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

Moriond day 3: The day of the Higgs

(I’m not skipping day 2, about heavy flavors and my own talk, but I think today’s topic merits a reshuffling)

It is no secret that Moriond EW 2012 celebrates the year of the “Higgs”. In December both CMS and ATLAS published preliminary results with the intention to present more complete analyses at this year’s Moriond, now the time has come at last.

Results aplenty: D0, CDF, CMS and ATLAS all presented limits consistent with “something” (*) around 120-130 GeV.

The plots below show upper limits on the Higgs mass from ATLAS and CMS. If you don’t know how to read one of these plots (I don’t blame you!) there is an excellent explanation on the ATLAS public site and in the Quantum Diaries.

The basic message is that if a Higgs particle exists we will see a peak, measured by the distance between the filled black line and the dashed line with green and yellow bands around it, that grows larger at a specific mass as we gather more data.

ATLAS and CMS limits. (scaled to match and presented by: Jean-Francois Grivaz)

Now, to the meat. The Higgs, or the scalar boson, as it is referred to at the moment. Is excluded for all masses up to around 600 GeV, except in the region around 120 to 130 GeV. What that indicates is that if the particle exists it is likely to be in that range. Now, why the vague language? In particle physics everything is statistics. We can see fluctuations in our data that can look like interesting physics; to avoid reading too much into these effect, we are fairly conservative when we see deviations from the standard model. What we require to claim a discovery is that we are 99.9999% sure that the “particle” is not a statistical fluctuation of the Standard Model playing tricks on us. With what we see at the moment we are over 95% confident but that still leaves room for error.

What is interesting is that both ATLAS and CMS see the same thing. Also the Tevatron experiments are in as much agreement as they can be with their available statistics. So it all looks like something at 125 GeV. Before the summer we are likely to know for sure. When the LHC starts in a few weeks time, it will “ramp” to 8 TeV. This slight increase in energy makes the possibility to see Higgs particles slightly higher and allows the accelerator to collide more particles per second than at 7 TeV.

With interesting results like these on the horizon, people are smelling a Nobel Prize. Will it go to Peter Higgs, his predecessors that originally described the electroweak symmetry breaking or perhaps the experiment(s) that discover(s) the particle? At the moment a lot of “positioning” is taking place to try and figure out who is responsible for this (potential) discovery. Semi-jokingly all the presentations feature titles like “SM Scalar boson” or “BEH Scalar boson” for Brout-Englert-Higgs to underline that the popular name is not necessarily describing the original creator. Hopefully it will be given to those deserving it with a nod towards the rest of the contributors, this is a large scale project after all, both experimentally and theoretically. What I find more worrying is that if the name sticks, we will have a fundamental particle named after a person, that seems somewhat odd compared to the rest of the Standard Model particles.

All in all a very exciting day, with a lot of discussion and emotion all around.

(*) I’m not a Higgs researcher and the opinions stated above do not necessarily represent the opinions of the ATLAS Collaboration.

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

Moriond day 2: Inverse time dilation

I work with crazy particles. Dark matter is pretty weird, so are neutrinos seemingly, but what I search for blows it all away. Tuesday was the day of my presentation. The format for these young scientist presentations are 5 minutes and time for a single question afterwards. Trying to present a full picture of any analysis in that short a time is impossible; instead the idea is more like handing out a business card telling the audience what you work on in the hope that some will be interested and contact you informally afterwards. For my presentation I wanted to introduce the general methodology we use and the latest results. Normally we can rely on the fact that most people in the audience know how we detect particles in ATLAS, but for the particles I look for it is slightly different.

R-Hadrons are composite particles with a heavy parton from some new theory like Supersymmetry and light standard model quarks

The particles we look for in my group are long-lived and carry color charge; this allows them to form into hadrons, like protons and pions. We call these particles R-Hadrons (R is from R-parity a concept from supersymmetry). An R-Hadron could be a supersymmetric gluino combined with two or three standard model quarks. Depending on the type of quarks the R-Hadron can be electrically charged or neutral. This is very important when we want to the detect the particles. We can only detect charged particles in our detectors and because these R-Hadrons don’t decay before they reach the detector we need them to be charged to find them. Now the method we use to distinguish R-Hadrons from say, muons and electrons is by their velocity. These particles must be very heavy. When we produce something heavy at the LHC there can’t be a lot of energy left to make them move fast.

Preliminary ATLAS Limit on R-Hadron Mass (from my talk)

The neat thing is that slow particles leaves a stronger signal in many of the detectors in ATLAS than fast particles, that way we can measure a difference. In fact, by measuring either the signal strength or the time of flight far away from the collision, we can calculate the mass of the particle directly. In that way we find R-Hadrons as well as their mass.

Now describing the method is in itself a full 45 min talk as we have around six different ways of measuring the velocity. The analysis that uses the velocity information can be fully as complicated (in fact more so, as the electric charge can change throughout the particles path in the detector), so giving a full description is impossible in five minutes.

What I ended up with was one slide on what we call these objects and what theories predict them. Another slide describing all the detection methods and a slide with the most recent result from one of the searches for R-Hadrons.

Energy loss of standard model particles at various momenta. Each band corresponds to a particle (from low p to high: pions, kaons and protons respectively) By knowing the momentum and the energy loss it is possible to estimate the mass of the particles. The same goes for R-Hadrons but at much higher momenta and mass.

For this presentation the most recent result was a new search using only the innermost detector system in ATLAS, the Pixel detector. The analysis was carried out by collaborators in our pixel performance group and was just approved for publication the day before the talk. The analysis finds no gluino (color-octet) R-Hadrons with a mass below 810 GeV a nice improvement from the first year’s limit of 650 GeV.

The interesting part for me personally was the discussions I had with people afterwards. I met a few theorists who work with these kinds of particles and we talked about including their specific models in our limits. Other common items on the search list are quasi-long-lived particles, R-Hadrons that decay with various lifetimes. All very interesting suggestions. While these searches are in the category of “blue sky research” for us experimentalists we look for them because we know how and because they form very interesting test subjects, it is also nice to hear that they are appreciated by theorists as well

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

Moriond day 1: The outer limits

Not many trips take you to all ends of the world in one day, but that was nevertheless how it felt after the first talks at Moriond. Sunday and Monday have mainly featured presentations on neutrino and dark matter physics. Many of these experiments are placed in remote regions or deep under ground. We have heard reports from the OPERA collaboration (of [perhaps] faster than light neutrino fame), Ice Cube, a wonderful neutrino detector located at the South-pole, the Japanese T2K neutrino experiment and many many others. For someone working on a LHC experiment it is very refreshing to learn about other experiments in the field, we might be the biggest game in town, but not the only one.

La Thuile, host of the 2012 Rencontres de Moriond

Monday morning was all about Dark Matter. A very interesting subject connecting astrophysics with particle physics. In short, we believe that some new, unexplained type of particle causes the galaxies to be five times heavier than expected from what we see from visible light alone. A possible explanation comes from Supersymmetry (SUSY), which predicts a stable particle that is supposed to interact only through the weak force and gravity, the lightest supersymmetric particle or LSP. Most of the dark matter experiments are reporting negative or inconclusive results, except one, DAMA. So the talks have been focused on how each experiment either confirms or disagrees with DAMA. With the ATLAS experiment, we also search for SUSY dark matter, but in a very different way than the dedicated experiments. In ATLAS, dark matter could show itself as “missing energy” that makes the rest of the particles coming from the collision not quite add up energy wise. Depending on the model, it is possible to search for dark matter indirectly by looking for this missing energy together with other particles decaying in a specific pattern.

The LHC talks have been moved to the end of the week to give us more time to approve the latests results from the 2011 run. Everyone I’ve talked to here (especially the theorists) are interested to learn what we might have discovered, patience can be difficult with potential discoveries just around the corner

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