Measurement of the angle between jet axes in pp collisions at $\sqrt{s} = 5.02$ TeV

This article reports measurements of the angle between differently defined jet axes in pp collisions at $\sqrt{s} = 5.02$ TeV carried out by the ALICE Collaboration. Charged particles at midrapidity are clustered into jets with resolution parameters $R=0.2$ and 0.4. The jet axis, before and after Soft Drop grooming, is compared to the jet axis from the Winner-Takes-All (WTA) recombination scheme. The angle between these axes, $\Delta R_{\mathrm{axis}}$, probes a wide phase space of the jet formation and evolution, ranging from the initial high-momentum-transfer scattering to the hadronization process. The $\Delta R_{\mathrm{axis}}$ observable is presented for $20 <~ {p_{\mathrm{T}}^{\mathrm{ch\; jet}}}<~ 100$ GeV/$c$, and compared to predictions from the PYTHIA 8 and Herwig 7 event generators. The distributions can also be calculated analytically with a leading hadronization correction related to the non-perturbative component of the Collins$-$Soper$-$Sterman (CSS) evolution kernel. Comparisons to analytical predictions at next-to-leading-logarithmic accuracy with leading hadronization correction implemented from experimental extractions of the CSS kernel in Drell$-$Yan measurements are presented. The analytical predictions describe the measured data within 20% in the perturbative regime, with surprising agreement in the non-perturbative regime as well. These results are compatible with the universality of the CSS kernel in the context of jet substructure.

 

Submitted to: JHEP
e-Print: arXiv:2211.08928 | PDF | inSPIRE
CERN-EP-2022-242
Figure group

Figure 1

Representation of different jet axes. The colored dashed lines represent particles comprising the jet in the initial sample constructed with the anti-$k_{\rm T}$ algorithm and $E$ recombination scheme. These define the ``Standard'' jet axis. The grooming procedure removes soft wide-angle radiation (represented by orange lines), and the resulting axis (``Groomed (SD)'') is defined by the remaining particles (red and blue lines). Finally, the ``WTA'' axis, which is determined with all the particles in the initial (ungroomed) jet, tends to be aligned with the most-energetic particle in the jet. The \DeltaR{} observable is determined from the angle between any pair of these axes.

Figure 2

Comparison between Standard\textendash{}SD measured distributions and Monte Carlo event generators for jets of $R=0.4$ (top) and 0.2 (bottom) in $40<\pTchjet<60$ \GeVc. Left: distributions with $\zcut=0.1$ and varying $\beta$. Right: distributions with $\beta=1$ and varying $\zcut$.

Figure 3

Comparison between WTA-relative measured distributions and Monte Carlo event generators for jets of $R=0.4$ (top) and 0.2 (bottom) in $40<\pTchjet<60$ \GeVc. Left: distributions with $\zcut=0.1$ and varying $\beta$. Right: distributions with $\beta=1$ and varying $\zcut$.

Figure 4

Distribution of the most-probable value of the WTA-Standard spectra as a function of \pTchjet{} for $R=0.2$ and $R=0.4$.

Figure 5

Comparison between measured distributions and analytic predictions for the difference between the WTA axis and the Standard (left), SD ($\zcut=0.2,\beta=1$) (center), and SD ($\zcut=0.3,\beta=1$) (right) axes for jets of $R=0.4$ (top) and 0.2 (bottom) in the transverse momentum range $40 < \pTchjet < 60$ \GeVc. The black markers correspond to the distributions determined from measured data. The vertical error bars correspond to the statistical uncertainties, and the rectangles correspond to the total systematic uncertainties. The colored curves correspond to the SCET-based analytic predictions corrected for charge and MPI effects using two event generators (PYTHIA 8 and Herwig 7). The vertical dashed line defines the approximate boundary between the non-perturbative and perturbative regions. Both the measured and analytic predictions are normalized so that the integral $\int_{\DeltaR^{\rm NP}}^{R/2}{\rm d}\DeltaR ({\rm d}\sigma/{\rm d}\DeltaR)=1$. The bottom panels show the data/SCET ratios. The colored rectangles correspond to the systematic uncertainty of the measured distribution. The size of the theoretical uncertainty in the analytic predictions is shown as a yellow band.

Figure A.1

Comparison between WTA-relative measured distributions and Monte Carlo event generators for jets of $R=0.4$ in $20<\pTjet<40$ \GeVc. Left: distributions with $\zcut=0.1$ and varying $\beta$. Right: distributions with $\beta=1$ and varying $\zcut$.

Figure A.7

Comparison between Standard$-$SD measured distributions and Monte Carlo event generators for jets of $R=0.4$ in $20<\pTjet<40$ \GeVc. Left: distributions with $\zcut=0.1$ and varying $\beta$. Right: distributions with $\beta=1$ and varying $\zcut$.