1. In-medium Modifications of Low-Mass Vector Mesons (LVM) in PHENIX at RHIC
Yuji Tsuchimoto
Hiroshima Univ. Quark Lab.
for the PHENIX collaboration
QM2009 in Knoxville, TN USA
Let me start my presentation: “In-medium Modifications of Low-mass Vector Mesons”.
I’m Yuji Tsuchimoto for the PHENIX collaboration from Hiroshima University.
* Current affrication : CNS Univ. of Tokyo

2. Motivation : the properties of QGP
A QGP has been created by relativistic heavy ion collisions at RHIC.
For the next task, we study the properties of the QGP.
Deconfinement of partons
Restoration of chiral symmetry
CEP? T mB Tc
deconfinement
chiral
QGP
hadron
RHIC
QCD phase
Recently, there are many experimental results suggest that the QGP has been created by Heavy Ion Collision at RHIC.
For the next task, we are going on stage to study the properties of the QGP.
This is a simplified QCD phase diagram.
There are 2 major phase transitions in the diagram.
One is deconfiment of quarks and gluons.
The other is a partial restoration of Chiral Symmetry.
The Braking of the Chiral Symmetry provides effective mass of quarks, corresponding to the hadron mass.
This effective mass will vanish in hot or dense matter. RHIC can reach this region.
Effective mass of quarks will vanish
in hot matter
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

3. Expected mass-modification of LVM
Hadron mass depends on temperature and density.
Mass-line-shape analysis is the most straightforward method.
needs large statistics
R. Rapp (Nucl. Phys A661(1999) 238c
Hadron mass depends on temperature and density.
Some model calculations predict mass modification of LVM in medium.
For example, this model predicts the modified mass spectra of omega and phi.
The higher temperature increase, the smaller mass become, and the wider width become.
This mass-line-shape analysis is the most straightforward and direct method.
But it needs large statistics.
It is quite difficult to apply for our data.
Some models predict mass and width modification.
Smaller and wider mass is predicted as higher temperature.
effects are larger for the w.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

4. Hadronic and electronic probes K+ K-  e+ e-
LVM has hadronic and leptonic decay mode.
Hadronic is dominant.
Hadrons loose initial information in the matter.
Leptons carry very clean information from LVM decays deep within the fireball.
The hadronic decay channel has an advantage of a larger branching ratio.
However, Hadron from deep within the hot matter is scattered by strong force, thereby loosing information of the original decay.
On the other hand, the electrons are not subject to strong interaction.
The leptonic probes can reconstruct information of the vector mesons decayed in medium.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

5. Gee/GKK may be sensitive
Branching ratio of f
RHIC
~ hadron mass
if f mass is modified in hot/dense matter with the decreasing of the <qq> condensate, them f mass is changed.
T.Hatsuda and S.Lee Phys.Rev.C
mKK mf
Small Q-value
Gee/GKK may be sensitive Mf
We are particular interested in the branching ratio of phi.
This is the other prediction of the in-medium modification of phi mass.
This shows the phi mass dependence on density, but similar behaviour is expected on temperature.
The vector meson mass will decrease in dense matter, also in hot matter. (top figure)
Kaonic decay is dominant branch of phi. Mainly phi decays into kaon pairs strongly.
And phi decays into electron pair in a few fraction of ten thousand.
For the Kaonic decay modes, the mass of Kaon pair is very close to mass of phi.
Due to the small Q-value of phi going to K+-, the branching ratio of phi may be sensitive to the mass modification.
For extreme example, If the mass of phi become smaller than the mass of kaon pair, this phi cannot decay into kaon pairs.
Then the branching ratio of phi is important.
Branching ratio of f is important
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

6. PHENIX central spectrometer
Full PID for p/K/p/e/g
ERT trigger for electrons (p+p & d+Au)
Run3/4/5 data is shown. × e- × × ×
B-Field × e+ × × × × × × × × K+ × K-
We measured phi and omega in both of electronic and hadronic probes using the PHENIX central spectrometer.
The PHENIX central spectrometer can measure fully identified hadrons, electrons and photons.
In this talk, I show data of run3 d+Au, run4 Au+Au and run5 p+p data.
We have already taken higher statistical data of run6 p+p, run7 Au+Au and run8 d+Au.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

7. Baseline measurement of f in p+p
LVM measurement in p+p collisions is an important baseline.
Clear peaks in e+e- and K+K- invariant mass.
r contribution is inseparable to w.
r,w,fee
fKK
fhh
These are invariant mass spectra of e+- and K+- in p+p collisions.
We can see clear peaks of omega and phi in e+- mass, and also peak of phi in K+- mass.
The solid line is the best fit of the mesons.
Relatively larger modification is predicted on omega, however, in the dielectron invariant mass, there is inseparable rho contribution in this mass region.
The phi is more cleaner probe, although a relatively small modification is expected, because there is no contribution of other resonances.
See poster #264 by K.M.Kijima
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

8. f in (run3) d+Au collisions
In d+Au case - more background.
PT=
r,w,fee
fKK
PT=
In case of d+Au, there is more combinatorial background.
Although limited statistics, we can see peaks of omega and phi in e+- and also phi in K+- mass spectra.
We counts number of phi and omega in these mass region, and correct acceptance and efficiency.
# “phi in Cold nuclear matter”
Background subtracted
fKK
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

9. Baseline established on f  e+e-/K+K-
d+Au
RdAu
p+p
Levy fit
PHENIX Preliminary
PHENIX Preliminary
We obtained corrected mT spectra of phi in both e+- and K+- decay channels in p+p and d+Au.
The spectra in both channels are consistent each other.
I fit these points in p+p by Levy function as solid line, and scaled by Ncoll in d+Au as dashed line.
The spectra in d+Au is agreed with scaled Ncoll scalling.
The right figure is R_dA, the points in d+Au divided by scaled p+p, which is dashed line.
The R_dA agreed with one and slightly above on one.
And we have established the analysis method more.
There are finalized and published soon.
# “Baseline established”
# no obvious modification observed
# タイトルを物理メッセージにする
# 絵をきちんと説明する
# 物理的に強調したい部分(下2つ)を
# Why Levy? Any function can be applied
We measured f  e+e-/K+K- in p+p and d+Au
Fit spectrum in p+p by Levy(solid) and scaled by Ncoll(dashed)
Baseline established
Both decay channels are consistent, Ncoll scaling holds
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

10. f in hot matter : mass spectra
w,f  ee
We observed peaks, despite of large background.
Electron selection is optimized for LVM
HBD will reduce B.G.
Au+Au Min bias
Min bias
f  ee
f  KK
In case of Au+Au, there is huge background.
We can see a peak of phi in K+- invariant mass spectra,
However, it is difficult to find peaks in e+- invariant mass spectra.
The peaks can be seen by accurate subtraction of the combinatorial background.
If we had much more statistics, we may apply line-shape analysis, however it is difficult due to statistical limit.
Mass Resolution～10MeV Line-shape analysis is difficult
by statistics(S/N)
Combinatorial Background
is Estimated by Event Mixing
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

11. f in hot matter : consistent to e+e-/K+K-
Suppression of  see talk by M. Naglis
(5D-2, Thu)
Min.Bias
0-20%
More statistics and careful analysis is established for central f→e+e-
20-40%
This is corrected mT spectra of phi in Au+Au collisions.
The both decay channel is consistent each other in Au+Au.
Phi yield is suppressed in most central Au+Au at least in K+- channel.
The details of the phi suppression is reported by another parallel talk by Maxim Naglis.
As I mentioned in introduction, the branching ratio of phi is important measurement.
The changing of the ratio can be seen as difference of these spectra.
More mesons may decay in the hot matter at low-momentum as naïve model.
We expect something interesting at low-momentum in the central collisions.
We need more statistics and careful analysis to discuss the difference in the central.
PHENIX Preliminary
40-92%
mT – m0 (GeV/c2)
No significant modification is observed
Hot Matter
Hot Matter
High mT
LVM
Low mT
LVM
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

12. w systematic measurements
w is also interesting probe, larger modification is expected.
We measured w  e+e-/p0g/p0p+p- in p+p, d+Au and Au+Au.
Assume r yield same as w.
w,f  ee
w  p0g
p+p
p+p
Remember omega is also an important probe and has a somewhat larger modification in many models.
We measured the omega in both the di-electron and hadron channels.
We can see clear peak of omega in e+- and pi0-gamma.
The rho contribution is estimated and subtracted, with assuming of rho yield as same as omega yield.
For the hadronic channels, these are 3 or 4 body photons in final state.
It is difficult to detect especially at low-momentum.
Backgrounds
Mee
Mass GeV/c2
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

13. w baseline in p+p & d+Au RdA
PHENIX Preliminary
PHENIX Preliminary
This is corrected mT spectra of omega in e+- and pionic channels.
There are small difference between the decay modes in preliminary results, however, the difference become smaller at the final results.
A Levy function fit to spectra in p+p is shown as solid line, and scaled to d+Au as dashed line.
As dividing the spectra in d+Au, the R_dA is obtained.
The R_dA is agreed with 1. The Ncoll scaling is applicable.
mT – m0 (GeV/c2)
pT (GeV/c2)
Systematic measurement of w  e+e-/p0g/p0p+p- in p+p and d+Au is established.
These agree with Ncoll scaling.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

14. w in hot matter : agreed with Ncoll
Au+Au
w  p0g
PHENIX Preliminary
The omega can be seen in Au+Au collisions in pi-zero gamma channel.
The mT spectra are obtained in limited mT for each centralities.
There are also agreed with Ncoll scaling, and suppression may be seen in the most central.
These(low-mT) points are e+- channels. Although no overwrap region, both of decay channels are agreed with scaled line.
Invariant Mass
mT – m0 (GeV/c2)
w  p0g is also measured in Au+Au collisions at high mT.
It is not applicable to low mT, but the range will cross over both channels.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

15. HBD will provides significant
Hadron Blind Detector (HBD) is installed and working!
Dominant electron source is p0 Daliz and conversion.
HBD can remove photonic electron background (small pair-mass)
At this moment, the RHIC run9 program is carrying out.
In this run, the Hadron Blind Detector is installed in the PHENIX, and working well.
The photonic background is dominant electron source.
The HBD can remove photonic electron background, which has small pair-mass.
The HBD may reduce background by about ten to hundred in near future.
The derails are shown at poster.
See poster by J. M. Durham
Electrons from LVM ~ few %
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

16. Summary
In-medium modification of LVM is the best probe to study chiral symmetry restoration.
We measured f  e+e-/K+K-, w  e+e-/p0g/p0p+p- in p+p, d+Au and Au+Au.
Systematic measurements are established.
f  e+e- and f  K+K- are consistent with each other in p+p, d+Au and (M.B.) Au+Au.
Higher statistics with HBD capability to reject p0 Dalitz, will enable measurements in central Au+Au.
Let us summarize my talk.
In-medium modification of LVM is one of the best probe to study chiral symmetry restoration in hot and dense matter.
We measured phi and omega in electronic and hadronic decay channel in p+p, d+Au and Au+Au collisions in PHENIX.
These systematic measurement methods are highly established.
The phi going to ee and KK are consistent in p+p, d+Au and minimum bias Au+Au.
These final results are in preparation of publish.
And higher statistics run with HBD which has capability to reject pi-zero dalitz background will enable measurements in central Au+Au collisions.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

17. PHENIX collaboration
I’m really glad to have a talk at the QM09, Thank you!
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

18. Timeline of HIC and electron probes for LVM
hadrons e+ e-
Thermal freeze-out
Chemical freeze-out
Hadronize
QGP
Collision
time
f,w
beam-axis
This is a famous diagram of space-time expansion.
The very hot matter, maybe QGP, is created at collision point immediately after the collision of heavy ion.
This QGP is hadronized and frozen chemically and kinematically.
Quarks in QGP is fragmented and become massive through the hadrnization phase.
If we could measure the mass of hadrons around the phase transition, it may be modified from these in vacuum.
But we can measure particles
The Low-Mass Vector Mesons are the best probe of the mass modification.
These has relatively short lifetime, the order of 10 fm/c. It is the same order of the lifetime of QGP.
And the LVM has leptonic branching mode.
Lepton carries properties of LVM decayed in the matter, because lepton is not subject to strong force. Au Au
We can measure particles only after the freezeout
In-medium LVM can be studied by electron pair
LVM lifetime ~ QGP lifetime
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

19. The invariant mass spectra for f mesons
f-> e+e- p+p
f-> e+e- d+Au
f->e+e- Au+Au f φ
f->K+K- (double PID) d+Au
f-> K+K- (double PID) Au+Au
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

20. The invariant mass spectra for φ mesons
f->K+K- (no PID) pp
f->K+K- (single PID) pp
f->K+K- (no PID) AuAU
f->K+K- (no PID) dAu
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

21. The invariant mass spectra for ω mesons
w-> e+e- p+p
w->e+e- d+Au
w->e+e- Au+Au ω ω
w->p0p+p- p+p
w->p0g p+p
w->p0g Au+Au
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

22.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

23. Yield and temperature for φ mesons
After several experimental checking, we found integrated
information is not enough to determine the difference between
e+e- and K+K-.
Need a careful check of mT spectra.
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

24. RdA and RAA of charged hadrons
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

25. RAA of f in Min.Bias Au+Au
Consistent RAA are obtained in electronic and hadronic decay channels.
Higher statistics may provides some difference around low-mT
Private Data, must not be shown!
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

26. Analysis – p+p, d+Au p+p, d+Au collisions p+p d+Au wφ  e+e- wφ  e+e-
w  p0g
p+p
w  p+p-p0
Both leptonic and
hadronic decay channel are measured
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

27. Analysis – Au+Au wφ  e+e- wφ conbinatorial BG backgrounds BG Signal
Reference: Nakamiya-san’s talk
wφ  e+e-
conbinatorial
backgrounds BG wφ
Signal BG
Extract
by Event Mixing BG
w,f→ee
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto

28. Analysis – A+A w  p0g (p0  2g) Subtract Backgrounds Mixed event BG2
Reconstructed invariant mass
Mixed event
Peripheral (cent 60~92 %)
3.5<pT<4.5[GeV/c]
BG2
BG3
Black: FG
count
BG1
Magenta:BG1+BG2+BG3
after subtraction of
BG1+BG2+BG3
[GeV/c^2]
Gaussian (Mean and With are Fixed
by Embedding simulation)
In-medium Modifications of Low-Mass Vector Mesons in PHENIX - Yuji Tsuchimoto