New ATLAS Higgs Diphoton Measurement

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.


Dark Matter Evidence: Lensing

Here’s another post continuing my discussion of dark matter.

Bullet Cluster
Image of the Bullet Cluster. Blue shows gravitational lensing measurements from Magellan and pink shows x-ray emission measurements from Chandra. The optical image is also from Magellan. Per NASA rules, image in public domain. Title: “The Matter of the Bullet Cluster” Composite Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

The previous two posts in this series dealt with two of the most important pieces of evidence for dark matter. Both galactic cluster and rotation curve data show a large disparity between the mass measured using the mass to luminosity ratio and the mass measured using kinetic energy measurements. However, you can easily imagine an alternative explanation: maybe gravity doesn’t really work the way we think it does on large scales. Maybe all we need to do is modify the equations of general relativity so that we retain the behavior at smaller scales that we can actually measure while giving us the correct forces at long scales to explain the apparent mass deficit in luminosity measurements. In recent years, some newer evidence has cropped up that presents a serious challenge to theories of modified gravity with no dark matter.

In general relativity, gravity acts on energy rather than just mass. Light, which has energy but no mass, follows geodesics – basically the equivalent of a straight line in a non-Euclidean geometry. When light passes by an object, the gravitational forces pull the light, deflecting it from its original direction. To a faraway observer, objects emitting light that is deflected by very massive objects will appear distorted in some way. Many types of spatial distortion can be seen. To give a couple examples, multiple copies of some objects can appear due to light being deflected toward us from several different directions. Einstein rings, where a point-like object appears to be a ring instead are observed as well. This is known as strong lensing. There is also weak lensing, where statistical methods must be used to find distortions. By measuring the lensing of objects in the background, the mass density of the object doing the lensing can be reconstructed.

The most significant lensing measurement relevant to dark matter research is that of the Bullet Cluster (1E0657-56) in the early 2000s. This led to the famous picture at the top of this post with data from the Chandra X-ray Observatory and the Magellan telescopes in Chile. The Bullet Cluster is believed to be the result of a collision between two galactic clusters. When two clusters collide, it is expected that the gas, which makes up most of the normal matter, will interact readily and clump together in the center. Galaxies act more like individual particles and are maybe deflected but continue moving on without interacting very much. So, it’s expected that most of the mass of normal matter will show up as a single diffuse cloud. Gas in a large gravitational potential is easily measured through x-ray emission. In several papers, Clowe, Markevitch et al. used x-ray data from Chandra to look at the distribution of gas (most of the normal matter) and optical data from telescopes like the VLT (Very Large Telescope) in the Atacama Desert in Chile to compare the mass distributions from x-rays and weak lensing. Optical measurements showed that the x-ray emitting gas was located in the center, as expected, and the galaxies were distributed in two lobes, again as expected. The lensing measurements, however, showed that most of the mass contributing to gravitational lensing was distributed with the two lobes of galaxies, not with the gas.

This result suggests that most of the mass exists as a diffuse cloud of non-interacting matter. When the clusters collided, this matter would just follow the galaxies since there are no interactions to cause it to cluster together as with gas (mostly hydrogen). This non-interacting matter neatly fits the description of dark matter. The dark matter hypothesis is a simple way to explain all three of these pieces of evidence*, while modified gravity (with no dark matter) struggles with at explaining these phenomena.

*Some have suggested that the Bullet Cluster and some similar objects are actually pretty difficult to model with the regular ΛCDM model. While this would suggest that our cosmological model is not entirely correct, the existence of dark matter would still be strongly favored in alternative models.

NHL Expansion?

Yesterday a number of news agencies reported that the NHL is considering expanding again, adding as many as four new teams. The NHL denies this. A replacement for the Whalers is not one of the possibilities being mentioned, so the I-91 corridor (Hartford, New Haven, & Springfield) will remain one of the most populous (maybe even the most populous) markets without a major league team.

The End of the Roseland Ballroom

Looks like the Roseland Ballroom is finally being demolished to make way for a new building. It was a pretty good venue for concerts but was in a location that was too good to stay around forever. It’s still sad to see another old venue go. I don’t think there are many venues left in Manhattan that can accommodate as many people as Roseland. Soon Disney will probably control almost all the performance venues in the theater district.

Salon on Extraterrestrial Intelligent Life

Salon has an interesting article reposted from Scientific American on whether or not we’ll ever find intelligent extraterrestrial life. It actually gives what seems like a reasonable argument, which is surprising given the amount of pseudoscience attached to anything involving aliens.

Unfortunately, the author’s view is not very satisfying. A few decades from now it’s likely that we won’t know any more than we do now. We can now find other planets, but even if they have intelligent life there’s no guarantee that we’ll ever find it. We don’t even have a good way to estimate how likely it is that such life exists or whether we’ll find it assuming it exists at all. Whether or not intelligent life outside Earth exists will probably remain a philosophical question for the foreseeable future.

New ATLAS Higgs Paper Posted to ArXiv

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).