$\sum ^{0}$ production in proton nucleus collisions near threshold

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

 ⁰ production in proton nucleus collisions near threshold

HADES Collaboration

J. Adamczewski-Musch

^d

, G. Agakishiev

^g

, O. Arnold

^i,j

, E.T. Atomssa

^o

, C. Behnke

^h

, J.C. Berger-Chen

^i,j

, J. Biernat

^c

, A. Blanco

^b

, C. Blume

^h

, M. Böhmer

^j

, P. Bordalo

^b

, S. Chernenko

^g

, C. Deveaux

^k

, A. Dybczak

^c

, E. Epple

^i,j

, L. Fabbietti

^i,j,∗

, O. Fateev

^g

,

P. Fonte

^b,1

, C. Franco

^b

, J. Friese

^j

, I. Fröhlich

^h

, T. Galatyuk

^e,2

, J.A. Garzón

^q

, R. Gernhäuser

^j

, K. Gill

^h

, M. Golubeva

^l

, F. Guber

^l

, M. Gumberidze

^e,2

, S. Harabasz

^c,e

, T. Hennino

^o

,

S. Hlavac

^a

, C. Höhne

^k

, R. Holzmann

^d

, A. Ierusalimov

^g

, A. Ivashkin

^l

, M. Jurkovic

^j

,

B. Kämpfer

^f,3

, T. Karavicheva

^l

, B. Kardan

^h

, I. Koenig

^d

, W. Koenig

^d

, B.W. Kolb

^d

, G. Korcyl

^c

, G. Kornakov

^e

, R. Kotte

^f

, A. Krása

^p

, E. Krebs

^h

, H. Kuc

^c,o

, A. Kugler

^p

, T. Kunz

^j,∗

,

A. Kurepin

^l

, A. Kurilkin

^g

, P. Kurilkin

^g

, V. Ladygin

^g

, R. Lalik

^i,j

, K. Lapidus

^i,j

, A. Lebedev

^m

, L. Lopes

^b

, M. Lorenz

^h

, T. Mahmoud

^k

, L. Maier

^j

, S. Maurus

^i,j

, A. Mangiarotti

^b

, J. Markert

^h

, V. Metag

^k

, J. Michel

^h

, C. Müntz

^h

, R. Münzer

^i,j

, L. Naumann

^f

, M. Palka

^c

, Y. Parpottas

^n,4

, V. Pechenov

^d

, O. Pechenova

^h

, V. Petousis

ⁿ

, J. Pietraszko

^d

, W. Przygoda

^c

, S. Ramos

^b

, B. Ramstein

^o

, L. Rehnisch

^h

, A. Reshetin

^l

, A. Rost

^e

, A. Rustamov

^h

, A. Sadovsky

^l

, P. Salabura

^c

, T. Scheib

^h

, K. Schmidt-Sommerfeld

^j

, H. Schuldes

^h

, P. Sellheim

^h

, J. Siebenson

^j

, L. Silva

^b

, Yu.G. Sobolev

^p

, S. Spataro

⁵

, H. Ströbele

^h

, J. Stroth

^h,d

,

P. Strzempek

^c

, C. Sturm

^d

, O. Svoboda

^p

, A. Tarantola

^h

, K. Teilab

^h

, P. Tlusty

^p

, M. Traxler

^d

, H. Tsertos

ⁿ

, T. Vasiliev

^g

, V. Wagner

^p

, C. Wendisch

^d

, J. Wirth

^i,j

, J. Wüstenfeld

^f

,

Y. Zanevsky

^g

, P. Zumbruch

^d

aInstituteofPhysics,SlovakAcademyofSciences,84228Bratislava,Slovakia

bLIP-LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,3004-516Coimbra,Portugal cSmoluchowskiInstituteofPhysics,JagiellonianUniversityof Cracow,30-059Kraków,Poland dGSIHelmholtzzentrumfürSchwerionenforschungGmbH,64291Darmstadt,Germany eTechnischeUniversitätDarmstadt,64289Darmstadt,Germany

fInstitutfürStrahlenphysik,Helmholtz-ZentrumDresden-Rossendorf,01314Dresden,Germany gJointInstituteofNuclearResearch,141980Dubna,Russia

hInstitutfürKernphysik,Goethe-Universität,60438Frankfurt,Germany iExcellenceCluster‘OriginandStructureoftheUniverse’,85748Garching,Germany jPhysikDepartmentE62,TechnischeUniversitätMünchen,85748Garching,Germany kII.PhysikalischesInstitut,JustusLiebigUniversitätGiessen,35392Giessen,Germany lInstituteforNuclearResearch,RussianAcademyofScience,117312Moscow,Russia mInstituteofTheoreticalandExperimentalPhysics,117218Moscow,Russia nDepartmentofPhysics,UniversityofCyprus,1678Nicosia,Cyprus

oInstitutdePhysiqueNucléaire(UMR8608),CNRS/IN2P3-UniversitéParisSud,F-91406OrsayCedex,France pNuclearPhysicsInstitute,AcademyofSciencesofCzechRepublic,25068Rez,CzechRepublic

qLabCAF.F.Física,Univ.deSantiagodeCompostela,15706SantiagodeCompostela,Spain

*

Correspondingauthors.

E-mailaddresses:Laura.Fabbietti@ph.tum.de(L. Fabbietti),Tobias.Kunz@tum.de(T. Kunz).

1 AlsoatISECCoimbra,Coimbra,Portugal.

2 AlsoatExtreMeMatterInstituteEMMI,64291 Darmstadt,Germany.

3 AlsoatTechnischeUniversitätDresden,01062 Dresden,Germany.

4 AlsoatFrederickUniversity,1036 Nicosia,Cyprus.

5 AlsoatDipartimentodiFisicaandINFN,UniversitàdiTorino,10125 Torino,Italy.

https://doi.org/10.1016/j.physletb.2018.02.043

0370-2693/©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received15November2017

Receivedinrevisedform17February2018 Accepted19February2018

Availableonline23February2018 Editor:D.F.Geesaman

Keywords:

Hyperons Strangeness Proton Nucleus

The production of⁰ baryons in the nuclearreaction p (3.5 GeV) + ^Nb (corresponding to √_s

N N = 3.18 GeV) is studiedwith the detectorset-up HADESatGSI, Darmstadt.⁰s wereidentified viathe decay ⁰→ γ with subsequent decays →^pπ⁻ in coincidence with a e⁺e⁻ pair from either external (γ →^{e+e−) or internal (Dalitz decay}γ^∗→^{e+e−) gamma} conversions. The differential ⁰ cross sectionintegratedoverthedetectoracceptance, i.e.therapidityinterval0.5<y<1.1,hasbeen extracted as σ_0=².3± (⁰.2)^stat±

+⁰.6

−⁰.6

sys

± (⁰.2)^normmb,yielding theinclusiveproductioncross sectioninfullphasespaceσ^total

⁰ =⁵.8± (⁰.5)^stat± +¹.4

−¹.4

sys

± (⁰.6)^norm± (¹.7)^extrapol mbbyaveraging overdifferentextrapolationmethods.Theall/⁰ ratiowithintheHADESacceptanceisequalto2.3± (0.2)^stat± (⁺₋⁰0^..⁶6)^sys.Theobtainedrapidityandmomentumdistributionsarecomparedtotransportmodel calculations.The⁰yieldagreeswiththestatisticalmodelofparticleproductioninnuclearreactions.

©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The study ofhyperon production in proton-induced collisions atbeamenergiesofafew GeVisimportantformanyopen ques- tions in the field of hadron physics. While several experimental results exist for  hyperons in p+^{p and p}+^{A reactions [1–6],} measurements of ⁰ production are scarce [4–6]. The dominant electromagneticdecay⁰→ +

γ

(BR≈^{100%)requirestheiden-} tificationofphotonswithEγ ^{80 MeV}coincident tothedetection of p

π

⁻ pairs from  decays.Our measurement is the first step towards gaining access tothe hyperon electromagneticform fac- tors [7]. Once the measurement of virtual photons in the Dalitz decay ⁰→ ^{e+e− (BR}<1%) is performed it can be separated from the decays involving a real photon and therefore provide complementary information on the nucleon and  baryon form factors[8].

Hadron collisions atenergies of a few GeV withhyperons in thefinalstate arealsosuitedtostudytheroleplayedbyinterme- diate hadronic resonances inthe strangeness productionprocess.

Indeed, non-strange resonances like N* and  have been found tocontribute significantly [9–13] via thechannels N^∗→ +^K+ and ⁺⁺→ (¹³⁸⁵)⁺+^{K+. In case of N*, up to seven reso-} nances with similar masses andwidths have been identified in- cludingtheoccurrenceofinterferenceeffectsamongthem[2,14].

Inthiscontext,thesimultaneousmeasurementofandhyper- ons becomes important tounderstand the interplaybetweenthe spin1/2 and3/2 statesoccurringinthestrongconversionprocess

+^N → +^{N. This process manifests itself as a peak struc-} tureontopofthesmooth+p invariant-massdistributionclose to the –N threshold and is known to be responsible for cusp effects [15].Hyperonproductioninnuclearreactionsgivesalsoac- cessto details of the hyperon–nucleoninteraction. The existence ofhypernucleiisargued asevidenceforan attractivepotential atrather large inter-baryon distances [16,17]. Theoretical models [18] tryingtodescribescatteringdata[19,20] withhyperonbeams postulate the presence of a repulsive core for the –N interac- tion.⁰ hypernuclei, ontheother hand,havenot beenobserved sofarduetodifficultiesimpliedbytheelectromagnetic⁰ decays andtherequirementoflargeacceptanceandhighresolutionelec- tromagneticcalorimeters.Sincealsoscatteringdataforhyperon beamsare scarce,constraintsonthe–Ninteractionare missing sofarandnewmeasurementsof⁰ productioninnucleartargets areessential.

Medium-energyheavy-ioncollisions producinghyperonsallow tostudytheirpropertieswithinadensebaryonicenvironment(up to

ρ

≈²−³

ρ

0)[21–24].Onequestionofinterestiswhethertheat- tractive–Ninteractioninvacuumoratnuclearsaturationmight

changeduetothepostulated appearance ofamoredominantre- pulsive coreat increased densities and short distances [25]. The questfordetailedinformationonsuchaspectsrequirestheknowl- edge of feed down effects from ⁰ productionand its corre- spondingbehaviourinbaryonicorevencoldnuclearmatter.

Experimental data forsimultaneous ⁰ andproduction are available for proton–protoncollisions either closeto the free NN productionthreshold(Eth=².518 GeVforandEth=².623 GeV for ⁰) [4,5] or at excess energies of ^{5 GeV and above [26].}

So far,nodataare availablefor⁰ hyperonsemerging frompro- ton+^{nucleuscollisionsystemsatfewGeVincidentbeamenergy.}

In this work we present the first measurement of ⁰ produc- tion in p + ^{Nb collisions atan incident kinetic beamenergy of} Ep=³.5 GeV.Ourpaperisorganisedasfollows.Insection 2,we describetheexperimentalset-up.Section3isdevotedto⁰ iden- tification andbackgroundsubtraction.Insection4themethodfor efficiencycorrection anddifferential analysisisshown. Insection 6theextractedcrosssectionsandyieldsarecomparedtodifferent models.Insections6wegiveasummaryandshortoutlook.

2. TheHADESexperiment

TheHigh-AcceptanceDi-ElectronSpectrometer(HADES)[27] lo- cated at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt (Germany) is an experimental facility for fixed target nuclear reactionstudies inthe fewGeVenergyregion. Thespec- trometer is dedicated to measure low-mass dielectrons originat- ing fromthedecayof vectormesons intheinvariant-mass range up to the φ massandoffers excellent identificationby means of charged hadrons such as pions, kaons and protons. The detector setup covers polar angles between 18^◦ to 85^◦ over almost the fullazimuthal rangedesignedtomatchthemid-rapidityregionof symmetric heavyioncollisions atE = ^{1–2 AGeV.A setofmulti-} wiredriftchamber(MDC)planesarrangedinasixfoldsegmented trapezoidal type structure,two layers in front andtwo behind a toroidal magnetic field, is used for charged-particle tracking and momentumreconstructionwithatypicalresolutionofp/p^3%.

An electromagneticshower detector(Pre-Shower) anda Time-Of- Flightscintillatorwall(TOFandTOFINO)buildtheMultiplicityand ElectronTriggerArray(META)detectorsystemusedforeventtrig- gerpurposes.Theenergyloss(dE/dx)signalsmeasuredintheTOF andMDCdetectorsareused forchargedparticleidentification. In addition,electrons andpositronsare identifiedovera largerange of momentawitha RingImagingCherenkov(RICH) detectorsur- roundingthetargetinanearlyfield-freeregion.

In the present experiment, a protonbeam accelerated by the SIS18 synchrotron to a kinetic energy of E_p=³.5 GeVhas been

Fig. 1. (Colour online.)Invariantmassdistributionofpπ⁻pairswithanadditional e⁺–e⁻pairinthesameevent.Thedashedcurveshowsacombinationofapolyno- mialbackgroundfitandagaussianfitappliedtothesignalarea. Inset: Four-particle invariantmassdistributionofaproton,pionanddielectronforpπ⁻pairsinthe signalregion.Measuredsignal(blackcrosses),combinatorialbackground(redhis- togram)andextractednetsignal(grayline)areshownincomparisontoaUrQMD simulation(orangehistogram)withscaled⁰production.

directed ona twelve-fold segmented ⁹³Nb target of2.8% nuclear interactionprobability.Fora TOF+^{TOFINOreactiontriggersetting} ofmultiplicity M≥^{3 and at typical beam} intensities of 2×¹⁰⁶ particles/sontarget,atotalof3.2×^{109eventshavebeenrecorded} andanalysed.

3. ⁰identificationandbackgroundsubtraction

Theidentification of⁰ hyperons was achievedvia thedecay channel⁰→ 

γ

(BR≈ ^100%[28])byreconstructing→^p

π

⁻

decays correlated with the emission of a dielectron from exter- nal pair conversion

γ

→^{e+e− or from the Dalitz decay} ⁰→

e⁺e⁻(BR<5·^{10−3[28]).Fig.1depictsthep}

π

⁻invariant-mass distribution of such events with a clear signature of a  con- tent in the data sample. Due to the low mass difference m_0− m≈^{77 MeV/c2 a} considerablefraction ofcoincident e^± candi- dateshavemomentabelowthespectrometeracceptancethreshold p_thr≈^{50 MeV/cneededforfulltrackandmomentum}reconstruc- tion.Forthisreason,dielectronshavebeenidentifiedbyrequiring twoRICHrings, atleastone fullyreconstructede^± trackandone neighbouringincompletetrackletdetectedinfrontofthemagnetic field in the first two MDCs. The missing momentum of the in- completetracklethasbeenestimatedbyapplyingamostprobable hypothesisasdescribedindetailin[29] which partlyexploitsre- sultsandconstraintsfromkinematically similar

π

⁰ Dalitzdecays.

Inthisway,theobservedincompletedielectronsarecombinedinto mostprobablephoton signalswitha resolutionofδ_Eγ(FWHM)= 57±^{2 MeV[29].}

Thecombinatorial backgroundhas beendetermined with two approaches. First, background yield and shape have been esti- mated from polynomial fits of the p

π

⁻ invariant mass in the sideband regions below and above the  peak, 1090 MeV/c²<

m^pπ⁻

inv <1105 MeV/c² and 1125 MeV/c²<m^pπ⁻

inv <1140 MeV/c² (see Fig. 1). The second approach aimed atthe suppression of a random peak structure. The momentum of the proton and pion wassmearedby2%such thattheresultinginvariantmassofpro- tonandpiondidnotshowanypeak.The obtaineddistribution was scaledto the sidebandofthe unsmeareddistributionshown

in Fig. 1 to evaluate the background in the signal region. Af- terweighting andnormalisation, both methodslead to the same backgroundyieldwithin 10%.Thesideband samplesusedtocon- structthe p

π

⁻ backgroundare combinedwiththereconstructed e^±pairstoobtainthebackgroundtothe⁰candidates(fordetails see[29]).

The inclusive four-particle p

π

⁻e^± invariant mass distribution isshownintheinsetofFig.1.Apeakstructurebecomesapparent at the ⁰ polemass with a width (FWHM) of 52±^{22 MeV/c2.} Theobserved FWHMismainly attributedtotheresolution ofthe

γ

reconstruction. Theestimated backgroundisshownby thered histogram.

FullscaleUrQMD [30,31] simulationshavebeencarriedoutand processedthrough Geantanda digitisation procedureto emulate the detector response. Subsequently the events have then been analysed in thesamemannerastheexperimental data.Then the simulationhasbeennormalisedtothe⁰yield.TheinsetinFig.1 shows that the simulated ⁰ mass distribution is in agreement with the measured distribution. A total of N_0=²²⁴±^24stat± 65^sys ⁰ candidateshasbeenextracted.

4. Efficiencycorrectionanddifferentialanalysis

After background subtraction, a differential analysis has been performed for the kinematic variables transverse momentum pt and rapidity y of the ⁰ candidates. Due to the limited event statistics, theexperimental yields are computedforthree equally spaced momentum bins between 240 MeV/c≤^pt ≤^{960 MeV/c} split in two rapidity bins 0.5< y < 0.8 and 0.8< y <1.1.

The acceptance and efficiency correction matrix for this phase space region has been obtained from simulations utilizing the UrQMD/Geant3 data set (see above) before and after ⁰ recon- struction. The systematic errors of these corrections stem from various sources.The uncertainty onparticle identificationofpro- tonsandpionsof^{5% isadoptedfromthehighstatisticsanalysis} ofinclusiveproduction[3].Theoveralluncertaintyforidentifi- cationoflow momentum e⁺/e⁻ partnersandpairreconstruction with two complete tracks is ^{25% as deduced in a previous} searchfordarkphotonswithhypotheticalmassesintheinterval 50–100 MeV[32].Theerrorinthebackgroundsubtractionisesti- matedfromacomparisonofthetwomethodsdescribedaboveand contributeswith^{8%. Other sources areof order10−2 andless.}

Thequadraticsumresultsinatotalsystematicerrorof≈^30%.The statisticalerrorsdidnotexceedvaluesof^10–30%.

The corrected reduced transverse-mass spectra (with mt =



p²_t +^m2_0) forthe⁰ candidates areshowninFig.2separately for both rapidity intervals. Towards smaller transverse momenta, the geometrical spectrometer acceptance does not cover the full region for at least one of the decay partners  or

γ

. To ex- trapolate to uncovered phase space regions we have assumed a thermal ⁰ phase space production. Hence, the differential dis- tributions have been fitted with a Maxwell–Boltzmann distribu- tion(1/m_t²)(d²N/(dmtdy))=^A(y)·^exp(−((^mt−^m⁰)c²)/(TB(y)), where A(y) is a rapidity dependent scaling factor and m_0 = 1192.642±⁰.024 MeV/c²[28].Theinverse-slopeparametersT_B= 82±^{23 MeV for the rapidity bin 0}.5<y<0.8 and T_B =⁷⁸± 22 MeV for themore forward region 0.8<y<1.1 can be com- paredwiththeaveragevalueof84MeVextractedforhyperons inthesamereaction[3].

Theexperimentalrapidity–densitydistributionsdN/dyobtained forbothhyperonsfromintegrationofthecorrespondingMaxwell–

Boltzmann distributions with the given parameters are depicted in the upper panel of Fig. 3. The calculation of minimum-bias multiplicitiesrequiresnormalisationofthe observedyieldsto the

Fig. 2. (Colour online.)Reducedtransversemassdistributionsof⁰scorrectedfor acceptanceanddetectionefficiency.Thedataareplottedfortworapiditybins.The reddashedlinesindicateMaxwell–Boltzmannfits.Seetextfordetails.

Fig. 3. (Colour online.) Top: Experimentalrapidity–densitydistributionsof(black) and⁰ (blue)hyperons.Thedistribution [3] referstoallexperimentallyidenti- fieds.Theshadedbandsdenotethesystematicerrors.Thedottedlinesrepresent modelcalculationsscaledtomatchthemeasured⁰yield(seetext). Bottom: The unscaledratioall/⁰.Colour andlinecodesasintoppanel.

total numberof reactions which we obtainedby multiplying the numberofM3 triggers(charged particlemultiplicity ≥^{3) witha} correction factorC. The latterhas beenextracted froma UrQMD simulation of the p+^{Nb reaction with impact parameters in the} range 0–8 fm and full Geant3 propagation of the events yield- ing C=¹/R^M3_{T rigger}^→M1 with R^M3_{T rigger}^→M1=⁰.58±⁰.06.Summationover bothrapiditybinsinFig.3givesthemultiplicityinsidetheaccep- tanceN_0= (².7± (⁰.2)^stat±

+0.7

−0.7

sys

± (⁰.2)^norm)×^{10−3/evt. and} Nall= (⁶.1± (⁺₋⁰0^..³3)^sys± (⁰.8)^norm×^{10−3/evt. Notethat the N}all

signal includes the feed down from heavier resonances, mainly from⁰ decays.Theproductionratioinsidetheacceptance0.5<

y<1.1 isfoundtobeall/⁰=².3± (⁰.2)^stat±

+⁰.6

−⁰.6

sys .

Table 1

Total⁰yieldsandcrosssectionsafterextrapolationunderthreeassumptions.

Shape ⁰yield per event σ_^total0 [mb]

-like 5.2×10⁻³ 4.4±0.4^stat±1.1^sys±0.5^norm GiBUU 7.3×10⁻³ 6.2±0.5^stat±1.5^sys±0.6^norm UrQMD 8.6×10⁻³ 7.3±0.6^stat±1.8^sys±0.8^norm

5. Crosssectionsandcomparisontomodels

The production crosssection hasthen been obtainedby mul- tiplying the multiplicity with the total interaction cross section

σ

pNb=⁸⁴⁸±^{126 mb for the p} + ^{Nb reaction} [33,34] and cor- rectingitforthetriggerbias.Theacceptanceintegratedcrosssec- tion 

σ

_0 which can be obtained from the experimental count ratesbymultiplicationwiththeluminosityisfoundtobeequalto



σ

_0=².3± (⁰.2)^stat±

+⁰.6

−⁰.6

sys

± (⁰.2)^normmbwithintherapid- ityinterval0.5<y<1.1.

Extrapolation to the uncovered rapidity region and extraction of an estimate for the total production cross section have been deduced with thehelp of transport model calculations. We have extracted ⁰ rapidity distributions fromUrQMD [30] and GiBUU [35,36] event generators and normalised them to match the ex- perimentaldatapoints.Thedistributions areplottedinFig.3and exhibit considerabledifferences. Those possibly indicate different weightsinthemodelsfortheimplementationoftheslowingdown of the⁰ which are initially producedat therapidity of theNN centre-of-mass system. While the data are well reproduced by UrQMD in the region above y>0.4, the extrapolation to target rapiditiesseemstobeambiguous.Undertheassumptionthatboth hyperonsexperiencecomparableemissionkinematicsduetotheir verysimilarmasseswecanprofitfromthelargerrapiditycoverage andsmaller binsizes ofthe reconstructed . Hence,asan alter- native guidance we have used the measured  rapidity density distribution (-like) aspublished in[3] and normalised itto the

⁰ distribution.Forcomparison, theresultingtotal⁰ yieldsand extrapolated productioncross sectionsof thescaled distributions arelistedinTable1.

The ⁰ production cross section has finally been calculated from amean ofthe -likeand UrQMDrapidity distributions re- sulting in

σ

_p^tot_+Nb(⁰)=⁵.8± (⁰.5)^stat ±

+¹.4

−¹.4

sys

± (⁰.6)^norm± (1.7)^extrapol mb. A ⁰ yield of N_0= (⁷±³)×^{10−3/evt for the} full phase space has been extracted in the same way. The ratio

all/⁰=².3±(⁰.2)^stat±(⁺₋⁰0^..⁷7)^sys±(⁰.7)^extrapol hasbeenobtained byusingtheratiowithintheacceptanceandanadditionalextrap- olationuncertaintystemmingfromthedifferencebetweenUrQMD and -like extrapolation methods. This can be justified by the rather flat distribution of experimental data as well as for the UrQMD and GiBUU simulations. The error on the extrapolation procedure introduces the largest uncertainty. The statistical and systematicerrorshavebeenaddedquadratically.Fig. 4showsour resultforthe totalnumber(i.e.,full phase spaceextrapolated)of

s not stemming from⁰ decays(that isthe numberof identi- fied s minusthe numberof s identified asdecayproducts of

⁰s)dividedbythenumberof⁰s, R=¹.3±⁰.6,togetherwith a compilation of the world data [4–6,26,37] and a data fit [37]

plottedasafunction ofexcessenergyabovethenucleon–nucleon threshold.TheresultsfromUrQMDare shownforcomparison. All datapointsbuttwostemfromproton–protoncollisions.Ourresult fortheproductionin aheavy nucleus(heavy bulletinFig.4) fits welltothesystematicsandmodelpredictions.Inthiscomparison, the multi-stepinteractionofthe ⁰ withone, two orevenmore nucleonshasbeenneglectedaswellastheFermimotion.

Fig. 4. (Colour online.)Experimentalexcitationfunctionof/⁰productioncross sectionratiosfromexclusivemeasurementsofσ(pp→p K)andσ(pp→p K⁰) reactions.The excessenergyaboveproduction thresholdreferstofree nucleon–

nucleoncollisions.Data(symbols)fromBNL[6],COSY[4,5,37],LB[26] andpresent work.Thethincurveisafitfrom[37].ThedottedandsolidcurvesexhibitUrQMD simulations.Fermimotionhasbeenneglected forp+^Acollisions.

Fig. 5. (Colour online.)ExperimentalhadronyieldsmeasuredbyHADES[39] incom- parisontoaTHERMUSstatisticalmodelfitw/o⁰.

We now compare our findings to the statistical model THER- MUS [38]. In this model, the total particle abundances strictly follow a distribution expected from hadron freeze-out at condi- tionsdeterminedbya temperatureT_f.o. andabaryochemicalpo- tential

μ

f.o.. Forthisscenario, particleyields are proportional to

e^(E−μf .o.)/T_{f .o.}. A THERMUS fit to measured particle yields [39],

excluding the ⁰, gives parameter values T_f.o.=^{100 MeV and}

μ

f.o.=^{620 MeV. For these parameters (see legend in Fig. 5),} the expected ⁰ yield slightly underestimates (1.5

σ

) the in- clusive experimental value presented in this work. Fig. 5 shows the corresponding THERMUS fit results. The THERMUS yield ra- tioall/⁰=³.9 isslightlyhigherthan that predictedbyGiBUU, UrQMD(R^{3) andour}measurement(R².3).Nevertheless,the overall agreement is surprising for proton induced nuclear colli- sionsatrelativelylowenergies,asalreadydiscussedin[39].

6. Summaryandoutlook

We havedemonstratedthecapability ofHADESto reconstruct the low energy

γ

→^{e+e− conversion processes in the detector} material via the identification of electrons and positrons. With this technique we were able to measure for the first time ⁰ hyperon production in proton-induced reactions off a heavy nu- cleusnearthreshold.Weprovide transversemassdistributions in two rapidity bins. Based on them, a ⁰ production cross sec- tion of

σ

p+Nb(⁰)=⁵.8 ± ².3 mb has been determined. The inclusive light hyperon production ratio is all/⁰=².3 ± ¹.1.

All uncertainties have been summed up quadratically. These ex- perimentalvaluescompare reasonablywell withtransport model calculationsandresultsfromastatisticalhadronisationscheme.In spiteofthelimitedspectrometeracceptancetheobtainedrelative productioncrosssectionsmayhinttoa slightlylarger production probability in nuclei as compared to expectations from proton–

protoncollisions, ^_⁰|p A>^_⁰|pp. Apossible measurementwitha lowmagneticfieldwillallowfullreconstructionofthedielectrons and therefore offer the possibility to determine electromagnetic transitionformfactors.Thecurrentlyongoing upgradeincludesan electromagnetic calorimeter which will significantly enhance the

γ

detection capabilities of HADES. Measurements able to sepa- ratethecontributionofp–pandp–nreactionsareplanned,which gobeyondtheaverage valuesextractednow fromproton-nucleus collisions.Thisopensuptheinvestigationofreactionchannelsin- volvingphotondecaysofhyperonsandother baryonicresonances produced in proton/pion–proton, proton/pion-nucleus andheavy- ioncollisionsandmightevengiveaccesstomeasurementsofelec- tromagnetictransitionformfactorsfortheseresonances.

Acknowledgements

The HADES collaboration gratefully acknowledges the sup- port by the grants VH-NG-823, TU Darmstadt (Germany);

BMBF05P15WOFCA,DFGEClust153,MLL,TUMünchen(Germany);

BMBF05P12RGGHM,JLU Giessen (Germany); CNRS/IN2P3,IPN Or- say (France);GACR13-06759S,MSMT LM2015049, Rez (CzechRe- public);BMBF05P15PXFCA,GSIWKAMPE1416,BUWuppertal(Ger- many); NCN2013/10/M/ST2/00042 (Poland); NSC 2016/23/P/ST2/

04066POLONEZ(Poland).

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