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A New Herwig7 Underlying Event Tune: from RHIC to LHC Energies

Umar Sohail Qureshi, Raghav Kunnawalkam Elayavalli, Luke Mozarsky, Helen Caines, Isaac Mooney

TL;DR

The paper tackles the mismatch between Herwig7's UE modeling and low-energy data by developing two dedicated tunes, Nashville and New Haven, optimized for RHIC energies and extended to LHC scales. Parameter tuning is performed with the Professor toolkit, using a third-degree polynomial surrogate for MC predictions and a Minuit-based $\chi^2$ minimization over MPI/UE parameters, including $p_T^{min}$ energy scaling $p_T^{min}(\sqrt{s})= p_{T,0}^{min} \left(\frac{b+\sqrt{s}}{E_0}\right)^c$ with $E_0 = 200$ GeV. The tunes show clear improvements over the default at $\sqrt{s}=200$ GeV, with Nashville excelling in charged-particle spectra and New Haven providing the best agreement for jet substructure, while maintaining competitive performance at higher energies up to $\sqrt{s}=7$ TeV. The work provides practical, mid-rapidity–oriented tunes for STAR, sPHENIX, ATLAS, ALICE, and CMS and motivates future global fits that include forward-rapidity data.

Abstract

We present parameter sets corresponding to new underlying event tunes for the Herwig7.3 Monte Carlo event generator. The existing Herwig tunes are in good agreement with LHC data, however, they are not typically designed for center-of-mass energies below $\sqrt{s}=300$ GeV. The tunes presented in this study can describe mid-rapidity data collected at the nominal RHIC energy of $\sqrt{s }=200$ GeV, as well as higher center-of-mass energies utilized by experiments elsewhere, such as the LHC. The base "New Haven" tune is developed by fitting minimum-bias simulations of proton-proton collisions to mid-rapidity identified hadron and jet data from the STAR experiment. The "Nashville" tune includes a separate set of parameters developed by tuning to Tevatron proton-antiproton data at $\sqrt{s}=300$, $900$ and $1960$ GeV from CDF, and LHC proton-proton measurements from CMS at $\sqrt{s}=7$ TeV, in addition to the STAR measurements. Both new tunes demonstrate significant improvements over the recommended default tune currently included in the latest version of Herwig for minimum bias production. As such, we advocate using these tunes for future simulation studies at mid-rapidity by the experimental collaborations at RHIC (STAR and sPHENIX) and the LHC (ATLAS, ALICE, CMS).

A New Herwig7 Underlying Event Tune: from RHIC to LHC Energies

TL;DR

The paper tackles the mismatch between Herwig7's UE modeling and low-energy data by developing two dedicated tunes, Nashville and New Haven, optimized for RHIC energies and extended to LHC scales. Parameter tuning is performed with the Professor toolkit, using a third-degree polynomial surrogate for MC predictions and a Minuit-based minimization over MPI/UE parameters, including energy scaling with GeV. The tunes show clear improvements over the default at GeV, with Nashville excelling in charged-particle spectra and New Haven providing the best agreement for jet substructure, while maintaining competitive performance at higher energies up to TeV. The work provides practical, mid-rapidity–oriented tunes for STAR, sPHENIX, ATLAS, ALICE, and CMS and motivates future global fits that include forward-rapidity data.

Abstract

We present parameter sets corresponding to new underlying event tunes for the Herwig7.3 Monte Carlo event generator. The existing Herwig tunes are in good agreement with LHC data, however, they are not typically designed for center-of-mass energies below GeV. The tunes presented in this study can describe mid-rapidity data collected at the nominal RHIC energy of GeV, as well as higher center-of-mass energies utilized by experiments elsewhere, such as the LHC. The base "New Haven" tune is developed by fitting minimum-bias simulations of proton-proton collisions to mid-rapidity identified hadron and jet data from the STAR experiment. The "Nashville" tune includes a separate set of parameters developed by tuning to Tevatron proton-antiproton data at , and GeV from CDF, and LHC proton-proton measurements from CMS at TeV, in addition to the STAR measurements. Both new tunes demonstrate significant improvements over the recommended default tune currently included in the latest version of Herwig for minimum bias production. As such, we advocate using these tunes for future simulation studies at mid-rapidity by the experimental collaborations at RHIC (STAR and sPHENIX) and the LHC (ATLAS, ALICE, CMS).

Paper Structure

This paper contains 6 sections, 4 equations, 11 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Tuning Methodology
  3. Data and Simulation
  4. New Herwig7 Tunes
  5. Comparisons with Data
  6. Discussion

Figures (11)

  • Figure 1: An illustration of a hadron-hadron collision in which a hard parton–parton collision has occurred, and the leading object is taken to be the charged particle of largest $p_\perp$ in the event. The black arrow in the toward direction indicates the leading object direction.
  • Figure 2: The energy extrapolations of $p_{\perp}^{\mathrm{min}}$ for the default, Nashville, New Haven, and Pythia Detroit tunes are shown as the solid blue, red, green, and purple lines, respectively. The black dashed line is drawn for reference at $\sqrt{s}=200$ GeV.
  • Figure 3: Minimum-subtracted $\chi^2$ profiles of the Nashville tune parameters in the vicinity of the best-fit value. The grey-shaded regions indicate the $1$-$\sigma$ uncertainties in the optimal parameters. The parameters in the figure correspond to those in Table \ref{['tab:tunedparams']}, where Power, Offset, pTmin0, InvRadius, PReco, and ladderMult correspond to $c$, $b$, $p_{\perp, 0}^{\text{min}}$, $\mu^2$, $p_{\text{reco}}$, and $N_{\text{ladder}}$ respectively.
  • Figure 4: Comparison of the default (green solid) and the new Nashville (red solid) and New Haven (blue solid) Herwig7 tunes with inclusive mid-rapidity $\pi^+$ invariant yields as a function of $p_\perp$ (top left), UE multiplicity as a function of leading jet $p_\perp$ (top right), SoftDrop groomed jet radius $R_g$ (bottom left), and SoftDrop groomed jet mass $M_g$ (bottom right) from $pp$ collisions at $\sqrt{s}=200$ GeV as measured by the STAR experiment. The bottom panels in each plot show the ratio of the Monte Carlo predictions to measured data and the yellow-shaded region indicates the experimental uncertainties.
  • Figure 5: UE observables as a function of leading jet $p_T$ from STAR measurements in proton-proton collisions at $\sqrt{s}=200$ GeV. The top left and right show the charge particle density in the towards and away regions, respectively. The bottom left and right figures show the charged particle density and average $p_T$ for the transverse and toward regions, respectively. The bottom panels in each plot show the ratio of the Monte Carlo predictions to measured data and the yellow-shaded region indicates the experimental uncertainties.
  • ...and 6 more figures