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.
LHCb has uploaded a new preprint presenting the discovery of two new resonances. Various media reports have been breathlessly declaring this to be the discovery of two new particles which, while not technically incorrect, is quite misleading. What they found was what looks like two new resonances (very short lived particles) related to a bound state of quarks. The headlines imply that these are some totally new phenomenon such as a new elementary particle, but in reality these look like excited states of an already-known composite particle.
In the analysis, LHCb looked at the center of mass energy of Ξb0 + π– final states in proton-proton collisions. The Ξb0 (the Ξ family are called xi baryons or cascade particles depending on who you talk to) is a baryon (like a proton or neutron) with an up-strange-bottom valence quark structure and an angular momentum (spin+rotational) of 1/2. Two peaks in the spectrum are seen with an energy that is very slightly different than the total mass of the final state particles. These peaks are generally interpreted as new bound states. These decays should occur via the strong force, and the width (inverse lifetime) of the heavier particle is large enough to be measured to a precision of around 20%.
Since the particles must decay strongly, the final state indicates that they have a down-strange-bottom valence structure, giving a charge of -1. One is identified as having an an angular momentum of 1/2 and the other with 3/2. This interpretation is based on predictions of various models but confirmation with data may come in the future when more data is analyzed. Measurements of the mass splittings and widths can help refine theoretical models of baryons, which are actually much more complex than just three quarks orbiting around each other.
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.
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.
A new Higgs measurement from ATLAS is out on the arXiv now. The paper presents the result of a measurement looking for the process H→ZZ*→4ℓ, where Z* is a virtual Z boson, since the Higgs isn’t heavy enough for H→ZZ to be possible.
The four lepton channel is one of the best channels for Higgs precision measurements. While ZZ*→4ℓ represents only a small fraction of ZZ* events, it has a number of advantages over other channels. Detectors can measure electrons and muons with much more accuracy than most particles. Electrons are stable while muons live long enough not to decay in the detector. No energy is carried away by neutral particles like neutrinos. By measuring the leptons, the invariant mass of the Z boson can , and the full invariant mass of the Higgs can be reconstructed for each event. The channel is also cleaner than most (has a good signal to background ratio), with the lepton reconstruction allowing for things like using the reconstructed Z mass from pairs of leptons to aid the selection. The paper also uses subsamples of the total data set to estimate the contributions from several different Higgs production mechanisms (like vector boson fusion and gluon fusion).
CMS posted a new preprint on a search for several types of exotic particles on Monday using 19.7 fb-1 of integrated luminosity at 8 TeV center-of-mass energy. The paper focuses on the theoretical models of WIMP dark matter, extra dimensions, and unparticles.
WIMP, or weakly interacting massive particle, dark matter is the most popular model of particle dark matter. It postulates that dark matter is composed of heavy particles that don’t interact much with regular matter, similar to what one might term a superheavy neutrino. Theories of extra dimensions postulate that there are additional spatial dimensions to the familiar three. If these dimensions are finite in size (“compactified:” like a tiny loop or torus, to give examples in 1D and 2D) and are small enough, we could see particles that seem equivalent to Standard Model particles but are much heavier. Unparticle physics is something that I don’t know very about, but it apparently proposes the existence of scale-invariant fields: where things like the mass and momentum all scale in the same way. This is very different from particles, where the mass is always constant regardless of the momentum.
The CMS paper looks for a particular event topology and uses this to set exclusion limits on models of these exotic particles. The channel that they look at is monojets with large missing transverse energy. The idea is that when quarks in the protons in the beams collide to create WIMPs, Kaluza-Klein particles (related to extra dimensions), or unparticles, they can emit a gluon prior to the interaction or can emit one as part of the interaction (if there is an interaction between gluons, the new particles and another particle mediating the interaction). A high energy gluon will fragment into a number of quarks, which themselves organize into hadrons that are measured by the detector. Thus, high energy gluons lead to a cone-shaped spray of hadrons called a “jet.” The interactions CMS searches for are “monojets” because they have a high energy jet and nothing else. If the jet points away from the beam direction, there will be a large imbalance if the momenta of all the particles in the transverse directions are added. Conservation of momentum requires that the total transverse momentum be basically zero, so this is a sign that an effectively noninteracting particle (or unparticle) carried away the excess momentum. Some models will predict cross sections for this process, so the experimental result can be compared to theoretical models.
Backgrounds for these interactions, according to the paper, mostly involve intermediate vector boson (W and Z) decays with associated jet production in the same interaction. The Z in particular can decay to neutrinos, which are invisible, so any jets in the event could appear like a monojet with large missing transverse energy. The results in all cases of the analysis are consistent with the Standard Model. So, CMS can set exclusion limits on the properties of these theoretical models. As in many such cases, the analysis contains a number of caveats, so the limits are for the specific models tested not for the most generic versions of these theories.