Magic Monday Journal Club

3d February 2014

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A running-running Journal Club before one of our magician (the present one) decided to jump on his plane for one week in the coolldddiissh Finland!! Even if there were some contestations during the election of the «paper of the week», and after counting and recounting the vote, it seems that the article of Steigman about CMB and BBN won the vote. Thanks for Filippo for his nice presentation of his recent work on chromo-EDM.

Light WIMPs, Equivalent Neutrinos, BBN, and the CMB

by G. Steigman and K.M. Nollett

This is in fact the proceeding corresponding of the articles written by the same authors on 31 of December (I just missed it, I really don’t know what I was doing this day....). It is sufficient to understand the main points. For people interesting to the original work, jump there. In their analysis, they combine the BBN constraint and CMB one by PLANCK satellite  to show that 1) without DM, the two classical scenario of Standard framework are excluded: the delta(Neff) =0 or 1 (sterile neutrino) seems to be excluded at 98% and 99% of CL respectively.  You can run (be careful to the steps) to the figures 3 and 4 to understand all their results. If you include the possibility of WIMP these combined analysis gives constraints on the mass of the DM which should be above 1 MeV, the best fit being for a WIMP between 5-10 MeV. A quick look at their Fig.5 will enlighten you. You can find an annotated version of the paper here.

A bound on the charm chromo-EDM and its implications

by F. Sala

If you are interested in models with non-negligible flavour and/or CP violation in the up-quark sector, now you have one more observable to worry about. The author in fact pointed out that the neutron electric dipole moment d_n constrains the chromo-EDM of the charm quark, at the level of 10^-22 cm. This is because of the chromo-EDM threshold contribution to the three-gluon Weinberg operator. This bound has strong implications for flavour violation in models where i) loop processes are mediated by NP associated with the third generation ii) a sizeable flavour violation is possible in the up-quark sector. In particular in the paper you will find discussed in detail, for many specific models, how this bound constrains the size of direct CP violation in D decays at a level below its current experimental upper limit, given by DeltaAcp. To obtain a visual summary of the message you can jump to Figure 1.

A New Look at Higgs Constraints on Stops

by J. Fan, M. Reece

Robust constraints on stop masses in the MSSM and beyond from Higgs data only. The authors give lower limits on stop masses the are independent  of the stop mixing parameter.  One interesting conclusion: a susy scenario with *both* stops having the mass of the order of the top quark is difficult to realize without tuning, independently of the model.

Sensitivity of CTA to dark matter signals from the Galactic Center

by M. Pierre, J. M. Siegal-Gaskins and Pat Scott

In this concise work, the authors look at the future sensitivity of the Cerenkov Telescope Array project. Some studies have already been done on the subject in the past, but this one is more complete, and take into account a specific likelihood way to determine the signal events, and also looked at different choices of dark matter profiles. To understand their procedure, look directly at the Fig.3 where they show the region they look at and their procedure (to subtract normalized events from the «OFF» region to the one from the «ON» region). Their results is nicely summarized in fig.5 and 10 where we see that they are quite complementary with the FERMI LAT instrument and even much better than the HESS telescope. For a reasonable final state and profile, they can exclude (or discover) thermal relic density after 500h of data processing.  The hungry guy can find an annotated version of the paper here.

Is the Higgs Boson Associated with Coleman-Weinberg Dynamical Symmetry Breaking?

by C.T. Hill

Some progress in the Coleman-Weinberg mechanism of  electroweak symmetry breaking. One interesting part is the formulation of the problem in the language of  all-order renormalization group Higgs potential. The other is the model realizing the CW mechanism using only new fermions (and no scalars), thanks to formally 2-loop effects.

Extended gamma-ray emission from coy Dark Matter

by C. Boehm, M.J. Dolan, C. McCabe, M. Spannowsky and C.J. Wallace

So... What to say. I appreciate a lot the authors of this article (even some personally). Their work in group or individually have always interesting ideas, and very easy to read for non-expert. However, in this case, I should say that using a «CDM» for «Coy Dark Matter» to sound as «cold» more than «shy» I can imagine) is not a new idea which needs a new name for it. Very anglo-saxon way. Articles of Thomas Schwetz et al. already studied this case of axial coupling with a regular Higgs,   and showed that such couplings makes direct detection impossible at LUX. In this paper, they reach the same conclusion with a light pseudoscalar  and a DM mass of around 30 GeV. The aim is to fit the little excess of FERMI at around 1 GeV from the DM annihilation into b bar final state through s-channel exchange of the pseudoscalar.  Their Fig.2 shows that it is possible to fit such an excess with their model. The fact that their candidate cannot be observed by LUX or future LZ experiments comes for gamma5-gamma5 coupling, proportional to the (very weak) exchanged energy q^2. Even if they affirm the contrary, I doubt that their model is not excluded by FERMI dwarf observation. Their fit is strongly dependent on   the coupling to the bottom which they set to a quite strong value arbitrarily. They also need a very steepy profile from the Galactic Center. You can find annotated version of the paper here.

The Earth’s velocity for direct detection experiments

by C. McCabe

Christopher published this paper before the one concerning the «Coy Dark Matter» (see above). Serious work where he computed in detail the velocity of the earth in the Galactic coordinates. He shows that the classical result of Lewin and Smith, used in a lot of computation is in fact not exact. Christopher confirms the result of Lee and Lisanti. Just jump on the table I (or try to hit your head on a tricky system of coordinates). In this table, you see that after 14 years,  there is 1.5 days of difference in the peak day for the dark matter detection. However effect on signals without modulation (like LUX or XENON) for instance. You can find an annotated version of the paper here.

Maximum Likelihood Signal Extraction Method Applied to 3.4 years of CoGeNT Data

by CoGeNT collaboration

CoGeNT used to give us surprising articles, full of certainties and self confident in their signal(s) claim. Already in the previous one (see the preceding MMJC) when they studied the modulated signal they seemed to put some security belt to their conclusions. Here the same. They decided to remake an analysis on the recent data obtained by the apparatus, using a different technique to deal with  the background events called «surface» ones, because generated on the surface of the detector (how logical is the world of words). They do not exclud them as usual using the different rising time (see the preceding  MMJC for details) but with a maximum likelihood method. Indeed, this background seems to develop an exponential shape which can be confused with a DM signal (see their Fig.6 for instance. As a clear consequence, is that their «signal» region completely shifts down to 10^-42 cm2 (it was 10^-41 with the previous method) and a WIMP mass passing from 8 to 12 GeV!! (see their Fig.11, all is summarized there). I don’t know what to say about such a change. It correspond to a 2.5 sigma «signal», which means clearly, no signal.  For an annotated version of the paper, swim there.

Démonstration touchant le mouvement de la lumière

by M. Roemer

Published in the «Journal des Sçavans» on Monday december 7th, 1676

In this communication to the French Science Academy was made by the Danish physicists, working for the French Observatory in 1676. This is the first known measurement of the velocity of light. To be more precise, the first proof that the velocity of light is finite with the idea of observing the eclipse of the Io satellite of Jupiter.