Tag Archives: Standard Model

New Article on Sterile Neutrinos

Symmetry has a fairly new article on sterile neutrinos, explaining some of the basic ideas about what they are and why we’re interested in them. Measurements of the Z peak in e+/e- collisions at LEP showed that there are only 3 neutrinos, but there are some caveats to that. The measurements showing that there are 3 neutrinos really mean that there are 3 neutrinos that (1) have less than half the Z mass (91.2 GeV) and (2) interact with Standard Model particles via the weak force.

If we can instead add some “neutrinos” that don’t interact through the weak force, then there’s room for more neutrinos. One of the main ways people search for them is to find problems with the standard picture of 3-neutrino oscillations. If there are sterile neutrinos that mix with the three usual flavors (electron, muon, and tau), then maybe we can find evidence of neutrinos oscillating into sterile neutrinos. That is, regular neutrinos seemingly disappearing altogether rather than changing from one type to another.

New e+/e- Collider Turns On

SuperKEKB, an upgraded version of the KEK B electron/positron collider at the KEK lab in Tsukuba has started up. There are no collisions yet, but they are starting to run beams around the main ring. The Belle II experiment will use he SuperKEKB facility to study B physics – that is, the physics of B mesons (and other particles involving b quarks). These are particularly interesting because the lifetimes of many of these particles decay weakly with long enough lifetimes to actually measure how far they travelled. B physics allows for high precision tests of various aspects of the Standard Model, including things like CP violation in the quark sector (i.e. matter/antimatter differences), particle spectroscopy (measuring the properties of the various kinds of composite particles), and searches for various kinds of new physics.

Finally Something Possibly Interesting…

The two big LHC experiments ATLAS and CMS just released some of their first results from LHC Run 2, which is the newest run and the first at close to the design energy of 14 TeV.

I missed streaming the talks live and haven’t had time to track down the videos or slides yet, but everyone is buzzing about one thing in particular: a small excess seen in both experiments in the diphoton channel at a center of mass energy near 1.5 TeV (note: going over the slides now and it looks like Nature has embarrassingly completely misunderstood the plots. The peak is at 750 GeV). The result is quoted at a few sigma significance for each experiment, so the combined result won’t be high enough to show anything definitive at will be lowered somewhat by the “look elsewhere” effect, where the true significance of an excess in such a global search is actually lower than the apparent local significance (i.e. if you make enough measurements you’ll always find some large deviation from your expectation).

A peak in the diphoton channel potentially points to a new heavy neutral boson. This could be one of the many predicted bosons in various models of beyond the Standard Model physics or it could be something completely new. If the result holds up and it really is some new elementary particle (it’s far heavier than even the top, so if the excess is real, it’s probably not a bound state of known particles), it would be very exciting news for the field. That would be the first truly new particle found since the development of the Standard Model. Figuring out what it is and where it comes from would then clearly be one of the highest priority tasks for experimentalists over the next few decades.

There still aren’t any signs of supersymmetric particles like squarks and gluinos, making it even harder to fit minimal supersymmetry with current measurements.

ATLAS Measures the Higgs Spin and Parity

ATLAS has a fairly new result out measuring the properties of the Higgs boson using leptonic and diphoton final states. In particular, they compare the expected results for a Standard Model JP = 0+ Higgs to several alternative models.  The measurement pretty clearly rules out things like a spin-2 Higgs and also sets upper bounds on the strengths of beyond the Standard Model Higgs tensor couplings. The spin-parity state can often be determined from looking at final state kinematic distributions. Things like the angular distribution will often be very different for different types of particles.

This kind of result is nice because it helps bolster the case that the ~125 GeV particle found at the LHC has the same fundamental properties as the Standard Model Higgs boson. Most other measurements are measuring cross sections or branching fractions, which tell us about the particle’s interactions with other particles. So far, everything looks remarkably like the Standard Model Higgs.

New CMS Search for a Beyond the Standard Model Higgs

The CMS collaboration posted a preprint for a search for a beyond the Standard Model Higgs boson decaying into pairs of intermediate vector bosons (Ws and Zs). In many common extensions of the Standard Model, such as in the MSSM (Minimal Supersymmetric Standard Model), there is an expanded Higgs sector that includes a number of scalar and pseudoscalar Higgs bosons. The extra Higgs bosons can have electric charges (and can even be doubly-charged).

This search looks for neutral Higgs bosons with masses that are quite a bit higher than the Higgs found a couple years ago. The analysis looks at several final states from Higgs decays to WW and ZZ, such as ZZ to four charged leptons. As usual, the general feature expected from a new particle is a bump in the invariant mass spectrum compared to the Standard Model prediction. The results are consistent with the Standard Model, so they set a limit on the cross section for these events, and can exclude a heavy Higgs up to around a 1 TeV mass (obviously this requires some assumptions on the couplings to other particles, so this isn’t a general exclusion for all scalar particles).

LHC To Restart This Weekend?

CERN is reporting that the Large Hadron Collider is ready to resume running. There was a minor problem with an electrical connection that has been fixed, so hopefully everything is ready for collisions at a center of mass energy of 13 TeV, just below the design energy of 14 TeV.

The beam is expected to start this weekend, as long as no other problems arise. Physics beam won’t happen quite yet though. It sounds like there is going to be an extended period of beam tuning, with actual collisions at full beam energy starting in June.

Higher energy allows for a great deal of new work to be done. In some important analyses, such as Higgs searches, the background can fall faster with energy than the signal, boosting the overall statistical power of results. Furthermore, higher energy increases the possible reach of searches for new physics. Many hope for new physics at the TeV scale in order to avoid fine-tuning in the electroweak sector of the Standard Model (such as from heavy quark loop corrections to the Higgs mass). In collisions at these energies, the protons can’t be treated as individual particles, but rather than collections of quarks and gluons (collectively called partons in this context). While the center of mass energy of a proton-proton collision will be 13 TeV, the actual amount of energy accessible in a collision will be smaller. If, for example, we take two typical quarks with about 30% of the total momentum, the center of mass energy would instead be around 4 TeV. Gluons typically carry even less energy (the actual momentum distributions of quarks and gluons in a proton are called “parton distribution functions”). So, in order to access TeV-scale physics, it would be useful to have an overall energy around an order of magnitude higher.

Dark Matter Candidates: Axions

In my last dark matter post, I described WIMPs. While WIMPs are a very attractive dark matter candidate, they are by no means the only one. Axions are a much more difficult particle to describe than WIMPs. As an experimental particle physicist, even I don’t understand a lot about them (though I admittedly have never studies them in detail). As usual, you can look at the PDG axion review for more detailed info (that’s where a lot of the information I’m using here is from).

Axions were first proposed by Peccei and Quinn to solve a very different problem in particle physics: the strong CP problem. It turns out that in a generic theory of quantum chromodynamics (QCD), we would expect there to a be a CP-violating term ΘGμνGμν where Θ determines the amount of CP violation. CP violation in strong interactions can be found by looking for things like the existence of the neutron electric dipole moment. If CP is preserved, this must be exactly 0. Experimental bounds have found no evidence for the existence of the neutron EDM, so the parameter Θ must be incredibly small. Such a coincidence would seem very strange to physicists, so this suggests that maybe there is something that forces this to happen. The axion is the particle arising from a scalar/pseudoscalar field added (using a new U(1) symmetry) to force QCD to preserve CP symmetry.

The axion is expected to interact with photons through an interaction term that looks like gφFμνFμν where the coupling constant g is related to the axion mass. This in particular, allows for the axion to be converted into a photon in the presence of a strong electromagnetic field. Most experimental searches try to find axions using strong magnetic fields in resonating cavities.

In the early universe, while axions can be formed through the usual thermal processes, these will lead to hot (relativistic) axions that probably won’t be a suitable dark matter candidate. However, a condensate of axions can be created during spontaneous symmetry breaking, generating a large number of non-relativistic axions. It turns out that for masses around 10 μeV the correct relic density is achieved. So, dark matter axions will have incredibly tiny masses, likely even less than neutrinos and, as for WIMPs, could be related to the solution for several open questions about the Standard Model.