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Underlying event measurements in $p+p$ collisions at $\sqrt s=$ 200 GeV at RHIC

The STAR collaboration
Phys.Rev.D 101 (2020) 052004, 2020.

Abstract (data abstract)
Particle production sensitive to nonfactorizable and nonperturbative processes that contribute to the underlying event associated with a high transverse momentum (pT) jet in proton+proton collisions at sqrt(s)=200 GeV is studied with the STAR detector. Each event is divided into three regions based on the azimuthal angle with respect to the highest-pT jet direction: in the leading jet direction ("Toward"), opposite to the leading jet ("Away"), and perpendicular to the leading jet ("Transverse"). In the Transverse region, the average charged particle density is found to be between 0.4 and 0.6 and the mean transverse momentum, <pT>, between 0.5 and 0.7 GeV/c for particles with pT >0.2 GeV/c at mid-pseudorapidity (|eta|<1) and jet pT >15 GeV/c. Both average particle density and <pT> depend weakly on the leading jet pT. Closer inspection of the Transverse region hints that contributions to the underlying event from initial- and final-state radiation are significantly smaller in these collisions than at the higher energies, up to 13 TeV, recorded at the LHC. Underlying event measurements associated with a high-pT jet will contribute to our understanding of QCD processes at hard and soft scales at RHIC energies, as well as provide constraints to modeling of underlying event dynamics.

  • Figure 2

    Data from Figure 2


    Average charged particle multiplicity densities for Toward, Away, and Transverse regions as functions of the leading jet pT, with charged...

  • Figure 3

    Data from Figure 3


    Transverse region average charged particle densities for pT>0.2 GeV/c (open symbols) and pT>0.5 GeV/c (filled symbols). Simulations are also shown...

  • Figure 4

    Data from Figure 4


    Charged particle for Toward, Away, and Transverse regions as functions of the leading jet pT, with charged particle pT>0.2...

  • Figure 5

    Data from Figure 5


    Transverse region charged particle as a function of the leading jet pT for pT>0.2 GeV/c (open symbols) and pT>0.5...

  • Figure 6

    Data from Figure 6


    Detector-level average charged particle multiplicity densities for TransMax and TransMin as functions of the leading jet pT. Points are measured...

  • Figure 7

    Data from Figure 7


    Transverse charged particle densities at various center-of-mass collision energies. Uncertainties smaller than the marker sizes.

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