A study of the polarization of Λ hyperons produced in inelastic pN reactions induced by the 450 GeV proton beam from the CERN SPS has been performed with the NA48 detector. The Λ hyperons were detected at a fixed angle of 4.2 mrad in the momentum range from 50 GeV/c to 200 GeV/c. The polarization changes from −0.053 ± 0.034 to −0.298 ± 0.074 for a transverse momentum range of the Λ between 0.28 GeV/c and 0.86 GeV/c. The antiΛ polarization is consistent with zero

A. Ceccucci

A. Formica

A. Gianoli

A. Gonidec

A. Kalter

A. Lai

A. Michetti

A. Nappi

A. Norton

A. Taurok

A. Tkatchev

A. Walker

B. Gorini

B. Hay

B. Panzer Steindel

B. Peyaud

B. Renk

B. Vallage

BIZZETI, Andrea

C. Biino

C. Cerri

C. Ebersberger

C. Simani

C. Talamonti

D. Coward

D. Cundy

D. J. Munday

D. Schinzel

E. Mazzucato

E. Menichetti

F. Costantini

F. Leber

F. Marchetto

F. Petrucci

F. Rossi

G. Anzivino

G. Barr

G. Bocquet

G. Fischer

G. Govi

G. Kesseler

G. Neuhofer

G. Oakland

G. Pierazzini

G. Tatishvili

G. Unal

H. Bluemer

H. Dibon

H. G. Becker

H. Parsons

H. Taureg

H. Wahl

I. Augustin

I. Kojevnikov

I. Mannelli

I. Mikulec

J. B. Cheze

J. Bremer

J. C. Chollet

J. Cogan

J. Duclos

J. Ocariz

J. P. Matheys

J. Scheidt

J. Schmidt

K. J. Peach

K. Kleinknecht

L. Bertolotto

L. Fayard

L. Gatignon

L. Iconomidou Fayard

L. Koepke

L. Musa

M. A. Parker

M. Bender

M. Calvetti

M. De Beer

M. G. Setzu

M. Holder

M. Jeitler

M. Lenti

M. Markytan

M. Martini

M. Pepe

M. Pernicka

M. Porcu

M. Savrie

M. Sozzi

M. Velasco

M. Ziolkowski

N. Doble

N. E. McKay

O. Boyle

O. Steinkamp

O. Vossnack

O. Zeitnitz

P. Buchholz

P. Calafiura

P. Cenci

P. Dalpiaz

English

V. Fanti

R. W. Moore

S. F. Takach

T. O. White

W. Funk

P. Grafstrom

W. Kubischta

V. Marzulli

S. Palestini

V. Kekelidze

Y.u. Potrebenikov

P. Hristov

P. L. Frabetti

T. Beier

U. Koch

S. Luitz

V. Schoenharting

R. Wilhelm

S. Crepe

P. Lubrano

R. Carosi

R. Casali

R. Fantechi

S. Giudici

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Lawrence Berkeley National LaboratoryRecent WorkTitleA measurement of the transverse polarization of $\Lambda$ -hyperons produced in inelastic pN-reactions at 450 GeV proton energyPermalinkhttps://escholarship.org/uc/item/6p1853jbJournalThe European Physical Journal C, 6(2)ISSN1434-6044AuthorFanti et al., VPublication Date1999DOI10.1007/s100529801045 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDOI 10.1007/s100529801045Eur. Phys. J. C 6, 265{269 (1999) THE EUROPEANPHYSICAL JOURNAL Cc© Springer-Verlag 1999A measurement of the transverse polarization of -hyperonsproduced in inelastic pN -reactions at 450 GeV proton energyV. Fanti, A. Lai, L. Musa, A. Nappi, M.G. SetzuDipartimento di Fisica dell’Universita e Sezione dell’INFN di Cagliari, I-09100 Cagliari, ItalyR.W. Moore, D.J. Munday, M.A. Parker, S.F. Takach1, T.O. WhiteHEP, Cavendish Lab., Physics Department, University of Cambridge2, GB-Cambridge CB3 OHE, UKG. Barr, G. Bocquet, J. Bremer, A. Ceccucci3, J. Cogan, D. Cundy, N. Doble, G. Fischer, W. Funk, L. Gatignon,A. Gianoli, A. Gonidec, P. Grafstro¨m, W. Kubischta, V. Marzulli, G. Kesseler, J.P. Matheys, A. Norton, S. Palestini2,B. Panzer-Steindel, D. Schinzel, H. Taureg, M. Velasco, O. Vossnack, H. WahlCERN, CH-1211 Geneva 23, SwitzerlandV. Kekelidze, I. Kojevnikov, Yu. Potrebenikov, P. Hristov, G. Tatishvili, A. TkatchevJoint Institute for Nuclear Research, Dubna, Russian FederationL. Bertolotto, O. Boyle, B. Hay4, N.E. McKay, G. Oakland, H. Parsons, K.J. Peach, C. Talamonti, A. WalkerPhysics Department, University of Edinburgh5, GB-Edinburgh EH9 3JZ, UKP. Dalpiaz, J. Duclos, A. Formica, P.L. Frabetti, M. Martini, F. Petrucci, M. Porcu, F. Rossi, M. Savrie, C. SimaniDipartimento di Fisica dell’Universita e Sezione dell’INFN di Ferrara, I-44100 Ferrara, ItalyA. Bizzeti, M. Calvetti, M. Lenti, A. MichettiDipartimento di Fisica dell’Universita e Sezione dell’INFN di Firenze, I-50125 Firenze, ItalyH.G. Becker, T. Beier, H. Blu¨mer6, P. Buchholz7, D. Coward8, C. Ebersberger, A. Kalter, K. Kleinknecht, U. Koch,L. Ko¨pke, F. Leber, S. Luitz4, B. Renk, J. Scheidt, J. Schmidt, V. Scho¨nharting, R. Wilhelm, O. ZeitnitzInstitut fu¨r Physik, Universita¨t Mainz, D-55099 Mainz9, GermanyJ.C. Chollet, S.Crepe, L. Fayard, L. Iconomidou{Fayard, J. Ocariz10, G. UnalLaboratoire de l’Accelerateur Lineaire, IN2P3-CNRS, Universite de Paris-Sud, F-91406 Orsay11, FranceG. Anzivino, P. Cenci, P. Lubrano, M. PepeDipartimento di Fisica dell’Universita e Sezione dell’INFN di Perugia, I-06100 Perugia, ItalyP. Calaura, R. Carosi, R. Casali, C. Cerri, F. Costantini, R. Fantechi, S. Giudici, B. Gorini, I. Mannelli, G. Pierazzini,M. SozziDipartimento di Fisica, Scuola Normale Superiore e Sezione INFN di Pisa, I-56100 Pisa, ItalyJ.B. Cheze, M. De Beer, P. Debu, E. Mazzucato, B. Peyaud, S. Schanne, O. Steinkamp12, R. Turlay, B. VallageDSM/DAPNIA - CEA-Saclay, F-91191 Gif-sur-Yvette, FranceI. Augustin, M. Bender, M. Holder, M. ZiolkowskiFachbereich Physik, Universita¨t Siegen, D-57068 Siegen13, GermanyR. Arcidiacono, C. Biino, G. Govi, P. Maas14, F. Marchetto, E. MenichettiDipartimento di Fisica Sperimentale dell’Universita e Sezione dell’INFN di Torino, I-10125 Torino, ItalyH. Dibon, M. Jeitler, M. Markytan, I. Mikulec, G. Neuhofer, M. Pernicka, A. TaurokO¨sterreichische Akademie der Wissenschaften, Institut fu¨r Hochenergiephysik, A-1050 Wien, AustriaReceived: 21 September 1998 / Published online: 19 November 1998Abstract. A study of the polarization of Λ hyperons produced in inelastic pN reactions induced by the450 GeV proton beam from the CERN SPS has been performed with the NA48 detector. The Λ hyperonswere detected at a xed angle of 4.2 mrad in the momentum range from 50 GeV/c to 200 GeV/c. Thepolarization changes from −0.053  0.034 to −0.298  0.074 for a transverse momentum range of the Λbetween 0.28 GeV/c and 0.86 GeV/c. The Λ polarization is consistent with zero.266 V. Fanti et al.: A measurement of the transverse polarization of Λ-hyperons at 450 GeV proton energy1 IntroductionThe polarization of  hyperons produced in inclusive re-actions with unpolarized protons on unpolarized targetshas been studied for more than 15 years. The polariza-tion has been studied over a wide range of reaction ener-gies and various production angles of the ’s. An experi-ment at the CERN ISR has obtained a signicant depen-dence of the polarization on the centre{of{mass energyfromps =53 GeV tops =62 GeV [1], while xed targetexperiments have shown no evidence for such dependencein the beam energy region from 13.3 GeV to 800 GeV [2{7]. The absolute value of the polarization has been foundto grow approximately linearly with pt and xF at least inthe region of pt < 1 GeV/c and xF < 0.5, where pt andxF are the transverse momentum and the fraction of thebeam energy carried by the , respectively.Some theoretical models attempt to describe experi-mental data on polarization [8{10] but the polarizationmechanism is still not understood.In this letter we present a measurement of the polar-ization of ’s produced by protons of 450 GeV in inclusivepN reactions at an angle of 4.2 mrad at the CERN SPS,using part of the NA48 detector [11{14].2 Experimental setup and data takingThe neutral beam containing the observed  hyperonswas produced on a 2 mm diameter, 400 mm long beryl-lium target by 450 GeV protons at a production angle of4.2 mrad in the vertical plane [13,14]. These protons are1 Present address: Department of Physics, Wayne State Uni-versity, Detroit, MI 48201, USA2 Funded by the UK Particle Physics and Astronomy ResearchCouncil3 Permanent address, Dipartimento di Fisica Sperimentaledell’Universita e Sezione dell’INFN di Torino, I-10125 Torino,Italy4 Present address: Cavendish Laboratory, University of Cam-bridge, Cambridge CB3 0HE, UK5 Funded by the UK Particle Physics and Astronomy ResearchCouncil6 Present address CERN, CH-1211 Geneva 23, Switzerland7 Present address: Institut fu¨r Physik, Universita¨t DortmundD-44221 Dortmund, Germany8 Present address: SLAC, Stanford, CA 94309, USA9 Funded by the German Federal Minister for Research andTechnology (BMBF) under contract 7MZ18P(4)-TP2, Ger-many10 Permanent Address: Departamento de Fsica, Universidadde los Andes, Merida 5101-A, Venezuela11 Funded by Institut National de Physique des Particules etde Physique Nucleaire (IN2P3), France12 Present address NIKHEF, 1009 DB Amsterdam, TheNetherlands13 Funded by the German Federal Minister for Research andTechnology (BMBF) under contract 056SI74, Germany14 Present address Dept. of Physics, Northwestern University,Evanston, IL, USAsplit o from a high intensity SPS beam (41011 protonsper spill) by channeling through a bent crystal [15], thentransported and focussed by a series of four quadrupolemagnets and deflected by two dipole magnets onto thetarget1. The target is followed by a 7.5 Tm dipole sweep-ing magnet (with magnetic eld directed horizontally andperpendicular to the beam axis), packed with tungsten-alloy inserts (in which the protons not interacting in thetarget are absorbed) and a 1.5 m long collimator reachingfrom 4.5 to 6.0 m downstream of the target, containing atapered hole. The beam dening aperture has a diameterof 3.6 mm at 4.8 m from the target. The exit of the col-limator is covered by a vetocounter with a 11 mm thicklead converter in front. It is followed by a 89 m long evac-uated tank and by a helium-lled tank which contains themagnetic spectrometer. Decays occurring in the upstreampart of the vacuum tank are accepted. The outer diameterof each of the detectors is typically 2.5 m.The analysis is based on data from a 40 h run in 1995with a proton beam intensity of about 1 107 ppp hittingthe target during the 2.4 s long SPS spill every 14.4 s.The detector elements used for the polarization analy-sis were: A magnetic spectrometer with two drift chambers infront and one behind a dipole magnet that producesa 267 MeV/c transverse momentum kick; each driftchamber is composed of four double planes with stag-gered wires to resolve left-right ambiguities; the wireorientations in the four views are horizontal, verticaland at 45 with respect to the horizontal plane; theresolution of a coordinate in a single drift chamber is110 m and the mean momentum resolution of thethree-chamber spectrometer is p=p = 0:6%, givinga resolution on the  invariant mass from  ! p−decays of 1.16 MeV/c2; A sampling calorimeter made of iron plates and scin-tillator planes designed to measure hadronic showerswith a readout in horizontal and vertical projections;a fast energy sum from the calorimeter is used in thetrigger; A scintillator hodoscope located in front of the cal-orimeter to trigger on charged particles; A muon veto system consisting of three planes of plas-tic scintillator each preceded by an 80 cm thick ironwall.A more complete description of the apparatus can befound elsewhere [11,12].3 Event selectionData for the polarization measurement were recorded witha two charged particle trigger. All events selected by thetrigger should have at least two hits in two diagonally1 The possibility of the protons being slightly polarized (<5 %) by their passage through the bent crystal of silicon hasbeen considered in [16]V. Fanti et al.: A measurement of the transverse polarization of Λ-hyperons at 450 GeV proton energy 267Fig. 1. Invariant mass distribution for Λ ! ppi− hypothesisopposite quadrants of the scintillator hodoscope, the en-ergy deposit in the calorimeter had to be greater than32 GeV, and both the vetocounter in the beam and themuon counters have been used in anticoincidence. Aftera full reconstruction of the events, the requirement thateach event contain at least two charged tracks led to asample of 4:1  105 events.To be sure that the same trigger conditions have beenrequired for data and Monte Carlo, a trigger requirementwith more severe cuts has been used for the analysis. Inthis simulation exactly two reconstructed tracks hit thehodoscope in two diagonally opposite quadrants and theenergy in the calorimeter was greater than 35 GeV. Thefollowing cuts have been applied to select the  ! p−, ! p+ and KS ! +− decay candidates:{ The selected events must have exactly two tracks (onepositive and one negative) with a reconstructed vertexin the interval from 6.07 m to 40 m downstream thetarget;{ The two tracks have to meet within 2.0 cm (closestdistance of approach);{ The momentum of each track has to be greater than10 GeV/c and the total momentum of both tracks hasto be between 50 GeV/c and 200 GeV/c;{ The angle between the axis of the neutral beam andthe total momentum vector of two tracks has to beless than 0.6 mrad to reject events coming from thecollimator.To select events consistent with a ,  or KS decaythe invariant mass of the two tracks was reconstructed un-der hypotheses of the decay into p− (see Fig. 1), p+ or+− , respectively. Events have been selected as() ! p−(p+) decays if the invariant mass was within3.5 MeV/c2 of the nominal () mass. KS ! +−events have been selected if the invariant mass was within10 MeV/c2 of the nominal KS mass.  ! p+ decayswhich satisfy also the KS mass hypothesis have been re-jected to suppress background from KS decays in the data set. Events satisfying the  selection criteria havebeen rejected from the KS sample.−0.5 0 0.5 −0.5 0 0.5 −0.5 0 0.5225002000017500150001250010007500500025001.21.151.11.0510.950.90.850.8cos coscos x y z()Fig. 2. W (cosθi) (points), A(cosθi) (histogram) and R(cosθi)distributions (lower half) for i = x, y, zThe number of  ! p+ events remaining after thesecuts is 88039; in addition 8810  ! p+ events have beenselected. 219309 KS ! +− events have been selectedto study detector geometry and systematic errors.4 Polarization analysis and resultsThe coordinate system is dened for each event as follows.The horizontal n^x axis is a normal to the  productionplane, n^x = −!pin  −!p=j−!pin  −!pj, where −!pin and −!p are themomentum vectors of the incident proton and the pro-duced , respectively. The n^z axis is along the directionof the produced , n^z = −!p=j−!pj; and the n^y axis is chosento form a right-handed coordinate system, n^y = n^z  n^x.The  polarization −!P (Px; Py; Pz) is determined by the an-gular distribution of the decay proton in the  rest frame,W (cosi) = A(cosi)  (1 + Picosi) = (1)A(cosi)  R(cosi);i = x; y; z;where cosi = n^i  k^ (k^ is the direction vector of the decayproton),  = 0.642 is the  -decay asymmetry parameter[17], and A(cosi) is the acceptance of the apparatus whichhas been determined with a Monte Carlo simulation. Thepolarization components Px; Py and Pz correspond to aleft-right decay asymmetry, an up-down decay asymmetryand a forward-backward asymmetry, respectively. Accord-ing to parity conservation in the strong interaction, Py andPz should be equal zero if the incident proton beam wasunpolarized. Thus the measurements of Py and Pz wereused for checking the systematic errors.The W (cosi) distributions together with the A(cosi)(histogram) distributions for i = x; y; z are shown in Fig. 2.268 V. Fanti et al.: A measurement of the transverse polarization of Λ-hyperons at 450 GeV proton energyTable 1. The mean values of pt, xF and Px measured in corre-sponding pt intervals (systematic errors are presented followingthe statistical errors)pt interval pt xF Px[GeV/c] [GeV/c]0.2 - 0.3 0.28  0.02 0.13  0.01 −0.053  0.034+0.001−0.0190.3 - 0.4 0.36  0.03 0.16  0.01 −0.066  0.018+0.007−0.0370.4 - 0.5 0.45  0.03 0.21  0.01 −0.114  0.017+0.006−0.0210.5 - 0.6 0.54  0.03 0.25  0.01 −0.155  0.025+0.010−0.0350.6 - 0.7 0.64  0.03 0.30  0.01 −0.208  0.035+0.015−0.0390.7 - 1.0 0.86  0.05 0.37  0.02 −0.298  0.074+0.026−0.077The acceptance corrected distribution R(cosi) is tted bya straight line to give the polarizationPx = −0:109  0:012; (2)averaged over the experimental pt and xF ranges. Thepolarization measurements in the y and z directions, asexpected, are consistent with zero: Py = 0:006  0:012and Pz = 0:007  0:011. In order to have an independentestimate of the systematic error on the polarization, thecosx and cosy distributions of the KS sample have beenanalysed. Fits to the acceptance corrected distributionsR(cosi) give the polarizations Px = 0:010  0:009 andPy = 0:003  0:009 which are consistent with zero. Themean of the errors on Pz and Py for the  decays,  0.011,has been taken as a measure of the systematic error in Px(2). This error is consistent with those obtained for Pxand Py for the KS , where the physics polarization mustbe zero due to the kaon being a spinless particle.The kinematic dependence of the polarization has beenstudied as a function of the transverse momentum pt of the relative to the proton beam line. Table 1 and Fig. 3 sum-marise the polarization results with statistical and system-atic errors. The polarization systematic errors presented inTable 1 are the root mean square sums obtained by vary-ing limits of the cuts used in the analysis. The combinederrors are given by the error bars in Fig. 3.The background in the  sample consisting of 0.8%misidentied KS ! +− events has no signicant con-tribution to the measured polarization.Finally, no polarization was observed for the selected events:Px = −0:014  0:037.Because of the xed  production angle, a denite xFvalue corresponds to each chosen pt. Thus the polariza-tion behavior presented in Fig. 3 reflects both pt { andxF { dependences. Some other xed target experiment re-sults [3,5,7] are presented in the same plot. They havebeen obtained at dierent beam energies and productionangles of ’s and consequently, represent dierent regionsof xF at the same pt interval. Our data indicate a steeperdependence of the polarization on pt than do the otherexperiments.Fig. 3. Polarization versus pt of the Λ samples involving bothdirect and indirect production (the statistical and systematicerrors are added in quadrature for this experiment)A fraction of the ’s detected in all experiments comesfrom other hyperon decays. They are produced in, for ex-ample, decays like 0 ! 0 and 0 ! γ. Due to theexperimental problems in distinguishing direct and indi-rect production, the polarization results are averaged overboth production processes. To estimate the polarizationof the directly produced ’s, the polarization and rate ofthe indirectly produced  hyperons have to be known.’s produced from 0 ! 0 decays have negativepolarization like the 0 hyperon itself [18]. An analysissimilar to that presented here [19] found 848 0 ! 0decays giving an impurity of (5.4  0.5)% in the  sampledue to this decay. This does not signicantly change themeasured polarization.A theoretical calculation of the polarization of ’s orig-inating from 0 ! γ decays gives a value related to the0 polarization, P = −1=3  P0 , which leads to a smallpositive  polarization [10]. Taking into account the rateof such ’s (39  4 %) measured at lower energies at BNL[20], it can be estimated that the absolute value of thepolarization of directly produced ’s is about 25% greaterthan the measured polarization in the  beam.In spite of the fact that theoretical models [8{10] canbe adjusted to explain large polarizations, a deeper under-standing of the  polarization phenomenon is still lacking.Acknowledgements. We would like to thank the technical staof the participating Laboratories, Universities and aliatedcomputing centres for their eorts in the construction of theNA48 detector, their help in the rst year of operation of theexperiment, and their help in the processing of the data.V. Fanti et al.: A measurement of the transverse polarization of Λ-hyperons at 450 GeV proton energy 269References1. S. Erhan et al., Phys. Lett. B 82, 301 19792. G. Bunce et al., Phys. Rev. Lett. 36, 1113 19763. K. Heller et al., Phys. Rev. Lett. 41, 607 19784. F. Abe et al., Phys. Rev. Lett. 50, 1102 19835. B. Lundberg et al., Phys. Rev. D 40, 3557 19896. B. E. Bonner et al., Phys. Rev. D 38, 729 19887. E. J. Ramberg et al., Phys. Lett. B 338, 403 19948. B. Andersson et al., Phys. Lett. B 85, 417 19799. J. Szwed, Phys. Lett. B 105, 403 198110. T. De Grand, H. Miettinen, Phys. Rev. D 24, 2419 198111. M. Holder, Status of the NA48 experiment, Proceedings ofWorkshop on Kaon Physics, Orsay, ed. by L.Iconomidou{Fayard, Editions Frontieres, p.77, 199612. I. Augustin, The NA48 experiment for the measurementof 0/, Proceedings of the 28th International Conferenceon High Energy Physics, Warsaw, Poland p.1679, 199613. C. Biino, N. Doble, L. Gatignon, P. Grafstro¨m, H. Wahl,The Simultaneous Long- and Short- Lived Neutral KaonBeams for Experiment NA48, Internal report: CERN-SL-98-033-EA, (to be published with a discription of theNA48 detector), 199814. C. Biino, N. Doble, L. Gatignon, P. Grafstro¨m, H. Wahl(presented at EPAC-98, Stockholm, 22{26 June, 1998)Preprint, CERN SL-98-041-EA, 199815. N. Doble, L. Gatignon, P. Grafstro¨m, Nucl. Instr. andMeth. B 119, 181 199616. V.M. Biryukov, V.A. Maisheev, V.M. Mikhailov, S.B. Nu-rushev, IHEP 91{38, Protvino, 199117. Review of Particle Properties, Phys. Rev. D 54, 1 199618. K. Heller et al., Phys. Rev. Lett. 51, 2025 198319. R. Wilhelm, Ph. D. Thesis: Messung der transversalenPolarization von Λ{Hyperonen aus inelastischen pN{Reaktionen bei 450 GeV Protonenergie; Institut fu¨rPhysik, Universita¨t Mainz, Mainz Germany, 199720. M. Sullivan et al., Phys. Rev. D 36, 674 1987

A measurement of the transverse polarization of Lambda-hyperons produced in inelastic pN-reactions at 450 GeV proton energy

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A measurement of the transverse polarization of Lambda-hyperons produced in inelastic pN-reactions at 450 GeV proton energy

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