First measurement of the forward rapidity dependence of boson transverse helicity fractions
Using 13 TeV proton-proton collision data from the LHCb experiment, this paper presents the first measurement of the forward rapidity dependence of boson transverse helicity fractions, confirming strong rapidity variation and agreement with Standard Model predictions.
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
In the subatomic world, particles do not merely exist; they spin. This intrinsic rotation, known as spin, gives particles a specific orientation in space, much like a spinning top has a direction it is leaning. When certain particles decay, the way their fragments fly out depends entirely on this spin orientation. For decades, physicists have understood that the force responsible for radioactive decay, called the weak force, has a distinct preference: it favors particles spinning in one direction over the other. This preference is a fundamental rule of nature, woven into the fabric of the Standard Model, the theory that describes how the universe's basic building blocks interact. However, while scientists have confirmed this rule in many settings, the universe is not uniform. The conditions under which particles collide change depending on where you look and how fast they are moving. Understanding how these spin preferences shift across different regions of space and energy is crucial. It acts as a stress test for our theories, revealing whether our understanding of the universe holds true in extreme or previously unexplored environments, or if hidden new physics might be lurking in the details.
A team of researchers at the Large Hadron Collider, using a detector called LHCb, has now mapped these spin preferences in a region of space that had never been explored before. They focused on the W boson, a heavy particle that carries the weak force and decays almost instantly into a muon and a neutrino. Because the neutrino escapes detection, the researchers could not see the full picture of the decay directly. Instead, they looked at the muon, the charged particle that remains, and studied how its speed and position changed depending on the direction the original W boson was traveling. By analyzing data from 5.1 inverse femtobarns of proton-proton collisions at an energy of 13 teraelectronvolts, they were able to reconstruct the spin state of the W boson as it moved forward, covering a rapidity range from zero to five. This forward region is unique because the collisions there are highly asymmetric, involving different types of particles inside the protons than those found in the center of the detector.
The researchers found that the spin of the W boson is not static; it changes dramatically as the particle moves faster in the forward direction. In the lowest energy range they measured, the W bosons were a mix of different spin states. But as the rapidity increased, the particles became almost entirely "left-handed," meaning their spin was aligned in a specific direction relative to their motion. For the positively charged W bosons, this left-handed fraction rose from about 23 percent to nearly 100 percent at the highest speeds. The negatively charged W bosons showed a similar, though less extreme, trend, staying mostly left-handed but shifting slightly as they moved. These results match the predictions of the Standard Model with high precision, confirming that the theory correctly describes how the weak force behaves even in these extreme, forward-moving collisions.
To reach this conclusion, the team had to separate the signal from a vast amount of background noise. They selected events where a single muon was produced with a specific momentum and direction, filtering out billions of other collision events. They used computer simulations to create templates of what the muon's path would look like if the W boson had a left-handed spin, a right-handed spin, or a spin perpendicular to its motion. By comparing the actual data from the detector against these templates, they could determine the proportion of each spin state present in the data. The analysis required careful calibration of the detector's measurements and a rigorous accounting of potential errors, such as the uncertainty in the momentum of the particles or the efficiency of the detectors in spotting them. Despite these challenges, the data showed a clear and strong dependence on the direction of travel, with the left-handed state becoming dominant as the particles moved further forward.
This measurement provides the first direct look at the transverse spin fractions of W bosons in the forward region. While previous experiments had studied these particles in other contexts, such as when they are produced alongside top quarks or at high speeds in the center of the detector, the forward region offered a new laboratory. Here, the collisions are dominated by the interaction of specific particles inside the proton, creating a unique environment where the separation between different spin states is predicted to be very pronounced. The fact that the observed data aligns so well with the theoretical predictions reinforces the current understanding of the weak force. It confirms that the chiral nature of the interaction, which favors left-handed particles, holds true even when the collision dynamics are skewed by the forward motion of the protons. This work serves as a new benchmark for future studies, ensuring that models of particle production are accurate in all directions, which is essential for searching for new physics in the years to come.
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