Title: Measurement of Two-Point Energy Correlators Within Jets in pp Collisions at Sqrt(s) = 200 GeV

PAs: Helen Caines (Yale), Raghav Kunnawalkam Elayavalli (Vanderbilt)
, Isaac Mooney (Yale), Andrew Tamis (Yale)
GPC:

Target Journal:  PRL

Abstract:

Jet substructure has often been used to elucidate the time-evolution of a jet scattered from a high-energy collision. However, many substructure observables focus on the hardest information within the jet, removing a non-perturbative regime that is of interest to study in its own right. A complementary observable that studies the transition between the perturbative and non-perturbative is the 2-point energy correlator (EEC), which looks at energy distribution as a function of all possible two track pairs in a jet. In this letter, the first measurement of the EEC at RHIC is presented, using data taken from 200 GeV pp collisions by the Solenoidal Tracker at RHIC (STAR) is presented. The EEC will be shown for several selections on full jet transverse momentum, jet radius, as well as for selections on the charge pairing of correlations. As this observable is infrared-and-collinear safe, direct comparisons with theory in the perturbative regime and simple models of the expected scaling behavior in the non-perturbative regime will be shown as well.  These comparisons reveal the EEC to have the expected clear separation into three angular regions: one dominated by perturbative QCD, one by non-perturbative QCD and a transition between them.

  PWGC Preview : Tamis - PWGC Preview 6_23_23 | The STAR experiment (bnl.gov)

Analysis Note: Link

Paper Draft: Link

Proposed Figures:

Figure 1: Corrected distributions of the normalized EEC plotted differential in R_L for R_jet=0.4 and R_jet=0.6 (scaled for comparison), for jet transverse momentum 30 < p_{T, jet} < 50 GeV/c. Random hadron scaling at low angles and next-to-leading-log-pQCD calculations at large angles are presented alongside Monte-Carlo predictions.

Figure 2: Corrected distributions of the normalized EEC within jets, plotted differential in  <p_{T, jet}> R_{L} at R_jet = 0.4 for several jet transverse momentum selections.

Figure 3: Corrected Distributions of the charged EEC ratio for R_jet=0.4 and R_jet=0.6, for a jet transverse momentum selection of 20  < p_{T, jet} <  30 GeV/c.  Comparisons with PYTHIA8: Detroit Tune and HERWIG7 are given.

Physics Messages:

-Figure 1

• Transition from geometrical random hadron scaling into the perturbative quark and gluon region, corresponding to onset of hadronization, is observed.
• Two regions determined via their scaling behavior: increasing linearly with angle at small angles and a downward trend predicted by perturbative QCD at large angles. 

This figure shows the ability of the EEC to separate angular scales out into both the non-perturbative and perturbative scales.  At small angles a prediction corresponding to uniformly distributed hadrons in phase-space is given, with a next-to-leading-log pQCD calculation given at large angles.  Comparisons with both HERWIG7.2 and PYTHIA8 Detroit Tune are given.  The observed EEC behavior is well described by the model prediction at low angles and the theory calculation at large angles: which highlights the non-perturbative transition region associated with hadronization.  The Monte-Carlo models, however, capture the location of this region well, showing that previous tuning to jet fragmentation allows them to describe energy flow in jets well.  the EEC with jet resolution parameters R_jet = 0.4 and R_jet = 0.6 are given, showing that the location of this transition regime does not depend strongly on jet radius, within the binning resolution available to this study.  However, the increased jet radius does allow for observation of the perturbative region to higher angles before geometrical effects due to the finite radius appear.

-Figure 2:

• Scaling the x axis by jet transverse momentum collapses the transition region to the same location regardless of momentum selection shows that transition region moves as a function of 1/jet momentum.

This shows the EEC for three different momentum selections scaled on the x-axis by the average p_{T, jet} within that momentum bin.  This is observed to collapse the transition region to a single point for each momentum selection, which supports a universal transition region that occurs at an angular scale inversely proportional to jet momentum.

-Figure 3:

• Distribution is shifted when selecting on opposite or like sign charged pairs, with opposite sign correlations moving to smaller angles and like sign moving to larger angles in general.
• Comparisons with HERWIG and PYTHIA  test effects of different hadronization models.

This figure shows the ratio of the charge-correlated EEC over the nominal EEC for R_jet = 0.4 and R_jet = 0.6.  This observable explores how the hadronization differences of like-sign and opposite-sign sign pairs differ as a function of angular scale.  An increased correlation moving from larger to smaller angles consistent with theory predictions is observed, however an increased de-correlation compared to Monte-Carlo models is observed at extremely small angles.

Conclusions:
In this letter, the first measurement of the 2-point energy correlator at STAR has been presented in pp collisions at sqrt(s) = 200 GeV, showing clear separation of the jet evolution into three regimes. Agreement with theoretical predictions seen in the region dominated by perturbative effects as well as scaling expectations of the free hadron regime: allowing for identification of a transition regime between the two. This transition region is seen to propagate at a scale inversely proportional to jet momentum, showing a universal hadronization scale that causes higher energy jets to hadronize at later times. Additionally, separating the track pairs with like-sign and opposite-sign correlations shifts the relative magnitude of perturbative and non-perturbative effects, implying that hadronization dynamics differ between the two samples. This study could also serve as a baseline in future measurements of the EEC in heavy-ion collisions. This result, particularly the collapse of the transition region, can only be compared reliably between experiments when taking into account the difference between momenta of full and charged jets as well as the previously mentioned quark/gluon fraction difference.  For example, due to the higher sqrt(s) at which the ALICE measurement was taken, 5 TeV, the fraction of jets that were gluon initiated increases significantly relative to this measurement due to being reported at a comparable p_{T, jet}, which would result in a transition present at a higher p_{T, jet}R_L.  However, within each experiment, the trend of the transition region being collapsed to a single point is recovered. These measurements, therefore, provide evidence for a universal transition from non-perturbative to perturbative effects for a given initiator flavor.  Additional studies done across experiments that directly involve the initiating parton, such as heavy flavor tagging, will then be extremely informative about the commonality of this transition between collision energies.  Since the EEC allows for time-scale separation in jets, proving useful in discriminating the evolution of a jet being modified by interactions with the QGP.