for the study of baryonlanti-baryon production asymmetries in the target (ZF < 0) and beam (ZF > 0) fragmentation regions. A priori, strong differences are expected for the asymmetries observed in each region where there is a different content of valence quarks in the produced particles relative to the anti-particles 1 with respect to the target and beam hadrons. Thus a growing asymmetry with I X F ~  is expected in Ao/A production in the target fragmentation region. A smaller or no asymmetry is expected in the beam fragmentation region, in which both A' and A share one valence quark with the pion. For E-/=+ pro- duction a growing asymmetry with I X F ~  is expected in both regions since the fi shares one valence quark with the sr- as well as with the target particles ( p  and n) whereas the E+ share none. A zero asymmetry is expected for 0-/0+ which have no valence quarks in common with either the target or the beam. 4 4 -- In the E791 experiment, da ta  were recorded from 500 GeV/c sr- interac- tions in five thin foils (one platinum and four diamond) separated by gaps of 1.34 to 1.39 cm. The E791 spectrometer4 is an upgrade of apparatus used in Fermilab charm experiments E516, E691 and E769. It is a large-acceptance, two-magnet spectrometer with eight planes of multiwire proportional cham- bers (MWPC) and six planes of silicon microstrip detectors (SMD) for beam tracking. Downstream of the target are a 17-plane SMD system for track and vertex reconstruction, 35 drift chamber planes, two MWPC's, two multicell threshold Cherenkov counters, electromagnetic and hadronic calorimeters and a muon detector. An important element of the experiment was its extremely fast da ta  acquisition system which was combined with a very open trigger to  record a da ta  sample of 20 x lo9 interactions. A''s were studied via their psr- decay mode. The distance of closest ap- proach between the proton and sr- tracks must be less that  0.7 cm to  be considered as candidates for A' decay and the candidate A' should have an invariant mass between 1.101 and 1.127 GeV/c2. In addition, the ratio of the momentum of the proton to  the pion is required to be larger than 2.5. The reconstructed A' decay vertex formed by the two tracks is required to  be downstream of the last target. In order to  avoid A''s coming from E decay, candidates must have an impact parameter with respect to  the primary vertex less than 0.3 cm for A''s decaying within the first 20 cm and 0.4 cm for A''s decaying downstream of 20 cm from the target. The reconstructed mass distri- bution of the obtained sample was fitted using a binned maximum likelihood with a Gaussian plus a linear background. We obtained 2,571,662 f 3,057 A''s and 1,668,950 f 2,627 A ' s ,  from approximately 6% of the total data  sample recorded in the experiment. 4 E's are selected via the decay mode E + A'sr and at the same time 0 ' s  are selected in the channel 0 + A'K. Starting with candidate A''s we add a third track as a possible pion/kaon daughter. The cuts applied to the A' 2 are the same quoted above except for the impact parameter cut, which is not applied in this case, and the third track must meet the following requirements: the track cannot be one of those that  made the A’, the invariant mass of the candidate E (calculated from the mass of the A’ and the net momentum of the two decays tracks plus the third track) must be between 1.290 and 1.350 GeV/c2 for E’s and 1.642 and 1.702 GeV/c2 for the 0’s .  In addition, E’s and 0’s  decaying upstream of the SMD and downstream of the A’ are not allowed and the efficiency of SMD’s for the candidate hyperon track must be larger than 0.75. An additional cut is applied for the third track: the Cherenkov probability of this track to be a kaon mut be larger than 0.13 if its momen- tum is smaller than 40 GeV/c and larger than 0.018 if its momentum is larger than 40 GeV/c. As with the lambda sample, we fitted the cascade and omega samples using maximum likelihood with a Gaussian plus a linear background. From approximately 2 /3  of the total E791 da ta  sample we found 570,535 f 1,386 E-; 401,156 f 1,166 E+; 7,401 f 132 0- and 6,598 f 97 0+ after back- ground substraction. For each XF and p$  bin we define an asymmetry parameter A as where NB (NE) is the number of baryons (anti-baryons) in the bin. Monte Carlo studies done with PYTHIA6 for A’ and A have shown that  efficien- cies for both particle and anti-particle are the same therefore no corrections are nedeed in this case. For E’s and 0’s  we are finishing the process of MC generation in order to study efficiency. The obtained asymmetries are shown as functions of XF and p$  in Fig. 1 for A ’ / P ,  E-/=+ and 0-/0+. The total asymmetries measured in the interval -0.12 5 XF 5 0.12 and in the backward and forward XF regions are shown in Table 1. In the backward XF region we observe an asymmetry growing with I X F ~  for both A’’s and E’s and an approximately constant and small asymetry for 0’s .  As can be seen in the figures, the asymmetry in the backward region is higher as more valence quarks are shared between the produced particle and protons and neutrons in the target. The higher asymmetry observed for the A’’s may be due to the ud diquark shared by the produced A’ and the protons and neutrons in the target. In the forward region the asymmetry for both A’’s and E’s is approximately constant. For the E-/=+ the observed asymmetry is compatible with the fact that  the E- shares one valence quark with the incident T -  whereas the E+ has no valence quarks in common with the T - ,  so some leading effect must be present. 4 3 A ( A o / p )  A(=-/=+) A(O-/O+) 4 The asymmetry measured for the Ao/A in the forward region can not be explained by a leading-particle effect because both A’ and A have one valence quark in common with the pion. For the three strange baryons we observe an approximately flat asymmetry, of the order of 0.2 for A’s and E’s and very small for O’s, in the complete p$  interval studied. 4 -0.12 5 ZF 5 0.12 0.213 f 0.001 0.246 f 0.001 0.138 f 0.002 0.174 f 0.002 0.186 f 0.002 0.131 f 0.004 0.057 f 0.012 0.058 f 0.015 0.056 f 0.017 -0.12 < ZF < 0 0 5 ZF < 0.12 Acknowledgments J.M. is supported by FAPERJ, FundaGao de Amparo A Pesquisa do Estado de Rio de Janeiro. References 1. S. Barlag e t  aE., Phys. Lett. B 325, 531 (1994) 2. D. Bogert e t  aE., Phys. Rev. D 16, 2098 (1977). 3. S. Mikocki e t  aE., Phys. Rev. D 34, 42 (1986), R.T. Edwards e t  aE., Phys. Rev. D 18, 76, (1978) and N.N. Biswas, Nucl. Phys. B 167, 41 (1980). 4. E.M. Aitala e t  aE., Phys. Rev. Lett. 76, 364 (1996) and references therein. 5. S. Amato et  aE., Nucl. Instr. Meth. A 324, 535 (1993). 6. H.-U. Bengtsson and T. Sjostrand, Computer Physics Commun. 46, 43 (1987) and T. Sjostrand, CERN-TH.7112/93 (1993). 4 -1 4.1 4.05 0 0.05 0.1 XF 0.4 1 -0.2 O C  -0.1 -0.05 0 0.05 0.1 x i  I F 1  E0.8 '0.6 2 0.4 0.2 0 -0.2 -0.1 -0.05 0 0.05 0.1 x, 0.6 0.4 O /  0 0.5 1 1.5 2 2.5 3 3.5 4 -0.2 ~ ' " " " " " " " " " " " " " " " " " " ' ' ~  p: (GeV2/c2) 0.6 0.4 -0.2 O L  0 0.5 1 1.5 2 2.5 3 3.5 4 p: (GeVZ/c2) 0.8 1 0.6 1 0.4 1 0 0.5 1 1.5 2 2.5 3 3.5 4 -0.2 ~ ' " " " " " " " " " " " " " " " " " " ' ' ~  p: (GeVZ/cZ) Figure 1: Preliminary results for 6% of the E791 data for A and 2/3 for E and 0. Statistical errors only. A o / P  (top), ?/E+ (middle) and 0-/0+ (bottom) asymmetries vs. XF (left) and p$ (right). 5 

Hyperon production asymmetries in 500 GeV/c pion nucleus interactions

Abstract

Similar works

Full text

Available Versions

UNT Digital Library