Improving results on transverse double spin asymmetries in the CNI region at STAR
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mproving Results on Transverse Double Spin Asymmetries in the CNI region at STAR1 D. Svirida for the STAR Collaboration Institute for Theoretical and experimental Physics, ul. B. Cheremushkinskaya 25, Moscow, 117218 Russia Abstract—Double spin effects in polarized ppelastic scattering in the Coulomb nuclear interference (CNI) region are sensitive to small contributions to the nuclear amplitude in addition to Pomeron exchange domi nating at high energies. Measurements of double spin asymmetries require external luminosity normalization using collision counts for all spin combinations. Several possible sources of such data from various STAR sub systems were thoroughly analyzed to make the best choice. BBC arrays were found to be free of double spin effects to the level of ~ 2 × 10–4 thus leading to the systematic uncertainty ~10–3 in the value of (ANN + ASS)/2. DOI: 10.1134/S1063779614011000 1
At high energies and very small momentum transfer protonproton elastic scattering is described by the interference of the Coulomb and nuclear amplitudes. The latter is believed to be totally dominated by the Pomeron exchange. In this case no double spin flip amplitude is present in the nuclear term and only very small double spin asymmetries can be expected due to the electromagnetic component. The manifestation of nonnegligible transverse double spin effects would point to possible contributions from other Reggeons, such as the hypothetical Odderon, to the scattering amplitude. Transverse double spin asymmetries ANN and ASS for the elastic protonproton scattering are expressed in terms of helicity amplitudes as [1]: 2 dσ 4π A NN = 2 { 2 φ 5 + Re ( φ *1 φ 2 – φ *3 φ 4 ) }, dt s dσ 4π A SS = 2 { Re ( φ 1 φ *2 + φ 3 φ *4 ) }. dt s
In each amplitude, both electromagnetic and nuclear terms should be taken into account in the CNI region. The largest contributions come from φ1 and φ3, the dominant nonspinflip amplitudes. The electromag netic part of φ5 is small due to a kinematic factor, while its hadronic part was measured to be small in [2]. Given that φ4 is also kinematically suppressed, one obtains approximate expectations for double spin parameters: A NN + A SS dσ 4π ≈ 2 Reφ 1 φ 2* , A NN – A SS ≈ 0. 2 dt s Thus ANN + ASS is a probe for ϕ2, while ANN – ASS is supposed to be small. 1 The article is published in the original.
For the transversely polarized beams the angular distribution of the scattered protons is given by the general formula: 2
d σ = dσ ( 1 + ( P + P )A cos ϕ 2π B Y N dtdϕ dt 2
2
+ P B P Y ( A NN cos ϕ + A SS sin ϕ ) ), where PB and PY are the signed values of the polariza tion of the two beams and the double spin term can be expressed in the form: ε NN ( ϕ ) = P B P Y ( ( A NN + A SS )/2 + ( A NN – A SS )/2 × cos 2ϕ ). Thus, while (ANN – ASS) can be extracted from the angular distributions, (ANN + ASS) effectively manifests only as the crosssection difference for parallel and antiparallel combinations of the beam spin direction
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