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Measuring high µB with resonances
Sascha Vogel
Resonance Workshop March 6th 2012, Austin, Texas, USA
Outline
• Quick UrQMD reminder
• Resonance kinematics
• How deep can we look into heavy ion collisions using resonances? (does high transverse momentum change anything?)
• Baryons @ low energies
• a1
• Conclusions
• Several physical effects are density driven, e.g.
• vector meson spectral function broadening
• chiral phase transition
• QGP phase transition
• quarkyonic matter
• ...
Motivation
The tool - UrQMD
• Ultra Relativistic Quantum Molecular Dynamics
• Non equilibrium transport model
• All hadrons and resonances up to 2.2 GeV included
• Particle production via string excitation and -fragmentation
• Cross sections are fitted to available experimental data or calculated via detailed balance or the additive quark model
• Does account for canonical suppression
Phys.Rev.C69:054907,2004Phys.Rev.C74:034902,2006
No explicit implementation of in-medium modifications!
nucleon � ⇥ ⌅ ⇤ ⇧N938 �1232 ⇥1116 ⌅1192 ⇤1317 ⇧1672
N1440 �1600 ⇥1405 ⌅1385 ⇤1530
N1520 �1620 ⇥1520 ⌅1660 ⇤1690
N1535 �1700 ⇥1600 ⌅1670 ⇤1820
N1650 �1900 ⇥1670 ⌅1775 ⇤1950
N1675 �1905 ⇥1690 ⌅1790 ⇤2025
N1680 �1910 ⇥1800 ⌅1915
N1700 �1920 ⇥1810 ⌅1940
N1710 �1930 ⇥1820 ⌅2030
N1720 �1950 ⇥1830
N1900 ⇥1890
N1990 ⇥2100
N2080 ⇥2110
N2190
N2200
N2250
The tool - UrQMD
0�+ 1�� 0++ 1++
⇥ ⇤ a0 a1
K K⇥ K⇥0 K⇥
1
� ⇧ f0 f1
�⇤ ⌅ f⇥0 f ⇤11+� 2++ (1��)⇥ (1��)⇥⇥
b1 a2 ⇤1450 ⇤1700
K1 K⇥2 K⇥
1410 K⇥1680
h1 f2 ⇧1420 ⇧1662
h⇤1 f ⇤2 ⌅1680 ⌅1900
The tool - UrQMD
Dileptonic and hadronic decays
ρ0 ρ0π-
π+
π+
π-
+
ρ0
ρ0
+
-
-
π-π+
ρ0
-
+
late stage integrated collision
hadronic decay leptonic decay
Rescattering
• well known effect, studied in➡ statistical hadronization models➡ transport models➡ hydrodynamical models
-4 -3 -2 -1 0 1 2 3 4y
10-1
2
5
100
2
5
101
2
5
102
2
dN
/dy
K0(892)(1520)(1232)
Bleicher, Aichelin, Phys.Lett.B530:81-87
UrQMD
100 120 140 160 180 200T [MeV]
10−1
R(1
520)
/(al
l R)
life[fm]
20
10
thermally produced
5
1
...
7
15
Torrieri, Rafelski, Phys.Lett.B509:239-245
Rescattering
• well known effect, studied in➡ experiment
Markert et al. [STAR], J.Phys.G35:044029,2008
/dychdN-100 0 100 200 300 400 500 600 700
reso
nanc
e/no
n-re
sona
nce
00.20.40.60.8
11.21.41.61.8
22.22.4
p+pA+A
= 200 GeVNNs
STAR preliminary
pp & AuAu x 2.9-K*/K
CuCu x 2.9-K*/K
pp & AuAu x 8.1- /Kq
Reach in density
• Normalized density spectrum
• Most resonances originate from very low density
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0/ 0
0
2
4
6
8
10
12
14
dN
/d(
/0)
[a.u
.]
K0(892)
**
A u + A u @ 200 A G e V p ro d
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0/ 0
0
1
2
3
4
5
6
7
8
9
10
dN
/d(
/0)
[a.u
.]
K0(892)
**
A u + A u @ 30 A G e V p ro d
30 AGeV 200 AGeV
Reconstruction probability
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0/ 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
fra
ctio
n
* El a b =30 G e V
* El a b =30 G e VEl a b =30 G e V
* E c m =200 G e V
* E c m =200 G e VE c m =200 G e V
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0/ 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
fra
ctio
n
El a b =30 G e VK*0
892 El a b =30 G e VEl a b =30 G e VEl a b =30 G e V
E c m =200 G e VK*0
892 E c m =200 G e VE c m =200 G e VE c m =200 G e V
• Probability to reconstruct resonances from a certain density
Baryons Mesons
pT dependence
0 1 2 3 4 5 6 7/ 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0<
p T>[G
eV
]
El a b = 30 A G e V < p T> a llE c m = 200 A G e V < p T> a llEl a b = 30 A G e V < p T> r e c .E c m = 200 A G e V < p T> r e c .
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0p T [ G e V]
10-3
10-2
10-1
100
101
102
103
104
105
1/p T
dN
/dp T
[Ge
V-2
]
E c m =200 G e V a ll d e c a y e dE c m =200 G e V r e c o nstr.
20% 49% 72%
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0p T [ G e V]
10-3
10-2
10-1
100
101
102
103
104
1/p T
dN
/dp T
[Ge
V-2
]
El a b =30 G e V a ll d e c a y e dEl a b =30 G e V r e c o nstr.
5% 19% 38%
• difference in pT spectrum between observable and all decayed
• percentage of reconstructable resonances produced at ρ>2ρ0
increases with pT
pT dependence
Δ,Σ*,Λ*,ρ,ω,K*,Φ
Δ,Σ*,Λ*,ρ,ω,K*,Φ
High pT resonances might shed some light on the dense phase of heavy ion collisions!
(but are they really what we want to measure?)
First conclusion
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0m [ G e V]
0.0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
dN
/dm
[Ge
V]-1
...............................................................................................................................
.........................................................................
1905017000
N*1720
0N*
15200
R 0 + 0
0. R 0
0 --> e + + e -
ρ meson in C+C @ 2AGeV
At low energies (~2 AGeV) contributions from baryon
resonance decays are dominant.
N*1520 contributes via the decay chain
N*1520 → N + ρ ρ → π+π- or ρ → e+e-
to the low mass part of the ρ meson mass spectrum.
SV, M. Bleicher, Phys.Rev.C74:014902,2006
ρ meson at higher energies
0 0.2 0.4 0.6 0.8 1
m [GeV]
1x10-5
1x10-4
1x10-3
0.01
0.1
1
10dN
/dm
[1/G
eV
] (n
orm
aliz
ed)
C+C 1 AGeV
C+C 2 AGeV
Au+Au 20 AGeV
Au+Au 30 AGeV
At higher energies the contribution from baryonic resonance decays become
less important.
Note: All curves normalized to the
770 MeV point.
ρ meson at higher energies
Due to the dependence on the baryon density the
mass of the ρ meson is rapidity dependent.
The ρ meson mass drops towards higher rapidity.
Controlling baryon kinematics is important
(otherwise some spectra seem more interesting than they are)
Second conclusion
Measuring Chiral Symmetry
• Can we observe a chirally restored phase? (and how?)
• What happens to the ρ meson in the medium? What happens to the a1 meson?
• What can we learn from reasonable hadronic dynamics (without a chirally restored phase)?
V. Koch, Int.J.Mod.Phys.E6:203-250,1997
a1 meson
The a1 meson mass is expected to be equal to the mass of the ρ meson, in case of
chiral symmetry restoration.
Problem:It is hard to measure.
a1 meson
The a1 meson mass is expected to be equal to the mass of the ρ meson, in case of
chiral symmetry restoration.
Problem:It is hard to measure.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
dN
/dm
[1/G
eV
]
30 G e V p + p20 G e V p + p
( a ll d e c a y c h a n n e ls)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0
20
40
60
80
100
120
dN
/dm
[1/G
eV
]
30 G e V A u + A u20 G e V A u + A u
( a ll d e c a y c h a n n e ls)
a1 meson
Idea: Check the mass distribution from the transport code.
p+p Au+Au
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.1
0.2
0.3
0.4
0.5d
N/d
m[1
/Ge
V]
A u + A u 30A u + A u 20p + p 30 (x200)p + p 20 (x200)
( a 1 )
a1 meson
Next: trigger on the decay channel a1 → γπ (assumed width = 640keV)
�i,j(M) = �i,jR
MR
M
⇤�pi,j(M)⇥�pi,j(MR)⇥
⌅2l+1 1.2
1 + 0.2�
�pi,j(M)⇥�pi,j(MR)⇥
⇥2l,
a1 meson
→ Mass dependent branching ratios
Low mass a1 favors γπ decay, not ρπ
Trigger on a1 → γπ = trigger on low mass a1 mesons
H. Sorge, Phys.Rev.C52:3291,1995
a1 meson
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
BR
a 1
a 1
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dN
/dm
[a.u
.]
a 1
Below 900 MeV γπ decay is dominant, ρπ is kinematically suppressed.
Branching ratio folded with BW distribution
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
BR
BW d istri b u ti o n (n or m a liz e d )a 1 d N / d m p p @ 20 A G e V (n or m .)BR a 1
BR a 1
a1 meson
Full model calculation
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0m [ G e V]
0.0
0.1
0.2
0.3
0.4
0.5d
N/d
m[1
/Ge
V]
A u + A u 30A u + A u 20p + p 30 (x200)p + p 20 (x200)
( a 1 )
a1 meson
Take home messages
• Experimentally reconstructable resonances are not sensitive to the high density region unless measured at high pT
• Beware of baryons kinematics
• a1 → γπ might not be the golden channel