First measurement of jet axis decorrelation with photon-tagged jets in pp and PbPb collisions at 5.02 TeV

This paper presents the first measurement of jet axis decorrelation in photon-tagged jets from PbPb and pp collisions at 5.02 TeV, revealing a narrowing effect for high-transverse-momentum jets in central PbPb collisions that is compared against theoretical models of parton energy loss in the quark-gluon plasma.

The Gist

The Big Picture: Smashing Heavy Balls to Make a "Super-Soup"

Imagine you have two giant, heavy bowling balls made of lead. You smash them together at nearly the speed of light. When they collide, they don't just bounce off; they melt into a tiny, super-hot, super-dense drop of liquid. Physicists call this the Quark-Gluon Plasma (QGP).

Think of the QGP as a thick, invisible "super-soup" made of the fundamental building blocks of matter (quarks and gluons). This soup existed right after the Big Bang, but today, we only see it for a split second in particle colliders like the one at CERN.

The goal of this paper is to figure out what happens when a fast-moving particle tries to swim through this super-soup.

The Experiment: The "Flashlight" and the "Jet"

To study this soup, the scientists at the CMS experiment (a giant particle detector) needed a way to see how the soup affects things.

1. The Flashlight (The Photon): In the collision, sometimes a particle of light (a photon) is shot out. Because light doesn't interact with the "soup" (it's invisible to the strong force), it flies out in a straight line without getting slowed down. It acts like a flashlight that tells us exactly how hard the initial crash was.
2. The Jet (The Particle Shower): On the opposite side of the crash, a high-energy particle (a quark) is knocked loose. As it tries to escape, it smashes into the soup, creating a spray of other particles. This spray is called a Jet.

By comparing the "Flashlight" (which didn't change) with the "Jet" (which did), scientists can measure how much the soup slowed down or scattered the Jet.

The New Tool: Measuring "Wobble" (Jet Axis Decorrelation)

In previous studies, scientists mostly looked at how much energy the Jet lost. But this paper introduces a new, clever way to look at the Jet: How much does it wobble?

Imagine a jet as a stream of water shooting out of a hose.

• The "Average" Direction (E-scheme): If you look at the whole stream, where is the center of mass? This is the average direction.
• The "Strongest" Direction (WTA scheme): If you only look at the single, most powerful droplet in the stream, where is it pointing? This is the "Winner-Take-All" direction.

In a perfect vacuum (empty space), these two directions are usually very close together. But if the stream has to push through thick mud (the QGP), the mud might knock the heavy droplets off course, or scatter the lighter droplets around. This causes the "Average" direction and the "Strongest" direction to wobble apart.

The scientists measured this "wobble" (called $\Delta_j$) to see how much the soup was messing with the jet's internal structure.

The Surprising Findings: The "Survivor Bias" Trap

The scientists looked at two groups of Jets:

1. Low Energy Jets (30–60 GeV): These are like small, weak streams.
2. High Energy Jets (60–100 GeV): These are like powerful, high-pressure hoses.

What they found:

• For the Low Energy Jets: The "wobble" looked exactly the same in the heavy lead collisions (PbPb) as it did in simple proton collisions (pp).
  • The Analogy: Imagine you are looking for survivors in a storm. The weak swimmers (low energy jets) get washed away or slowed down so much that they don't make it to the finish line. The only ones you see are the ones that were lucky enough to take a straight path. Because you only see the "lucky" ones, the data looks normal, even though the soup is there. This is called Survivor Bias.
• For the High Energy Jets: Here, they saw something interesting. In the most central (densest) collisions, the "wobble" got narrower. The strong hoses stayed straighter than expected.
  • The Analogy: The powerful hoses are so strong they can push through the mud. However, the ones that do get scattered (wobble a lot) lose so much energy that they drop below the "high energy" threshold and disappear from the data. The ones that remain are the ones that managed to stay straight. So, the data shows a "narrowing" effect, not because the soup is gentle, but because the "wobbly" high-energy jets were filtered out.

Checking the Theory: The "Video Game" Models

The scientists compared their real-world data to three different computer simulations (theories) that try to predict how particles behave in the soup:

1. JEWEL: This model did a great job. It predicted that the "wobble" would be relatively unchanged by the soup's "wake" (the trail left behind), but very sensitive to direct collisions (elastic scattering).
2. HYBRID: This model also worked well, confirming that the "wobble" is mostly caused by particles bouncing off the soup molecules, not by the soup flowing around them.
3. PYQUEN: This model predicted too much "wobble" (broadening), suggesting it might be overestimating how much the soup spreads the particles out.

The Bottom Line

This paper is like a detective story. The scientists used a "Flashlight" (photon) to tag the "Jet" (particle spray) and measured how much the Jet's internal direction wobbled.

They discovered that:

1. Low-energy jets are so easily filtered out by the soup that we can't see the soup's effect on them (Survivor Bias).
2. High-energy jets show a "narrowing" effect, which is actually a sign that the soup is filtering out the messy, scattered jets, leaving only the straight ones.
3. The JEWEL and HYBRID models are the best "maps" we have so far for understanding how particles bounce off the super-hot soup of the early universe.

In short, by measuring how much a particle stream "wobbles," scientists are learning exactly how the universe's "super-soup" interacts with matter, helping us understand the laws of physics at their most extreme.

Technical Summary

1. Problem Statement and Motivation

The primary goal of this research is to investigate the properties of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. Specifically, the study aims to probe jet quenching—the energy loss and momentum broadening of high-energy partons as they traverse the QGP medium.

• The Challenge of Survivor Bias: Traditional inclusive jet measurements suffer from "survivor bias." In heavy-ion collisions, wider jets or those with more constituents interact more strongly with the medium, lose more energy, and often fall below the detection threshold ($p_T$ cut). This biases the observed sample toward narrower, less quenched jets, potentially obscuring the true medium effects.
• The Solution: The authors utilize photon-tagged jets. Since the isolated photon is colorless and does not interact strongly with the QGP, its transverse momentum ($p_T^\gamma$) serves as a proxy for the initial energy of the recoiling parton. By selecting events based on the photon's $p_T$, the analysis reduces the bias associated with the final jet energy loss, allowing for a cleaner comparison between proton-proton (pp) and lead-lead (PbPb) collisions.
• The Observable: The paper introduces the jet axis decorrelation ($\Delta_j$), defined as the angular difference between two distinct definitions of the jet axis:
  1. E-scheme (Energy-weighted): The standard anti-$k_T$ axis, sensitive to the average energy flow.
  2. WTA (Winner-Take-All): Determined by reclustering with the Cambridge-Aachen algorithm and following the hardest subjet at each step. This axis is less sensitive to soft radiation and medium response but more sensitive to hard scattering (Molière scattering).

2. Methodology

Data Samples:

• Collisions: Proton-proton (pp) and Lead-Lead (PbPb) at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{NN}} = 5.02$ TeV.
• Dataset: Collected by the CMS experiment in 2017 (pp) and 2018 (PbPb).
  • PbPb integrated luminosity: $1.69 \pm 0.03 \text{ nb}^{-1}$.
  • pp integrated luminosity: $302 \pm 6 \text{ pb}^{-1}$.

Event Selection and Reconstruction:

• Trigger: Events required a photon candidate with $p_T^\gamma > 40$ GeV.
• Photon Selection: Isolated photons with $60 < p_T^\gamma < 200$ GeV in the barrel region ($|\eta_\gamma| < 1.44$). Isolation is defined by the energy sum in a cone ($\Delta R = 0.3$) around the photon, with underlying event (UE) subtraction for PbPb.
• Jet Selection: Jets reconstructed using the anti-$k_T$ algorithm with radius $R=0.3$. Selected jets must have $30 < p_T^{jet} < 100$ GeV and $|\eta_{jet}| < 1.6$.
• Pairing: Jets are paired with the leading photon if $|\Delta \phi_{j\gamma}| > 2\pi/3$.

Analysis Techniques:

• Background Subtraction:
  • Uncorrelated Jets: In PbPb, uncorrelated jets from UE fluctuations are subtracted using a mixed-event technique (pairing photons from one event with jets from independent minimum-bias events of similar centrality).
  • Non-prompt Photons: A "purity subtraction" technique using electromagnetic shower shape ($\sigma_{\eta\eta}$) and isolation variables removes background from neutral meson decays.
• Unfolding: The measured distributions are unfolded to the particle level to correct for detector smearing and resolution effects using the D'Agostini iterative method. Response matrices were generated using PYTHIA 8 (pp) and PYTHIA 8 + HYDJET (PbPb).
• Systematic Uncertainties: Major sources include Jet Energy Corrections (JEC), Jet Energy Resolution (JER), photon purity, and unfolding priors.

Theoretical Comparisons:
Results are compared against three theoretical models describing parton energy loss:

1. JEWEL: Dynamic framework based on perturbative QCD.
2. HYBRID: Combines perturbative QCD with gauge/gravity duality (strong coupling) for soft exchanges.
3. PYQUEN: Calculates accumulated energy loss via gluon radiation.

3. Key Contributions

1. First Measurement: This is the first measurement of jet axis decorrelation ($\Delta_j$) specifically for photon-tagged jets. Previous measurements (e.g., by ALICE) were performed on inclusive jets.
2. Mitigation of Survivor Bias: By using photon tagging, the study demonstrates a method to isolate medium-induced modifications from selection biases inherent in inclusive jet spectra.
3. Differential Analysis: The analysis splits the data into two jet $p_T$ intervals ($30-60$ GeV and $60-100$ GeV) to explicitly study the interplay between jet quenching and survivor bias.
4. Model Discrimination: The study provides critical constraints on theoretical models by testing their ability to reproduce the specific angular decorrelation patterns, distinguishing between effects caused by elastic scattering, wake effects, and wide-angle radiation.

4. Results

Jet Axis Decorrelation ($\Delta_j$) Distributions:

• General Shape: The $\Delta_j$ distribution peaks between $0.01$ and $0.04$ and falls steeply. A dip at $\Delta_j \approx 0$ is observed due to the rarity of E-scheme and WTA axes aligning perfectly in multi-particle jets.
• $p_T$ Dependence: Higher $p_T$ jets ($60-100$ GeV) exhibit a narrower $\Delta_j$ distribution compared to lower $p_T$ jets ($30-60$ GeV), consistent with vacuum expectations where higher boost leads to collimation.

Comparison: PbPb vs. pp:

• Low $p_T$ Jets ($30 < p_T < 60$ GeV): The ratio of PbPb to pp distributions is consistent with unity across all centralities. This suggests that for lower energy jets, the effects of medium-induced broadening are balanced by survivor bias (wider jets are quenched out of the sample).
• High $p_T$ Jets ($60 < p_T < 100$ GeV): A narrowing of the $\Delta_j$ distribution is observed in central PbPb collisions (0-10%) compared to pp. The ratio shows an enhancement at small $\Delta_j$.
  • Interpretation: This narrowing is attributed to survivor bias. Wider jets in this high-$p_T$ bin lose more energy and fall below the 60 GeV threshold, leaving a surviving sample enriched with narrower jets. This effect is less pronounced in the lower $p_T$ bin where the spectrum is flatter due to the photon tag.

Theoretical Model Comparisons:

• HYBRID Model:
  • Successfully describes the data when both elastic scattering and QGP wake effects are included.
  • The $\Delta_j$ observable is found to be highly sensitive to elastic scattering (Molière scattering) but relatively insensitive to the QGP wake contribution regarding the shape of the distribution (though the wake affects the overall yield).
• JEWEL Model:
  • Provides a reasonable description of the data, particularly the narrowing at high $p_T$ and broadening at low $p_T$.
  • Indicates that $\Delta_j$ is relatively insensitive to medium recoil (wake) effects.
• PYQUEN Model:
  • Fails to describe the data properly. It overestimates medium-induced broadening effects.
  • Even with the inclusion of wide-angle radiation, it cannot simultaneously describe the pp baseline and the PbPb modification, suggesting its treatment of radiative energy loss is insufficient for this observable.

5. Significance

This paper represents a significant advancement in heavy-ion physics by:

1. Validating Photon Tagging: Confirming that photon-tagged jets provide a less biased window into QGP properties compared to inclusive jets, allowing for a more direct study of parton energy loss mechanisms.
2. Disentangling Mechanisms: The measurement of $\Delta_j$ helps distinguish between different energy loss mechanisms. The results strongly support the importance of elastic scattering (Molière scattering) in modifying jet substructure, a finding that complements studies on radiative energy loss.
3. Refining Theoretical Models: The data places stringent constraints on jet quenching models. The success of the HYBRID and JEWEL models in describing the specific angular decorrelation, and the failure of PYQUEN, guides future theoretical developments in understanding the interplay between weak and strong coupling regimes in the QGP.
4. Understanding Survivor Bias: The study quantitatively demonstrates how survivor bias can mimic or mask medium effects, emphasizing the necessity of using tagged jets or substructure observables to extract true medium properties.

In conclusion, the CMS Collaboration has successfully measured the jet axis decorrelation in photon-tagged events, revealing that while survivor bias dominates the low-$p_T$ region, the high-$p_T$ region shows clear signs of medium-induced narrowing driven by the preferential quenching of wide jets. The results provide strong evidence for the role of elastic scattering in jet quenching.