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

What is the deal about them at low energies?

Baryons

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

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

Thanks!