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

Particle production sensitive to nonfactorizable and nonperturbative processes that contribute to the underlying event associated with a high transverse momentum ($p_T$) jet in $\mathrm{proton}+\mathrm{proton}$ collisions at $\sqrt{s}=200\mathrm{GeV}$ is studied with the STAR detector. Each event is divided into three regions based on the azimuthal angle with respect to the highest-$p_T$ 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, $⟨p_T⟩$, between 0.5 and $0.7\mathrm{GeV}/c$ for particles with $p_T>0.2\mathrm{GeV}/c$ at mid-pseudorapidity ($|\eta |<1$) and jet $p_T>15\mathrm{GeV}/c$. Both average particle density and $⟨p_T⟩$ depend weakly on the leading jet $p_T$. 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-$p_T$ 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 1

Projections of the four-dimensional response matrix for (a) the leading jet $p_T$ and (b) Transverse charged particle multiplicity $N_{\mathrm{ch}}$, each with the other variable integrated. X-axis is the generator-level variable and the Y-axis is the detector-level variable. The bins along the negative axes are for the cases when a matched pair is not found.

Figure 2

Average charged particle multiplicity densities for Toward, Away, and Transverse regions as functions of the leading jet $p_T$, with charged particle $p_T>0.2\mathrm{GeV}/c$. The wide curves are pythia 6 (STAR). The middle width curves are default pythia 6 Perugia 2012 tune. The thin curves are pythia 8 Monash 2013 tune. The solid curves are the Toward region. The sparse dashed curves are the Away region. The dense dashed curves are the Transverse region.

Figure 3

Transverse region average charged particle densities for $p_T>0.2\mathrm{GeV}/c$ (open symbols) and $p_T>0.5\mathrm{GeV}/c$ (filled symbols). Simulations are also shown as curves. The wide curves are pythia 6 (STAR). The middle width curves are default pythia 6 Perugia 2012 tune. The thin curves are pythia 8 Monash 2013 tune.

Figure 4

Charged particle $⟨p_T⟩$ for Toward, Away, and Transverse regions as functions of the leading jet $p_T$, with charged particle $p_T>0.2\mathrm{GeV}/c$. Simulations are also shown as curves. The wide curves are pythia 6 (STAR). The middle width curves are default pythia 6 Perugia 2012 tune. The thin curves are pythia 8 Monash 2013 tune. Note the three curves overlap for the Transverse region calculations.

Figure 5

Transverse region charged particle $⟨p_T⟩$ as a function of the leading jet $p_T$ for $p_T>0.2\mathrm{GeV}/c$ (open symbols) and $p_T>0.5\mathrm{GeV}/c$ (filled symbols). Simulations are also shown as curves. The wide curves are pythia 6 (STAR). The middle width curves are default pythia 6 Perugia 2012 tune. The thin curves are pythia 8 Monash 2013 tune.

Figure 6

Detector-level average charged particle multiplicity densities for TransMax and TransMin as functions of the leading jet $p_T$. Points are measured data. Histograms are calculated values assuming TransMax and TransMin are derived from the same parent distribution. See text for details.

Figure 7

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