TOP 2013: over the TOP with top quarks!

26 Sep 2013 - 23:07:17 by in Conference

Starting on Sunday September 14th I attended the five-day international workshop on top quark physics: TOP2013 in Durbach, Germany. Last year I was chairing the local organizing committee of TOP2012 in beautiful Winchester and that was quite a lot of hard work, so I was looking forward to enjoying the workshop this year. The tradition with the TOP workshop is that the chair of the local organizing committee becomes the chair of the International Advisory Committee for the following edition. So this year, my job was to provide some moral support to this year's LOC and also chair the discussion over the scientific programme, as well as invite all the theory speakers. Sounds like a lot of work, but this was infinitely easier than last year's madness!

The workshop this year was again quite a success: having all the experts on top quark physics from around the world in one room for talks after talks after discussion on our favourite subject: the top quark, pure heaven! Of course there is some excitement to compare ATLAS and CMS results. Whether it is about the respective size of their systematic uncertainties or the choice of new physics models to investigate, there is plenty to discuss. Top quark phenomenologists also attend the workshop and so for example this year we got to hear all the details of the calculation of the top anti-top quark production cross section at Next-to-Next-to-Leading-Order (NNLO). What this NNLO means is that the calculation takes into account Feynman diagrams that contain up to two virtual loops, this doesn't sound like much, but just thinking of all the possible diagrams, well my head hurts and these guys calculated the whole thing, wow, hat off!

Like in any edition of TOP, we discussed at lenght whether the interesting top quark Forward Backward production asymmetry from the Tevatron is a sign of new physics or just a sign that the theoretical calculation of this subtle effect might need some hmmm.... work? We need the final D0 analysis using the full dataset to get a clearer picture. What makes the situation even murkier is the fact that the forward backward asymmetry can not be measured at the LHC because it is a proton-proton collider (instead of proton-anti-proton like the Tevatron). Instead ATLAS and CMS have results on the the top quark charge asymmetry and these agree quite well with the Standard Model calculations. It is still possible that there could be new physics models that predict a Tevatron-like forward backward asymmetry while at the same time predicting a Standard Model-like charge asymmetry, but that is a bit unlikely, so the suspense continues....

Another topic that is always hot at TOP is agonizing over the definition of the top quark mass: you would think that mass is a pretty fundamental quantity, but actually it turns out that trying to define the mass of a coloured object is quite tricky. But this year Michelangelo Mangano himself set us straight, so I think we are set... well I thought I was while I listened to him, but then I discussed with colleagues, and now I am not so sure... oh well I'll sort it out at TOP2014!

This year the really exciting topic we started discussing is the fundamental relationship between the top quark and its best mate: the Higgs boson himself! And also the fact that there is another member of the gang that is still not joining the party: the stop quark. Interestingly Michail Shaposhnikov gave us all the details about how from the measurement of the top quark mass and the Higgs mass we seem to be in a "metastable" situation. This describes the shape of the Higgs vacuum: if the shape is that we are at the bottom of the Higgs energy potential well, then because we are at the minimum, this is a "stable" situation, and that doesn't really tell us anything about whether new physics should be present below the Planck scale. If instead we are in a local minimum and the real minimum is at - infinity, then this would be an "unstable" situation and would imply that there MUST be new physics at the TeV scale to modify this vacuum, otherwise the universe would have decayed to the lower energy minimum and we would not be here to discuss these things. So the current measurements of the top and Higgs mass tell us that we are in between these two situations: our minimum is a local minimum of potential energy, so there is a lower minimum which eventually the Universe will decay into. Somone from the audience asked how this would manifest itself and the answer was: just like a kettle about to boil, there would be a nucleation bubble that will form somewhere in the Universe and once it forms it will propagate at the speed of light over the entire universe (and hence kill us since there wont be any such things as atoms anymore...). This sounds pretty scary, but the lifetime of our current minimum is 10^100 times the age of the Universe, so... essentially you should be able to attend your party this week-end... that is if the theorists calculations are right of course...

Other than the great talks and lively discussions, we had Rhine valley wine to taste, a poster session to attend accompanied by more wine and also the Black Forest at our door. I actually went for a jog one lunch time into said Black Forest, and then it started raining (again) and then, well I got lost... and I didn't even know the name of the hotel (I now know it was the Four Seasons... in German), so I was a bit worried, not having any phone or anything on me. Eventually with the help of some very nice local people I managed to get back to my hotel, only an hour and a half later... Next time I eat a Black Forest cake I will remember this.

The location of TOP2014 has already been announced: we will enjoy San Rafael in the South of France, the photos look lovely and I can't wait to hear more about the top quark!

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Topic revision: r1 - 26 Sep 2013 - VeroniqueBoisvert

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