Soft Unclustered Energy at 13 TeV: A First Search in Proton-Proton Collisions
Introduction to Hidden Valley Models and SUEPs
Many physics studies aim to experimentally uncover exotic phenomena extending beyond the Standard Model (SM), as outlined by theoretical frameworks. Among these are hidden valley models, which propose a dark sector where particles interact via a strong, dark force. These models predict particles and interactions with unique decay characteristics.
CMS Collaboration's Groundbreaking Search for SUEPs
In a recent publication in Physical Review Letters, researchers from the CMS (Compact Muon Solenoid) collaboration at CERN reported the results of the first search for soft unclustered energy patterns (SUEPs), a unique signal predicted by hidden valley models in high-energy particle collisions.
SUEPs and Their Role in Extending the Standard Model
"SUEPs belong to a broader class of theories aimed at extending the Standard Model to address unresolved phenomena in universe, such as dark matter and matter-antimatter asymmetry," said Luca Lavezzo of the CMS search team in an interview with Phys.
Theoretical Foundations of Hidden Valley Models
Specifically, these phenomena are among the predictions derived from hidden valley theories. Introduced nearly two decades ago by Matt Strassler and Kathryn Zurek, these theories propose a "Dark Sector" distinct from the Standard Model, characterized by its own strong, confining force, analogous to the Standard Model's strong force that binds quarks and gluons into hadrons such as protons and neutrons.
Challenges in Validating Hidden Valley Predictions
Many of the fascinating predictions made by hidden valley models have yet to undergo experimental validation. When these theories were first proposed, the technological limitations of the time rendered searches for the predicted dark sectors impractical, deferring such efforts to future studies.
Revisiting Hidden Valley Theories with Colliders
"Several years ago, as interest in investigating complex dark sectors grew within the scientific community, theorists and experimentalists revisited the unusual predictions of hidden valley theories, realizing that some could now be explored using colliders," said Lavezzo.
New Search Strategies for SUEPs and Other Phenomena
Soft unclustered energy patterns (SUEPs), semivisible jets, and emerging jets represent the initial set of searches aimed at validating specific predictions from hidden valley models, all published within the last few years.
Characterizing SUEPs in High-Energy Collisions
Hidden valley models suggest that high-energy particle collisions might produce distinct signatures, such as SUEPs, characterized by numerous low-momentum particles arranged in a spherical pattern within particle colliders like those used in the CMS experiment.
Challenges in Identifying SUEPs in Collider Events
"This is a highly distinct signature compared to Standard Model predictions. However, identifying a SUEP in a typical collider event is challenging due to the presence of several dozen simultaneous collisions, each generating numerous low-energy particles," Lavezzo explained.
Refining Trigger Mechanisms to Capture SUEP Events
"Additionally, our trigger mechanisms—criteria determining which proton—proton collisions are deemed noteworthy—are specifically configured to capture events involving high-energy particles, making it challenging to select those with naturally low energy."
New Strategies in Search of SUEPs
To overcome the challenges that hindered previous searches for these particles, the CMS Collaboration first ensured that the particle responsible for generating a SUEP—acting as the 'Portal' between the Standard Model and Hidden Valley models—recoiled against an SM particle, specifically a jet in their experiment. This recoil results in an event where both particles exhibit substantial yet balanced energy, enabling the event to be triggered on the SM jet.
Differentiating Between SUEPs and Standard Model Jets
"By employing this strategy, the SUEP's structure shifts from a spherical pattern to one resembling a broader version of an SM jet—s shower of particles from a quark," explained Lavezzo.
Challenges in Comparing Predictions to Experimental Observations
"The challenge now is to differentiate between SM jets and SUEPs. However, obtaining reliable predictions through our traditional methods proves difficult in these complex environments and events, which is essential for comparing our measurements to theoretical models and determining if there is any evidence of SUEPs or if the observations align with Standard Model expectations."
Innovative Methods for Estimating SM Contributions
The CMS collaboration chose to estimate the contribution of SM events directly from the data they gathered during their search. This was done by utilizing the extended-ABCD method, a method that helps assess the SM contribution in the signal region.
Successful Exclusion of SUEP Theorie's Phase Space
"We are the first team to conduct a search for SUEPs at colliders, and we successfully excluded a significant portion of the available phase space for SUEP theories. Additionally, we've established a set of methods that we hope will be further developed in future studies," stated Lavezzo. "The response from theorists, including Matt Strassler who originally proposed SUEPs, was incredibly positive. They were excited about our experimental findings, as it opens the door for testing more hypotheses."
Future Directions and Open Questions in SUEP Research
The recent search undertaken by this research group has provided new constraints that will inform future strategies for detecting SUEPs in particle colliders. Hidden Valley models suggest that SUEPs should be fully visible, meaning that all dark sector particles decay to Standard Model particles. However, this assumption may not necessarily apply.
The Possibility of Stable, Undetectable, SUEPs
"SUEPs may decay into the Standard Model after a certain lifespan, or some could remain stable and undetectable, leading to distinct signatures that previous searches might have missed," explained Lavezzo. "Further focused searches could be conducted in areas where our approach was not optimized; notably, low-mass portals remain largely unconstrained.
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