Probing the Effects of Strong Electromagnetic Fields with Charge-Dependent Directed Flow in Pb-Pb Collisions at the LHC
S. Acharya, D. Adamová, A. Adler, Jonatan Adolfsson, Madan Mohan Aggarwal, G. Aglieri Rinella, M. Agnello, N. Agrawal, Z. Ahammed, S. Ahmad, S. U. Ahn, A. Akindinov, M. Al-Turany, S. N. Alam, D. S. D. Albuquerque, D. Aleksandrov, B. Alessandro, H. M. Alfanda, R. Alfaro Molina, B. Ali, Y. Ali, A. Alici, A. Alkin, J. Alme, T. Alt, L. Altenkamper, I. Altsybeev, M. N. Anaam, C. Andrei, D. Andreou, H. A. Andrews, A. Andronic, M. Angeletti, V. Anguelov, C. Anson, T. Antičić, F. Antinori, P. Antonioli, R. Anwar, N. Apadula, L. Aphecetche, H. Appelshäuser, S. Arcelli, R. Arnaldi, M. Arratia, I. C. Arsene, M. Arslandok, A. Augustinus, R. Averbeck, S. Aziz, M. D. Azmi, A. Badalà, Y. W. Baek, S. Bagnasco, Xiaozhi Bai, R. Bailhache, R. Bala, A. Baldisseri, M. Ball, Samah Balouza, R. Barbera, L. Barioglio, G. G. Barnaföldi, L. S. Barnby, V. Barret, P. Bartalini, K. Barth, E. Bartsch, F. Baruffaldi, N. Bastid, S. Basu, G. Batigne, B. Batyunya, D. Bauri, J. L. Bazo Alba, I. G. Bearden, C. Bedda, N. K. Behera, I. Belikov, A. D. C. Bell Hechavarria, F. Bellini, R. Bellwied, V. Belyaev, G. Bencedi, S. Beolè, A. Bercuci, Y. Berdnikov, D. Berényi, R. A. Bertens, D. Berzano, M. G. Besoiu, L. Betev, A. Bhasin, I. R. Bhat, M. A. Bhat, H. Bhatt, B. Bhattacharjee, A. Bianchi, L. Bianchi, N. Bianchi
Abstract
The first measurement at the LHC of charge-dependent directed flow (${v}_{1}$) relative to the spectator plane is presented for Pb-Pb collisions at $\sqrt{{s}_{\mathrm{NN}}}=5.02\text{ }\text{ }\mathrm{TeV}$. Results are reported for charged hadrons and ${\mathrm{D}}^{0}$ mesons for the transverse momentum intervals ${p}_{\mathrm{T}}>0.2\text{ }\text{ }\mathrm{GeV}/c$ and $3<{p}_{\mathrm{T}}<6\text{ }\text{ }\mathrm{GeV}/c$ in the 5%--40% and 10%--40% centrality classes, respectively. The difference between the positively and negatively charged hadron ${v}_{1}$ has a positive slope as a function of pseudorapidity $\ensuremath{\eta}$, $\mathrm{d}\mathrm{\ensuremath{\Delta}}{v}_{1}/\mathrm{d}\ensuremath{\eta}=\phantom{\rule{0ex}{0ex}}[1.68\ifmmode\pm\else\textpm\fi{}0.49(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.41(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. The same measurement for ${\mathrm{D}}^{0}$ and ${\overline{\mathrm{D}}}^{0}$ mesons yields a positive value $\mathrm{d}\mathrm{\ensuremath{\Delta}}{v}_{1}/\mathrm{d}\ensuremath{\eta}=[4.9\ifmmode\pm\else\textpm\fi{}1.7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.6(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}$, which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed $\mathrm{\ensuremath{\Delta}}{v}_{1}$ of charged hadrons and ${\mathrm{D}}^{0}$ mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of $\mathrm{d}\mathrm{\ensuremath{\Delta}}{v}_{1}/\mathrm{d}\ensuremath{\eta}$ for both light flavor and charmed hadrons.