Associated + charm production: indications for PDF analysis
This paper calculates the next-to-leading order production of bosons in association with charmed mesons to demonstrate that recent ATLAS data favors a zero strangeness asymmetry in the proton, thereby challenging PDF sets like MSHT20NLO and NNPDF4.0NLO that predict a positive asymmetry.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep within the heart of matter, protons are not solid, indivisible spheres but rather bustling cities of smaller particles called quarks. Among these, the "strange" quark is a particularly elusive resident. While scientists have mapped the locations of other light quarks with great precision, the strange quark and its antimatter twin, the antistrange quark, remain somewhat mysterious. A central question in modern physics is whether these two exist in equal numbers or if one outnumbers the other inside the proton. This imbalance, known as asymmetry, is difficult to measure directly because the strange quark rarely reveals itself in isolation. To solve this puzzle, researchers look for rare events where protons collide at incredibly high speeds, creating new particles that carry the signature of these hidden quarks. One such event involves the creation of a W boson, a heavy carrier of the weak nuclear force, alongside a charm quark. Because the strange quark is the most likely partner to produce this specific combination, studying these collisions offers a unique window into the proton's internal structure.
A team of physicists from the University of Jyväskylä in Finland has taken a fresh look at this process to refine our understanding of the proton's contents. They focused on a specific type of collision recorded by the ATLAS experiment at the Large Hadron Collider, where protons smash together at an energy of 13 trillion electron volts. In these collisions, a W boson is produced alongside a charmed meson, a particle containing a charm quark. The researchers calculated the expected rate of these events using advanced computer simulations that account for the complex interactions of quantum mechanics. Crucially, they did not just look at the total number of events; instead, they compared the production of positively charged W bosons paired with negatively charged charmed mesons against the production of negatively charged W bosons paired with positively charged mesons. By examining the ratio between these two outcomes, the team found that many of the usual theoretical uncertainties cancel out, leaving a clear signal that depends directly on the balance of strange and antistrange quarks inside the proton.
When the team compared their calculations with the actual data collected by ATLAS, a distinct pattern emerged. The experimental measurements matched the predictions of a theoretical model that assumes the strange and antistrange quarks are present in equal numbers, meaning there is no asymmetry. However, the data showed a noticeable tension with other popular models that suggest there are more strange quarks than antistrange quarks in the relevant energy range. The researchers found that as the momentum of the particles increased, the gap between the data and the models with an asymmetry grew wider. This suggests that the models assuming an imbalance are likely overestimating the difference between the two types of quarks.
To understand why this matters, the team performed a simplified analysis of how the different quark distributions behave. They found that the discrepancy in the data could be traced directly to the assumption of a non-zero difference between strange and antistrange quarks. When they adjusted their calculations to remove this assumed difference, the theoretical predictions aligned much more closely with the experimental observations. This does not prove definitively that the asymmetry is zero, but it strongly indicates that previous models may have exaggerated the imbalance. The study suggests that the strange quark content of the proton is more symmetric than some current theories propose, offering a clearer picture of the fundamental building blocks of our universe. By focusing on the ratio of these specific particle pairs, the researchers have provided a sensitive tool that future experiments can use to further constrain our knowledge of the proton's hidden interior.
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