Monday, June 9, 2014

Interesting LHCP 2013 (Barcelona) Higgs Conference Abstracts

* Francesco Riva, "The Higgs: supersymmetric partner of the neutrino"
Recent LHC searches have provided strong evidence for the Higgs, a boson whose gauge quantum numbers coincide with those of a SM fermion, the neutrino. This raises the question of whether Higgs and neutrino may be related by supersymmetry. I will show explicitly the implications of models where the Higgs is the sneutrino: from a theoretical point of view an R-symmetry, acting as lepton number is necessary; on the experimental side, squarks exhibit novel decays into quarks and leptons, allowing to differentiate these scenarios from the ordinary MSSM.
The idea that the Higgs boson itself could be a superpartner of the neutrino is novel and interesting.  A related pre-print discusses the issue.  This model is quite interesting, effectively proposing a SUSY model that is even more minimal than the MSSM:

We have shown that the phenomenology of this model is quite different from that of the MSSM. In the Higgs sector, a sizable (∼ 10%) invisible branching ratio for Higgs decays into neutrinos and gravitinos is possible, together with small deviations in the Higgs couplings to gluons and photons, due to loop effects if the stop t˜R is light. These effects are not yet favored nor disfavored by the present LHC Higgs data, but could be seen in the near future by measuring a reduction of the visible Higgs BRs. Higgsinos are absent in this model, and gauginos must get Dirac masses above the TeV. Only third-generation squarks are required, by naturalness, to be below the TeV. We have shown that the R-symmetry implies that squarks decay mainly into quarks and either leptons or gravitinos. Therefore, evidence for models with the Higgs as a neutrino superpartner can be sought through the ongoing searches for events with third-generation quarks and missing energy (tailored for the MSSM with a
massless neutralino) or through leptoquark searches for final states with heavy quarks and leptons. In the stop decays into tops and neutrinos, the determination of the top helicity will be crucial to unravel these scenarios.

Existing LHC searches rule out stops of less than 530 GeV and sbottoms of less than 500 GeV in this model.

* Juan Rojo Chacon, "Parton Distributions in the Higgs Boson Era"
With the recent discovery of the Higgs boson at the LHC, particle physics has entered a new era, where it is of utmost importance to provide accurate theoretical predictions for all relevant high energy processes for signal, bacground and New Physics production. Crucial ingredients of these predictions are the Parton Distribution Functions, which encode the non-perturbative dynamics determining how the proton’s energy is split among its constituents, quarks and gluons.

To bypass the drawbacks of traditional analyses, a novel approach to PDF determination has recently been proposed, based on artificial neural networks, machine learning techniques and genetic algorithms. In this talk we motivate their relevant of PDFs for LHC phenomenology and describe the latest developements of PDFs with LHC data.
The clear description of what the PDF is and why it is important provides useful background context.

* Daniele Barducci, Alexander Belyaev, Stefano Moretti and Stefania De Curtis, "The 4D Composite Higgs model and the 125 GeV Higgs like signal at the LHC"
General Composite Higgs models provide an elegant solution to the hierarchy problem present in the Standard Model (SM) and give an alternative pattern leading to the mechanism of electroweak symmetry breaking (EWSB). We present a recently proposed realistic realization of this general idea analyzing in detail the Higgs production and decay modes. Comparing them with the latest Large Hadron Collider (LHC) data we show that the 4D Composite Higgs Model (4DCHM) could provide a better explanation than the SM to the LHC results pointing to the discovery of a Higgs like particle at 125 GeV.
It is hard to tell from the abstract what the gist of the 4D Composite Higgs Model is, but it is worth looking at preprints to find out. Alas, the larger picture looks abysmally complex and hence less plausible: "Besides the SM particles the 4DCHM present in its spectrum 8 extra gauge bosons, 5 neutral called Z' and 3 charged called W', and 20 new quarks: 8 with charge +2/3, 8 with charge −1/3 and 2 respectively with charge 5/3 and −4/3; these coloured fermions are called t', b', T˜ and B˜, respectively."

* Jernej Kamenik, "On lepton flavor universality in B decays"
Present measurements of b->c tau nu and b->u tau nu transitions differ from the standard model predictions of lepton flavor universality by almost 4 sigma. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super)B- factories are presented.
Some of the strongest evidence for beyond the Standard Model behavior involves Lepton flavor violations where decays to electrons are about 25% more common than decays to muons, contrary to a Standard Model expectation of equal frequencies. This is a promising place to look for new physics. Motl discusses the results in a post here.

* Jorge de Blas Mateo, "Electroweak constraints on new physics"
We briefly review the global Standard Model fit to electroweak precision data. After that we analyze the electroweak constraints on new interactions, following a model-independent approach based on a general dimension-six effective Lagrangian. Finally, we also discuss the limits on several common new physics additions.
The abstract again says little, but this methodology is a window into higher energies than direct searches can reveal so it is always interesting to follow up upon at some point in the pre-print. Alas, however, the paper has so little analysis and so many undefined quantities (since it basically updates prior papers on the same topic with new numbers) that it is virtually impossible to make any sense of them.

* Carlos Lourenco, "Quarkonium production and polarization"
All the three frame-dependent polarization parameters (lambda_theta, lambda_phi and lambda_thetaphi), plus the frame-invariant parameter lambda_tilde, are measured in three different polarization frames, in five transverse momentum bins and two rapidity ranges, significantly extending the pT and rapidity ranges probed by previous experiments. The observations are in disagreement with the available theoretical expectations.
The excerpt of the abstract above captures the key point. Once again, the observed properties of quarkonium have failed to conform to theoretical expectations.

From here.

* The Conference also, according to Matt Strassler, showed convergence in the ATLAS mass measurements of the Higgs boson, but the exact new Higgs boson mass numbers from ATLAS aren't available in the abstracts or his commentary at his blog:

[Y]ou may recall a tempest in a teapot that erupted in late 2012, when ATLAS’s two measurements of the Higgs particle’s mass disagreed with each other by more than one would normally expect. This generated some discussion over coffee breaks, and some silly articles in on-line science magazines, even in Scientific American. But many reasonable scientists presumed that this was likely a combination of a statistical fluke and a small measurement problem of some kind at ATLAS. The new results support this interpretation. ATLAS, through some hard work that will be described in papers that will appear within the next couple of days, has greatly improved their measurements, with the effect that now the discrepancy between the two measurements, now dominated by statistical uncertainties, has gone down from nearly 3 standard deviations to 2 standard deviations, which certainly isn’t anything to get excited about. Experts will be very impressed at the reduction in the ATLAS systematic uncertainties, arrived at through significantly improved energy calibrations for electrons, photons and muons.

Experts: More specifically, the measured mass of the Higgs in its decay to two photons decreased by 0.8 GeV/c², and the systematic uncertainty on the measurement dropped from 0.7 GeV/c2 to 0.28 GeV/c2. And by the way, the rate for this process is now only 1 standard deviation higher than predicted for the simplest possible type of Higgs (a “Standard Model Higgs“); it was once 2 standard deviations high, which got a lot of attention, but was apparently just a fluke.

Meanwhile, for the decays to two lepton/anti-lepton pairs, the systematic error has dropped by a factor of ten — truly remarkable — from 0.5 GeV/c2 to 0.05 GeV/c2. The Higgs mass measurement itself has increased by 0.2 GeV/c2.

UPDATE: Analysis of these facts in a comment. My previous analysis overlooked statistical error.  The report is here at page 15.

* Beyond the scope of the Conference, I also note that two recent preprints have suggested that the combined ATLAS and CMS data reveal a potential signal of a 200 GeV mass stop sparticle and that there could be several other light sparticles that were overlooked in earlier searches, a possibility which Motl disucsses in an upbeat way at his blog.

Given the number of searches made by ATLAS and CMS combined and the fact that joint data from the two studies is necessary to produce even a 3 sigma effect, my money is on the possibility that this is really just a statistical fluke.  I find it highly unlikely all previous searches would have overlooked multiple supersymmetric (i.e. SUSY) particles and that these particles would only show up in this single isolated channel in such a weak way, despite the high energies of the two experiments.  It looks to me like a rifle shot hole in the general exclusion of such particles for almost all other parameters.


andrew said...

The old Atlas diphoton mass was 126.8 +/ 0.2 +/- 0.7, so this should now be 126.0 +/- 0.2 +/- 0.28 (combined MOE +/- 0.344)

The old Atlas four lepton mass was 124.3 +0.6/-0.5+0.5/-0.3. So this should now be 124.5 +0.6/-0.5 +/- 0.05 (combined MOE +/- 0.55)

The old combined number for ATLAS was 125.5 +/- 0.2 +0.5/-0.6. The weighted average combined new number for ATLAS should be 125.4 but I don't know how to computer the MOE (the weighted average of the MOEs is 0.42)

The CMS diphoton number was 125.4 +/- 0.5 +/- 0.6. The CMS four lepton mass was 125.8 +/- 0.5 +/- 0.2. The combined CMS number was 125.7 +/- 0.3 +/- 0.3 (combined MOE 0.42)

Combining the New weighted average combined ATLAS result and the CMS combined result on an unweighted basis gives 125.55 GeV, which makes sense if the MOEs of the combined results are very nearly the same anyway.

Shorter: my current best guess of Higgs boson combined mass is 125.6 +/- 0.4 GeV (within 1 sigma of the 2H=2W+Z value).

andrew said...

Majorana mass for the neutrino implies a much higher electric dipole moment for the electron. In the Dirac neutrino case the EDM of the electron is negligible. In one version of the Majorana case, the EDM of the electron is close to the current experimental threshold (which is stated in terms of less than X). This provides an alternative to precision neutrino experiments to distinguish between Majorana and Dirac neutrino masses. But, it is model specific.