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.

A new particle is born, but who is the father?

ATLAS Higgs Combination Confidence Plot.

We have discovered a particle. It is perhaps the particle everybody has been looking for but, for now, let us just call it a particle possibly known as the Higgs.

I’m 28 years old, I have spent 4 of them in the ATLAS family and particle physics in general. That means this is the first particle discovery I have taken part in, and that is exciting even for the most level-headed of us.

Ironically the discovery made me think of how it must have been to discover the W and Z particles in UA2 and UA1 the same year I was born. Many of the slightly older members of ATLAS remember those days – I have only been told bed-time stories by my professors.
What strikes me is the magnitude of these discoveries. Did they realise these were the long-sought Weak bosons predicted by the Standard Model? Did they admit to believe it and — perhaps more interesting — When? The titles of the published articles implies a level of caution:

“Experimental observation of isolated large transverse energy electrons with associated missing energy at sqrt(s)= 540 GeV”
- The UA1 Collaboration

“Observation of single isolated electrons of high transverse momentum in events with missing transverse energy at the CERN p-pbar collider”
- The UA2 Collaboration

But, they had just discovered the W boson! Better not be too brash too early, but the titles are vague (and notice the length of the author lists)!

So, now we discovered a particle as well. Is it a Higgs, and if so, which one? Currently I’d say that this question is a bit academic, we only have one really precise theory that has been tested many times over, and that theory usually comes with one specific Higgs boson. The rest must be manifestations of a hopeful dreamer. Now I like to indulge in daydreaming, in fact that is what I’m paid to do, so let us explore what else is out there and why it is very likely that what we found is a standard model Higgs.

SUSY, Technicolor and other Dream-states

Our theory-minded friends have had plenty of time to dream up beautiful alternatives to the Standard Model. Some hope to supplant the status quo with more mathematically appealing models unifying the particle world in Grand Unification Theories (GUT). Others are going for the deeper truths combining the microcosm of particles with gravity into what are affectionately called TOE(s) or Theories of Everything. Some of these theories lean against the Standard Model’s solution for adding mass to otherwise mass-less particles. Others might use entirely different schemes. But common to even the more obvious alternatives is that they provide quantifiable(/falsifiable) predictions.

One way to tell different models apart is by looking at how the models vary in their predictions. Common (I think) to all Higgs look-a-likes is that they are unstable and decay fast in various ways. We call the probability for a specific decay the decay-mode branching-ratio. For example, the results shown from ATLAS and CMS both rely on two main decay-modes: one in which the Higgs decays into two photons and another in which the Higgs decays to two Z bosons that each again decay to either two muons or two electrons. For the standard model and supersymmetry (SUSY) some of my friends and I published a small report with the predicted branching ratios. Let’s compare:

Top: Standard Model branching ratio predictions. Bottom: SUSY MSSM predictions of a light higgs.

For a few reasons, the figures are not directly comparable. The top one shows that SUSY, even constrained to the Minimal Supersymmetric Model (MSSM), which is a small subset of all possible ways to describe Supersymmetry, still has a lot of freedom in terms of free parameters. In the plot, one of them tan(beta) is fixed at 10, but this value could be something else in reality. The other difference is that MSSM actually requires FIVE Higgs particles to give mass to both “up” and “down” type quarks, as well as charged leptons.

Luckily just the mass of one of the Higgs together with tan(beta) is enough to estimate the mass of the rest. What is relevant is that the branching ratios are different for the Standard Model Higgs and the SUSY Higgs. So, as we see our measured particle decay to two photons and two Zs, it is already in better agreement with the SM Higgs than the SUSY one in the figure.

Branching ratio for a composite Higgs in Technicolor

What can happen, will happen, but how often?
Another way to measure the difference is simply to look at the overall production cross-section, or how many times per proton-proton collision do we find a Higgs particle with any decay-mode?

In the presentations made by the two collaborations we see that the expected Standard Model Higgs cross-section is a bit less than what has been measured. But, there is nothing alarming here. The analyses are based on very few Higgs candidates, so we might just have been lucky to produce a bit more than expected statistically, and it might even out when more data is collected.

Spin it
Is it a Higgs at all? One of the most fundamental observables that separates a Standard Model Higgs from other particles is its spin, or rather its lack of spin. Measuring the spin of the Higgs particle requires a lot of statistics, hundreds of times more collisions than we have collected so far. The result from that measurement will be well worth waiting for, as the Higgs particle is the only particle in the Standard Model with zero spin, something not observed before in any elementary particle.

It might be too early to tell who the father is, but based on the baby’s hair colour I’d not be too worried about the postman yet!

Morten Dam Jørgensen is a PhD fellow at the Niels Bohr Institute in Copenhagen, Denmark. He is currently working on searches for long-lived particles and general model independent searches for deviations from the Standard Model. You can find more information at http://mdj.dk

Mystical Moriond

As a young physicist not many conferences have the same mystical status as Rencontres de Moriond. This gathering of physicists from all areas of particle physics is one of most anticipated events of the year. More a gathering than a conference, Moriond started in 1966 and has inspired many similar events. Presentations, time for discussion and recreation is combined to inspire and foster collaboration and new ideas. Another element is the meeting between young and more experienced scientists. Nearly half of the talks are given by young participants below 35 like myself.
I was invited by the ATLAS collaboration to present our latest results on a search for a type of long-lived particles that has meant a lot to me for the last two years.

The particles are called R-Hadrons, or perhaps they will be – because at the moment they are just an idea about what Nature can potentially give us if the world is super-symmetric or contains extra dimensions. These particles are pretty crazy, not only are they very very heavy (much heavier than the top quark or the yet to be discovered Higgs boson) but they also live longer than most particles. Even stranger they can “flip” their electrical charge if they pass through material. So all in all some very strange particles, but also very interesting to look for.

In ATLAS there are quite a few of us working on this kind of search, so presenting the work is not simply a personal effort; the results are made in collaboration between many people creating the analysis, not to mention all the work that goes into running the experiment. Because we always publish together, presentations like mine must be approved and agreed upon by the rest of the collaboration, meaning that they have to be thoroughly worked out before the talk. In the next post I will talk about the preparations and my first impressions of the place itself, now I have to catch my flight!

Morten Dam Jørgensen is a PhD fellow at the Niels Bohr Institute in Copenhagen, Denmark. He is currently working on searches for long-lived particles and general model independent searches for deviations from the Standard Model. You can find more information at http://mdj.dk

7 or 8 TeV, a thousand terabyte question!

Event Pile-Up in the New Year?

A very happy new year to the readers of this blog. As we start 2012, hoping to finally find the elusive Higgs boson and other signatures of new physics, an important question needs to be answered first – are we going to have collisions at a center of mass energy of 7 or 8 TeV. While that may not feel like a such a drastic step up, certainly not like going to the full design collision energy of 14 TeV, it does bring its own sets of challenges for ATLAS. Understanding the detector performance is crucial for doing physics with our data, and we will have to make sure all the good work done during 7 TeV collisions can be extended if we run at 8 TeV. More collision energy means more pileup interactions; these occur when our detector can not distinguish between two separate collision events and thus considers them part of the same collision. We need to disentangle the pileup contribution to look at the real single collision event, and while a lot of work has been done in this direction, an increase in pileup is always a cause for concern. However, as someone working closely with Monte-Carlo tuning and production, I know firsthand how big of an issue this is going to be for us.

We need Monte-Carlo samples, or simulated data sets for every analysis, to calculate detector efficiency, backgrounds and what not. Also, a lot of times these Monte-Carlo event generators reflect our best understanding of certain processes, and we want to make sure they are predicting the behavior of real data closely. At times when they do not, we turn the knobs in the Monte-Carlo generators and tune them to match the data. Up until now, this tuning has been done mostly with 7 TeV collision data – although we tried to get the energy extrapolation right by using lower energy Tevatron and ATLAS data. We believe the simulation will do a good job at describing 8 TeV collision data – but we can’t be sure unless we actually compare, and most analysis groups will already want the latest and the best Monte-Carlo samples by the time the data starts coming in!

Then there is the question of size. The combined size of ATLAS 7 TeV Monte-Carlo samples is at least a few thousand terabytes. A very conservative estimate suggests we will need a few of months to produce the 8 TeV samples – the caveat is we can’t start producing them until the decision is actually made to switch to 8 TeV. This will happen immediately after the annual Chamonix meeting in the beginning of February, when the CERN management, engineers and experiment representatives meet to decide. As ramping up the energy results in higher cross-sections for the rare process we want to look at, from a physics perspective it is definitely beneficial, but we have to be ready to utilize this if and when it happens.

With input from Borut Kersevan.

Deepak Kar is a postdoctoral research fellow with University of Glasgow. His physics interest is soft-quantum chromodynamics, and he is currently involved in underlying event analysis activities and Monte Carlo tuning in ATLAS.