ATLAS detector – Close Read

My heart of physics

Every July 4, I have occasion to remember two things: the discovery of the Higgs boson, and my first published byline for an article about the discovery of the Higgs boson. I have no trouble believing it’s been eight years since we discovered this particle, using the Large Hadron Collider (LHC) and its ATLAS and CMS detectors, in Geneva. I’ve greatly enjoyed writing about particle physics in this time, principally because closely engaging with new research and the scientists who worked on them allowed me to learn more about a subject that high school and college had let me down on: physics.

In 2020, I haven’t been able to focus much on the physical sciences in my writing, thanks to the pandemic, the lockdown, their combined effects and one other reason. This has been made doubly sad by the fact that the particle physics community at large is at an interesting crossroads.

In 2012, the LHC fulfilled the principal task it had been built for: finding the Higgs boson. After that, physicists imagined the collider would discover other unknown particles, allowing theorists to expand their theories and answer hitherto unanswered questions. However, the LHC has since done the opposite: it has narrowed the possibilities of finding new particles that physicists had argued should exist according to their theories (specifically supersymmetric partners), forcing them to look harder for mistakes they might’ve made in their calculations. But thus far, physicists have neither found mistakes nor made new findings, leaving them stuck in an unsettling knowledge space from which it seems there might be no escape (okay, this is sensationalised, but it’s also kinda true).

Right now, the world’s particle physicists are mulling building a collider larger and more powerful than the LHC, at a cost of billions of dollars, in the hopes that it will find the particles they’re looking for. Not all physicists are agreed, of course. If you’re interested in reading more, I’d recommend articles by Sabine Hossenfelder and Nirmalya Kajuri and spiralling out from there. But notwithstanding the opposition, CERN – which coordinates the LHC’s operations with tens of thousands of personnel from scores of countries – recently updated its strategy vision to recommend the construction of such a machine, with the ability to produce copious amounts of Higgs bosons in collisions between electrons and positrons (a.k.a. ‘Higgs factories’). China has also announced plans of its own build something similar.

Meanwhile, scientists and engineers are busy upgrading the LHC itself to a ‘high luminosity version’, where luminosity represents the number of interesting events the machine can detect during collisions for further study. This version will operate until 2038. That isn’t a long way away because it took more than a decade to build the LHC; it will definitely take longer to plan for, convince lawmakers, secure the funds for and build something bigger and more complicated.

There have been some other developments connected to the current occasion in terms of indicating other ways to discover ‘new physics’, which is the collective name for phenomena that will violate our existing theories’ predictions and show us where we’ve gone wrong in our calculations.

The most recent one I think was the ‘XENON excess’, which refers to a moderately strong signal recorded by the XENON 1T detector in Italy that physicists think could be evidence of a class of particles called axions. I say ‘moderately strong’ because the statistical significance of the signal’s strength is just barely above the threshold used to denote evidence and not anywhere near the threshold that denotes a discovery proper.

It’s evoked a fair bit of excitement because axions count as new physics – but when I asked two physicists (one after the other) to write an article explaining this development, they refused on similar grounds: that the significance makes it seem likely that the signal will be accounted for by some other well-known event. I was disappointed of course but I wasn’t surprised either: in the last eight years, I can count at least four instances in which a seemingly inexplicable particle physics related development turned out to be a dud.

The most prominent one was the ‘750 GeV excess’ at the LHC in December 2015, which seemed to be a sign of a new particle about six-times heavier than a Higgs boson and 800-times heavier than a proton (at rest). But when physicists analysed more data, the signal vanished – a.k.a. it wasn’t there in the first place and what physicists had seen was likely a statistical fluke of some sort. Another popular anomaly that went the same way was the one at Atomki.

But while all of this is so very interesting, today – July 4 – also seems like a good time to admit I don’t feel as invested in the future of particle physics anymore (the ‘other reason’). Some might say, and have said, that I’m abandoning ship just as the field’s central animus is moving away from the physics and more towards sociology and politics, and some might be right. I get enough of the latter subjects when I work on the non-physics topics that interest me, like research misconduct and science policy. My heart of physics itself is currently tending towards quantum mechanics and thermodynamics (although not quantum thermodynamics).

One peer had also recommended in between that I familiarise myself with quantum computing while another had suggested climate-change-related mitigation technologies, which only makes me wonder now if I’m delving into those branches of physics that promise to take me farther away from what I’m supposed to do. And truth be told, I’m perfectly okay with that. 🙂 This does speak to my privileges – modest as they are on this particular count – but when it feels like there’s less stuff to be happy about in the world with every new day, it’s time to adopt a new hedonism and find joy where it lies.

Prospects for suspected new fundamental particle improve marginally

This image shows a collision event with a photon pair observed by the CMS detector in proton-collision data collected in 2015 with no magnetic field present. The energy deposits of the two photons are represented by the two large green towers. The mass of the di-photon system is between 700 and 800 GeV. The candidates are consistent with what is expected for prompt isolated photons. Caption & credit © 2016 CERN

On December 15 last year, scientists working with the Large Hadron Collider experiment announced that they had found slight whispers of a possible new fundamental particle, and got the entire particle physics community excited. There was good reason: should such a particle’s existence become verified, it would provide physicists some crucial headway in answering questions about the universe that our current knowledge of physics has been remarkably unable to cope with. And on March 17, members of the teams that made the detection presented more details as well as some preliminary analyses at a conference, held every year, in La Thuile, Italy.

The verdict: the case for the hypothesised particle’s existence has got a tad bit stronger. Physicists still don’t know what it could be or if it won’t reveal itself to have been a fluke measurement once more data trickles in by summer this year. At the same time, the bump in the data persists in two sets of measurements logged by two detectors and at different times. In December, the ATLAS detector had presented a stronger case – i.e., a more reliable measurement – than the CMS detector; at La Thuile on March 17, the CMS team also came through with promising numbers.

Because of the stochastic nature of particle physics, the reliability of results is encapsulated by their statistical significance, denoted by σ (sigma). So 3σ would mean the measurements possess a 1-in-350 chance of being a fluke and marks the threshold for considering the readings as evidence. And 5σ would mean the measurements possess a 1-in-3.5 million chance of being a fluke and marks the threshold for claiming a discovery. Additionally, tags called ‘local’ and ‘global’ refer to whether the significance is for a bump exactly at 750 GeV or anywhere in the plot at all.

And right now, particle physicists have this scoreboard, as compiled by Alessandro Strumia, an associate professor of physics at Pisa University, who presented it at the conference:

750_new

Pauline Gagnon, a senior research scientist at CERN, explained on her blog, “Two hypotheses were tested, assuming different characteristics for the hypothetical new particle: the ‘spin 0’ case corresponds to a new type of Higgs boson, while ‘spin 2’ denotes a graviton.” A graviton is a speculative particle carrying the force of gravity. The – rather, a – Higgs boson was discovered at the LHC in July 2012 and verified in January 2013. This was during the collider’s first run, when it accelerated two beams of protons to 4 TeV (1,000 GeV = 1 TeV) each and then smashed them together. The second run kicked off, following upgrades to the collider and detectors during 2014, with a beam energy of 6.5 TeV.

Although none of the significances are as good as they’d have to be for there to be a new ‘champagne bottle boson’moment (alternatively: another summertime hit), it’s encouraging that the data behind them has shown up over multiple data-taking periods and isn’t failing repeated scrutiny. More presentations by physicists from ATLAS and CMS at the conference, which concludes on March 19, are expected to provide clues about other anomalous bumps in the data that could be related to the one at 750 GeV. If theoretical physicists have such connections to make, their ability to zero in on what could be producing the excess photons becomes much better.

But even more than new analyses gleaned from old data, physicists will be looking forward to the LHC waking up from its siesta in the first week of May, and producing results that could become available as early as June. Should the data still continue to hold up – and the 5σ local significance barrier be breached – then physicists will have just what they need to start a new chapter in the study of fundamental physics just as the previous one was closed by the Higgs boson’s discovery in 2012.

For reasons both technical and otherwise, such a chapter has its work already cut out. The Standard Model of particle physics, a theory unifying the behaviours of different species of particles and which requires the Higgs boson’s existence, is flawed despite its many successes. Therefore, physicists have been, and are, looking for ways to ‘break’ the model by finding something it doesn’t have room for. Both the graviton and another Higgs boson are such things although there are other contenders as well.

The Wire
March 19, 2016

The hunt for supersymmetry: Reviewing the first run – 2

I’d linked to a preprint paper [PDF] on arXiv a couple days ago that had summarized the search for Supersymmetry (Susy) from the first run of the Large Hadron Collider (LHC). I’d written to one of the paper’s authors, Pascal Pralavorio at CERN, seeking some insights into his summary, but unfortunately he couldn’t reply by the time I’d published the post. He replied this morning and I’ve summed them up.

Pascal says physicists trained their detectors for “the simplest extension of the Standard Model” using supersymmetric principles called the Minimal Supersymmetric Standard Model (MSSM), formulated in the early 1980s. This meant they were looking for a total of 35 particles. In the first run, the LHC operated at two different energies: first at 7 TeV (at a luminosity of 5 fb^-1), then at 8 TeV (at 20 fb^-1; explainer here). The data was garnered from both the ATLAS and CMS detectors.

In all, they found nothing. As a result, as Pascal says, “When you find nothing, you don’t know if you are close or far from it!”

His paper has an interesting chart that summarized the results for the search for Susy from Run 1. It is actually a superimposition of two charts. One shows the different Standard Model processes (particle productions, particle decays, etc.) at different energies (200-1,600 GeV). The second shows the Susy processes that are thought to occur at these energies.

Cross sections of several SUSY production channels, superimposed with Standard Model process at s = 8 TeV. The right-handed axis indicates the number of events for 20/fb.

The cross-section of the chart is the probability of an event-type to appear during a proton-proton collision. What you can see from this plot is the ratio of probabilities. For example, stop-stop* (the top quark’s Susy partner particle and anti-particle, respectively) production with a mass of 400 GeV is 10¹⁰ (10 billion) less probable than inclusive di-jet events (a Standard Model process). “In other words,” Pascal says, it is “very hard to find” a Susy process while Standard Model processes are on, but it is “possible for highly trained particle physics” to get there.

Of course, none of this means physicists aren’t open to the possibility of there being a theory (and corresponding particles out there) that even Susy mightn’t be able to explain. The most popular among such theories is “the presence of a “possible extra special dimension” on top of the three that we already know. “We will of course continue to look for it and for supersymmetry in the second run.”

Higgs boson closer than ever

The article, as written by me, appeared in The Hindu on March 7, 2013.

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Ever since CERN announced that it had spotted a Higgs boson-like particle on July 4, 2012, their flagship Large Hadron Collider (LHC), apart from similar colliders around the world, has continued running experiments to gather more data on the elusive particle.

The latest analysis of the results from these runs was presented at a conference now underway in Italy.

While it is still too soon to tell if the one spotted in July 2012 was the Higgs boson as predicted in 1964, the data is convergent toward the conclusion that the long-sought particle does exist and with the expected properties. More results will be presented over the upcoming weeks.

In time, particle physicists hope that it will once and for all close an important chapter in physics called the Standard Model (SM).

The announcements were made by more than 15 scientists from CERN on March 6 via a live webcast from the Rencontres de Moriond, an annual particle physics forum that has been held in La Thuile, Italy, since 1966.

“Since the properties of the new particle appear to be very close to the ones predicted for the SM Higgs, I have personally no further doubts,” Dr. Guido Tonelli, former spokesperson of the CMS detector at CERN, told The Hindu.

Interesting results from searches for other particles, as well as the speculated nature of fundamental physics beyond the SM, were also presented at the forum, which runs from March 2-16.

Physicists exploit the properties of the Higgs to study its behaviour in a variety of environments and see if it matches with the theoretical predictions. A key goal of the latest results has been to predict the strength with which the Higgs couples to other elementary particles, in the process giving them mass.

This is done by analysing the data to infer the rates at which the Higgs-like particle decays into known lighter particles: W and Z bosons, photons, bottom quarks, tau leptons, electrons, and muons. These particles’ signatures are then picked up by detectors to infer that a Higgs-like boson decayed into them.

The SM predicts these rates with good precision.

Thus, any deviation from the expected values could be the first evidence of new, unknown particles. By extension, it would also be the first sighting of ‘new physics’.

Bad news for new physics, good news for old

After analysis, the results were found to be consistent with a Higgs boson of mass near 125-126 GeV, measured at both 7- and 8-TeV collision energies through 2011 and 2012.

The CMS detector observed that there was fairly strong agreement between how often the particle decayed into W bosons and how often it ought to happen according to theory. The ratio between the two was pinned at 0.76 +/- 0.21.

Dr. Tonelli said, “For the moment, we have been able to see that the signal is getting stronger and even the difficult-to-measure decays into bottom quarks and tau-leptons are beginning to appear at about the expected frequency.”

The ATLAS detector, parallely, was able to observe with 99.73 per cent confidence-level that the analysed particle had zero-spin, which is another property that brings it closer to the predicted SM Higgs boson.

At the same time, the detector also observed that the particle’s decay to two photons was 2.3 standard-deviations higher than the SM prediction.

Dr. Pauline Gagnon, a scientist with the ATLAS collaboration, told this Correspondent via email, “We need to asses all its properties in great detail and extreme rigour,” adding that for some aspects they would need more data.

Even so, the developments rule out signs of any new physics around the corner until 2015, when the LHC will reopen after a two-year shutdown and multiple upgrades to smash protons at doubled energy.

As for the search for Supersymmetry, a favoured theoretical concept among physicists to accommodate phenomena that haven’t yet found definition in the Standard Model: Dr. Pierluigi Campana, LHCb detector spokesperson, told The Hindu that there have been only “negative searches so far”.