TOP 2012 – Part 2

Welcome back, dedicated top quark enthusiast. I’m sure you’ve all been waiting on the edge of your seats for an update from TOP 2012, and I can now confirm that a combined team of LHC & Tevatron physicists narrowly beat a mixed team of physicists from LHC & Tevatron at croquet.

Congratulations to both colliders on their wins and losses, and to the theorists, whichever team you ended up on! How many jokes were there about ‘conservation of angular momentum’ and ‘pileup’ you might be wondering? Too many, and we’ve all agreed not to speak of it again.

A big mass with a small error

‘Finally, some physics,’ I hear you cry! I will surely be doing an injustice to many by singling out any one result, and as a young PhD student I probably won’t even pick out the most significant, but I can’t help but look at the LHC and Tevatron mass combinations and think, ‘Cool!’

Performing a top mass measurement is far from easy, even in the most agreeable of decay channels. Presented at the conference were nothing short of 20 different mass measurements[1] that not only have overcome the huge challenges unique to each analysis, but have managed to combine these results to obtain a measurement of staggering sensitivity. This may sound easy to the uninitiated but when done correctly these combinations are a vast undertaking.

The Tevatron plot you can see above has an uncertainty of less than 1% on the final combination. One percent! Perhaps even more exciting is that the Tevatron hasn’t finished analysing all of its data. Many of these results will be updated to the final dataset, and of course the LHC is catching up fast! Perhaps if we all donate the required offerings of coffee and Red Bull, and pray to the gods in charge of the grid, we may just see a world combination before too long.

World leading measurements!

Let’s face it, we all have a barely completely justifiable sense of loyalty to our experiments. But top quark physics is a broad field, and it would be criminal not to point out a few of my favourite highlights from all of the experiments.

ATLAS: Spin correlation

Convenors would probably argue about this choice for ATLAS, but I think it’s a great little analysis, with some really talented blogger/physicists working on it. These types of angular distributions are excellent probes for non-resonant new physics and a lot of attention has been paid to them at the conference. Though I picked out ATLAS, new results are also available from CMS (also pictured), and DZero and CDF are well worth a read.

CMS: W associated single top production

Hot off the press, this analysis was approved just in time for this conference. Soon to be submitted to Physical Review Letters (PRL), this cross section represents the best known evidence for the W boson associated single top production channel. This channel plays an important role in many analyses and understanding this production mechanism will be crucial in later results.

DZero & CDF: Harder, better, faster, stronger, forward, backward!

When I asked a DZero colleague which was his favourite analysis, he laughed and bought me a beer. I took this to mean that he likes them all and therefore I picked my favourite. Combined with CDF, both experiments provide fascinating results on one of the most interesting anomalies observed at the Tevatron, the top forward backward asymmetry.

This is a phenomenon unique to the Tevatron collider[2], as the Standard Model predicts practically no asymmetry. Nevertheless, CDF and DZero have consistently measured an unexplained asymmetry for several years and the question on everyone’s minds is: Could this be an effect from new physics? Time will tell…

TOP 2013:

Hopefully you now have a nice overview of what has been a fantastic conference. There were far too many results to do them all justice, but if you’re after a detailed description of each talk (presented in a sarcastic, British style), then feel free to fly me out to your institute or nearest luxury spa/resort at your convenience. For those not blessed with a limitless travel budget, find your nearest top quark physicist and ask them to explain it to you over a beer.

If you wanted a ‘what to look out for’ summary, personally I’d say angular correlations, mass combinations, and interplay with the Higgs sector. Of course that’s from an experimentalist point of view and I’ve barely touched on the theory. Keep an eye out for the public results, and for those with Indico access you can find the slides from all talks here.

We now have a long year to wait until TOP 2013, and I for one am counting the days. See you all in Durbach, Germany!

[1] Tevatron – 12 (3 new), LHC – 8 (4 new)

[2] The LHC is a charge symmetric proton-proton collider and therefore has no ‘forward-backward’ asymmetry. One can define a ‘forward-central’ asymmetry that probes the same physics and a great deal of attention is paid to this at ATLAS, CMS and now also LHCb.

James Howarth is an experimental PhD student studying at the University of Manchester, UK. His physics interests centre on top quark physics in general and on top properties in particular.

TOP 2012 – Part 1

Greetings from the TOP 2012 conference, Winchester UK! What’s a ‘Winchester’ I hear you asking? A type of gun? Indeed yes, though sadly not of the smoking variety that we’re all so keen to find. However in this particular case Winchester is a historical town in the south of England, complete with the typical rolling green fields, a cathedral, and the not so typical contingent of visiting physicists!

This is the highlight of the conference year if you’re a top quark physicist. Better than ICHEP (unless of course, you actually got to go to ICHEP) and more relaxed than Moriond QCD. Five full days in a four star hotel in the English countryside with only the top quark on our minds, bliss! There’s also a pool and a spa, but who cares about such trivialities…

The best ‘little’ particle there is!

In the year of the Higgs discovery, what might we expect to see in the top sector? Well intrepid reader, that’s a mighty fine question and one that has an agreeable sense of symmetry to it. Just as we’re beginning to probe the boson responsible for bestowing particles with mass, we’re well and truly entering into an era of precision measurements of the standard model’s (SM) most massive particle! In this year’s conference all four five[1] hadron collider experiments involved in top quark physics will present cutting edge, state of the art results on everything from cross sections and properties, to resonances and exotic particle searches. This aptly named particle is, for lack of a more articulate argument, AWESOME [2]!

Conference goody bag win!

Now it’s only the first day of the conference, and we’ve plenty of reason to expect some very exciting results over the next few days, but what has got me so excited on day one? Is it the promise of some ‘friendly’ games of football and croquet with the theorists? No of course not, it’s the simply amazing conference goody bag, in which you’ll find this little gem!

Not only is it a laser pointer (never again will I have to gesture wildly from the audience at a dubious data point on a plot that the speaker pretends not to be able to see), it is also a more retro mechanical pointer, with a pen on the end as a finishing touch! Take that ICHEP!

Why do we care?

So why should you care about top quark physics? If the particle found is indeed the fabled Higgs Boson, then the top quark is the highest mass particle known to exist and may play a special role in electroweak symmetry breaking. Furthermore, there’s barely a new physics model out there that doesn’t involve the top quark in one way or another and top events are fantastic probes for new physics particles such as Z’, axi-gluons and certain flavours of SUSY.

Aren’t you going to show us some plots?

Since many of the physics analysts are taking advantage of last minute editorial board approvals, some of the more interesting and controversial plots are not available for me to post (at least not yet). But there are a few highlights that we can be pretty sure are on the way. Almost certainly there will be a lot of interest in the latest and greatest LHC top cross section combination and time will surely be allotted to the most current forward-backward asymmetry results from CDF and DØ. But with the full 7TeV data sets under their belts, and 8TeV on the way, it will be exciting to see what ATLAS and CMS have been doing with the forward-central asymmetries and lepton charge asymmetries.

If you’re not familiar with these terms, I could easily spend several blog entries explaining why this is a really cool measurement, and why it tantalisingly offers hints of beyond the standard model physics. But I suspect if you’ve read this far on a blog entitled “TOP 2012” then you’ve probably already heard of it. For those that haven’t, the following link is a very nice summary paper.

It’s a theory paper but try not to panic, and stay tuned for the latest results as they are presented this week.

[1] Welcome to top physics LHCb!

[2] Christian Schwanenberger, DØ Physics co-ordinator – ad verbatim, too many times to count and in various accents.

James Howarth is an experimental PhD student studying at the University of Manchester, UK. His physics interests centre on Top Quark Physics in general and on Top Properties in particular.

Needle in a haystack

The LHC is designed to collide bunches of protons every 25 ns, i.e., at a 40 MHz rate (40 million/second). In each of these collisions, something happens. Since there is no way we can collect data at this rate, we try to pick only the interesting events, which occur very infrequently; however, this is easier said than done. Experiments like ATLAS employ a very sophisticated filtering system to keep only those events that we are interested in. This is called the trigger system, and it works because the interesting events have unique signatures that can be used to distinguish them from the uninteresting ones.

The ATLAS Trigger and Data Acquisition System

The ATLAS trigger system is a combination of electronic circuit boards and software running on hundreds of computers and is designed to reduce the 40 MHz collision rate to a manageable 200-400 events per second. Each event is expected to be around 1 Mbyte (for comparison, this post corresponds to about 4-5 kilobytes), so you can see that we are dealing with a lot of data. And, all this has to be done in real time. In a previous post, Regina Caputo gave an overview of triggers. Here I expand on that.

Before I get to the numbers of events that we collect, let me first explain a couple of concepts: cross-section of a particular process and luminosity. Cross-section is jargon; basically, it gives you a measure of the probability of a certain kind of event happening, and is a function of the energy of the collision. In general, higher the collision energy, higher is the cross-section of a process, especially if we are producing a heavy particle (there are some subtleties that I won’t get into now). Luminosity is a measure of the “intensity” of the beam. The product of Luminosity and Cross-section gives the number of events that are produced for a given process. The beauty of the trigger system is that it can be configured to pick the kinds of events we want to study.

One common kind of event happens when two protons “glance” off each other, without really breaking up; these are called Elastic Collisions”. Then you have protons colliding and breaking up, and producing “garden-variety” stuff, e.g., pions, kaons, protons, charm quarks, bottom quarks, etc; these are labelled Inelastic Collisions. The sum of all these processes is the “total cross-section”, and is about 70-80 millibarns at a collision energy of 7 TeV, i.e., 1/12th of barn; the concept of a “barn” probably derives from the expression “something is as easy as hitting the side of a barn”! So, a cross-section of 80 millibarns implies a very, very large probability (1 barn = 10^-24 cm² ). At collision energies of 14 TeV, this might increase by about 10-20%.

In contrast, the cross-section for producing a Higgs boson (with mass = 150 GeV, i.e., 150 times the mass of a proton) in 7 TeV collisions is approximately 8 picobarns (8*10^-12 barns), i.e., approximately 10 billion times less than the “total cross-section”. The cross-section for producing top quarks is about 170 picobarns. Events containing a Higgs or top quarks have some unique signatures that are exploited by the trigger algorithms. (At 14 TeV, the cross-section for these interesting events can increase by as much as a factor of five, so you can see why we want to keep increasing the energy of these collisions.)

The LHC is designed to have a luminosity of 10³⁴ , i.e., looking head-on at the beam there are 10³⁴ protons/square cm/second. In reality, each colliding bunch only has about 10¹¹ protons, but they are squeezed into a circle with a radius of 0.003 cm, and come about 40 million times/sec. So, taking the product of cross-section and luminosity, we estimate that we will get approximately 10⁹ “junk events”/second and 0.1 Higgs events/second! Of course, there are other interesting events that we would like to collect, e.g., those containing top quarks that come at a rate of 2 Hz. We also record some of the “garden-variety” events, because they are very useful in understanding how the detector is working. So, this is what the trigger does, separate what we want from what we don’t want, and all in “real time”.

As mentioned above, we plan to write to disk approximately 200-400 events per second, with each event being 1 MB in size. If we run the accelerator continuously for a year, we will collect (6-12)*10¹⁵ bytes of data, i.e., 6-12 petabytes; this will fill about 38,000-76,000 IPods (ones with 160 GB of storage)! Each event is then passed through the reconstruction software (see the ATLAS Blog “From 0-60 in 10 million seconds! – Part 1“), which only adds to its size; talk about standing in front of a fire hose!

–Vivek Jain, Indiana University

p.s. For fun facts about ATLAS, check out the ATLAS pop-up book! You can find it on Facebook, watch a video on YouTube, and purchase it on Amazon.

Vivek Jain is a Scientist at Indiana University, Bloomington. His current interests range from understanding various aspects of tracking to R-parity violating Supersymmetry. More information about his interests can be found at http://www.indiana.edu/~iubphys/faculty/jain2.shtml