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 Search for New Physics

ATLAS released a new preprint yesterday on a search for new physics using monophoton events with missing transverse momentum. The paper mentions that this type of search is sensitive to various models of new physics, including some variations of large extra dimensions, dark matter, and supersymmetry.

In particular, these events could indicate the collision of two quarks leading to invisible particles, with a photon coming from initial state radiation. Final state radiation generally won’t be allowed in many of these models since an invisible particle won’t interact with photons at tree level. Collisions have basically no transverse momentum, so a high energy photon without anything else to balance out the transverse momentum is a strong indication that some invisible particle was also present in the final state. This channel is not free of backgrounds. As the paper notes, one of the most important backgrounds is Z production where the Z decays to neutrinos. If a photon is emitted in conjunction with this, then the even will look identical to the signal event from new physics. This represents an irreducible background since even a perfect reconstruction process can’t eliminate it. Instead, new physics must be found on top of this background (as well as others).

As with more or less every other paper from the LHC so far, no significant deviation from the Standard Model is found, so ATLAS is able to set exclusion limits for a number of different models.

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).