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Probing cold nuclear ma0er with virtual photons
1
Michael Weber, Manuel Lorenz, Anar Rustamov and Pavel Tlusty for the HADES collaboraAon
HADRON 2011 -‐ 17.06.11
Outline: • Virtual photons in cold nuclear ma0er • HADES • Slow and fast e+e-‐ pairs • Nuclear modificaKon factor • Conclusion
Virtual photons in cold nuclear ma0er
• EM structure / Vector mesons in ma0er • Decay in e+e-‐:
– Weak interacKons with surrounding medium – Other sources (conversion, Dalitz decays)
• Experiments: – Shape measurements (spectral funcKon) – AbsorpKon (total width)
• Results (Spring8, KEK -‐ E328, JLab – CLAS, CB/TAPS@ELSA, ANKE) – Broadening – Pole mass shi`? – Difficult interpretaKon (Differences in Background determinaKon, ρ contribuKon, decay kinemaKcs, …)
2
e+
e-
Heavy nucleus γ*
HADRON 2011 -‐ 17.06.11
HADES@GSI
HADRON 2011 -‐ 17.06.11 3
Observable Detector p MDC (Magnet) β TOF(ino)
dE/dx MDC TOF(ino)
e+/e-‐ RICH Pre-‐Shower
High Acceptance Di-‐Electron Spectrometer[ EPJ A41 243 ]
Data sets: protons with Ekin = 3.5 GeV • pNb (cold nuclear ma0er) • pp (reference)
HADES@GSI
HADRON 2011 -‐ 17.06.11 4
High Acceptance Di-‐Electron Spectrometer[ EPJ A41 243 ]
Data sets: protons with Ekin = 3.5 GeV • pNb (cold nuclear ma0er) • pp (reference)
See also: • „In-‐medium modificaKon
of hadrons”, Piotr Salabura
• „Neutral kaon producKon in pp and pNb collisions”, Kirill Lapidus
e+e-‐ Invariant Mass
• Signal = all e+e-‐ pairs – same event Like Sign CB • S/B ~ 10 in vector meson mass region • Mass resoluKon: σω~15 MeV/c2
• 2 independent PID: Hard Cuts and MulK Variate Analysis HADRON 2011 -‐ 17.06.11 5
)2 (MeV/ceeM
0 200 400 600 800 1000 1200
)-1 )2 ((
MeV
/cee
dN/d
M
-110
1
10
210
310
410
- e+ eCB (LS)
p+Nb (3.5 GeV) > 9 ̊
Signal
α
Reference: pp (Ekin = 3.5 GeV)
HADRON 2011 -‐ 17.06.11 6
Separate 4 mass regions: • π0 :
0.00 < Mee (GeV/c2) < 0.15 • η :
0.15 < Mee (GeV/c2) < 0.47 • Δ, ρ, ω Dalitz :
0.47 < Mee (GeV/c2) < 0.70 • ω, ρ, φ :
0.70 < Mee (GeV/c2) < 1.20
PRELIMINARY
Cold nuclear ma0er: pNb (Ekin = 3.5 GeV)
HADRON 2011 -‐ 17.06.11 7
• NormalizaKon: – pp: elasKc collisions – pNb: negaKve pion yield
• Pion scale with ~ A0.7 – Expected for surface
producKon
• Shape analysis: – RelaKve to π0 yield
• Nuclear modificaKon factor PRELIMINARY
]2 [GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2
)]2 [1
/(GeV
/cee
dN
/dM
0π
1/N
-510
-410
-310
-210
-110
1p+Nb 3.5 GeV
p+p 3.5 GeV
Comparison: pNb vs. pp
HADRON 2011 -‐ 17.06.11 8
• Shape analysis • Scaled with the number of π0
• Different producKon and/or absorpKon processes dominant
for different e+e-‐ sources
PRELIMINARY
Pair momenta
HADRON 2011 -‐ 17.06.11 9
HS
D S
imulation
Momentum cut
• Enrich in-‐medium decays of vector mesons
Pair momenta
HADRON 2011 -‐ 17.06.11 10
• Enrich in-‐medium decays of vector mesons • HADES: Significant e+e-‐ yield with pee< 800 MeV/c ( ~ 35 % in VM mass region)
S.Leupold, V.Metag and U.Mosel, nucl-th 0907.2388
JLAB-CLAS HADES
]2 [GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2
)]2 [1
/(GeV
/cee
dN
/dM
0π
1/N
-510
-410
-310
-210
-110
1p+Nb
p+p
]2 [GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2
)]2 [1
/(GeV
/cee
dN
/dM
0π
1/N
-510
-410
-310
-210
-110p+Nb
p+p
Slow and fast pairs
HADRON 2011 -‐ 17.06.11 11
pee> 800 MeV/c pee< 800 MeV/c
PRELIMINARY PRELIMINARY
Slow and fast pairs
HADRON 2011 -‐ 17.06.11 12
pee> 800 MeV/c pee< 800 MeV/c
• High p: free p+p producKon • Low p: overshoot over p+p -‐ different for ρ, ω, and φ
PRELIMINARY PRELIMINARY
Nuclear modificaKon factor
HADRON 2011 -‐ 17.06.11 13
RpA =Apppart !dN / dp
pNb
ApNbpart !dN / dp
pp
=Apppart !! reaction( pp) !d! / dp
pNb
ApNbpart !! reaction( pNb) !d! / dp
pp
! reaction( pNb)
! reaction( pp)
= 20,5= (A)0.67
HADRON 2011 -‐ 14.06.11
PRELIMINARY RpA = 1: No In-‐Medium effects
RpA > 1: Excess
RpA < 1: Suppression
[GeV/c]eeP0 0.5 1 1.5
pAR
0
1
2
3
4 <0.15 eeM<0.47 ee0.15<M<0.7 ee0.47<M
ee0.7<M
ωidentified
Nuclear modificaKon factor
HADRON 2011 -‐ 17.06.11 14
At low pair momenta: • Pions ~ const
• Strong excess of ρ, Δ type sources – Secondary reacKons – Isospin dependence
• ω: Trend to absorpKon
PRELIMINARY
Transport model calculaKons
HADRON 2011 -‐ 17.06.11 15 J. Weil, GIBUU [hCp://arxiv.org/abs/1106.1344]
pee> 800 MeV/c pee< 800 MeV/c
PRELIMINARY PRELIMINARY
Transport model calculaKons
HADRON 2011 -‐ 17.06.11 16 J. Weil, GIBUU [hCp://arxiv.org/abs/1106.1344] Pee (GeV/c)
R pA
R pA
Transport model calculaKons
HADRON 2011 -‐ 17.06.11 17 J. Weil, GIBUU [hCp://arxiv.org/abs/1106.1344]
10-1
100
101
102
103
0 0.2 0.4 0.6 0.8 1
d!/dµ
[µb/
GeV
]
dilepton mass mee [GeV]
all momenta
datano mesonic FSI
FSI, K" = 1K" = 2K" = 3K" = 4K" = 5
CB/TAPS@ELSA
• RelaKve yield
• Fast and slow pairs
• Nuclear modificaKon factor
• Have to understand producKon mechanisms in primary and secondary reacKons: – Transport models – π beam experiments
Conclusion
18
In π0/ω region ~ 1.0 Other sources ~ 1.5
High p: p+Nb = p+p Low p: excess (secondary reacKons)
ω absorpKon?
High p: RpNb = 0.8 – 0.9 Low p: RpNb = 0.8 – 2.5
HADRON 2011 -‐ 17.06.11
BACKUP
HADRON 2011 -‐ 17.06.11 19
[GeV/c]tP0 0.2 0.4 0.6 0.8 1 1.2
[1/(G
eV/c
)]t
dN
/dP
0 π1/
N
-510
-410
-310
-210
-110
1
2<0.150 GeV/ceeM
[GeV/c]tP0 0.2 0.4 0.6 0.8 1 1.2
[1/(G
eV/c
)]t
dN
/dP
0 π1/
N
-510
-410
-310
-210
2<470 GeV/cee0.150 <M
[GeV/c]tP0 0.2 0.4 0.6 0.8 1 1.2
[1/(G
eV/c
)]t
dN
/dP
0 π1/
N
-610
-510
-410
-310
2<700 GeV/cee0.470 <M
[GeV/c]tP0 0.2 0.4 0.6 0.8 1 1.2
[1/(G
eV/c
)]t
dN
/dP
0 π1/
N
-510
-410
p+Nb
p+p
2>700 GeV/ceeM
Transverse momenta
HADRON 2011 -‐ 17.06.11 20
PRELIMINARY PRELIMINARY
eeY0 0.5 1 1.5 2 2.5
dN
/dY
0 π1/
N
-410
-310
-210
-110
2<0.150 GeV/ceeM
eeY0 0.5 1 1.5 2 2.5
dN
/dY
0 π1/
N
-510
-410
-310
-210
2<470 GeV/cee0.150 <M
eeY0 0.5 1 1.5 2 2.5
dN
/dY
0 π1/
N
-510
-410
2<700 GeV/cee0.470 <M
eeY0 0.5 1 1.5 2 2.5
dN
/dY
0 π1/
N
-610
-510
-410
p+Nb
p+p
2>700 GeV/ceeM
RapidiKes
HADRON 2011 -‐ 17.06.11 21
PRELIMINARY PRELIMINARY
Isospin dependence
• Dielectron cross sections in p+p and p+d at beam energies from 1.04 to 4.88 GeV measured with DLS
• Decreasing mass dependence of pd/pp with increasing beam energy
• pd cross section becomes approximately twice the pp cross section at all masses with increasing beam energy
W.K.Wilson et al., PRC 57 (1997) 1865
HADRON 2011 -‐ 17.06.11 22
Transparency ratio ? γ + A à ω/ρ + X p + A à ω/ρ + X
Low interaction probability à Production in whole volume à ~ Aα, α = 1
Strong interaction à Production on surface à ~ Aα, α = 2/3 BUT: secondary proction via pions à ~ Aα, α > 2/3
TA à absorption in nucleus à in medium width
TA à disentanglement of production and absorption à Model dependence
g A
p A
π
HADRON 2011 -‐ 17.06.11 23