Polarisation Observables from Strangeness Photoproduction on a Frozen Spin Target with CLAS at Jefferson Lab Stuart Fegan Nuclear Physics Group Seminar November 10th, 2011

Outline Introduction Previous Measurements Experimental Setup Analysis Motivation Polarisation Observables Previous Measurements Experimental Setup Jefferson Lab Hall B and CLAS The FROST Target and the g9a Experiment Analysis Particle Identification Channel Identification

Outline Extraction of Observables Results Summary and Outlook From Beam Asymmetries The Double Asymmetry Technique Carbon Scaling and Dilution Results Beam Polarisation Observable (Σ) Comparison with Previous CLAS Data The G Observable Summary and Outlook

Introduction Previous Data Experiment Analysis Observables Results Motivation Baryon spectroscopy is the study of excited states of the nucleon These excited states, or resonances, aid the study of the internal structure of the nucleon and the quark interactions therein Not only should we measure these states, but also their physical properties and quantum numbers Meson photoproduction cross section on the proton shows resonant behaviour in several reactions Resonances can be predicted theoretically by models describing the nucleon At the energy regime where many resonances exist, these phenomenological models are based on non-perturbative QCD

Introduction Previous Data Experiment Analysis Observables Results Motivation Phenomenological quark models describe the nucleon in terms of three constituent valence quarks, interacting through a potential By considering permutations of the allowed degrees of freedom of the model, a series of resonances can be predicted Many of these predicted resonances have been observed experimentally, but not all Symmetric Quark Model Two competing types of model dominate; symmetric quark model and diquark model Key difference is the presence of a bound quark pair in the di-quark model Both predict a range of resonances consistent with the existing data, but the symmetric quark model predicts more resonances than the di-quark model This is known as the “Missing Resonance” problem Diquark Model

Polarisation Observables Introduction Previous Data Experiment Analysis Observables Results Polarisation Observables Cross-section measurements alone are a fairly blunt probe of the resonance spectrum States are wide and overlap, making them difficult to resolve Some states couple more strongly to certain reactions, such as the strangeness channels Even high quality, high statistics data is insufficient to resolve the full resonance spectrum purely from cross-section measurements Polarisation of the reaction particles can be exploited to probe resonances in more detail by measuring Polarisation Observables

Polarisation Observables Introduction Previous Data Experiment Analysis Observables Results Polarisation Observables Polarisation observables are a property associated with the polarised particles in a reaction For photoproduction reactions these are the photon, the target nucleon, and the recoiling baryon (strangeness reactions are self-analysing; i.e. can measure hyperon polarisation without a polarimeter) The observables arise from the study of the scattering amplitudes associated with a photoproduction reaction These amplitudes can be expressed in terms of transversity amplitudes; a set of four complex amplitudes completely describing the photoproduction process Taking bilinear combinations of the amplitudes allow 16 polarisation observables to be defined

Polarisation Observables Introduction Previous Data Experiment Analysis Observables Results Polarisation Observables The 16 polarisation observables are grouped into single and double types, accessible depending on which particles in the reaction carry polarisation The four single polarisation observables arise from polarisation in one particle Double polarisation observables can be measured when pairs of particles are polarised (beam-target, beam-recoil and target-recoil) Barker, Donnachie, Storrow, Nucl. Phys. B95 347 (1975) Due to their definition from the four transversity amplitudes, the observables contain redundant information and can be related to one another via a series of algebraic expressions For this reason, a “complete measurement”, which enables the amplitudes to be defined unambiguously, should be possible using a carefully chosen subset of observables

Polarisation Observables Introduction Previous Data Experiment Analysis Observables Results Polarisation Observables Each observable contributes to the overall differential cross-section, scaled by the relevant degrees of polarisation: 𝑑σ 𝑑Ω = σ 0 {1− 𝑃 𝑙𝑖𝑛 Σcos2φ+ 𝑃 𝑥 𝑃 circ 𝐹+ 𝑃 𝑙𝑖𝑛 𝐻𝑠𝑖𝑛2φ + 𝑃 𝑦 𝑇− 𝑃 𝑙𝑖𝑛 𝑃𝑐𝑜𝑠2φ + 𝑃 𝑧 𝑃 circ 𝐸+ 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2φ + σ 𝑥 ′ [ 𝑃 circ 𝐶 𝑥 + 𝑃 𝑙𝑖𝑛 𝑂 𝑥 sin2φ+ 𝑃 𝑥 𝑇 𝑥 − 𝑃 𝑙𝑖𝑛 𝐿 𝑧 cos2φ + 𝑃 𝑦 𝑃 𝑙𝑖𝑛 𝐶 𝑧 sin2φ− 𝑃 circ 𝑂 𝑧 + 𝑃 𝑧 𝐿 𝑥 + 𝑃 𝑙𝑖𝑛 𝑇 𝑧 cos2φ + σ 𝑦 ′ [𝑃+ 𝑃 𝑙𝑖𝑛 𝑇𝑐𝑜𝑠2φ+ 𝑃 𝑥 𝑃 circ 𝐺− 𝑃 𝑙𝑖𝑛 𝐸𝑠𝑖𝑛2φ + 𝑃 𝑦 Σ− 𝑃 𝑙𝑖𝑛 cos2φ + 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐹𝑠𝑖𝑛2φ+ 𝑃 circ 𝐻 + σ 𝑧 ′ [ 𝑃 circ 𝐶 𝑧 + 𝑃 𝑙𝑖𝑛 𝑂 𝑧 sin2φ+ 𝑃 𝑥 𝑇 𝑧 + 𝑃 𝑙𝑖𝑛 𝐿 𝑥 cos2φ + 𝑃 𝑦 − 𝑃 𝑙𝑖𝑛 𝐶 𝑥 sin2φ− 𝑃 circ 𝑂 𝑧 + 𝑃 𝑧 𝐿 𝑧 + 𝑃 𝑙𝑖𝑛 𝑇 𝑥 cos2φ ] The analysis presented aimed to measure the Σ and G observables using a linearly polarised beam and a longitudinally polarised target for the strangeness reactions γp → K+Λ and γp → K+Σ0 In this case, terms not involving linear photon polarisation and longitudinal target polarisation are zero and the differential cross-section reduces to something much simpler: 𝑑σ 𝑑Ω = σ 0 1− 𝑃 𝑙𝑖𝑛 Σcos2ϕ+ 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2ϕ

Polarisation Observables Introduction Previous Data Experiment Analysis Observables Results Polarisation Observables Measuring observables is useful because theoretical predictions of the observables vary dependant on the resonances included in the model used for the prediction The blue line shows the SAID partial wave analysis solution, the red dotted curve is Saghai's model [1] and the pink dashed curve represents the Mart-Bennhold model [2], which unlike the others, includes a D13(1960) resonance Measuring observables provides constraints to theoretical models, and allows these models to be tested and improved upon [1] Invited talk, International Symposium on Hadrons and Nuclei, Seoul (2001)[2]Phys. Rev. C61, 012201 (2000)

Previous Measurements Introduction Previous Data Experiment Analysis Observables Results Previous Measurements Since the 1990's, several facilities have embarked on experimental programs aiming to measure polarisation observables from strangeness photoproduction With large acceptance detector systems, high luminosity polarised photon beams and polarised targets, a “complete measurement” of the associated polarisation observables was now within reach

Jefferson Lab Introduction Previous Data Experiment Analysis Observables Results Jefferson Lab

Introduction Previous Data Experiment Analysis Observables Results Jefferson Lab Jefferson Lab is a US Department of Energy National Facility, located in Newport News, Virginia Research efforts are primarily focussed on studies of the atomic nucleus at the quark an gluon level, at the energy scale where nuclear physics meets high energy physics The lab's 6 GeV superconducting continuous wave accelerator, CEBAF, provides beam simultaneously to three experimental halls With the 6 GeV era coming to an end in the near future, work is already in progress to upgrade the energy to 12 GeV and build a fourth experimental hall

Introduction Previous Data Experiment Analysis Observables Results Hall B Hall B is the smallest of the three existing halls, and is home to the CEBAF Large Acceptance Spectrometer (CLAS) CLAS is a multi-layered and segmented arrangement of particle detectors around six superconducting coils which generate a toroidal magnetic field Polarised photon beams for photonuclear experiments are produced from the CEBAF electron beam via the bremsstrahlung process Energy degraded electrons are detected by the tagging spectrometer and used to determine the energy of the photon beam

Introduction Previous Data Experiment Analysis Observables Results Hall B The photon beam interacts with a target, positioned in the centre of CLAS Reaction products are detected with large acceptance over the full 4π solid angle Charged particles are detected from their curved trajectories through the drift chambers, under the influence of the toroidal magnetic field Combined with time-of-flight measurements, particle identification can be made Cerenkov counters and electromagnetic calorimeters provide additional information for forward focussed particles

Introduction Previous Data Experiment Analysis Observables Results Targets Liquid hydrogen (LH2) is typically used as the target in CLAS photonuclear experiments, which maximises the density of atomic protons available for reactions To access the G polarisation observable, a polarised target is required Materials can be polarised using a high magnetic field, but this has two problems; at the operating temperatures of the CLAS LH2 target, hydrogen is unpolarisable, and polarising magnets would obscure a large part of the angular coverage of the detector Resolving these issues has led to the development of the FROST target at Jefferson Lab

Introduction Previous Data Experiment Analysis Observables Results The FROST Target FROST, which stands for FROzen Spin Target, is the name of the polarised proton target designed and built by the Jefferson Lab Target Group Combining Dynamic Nuclear Polarisation (DNP) of a solid (butanol) target with millikelvin cooling enables the target to operate in “frozen spin” mode, eliminating the need for a large continuous polarising field DNP enables target nuclei to be polarised by polarising free electrons in the target first, then transferring the polarisation to the nucleon by applying microwaves

Introduction Previous Data Experiment Analysis Observables Results The FROST Target Once the target has been polarised, the polarising magnet and microwaves are switched off, and it is cooled to around 40 mK A smaller holding magnetic field is switched on, and in this “frozen spin” mode, the polarisation can be maintained for periods of several days During this time, the target is positioned inside CLAS and data is taken with the polarised photon beam After several days, the target is removed from CLAS and repolarised Time Polarisation

Introduction Previous Data Experiment Analysis Observables Results The g9a Experiment The g9 experiments utilised the FROST target in two experimental run periods; g9a (Nov 2007 – Feb 2008), and g9b (Mar – Jul 2010) For the g9a run, the target operated in longitudinal polarisation mode, and performed exceptionally well, exceeding design specifications in polarisation achieved, relaxation time and target cooling power Around 10 billion triggers were recorded for circularly and linearly polarised photon beams at a range of energies

Particle Identification Introduction Previous Data Experiment Analysis Observables Results Particle Identification Initial particle identification realised via charge and time-of-flight calculated mass Potential events for the channels of interest are selected from possible combinations of final state particles γ𝑝→ 𝐾 + Λ→ 𝐾 + 𝑝 π – γ𝑝→ 𝐾 + Σ→ 𝐾 + Λγ→ 𝐾 + 𝑝 π – γ Require one proton and one kaon, with the option of detecting the pion Photon selection is important to reduce particle misidentification from the photon to particle timing difference

Particle Identification Introduction Previous Data Experiment Analysis Observables Results Particle Identification Momentum dependent timing cuts are employed to increase the number of misidentified particles removed from the event selection Some misidentified particles will remain at this stage, but more of these will be removed by the channel identification cuts Cuts on the low acceptance regions of CLAS are also applied, as are energy corrections to account for the energy lost by particles as they pass through the target

Channel Identification Introduction Previous Data Experiment Analysis Observables Results Channel Identification There are two options for channel identification; exclusive identification of the final state products (fewer events) or non-exclusive, reconstructing undetected particles via the missing mass technique (susceptible to particle misidentification) Proceeding via the missing mass technique, with additional studies made to minimise the effects of misidentified particles Target selection cuts chosen based on target geometry and the ability to resolve the individual target cells in FROST (butanol, carbon and polythene) Only the butanol is polarised, with the other targets used to account for nuclear background and asymmetry dilution, as well as to test the measurement of observables on molecular targets

Channel Identification Introduction Previous Data Experiment Analysis Observables Results Channel Identification The K+Λ and K+Σ0 reactions are then identified from a plot of the kaon missing mass versus the invariant mass of the proton and reconstructed pion Λ Σ 0 Preliminary cuts are applied to select the channels, which are verified after account is taken of the carbon background in the butanol data

Extracting Observables Introduction Previous Data Experiment Analysis Observables Results Extracting Observables Recall that polarisation observables contribute to the differential cross-section 𝑑σ 𝑑Ω = σ 0 1− 𝑃 𝑙𝑖𝑛 Σcos2φ+ 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2φ They can also be expressed as an asymmetry of cross-sections between polarisation states Σ= σ ⊥,0,0 −σ ∥,0,0 σ ⊥,0,0 +σ ∥,0,0 𝐺= σ π 4, +𝑧,0 −σ π 4, −𝑧,0 σ π 4, +𝑧,0 +σ π 4, −𝑧,0 By equating the reduced cross-section expression with an asymmetry of beam polarisation states, polarisation observables can be measured Asymmetries are made using kaon azimuthal angle distributions for the PARA and PERP beam polarisation states, and sinusoidal functions are fitted to the resulting distribution to extract observables

Extracting Observables Introduction Previous Data Experiment Analysis Observables Results Extracting Observables For example, PγΣ can be extracted from the magnitude of a cos(2Φ) function fitted to the PARA/PERP asymmetry of the kaon azimuthal angle for an unpolarised target PARA Asymmetry PERP

Extracting Observables Introduction Previous Data Experiment Analysis Observables Results Extracting Observables For the polarised butanol data, the resulting asymmetry is not just a measurement of Σ – it also contains a contribution from the G observable 𝑑σ 𝑑Ω = σ 0 1− 𝑃 𝑙𝑖𝑛 Σcos2φ+ 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2φ The G observable can be seen in the difference between these distributions for positive and negative longitudinal target polarisations The positive (top) and negative (bottom) target polarisation distributions show a phase shift due to the change in target polarisation The Σ and G observables can be extracted by fitting a cos(2Φ) + sin(2Φ) function to the PARA/PERP asymmetry for each target state PγΣ is the amplitude of the cos(2Φ) term, and the amplitude of the sin(2Φ) term is a measure of PγPTARGETG

Introduction Previous Data Experiment Analysis Observables Results The Double Asymmetry Measurements of G from the beam asymmetry are difficult to separate from other phase offset effects It is also difficult to obtain a consistent measurement between target polarisation states The double asymmetry technique was developed in an attempt to find a way of combining the two sets of target data into one consistent measurement It extends the beam asymmetry method, by taking the asymmetry of beam asymmetries for each state of target polarisation 𝐴 +𝑧 = 𝑃 𝑙𝑖𝑛 Σcos2ϕ+ 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2ϕ 𝐴 −𝑧 = 𝑃 𝑙𝑖𝑛 Σcos2ϕ− 𝑃 𝑧 𝑃 𝑙𝑖𝑛 𝐺𝑠𝑖𝑛2ϕ 𝐴 𝑑𝑜𝑢𝑏𝑙𝑒 = 2P γ 𝑃 𝑧 𝐺sin2ϕ 2P γ Σcos2ϕ = 𝑃 𝑧 𝐺 Σ tan2ϕ

Introduction Previous Data Experiment Analysis Observables Results The Double Asymmetry The fit function used to extract G from the double asymmetry is constrained by the measured polarisations and the Σ observable extracted from the beam asymmetry method

Introduction Previous Data Experiment Analysis Observables Results Carbon Scaling Quantify how much carbon (unpolarised nuclei) are present in the butanol, in order to account for its effect on asymmetries and isolate hydrogen (protons) Determine a carbon scaling factor by dividing kaon missing mass for butanol by the same histogram for carbon Scaling factor is then used subtract scaled carbon spectrum from the butanol, verifying the hyperon selection cuts Can also provide an estimate of the number of carbon events in butanol when diluting asymmetries

Introduction Previous Data Experiment Analysis Observables Results Observable Dilution The extracted PγΣ from butanol is actually a measure of two things; PγΣ (proton) and PγΣ (carbon), with each term diluted by the respective relative amounts of carbon and hydrogen in the target PγΣ for the proton can be approximated from measurements of PγΣ on the butanol and carbon targets by; 𝑃 γ Σ 𝑃𝑅𝑂𝑇𝑂𝑁 = 1 𝑁 𝑃𝑅𝑂𝑇𝑂𝑁 ∗ 𝑁 𝐶𝐻2 𝑃 γ Σ 𝐶𝐻2 − 𝑁 𝐶𝐴𝑅𝐵𝑂𝑁 𝑃 γ Σ 𝐶𝐴𝑅𝐵𝑂𝑁 Carbon is unpolarisable, so dilution is simpler for measurements of G, and PγPTARGETG for the proton is given by; 𝑃 γ 𝑃 𝑇𝐴𝑅𝐺𝐸𝑇 𝐺 𝑃𝑅𝑂𝑇𝑂𝑁 = 𝑁 𝐵𝑈𝑇𝐴𝑁𝑂𝐿 𝑁 𝑃𝑅𝑂𝑇𝑂𝑁 ∗ 𝑃 γ 𝑃 𝑇𝐴𝑅𝐺𝐸𝑇 𝐺 𝐵𝑈𝑇𝐴𝑁𝑂𝐿

Results Beam asymmetry (Σ) for γp → K+Λ Introduction Previous Data Experiment Analysis Observables Results Results Beam asymmetry (Σ) for γp → K+Λ

Results Comparison of beam asymmetry for γp → K+Λ with g8b data Introduction Previous Data Experiment Analysis Observables Results Results Comparison of beam asymmetry for γp → K+Λ with g8b data

Results G observable for γp → K+Λ from beam asymmetries Introduction Previous Data Experiment Analysis Observables Results Results G observable for γp → K+Λ from beam asymmetries

Results G observable for γp → K+Λ from the double asymmetry Introduction Previous Data Experiment Analysis Observables Results Results G observable for γp → K+Λ from the double asymmetry

Summary First, preliminary measurements of the Σ and G observables have been made for γp → K+Λ and γp → K+Σ0 The Σ results, and their agreement with previous CLAS measurements, show that polarisation observables can be measured on a molecular proton target and the associated background and dilution effects can be accounted for Two techniques have been used to measure G; an extraction from the phase shift present in a beam asymmetry, and a double asymmetry technique intended to combine all the available data into a single measurement Inconsistencies between the G measurements for each target polarisation state highlight the difficulty in an extraction from beam asymmetries The double asymmetry technique is especially hampered by insufficient constraint of the fit parameters from the limited available data Further studies of the associated systematics is expected to improve these measurements, as will further investigation of extraction methods