1. Soft Physics at Forward Rapidity
I.G. Bearden
Niels Bohr Institute
Københavns Universitet
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

2. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
Outline
Introduction
BRAHMS experimental details
K/p ratio
Why is it interesting?
How does it look vs. rapidity? vs. pbar/p?
Stopping
What is it? Why is it interesting?
BRAHMS results +systematics
Summary
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

3. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
early universe
QCD phase diagram
quark
gluon
plasma
SIS
AGS
SPS
RHIC T
TC~170 MeV
crab nebula
neutron stars
temperature
hadron gas
Nuclear
Matter
baryon chemical potential mB
940 MeV
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

4. how can we probe the plasma?
e, pressure builds up
System expands & cools
g, g* e+e-, m+m-
Real and virtual photons emitted as thermal radiation.
Hard scattered partons,
Heavy q’s probe
plasma
p, K, p, n, f, L, D, X, W, d,
Hadrons reflect (thermal) properties when inelastic collisions stop (chemical freeze-out).
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

5. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
The BRAHMS Experiment
It’s Broad RAnge Hadron Magnetic Spectrometers!
90°
30°
and 30° to 90° (MRS)
Two small solid angle
spectrometers (FS and MRS)
2.3°
30°
that can rotate from 2.3° to 30°
provide excellent PID over
broad range in y-pT
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

6. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
The BRAHMS experiment
FS: TPC, TOF, Cherenkov
deg.  =
MRS: TPC, TOF
deg =
Global:
|| < 5
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

7. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
A BRAHMS Event T2 D2
FFS at 6 degrees T1 D1
collision point
(vertex)
beam
beam
TPM1 D5
TPM2
Reconstructed tracks
MRS at 90 degrees
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

8. Particle Identification
TIME-OF-FLIGHT
RICH: Cherenkov light focused
on spherical mirror ; ring on image plane
Ring radius vs momentum gives PID
p / K separation 20 GeV/c
Proton ID up to 35 GeV/c
CHERENKOV
(2 settings)
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

9. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
Particle Spectra
After appropriate corrections, we combine all data sets to obtain
final invariant yields over a broad range of rapidity and pT p K p
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

10. Particle Spectra (200 GeV Au+Au)
PRL 94, (2005)
Top 5% central collisions
Pions: power law
Kaons: exponential
Protons: Gaussian
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

11. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
Outline
Introduction
BRAHMS experimental details
K/p ratio
Why is it interesting?
How does it look vs. rapidity? vs. pbar/p?
Stopping
What is it? Why is it interesting?
BRAHMS results +systematics
Summary
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

12. Why look at ’low’ pT K and pi?
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

13. Pt spectra from 0-10% central Au+Au collisions
Pion spectra fitted with a power law at mid-rapidity and mt exponentials at forward rapidities.
The kaon spectra fitted with mt exponentials at all rapidities.
Quark Matter, 4-10 February, Jaipur, India

14. Anti-particle/particle ratios vs. rapidity
At y~0
pbar/p ~0.47
K⁻/K⁺ ~ 0.85
π⁻/π⁺ ~ 0.98
At y~3
pbar/p~0.02 !!
K⁻/K⁺ ~ 0.6
π⁻/π⁺ ~ 1.1
Quark Matter, 4-10 February, Jaipur, India

15. K⁻/K⁺ vs baryo-chemical potential
The blue curve is a statistical model calculation with a chemical freez-out temperature fixed at 170 MeV but allowing the baryo-chemical potential to vary.
I.G.Bearden et al., BRAHMS Collaboration, Phys.Rev.Lett. 90 (2003)
F.Becattini et al., Phys.Rev. C64 (2001)
Quark Matter, 4-10 February, Jaipur, India

16. K⁻/K⁺ vs baryo-chemical potential
pbar/p
K⁻/K⁺ vs baryo-chemical potential
The blue curve is a statistical model calculation with a chemical freez-out temperature fixed at 170 MeV but allowing the baryo-chemical potential to vary.
I.G.Bearden et al., BRAHMS Collaboration, Phys.Rev.Lett. 90 (2003)
F.Becattini et al., Phys.Rev. C64 (2001)
Quark Matter, 4-10 February, Jaipur, India

17. Integrated dN/dy vs rapidity
UrQMD references:
S.A.Bass et al., Prog.Part.Nucl.Phys.41 (1998)
M.Bleicher, E.Zabrodin et al., J.Phys.G25 (1999)
AMPT references:
B.Zhang et al., Phys.Rev.C61 (2000)
Z.-w. Lin et al., Nucl.Phys.A698 (2002)
The dN/dy distributions were fitted with gaussian functions with fixed centroids at y=0.
The different kaonic widths reflect their different production mechanisms.
Quark Matter, 4-10 February, Jaipur, India

18. Quark Matter, 4-10 February, Jaipur, India
K/π ratios vs. Rapidity
UrQMD and AMPT models fail to fit the K⁺/π⁺ ratio behaviour at forward rapidity.
The negative ratio is reasonably explained.
Quark Matter, 4-10 February, Jaipur, India

19. Quark Matter, 4-10 February, Jaipur, India
K/π vs pbar/p
I.G.Bearden et al., BRAHMS Collaboration, Phys.Rev.Lett.94 (2005)
I.Arsene et al., BRAHMS Collaboration, Nucl.Phys.A757 (2005) 1-27
Quark Matter, 4-10 February, Jaipur, India

20. WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute
Outline
Introduction
BRAHMS experimental details
K/p ratio
Why is it interesting?
How does it look vs. rapidity? vs. pbar/p?
Stopping
What is it? Why is it interesting?
BRAHMS results +systematics
Summary
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

21. Stopping or Transparency ?
Before collision
After collision
Stopping
Transparency y
dN/dy (p-pbar) yb δy y y
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

22. Stopping: where are the baryons?
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

23. What you can do with 100 times more data…
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

24. Stopping: where are the baryons?
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

25. Measure @ 62 A GeV: Protons and antiprotons
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

26. How many ’net’ protons :
BRAHMS preliminary
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

27. Rapidity loss: 6th order polynomial
Gaussians in pz:
2.03  0.16
2.00  0.10
6 order polynomial
Total E=25.72.1TeV
Rapidity loss:
22. Oktober 2004
Forward Physics Workshop, KU
I.G.Bearden, Niels Bohr Institute

28. Systematics of stopping
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

29. Projectile net-baryons:
Subtract ”target” contribution
(characterized by exp(-y)[p+p] exp(-y/2)[ZPhysC43(1989)241]
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute

30. ’Net’ baryon vs. rapidity as a function of beam energy
AGS √snn=5 GeV
SPS √snn=17 GeV
RHIC √snn=62 GeV
RHIC √snn=200 GeV
LHC √snn=5500 GeV
6. April 2008
WNND08, South Padre Island I.G.Bearden, Niels Bohr Institute