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The measurement of in-medium modification ofη’ meson properties at LEPS2/BGOegg experiment

(LEPS2/BGOegg 実験における原子核中でのη’ メソンの性質変化測定)

Yuji Matsumura

for the LEPS2/BGOegg collaboration

Research Center for Electron Photon Science , Tohoku Univ.

Dec 2, 2016

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Contents

・physics motivation

・calibration

・mass resolution

・summary

・LEPS2/BGOegg experiment

・ mass

An meson has larger mass than otherpseudo scalar mesons due to UA(1) anomaly.→ The mass of meson is expected to

decrease at the nuclear density.

・line shape analysisA direct measurement of mass spectra

c.f.

physics motivation

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Nagahiro et al., PRC74, 045203(2006)

(based on NJL model + KMT interaction) ee 0

R.Muto et al. PRL98, 042501(2007) M.Thiel et al. Eur.Phys.J.A49, 132(2013)

・ width

in a vacuum

in-medium width : from the measurement of transparency ratio @ CBELSA

→ decay mainly inside nuclei

・・no hadrons in the final state → pure information of

in-medium mass・

・distortion by final state interaction・higher mass resolution by kinematical fit

physics motivation

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fm 10 , keV 200 3 c

MeV 2515)( 0

(M.Nanova et al. PLB710, 600(2012))

-4

0 10)(/

00

SPring-8/LEPS2 beamline

・ 1.3 – 2.4 GeV tagged photon beam→ nearly recoilless can be produced

・ highly polarized・ ~2 Mcps beam intensity

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Energy spectrum of BCS

BGOegg calorimeter

・1320 BGO crystals ・polar angle : 24° 144°

azimutial angle : 360°・homogenious・no housing material・energy resolution : 1.3% @ 1GeV・position resolution : 3.1mm @ 1GeV

～

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20 mm thick C target

Energy measurement

： gain factor： ADC ch： pedestal ch

Energy calibration

)( 0iiii AAE

i

iAiA0

Clustering

: crystal with time information

・ Adjust gain factors iteratively so that mass agree with PDG value.0

invariant mss

MeV 98.134 mmi

i

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Leakage correction

・When reconstructing a cluster, a few % of total energyleaks out the cluster→ estimate leakage ratio as a function of energy

and polar angle by MC simulation

Red ：Before correctionBlue：After correction

Leaked enery ratio Momentum dependence of mass 0

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・slewing correction

Timing calibration

・ timing resolution for a hit

・ timing cut (±1 ns)separate neutron hitseliminate accidental hits (ex. cosmic rays)

：σ = 0.23 ns MeV) 200( 　E

・timing distribution with respect to RF timing

Red ：Before cutBlue：After cut

・ peak : mean : 135.1 MeVσ : 7.2 MeV

・ peak : mean : 547.4 MeVσ : 14.5 MeV

2γ invariant mass distribution

0

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MeV 957.78

MeV 86.547

MeV 98.134

m

m

m

(PDG mass)

0

・ peak : mean : 560.5 MeVσ : 20.9 MeV

・672 NaI(Tl) / 528 BaF crystals・20° 160°/ 4.5° 30°

Comparison of resolution

BGOegg Crystal Barrel Crystal Ball/TAPS・1320 BGO crystals・24° 144°～

・1230 CsI(Tl) crystals・30° 168°～ ～

= 7.2 MeV

・ , mass resolution( invariant mass , overall )

0

(I.Jaegle, Ph.D.thesis, Basel Univ.(2009))

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～

(A.Neiser et al., J.Phys:Conf.Ser. 587 012041(2015))

= 14.5 MeV

= 9.9 MeV

= 16.6 MeV

= 8.1 MeV

= 18.7 MeV

events : mode

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2

・2 neutral clustersno charged cluster @ BGOegg

・～6000 events(Apr 2015 – Jul 2015)

・mass resolution

2.2%

・background

, etc.

→ wrong combination of pairs cluster combine

m/

400

40

2

Background reduction

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・Photons emitted close to each other are sometimes combined→ generate large clusters

・Such clusters can be distinguished by “cluster size”

：

cluster cls

cls

i

ii

E

ErrR

R R

2 400

simulation

Background reduction

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・Cut by R<20

cluster cls

cls

i

ii

E

ErrR

Red : beforeBlue : afterBlack : rejected

・S/N ratio is much improved・～46% of background events are rejected

events : mode

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00

・kinematical fit・select best combination

of 6 neutral clusters

・constraintsfor 2 pairsfor 1 pair

・select probability > 0.1

0 mm

mm

2

・mean : 958.0 ± 0.4 MeVσ : 7.3 ± 0.4 MeV

σ/m = 0.76%

MeV 957.78 m(PDG mass)

CBELSA/TAPS

mean : 957.1 ± 0.4 MeVσ : 11.9 ± 0.3 MeV

σ/m = 1.24%

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Summary

・We are aiming to make a direct measurement of in-medium properties of meson by detecting

decay inside a nucleus with an electro-magneticcalorimeter BGOegg.

・Energy and timing calibration has been performedand , , mesons were clearly observed with high mass resolutions.

・ mass resolution of 2.2% ( mode) and 0.8% ( mode) was achieved.A line shape analysis of is on-going withextremely high mass resolution of BGOegg.

2

m/ m/ 00

0

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