Tamis publishes results on the evolution of jets through time
Jet undergoing a parton shower of quarks (straight lines) and gluons (curvy lines), hadronization, and hadronic scatterings/decays. (E. Pottebaum)
Yale Physics graduate student Andrew Tamis, who is a member of Wright Lab’s Relativistic Heavy Ion Group (RHIG), recently had a set of results from his thesis studies characterizing the evolution of jets through time to better understand the strong force published in Physical Review Letters (PRL).
The paper, “Measurement of Two-Point Energy Correlators within Jets in 𝑝 +𝑝 Collisions at √𝑠 =200 GeV,” includes data from the Solenoidal Tracker at RHIC (STAR) experiment of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab (BNL). RHIG associate research scientist Isaac Mooney, Horace D. Taft Professor of Physics Helen Caines, and the STAR Collaboration are co-authors of the paper. Additional RHIG researchers in the STAR collaboration include D. Allan Bromley Professor Emeritus of Physics John Harris, assistant professor Laura Havener, and graduate students Ryan Hamilton, Iris Ponce Pinto, and Emily Pottebaum.
More information about the science in the paper is explained, below.
Studying the beginnings of the Universe by colliding particles at high energies
The component particles of protons and neutrons—quarks and gluons (also called partons)—are normally held together by the strong nuclear force in subatomic composite particles called hadrons. The Relativistic Heavy Ion Collider (RHIC) is an accelerator that creates high energy collisions of protons or heavy ions (which are atoms which have had their electrons removed) at high speeds. This enables the partons to break away from the strong force holding them together in the hadrons and separate into a strongly interacting “soup” of particles and be studied. The resulting state of matter is called the quark-gluon plasma (QGP). It is predicted by the Standard Model of particle physics, in particular Quantum Chromodynamics (QCD), to have existed ten millionths of a second after the Big Bang and may exist in the cores of very dense stars.
Therefore, the collisions at RHIC create an experimental ground for scientists to study conditions of the very beginning of the Universe before these particles bound together when the Universe expanded and cooled.
The Solenoidal Tracker at RHIC (STAR) is designed to track the particles produced by the ion collisions at RHIC, and especially to search for and characterize QGP generated by the collisions.
Investigating the behavior of observable jets to learn more about invisible processes
When heavy ions collide, there is a rare chance for partons to directly interact with each other, scatter, and leave the collision in a spray or parton “shower”. This is called hard scattering. The end result of hard scattering is that the partons recombine back into hadrons (a process called hadronization). These hadrons further cluster into objects called “jets,” which scientists can search for using jet-finding algorithms. Studying the observable jets can therefore enable scientists to learn more about the nature of the otherwise invisible parton shower and hadronization.
Tamis explained this process using the language of QCD, or color dynamics. He said, “just as the electromagnetic force has charge, the strong force has what scientists call color, which behaves like charge. Color is not visible, but there are three colors named Red, Green, and Blue. The Universe wants to be very color-neutral, and the strong force only gets stronger when you try to pull a color and its anti-color apart, so we can only isolate these in very high energy scenarios.”
Tamis said, “Because the partons in a proton-proton collision have color, they don’t like to exist by themselves. And at high energies, they exist at a mass they should not and will split into particles to get rid of that. As they are flying apart, the color string that connects them will eventually break and create anti-color/color pairs. This process, called string breaking, leads to a shower of many hadrons that can be picked up in our detectors. We call these jets.”
While some scientists measure how many jets there are or study how jets are produced, Tamis studies how jets behave, for example, how stuff is distributed in the final part of the jet; at what distance energy is carried from hadron to hadron; how the partons combine into hadrons we can see; and how color-neutral objects find each other.
Distributions of the normalized EEC within jets, scaled by jet momentum on the x-axis to collapse the peak for several jet momentum selections.
Methods
Tamis is at the forefront of a new trend to look at jets in terms of their evolution through time. Detectors can only pick up the jet at the end of its lifespan, so Tamis used a measurement for his analysis called an Energy-Energy Correlator (EEC) to determine how the energy is distributed throughout the jet’s lifetime and learn more about the jet’s past history. Similarly, cosmologists use energy correlators of the cosmic microwave background (CMB)—the radiation left over from the Big Bang—to learn about particles present at the beginning of the Universe.
Tamis said, “For the main EEC, we show that when certain particles become confined into hadrons, it happens at a similar scale. We can define that as energy times distance. Normally, this confinement happens at smaller distances the more energetic the particle is. As these jets fragment, the splittings go on and happen at shorter and shorter distances. We can then relate and say if we are looking at this confinement into hadrons happening at shorter distances, it happened at later times.”
Charge-weighted ratio of the EEC, with a more negative value corresponding to more correlation. Both Monte-Carlo models fail to describe data.
Tamis further tunes his analysis with a charge selection that shows how charge is distributed in the final state jet. If there is color string breaking, the observed data should show a positive particle and negative anti-particle pair.
Tamis said, “The method of hadronization will affect the charge distribution in ways separate from the energy. The current leading simulation tools use different models: a simple string-breaking model (PYTHIA) and a cluster model (HERWIG), and neither accurately describe the data, which gives us an opportunity to improve our understanding of existing models or even possibly find new physics.”
Impact
Mooney said, “What is so exciting about this measurement is that it opens more doors than it closes. Beyond the observed first-order universality of the transition region between partonic and hadronic states, there are many subleading effects that can be teased apart in future measurements, including the effect on this transition due to the identity of the hard parton which initiates the jet. Additional avenues for further exploration are extractions of fundamental physical quantities such as the anomalous dimension and the strong coupling of QCD, and improvements to the hadronization mechanisms of state-of-the-art models.”
Mooney continued, “Andrew has been nimble in responding to challenges that have arisen with this novel measurement, and adopted new analysis techniques as necessary. He has become an expert in both the conceptual basis and experimental methodology of energy correlators, and is now helping to push the field forward into a much more difficult territory by performing a similar measurement in heavy-ion collisions, in which a large and fluctuating background must be handled carefully.”
Tamis said: “Working on this project has been an amazing experience, thanks to the endless scientific and moral support from RHIG, who made working every day with it fun, and helped me break through every dead end. It’s exciting to work with such fundamental interactions of physics as the behavior of the strong force, and I think the most exciting aspect of the work is pushing up against the boundary of what can be described by our current perturbative understanding of QCD, and when it necessarily has to transition to non-perturbative effects.”
Tamis continued, “Energy correlators are a great tool for accomplishing this, and I’m hoping to use them further to study the behavior of the quark-gluon plasma in heavy-ion collisions at RHIC. This being published both feels like closing a chapter on the project I began with when I first started at Yale, but also only the beginning, as everything I learned during the process has left me with endless other investigations still to be done that leave me excited about my future in physics.”