SUSY Exclusions Summarized and Photoproduction of Pions

SUSY Exclusions at ATLAS

A new preprint summarizes the ATLAS SUSY exclusions to date (there are no anomalies or hints in the searches pointing to SUSY particles). 

Gluinos are excluded below about 2 TeV, while stop and sbottom squarks are excluded up to the high hundreds of GeVs.

Somewhat disappointingly, even though Figure 1 of the paper points out the one of the most important arguments for SUSY is that it modifies the beta functions of the Standard Model coupling constants (i.e. it causes Standard Model forces to become stronger or weaker at high energies in a different way that the Standard Model does), there are no reports on the measurements the LHC has made of the running of the Standard Model coupling constants with energy scale even tough it should have enough data at this point to make preliminary reporters on this question.

Pion Photoproduction

If you hit a proton with an X-ray (i.e. a high energy photon), one thing that can happen is that it can spit out a proton and a neutral pion, which in turn decays to a positron, an electron, and a photon. The CLAS experiment did this and compared the results to the predictions of the Standard Model and in particular, to the strong force part of the Standard Model called QCD.

The neutral pion is special. As the paper cited below explains in its introduction:

[T]here are properties of π⁰ that make this particle very special for our understanding of Quantum Chromodynamics (QCD). To name a few: it is the lightest element of all visible hadronic matter in the Universe; according to its qq^¯ content the π⁰ has a mass much less than one would expect from a constituent quark mass, m ≈ 350 MeV and it has an extremely short life time, τ ≈ 10⁻¹⁶ s. Its main decay mode, π⁰ → γγ, with a branching ratio ≈ 99%, played a crucial role in confirming the number of colors in QCD and in establishing the chiral anomaly in gauge theories. With all this being said, the structure and properties of π⁰ are not completely understood.

Certain predictions of QCD were accurately reproduced. Others, based upon a Parton Distribution Function (PDF) used to make calculations in QCD which was empirically established at a lower energy scale was quite a bit off the mark, suggesting that PDFs are more energy scale dependent than expected.

While we think we know the exact "laws of the universe" for QCD, except for the exact value of some experimentally measured physical constant (especially the QCD coupling constant), the math necessary to turn those equations into actual predictions is so hard that scientists have to approximate and the task of figuring out which approximations to use in which circumstances continues to be on ongoing project established by trial and error.

The abstract of the preprint and it citation are as follows:

"Exclusive photoproduction cross sections have been measured for the process γp→pπ0(e+e−(γ)) with the Dalitz decay final state using tagged photon energies in the range of Eγ=1.275−5.425 GeV. The complete angular distribution of the final state π0, for the entire photon energy range up to large values of t and u, has been measured for the first time. The data obtained show that the cross section dσ/dt, at mid to large angles, decreases with energy as s−6.89±0.26. This is in agreement with the perturbative QCD quark counting rule prediction of s−7. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy s=11 GeV2. At the same time, the Regge exchange based models for π0 photoproduction are more consistent with experimental data."

Kunkle, et al., "Exclusive photoproduction of π0 up to large values of Mandelstam variables s,t and u with CLAS" (December 29, 2017).

The data was actually collected in 2008.