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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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Exploring ¹⁰⁰Sn: Strong Evidence for its Rare Doubly Magic Properties

Introduction

New experiments at CERN have provided valuable insights into the nuclear characteristics of atomic nuclei, which make up the majority of atomic mass. A major focus has been on understanding Tin-100 (¹⁰⁰Sn), a rare isotope with an equal number of protons and neutrons—50 each.

Magic Numbers in Nuclear Physics

In nuclear physics, these particular counts of protons and neutrons are termed 'magic numbers.' This designation indicates that the isotope possesses fully filled proton and neutron shells, resulting in an exceptionally stable nuclear configuration.

Breakthrough Findings on Tin-100

A team of researchers from MIT, the university of Manchester, CERN, KU Leuven, and other institutions recently presented compelling evidence indicating that Tin-100 (¹⁰⁰Sn) exhibits a doubly magic nucleus. Published in Nature Physics, their findings pave the way for groundbreaking research to test and refine nuclear theories.

Challenges in Understanding Tin-100

Dr. Jonas Karthein, lead author of the paper, "Exploring the nuclear properties near Tin-100 (¹⁰⁰Sn)—believed to be the heaviest doubly magic nucleus with equal proton (Z=50) and neutron (N=50) numbers—has posed a significant challenge for both experimental and theoretical nuclear physics for many years."

"In recent decades, numerous experimental campaigns at leading radioactive beam facilities worldwide have focused on studying isotopes near Tin-100 (¹⁰⁰Sn)."

Isotopes like Tin-100 (¹⁰⁰Sn), with extremely short lifetimes (around one second or less), must be synthesized artificially. Consequently, physicists have only managed to produce them at very low rates, leading to conflicting and inconclusive findings about their structure in past studies.

Advancements in Experimental Techniques

Investigating Nuclear Structure

"Before our study, there was little experimental understanding of the changes in nuclear size and shape as we approach Tin-100 (¹⁰⁰Sn)," remarked Karthein.

"Indium isotopes (Z=49), possessing only one fewer proton than tin, serve as an excellent platform for investigating the evolution of nuclear structure properties near Tin-100 (¹⁰⁰Sn). Recent advancements in indium isotope production at CERN, coupled with our progress in highly sensitive laser spectroscopy, have facilitated the initial measurements in proximity to ¹⁰⁰Sn."

Significant Progress in Nuclear Theory

Recent years have seen notable progress in nuclear theory concerning heavy isotopes like ¹⁰⁰Sn. By gathering extensive experimental evidence regarding the electromagnetic characteristics of ¹⁰⁰Sn, Karthein and his colleagues have confirmed certain theoretical predictions while creating a stringent standard for the advancement of nuclear models.

Prof. Ronald Garcia Ruiz, a co-author of this study, explained, "The recent advancements in the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at CERN-ISOLDE, along with the production of exotic indium isotopes at the facility, enabled us to conduct precision laser spectroscopy on the atomic energy levels of the indium atom, allowing for the extraction of their nuclear electromagnetic properties."

Implications of the Research

Examining Neutron Number Variability

By examining short-lived indium nuclei with varying neutron numbers in comparison to their stable counterparts, we successfully investigated how nuclear shape and size evolve with changes in neutron number, ranging from the naturally occurring isotopes ¹¹³In and ¹¹⁵In to the neutron-deficient ¹°¹In and the neutron-rich ¹³¹In.

Confirming Doubly Magic Nature of ¹⁰⁰Sn

The results obtained by the researchers provide compelling evidence for the doubly magic nature of ¹⁰⁰Sn, a phenomenon predicted by contemporary nuclear theories but not yet definitively confirmed through experimental data. In addition, Karthein and his team conducted comprehensive nuclear calculations employing advanced methodologies, which clarified the structural characteristics of ¹⁰⁰Sn atomic nuclei.

Karthein remarked, "Our results strongly indicate the doubly magic nature of ¹⁰⁰Sn, delivering essential experimental data that aids in elucidating this crucial region of the nuclear chart and resolving inconsistencies from spectroscopy studies across various international facilities. The relatively simple structure of these nuclear systems provides an ideal framework for advancing our theoretical insights into atomic nuclei."

Future Research Directions

The recent work conducted by this research team has the potential to pave the way for significant new directions in the study of atomic nuclei. For example, it will inform forthcoming experiments at large-scale and next-generation research facilities, such as the U.S. Department of Energy's Facility for Rare Isotope Beams (FRIB).

Upcoming Initiatives

These initiatives will facilitate highly precise investigations of ¹⁰⁰Sn and its neighboring isotopes, further illuminating their nuclear properties. Additionally, they will provide theoretical physicists the opportunity to evaluate current theories and models of nuclei in extreme regions far from stability.

Karthein indicated that the CRIS collaboration at CERN intends to push these measurements further into the neutron-deficient isotopes ⁹⁹In and ¹⁰⁰In. He added that the recent independent mass measurements obtained at CERN-ISOLDE highlight the critical need to examine their nuclear electromagnetic properties.

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