Evidence for Nonlinear Gluon Effects in QCD and Their Mass Number Dependence at STAR
M. S. Abdallah, B. E. Aboona, J. Adam, L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, Y. Hu, M. M. Aggarwal, Z. Ahammed, I. Alekseev, D. M. Anderson, A. Aparin, E. C. Aschenauer, M. U. Ashraf, F. G. Atetalla, A. Attri, G. S. Averichev, V. Bairathi, W. Baker, J. Ball, K. N. Barish, A. Behera, R. Bellwied, P. Bhagat, A. Bhasin, J. Bielčík, J. Bielčíková, I. G. Bordyuzhin, J. D. Brandenburg, A. V. Brandin, I. Bunzarov, X. Z. Cai, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, I. Chakaberia, P. Chaloupka, B. K. Chan, F-H. Chang, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, Surajit Chattopadhyay, D. Chen, J. Chen, J. H. Chen, X. Chen, Z. Chen, J. Cheng, M. Chevalier, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csanád, M. Daugherity, Т. Г. Дедович, I. M. Deppner, A. A. Derevschikov, A. Dhamija, L. Di Carlo, L. Didenko, P. Dixit, X. Dong, J. L. Drachenberg, E. Duckworth, J. C. Dunlop, N. Elsey, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, F. M. Fawzi, S. Fazio, P. Federic, J. Fedorišin, C. Feng, Y. Feng, P. Filip, E. Finch, Y. Fisyak, A. Francisco, C. Fu, Ł. Fulek, C. A. Gagliardi, T. Galatyuk, F. J. M. Geurts, N. Ghimire, S. M. Gibson, K. Gopal, X. Gou, D. Grosnick, A. Gupta, W. Guryn, A. I. Hamad, A. Hamed
Abstract
The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-${\ensuremath{\pi}}^{0}$s produced at forward pseudorapidities ($2.6<\ensuremath{\eta}<4.0$) in $p+p$, $p+\mathrm{Al}$, and $p+\mathrm{Au}$ collisions at a center-of-mass energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back ${\ensuremath{\pi}}^{0}$ pairs in $p+\mathrm{Al}$ and $p+\mathrm{Au}$ collisions compared to the $p+p$ data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in $p+\mathrm{Au}$ relative to $p+\mathrm{Al}$ collisions exhibits a dependence of the saturation scale ${Q}_{s}^{2}$ on the mass number $A$. A linear scaling of the suppression with ${A}^{1/3}$ is observed with a slope of $\ensuremath{-}0.09\ifmmode\pm\else\textpm\fi{}0.01$.