p( g ,n p + g / ) reaction measured with the Crystal Ball at MAMI

p(,n reaction measured with the Crystal Ball at MAMI

Dan Watts, Derek GlazierUniversity of Edinburgh

Richard Codling, John AnnandUniversity of Glasgow

Crystal Ball Collaboration meeting, Mainz, 2007

Why measure p(,n’

• Independent test of theoretical treatment of reaction amplitudes and rescattering effects in radiative photoproduction

• radiated from + lines (rather than proton lines as in p0’) – brem production has different strength/angular behaviour

• Give additional sensitivity to MDM?

Blue lines : + p →n + + + 'Black lines : + p →p + 0 + '

Theoretical predictions p(,n+’)

• Predictions presently available in unitary model (and EFT presently in development)

Main features:

1) Cross sections ~5x larger than p(,p0’) 2) Linear asymmetries large and positive3) Sensitivity to MDM marginal (in sampled kinematics)4) But helicity asymmetry shows promise as complimentary determination of MDM

Tree levelUnitary model

+ detection in the Crystal Ball:Achieving good energy determination

Utility of Crystal Ball for detection well understood but energy determination unexploredExpect some challenges:

1) Separation from proton/electron events 2) Hadronic/nuclear interactions 3) Unstable decay products }GEANT simulation to indicate

CB response

Particle-ID detector

● (~26 ns)

ee (~2 s)

Michel spectrumof e+ energies

● Use shower shape to help identify event types

● Reject many of , NI events with simple restriction on Ncryst<=4

Good Event

Muon decay event

Nuclear interaction

Geantsimulation: + shower shapes

Geant simulation: 150 MeV + signals in the CB

Cou

nts

Energy contained in cluster (GeV)

Cou

nts

Energy contained in cluster (GeV)

Split off clusters

Muon decay

Hadronicinteractions

No shower size restriction <=4 crystals in the shower

p(,n+’) : Outline of data analysis

Accept events with: 1+, 2 neutral clusters in CB/TAPS1+, 1 neutron TAPS, 1 other neutralp(,n+’) total 4-mom kinematic fit (CL>10-1)

If two neutrals assume either is photon or neutron, analyse both combinations

Reject events with:2 neutrals pass M0 kinematic fit (CL>10-3) - p0,n+0

M+miss = Mn Kin. Fit (CL>10-3) - n+

n+ Total 4 momentum fit (CL>10-2) - n+

+ shower condition <=4 crystals

Data used in next plots: all MDM data at Ee=885 MeV July/Sep/JanTotal p(,n+’) events – 70,000

p(,n+’) : Simulation data

• Run event generators through Monte Carlo of CB/TAPS

• Predicted energy deposits smeared according to observed experimental energy resolutions

Event generators:p(,n+p(,n+- split off clusters from n/+p(,n+0– Missed/combined from 0 decay

All phase space distributions at the moment!’) :

p(,n+’) : Analysis results

N.B

. K

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to r

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backg

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nd

rela

xed

in

th

ese p

lots

!!ExperimentSimulated n+Simulated n+

Simulated no

Mmass of the mass of the system recoiling system recoiling

from the pionfrom the pionminusminus the neutron the neutron

massmass

M M

M M


ExperimentSimulated n+Simulated n+

Simulated no

p(,n+’) : Linear asymmetry E= 360 ± 20 MeV

CM = 0o-70o

CM = 70o-110o

CM = 110o-180o

= 50-80 MeV = 80-110 MeV

= 110-140 MeV

p(,n+’) : Linear asymmetry E=420 ± 20 MeV = 50-80 MeV = 80-110

MeV = 110-140 MeV

CM = 0o-70o

CM = 70o-110o

CM = 110o-180o

E = 320 ±20 MeV

E = 360 ±20 MeV

E = 420 ±20 MeV

o(CM) < 110o

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr

y p(,n+’) : Analysis results (Linear Asymmetry)

Unitary model (=2)

Unitary model normalised to agreein soft photon limit

Rescattering not included

E = 320 ±20 MeV

E = 360 ±20 MeV

E = 420 ±20 MeV

o(CM) < 70o

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr


Unitary model (=2)



E = 320 ±20 MeV

E = 360 ±20 MeV

E = 420 ±20 MeV

o(CM) < 180o

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr


Unitary model (=2)



p(,n+’) : Helicity dependence E=420 ± 20 MeV

CM = 0o-70o

CM = 70o-110o

CM = 110o-180o

= 50-90 MeV = 90-130 MeV

= 130-170 MeV

in CM framez = beam

y = x beam

= 50-90 MeV = 90-130 MeV

= 130-170 MeV

CM = 0o-70o

CM = 70o-110o

CM = 110o-180o


CM = 0o-70o

CM = 70o-110o

CM = 110o-180o

= 50-90 MeV = 90-130 MeV

= 130-170 MeV


p(,n+’) : Analysis results (Helicity dependence)

Helicity shows sin (dependence

Assumption:Fit distributions with sin() - extract amplitude to give helicity asymmetry at phi =90o


Unitary model = 1 = 3 = 5

Experimental data:E = 420±20 MeVAll (CM)(CM) = 90o

CM = 110o-180o

CM = 70o-110oCM = 0o-70o

Unitary model integratedover appropriate (CM) ranges

(at fixed (CM) = 90o)

cir

c

cir

c

cir

c



Experimental data:E = 470±20 MeVAll (CM)(CM) = 90o

CM = 110o-180o

CM = 70o-110oCM = 0o-70o


(at fixed (CM) = 90o)

cir

c

cir

c

cir

c

Summary

• We see a promisingly clean p(,n+’) signal

• Extracted linear polarisation observables will give important constraints on the theoretical modelling of radiative pion photoproduction

• Helicity asymmetry may show promising additional route to gain sensitivity to MDM - future dedicated beamtime ?

• Need to pass theoretical predictions through detector acceptance before publication (Unitary, CEFT?)


E = 470±20 MeV(CM) = 90±??o

(CM) = 90o


CM = 0o-70o CM = 70o-110o

CM = 110o-180o Unitary model

integratedover appropriate (CM) ranges


Only keep data which haveoverall p(,n+’) 4-momentum

with confidence level > 0.1

All plots: E = 400 ± 20 MeV

Importance of MDM determination of (1232)

Present knowledge

CB@MAMI

Outline

● Motivation

● Count rate estimate

● n (Deuterium data)

● + detection – preliminary analysis of experimental data

Count rate estimate● Detection efficiencies +

~25% n~30% ~90%

(p0 0~85% p~70% ~90% )

● Electron count rate 5x105 s-1MeV-1

● Tagging efficiency ~50%

● Tagged photon flux 2.5x105 s-1MeV-1

● 5cm long proton target 2.1x1023 cm-2

● Data acquisition live time ~70%

● d/dE ~0.5 nb/MeV

● Total count rate ~0.7x105 events (with '=30-150 MeV Eg=340-490 MeV)



E = 420±20 MeV(CM) = 90 ±?? o

(CM) = 90o

CM = 110o-180o

CM = 70o-110oCM = 0o-70o


+ detection in the Crystal Ball : Tracker & Particle-ID detector

~ 1.5o

~ 1.3o

• Two cylindrical wire chambers• 480 anode wires, 320 strips

2mm thickEJ204 scintillator

320m

m


E(MeV)

(b

arn

s)*

10

-6A

ccep

tan

ce x

10

-3

E(MeV)

E(MeV)E(MeV)

Accep

tan

ce

Accep

tan

ce x

10

-3

CB – data analysis parameters ● Threshold for cluster finding = 5 MeV

● Individual crystal threshold given by TDC (~1.5 MeV).

● Do not include clusters near to edge of CB - 30 - 150 deg

● Require PID hit within =±10 deg of cluster centre

● 2-D region cut on plot of PID energy versus CB cluster energy

Energy of cluster in CB(MeV)E

nerg

y de

posi

ted

in P

ID

Pion cut

Protons

MWPC & Particle-ID in situ


E = 470±20 MeV(CM) = 90±??o

(CM) = 90o


CM = 0o-70o CM = 70o-110o

CM = 110o-180o Unitary model

integratedover appropriate (CM) ranges

+ - Selection of energy tagged events

● Use two-body kinematics + p → n + +

● Select n and + events back-to-back in phi plane

● Calculate + energy from pion angleand E

●Note that wire chamber tracking NOT included – uncertainty from reaction vertex

Good angular and energy resolution, close to 4acceptance

Setup at MAMI

Tracker & Particle-ID

GeV)

~41cm

~25cm

sin

Preliminary + signals

● Ecalculated – E

Measured

● No restriction on shower size

0-2525-5050-7575-100

100-125125-150150-175175-200



Measured

● 4 or less crystals in the + shower

0-2525-5050-7575-100

100-125125-150150-175175-200



Measured

● 2 or less crystals in the + shower

0-2525-5050-7575-100

100-125125-150150-175175-200

Energy resolution

●Includes uncertainties in reaction vertex, energy loss … as well as intrinsic CB resolution

Fraction with good energy determination

●Look at fraction of events within

Conclusions

●+ p → n + + events identified

● Energy tagged + events indicate CB gives reasonable energy signal

● MWPC software now implemented – further studies

● Develop improved shower shape algorithm which exploits correlation of energy deposits and shape in pion induced shower.

● Look at sampling after pulse - see time dependence of positron decays?

Magnetic moment of the + via the + p n + + + ' reaction

Daniel Watts – University of EdinburghPh.D student Richard Codling – University of Glasgow

p n

+

Preliminary + signals in CB

●Plot Ecalculated - E

Measured

● Shift of peak - energy losses?

● Simple shower shape restrictions give noticeable effect on response shape

● Development of better shower algorithms underway

No. cryst <4 No. cryst < 16

0-2525-5050-7575-100

100-125125-150150-175175-200

Michel spectrum

+ - Comparison of calculated and measured energies

● Rough tagger random subtraction included

● All angles summed over

Incident + energy (GeV)

Hig

hest

clu

ster

ene

rgy

(GeV

)

No restriction on shower sizeNcryst<3 & no neighbours

+ decay

Nuclear interaction

Geant simulation: + signals in the CB

Theoretical background● m - quark spins & currents.

● Test validity of theoretical hadron description in NPQCD

● Long lived particles - precession in B-field

● Short lived - Radiative decay

● Pioneered in p++p D++ D++g'

● TAPS@MAMI - proof of principle g+p D+ D+g' pp0

Energy

s

pp+

Theory mD+ / m

N

LQCD 2.20 0.4QCDSR 2.19 0.5Latt 2.26 0.31XPT 2.40 0.2RQM 2.38 NQM 2.73XQSM 2.19XB 0.75

Theoretical Background

● Reaction has important background terms

● Different for pp0 and np+ final states

● Simultaneous measurement also tests pN rescattering

D terms

Born terms

Black lines : g + p ->p + p0 + g' Blue lines : g + p ->n + p+ + g'

w exchange

Theoretical model● Effective lagrangian

● Integral s : sensitivity to mD+

● Kinematics can suppress brem.

● Simultaneous unitarised description

Experiment● CB : 672 * 0.5m NaI

TAPS : 540 * 0.25m BaF2

● Tracker: MWPC

● PID: 2mm plastic scint. Barrel

● >1 cluster trigger: Measure g + p ->n + p+ + g' and g + p ->p + p0 + g' (Expt. A2-1-02) simultaneously.

Neutron detection

● Neutron detection capabilities of CB established (BNL-AGS) p- p p0n

● en~10-40%

● Dqn< 10o; Df

n< 20o

Stanislaus, Koetke et. al., NIM

A462 463 (2001)

p+ decay● p+ m+ + nm (~26

ns)e+ n

e nm (~2 ms)

● NaI: t ~1ms tr~

0.1ms

Energy of positron (MeV)500

e+

n

en

m

Michel spectrum

No.

of

coun

ts

e+ ne nm (~2 ms)

+ signals in Crystal Ball

● 150 MeV + - isotropic

● Spectra sensitive to time over which energy deposits are recorded

● See signal at Tp.......but with

background

Michel spectrumt~infinite

Energy deposited in Ball (GeV)

Nuclear intn.+ absorbed

Nuclear intn.

t<1ms!!

0 150 300

4000

14000

0 150 300

Neutron detection in the CB

Neutron kinetic energy (MeV)

Dete

ctio

n e

ffici

en

cy

Neutron difference (deg)

~5o

E = 320 ±20 MeV

E = 360 ±20 MeV

E = 420 ±20 MeV

o(CM) < 70o

o(CM) < 110o

o(CM) < 180o

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr

y

Lin

ear

Asym

metr


p+ signals in CB

● Simple cut on shower size. N

cryst (HE clust) <3 &

No neighbouring clusters

● Get peak with manageable background!

● Eff ~25% at 100MeV

Summary

● Simultaneous measurement of np+g' with pp0g' improves confidence in model dependent extraction of mD+

● Measurement requires no extra beam time

● Establishing p+ detection capabilities of CB - opens perspectives for other future measurements

Documents

p( g ,n p + g / ) reaction measured with the Crystal Ball at MAMI