Meson photoproduction

- Associated strangeness photoproduction K
^{+}Λ photoproduction - K
^{0}Λ photoproduction -
K
^{0}Σ^{+}photoproduction

Figure 3.13: Differential cross sections for *γp → K ^{+} Λ*.
Closed circles, open squares and open triangles are LEPS [44], SAPHIR [40] and CLAS [41, 42] data respectively.
The dotted and dash-dotted curves are Regge models with only

The search for missing resonances, and the goal of a complete, model independent measurement have provided motivation for the most extensively measured associated strangeness channel, *γp → K ^{+} Λ*.
Differential cross section data with high statistics have been measured with the SAPHIR detector [39, 40] at ELSA and CLAS detector [41, 43] at Jefferson Lab.
Discrepancies between these data however leave ambiguities to the contributing

*γp → K ^{+} Λ* differential cross section measurements,
and the polarisation observables

Figure 3.14:

Determining *s*-channel resonances contributing to the *γp → K ^{+} Λ* spectrum is complicated by large

The BGO-OD experiment will measure differential cross sections and polarisation observables for this channel, vastly improving the limited data set.
Statistics will be greatly improved over previous experiments, as the BGO-OD experiment is able to reconstruct K^{0} via both neutral and charged pion decay modes.
Recoil polarisation, *P* , will be measured from the self analysing *Λ* weak decay.
A linearly polarised beam will be used to measure the beam asymmetry, *Σ*, and the double beam-recoil polarisation observables, *O _{X} *and

The BGO-OD experiment will study the channel *γp → K ^{0} Σ^{+}* and provide complementary data to the differential cross sections,
beam asymmetry and recoil polarisations measured with the Crystal Barrel (analyses page).
The beam asymmetry will be measured over the

**Excited hyperon photoproduction** Constituent quark models with three quenched quark states cannot accurately describe the mass differences of excited hyperons.
The lowest excitation energy of a baryon of approximately 450 MeV is too high to describe the *Λ(1405)*, and the mass difference to the spin orbit partner, *Λ(1520)* is too large.
It is also difficult to reconcile the *Λ(1405)* mass to be lower than the non-strange *N(1535)*.
These mass differences however can be described using unquenched quark models with five components [50, 51, 52].
These can either be interpreted as a meson cloud or a pentaquark molecule with di-quark structure.
The *N ^{∗}(1535)* for example can be described as a bound

Figure 3.15: Partial wave analysis solutions for beam asymmetry for *γp → K ^{0} Σ^{+}* for centre of mass energy 1900 and 2050 MeV.
Discriptions of the two solutions, BG2011 02 and BG2011 01 are in [49], references therein and the A.2 scientific program of this proposal.
Figure created from [49].

Old data (pre 1980) for *K ^{−} p → Λπ^{+} π^{−}* [53]
has been re-examined in a search for

Wu, Dulat and Zou [50] proposed that the existence of a *Σ ^{∗}* (1/2

Recent analyses of previous Crystal Ball data for *K ^{−} p → π^{0} Λ* [56, 57] fitted effective Lagrangians to differential cross sections and Λ polarisation data. After including

Recent advances in chiral unitary formalism for meson baryon interactions [58, 59, 60, 61] described the *Λ(1405)* as a two-pole structure, with the contributions interfering on the real energy axis. The *Λ(1405)* can only be observed via the decays: *Λ(1405) → πΣ* with *I* = 0, however, it was found that there is different coupling of the two poles to different meson-baryon channels, leading to a difference in the *Λ(1405)* line shape depending upon the decay it is observed via.

The *Λ(1405)* mass was reconstructed from all three decay modes using recent data from the CLAS detector [62]. It is clear that the line shape differs depending upon the decay mode, however they do not agree with theoretical predictions where the *Σ ^{−} π^{+}* line shape is at a higher mass.

Due to the limited data on excited hyperons and the ambiguity as to their structure, a detailed search with the BGO-OD experiment is proposed.
*Σ ^{∗}* resonances close in mass to

** K^{∗} photoproduction** can be used to search for contributions of the

Fig. 3.16(a,b,c) are examples for *γp → K ^{∗0} Σ^{+}* differential cross section with two model fits,
including

Figure 3.16: (a,b,c) Differential cross sections for

Due to the different parity of the *κ* and the pseudoscalar *K*, a parity exchange asymmetry, *P _{σ}* can be extracted (given in [63]).
At forward angles and high energies (to limit

The BGO-OD experiment will measure both *K ^{∗+} Λ* and

**Simulated analysis of proposed experiments** Fig. 3.17 shows simulated data analysis of particles with strangeness (descriptions given in the caption). These identification procedures will form the basis of all associated strangeness experiments described in the BGO-OD physics proposal.

** ω and φ vector mesons** The interest in vector meson photoproduction is strongly motivated by the problem of the so-called “missing resonances” of the baryon excitation spectrum. Recently, there is evidence (

Figure 3.17: Simulated analysis of associated strangeness channels (analysed channels inset). Missing mass from

*ω* photoproduction off the proton has been investigated at several facilities ([67]-[74],[2]).
Measurements of the differential cross sections revealed pomeron and *π ^{0} t*-exchange dominance at small momentum transfers.
The relative increase of the differential cross section at higher

Theoretical interpretations of the available experimental cross section and beam asymmetry results are strongly model dependent ([75]-[78], [16]).
New high precision polarisation results will constrain these models.
It will be essential to completely understand the *t*-exchange mechanisms.
This shall be achieved by simultaneous study of *φ* photoproduction.
Here, (almost) pure *t*-exchange dynamics is probed, since the *φ* meson is a pure *ss–* state and its direct coupling to the nucleon is strongly inhibited (OZI rule [79]).
Due to this, the total cross section for *φ* photoproduction is over an order of magnitude smaller than the one of *ω* photoproduction (*σ(γp → ωp) ≅ 8−9µb* at threshold; *σ(γp → φp) ≅ 0.2−0.3µb* at threshold).
The differential cross sections ([80] and [81]) show the diffractive behavior typical for vector meson photoproduction which is associated with pomeron exchange.
LEPS results on the decay angular distributions [82] reveal additional contributions of unnatural parity exchange terms close to threshold, as would be expected from *π ^{0}* or

Figure 3.18: Energy dependence of the differential cross section of the reaction *γ + p → φ + p* (figure from Ref.[81]).
Data from LEPS (full circles) show a bump structure at *Eγ* ≅ 2 GeV. The solid curve represents the prediction from Ref.[83], including Pomeron trajectory and *π* and *η*-exchange.

It is a further goal of our investigation, to better understand the mechanism of *φ* photoproduction in this energy regime.
An experimental way to disentangle the role of *π ^{0}*-exchange is offered by coherent photoproduction off (isoscalar) deuteron targets (

From these considerations it appears necessary to study *ω* and *φ* photoproduction simultaneously.
For both channels, *ω* and *φ*, it will be important to investigate also the photoproduction off the neutron.
Preliminary GRAAL results on the beam asymmetry [74] show important differences between *ω* photoproduction off the proton and off the neutron.
New higher precision polarisation measurements over a larger energy range are necessary to pin this down.
Similarly, data on *φ* photoproduction off the neutron is still scarce.

The BGO-OD set-up is ideal to study *ω* and *φ* photoproduction.
The high momentum resolution in forward directions combined with charged particle tracking and photon spectroscopy with BGO ball and inner MWPC in the central angular region gives access to complex multi-particle final states.
The *ω* meson will be identified by both, the *ω → π ^{+} π^{0} π^{−}* (B.R.: 89.2 %) and the radiative

The goal is to measure the vector meson decay angular distributions using linearly and circularly polarised beams with sufficient precision to determine the relevant polarisation density matrix elements for the photoproduction off proton, neutron and deuteron. Detailed Monte-Carlo simulations to study the achievable sensitivity are presently ongoing. Unpolarised photon beams – obtained by averaging over both helicities of circularly polarised beams – will yield the polarisation density matrix elements

*ρ*,^{0}_{00}*ρ*and^{0}_{1−1}*Re(ρ*;^{0}_{10})

evaluating the circular polarisation of the beam will allow access to- •
*ρ*,^{3}_{00}*ρ*and^{3}_{1−1}*Re(ρ*;^{3}_{10})

and with linear polarisation - •
*ρ*,^{1}_{00}*ρ*and^{1}_{1−1}*Re(ρ*^{1}_{10})

can be obtained, as well as the photon beam asymmetry, Σ, which represents a superposition of density matrix elements.

** η′ meson** The CLAS and CBELSA/TAPS experiments have produced a rich amount of cross section data for

- in a relativistic meson-exchange model of hadronic interactions [90], Nakayama and Haberzettl consider
*t*-channel mesonic (*ρ*and*ω*) together with*s*- and*u*-channel nucleon and resonance contributions.*S*,_{11}(1535)*P*,_{11}(1710)*D*and_{13}(1520)*P*resonances were included, the latter two required to reproduce details of the angular distributions._{13}(1720) - in a reggeized model for
*η*and*η’*photoproduction [91] of Tiator and co-workers the*t*-exchanges are treated in terms of Regge trajectories to reproduce the correct high-energy behavior. Both approaches give a reasonable description of the data. They can not be distinguished on the sole basis of cross sections, nor the resonances parameters unambiguously determined. This requires polarisation observables, in particular beam and/or target asymmetries.

The BGO-OD setup is ideally suited for the measurement of both, charged and neutral, decay channels, e.g. (a) *η ′ → π ^{+} π^{−} η → π^{+} π^{−} 2γ (BR ≅ 17.5%)*, (b)

Using a 6 cm long liquid hydrogen target and a tagged photon rate of *N _{γTot }≅ 5 · 107 s^{−1}* we detect about 103

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