Electron pair spectrometer
at J-PARC 50GeV-PS
to mesure the meson modification

1

Satoshi Yokkaichi , RIKEN&Kyoichiro Ozawa, CNS, U-Tokyo●

Physics : vector meson modification & chiral symmetry●

expected experimental signature in p+A reaction●

performed experiment KEK-PS E325●

Recent E325 Results :●Proposed experiment at J-PARC

φ → e+e- invariant mass spectra modification

HS05 @ KEK , 05Dec02 S.Yokkaichi

2Chiral symmetry restoration in dense matter●In hot/dense matter, chiral symmetry is expected to restore– hadron modification is expected in such matter●

quark-antiquark condensate (order parameter)~2/3 even at the normal nuclear density, T=0–●

Achievable at KEK-PS in use ofnuclear medium of target nuclei themselves.

Many theoretical predictions of vector meson (mass/width)modification in dense medium, related (or not related) with CS–Brown & Rho ('91) : m*(ρ)/m0 ~ fπ*/ fπ ~ 0.8 at ρ=ρ0–

Hatsuda &Lee ('92) , Klingle, Keiser &Weise ('97), Muroya,Nakamura & Nonaka('03), etc.
HS05 @ KEK , 05Dec02 S.Yokkaichi3Hatsuda and Lee, 92,96
linear dependence on density
m*/m0 = 1- k ρ/ρ0mass decreasing- 16(±6)%for ρ/ω
- 0.15(±0.05)*y=2~4%
for φ(for y=0.22)
at the normal nuclear density

Klingle,Keiser,Weise, 97ρω

Muroya, Nakamura, Nonaka, 03

φHS05 @ KEK , 05Dec02 S.Yokkaichi

4+ -Expected Inv. e e mass spectra in p+A●smaller FSI in e+e- decay channel rather thanhadronic decay channel●double peak (or tail-like) structure–second peak is made by inside-nucleus decay(modified meson) : amount depend on thenuclear size and meson velocity–

enhanced for slower mesons & larger nuclei
longer-life meson (ω(Γ=8.4ΜeV ∼ 24 fm) & φ (Γ=4.3ΜeV ∼ 46 fm) ) cases :outside decay(natural)+inside decay(modified)

expected
to be observed=
HS05 @ KEK , 05Dec02 S.Yokkaichi

5+ -Expected Inv. e e mass spectra in p+A●

smaller FSI in e+e- decay channel rather thanhadronic decay channel●

double peak (or tail-like) structure–

second peak is made by inside-nucleus decay(modified meson) : amount depend on thenuclear size and meson velocity–enhanced for slower mesons & larger nuclei

shorter-life meson( ρ: Γ=150ΜeV ∼ 1.3fm) cases
outside decay
inside decay(natural)(modified)+

expected
to be observed=
HS05 @ KEK , 05Dec02 S.Yokkaichi

6Experiment KEK-PS E325●

12GeV p+A → ρ/ω/φ +X ( ρ/ω/φ → e+e- , φ → K+K- )●Experimental key issues:– Very thin target to suppress the conversion electron background(typ. 0.1% interaction/0.2% radiation length of C)– To compensate the thin target, high intensity proton beam to collecthigh statistics (typ. 109 ppp → 106Hz interaction)– Large acceptance spectrometer to detect slowly moving mesons,which have larger probability decaying inside nuclei (1<βγ<3)Collaboration
J. Chiba, H. En'yo, Y. Fukao, H. Funahashi, H. Hamagaki, M. Ieiri, M. Ishino, H. Kanda,
M. Kitaguchi, S. Mihara, K. Miwa, T. Miyashita,T. Murakami, R. Muto, T. Nakura,
M. Naruki, K.Ozawa, F. Sakuma, O. Sasaki, H.D.Sato, M.Sekimoto,T.Tabaru, K.H. Tanaka, M.Togawa, S. Yamada, S.Yokkaichi, Y.Yoshimura(Kyoto Univ. , RIKEN, KEK, CNS-U.Tokyo, ICEPP-U.Tokyo, Tohoku-Univ.)
HS05 @ KEK , 05Dec02 S.Yokkaichi

7(Cont'd)●History of E325

E325 spectrometer
located at KEK-PS EP1-B primary beam line

– 1993 proposed– 1996 const. start– '97 data taking start– '98 first ee data
●PRL86(01)5019– 99,00,01,02....
●x100 statistics– nucl-ex/0504016– nucl-ex/0511019– '02 completed– spectrometer paper●NIM A516(04)390

HS05 @ KEK , 05Dec02 S.Yokkaichi

8Experimental setup●Spectrometer Magnet––●

0.71T at the center
0.81Tm in integral

particle track

BTargets–●

schematic plan view of spectrometerat the center of theMagnet–

C & Cu are usedtypically–

very thin: ~0.1%
interaction lengthbeam

Primary proton beam–

12.9 GeV/c–~1x109 in 2sec
duration, 4sec cycle

targetHS05 @ KEK , 05Dec02 S.Yokkaichi

9Experimental setup - Detectors
Electron ID counters
Gas Cherenkov &Lead Glass EMC
total 3x10-4 π rejection
with 78% e efficiency
in two-stage operation
TrackerThree Drift Chambers
Kaon ID counters
Aerogel Cherenkov& TOF
HS05 @ KEK , 05Dec02 S.Yokkaichi

10●

Typical e+e- Event– blue:electron– red : other– invariant mass of

eletron pair iscalculated

HS05 @ KEK , 05Dec02 S.Yokkaichidilepton mesurement

Vector meson measurements in the world–

HELIOS (ee, µµ)450GeV p+Be / 200GeV A+A–

CERES (ee)450GeV p+Be/Au / 40-200GeV A+A–E325(ee,KK)

12GeV p+C/Cu–NA60(µµ)

400GeV p+A/158GeV In+In–

PHENIX (ee,KK)

p+p/Au+Au–

HADES (ee)4.5GeV p+A/ 1-2GeV A+A

–

CLAS

1~2 GeV γ+A–

J-PARC (ee)30/50GeV p+A/ ~20GeV A+A–

CBM/FAIR(ee)20~30GeV A+A

–

TAGX

(ππ)

~1 GeV γ+A–

STAR(ππ,ΚΚ)

p+p/Au+Au–

LEPS(KK)

1.5~2.4 GeV γ+A–

CBELSA (π γ)(ee)00.64-2.53 GeV γ + p/Nb

11

already state 'modified'running/in analysis
future plan
HS05 @ KEK , 05Dec02 S.Yokkaichi

12Result (1)ee invariant mass spectra
M. Naruki et al.,nucl-ex/050416

HS05 @ KEK , 05Dec02 S.Yokkaichi

13+ -Observed e e invariant mass spectra●from 2002 run data (~70% oftotal data)●C & Cu target●

clear resonance peaks●

m<0.2 GeV is suppressed bydetector acceptance●acceptance uncorrectedCω(783)

φ(1020)→ fit the spectra with known sources
HS05 @ KEK , 05Dec02 S.Yokkaichi

14

Fitting with known sources●Hadronic sources of e+e-:–ρ/ω/φ → e+e-, ω → π0e+e- ,η → γ e+e-–

relativistic Breit-Wigner shape ( withoutany modifications )–Geant4 detector simulation●

multiple scattering and energy loss ofe+/e- in the detector and the target
materials●

chamber resolutions●

detector acceptance, etc.

●

Combinatorial background : event
mixing method●

Relative abundance of these components
are determined by the fitting

estimated spectrum using GEANT4

HS05 @ KEK , 05Dec02 S.Yokkaichi

15

Fitting results●1) excess at the low-mass side of ω●●

To reploduce the data by the fitting, we have to exclude the excess
region : 0.60∼0.76 GeV2) ρ−meson component seems to be vanished !

HS05 @ KEK , 05Dec02 S.Yokkaichi

16

Fitting results (BKG subtracted)
ρ/ω < 0.15 (for C),●< 0.31(for Cu)

However, ρ/ω =1.0±0.2 in former experiment (p+p, 1974)...suggests that the origin of excess is modified ρ mesons.
HS05 @ KEK , 05Dec02 S.Yokkaichi

17

Toy model M.C. including modification●

Assumptions to include the nuclear size effect in the fitting shape–mesons fly through the nucleus, decay with modified mass if the decaypoint is inside nucleus●

meson production point : incident surface of nucleus– measured α~ 0.68 for ω●

meson momentum :– measured distribution in our experiment~0.8 GeV < p < ~2.4GeV for ω● nuclear density distribution : Woods-Saxon type●●modification form :m*/m0 = 1- k ρ/ρ0for ρ & ω(k=0.16±0.06 in Hatsuda & Lee, '92,'96)●( width modification & momentum dependence of modification
are not taken into account this time)
HS05 @ KEK , 05Dec02 S.Yokkaichi

Fitting results by the toy model18

Free param.: - scales of background and hadron components for each C & Cu
- modification paramter k for ρ & ω is common for C & Cu

From the fit : k=0.092 ± 0.002: ~ 9 % reduced at normal nuclear density

ρ/ω ratio : 0.7± 0.1 (C), 0.9±0.2 (Cu) : ... ρ meson returns.
HS05 @ KEK , 05Dec02 S.Yokkaichi

19Result (2)ee invariant mass C
spectra of φ
(R. Muto et al.,nucl-ex/0511019)

ω(783)

φ(1020)HS05 @ KEK , 05Dec02 S.Yokkaichi

φ → e e invariant mass spectra+ -●from 2001/02 run data●

C & Cu target●

acceptance uncorrected●mass resolution :10.7MeV●

fit with

20φ(1020)– simulated mass shape of φ●(evaluated as same as ρ&ω)

– polinomial curve background

HS05 @ KEK , 05Dec02 S.Yokkaichi

φ → e e invariant mass spectra+ -●from 2001/02 run data●

C & Cu target●

acceptance uncorrected●mass resolution :10.7MeV●

fit with– simulated mass shape of φ●(evaluated as same as ρ&ω)

21CCu

– polinomial curve background●examine the 'excess' is significant or not.
–→ see the βγ dependence : excess could beenhanced for slowly moving mesons
HS05 @ KEK , 05Dec02 S.Yokkaichi

e e spectra of φ meson (divided by βγ )+ -βγ<1.25 (Slow)●

1.25<βγ<1.75

22

1.75<βγ (Fast)

Only slow/Cu is not reproduced in 99% CL.
HS05 @ KEK , 05Dec02 S.Yokkaichi

23Amount of excess●To evaluate the amount of excess (Nexcess ), fit again excluding theexcess region (0.95~1.01GeV) and integrate the excess area.

excess region

HS05 @ KEK , 05Dec02 S.Yokkaichi

24Amount of excess●To evaluate the amount of excess (Nexcess ), fit again excluding theexcess region (0.95~1.01GeV) and integrate the excess area.

●Model calculation reproduces the tendency of Nexcess / (Nexcess + Nφ)excess region

HS05 @ KEK , 05Dec02 S.Yokkaichi

Toy model again for φ meson●

Toy model like ρ/ω case, except for25βγ<1.25 (Slow), w/ unmodified– uniformly made in nuclei●–measured α of φ production ~ 1

m*/m0 = 1- k1 ρ/ρ0(k1=0.04, Hatsuda & Lee, '92,'96)–To reproduce such amount of excess, lineardependent width broadning is adopted :Γtot*/Γtot 0 = 1+ k2 ρ/ρ0(k2=10, it means Γtot*=~47MeV at ρ0 )(predicted value by Klingl et al., '98)
●e+e- branching ratio is not changed– Γ e+e*

Γ*tot = Γ0e+e- /Γ0tot 0/– k1 & k2 is not free param., but fixed.

HS05 @ KEK , 05Dec02 S.Yokkaichi

Toy model result for φ meson●modified (model) shapes well
reproduce the data, even slow/Cu●

modified shapes are analyzed withunmodified shape to evaluate theNexcess / (Nexcess + Nφ)26βγ<1.25 (Slow), w/ modified

HS05 @ KEK , 05Dec02 S.Yokkaichi

Summary (1)●●27KEK-PS E325 measured the e+e-(&K+K- ) decay of slowly moving
vector mesons in nuclei produced by 12-GeV proton beam, to explore
the chiral symmetry restoration at the normal nuclear density.
Observed e+e- invariant mass spectra have excesses below the ω meson
peak, which cannot be explained by known hadronic sources in normal(unmodified) shape. These suggest modification of (at least) ρ meson.
– Simple model calculation including predicted modification of ρ & ω

reproduces the observed spectra.
●

φ → e+e- also have excess, for the larger target, slowly moving φ– First observation of the modification of φ meson– World best mass resolution for φ → e+e–model calc. including mass shift and width broadning in nuclei also
reproduces the data.
HS05 @ KEK , 05Dec02 S.Yokkaichi

28

ProposedExperiment atJ-PARC

HS05 @ KEK , 05Dec02 S.YokkaichiNext generation experiment at J-PARC●Same concept as E325
●

●

29

thin target / primary beam (109 ~1010 ppp)/ slowly moving mesons

Main goal : collect 104 ~105 φ → ee for each target in ~5 weeks●

10-100 times as large as E325
– velocity dependence of 'modified' component–●●

new nuclear targets : proton (CH2 -C subtract), Pb●

narrow width : sensitive to modification●

mass resolution : less than ~10 MeV/c2

ω, ρ and J/ψ can be collected at the same time–higher statistics of ω, ρ than E325 with differ nuclear targets–100-1000 J/ψ are expected in 50GeV operation

Normal nuclear density (p+A)–but also high matter density (A+A, ~20GeV/u)

HS05 @ KEK , 05Dec02 S.Yokkaichi

30High momentum Beamline

High momentum beamline
HS05 @ KEK , 05Dec02 S.Yokkaichi

31high statistics●

Main goal : collect 104 ~105 φ → ee for each target in 5 weeks●

100 times as large as E325
– velocity dependence of 'modified' component

x 10 stat.

HS05 @ KEK , 05Dec02 S.Yokkaichi

32high statistics●

Main goal : collect 104 ~105 φ → ee for each target in 5 weeks●

100 times as large as E325
– velocity dependence of 'modified' component

x 10 stat.

error bars can be shrinked (data points are moved around the model curve)
HS05 @ KEK , 05Dec02 S.Yokkaichi

33high statistics●

Main goal : collect 104 ~105 φ → ee for each target in 5 weeks●

100 times as large as E325
– velocity dependence of 'modified' component

x 10 stat.

βγ bin can be divided
HS05 @ KEK , 05Dec02 S.Yokkaichi

34high statistics●

Main goal : collect 104 ~105 φ → ee for each target in 5 weeks●

100 times as large as E325
– velocity dependence of 'modified' component

x 100 stat.

error bars are shrinked again
HS05 @ KEK , 05Dec02 S.Yokkaichi

35high statistics●

Main goal : collect 104 ~105 φ → ee for each target in 5 weeks●

100 times as large as E325
– velocity dependence of 'modified' component

We can compare the datawith theoretical predictions
more precisely, and we canapproach the puzzle thatthe modification is due to thechiral symmetry restoration
or not.

x 100 stat.

Advance of the theoreticalworks is also needed tosolve the puzzle....
HS05 @ KEK , 05Dec02 S.Yokkaichi

high mass resolution●36mass resolution should be less than ~10MeV

FastSlow(model calc. with k=0.05/Γ=x10/E325 spectrometer)

HS05 @ KEK , 05Dec02 S.Yokkaichi

37

To collect the high statisticsA) Reuse of E325 spectrometerorB) New larger acceptance spectrometer( coverage : ±45o (vert.), ±12o ~132o (holi.) )spectrometer acceptance φ−>ee(estimated by JAM)

Further, for 10 times higher intensity beam (1010)(i.e. high interaction rate : 10MHz)
to collect higher statistics ( 100 times of E325 =105 φ), (B) is needed
HS05 @ KEK , 05Dec02 S.Yokkaichi

38

Proposed New spectrometer●

Cover the large acceptance–●●Tracking Device–Drift Chamber–

GEM(Gas electron multiplier)Two-stage Electron ID––●

Gas Cherenkov●

PMT+2 mirrors●

GEM+CsI photocathode

Schematic view of spectrometer

Leadglass EMC~30K Readout Channels (in 20 units)–●±45o (vert.), ±12o ~132o (holi.)

E325: 3.6K, PHENIX:~300K

Cost : ~$5M (including $2M electronics)

segment w/GEMGEM

HS05 @ KEK , 05Dec02 S.Yokkaichi

Proposed New spectrometer (2)●Detector

Focus to the very backward region:(option C)–rotate the spectrometer (B) with 90 degree–2π acceptance–acceptance is 2 times as E325(A) in βγ<1.25–

can be operated in more higher rate?
●●39Polebeam

Cross section (C)βγ acceptance

operation capability with the dimuon
experiment : 1012 ppp beam

Mass resolutions are less than ~10 MeV/c2 in the simulation(C)
σ=8.8MeV(B)(B)
σ=10.1MeV(C)(A)

HS05 @ KEK , 05Dec02 S.Yokkaichi

40Detector R&D item●●Environment with primary beam (109~1010 ppp )–high interaction rate (1~10MHz)–beam halo is origin of trigger background & saturation of forward detector

Tracker should cope with high beam / interaction rate–●

GEM tracker in forward region : works 10KHz/mm2

Electron ID : 10−4 π rejection–

lead glass EMC (recycled from TRISTAN) : < 10-1–

Gas Cherenkov : ~ 10-3–

HBD (Hadron Blind Detector )GEM●GC with GEM/CsI photocathode & pad readout / CF4 radiator/amplification gas●No mirror, No segment, No window : No photon loss with reflection efficiency●Flexible trigger capability
HS05 @ KEK , 05Dec02 S.Yokkaichi

HBD (Hadron Blind Detector)●

HBD : Thr. type Gas Cherenkov Counter–CsI photocathode : UV photon sensitive–Triple GEM with pad readout●–●

low granularity/low gain

Ionized electrons are collected by mesh●photoelectrons are amplified by 3 stages●

ionized electrons are amp. by only last 2 stages●●41

Concept of HBD-> can detect only particles with cherenkov photon.

Joint project with Weitzman Institute–originally for PHENIX upgrade plan–

GEM with CsIBeam Test was done in 2004 at KEK-PS–

NIM A 546(2005)466

Proposed structure for PHENIX HBD
HS05 @ KEK , 05Dec02 S.Yokkaichi

42

Development of GEM in CNS, U-Tokyo
We succeeded in fabricating a new type of GEM(CNS-GEM) using a dry etching method.
Etching
method

CERN-GEM

CNS-GEMwet etching

dry etching

Thecrosssectionof a hole double-conicalcylindrical shapeshape●Gain stability of CNS-GEM is better than that ofCERN-GEM.
––The shape of holes of GEM significantly affect gainstability.
stability for more longer time range (~1week) is also
under study

HS05 @ KEK , 05Dec02 S.Yokkaichi

43Summary (2)●Vector meson measurements in e+e- channel at J-PARC–

use primary proton beam (1x109 ~1x1010 /sec) on thin targets (~0.1%int.length) to reduce electron background–especially collect 104 ~105 φ → e+e- in p+A reaction in 100 shift (~5weeks)
operation●●●

10 ~100 times as large as E325's statistics

New spectrometer using new technology (GEM tracker/HBD)–mass resolution : less than 10 MeV/c2–

larger incident energy/larger acceptance → 10 times larger statistics.

–higher rate capability → more 10 times stat. using higher intensity beam

Prototype Detector with new technology is being developed.
–GEM made in Japan works well–

CsI photocathord & CF4 operation are next hurdle in Japan

HS05 @ KEK , 05Dec02 S.Yokkaichi