Letter of Intent for the BGOegg Experiment · Letter of Intent for the BGOegg Experiment 4 We will...

Letter of Intent for the BGOegg Experiment

Spokesperson : Hajime Shimizu

Research Center for Electron Photon Science, Tohoku University1-2-1 Mikamine, Taihaku, Sendai, Miyagi 982-0826, Japan

Tel: 81-22-743-3423, FAX: 81-22-743-3401e-mail: [email protected]

November 19, 2012

Contents

1 Introduction 2

2 Physics Programs 22.1 η

′-mesic Nuclei and Related Topics . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Highly Excited Baryon Resonances . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Light-Flavor Mesons at 1–1.5 GeV/c2 . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Experimental Setup 53.1 Photon Detector BGOegg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 Forward Proton Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Charged Particle Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4 Liquid Hydrogen / Deuterium Target System . . . . . . . . . . . . . . . . . . . 103.5 Data Acquisition System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Facility and Available Beam 13

5 Experimental Group 15

6 Schedule 16

Abstract

We propose a hadron photoproduction experiment to be conducted at the SPring-8/LEPS2 beamline, making full use of BGOegg, a recently constructed 4π electro-magneticcalorimeter. The physics program concerning the proposed experiment will focus on thefollowing subjects: 1) search for precursory phenomena of the chiral transition in nuclei,such as character change of mesons and η

′-mesic nuclei; and 2) study of baryon resonancesthrough multi-meson production channels and/or with hidden strangeness. Our proposalwill provide one of the most fruitful photoproduction experiments which cannot be real-ized without combining the LEPS2 beam, the world-cleanest high-intensity GeV photonsand the BGOegg detector with the world-best energy resolution at 1 GeV.

Letter of Intent for the BGOegg Experiment 2

1 Introduction

Hadron photoproduction experiments play an important role to investigate new aspects ofhadron description because photons, coupling to quark and anti-quark pairs, work as a uniquehadron beam with flavor blindness. The LEPS2 project at SPring-8 has started to advancenew generation experiments in this field using a high intensity photon beam and large solid-angle detector systems. In this document, we propose a new experiment with a high resolutionelectro-magnetic (EM) calorimeter, called BGOegg, as an initial program of the LEPS2 project.

The BGOegg experiment aims to promote high precision ”quark-nuclear physics”, in whichwe investigate the origin of hadron mass, the hadron structure beyond a constituent quarkmodel, and so on. As widely discussed, dynamical mass generation coming from spontaneousbreaking of chiral symmetry can be studied by the nuclear medium modification of meson mass.For this kind of physics, we plan intensive studies on the η′ meson, which is also related to theUA(1) anomaly. In this case, it would be the key issue to find bound-η′ signal events in thehuge background, so that the BGOegg plays an essential role to reduce the background bydetecting η coming from the η′ decay. On the subject of quark configuration inside a hadron,it is effective to study high excitation states of baryon resonances, which are not well describedby the constituent quark model in contrast to the case of low lying baryons. New degree offreedom in the hadron structure would appear at the high excitation states. The LEPS2 energyand the large acceptance of the BGOegg detector are suitable for inspecting such high massresonances, which likely to decay by emitting multiple mesons.

The BGOegg calorimeter has been assembled at Research Center for Electron Photon Sci-ence (ELPH), Tohoku University. We are ready to move the detector system to the LEPS2experimental building at SPring-8 for installation. In the end of FY2012, a photon beam will beavailable at the LEPS2 beamline, where the beam intensity is expected to be one-order higherthan that of the preceeding LEPS experiments. We plan to start the BGOegg experiment soonafter the beam commissioning, and continue for ∼2 years until the construction of anotherdetector system for charged particles (LEPS2 spectrometer) finishes.

The tagged photon beam energy of LEPS2 is higher than the energy available by thebremsstrahlung photon beam at ELPH, so that the experiments at LEPS2 and ELPH arecomplementary to each other. We can explore a wide energy range of hadron photoproductionusing the data from the two sites. Hadron photoproduction experiments in the energy regionclose to the LEPS2 project are carried out at BONN-ELSA and J-Lab using bremastrahlungphoton beams. The CB-ELSA experiment at BONN is equipped with an EM calorimeter,while the CLAS experiment at J-Lab uses a spectrometer for charged-particle detection. TheBGOegg experiment would compete with them, however, there are many subjects to be ex-plored due to a diversity in hadron physics. And more detailed data are also required fromrecently developed theoretical works, such as coupled-channel analysis of nucleon resonances.We have a great advantage of being able to use a high-intensity polarized photon beam withhigh polarization thanks to Laser Compton Scattering.

2 Physics Programs

2.1 η′-mesic Nuclei and Related Topics

It is widely accepted to consider that the effective mass of the hadron is generated due tospontaneous chiral symmetry breaking which originates from quark and anti-quark condensatein the QCD vacuum. The variation of environmental temperature and density may cause the


Figure 1: Calculated spectra of the 12C(γ, p)11B ⊗ η′ reaction at Eγ = 2.7 GeV as a functionof the excited energy Eex. E0 is the η′ production threshold energy.

change of the vacuum condensate, resulting in changing the effective mass. Several theoreticalpredictions show the mass modification of mesons even in the normal nuclear density.

If U(3)L×U(3)R chiral symmetry is spontaneously broken, 9 Nambu-Goldstone (NG) bosonswould appear in the pseudo-scalar meson sector. Their masses are expected to be less than√

3mπ [1]. However, the experimental value of η′ meson mass is 958 MeV/c2, which is 4 timeslarger than

√3mπ. This problem is well known as the influence of the UA(1) anomaly, which

has been an important topic since 1970’s. The UA(1) symmetry is explicitly broken in QCDeven at the chiral limit due to a quantum effect. Our interest is in the question what happenson the mass of η′ when chiral symmetry is restored. According to an effective theory [2] usingthe NJL model with a UA(1) breaking term, the η′ mass is possibly modified under the finitetemperature. Recently, Nagahiro et al. [3] predicted a ∼150 MeV/c2 mass reduction of theη′ meson in the normal nuclear density, concluding to the existence of a bound state with anucleus (η′-mesic nuclei) as shown in Fig. 1. We are ready to shed light on the UA(1) anomalyeffect to hadron mass by improving the current poor experimental situation on this subject.

We will measure this η′ mass modification using a nuclear target with the BGOegg detectorsystem and the LEPS2 high intensity photon beam. We plan to require the detection of η′

meson in its decay to γγ, π0π0η, etc, in addition to analyzing the momentum of a forwardlyproduced proton in the A(γ, p) reaction. All final-state particles will be identified to eliminatea large amount of background processes. Signal peaks corresponding to the η′-mesic nucleiare searched for in the proton missing mass spectrum to evaluate the binding energy. The η′

mass reduction would be also seen in the invariant mass spectrum of its decay. Therefore, theBGOegg experiment requires a high-resolution calorimeter for the η′ decay and a high-resolutioncharged-particle detector for the extremely forward proton.

We also plan to investigate the elementary process of η′ photoproduction from a liquidhydrogen or deuterium target in addition to the study of η′ behavior in the nuclear medium.


We will measure differential cross sections, polarization observables, an η′N scattering length,and so on.

The mesic nuclei are predicted for vector mesons, too. The mass reduction of these mesonsis just due to the partial restoration of chiral symmetry, while the case of η′ is related alsoto the U(1) anomaly. The BGOegg experiment can access to this subject through the ωmeson photoproduction process in the similar way as the η′-mesic nucleus search. The recoilesscondition of the ω meson comes about at Eγ=2.75 GeV, where the photon beam intensity willincrease at LEPS2. The use of a photon beam also has an advantage to reach deeply inside anucleus for forming a bound state.

2.2 Highly Excited Baryon Resonances

The experimental study of baryon resonances is one of the keys to understand the quark config-uration inside a hadron, which confines quarks in the non-perturbative QCD region. Generally,the constituent quark model explains the mass, width, magnetic moment, etc of low lyingbaryons in the framework using three valence quarks with the u, d, and s flavors. However,high excitation states are not well reproduced by the quark model, and it is also known thatthere are ”missing” baryon resonances, which have not been found mainly by the partial-waveanalyses in elastic πN scattering experiments. In addition, the candidates of exotic hadronslike pentaquark baryons have been reported from the LEPS, FOREST, and other experiments.The LEPS2 energy is suitable for the study of such baryons, so that we plan to search for newresonances in the BGOegg experiement.

The multi-meson photoproduction process provides important information on highly excitedbaryon states, which usually have a large branching ratio to multi-meson decay channels. Weinvestigate double-meson photoproduction processes such as π0π0, π±π0, ηπ0, ηπ±, ηη channels

0

0.02

0.04

0.06

0.08

0.1

dσ/

dΩ (µ

b)

γp → pη γp → pη′

0

0.1

0.2

0.3

0.4

2 2.1 2.2 2.3

γp → pω

2 2.1 2.2 2.3W (GeV)

γp → pπ0

Figure 2: Differential cross sections for η, η′, ω and π0 photoproductions plotted as a functionof the total energy (W) at −0.8 ≤ cos Θc.m. ≤ −0.7.


and so on. Different combinations of the two mesons select the properties of excited high-massbaryons produced as an intermediate state. A wide angular coverage of the detector is necessaryfor the multi-particle detection, which will help in application of partial-wave analyses. TheBGOegg calorimeter is used not only for photon detection but also for charged particle detectionin combination with an inner cylindrical drift chamber (CDC) and plastic scintillators.

The detection of an η meson at the final state would be a good probe for a hidden strangecomponent inside a baryon. At the LEPS experiment, the differential cross sections for π, η,ω, and η′ photoproduction from a liquid hydrogen target were measured as shown in Fig. 2.A bump structure was found above 2 GeV in the total energy for the η channel [4]. However,the bump position depends on the η emission angle, and multiple resonances may contributeto the observed bump. The measured angular region is limited to the backward for mesonproduction because only a forward proton is detected for the missing mass analysis. In theBGOegg experiment, the η meson is directly detected in the large solid angle, and the detailsof the bump structure will be examined. In addition, new resonances will be searched for withthe energy-extended beam up to 2.9 GeV. The ηN and η∆ channels will be investigated in thissearch, where a neutron target is also employed.

2.3 Light-Flavor Mesons at 1–1.5 GeV/c2

The LEPS2 beam energy allows to produce light quark mesons up to ∼1.6 GeV/c2. Thehigh-mass mesons above ∼1 GeV/c2 typically possess a large width. There still remain manyopen questions in the meson sector of quark configuration. For example, the π1(1400) has anexotic quantum number JPC=1−+, so that it is considered as a candidate of the hybrid meson,which is a gluonic excitation of meson. This meson can be studied through the π0η channelin the BGOegg experiment. Also it is interesting to study glueballs, which consist of gluonsonly. Currently, the f0(1500) meson is considered as a glueball candidate, while the f0(1370)dominantly shows a usual meson configuration. Since the glueballs do not couple with photons,the branching fraction ratio of the γγ channel to the ηη channel may be a good probe to bemeasured in order to investigate the nature of those f0 mesons. In addition, the axial-vectormesons a1(1260) and K1(1270) are accessible in the 3π and K0

Sππ channels, respectively. Themass and width of these mesons are not well determined up to date, and we may contribute totheir measurements.

3 Experimental Setup

3.1 Photon Detector BGOegg

The main detector for the proposed experiment is a new EM calorimeter, BGOegg. Thiscalorimeter is an assembly of 1,320 Bi4Ge3O12 (BGO) crystals, covering the polar angle from24 to 144. The shape of each crystal is a frustum of isosceles trapezoid with a length of∼220 mm, corresponding to 20 radiation lengths. The front and rear faces of each crystal havecircumscribed spheres with a center (0, 0, 0). Not to make the area of the front face too small,the center of front face is placed on a prolate spheroid for the forward crystals, which are setat the polar angle smaller than 90. As for the backward crystals, the center of front face isplaced on the sphere like the LEPS Backward Gamma detector, which is a part of the FORESTdetector at ELPH. Thus, the BGOegg calorimeter takes a shape like an ”egg”. Figure 3 showsthe cross-sectional and schematic view of the BGOegg. Here, two types of photo-multipliertubes are used for the readout of signals: a metal-packaged H11334 for the central 960 crystalsand a head-on 3/4 inch H6524 for the peripheral 360 crystals.


z

x/y

+400−200

420420

200200

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hhw1w1

w2w2

H6524 (R4125)

H11334 (R11265)

photon beam

target

γγ

γγ

η'η'

γγ

pp

Figure 3: Cross-sectional (upper panel) and schematic (lower panel) views of the new EMcalorimeter BGOegg. The center of the crystal front faces is placed on a prolate spheroid anda sphere for the forward and backward hemispheres, respectively. The arrow indicates theincident photon beam.

Since each BGO crystal possesses a tapered shape, the significant longitudinal non-uniformityof light output is observed due to the focusing effect when all the crystal surfaces are polished.To uniform the light output as a function of the distance from the PMT window, the surfacesexcept for the front and rear faces are etched.

The performance study of a prototype calorimeter was made at the positron beamline at


ELPH [5]. The prototype calorimeter was made up with 25 BGO crystals, which were alignedin a 5 × 5 matrix. The 01B, 00B, 00F, 01F, and 02F type crystals, indicated in Fig. 3, wereused in this test. Momentum-analyzed positrons were used as an incident beam with the energyranging from 100 to 800 MeV/c. A beam profile monitor (BPM) was used to specify the positionof the incident positrons and make a trigger for the data acquisition. The BPM consists of twolayers of scintillating fiber (SciFi) hodoscopes, each of which is made by 16 SciFi’s with a crosssection of 3 × 3 mm2. The measured energy resolution is given by

(σE

E

)2

= (0.63%)2 +

(

1.06% ± 0.04%√

E/GeV

)2

+

(

0.45% ± 0.03%

E/GeV

)2

. (1)

Therefore, a 1.3% energy resolution is obtained for 1 GeV positrons. Simultaneously, theposition resolution was examined from the weights of energy deposites in neighboring crystals.The measured position resolution is expressed by

( σ

mm

)2

= (3.07 ± 0.03) (E/GeV)−0.202±0.008 , (2)

where the position resolution of 3 mm corresponds to the angular resolution of 1. Figure 4shows the energy and position resolution obtained for the prototype BGO calorimter as afunction of the incident positron energy. The BGOegg has the world’s highest resolution forseveral hundrend MeV photons.

0

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Incident Energy (MeV)

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y R

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Incident Energy (MeV)

Posit

ion R

esolu

tion (

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)

Figure 4: Energy resolution (left panel) and position resolution (right panel) obtained for theprototype BGO calorimeter as a function of the incident energy. The energy resolution dataare compared with eq (1). The position resolution data for the central modules are indicatedby the violet points and are fitted with eq (2). Those for the peripheral modules are indicatedby the blue points and are compared with 3.7× (E/GeV)−0.154. Here, the central module standfor the module that have all the adjacents, and the peripheral denotes the one that does nothave some of the adjacents.


3.2 Forward Proton Detector

We plan to detect a proton produced at extremely forward angles in the mesic nuclei search. Inany physics program, a detector for charged particles is useful to cover the forward acceptancehole of the BGOegg calorimeter. Currently, we plan to perform high-precision time-of-flightmeasurement using a resistive plate chamber (RPC), which provides a time resolution of ∼60psec. We will take a long distance of 10–15 m between the RPC and the BGOegg for the precisemomentum measurement. Alternatively, we have an option to use a bending magnet and driftchambers for the momentum measurement with high resolution.

3.3 Charged Particle Detector

A cylindrical drift chamber (CDC) and plastic scintillation counters will be placed inside theBGOegg. Figure 5 shows the structure of CDC. The chamber has 11 cylindrical layers and atotal of 288 drift cells to measure z-coordinate position. A reconstruction of charged particletracks makes it possible to select events produced at a target. The z information is providedby the stereo wires at four drift layers, where the stereo angles are set to 5.48, 5.88, −6.27 and−6.67 degrees. Detail parameters of the wire configuration are listed in Table 1. Expectedresolution of z-coordinate position is less than 2 mm. The chamber covers a large solid angle,corresponding to the polar angle region from 13 to 160. The plastic scintillation counterswill be placed around the chamber. The charged particle identification of π±/K±/p will bedone by dE/dx measurements of the plastic scintillation counters. In addition to the particleidentification, the plastic scintillators provide stop signals for TDCs of the chamber signals.

Table 1: Configuration of the CDC wires.

radius (cm) type offset angle (degree) Stereo angle (degree) number of wires6.40 Guard 2.5 +5.09 726.65 Field 0.0 +5.30 1446.90 Field 2.5 +5.48 726.90 Sence 0.0 +5.48 727.15 Field 0.0 +5.68 1447.40 Field 0.0 +5.88 727.40 Sence 2.5 +5.88 727.65 Field 0.0 −6.08 1447.90 Field 2.5 −6.27 727.90 Sence 0.0 −6.27 728.15 Field 0.0 −6.47 1448.40 Field 0.0 −6.67 728.40 Sence 2.5 −6.67 728.65 Field 0.0 −6.86 1448.90 Guard 0.0 −7.06 72


x (mm)35 40 45 50 55 60 65 70 75

y (m

m)

35

40

45

50

55

60

65

70

75Sence wire

Field wire

Guard wire

Figure 5: Upper Panel : Schematic view of the cylindrical drift chamber (CDC) for detectingcharged particles. Lower Panel : Cell structure of the CDC.


3.4 Liquid Hydrogen / Deuterium Target System

At the first stage of our experiment, we devote ourselves to the measurements of elementaryprocesses. While the detector performance and acceptance will be checked by those processes,we will investigate highly excited baryons, the fundamental parameters for the η′-N interaction,and so on. For these purposes, a liquid hydrongen and deuterium target system will be prepared.

The similar system has been used by the FOREST experiment at ELPH. Figure 6 showsthe schematic view of the cryogenic H2/D2 target system for the FOREST experiment. Boththe FOREST and BGOegg EM calorimeters have a large solid angle, and the refrigeratorshould be placed outside the calorimeter to minimize deterioration of the resolution due tomaterials in front of detectors. The target pipe is made of 4N pure aluminum whose thermalconductivity is exetremely high at the low temperature (∼ 10 K). This is approximately 10times higher than that of an ordinal aluminum alloy. The length of the target pipe is 1000mm, and the outer and inner diameters are 65 and 61 mm, respectively. A target cell with athickness of 40 mm is located at the end of the target pipe. Two aramid films with a thickness of12.5 µm are located upstream and downstream of the target cell. A two-stage Gifford-McMahonrefrigerator (Sumitomo Heavy Industries SRDK-415D-W71C) is used to cool down the targetcell. The cooling power of the refrigerator is 35 W at 50 K and 1.5 W at 4.2 K for the firstand second stages, respectively. Because of a high cooling power of the refrigerator and goodthermal conductivity of 4N pure-aluminium, achieved temperature of the target cell is 4.5 K.A pre-cooled pipe combines the target cell with compressed H2/D2 gas cylinders. It takes onlytwo hours to make up a liquid hydrogen target after 2.5-hour precooling of the target pipe,and takes only 1.5 hours to vaporize the target. The developed target system enables us tochange targets within only six hours. The temperature of the target cell is controlled by smalltemperature sensors and a heater, and we can keep the target stable for a long time.

PhotonBeam

Target CellRadiation Shield

Vacuum Pipe

Figure 6: A schematic view of the cryogenic H2/D2 target system for the FOREST experimentat ELPH. A similar system will be prepared for the BGOegg experiment.


3.5 Data Acquisition System

We plan to use the same data acquisition (DAQ) software framework as the FOREST exper-iment at ELPH. The framework consists of collector, event-builder, and recorder subsystems.The collector subsystem reads out the digitized data from the front-end electronics, and convertsthe data to a single data fragment. The event-builder subsystem gathers the data fragmentsfrom all the collector subsystems, and construct a physics event from the fragments. The physicsevent is transfered to the recorder subsystem, and stored on hard disk. The communicationbetween different subsystems are made by using the Transmission Control Protocol/InternetProtocol (TCP/IP). Three independent networks will be constructed:

1. between the collector and event-builder subsystems for the data transfer,

2. between the event-builder and recorder subsystems for the data transfer, and

3. between all the subsystems for the command transfer.

Figure 7 shows the schematic view of the DAQ framework and networks for the first-stageBGOegg experiment.

We have decided to use LeCroy 4300B Fast Encoding Readout ADC (FERA) modules,which were originally used for the BNL-E949 experiment, for the energy measurements ofBGO crystals due to a limited budget. The digitized data of FERA modules in each collectorsubsystem are collected by using a LeCroy 4301 FERA driver and a universal I/O (UIO)module. Both the LEPS experiment at SPring-8 and the FOREST experiment at ELPH adoptthe similar readout system for the energy measurement. The response timings of BGO crystalsand tagging detectors are measured with CAEN V1190A 128 Channel Multihit TDC modules.The FOREST experiment uses the similar readout system for the timing measurement. Thefront-end electronics for the other peripheral detectors have not been decided yet. The numberof readout words is expected to be approximately 2 k words for each event. To reduce thedead time for processing an event, the parallel readout of the digitized data will be made byseveral collector subsystems. Figure 8 shows the configuration of front-end electronics for thefirst-stage BGOegg experiments. To acquire two thousand events in a second, the processing

…eggc01 eggc08 eggc09 eggc10

eggr01 eggr02

eggc02 eggc07

eggebdata transfer network C

collector subsystems

event builder subsystem

data transfer network R

command transfer network (local DHCP)

14 FERA/CPU 14 FERA/CPU 6 V1190A/CPU 6 V1190A/CPU

Tagger

3 V1190A/CPU?14 FERA/CPU14 FERA/CPU 14 FERA/CPU 6 V1190A/CPU 6 V1190A/CPU

Tagger

2 V1190A/CPU14 FERA/CPU

recorder subsystems

Figure 7: A schematic view of the DAQ framework and networks for the first-stage BGOeggexperiment. The framework consists of collector, event-builder, and recorder subsystems. Threeindependent networks are prepared for the data and command transfer processes.


rate of 32 Mbit/s is required for recording on hard disks and data transfer in the networks.The rate can be achieved easily by using ordinary hard disks and Gigabit Ethernet.

VME

TDC

Event Builder

Recorder

VME

TDC + Scaler

VME

TDC

VMEVMEVMEVMEVMEVMEVME

FERA x 16

FERADriver

UIO

CAMAC

VMEVMEVMEVME VMEVMEVMEVMEVMEVMEVME

FERA x 16

FERADriver

UIO

CAMAC


FERA x 16

FERADriver

UIO

CAMAC

VMEVMEVMEVMEVME VMEVMEVMEVMEVMEVMEVME

FERA x 16

FERADriver

UIO

CAMAC


FERA x 16

FERADriver

UIO

CAMAC

VMEVMEVMEVMEVME VMEVMEVMEVMEVMEVMEVME

FERA x 16

FERADriver

UIO

CAMAC


FERA x 16

FERADriver

UIO

CAMAC

VMEVMEVMEVME

Tagging system

Figure 8: Configuration of front-end electronics for the first-stage BGOegg experiment.


4 Facility and Available Beam

We plan to install the BGOegg detector at the upstream space of LEPS2 experimental building,as shown in Fig. 9. Since the signal output from a BGO crystal with a bialkali photomultipliertube is sensitive to temperature with a rate of about −1.5%/C, the whole detector system iscovered by a thermostatic booth, whose dimension is W5200 mm × D4200 mm × H3000 mm.Cryogenic equipment for a liquid hydrogen or deuterium target will be also set up inside thebooth with an appropriate exhaust duct. We need to place six 19-inch racks for data acquistionand 1,320 sets of 30 m-long delay cables at the space surrounding the booth. The RPC willbe set up at the most downstream part of the experimental building, so that the time-of-flightmeasurement for a charged particle in the forward acceptance becomes possible with a 10–15m flight length. Most part of the experimental building will be inter-locked for radiation safetyduring data collection.

At the LEPS2 beamline, we expect that the photon beam intensity increases nearly one-order higher than that of the LEPS experiments [6]. Figure 10 shows the upstream part ofBL31LEP, where a photon beam is produced for the LEPS2 experiment. We will prepare 16 Wor 24 W ultra-violet (UV) lasers and 2 W deep ultra-violet (DUV) lasers, whose wavelengthsare 355 nm and 266 nm, respectively. We plan to operate four sets of those high power laserssimultaneously. The four laser beams will be merged by using prism mirrors at the laser room,and will be injected into the 8 GeV electron storage ring from the side wall of the acceleratorhall. Laser light is focussed to the electron beam size at the 30 m-long straight section, so thata photon can obtain high energy by backward Compton scattering. The injection of UV andDUV lasers provides the maximum photon energies of 2.4 and 2.9 GeV with expected intensitiesof ∼107 and ∼106/sec, respectively. Since electron beam divergence at the long straight sectionis small, the backscattering photon beam is well collimated. Therefore, we extract the photonbeam to the LEPS2 experimental building, which is located 150 m downstream. A recoilelectron will be tagged event by event at the downstream of an accelerator bending magnet inorder to measure its momentum and determine the photon energy.

Figure 9: A plan view of the LEPS2 experimental building.


Figure 10: An upstream part of the LEPS2 beamline.


5 Experimental Group

The LEPS2/BGOegg experiment is conducted under the collaboration listed below. Corre-sponding responsibilities are also summarized. RCNP, Osaka University and ELPH, TohokuUniversity will cohost the LEPS2 group and cooperate for the operation and maintenance ofthe facility.

• Research Center for Electron Photon Science (ELPH), Tohoku University (H. Shimizu,N. Muramatsu, H, Yamazaki, T. Ishikawa, M. Miyabe, Y. Tsuchikawa, Q.H. He) : Super-vision of the BGOegg experiment, Construction and operation of the BGOegg detector,Preparation of the data acquisition system, Preparation of a liquid hydrogen and deu-terium target, and Generation of the LEPS2 high intensity photon beam.

• Research Center for Nuclear Physics (RCNP), Osaka University (T. Nakano, M. Yosoi,S. Ajimura, T. Hotta, T. Yorita, Y. Sugaya, H. Kohri, J.Y. Chen, Y. Morino, M. Oka,C.J. Yoon, T. Sawada, Y. Kon, Y. Kasamatsu) : Cooperation of the LEPS2 experiment,Construction of the BL31LEP beamline, Preparation of experimental facilities (the LEPS2experimental building, electric power lines, a cooling water system, etc), Preparationof the tagging detector, Cooperative preparation of the data acquisition system, andGeneration of the LEPS2 high intensity photon beam.

• Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Or-ganization (KEK) (K. Ozawa) and Department of Physics, University of Tokyo (S. Ma-sumoto) : Construction and operation of the BGOegg detector, Construction of a cylin-drical drift chamber (CDC) and plastic scintillator hodoscopes in front of BGO crystals.

• Department of Physics, Yamagata University (Y. Tajima, H. Yoshida) and RIKEN SPring-8 Center (Y. Asano) : Preparation of the photon beam monitor.

• Japan Synchrotron Radiation Research Institute (JASRI), SPring-8 (H. Ohkuma, S. Suzuki,Y. Ohashi, S. Date) : Cooperative construction of the BL31LEP beamline, Cooperativegeneration of the LEPS2 high intenisy photon beam.

• Department of Physics, Kyoto University (M. Niiyama, Y. Nozawa, N. Tomida) andRIKEN Nishina Center (H. Onishi) : Development and construction of the resistive platechamber (RPC).

• Other group members of the LEPS2 collaboration from Gifu University, Wakayama Med-ical University, Academia Sinica (Taiwan), Pusan National University (Korea), etc jointhe LEPS2/BGOegg experiment for data collection and analyses.


6 Schedule

Our current plan of the BGOegg experiment is as follows:

2012 November The construction of BGOegg calorimeter is completed at ELPH,Tohoku University. The thermostatic booth is constructed atthe SPring-8 LEPS2 experimental building.

December The BGOegg calorimeter is transferred to SPring-8 and placedinside the thermostatic booth. The data acquisition system forforward 300 channels is prepared.

2013 January The construction of LEPS2 beamline is completed at SPring-8,and the commissioning of photon beam and the BGOegg detectorstarts immediately. We start data collection with the BGOegg.

September The BGOegg detector and data acquisition system is fullyprepared for all the 1,320 channels. We operate the experimentwith the full experimental setup.

References

[1] S. Weinberg, Phys. Rev. D 11, 3583 (1975).

[2] T. Kunihiro, Phys. Lett. B219 (1988).

[3] H. Nagahiro, M. Takizawa, and S. Hirenzaki, Phys. Rev. C 74, 045203 (2006).

[4] M. Sumihama et al. (LEPS Collaboration), Phys. Rev. C 80, 052201(R) (2009).

[5] T. Ishikawa et al., ELPH Annual Report Vol.1, 61 (2012).

[6] N. Muramatsu, arXiv:1201.4094 (2012).

Letter of Intent for the BGOegg Experiment · Letter of Intent for the BGOegg Experiment 4 We will...

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