Tag Archives: Particles

When Size Matters: Story of the Incredible Shrinking Proton!

The size of the proton matters in the field of the ultra-small and it seems that no one can agree on the correct value. The answer was long believed to be well-known, but the puzzle seems to be back to haunt the physics community. The proton seems to have suddenly shrunk in size.


How do we look?

The radius of the proton is found out by shooting high energy electrons at it and then finding how it forms a bound state. It’s very much like forming an atom, except that this atom is much smaller than the normal atoms which make up matter. Energetic electrons fired at protons often get bound to the proton, and form a hydrogen-like object. However, since the electron has a lot more energy than the ordinary hydrogen atom electron, it is attached much closer to the proton than the normal hydrogen electron. As a result, the proton can no longer be treated as a point particle, but its spatial extent become important.

So we can form a bound state and then measure the minute transition between energy levels and these now have an imprint of the proton magnetic moment and the proton radius. And thus, the proton radius can be determined.
For a long time, physicists were safe in their determination of the proton radius and their value was 0.8768 femtometers (a femtometer is a millionth of a billionth of a meter, or a meter divided by 10^15). Case closed, right? Wrong…

New experiment

A new experimental result threatens to blow this question of the radius wide open again. The muon is a close cousin of the electron. It has a negative charge and behave very much like the electron in a magnetic field, except that it is 200 times heavier than an electron. Recent experiments shoot these heavy electrons – or muons – at protons and these now form a bound state. The higher mass of the muon (by a factor of 200) means that at same energies, the muon is much closer to the proton (by a factor of 800 million). It can ‘see’ the proton much better and measure the radius to greater accuracy.

However, this has produced a shocking reduction in the accepted value – 0.84087 femtometers – a reduction of 4%. That is huge, well above the experimental uncertainties.

So, what’s going on?

Physicists are not very sure what’s going on. Why should the muon behave any differently from the electron? Is the muon, being closer to the proton experiencing some short range force, other than the usual long ranged electromagnetic and the short ranged weak force, that we just don’t know about? Is a new force of nature at work here? Is there new physics, something beyond the Standard Model of particle physics?

The muon measurements were made by a group of scientists at the Max Planck Institute of Quantum Optics, led by Randolf Pohl. Of course, the crudest explanation to all of this is that the experimentalists simply bungled and got the value wrong. No one’s ruling that explanation out right now, but other avenues are also being explored.

Muon scattering experiments like MuSE will only be ready in a few years, so this debate will continue for some time. When size does matter, we just don’t want it to change.

New Particle Discovered At Fermilab; Existence Confirmed

This time it appears to be genuine. Fermilab has indeed detected a new particle the Xi-sub-b baryon.

A few days ago Fermilab got the entire physics community excited by announcing that it was almost sure of a discovery of a new particle the W/Z-bar boson. We covered the sensational news here. It turns out that it was not the case.

The CDF detector, where the new particle was detected

The particle

A baryon is a particle (technically, a bound state’) of three quarks. A proton is an example and is made up of two up and one down quark. Apart from these two types, there are four more types of quarks top(some people call it truth’), bottom (some prefer beauty’), strange and charm. The top quark is the most massive, followed by bottom, charm and strange in that order. The Xi-sub-b baryon has an up quark, a strange quark and a bottom quark. The discovery was made yesterday and reported today in Fermilab’s press release.

The standard model list for the quarks and all other fundamental particles.

The Xi-sub-b baryon is extremely unstable, being able to travel just a fraction of a millimetre before disintegrating into lighter particles. The particle was detected in the CDF detector.

Confidence levels

There have been 25 isolated detections, or events, that confirm the existence of the new particle. As we mentioned in earlier posts, the criteria for a discovery is confidence level’. We need a minimum of five-sigma confidence level, or a confidence of more than 99.997% that a certain detection is genuine. Here, Fermilab, after data analysis, puts the confidence level at seven sigma, much higher than the threshold!

The details of the discovery have been sent to Physical Review Letters (PRL) for publication.

Miniature Particle Accelerator Can Solve World’s Energy Crisis (And Cure Cancer)

It’s a pocket-sized power plant, just 10 m across. A miniature particle accelerator, small enough to be stashed away in your basement, can be used to produce nearly unlimited amounts of nuclear energy, in a controlled manner, using the radioactive element thorium.

One Wonder…

The accelerator, cheekily named Electron Machine with Many Applications or EMMA, is a ring particle accelerator, capable of accelerating electrons to about 10 or 20 MeV. That means that it can drive an electron through a voltage difference of 10 to 20 million volts, which, though large by everyday standards, is modest in terms of the particle accelerators of today. However, this is all that’s needed and the portability allows for diverse usage.

The beautiful EMMA. Cryogenics engineer Racheal Buckley looks quite pretty herself as she provides an estimate of size. (Credits: Neale Haynes/Reuters)

… And Another…

Scientists have long looked at a radioactive element other than Uranium as a potential candidate for producing nuclear power Thorium. Thorium research is, however, at its infancy, given that the focus of all nuclear research has focussed on Uranium. Thorium is found widely, easily refined, extremely safe to handle and leaves no residue after fission. Thorium produces about 200 times the energy produced by Uranium and produces no carbon dioxide. The only catch is that, for fission, it needs to be initiated by bombardment with charged particles, such as electrons. This is a happy handicap, since a Thorium reactor would be incapable of a meltdown. After Fukushima, this maybe the new global fool-proof safety standard.

[summary float = “right”]

… Get Married

Enter EMMA. EMMA can deliver the required accelerated electrons onto a Thorium core, producing energy in a process that is much more controlled than Uranium fission. It’s a free lunch, as a nation powered by only Thorium based nuclear energy would leave no carbon footprint or nuclear waste and would not run out of energy in the next 10,000 years.

Getting Slightly Technical

The miniature machine is lined with quadrupole magnets (magnets with four parts) in order to keep the particles in the beam focussed in a narrow region. It’s also a Fixed Field Alternating Gradient Accelerator (FFAG), meaning it has an alternating gradient quadrupolar magnetic field to constraint the beam to a ring. As the particles are accelerated, they tend to move in orbits of slightly larger radius, but they encounter higher magnetic fields further out. This keeps the particles confined in a narrow beam in the orbital ring. EMMA is in conjunction with a particle injector named ALICE. ALICE (no relation with the detector at LHC, CERN) produces copious amounts of electrons and then injects them into EMMA using an injector or ‘septum’ tube at an angle of 65 degrees to the ring, which accelerates them to prescribed energies and allows it to impinge on a Thorium target.

The blueprint for EMMA. Note the particle injector ALICE, which is kept under ultra-vacuum and at a constant -271 degree Celsius.

Further, EMMA could be used for medical purposes, as envisioned in the Particle Accelerator for Medical Application (or Pamela, don’t you love these abbreviations?) project. It will focus on Cancer research.


EMMA could usher in new-generation, clean and danger-free nuclear power plants, something that is considered an oxymoron today. EMMA, developed (and currently housed) by researchers at Daresbury science park in Manchester, Britain, is also a research prototype, paving the way for more such miniature, but powerful, accelerators for heavier particles such as protons and helium nuclei.

In order to unleash the thunder in an element named after the Norse God of Thunder, Thor, what is needed is a non-polluting non-energy guzzling, miniature particle accelerator of moderate energy. Sometimes, Nature is simpler than you think.

Eureka Moment Claims Rejected: No New Particle Discovered At Fermilab

It is disappointing news for the particle physics community coming out of Fermilab, we’re afraid. Fermilab has confirmed that the earlier bump seen in the data, presumed to be a new particle, is not significant enough to be considered a detection. We broke the news in emphatic fashion of a new particle discovered in Fermilab in an earlier post.

Story So Far

We had reported that there was a bump found at about 145 GeV with a 5 GeV spread. Data acquired from proton-antiproton collisions with semi-leptonic dijet emissions showed a peak at 145 GeV. The curve was Gaussian in nature with a spread of 5 GeV on either side of the peak. Initial analysis showed that the curve had a three-sigma confidence level (More on confidence levels later). There was thus a strong possibility that a new particle was on the way, since no boson is known having a mass of 145 GeV. The new particle was named as the Z’ or the W’ (Z-primed or W-primed) boson. The Standard Model, wildly successful in particle physics, did not predict this and to fit this in would have required a serious rethinking of known physics. Physicists were naturally excited.

This detection was made at Fermilab at their CDF detector.

The CDF detector

Fermilab was the biggest particle accelerator till the Large Hadron Collider came onto the scene. It has been a major progressive force for particle physics over the last three decades, also serving to etch the American superiority in the particle physics arena. It is however expected to be closed down forever late this year. Data recovered from it over the years is still being analysed, and as such will continue for the next five years. One of the two detectors at Fermilab the CDF had detected the anomalous bump of our present interest.

So What’s Wrong?

There are two problems with the CDF data it cannot be corroborated and it falls outside the required confidence levels.

Problem 1:

The DZero Detector

The other detector at Fermilab, named DZero, repeated the experiment, but failed to come up with any conclusive evidence of detection. The negative DZero result would definitely cast shadows over the CDF discovery. Scientists are now baffled as to how the two detectors extremely alike could give such widely varying results under the same experimental conditions. However this is a very good safeguard.


Problem 2: Remember that earlier we had said something about a three-sigma confidence level? It means that the data is reliable and the chances of it being wrong are one-in-a-thousand (99.9% accurate). Confidence levels measure reliability of data. For a discovery to be accepted by the scientific community, the event must have at least a five-sigma confidence level or higher, which means that doubts must reduce to less than one-in-a-million. The problem with the current bump is that it lies just below the five-sigma confidence level.

Graph for the DZero Results.

Take a look at the above graph. Never mind the mathematics and abstruse symbols. Know that the horizontal axis represents the mass of the particles and the vertical axis represents the number of particles detected. At the 145-150 GeV range (point 145 GeV on the horizontal axis), you’d have expected a curve if the previous CDF results were replicated. This is marked with the dotted curve. There is nothing there as far as DZero is concerned. The red regions represent detections and these are in complete agreement with the Standard Model. There is no anomaly to be seen anywhere.

On both counts, the bump is rejected as a new discovery.

What changes then?

Practically nothing changes. The 145 GeV particle, if discovered, would have been interesting, as the Standard Model doesn’t predict it. Further, it could have provided a mechanism for particles acquiring mass without the need of the Higgs boson (essentially becoming the new God particle’). With it being ruled out, the Standard Model stands as it is with the Higgs mechanism being the most favoured mechanism for mass generation.

The discovery would have been exciting, but the field’s exciting even without it. After all, science is like this. DZero spokesperson Stefan Soldner-Rembold  put it approproiately in a Fermilab press conference:

This is exactly how science works. Independent verification of any new observation is the key principle of scientific research.

So very true!