Total and Differential Cross Sections for the pp-->pp eta-prime Reaction Near Threshold

0.21 MB·English

by  A. Khoukaz

EPJ manuscript No.
(will be inserted by the editor)
Total and Differential Cross Sections for the pp ppη Reaction
′
→
Near Threshold
A. Khoukaz1,a, I. Geck1, C. Quentmeier1, H.-H. Adam1, A. Budzanowski2, R. Czyz˙ykiewicz3, D. Grzonka4, L.
4 Jarczyk3, K.Kilian4, P.Kowina4,5,N. Lang1,T. Lister1,P. Moskal3,4, W. Oelert4, C. Piskor-Ignatowicz3,T. Roz˙ek5,
0 R. Santo1, G. Schepers4, T. Sefzick4, S. Sewerin4, M. Siemaszko5, J. Smyrski3, A. Strza lkowski3, A. T¨aschner1, P.
0 Winter4, M. Wolke4, P. Wu¨stner4, and W. Zipper5
2
n 1 Institut fu¨r Kernphysik,Westf¨alische Wilhelms-Universit¨at, D-48149 Mu¨nster, Germany
a 2 Instituteof Nuclear Physics, Pl-31-342 Cracow, Poland
J 3 Instituteof Physics, Jagellonian University,PL-30-059 Cracow, Poland
1 4 IKPand ZEL, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany
1 5 Instituteof Physics, Universityof Silesia, PL-40-007 Katowice, Poland

1
Received: date/ Revised version: date
v
1
Abstract. The η′ meson production in the reaction pp → ppη′ has been studied at excess energies of Q
1
0 = 26.5, 32.5 and 46.6 MeV using the internal beam facility COSY-11 at the cooler synchrotron COSY.
1 The total cross sections as well as one angular distribution for the highest Q-value are presented. The
0 excitation function of thenearthreshold datacan bedescribed bya pures-wave phasespace distribution
4 with theinclusion of theproton-proton final state interaction and Coulomb effects. The obtained angular
0 distribution of the η′ mesons is also consistent with pure s-waveproduction.
/
x
PACS. 13.60.Le Meson production – 13.75.-n Hadron-induced low- and intermediate-energy reactions
e
- and scattering (energy≤ 10GeV) –13.85.Lg Total cross sections –25.40.-h Nucleon-inducedreactions –
l 29.20.Dh Storage rings
c
u
n
: 1 Introduction 21]. The gluonic contribution to the cross section for the
v NN NNη′ reactionmaybeinferredbythecomparison
Xi Measurements on the production of η′ mesons, the heavi- of th→e η′ production in different isospin channels [18,19,
r estrepresentativeofthemultipletofpseudoscalarmesons, 22].
a Recently,detailedmeasurementsonthe η′ mesonproduc-
allowtostudythepropertiesandthestructureofthisiso-
tion in the reaction channel pp ppη′ have been per-
scalar meson, which are still far from being well known. →
formedinthepreviouslyunexploredregionclosetothresh-
SincestateswiththesamequantumnumbersIJP canmix,
thephysicallyobservableparticlesη andη′ areconsidered old up to an excess energy of Q = 24 MeV [23,24,25,26]
as well at a higher excess energy of Q = 144 MeV [27].
to be mixed states of the I = 0 members of the ground
Inthispublicationwepresentnewresultsonthisreaction
statepseudoscalaroctetandsinglet,commonlydenotedas
channel at intermediate excess energies of Q = 26.5, 32.5
η8 and η1. In case of an ideal mixing, the η meson would
have a pure non-strange content (uu¯+dd¯), while the η′ and 46.6 MeV, filling the gap between the available data
sets.
wouldshowupasapuress¯state,correspondingtoamix-
ing angle of θ = arctan√2 54.7◦. This value is Due to the small relative momenta of the ejectiles in the
ideal
− ≈ − regionoflowexcessenergies,onlypartialwavesofthelow-
in contrast to the experimentally still inaccurately deter-
est order participate in the exit channel. Therefore, total
mined mixing angleθ , whichhas been subject of several
P
anddifferentialcrosssectiondatayieldnearlyunscreened
investigations[1,2,3,4,5,6,7,8,9,10,11,12] andwasfound
◦ informationonrelevantproductionmechanismsandallow
to be between 9 and 20 .
Furthermore, s−tudies o−n the production of η′ mesons are to study finalstate interactions (FSI) of the participating
particles.Consequently,thesenewdatabecamesubjectof
also important with respect to still controversially dis-
several model calculations and comparisons with the re-
cussed topics like possible cc¯ or gluonic components in
the structure of the η′ meson [8,13,14,15,16,17,18,19] or latedreactionchannelsontheπ0 andη mesonproduction
[28,29,30].
the understanding of the unexpected high mass, which
Incontradistinctiontothecorrespondingηmesonproduc-
is discussed in the context of the U(1) anomaly [17,20,
A
tion channel, whose production amplitude is assumed to
a email: [email protected] be dominated by the excitation of the S11 nucleon reso-
2 A. Khoukazet al.: Total and Differential Cross Sections for thepp→ppη′ Reaction Near Threshold
nance N∗(1535), the relevance of possible production di-
agrams to describe the η′ meson formation is still contro- monitor
detector
versially discussed. A prediction for both the shape and
theabsolutescaleofthenearthresholdexcitationfunction
of the η′ production via the reaction pp ppX has been dipole COSYbeam
COSY
→
determined by Hibou et al. [23], comparing the ppη and beam
ppη′ channelswithin anone-pionexchangemodelandad- target
justing anoverallnormalizationfactorto fit the ppη total
cross section data. The obtained prediction for the shape
of the η′ excitation function is able to describe the data
well.However,theabsolutescaleofthetotalcrosssections
S2
is underestimatedby afactorof2-3by these calculations, S3
whichmightbeinterpretedasasignalfortherelevanceof DC1
exchange diagrams of heavier mesons (e.g. ρ) [23] or the
DC2 protons
importance of the gluonic contact term in the η′ produc-
S1
tion [19].
Contrary, model calculations by Sibirtsev et al. [31], also
based on the one-pion exchange diagram including the Fig. 1. Sketch of the internalbeam facility COSY-11.
proton-proton final state interaction, have been found to
be able to describe both the shape as well as the abso-
lute scale of the near threshold total cross section data.
alargescintillationwall(S3)atadistanceof 9.3m,act-
Thisresultisarguedtoindicateeitheronlynegligiblecon- ∼
ing as a stop detector. By measuring the momentum and
tributions of the exchange of heavier mesons or a mutual
thevelocity,particlesareidentifiedviainvariantmass,i.e.
cancellationoftheircontributions.Itshouldbenotedthat the four momentum vectors P of positively charged ejec-
i
in both models contributions of initial state interactions
tiles are fully determined.
of both protons have been neglected, which have been re- The event selection for the reaction pp ppη′ was per-
portedtoscaletheabsolutesizeofthetotalcrosssections →
formedby acceptingevents withtworeconstructedtracks
by a factor of f = 0.2 [32] and f = 0.33 [33] in the near
inthe driftchambers,requiringbothparticlesbeing iden-
threshold region for the η and η′ mesons, respectively.
tified as protons. The four-momentum determination of
Recently, the η′ meson production has been investigated
the positively charged ejectiles yields a full event recon-
theoretically by Nakayama et al. [33] within a relativistic struction for the reaction type pp ppX and allows
meson exchange model, considering the exchange of π, η, →
an identification of the X-particle using the missing mass
ρ, ω, σ, a0 mesons and including effects of the proton- method
proton initial and final state interaction. These calcula-
tions include contributions of nucleonic and mesonic cur-
rents as well as contributions of nucleon resonances, de- mx =|Pbeam+Ptarget−Pproton1−Pproton2| (1)
noted as S11(1897) and P11(1986), which have been ob-
served in multipole analyses of η′ photoproduction ex- and to study angular distributions of the ejectiles. This
periments off protons [34]. Though the nucleonic and the situation is demonstrated in fig. 2 for a beam momentum
mesoniccurrentsarefoundtoreproducetheobservedcross of pbeam = 3.356 GeV/c, corresponding to an excess en-
sections,alsocontributionsoftheS11(1897)resonancealone ergy of Q = 46.6 MeV above the η′ meson production
arereportedtobesufficienttodescribethedata.However, threshold. In the raw spectrum (upper figure) a signal
to determine the relative magnitude of these currents,to- of the η′ meson production is clearly visible on a back-
tal and differential cross section data at higher excess en- ground arising from multi pion production channels. The
ergies are needed. lower spectrum presents the η′ missing mass peak with
a content of N 13000 events after subtraction of the
∼
background,which was fitted by a first order polynomial.
2 Experiment The observed mean position of the missing mass peak,
m =958.1MeV/c2,differsby0.3MeV/c2 fromthenom-
X
Measurements on the reaction pp ppη′ have been per- inal value (mη′ = 957.78 0.14 MeV/c2 [38]), reflecting
→ ±
formedattheinternalbeamfacilityCOSY-11[35]atCOSY- the precision of the experimental method and the qual-
Ju¨lich[36],using ahydrogencluster target[37]infrontof ity of the accelerator beam. The missing mass resolution
a COSY-dipole magnet, acting as a magnetic spectrom- amounts to σ = 3 MeV/c2 (Γη′ = 0.202 0.016 MeV/c2
±
eter. Tracks of positively charged particles, detected in a [38]), consistently with Monte-Carlo simulations (fig. 2,
set of two drift chambers (DC1 and DC2, fig. 1), can be dashed line) based on GEANT 3.21 [39].
traced back through the magnetic field to the interaction To extract total and differential cross section data it is
point, leading to a momentum determination. The veloc- important to investigate the phase space coverage of the
ities of these particles are accessible via a time-of-flight detection system. For the highest energy data point pre-
path behind the drift chambers, consisting of two scintil- sented in this paper this situation is illustrated in fig. 3
lation hodoscopes (start detectors S1 and S2) followedby presentingthesquaredinvariantmassoftheproton-η′sys-
A. Khoukazet al.: Total and Differential Cross Sections for thepp→ppη′ Reaction Near Threshold 3
] atanexcessenergyofQ=46.6MeVthewholeDalitzplot
1
- )9000 iscovered.The overalldetectionefficiencies,requiringthe
V
e detection of both protons, were determined to be
M8500 polynomial fit ε(Q = 26.5 MeV) = (2.0+0.4) 10−2,
m [ (0.5 8000 εεE((sQQpe==cia34l26ly..56aMMt eehVVig))h==en((e19r..g52i−+−+−e03002s.....42331t))h×××e11u00n−−c23e,.ratanidnty in the deter-
d7500 mination of the detection efficiency is mainly given by
/
N application of different models for the proton-proton FSI
d7000
[40,41,42,43,44].
To receive an η′ meson angular distribution, the range
6500
of scattering angles in the center of mass system was di-
vided into eight angular bins and missing mass spectra
6000
havebeenextractedforeachbin.Thecontentsofthemiss-
m(h’)
ing mass peaks havebeen acceptance correctedby results
5500
from phase space Monte-Carlo simulations including the
pp FSI, according to [40,45]. The inclusion of the final
5000
930 940 950 960 970 980 990 1000 1010
2
missing mass [ MeV/c ]
]
-1 V) 1200 2 V ] 3.8 2
e 10
M real data Ge
0.5 1000 MC simulation , [ h3.75
( p
[ S
m 800 10
3.7
d
/
N
d 600
3.65
1
400
3.6
200 3.55
3.5 3.55 3.6 3.65 3.7
2
0 Spp [ GeV ]
930 940 950 960 970 980 990 1000 12010 2 V ] 3.8
missing mass [ MeV/c ] e
Fig. 2. Missing mass distribution of the selected two track G
eMveeVnt)s. Twhitehlobwoethr fipgaurrteiclpersesiednetnstitfiheedeaxspeprrimoteonntsal(dQat=a a4f6te.6r , [ h3.75 10
p
subtraction of the background (see upperfigure). In addition, S
the resulting η′ signal (solid line) is compared with expecta- 3.7
tions according to Monte-Carlo simulations (dashed line).
1
3.65
temSpη′ asafunctionofthesquaredinvariantmassofthe
proton-protonsystem S for Monte-Carlo events1. -1
pp 3.6
10
As expected, the Dalitz plot displaying all generated
events (upper figure) is homogeneously filled, while the
corresponding plot for accepted and reconstructed events 3.55
3.5 3.55 3.6 3.65 3.7
(lower figure) presents an inhomogeneous structure, re-
flecting the acceptance ofthe COSY-11 detectionsystem. 2
Spp [ GeV ]
However,fromthe latter spectrum it is obvious that even
Fig. 3. Dalitz plots for generated (upper figure) and by the
w1ithTthheesfqouuarr-medominevnatruiamntvemcatossrsSPiji aonfdtwPoj ipsagrtivicelnesbiyaSnijd=j CCOarSloY-e1v1endtsetoefcttihone rseyasctteimon apcpce→ptepdpη(′loawteranfigexucrees)sMenoenrtgey-
|Pi+Pj|2. of Q = 46.6 MeV.
4 A. Khoukazet al.: Total and Differential Cross Sections for thepp→ppη′ Reaction Near Threshold
state interactionitself in the eventgeneratoris motivated Table 1. Total cross sections for thereaction pp→ppη′.
by the use of overall detection efficiencies in the analysis.
excess energy absolute cross section
Q [MeV] σ [nb]
3 Results 26.5 ± 1.0 130.0 ± 13.8 (stat.) +21.2 (syst.)
−24.8
32.5 ± 1.0 174.1 ± 20.2 (stat.) +34.3 (syst.)
In fig. 4 the resulting angular distribution of the emitted −45.8
η′ mesonsintheoverallcenterofmasssystemispresented
46.6 ± 1.0 314.9 ± 17.3 (stat.) +81.9 (syst.)
for an excess energy of Q = 46.6 MeV. The quoted errors −116.5
]
sr 50 const.) modified by the proton-proton final state interac-
b/45 phase space + FSIpp tion(FSI)andCoulombeffects,scaledtofitthedata[28]:
n 2
[ 40 a + b • cos(J)
W 35 Nakayama qmax
ds/d2350 σ ∝Z0 kNNq2|M0|2·|MFSI,Coulomb|2dq. (2)
20
15 In this notation q and kNN represent the momentum of
10 the meson in the CMS and the momentum of either nu-
5 cleon in the rest frame of the NN-subsystem. Within the
0-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 experimental errors this fit is able to describe the whole
set of existent data. Therefore, one can conclude that no
Fig. 4. Angular distribution of the emitted η′ mesoncoisn(Jt)hhe, further assumptions like a significantη′-protonfinal state
center of mass system at an excess energy of Q = 46.6 MeV. interaction are needed in order to describe the excitation
Newest results from [48] are represented by the solid line. For function. In particular, distinct effects of higher partial
comparison, thedashedandthedottedlines indicateexpecta- wavescanbe rejecteddue tothe isotropyofthepresented
tionsaccordingtophasespaceconsiderationswithandwithout angulardistributionandthe onefromDISTO atQ=144
theinclusion of a cos2(ϑ) term. MeV [46].
The dotted curve of fig. 5 represents calculations from
Sibirtsevetal.[31]basedonaone-pionexchangediagram
include statistical and systematical errors except contri- including the pp final state interaction. While for excess
butions fromoverallsystematicaluncertainties(e.g.lumi- energiesbelow Q = 25MeV the observedexcitationfunc-
nosity determination). The differential cross sections are tion is described well, the new data from COSY-11 (filled
compatible (χ2 = 0.92) with an isotropic emission (dot- circles)aswellasthecrosssectionfromDISTO aresome-
ted line), indicating a dominance of S-waves in the final what overestimated.
state, consistent with results obtained from the DISTO Additionally, fig. 4 and fig. 5 show the newest calculation
collaboration at an excess energy of Q = 144 MeV [46]. from Nakayama [48] for the near threshold η′ meson pro-
However,fig.4mightalsoindicatecontributionsofhigher duction in proton-proton collisions (dashed lines), based
partialwaves,i.e.D-waves.Aninclusionofacos2(ϑ)term ona relativisticmesonexchangemodel.Since the relative
to account for higher partial waves leads to an adequate strengths as well as the absolute scales of the considered
descriptionoftheangulardistributionasdemonstratedby mesonic,nucleonicandnucleonresonancecurrentsarenot
the dashed line (χ2 = 0.45). known, different combinations reproducing the available
Theintegratedluminositieshavebeendeterminedbycom- total cross sections are possible. However, in a combined
paring the differential counting rates of elastically scat- analysis of the η′ meson production in pp and γp inter-
tered protons with data obtained by the EDDA collabo- actions Nakayama succeeded to describe both reactions
ration[47].Forbeammomentaofp=3.292GeV/c,3.311 within his model consistently [48]. In this approach the
GeV/cand3.356GeV/c,integratedluminositiesof Ldt contribution of the mesonic exchange current has been
= 908 nb−1, 841nb−1 and 4.50 pb−1 (all: 1% (staRt.) fixed by the photoproduction data and appeared to be
± ±
5% (syst.)) have been determined. much smaller than assumed in earlier calculations [33].
The obtained values of total cross sections are listed in On the contrary, contributions of at least an S11 nucleon
table 1. The overallsystematical errorarises from the de- resonance in the mass region of 1650 MeV/c2 and prob-
tection efficiency determination, the calculationof the lu- ably a P11 resonance in the mass region of 1880 MeV/c2
minosity, the uncertainty of the COSY beam momentum werefoundtobenecessaryinordertodescribetheenergy
(∆p/p 0.1%) as well as from acceptance corrections of dependence of the total cross sections of both the γp an
≈
the differential cross sections. pp data. Furthermore, in proton-proton collisions contri-
Infig.5theresults(filledcircles)arecomparedwithexist- butions of the nucleonic exchange current were estimated
ing data. The solid line represents an s-wave phase space to be comparatively small in order to describe within the
calculation (meson production matrix element M0 2 = given uncertainties both the excitation function a well as
| |
A. Khoukazet al.: Total and Differential Cross Sections for thepp→ppη′ Reaction Near Threshold 5
]

b
n
3

[10 DISTO
n
o SPES-III
i
t
c COSY-11
e
s present work

s
os 2
r10
c
phase space + FSI
10 pp
Sibirtsev
Nakayama
1
2
1 10 10
excess energy [ MeV ]
Fig.5. Totalcrosssectionsforthereactionpp→ppη′asafunctionoftheexcessenergyQ.Filledcirclescorrespondtothedata
presentedinthispaperandtriangles, opencirclesandstarscorrespondtodatafrom[23,24,25,46],respectively.Thecurvesare
explained in thetext.
the observed angular distributions of emitted η′ mesons production, similarly to the η meson case but on a lower
presented in [46] and in this work (fig. 4). scale.
5 Acknowledgements
4 Summary
We thank K. Nakayama for valuable discussions on the
At the COSY-11facility the near thresholdη′ mesonpro- mechanismoftheη′production.Thisresearchprojectwas
duction in the reaction channel pp ppη′ has been stud- supportedinpartbytheBMBF(06MS881I),theBilateral
→
iedatexcessenergiesofQ=26.5,32.5and46.6MeV.The Cooperation between Germany and Poland represented
obtainedtotalcrosssectiondatafilltheregionofinterme- by the Internationales Bu¨ro DLR for the BMBF (PL-N-
diate excess energies between the low energy data [23,24, 108-95)andby the KomitetBadan´ NaukowychKBN,the
25,26,27]andone datapoint ata highexcessenergy[46]. EuropeanCommunity-AccesstoResearchInfrastructure
It was demonstrated that within the quoted errors the action of the Improving Human Potential Programmand
complete available excitation function can be adequately by the FFE grants (41266606 and 41266654) from the
describedbycalculationsonbasisofthethreebodyphase ForschungszentrumJu¨lich.
space behaviour including effects of the pp FSI only, and
nodistinctcontributionsfromtheη′-protoninteractionor
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