Antiproton-proton elementary cross sections:
lack of data at low momentum
• Total cross section
– Measurements exist down
to 220 MeV/c
• Annihilation cross section
pp total
cross
section
– Measurements by OBELIX
exist even below 100 MeV/c
• Elastic cross section
– Measurements down to 181
MeV/c
– No way to determine el
below this value
pp
annihilation
cross
section
• Lack of measurements in the
low momentum region
• Why is the elastic cross section
so interesting? Evidences of an anomaly observed in np interactions
Total vs annihilation np cross sections
An odd dimp-bump effect is
observed in (np)TOT close
to 65-80 MeV/c


Regular and smooth trend of
(np)ANN in the same
momentum region
Impossible to find a set of
parameters able to describe
correctly at the same time both
T and ann


Bad fits of T with ER
expansions, of different kinds
Total cross section
Annihilation xsec: smooth
The elastic cross section is
most likely responsible for the
unexpected irregular trend of
T
Due to a NN bound state close to threshold?
I=0
vs
I=1 interactions
• From the ratio between T(pp) and T(np) one can deduce the
contribution to annihilation of the I=0 and I=1 sources of the NN
interaction:
T (pp) T (I  0)  T (I  1)
R

T (np)
2T (I  1)

Strong dominance of the I=0 component at low momentum:

T(I=0)/T(I=1):




Effect explained as a manifestation of the coherence of meson exchanges in
the central and tensor terms of the NN medium range force (Dover et al.)
For ann the same rule is valid: ann(np) < ann(pp)

ann(I=0)/ann(I=1):


(2.50.4) @ 70 MeV/c
(1.10.1) @ 300 MeV/c
(2.4 0.4) @ 70 MeV/c
(I=1) always lower than (I=0) BUT 1.5 @ ~ 700 MeV/c!