Magic Monday Journal Club

4th of November 2013

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Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron

ACME Collaboration

The ACME collaboration performed a search for the electric dipole moment of the electron  using the thorium monoxide (ThO) molecule. They obtained the limit |de| < 8.7 x 10^-29 e cm at 90% CL, which  represents an improvement of an order of magnitude over the previous best limits. We discussed the significance of this result for new physics at the TeV scale with new sources of CP violation.

Charming the Higgs

  1. C.C. Delaunay, T. Golling, G. Perez, Y. Soreq

The authors consider the possibility of the Higgs boson decaying to charm quark pairs with a branching fraction significantly larger than the one in the SM (3%). From a global fit to the Higgs data they find that the Higgs coupling to charm could be up to 7 times larger than in the SM, leading to the branching fraction of order 50%. The corrolary would be a largery reduced branching fraction into bottom quarks, which would make very difficult the searches for  this decay mode in the VH production channel. However, they point out that in such a case the decay to charm may be observable in the same production channel, thanks to the recent advancement in charm tagging at the LHC.

First results from the LUX dark matter experiment at the Stanford Underground Research Facility

by LUX collaboration

The first results of LUX collaboration seem quite impressive. They obtained the strongest limit on direct detection cross section, reaching the zeptobarn level of sensitivity (10^-18 barn). They also exclude the DAMA/CoGENT/CRESST/CDMSSi interperetation of light dark matter. It seems that even playing with astrophysical uncertainties or the nature of dark matter (Xenophobic) it will be difficult to reconciliate light dark matter scenario with the LUX data. The Fig.5 summarizes (almost) clearly their results. We can expect infinite discussions about low energy efficiency of XENON response in the next days/weeks. This will give the opportunity to write and read several papers to present in the next MMJC.  You can find the annoted version of the paper here and a more complete discussion there.

Phenomenology of a very light scalar (100 MeV < mh < 10 GeV) mixing with the SM Higgs

by J.D. Clarke, R. Foot, R.R. Volkas.

In this work, the authors compiled different colliders constraints from LEP to LHC passing through CHARM or BaBar to restrict the mixing angle between a SM Higgs and a light scalar. They obain values as small as 10^-10 for mh of the order of 300 MeV. Their Fig.5 gives a good compilation of their results, showing that LHC searches at LHCb are competitive with previous low energy analysis You can find the annoted version of the paper here.

Quantifying the reheating temperature of the universe

by A. Mazumdar and B. Zaldivar.

In this nice article, the authors show that if you relax the classical hypothesis of instant thermalization, ie if you do not suppose that the Standard Model (SM) bath is automatically in thermal equilibrium after being produced through inflaton decay, solving the set of the coupled Boltzmann equations give solutions with much lower temperature than when instant reheating is considered. It can even reach the TeV scale, depending on the inflation mass/coupling to the SM particles. The effect is the strongest when the thermalization is attained during the radiation era. You can find the annoted version of the paper here.

The scattering of α and β particles by  matter and the structure of the atoms

by E. Rutherford (1911)

Rutherford, already Nobel prize of Physics since 1908 for his work on the decay of the Thorium he made at Mac Gill came back to Manchester. After given a work to his student (Geiger) and the student of Geiger (Mardsen) they observed a large deflection of the α particle (above 90 degrees) when send on a foil of gold. He computed the probability of these events to occur, which should be inexistent in the atomic model of Thomson. In Thomson vision, atoms are a collection of negative electrons plunged in an uniform positively electrified sphere. He then proves that if the atom is supposed to be composed of a hard small central charge surrounding by electrons, like a planetary system, the deflexion formula fit the data obtained by Geiger and Mardsen.   You can find the original paper here.