Measurement of the invariant mass distributions for the pp -> ppeta' reaction at excess energy of Q = 16.4 MeV

0.16 MB·English

by  P. Klaja

Measurement of the invariant mass distributions for the
pp ppη reaction at excess energy of Q = 16.4 MeV
′
→
P.Klajaa,b,c,P.Moskala,b,E.Czerwin´skia,b,R.Czyz˙ykiewicza,A.Deloffd,D.Gila,D.Grzonkab,
L.Jarczyka,B.Kamysa,A.Khoukaze,J.Klajaa,b,K.Nakayamaf,W.Oelertb,J.Ritmanb,
T.Sefzickb,M.Siemaszkog,M.Silarskia,J.Smyrskia,A.Ta¨schnere,M.Wolkeb,J.Zdebika,
M.Zielin´skia,b,W.Zipperg
0 aInstituteofPhysics,JagellonianUniversity,PL-30-059Cracow,Poland
1 bInstitutfu¨rKernphysik,ForschungszentrumJu¨lich,D-52425Ju¨lich,Germany
0 cPhysikalischesInstitut,Universita¨tErlangen–Nu¨rnberg,D-91058Erlangen,Germany
2 dInstituteforNuclearStudies,PL-00-681Warsaw,Poland
eInstitutfu¨rKernphysik,Westfa¨lischeWilhelms–Universita¨t,D-48149Mu¨nster,Germany

n fDepartmentofPhysics,UniversityofGeorgia,GA-30602Athens,USA
a gInstituteofPhysics,UniversityofSilesia,PL-40-007Katowice,Poland
J

8
2

]
x Abstract
e
Theproton-protonandproton-η invariantmassdistributionshavebeendeterminedforthepp
- ′
l ppη reactionatanexcessenergyofQ=16.4MeV.Themeasurementwascarriedoutusingt→he
c ′
u COSY-11detectorsetupandtheprotonbeamofthecoolersynchrotronCOSY.Theshapesofthe
n determinedinvariantmassdistributionsaresimilartothoseofthe pp ppηreactionandreveal
[ →
anenhancementforlargerelativeproton-protonmomenta.Thisresult,togetherwiththefactthat

1 theproton-ηinteractionismuchstrongerthattheproton-η′interaction,excludesthehypothesis
v thattheobservedenhancementiscausedbytheinteractionbetweentheprotonandthemeson.
4
7 Keywords: pseudoscalarmesons,differentialdistributions,nearthresholdmesonproduction,
1 interaction
5
PACS:13.60,13.75.-n,14.40.-n,25.40.-h
.
1
0
0
1
1. Introduction
:
v
i Theunderstandingofthemeson-nucleoninteractionaswellasstudiesofmesonstructureand
X
productionmechanismsconstituteone ofthe basic issuesof the contemporaryhadronphysics.
r
a The η andη mesonsconstitutea mixtureof the SU(3)singletand octetstates with almostthe
′
samerelativecontributionsofvariousquarkflavours. Nevertheless,theyhaveunexpectedlydif-
ferentproperties,e.g.asregardsthemass[1],branchingratios[2,3]orproductioncrosssections
[4]. Thesedifferencesindicatethatalsotheinteractionofηandη mesonswithnucleonsmaybe
′
different.
Up to now, the proton-η interaction was studied intensively but still rather large range of val-
ues of the scattering length is reported depending on the analysis method [5]. The proton-η
′
Emailaddresses:[email protected](P.Klaja),[email protected](P.Moskal)
PreprintsubmittedtoElsevier January28,2010
interaction is much less known. It is only qualitatively estimated (based on the pp ppη
′
→
excitation function) to be much weaker than for the proton-η system [6]. In principle, stud-
ies of pp ppmeson reactions permit informationabout the proton-mesoninteraction to be
→
gained not only from the shape of the excitation function but also from differential distribu-
tionsofproton-protonandproton-mesoninvariantmasses. Therefore,inordertoinvestigatethe
proton-η interaction the COSY-11 collaboration performeda measurement [4] of the proton-η
and proton-protoninvariant mass distributions close to the threshold at Q = 15.5 MeV, where
the outgoing particles possess small relative velocities. Indeed a large enhancementin the re-
gionofsmallproton-ηandlargeproton-protonrelativemomentawasobserved1. However,the
observedeffectcannotbeunivocallyassignedtotheinfluenceoftheproton-ηinteractioninthe
finalstate [8, 9], since itcan also be explainedbythe admixtureof higherpartialwavesin the
proton-protonsystem[10],orbytheenergydependenceoftheproductionamplitude[11,12].
Theendeavortoexplaintheoriginoftheobservedenhancementmotivatedthemeasurement
of the proton-proton and proton-η invariant mass distributions for the pp ppη reaction
′ ′
→
presentedinthisarticle.
Iftheenhancementobservedintheproton-protoninvariantmassdistributionsforthe pp
→
ppηreactionwouldbeduetotheproton–ηinteractionthenitisexpectedtobesignificantlylower
forthe pp ppη reactionsincetheproton-ηinteractionisstrongerthantheproton-η [6].
′ ′
→
Inordertomakeamodelindependentcomparisonofthespectraintheppηandppη systems
′
weperformedameasurementofthepp ppη reaction,nominallyatthesameexcessenergyas
′
→
previouslymeasuredforthe pp ppηreaction. Invariantmassspectradeterminedatthesame
→
excessenergiesallowforthecomparisonwithoutaneedforacorrectionofkinematicalfactors
intheoutgoingsystem.
Itisimportanttostressthattheinvariantmassdistributionsforthe pp ppη reactionhave
′
→
not been measured so far. This is because the total cross section for the η meson production
′
in hadroncollisionsis by morethan a factor of 30 smaller thanthe onefor the η mesonat the
sameexcessenergyandadditionallythetotalcrosssectionfortheproductionofthemulti-pion
backgroundgrowsbythreeordersofmagnitudewhenthebeamenergyincreasesfromtheηto
theη productionthreshold[13]. Adeterminationoftheinvariantmassspectrareportedinthis
′
letterwasmadepossibleduetostochasticcoolingoftheprotonbeamoftheCOSYsynchrotron
[14,15,16,17]andthegoodmomentumresolution(σ=4MeV/c)[4]achievedwiththeCOSY-
11 detector setup designed especially for measurementsnear the kinematicalthreshold. Suffi-
cientstatisticswerecollectedtoallowthebackgroundtobesubtractedfromtheinvariantmass
distributions.
Detailed description of the COSY-11 method used for the pp ppη reaction measure-
′
→
mentscanbefoundin[18,19,20],therefore,hereweconcentrateonthebackgroundsubtraction
procedurecrucialforthedeterminationofinvariantmassdistributions.
2. Experimentanddataanalysis
The measurementof the pp ppη reaction was conductedusing the cooler synchrotron
′
→
COSY [14] and the COSY-11 detector setup [21, 22, 23] at the proton beam momentum of
p = 3.260MeV/c, which correspondsto an excessenergyof Q = 16.4MeV. It was basedon
B
theregistrationofthetwooutgoingprotonsandreconstructionoftheirmomenta. Theη meson
′
1ThesameenhancementwasalsoseeninindependentmeasurementsbytheCOSY-TOFgroup[7].
2
isidentifiedusingthemissingmasstechnique.
Figure1illustratesaschematicviewoftheCOSY-11apparatuswithatopologyofthepp ppX
→
reaction. Twooutgoingprotonspossessingsmallermomentathanthebeammomentumarebent
inthedipolemagneticfieldtowardsthedetectorsystemleavingthevacuumchamberthroughthe
exitwindow.Afterwards,theyaredetectedinthetwodriftchambers,D1andD2,inthescintilla-
torhodoscopesS1andS2,andinthescintillatorwallS3.Thetarget2usedduringtheexperiment,
wasrealizedasabeamof H moleculesgroupedinsideclustersofuptoabout106 atoms. The
2
averagedensityofthetargetwasaround5 1013atoms/cm2[24]. Itwasinstalledinfrontofthe
·
dipolemagnetasitcanbeseenschematicallyinFigure1.
Inordertodeterminetheabsolutevaluesofthedifferentialcrosssections,thetimeintegrated
luminosity(L)hasbeenestablishedbytheconcurrentmeasurementoftheangulardistributionof
theelasticallyscatteredprotons[25]. Theextractedvalueoftheintegratedluminosityamounts
toL = (5.859 0.055)pb 1[26].
−
±
Inordertosearchforsmalleffectslikeproton-mesoninteractionitisofimportancetoaccount
for smearing of the measured distributions due to the finite resolution of the detector system,
whichmayaltertheshapeofthespectrumespeciallyclosetothekinematicallimit. Therefore,
ashasbeendonepreviouslyintheanalysisofthe pp ppηreaction[4],akinematicalfitting
→
of the data has been performed[26] in order to improveresolution. To this end, the momenta
oftheprotonswerevarieddemandingthatthemissingmassoftheunregisteredparticleequals
the knownmass of the η meson exactly. Furthermore,as a result of the fit, only those proton
′
momentumvectorswhichwereclosesttothevectorsdeterminedfromtheexperimenthavebeen
chosen. Theinverseofthecovariancematrixwasusedasametricforthedistancecalculation.
Thekinematicalfitimprovestheresolutionbyafactorof1.5,ascanbeseeninFigure2.Afterthe
kinematicalfiteacheventcanbecharacterizedbyboth: experimentallydeterminedmomentum
vectorsand kinematicallyfitted momenta. In the subsequentanalysis the fitted momentawere
usedtogroupeventsintos and s intervalsandtheninordertosubtractthebackground,for
pp pη
′
eachgroupseparately,amissingmassspectrumwasdeterminedfromexperimentalmomentum
vectors. The available range of s and s was divided into 22 bins. The width of the bins
pp pη
′
(0.003 GeV2/c4) was chosen as a compromise between statistics and the experimental resolu-
tion. Then, for each bin, the missing mass spectrum was reconstructedand the numberof the
pp ppη eventswascalculatedseparatelyforeachintervalofs ,s .
′ pp pη
→ ′
In Figure 3, examplesof missing mass spectra for two intervalsof the invariantproton-proton
massarepresented. Aclearsignalfromtheη mesonproductionisseenontopofacontinuous
′
spectrum fromthe multi pionbackground. A smooth behaviourof the backgroundallowsone
to interpolateits shape underthe η peak with polynomialfunctionsthatmatchthe data below
′
andabovethepeak.Thesmoothbehaviourofthemulti-pionbackgroundinthepeakregionwas
verifiedbyMonteCarlosimulations[26]. InbothpanelsofFigure3thedottedlinescorrespond
to the result of the fit of the second order polynomial. An equally good approximationof the
backgroundwas also achievedby a fit of the sum of two Gaussian distributions[26]. The pa-
rameterizationswere performedinthe entirerangeofmissingmass outsideofthe pp ppη
′
→
signal,andobviouslyreproducethebackgroundverywell.
Thesituationismorecomplicatedforthesemissingmassspectrawhenthesignaliscloseto
thekinematicallimit(seeFig. 4). Inthiscasetheshapeofthebackgroundontherightsideof
2TheH2clustertargetspecificationsaredescribedinreferences[22,24].
3
thepeakcannotbeeasilypredicted. Suchspectraareobtainedforkinematicalregionsofhigher
squaredinvariantproton-protonmassesandrelativelylowsquaredinvariantproton-η masses. In
′
ordertodescribetheshapeofthe backgroundinthese regions,the pp pp2πη, pp pp3π
→ →
and pp pp4πreactions3 havebeengeneratedandthesimulatedeventswereanalysedinthe
→
same way as for the experimentaldata. The result of these simulations (dashed lines) is com-
pared to the experimentaldata in Figure 4. The simulations of the different reaction channels
were performedwith a distribution based on an equal populationof phase space includingthe
proton-protonfinalstateinteraction[6,27]. Alinearsumofthesimulatedmissingmassspectra
wasmatchedtothedatawiththerelativemagnitudesastheonlyfreeparameters.Inbothexam-
ples,simulationsareinagoodagreementwiththeexperimentalbackgrounddistributionsbelow
theη peak. Moreoverthebehaviourofthesimulatedbackgroundmatchesthekinematicallimit
′
ofthemissingmassdistributions.
Forthedynamicsofthepionproduction,ithadbeenassumedthatpionsareproduceduniformly
over the available phase space. As described in reference [27] the shape of the missing mass
spectrumdoesnotchangesignificantlyattheedgeofthekinematicallimitifoneassumesreso-
nantordirectpionproduction.
Inordertoincreasetheconfidenceintheestimationofthebackgroundbehaviournearthekine-
maticalboundaryandtoestimatethesystematicuncertaintiesduetothechoiceofthebackground
parameterizations,thesedistributionsweredescribedinanindependentwaywithasecondorder
polynomialdividedbytheFermifunctionforthedescriptionoftherapidslopeattheendofthe
distributions.Tothisend,thefollowingformulawasapplied:
F(mm,a,b,c,d,g) = (a + b mm + c mm2)/(1 + e(mm d)/g), (1)
−
· ·
wherea,b,c,dandgarefreeparameters.
TheresultsarepresentedinFigure4asdottedlines. Itisseenthatundertheη peaktheresult
′
of Equation1 agrees well with the backgrounddeterminedfrom the simulationsand that both
reproducetheshapeoftheslopequitewell.
A furthercheckofthebackgroundwasperformedbyextractingthemissingmassdistributions
for regionsof the squaredinvariantmasses of proton-protonand proton-mesonwherethe η is
′
notproduced. TheresultingmissingmassdistributionsareshowninFigure5andrepresentthe
regionsoflowsquaredinvariantmassesoftheproton-mesonsubsystem(left)andhighsquared
invariantmassesoftheproton-protonsubsystem(right). Forsuchvaluesof s or s thepro-
pp pη
′
ductionoftheη mesonisnotkinematicallyallowedbecause s < (m +m )2 and s istoo
′ pη p η pp
′ ′
large leaving insufficient energy to create the η meson. The simulations reproduce the back-
′
groundverywellasonecanseeinFigure5.
Themaincontributiontothesystematicuncertaintyofthedetermineddifferentialcrosssections
comesfromtheuncertaintyoftheestimationoftheyieldoftheη eventswhich,inturn,isdue
′
to the assumption of the shape of the background. In order to estimate these errors, the num-
bers of backgroundevents extracted under the two differentassumptions were compared. For
the missing mass spectra with the signal far from the kinematicallimit, the backgrounddeter-
minedbyGaussian distributionswascomparedtothe backgroundestimatedbya secondorder
polynomial. For the spectra close to the kinematical limit, the background determination by
3Thesereactionchannelswerechosenasarepresentation ofthepossiblemulti-pionproductionbackground, since
contributionfromthepp pp5π,6π,7πreactionscanbeneglected[18]andthemissingmassspectrumfrom2πhasa
→
similarshapeasthosefor3πand4πintherelevantkinematicalregion.
4
Monte-CarlosimulationswascomparedtoasecondorderpolynomialdividedbytheFermidis-
tribution. Theuncertaintyofthebackgroundestimationconstitutesthemaincontributiontothe
systematicerrorofthedifferentialcrosssections. Thedifferencesinthenumberofbackground
events obtained by applying the above described different fit procedures are below 3% of the
backgroundvalue,whichcorrespondstoabout20%ofthesignal,ascanbeseenforinstancein
Figure4.
Finally,themeasureddistributionswerecorrectedfortheacceptanceaccordingtothemethod
describedelsewhere[4].Here,itisimportanttostressthatatthebeammomentumofp =3.260
B
GeV/ctheCOSY-11acceptanceforthe pp ppη reactionisfiniteovertheentireareaofthe
′
→
s vss Dalitzplot. ThisisshowninFigure6,whereonecanseethatthefullphasespacefor
pp pη
′
the pp ppη reactioniscovered.
′
→
3. Results
A comparison of the spp and √sp meson distributions between the pp ppη′ and the
− →
pp ppη reactions is presented in Figure 7. For the proton-meson system the comparison
→
was performed for the kinetic energy (√sp meson mp mmeson) and not as a function of
s becausethe rangeof the s and s− are d−ifferent−due to the differentmasses of the η
p meson pη pη
− ′
andη′mesons.Buttherangeof(√sp meson mp mmeson)isthesamesincethemeasurements
fortheηandη productionwereperfo−rmeda−tabou−tthesameexcessenergy4.
′
Inbothpanelsitisseenthattheshapeofthedistributionforthe pp ppηmeasurement(open
→
squares)isinagreementwiththatforthe pp ppη reaction(closedsquares)withintheerror
′
→
bars, thus, showingthe same enhancementin the regionof largeproton-protoninvariantmass.
Therefore,iftheη-protoninteractionisindeedmuchsmallerthantheη-protonasinferredfrom
′
theexcitationfunction[4,6],then,onewouldhaveobservedasignificantlysmallerenhancement
inthecaseoftheη meson.Hence,thespectrapresentedheretendtoexcludethehypothesisthat
′
theenhancementisduetothemeson-protoninteraction.
Theabsolutevaluesofthecrosssectionforthe pp ppη reactiondeterminedasafunctionof
′
→
s ands aregiveninTables1and2,andareshowninFigure8. Thedetermineddistributions
pp pη
′
differsignificantlyfromthepredictionsbasedonthehomogeneousphasespacepopulation(thick
solidline). Also,theresultsofcalculationsincludingtheFSI (dottedline)donotdescribethe
pp
dataunderestimatingthecrosssectionsatlargevaluesof s andlowvaluesof s . Similarlyto
pp pη
thecaseofηmeson,abetteragreementwiththeexperimentaldatacanbeachievedwhentaking
into account contributions from higher partial waves. The calculations depicted as solid lines
resultfromacombinedanalysis(basedontheeffectiveLagrangianapproach)oftheproduction
ofηandη mesonsinphoto-andhadro-inducedreactions[10,28,29,30]. Sincethecalculation
′
is done in the plane-wave basis, not onlythe 3P 1S s butalso the 1S 3P s (and all the
0 0 0 0
→ →
higherpartial-waves)transition5contributesattheexcessenergyof16.4MeV.
On the other hand, one can explain the enhancement seen in the distributions by an energy
dependentproductionamplitude[11]. Theresultindicatedbythedashedlineswasobtainedal-
lowingforalinearenergydependenceofthe3P 1S spartialwaveamplitudeandneglecting
0 0
→
4ThenominalbeamenergycorrespondstoanexcessenergyofQ=15.5MeV,consistentwiththedatacollectedfor
thepp ppηreaction.However,therealvaluewasdeterminedtobe16.4MeV[26].Thisdifferenceiswellwithinthe
precisio→noftheabsolutebeammomentumadjustmentoftheCOSYsynchrotronamountingtoaboutδp/p=10 3[15]
−
5
otherpartialwavestransitions[11]. Also,Cecietal.[12]haveshownrecentlythatthediscussed
enhancementintheinvariantmassspectracanbewelldescribedbytheenergydependenceofthe
productionamplitudewhenthe negativeinterferencebetweenthe π andthe ηmesonexchange
amplitudesisassumed.
Within the statistical and systematic error bars both model of Nakayama et al.[10] and of
Deloff [11] describethe data well althoughtheydiffer slightly in the predictedshapes. Taking
intoaccountstatisticaluncertaintiesonly,oneobtainsχ2 =2.1andχ2 =4.7forthecomparison
of thedashedline andsolidline to thedata, respectively. Thisindicatesthatperhaps,notonly
higherpartialwavesbutalsotheenergydependenceoftheproductionamplitudeshouldbetaken
intoaccount.
4. Summary
Using the COSY-11 detector setup and the proton beam of the cooler synchrotron COSY
theproton-protonandproton-η invariantmassdistributionshavebeendeterminedforthe pp
′
→
ppη reactionatanexcessenergyofQ=16.4MeV.
′
Similartotheearlierobservationfortheηmesonproductionthemeasureddifferentialcross
sectiondistributions(s ands )forthe pp ppη reactionatQ=16.4MeVstronglydeviate
pp pη ′
′ →
fromthepredictionsbasedonahomogeneouspopulationofeventsovertheallowedphasespace.
Also, the inclusion of the proton-proton final state interaction is not sufficient to explain the
enhancementseenintherangeoflarges values.
pp
Withinthe achieveduncertainties,the shapeof theproton-protonandproton-mesoninvari-
ant mass distributions determined for the η meson is essentially the same to that established
′
previously for the η meson. Since the enhancementis similar in both cases, and the strength
of proton-η and proton-η interaction is different [6, 26], one can conclude that the observed
′
enhancementisnotcausedbyaproton-mesoninteraction.
Finally, calculations assuming a significant contribution of P-wave in the final state [10],
and models including energy dependence of the production amplitude [11, 12], reproduce the
datawithinerrorbarsequallywell. Therefore,onthebasisofthepresentedinvariantmassdis-
tributions, it is not possible to disentangle univocally which of the discussed models is more
appropriate. As pointed out in [10], future measurements of the spin correlation coefficients
shouldhelpdisentanglethesetwomodelresultsinamodelindependentway.
Acknowledgements:
The work was partially supportedby the EuropeanCommunity-ResearchInfrastructureActiv-
ityundertheFP6andFP7programmes(HadronPhysics,RII3-CT-2004-506078,PrimeNetNo.
227431.),bythePolishMinistryofScienceandHigherEducationundergrantsNo.3240/H03/2006/31
and1202/DFG/2007/03,bytheGermanResearchFoundation(DFG),bytheFFEgrantsfromthe
ResearchCenterJu¨lich,andbythevirtualinstitute”SpinandstrongQCD”(VH-VI-231).
whichcorrespondsto 1MeVuncertaintyinQ.
5Thetransitionbetw∼eenangularmomentumcombinationsoftheinitialandfinalstatesaredescribedaccordingtothe
conventionalnotation[31]inthefollowingway:
2Si+1LiJ→2S+1 LJ,l (2)
where, superscript“i”indicates theinitial statequantities. S denotesthetotalspinofnucleons, and J standsforthe
overallangularmomentumofthesystem.Landldenotetherelativeangularmomentumofnucleon-nucleonpairandof
themesonrelativetotheNNsystem,respectively.
6
[1] C.Amsleretal.,Phys.Lett.B667(2008)1.
[2] C.P.Jessopetal.,Phys.Rev.D58(1998)052002.
[3] G.Brandenburgetal.,Phys.Rev.Lett.75(1995)3804.
[4] P.Moskaletal.,Phys.Rev.C69(2004)025203.
[5] A.M.Green,S.Wycech,Phys.Rev.C71(2005)014001.
[6] P.Moskaletal.,Phys.Lett.B482(2000)356.
[7] M.Abdel-Baryetal.,Eur.Phys.J.A16(2003)127.
[8] A.Fix,H.Arenho¨vel,Phys.Rev.C69(2004)014001.
[9] A.Fix,H.Arenho¨vel,Nucl.Phys.A697(2002)277.
[10] K.Nakayamaetal.,Phys.Rev.C68(2003)045201.
[11] A.Deloff,Phys.Rev.C69(2004)035206.
[12] S.Ceci,A.Sˇvarc,B.Zauner,ActaPhys.Pol.BSuppl.2(2009)157.
[13] M.Zielin´ski,DiplomaThesis,e-printarXiv:hep-ex/0807.0576.
[14] R.Maier,Nucl.Instr.&Meth.A390(1997)1.
[15] D.Prasuhnetal.,Nucl.Instr.&Meth.A441(2000)167.
[16] H.Stockhorstetal.,IKPAnnualReport(1997)160.
[17] H.Stockhorst,ShriftendesFZ-Ju¨lich,MatterandMaterials11(2002)176.
[18] P.Moskaletal.,Phys.Rev.Lett.80(1998)3202.
[19] P.Moskaletal.,Phys.Lett.B474(2000)416.
[20] A.Khoukazatal.,Eur.Phys.J.A20(2004)345.
[21] S.Brauksiepeetal.,Nucl.Instr.&Meth.A376(1996)397.
[22] H.Dombrowskietal.,Nucl.Instr.&Meth.A386(1997)228.
[23] P.Klajaetal.,AIPConf.Proc.796(2005)160.
[24] A.Khoukazetal.,Eur.Phys.J.D5(1999)275.
[25] P.Moskaletal.,Nucl.Instr.&Meth.A466(2001)448.
[26] P.Klaja,PhDthesis,e-Print:arXiv:0907.1491.
[27] P.Moskaletal.,J.Phys.G32(2006)629.
[28] K.Nakayama,Y.Oh,H.Haberzettl,ActaPhys.Pol.BSuppl.2(2009)23.
[29] K.Nakayama,H.Haberzettl,Phys.Rev.C69(2004)065212.
[30] K.Nakayama,Y.Oh,H.Haberzettl,e-print:arXiv:hep-ph/0803.3169.
[31] H.O.Meyeretal.,Phys.Rev.C63(2001)064002.
7
s [GeV2/c4] dσ [µb/GeV2/c4]
pp dspp
3.5215 1.01 0.11 0.12
stat sys
± ±
3.5245 4.28 0.25 0.56
stat sys
± ±
3.5275 4.06 0.26 0.59
stat sys
± ±
3.5305 3.78 0.26 0.55
stat sys
± ±
3.5335 3.52 0.26 0.52
stat sys
± ±
3.5365 2.74 0.25 0.46
stat sys
± ±
3.5395 1.99 0.23 0.40
stat sys
± ±
3.5425 2.75 0.25 0.40
stat sys
± ±
3.5455 2.23 0.23 0.37
stat sys
± ±
3.5485 2.66 0.25 0.39
stat sys
± ±
3.5515 1.96 0.21 0.30
stat sys
± ±
3.5545 2.18 0.23 0.34
stat sys
± ±
3.5575 1.93 0.22 0.31
stat sys
± ±
3.5605 1.62 0.22 0.33
stat sys
± ±
3.5635 1.76 0.20 0.26
stat sys
± ±
3.5665 1.66 0.19 0.24
stat sys
± ±
3.5695 1.76 0.19 0.26
stat sys
± ±
3.5725 1.39 0.16 0.22
stat sys
± ±
3.5755 1.12 0.14 0.19
stat sys
± ±
3.5785 1.07 0.11 0.11
stat sys
± ±
3.5815 0.72 0.09 0.09
stat sys
± ±
3.5845 0.013 0.004 0.002
stat sys
± ±
Table 1: Differential cross section as a function of the squared invariant mass of the proton-proton system, for the
pp ppη reactionatQ=16.4MeV.
→ ′
8
s [GeV2/c4] dσ [µb/GeV2/c4]
pη′ dspη′
3.5945 0.14 0.02 0.02
stat sys
± ±
3.5975 0.76 0.06 0.09
stat sys
± ±
3.6005 1.25 0.09 0.14
stat sys
± ±
3.6035 1.32 0.10 0.17
stat sys
± ±
3.6065 1.62 0.12 0.24
stat sys
± ±
3.6095 1.28 0.13 0.28
stat sys
± ±
3.6125 1.69 0.15 0.33
stat sys
± ±
3.6155 1.91 0.16 0.37
stat sys
± ±
3.6185 1.87 0.15 0.36
stat sys
± ±
3.6215 1.94 0.15 0.38
stat sys
± ±
3.6245 2.62 0.18 0.44
stat sys
± ±
3.6275 2.33 0.18 0.48
stat sys
± ±
3.6305 3.37 0.19 0.51
stat sys
± ±
3.6335 4.96 0.22 0.54
stat sys
± ±
3.6365 3.93 0.21 0.45
stat sys
± ±
3.6395 2.21 0.17 0.40
stat sys
± ±
3.6425 2.93 0.16 0.32
stat sys
± ±
3.6455 3.34 0.18 0.48
stat sys
± ±
3.6485 2.96 0.15 0.38
stat sys
± ±
3.6515 2.30 0.13 0.35
stat sys
± ±
3.6545 1.27 0.08 0.22
stat sys
± ±
3.6575 0.22 0.03 0.04
stat sys
± ±
Table2:Differentialcrosssectionasafunctionofthesquaredinvariantmassoftheproton-η system,forthepp ppη
′ → ′
reactionatQ=16.4MeV.
9
Si
S2
Figure1: SchematicviewoftheCOSY-11detectorfacility[21]. Protonsoriginatingfromthepp ppXreactionare
→
bentinthedipolemagneticfield,andleavethevacuumchamberthroughtheexitwindow.Afterwardstheyaredetected
inthetwodriftchambers D1andD2, inthescintillator hodoscopes S1andS2, andinthescintillator wallS3. The
scintillationdetectorS4andthesiliconpaddetectorSiareusedincoincidencewiththeD1,D2andS1detectorsforthe
registrationoftheelasticallyscatteredprotons.
60000
50000
nts40000
u
co30000
20000
10000
0
-0.02 0 0.02
D p [ GeV/c ]
Figure 2: Spectrum of the difference between the simulated and reconstructed proton momentum. The dashed line
denotesthespectrumbeforekinematicalfitandthesolidlinecorrespondstothesituationafterthefittingprocedure.
10