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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)
Stephen Kay University of Edinburgh NP 2015 3 / 13
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)
Stephen Kay University of Edinburgh NP 2015 4 / 13
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)
Stephen Kay University of Edinburgh NP 2015 5 / 13
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
Stephen Kay University of Edinburgh NP 2015 6 / 13
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
Stephen Kay University of Edinburgh NP 2015 7 / 13
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
Stephen Kay University of Edinburgh NP 2015 8 / 13
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
Stephen Kay University of Edinburgh NP 2015 9 / 13
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
Stephen Kay University of Edinburgh NP 2015 10 / 13
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
Stephen Kay University of Edinburgh NP 2015 11 / 13
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
Stephen Kay University of Edinburgh NP 2015 13 / 13
Thanks for listening, anyquestions?