T2K released a preprint today measuring the charged-current quasielastic scattering cross section of muon neutrinos on carbon using the ND280 off-axis near detector. The measurement reports the total cross section, the double-differential cross section using muon kinematic variables (muon momentum and angle with respect to the beam direction), and also reports a fit to the axial mass parameter of a theoretical model. There is good agreement between the measured flux-averaged cross section and the expectation from theory, and agreement between the measured and expected differential cross sections are also mostly good. The axial mass extraction is also reasonably consistent with expectations, so overall, everything seems to fit pretty well to expectations from the NEUT Monte Carlo generator and also from previous measurements.
On Thursday, the EXO-200 experiment released a preprint on a search for exotic double beta decay modes involving no neutrinos but with emission of one or more “Majorons,” which are bosons related to the violation of lepton number required by these decay modes.
EXO-200 is an experiment at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM. It consists of a time projection chamber (TPC) using highly enriched liquid xenon as the target material. The xenon is enriched with a higher fraction of a particular isotope that is expected to undergo double beta decay – which happens when regular beta decay is disallowed due to energy constraints but double beta decay, where two electrons (or positrons) are emitted at once is allowed. The main goal of EXO is to search for neutrinoless double beta decay, where the two neutrinos created along with the two electrons basically cancel one another. The existence of neutrinoless double beta decay requires that neutrinos be Majorana fermions, where they are their own antiparticles, instead of Dirac fermions like all the other known fermions. This paper looks at more complicated decay mode than the standard one of just two electrons and a nucleus in the final state.
The paper sets lower limits on the lifetime of xenon-136 using different models of neutrinoless double beta decays with Majorons in the final state. It also sets equivalent upper limits on the Majoron-electron coupling for some of these models.
The Planck collaboration has released a new measurement of the power spectrum of polarized light from dust. It’s been getting a lot of press because the measurement seems to present a challenge to the earlier (and much-touted) BICEP2 result that claimed to see evidence of primordial gravitational waves in polarized light from the Cosmic Microwave Background (CMB).
Sean Carroll has a summary here. It seems that there is a good chance that BICEP’s measurement was good but their interpretation of it as gravitational waves was not correct. However, a member of the Planck collaboration shows up in the comments to say that Carroll is probably overstating the case that the BICEP2 result was spurious. The two groups are apparently working on a combined result that will hopefully clear up the current confusion over what was actually seen.
PRL published a couple new papers from AMS today, including this one, which shows an updated plot of the positron fraction (e+/(e+ + e-)) as a function of energy. I don’t think these have been posted to the arXiv yet, which is unusual for our field. Unfortunately, PRL must be accessed either through a network with access or with an account, which requires money, so if you’re not at a university and aren’t an APS member you might have trouble looking at the paper. In conjunction with the papers being released, there was a seminar at CERN earlier today. I didn’t find out about the seminar until it was too late for me to try to call in remotely.
AMS is a large multipurpose detector based at the International Space Station that studies cosmic rays, which are high energy particles flying around in space. The search for dark matter is one of the main purposes of the experiment, but it can study many things about cosmic rays, such as their composition, energy spectra, directions, etc.
This paper shows the positron fraction up to 500 GeV, which is a bit higher than in previous measurements. This measurement is useful for looking for dark matter annihilation to electron-positron pairs. The expected signal is for the positron fraction to increase at some energy and then peak and drop fairly sharply at an energy of approximately the dark matter mass. Previous measurements from AMS, Fermi, PAMELA and ATIC have all seen an increase in the ratio and have even seen the spectrum seem to level off. This new measurement shows that the leveling off continues, and the spectrum may even be starting to fall in the highest energy bin. However, the measurement is limited by both statistics and systematics at the highest energies, so the apparent decrease in the highest bin is not going to be statistically significant at this point in time. That may change with more events in the future and hopefully a better understanding of the detector and of astrophysical models for the non-dark matter background.
The Particle Data Group has announced that the 2014 edition of the PDG’s Review of Particle Physics is now available online. As usual, you can find it here. The PDG provides exhaustive reviews of all known particles as well as short review articles on various topics in particle physics. It tends not to change much from year to year but is a very useful reference for both students and professionals.
ATLAS has yet another Higgs measurement on the arXiv, this time looking at the diphoton channel. This is the Higgs decay H→2γ.
Higgs couplings to other particles are related to those particles’ masses. Couplings to fermions are proportional to the fermions’ masses and couplings to bosons are proportional to the squares of the masses. The photon, however, is massless, so it does not couple to the Higgs. Then how does this channel even exist? The Higgs couples directly to particles that also couple to photons. Using Feynman diagrams, the Higgs can couple to photons via a loop of some massive particle like a heavy quark. This leads to an indirect Higgs-photon coupling but the diagram (representing the mathematical equations to calculate the decay width) is more complicated. As a result, the Higgs to two photon decay exists, but it is not particularly common. It is still a very useful channel for searches because the energies of photons can be measured very well, allowing for an accurate reconstruction of the Higgs energy, and because the two photon channel is much cleaner than most channels. There aren’t many processes that will result in two high energy photons with a large transverse momentum, so while the signal is small, the background is small as well. A statistically stronger measurement can be obtained from the relatively few events in this channel. Higgs production via gluon fusion is basically the opposite process – just with gluons rather than photons – as this decay,
As with the earlier paper on the four lepton final state, this paper measures the total number of H→2γ events at 7-8 TeV and also tries to separate the sample into various Higgs production channels. Not surprisingly, no significant deviations from Standard Model predictions are found.
The PandaX group has released their first dark matter result, which can be found here. PandaX is a dual-phase liquid xenon detector, similar to XENON-100 and LUX. The actual result is not particularly interesting, but PandaX is an interesting experiment because it is one of the first particle physics experiments to be hosted in China. The collaboration is mostly from China but includes a few people from institutions in the US.