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Register a new userMeasurement of Groomed Jet Substructure Observables in pp Collisions at $sqrt{s} = 200$ GeV with STAR
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Raghav Kunnawalkam Elayavalli
Publication date
2020
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English
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In this letter, measurements of the shared momentum fraction ($z_{rm{g}}$) and the groomed jet radius ($R_{rm{g}}$), as defined in the SoftDrop algorihm, are reported in pp collisions at $sqrt{s} = 200$ GeV collected by the STAR experiment. These substructure observables are differentially measured for jets of varying resolution parameters from $R = 0.2 - 0.6$ in the transverse momentum range $15 < p_{rm{T, jet}} < 60$ GeV$/c$. These studies show that, in the $p_{rm{T, jet}}$ range accessible at $sqrt{s} = 200$ GeV and with increasing jet resolution parameter and jet transverse momentum, the $z_{rm{g}}$ distribution asymptotically converges to the DGLAP splitting kernel for a quark radiating a gluon. The groomed jet radius measurements reflect a momentum-dependent narrowing of the jet structure for jets of a given resolution parameter, i.e., the larger the $p_{rm{T, jet}}$, the narrower the first splitting. For the first time, these fully corrected measurements are compared to Monte Carlo generators with leading order QCD matrix elements and leading log in the parton shower, and to state-of-the-art theoretical calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with parameters tuned to reproduce RHIC measurements is able to quantitatively describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are unable to provide a simultaneous description of both $z_{rm{g}}$ and $R_{rm{g}}$, resulting in opportunities for fine parameter tuning of these models for pp collisions at RHIC energies. We also find that the theoretical calculations without non-perturbative corrections are able to qualitatively describe the trend in data for jets of large resolution parameters at high $p_{rm{T, jet}}$, but fail at small jet resolution parameters and low jet transverse momenta.
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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.
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.
We report our measurement on longitudinal spin transfer, D_LL, from high energy polarized protons to $Lambda$ and $bar{Lambda}$ hyperons in proton-proton collisions at $sqrt{s} = 200 GeV$ with the STAR detector at RHIC. The current measurements cover $Lambda$, $barLambda$ pseudorapidity $|eta| < 1.2$ and transverse momenta $p_T$ up to $4GeV/c$ using the data taken in 2005. The longitudinal spin transfer is found to be D_LL= -0.03pm 0.13 (stat) pm 0.04(syst)$ for inclusive $Lambda$ and $D_{LL} = -0.12 pm 0.08(stat) pm 0.03(syst)$ for inclusive $bar{Lambda}$ hyperons with $<eta> = 0.5$ and $<p_T> = 3.7 GeV/c$. The prospects with 2009 data and the future measurements are also given.
We report the first measurement of the longitudinal double-spin asymmetry $A_{LL}$ for mid-rapidity di-jet production in polarized $pp$ collisions at a center-of-mass energy of $sqrt{s} = 200$ GeV. The di-jet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. $A_{LL}$ results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. The measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-$x > 0.05$.
We report the measurement of charged $D^*$ mesons in inclusive jets produced in proton-proton collisions at a center of mass energy $sqrt{s}$ = 200 GeV with the STAR experiment at RHIC. For $D^{*}$ mesons with fractional momenta $0.2 < z < 0.5$ in inclusive jets with 11.5 GeV mean transverse energy, the production rate is found to be $N(D^{*+}+D^{*-})/N(mathrm{jet}) = 0.015 pm 0.008 (mathrm{stat}) pm 0.007 (mathrm{sys})$. This rate is consistent with perturbative QCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.
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