Some ideas and possibilities for nuclear target measurements with new generation electron

beams

Dan Watts

University of Edinburgh

Outline

• Many proton knockout

• Nuclear tagging using nuclear gamma decay photons

• Recoil nucleon spin polarimetry – first crude look at possibilities

Many nucleon knockout from nuclei

• We have carried out some initial studies of many proton knockout from nuclei (up to >6 protons)

• Central physics motivation - test transport models for proton/nucleon production in nuclei (e.g. GiBUU).

• These models are also used for neutrino detection using heavy nuclei - neutrino interaction and energy is reconstructed from hadronic final state

• Assess if exotic isotopes could be produced and at what rate.

Many proton knockout at JLAB• Use data from e2 run. Utilising Data Mining Framework. • 5 GeV electron beam on 208Pb target. • Reconstruct mass of recoiling system from scattered electron and proton

• Good test of theoretical models – can look for events where residual nucleus near it’s ground state

Path of r-process

P. Seeger, W. Fowler & D. Clayton , Astrophys. J. Suppl. 26, 231-312 (1965)

Path of r-process below closed shell – waiting points in neutron capture

Kink starts ~8 protons south of 208Pb.

Terra incognita No knowledge of existence/mass of nuclei 3 protons south of 208Pb

http://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html

Likely a range of mechanisms can contribute. • Earler work on (g,3N) in light nuclei indicated the main mechanism for 3N

knockout is Meson production from a nucleon plus reabsorption (meson,NN) (meson,NNN)

• Smaller contribution of direct 3N knockout (photon coupling to 3N current) and 2N + final state interaction

(e.g D.P. W et. al. Phys.Lett. B553 (2003) 25-30)

• At higher energies of CLAS data initial 2-meson and 3-meson production may contribute leading to possibilities for high multiplicity emission

What are the mechanisms of many-proton knockout?

meson

12C(g,ppn) reaction at Mainz

Dominant mechanisms giving strength at low missing energy2N+FSI, direct 3N, Np+ABSORPTION

Clear peak from intermediateexchange pion (close to mass shell)

D.P.W et. al. Phys.Lett. B553 (2003) 25-30)

CLAS analysis – study of low missing mass fragments

208Pb(e,e’p)

Reconstruct mass of recoiling systemusing detected electron and proton(s) 4-vectors

Low missing mass fragment particularly interesting

First crude analysis – fit near threshold using gaussian having width of CLAS resolution

Simple guess for background shape

Mass (GeV)

Coun

ts

Mass (GeV)

Coun

ts

208Pb(e,e’pp) 208Pb(e,e’ppp)

Mass (GeV)

Coun

ts

Mass (GeV)

Coun

ts208Pb(e,e’pppp)208Pb(e,e’ppppp)

low missing mass high missing mass

CLAS analysis - Relative yield for different multiplicities

Not detector acceptance corrected

202.5 203 203.5 204 204.5 205 205.5 206 206.5 207 207.5-150

-100

-50

0

50

Fit - Exp/Theory Mass Residuals

Sigma theory

Sigma Simulation

Mass number

Mea

sure

d m

ass

- exp

/ th

eory

(MeV

)

Sigma ineq.

CLAS analysis – study of low missing mass fragments

Assuming centroid of gaussian gives the mass of daughter system – compare with ground state masses. reasonable agreement with exp/theory

Will calibrate using proton knockout from 56Fe – masses measured up to 8 protons south

Next steps

Analyse data from wide range of targets

Pass theoretical models through the detector acceptance and compare

Preliminary GiBUU result

Recoil nucleon spin polarimetry

• Coupling of large acceptance nucleon spin polarimetry to large acceptance detectors would provide important new data to the future nucleon/deuteron physics programmes

(e.g meson spectroscopy, DVCS?

• What (if anything) is possible/feasible?

Large acceptance spin polarimetry

• It is possible to get spin information on recoiling nucleons from study of their scattering in an analysing medium

• Recent work at MAMI has shown that good results can be obtained using a kinematic reconstruction of this scatter – without the need for large acceptance nucleon tracking systems

DataG4 totalG4 no nuclear int

Proton scattering angle in graphite

Analysing power of scatterer

Unpolarised polar angle distributiion x and y (transverse) components

of nucleon polarisation

No. nucleons scattered In the direction , q f

n( ,q f) =no(q){1+A(q)[Pycos(f)–Pxsin(f)]

Large acceptance spin polarimetry at MAMI

Cx* for p(g,p0) from CB@MAMI polarimeter

Sikora, DPW, Glazier PRL12 022501 (2014)

Fit B Juelich PWA

Changes in P11 partial wave and P11(1710) pole positions with new fitNew fit shows features more consistent with SAID PWA using only pN scattering Only for one PWA framework – but Illustrates utility of new observables – even when poorer stat accuracy

Cx* for p(g,h) from CB@MAMI polarimeter

Fit to world data(including beam-target)Forced fit to Cx* data

Harder Forced fit to Cx* data

PRELIMINARY

Ongoing work using CLAS data

Have large dataset of D(g,pn) from g13 run period – can we obtain informationon nucleon spin polarization from nucleon scatters in the start counter?

First study – look at (n,p) charge exchange reactions. Get first info on final state neutron polarizations. Check HHC, .. models. Further constraints on proposed d* state

Reconstruct incident neutron track kinematically - track proton in CLAS

Measured (n,p) analyzing powers on light nuclei close to (established) free nucleon

N. Zachoriou (Edinburgh)

Scintillator

Target cell

qsc distributionsas expected

jsc distributions - strong acceptance effects- but cancel to first order when constructing beam asymmetry

cos(2j) fit to extract transferred polarisation

A ~0.1 : Needs detailed modelling to obtain accurate valueSimilar analysis using start counter with CB@MAMI in progress (S. Kay Edinburgh)

D(g,pn)

Polarimetry in new energy regime

• Central programmes at CLAS 12 in area of meson spectroscopy and nucleon structure (DVCS,DVMP)

• What is possible for these programmes

(quick discussion)

Glue-X/MesonEx production kinematics

|t| (GeV2)

P

roto

n k

inet

ic e

ner

gy

(GeV

)

g + p → X + pMX=2 GeVEg=9 GeV

P

roto

n a

ng

le (

deg

rees

)

Heavy mass meson photoproduction ~ MX~2 GeV

nucleon energiesfor low t in similar regionto previous analyses

|t| GeV2 |t| (GeV2)

Polar angles in region of 40 degrees

Glue-X/MesonEx production kinematics

Pro

ton

kin

etic

en

erg

y (G

eV)

|t| (GeV2)

g + p → X + pMX=0.78 GeVEg=9 GeV

|t| (GeV2)

P

roto

n a

ng

le (

deg

rees

)

Light vector meson mass ~ 0.8 GeV

Low tPolar angles ~45-75

DVCS

Nucleons have low energies

The CND in CLAS12 – A MAKESHIFT POLARIMETER?

Complicated: spin precession in B field, extracting scatter ,q y, acceptance, efficiency, analysing powers, ..

Rule of thumb – error bars for beam recoil observables larger than beam target by: factor ~8 from analysing power (0.1 – 0.3 compared with 0.8 for polarised target) factor ~6 from reduced number of analysable events (scales as sqrt) Need theoretical steer to justify the effort – are beam-recoil observables (with poorer statistical accuracy than the beam target observables) worthwhile for the nucleon structure meson spectroscopy physics programmes ?? Calculations needed…

Bars 3cm thick, 3.5 cm wide

Very roughly 3% per layer probablity for (n,p) or (p,p)

Acceptance for polar angles <~40 degrees

Would need extensive simulation programme to establish capabilities. First test simulations to see if anything is feasible are underway (G. Smith, L.Zana, Edin

• Thank you for your attention

• Phoswich design - good timing properties of LaBr to be combined with high stopping power, lower cost scintillators

• Different timings of the scintillator output - allow DE-E ?? → Need Flash ADC or dual gated ADC readout

Crystal tests with possibilities for CLAS12

First tests of LaBr/CsI phoswich

• Investigate phoswich: particle ID capabilities, timing etc. (In collaboration with Univ. York (UK). Plans for portable array for use at SPIRAL - shared use at JLAB ?

• Explore LaBr / hybrid modules alongside other crystal possibilities for forward calorimeter

• Grant request (approved but pending) - additional RA support and further money for prototype crystals

Future plans

Hybrid meson production • Coherent and incoherent nuclear production processes suppress the s-channel nucleon resonance contributions → greatly simplifies the PWA for hybrid searches

• Could the Jp of residual nuclear states provide new information? Tag by detecting nuclear decay photon in coincidence

• Example: 0+ to 2+ transition emphasizes tensor like exchanges in t-channel → favourable production mechanism for hybrids in flux tube model

12C 12C(2+;T=0; 4.4MeV)

Also use in nuclear DVCS?e.g. select highertwist effects?

Hypernuclei?

12C 12C(2+;4.4MeV)

• Coherent proposal 4He at JLAB: virtual photon tagger with TPC estimate ~60k hybrid mesons produced

• Incoherent

→ Rate not limited by TPC ~102 increase in g flux

→ No minimum limit on t

→ Will lose factor ~10 in cross section due to loss of coherence in amplitudes, form factor effects

Incoherent hybrid meson production

A. Donnachie arXiv. 0806. 3698(2008)

p1(1600)

p1(1400)a2(1320)

TPC detection limit

0.30.20.1 t(GeV/c2)

1

10

100

0.1

12C( ,g J/Y)12C

CoherentSum incoherentIncoherent 2+ level

Rat

io to

cro

ss s

ectio

n o

n nu

cleo

n at

q=

0

• As well as the Jp of the nuclear state the angular distribution of the nuclear decay photons tells you about the alignment of the residual nucleus

• Polar distribution wrt mom transfer gives sensitivity to the spin dependence of the production amplitude (next slide) (Tryasuchev and Kolchin Phys. At. Nuc. 70 827 (2007))

• Azimuthal distribution allows information on meson-nucleon interaction to be extracted. e.g. J/Y photoproduction from 12C (V.L. Korotkikh and N.I. Strakov, Yad. Phys. 37 1030 (1983)

Nuclear decay photons

• t-channel amplitudes of (Sandy Donnachie, Univ. Manchester, UK) being incorporated into model of nuclear photoproduction (Helmy Sherif, Univ. Alberta, Canada)

• First step - Plane wave calculations for t-channel eta production. Calculations for further channels e.g. ao, fo in progress

New calculations in progress

Detector issues – next steps

• Can LaBr readout be in a region where conventional PMTS could be used? – Depend on necessary solenoid, shielding etc

• Edinburgh group applied for R&D funds for LaBr and SenSL avalanche PMT readout and 1 year RA

•

Device would need upwards of ~60 crystals

Crystals should be ~10cm depth to give~80% photopeak efficiency up to 10 MeVSOLENOID

Nuclear decay Photon calorimeter

CND possibilities