Lambda Hyperon Production Research Articles

Lambda hyperon production and polarization in collisions of p(3.5 GeV)+Nb

Results on $ \Lambda$ hyperon production are reported for collisions of p(3.5 GeV) + Nb , studied with the High-Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. The transverse mass distributions in rapidity bins are well described by Boltzmann shapes with a maximum inverse slope parameter of about 90 MeV at a rapidity of y = 1.0, i.e. slightly below the center-of-mass rapidity for nucleon-nucleon collisions, $ y_{cm}=1.12$ . The rapidity density decreases monotonically with increasing rapidity within a rapidity window ranging from 0.3 to 1.3. The $ \Lambda$ phase-space distribution is compared with results of other experiments and with predictions of two transport approaches which are available publicly. None of the present versions of the employed models is able to fully reproduce the experimental distributions, i.e. in absolute yield and in shape. Presumably, this finding results from an insufficient modelling in the transport models of the elementary processes being relevant for $ \Lambda$ production, rescattering and absorption. The present high-statistics data allow for a genuine two-dimensional investigation as a function of phase space of the self-analyzing $ \Lambda$ polarization in the weak decay $ \Lambda\rightarrow p\pi^{-}$ . Finite negative values of the polarization in the order of 5-20% are observed over the entire phase space studied. The absolute value of the polarization increases almost linearly with increasing transverse momentum for p t > 300 MeV/c and increases with decreasing rapidity for y < 0.8 .

QCD analysis of Lambda hyperon production in DIS target-fragmentation region

We consider Lambda-hyperon production in the target-fragmentation region of semi-inclusive deep-inelastic scattering within the framework of fracture functions. We present a first attempt to determine the flavour and energy dependences of these non-perturbative distributions through a simultaneous QCD-based fit to available neutral- and charged-current semi-inclusive-DIS cross sections. Predictions based on the resulting nucleon-to-Lambda fracture functions are in good agreement with data and observables not included in the regression. The successful prediction of the $Q^2$ dependence of the Lambda multiplicity notably represents the first validation of the perturbative framework implied by fracture functions.

Manifestation of s → Λ fragmentation matrix elements via transverse Λ polarization in unpolarized e−e + annihilation

Making use of the collinear expansion technique developed by Ellis, Furmanski and Petrozio, and the special propagator concept invented by Qiu, we present a factorization approach to the photon fragmentation tensor for the inclusive Lambda hyperon production. As a result, the structure function F , which is related to the transverse polarization of the inclusive Lambda hyperon in unpolarized electron-positron annihilation, is expressed as a combination of four parton fragmentation matrix elements. Since the inclusive Λ production we considered is theoretically the simplest, our study can be taken as a question to the operatability and practability of the first nonleading QCD factorization theorem in the transverse Λ polarization phenomena.