Letter of Intent for Alpha Knockout from Nuclei in Hard Exclusive Processes with ALERT
This Letter of Intent proposes using the CLAS12 spectrometer with the ALERT detector at Jefferson Lab's 12 GeV facility to perform the first simultaneous measurements of quasi-elastic alpha knockout, coherent DVCS, and coherent vector-meson production on light alpha-conjugate nuclei, aiming to determine whether the quark and gluon distributions of bound alpha clusters differ from free helium-4 and how these modifications correlate with alpha-clustering strength.
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Nuclear physics has long relied on a simple picture of the atom's core: heavy nuclei are built from protons and neutrons, which in turn are made of smaller particles called quarks. For decades, scientists have known that when these protons and neutrons are packed tightly together inside a nucleus, their internal makeup changes slightly compared to when they are floating freely. This phenomenon, known as the EMC effect, suggests that the environment of the nucleus alters the behavior of the fundamental particles within. However, a deeper mystery remains. Some light nuclei, such as carbon and oxygen, appear to be organized not just as a random soup of protons and neutrons, but as clusters of four-particle groups called alpha particles. The question driving a new proposal from researchers at the Jefferson Lab is whether these alpha clusters retain their unique internal structure when bound inside a larger nucleus, or if the surrounding nuclear environment reshapes them at the level of quarks and gluons.
A team of physicists led by researchers at Argonne National Laboratory and Temple University has outlined a plan to answer this question by firing high-energy electrons at a series of light gas targets. Their goal is to perform a precise "knockout" experiment, where an electron strikes a nucleus and ejects a single alpha particle, allowing scientists to study the ejected particle's properties in detail. They intend to measure this process on three different types of nuclei—carbon, oxygen, and neon—which are known to have varying degrees of alpha-cluster organization. By comparing the results from these heavier nuclei against a baseline measurement of a free, unbound alpha particle (helium-4), the team hopes to see if the internal structure of the alpha particle changes depending on how tightly it is clustered within its host nucleus.
The experiment relies on a specialized detector called ALERT, which was recently installed in the CLAS12 spectrometer at the Jefferson Lab. This device is uniquely capable of catching the slow-moving alpha particles that recoil after being struck, a task that previous equipment could not perform with the necessary precision. The researchers plan to use a 12-GeV electron beam to hit targets containing helium, oxygen, neon, and ethane gas. They will focus on three specific types of interactions. First, they will measure the basic probability of knocking an alpha particle out of the nucleus. Second, they will study how the alpha particle scatters light, a process that reveals the distribution of quarks inside the cluster. Third, they will observe how the alpha particle produces heavy particles called vector mesons, which acts as a probe for the distribution of gluons, the particles that hold quarks together.
The significance of this work lies in the comparison of ratios. Instead of looking at absolute numbers, the team will compare the behavior of alpha particles inside carbon, oxygen, and neon against the behavior of free helium-4. If the internal structure of the alpha particle is modified by its nuclear environment, these ratios will shift in a way that correlates with how strongly the alpha clusters are formed in each nucleus. For instance, if the quark and gluon distributions change as the clustering strength increases from carbon to neon, it would suggest that the nuclear environment directly impacts the fundamental building blocks of matter. Conversely, if the ratios remain constant, it would imply that the alpha particle's internal structure is robust and unaffected by its surroundings.
The researchers have designed the experiment to minimize uncertainty by using the same detector and analysis methods for all targets. They will run the experiment for approximately 85 days, collecting data on the scattered electrons and the recoiling alpha particles. By matching the statistical precision of previous successful runs, they aim to produce clear, high-quality data that can distinguish between different theoretical models of nuclear structure. This approach allows them to test whether the modifications seen in the EMC effect are driven by the average density of the nucleus or by the specific, long-range correlations that form alpha clusters.
Ultimately, this proposal seeks to determine if the rules of quantum chromodynamics, the theory describing the strong force, apply differently to particles when they are part of a cluster versus when they are free. The outcome will either confirm that the alpha particle is a rigid, unchangeable unit within the nucleus or reveal that its internal quark and gluon landscape is fluid and responsive to its neighbors. Regardless of the result, the measurement will provide a direct, partonic-level view of how light nuclei are constructed, offering a new perspective on the fundamental forces that hold the visible universe together.
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