ATLAS Experiment probes the quark-gluon plasma in a new study of photo-produced muon pairs

The circulations of k⊥ in various midpoint periods, where the portion worths show the portion of occasions in a provided midpoint class beginning with the most main occasions (0-5%). The bottom left panel represents the UPC occasions. The pT of both muons in the set is needed to be bigger than 4 GeV . The circulations are stabilized such that the essential of the circulations in each midpoint class amounts to the ratio of dimuon pairs in that midpoint class to the overall number of dimuon pairs throughout all midpoints (consisting of the UPC occasions). The red lines show parameterized fits to the circulations. Credit: ATLAS Collaboration/CERN

At the Large Hadron Collider (LHC) at CERN, the electro-magnetic fields of Lorentz-contracted lead nuclei in heavy-ion crashes function as extreme sources of high-energy photons, or particles of light. This environment enables particle physicists to study photon-induced scattering procedures, which can not be studied somewhere else.

A crucial procedure taken a look at by ATLAS Experiment physicists includes the annihilation of photons into pairs of oppositely charged muons. Such electro-magnetic “two-photon processes” are normally studied in “ultra-peripheral collisions” (UPC), where the transverse separation in between the clashing lead nuclei is higher than the amount of their radii, resulting in no direct strong interactions in between the clashing nuclei. This supplies a tidy environment for the study of electro-magnetic interactions at high energy and strength. Nevertheless, these two-photon procedures are likewise present in crashes where the 2 nuclei overlap (“central collision events”) and produce quark-gluon plasma. The produced muons can, in concept, engage with the charges in the plasma, making the muon pairs produced in two-photon procedures a possibly important probe of the electro-magnetic fields in the plasma.

The ATLAS Collaboration just recently launched a new, detailed measurement of the circulations of muon pairs from two-photon annihilation procedures, in UPC and non-UPC accident occasions. The measurement uses the big dataset tape-recorded throughout the 2015 and 2018 heavy-ion runs of the LHC.

ATLAS physicists discovered that the circulations of muon pairs differed methodically depending upon the “centrality” of the accident (a procedure of how head-on 2 nuclei clash). This habits is measured by the observable k which represents the transverse momentum of the dimuon set perpendicular to the muon instructions. The figure programs the circulation of numerous various midpoint classes, varying from UPC occasions to main accident occasions.

A substantial modification in the circulations is observed from UPC to peripheral to main accident occasions. In specific, for the UPC occasions, the 2 muons are probably to be produced back-to-back, resulting in the k circulations peaking at k = 0 MeV. Nevertheless, in more main crashes with hadronic interactions, the 2 muons are most likely to have a minor shift from being simply back-to-back, resulting the k circulations to have a most possible worth bigger than no. The most possible worth of k⊥ shifts, depending upon the midpoint of the accident occasion, from k = 0 MeV in UPC occasions to k = 36 ± 1 MeV in the 0-5% most main crashes.

These measurements supply new insight into the possible interaction of the outbound muons with electro-magnetic charges or fields present in the quark-gluon plasma. Nevertheless, current estimations recommend that results comparable to those seen in the information might arise from a mix of the preliminary state widening of the photon transverse momenta and from the production procedure itself. Future analyses and extra measurements are required to develop the system(s) accountable for the includes observed in the information.

Zooming in on top-quark production

More details:
Measurement of non-exclusive dimuon pairs produced through γγ scattering in lead-lead crashes at 5.02 TeV with the ATLAS detector (ATLAS-CONF-2019-051): … ATLAS-CONF-2019-051/

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ATLAS Experiment probes the quark-gluon plasma in a new study of photo-produced muon pairs (2019, November 27)
obtained 27 November 2019

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