Title: Observation of jet quenching in O+O collisions at 200 GeV in STAR

PAs: Sijie Zhang, Yang He, Maowu Nie, Rongrong Ma, Li Yi

Contact information: sijiezhang@mail.sdu.edu.cn; maowu.nie@sdu.edu.cn; marr@bnl.gov; li.yi@sdu.edu.cn

Target journal: Physics Review Letters

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Paper Draft:
Current Version: Link
version 1: Link
version 2: Link

Analysis Note:
Current Version: Link
Version 1: Link
Version 2: Link

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 Talks in PWG meeting:
https://drupal.star.bnl.gov/STAR/system/files/OOh-jet_pwg_240919.pdf
https://drupal.star.bnl.gov/STAR/system/files/OO_jet_pwg_250313.pdf
https://drupal.star.bnl.gov/STAR/system/files/OO_jet_pwg_250327.pdf
https://drupal.star.bnl.gov/STAR/system/files/OO_jet_pwg_250626_2.pdf

Talks in collaboration meeting:

https://drupal.star.bnl.gov/STAR/system/files/241023_coll_meeting.pdf
https://drupal.star.bnl.gov/STAR/system/files/250305_coll_meeting_parallel_v1.pdf
https://drupal.star.bnl.gov/STAR/system/files/250307_coll_meeting_plenary.pdf

Abstract:
The STAR experiment at Relativistic Heavy Ion Collider presents measurements of correlations between charged hadron triggers of high transverse momenta ($7 < p_{\rm T} < 30$ GeV/$c$) with recoiling charged hadrons ($3 < p_{\rm T} < 7$ GeV/$c$) or charged-partilce jets ($p_{\rm T, jet} > 5$ GeV/$c$) in O+O collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair. Yields of associated hadrons and jets, normalized by the number of trigger hadrons, are suppressed by about 20\% in central collisions compared to those of peripheral collisions, with a significance of more than 5$\sigma$. These results constitute clear evidence for jet quenching in central O+O events, providing important new insight into the limits of Quark-Gluon Plasma formation in small collision systems. 

Figures:

Figure 1: Self-normalized distributions of the number of MIPs measured in the EPD for MB O+O events (open symbols) and those containing a high-\pT\ trigger particle (filled symbols). Correction for EPD detection efficiency is not applied. Vertical bars around data points represent statistical errors. Vertical dashed lines indicate selected percentile intervals in EA.

Figure 2: Top: raw di-hadron correlations as a function of azimuthal separation $\Delta\phi=\phi^{\rm trig}-\phi^{\rm assoc}$. The horizontal dashed lines show background fit within $0.8 < |\Delta\phi| < 1.6$, and the vertical dashed lines show signal ranges. Bottom: distribution of recoil charged-particle jets as a function of \pTjetReco. The gray filled histogram represents a similar distribution from mixed events as an estimate of combinatorial background, and the hatched area denotes the range where the mixed-event distribution is normalized. Ratio of the recoil jet distribution to that from mixed events after normalization is shown in the sub--panel.

Figure 3: Recoil jet \Icp\ between 0-10\% and 40-60\% EA classes as a function of \pTjet\ for two jet radii of $R=0.2$ and 0.5. Vertical bars and shaded boxes around data points display statistical and systematic uncertainties, respectively.

Figure 4: \Icp\ for associated hadrons in near-side and recoil peaks (top) and recoil jets (bottom) as a function of EA, with 40-60\% EA population used as reference. Vertical bars and shaded boxes around data points display statistical and systematic uncertainties, respectively.

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Summary:

In summary, the STAR experiment at RHIC presents the latest search for the jet quenching signal in a small system of O+O collisions at \sqrtsnn\ = 200 GeV. Utilizing high-\pT\ trigger particles within $7<p_{\rm T}^{\rm trig}<30$ \gev, a clear suppression of the yields for charged hadrons ($7.1\sigma$) and charged-particle jets ($5.3\sigma$) opposite to the trigger particles azimuthally is observed in 0-10\% high-EA collisions compared to those in 40-60\% low-EA events. Corroborated by the absence of suppression for near-side associated hadrons, such a suppression provides strong evidence for the presence of jet quenching in high-EA O+O collisions, the smallest collision system where the jet quenching signal is observed unambiguously so far. In addition, recoil jets with radii of both $R=0.2$ and 0.5 are suppressed, suggesting that lost energies by energetic partons in the medium are distributed beyond a radius of 0.5. These results will provide new insights to the limit of system size for jet quenching to be observable and stringent constraints on how energetic partons interact with the medium produced in heavy-ion collisions. 
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Response to Isaac: Link