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Target journal: Physics Letters B
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Paper Draft:
Current Version: Link
version 1: Link
version 2: Link

Analysis Note:
Current Version: Link

Diff pdf file between v2 and v1: Link

Abstract:
The semi-inclusive distribution of hadron-triggered recoil jets (h+jet) serves as an effective probe for studying the properties of the quark-gluon plasma (QGP) through the jet quenching phenomenon. This observable is increasingly being utilized in the heavy-ion field due to its unique capability to measure jets with low transverse momenta (p_{T}) and large radii — a regime critical for probing the jet-QGP interactions. In this paper, we report on the measurement of charged jet yields recoiling from high-p_{T} charged hadrons in Ru+Ru and Zr+Zr collisions at the center-of-mass energy per nucleon pair of 200 GeV by the STAR experiment at RHIC. Trigger hadrons are selected within 7<p_{T}<25 GeV/c, while jets of radii (R) ranging from 0.2 to 0.5 are measured. Ratio of recoil jet yield in central collisions to that in peripheral collisions (I_{CP}) is less than one at low jet p_{T}, indicating parton energy loss in the QGP, and shows a rising trend with increasing jet p_{T}. The ratio of I_{CP} for jets of R = 0.2 to 0.5 is smaller than one within jet p_{T}>10 GeV/c, providing an evidence for medium-induced intra-jet broadening.

Motivation:
Semi-inclusive hadron jet measurements in intermediate-size collision systems: Zr+Zr and Ru+Ru collisions. Semi-inclusive hadron jets refer to the jets recoiling from high-pT triggers.
By requiring trigger particles, we can select on hard scatterings and largely suppress the background jet contribution.
Combined with the mixed-event technique, we can explore jet energy loss over a broad jet pT range and study modification to jet fragment.

Figures:

Figure 1: Recoil charged-particle jet \pT\ distributions for SE (red points) and ME (gray shaded regions). Top panels: jets with $R$ = 0.2 (a) and 0.5 (b) in 0-10$\%$ Zr+Zr collisions. Bottom panels: jets with $R$ = 0.2 (c) and 0.5 (d) in 0-10$\%$ Ru+Ru collisions. Only statistical uncertainties as shown.

Figure 2: Fully-corrected h+jet \pT\ spectra for Zr+Zr (circles) and Ru+Ru (squares) collisions at \sqrtsNN = 200 GeV. Top panels: jets with $R$ = 0.2 (a) and 0.5 (b) in 0-10$\%$ centrality. Bottom panels: jets with $R$ = 0.2 (c) and 0.5 (d) in 60-80$\%$ centrality. Statistical and systematic uncertainties are indicated by error bars and shaded boxes.

Figure 3: h+jet \ICP\ as a function \pTjetch\ combining Zr+Zr and Ru+Ru systems at \sqrtsNN = 200 GeV, in comparison to STAR results of $\pi^{0}$-triggered ($9<\pTtrig <11$ GeV/$c$) charged-particle jets \IAA\ in 0-15$\%$ centrality interval~\cite{STAR:2023ksv, STAR:2023pal}. Different sub-panels correspond to jet radii $R$ = 0.2 (a), 0.3 (b), 0.4 (c), and 0.5 (d). Statistical and systematic uncertainties are indicated by error bars and shaded boxes.

Figure 4: Ratios of \ICP\ between small-$R$ jets and $R = 0.5$ jets combining Zr+Zr and Ru+Ru collisions at \sqrtsNN = 200 GeV. Top panel: ratios of \ICP\ for $R = 0.2/ R = 0.5$ (green), $R = 0.3/R = 0.5$ (red), and $R = 0.4/R = 0.5$ (blue). Bottom panel: ratio of \ICP\ for $R = 0.2/ R = 0.5$, compared to STAR results of the \IAA\ ratio for $\pi^{0}$-triggered ($9<\pTtrig <11$ GeV/$c$) charged-particle jets in 0-15$\%$ centrality interval.

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Summary:

This letter presents measurements of semi-inclusive hadron-jet distributions in Zr+Zr and Ru+Ru collisions at \sqrtsNN = 200 GeV, with data from the STAR experiment at RHIC. The h+jet \pT\ spectra are reported for both collision systems in 0-10% central and 60-80% peripheral events, with jet resolution parameters of R=0.2 and 0.5. Within experimental precision, no significant difference is observed between the Ru+Ru and Zr+Zr systems, and therefore the two datasets are combined. The combined results reveal a jet quenching signature characterized by $\ICP < 1$ in the low \pTjetch\ region, while an apparent increase of \ICP\ to values approaching and even exceeding unity emerges at high \pTjetch\, which could still be consistent with jet energy loss effects in the QGP as suggested by a model study. To probe the energy redistribution within jets, the ratio of \ICP\ between $R = 0.2$ and $R = 0.5$ is analyzed. The observed stronger suppression for smaller $R=0.2$ jets provides direct evidence that jet-medium interactions redistribute energy from the jet center to larger angular regions, highlighting the influence of the QGP on the jet substructure. Compared to \IAA\ measurements in 200 GeV Au+Au collisions, the observed suppression in Zr+Zr and Ru+Ru is weaker, particularly at small jet radii, reflecting the reduced medium effects in smaller collision systems.

These precision measurements offer new insights into the transport properties of the QGP formed in Ru+Ru and Zr+Zr collisions, and provide critical constraints on theoretical modelling of system size dependence of in-medium jet modification.

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Paper proposal: Semi-inclusive hadron+jet measurement in Ru+Ru and Zr+Zr collisions at 200 GeV in STAR