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

Melbourne Dispatch: A First Coming To Terms with Discovery

Melbourne Convention and Exhibition Centre

Where to begin? The 4th of July, 2012 will remain burned in the memories of those of us fortunate to be delegates at this historic 36th International Conference on High Energy Physics (#ICHEP2012) in beautiful Melbourne, Australia. My day began in a Boeing 747, dodging tropical clouds high above the Pacific Ocean. One connection in Sydney, two shuttle buses, and one shower later, I found myself in the plenary hall at the stylish and serene Melbourne Convention and Exhibition Centre.

Projected onto the giant Melbourne screen was a live video connection to the CERN amphitheatre in Geneva, where the much awaited Higgs boson search update talks, timed for ICHEP, were about to commence. My sentiments immediately turned to Dec. 13, 2011, when I was at CERN to live-tweet the “tantalizing Higgs hints” talks for McGill. This time around, I was among a handful of physicists who’d agreed to help tweet the Melbourne conference on behalf of CERN.

Your Melbourne correspondent, with other ATLAS colleagues, including former and future spokespeople Peter Jenni and Dave Charlton. Credit: Claudia Marcelloni

The seminars that followed have already been widely discussed in blogs, newspapers, and other media, right down to the choice of font used. Through my tweets, under the handle @AWarb and hashtag #ICHEP2012, I endeavoured to convey minute-by-minute happenings while maintaining some editorial restraint. It felt a little like being a play-by-play broadcaster at a momentous sporting event, but one in which I played for one of the teams, the score was basically tied, and the teams were both separately and mutually victorious.

Minutes before the talks began, there was a distinct air of anticipation amongst the Melbourne delegates. On our giant screen, we watched as the CERN camera zoomed to one of the Geneva attendees, a placard reading “Ciao Mamma!” visible on his desk. The audience in Melbourne chuckled nervously. Then we saw Professor Higgs and the other living progenitors of the theoretical Higgs-mechanism concept entering CERN’s amphitheatre, to loud applause. Melbourne reacted as well. But when CERN’s Director-General Rolf-Dieter Heuer entered the room and prepared to initiate the proceedings, our Melbourne hall fell utterly silent.

Joe Incandela, spokesperson for the CMS collaboration, began with the first talk. Excitement at CERN and in Melbourne mounted as the results, initially articulated carefully for each of the studied decay channels, culminated in a discovery-level combination to reveal, unequivocally, that we had a new particle on our hands. The CMS team had even analyzed its data enough to be able to quote the new particle’s mass with an impressive precision of under 0.5%. Moments later, I tweeted “Day’s big question: are these Standard Model #Higgs bosons, or something else? There’s something there, but what is it exactly?”

Then, it was spokesperson Fabiola Gianotti’s turn to present our ATLAS collaboration’s Higgs search results. We ATLAS collaborators already knew our outcome, but it was nevertheless thrilling to see it presented in this electrified forum. When Fabiola’s talk climaxed with our combined-channel statistical significance, the Melbourne delegates erupted in sudden loud applause followed by an immediate, eerily stunned, silence. Appreciation for the historic importance of this amazing day had now fully gripped the hall.

Meanwhile, an interesting sequence of events had occurred. CERN’s press release had been embargoed until the start of the second hour of the event, during Fabiola’s ATLAS talk. The moment it went public, my Twitter feed began to light up with scientific commentators, news organizations, and others heralding the discovery. Because of the timing, the ATLAS discovery punchline actually got sent to the rest of the planet before Dr. Gianotti reached it in her talk to the CERN and Melbourne scientific audiences.

The ICHEP meeting is now in full swing here in Melbourne. We’ve had detailed talks and discussions about the key ingredients of this new particle discovery. Other delegates and I have been tweeting updates with both Higgs and non-Higgs news. We’ve also been getting interesting questions from the worldwide public, some scientifically detailed, some more in the category of outreach. We’re gradually getting better at putting Greek and other scientific symbols into tweets! G’day mates, from Down Under.

Andreas Warburton tweets as @AWarb and is an associate professor in the Department of Physics at McGill University. For information about his research and other interests, see http://twitter.com/#!/AWarb.

Very exciting day at CERN about the Higgs ??!

Good morning science addicts and everyone !

What a special day at CERN today ! Indeed, the ATLAS and CMS experiments have just released some outstanding results and observations about the search for the Higgs boson, and the ATLAS and CMS spokespersons (Fabiola Gianotti, and Joe Incandela) just presented those results in the main auditorium at 9 a.m (CERN time).

Just before we get to the big news, let me describe a little bit the atmosphere we could feel around. Knowing the seminar would be at 9 am, and the doors opening at 7:30 am, I thought that arriving at 7 am would make it (to get a sit in the main auditorium). What a fool !! Arriving at the main restaurant, the queue was already going till the new building of restaurant 1 ! Then, approaching the room, I heard that some students had the good idea to spend the whole night in front of the doors of the main auditorium. Maybe those who arrived around 5:30 am this morning could enter at the very last minute, as the last lucky persons inside. Obviously this was not a big deal for the (very special) persons invited (did you hear that Peter Higgs was there ?)…

Thankfully, there were several webcast retransmissions, for instance in the council chamber (reserved for the press), all the meeting rooms in building 40, the globe, TH, IT and TE auditoriums, as well as Prévessin. In short, pretty much everywhere we meet in CERN.  In addition, two live channels were webcast (one for CERN users, and one totally open to the public). So, as you could expect, I couldn’t get into the main auditorium, but went back to the office to watch the CERN webcast.

ATLAS Higgs Combination Confidence Plot. Note the peak at 126.5 GeV!

9 a.m. Joe started. Mmm impressive atmosphere, with a huge tension. Even Joe admitted that, joking that

he didn’t know why … Then, he started one of his 170 slides or so… saying that this was still not enough to explain, show, and thank the huge amount of work, and all the persons involved in this extraordinary adventure !

Now let me get to the main point: 9:30 – CMS has observed a new boson with a mass of 125.3 +/- 0.6 GeV with 4.9 sigma significance. Tremendous applause. We’ll come to the details a bit later…

About one hour later, Fabiola announced the remarkable results obtained by ATLAS: Combining results on various decay modes

also observe a boson of 126.5 GeV mass with a local significance of 5 sigma!! We found it ! And what a good agreement between the 2 experiments !

As the speakers said, we should thank (and think about) all the people involved in the different fields (accelerators, detectors, theory, computing, labs, personnel, engineers, technicians). We can be all proud of these great and amazing scientific achievements.

As our Director General (Rolf Heuer) said, this is a historical milestone. Fabiola also reminded us that this is also the start of the Higgs era. Some other questions are already on our mind: Is this a Standard Model Higgs ? What are its properties ? Can we find other particles ? What about the Higgs mechanism…? Such an exciting time.

As this is very interesting, I’ll try to write very soon (in the coming days) a complete article about the Higgs discovery, with more details about the decay channels, and some historical references.

Best wishes, Marc

Dr. Marc Goulette is a particle physicist working on the ATLAS experiment for the University of Geneva. His research interests in Particle Physics are: Standard Model and Electroweak Physics (W, Z, electrons, photons), MC Generators, structure functions, Di-boson Physics, Higgs searches, Neutrinos, W’, Z’, new physics searches in general, and outreach activities.