a0+(980)-resonance production in the reaction pp -> dpi+eta close to the K(bar(K)) threshold

0.3 MB·English

by  P. Fedorets

a+(980)-resonance production in the reaction pp → dπ+η close to the KK¯ threshold
0
P. Fedorets,1,2 M. Bu¨scher,1 V.P. Chernyshev,2 S. Dymov,1,3 V.Yu. Grishina,4 C. Hanhart,1
M. Hartmann,1 V. Hejny,1 V. Kleber,5 H.R. Koch,1 L.A. Kondratyuk,2 V. Koptev,6
A.E. Kudryavtsev,2 P. Kulessa,1 S. Merzliakov,3 S. Mikirtychiants,6 M. Nekipelov,1,6
H. Ohm,1 R. Schleichert,1 H. Stro¨her,1 V.E. Tarasov,2 K.-H. Watzlawik,1 and I. Zychor7
1Institut fu¨r Kernphysik, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany
2Institute for Theoretical and Experimental Physics,
Bolshaya Cheremushkinskaya 25, 117218 Moscow, Russia
3Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, 141980 Dubna, Russia
4Institute for Nuclear Research, 60th October Anniversary Prospect 7A, 117312 Moscow, Russia
5Institut fu¨r Kernphysik, Universit¨at zu K¨oln, Zu¨lpicher Str. 77, 50937 K¨oln, Germany
6High Energy Physics Department, Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia
7The Andrzej Soltan Institute for Nuclear Studies, 05400 Swierk, Poland
5
0 Thereaction pp→dπ+η hasbeenmeasuredatabeamenergyofTp=2.65GeV(pp=3.46GeV/c)
0 usingtheANKEspectrometerat COSY-Ju¨lich. Themissingmassdistribution ofthedetecteddπ+
2 pairsexhibitsapeakaroundtheη massontopofastrongbackgroundofmulti-pionpp→dπ+(nπ)
n events. The differential cross section d4σ/dΩddΩπ+dpddpπ+ for the reaction pp → dπ+η has been
a determined model independently for two regions of phase space. Employing a dynamical model
J for the a+ production allows one then to deduce a total cross section of σ(pp → da+ → dπ+η) =
8 (1.1±0.30stat ±0.7sys) µb for the production of π+η via the scalar a+0(980) resonanc0e and σ(pp→
2 dπ+η) = (3.5±0.3stat ±1.0sys) µb for the non-resonant production. Using the same model as for
theinterpretationofrecentresultsfromANKEforthereactionpp→dK+K¯0,theratioofthetotal
1 cross sections is σ(pp→d(K+K¯0)L=0)/σ(pp→da+0 →dπ+η)=0.029±0.008stat±0.009sys,which
v is in agreement with branchingratios in theliterature.
7
2
0 I. INTRODUCTION chain φ → γa /f → γπ0η/π0π0. In pp collisions the
0 0
1 a (980)resonancehasbeenmeasuredatp =450GeV/c
One of the primary goals of hadronic physics is the 0 p
0 via f (1285) → a±π∓ decays [13] and in inclusive mea-
5 understanding of the internal structure of mesons and 1 0
surements of the pp → dX+ reaction at p = 3.8, 4.5,
0 baryons, and their production and decay, in terms of p
and 6.3 GeV/c [14]. Despite these many experimental
/ quarks and gluons. The non-perturbative character of
x results, the properties of the a (980) resonance are still
the underlying theory — Quantum Chromo Dynamics 0
e
far from being established. The Particle Data Group
- (QCD) — hinders straightforward calculations. QCD
cl can be treated explicitly in the low momentum-transfer givesamass ofma0 =(984.7±1.2)MeV/c2 anda width
of Γ = (50−100) MeV/c2 [3]. The main decay chan-
u regime using lattice techniques [1], which are, however, a0
n not yet in the position to make quantitative statements nels, πη and KK¯, are quoted as “dominant” and “seen”
v: about the light scalar mesons. Alternatively, QCD- respectively.
An experimental programme has been started at the
i inspired models, which employ effective degrees of free-
X Cooler Synchrotron COSY-Ju¨lich [15] aiming at exclu-
dom, can be used. The constituent quark model is one
r of the most successful in this respect (see e.g. [2]). This sive data on the a0/f0 production from pp, pn, pd and
a ddinteractionsatenergiesclosetotheKK¯ threshold[16].
approachtreatsthelightestscalarresonancesa /f (980)
0 0
The final goal of these investigations is the extraction of
as conventional qq¯states.
However, more states with quantum numbers JP=0+ thea0/f0-mixingamplitude,aquantitywhichisbelieved
to shed light on the nature of these resonances [17, 18].
have been identified experimentally than would fit into
As a first stage the reaction pp → dK+K¯0 has been
a single SU(3) scalar nonet: the f (600) (or σ), f (980),
0 0
measured at T =2.65 GeV, corresponding to an excess
f (1370), f (1500) and f (1710) with I=0, the κ(800) p
a0nd K∗(14030) (I=1/2), 0as well as the a (980) and energy of Q=46 MeV above the K+K¯0 threshold, using
0
a (1450) (I=1) [3]. Consequently, the a /f (980) have theANKEspectrometer[19]. Thedata,whichhavebeen
0 0 0
also been associated with KK¯ molecules [4] or compact decomposedintopartialwaves,showthatmorethat80%
qq-q¯q¯states [5]. It has even been suggested that a com- of the kaons are produced in a relative s–wave, corre-
plete nonet of four-quark states might exist with masses sponding to the a+0 channel[20]. In this paper we report
below 1.0 GeV/c2 [6]. on the analysis of the reaction pp → dπ+η, which was
measured in parallel.
Thefirstclearobservationoftheisovectora (980)res-
0
onancewasachievedinK−pinteractions[7],andinsub-
II. THE ANKE SPECTROMETER AND DATA
sequentexperiments ithas alsobeenseeninpp¯annihila-
tions[8],inπ−pcollisions[9],andinγγ interactions[10]. ANALYSIS
Experiments on radiative φ-decays [11, 12] have been Fig. 1 shows the layout of ANKE. It consists of three
analysed in terms of the a /f production in the decay dipole magnets (D1 – D3), installed at an internal tar-
0 0
2
get position of COSY. D1 and D3 deflect the circulat- lators. Twodistinct bandscorrespondingto protonsand
ing COSY beam from and back into the nominal or- deuterons are seen in this distribution (Fig. 2b). The
bit. The C-shaped spectrometer dipole D2 separates selection of deuterons was achieved by cutting along the
forward-going reaction products from the COSY beam. deuteron band.
The angular acceptance of ANKE covers |ϑ | ≤ 10◦
h
horizontally and |ϑ | ≤ 3◦ vertically for the detected
v
deuterons (p > 1300 MeV/c), and |ϑ | ≤ 12◦ and
d h
|ϑ | ≤ 3.5◦ for the pions. An H cluster-jet target [21],
v 2
placed between D1 and D2, has been used, providing
areal densities of ∼5×1014cm−2. The luminosity has
been measured using pp elastic scattering, recorded si-
multaneously with the dπ+ data. Protons in the angu-
lar range ϑ = 5.5◦ − 9◦ have been selected, since the
ANKE acceptance changes smoothly in this region and
the elastic peak is easily distinguished from the back-
FIG. 2: a) Time-of-flight of particles detected in TOF start
ground in the momentum distribution. The average lu-
counter 18 and stop counter 4; b) time difference ∆t be-
minosity during the measurements has been determined tween fast forward-going particles in the FD and π+ mesons
asL=(2.7±0.1stat±0.7syst)×1031s−1cm−2,correspond- as a function of the momentum of the forward particle. The
ing to an integrated value of Lint =3.3pb−1 at a proton dashed lines indicate thecriteria for deuteron identification.
beam intensity of up to ∼4×1010.
The tracking efficiency for pions in the SD MWPCs
wascalculatedastheratioofparticlesintheproperTOF
D2
x range with and without requiring a reconstructed track.
Negative ejectiles The efficiency depends on the SD stop-counter number
(i.e. π+ momentum) and varies between 53% and 76%.
z
y
For the FD MWPCs the efficiency has been determined
1m foreachofthesixsensitiveplanes(twoperchamber). At
D1 D3 least two vertical and horizontal planes were demanded
Target MWPC3-S5cintillator for track reconstruction and for the calculation of the
hodoscope intersection point with the remaining plane. Each plane
1 23 COSY beam has been divided in 20×20 subcells, and the efficiency
TOF-Start FD d of each cell has been calculated from the presence and
MWPC1,2 absence of a hit in the reconstructed intersection. The
SD #6
averageFDMWPCtrackefficiencyfordeuteronsis73%.
π+
#1 The efficiencies of the scintillators and TOF criteria are
Focal-surface telescopes
TOF-Stop larger than 99% [23]. The efficiency correction is done
Positive ejectiles on an event-by-event basis.
FIG. 1: Top view of the ANKE spectrometer and simulated
tracks of pions and deuterons from pp→dπ+η events.
III. RESULTS FOR THE REACTION pp→dπ+η
Two charged particles, π+ and d, were detected in co-
incidence. Theirtrajectories,simulatedwiththe ANKE- The missing mass distributions mm(pp,d) and
GEANT program package [22], are shown in Fig. 1. mm(pp,dπ+) for the selected dπ+ pairs are presented in
Positively charged pions in the momentum range p = Fig.3. Inthe (pp,dπ+)missingmassdistributionaclear
π
(600−1100)MeV/c were identified in the side detection peak is observed around m(η)=547 MeV/c2 with about
system (SD) [19, 23], consisting of one layer of 23 start 6200 events. The peak sits on top of a smooth back-
time-of-flight (TOF) scintillation counters, two multi- ground from multi-pion production pp →dπ+(nπ) (n ≥
wire proportional chambers (MWPCs) and one layer of 2). After selecting the mass range (530−560) MeV/c2
6 counters for TOF-stop. Pions were selected by a time- aroundtheηpeak,themissingmassspectrummm(pp,d)
of-flight technique (Fig. 2a). The fast deuterons with exhibits a shoulder at 980 MeV/c2 (Fig. 3a, dotted),
momenta p = (1300 − 2800) MeV/c (and elastically where the peak from the a+(980) resonance is expected.
d 0
scattered protons with p ≈ 3400 MeV/c) hit the for- TableIpresentsthedifferentialcrosssectionsforpp→
p
warddetectionsystem(FD)[19,24],whichincludesthree dπ+ηfortworegionsofphasespacewheretheacceptance
MWPCs and two layers of scintillation counters. Two- of ANKE is essentially 100% for this reaction. Six vari-
dimensional distributions ∆t vs. momenta of forward ables in the lab. system have been chosen to describe
particles have been used for the deuteron identification. these rectangular areas: the vertical (θ ) and horizon-
y
∆t was calculatedas the time difference betweenthe de- tal (θ ) angles and momenta of the two detected par-
x
tection of a pion in the TOF-stop counter and a fast ticles, deuteron and π+. These angles are defined as
forward-goingparticle inthe firstlayerofthe FD scintil- tan(θ )=p /p , tan(θ )=p /p .
y y z x x z
3
u.] while, for the non-resonant π+η production, different
m [a. 12x103 a) 4x103 b) predictions exist, ranging from (0.6−1.4) µb [30] and
N/d 3x103 (1.6−3.3)µb[31]uptooneorderofmagnitudemore[26].
d 8x103 ThedifferentialcrosssectionsfromTableIcontaincon-
2x103 tributions from both the resonant and the non-resonant
4x103 x 6 reactions. We have performed a model-dependent anal-
103
ysis using the calculated momentum distributions of the
0 0 produceddeuterons(seeFigs.4a,b). Thecalculationsare
900 1000 200 400 600 800 based on models describing the resonant process within
mm(pp,d) [MeV/c2] mm(pp,dπ+) [MeV/c2]
the Quark-Gluon Strings Model (QGSM) (Fig. 5a) [32]
FIG. 3: Missing mass distributions (a) mm(pp,d), (b) and the non-resonant (Fig. 5b) production via N∗- and
mm(pp,dπ+) for the reaction pp → dπ+X. The dotted
∆-resonanceexcitation[29, 31, 33]. The momentum dis-
histogram in mm(pp,d) (scaled by factor 6) corresponds to
tribution for the resonant reaction is narrower, because
the selected area around the η peak (530−560 MeV/c2) in
mm(pp,dπ+) (indicated byarrows). low(π+η)massesaresuppressedfora+0 production. The
momentum distributions within the ANKE acceptance
are shown in Figs. 4c,d.
d4σ/dΩddΩπ+dpddpπ+ variables, lab. system
µb/(srGeV/c)2 θy, deg.deuteθrxo,ndveagr.iablesp, GeV/c dp [a.u.] 56xx110044 a) pp->da+0 4x104 b) pp->dπ+η
71±6stat±20sys (−3◦,+3◦) (−3.5◦,+3.5◦) (1.4,1.6) dN/ 4x104 3x104
pion variables 3x104 2x104
(−4◦,+4◦) (−11◦,−3◦) (0.65,0.95) 2x104
1x104
1x104
0 0
1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5
deuteron variables p(d) [GeV/c] p(d) [GeV/c]
30±4stat±9sys ((−−34◦◦,,++34◦◦)) p(i(−o−n161v◦◦a,,r−−ia23b◦◦l)e)s (0(.16.85,,02..975)) dp [a.u.] 210500 pApN-K>dEa+0 c) 330500 d) pApN-K>dEπ+η
N/ acceptance 250 acceptance
d
200
100
150
TABLE I: Differential production cross sections for the re-
action pp → dπ+η. In both regions of phase space a+ and 50 100
0
non-resonant π+η productions contribute. For the momen- 50
tumrangepd =(1.4−1.6) GeV/cthenon-resonantπ+η pro- 01 1.5 2 2.5 3 3.5 01 1.5 2 2.5 3 3.5
duction should be dominant, because this momentum range
corresponds to low masses of the π+η system, where the a+ p(d) [GeV/c] p(d) [GeV/c]
0
production is suppressed. FIG.4: Simulatedacceptanceforthedeuteronmomentumfor
theresonant(a,c)andthenon-resonant(b,d)π+ηproduction.
Upper row: the initial model distributions [29, 31, 32, 33];
lowerrow: momentumdistributionswithintheANKEaccep-
Unfortunately, due to the limited phase-space cover- tance.
age of ANKE, a partial wave decomposition of the type
performed in Ref. [20] is not possible in this case. Thus, π+
in order to interpret the data in terms of a+ and non– p q a+ ∆++ (∆+)
0 0 p
resonant π+η production, we need to employ a model.
p(n)
This investigation will be described in the next section. d
a) b) π+(π0)
n(p)
p
IV. INTERPRETATION OF THE RESULTS p 5q d
η
FIG. 5: Diagrams describing the resonant π+η production
The dπ+η final state can be produced via the a+0 res- within the QGSM [32] (a) and the non-resonant via N∗ and
onance, pp → da+ → dπ+η (resonant production), or
0 ∆ excitations [29, 31, 33](b).
through the direct reaction pp → dπ+η (non-resonant
production). The production mechanism for the a+ has Withinthe modelsthenon-resonantbackgroundhasa
0
been studied theoretically in Refs. [25, 26, 27, 28, 29]. negligible πη s–wave contribution and thus does not in-
According to Refs. [25, 26], the cross section for a+ terfere with the resonant amplitude. As a consequence,
0
production at T=2.65 GeV is expected to be ∼ 1µb in the range p =(1.4−1.6) GeV/c, the contribution of
d
4
thenon-resonantprocessdominateswhereastheresonant u.] 103 x 102
pTahruts,isthneegclrigoisbslysescmtioanllo(sfeteheshnaodne-dreasroenaasnitncoFnigtrsi.b4uct,ido)n. dm [a. 102 a) sum 3π 4π 125000 b)gexeapn.t (sum)
in this momentum range can be defined by fitting the η N/ 5π
peak in mm(pp,dπ+) (Fig. 6a) and extracting the num- d nonres. 100
ber ofdπ+η events. The missing massspectrahavebeen 10
50
described by the sum of a Gaussian distribution and a a
0
4th order polynomial. 1 0
0.85 0.9 0.95 1 1.05 0.85 0.9 0.95 1 1.05
mm(pp,d) [GeV/c2] mm(pp,d) [GeV/c2]
u.] 1000 4x103
a. a) b) FIG. 7: a) GEANT simulations for multi-pion background
m [ 800 χ2/ndf=0.9 3x103 χ2/ndf=1.1 (pp→dπ+(nπ) (n≥2)), and for resonant and non-resonant
N/d production of the dπ+η final state in the ANKE acceptance.
d 600 b) comparison between the simulated missing mass distribu-
2x103
400 tion mm(pp,d) and experimental data.
1x103
200
in accordance with the predictions of the models dis-
0 0 cussed above, the ratio of the total cross sections is
200 400 600 800 200 400 600 800 R = σ(a+ → (K+K¯0) )/σ(a+ → π+η) = 0.029 ±
mm(pp,dπ+) [MeV/c2] mm(pp,dπ+) [MeV/c2] 0 L=0 0
0.008 ±0.009 .
FIG. 6: Missing mass distribution mm(pp,dπ+) for the mo- stat sys
mentum ranges p = (1.4−1.6) GeV/c (a) and p = (1.6−
d d
2.8) GeV/c (b).
o
ati
R
Inthemomentumrangep =(1.6−2.8)GeV/c,where
d
both the resonant and the non-resonant reactions con-
tribute, it is possible to calculate the number of events 10-1
from the non-resonant production taking into account
the differentacceptances. Thenthe number ofa+ events
0
ANKE
is the difference between the total number of events un-
T =2.65 GeV
p
der the η peak (Fig. 6b) and the calculated amount of Q=46 MeV
non-resonant π+η events. -2
10
Including all corrections, total cross sections σ =
a0
(1.1 ± 0.3 ± 0.7 ) µb for the a+ production and
stat sys 0
σ =(3.5±0.3 ±1.0 )µbforthenon-resonantπ+η
n.r. stat sys
production have been obtained. It is obvious, that these
numberscouldchangeifdifferentassumptionsweremade 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Q ,GeV
onthe partialwavedecompositionofthe (non–)resonant
contributions. However, we here refrain from investigat- FIG. 8: Ratio of the total cross sections σ(a+0 →
ingfurthertheuncertaintyinducedbythemodelassump- K+K¯0)/σ(a+0 →π+η)asfunctionof Q. Theerrorbarshows
thestatistical error only.
tions.
Figure 7a presents the results of GEANT simulations
oftheANKEacceptanceformultipionbackground(pp→ For measurements at an excess energy Q >> Γ ∼
dπ+(nπ) (n ≥ 2)), and for resonant and non-resonant (50 − 100) MeV, the ratio of the total cross sectai0ons
production of the dπ+η final state. The number of ini- R = σ(a+ → K+K¯0)/σ(a+ → π+η) should not depend
0 0
tialeventsforeachreactionisproportionalto theknown uponQ. However,whenQisoftheorderofΓ ,thecon-
cross sections for multipion production [34] and the val- tributionfromthea+ →K+K¯0 decaychanneal0decreases
0
ues of the total cross sections for the resonant and non- morestronglyduetothe proximityoftheK+K¯0 thresh-
resonant π+η production given above. The shape of the old and the consequently limited available phase space.
simulatedmissingmassdistributionmm(pp,d)including The calculated ratio of the two cross sections as a func-
all channels is in good agreement with the experimental tion of Q is presented in Fig. 8. The calculation is nor-
data (Fig. 7b). malisedto R((KK¯)/(πη))=0.23±0.05,which has been
Data for the second a+ decay channel, a+ → K+K¯0, measured at Crystal Barrel in the reaction pp¯→ a π at
0 0 0
have been obtained at ANKE in the reaction pp → Q = 768 MeV [35]. The parameters of the Flatt´e distri-
da+ →dK+K¯0, simultaneously with the π+η data. The bution[36]m =999±2MeV/c2,g =(324±15)MeV
0 0 πη
measuredtotal crosssectionis σ(pp→dK+K¯0)=(38± and r = g2 /g2 = 1.03 ± 0.14 are also taken from
2stat±14sys) nbandthe contributionofthe (K+K¯0)L=0 Ref. [35]. TKhKe errπoηrs of the Flatt´e parameters define the
channel is ∼ 83% [20]. Assuming that s–wave (K+K¯0) range of possible R values (shaded area in Fig. 8). Our
production proceeds fully via the a+(980) resonance, result is in agreement with the calculated value.
0
5
V. CONCLUSIONS production in pp collisions, obtained in a simultaneous
exclusive measurement of both a+ decay channels.
0
To summarize, data on the reaction pp → dπ+η at
T = 2.65 GeV are presented. Differential cross sections
p
for limited regions of phase space, corresponding to for- We are grateful to C. Wilkin for critical discussions
wardemissionof the d and π+ in the laboratorysystem, and a careful reading of the manuscript. We would like
have been extracted. Dynamical models for the reaction to thank to W. Borgs,C. Schneider and the IKP techni-
were employed to obtain the total cross sections for the cians for the support during the beamtime. One of the
resonant π+η production via the a+(980) and for the authors (P.F.) acknowledges support by the COSY-FFE
0
non-resonant π+η channel. For a+ production the value program (grant FFE-41520733). The work at ANKE
0
for the total cross section pp→da+ →dπ+η is in agree- has partially been supported by: BMBF (grants WTZ-
0
ment with theoretical predictions based on the value for RUS-686-99, 211-00, 691-01, GEO-001-99, POL-007-99,
the pp → d(K+K¯0) reaction recently measured at 015-01), DFG (436 RUS 113/444, 630, 733,787), DFG-
L=0
ANKE [20] and the branching ratio BR(KK¯/πη) from RFBR03-02-04013,02-02-04001(436RUS113/652),Pol-
the literature. ish State Committee for Scientific Research(2 P03B 101
Our results, together with those from ANKE on the 19), Russian Academy of Science (RFBR 06518, 02-02-
decaychannela+ →K+K¯0,arethefirstevidencefora+ 16349),ISTC (1861, 1966).
0 0
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