# Is There Lepton Universality?

Scientific American has an overview of some interesting recent results looking at semileptonic decays of neutral B mesons. Several experiments see what looks like a possible excess of events with tau particles in the final state, which is not allowed in the Standard Model. The Standard Model includes a weak force that has the same couplings to all leptons, so the only differences that we should see between semileptonic decays with electrons, muons or taus are due to phase space and not due to any fundamental interactions. LHCb and Belle both see a fairly large excess of tau events compared to muon events, and earlier results by BaBar also see this kind of effect as well. None of the effects are particularly large, but the combined significance is reported at “3.9 sigma,” which is getting pretty close to the typical threshold for making a discovery of some new effect.

The article also mentions that if this is real, it potentially points toward some exciting new physics like charged Higgs particles (which are predicted in theories like supersymmetry) or leptoquarks (quark-lepton composite particles). Of course, it could still just be nothing, so we’ll just have to wait to see if it holds up.

# 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).

# New Combined ATLAS/CMS Higgs Mass Result

The ATLAS and CMS collaborations have finally released a joint result on their Higgs analyses. The paper uses the two channels where the final state invariant mass is most easily calculated and where backgrounds are expected to be small (Higgs to two gammas or Higgs to four charged leptons). These two channels don’t have have very many signal events, which limits the ability of the two experiments to make a precise measurement.

Combining results from different experiments is always tricky, since the analyzers have to be sure that systematic uncertainties are treated properly. Some systematics will be completely separate for each experiment, while others may be correlated. Regardless, the new result is mH = 125.09±0.24 GeV. The measurement is still statistics-limited, but is getting closer and closer to being limited by systematics. This is quite good, but still nowhere near the kind of precision to which the Z and W masses are known. To do that at the LHC, the experiments would need to improve systematics by more than an order of magnitude as well as collect many times more data. If we realistically hope to know the Higgs mass as precisely as for the W and Z bosons, we’ll need an e+e Higgs factor collider (i.e. the ILC or something similar).

# ATLAS Paper on Higgs to Tau Coupling

The ATLAS experiment at the LHC has released a lengthy paper presenting results from an analysis looking for Higgs bosons decaying to tau leptons (H→ττ). This decay can occur due to the Yukawa coupling (a 3-particle vertex, so no virtual particles are needed). The Yukawa coupling combined with the non-zero vacuum expectation value of the Higgs generates the mass of the tau. Similar couplings are expected to exist for the other fermions. Because of the relationship between the mass and the coupling, the Standard Model provides a prediction for the coupling: $m_\tau / vev$. Thus, the heavier the fermion, the larger the decay width and higher the branching fraction.

The tau channel is a pretty difficult channel to analyze in Higgs searches. Taus can’t be seen directly. Only their decay products (jets or other leptons) are seen, which makes it much harder to reconstruct things like the momentum. These particles are found in many other processes as well: direct production of leptons and jets, decay products of other particles like W and Z bosons, and even other decays of the Higgs. Furthermore, Z→ττ presents in irreducible background since it has the same final state. So, in order to make this measurement, these backgrounds must be accurately modeled.

The final result shows a value consistent with the Standard Model prediction. The statistical strength of the result isn’t quite high enough to call this a true measurement of the Higgs to tau Yukawa coupling, but it is enough to present the measurement as evidence of a nonzero coupling. With more data (and also an ever more thorough understanding of the background physics and detector), the analysis of this channel should continue to get stronger.

# 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.

# 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).