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Standard Model Soft and Hard QCD at ATLAS and CMS

This paper presents a selection of recent ATLAS and CMS measurements exploring Quantum Chromodynamics across a wide range of energy scales, from soft hadronic interactions and event-shape observables to hard perturbative processes like di-jet production, double-parton scattering, and diffractive events measured by forward proton detectors.

Original authors: Maciej Trzebinski

Published 2026-07-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Maciej Trzebinski

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smasher. Inside its massive ring, scientists crash protons together at nearly the speed of light. This paper is a report card from two giant detectors, ATLAS and CMS, on how well our current understanding of the "glue" that holds matter together—called Quantum Chromodynamics (QCD)—matches what actually happens when these protons collide.

Think of QCD as the rulebook for how tiny building blocks (quarks and gluons) stick together to form protons and other particles. The paper looks at these rules in two very different ways: the "soft" way (gentle, messy interactions) and the "hard" way (violent, high-energy crashes).

Here is a breakdown of their findings using simple analogies:

1. The "Soft" Side: The Crowd at a Concert

The Concept: When protons collide, they don't always smash into each other head-on. Sometimes they just graze each other, creating a messy spray of particles. This is called "soft QCD."
The Experiment: The CMS team looked at these "minimum-bias" events (basically, just letting collisions happen without picking specific ones). They measured the shape of the particle spray, like checking if the crowd at a concert is standing in a neat circle or a chaotic blob.
The Result: They compared the real data to computer simulations (like a video game trying to predict the crowd's behavior).
The Problem: The computer models consistently got it wrong. The real particle sprays were more "round" and spread out (isotropic) than the simulations predicted. It's like the video game thinks the crowd stays in a tight huddle, but in reality, they spread out all over the floor. This tells the scientists that their "rulebook" for how particles are born and spread out needs a serious update.

2. The "Hard" Side: The High-Speed Crash Test

The Concept: When protons smash head-on with huge force, they break apart into jets of particles. This is "hard QCD," where the laws of physics are more predictable and calculable.
The Experiment:

The Jet Cross-Section: ATLAS and CMS measured how often these jets appear at different energies. It's like counting how many cars crash at a specific speed on a highway.
The Result: Generally, the math (perturbative QCD) works very well. The predictions match the crashes. However, there are some bumps in the road. At certain angles and speeds, the math predicts too many crashes or the wrong amount. It's like a weather forecast that is usually right but gets confused when the wind blows from a specific direction.
The "Dead Cone" Effect: This is a cool discovery about heavy particles (like bottom quarks). Imagine a lighthouse beam. If the lighthouse is light, the beam shines everywhere. But if the lighthouse is heavy, it casts a "shadow" or a "dead zone" directly in front of it where no light (gluons) can go.
- The Finding: The CMS team saw this shadow in the data. Heavy quarks stop radiating energy at very sharp angles, exactly as the theory predicted. It's like seeing a heavy truck leave a clear, empty space in the dust cloud behind it, while a light bicycle kicks up dust everywhere.

3. The "Double Trouble": Two Collisions at Once

The Concept: Sometimes, in a single proton-proton collision, two separate things happen at the same time. It's like two people in a crowded room bumping into two different pairs of people simultaneously. This is called "Double-Parton Scattering."
The Experiment: The ATLAS team looked for a specific, rare event where two "W" particles (which carry force) are created with the same electric charge. This is a very clean signature, like finding two identical red balloons in a sea of blue ones.
The Result: They successfully isolated these double-collision events and calculated how likely they are to happen. This helps scientists understand how the "traffic" of particles is arranged inside the proton. It's like figuring out if the people in the room are standing in random clusters or if they are paired up in specific ways.

4. The "Side Eyes": Forward Detectors

The Concept: Most detectors look straight at the crash. But some particles (protons) get kicked sideways at very shallow angles, almost skimming the edge of the beam pipe.
The Experiment: ATLAS and CMS have special "forward" detectors (AFP and PPS) that act like security cameras watching the exits of the room.
The Result: These detectors catch protons that barely interacted. By catching these "side-glance" protons, scientists can study rare events that happen without a messy central crash, like "exclusive production" where the protons stay intact but exchange energy. It's like watching a magician's assistant walk out the back door to see what trick was performed without seeing the main stage.

Summary

In short, this paper says:

We are getting better at understanding the messy, low-energy collisions, but our computer models still struggle to predict exactly how the particles spread out.
Our math for high-energy crashes is mostly correct, but we found specific spots where the rules get a little fuzzy.
We confirmed a weird "shadow" effect around heavy particles, proving the theory right.
We are learning how to spot when two collisions happen at once, giving us a better map of the inside of a proton.
New "side-view" cameras are opening up a whole new way to watch these particle interactions.

The scientists are using the massive amount of data from the LHC's latest run to sharpen these tools, hoping to eventually find cracks in the Standard Model that could lead to new physics.

Technical Summary: Standard Model Soft and Hard QCD at ATLAS and CMS

Problem and Scope
This contribution addresses the comprehensive exploration of Quantum Chromodynamics (QCD) across a vast spectrum of energy scales at the Large Hadron Collider (LHC). The central challenge lies in simultaneously characterizing the non-perturbative regime of strong interactions—governing soft hadronic activity, hadronisation, and multi-parton interactions—and rigorously testing perturbative QCD (pQCD) predictions at the highest accessible energies. The paper aims to present a selection of recent measurements from the ATLAS and CMS experiments using proton–proton collision data from LHC Run 2 ( $\sqrt{s} = 13$ TeV) and early Run 3. These studies are critical for validating theoretical predictions, constraining Monte Carlo (MC) event generators, and refining the modeling required for precision Standard Model (SM) measurements and Beyond the Standard Model (BSM) searches.

Methodology
The paper synthesizes results from diverse analyses employing distinct methodologies tailored to specific QCD regimes:

Soft QCD (Event Shapes): The CMS Collaboration analyzed minimum-bias events collected at $\sqrt{s} = 13$ TeV under special low-pile-up conditions ( $\mu \approx 1$ ) with an integrated luminosity of 64 $\mu$ b $^{-1}$ . The study measured global event-shape observables (multiplicity, sphericity, thrust, broadening, etc.) and unfolded the distributions to the particle level. These were compared against predictions from various MC event generators to test phenomenological models of soft particle production.
Hard QCD (Jet Production):
- Inclusive Di-jets: ATLAS utilized 140 fb $^{-1}$ of data at 13 TeV, while CMS used 24.7 pb $^{-1}$ at 5.02 TeV. Both collaborations measured double-differential inclusive di-jet cross-sections as a function of di-jet invariant mass ( $m_{jj}$ ) and rapidity separation ( $y^*$ ). Results were corrected for detector effects via unfolding to enable direct comparison with particle-level theoretical predictions (NLO and NNLO).
- Upsilon Fragmentation: CMS investigated $\Upsilon(nS)$ production within jets at 13 TeV, measuring the distribution of bottomonium states relative to jet observables to constrain heavy-quark fragmentation models.
- Dead-Cone Effect: Using $t\bar{t}$ events at 13 TeV (59.8 fb $^{-1}$ ), CMS measured the differential emission density of b-quark jets as a function of angular variables to observe the suppression of collinear gluon radiation from massive quarks.
Double-Parton Scattering (DPS): ATLAS performed a measurement of DPS in same-sign WW production using the full Run 2 dataset. A multivariate analysis was employed to isolate the DPS signal from backgrounds, extracting the effective cross-section ( $\sigma_{eff}$ ) via a fit to the data.
Forward Proton Detectors: The paper reviews the capabilities of the AFP (ATLAS) and PPS (CMS) forward detectors, which tag protons scattered at small angles to study diffractive and photon-induced processes.

Key Contributions and Results

Soft QCD Modeling Deficiencies: The measurement of charged-particle event-shape variables revealed that none of the considered MC models provide a satisfactory simultaneous description of all observables. A systematic trend was observed where experimental data are more isotropic than simulations, highlighting persistent deficiencies in modeling soft particle production and multi-parton interactions.
Precision Tests of pQCD:
- Di-jet Cross-sections: Measured cross-sections are generally well-described by NLO and NNLO calculations with electroweak corrections. However, the ATLAS measurement identified a non-trivial dependence of the data-to-theory ratio on $m_{jj}$ and $y^*$ , with theoretical predictions overestimating data at low $y^*$ and deviations increasing at higher rapidities and masses. The CMS study demonstrated that theoretical uncertainties are strongly dependent on scale choices ( $\mu_R, \mu_F$ ), with NNLO calculations reducing uncertainties at high transverse momentum but showing increases in some low- $p_T$ regions.
- Upsilon Fragmentation: The results provided constraints on heavy-quark fragmentation into bound states and offered data to discriminate between production mechanisms, contributing to the understanding of the "quarkonia polarisation puzzle."
- Dead-Cone Effect: The analysis confirmed the suppression of collinear radiation in b-quark jets compared to massless parton shower predictions. Simulations incorporating mass effects provided a significantly improved description of the data, while light-quark baselines showed substantially stronger emission at small angles.
Double-Parton Scattering: The ATLAS measurement extracted an effective cross-section of $\sigma_{eff} = 10.6 \pm 1.8$ mb for same-sign WW production. This result is consistent within uncertainties with previous measurements in different final states, offering new insight into spatial and momentum correlations among partons.
Forward Detectors: The review highlights that AFP and PPS enable the study of color-singlet exchange and central exclusive production, providing unique background suppression for rare exclusive final states. Analysis of Run 3 data from these detectors is noted as having just commenced.

Significance and Claims
The paper asserts that these results collectively provide stringent tests of theoretical predictions and MC event generators. The findings underscore the necessity for improved modeling of soft dynamics and multi-parton interactions to support precision SM measurements and BSM searches. The authors claim that the increased center-of-mass energy and data volume of Run 3 promise even more rigorous tests of QCD, leading to a deeper understanding of proton structure. Specifically, the paper notes that forward detector data will provide further input for precision SM measurements and enable new types of BSM searches. The work is presented as a contribution to the ongoing LHC physics programme, emphasizing the transition from Run 2 results to the extended kinematic reach and precision of Run 3.

1. The "Soft" Side: The Crowd at a Concert

2. The "Hard" Side: The High-Speed Crash Test

3. The "Double Trouble": Two Collisions at Once

4. The "Side Eyes": Forward Detectors

Summary

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