Proposal
Electron pair spectrometer at the J-PARC
50-GeV PS to explore the chiral symmetry in
QCD
April 28, 2006
June 07, 2006 rev.1
S. Yokkaichi1, H. En’yo, M. Naruki, R. Muto, T. Tabaru
RIKEN

K. Ozawa, H. Hamagaki
Center for Nuclear Study, Graduate School of Science, University of Tokyo

K. Shigaki
Graduate School of Science, Hiroshima University

S. Sawada, M. SekimotoHigh Energy Accelerator Research Organization (KEK)F. Sakuma, K. Aoki
Department of Physics, Kyoto University

1

spokesperson, yokkaich@riken.go.jp

Contents
1 Introduction

3

2 Physics
5
2.1 Predecessor experiment KEK–PS E325 . . . . . . . . . . . . . 6
2.2 Goals and physics impacts . . . . . . . . . . . . . . . . . . . . 10
3 Experimental Apparatus
3.1 Beam line . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Spectrometer design . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Yield and acceptance . . . . . . . . . . . . . . . . . . .
3.2.2 Counting rate . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Momentum and mass resolution . . . . . . . . . . . . .
3.2.4 Collision geometry resolution . . . . . . . . . . . . . .
3.3 Detector elements . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Magnet . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Target . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Central tracking device . . . . . . . . . . . . . . . . . .
3.3.4 Gas Čerenkov counter . . . . . . . . . . . . . . . . . .
3.3.5 Electro-Magnetic calorimeter . . . . . . . . . . . . . . .
3.3.6 Outside tracker . . . . . . . . . . . . . . . . . . . . . .
3.4 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Trigger and background . . . . . . . . . . . . . . . . . . . . .
3.6 Cost estimation . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Summary

33

2

1

Introduction

The spontaneous breaking of the chiral symmetry is playing an essential
role in QCD. Effective quark mass which constitutes hadrons is due to thebraking. In hot (finite temperature) and/or dense (finite density) matter, thechiral phase transition takes place and the broken symmetry is restored. It
is predicted by Lattice QCD that the deconfinement of quark and the chiralphase transition would occur at the same temperature. Quark condensate
hq̄qi is an order parameter of the chiral phase transition. As temperature(density) increases, the absolute value of hq̄qi decreases and finally goes tozero at the critical temperature (density). Unfortunately hq̄qi itself is not an
observable, however, the spectral properties of hadrons like the mass and the
decay width are expected to change sensitively to the hq̄qi value. Namely theproperty of hadrons in the hot/dense matter is a good measure of the chiralsymmetry restoration in the matter.
There have been many theoretical approaches to this problem[1] and someexperiments already detected the modification of hadron in nuclear matter.
At high temperature, the CERES experiment at CERN-SPS observed thechange of low-mass e+ e− spectra which suggests the modification of vector
mesons in Pb-Au collision[2]. Also at CERN-SPS, the NA60 experimentmeasured low-mass µ+ µ− spectra and recently reported the modification ofρ meson in In-In collision[3]. The KEK-PS E325 experiment [4, 5, 6, 7], which
was carried out by a collaboration including a part of the authors, measured
the e+ e− and K + K − decay of light vector mesons (ρ/ω/φ) made by the 12–
GeV proton induced reaction in the target nucleus. They also detected themodification of the vector-meson invariant mass spectra in the e+ e− channel
with higher mass resolution than the CERES and NA60 experiments. The
results of E325 are explained in the following section.
In the lower energy region as several GeV, TAGX at INS (KEK-TANASHI)
detected the ρ meson modification in π + π − channel in γ + 3 He/12 C reaction[8].
In π + π − channel, the data measured by CHAOS at TRIUNF[9] suggested
the modified σ meson in various nuclear targets made by the pion induced
reaction. CB at AGS[10] also reported similar effect in π 0 π 0 channel. The
LEPS group at Spring-8 measured φ → K + K − and the A-dependence of the
φ-production cross section in γ + A reaction. They reported the medium
effect against φ production and φ-N interaction. The spectral modificationof ω meson was measured by the CBELSA/TAPS experiment in π 0 γ decay
channel[12]. Dielectron measurement experiment in the energies are ongoing
by HADES at GSI and CLAS-G7 at JLab.
It can be said that the existence of the hadron modification in medium
has been established by these experimental results. However, the origin of the3

modification is not clarified yet. There are also many explanation unrelated
to the chiral symmetry. To investigate the problem, we propose to construct
new spectrometer at “multi-purpose high momentum beam line” at J-PARC
50-GeV PS. The main aim of the spectrometer is a measurement of e+ e− pairfrom the p+A reaction to investigate the chiral symmetry around the normal
nuclear matter density as same as E325, with definitely higher statistics andpossibly improved mass resolution.
In section 2 we discuss our physics motivation, related experiments andthe main goal of the proposed experiment. At the first stage, we aim to collectaround 105 of φ meson in e+ e− channel for each nuclear target. The statistics
are 100 times as large as the KEK–PS E325 experiment collected. When wecollect such amount of φ meson , we can also collect ρ/ω meson about 10
times as much as φ meson in e+ e− channel at the same time. Furthermore,
approximately 1000 of J/ψ can also be collected when 50–GeV proton beamis used.
To collect such statistics in 100 shifts (33 days) of beam time, we have to
use the high intensity primary proton beam of which intensity is 1010 proton
per spill and energy is 30 or 50 GeV with thin target around 0.1% interaction
length. To cope with 107 Hz (10 MHz) of interaction rate at the target, wehave to construct a new spectrometer using new technologies. The design ofour proposed spectrometer and experimental details are described in section
3. The design work is performed on the basis of the experimental experiencesof the E325 experiment.
At the next stage of the proposed experiment, measurement of J/ψ , K∗ ,D, σ and so on using this spectrometer with some upgrading should be considered. When heavy-ion beam is available in J-PARC, we can explore thechiral symmetry in higher density, which may be the highest density in humankind have ever made on earth. By such measurements of various hadrons
in same density environment, we will have deeper insight into modificationof hadrons and further, the chiral symmetry and QCD itself.

4

2

Physics

As described in previous section, both the finite temperature and the finite density environment are important to investigate the chiral symmetry
in QCD. In J-PARC, we put emphasis on the latter environment. The proton (or secondary beam hadron) induced reactions make mesons in target
nucleus, which bring the information of the environment around normal nuclear density. Heavy ion collisions around 20∼25 GeV/nucleon are expectedto achieve the high baryon density 5∼10 times as large as the normal nuclear
density. Of course such energy region was investigated by the AGS (∼ 12
GeV/nucleon) and recently SPS (20 ∼ 30 GeV/nucleon) heavy ion experiments, however, these experiments did not use the electron probe, which lessinteract in final state. Only CBM experiment (GSI future project) [19] iscompeting with us in this energy region.

M( ρ ) / M( ρ=0 )

y=0.12

1.0

φ mesons
y=0.220.9

0.8

ρ, ω mesons

0.7

0.6

0

1

0.5

ρ / ρ0Figure 1: mass modification of vector mesons in finite density predicted by
Hatsuda and Lee [13](left) and Klingl et al. [15] (right: upper is for ρ, middle
is for ω and lower is for φ meson ).

5

The chiral symmetry restoration in finite density has been studied inmany theoretical methods. Hatsuda and Lee studied using the QCD sum
rule and calculate the mass of ρ/ω/φ mesons and predicted 10∼20% decreasing for ρ/ω and 2∼4 % for φ at normal nuclear density[13] as shown in Fig1.
Hayashigaki studied the mass of J/ψ and D meson also using QCD sum ruleand predicted about 0.1∼0.2% decreasing for J/ψ and 2∼3% for D meson
also at the normal nuclear density[14]. Klingl et al. calculated the downward
mass-shift and even mass broadening of ρ/ω/φ in dense matter[15]. Furthermore, they also predicted the width-broadening of φ meson by a factor often[16] at normal nuclear density, by including the in-medium kaon spectralchange. Some trials using lattice QCD have been made recently althoughsuch approaches using lattice for the finite density is more difficult than for
the finite temperature. For example, Muroya, Nakamura and Nonaka calculated the mass of π and ρ mesons in finite chemical potential, and found themass of ρ decreases as the chemical potential increases in the color-SU(2)QCD[17].
In the following subsections, we review the predecessor experiment KEK–PS E325 and our goal in the proposed experiment.
FTOF Counter
Aerogel Cherenkov
KDC
HodoscopeCounterb)

Aerogel Cerenkov

Start
TimingCountertargetchamber

SLG
BDC
FGC
CDC

RGC
RLG

Start Timing
Counter

RGC
RLG

FGC

FTOF
Counter
BDC

beam

1000mmCDC

target3980mma)

KDC

Hodoscope Counter

Figure 2: E325 spectrometer: top view and vertical cross section.

2.1

Predecessor experiment KEK–PS E325

The experiment KEK–PS E325 was proposed in 1993 and the construction
was started in 1996 at the EP1–B primary beam line at KEK–PS. Datataking was started on the May, 1997 and completed on the March, 2002.
The spectrometer[18] was designed to detect the relatively slowly-moving
vector mesons, because such mesons expected to spend longer time in nuclei
after they are produced in proton-induced reaction and to have larger probability of decay inside nuclear matter. Possible spectral change depends on6

the number of decayed meson in medium, thus slowly moving mesons andlarger size nuclei are desired. The spectrometer mainly covered the backward
region in the center-of-mass system. Schematic picture of the spectrometeris shown in Fig.2
In order to reduce the electron background from the γ conversion in thetarget material, typical radiation length of each target is set around 0.5%
radiation length in order to balance the number of e+ e− pair from π 0 Dalitz
decays and that from the γ conversion in the target material.

Figure 3: Invariant mass spectra in e+ e− channel observed by E325. left
is for carbon target and right is for copper target, after the backgroundsubtraction.
In order to compensate such thin targets and also small branching ratio
into lepton channels (order of 10−4 ∼ 10−5 ), high intensity primary beam wasused to collect the high statistics. Typical beam intensity was 109 proton per
spill and typical interaction rate on the targets was around 1 MHz, because
the interaction length of such targets were around 0.1∼0.2%.
The experiment has been detected the modification of vector-meson spectra using e+ e− decay. Here we explain the recent results, mainly from thedata taken in 2001∼2002. As shown in Fig. 3 based on the analysis in thereference[5], significant excesses are found in the low-mass side of the usual
ω(783) peak, which cannot be explained using known hadronic sources. Meson mass shape used in the fit are relativistic Breit-Wigner shape and distorted by the detector simulation taking account of the experimental effectlike the energy loss and multiple scattering of electrons in the detectors,
7external bremsstrahlung, detector acceptances, etc. Distortion due to theinternal radiative correction for the leptonic decay is also taken into account.
Combinatorial background is evaluated by the event mixing method.
The ω mesons have a longer life (∼24 fm) relatively than ρ meson (∼1.3
fm). Therefore more than 90% of ω are expected to decay in free spaceoutside nucleus in which it is created and make such clear peak around 780
MeV/c2 . On the other hand, ρ meson amplitude is very small as a result of
the fit. It should be noted that the fit is performed with the hadronic shapein vacuum, in other word, unmodified. Therefore, the excess could be madeby ’modified’ ρ mesons decayed in nuclei.
22

events[/10 MeV/c ]events[/10 MeV/c ](a) C(b) Cuwithmass
modification

with
mass
modification

fit result+ ρ→e e+ ω→e e2

fit result+ ρ→e e+ ω→e e2

invariant mass [GeV/c ]invariant mass [GeV/c ]Figure 4: Model-fitting results of the invariant mass spectra in e+ e− channel
observed by E325. a) is for carbon and b) is for copper targets. Solid lines
are best fit results.
To understand the shape observed including the nuclear size effect andvelocity dependence, it is performed that the Monte Carlo type model calculation based on the prediction by Hatsuda and Lee[13]. Mesons are generated at the incident surface of the nucleus, having the measured momentum distribution, fly through the nucleus and decay following own life. If
the decay point is inside nuclei, the mass is decreased by the predicted form:m∗ /m0 = 1−k(ρ(r)/ρ0 ). The nuclear matter density ρ(r) follows the WoodsSaxon formula. The predicted value of the shift parameter k = 0.16 ± 0.06.
And m∗ is modified mass, m0 is mass in vacuum and ρ0 is a normal nuclear
density. By this model calculation, the spectra are explained like as shownin Fig.4 with the modified shapes of ρ and ω. The shift parameter k is determined as 0.092 ± 0.002 by the fit, where k is assumed as common for C
8and Cu target.

Figure 5: Invariant mass spectra of φ → e+ e− observed by E325. Upper
panels are for carbon and lower are for copper targets. The data are divided
by the meson velocity, slower (βγ < 1.25 ), middle (1.25 < βγ < 1.75) andfaster (1.75 < βγ ) component as shown in left, middle and right panels,
respectively. Only the data shown in the lower-left panel have significant
excess over the φ meson shape and the quadratic background curve.
For φ meson case, similar analysis was performed and the excess is observed only the slowly moving component in larger nuclear target as shownin Fig.5 from the reference [6]. The result is consistent with a view that almost all mesons, which are produced in nuclei with a few GeV of momentum,
should decay outside the nucleus or nuclear matter and only limited number of mesons could decay inside nucleus with mass-modification by density
effect, because the mean life is relatively longer than the nuclear size. The
velocity-dependence analysis for ρ/ω meson case is also desired and ongoing.
The number of mesons in the excess is calculated by subtracting unmodified shape distorted by the detector simulation from the data and plotted in
the left panel of Fig. 6. Supposing the excess consists of modified φ meson ,the ratio Nexcess/(Nexcess + Nφ ) means the fraction of φ → e+ e− decay inside
nucleus among the total φ → e+ e− decays. The lines in the figure are the90.3

Nexcess /(Nexcess+Nφ)

Nexcess /(Nexcess+Nφ)results of the toy model calculation similar the ρ/ω meson case, including
not only the mass decreasing but also the width broadening. Because thestatistics are limited, it is difficult to distinguish the two value of the OZI
breaking parameter y (see fig1) in the theoretical prediction[13].
Cu Model Calc. k=0.04
Cu Model Calc. k=0.02
C Model Calc. k=0.04
C Model Calc. k=0.02

0.2

0.1

0

-0.1

0.3

Cu Model Calc. k=0.04
Cu Model Calc. k=0.02
C Model Calc. k=0.04
C Model Calc. k=0.02

0.2

0.1

0

1

1.5

2

2.5

3

-0.1

βγ

1

1.5

2

2.5

3

βγ

Figure 6: Left panel shows the βγ dependence of the ratio Nexcess/(Nexcess +Nφ ) observed in e+ e− spectra measured by E325. Red circle is for copper andblue square is for carbon target data. Solid and dashed lines show the resultof model calculation based on the theoretical prediction[13], red is for copperand blue is for carbon. Solid and dashed are show the two cases of parameterin the prediction. Right panel shows the effect of statistics expected in the
proposed experiment. Statistical error bars will be shrunk as shown in theright panel by 100 times as large statistics as E325 which is shown in the left
panel.

2.2

Goals and physics impacts

The main goal of the proposed experiment is to unambiguously measure themass modification of mesons in cold (and hot, if heavy ion beams become
available at J-PARC) nuclear matter, and approach how the hadron mass
originates by spontaneous chiral symmetry breaking. With the high intensity
proton beam at J-PARC, the statistics alone is expected to improve by twoorders of magnitude compared to the predecessor experiment KEK–PS E325.
It will allow us highly systematic approaches to how the meson mass behavesin matter.
The E325 has reported an interesting phenomenon that slower mesons
generated in heavier nuclear targets exhibit higher fractions to have their
10

mass modified from the in-vacuum property. It suggests, or at least is consistent with, the picture that information of the modified meson mass is carriedout to detection only in case the parent meson decays before escaping fromthe surrounding nuclear matter. That is, slower mesons taking longer timein nuclei are to decay more in the matter (and possibly modified stronger)
than faster mesons. Further systematic studies are awaited to investigate
the picture, and it is vital to study the collision geometry dependence of themeson mass spectrum both on the target mass and collision centrality axes,
in addition to the meson velocity dependence.
When the proposed experiment collect the 100 times as large statistics
as E325, the statistical error bars can be shrunk and the data binning in
the meson-velocity (βγ ) axis can be divided much more as shown in theright panel of Fig. 6. By such analysis, we will be able to approach more
precise comparison with several theoretical predictions of the spectral change
of vector mesons in nuclear matter.
σ=5MeV

σ=20MeVβγ <1.5βγ > 3σ=11MeVFigure 7: The expected modified invariant mass distributions of φ → e+ e−from Cu target with different experimental mass resolutions. Upper panels
show the spectra by fast moving mesons and lowers show by slowly moving
mesons.
As for the target mass dependence, a hydrocarbon target and a carbon
one are planned to be bombarded by the primary proton beam, as well as
heavier targets, as lead, up to A∼200. Meson mass spectrum in proton-onproton collisions will be obtained by subtracting the contribution of carbon
nuclei target from the hydrocarbon data, which was given up in E325 because11

of the statistical limitation. The high-statistics capability in the proposed
experiment enable us to use such new targets in various size with reasonablestatistical precision. The expanding the lever arm of the target mass from
A=1 to A∼200, in contrast to from A=12 to A∼64 as in KEK–PS E325.
On the other hand, the collision geometry in p+A reaction can be measured via the charged particle multiplicity in the mid- to backward-rapidity
region in the center-of-mass system, and/or via the number of so-called“grey” (slow) protons [20]. The meson velocity, the target mass, and thecollision centrality dependences will play as “three arrows” and assure thesystematics which is required to unambiguously understand the underlying
mechanism of how the hadron mass is modified in the nuclear matter, andultimately how it originates.
Another key to reduce systematic uncertainties is a good invariant mass
resolution for the electron pairs. It enables to distinguish the possible slighttail of modified meson mass spectrum from the background. The effect ofexperimental invariant mass resolution on how the modified φ meson mass
distribution looks is shown in Fig. 7. The result shows that at least approximately 10 MeV in r.m.s. at the φ mass, the same level as in KEK–PS E325,
needs to be achieved in the proposed experiment.

12

3

Experimental Apparatus

3.1

Beam line

Construction of a high-momentum beam line is needed. Primary proton of
50 GeV is delivered to the beam line and beam intensity of 1010 per spill isneeded for the proposed experiment. The beam line is originated from theSM1 separation point and the beam loss of 15 kW is allowed from radiation
protection point of view.
Figure 8 shows a schematic view of the beam line. The high-momentum
beam line will be constructed as the C line which originates from SM1, while
the A line is the major primary beam line which has the T0 target point (fortest beam lines) and the T1 target point.

Figure 8: High-Momentum Beam Line (red line)
The beam spot size of less than 10 mm is acceptable and around 1 mm is
desirable from a view point of the target(s) configuration. Less than 1 mm
is not necessary taking account of heat deposit on thin targets.
The spill structure and the long time stability of intensity are also theissue. Since the experiment is performed with the high rate limit of thedetector operation, stability of the beam and flat structure of the spill arevery important.
The spectrometer is placed at the final focus point of the beam line asshown in Fig. 9. This spectrometer covers mainly the target region to detect
the slowly moving mesons, thus the forward region is opened. Therefore
the proposed spectrometer can share the beam line with other spectrometercovers the forward region. It is not favored to share the beam line withother spectrometer located just upstream our spectrometer, since the main
background of the trigger is made by electrons from upstreams.

13

5RGEVTQOGVGT

Figure 9: Counter Hall and spectrometer (blue)

3.2

Spectrometer design

The experiment is based on the same concept as E325 experiment: using thintargets (∼ 0.1 % interaction) to reduce the electron background from the γconversion in target materials, high intensity beam to cope with the thintarget and small branching ratio, large acceptance spectrometer covering the
backward in CM system to detect the slowly moving mesons. The proposed
spectrometer keeps mass resolution and electron identification capability of
E325 and has 5 times larger acceptance and works with the 10 times higher
beam intensity than E325.
Name
GEM Tracker1
GEM Tracker2
GEM Tracker3Čerenkov Counter
Outside Tracker
EM Calorimeter

Radius [mm]
200
400
600
600 – 1100
1120
1140 – 1700

Table 1: List of detectors
We propose to build a new spectrometer to cover wider geometrical acceptances as shown in Fig.10. The vertical acceptance is enlarged to ±45◦ .
The horizontal acceptance is 0◦ ∼ ±135◦ except the very forward region of14

0◦ ∼ ±12◦ both vertically and horizontally to avoid beam halo. Schematic
view of the geometrical acceptances are shown in Fig.11.

beam
return
yoke

coil
pole
piece

Cherenkov
radiator

TargetCsI +GEM
GEM
Pad chamber

NC[GTU)'/VTCEMGTLeadGlassCalorimeter

tracker)CU%JGTGPMQX.GCF)NCUU%CNQTKOGVGT%U+ )'/1WVUKFG
6TCEMGT

Figure 10: Schematic view of the proposed new spectrometer (planview/beam view). Green and red area represents pole piece and coil, respectively.
The list of detectors is shown in Table 1. The spectrometer has a large
magnet and detectors. We will use the same magnet used by E325. The tracking device consists of 3 layers of GEM trackers and outside tracker. Particle
momentum is mainly determined by Gas Electron Multiplier(GEM) trackers.
The GEM tracker is originally developed for the COMPASS experiment[21]for high rate counting and is also used at RCNP[22]. The outside tracker
15

Geometrical (horizontal & vertical) coverageof the spectrometer(left)+135 (degree)
+45
+23

0

(very forward)
0(right)
−135

+90

+12

(E325 coverage)−12−45
beam pipe+12−12 (degree)

Figure 11: Schematic view of the geometrical acceptances of the spectrometer.
is pad chamber and is used for track finding and charge veto for γ measurements. Electron ID counters consist of highly segmented gas Čerenkov
counter and EM calorimeter. As photon detector for the gas Čerenkov
counter, CsI photo-cathode and the GEM are used. CsI photo-cathode and
the GEM are being developed for the PHENIX upgrade plan[23]. Lead Glasscounter is used as EM Calorimeter. The EM calorimeter segmented finer thanE325 gives us a γ identification capability, thus the detection of the radiative
decay of K∗ and σ mesons comes into the scope.
To avoid the background from miss identification of π mesons, the rejection factor of 4.0×10−4 for π mesons, which is achieved at E325, is required in
the momentum region of electrons from φ meson decays. The gas Čerenkov
counter and the EM calorimeter have the rejection factor of 1.0×10−2 and
4.0×10−2 , respectively.
Multiplicity of charged particles into the tracking detectors is to be used
as a measure for the collision geometry. Measurement of “grey” (slow) protons, e.g. below 1 GeV/c, is another option to access the collision centrality
information with a higher accuracy expected. The resolution for the collisiongeometry is evaluated in Section 3.2.4, based on charged particle multiplicity
and grey proton measurements. With the presently proposed spectrometer
design and the detector configurations, the spectrometer acceptance is large
enough to have a few to several grey protons, depending on the target nu16

clei and collision centrality, while the particle identification capability stillrequires some further consideration. In case the timing resolution of ∼ 1 ns
in r.m.s. of the electro-magnetic calorimeter is insufficient, a detector arraybased on a scintillation counter technology could be placed right in front of
the calorimeter, to provide both the multiplicity and time-of-flight information for charged particles, without losing tracking and identification efficiency
for electrons. The expected reaction rate of ∼ 107 Hz is within the reach of
conventional scintillation detectors.
3.2.1

Yield and acceptance

We estimated a yield of φ mesons in e+ e− decay mode. While the estimation is done for φ mesons, the yield of the ρ/ω mesons is expected to beabout 10 times as much as φ. We used the nuclear cascade code JAM[24]to estimate the production cross section and the kinematic distribution of φmeson in p+A reactions. It should be noted that the predictions by JAM areconsistent with the results of 12 GeV p+A reaction measured by E325[7], inthe kinematic distribution of φ meson and even in the absolute production
cross section within factor 2. It was also confirmed that the incident-energy
dependence of the predicted cross section by JAM is consistent with the phenomenological scaling law about production cross section of narrow vector
mesons in pp reaction[25].
beam energy
production cross section
detector acceptancenormalized yield by E325

12 GeV
1.0 mb
E325
8.8%
J-PARC 45%
E325
1
J-PARC 5.1

30 GeV 50 GeV
3.0 mb 5.1 mb(6.0%) (4.5%)
31%
23%
(2.0)
(2.6)
10.0
12.7

Table 2: The φ meson yield in p + Cu reactions for two detector geometries.
Here, we discuss p + Cu reaction at 12 GeV, 30 GeV and 50 GeV ofincident kinetic energies. The beam energy 30 GeV is planned at the Phase
1 of J-PARC, while 50 GeV is available at the Phase 2. Cross sections are
scaled by the measured value at 12 GeV. The results are shown in Table 2 andFig. 12. At 30 or 50 GeV the production cross sections are 3∼5 times larger
than 12 GeV, while the Lorentz boost decreases the detector acceptances forthe φ → e+ e− detection. As a result, the yield is 10 (13) times as large as
E325, for 30 (50) GeV of beam energy.

17

It should be noted that only the geometrical factor of the spectrometers and cross sections are considered here, in other words, the same detector/trigger/analysis efficiency, the same beam intensity (∼ 1 × 109 ) and thesame data acquisition periods (100 shifts) as E325 are assumed. The proposed spectrometer is designed to cope with the ∼ 1×1010 proton/spill and∼ 1×107 interaction/spill. Thus, the 100 times of φ meson yield as large as
E325 can be collected in the proposed experiment.

rapidity(CM)βγ(Lab)Figure 12: The φ meson acceptance in 50 GeV p + Cu reactions for two
detector geometries. Left panels are for βγ of φ meson in Lab system andright panels are for rapidity of φ meson in CM system. Black lines are forthe φ meson distribution generated by JAM. Red lines are for the φ meson
accepted by proposed new spectrometer geometrically. Green lines are forthe φ meson accepted by E325 spectrometer geometrically.

3.2.2

Counting rate

The main background in E325 trigger was electrons flying from upstream
whose origin may be collisions between the beam halo and materials aroundthe beam line. For 1 × 109 /spill of intensity, particle density at the point
20 mm apart from the beam line was about 350 kHz measured by the mostforward cell of a E325 drift chamber with 3.5 mm of the cell width and 220
mm of height. The most forward cell located at radius of 200 mm and thehorizontal angle of 6◦ .
The most inner layer of the new tracking device is placed at the sameplace as the above drift chamber. Taking account the 10 times increasingof the beam intensity and 1 sec of duration in 3 sec accelerator cycle, themaximum counting rate becomes 3.5 MHz. Note: the rate could be reduced,
18

since the most forward cell of the proposed chamber has the large horizontal
angle of 12◦ and placed at the point 40 mm apart from the beam line. To
cope with such high rate, the tracker has to have the 0.7 mm of the cellwidth and 100 mm of height. It is 10 times finer segments than the above
E325 drift chamber and the counting rate per one cell reduce to 350 kHz,
which corresponds to 5 kHz/mm2 . Such high rate counting can be achieved
with GEM tracker and strip readout of 700 µm pitch. According to the
COMPASS experiment, the detector is working up to 25 kHz/mm2 withstrips of 400 µm pitch. Taking account of the difference of the strip pitch,
The proposed detector works up to 14 kHz/mm2 .
Another measurement of counting rate is done by E325 using a scintillation counter which was placed at the radius of 400 mm from 12◦ to 60◦
and has a width of 40 mm and a height of 400 mm. The courting rates ofthe scintillation counter at 12◦ and 60◦ are 1.8×106 /spill and 0.4×106 /spill,
respectively.
The radius of the above scintillation counter is the same as the second
layer of the proposed GEM tracker. Taking account of the 10 times increasingof the beam intensity and 1 sec of duration in 3 sec accelerator cycle, thecounting rate becomes 4 MHz at the horizontal angle of 60◦ . Even with thedrift cell whose size of 4 mm(W)× 200 mm(H) at the radius = 400 mm (forthe second layer), the counting rate is 200 kHz, and for the first layer at theradius = 200 mm , 400 kHz for 4 mm(W) x 100 mm(H) of drift cell. The
rates are marginal for the operation of a drift chamber, even such backward
as 60◦ . The 4 mm of cell width is also marginal for the design of the drift
chamber. Thus, we decided to use GEM trackers for all the horizontal angle
region and all three layers.
3.2.3

Momentum and mass resolution

Momentum and mass resolution are evaluated using a brief simulation for the
proposed spectrometer and the E325 spectrometer. The purpose of the calculation for E325 is to check the accuracy of the calculation. The simulation
contains position resolution of the tracking devices and multiple scattering
effects. The calculations for the proposed experiment are done with 4 position resolutions, such as 300 µm, 200 µm, 100 µm, and 50 µm. The position
resolution of 300 µm is achieved with the E325 spectrometer and used in
the calculation for E325. The same vertex resolution is used both for the
proposed spectrometer and E325 calculation. The radiation length of thespectrometer is based on the E325 spectrometer. One layer of GEM tracker
has 0.07% of radiation length. The increase of the radiation length from
GEM is taken into account in the calculation. The magnetic field map as19

sumes the axial symmetry and is calculated by POISSON. The same current
density is used as E325.
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Figure 13: Momentum resolution (δp/p) as a function of momentum for E325
(green), position resolution of 300µm(blue), 200 µm(purple), 100 µm(red)and 50 µm(cyan). Black line represents achieved momentum resolution for
E325.

20

Figure 13 shows results for momentum resolution as a function of momentum. The simulated resolution for E325 is consistent with the real value.
The improvement of the chamber resolution dramatically improves the momentum resolution.
20

20

18

18

16

16

14

14

12

12

10

10

8

8

6

6

4

4

2

2

0
0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.05

0.1

0.15

0
0

0.05

0.1

0.15

0.25

0.3

0.35

0.4

0.2

0.25

0.3

0.35

0.4

20
18
16
14
12
10
8
6
4
2
0
0

0.2

Figure 14: Mass resolution as a function of chamber resolution for all kinematics region of φ (Top left), βγ < 1.25 (Top right), and βγ < 0.8 (Bottom).
Green points represent for E325 and purple points represents for the proposed
spectrometer.
Figure 14 shows mass resolution as a function of chamber resolution.
The simulated resolution for E325 (green, 8.2 MeV) is better than real value(10.7 MeV). The difference between simulation and real values is caused by
the difference of magnetic field map and calibrations.
To achieve the same mass resolution as E325, the chamber resolution of
200 µm is required.

21

3.2.4

Collision geometry resolution

The resolution for the collision geometry, based on charged particle multiplicity and grey proton measurements, is evaluated via simulations with theJAM event generator, assuming the presently proposed spectrometer design
and the detector configurations. Figure 15 shows (a) number of charged particles emitted into the aperture of the spectrometer, with an approximate
low-momentum cutoff at 0.3 GeV/c by the magnetic field, (b) the same withan additional high-momentum cutoff at 1.0 GeV/c, and (c) number of protons into the aperture within the momentum range (from 0.3 to 1.0 GeV/c),in p+C and p+Pb collisions with the incident proton energy at 50 GeV, asa function of the impact parameter. Note that the selection (b) is easily
achievable with the tracking system alone, while the (c) requires a moderateparticle identification capability. The vertical bars indicate the dispersions,i.e. the distribution widths of numbers of particles. A rough classificationof collision geometry, resulting in a few centrality bins, would be feasible
with heavy nuclear targets such as 208 Pb, while it would be challenging todistinguish collision geometry with light ones as 12 C.
p+Pb

7

Accepted Particles per CollisionAccepted Particles per Collision

p+C

6
5
4
3
2

18
16
14
12
10
8
6
4

1
0

20

2
0

0.5

1

1.5

0

2
2.5
3
Impact Parameter [fm]

0

1

2

3

4
5
6
Impact Parameter [fm]Figure 15: Evaluation of collision geometry resolution in p+C (left) and
p+Pb (right) collisions at 50 GeV. Closed black circles show number ofcharged particles emitted into the aperture of the spectrometer with a lowmomentum cutoff at 0.3 GeV/c; open blue squares are with an additional
high-momentum cutoff at 1.0 GeV/c; open red circles show number of protons alone with the acceptance and momentum cuts. See text for the details.

22

3.3

Detector elements

3.3.1

Magnet

The magnet of the E325 spectrometer is reused for the proposed experiment. The pole piece and coil are modified to fit the larger acceptance. The
schematic view of the magnet is shown in Fig. 16. The calculated magnetic
Dipole Magnet Problem
200

200

180

180

160

160

140

140

120

120

100

100

80

80

60

60

40

40

20

20

0

0
0

50
100
150
200
250
C:\Documents and Settings\Kyoichiro Ozawa\My Documents\POISSON\JPARC3_Dipoles\DIPOLE1.AM 4−23−2006 0:51:32

Figure 16: Schematic view of cross section of the magnet and magnetic field.
field is also shown in in Fig. 16 as red lines.
The calculation assumes the axial symmetry and is done by POISSON.
The same current density is assumed as E325. Figure 17 shows field strength
for the proposed and E325 spectrometer as a function of radius along y=0
plane. The change of shape of the pole piece gives us two times larger fieldstrength than E325 at the center of the magnet.
3.3.2

TargetHigh statistics capability in this experiment gives us an opportunity for thesystematic study of nuclear size dependence of the mass modification by using
the Pb target and effective proton target by CH2 –C subtraction,
23

Figure 17: Field strength of y (Green) and r (Red) direction as a functionof radius along y=0 plane for the proposed spectrometer (Top) and E325
(Bottom). Return yoke is located at r = 190 cm.
Typical thickness of each target foil should be around 0.5% radiation
length. In such thickness of target, two main origins of the e+ e− background have almost same number. One is the e+ e− pair from π 0 Dalitz
decays(Br=∼1%) and other is the e+ e− from the γ conversion in the target
material, where γ is from the π 0 → 2γ decays(Br=∼99%). It should be
noted that the main background in e+ e− spectra is the combinatorial background such that the combination of electron and positron each comes from
uncorrelated two e+ e− pairs.
Three target foils are aligned on the beam line, 20mm apart from eachother, and used at the same time. Total interaction length is to be kept
around 0.1∼0.2%. For example, carbon foil is located at the center position
and up/down stream targets are pairs of CH2 , copper or lead foils whose
thicknesses are shown in the Table 3.
24

nuclei
C
CH2
Cu
Pb

interaction
length(%)
0.05
0.05
0.05
0.01

radiation
length(%)
0.1
0.1
0.5
0.3

thickness [µm]
200
400
80
20

Table 3: target spec
Target chamber is located at the center of magnet and filled by Helium.
Beam pipe from upstream is kept as vacuum and terminated by mylar window at the just upstream of the target chamber. Helium-filled pipe is locateddownstream the target chamber and terminated by mylar window at just outside of the spectrometer. From here to the beam dump, vacuum beam pipe
is also adopted to reduce the background from the beam–gas or beam–air interaction. Target chamber should be also vacuum to reduce the background,however, large window of the target chamber to adopt the large acceptancemakes it more difficult to keep the vacuum. Thin window should be used forthe target chamber filled by 1 atm gas.
3.3.3

Central tracking device

GEM-based micro-pattern gas detector is used for tracking. The detectors
has three layers of GEM detectors. The GEM detectors have the similar
structure as developed at COMPASS [21], as shown in Fig. 18.
The GEM detectors are placed at the radius of 200mm, 400mm, and
600mm. One layer of the detector consists of three GEM foils, stacked on
top of each other and separated by thin spacer grids. The electrical cloud
emerging from the last GEM is collected on a readout anode comprising two
perpendicular sets of strips of 700µm pitch. The total number of channel is∼ 46440 for 27 segments.
The detector is divided into 9 segments horizontally and 3 segments vertically, as shown in Fig. 10 and lean to the target to make the spectrometersmaller. One segment at the most inner layer has 100mm × 100mm area.
The position resolution of 200µm can be achieved with strips√ of 700µm
pitch. Even if there is no charge sharing between strips, 700/ 12 ∼ 200
is obtained as a spatial resolution. When the charge sharing is taken into
account, the resolution is improved. In fact, the COMPASS detector achieved
71.6µm with Ar/CO2 gas and strips of 400µm pitch. Also, developed TPCGEM at CNS has 80µm of the resolution with Ar/CH and 1.3mm pitch
25

Figure 18: Exploded view of a COMPASS triple GEM detector[21].
strips[26]. Thus, we can expect better resolution than 200µm.
3.3.4

Gas Čerenkov counter

The concept of the proposed Gas Čerenkov counter (GC) is a windowless
and mirror-less Čerenkov counter, operated with pure CF4 in a proximity
focus configuration. The detector consists of a 50 cm long radiator directly
coupled to a triple-GEM detector which has a CsI photo-cathode evaporated
on the top face of the first GEM foil and a pad readout at the bottom of
the GEM stack, as shown in Fig. 19. The detector anode pad is a hexagon
side of a = 15.8 mm and there are a total of 12420 pads for 27 segments.
In this scheme the Čerenkov light from particle passing through the radiator
is directly collected on the photo-cathode forming a circular blob image.
Several kinds of R&D are already done for the PHENIX upgrade plan[23]using the same configuration and PHENIX will install the new detector inthis year.
The CsI photo-cathode has large quantum efficiency in ultra violet region(100 ∼ 200 nm, 6 ∼ 11.5 eV), as shown in Fig. 20. Such large quantum
26

Figure 19: Schematic view of the photon detector of Čerenkov counter
efficiency gives a figure of merit N0 =822 cm−1 and ∼ 36 photoelectrons overa 50 cm long radiator. Hadron rejection factor of 100 is achieved at thethreshold of 16 electrons, as shown in Fig. 21, while preserving an electron
detection efficiency larger than 90%.
3.3.5

Electro-Magnetic calorimeter

We would like to use the lead glass calorimeter (LG) modules which wereused in TOPAZ experiment and has been preserved at KEK after the deconstruction of the TOPAZ spectrometer. About 650 blocks are needed to cover
the acceptances of our spectrometer. They were also used in the E325 spectrometer and the rejection factor of 4.0×10−4 for π mesons are achieved. The
rejection value is sufficient for the experiment. In addition, approximately
15% of energy resolution for 1 GeV electron was achieved. This bad resolution was due to the usage such that the particles come from the side of lead
glass block in order to reduce the readout channels. In this time we expect
to achieve the original energy resolution (∼10% for 1 GeV electron) becauseof the original usage as the head-on injection to the block. By this usage,
the area of trigger tile is decreased and fake electron trigger by accidental
coincidence between Gas Čerenkov counter and lead glass counter could be
reduced. Also, photon-measurement capability is added our spectrometerusing with the charge veto described following subsection.

27

Figure 21: Hadron rejection factor using the PHENIX HBD in the test[23]Figure 20: CsI quantum efficiency as afunction of photon energy. Transmissive type is chosen at the current detector.
3.3.6

Outside tracker

Cathode strip chamber is used as the outside tracker to help track finding.
According to E325 analysis, outside tracker greatly reduces miss trackings,
while it does not contribute significantly to the momentum resolution. In
addition, the detector is used for the charge veto to measure photons. The
total number of channel is ∼ 10800 for 27 segments.

3.4

Electronics

For the readout electronics of GEM, one candidate is a hybrid preamp developed at BNL for the PHENIX experiment. The hybrid preamp, the IO1195-1,
developed by the Instrumentation Division at BNL. The schematic diagram
is shown in Fig. 22 It drives a differential output to a 100 ohm cable which
delivers the signal to a shaper located in the Front End Module. The gainof the preamp is 6.25 mV/fC and the average output signal is 100 mV. The
28

Figure 22: The IO1195-1 hybrid preamp.
noise of the preamp and current configuration is 1100 e’s, which gives signalto-noise ratio of ∼ 100:1 for the average input signal.
Since we have so many channels ∼ 70000, the dedicated front end module
should be developed. The front-end module has the function of receiving thepreamp signals, generating trigger primitive signal, waiting global trigger
signal and digitize the preamp signals.

3.5

Trigger and background

Here we estimate whether we can conserve the first-level trigger rate as thatof the E325 experiment, which was 1∼2 kHz, under the 10 MHz of interaction
rate which is 10 times as large as that of the E325. Our goal of the DAQ
capability is several kHz with 90% of live time, which is achieved performanceby the PHENIX experiment. Thus 1∼2 kHz is reasonable requirement.
In the proposed experiment, main background to trigger the electronpositron pair are considered the electrons come from the upstream of thebeam line and also the accidental coincidence of the electron ID counters.
To trigger the electron-positron pair, first we define the electron candidateby a coincidence of a hit on Gas Čerenkov Counter(GC) and a hit on the
EM calorimeter (LG) which located just after the fired pad of GC. Thresholdenergy of LG should be set to 0.4 GeV. Moreover we require a hit on the mostouter layer of the Central tracker which roughly matches the line connecting
the target and the fired LG. The rough hit position is read out from the third
GEM foil of the triple-stacked structure of the GEM Tracker 3 (located mostouter) which is segmented to the size of 100mm x 100mm. Lastly we require29

two electron candidates which have an opening angle is larger than 60 degree
at the GC pad to suppress the very low mass pair from γ conversions and π 0Dalitz decays.
Comparing with E325 experiment, using the 10 times finer vertical information at the Central tracker and matching the line connecting the LG andthe target, we can reduce the background not come from the target like as theelectrons from upstream the beam line. Also 10 times finer vertical segmentation of GC and 4 times finer segmentation in LG can reduce the accidental
coincidence rate of GC and LG by a factor of 40. In the proposed experiment,
10 MHz of interaction rate can cause the 10 times as high single counting
rate of the electron ID counters as that of the E325 experiment. Two-fold
accidental coincidence rate of the electron ID counters, which is used as the
single electron candidate in the E325, could increase by 100 times. However,
we have the 40 of suppression factor described above and we require eventhree-fold coincidence in use of the Central Tracker. Thus we can conserve
the counting rate of the fake single electron and also fake pair event as sameor less than the E325 experiment case, even under the 10 MHz-interaction
environment.
The main background on the final e+ e− invariant mass spectrum is the
combinatorial background : the random combinations of electron and positronfrom the independent sources. For example, a combination of an electronfrom the γ-conversion in the target and a positron from a π 0 Dalitz decay.
The miss-identified pions are also the origin of this background.
We estimated the combinatorial background originated from neutral andcharged pions with the proposed pion rejection factor 1.0×10−4 and 0.5%
radiation-length of target. Here, η → γγ is neglected because the production
cross section is smaller than that of π 0 in the order of one or two. Other
hadronic sources as η → eeγ, ω → eeπ 0 , etc. are also neglected because theoverall effect of these hadronic sources are expected to be less than 10% of
the total expected background in phi-meson mass region. The cascade code
JAM is used to generate the pions in 50 GeV p + Cu interaction.
As a result, number of the random combination in our proposed detector
is about 15 times as many as that of E325 with same induced-proton rate.
In this acceptance and incident energy, the expected yield of φ meson is 10
times as large as that of E325 as described before, thus Signal-to-Noise ratio
is expected to be better than the half of that of E325. It means that in the
proposed experiment using the 1010 /spill of beam intensity, combinatorial
background in the final e+ e− spectrum comes to 150 times and number of φcomes to 100 times as that of the E325 case in the same length of machine
time. And the number of combination accepted by the trigger is less than
200/spill for 10 M interaction/spill. In other words, not only accidental
30

background but also physical background cannot be negligible in the trigger
rate.

3.6

Cost estimation

A brief cost estimation is shown in Table 4. In total, about five million
dollars (∼600 M Yen) is required to construct the proposed spectrometer at
J-PARC.
The detectors consist of 27 segments. The ±135◦ of horizontal coverageof the spectrometer is divided into nine and the ±45◦ of vertical coverage
is divided into three. Thus each segment covers 30◦ × 30◦ area. The area
sizes of GEM Tracker 1, 2, 3 and CsI-GEM photon detector of Gas Čerenkov
counter in a segment are 10x10[cm2 ], 20x20[cm2 ], 30x30[cm2 ] and 55x55[cm2 ]respectively. Each GEM detector consists triple-stacked GEM foils. Only the
first foil of GC is required to be CsI-coated as a photo-cathode.
Although the most forward segment, which is around the beam pipe, hassmaller acceptance than other segments, the same number of channels are
assumed for this segment in this estimation.

31

Detector

element

description

cost [M Yen]
cost[Yen]( /segment)

GEM
TrackerČerenkov
CounterFrame
GEM foil(10×10[cm2 ] foil)
readout strip board(10×10[cm2 ])electronics
Frame
GEM foil(11×11[cm2 ])CsI coatreadout pad board
electronics

OutsideTrackerFrame
readout strip board
electronics

20 k ×(1+4+9) ×3

820k

100 k ×14

1.4M

3 k/ch ×(860 ×2)chs

5.2M

20 k ×25 ×3

1.5M

40 k ×25 foils

1M

3 k/ch ×460 chs

3 k/ch ×400 chs

subtotal

Magnet

500k
1.2M
13.5M

lead glass and PMTEM
Calorimeter

500k
1.4M

50
23
(1134 foils)
38(378 boards)
140(46440 chs)
50
41(2025 foils)
27(675 foils)
14
38(12420 chs)
20
14
33(10800 chs)

Frameelectronics

3 k/chReturn yoke
Pole piece
Coilreuse from
TRISTAN/TOPAZ

reuse from E325
transfer from KEK
modification
repair

Total
Table 4: cost estimation

32

0
20
2
(650 chs)
0
30
20
30
590

4

Summary

We proposed the construction of the spectrometer at “multi-purpose highmomentum beam line” at J-PARC 50-GeV PS, to measure the electron pair
to investigate the chiral symmetry restoration in the finite density matter
through the meson modification. At the first stage, proton induced reactions
are used to investigate around the normal nuclear density. As the beamenergy, both of 30 and 50 GeV are acceptable for the proposed experiment.
We aim to collect around 105 of φ meson in e+ e− decay channel and also
ρ/ω → e+ e− more 10 times as large as φ. The statistics is improved by
two orders of magnitude in comparison with the former experiment KEK-PS
E325.
With such high statistics, we can investigate the velocity dependence ofthe mass spectra of vector mesons more precisely than E325, compare withvarious theoretical predictions and discuss the origin of the hadron massmodification in nuclear matter. We are also able to use larger and smaller
size of nuclear targets as lead and proton, which cannot be used by E325
because of the statistical limitation. Then we can investigate the nuclear
size effect on the mass spectra in larger and smaller target cases than thatwere measured in E325. By measurement of the charged particle multiplicity
in the spectrometer, collision centrality can be estimated in larger target case.
The dependencies of the mass spectra on the nuclear size and the collisioncentrality also helps to discuss the origin of the mass modification.
For definitely higher statistics and higher mass resolution than that achievedby E325, it is necessary to construct the new electron ID counters and trackers to cope with the expected high intensity beam, high interaction rate andhigh multiplicity interaction. For the reaction classification with the collisiongeometry, measurements of hadrons like grey protons are also important. In
this proposal, we discussed the feasibility of the proposed spectrometer being
operated under the 10 MHz of interaction-rate environment being based on
the experimental experiences by the experiment KEK–PS E325.
At the next stage, other mesons like J/ψ , σ,K∗ , D etc. and possibly themodification of them can be measured with some upgrades of detectors andtriggers. Furthermore, heavy-ion beam is used to explore the high baryondensity state in future.

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