Measurement of the tensor analyzing power T20 in the dd->^3Hen and dd->^3Hp at intermediate energies and at zero degree

The data on the tensor analyzing power T20 in the dd->^3Hen and dd->^3Hp reactions at 140, 200 and 270 MeV of the deuteron kinetic energy and at zero degree obtained at RIKEN Accelerator Research Facility are presented. The observed positive sign of T20 clearly demonstrates the sensitivity to the D/S wave ratios in the ^3He and ^3H in the energy domain of the measurements. The T20 data for the ^3He-n and ^3H-p channels are in agreement within experimental accuracy.

The spin structure of the light nuclei has been extensively investigated during the last decades using both electromagnetic and hadronic probes. The main purposes of these studies at intermediate and high energies are to obtain the information on the highmomentum components of light nuclei affected by the relativistic effects and to search the manifestation of non-nucleonic degrees of freedom. Especially, three nucleon bound states are of interest, because even a fundamental constant as the binding energy in the system cannot be reproduced by calculations with modern pairwise nucleon-nucleon potentials [1]. Since the binding energy is known to have a strong correlation with strength of spin-dependent forces such as tensor force and/or three-nucleon force, experimental study of the spin structure of three-nucleon bound system is expected to provide a clue to understand the source of the missing energy.
The non-relativistic Faddeev calculations [2] for three-nucleon bound state predict that the dominant components of the 3 He ground state are a symmetric S-state, where the 3 He spin due to the neutron and two protons are in a spin singlet state and a D-state, where all three nucleon spins are oriented opposite to the 3 He spin. The S-state is found to dominate at small momenta while D-state dominates at large momenta. The relative sign of D-and S-wave in the momentum space is positive at small and moderate nucleon momenta [3].
The sensitivity to the different components of 3 He can be observed in polarization observables in both hadronic and electromagnetic processes.
Polarized electron scattering on polarized 3 He target, 3 He( e, e ′ )X, can be used to study the different components of the 3 He wave function [2]. However, it is necessary to take into account final state interaction (FSI) and meson exchange currents (MEC) in addition to the plane wave impulse approximation (PWIA) to describe the experimental results [4] obtained at different relative orientations of electron and 3 He spins. Recent CEBAF data on the transverse asymmetry A T ′ [5] at Q 2 values of 0.1 and 0.2 GeV/c 2 have been described using full Faddeev calculation with the MEC effects.
The 3 He( p, 2p) and 3 He( p, pn) breakup reactions were studied at TRIUMF in quasielastic kinematics at 200 [6] and 290 MeV [7] of incident proton energy. In the last experiment, spin observables A on , A on and A nn were measured up to q ∼ 190 and ∼ 80 MeV/c for 3 He( p, 2p) and 3 He( p, pn) reactions, respectively. The results indicate that analyzing powers A no , A on and A nn are close to the PWIA calculations for the 3 He( p, 2p) reaction, while for the 3 He( p, pn) there is a strong disagreement with these predictions. The same observables were recently measured at 197 MeV at IUCF Cooler Ring [8] up to q ∼ 400 MeV/c. It was observed that the polarization of the neutron and proton at zero nucleon momentum in 3 He are P n ∼ 0.98 and P p ∼ −0.16, respectively, that is in good agreement with the Faddeev calculations [2]. However, at higher momenta there is the discrepancy, which can be due to the uncertainty of the theoretical calculations, as well as to large rescattering effects.
One Nucleon Exchange (ONE) reactions, like dp → pd, d 3 He → p 4 He or d 3 He → 3 Hed, are the simplest processes with large momentum transfer and, therefore, can be used as an effective tool to investigate the structure of the deuteron and 3 He at short distances. In the framework of the ONE approximation [9] the polarization observables of the above reactions are expressed in terms of the D/S-wave ratios of these nuclei. For instance, tensor analyzing power T 20 for the dp → pd reaction in the collinear geometry is expressed in terms of D and S wave of the deuteron in the momentum space where r is the D/S-wave ratio in the deuteron at the corresponding internal momentum [9]. A significant amount of the data devoted to the investigation of the deuteron and 3 He( 3 H) spin structure at short distances have been accumulated in the last years. Recently, the tensor analyzing power T 20 and polarization transfer coefficients in backward elastic scattering, dp → pd, have been measured at Saclay, Dubna and RIKEN [10,11,12,13]. Another binary reaction, d 3 He → p 4 He, has been investigated at RIKEN using both polarized deuteron and 3 He up to 270 MeV [14,15,16]. All the data show the sensitivity to the deuteron spin structure at short distances. For instance, T 20 for both dp → pd [10,12,13] and d 3 He → p 4 He [15,16] reactions at intermediate energies has a large negative value reflecting the negative sign of the D/S-wave ratio in the deuteron in the momentum space.
As concerns 3 He spin structure, the tensor analyzing power T 20 in the d 3 He backward elastic scattering has been measured at 140, 200 and 270 MeV [17]. The sign of T 20 is found to be positive in accordance with the positive sign of D/S-wave ratio in the 3 He [3].
The data sensitive to the three-nucleon bound state spin structure are scarce, and, new polarization data, especially, at short distances is of great importance.
The dd → 3 Hp( 3 Hen) process is the simplest ONE reaction where three nucleon structure is relevant. The theoretical analysis of the polarization phenomena for this reaction in the collinear geometry has been performed [18,19]. It has been shown that the tensor analyzing power T 20 due to polarization of the incident deuteron can be expressed in the terms of the D/S-wave ratio in the 3 H( 3 He), when three-nucleon bound state is emitted in the forward direction in the cms. A new experiment has been proposed [20] to measure the energy and angular dependence of the tensor analyzing powers in the dd → 3 Hp( 3 Hen) process at RIKEN.
In this paper the data on the tensor analyzing power T 20 due to the incident deuteron polarization in the dd → 3 He(0 • )n and dd → 3 H(0 • )p reactions at 140, 200 and 270 MeV of the deuteron kinetic energy are presented.
The experiment has been performed at RIKEN Accelerator Research Facility (RARF). A polarized deuteron beam was produced by the high-intensity polarized ion source (PIS) [21] and accelerated by AVF and Ring cyclotrons up to the required energy. The direction of the beam polarization axis was controlled with a Wien filter located at the exit of the PIS. The polarization of the beam has been measured with the beam-line polarimeters based on the asymmetry measurements in dp-elastic scattering reaction, which has large values of tensor and vector analyzing powers [22,12]. New values of the analyzing powers for dp-elastic scattering at 140 and 270 MeV obtained during absolute calibration of the deuteron beam polarization via the 12 C(d, α) 10 B * [2 + ] reaction [23] have been used.
SMART (Swinger and Magnetic Analyzer with a Rotator and a Twister) spectrograph [24] has been used for the measurements. In this system, the incident beam can be swung by rotating the swinger magnet, while the magnetic analyzer is fixed. The measurement of the particle's momentum and separation from the primary beam was achieved by the magnetic system of SMART spectrograph consisting of two dipole and three quadrupole magnets (Q-Q-D-Q-D).
The beam intensity during the experiment was about 1-2 nA, the solid angle was 10 −2 sr. The live time of DAQ system [25] was more than 80% at the trigger rate of few thousands per second. A CD 2 thin sheet [26] has been used as a deuterium target. The measurements on CD 2 and carbon targets were made for each setup setting to obtain the contribution from deuterium via the CD 2 −C subtraction. The thicknesses of the CD 2 and carbon targets used were 54 mg/cm 2 and 34 mg/cm 2 , respectively.
Three plastic scintillators viewed by photomultipliers from the both sides were used in coincidence for the trigger and to provide the information about time-of-flight and energy losses of the particles. The distance between the target and detection point is about 17 m, which is enough to separate tritons, deuterons and protons with the same momentum. The time difference between the appearence of the trigger signal and radio-frequency signal of cyclotron was used as the time-of-flight information (TOF). The information about the charge of the particles was used at the trigger level. Protons and deuterons were partly suppressed in the cases of 3 He and 3 H detection by rasing threshold levels of the discriminators.
The event was used in the following analysis only in the case when information on the pulse height in all tree scintillation counters was consistent with the energy losses for particle of interest.
The set of multiwire drift chambers has been used to obtain the information on the hit position in the focal plane and, therefore, on the momentum of the particle and emission angle from the target. The energy resolution provided by the tracking system was ∼ 300 keV. The typical track reconstruction efficiency was better than 99%.
In the experiment the data for 3 He-n channels have been obtained at 140, 200 and 270 MeV, while for the 3 H-p channel at 140 and 200 MeV only, because the momentum of the 3 H at 270 MeV is higher than the maximal rigidity of SMART [24].
The quality of the CD 2 −C subtraction procedure for the dd → 3 He(0 • )n reaction at 270, 200 and 140 MeV is demonstrated in Fig.1 a), b) and c), respectively. The spectra are plotted versus excitation energy E X defined as following where P 0 is the incident momentum; E 0 = 2M d + T d is the total initial energy; E 3N and P 3N are energy and momentum of the three-nucleon system, respectively; M N is the nucleon mass. Peaks at E X = 0 MeV correspond to 3 He from the dd → 3 Hen reaction, while the shadowed areas show the carbon contribution. For the 3 H detection case the yield from carbon under peak at E X = 0 MeV is negligibly small. The peak for the binary reaction on deuterium, dd → 3 Hp, is separated from the peaks for the reaction d 12 C → 3 HX by ∼ 5 and ∼ 10 MeV at 200 and 140 MeV, respectively.
The events considered for the analysis were selected within polar angle acceptance ≤ 1.4 • .
Only the events for unpolarized spin mode and two spin modes with the ideal values of the tensor polarization p zz = −2 and p zz = +1 [21] were used for the analysis. Actual values of the beam polarization were ∼75%, ∼50% and ∼15% of the ideal values at 270 MeV, 200 MeV and 140 MeV, respectively.
Tensor analyzing power T 20 for each polarized spin mode was calculated from the following expression where p ii is the corresponding tensor polarization of the beam, σ pol and σ 0 are yields in polarized and unpolarized spin modes obtained via CD 2 −C subtraction after corrections of dead-time, detection efficiency and beam intensity. The analyzing power T 20 was taken as weighted averaged for two spin modes.
The results on the tensor analyzing power T 20 for the dd → 3 He(0 • )n and dd → 3 H(0 • )p reactions are given in the Table 1. The systematic error due to uncertainty in the beam polarization and statistical error are added in quadrature.
These data are also shown in Fig.2. The open triangles and full circles correspond to the 3 H-p and 3 He-n channels, respectively. The T 20 values obtained for the both charge symmetrical 3 H-p and 3 He-n channels at 140 and 200 MeV in this experiment are in good agreement within achieved experimental accuracy. No evidence of possible charge symmetry breaking is found in these processes.
The positive sign of T 20 values is in a striking contrast to negative T 20 for dp → pd or other reactions where deuteron structure is relevant. The energy dependence of T 20 demonstrates the increasing of T 20 magnitude with the energy. This behaviour can be understood in terms of D/S wave ratio in the 3 He( 3 H) with the help of ONE.
Within ONE approximation tensor analyzing power T 20 for the dd → 3 Hp( 3 Hen) reaction in the collinear geometry is expressed in terms of D and S wave ratio r of the 3 H( 3 He) [18,19,20] (see expression (1)). The positive sign of T 20 in the explored energy domain reflects the positive sign of the D/S wave ratio in the 3 He( 3 H) in the momentum space [2,3]. In this respect one can conclude that our data are sensitive to the D-state in the 3 He( 3 H).
The solid, dashed and dotted curves in Fig. 2 are the results of non-relativistic ONE calculations [20] using Urbana [27], Paris [28] and RSC [29] (with the parametrization from [30]) 3 He wave functions. Paris deuteron wave function [31] was used in the calculations. The data are in a qualitative agreement with the ONE calculations [20].
The behaviour of our data is consistent with the behaviour of T 20 for other reactions sensitive to the 3 He spin structure. In Fig.3 the data on T 20 in the dd → 3 He(0 • )n are plotted versus internal momentum k along with the data obtained for the d 3 He → 3 Hed reaction [17]. The data for the both processes demonstrate the universality in the kbehaviour. The solid curve is the result of ONE calculation using Urbana 3 He wave function [27] according to Eq.(1). The relativistic effects are taken into account by the minimal relativization scheme [32].
The discrepancy between the data and the calculations can be due both to the contribution from the reaction mechanism other than ONE and to the nonadequate description of the short-range 3 He spin structure. Concerning the reaction mechanism, the virtual excitation to the other channels, for example, excitation to ∆-isobar, is considered phenomenologically in Ref. [17] to reproduce the energy dependence of T 20 for the d 3 He → 3 Hed process. The microscopic calculation by Laget et al. [28] shows that the ∆-isobar contribution to the dd → 3 Hen process is less than 10% at energies lower than 300 MeV. At higher energies, in GeV region, it is shown that the coherent sum of ONE and the ∆-isobar excitation reproduces the cross section data reasonably. Thus it is expected that the uncertainty in the reaction mechanism is too small to explain the descrepancy between the data and the ONE calculation. Further theoretical investigations of the short-range 3 He spin structure as well as the reaction mechanism of the dd → 3 Hen process are needed to understand the T 20 data presented here.
The extension of the T 20 measurements to the higher energies, namely to larger internal momenta, is of great interest. In particular, the measurement of T 20 in the vicinity of k ∼0.4 GeV/c, where the changing of the T 20 sign is expected, could distinguish different models of the short range 3 He spin structure description.
The results can be summarized as following. The data on the tensor analyzing power T 20 in the dd → 3 Hp and dd → 3 Hen reactions at intermediate energies and in collinear geometry are obtained. The sign of T 20 is positive being in the agreement with the results on T 20 in the d 3 He → 3 Hed reaction [17], on the one hand, and with the ONE calculations using standard 3 He wave functions, on the other hand.
In general, ONE reproduces the global feature of the T 20 energy dependence. However, the deviation of the experimental data from the ONE calculations can be due to not only the nonadequate description of the short range 3 He spin structure, but also the influence of the mechanisms additional to ONE. To improve the description of the obtained data further theoretical calculations are required.
The extension of T 20 measurements to a GeV energy range is important to explore the short range 3 He spin structure.    [20] for the forward emission of the 3 He( 3 H) in the cms, using Urbana [27], Paris [28] and RSC [29] 3 He wave functions, respectively. Paris deuteron wave function [31] was used for the deuteron structure description.