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

Tweeting live #Higgs boson updates from #CERN

My view of CERN's auditorium, 2:15h before seminars began.

“If it’s just a fluctuation of background, it will take a lot of data to kill.”

Dr. Fabiola Gianotti, spokesperson for the ATLAS collaboration, made this statement on Dec. 13, 2011 during a special seminar I attended at CERN. Within the minute that followed, I hurriedly concocted a tweet, tacked on #Higgs and #CERN hashtags, and sent Fabiola’s weighty comment out onto the WWW.

CERN, where the WWW was invented, is the main European particle physics laboratory. I was at the lab for a week to discuss physics and the  performance of the ATLAS detector, a 7000-ton apparatus used to examine remnants of high-energy proton collisions delivered by CERN’s 27-km Large Hadron Collider (LHC), straddling the Franco-Swiss border.

This turned out to be no ordinary week. The 2011 LHC program had yielded a fecund data sample, and we needed to take stock of our most promising new-phenomena searches. By far the most anticipated were those of the Higgs boson, hypothetical pieces in the emerging puzzle of the tiniest known subatomic particles. Signal rumours had been swirling around the planet in blogs and other media. I was asked by the media relations department at my home institute, McGill University, to live tweet the Higgs update event. I already knew our ATLAS measurements, but was keen to see results by our competitors, the CMS collaboration, running a complementary detector on the opposite side of the LHC. Exciting times!

We owe much of this excitement to Ernest Rutherford who, while a McGill professor of Experimental Physics early last century, unwittingly helped to kick off the Higgs hunt through his Nobel Prize work on radioactivity. Modern theories that posit the existence of one or more types of Higgs particles seek to unify – into a more symmetric and fundamental theory – two basic forces: 19th-century electromagnetism and Rutherford’s 20th-century radioactivity. As if that weren’t enough, observing Higgs particles would also help to reveal a mechanism by which various fundamental particles are endowed with their non-zero mass values. This gets at the very essence of the physical universe.

More recently, my McGill colleagues and I have taken part in the search for Higgs bosons using the Fermilab Tevatron matter-antimatter collider near Chicago. Just last summer at McGill University, Dr. Adrian Buzatu defended a PhD thesis using Tevatron data to set the world’s best limits on Higgs boson production in the low-mass region that is now revealing hints at the LHC. Adrian recently took up a postdoctoral position in our ATLAS collaboration, working with the University of Glasgow group.

In December, I entered CERN’s auditorium three hours before the seminar was to begin. Within about 30 minutes, all available seats and aisles were jammed. A mob formed outside the auditorium door, but security guards were able to maintain control. Unable to work in all the nervous energy and jostling, I tweeted, “You’d think it was John Lennon coming to CERN today.”

You’d think it was John Lennon coming to CERN today. #Higgs #CERN

— Andreas Warburton (@AWarb) December 13, 2011

At long last the ATLAS and CMS talks began. Both collaborations had searched extensively for several different signatures, scenarios by which a Higgs particle could be created from LHC collision energy before disintegrating into lighter particles.

Given our detectors’ sensitivities, and the colossal Higgs-mimicking backgrounds, we knew in advance that our samples wouldn’t suffice for a statistically robust 2011 discovery. Nevertheless, both ATLAS and CMS showed suggestive traces in a variety of Higgs signatures. Enticingly, both collaborations ruled out overlapping mass ranges and recorded hints at similar masses.

These indications are thrilling. In this kind of science, discoveries take time and are often preceded by whiffs. We’re also cautious. Our 2011 results could be chance background fluctuations, tantamount to tossing six coins and getting tails every time. Only when our observations are flukier than tossing 20 tails in a row will we claim a discovery.

The 2012 data will likely enable us to observe or rule out the Higgs boson. Either of these outcomes would constitute exhilarating, 21st-century science. I look forward to tweeting about it.

For the event’s archived tweets, which contain links to further information, go to this page.

The above blog posting was adapted from an outreach article requested by the McGill Reporter newspaper. The ATLAS and CMS experiments have now submitted their Higgs search papers based on the 2011 data set.

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