Measurement of double polarization observables in meson photoproduction off the proton with the CBELSA/TAPS experiment

0.21 MB·

by  Jan Hartmann

Measurement of double polarization observables in
meson photoproduction off the proton with the
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CBELSA/TAPS experiment
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a
J

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2 JanHartmann∗

FortheCBELSA/TAPSCollaboration
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x HISKP,BonnUniversity
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E-mail: [email protected]
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Oneoftheremainingproblemswithinthestandardmodelistogainagoodunderstandingofthe
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low energy regime of QCD, where perturbative methodsfail. One key towards a better under-
[

standingisbaryonspectroscopy. Unfortunately,inthepastmostbaryonspectroscopydatahave
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beenobtainedonlyusingp Nscattering.Togainaccesstoresonanceswithsmallp Npartialwidth,
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8 photoproductionexperiments,investigatingvariousfinalstates,provideessentialinformation.In
9 ordertoextractthecontributingresonances,partialwaveanalysesneedtobeperformed.Here,the
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completeexperimentisrequiredtounambiguouslydeterminethecontributingamplitudes. This
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. involvesthemeasurementofcarefullychosensingleanddoublepolarizationobservables.
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TheCrystalBarrel/TAPSexperimentwithalongitudinallyortransverselypolarizedtargetandan
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4 energy tagged, linearly or circularly polarized photon beam allows the measurementof a large
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set of polarizationobservables. Due to its goodenergyresolution, highdetectionefficiencyfor
:
v photons,andthenearlycompletesolidanglecoverage,itisideallysuitedforthemeasurementof
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X thephotoproductionofneutralmesonsdecayingintophotons.
r PreliminaryresultsforthetargetasymmetryT,recoilpolarizationPandthedoublepolarization
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observableH arediscussedforp 0andh photoproductionofftheproton.
PACS: 13.60.Le, 13.88.+e, 14.20.Gk
XVInternationalConferenceonHadronSpectroscopy
4-8/11/2013
Nara,Japan
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
MeasurementofdoublepolarizationobservableswiththeCBELSA/TAPSexperiment JanHartmann
1. Introduction
In order to extract the contributing resonances in photoproduction experiments, partial wave
analyses need to be performed. A complete experiment is required to determine the contributing
amplitudes. Thisinvolves themeasurementofsingleanddouble polarization observables.
Forsinglepseudoscalar mesonphotoproduction usingalinearly polarized photonbeamanda
transversely polarized target, thecrosssection canbewrittenintheform
ds ds
dW =(cid:18)dW (cid:19) 1−PgS cos(2f )−PxPg Hsin(2f )−Py PgPcos(2f )−T (1.1)
0(cid:2) (cid:0) (cid:1)(cid:3)
where ds istheunpolarizedcrosssection,S ,T,P,andH aretheoccurringpolarization observ-
dW 0
ables[1(cid:0)],P(cid:1)g isthedegreeoflinearphotonpolarization, andf istheazimuthal angleofthephoton
polarizationplanewithrespecttothereactionplane. P andP arethedegreesoftargetpolarization
x y
inthereactionplaneandperpendicular toit,respectively.
In this paper, preliminary new data on the observables T, P, and H will be presented for
the reactions ~g~p → pp 0 and ~g~p → ph , thus providing an important step towards the complete
experiment.
2. Data analysis
ThedatapresentedhavebeenobtainedwiththeCBELSA/TAPSexperimentatELSA[2]. The
detector system consists oftwoelectromagnetic calorimeters, the Crystal Barrel[3]and theMini-
TAPSdetector [4], together covering the polar angle range from 1◦ to 156◦ and the full azimuthal
angle. For charged particle identification, a three-layer scintillating fiber detector [5] surrounding
the target, and plastic scintillators in forward direction were used. The frozen spin butanol target
[6]wasoperated withasuperconducting saddle coilproviding ahomogeneous magnetic fieldper-
pendicular to the beam direction, reaching anaverage target polarization of 74%. Datahave been
takenwithtwoopposite settingsofthetargetpolarization direction (named↑and↓).
Forthisanalysis, adatasetobtainedwithaprimaryelectronenergyof3.2GeVwasused. The
energy tagged photon beam was linearly polarized by means of coherent bremsstrahlung [7] with
amaximumpolarization of65%atEg =850MeV. Twoperpendicular settings ofthepolarization
planewereused(namedkand⊥).
Thedatasamplewasselected foreventswiththreedistinct calorimeter hits. Firstallthreeare
treated as photon candidates, three 2g invariant masses are formed and a cut on the gg invariant
mass is applied to select the reactions ~g~p → pp 0 → pgg and~g~p → ph → pgg . Then, with the
remainingcalorimeterhitastheprotoncandidate, additional cutstoensureenergyandmomentum
conservation are applied. Thisresults in afinalevent sample containing atotal of 1.4 million pp 0
and 140000 ph events with a background contribution of < 1% and < 2%, respectively. The
selected events for each of the four combinations of beam and target polarization directions were
normalized w.r.t. theaveragebeamandtarget polarization andtheintegrated photon flux.
The butanol target contains unpolarized C and O nuclei. The dilution factor d takes into
account that photoproduction of p 0 orh offunpolarized, bound nucleons in Cor O nuclei cannot
becompletelyseparatedfromthereactionsonthepolarized, freeprotons. d isafunctionofEg and
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MeasurementofdoublepolarizationobservableswiththeCBELSA/TAPSexperiment JanHartmann
NT1700 MeV < Eg < 732 MeV y (a) NT11049 MeV < Eg < 1154 MeV y (b) NBT1700 MeV < Eg < 732 MeV y (c)
D0.5 a r D0.5 a r D0.5 a r
n n n
mi mi mi
0 eli 0 eli 0 eli
p r p r p r
-0.5 -0.5 -0.5
b f [(cid:176)] b f [(cid:176)] a b f [(cid:176)]
-1 -1 -1
0 90 180 270 360 0 90 180 270 360 0 90 180 270 360
Figure 1: Examples for measured azimuthal yield asymmetries (integrated over the polar angle) used to
extractthetargetasymmetryT inp 0(a)andh photoproduction(b),andPandH inp 0photoproduction(c).
cosq , which isdetermined using data for whichthe butanol target wasreplaced byacarbon foam
target insidethecryostat.
Sincethedetector acceptance isidentical forallpolarization settings, thecrosssection canbe
replaced by the normalized yield N, and the target asymmetry T is determined from a fit to the
azimuthal yieldasymmetry
1 N −N N −N
D N(f )T= · ↑ ↓ =T·sin(b −f ); d(Eg ,q )= butanol carbon (2.1)
d·P N +N N
t ↑ ↓ butanol
with average target polarization P and b =100◦ being the direction of the target polarization in
t
the ↑setting. Atypical example forsuch afitisshown inFig. 1(a) andFig. 1(b). Twoadditional
observables are accessible by also considering the linear polarization of the photon beam. In ad-
dition totheobservable H,onealsohasaccess totherecoil polarization P,withoutthe difficulties
associated with a direct measurement of the recoil proton polarization. In order to extract both
observables fromthedata,allfourcombinations ofbeamandtargetpolarization settingsareused:
1 (N −N )−(N −N )
D N(f ) = · ⊥↑ ⊥↓ k↑ k↓ (2.2)
BT
d·Pg Pt (N⊥↑+N⊥↓)+(Nk↑+Nk↓)
=Psin(b −f )cos(2(a −f ))+Hcos(b −f )sin(2(a −f ))
withaveragebeampolarizationPg anda =45◦beingthedirectionofthepolarizationplaneinthek
setting. Again,theobservables aredetermined byafittotheazimuthalyieldasymmetry, asshown
bytheexampleinFig.1(c).
3. Preliminary results
Preliminary results for the observables T, P, and H are shown in Fig. 2 for p 0 photoproduc-
tion. Theagreement withprevious measurements ofT performed attheDaresbury 5GeVelectron
synchrotron [14] is quite good. If compared to the predictions of different state of the art PWA,
discrepancies are observed between thedifferent predictions and the data, especially athigher en-
ergies. Thisindicates anotyetsatisfactory understanding ofp 0 photoproduction.
VerypreliminaryresultsforthetargetasymmetryT inh photoproduction areshowninFig.3.
Theresultsseemtobeinconsistent withprevious measurementsdoneatELSA[12],butadetailed
study of the systematic uncertainties needs to be done before a final conclusion can be drawn. A
comparison of the PWA predictions to the data shows significant discrepancies. This data will
without doubt contribute toabetter understanding ofh photoproduction. Itprovides animportant
nextsteptowardsthecompleteexperiment.
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MeasurementofdoublepolarizationobservableswiththeCBELSA/TAPSexperiment JanHartmann
T 11 641 MeV < Eg < 667 MeV 1 700 MeV < Eg < 731 MeV 1 768 MeV < Eg < 798 MeV 1 833 MeV < Eg < 867 MeV 1 y901 MeV < Eg < 933 MeV
0.50.5 0.5 0.5 0.5 0.5
r
00 0 0 0 0
-0.5 -0.5 -0.5 a -0.5
-1-1 -1 -1 -1 -1
11-1 974 M-0.5eV < Eg0 < 10150.5 MeV 1-111058 -M0.5eV < E0g < 11000.5 MeV 1-11 1154 -M0.5eV < E0g < 11907.5 MeV 1n-111246 -M0.5eV < E0g < 1300.15 MeV1-111353 -M0.5eV < E0g < 1400.35 MeV 1
0.50.5 0.5 0.5 0.5 0.5
00 0 0 0 0
i
-0.5 -0.5 m -0.5 -0.5
-1-1 -1 -1 -1 -1
11-11465 -M0.5eV < E0g < 15305.5 MeV1-111535 -M0.5eV < E0g < 16001.5 MeV1-111601 -M0.5eV < E0g < 1700.05 MeV1-111700 -M0.5eV < E0g < 1790.95 MeV1-111799 -M0.5eV < E0g < 1890.75 MeV 1
0.50.5 0.5 0.5 0.5 0.5
i
00 0 0 0 0
l
-0.5 e -0.5 -0.5 -0.5
-1-1 -1 -1 -1 -1
P 11-1 641 -M0.5eV < E0g < 6670 .5MeV 1-11 700 -M0.5eV < E0g < 7310 .5MeV 1-11 768 -M0.5eV < E0g < 7980. 5MeV 1-11 833 -M0.5eV < E0g < 8670. 5MeV 1-11 901 -M0.5eV < E0g < 9330. 5MeV 1
0.50.5 r0.5 0.5 0.5 0.5
00 p 0 0 0 0
-0.5 -0.5 -0.5 -0.5
-1-1 -1 -1 -1 -1
H 11 641 MeV < Eg < 667 MeV 1 700 MeV < Eg < 731 MeV 1 768 MeV < Eg < 798 MeV 1 833 MeV < Eg < 867 MeV 1 901 MeV < Eg < 933 MeV
0.50.5 0.5 0.5 0.5 0.5
00 0 0 0 0
-0.5 -0.5 -0.5 -0.5
-1-1 -1 -1 -1 -1
--11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 11
cos q
p0
Figure 2: Preliminary distributions of T (top 3 rows), P (4th row), and H (bottom row) for the reaction
~g~p→pp 0 (black)asafunctionofthep 0 CMSangle,comparedtopreviousmeasurements[8]ofT andP
(gray)andthepredictionsoftheBnGa [9](solid),SAID [10] (dotted),andMAID[11] (dashed)analyses.
Onlyeverysecondenergybinisshownhere.
y
r
T 11 700 MeV < Eg < 750 MeV 1 750 MeV < Eg < 798 MeV 1 798 MeV < Eg < 851 MeV 1 851 MeVa < Eg < 901 MeV 1 901 MeV < Eg < 949 MeV
n
0.50.5 0.5 0.5 0.5 0.5
i
00 0 0 m 0 0
-0.5 l -0.i5 -0.5 -0.5
e
-1-1 -1 r -1 -1 -1
11 949 MeV < Eg < 1049 MpeV 1 1049 MeV < Eg < 1154 MeV 1 1154 MeV < Eg < 1246 MeV1 1246 MeV < Eg < 1353 MeV1 1353 MeV < Eg < 1451 MeV
0.50.5 0.5 0.5 0.5 0.5
00 0 0 0 0
-0.5 -0.5 -0.5 -0.5
-1-1 -1 -1 -1 -1
--11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 -11 -0-0..55 00 00..55 11
cos q h
Figure3: VerypreliminarydistributionsofT forthereactiong~p→ph (black)asafunctionoftheh CMS
angle, comparedto previousmeasurements[12] (gray)and the predictionsof the BnGa [9] (solid), SAID
[13](dotted),andMAID[11](dashed)analyses.
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MeasurementofdoublepolarizationobservableswiththeCBELSA/TAPSexperiment JanHartmann
4. Summary
Data have been taken with the CBELSA/TAPSexperiment using the newly developed trans-
versely polarized target and a linearly polarized photon beam. The preliminary results show the
excellent quality of the data. Further measurements are planned to increase the statistics to inves-
tigate other reactions and higher energies. Together with the measurements with a longitudinally
polarized target and both linearly [15] or circularly [16] polarized photon beams this is an impor-
tantsteptowardsthecompleteexperiment andwillprovide furtherconstraints forthepartialwave
analysis.
ThisworkwassupportedbytheDeutscheForschungsgemeinschaft withinSFB/TR-16andthe
Schweizerischer Nationalfonds.
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