1. Research Center Dresden-Rossendorf Technical University Dresden
Virtual Bremsstrahlung*
B. Kämpfer
Research Center Dresden-Rossendorf
Technical University Dresden
1. goal 2004: tagging
2. goal 2008: shoulder in np at M ~ 450 MeV
Price for 1.: rough treatment of pp (not adjusted to DLS
but „oriented“ at Shyam-Mosel, PRC 2003)
Result for 2.: no significant shoulder in np if
pp stays shoulder-free
*) work by L.P. Kaptari

2. Motivation HADES Workshop for Advanced Detectors Rossendorf 2001 2008
Zschocke 2002

3. DLS Puzzle Solved by Bremsstrahlung?
(known since 111 years)
Barz et al.
Bratkovskaya, Cassing
C(1 AGeV) + C
C(1 AGeV) + C
C(1 AGeV) + C
M [GeV]
1997: bremsstrahlung ... contribution
was found to be small
2007: DLS puzzle... may be solved
when incorporating a stronger
bremsstr. contribution
Aichelin et al. 2008: w/ bremsstrahlung
Santini et al. 2008: w/o bremsstrahlung
no DLS puzzle at all?

4. S N e+ e- 36 + ISI + FSI Kroll-Ruderman isovector (KK) or
isoscalar (Shyam-Mosel `08)
Aichelin et al., Rossendorf BUU

5. Di-Electron Spectrum Pure Bremsstrahlung require seagulls for q.J = 0
no information on y, pT left N M
Pure Bremsstrahlung
require seagulls
for N M

6. Exchange of Neutral Mesons in pp+
q.J = 0
to be done: check
charge
an. mag. mom.
off-shell N:
8 FF‘s
or loop corrections
 VMD
KK:

7. Exchange of Charged Mesons in np+ +
q.J = 0 w/o FF‘s n p
to fulfill
WT id.
only for ps coupling sufficient
not unique N M
derivative couplings: seagull
from minimal coupling

8. parameters (similar to Shyam-Mosel 2003 [2])
nucleons only
parameters (similar to Shyam-Mosel 2003 [2])
differ slightly from KK 2006
p. 348: „our effective parameters have been fixed in such a way as to reproduce
reasonably well the results of the study [2] ...
Our OBE parameters (listed in Table 1) and their energy dependence have been
taken as in Ref. [2].“
from exchange of charged mesons + seagulls
real photons:

9. Gauge Invariance Restoration is Not UniqueK +
+ ... N
Usov & Scholten: fixed parameters

10. Baryon Resonances N M
Pascalutsa PRD 1998:

11. our choice -1: common choice error bars  range of (compatible with
helicity amplitiudes,
e.m.decay width,
pion-photoproduction)

12. with rho, pion
exchange
and seagull
... pure bremsstrahlung
--- Delta contribution
coherent sum

13. (i)
(ii)
(i)

17. results: see below

18. planar kinematics: M purely real or purely imaginary
our choice

19. once more: on-mass vs. off-mass shell decays

20. all together at kin. limit: FSI matters -.. 1440 -. 1535 ... brems
- total (w/ Delta)
at kin. limit: FSI matters

21. solid: total
broken: brems

22. dashed: incoherent sum
interference matters
HADES measured ratio
solid: coherent sum
dashed: incoherent sum
disappointing/interesting: neither pp nor np HADES data are described
to be done: pion production within same framework

23. Why should it fail for bremsstrahlung?
OBE Approach is Fine for Mesons
Why should it fail for bremsstrahlung?

24. check by
HADES
possible
x = 0.45
x = 0.50
x = 0.55

25. Spectator Distribution
1.25
1.90
3.50
HADES:
data for pp and pn for 1.25 GeV are at our disposal

26. factorization works:
VDM
seems not to help
sub-threshold
production:

27. Transition Form Factors
= hadronic matrix elements

28. Summary bremsstrahlung:
tree level diagrams FF‘s + gauge inv. + FSI
- HADES puzzle: approach seems to fail in detail
for pp, Dp(np) at 1.25 GeV:
„an understanding of the elementary channels
remains challenging“ at least for KK
why we fail for pp? check pion production
check various gauge inv. restoration procedures
unitary approach is needed