Meson Assisted Baryon-Baryon Interaction Hartmut Machner Fakultät für Physik Universität Duisburg-Essen Why is this important? NN interactions  Nuclear potential, nuclear structure NY interactions  Hypernuclear potential, hypernuclear structure, neutron stars

Baryon-Baryon Interactions

Baryon-baryon interaction Fäldt & Wilkin derived a formula ( for small k) From this follows, that from a the cross section of a known pole (bound or quasi bound) the continuum cross section is given [N({1+2})  N(1+2) t  N(1+2) s ]. The fsi is large for excitation energies Q of only a few MeV. Standard method: elastic Scattering: Often it is impossible to have either beam or target. Way out FSI in (at least) three body reaction. If the potential is strong enough one has resonances or even bound states.

Example: neutron-neutron scattering Bonn neutron and proton coincidences different sides of the beam TUNL neutron-neutron coincidences same side of the beam

neutron-neutron scattering Bonn equipment at TUNL yielded: Obviously ist the geometry which makes the difference. Three body effects? Better method: a meson in the final state:

Problem Why meson assisted? factorisation: with leads to baryon-baryon interaction strong meson-baryon interaction weak

resonances Strategy: choose beam momentum so that no resonance is close to the fsi region. Note: different sign conventions in a. a<0 unbound

The pp  0 pp case Elastic pp scattering, Coulomb force seems to be well under controle: a pp =-7.83 fm However: an IUCF group Claimed „…the data require a pp =-1.5 fm.“ They questioned the validity of the factorization. Experiment at GEM, differential and total cross sections. FIT Ss, Pp (Ps from polarisation experiments) no  resonance with  resonance, usual fsi with  but fsi with half the usual pp scattering length No need to change the standard value!

Saclay pp  + (pn) 1000 MeV pp   (pn) dp  p(pn) triplet (from fit to all) singlet (from deuteron)

Data from Uppsala and GEM p= MeV/c

Triplet FSI absolute a t and r t close to literature values

Singlet FSI absolute No singlet state! singlet fractionRef. 0.40±0.05Boudard et al. < 0.10Betsch et al. <0.10Uzikov & Wilkin < 0.10Abaev et al. < 0.003GEM

More experiments Fäldt-Wikin relation

Full 3 body calculation Relativistic phase space Reid soft core potential

 p elastic scattering events in a 82 cm bubble chamber fitasas rsrs atat rtrt A B F

pp  pK + Simultaneous fit:

  production

Resonances without/with resolution folding Upper limits (99%) solid  =1 MeV dashed 0.5 MeV dotted 0.1 MeV Aerts and Dover

 deuteron? FW-theorem:  nb Exp.: 75±3 nb  2 /dof = 1.3

peak Peak below threshold:  -deuteron Peak at threshold: cusp Peak above threshold: Resonance (dibaryon?) Shaded: HIRES only  p; dots TOF (submitted)

Peak analysis lower mass peak at  + n threshold higher mass peak or shoulder =???

Flatté analysis

Elastic  p scattering

Potential models - bound state (deuteron like  N): 3 D 1 phase passes through 90°: Nijmegen NSC97f, Nijmegen NF, Jül89 S1 phase passes through 90°: Nijmegen ESC04, Toker&Gal&Eisenberg -inelastic virtual state = peak direct at threshold = genuine cusp, none of the relevant phases passes through 90°: Nijmegen NSC89, ND, ESC08, Jülich05, EFT

Summary Meson assisted baryon-baryon interaction is a powerful tool to study bb- potential. Low energy interaction can be studied via fsi and resonances as well as bound states. pp  0 pp: indicates factorization is valid. No different parameters than in pp  pp. pp  + pn: only triplet scattering. Why? pp    p: mostly singlet scattering. Why? The potential is to week to form a bound state. No dibaryon resonance! Strong enough to form a resonance? pp  K +  N: potential strong enough to bind? But no fsi visible. Thank you to GEM & HIRES Johann Haidenbauer, Frank Hinterberger, Jouni Niskanen, Andrzej Magiera, Jim Ritman, Regina Siudak

FSI approaches A lot of studies made use of a Gauss potential. However the Bargman potential is the potential which has the effective range expansion as exact solution: a, r  .  defines the pole position (positive ↔ bound, negative ↔ unbound). All with Gamow factor

p(p,X) P beam =2735 MeV/c vetoed with cherenkov signal cherenkov signal