Estimate of background baseline and upper limit on the chiral magnetic effect in isobar collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>200</mml:mn></mml:mrow></mml:math> GeV at the BNL Relativistic Heavy Ion Collider
M. I. Abdulhamid, B. E. Aboona, J. Adam, J. R. Adams, G. Agakishiev, Y. Hu, M. M. Aggarwal, Z. Ahammed, A. Aitbaev, I. Alekseev, E. Alpatov, A. Aparin, S. Aslam, J. Atchison, G. S. Averichev, V. Bairathi, J. Ball, K. N. Barish, P. Bhagat, A. Bhasin, S. Bhatta, S. R. Bhosale, I. G. Bordyuzhin, J. D. Brandenburg, A. V. Brandin, C. Broodo, X. Z. Cai, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, J. Ceska, I. Chakaberia, B. K. Chan, Z. Chang, A. Chatterjee, D. Chen, J. Chen, J. H. Chen, Z. Chen, J. Cheng, Y. Cheng, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, G. Dale-Gau, Arpita Das, T. G. Dedovich, I. M. Deppner, A. A. Derevschikov, A. Dhamija, P. Dixit, X. Dong, J. L. Drachenberg, E. Duckworth, J. C. Dunlop, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, O. Eyser, R. Fatemi, S. Fazio, C. Feng, Y. Feng, E. Finch, Y. Fisyak, F. Flor, Changbo Fu, Ting Gao, F. J. M. Geurts, N. Ghimire, S. M. Gibson, K. Gopal, X. Gou, D. Grosnick, A. Gupta, A. Hamed, Y. Han, M. D. Harasty, J. W. Harris, H. Harrison-Smith, Weihua He, X. H. He, Yan He, C. Hu, Q. Hu, Y. Hu, R.G. Huang, H. Z. Huang, S. Huang, Tao Huang, X. Huang, Yanyan Huang, Y. Huang, T. J. Humanic, M. Isshiki, W. W. Jacobs, A. Jalotra, C. Jena
Abstract
For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions ($_{44}^{96}\mathrm{Ru}+_{44}^{96}\mathrm{Ru}, _{40}^{96}\mathrm{Zr}+_{40}^{96}\mathrm{Zr}$) obtained through a blind analysis. The ratio of results in $\mathrm{Ru}+\mathrm{Ru}$ to $\mathrm{Zr}+\mathrm{Zr}$ collisions for the CME-sensitive charge-dependent azimuthal correlator ($\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$), normalized by elliptic anisotropy (${v}_{2}$), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, $Y=\frac{{(\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}/{v}_{2})}^{\text{Ru}}}{{(\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}/{v}_{2})}^{\text{Zr}}}$, is naively expected to be $\frac{{(1/N)}^{\mathrm{Ru}}}{{(1/N)}^{\mathrm{Zr}}}$; however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to $Y$ from those correlations, utilizing both the isobar data and hijing simulations. After including those contributions, we arrive at a final background baseline for $Y$, which is consistent with the isobar data. We extract an upper limit for the CME fraction in the $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$ measurement of approximately $10%$ at a $95%$ confidence level on in isobar collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$, with an expected 15% difference in their squared magnetic fields.