Abstract
We report on the first measurements of production at very low transverse momentum () in hadronic collisions at and collisions at . Remarkably, the inferred nuclear modification factor of at midrapidity in () collisions reaches about 24 (52) for in the 60%–80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the distribution of for the very low range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semicentral collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of yield observed at extremely low originates from coherent photon-nucleus interactions. In particular, coherently produced ’s in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.
- Received 25 April 2019
- Revised 21 July 2019
DOI:https://doi.org/10.1103/PhysRevLett.123.132302
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.
Published by the American Physical Society