Alexander Mitov

It was quiet on my blog during the last 1 year. But the reason is that there were some major developments in top physics and the "saga" is now complete. I am happy to say that in a series of papers with my esteemed collaborator Michael Czakon and his students Peter Barnreuther and Paul Fiedler, we managed to complete the calculation of the total inclusive top quark pair cross-section at hadron colliders at NNLO (Next-to-Next-to-Leading order).

This result is an ultimate example of precision in particle physics; read my earlier article about the role of precision.

The non-expert reader might wonder if this result is a big deal. And the short answer is "YES". There are many reasons:

• First and foremost, this calculation has the distinction of having two "first ever": first ever hadron collider calculation at NNLO with 1) massive fermions and 2) more than two color partons at Born level (it is a bit technical, I know).
• If you are a more formal theorist, you might want to look at my April 24 talk at CERN (see Talks at the top of the page) where I'll discuss at length why top physics is a treasure not only for experimentalist and phenomenologists but also for formal theorists.
• The work is important phenomenologically. That means that its implications for what we can conclude from the LHC measurements about the validity of our models (the Standard Model) is broad and powerful. And this is rare. We have studied this in detail already in a paper with Michelangelo Mangano and Juan Rojo.
• It has direct  implications to searches for physics beyond the Standard Model. We have some ongoing work which we hope to report soon, as well as some already reported in the paper mentioned above.

There were many new developments in the last 1 year (see for example the program of the conference CKM 2012). In particular, about one month ago (i.e. March 2013), the LHCb collaboration published new measurement of the same observable that "does not confirm the evidence for direct CP violation in the charm sector reported in other analyses".

To elaborate, LHCb has updated their original analysis and now finds value which is closer to the Standard Model. They also present a brand new (independent) analysis which leads to a very different value for the measured CP asymmetry. Taken together, the two values are consistent with the naive predictions of the SM.

Recently the LHCb collaboration here at CERN presented first evidence for CP violation in charm D meson decays (read more about D-mesons and CP violation). The result is very impressive and currently has statistical significance of 3.5σ. The interesting part is that the measured signal is 20-30 times larger than the naive theoretical predictions based on the Standard Model.

Of course, in cases like this, one should have a second look at the problem. The particular decay modes that were measured by the LHCb collaboration (D⁰ → K⁺K^- and D⁰ → π⁺π^-) are of the so-called exclusive type which are notoriously hard to predict and, at present, impossible to calculate from first principles. This is true in the Standard Model but also in all models with New Physics.

To discuss these issues a mini-workshop was organized this week at CERN. What became clear to me as a result of the presentations and discussions is that assuming reasonable sizes for the non-perturbative (i.e. non-calculable) contributions, one can bring the SM prediction within a factor of 2 or so from the measurement. Given the existing experimental uncertainties this implies that at present there is no significant case for deviation from the Standard Model.

Of course this doesn't yet imply that the Standard Model explains the measurement; for that we will have to demonstrate that the non-perturbative contributions indeed have the size they need to have in order to explain the measurement. On the other side, the LHCb measurements will be improving, so things are bound to get more interesting (hopefully this year!).

What a great time to be at CERN!

The news this week from CERN is that, finally, the Standard Model Higgs particle might have been found! What both CMS and ATLAS see is a signal consistent with Standard Model Higgs of mass around 124 GeV (i.e. 124 times heavier than the proton). Moreover they see that equally well in all important Higgs decay channels. We will see in 2012 what it actually is, but it definitely smells like the Higgs! Here is a link to the presentations from CMS and ATLAS.

This would be an incredible result, if confirmed next year, when much more data should be collected. The reason is that the Higgs is not just another particle; it is a 'fundamental' particle of a kind never seen in Nature before. Its discovery, in a mass range that is favored by the electro-weak precision fits, will be a tremendous success for particle physics (and modern humanity) in general and the Standard Model in particular. In our quest for new physics we often forget that less is better and the immense (and consistent) successes of the Standard Model need to be better understood. Clearly, SM has a lot to offer! OK, back to work :)

A very well written, well balanced popular review regarding the Higgs boson can be found here.

A new paper, in collaboration with Matteo Cacciari, Michael Czakon, Michelangelo Mangano and Paolo Nason appeared yesterday. I am very excited about it! It has been in the pipeline for a long time and now, looking back, I can definitely say its is a paper that I am very happy with.

The paper fully quantifies the question of how well we know the total inclusive top-pair cross-section at present. In the last few years, results based on the so-called approximate NNLO approach have been explored as a solution to precision top physics. The problem with working with approximations, however, is that one needs to understand the underlying physics very well in order to be able to quantify the full theoretical uncertainty. This is precisely what we have done in this paper.

Our findings are quite natural: based on the soft approximation alone, one does not obtain a significant improvement in the theoretical predictions with respect to the long known NLO/NLL results. The main remaining sensitivity is with resect to the unknown intrinsic NNLO corrections that are beyond any known approximation. Deriving them, and thus settling the outstanding questions in top physics, would require the full NNLO result.

The derivation of the exact NNLO top-pair cross-section is one of my current projects. It is a great problem to work on, especially with a collaborator like Michal Czakon, and I hope to report it very soon. Stay tuned!

In the meanwhile with Michal Czakon we are releasing the program Top++ for the numerical computation of the total inclusive cross-section. The program is the only one publicly available program able to perform soft-gluon approximation in this observable. Please note that, as explained in detail in our new paper, it is much preferred to use the resummed result over the approximate NNLO one.

If you want to give our program Top++ a try, and explore its many options, please visit the program's webpage.

I just published a paper on the dimuon asymmetry at the Tevatron that was measured by the DO collaboration, and which deviates substantially from the predictions of the Standard Model. (I mentioned this very interesting measurement earlier in my Blog). My findings are that B-meson production effects that were not considered previously and now I study for the first time, cannot explain the asymmetry. By far. And the reason is that the discrepancy is just too big. So it may indeed be that one needs to have a second look at the physics that's going on in the neutral B meson sector.

My results give substantial corrections to the extraction of the so-called flavor specific asymmetry for the B_s meson. If new physics is the explanation behind this discrepancy then my results provide and additional hint for the right direction. If you are even slightly interested in the subject, then you also need to have a look at the plots in the paper (in pdf format).

My papers now have more than 1000 citations!

The state of science in Bulgaria has deteriorated dramatically in the last year. According to the bulgarian scholarly community:

"The government of Bulgaria has declared war on the main research centre of the country, the Bulgarian Academy of Sciences, and has announced its intention to, de facto, liquidate it, an unprecedented arbitrary act in its 141-year history".

Here is the full text of the appeal of the Civil Movement for the Support of Science and Education in Bulgaria to the international scientific community:  http://www.science.nauka2010.com

By now the appeal has been signed by almost 7000 people!

A few months ago I finally parted with my PC and switched to a Mac. It wasn't easy: I was a devoted fan of IBM's ThinkPads for many years. But the quality of my latest (Lenovo) model was disappointing and after a frustrating search I realized one can't even find a good old second-hand IBM anymore.

So, all I can say is that my new Mac (MacBook Pro 15'') is incredible! I am impressed not only by its multimedia side (obvious), but also by its computational power and stability. I found the preinstalled C++ compiler not 100% usable. So I installed the entire GNU compiler collection and now am as free as a bird to compute. The installation is non-trivial; FINK comes very handy.

The most intriguing measurements at particle accelerators are the ones that deviate from the theoretical predictions. Such deviations can signify:

1. The discovery of fundamental new physics (think Nobel prize)
2. A mis-interpretation of the ongoing physics
3. Incorrect measurement or theoretical prediction