Journal Club

Seminar Room, Mondays at 14:00

Wednesday 12th of June, 2013

 Presented by J. Santiago

Far from standard Higgs couplings

In this short note we consider a permitted region in the space of couplings of the 126 GeV boson that extends far away from the standard model Higgs couplings. This region is of interest for more natural models of electroweak symmetry breaking. Stronger evidence of vector boson fusion and/or associated production is needed to eliminate this possibility.
Comments: 5 pages, 10 figures
Subjects: High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:1306.1564 [hep-ph]
  (or arXiv:1306.1564v1 [hep-ph] for this version)

Submission history

From: Bob Holdom [view email
[v1] Thu, 6 Jun 2013 21:48:36 GMT (659kb,D)

Presented by Pérez-Victoria

The hierarchy problem of the electroweak Standard Model revisited

A careful renormalization group analysis of the electroweak Standard Model reveals that there is no hierarchy problem in the SM. In the broken phase a light Higgs turns out to be natural as it is self-protected and self-tuned by the Higgs mechanism. It means that the scalar Higgs needs not be protected by any extra symmetry, specifically super symmetry, in order not to be much heavier than the other SM particles which are protected by gauge- or chiral-symmetry. Thus the existence of quadratic cutoff effects in the SM cannot motivate the need for a super symmetric extensions of the SM, but in contrast plays an important role in triggering the electroweak phase transition and in shaping the Higgs potential in the early universe to drive inflation as supported by observation.
Comments: 7 pages
Subjects: High Energy Physics - Phenomenology (hep-ph)
Report number: DESY~13-093,HU-EP-13/25
Cite as: arXiv:1305.6652 [hep-ph]
  (or arXiv:1305.6652v1 [hep-ph] for this version)

Higgs Couplings and Naturalness

Many extensions of the Standard Model postulate the existence of new weakly coupled particles, the top partners, at or below the TeV scale. The role of the top partners is to cancel the quadratic divergence in the Higgs mass parameter due to top loops. We point out the generic correlation between naturalness (the degree of fine-tuning required to obtain the observed electroweak scale), and the size of top partner loop contributions to Higgs couplings to photons and gluons. If the fine-tuning is required to be at or below a certain level, a model-independent lower bound on the deviations of these Higgs couplings from the Standard Model can be placed (assuming no cancellations between contributions from various sources). Conversely, if a precise measurement of the Higgs couplings shows no deviation from the Standard Model, a certain amount of fine-tuning would be required. We quantify this connection, and argue that a measurement of the Higgs couplings at the per-cent level would provide a serious and robust test of naturalness.
Comments: 17 pages, 6 figures
Subjects: High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Experiment (hep-ex)
Cite as: arXiv:1305.6068 [hep-ph]
  (or arXiv:1305.6068v1 [hep-ph] for this version)

Presented by Donati

On the predictivity of the non-renormalizable quantum field theories

Roberto Pittau (Granada U., Theor. Phys. Astrophys. & CAFPE, Granada)
Following a recently introduced approach to ultraviolet divergences, I extend the concept of predictivity to non-renormalizable quantum field theories. The idea of topological renormalization is introduced, which allows to keep a finite value for the parameters of the theory. One additional measurement is then sufficient to systematically compute quantum corrections at any loop order.
Comments: Discussion on the tensors modified, text expanded, references added
Subjects: High Energy Physics - Phenomenology (hep-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1305.0419 [hep-ph]
  (or arXiv:1305.0419v2 [hep-ph] for this version)


Presented by Cerezo

Cold dark matter: controversies on small scales

The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way's dwarf galaxy satellites. We review the current observational and theoretical status of these "small scale controversies." Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: recent numerical simulations and analytic models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star-formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.
Comments: 7 pages, 4 figs. Short review based on panel discussion at NAS Sackler Symposium on Dark Matter. Submitted to PNAS, to appear in Sackler Symposium proceedings
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1306.0913 [astro-ph.CO]
  (or arXiv:1306.0913v1 [astro-ph.CO] for this version)


Presented by Setzer

Higgs Search in b bbar Signatures at ATLAS and CMS

We report on searches for the Standard Model Higgs boson decaying to b bbar in pp collisions at the ATLAS and CMS experiments. These rely on signatures where the Higgs boson is produced in association with a vector boson or in association with a top-quark pair. Results are presented, based on 5.0 fb^-1 of pp collision data at \sqrt{s} = 7 TeV and up to 13.0 fb^-1 of data at \sqrt{s} = 8 TeV collected by the LHC accelerator. These are expressed in terms of 95\% confidence level upper limits on the production cross section of the Standard Model Higgs boson times the branching ratio to decay to b-quark pair.
Comments: 8 pages. 14th International Conference on B-Physics at Hadron Machines (Beauty 2013), 8-12 April 2013, Bologna, Italy
Subjects: High Energy Physics - Experiment (hep-ex)
Report number: ATL-PHYS-PROC-2013-139
Cite as: arXiv:1306.1784 [hep-ex]
  (or arXiv:1306.1784v1 [hep-ex] for this version)