Λ and Σ photoproduction on the neutron

Pawel Nadel-Turonski

The George Washington University

for the CLAS Collaboration

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 2

Outline

Exclusive Measurement of γn(p) → K0Λ(p) → π+π-π-p(p) γn(p) → K0Σ0(p) → K0Λγ(p)

Final State Interactions K-N, Y-N, and Y*-N scattering

The Future Is Polarized

100%

44%

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Calculations by C. Bennhold et al.

PRELIM

INARY

isobaric: H. Yamamura et al., Phys. Rev. C 61, 014001 (2000) coupled-channels: A. Waluyo, private communication (2005)

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The g10 Experiment at Jefferson Lab Performed in Hall B Tagged photon beam of 1 – 3.6 GeV (unpolarized) Liquid deuterium target Charged particles detected in CLAS

(CEBAF Large Acceptance Spectrometer) Two track (sector) trigger 4.71 · 109 events taken with low magnetic field

(giving larger acceptance for negative particles)

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K0 and Λ Invariant Masses Only particle ID

cuts have been applied here.

Missing mass cuts significantly reduce the background.

All the following plots will have 3σ cuts on the K0 and Λ invariant masses.

3σ cut

3σ cut

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Λ and Ks Lifetimes PDG values:

λΛ = 1/τΛ = 3.8 ns-1

λK = 1/τK = 11.2 ns-1

As expected from the CLAS resolution, there is a slight deficit of events with short path lengths.

The measured Ks lifetime agrees with the PDG value, and the Λ is within 3%.

Vertex reconstruction and event selection are satisfactory.

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Missing Masses: Λ – Σ0 separation mm(K0Λ): mass of the

‘spectator’ proton. mm(K0): mass of the

hyperon. Fermi motion is ignored as in an inclusive measurement.

The Λ and Σ0 are well separated in 2D.

The projections show simple double Gaussian fits to regions of interest.

Fitting gives a lower uncertainty in the yields than 2D cuts.

Different limits on mm(K0) can be used to check the fitting procedure.

A missing momentum cut has been applied.

PRELIMIN

ARY

Σ0

Λ

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Missing Momentum: spectator selection The ‘missing momentum’

is the momentum of the ‘spectator’ proton if there were no additional particles.

The undetected γ from the Σ0 is boosted, making the spectators seem forward peaked.

True spectators can be selected by a cut on the missing momentum (magenta colored distribution on lower left).

The spectator protons have an isotropic angular distribution.

The detection threshold for protons in CLAS is just below 0.3 GeV/c.

PRELIMINARY

Inclusive Λ

Σ0Λ

extra π

Λ2p detected

Inclusive Λ

spectators

spectator cut

Exclusive Σ0

Exclusive Σ0

Exclusive Λ

FSI ?

Exclusive Λ

Inclusive Λ

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 9

Σ-(1385) rescattering on the proton

pmin: minimum momentum of the ‘spectator’ proton before the rescattering, assuming that it propagated on shell.

-t > 0.5, W > 2.0, 0.4 < pmiss < 0.6 Peak at zero predicted

J.-M. Laget, nucl-th/0507035 See NSTAR 2005 poster by

Rebecca Davis for more details.

deuteron

Σ -*Λ

?

p π -

p

π -

K+

n

p

γ

PRELIM

INARY

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 10

Very Preliminary! Not corrected for

photon flux or the CLAS acceptance!

Most high energy photons produce additional particles.

When calculated from the final state, W is not sensitive to Fermi motion.

Will be more interesting when corrected and appropriately binned.

Yields

PRELIMINARY

2p detected

Inclusive Λ

Exclusive Σ0

Exclusive Λ

2p detected

Inclusive Λ

Exclusive Σ0

Exclusive Λ

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Polarization Unpolarized photon beam and target

Cross section and recoil polarization (P) Analysis in progress

Polarized beam and unpolarized LD2 target Circularly polarized beam + recoil polarization: Cx, Cy

Circular beam polarization available in smaller g2 data set Linearly polarized beam + recoil polarization: Ox, Oy, T g8b recently achieved very high linear photon polarization No CLAS deuterium data with linearly polarized beam!

Polarized beam and polarized target Frozen spin target (FROST) to be ready in summer of 2006

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Λ Recoil Polarization

NN

NNP

013.0642.0

2

Θp is the angle in the rest frame of the Λ between the momentum of the decay proton and n = pγ pK.

N± is number of events with the sign of the cosine given by its index.

Not corrected for photon flux or acceptance.

Will be binned in ΘK.N+N-

PRELIMIN

ARY

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Polarized FSI Recent calculations by Miyagawa,

Mart, Bennhold, and Glöckle for circularly polarized photons show Cz to be most sensitive to final state interactions.

Calculations for K0 channels using a linearly polarized photon beam are under way.

Kaon and hyperon angles – the latter is defined with respect to the momentum of the Y-N system.

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Summary Analysis of the cross sections and recoil polarization

for the γn → K0Λ and γn → K0Σ0 channels is in progress.

Studies of Y-N scattering bring new fundamental understanding and insight into the final state interactions.

We intend to propose a new experiment, using a linearly polarized photon beam and a LD2 target, to measure the polarization transfer.

In the future, the frozen spin target will give access to more polarization degrees of freedom.