Transmutation of $^{16}$O and $^{20}$Ne at the Large Hadron Collider

TL;DR

The study examines the potential transmutation of ultrarelativistic beams of $^{16}$O and $^{20}$Ne at the LHC into lighter isotopes that can continue circulating as contaminants. It uses the Trajectum framework with an augmented spectator–cluster model and GEMINI decay to estimate production cross-sections for isotopes with lifetimes $\tau>1~\mathrm{s}$ and to map their momentum distributions under LHC optics, finding total hadronic cross-sections of $\sigma_{\rm had}\approx 1.42~\mathrm{b}$ for OO and $\sigma_{\rm had}\approx 1.85~\mathrm{b}$ for NeNe. A dominant circulating component is $^{4}$He, with a characteristic proton peak near $\delta p/p\approx-0.5$ from Fermi motion, and only isotopes within $|\delta p/p|<0.034\%$ can remain in the beam, implying that optics largely set the circulating yields. The work also shows that transmuted species influence observables such as multiplicity $N_{\rm ch}$ and mean transverse momentum $\langle p_T \rangle$, offering a potential experimental handle to study $^{16}$O+$^{4}$He collisions and to explore fragmentation-region physics, albeit with notable systematic uncertainties tied to nucleon momenta and cluster formation.

Abstract

In July 2025 the Large Hadron Collider (LHC) will collide $^{16}$O$^{16}$O and $^{20}$Ne$^{20}$Ne isotopes in a quest to understand the physics of ultrarelativistic light ion collisions. One particular feature is that there are many smaller isotopes with the exact same charge over mass ratio that potentially can be produced and contaminate the beam composition. Using the Trajectum framework together with the GEMINI code we provide an estimate of the production cross-section and its consequences. A potential benefit could be the interesting measurement of the multiplicity and mean transverse momentum of $^{16}$O$^{4}$He collisions.

Figures (7)

• Figure 1: We show cross-sections of isotope productions as a function of momentum (relative to their 'beam' momentum) by hadronic interactions for $^{16}$O$^{16}$O (top two rows) and $^{20}$Ne$^{20}$Ne collisions (bottom two rows) for isotopes that have a lifetime greater than one second. We assume that isotopes with $|\delta p/p|<0.034\%$ can keep circulating in the LHC. The total hadronic $^{16}$O$^{16}$O and $^{20}$Ne$^{20}$Ne cross-section is about $1.42\,$b and $1.85\,$b respectively for our model. The production of helium and other isotopes is enhanced by about 14% more than a naive scaling with the cross-section would have given.
• Figure 2: We show the $N_\text{ch}$ distributions for several light ion configurations.
• Figure 3: We show the mean transverse momentum (top) and fluctuations thereof (bottom) as a function of $N_\text{ch}$ (top, defined as in \ref{['fig:ntrackdistributions']}) and centrality (bottom) for several light ion configurations. Notably there is a large difference in $\langle p_T \rangle$ when comparing different systems at fixed multiplicity.
• Figure 4: We show increase in mean $p_T$ as a function of $N_{\rm ch}$ (same cuts as in Fig. \ref{['fig:ntrackdistributions']}) that would occur if 0.073% (0.088%) of transmuted deuteron ($^{4}$He) would be present in the beam.
• Figure 5: We show the equivalent figure as in \ref{['fig:oxygentransmutation']}, but with all initial nucleon momenta in the nucleus halved, as well as their maximum distance, $\Delta p_\text{max}$, to be combined into a cluster. The smaller momentum leads to an approximate doubling of the transmuted isotopes in the beam.