First measurements of final state neutronpolarisation in deuterium photodisintegration

Stephen Kay

University of Edinburgh

NP Summer School 2015

Overview

- Deuterium Photodisintegration

- Motivation - the d* 2380 Resonance

- Experimental Facility - Crystal Ball at MAMI

- Analysis Progress

Stephen Kay University of Edinburgh NP 2015 2 / 13

Deuterium Photodisintegration - An Unexpected Result

- Work by Kamae[1][2] et al. in 1977 discovered an unexpected result forthe spin polarisation of protons from deuterium photodisintegration

- At√s ≈ 2380 MeV protons from photodisintegration highly polarised

- The neutron polarisation was not measured in this experiment

- This result cannot be explained by standard theories of deuteriumphotodsintegration. Kamae[2] speculated that this result could indicate apossible Jπ = 3+ resonant state[1] - PRL 38, 9, PP468-471 (1977), [2] - PRL 38, 9, PP471-475(1977)

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Hadron Production Channels

- Various experiments looking at hadron production channels have seenanomalous structure at

√s ≈ 2380 MeV

Figure: Left[1] - A plot of σ as a function of√s for the shown reaction

from WASA at COSY. Right[2] - The analysing power as a function of√s

for polarised neutron-proton scattering from WASA at COSY

[1] - PRL 106, 242302 (2011), [2] - PRL 112, 202301 (2014)

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The Nature of the Resonance

- Observations so far have suggested the resonance is found at√s ≈ 2380 MeV with a width of Γ ≈ 70 MeV and Jπ = 3+

- What is this resonance? Difficult to explain using standardnucleon resonances. This width is far narrower than would beexpected for a ∆ resonance for example

- Some propose the exciting interpretation of the resonance[1][2] asa d* 2380 “dibaryon”. A “dibaryon” is a six quark object, in thiscase consisting of 3 u quarks and 3 d quarks

- Key expectation from a genuine 3+ resonance is that both theproton and neutron would show a high degree of polarisation

- As mentioned there is no previous data on the neutronpolarisation, this needs to be measured

[1] - arXiv:1308.6404 [hep-ph], [2] - PRL 38, 9, PP471-475(1977)

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The MAMI Facility

- MAMI is an electron beam facility in Mainz, Germany

- The Edinburgh group work in the A2 hall which houses thecrystal ball detector

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The Crystal Ball at MAMI

- The crystal ball detector consists of 672 NaI(Tl) scintillationdetectors covering ∼ 94 % of 4π

- Within the crystal ball is the PID which surrounds the target

- We observe neutrons via (n,p) scattering in the PID - this has alow probability (roughly 0.5%), these events can be analysed toobtain the neutron polarisation however

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Current Analysis

- The current analysis effort focuses on examining data taken inMarch 2013

- Particles are identified by comparing the energy deposited in thePID compared to that deposited in the crystal ball

Figure: A typical E dE plot for A2 data

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Analysis Progress

- With various cuts in place the proton and neutron from thephotodisintegration have been identified

- A plot of the missing massfrom the perspective of theproton and E dE plots for theprotons and neutrons identified

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Analysis Progress

- A Monte Carlo simulation of the detector setup is also available

- The real data was compared to the output of this simulation

Figure: A comparison of the EdE plots for the neutrons as seen in real(left) and MC (right) data

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Analysis Progress

- The MC simulation allowed us to test whether the two differentregions the neutrons were found it corresponded to scattering fromdifferent materials

Figure: A comparison of the EdE plots for the neutrons in the MC datawith (left) and without (right) deuterium gas filling the target cell

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Analysis Progress

- Once identified the neutrons are rotated to a new frame

- In this frame the polarisation, P, is related to the angle φ of theparticles in this frame via

dσ

dΩ=

dσ0dΩ

(1 + AyP cosφ)

- Therefore the polarisation can be determined by fitting a cosineto the measured φ distributions

Figure: An illusatration of the frame rotation and an example φdistribution with a cosine fitStephen Kay University of Edinburgh NP 2015 12 / 13

Summary

- Current analysis has identified p,n photodisintegration events

- MC Analysis has shown that two regions of detected neutronsappear to be due to scattering from two different materials

- Adjustments to the real data based upon observations in thesimulated data will be carried out

- Polarisation results should be available very soon

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Thanks for listening, anyquestions?