Helicity Density Matrix Elements Research Articles

Off-diagonal helicity density matrix elements for vector mesons produced in polarized $e^+e^-$ processes

Final-state $q \bar q$ interactions give origin to nonzero values of the off-diagonal element $\rho_{1,-1}$ of the helicity density matrix of vector mesons produced in $e^+ e^-$ annihilations, as has been confirmed by recent OPAL data on $\phi$ , $D^*$ , and $K^*$ . New predictions are given for $\rho_{1,-1}$ of several mesons produced at large $x_{_\mathrm{E}}$ and small $p_\mathrm{T}$ – i.e., collinear with the parent jet – in the annihilation of polarized $e^+$ and $e^-$ ; the results depend strongly on the elementary dynamics and allow further nontrivial tests of the standard model.

A study of spin alignment of $\rho(770)^{\pm}$ and $\omega(782)$ mesons in hadronic Z $^0$ decays

The helicity density matrix elements $\rho_{00}$ of $\rho(770)^{\pm}$ and $\omega(782)$ mesons produced in Z $^0$ decays have been measured using the OPAL detector at LEP. Over the measured meson energy range, the values are compatible with 1/3, corresponding to a statistical mix of helicity $-1$ , 0 and +1 states. For the highest accessible scaled energy range 0.3 $<$ $x_E$ $<$ 0.6, the measured $\rho_{00}$ values of the $\rho^{\pm}$ and the $\omega$ are 0.373 $\pm$ 0.052 and 0.142 $\pm$ 0.114, respectively. These results are compared to measurements of other vector mesons.

Off-diagonal helicity density matrix elements for vector mesons produced in polarized

Final state quark-antiquark interactions give origin to non zero values of the off-diagonal element rho_{1,-1} of the helicity density matrix of vector mesons produced in e+e- annihilations, as confirmed by recent OPAL data on Phi, D^* and K^*'s. New predictions are given for rho_{1,-1} of several mesons produced at large x_E and small p_T -- i.e. collinear with the parent jet -- in the annihilation of polarized e+ and e-; the results depend strongly on the elementary dynamics and allow further non trivial tests of the Standard Model.

Off-diagonal helicity density matrix elements for heavy vector mesons inclusively produced in NN, γN and ℓ N interactions

Final state interactions in quark fragmentation may give origin to non zero values of the off-diagonal element ρ 1,−1 of the helicity density matrix of vector mesons V produced in current jets, with a large energy fraction x E ; the value of ρ 1,−1( V) is related to the hard constituent dynamics and tests unusual properties of it. Some recent data on φ, K ∗ and D ∗ produced in e + e − annihilations at LEP show such effects. Predictions are given here for ρ 1,−1 of heavy mesons produced in nucleon-nucleon, γ-nucleon and ℓ-nucleon interactions.

Off-diagonal helicity density matrix elements for vector mesons produced at LEP

Final state $q\bar q$ interactions may give origin to non-zero values of the off-diagonal element $\rho_{1,-1}$ of the helicity density matrix of vector mesons produced in $e^+ e^-$ annihilations, as confirmed by recent OPAL data on $\phi$ and $D^*$ 's. Predictions are given for $\rho_{1,-1}$ of several mesons produced at large $z$ and small $p_T$ , i.e. collinear with the parent jet; the values obtained for $\phi$ and $D^*$ are in agreement with data. The fragmentation functions of quarks and gluons into photons are studied beyond the Leading Logarithm approximation. We address the nature of the initial conditions of the evolution equation solutions and study problems related to factorization scheme invariance. The possibility of measuring these distributions in LEP experiments is discussed, and a comparison with existing data is made.

Spin alignment of leading K∗(892)0 mesons in hadronic Z0 decays

Helicity density matrix elements for inclusive K ∗ (892) 0 mesons from hadronic Z 0 decays have been measured over the full range of K ∗ 0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x p values above 0.3, with the matrix element ϱ 00 rising to 0.66 ± 0.11 for x p > 0.7. The values of the real part of the off-diagonal element ϱ 1 - 1 are negative at large x p , with a weighted average value of −0.09 ± 0.03 for x p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the q q system from the Z 0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x p range. The K ∗ 0 fragmentation function has also been measured and the total rate determined to be 0.74 ± 0.02 ± 0.02 K ∗ (892) 0 mesons per hadronic Z 0 decay.

Off-diagonal helicity density matrix elements for vector mesons produced at LEP

Final state quark-antiquark interactions may give origin to non zero values of the off-diagonal element rho_{1,-1} of the helicity density matrix of vector mesons produced in e+e- annihilations, as confirmed by recent OPAL data on phi and D^*'s. Predictions are given for rho_{1,-1} of several mesons produced at large z and small p_T, i.e. collinear with the parent jet; the values obtained for phi and D^* are in agreement with data.

Study of $\phi(1020)$ , ${\rm D}^{*\pm}$ and B $^*$ spin alignment in hadronic Z $^0$ decays

Measurements of helicity density matrix elements have been made for the φ(1020), D*± and B* vector mesons in multihadronic Z0 decays in the OPAL experiment at LEP. Results for inclusive φ produced with high energy show evidence for production preferentially in the helicity zero state, with ρ00 = 0.54 ± 0.08, compared to the value of 1/3 expected for no spin alignment. The corresponding element for the D*± has a value of 0.40 ± 0.02, also suggesting a deviation from 1/3. The B* result, with ρ00 = 0.36 ± 0.09, is consistent with no spin alignment. Off-diagonal elements have been measured for the f and D* mesons; for the D* the element Re ρ1−1 is non-zero, indicating non-independent fragmentation of the primary quarks.

Top quark production at LEP energies

Top quark pair production is analyzed for e +e − annihilation at LEP energies. γ−Z interference and the heavy mass of the quark cause the non-diagonal helicity density matrix elements of the produced quark to be sizeable. The detection of this feature through the angular distribution of the decay products is discussed. Coherence effects in the fragmentation process are also considered.

General relations between photoproduction of pions and vector meson production by πN collision based on the photon-vector-meson analogy model

The relations among differential cross section, polarization and helicity density matrix elements in γN → πN and πN → VN processes are given for the photon-vector-meson analogy model. For example, the differential cross sections of γN → πN processes are given in terms of density matrix elements and differential cross sections of πN → ϱN processes, and the relations fit the experimental data fairly well.