- Email: [email protected]
LEPS
Nuclear Physics A72 1 (2003) 739c-742~ www.elsevier.com/locate/npe
Threshold production of 4 mesons off protons by linearly polarized photons at Spring-8/LEPS W.C. Chang a and T. Mibe b ’ for the LEPS collaboration aInstitute of Physics, Academia Sinica, Taipei 11529, Taiwan bResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan CAdvanced Science Research Center, JAERI, Ibaraki 319-1195, Japan The LEPS/SPring-8 experiment measured the reaction of yp -+ 4~ at E-, = 1.5 2.4 GeV from December 2000 to June 2001. Preliminary results of azimuthal K+ decay angle distribution show that natural-parity exchange is relatively stronger than unnaturalparity exchange at energy E7 = 2.3 GeV in the forward direction. 1. Introduction The cross section of photo-production of vector mesons like p, w, and 4, is dominated by peripheral diffractive scattering which is well described within the vector meson dominance model (VDM) [l]. In this model, the incident photon fluctuates into virtual vector mesons of quark and anti-quark pairs which interact with the quarks in the nucleons by t-channel exchange. The energy dependence of production cross section can be parameterized in the following way [2]: o = &+0.08 +
BS-0.45,
(1)
where s is the center of mass energy. These two components with opposite energy dependence can be understood in terms of two Regge trajectories; one associated with (soft) Pomeron exchange and the other with meson exchange. Pomeron exchange, describing the universal rise of hadronic cross sections at high energies, is made of gluon origin and a spin-conserving process of natural parity. On the other hand, meson exchange is made of quark contents with an unnatural-parity for exchange of pseudoscalar particles like r and v and a natural-parity for that of scalar particles like aa and fe. Because of the negative power of the energy dependency, meson exchange dominates in the photoproduction of vector mesons near the threshold energy except for the case of 4 mesons. Because of the ss quark nature of the 4 mesons, its interaction with the u and d inside the nucleons via meson (quark) exchange is strongly suppressed due to the OkubaZweig-Iizuka (OZI) rule. Thus, the 4 photo-production is an unique channel to reveal the behavior of gluon exchange near the threshold. For example, it has been suggested that a precise measurement of unpolarized cross section near production threshold will clarify the O+ glueball contribution [3]. Furthermore, with usage of polarized photon beams 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V doi:lO.l016/S0375-9474(03)01170-9
All rights reserved.
74oc
KC. Chang, T. Mibe/Nuclear Physics A721 (2003) 739c-742~
and/or polarized targets, the contribution of processes with different parity properties in the t-channels can be disentangled [4]. And the polarization observables are found to be sensitive to a rather small strangeness content in the nucleon via the exotic ss-knockout process [5]. So far, experimental data for photo-production of 4 mesons near the threshold energy has been rather scarce and only one measurement of polarization observables with meager statistics exists [6]. The LEPS experiment has focused on the measurement of 4 photo-production at production threshold using GeV polarized photons [7]. A high energy photon beam is produced by laser-induced backward Compton scattering off 8 GeV electrons inside the Spring-8 storage rings. The maximum photon energy available is about 2.4 GeV. In the following sections, we will briefly describe the LEPS spectrometer and the detection of e!~ events. Preliminary results of the azimuthal angular distribution of K+ decayed from 4 mesons relative to the polarization direction of photons are presented.
2. Detectors
and Particle
Identification
The LEPS spectrometer is optimized for the measurement of 4 photo-production in the forward direction. It consists of Aerogel detector (AC), silicon strip vertex detector (SVTX), multi-wire drift chambers (DC), a dipole magnet and a time-of-flight wall (TOF). The e+e- pair production is vetoed at the trigger level using the AC. The SVTX and DC are used to measure and momentum-analyze charged tracks in the dipole field. Combined with the time-of-flight measurement, particle identification is performed. A 3-sigma separation of kaons and pions in mass is achieved up to momenta of 1.5 GeV/c. For more details about the detectors and the way to generate laser electron photons, see Ref. [7]. The d events are identified from the charged kaons (K+K-) decay mode. In principle, a detection of at least any two particles out of the three particles (K+, K-,p) in the final state is sufficient for charactering the event using cuts on invariant mass and missing mass of the detected pair.
3. Preliminary
Results
In this section, we present preliminary results for 4 photoproduction off 50-mm liquidHydrogen target. The beam energy coverage is from 1.5 to 2.4 GeV and the acceptance is limited to the forward direction of It] < 0.5(Gev/c)2 where t is the Mandelstam variable for momentum transfer. In total about 3000 events are reconstructed from the detected K+Kpairs produced by linearly polarized photon beams. 311. Invariant Mass and Missing Mass Spectra The invariant mass distribution of identified K+K- pairs is shown in Fig. 1 and fitted with a relativistic Breit-Wigner distribution of a fixed width (4.4 MeV) plus a background distribution. The fitted peak position of 1019.4 f 0.1 MeV is consistent with the worlddata average. The missing-mass spectrum peaks at the mass of protons as shown in Fig. 2 with a width of 10 MeV.
KC. Chang, T. Mibe/Nuclear Physics A721 (2003) 739c-742~
2
1600
: 0
1400
741c
1200 1000 800 600 400 200
1
Invariant
1.025
Mass
1.05
1.075
of K ’K-
1.1
(GeV)
Figure 1. The invariant mass distribution of identified K+K- pairs.
n”
1
0.8
Missing
Mass
of K ’K-
1.2
(GeV)
Figure 2. The missing-mass spectra of identified K+K- pairs.
3.2. Azimuthal Decay Angle of K+ The decay angular distribution (W) for vector mesons produced by linearly-polarized photons can be expressed in terms of nine measurable spin density matrix elements (pPlc) and the degree of photon polarization (P7) as a function of 6~, q5K and QT, being the polar and azimuthal angle of decayed kaons in the 4 meson rest frame and the polarization direction of the photons, respectively [8]. In the very forward direction, this angular distribution could be much simplified under the condition of no presence of any spin-flip processes. The scattering amplitude would be composed of spin-conserving amplitudes: that of natural-parity (TN) and that of unnatural-parity(TUN): 1 - a? T totat zz TN+TNU,---- ITN12 = ITUN12 cx2
where a is the relative weight of unnatural-parity amplitude to natural-parity one. With the increase of a, i.e. contribution from the unnatural-parity exchange in the total scattering amplitude, the angular distribution varies from (1 + PY cos 2(4K - CL,)) to (1 - py cos 2(&c - $))> corresponding to a change of peak position of (4K - @?) distribution from O(and 180) degree to 90(and 270) degree. Fig. 3 shows the measured azimuthal decay-angle distribution of the K+ relative to the photon polarization in the helicity system for 2.2 < ET < 2.4 GeV and -0.2 < t < 0.0 (GeV/c)2 (left-hand side: horizontal polarization; right-hand side: vertical polarization). No acceptance correction is applied which is however expected to be minor for
WC. Chang, I:
742~
M ibe/lVucIear Physics A721 (2003) 73%~742~
the chosenE7 and t region. It is clear that the contribution from natural-parity exchangecomposesa larger fraction in the total scattering amplitude at the energy as low as ET = 2.3 GeV sincethe peak positions are measuredto be at 0 and 180 degree. a0 70 60 50
5(
40
40
30
30
20
20
10
Figure 3. The distribution of azimuthal decay-angleof K+ (I#K) relative to the photon polarization (@?)in the helicity system. The left (right) plot is for eventsof linearly horizontal(vertica1) photons. Line is a fit of N (1 -t a * cos(2(& - @ -,))) to guide the eyes where a is the fitting parameter.
4. Cancluaion
We emphasizethe importanceand uniquenessof measuringI$photoproduction near the thresholdenergyin understandingthe behaviorof gluon exchangeat low energy. The prelim inary results of azimuthal decay-angledistribution show that natural-parity exchange is relatively stronger than unnatural-parity exchangeat energynear E-, = 2.3 GeV in the forward direction. Further analysisof the LEPS data with the experimentalacceptance correction will provide results for the spin density matrix elements. REFEFtENCES
1. T.H. Bauer, R.D. Spital, D. R. Yennie and F.M. Pipkin, Rev. Mod. Phys. 50, 261 (197%). 2. A. Donnachieand P.V. Landshoff,Phys. Lett. B396, 227 (1992). 3. T. Nakanoand H. Toki, Proceedingof the International Workshopon Exciting Physics with New AcceleratorFacilities,World Scientific,p.48 (1998). 4. A. 1. Titov et al., Phys. Rev. CSQ, 2993 (1999); Phys. Rev. CQO, 035205 (1999). 5. A. I. Titov, Y. Oh, and S. N. Yang, Phys. Rev. Lett. 79, OlKM (1997). 6. J. Ballam et al., Phya. Rev. D7, 3150 (1973). 7. T. Nakano for LEPS coilaboration, these proceedings; Nucl. Phys. A684, 71~ (2001).
8. K. Schilling,P. Seyboth, and G. Wolf, Nucl. Phys. B15, 397 (1970).
Threshold production of 4 mesons off protons by linearly polarized photons at Spring-8/LEPS W.C. Chang a and T. Mibe b ’ for the LEPS collaboration aInstitute of Physics, Academia Sinica, Taipei 11529, Taiwan bResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan CAdvanced Science Research Center, JAERI, Ibaraki 319-1195, Japan The LEPS/SPring-8 experiment measured the reaction of yp -+ 4~ at E-, = 1.5 2.4 GeV from December 2000 to June 2001. Preliminary results of azimuthal K+ decay angle distribution show that natural-parity exchange is relatively stronger than unnaturalparity exchange at energy E7 = 2.3 GeV in the forward direction. 1. Introduction The cross section of photo-production of vector mesons like p, w, and 4, is dominated by peripheral diffractive scattering which is well described within the vector meson dominance model (VDM) [l]. In this model, the incident photon fluctuates into virtual vector mesons of quark and anti-quark pairs which interact with the quarks in the nucleons by t-channel exchange. The energy dependence of production cross section can be parameterized in the following way [2]: o = &+0.08 +
BS-0.45,
(1)
where s is the center of mass energy. These two components with opposite energy dependence can be understood in terms of two Regge trajectories; one associated with (soft) Pomeron exchange and the other with meson exchange. Pomeron exchange, describing the universal rise of hadronic cross sections at high energies, is made of gluon origin and a spin-conserving process of natural parity. On the other hand, meson exchange is made of quark contents with an unnatural-parity for exchange of pseudoscalar particles like r and v and a natural-parity for that of scalar particles like aa and fe. Because of the negative power of the energy dependency, meson exchange dominates in the photoproduction of vector mesons near the threshold energy except for the case of 4 mesons. Because of the ss quark nature of the 4 mesons, its interaction with the u and d inside the nucleons via meson (quark) exchange is strongly suppressed due to the OkubaZweig-Iizuka (OZI) rule. Thus, the 4 photo-production is an unique channel to reveal the behavior of gluon exchange near the threshold. For example, it has been suggested that a precise measurement of unpolarized cross section near production threshold will clarify the O+ glueball contribution [3]. Furthermore, with usage of polarized photon beams 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V doi:lO.l016/S0375-9474(03)01170-9
All rights reserved.
74oc
KC. Chang, T. Mibe/Nuclear Physics A721 (2003) 739c-742~
and/or polarized targets, the contribution of processes with different parity properties in the t-channels can be disentangled [4]. And the polarization observables are found to be sensitive to a rather small strangeness content in the nucleon via the exotic ss-knockout process [5]. So far, experimental data for photo-production of 4 mesons near the threshold energy has been rather scarce and only one measurement of polarization observables with meager statistics exists [6]. The LEPS experiment has focused on the measurement of 4 photo-production at production threshold using GeV polarized photons [7]. A high energy photon beam is produced by laser-induced backward Compton scattering off 8 GeV electrons inside the Spring-8 storage rings. The maximum photon energy available is about 2.4 GeV. In the following sections, we will briefly describe the LEPS spectrometer and the detection of e!~ events. Preliminary results of the azimuthal angular distribution of K+ decayed from 4 mesons relative to the polarization direction of photons are presented.
2. Detectors
and Particle
Identification
The LEPS spectrometer is optimized for the measurement of 4 photo-production in the forward direction. It consists of Aerogel detector (AC), silicon strip vertex detector (SVTX), multi-wire drift chambers (DC), a dipole magnet and a time-of-flight wall (TOF). The e+e- pair production is vetoed at the trigger level using the AC. The SVTX and DC are used to measure and momentum-analyze charged tracks in the dipole field. Combined with the time-of-flight measurement, particle identification is performed. A 3-sigma separation of kaons and pions in mass is achieved up to momenta of 1.5 GeV/c. For more details about the detectors and the way to generate laser electron photons, see Ref. [7]. The d events are identified from the charged kaons (K+K-) decay mode. In principle, a detection of at least any two particles out of the three particles (K+, K-,p) in the final state is sufficient for charactering the event using cuts on invariant mass and missing mass of the detected pair.
3. Preliminary
Results
In this section, we present preliminary results for 4 photoproduction off 50-mm liquidHydrogen target. The beam energy coverage is from 1.5 to 2.4 GeV and the acceptance is limited to the forward direction of It] < 0.5(Gev/c)2 where t is the Mandelstam variable for momentum transfer. In total about 3000 events are reconstructed from the detected K+Kpairs produced by linearly polarized photon beams. 311. Invariant Mass and Missing Mass Spectra The invariant mass distribution of identified K+K- pairs is shown in Fig. 1 and fitted with a relativistic Breit-Wigner distribution of a fixed width (4.4 MeV) plus a background distribution. The fitted peak position of 1019.4 f 0.1 MeV is consistent with the worlddata average. The missing-mass spectrum peaks at the mass of protons as shown in Fig. 2 with a width of 10 MeV.
KC. Chang, T. Mibe/Nuclear Physics A721 (2003) 739c-742~
2
1600
: 0
1400
741c
1200 1000 800 600 400 200
1
Invariant
1.025
Mass
1.05
1.075
of K ’K-
1.1
(GeV)
Figure 1. The invariant mass distribution of identified K+K- pairs.
n”
1
0.8
Missing
Mass
of K ’K-
1.2
(GeV)
Figure 2. The missing-mass spectra of identified K+K- pairs.
3.2. Azimuthal Decay Angle of K+ The decay angular distribution (W) for vector mesons produced by linearly-polarized photons can be expressed in terms of nine measurable spin density matrix elements (pPlc) and the degree of photon polarization (P7) as a function of 6~, q5K and QT, being the polar and azimuthal angle of decayed kaons in the 4 meson rest frame and the polarization direction of the photons, respectively [8]. In the very forward direction, this angular distribution could be much simplified under the condition of no presence of any spin-flip processes. The scattering amplitude would be composed of spin-conserving amplitudes: that of natural-parity (TN) and that of unnatural-parity(TUN): 1 - a? T totat zz TN+TNU,---- ITN12 = ITUN12 cx2
where a is the relative weight of unnatural-parity amplitude to natural-parity one. With the increase of a, i.e. contribution from the unnatural-parity exchange in the total scattering amplitude, the angular distribution varies from (1 + PY cos 2(4K - CL,)) to (1 - py cos 2(&c - $))> corresponding to a change of peak position of (4K - @?) distribution from O(and 180) degree to 90(and 270) degree. Fig. 3 shows the measured azimuthal decay-angle distribution of the K+ relative to the photon polarization in the helicity system for 2.2 < ET < 2.4 GeV and -0.2 < t < 0.0 (GeV/c)2 (left-hand side: horizontal polarization; right-hand side: vertical polarization). No acceptance correction is applied which is however expected to be minor for
WC. Chang, I:
742~
M ibe/lVucIear Physics A721 (2003) 73%~742~
the chosenE7 and t region. It is clear that the contribution from natural-parity exchangecomposesa larger fraction in the total scattering amplitude at the energy as low as ET = 2.3 GeV sincethe peak positions are measuredto be at 0 and 180 degree. a0 70 60 50
5(
40
40
30
30
20
20
10
Figure 3. The distribution of azimuthal decay-angleof K+ (I#K) relative to the photon polarization (@?)in the helicity system. The left (right) plot is for eventsof linearly horizontal(vertica1) photons. Line is a fit of N (1 -t a * cos(2(& - @ -,))) to guide the eyes where a is the fitting parameter.
4. Cancluaion
We emphasizethe importanceand uniquenessof measuringI$photoproduction near the thresholdenergyin understandingthe behaviorof gluon exchangeat low energy. The prelim inary results of azimuthal decay-angledistribution show that natural-parity exchange is relatively stronger than unnatural-parity exchangeat energynear E-, = 2.3 GeV in the forward direction. Further analysisof the LEPS data with the experimentalacceptance correction will provide results for the spin density matrix elements. REFEFtENCES
1. T.H. Bauer, R.D. Spital, D. R. Yennie and F.M. Pipkin, Rev. Mod. Phys. 50, 261 (197%). 2. A. Donnachieand P.V. Landshoff,Phys. Lett. B396, 227 (1992). 3. T. Nakanoand H. Toki, Proceedingof the International Workshopon Exciting Physics with New AcceleratorFacilities,World Scientific,p.48 (1998). 4. A. 1. Titov et al., Phys. Rev. CSQ, 2993 (1999); Phys. Rev. CQO, 035205 (1999). 5. A. I. Titov, Y. Oh, and S. N. Yang, Phys. Rev. Lett. 79, OlKM (1997). 6. J. Ballam et al., Phya. Rev. D7, 3150 (1973). 7. T. Nakano for LEPS coilaboration, these proceedings; Nucl. Phys. A684, 71~ (2001).
8. K. Schilling,P. Seyboth, and G. Wolf, Nucl. Phys. B15, 397 (1970).