Cascade baryon spectroscopy at J-PARC
high-momentum beamline
Megumi Naruki (Spokesperson)∗1 , Kotaro Shirotori (co-Spokesperson)2 ,Kazuya Aoki3 , Wen-Chen Chang4 , Takatsugu Ishikawa2 , Hiroyuki Noumi2,3 ,Hiroaki Ohnishi5 , Kyoichiro Ozawa3 , Fuminori Sakuma6 , Tomonori
Takahashi7 , and Natsuki Tomida1
1

Dept. of Physics, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto,606-8502, Japan
2
Research Center for Nuclear Physics (RCNP), Osaka University, 10-1
Mihogaoka, Ibaraki, Osaka 567-0047, Japan
3
Institute of Particle and Nuclear Studies, High Energy Accelerator
Research Organization KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
4
Institute of Physics, Academia Sinica, Institute of Physics, Academia
Sinica, Taipei 11529, Taiwan
5
1-2-1 Mikamine, Taihaku, Sendai, 982-0826, Miyagi, Japan, ResearchCenter for Electron-Photon Science(ELPH), Tohoku University
6
2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, RIKEN, Advanced
Science Institute (ASI)
7
Wako, Saitama 351-0198, Japan, Nishina Center for Accelerator-Based
Science, Institute of Physical and Chemical Research (RIKEN)
December 21, 2022

Executive Summary
We propose to perform cascade baryon spectroscopy at the high-momentum secondary beamline of J-PARC. The physics goals is to determine Ξ∗0 states up to
2.3 GeV/c2 with 7σ significance for the first time. We will measure the missing-mass
spectrum of K− p→K∗0 Ξ∗0 reaction and obtain production cross sections of K− p →K∗0 Ξ∗0 reactions at the beam momentum of 8 GeV/c. The J-PARC E50 spectrometeris used to measure scattered K∗0 together with decay products of Ξ∗0 . The expected
results will shed a light on an effective degree of freedom of baryon, especially the roleof (sq) diquarks.
∗

contact person

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Contents
1 Introduction
1.1 Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Scientific goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2
3
4

2 Experimental method
2.1 Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 RICH detector for beam particle . . . . . . . . . . . . . . . . . . . . . . . . .

667

3 Expected Results
3.1 Schedule and Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Beam Time Request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9
11
12

4 Impact for other baryon spectroscopic studies at J-PARC

13

1

Introduction

The first observation of cascade baryon Ξ was announced in 1950’s. The charged Ξ was found
for the first time in the distinct cascade decay recorded in the cloud chamber, as shown in
Fig.1 [1]. Basic information such as mass and decay modes was studied mainly in bubble

Figure 1: First observation of Ξ− decayed in the cloud chamber [1]. The names of particle
are added by the author.
chamber experiments in the early days. In 1980’s, more significant measurements were
performed using hyperon beams at FNAL and CERN SPS. They successfully measured abranching ratio of ground state and the production of light excited state Ξ(1690), respectively[2], [3]. Figure 2 shows the invariant mass spectrum of Ξ− π + measured at the CERN hyperon
beam. Above the clear peak of Ξ(1530), Ξ(1690) was observed with 4.7 σ significance [4].
They also reported that the ratio of the product of the production cross section and the2branching ratio of Ξ(1690) to Ξ(1530) is 2.2±0.5%. The small decay ratio is indicative of an
specific internal structure of Ξ(1690). Besides the hyperon beam experiments, Ξ resonanceswere also measured in the Kp reaction at the medium energy separated beamline at AGS,
and also in the LASS experiment at SLAC. Ξ∗ above 2 GeV/c2 were systematically searched
for through the missing mass technique as shown in Fig.3. However, not much information

Figure 2: a) Invariant mass spectrum ofΞπ b) its closeup and c) the spectrum after subtracting the background measured
in the CERN hyperon beam experiment[4]. Names of state are added by the author.

Figure 3: Missing mass squared of 5 GeV/c
K − p →K+ X reaction, measured at AGS [5].
Names of state are added by the author.

has been added since then. The current status of Ξ resonances is summarized in Fig.4. Only
two ground states are listed as a four-star state, whose existence is thought to be certain,
as classified by the Particle Data Group (PDG) [6]. This is mainly because the production
cross sections are small for Ξ, which has two strange quarks, and there has been a lack ofintense kaon beams.

1.1

Theoretical background

One of the main questions in baryon spectroscopy is what are the effective degrees of freedom to describe the spectrum of baryons. Baryon spectroscopy for light baryons has been
intensively performed at JLab, well summarized in [7]. They established newly 8 N* states
as three- and four-star states in the past 10 years. For a comprehensive review of baryon
spectroscopy, see [8]. The quark models are quite successful to describe electromagnetic
observable such as magnetic moments and form factors. However, they lead to the problemof missing resonances. One of the solution would be given by diquark models, introducing
3

Figure 4: Present status of Ξ. Summary table from PDG 2022 [6].
a pair of quark as the effective degree of freedom to describe baryons. All observed baryons
seems to be described in terms of a quark-diquark picture, and ”missing resonances” arenaturally forbidden in diquark models since the colour of diquark is restricted to be 3̄.
The Ξ is a baryon made of one light quark and two strange quarks, its quark configuration
is denoted by qss. They form isospin doublets in the baryon octet and decuplet, as shownin Fig.5. The quark model level diagram predicts 44 states up to 2.3 GeV/c2 as shown

Figure 5: Baryon multiplet [9].
in Fig.6, whereas only 2 four-star states and 4 three-star states were actually measured inexperiments. The level structure of Ξ is also calculated base on the quark-diquark model [10].
The spectrum displays a character different from that of three-quark models. As mentioned
above, since there is not much information about Ξ, still there are some missing resonanceseven in the comparison with the quark-diquark model.

1.2

Scientific goal

We aim at measuring the production of Ξ∗ up to 2.3 GeV/c2 in the K − p reaction systematically through the missing mass technique. We examine the existence of Ξ(1690) and othermissing resonances theoretically predicted. It will enable us to obtain the level structureof Ξ for the best quality in the world. For the first time the existence of Ξ∗ states aredemonstrated with more than 5σ significance for each states. The masses of each state is4

Figure 6: Quark model calculation for Ξ∗ masses [11].

Figure 7: The result of quark-diquark model calculation of Ξ∗ and Ω masses. Bluue boxes
are experimental masses from PDG [10].

5Figure 8: Schematic view of the J-PARC E50 Spectrometer.
determined with the accuracy of 0.1 MeV/c2 . Also the production cross sections in the K − preaction are determined. It will contribute to the future expansion of the Ξ∗ , Ω spectroscopies which will be performed at K10 beamline in the Extension project of the J-PARC
Hadron Experimental Facility [12].

2Experimental method

Ξ are produced in the K − p reaction at the high-momentum secondary beamline, which will
be utilized in the near future. The 8 GeV/c K− beam is irradiated into a liquid hydrogentarget with the thickness of 4 g/cm2 . The expected beam intensity is 6 × 105 kaons/spill,assuming that the production target of 5kW loss will be installed. The missing mass of thereaction K − p →− K ∗0 Ξ∗0 is measured to establish signals of Ξ∗ with 7σ. In addition, themissing mass of decays, Ξ∗0 → Σ+ K − /Ξ− π + , are measured by detecting K − /π + to suppress
the background.

2.1

Spectrometer

The beam, scattered and decay particles are measured with the J-PARC E50 spectrometer.
It can be regarded as a multi-purpose spectrometer providing a huge possibility to study all
strange baryons such as hyperons, Ξ∗ , Ω∗ and charm baryons. The schematic view of thespectrometer is shown in Fig.8. The detector R&D have been intensively performed so far.
The detail and current status of R&D of the spectrometer is well summarize in [13], [12] andreferences therein. A brief summary and relevant devices are described here.
6

The spectrometer dipole magnet for the J-PARC E16 experiment will be reused witha minor change of pole pieces. It enable us to analyze momenta of scattered and decay
particles with large geometrical coverage. The experimental target is located just upstream
from the spectrometer magnet.
A detection section for beam particles is located upstream the target. A series of scintillating fiber trackers [14] is used for the momentum analysis of beam particles. The time-zerois defined with a fine-segmented acrylic Cherenkov counter placed in front of the target [15].
Since the secondary beam particles are not specially separated, we introduce a RICH-typeparticle identification detector for beam particles (b-RICH). It is a key device for this measurements to separate beam kaons actively from pions. The detail of the b-RICH is described
below.
Decay products of scattered K ∗0 are detected with the forward detectors located downstream the target. Scintillating fiber hodoscopes and three planer drift chambers are combined for momentum analysis of K+ and π − to cope with the high interaction rate. The
momentum of these particles ranges from 0.8 to 2.3 GeV/c. For particle identification, theTime-Of-Flight and Ring Imaging Cherenkov measurements are combined. The TOF detector is composed of plastic scintillators. In the higher momentum region, K/π identificationis accomplished with a RICH together with a threshold-type Cherenkov detector. There
has been a substantial progress in the R&D of the Cherenkov detector [16]. Performances ofphoto sensors, mirrors and light-collecting cones have been minutely inspected and the study
led to the development of a transparent radiator of aerogel applicable to the momentum region of 2 to 4 GeV/c. It is concluded that the required efficiency of 95% will be accomplishedwith the agerogel radiator.
For the decay measurement, barrel-shaped drift chambers together with resistive-platechambers (RPCs), originally developed for the LEPS/LEPS2 experiments [17], will be installed into the sideway of a pole gap of the spectrometer magnet. The tracking devices ofthe fiber hodoscopes and the inner drift chambers placed just after the target are commonlyused with the forward measurement.
In the E50 experiment, a trigger-less DAQ will be implemented. This is called as a
streaming DAQ which enable us to realize the DAQ efficiency of 100%. The performance of
the DAQ system is evaluated with a prototype recently and it demonstrated the feasibility
successfully [18].

2.2

RICH detector for beam particle

Since the intensity of beam pions is larger by two orders of magnitude compared with that ofkaon’s, it is crucial to develop a detector which actively identifies kaons. We have developed
the RICH type detector named as b-RICH, which is able to identify kaons up to 10 GeV/c[19]. The conceptual design is shown in Fig.9. We use aerogels with index of 1.02 as radiator
which are suitable to separate kaons from pions for momenta greater than 5 GeV/c. The
ring image is once reflected at a concave mirror with radius of curvature of 1.0 m andfocused onto MPPC arrays. The performance of the prototype detector was evaluated using
electrons at the SPring-8 LEPS beamline, as summarized in [20]. The measured resolutionof the Cherenkov angle is shown in Fig.10. With the depth of 10 cm agerogels in total,
the enough Cherenkov angle resolution of 2.1 mrad has been achieved. Since the number of7

Figure 9: conceptual design of RICH detector for beam particles.

Figure 10: Measured Cherenkov angle for the prototype b-RICH.

8

photo sensor will be doubled, the angle resolution is expected to be improved to 1.5 mradin the case of the real device. Keeping the rejection of pions to 3%, the efficiency for kaon is
expected to be 95%.

3

Expected Results

We demonstrate the sensitivity of Ξ∗ measurement in Fig.11. The expected missing-massresolution is 6.6 MeV/c2 , enough good to separate each states. The signal yields is estimatedto be 1.3 × 104 for Ξ(1530) to Ξ(1950) states, 8.4 × 103 for Ξ(2030) and 4.5 × 103 for higher
excited states, respectively, with 30-day beamtime. The achievable sensitivity is discussed inSec.3.2. It should be noted that the S/N is improved by 30 times applying the decay event
measurement especially for higher resonances. Here we assume the following conditions:• The beam intensity of 8 GeV/c kaon is estimated to be 6 × 105 /spill based to the
Sanford-Wang parameterization assuming that;– the primary beam power of 30 kW– A 5kW-loss production target for the high-momentum secondary beamline– The intensity of pion is less than 6 × 107 , which is acceptable with the E50
spectrometer– repetition : 5.2 sec– spill length : 2.0 sec• The thickness of the liquid hydrogen target is 4.0 g/cm2• the geometrical acceptance of the spectrometer is estimated using Geant4 simulation.
• The background estimation is performed based on the event generator of nuclear cascade code JAM [21] that reproduces well non-strange and strange baryon productions
at the corresponding energy region. The detector effects are taken into account throughGeant4 simulation.
• The resolution of the missing mass is estimated with Geant4 using the measured position resolution of fiber trackers, 150µm and 250 µm for drift chambers which isreasonably achievable on the basis of past experiences. The effect of energy loss caused
by detector materials, especially of the experimental target is taken into account in thesimulation.
• Efficiencies of detectors and analysis are evaluated through the detector R&D and pastexperience, as summarized in Tab.1.
• Production cross sections for each Ξ∗ , σΞ∗ , are summarized in Tab.2. Since the crosssection with K∗ scattering is one third of that with K in hyperon productions, we
expect that σΞ∗ are scaled by 1/3 compared with the measured values [5].

9Figure 11: Expected missing mass spectra with event tagging for (top) Ξ∗0 → Σ+ K − and(bottom) Ξ∗0 → Ξ− π + decays.

10• Widths of Ξ∗ are not well determined. Data are spread in a wide range with large
errors, as summarized in Tab.3. The most large value is adopted for each resonances.
• Branching ratios of Ξ∗0 → Σ+ K − and Ξ∗0 → Ξ− π + are assumed to be 0.1.
• The beamtime is assumed to be 30 days.

Table 1: Expected efficiencies considered in estimation of yields.
item
efficiency
fiber tracker 98% [14]drift chamber 98%
PID detector 95% [16]DAQ
99% [18]
b-RICH
95% [19]
analysis70%

Table 2: Assumed cross sections of all appeared Ξ∗ states in PDG. The measured crosssection in the 5 GeV/c Kp → K+ X reaction [5] is also listed for reference.
cross section (µb)
statemeasured assumed
Ξ(1320) 7.2±0.6
2.0Ξ(1530) 2.8±0.60.80
Ξ(1620) < 10.80
Ξ(1690) < 10.80
Ξ(1820) 3.1±0.5
0.80
Ξ(1950) < 0.8
0.80
Ξ(2030) 1.7±0.4
0.48
Ξ(2120) < 1.1
0.28
Ξ(2250) 1.0±0.3
0.28
Ξ(2370) 0.9±0.30.28
Ξ(2500) 1.0±0.5
0.28

Figure 12 shows the missing mass spectrum of the K− p→K∗0 Ξ∗0 reaction, without decay
tagging. States below 1.9 GeV/c2 are well reconstructed with good S/N of 2 without the
decay measurement. The signal yield in this case is enlarged to be 1.7 × 105 .

3.1

Schedule and Cost

The proposed experiment will be performed at the high-momentum secondary beamline
which provides 5 GeV/c or more higher momentum kaons. The schedule depends on theconstruction schedule of the beamline. The construction of the E50 spectrometer is at almost
11

Table 3: Width of Ξ∗ . Data are taken from PDG [6].
width (MeV/c2 )statemeasured assumed
Ξ(1530) 9.1 ± 0.5 9.1
Ξ(1620) 21 ± 73040 ± 15Ξ(1690) 20 ± 1530Ξ(1820) 24 ± 524
Ξ(1950) 25 ∼ 140 60
Ξ(2030)20
Ξ(2120) 25 ± 1225
Ξ(2250) 46 ± 2747130 ± 80
Ξ(2370) 80
8080 ± 2575 ± 69Ξ(2500) 150+60
150−40
59 ± 27

the final stage. The construction budget for main components of spectrometer together with
b-RICH has been secured. The b-RICH will be completed till the end of FY2023. The last
big element of RICH detector for scattered particles is now being developed and will becompleted by FY2026 at the latest.

3.2

Beam Time Request

We request the beamtime of 30 days. We aim at observing the Ξ∗ up to 2.3 GeV/c2 with morethan 7σ significance. With 30-day beamtime, the yields of Ξ(1530) to Ξ(1950) is estimatedto be 1.3 × 104 . The background level is equal or less than the signal yield, therefore we areable to realize the significance of 10σ for these resonances. For Ξ(2030) to Ξ(2250), the yieldis decreased to 4.5 × 103 mainly because of the small cross section. However the S/N is kept
to be greater than 0.8, therefore the significance for these resonances is estimated to be 7σ.
Depend upon schedule, we kindly request additional beamtime for beamline and detector
commissioning. Since the production cross section of Ξ∗ is expected to be larger by an orderof magnitude than that of charmed baryons, the proposed experiment is naturally expected
to be performed before the E50. Moreover the outputs will give a good input for the E50,
as described in the next section. Therefore it will take a significant time for commissioning
including beam momentum optimization, targeting to optimize b-RICH operation, detector
timing and threshold optimization and obtaining control data samples. We expect that it
will take 4 days to start up completely new beamline and spectrometer.

12Figure 12: Expected missing mass spectrum of K− p→K∗0 Ξ∗0 reaction.

4

Impact for other baryon spectroscopic studies at JPARC

In this program, the production cross section of Ξ∗ up to 2.3 GeV/c2 will be measured.
Also the strange production, especially K+ production can be studied from the background
analysis. Since the strangeness production in hadronic reactions at this energy region is
not well established, it would be useful for estimating the background in the production ofcharmed baryon before the J-PARC E50 based on real measurements. Also the knowledge
obtained from this experiment will provide basic information for decays of Ω baryons, which
will be mainly studied at the K10 beamline in the future extension project of the J-PARC
Hadron Experimental Facility. The kaon intensity at the K10 will be increased by an order ofmagnitude compered with that of the high-momentum secondary beamline, therefore spinparity determination for Ξ∗ will be possible. It will give us the comprehensive understandingof key role of (sq) diquark possibly realized in Ξ baryons.

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