Category Archives: LHC

New CMS Dark Matter Paper

CMS has posted a new paper onto the arXiv that gives some new limits on theories like dark matter and unparticles. This paper looks for a signal of two charged leptons (with a reconstructed mass consistent with a Z boson) with a lot of missing transverse energy from invisible particles like WIMPs. As usual, they get results consistent with Standard Model prediction and thus set various limits on the theoretical models being studied.


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.

LHCb Discovers New Resonances

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.