# Why should the X17 be a force carrier?

A preprint has been recently published on arXiv about another experimental evidence of the existence of the X17 particle, a 17 MeV boson that would be a potential force carrier. Now, I don't have a strong subnuclear background, but I was able to read the article and understand how the experiment was performed and how accurate the results were. But why finding such a boson would mean another fundamental force? Couldn't it be just a resonance? My guess is for the low invariant mass, but I'm probably wrong.

• a resonance made out of what? – JEB Nov 26 '19 at 18:20
• Related, but the emphasis is on whether the effect is real rather than on whether it's a force: physics.stackexchange.com/q/258214/44126 – rob Nov 26 '19 at 19:13

One of the guiding principles of physics is that anything not forbidden is mandatory. If it's possible for something to happen, then somewhere in the universe it happens spontaneously.

In particle physics, this means that if there's some particle that you can produce resonantly in some well-chosen reaction, that particle is a background contributor to other reactions. For instance, when we are teaching people about the Standard Model we might say something like "electrons don't carry the color charge and therefore don't participate in the strong interaction." Which is approximately true. But if you collide electron beams with many GeV of interaction energy, the electric field at the interaction point is strong enough to make electrically-charged hadrons, which do participate in the strong interaction, appear out of the vacuum and reach your detector. Those hadron jets may suddenly appear in your detector above a threshold beam energy, but the selection rules which produce them still apply even at energies below the threshold, and electron-electron scattering is modified by hadronic interactions with virtual particles in the vacuum even when there is not enough energy to make any real hadrons. The statement that "electrons don't participate in the strong force because they don't have color charge" is only true at "tree level"; it breaks down when you start to consider Feynman diagrams with "loops" that describe virtual particles, in a manner that varies with energy and angle in a calculatable way.

Likewise, some people explain that the weak interaction isn't a "force" so much as a flavor-changing mechanism that causes beta decay. (Those people get quiet when you ask then about the weak neutral current.) If you want to produce real weak bosons to send signals to a detector, you have to have hundreds of GeV of energy at play in your interaction. But the because the weak interaction obeys a different set of symmetries than the strong and electromagnetic interactions, we can look for parity-violating corrections to those interactions and tease out information about the weak force. I did my PhD looking at evidence of the weak force as a contributor to the interaction between protons at rest and neutrons at rest.

The so-called X17 has been hinted at in interactions where few-nucleon systems are combined to make very energetic photons, some of which turn into $$e^+e^-$$ pairs, because at some energies and angles there is an excess of the lepton pairs. That means that this particle couples to the nucleon field, the photon field, and/or the electron field. By the arguments above, every calculation of nucleon/photon/electron interactions should therefore be modified, if only a little, by the presence of virtual X17s. That's what forces do: they modify interactions.

As to whether the X17 is a new fundamental particle or not, we can't really answer that question until we have confirmation from another group that it exists. But its existence reveals that something important is missing from the standard model. It's a factor or ten lighter than the pion, which is the "massless" Goldstone boson of QCD, and a factor of 10,000 lighter than the massive electroweak bosons. We know that our current Standard Model is missing phenomena that occur in nature because the current Standard Model fails to predict important observed phenomena like our matter-dominated universe. Our current Standard Model also fails to predict any 20-MeV-ish scalar or pseudoscalar bosons. Even if it's "just a resonance," it's new physics.

If it's real. Nothing resembles a new effect quite so much as a mistake.

• This was very clear, thank you, especially the last paragraph brightly answered my question. But I don't understand what you mean by "forces modify interactions". I thought they WERE interactions. – Mauro Giliberti Nov 27 '19 at 13:29
• That's a question about vocabulary, and I'm not sure I can clarify it without going in a circle. I guess that a force is a type of interaction, one where the momenta of the participants is modified. That would appear to leave open the possibility of other types of interactions (like when I say "good morning" to a stranger but they don't respond, and their momentum doesn't change because I haven't pushed them). The long part of this answer is that it still make sense to refer to such interactions as "forces" anyway. (I could knock someone over by saying "good morning" vigorously enough.) – rob Nov 27 '19 at 16:07

Bosons tend to be force carriers - all of the currently-known bosons are force-mediating particles. So, it seems natural to infer that, if this new particle is a boson, it would imply the existence of a previously unrecognized force of nature.

• This makes sense to me, but what you're saying only applies to fundamental bosons. The pion, that was thought to be fundamental (and therefore a force carrier) turned out to be composite, made of fermions. Couldn't this be another pion-like situation? – Mauro Giliberti Nov 27 '19 at 13:18