DIS2003Erika Garutti1 Electro-production of f 0 (980) scalar meson at HERMES Erika Garutti (DESY) On behalf of the HERMES collaboration Physics introduction on scalar mesons - present knowledge on f 0 (980) Observation of f 0 (980) in the semi-inclusive     channel - mass peak determination - study of kinematics dependences - extrapolation to 4 

DIS2003Erika Garutti2 Why are scalar mesons interesting? Scalar mesons in Constituent Valence Model: - qq state with J PC = 0 ++  S=1, L=1 - expected mass ~ 1.2 – 1.5 GeV  but f 0 (980), a 0 (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)

DIS2003Erika Garutti3 Present knowledge on f 0 (980) E687:  N      X OPAL:        X NOMAD: C      X PDG: M f0 = ±0.010 GeV  f0 = GeV E  = 220 GeV M f0 = 0.968± GeV  f0 = 0.050± GeV M f0 = 0.957±0.006 GeV Study also     channel E = GeV Comparison to JETSET where f 0 is a fragmentation product made of uu+dd

DIS2003Erika Garutti4 HERMES experiment  P/P < 2.6 %,  mrad detector eff. 99% (e+), RICH eff. 98%  p)      nucl/cm -2, gas: 1 H, 2 H, 3 He, 4 He, 14 N, 20 Ne, 84 Kr Tracking: Particle ID: Target: E beam = 27.5 GeV DIS selection: W 2 > 4 GeV 2 y < 0.85 E calo (e + ) > 3.5 GeV Hadron selection: P  > 2.5 GeV

DIS2003Erika Garutti5 Invariant mass distribution Channel: f       Target: hydrogen Event type: ep   e     X M f0 = 0.980±0.007 GeV  f0 = 0.039±0.015 GeV Significance: 4.1   K s  0 f0f0

DIS2003Erika Garutti6 Kinematics dependences Studied dependences on: Q 2, P t 2, z and in the HERMES acceptance Comparison to PYTHIA / JETSET : f 0 (980) produced in uu + dd fragmentation  good agreement with MC  peaking of data at  0.5

DIS2003Erika Garutti7 Acceptance corrected results DIS processes  (1/Q 2 ) 2  f 0 -production at Q 2 > 1 GeV 2  = 1.8  0.3 consistent with DIS prod. Exponential fit to P t 2 -dependence  slope parameter b= 4.4  1.2 GeV -2 consistent with NOMAD b= 5.3  0.2 GeV -2 Using JETSET for acceptance corr.  model dependent results !!!

DIS2003Erika Garutti8 Conclusions First observation of f 0 (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 4  using MC  strongly model dependent - Q 2 -dependence of f 0 acc. corr. yield consistent with DIS production mechanism - P t 2 -dependence of f 0 acc. corr. yield described by exponential behavior consistent with NOMAD findings No MC exist for comparison to alternative non-qq interpretations

DIS2003Erika Garutti9 Outlook Optimization of MC tune + increased MC statistics Extraction of multiplicities from acceptance corrected yields Extraction of f 0 (980) on nuclear targets ( 2 H, N, Ne, Kr)  study A-dependence of yield and peak position Investigate      decay channel Comparison to other MC predictions (when available!)