References - Bogoliubov Laboratory of Theoretical Physics - JINR

of electroproduction from polarized targets is richer than that for polarized lepton beams
and unpolarized targets, and the formalism was generalized for the case of polarized targets.
Recently a new general formalism for the description of vector-meson SDMEs was
proposed in [3]. From the symmetry properties [1–3] of the helicity amplitudes it follows
that there are 18 independent amplitudes. Since the SDMEs depend on ratios of these
amplitudes, there are 34 independent real parameters needed to determine all the SDMEs.
However the number of SDMEs for a given measurement can exceed 34 as, for example,
in the case of the 45 [3] SDMEs observable from electroproduction from a transversely polarized
target with an unpolarized lepton beam. This means that the SDMEs themselves
are not independent and amplitude ratios Ai provide a more economic basis for fitting
the angular distributions of the decay particles. The hierarchy of amplitudes established
at HERMES kinematics [4], namely
|T00| 2 ∼|T11| 2 ≫|U11| 2 > |T01| 2 ≫|T10| 2 ∼|T1−1| 2 , (2)
permits one to reduce the number of essential free amplitude ratios to 5: A1 =Re(T11/T00),
A2 =Im(T11/T00), A3 =Re(T01/T00), A4 =Im(T01/T00), and A9 = |U11/T00|. The ratios
A5 − A8 are found to be very small, where A5 =Re(T10/T00), A6 =Im(T10/T00),
A7 =Re(T1−1/T00), A8 =Im(T1−1/T00). The quantities TλV λγ are a shorthand notation
for the amplitudes TλV 1 1
λγ
2 2
for natural-parity exchange (NPE) without nucleon spin flip;
| 2 +
the amplitude of unnatural-parity exchange (UPE) is UλV νN λγλN and |U11| 2 = |U 1 1
2
|U 1− 1
2
1 1
2
| 2 . These NPE and UPE amplitudes are related to helicity amplitudes FλV νN λγλN
by the equations [1–3] TλV νN λγλN =(FλV νN λγλN +(−1)λV −λγ F−λV νN −λγλN )/2, UλV νN λγλN =
)/2. In Regge phenomenology, the NPE amplitudes
(FλV νN λγλN − (−1)λV −λγF−λV νN −λγλN
correspond to exchanges of reggeons of natural parity with P =(−1) J (Pomeron, ρ, f2,
a2, ...), whereas exchanges of reggeons of unnatural parity with P = −(−1) J (π, a1, b1,
...) contribute to the UPE amplitudes.
2 Selection of Exclusive Vector Meson Events
The ρ 0 and φ events used for the SDME analayis were produced in DIS of longitudinally
polarized electrons and positrons with an energy 27.57 GeV from unpolarized hydrogen
and deuterium targets; the ω events were collected from DIS with a transversely polarized
proton and unpolarized beam. The scattered electron (positron) and particles from the
decays ρ 0 → π + π − , φ → K + K − , ω → π + π − π 0 (π 0 → 2γ) were detected and identified
by the HERMES spectrometer [5]. In the period 1996-2000 particles were identified
using a threshold Čerenkov detector, lead glass electromagnetic calorimeter and preshower
detector; since 2000 a dual-radiator RICH detector [6] replaced the threshold detector.
The constraints 0.6