SciPost Submission Page
Physics case for low-$\sqrt{s}$ QCD studies at FCC-ee
by David d'Enterria, Pier Francesco Monni, Peter Skands, Andrii Verbytskyi
This is not the latest submitted version.
Submission summary
| Authors (as registered SciPost users): | Andrii Verbytskyi |
| Submission information | |
|---|---|
| Preprint Link: | https://arxiv.org/abs/2503.23855v1 (pdf) |
| Date submitted: | April 9, 2025, 5:37 a.m. |
| Submitted by: | Andrii Verbytskyi |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Experimental, Phenomenological |
Abstract
Measurements of hadronic final states in $e^{+}e^{-}$ collisions at centre-of-mass (CM) energies below the Z peak can notably extend the FCC-ee physics reach in terms of precision quantum chromodynamics (QCD) studies. Hadronic final states can be studied over a range of hadronic energies $\sqrt{s_\mathrm{had}} \approx 20\mbox{--}80\,\mathrm{GeV}$ by exploiting events with hard initial- and final-state QED radiation (ISR/FSR) during the high-luminosity Z-pole run, as well as in dedicated short (about one month long) $e^{+}e^{-}$ runs at CM energies $\sqrt{s} \approx 40\,\mathrm{GeV}$ and $60\,\mathrm{GeV}$. Using realistic estimates and fast detector simulations, we show that data samples of about $10^{9}$ hadronic events can be collected at the FCC-ee at each of the low-CM-energy points. Such datasets can be exploited in a variety of precision QCD measurements, including studies of light-, heavy-quark and gluon jet properties, hadronic event shapes, fragmentation functions, and nonperturbative dynamics. This will offer valuable insights into strong interaction physics, complementing data from nominal FCC-ee runs at higher center-of-mass energies, $\sqrt{s} \approx 91, 160, 240,$ and $365\,\mathrm{GeV}$.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Current status:
Reports on this Submission
Strengths
1- timely 2- predicts important input for FCC program 3- written clearly, briefly and comprehensively
Report
The authors offer two possible avenues to achieve these data sets at the FCC-ee. One is the usage of events with initial- or final state photon radiation at the dedicated high luminosity run at the Z pole. Reasonable simulation of events shows that selected data sets will contain still well over a billion events at a number of low energy bins of 5 GeV size, which will cover the CM energy range between 20 and 91 GeV. The second avenue would be to have dedicated runs at lower energies. The latter option would clearly benefit from pure data samples that might be easier to compare systematically to existing observables from LEP and other colliders.
Still, the study shows that both avenues should lead to very interesting physics results and would give very important input to understanding QCD effects of various kinds.
Recommendation
Publish (easily meets expectations and criteria for this Journal; among top 50%)
Strengths
2-Two different avenues (ISR/FSR and low energy runs) are explored
Weaknesses
Report
The projections are based on the extrapolation of an L3 paper for the ISR/FSR approach and on a some assumption for the FCC-ee luminosity at low energies. In both cases the sensitivities are assessed merely in terms of the HFS event statistics but no detailed studies involving simulated detector performance have been carried out.
So, I would say this paper is interested for getting a first idea of the sensivity of FCC-ee for QCD studies at intermediate energies but clearly more research using full simulation is necessary to establish the physics case. I would judge that the acceptance criterium "Opens a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work" is met. The paper itself is well written (clarity, level of detail, relevant references, abstract/conclusion, reproducibility ). I therefore recommend publication after the detailed comments below have been considered by the authors.
Requested changes
1-p. 5, Table 2: Are the expected yields at FCC-ee for one experiment, for all for 4 together? This should be clarified.
2-p. 8, last paragraph: Luminosity scales as sqrt(s). As this is a key assumption for the results you present in this section, is there no way to find a more rigorous justification than a 'private communication'?
3-p. 9, first paragraph: It should be added that the time estimate of 1-3 weeks plus 1 week for setup is *for every c.m. energy point*.
Recommendation
Ask for minor revision
Dear Referee, dear Editors,
please find the answers to the Referee report attached.
Sincerely,
Andrii Verbytskyi for the authors
Attachment:
Dear Editors,
please see the answer to the Report below and also attached in a form of PDF file.
Best regards,
Andrii Verbytskyi for the Authors
Response to Referee Report on ”Physics case for low-√s QCD studies at FCC-ee” The Authors September 24, 2025
Introduction We are pleased that the referees find our manuscript well written and recognize its potential to open a new pathway for QCD studies at intermediate energies using e+e− collisions at the future FCC-ee collider. Below, we address the specific concerns raised.
1 Report 1 by Referee 1 on 2025-6-10 (Invited Report) Report 1 by Anonymous (Referee 1) on 2025-6-10 (Invited Report) Strengths 1-First idea of FCC-ee sensitivity for QCD studies in the intermediate energy range (30-80 GeV) 2-Two different avenues (ISR/FSR and low energy runs) are explored Weaknesses 1-Lack of detailed studies to build an actual physics case: The ISR/FSR sensitivity is based on L3 extrapolation but it’s not clear if this is a valid assumption for an actual FCC-ee detector. Also the actual feasibility of low energy runs (luminosity, requirements for the detector) are not studied. Report Hadronic final states in e+e- collisions in the intermediate energy range (roughly between the Y(4S) and the Z poles) have not been studied in recent experiments and available statistics are low. This paper explores how this energy range could be accessed at the future FCC-ee, either in ISR/FSR events while running at the Z pole, or in dedicated low energy runs. The projections are based on the extrapolation of an L3 paper for the ISR/FSR approach and on a some assumption for the FCC-ee luminosity at low energies. In both cases the sensitivities are assessed merely in terms of the HFS event statistics but no detailed studies involving simulated detector performance have been carried out. So, I would say this paper is interested for getting a first idea of the sensivity of FCC-ee for QCD studies at intermediate energies but clearly more research using full simulation is necessary to establish the physics case. I would judge that the acceptance criterium "Opens a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work" is met. The paper itself is well written (clarity, level of detail, relevant references, abstract/conclusion, reproducibility ). I therefore recommend publication after the detailed comments below have been considered by the authors. Requested changes 1-p. 5, Table 2: Are the expected yields at FCC-ee for one experiment, for all for 4 together? This should be clarified. 2-p. 8, last paragraph: Luminosity scales as sqrt(s). As this is a key assumption for the results you present in this section, is there no way to find a more rigorous justification than a ’private communication’? 3-p. 9, first paragraph: It should be added that the time estimate of 1-3 weeks plus 1 week for setup is for every c.m. energy point. Recommendation Ask for minor revision
1.1 Future detector modeling and feasibility of low-energy runs in terms of detector requirements
Our studies are based on fast (parametrized DELPHES) simulations of the current FCC-ee IDEA detector concept. Therefore, despite its intrinsic limitations, this is a state-of-the-art approach to account for the instrumental effects of a typical FCC-ee detector on the event selection. The studies will be updated in lockstep with the developments of future FCC-ee detectors and will be redone once their full simulations are available. In the performed studies, no detector requirements for the low-energy runs were explicitly considered, as one of the key aspects of the proposal is to make the low-energy dedicated runs a low-budget extension of the main FCC-ee program. Therefore, the baseline assumption is that no dedicated modifications should be made to the detector proposals to accommodate such runs. (If anything, in the case of dedicated low-√s runs, one could consider lowering the magnetic field of the detectors, which would lead to further improved low-momentum charged-particle tracking performances).
1.2 Comparison to L3 detector performance The experience from the existing L3 measurements provides a reliable baseline for the expected event selection at future experiments. The reason is that while detectors built for the FCC-ee will benefit from major advances in detector technologies, these improvements will primarily enhance the resolution of reconstructed quantities and enable data acquisition at rates several orders of magnitude higher, but the purity and selection efficiency of ISR/FSR samples at LEP were fundamentally limited by the underlying physics. A textbook example is the fact that even with state-of-the-art 4π solid angle tracking and calorimeter systems, a fraction of the energy escapes the detector (e.g. from the produced neutrinos and very low-angle ISR photons) and therefore the total registered energy will differ from 2 × Ebeam.
1.3 Further studies to establish the physics case more rigorously Studies are currently ongoing to make a stronger physics case by exploiting a concrete case: comparing the precision achieved for measurements of event shape observables obtained via ISR/FSR at the Z pole and from dedicated low-energy runs. However, the quality of reconstruction of those quantities is closely related to the quality of reconstruction of the HFS (e.g. the invariant mass of HFS), already presented in the current study.
1.4 Requested changes 1 Yes, the luminosity is given for the 4 interaction points combined. The text has been updated as follows: • “FCC-ee” −→ “FCC-ee (4 IPs combined)”
1.5 Requested changes 2 The performance of the accelerator running at lower energies is indeed a more complex topic. However, at our request, parametric simulations were performed by the accelerator experts (K. Oide and F. Zimmermann), following similar early estimates for the FCC-ee Feasibility Report, keeping an upper limit on the beam current equal to that at √s = 91 GeV. Those results were used in the estimations of the run duration in this study (Table 4). Based on the beam parameters setup of Table 4, the accelerator experts conclude that the textbook assumption that the luminosity follows the standard Lorentz γL-dependence of the beam energies (i.e. the L ∝ √s scaling of luminosity) is reasonable for the conditions they used. The text has been updated as follows: • “the luminosity follows the standard Lorentz γL-dependence of the beam energies, namely L ∝ √s [43].” −→ “the luminosity follows the standard Lorentz γL-dependence driven by the beam energies[footnote: Such a dependence is approximately confirmed with the simulated collider parameters of Table 4.], namely L ∝ √s.”
1.6 Requested changes 3 Text updated as follows: • “plus 1-week of additional beam-setup time [43].” −→ “plus 1-week of additional beam-setup time [43], for each single CM-energy point.”
2 Report 2 by Referee 2 on 2025-6-12 (Invited Report) Report 2 by Anonymous (Referee 2) on 2025-6-12 (Invited Report) Strengths 1- timely 2- predicts important input for FCC program 3- written clearly, briefly and comprehensively Report The authors report on two interesting options to generate data samples for lower-than LEP CM energy hadronic final states in electron position annihilation events at the FCC-ee. Input from existing data sets is very limited and the input from these data to tuning Monte Carlo event generators and understanding QCD effects is highly desirable. The study is a contribution to the 2025 update of the European strategy for Particle Physics. The authors offer two possible avenues to achieve these data sets at the FCC-ee. One is the usage of events with initial- or final state photon radiation at the dedicated high luminosity run at the Z pole. Reasonable simulation of events shows that selected data sets will contain still well over a billion events at a number of low energy bins of 5 GeV size, which will cover the CM energy range between 20 and 91 GeV. The second avenue would be to have dedicated runs at lower energies. The latter option would clearly benefit from pure data samples that might be easier to compare systematically to existing observables from LEP and other colliders. Still, the study shows that both avenues should lead to very interesting physics results and would give very important input to understanding QCD effects of various kinds. Recommendation Publish (easily meets expectations and criteria for this Journal; among top 50%)
2.1 Feedback We are pleased that the relevance, clarity, and potential impact of our study have been appreciated. We agree that both proposed approaches (radiative return and dedicated low-energy runs) offer complementary paths toward enriching the FCC-ee physics program and improving our understanding of QCD in a previously underexplored regime.
Sincerely, The Authors
Author: Andrii Verbytskyi on 2025-12-05 [id 6108]
(in reply to Report 2 on 2025-06-12)Dear Referee, dear Editors,
please find the answers to the Referee report attached.
Sincerely,
Andrii Verbytskyi for the authors
Attachment:
Low_sqrt_s__FCC_ee-ref2.pdf