Tag: SUSY

Hopes for a new particle at the LHC offset by call for more data

At a seminar at CERN on Tuesday, scientists working with the Large Hadron Collider provided the latest results from the particle-smasher at the end of its operations for 2015. The results make up the most detailed measurements of the properties of some fundamental particles made to date at the highest energy at which humankind has been able to study them.

The data discussed during the seminar originated from observations at two experiments: ATLAS and CMS. And while the numbers were consistent between them, neither experimental collaboration could confirm any of the hopeful rumours doing the rounds – that a new particle might have been found. However, they were able to keep the excitement going by not denying some of the rumours either. All they said was they needed to gather more data.

One rumour that was neither confirmed nor denied was the existence of a particle at an energy of about 750 GeV (that’s about 750x the mass of a proton). That’s a lot of mass for a single particle – the heaviest known elementary particle is the top quark, weighing 175 GeV. As a result, it’d be extremely short-lived (if it existed) and rapidly decay into a combination of lighter particles, which are then logged by the detectors.

When physicists find such groups of particles, they use statistical methods and simulations to reconstruct the properties of the particle that could’ve produced them in the first place. The reconstruction shows up as a bump in the data where otherwise there’d have been a smooth curve.

This is the ATLAS plot displaying said bump (look in the area over 750 GeV on the x-axis):

ATLAS result showing a small bump in the diphoton channel at 750 GeV in the run-2 data. Credit: CERN
ATLAS result showing a small bump in the diphoton channel at 750 GeV in the run-2 data. Credit: CERN

It was found in the diphoton channel – i.e. the heavier particle decayed into two energetic photons which then impinged on the ATLAS detector. So why aren’t physicists celebrating if they can see the bump?

Because it’s not a significant bump. Its local significance is 3.6σ (that’s 3.6 times more than the average size of a fluctuation) – which is pretty significant by itself. But the more important number is the global significance that accounts for the look-elsewhere effect. As experimental physicist Tommaso Dorigo explains neatly here,

… you looked in many places [in the data] for a possible [bump], and found a significant effect somewhere; the likelihood of finding something in the entire region you probed is greater than it would be if you had stated beforehand where the signal would be, because of the “probability boost” of looking in many places.

The global significance is calculated by subtracting the effect of this boost. In the case of the 750-GeV particle, the bump stood at a dismal 1.9σ. A minimum of 3 is required to claim evidence and 5 for a discovery.

A computer’s reconstruction of the diphoton event observed by the ATLAS detector. Credit: ATLAS/CERN
A computer’s reconstruction of the diphoton event observed by the ATLAS detector. Credit: ATLAS/CERN

Marumi Kado, the physicist who presented the ATLAS results, added that when the bump was studied across a 45 GeV swath (on the x-axis), its significance went up to 3.9σ local and 2.3σ global. Kado is affiliated with the Laboratoire de l’Accelerateur Lineaire, Orsay.

A similar result was reported by James Olsen, of Princeton University, speaking for the CMS team with a telltale bump at 745 GeV. However, the significance was only 2.6σ local and 1.2σ global. Olsen also said the CMS detector had only one-fourth the data that ATLAS had in the same channel.

Where all this leaves us is that the Standard Model, which is the prevailing theory + equations used to describe how fundamental particles behave, isn’t threatened yet. Physicists would much like it to be: though it’s been able to correctly predict the the existence of many particles and fundamental forces, it’s been equally unable to explain some findings (like dark matter). And finding a particle weighing ~750 GeV, which the model hasn’t predicted so far, could show physicists what could be broken about the model and pave the way for a ‘new physics’.

However, on the downside, some other new-physics hypotheses didn’t find validation. One of the more prominent among them is called supersymmetry, SUSY for short, and it requires the existence of some heavier fundamental particles. Kado and Olsen both reported that no signs of such particles have been observed, nor of heavier versions of the Higgs boson, whose discovery was announced mid-2012 at the LHC. Thankfully they also appended that the teams weren’t done with their searches and analyses yet.

So, more data FTW – as well as looking forward to the Rencontres de Moriond (conference) in March 2016.

On the shoulders of the Higgs

On July 4, 2012, when CERN announced that a particle that looked a lot like the Higgs boson had been spotted, the excitement was palpable. A multibillion-dollar search for an immensely tiny particle had paid off, and results were starting to come in.

On March 6, 2013, when CERN announced through a conference in Italy that the particle was indeed the Higgs boson and that there were only trivial indications as to otherwise, there was closure. Textbook-writers and philosophers alike could take the existence of the Higgs boson for granted. People could move on.

The story should’ve ended there.

It would’ve, too, but for a theory in particle physics that many physicists aren’t quite fond of, called the Standard Model.

For particle physicists, everything in the universe is made up of extremely tiny particles. Even the protons, neutrons and electrons that make up atoms are made up of tinier particles even though we don’t encounter them or their effects in our daily lives.

And those tiny particles, of tinier particles, until particle physicists are satisfied they’ve hit upon the “stuff” of the universe, as it were.

This “stuff”, physicists have found, comes in three types: Leptons, bosons and quarks. They are the ingredients of the universe and everything within it.

Leptons are the lightest of the lot. One example of the lepton is a neutrino, whose mass is so low that it has no problems travelling at very near to the speed of light.

Bosons are the force-carriers. When two particles exert a force on each other, particle physicists imagine that they’re simple exchanging bosons. What kind of boson is being exchanged throws light on the nature of the acting force. Examples include the massless photon, the W and Z bosons, and gluons.

Quarks are the proverbial building blocks of matter. They come in six types, each called a flavour. Unlike leptons and bosons, quarks can be stuck together using gluons to form heavier composite particles. For example, two up-flavoured quarks and one down-flavoured quark together make up the proton.

In the universe as a cauldron, these ingredients come together to make up different phenomena we perceive around us. While each particle sticks to its properties while mixing with others, its behavior is continuously modified by other particles around it. If there are too many particles in the fray, which is natural, matters can get complex for physicists studying them. But after studying them over long periods of time, they found that there are patterns and a few rules that aren’t ever broken.

The set of all these patterns and rules is called the Standard Model. In fact, the Higgs boson was the last remaining piece of this framework, and now that it’s been found, the Model is good as true.

So far, so good.

Where the Model really flounders is when physicists asked why it was the way it was. Why are there six quarks and not five or seven, why are there three kinds of leptons – electron, muon, tau – and now two or four, why can one quark only always be found in the company of another quark and never alone… such questions were just the beginning.

These are the real questions that physicists want the answers to – the ultimate “why” is the goal of all scientific studies.

Many pinned their hopes on CERN’s Large Hadron Collider (LHC), which led the search for the Higgs boson by smashing protons together and open at high energies to see if a Higgs boson popped out. Based on preliminary calculations and simulations, they speculated that something more significant would pop out with the Higgs boson.

There has been nothing so far, i.e. the frustratingly familiar Standard Model is all we have.

But physicists took heart. “The Model is all we have… for now,” they said. Every time a new ingredient of the universe was hoped for and there was none, physicists only believed the hypothetical particle didn’t exist at the energy they were combing through.

Like this, with only negative results to show over hundreds of trials across a swath of energies, physicists have put together a stack of upper and lower limits between which new particles, crucial to the future of particle physics, can be spotted.

A good place to start was that the conditions for these “new” ingredients all were tied in with the conditions in which the Higgs boson could show itself. So, the converse must also be true: the conditions in which the Higgs boson showed itself could contain traces of the conditions for “new physics” to show itself. All physicists would have to do is ask the right questions.

One example is an instance of the fermiophobic Higgs. Because it’s so heavy and energetic, the Higgs boson quickly breaks down into lighter particles like photons, W and Z bosons, leptons and quarks. Of them, leptons and quarks are collectively titled fermions. True to its name, a fermiophobic Higgs doesn’t decay into fermions.

As a result, it will have to decay into the other kinds of particles more often. While they had the resources, physicists were able to determine that a fermiophobic Higgs didn’t exist in the energy range 110-124.5 GeV, 127-147.5 GeV, and 155-180 GeV with 99 per cent confidence.

Another example, this one more favoured in the scientific community, is called suppersymmetry, SUSY for short. Its adherents posit that for every fermion, there is a heavier partnering boson that we haven’t found yet, and vice versa, too. Thus, the Higgs boson has a hypothetical partnering fermion, tentatively called the Higgsino.

When physicists tried to incorporate the rules of SUSY into the Standard Model, they saw that five Higgs bosons would be necessary to explain away some problems. Of these, three would be neutral, and collectively denoted as Φ, and two would be charged, denoted H+ and H-. Moreover, the Φ Higgs would have to decay into two particular kinds of quarks a whopping 90 per cent of the time.

While running experiments to verify this decay rate, tragedy struck: the Standard Model stood in the way. It predicts that the Higgs will decay into the two quarks only 56.1 per cent of the time… And the results swore by it.

The never-say-die faith persisted: Now, physicists wait for 2015, when the LHC will reawaken with doubled energy, possibly bringing them closer to the very high energies at which SUSY might be thriving.

There are other models like these, such as one that suggests there are hidden fermions we haven’t found yet and one that suggests that the Higgs boson decays to lighter, undetected versions of itself before coming into a form we can study. But until 2015, they will be the stuff of science fiction, the Standard Model will rule as a tolerable tyrant, and we will be no closer to understanding the stuff of the universe.