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Register a new userInvariant Jet Mass Measurements in $pp$ Collisions at $sqrt{s} = 200$ GeV at RHIC
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We present the first inclusive measurements of the invariant and SoftDrop jet mass in proton-proton collisions at $sqrt{s}=200$ GeV at STAR. The measurements are fully corrected for detector effects, and reported differentially in both the jet transverse momentum and jet radius parameter. We compare the measurements to established leading-order Monte Carlo event generators and find that STAR-tuned PYTHIA-6 reproduces the data, while LHC tunes of PYTHIA-8 and HERWIG-7 do not agree with the data, providing further constraints on parameter tuning. Finally, we observe that SoftDrop grooming, for which the contribution of wide-angle non-perturbative radiation is suppressed, shifts the jet mass distributions into closer agreement with the partonic jet mass as determined by both PYTHIA-8 and a next-to-leading-logarithmic accuracy perturbative QCD calculation. These measurements complement recent LHC measurements in a different kinematic region, as well as establish a baseline for future jet mass measurements in heavy-ion collisions at RHIC.
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Jets are algorithmic proxies of hard scattered partons, i.e. quarks and gluons, in high energy particle collisions. The STAR collaboration presents the first measurements of substructure observables at the first, second and third splits in the jet clustering tree via the iterative SoftDrop procedure. For each of these splits, we measure the fully corrected groomed shared momentum fraction (zg) and groomed jet radius (rg). We discuss the evolution of jet substructure in both the angular and momentum scales which allows for a self-similarity test of the DGLAP splitting function. We compare the fully corrected data to Monte Carlo models, providing stringent constraints on model parameters related to the parton shower and non-perturbative effects such as hadronization.
Particle production sensitive to non-factorizable and non-perturbative processes that contribute to the underlying event associated with a high transverse momentum ($p_{T}$) 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-$p_{T}$ 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, $langle p_{T}rangle$, between 0.5-0.7 GeV/$c$ for particles with $p_{T}$$>$0.2 GeV/$c$ at mid-pseudorapidity ($|eta|$$<$1) and jet $p_{T}$$>$15 GeV/$c$. Both average particle density and $langle p_{T}rangle$ depend weakly on the leading jet $p_{T}$. Closer inspection of the Transverse region hints that contributions to the underlying event from initial- and final-state radiation are significantly smaller in these collisions than at the higher energies, up to 13 TeV, recorded at the LHC. Underlying event measurements associated with a high-$p_{T}$ jet will contribute to our understanding of QCD processes at hard and soft scales at RHIC energies, as well as provide constraints to modeling of underlying event dynamics.
We present the first measurements of the longitudinal double-spin asymmetry $A_{LL}$ for dijets with at least one jet reconstructed within the pseudorapidity range $0.8 < eta < 1.8$. The dijets were measured in polarized $pp$ collisions at a center-of-mass energy $sqrt{s}$ = 200 GeV. Values for $A_{LL}$ are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction $x$ down to $x sim$ 0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on $Delta g(x)$ in this poorly constrained region when included in future global analyses.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured low mass vector meson, $omega$, $rho$, and $phi$, production through the dimuon decay channel at forward rapidity ($1.2<|y|<2.2$) in $p$$+$$p$ collisions at $sqrt{s}=200$ GeV. The differential cross sections for these mesons are measured as a function of both $p_T$ and rapidity. We also report the integrated differential cross sections over $1<p_T<7$ GeV/$c$ and $1.2<|y|<2.2$: $dsigma/dy(omega+rhorightarrowmumu) = 80 pm 6 mbox{(stat)} pm 12 mbox{(syst)}$ nb and $dsigma/dy(phirightarrowmumu) = 27 pm 3 mbox{(stat)} pm 4 mbox{(syst)}$ nb. These results are compared with midrapidity measurements and calculations.
The differential cross section for the production of direct photons in p+p collisions at sqrt(s)=200 GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider. Inclusive-direct photons were measured in the transverse-momentum range from 5.5--25 GeV/c, extending the range beyond previous measurements. Event structure was studied with an isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good description of the spectrum. When the cross section is expressed versus x_T, the PHENIX data are seen to be in agreement with measurements from other experiments at different center-of-mass energies.
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