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STAR results and perspectives on transverse spin asymmetries | | N. Poljak
; STAR collaboration
; | | Date: |
3 Nov 2011 | | Abstract: | The STAR collaboration reported precision measurements on the transverse
single spin asymmetries for the production of forward $pi^0$ mesons from
polarized proton collisions at $sqrt{s}=$ 200 GeV. To disentangle
contributions to measured forward asymmetries one has to look beyond inclusive
$pi^0$ production to the production of forward jets or direct photons. In
2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8
pb$^{-1}$ of forward data with an average polarization of 60%. FPD++ had
sufficient acceptance for "jet-like" objects, which are clustered responses of
an electromagnetic calorimeter primarily sensitive to incident photons,
electrons and positrons. For these objects, the angle of the outgoing leading
$pi^0$ with respect to the fragmenting parton was reconstructed, thus enabling
us to disentangle the contributions to the forward $pi^0$ asymmetries. The
simulated data set shows that on average there are approximately 2.5
fragmenting mesons per one "jet-like" object, making them reasonably "jetty".
Preliminary results provide no evidence of measured contributions to the
asymmetry from jet fragmentation, implying the Sivers distribution functions
play a substantial role in producing the large inclusive forward $pi^0$
asymmetries. A similar effort was made in the mid-rapidity $(|eta|<1)$ region
of the STAR detector, where 2.2 pb$^{-1}$ of data was collected. We present
progress made by making measurements of the azimuthal asymmetry of leading
charged pions in jets produced by transversely polarized proton collisions. | | Source: | arXiv, 1111.0755 | | Services: | Forum | Review | PDF | Favorites | |
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