riken, kyoto univ. a, kek b, cns univ. of tokyo c, icepp univ. of tokyo d,tohoku univ. e ryotaro...
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RIKEN, Kyoto Univ.a , KEKb, CNS Univ. of Tokyoc,
ICEPP Univ. of Tokyod,Tohoku Univ.e
Ryotaro Muto, RIKEN, Japan
J. Chibab, H. En’yo, Y. Fukaoa, H. Funahashia, H. Hamagakic, M. Ieirib,M. Ishino
d, H. Kandae , M. Kitaguchia, S. Miharad, K. Miwaa, T. Miyashitaa,
T. Murakamia, T. Nakuraa, M. Naruki, M. Nomachib, K. Ozawac, F. Sakumaa,
O. Sasakib, H.D. Satoa, M. Sekimotob, T. Tabaru, K.H. Tanakab,
M. Togawaa, S. Yamadaa, S. Yokkaichi, Y. Yoshimuraa
(KEK-PS E325 Collaboration)
First observation of First observation of meson meson mass-modification in nuclear mass-modification in nuclear
medium (KEK-PS E325) medium (KEK-PS E325)
ContentsContents
Physics Motivation Experimental Setup Result of e+e- analyses
e+e- Invariant Mass Distribution
Our experiment, KEK-PS E325, measured e+e- and K+K- invariant mass spectgra to investigate possible in-medium mass modification of vector mesons.
Physics MotivationPhysics Motivation
Spontaneous Breaking of Chiral Symmetry
Hadrons ~1GeV/c2
Constituent quarks ~300MeV/c2
Current quarks ~5MeV/c2
In Vacuum In Hot/Dense Matter
Tqq
,
How to detect?
Vector meson massat normal nuclear density
u
ud
u
ud
Vector MesonsVector Mesons
Mass modification 20 ~ 40MeV/c2 : relatively smallSmall decay width (4.4MeV/c2), no other resonance nearby : sensitive to small mass modification
Hatsuda & Lee PRC 46, R34(1992)
predictions of vector meson modification in medium
Brown, Rho(1991), Hatsuda, Lee(1992),
Klingl, Kaiser, Weise(1997), etc.
Large mass modification ~120MeV/c2 at = 0
Large cross sectionDifficult to separate and on e+e- mass spectrum
Poster No.194 by M. Naruki
E325 ExperimentE325 Experiment
Slowly moving (plab~2GeV/c) Large Acceptance Spectrometer
measures Invariant Mass of e+e-, K+K-
in 12GeV p + A , , + X reactions Measurement of e+e- almost free from FSI Mass Resolution for e+e- : 8.0MeV/c2 for 10.7MeV/c2 for
Primary proton beam (~109/spill/1.8s)
Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress conversion
Beam
Target
Targets C Cu
Interaction length(%) 0.2% 0.05%X4
radiation length(%) 0.4% 0.5%X4
Forward LG Calorimeter
Rear LG Calorimeter
Side LG Calorimeter
Front Gas Cherenkov
Rear Gas Cherenkov
Barrel Drift Chamber
Cylindrical DC
Vertex Drift chamber
1m
Detector SetupDetector Setup
12GeV proto
n
beam
B
Hodoscope
Aerogel Cherenkov
Forward TOF
Start Timing Counter
ee++ee-- Invariant Mass Distributions Invariant Mass Distributions
C Cu
ee++ee-- Invariant Mass Distributions Invariant Mass Distributions
C Cu
On the FitOn the FitBack Ground : Quadratic curve Resonance Shape : Breit-Wigner shape experimental effect estimated by Geant4 simulation – energy loss, mass resolution, mass acceptance etc.
•Blue histogram : Detector Simulation
•Red line : Breit-Wigner (gaussian convoluted) fitting result
Experimental effects are fully taken into account
detector simulation for
Fit ResultsFit Results
C Cu
??
To see To see dependence dependenceSlowly moving mesons have a larger probability to decay inside the target nucleus.We divided the data into three by ; <1.25, 1.25<<1.75 and 1.75<.
distribution
Fit Results (divided by Fit Results (divided by ))<1.25 (Slow) 1.25<<1.75 1.75< (Fast)
Lar
ge
Nu
cleu
sS
mal
l N
ucl
eus
Data cannot be reproduced (99% C.L.)
The amount of the ExcessThe amount of the Excess• Fit again excluding the region where the excess was seen; 0.95~1.01GeV/c2
• Integrate the amount of the excess in the above region (0.95~1.01 GeV/c2) Nexcess
excluded from the fitting
• mesons are generated uniformly in target nucleus
• momentum distribution: measured• pole mass: m*/m = 1 – k1 /0
(from Hatsuda, Lee)
We set k1 = 0.04 • decay width: */ = 1 + k2 /0
We set k2 = 10
(at =0, *~48MeV (from Klingl et.al ))• density distribution
– Woods-Saxon– radius: C:2.3fm/Cu:4.1fm
Model Calc. Model Calc. including Mass Modificationincluding Mass Modification
e+
e+
e-
e-
Results of Model Calc.Results of Model Calc.with kwith k11=0.04, k=0.04, k22=10=10
<1.25 (Slow)Amount of Excess
by Model Calc.
SummarySummary• KEK PS-E325 measured e+e- and K+
K- invariant mass distributions in 12GeV p + A reactions.
• Significant enhancement was seen on the e+e- invariant mass distributions at the low-mass side of the meson peak in the Cu data, in < 1.25 region.
• This observation is comparable to the model calculation including in-medium mass modification.
Cu
SummarySummary
Result of e+e-
Cu
Cu • KEK PS-E325 measured e+e- and K+
K- invariant mass distributions in 12GeV p + A reactions.
• Significant enhancement was seen on the e+e- invariant mass distributions at the low-mass side of the meson peak in the Cu data, in < 1.25 region.
• This observation is comparable to the model calculation including in-medium mass modification.
• We observed the signature of the in-medium mass modification both in and mesons.
( results were presented in the poster session 1 by M. Naruki)
Backup slides
Kinematical DistributionsKinematical Distributions
y
pTpTvs y
Check on Detector SimulationCheck on Detector Simulation p -
Mass spectra are well reproduced by the simulation
Ks + -
M =496.8±0.3(MC 496.9±0.1) MeV/c2
= 3.9±0.4(MC 3.5±0.1) MeV/c2
M =1115.71±0.02(MC 1115.53±0.01) MeV/c2
= 1.73±0.02(MC 1.62±0.01) MeV/c2
Results of Model Calc.Results of Model Calc.<1.25 (Slow) 1.25<<1.75 1.75< (Fast)
Lar
ge
Nu
cleu
sS
mal
l N
ucl
eus
Target ConfigurationTarget Configuration
material beam intensity(p/spill)
Interaction length(%)
radiation length(%)
C ~1x109 0.2% 0.4%
CuX4 ~1x109 0.05%X4 0.5%X4
23mm
CCu
Very thin target with clean and high intensity beam
Vertex Distribution
Beam
Results of Model Calc.Results of Model Calc.with kwith k11=0.02&0.04, k=0.02&0.04, k22=10=10
Invariant Mass Spectrum of e+e- (background subtracted)Invariant Mass Spectrum of e+e- (background subtracted)
ratio is consistent with zero=0.0±0.02(stat.)±0.2(sys.) 0.0±0.04(stat.)±0.3(sys.)
C Cu
It is pretty much surprising because the is known to be unity in pp reactions
the region 0.60-0.76 GeV/c2 is excluded from the fit.