mersenneforum.org - Official "Science News" Thread

I wasn't able to find the paper published in the PRL, but the article Mike posted did include [URL="http://arxiv.org/abs/1608.03591"]an arXiv link[/URL]. Its conclusion is here, which is really more of a summary than a conclusion, but it's also omits tons of technical details and provides an excellent overarching map of what the anomaly may mean.

I'm too lazy to fix the copy and paste issues, you can go see the paper if you want it better. The summary of the summary is that there is a 6.8 sigma anomaly, the sort of thing that doesn't disappear overnight. Many possible interpretations are excluded by this paper, with a couple of categories left open and focus on the category they view as most likely/best fitting, presenting a couple of different SM extensions within the category. Among other things, they describe what the LHC and several other near future experiments can do to exclude the other categories and one or the other of the options within their focused category. They also point out that the two most likely options they explored also have the potential to explain several other as yet unexplained anomalies in particle physics.

@xilman they focus on vector gauge bosons as the category of interest.

[quote]X. CONCLUSIONS
The 6.8σ anomaly in 8Be cannot be plausibly explained as a statistical fluctuation, and the
fit to a new particle interpretation has a χ
2/dof of 1.07. If the observed bump has a nuclear
physics or experimental explanation, the near-perfect fit of the θ and mee distributions to
the new particle interpretation is a remarkable coincidence. Clearly all possible explanations
should be pursued. Building on our previous work [7], in this study, we presented particle
physics models that extend the SM to include a protophobic gauge boson that explains the
8Be observations and is consistent with all other experimental constraints.
To understand what particle properties are required to explain the 8Be anomaly, we first
presented effective operators for various spin-parity assignments. Many common examples
of light, weakly coupled particles, including dark photons, dark Higgs bosons, axions, and
B − L gauge bosons (without kinetic mixing) are disfavored or excluded on general grounds.
In contrast, general gauge bosons emerge as viable candidates.
In Ref. [7] we determined the required couplings of a vector gauge boson to explain the
8Be anomaly assuming isospin conservation, and found that the particle must be protophobic.
In this work, we refined this analysis to include the possibility of isospin mixing in the
8Be∗
and 8Be∗0
states. Although isospin mixing and violation can yield drastically different
results, these effects are relatively mild once one focuses on protophobic gauge bosons. It
would be helpful to have a better understanding of the role of isospin breaking in these
systems and a quantitative estimate of their uncertainties. The presence of isospin mixing
also implies that the absence of an anomaly in 8Be∗0 decays must almost certainly be due to
kinematic suppression and that the X particle’s mass is above 16.7 MeV. Combining all of
these observations with constraints from other experiments, we then determined the favored
couplings for any viable vector boson explanation.
We have presented two anomaly-free extensions of the SM that resolve the 8Be anomaly.
In the first, the protophobic gauge boson is a U(1)B gauge boson that kinetically mixes
with the photon. For gauge couplings and kinetic mixing parameters that are comparable in
size and opposite in sign, the gauge boson couples to SM fermions with approximate charge
Q − B, satisfying the protophobic requirement. Additional matter content is required to
cancel gauge anomalies, and we presented a minimal set of fields that satisfy this requirement.
33
In the second model, the gauge boson is a U(1)B−L gauge boson with kinetic mixing, and the
SM fermion charges are Q − (B − L). Additional vectorlike leptons are needed to neutralize
the neutrino if we consider only a single U(1) gauge group. Both models can simultaneously
resolve the (g − 2)µ anomaly, have large electron couplings that can be probed at many near
future experiments, and include new vectorlike lepton states at the weak scale that can be
discovered by the LHC.
One may speculate that the protophobic gauge boson may simultaneously resolve not only
the 8Be and (g −2)µ anomalies, but also others. Possibilities include the NuTeV anomaly [14]
and the cosmological lithium problem mentioned in Sec. VIII C. Another possibility is the
π
0 → e
+e
− KTeV anomaly, which may be explained by a spin-1 particle with axial couplings
that satisfy

g
u
A − g
d
A

g
e
A

20 MeV
mX
2
≈ 1.6 × 10−7
, (99)
which is roughly consistent with the vector couplings we found for a protophobic gauge
boson [137]. Independent of experimental anomalies, a spin-1 boson with purely axial
couplings is a promising candidate for future study. Such bosons need not be protophobic
because their suppressed contributions to neutral pion decays relax many constraints that
existed for vector bosons. We note, however, that some bounds become stronger for the axial
case. For example,the decay φ → η(X → e
+e
−) used in deriving the KLOE constraints [64]
is an s-wave in the axial case, implying a stronger bound than the p-wave–suppressed one in
the vector case. Another example is (g − 2)e [138], for which an axial vector makes larger
contributions than a vector, for couplings of the same magnitude. In addition, there are very
stringent bounds, for example, from atomic parity violation, on gauge bosons with mixed
vector and axial vector couplings [139].
Finally, if the 8Be anomaly is pointing toward a new gauge boson and force, it is natural to
consider whether this force may be unified with the others, with or without supersymmetry.
In the case of U(1)B−L, which is a factor of many well-motivated grand unified groups, it
is tempting to see whether the immediately obvious problems—for example, the hierarchy
between the required U(1)B−L gauge coupling and those of the SM—can be overcome, and
whether MeV-scale data may be telling us something interesting about energy scales near
the Planck scale.[/quote]