At the LHC, lead ions are smashed together at extremely high speeds to create a unique state of matter called the quark gluon plasma. Normally, quarks and gluons, such as those that make up lead ions, are confined within protons and neutrons. However, when heavy ions collide at extreme energies, the protons and neutrons “melt”, and a resulting soup of quarks and gluons becomes free to move in a liquid-like manner. This process is similar to the melting of ice: as ice is heated, the neat crystal lattice of H2O molecules collapses, and water is formed.
The quark gluon plasma produced in the lab is ephemeral - it only exists for 10-22 seconds, which means it cannot be probed with any external tools. Luckily a natural probe is sometimes formed in the collisions. High energy quarks and gluons within the ions can interact very energetically during the initial collision, forming new particles which fly off in opposite directions. As these particles travel, they split and fragment into something called a jet, which is a narrow cone of particles that emerges from the collision.
Jets can be formed even in collisions where the quark gluon plasma is not produced, like proton-proton collisions, which means scientists can study how they are modified by the quark gluon plasma’s presence. As jets pass through the quark gluon plasma, they interact with it via the strong force, which results in energy loss and modification of their evolution, as depicted in Fig. 1. This process is analogous to how a pellet slows down passing through water. And just like pellets, jets can also be deflected by the medium and create waves.
Figure 1: Image of two jets back to back in the CMS detector. Jet 1 has lost energy from passing through the quark gluon plasma, and Jet 0 is relatively unmodified.
Understanding jet deflection and wake effects can shed light on energy loss mechanisms in the quark gluon plasma, as well as its fundamental behavior. One way to characterize jet deflection is to study the jet axes in different ways:
- Winner-take-all axis: This axis follows the most energetic particle in the jet, which is sensitive to deflection due to scatterings from quark gluon plasma constituents.
- Energy-weighted axis: This axis follows the overall direction of the jet, which is relatively unmodified by interaction with the quark gluon plasma.
By analyzing how these axes differ, scientists can gain insights into the properties of the QGP and how it affects the particles moving through it. “Kicks” from quark gluon plasma constituents may cause the jet axes to decorrelate from each other.
However, since jets lose energy to the quark gluon plasma, most measurements will suffer from a “survivorship bias”; this bias emerges because some jets lose more energy than others due to their inherent shape or number of constituents. If a jet is wider or has more constituents, more color charges can be resolved by the medium resulting in stronger quenching. Analogously, one can imagine how a large flat disk shot through water would lose energy faster than an aerodynamic pellet.
This study focused on data from proton-proton and lead-lead collisions measured with the CMS detector. To mitigate the “survivorship bias” effects, the study investigated jets recoiling from photons, which are colorless and don’t interact via the strong force. The jet and photon start with similar energies because of energy conservation, since they are produced at the same time and travel in opposite directions. Then, while the jet loses energy interacting with the quark gluon plasma, the photon doesn’t, and hence “tags” the initial energy of the jet, allowing a less biased comparison between lead-lead and proton-proton collisions.
The analysis was performed by Molly Park, a graduate student at MIT, under the guidance of postdoc Dr. Christopher McGinn. The measurement looked at the angular difference, Δj, between the energy-weighted and winner-take all jet axes in jets “tagged” with a high-energy photon. The study revealed that the shapes of the Δj distributions in pp and PbPb collisions were similar. Figure 2 shows the ratio of the PbPb Δj distribution shape to the pp shape. The compatibility of the Δj distribution shapes is thought to arise due to competing effects from the remaining “survivorship bias”, which would tend to select narrow jets and lead to a narrower Δj distribution in PbPb collisions, and scattering from quark gluon plasma constituents, which would decorrelate the jet axes and lead to a wider Δj distribution in PbPb collisions.
The results are compared with the HYBRID theoretical model, which can variably include the quark-gluon plasma wake and deflections from medium particles. The HYBRID model shows that the jet axis difference is relatively insensitive to medium wake effects, but is very sensitive to potential scatterings from the quark gluon plasma particles. The inclusion of scattering effects is found to significantly improve agreement with data.
Figure 2: Ratio of photon-tagged jet axis decorrelation shapes in PbPb and pp collisions, shown for collisions with the greatest degree of overlap. The results are compared to theoretical predictions from the HYBRID model, which can variably include quark-gluon plasma wake and scattering (“elastic”) effects.
Studying the jet axis decorrelation is interesting, because it provides a new way to probe the properties of the quark gluon plasma. The findings from this study also offer benchmarks for testing and improving theoretical models which describe jet quenching and interactions within the quark gluon plasma. Studying quark-gluon plasma is crucial because it offers insights into how fundamental forces behave under extreme conditions, and allows us to understand how this strange primordial liquid can emerge from the most fundamental building blocks of matter.
Read more about these results:
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CMS Physics Analysis Summary (HIN-21-019): "First measurement of jet axis decorrelation with photon-tagged jets in pp and PbPb collisions at 5.02 TeV"
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Display of collision events: CERN CDS
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