LOGO The η -mass measurement with the Crystal Ball at MAMI A. Nikolaev for the Crystal Ball @ MAMI and A2 Collaborations Helmholtz Institut für Strahlen-

LOGO

The η-mass measurement with the Crystal Ball at MAMI

The η-mass measurement with the Crystal Ball at MAMI

A. Nikolaev for the Crystal Ball @ MAMI and A2 CollaborationsHelmholtz Institut für Strahlen- und Kernphysik, University Bonn

A.Nikolaev, Bonn CB Meeting, Basel 10/5/06 2

The mass measurement at MAMI

The production threshold measurement from γp→pη. The mass of is calculated from relation:

where mp is the proton mass, Ethr – production threshold.

22 2

c

Emmmm

thr

ppp

Recently published measurements:

A. Duane et al.: Phys. Rev. Lett. 32 (1972) π -p → n neutrals

F. Plouin et al.: Phys. Lett. B276 (1992) dp → 3 He ηB. Krusche et al.: Z. Phys. A351 (1995) γp → pη

A. Lai et al.: Phys. Lett. B533 (2002) η → 3π 0

M. Abdel-Bary et al.: Phys. Lett. B 619 (2005) dp → 3 He ΧE. Ambrosino et al.: Acta Phys. Slov. 56 (2006) φ → γη


Crystal Ball/TAPS detector

Crystal Ball detector: 672 NaI crystals

measures Eγ and Өγ

TAPS 510BaF2 detectors

Particleidentificationdetector

Crystal Ball672 NaI detectors

Particle identification detector: 24 plastic bars of size 30 cm x 2 cm 0.2 cm marks charged particles

Forward wall detector TAPS: 510 BaF2 single plastic detectors with

individual vetos


The tagging facility

The energy E of the photon is determined (tagged) by:

Incoming MAMI electron beam energy E0 is known with an accuracy of = 68 keV.

Scattered electron energy Ee- is known with an accuracy of e = 78 keV.

eEEE 0

222

0

eEEE


Tagger microscope detector

An array of 96 plastic scintillator fibres (3 mm x 2 mm).

Each single fibre overlaps by 1/3 with its neighbor. The overlap region defines microscope detector channel

μch (191 channels in total). The energy resolution is 0.3 MeV per

microscope channel μch. Tagger microscope is positioned to

cover electron energies 153 to 209 MeV. At a beam energy of 883 MeV this corresponds to tagged photons between 674 MeV and 730 MeV (η threshold ~707 MeV).


Tagger microscope energy calibration

Direct calibration: Direct position measurement of the MAMI electron beam for the

energies Ee- = 180.1 MeV, 195.2 MeV and 210.2 MeV using the same

dipole magnetic field as in the experiment (Bcal = Bexp = 1.049 T).

Scan beam through microscope varying the dipole field: Increase tagger dipole magnetic field Bcal in small steps.

Measure new electron beam position supposing equivalent electron beam energy for a given field setting Bcal :

cal

exp

e B

BEE 0


Tagger microscope energy calibration

# Date Beam energy, MeV Dipole field, Tesla

1 12.12.2003 180.12 ± 0.16 1.01-1.072 20.01.2004 180.10 ± 0.16 0.89-1.083 21.01.2004 210.18 ± 0.16 1.05-1.214 23.01.2004 180.21 ± 0.16 0.90-1.215 27.05.2004 195.24 ± 0.16 0.98-1.216 07.12.2004 185.90 ± 0.16 0.92-1.23

Blue arrows show direct position measurements of the electron beam of energies 180.1, 195.2 and 210.2 MeV.

180.1 MeV1.049 T

210.2 MeV1.055 T

195.2 MeV1.049 T

Expected η threshold


Total energy measurement uncertainty

Standard deviation of data points from fit:

was calculated for

each scan individually:

N

ie iEbaich

N 1

2]}[{][2

1

Average uncertainty: = 0.27 ch = 78 keV (electron energy Ee-).

Add electron beam energy E0 determination uncertainty of MAMI 68 keV ()

# Date N data points ch1 12.12.03 18 0.3032 20.01.04 26 0.2113 21.01.04 33 0.2014 23.01.04 11 0.3985 27.05.04 32 0.1906 07.12.04 19 0.224

(threshold) = 103 keV(m) = 65 keV

:222

0

eEEE


The mass beamtimes

7.12.2004-13.12.2004 11.01.2005-20.01.2005Tagger dipole field 1.0494 Tesla 1.0490 TeslaMAMI beam current 35 nA 12 nAHours 57 h 114 hN of →2 decays ~725 000 ~638 000N of →3 ° decays ~218 000 ~198 000MAMI beam energy 4 measurements with mean: 3 measurements with mean:

883.19 ± 0.16 MeV 883.24 ± 0.16 MeV

Two beamtimes for the mass measurement:


Identification of meson

mesons are identified via their uncharged decay modes.

Events with 2 photons and 6 photons (with or without proton) are investigated.

Photons are identified with Crystal Ball/TAPS detector.

Particle identification detector serves as a veto for charged particles in CB.

TAPS crystals have individual veto detectors for marking charged particles.

→2γ and →3π0 decays are analyzed independently.

)%29.051.32(3

)%26.043.39(20

Particleidentificationdetector (PID) Crystal Ball

672 NaI crystals

TAPS 510 BaF2 crystals with individual vetos


Identification of photons and protons

Crystal Ball Uncharged particles = photons.

Charged particles: ΔE(PID) vs. ECB:

cut the proton.

TAPS forward wall Vetos: charged particles. Pulse shape analysis: cluster energy

deposition in short Eshort vs. in long

Elong integration interval.

Time of flight:

ETAPS cl. vs. ( tTAPS cl. – tCB photons ).

protons

protons, neutrons

protonsphotons

photons


CB time walk correction

Investigate γp→pπ0 with γγp final state. For each of 672 NaI crystals:

ECB hit vs. ( tCB photons – tPID proton ) fit the peaks with t(E) function.

7-13 Dec 2004 #4924-5088: faulty crystals #: 168, 183, 194, 205, 221, 352.

11-20 Jan 2005 #5200-5452: faulty crystals #: 41, 168, 205.

MicroscopeFWHM = 1.7 nsSigma = 0.7 ns

LadderFWHM = 2.3 nsSigma = 1.0 ns

33.02

1)(E

ppEt


Invariant mass distributions

2 (with or without proton) final state invariant mass distribution.

30 (6with or without proton) final state invariant mass distribution.

Cut on invariant mass distribution. Time coincidence peak between

tagger microscope and detector: cut the coincident events.

Cut on lost proton missing mass.

Mean = 568 MeVSigma = 20 MeV



FWHM = 1.7 nsSigma = 0.7 ns

η

π0


Tagger dipole field / Jan 2005

NMR = 1.049057±0.000535 T (± 0.01 MeV in Ee). Very good!


Tagger dipole field / Dec 2004

NMR = 1.049527±0.015 T (± 2.78 MeV in Ee).

Need to refine data with faulty NMR!


Microscope channel efficiency

Investigate γp→pπ0 with 2 photons and 1 proton final state using tagger.

Prompt - random time windows. In microscope region (tagger

channels 55-85) “pion yield” looks constant.

Investigate the same with microscope.

Microscope channels have different efficiencies!

Use “pion yield” distribution as microscope channels efficiency to correct the η yield!

Microscope region

pion

yie

ld in

mic

rosc

ope

p

ion

yiel

d in

tagg

er


Eta yield for reaction γp→pη (η→2γ)

Fit range: 0-75 μch

Without acceptance correction!

Fit function: f(x)=a0+a1 x+a2 x2+a3 x3

η→2γ

Eγ


Eta yield for reaction γp→pη (η→3π0)

Without acceptance correction!

Fit function: f(x)=a0+a1 x+a2 x2+a3 x3

η→3π0

Fit range: 0-75 μch

Eγ


Very preliminary

Result dependence on fit rangeη→2γ η→3π0

1. η→2γ: mη=547.683 ± 0.013fit MeV

2. η→3π0: mη=547.743 ± 0.013fit MeV

Fit range: 0 to 75 μch10 to 75 μch20 to 75 μch30 to 75 μch40 to 75 μch50 to 75 μch60 to 75 μch70 to 75 μch



Conclusions

New threshold data on γp→pη. The η identified by η→ γγ and η→3π0.

Energy calibration for Ee- (tagger microscope) done.

Energy calibration E0 (Mainz Microtron) final checks for

systematical error underway.

22 2

c

Emmmm

thr

ppp

eEEE 0


MAMI energy determination uncertainty

B = 1.3260 T ± 0.13 mTΔE = ~7.81 ± 0.02 MeVD73 = 3768.45 ± 0.4 mm

RTM 3

D73

turn 73 Cavity 2

turn 90 Cavity 1

ΔE

E73 = 751.649 ± 0.107 MeVExtrapolate to end energy:Eout = 883.X ± 0.16 MeV (FWHM)

σEout = 68 keV

2

DBceE

Eout = Ein + z·ΔE, z = 90 Ein = 180 MeV

Dipole 1 Dipole 2

where e is electron charge, B is dipole magnetic field, D - electron track diameter.


Proton missing mass distributions p→p

p→p (→2)

p→p (→2)

p→p (→3)

p→p (→3)

Microscope Microscope

Main tagger Main tagger

Missing mass / MeV Missing mass / MeV

Missing mass / MeV Missing mass / MeV

hits

hits

hits

hits


Photon beam position Jan 2005


The mass beamtimes

7.12.2004-13.12.2004 11.01.2005-20.01.2005Tagger channels 1 to 80 1 to 284Microscope channels 1 to 191 1 to 191Radiator Diamond DiamondCollimator 3 mm 4 mmTagger dipole field 1.049 Tesla 1.049 TeslaMAMI beam current 35 nA 12 nAHours 57 h 114 hN of →2 decays ~725 000 ~638 000N of →3 ° decays ~218 000 ~198 000MAMI beam energy 4 measurements with mean: 3 measurements with mean:

883.19 ± 0.16 MeV 883.24 ± 0.16 MeV

Two beamtimes for the mass measurement:


MAMI energy measurements

7.12.2004-13.12.2004 11.01.2005-20.01.2005Date Beam energy Date Beam energy7.12.2004 883.17 MeV8.12.2004 883.20 MeV 10.01.2005 883.23 MeV10.12.2004 883.22 MeV 11.01.2005 883.26 MeV13.12.2004 883.16 MeV 13.01.2005 883.24 MeV

Mean 883.19 ± 0.16 MeV Mean 883.24 ± 0.16 MeV



1. η→2γ: σ = 0.023 MeV (in mη)

2. η→3π0: σ = 0.025 MeV (in mη)

Function fits the points very good!

0.1

MeV





1. η→2γ: σ = 0.023 MeV (in mη)

2. η→3π0: σ = 0.100 MeV (in mη)

Function fits the points good!0.1

MeV



Documents

LOGO The η -mass measurement with the Crystal Ball at MAMI A. Nikolaev for the Crystal Ball @ MAMI and A2 Collaborations Helmholtz Institut für Strahlen-