Measurement of the Λ Spin-flip B(M 1) Value in
Hypernuclei
Y. Sasaki1 M. Agnello2,3 Y. Akazawa1, N. Amano4, K. Aoki5, E. Botta6,3, N. Chiga1, H. Ekawa4, P. Evtoukhovitch7, A. Feliciello3, T. Haruyama5, S. Hasegawa9, S. Hayakawa9, R. Honda1, K. Hosomi8, S. H Hwang8,
Y. Ichikawa4, Y. Igarashi5, K. Imai8, S. Ishimoto5, R. Iwasaki5, S. Kanatsuki4, K. Kasami5, T. Koike1, J. Y. Lee11, S. Marcello6,3, K. Miwa1, T. Nagae5, S. Nagao1, M. Nakagawa9, M. Naruki5, A. Sakaguchi9, H. Sako9, V. Samoilov7, S. Sato8, T. Shiozaki1, K. Shirotori11,
H. Sugimura4, S. Suzuki5, T. Takahashi5, H. Tamura1, K. Tanabe1, K. Tanida10, Z. Tsamalaidze7, M. Ukai1, T. F Wang12, T. O. Yamamoto1, Y. Yamamoto1, and S. B. Yang10
1 Department of Physics, Tohoku University, Sendai 980-8578, Japan
2 Dipartimento di Scienza Applicate e Tecnologia, Politecnico di Torino, corso Duca degli
Abruzzi, 10129 Torino, Italy
3 INFN, Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy 4 Department of Physics, Kyoto University, Kyoto 606-8502, Japan
5 Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research
Organization (KEK), Tsukuba, 305-0801, Japan
6 Dipartimento di Fisica, Universita‘ di Torino, Via P. Giuria 1, 10125 Torino, Italy 7 Joint Institute for Nuclear Research, Dubna ,Moscow Region 141980, Russia 8 Advanced Science Research Center (ASRC), Japan Atomic Agency (JAEA), Tokai,
Ibaraki 319-1195, Japan
9 Department of Physics, Osaka University, Toyonaka 560-0043, Japan
10 Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea 11 Research Center of Nuclear Physics, Osaka University, Ibaraki 567-0047, Japan
12 Research Center of Nuclear Science and Technology (RCNST) and School of Physics
and Nuclear Energy Engineering, Beihang University, Beijing 100191, China E-mail: ysasaki@lambda.phys.tohoku.ac.jp
(Received October 17, 2014)
A hypernulear γ-ray spectroscopy experiment (E13) will be performed using K− beam at the K1.8 beam line of the J-PARC Hadron Experimental Facility in 2015. In this experiment, we will determine the B(M 1) value of the Λ spin-flip transition of the ground state doublet in 19
ΛF to investigate the magnetic moment of a Λ hyperon in nuclei. In order to check the performance of the whole spectrometer system of E13, a commissioning run was performed at the K1.8 beam line in 2013. We confirmed that the E13 setup is almost ready for the next beam time in 2015. In particular, new detectors for suppressing events from the two main
K− decay modes in the on-line and off-line analyses are installed. As a result, the signal to noise ratio of gamma spectra is drastically improved, and the sensitivity to the B(M 1) measurement is expected to increase.
KEYWORDS: Hypernuclei, γ ray spectroscopy, medium effect of hadron
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©2015 The Physical Society of Japan http://dx.doi.org/10.7566/JPSCP.8.022013
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Proc. 2nd Int. Symp. Science at J-PARC — Unlocking the Mysteries of Life, Matter and the Universe —
1. Introduction
Properties of hadrons in nuclear matter such as the magnetic moment of a Λ hyperon in hypernuclei may be different from those in free space. Such nuclear medium effects on hadrons are expected to be caused mainly by partial restoration of the chiral symmetry in QCD vacuum. The g-factor of a Λ in nuclei can be derived indirectly by measuring a reduced transition probability between spin-doublet states [Λ spin-flip B(M 1)] in hypernuclei. Figure 1 shows a schematic illustration of the spin-doublet states of a hypernucleus and a Λ spin-flip M1 transition. Jc (Jlow) Jc+1/2 Jc-1/2 (Jhigh) (Jc !0)
Fig. 1. Spin-doublet states of a hypernucleus and a spin-flip M 1 transition
The B(M 1) value of such a Λ spin-flip M 1 transition can be expressed as [1],
B(M 1) = (2Jup+ 1)−1| hψlow||µ||ψupi |2 = 3 8π 2Jlow+ 1 2Jc+ 1 (gc− gΛ)2 = 9 16π 1 E3 γ 1 τ (1)
where gc and gΛ denote the g-factors of the core nucleus and Λ, respectively. Jc and Jlow are
the spins of the core nucleus and of the lower state of the spin doublet, while Eγ is a M 1
transition energy, as shown in Fig. 1. The B(M 1) value can be derived from the lifetime τ of the upper state of the spin doublet. The lifetime τ can be measured by analyzing a partly Doppler-broadened γ ray peak shape measured with Ge detectors.
The energy spacings of spin doublets of hypernuclei are typically several hundred keV, but sometimes of a few ten keV. Therefore, We need to use Ge detectors with an excellent energy resolution to resolve the spin doublet.
The recoil momentum of a hypernucleus produced via the (K−, π−) reaction is in the range of 0.1–0.6 GeV/c for pK= 1.8 GeV/c. When a γ ray is emitted from the recoil
hy-pernucleus in motion in the target material, the γ ray energy is shifted according to the recoiling velocity due to the Doppler effect. The de-excitaion lifetime can be extracted by comparing the γ ray spectral shape between the experimental data and a simulation. This method is called the Doppler shift attenuation method (DSAM), which is applicable when the lifetime of the γ-transition and the stopping time of the hypernucleus are of the same order of magnitude.
Several γ-ray spectroscopy experiments using a Ge detector array have revealed strengths of the ΛN spin-dependent interactions from precise measurements of the energy levels of p-shell hypernuclei [2]. They successfully determined the values of parameters in the effective ΛN interaction [4]. On the other hand, the g-factor of a Λ in nuclei is yet to be measured due to restriction in the stopping time of the recoiling hypernucleus determined by the target density and the recoil momentum as well as a need for large statistics. In the E13 experiment at J-PARC, γ-ray spectroscopy study of 19ΛF hypernucleus via the (K−, π−) reaction will be performed to investigate the effective ΛN spin-spin interaction in sd-shell hypernuclei. In this experiment, the Λ spin-flip B(M 1) value of 19
ΛF will also be obtained.
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The M 1 transition energy of 19
ΛF(3/2+→1/2+) and the lifetime of the 3/2+ state are predicted to be 300 keV [4] and 6 ps, respectively. The yield of 19ΛF(3/2+→1/2+) is expected to be maximum at pK− = 1.8 GeV/c [6]. Carbon tetrafluoride (CF4) (ρ = 1.6 g/cm3) is planned to be used as a hypernuclear production target. In these conditions, DSAM can be applied because the stopping time of 19ΛF in the target is estimated to be ∼2.4 ps by the SRIM code [5]. 2. E13 Experiment BH1 Mass slit Beam line spectrometer 0 5 m BFT D4 Q12 Q13 BC3,4 Q10 Q11 BH2 SksMinus spectrometer SKS magnet BAC1,2 SAC1 Target Hyperball-J SDC1,2 SP0 TOF SDC3,4 SFV SAC3 Iron block SMF
Fig. 2. E13 experimental setup
The E13 experiment will be performed at the K1.8 beam line [7] using the Supercon-ducting Kaon Spectrometer (SKS) [8] in the spectrometer configuration called SksMinus to-gether with a large-acceptance Ge detector array (Hyperball-J). These spectrometers are shown in Fig. 2. Each incident K−is momentum-analyzed by the K1.8 beam line spectrometer and irradi-ated on a CF4 target (20 g/cm2). The outgoing π− is analyzed by SksMinus. These mesons are identified by aerogel ˇCerenkov counters with a refractive index is 1.03 installed around the tar-get. Production of a 19ΛF hypernucleus is iden-tified in the missing mass spectrum. In coinci-dence with the (K−, π−) reaction, γ rays are measured by Hyperball-J installed surrounding the target.
K− decay events in the target region via
K−→ µ−ν and K¯ −→ π−π0 cannot be discrim-inated from (K−,π−) reaction events with the aerogel counters. They produce a huge number of fake triggers and remain in the missing mass spectrum at the most significant background even after the offline analysis. In order to sup-press those K− decay events, the Muon Filter (SMF) and π0 veto counter (SP0) are designed and used. SMF consists of hodoscope counters and iron. The decay µ− is detected after passing the iron blocks. SP0 consists of lead plates and plastic scintillators and installed at the entrance of the SKS magnet. High energy γ rays produced from π0 decay are detected as an electromagnetic shower in SP0. The detection efficiencies for K− → µ−ν and K¯ − → π−π0 decays are 99% and 60%, respectively. The de-tection efficiency of K− decay products is 80%. This is the first time to use such beam-decay suppression counters for hypernuclear γ ray spectroscopy.
The E13 commissioning data were taken in order to check the performance of the detector system under a realistic K− beam condition (300 k/spill, beam spill occurs every 6 s ) at the K1.8 beam line from March to May, 2013. Figure 3 shows a missing mass spectrum (left) and a γ spectrum (right) for the (K−, π−) reaction on a CH2 target. Spectra after taking anti-coincidence with the background suppressors are also shown. The signal to noise ratio of both spectra is drastically improved. In the γ spectrum, background level is reduced to 30% at around 300 keV, which improves the S/N ratio in the γ spectrum by 3.3 times.
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100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 GeE 1.14 1.16 1.18 1.2 1.22 1.24 0 100 200 300 400 500 600 700 Target : CH2 Black : ALL Red : SP0, SMF cut !19F (3/2+!1/2+) expected Energy [keV] C o u n ts C o u n ts Target : CH2 Black : ALL Red : SP0, SMF cut Mass [GeV]
!
+Fig. 3. Missing mass spectrum (left) and γ ray spectrum (right) before and after suppression of K− beam decay events. An expected M 1 transition energy of19
ΛF(3/2+→1/2+) of 300 keV is indicated.
3. Summary and Future Plans
The hypernuclear γ-ray spectroscopy experiment (E13) will be performed at the K1.8 beam line at J-PARC in 2015, where we will produce 19ΛF hypernuclei via the (K−, π−) reaction with 1.8 GeV/c K− beam to measure the B(M 1) value of 19ΛF(3/2+→1/2+). The E13 commissioning was performed at the K1.8 beam line in 2013 to check the performances of all the spectrometers and detectors. The E13 setup is almost ready for physics run in the next beam time in 2015.
References
[1] R. H. Dalitz and A. Gal, Ann : Phys.: 116, 167 (1978); J. Phys G6 (1978) 889.
[2] O. Hashimoto, H. Tamura : Progress in Particle and Nuclear Physics 57 (2006) 564-653. [3] H. Tamura, M. Ukai, T. O. Yamamoto and T. Koike : Nucl. Phys. A 881 (2012) 310. [4] D. J. Millener : Nucl. Phys. A 914 (2013) 109-118.
[5] http://www.srim.org/.
[6] H. Tamura et al.: “A Revised Run Plan of J-PARC E13”(2011). [7] T. Takahashi et al.: Prog. Theor. Exp. Phys. 2012 (2012) 02B010. [8] T. Fukuda et al.: Nucl. Inst. Meth. A 361 (1995) 485.
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