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Can humanity’s neutrino eyes, observatories like the IceCube in Antarctica, perceive neutrinos from distant space? The answer is starting to emerge from studies at the Large Hadron Collider (LHC), which is studying the interior structure of protons, among other things. According to the most recent model developed by IFJ PAS researchers, this structure seems to be richer in charm particles, making it harder for terrestrial neutrino viewers to comprehend what they see.
Contrary to conventional belief, the proton may be made up of not three, but five quarks. A quark and antiquark generated by the interactions of gluons within the proton then form an extra pair. It has long been assumed that these ‘extra’ pairs may be formed of heavy quarks and antiquarks like charm. It has now been discovered that taking into consideration the inherent charm of protons helps us to more precisely characterize the path of occurrences recently observed in one of the LHCb detector’s low-energy experiments. The appropriate theoretical model is provided in Physical Review D by physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow.
The proton is depicted in school textbooks as a simple agglomeration of three quarks: two up quarks and one down quark, held together by strong forces conveyed by gluons. Such a simple model has not had a lengthy career in physics. It was discovered towards the end of the 1980s that in order to explain the observed occurrences, light quarks from the nucleon’s meson cloud had to be included (these are so-called higher Flock states). Surprisingly, the impact is not minor and may potentially represent a 30% adjustment compared to the basic three-quark model. Unfortunately, it has not been feasible to estimate the magnitude of a similar contribution from charm quarks so far.
“Our previous models of charm creation have consistently shown agreement with experiments.” We were able to characterize the creation of pairs including charm quarks and antiquarks rather effectively at high energy of proton collisions, when two opposing beams of protons experienced mutual interactions at the LHC. The problem is that, although they were generated after proton collisions, they did not originate from the interior of protons. “They were formed as a consequence of the fusion of gluons produced by protons a little earlier,” explains Prof. Antoni Szczurek (IFJ PAN).
Recent data in the LHCb detector with a single proton beam focused at a stationary helium or argon gas target provided hope for progress in tracking the charm within the protons themselves.
“When collisions occur at the LHC’s greatest energy, a considerable fraction of proton collision products flow in the ‘forward’ direction, along the proton beams.” As a consequence, they find themselves in a region with no detectors for technological reasons. However, the collisions of protons with helium nuclei that we have just studied occurred at energy that were many tens of times lower than the LHC’s greatest energies. Collision products bounced about at bigger angles, more sideways, and as a consequence were detected in detectors, which we could examine,” says Dr. Rafa Maciua (IFJ PAN).
The Cracow-based scientists utilized a model enhanced by the probability of a charm quark or antiquark bursting out from within the proton to understand the findings from the LHCb detection experiment. It was not feasible to calculate the likelihood of such a process from fundamental principles. As a result, the researchers chose to investigate what probability values would result in the greatest agreement between the model predictions and the actual data. The results indicated that the contribution of charm pairs inside the proton is no more than roughly 1%.
After breaking out from the proton’s interior, the charm quark-antiquark pair immediately transforms into short-lived D0 mesons and antimesons, which create other particles, including neutrinos. This feature prompted IFJ PAN researchers to test the novel model against data from Antarctica’s IceCube neutrino observatory.
Nowadays, owing to the methods utilized, IceCube scientists are certain that if they detect a neutrino with a high energy (on the scale of hundreds of teraelectronvolts), the particle originated in deep space. It is also thought that neutrinos with somewhat lower but still very uncommon high energy are cosmogenic in nature. This interpretation may be questioned if a charm quark-antiquark pair is pushed out of the proton interior and decays in a cascade involving high-energy neutrinos. Indeed, neutrinos in a certain energy range that are now being recorded may have originated not from space, but rather from cascades launched by collisions between primary cosmic radiation particles and atmospheric gas nuclei. An essay delving into this possibility has been accepted for publication in the European Physical Journal C.
“When analyzing the IceCube observatory data, we used the following strategy.” Assume that almost all presently documented neutrinos in the energy range we’re looking at come from the environment. What would the contribution of charm quark-antiquark couples within the proton have to be for our model to match with the data thus far? Consider that we received a value of one percent, which is almost equal to the value derived from the model explaining proton-helium collisions in the LHCb detector!” Dr. Maciua states
The convergence of estimates in each of the above situations necessitates extreme care in determining the origins of neutrinos detected by current observatories. However, the Cracow researchers emphasize that their findings place just an upper limit on the contribution of charm quarks and antiquarks to proton structure. If it turns out to be smaller, at least some of the high-energy neutrinos that have been identified so far will preserve their cosmic character. If the top limit is true, our view of their origins will have to shift dramatically, and IceCube will turn out to be more than just an astronomical observatory, but also… atmospheric.