Electro-production of f0(980) scalar meson at HERMES

1
Electro-production of f0(980) scalar meson at
HERMES
Erika Garutti (DESY) On behalf of the HERMES
collaboration

• Physics introduction on scalar mesons
• - present knowledge on f0(980)
• Observation of f0(980) in the semi-inclusive
  pp- channel
• - mass peak determination
• - study of kinematics dependences
• - extrapolation to 4p

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Why are scalar mesons interesting?

• Scalar mesons in Constituent Valence Model
• - qq state with JPC 0 ? S1, L1
• - expected mass 1.2 1.5 GeV
• ? but f0(980), a0(980)

• Theoretical prediction for lightest glueball
  - scalar quantum numbers ?
  mixing with scalars - mass between 0.9 1.7 GeV

• Requirements to identify gluonic state
  - good knowledge of light
  scalar meson nonet (Thoma)
  - one isoscalar state in excess for the nonet
  (Ableev et. al)

• Alternative non-qq interpretations for scalar
  mesons - qqqq state (Jaffe, Alford)
  - vacuum
  scalar state (Dokshitzer,Gribov,Nyiri)
  - KK molecule (Close,
  Isgur, Khoze, Ryskin)

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Present knowledge on f0(980)
E687 gN ? pp- X
OPAL Z0 ? pp- X
PDG Mf0 0.980 0.010 GeV Gf0 0.040
- 0.100 GeV
Eg 220 GeV Mf0 0.9680.0005 GeV Gf0
0.0500.0024 GeV
NOMAD nC ? pp- X
Mf0 0.9570.006 GeV Study also KK- channel
En 30 - 60 GeV Comparison to JETSET where f0
is a fragmentation product made of uudd
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HERMES experiment

• Ebeam 27.5 GeV
• DIS selection
• W2 gt 4 GeV2
• y lt 0.85
• Ecalo (e) gt 3.5 GeV
• Hadron selection
• Pp gt 2.5 GeV

dP/P lt 2.6 , dq lt 0.6 mrad detector eff. 99
(e), RICH eff. 98 (p), 85 (K), 73 (p) r
1015-1016 nucl/cm-2 , gas 1H, 2H, 3He, 4He, 14N,
20Ne, 84Kr

• Tracking
• Particle ID
• Target

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Invariant mass distribution
Ks r0
Channel f0 ? pp- Target hydrogen Event
type ep ? epp- X Mf0 0.9800.007
GeV Gf0 0.0390.015 GeV Significance
4.1s
f0
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Kinematics dependences

• Studied dependences on
• Q2, Pt2, z and n
• in the HERMES acceptance
• Comparison to PYTHIA/JETSET
• f0(980) produced in uu dd
• fragmentation
• good agreement with MC
• peaking of data at ltzgt ? 0.5

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Acceptance corrected results
Using JETSET for acceptance corr. ? model
dependent results !!!

• DIS processes ?? (1/Q2)2
• f0-production at Q2 gt 1 GeV2
• a 1.8 ? 0.3
• consistent with DIS prod.

• Exponential fit to Pt2-dependence
• slope parameter
• b 4.4 ? 1.2 GeV-2
• consistent with NOMAD
• b 5.3 ? 0.2 GeV-2

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Conclusions

• First observation of f0(980) scalar meson
  electro-production
• Mass and width consistent with PDG and previous
  measurements
• Kinematics dependences in acceptance reproduced
  by PYTHIA/JETSET
• - z dependence more peaked in the data
• Extrapolation to 4p using MC ? strongly model
  dependent

- Q2 -dependence of f0 acc. corr. yield
consistent with DIS production mechanism -
Pt2-dependence of f0 acc. corr. yield described
by exponential behavior consistent with NOMAD
findings

• No MC exist for comparison to alternative non-qq
  interpretations

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Outlook

• Optimization of MC tune increased MC
  statistics
• Extraction of multiplicities from acceptance
  corrected yields

• Extraction of f0(980) on nuclear targets (2H, N,
  Ne, Kr)
• ? study A-dependence of yield and peak position
• Investigate p0p0 decay channel

• Comparison to other MC predictions (when
  available!)