Alexander's Blog

The new kid on the block: CP violation in charm (D meson) decay

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!).

Last Updated ( Friday, 13 January 2012 20:05 )

Did we just find the Standard Model Higgs boson?

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.

Last Updated ( Thursday, 15 December 2011 15:27 )

New paper on top physics at hadron colliders

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.

Last Updated ( Tuesday, 06 December 2011 09:18 )

New paper on the dimuon asymmetry at the Tevatron

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).

Last Updated ( Friday, 18 February 2011 13:57 )

1000 Citations

My papers now have more than 1000 citations!

Last Updated ( Tuesday, 28 December 2010 12:07 )

The state of science in Bulgaria

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!

Last Updated ( Saturday, 25 December 2010 00:07 )

My new Mac!

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.

Last Updated ( Sunday, 26 September 2010 10:44 )

Measurements that point to deviation from the established theoretical models

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

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

I will not dwell on 3. Clearly everyone hopes for 1. But in reality it is very hard to eliminate 2., i.e. to get a clear shot at 1.

Here are few of the most recent examples and what we have learned from them.

Last Updated ( Wednesday, 24 November 2010 00:19 ) Read more...

On the problem of Dark Matter

Perhaps the most talked about problem in theoretical physics these days is the so called Dark Matter problem. Put simply, Dark Matter is an unknown that parameterizes our inability to describe the observed Universe (at Galactic scales and up). The most compelling evidence for the existence of Dark Matter is the inability of the Newtonian gravity to describe the dynamics of the rotation of Galaxies. The term Dark Matter naturally derives from this context since, assuming that the Newtonian mechanics describes the dynamics, the rotation only depends on the distribution of the gravitational masses in the Galaxies. At present the astronomers believe that we are able to determine fairly well the amount of matter in the Galaxies. Therefore, to reconcile the discrepancy between experiment and theory, we need to 'add' more gravitational matter. And this additional matter has to be 'dark' in the sense that we do not detect it even in our most advanced telescopes.

The interpretation of Dark Matter as some sort of non-standard matter is the most popular nowadays. Such interpretation is appealing from the following point of view: all theoretical models of physics beyond the Standard Model predict new particles and forces which, for a variety of reasons, are presently undetectable. Some of these 'new' particles may even be stable or long lived and could constitute the Dark Matter.

While the above interpretation might well be correct, it is not perfect. I give brief account below. But first let me mention that matter interpretation of Dark Matter is not the only possible solution; the other option is to have a modification to the Newtonian (and Einstein to that effect) dynamics of gravity. While at first this appears as a rather radical approach, it turns out it is both viable and in some sense more natural. I'll return to that below.

Last Updated ( Wednesday, 24 November 2010 00:20 ) Read more...

Science and society

What is the driver of science: the scientific merit or the all too natural for humans desire for personal gain? No answer here, just the following story:

Grigori Perelman: the man that solved the Poincaré conjecture but declined the Fields Medal and the $1M Millennium Prize.

The story sounds as no less than a Hollywood blockbuster. It clearly makes anyone's imagination run wild in the search for a rational (or more likely irrational) explanation. Even the word genius seems not to have enough weight to describe the picture. I was myself in a search of a "rational" explanation of how something like that might be possible, until ... I read the comments by Perelman himself quoted in the New Yorker (see also the Wikipedia article on Perelman):

“... there are many mathematicians who are more or less honest. But almost all of them are conformists. They are more or less honest, but they tolerate those who are not honest.”

Last Updated ( Sunday, 26 September 2010 21:58 )

The wisdom of calculating higher-order perturbative corrections

Recently I stumbled upon a post in a physics blog. I noticed the following comment there: "Admittedly, I find the perspective of a 'physics culture' that produces 'solid' Next-to-next-to-next-to-next-to leading order calculations somewhat depressing" referring to "a field that is several decades old, and hasn't seen very much novelty lately".

The post in question is about the US LHC Theory Initiative (I am its inaugural fellow); naturally that gave me the additional motivation to comment on a misconception I have witnessed numerous times before.

One can hardly argue that excesses are useful (anywhere). But I will argue that, at its core, higher order calculations are a field of solid depth and one that is becoming more and more likely to play a crucial role in the future of particle physics. On a personal level, I switched the focus of my research entirely to perturbative QCD applications from Model Building because I believe that currently this is the field where one can both ask and answer deep, experimentally relevant physics questions.

To get the point across, here are three not-well-known or appreciated in the wider community examples:

The String Theory connection.
The connection to Experiment: The Foundation of all Physics
Developing perturbative QCD to the level of learning about non-perturbative physics (think confinement).

(to be continued)

Last Updated ( Saturday, 02 October 2010 17:53 )

Alexander Mitov