Found: Elusive particle!

[Dannyalcatraz]

Mysterious Particle Found After Decades of Searching - Yahoo! News

Embarrassingly, though, the picture accompanying the article shows the orange balls of the detector...kinda personal, I'm thinking.

 

[Relique du Madde]

Does that mean they can finally find out if Schrodinger's cat is alive or dead without opening the damned box?

 

[Mustrum_Ridcully]

Mysterious Particle Found After Decades of Searching - Yahoo! News

Embarrassingly, though, the picture accompanying the article shows the orange balls of the detector...kinda personal, I'm thinking.

Well, the detector isn't a person, so I think it doesn't apply.

Also, the description text for the image is inconsistent with the explanation of the article itself.

The researchers think they have created majorana fermions, which have identical antiparticles but which don't annihilate each other

Bosons, however, are particles that are their own antiparticle, and they don't annihilate when they touch each other. Majorana fermions are like photons in that respect, as they act as their own antiparticles. But unlike photons, Majoranas will still annihilate when they meet their antimatter cousins. (Neutrinos may also be like this, but it is not clear yet and is an active area of research).

Okay, so they are their own anti-particle. But do they destroy each other (like Fermions do) or not (like Bosons do).
I believe they destroy each other, though, since further Wikipedia research suggests that Neutrinos are a possible candidate for being Majorana Fermions, and if so, there were possible forms of beta decay that would be neutrino-less. Which to me indicates that the neutrino and anti-neutrino destroy each other, leaving no neutrinos behind.

Does that mean they can finally find out if Schrodinger's cat is alive or dead without opening the damned box?

Yes, but that has nothing to do with the article. They quantum entangled the cat with a mouse in a different box. Now, if the cat dies, the mouse lives. So you only have to open the mouse box, and can keep the cat box closed!

This is a considerably improvement, as the mouse box is much smaller and can be transported more easily to any scientific event where the question needs to be resolved.

Also, in case the cat is alive, she was usually reported to be very angry, clawing after the scientists opening the box. The mouse just runs away*.

*) It only runs away when the cat is dead, of course. When the cat is alive, the mouse is dead.

 

[Umbran]

Yes, but that has nothing to do with the article. They quantum entangled the cat with a mouse in a different box. Now, if the cat dies, the mouse lives. So you only have to open the mouse box, and can keep the cat box closed!

If you entangle the cat and mouse, then, effectively, there is only one box.

If I recall correctly, the image description is off:

Normal (Dirac) Fermion: Particle and antiparticle are not the same, they annihilate on contact.

Normal Boson: Particle and antiparticle are the same, they do not annihilate on contact.

Majorana Fermion: Particle and antiparticle are the same, they do annihilate on contact.

It is important to note, however, that these people did not find a new fundamental particle, but a "quasiparticle" - an arrangement in matter that you can think of acting like a particle, but isn't a physical object. In solid state physics, for example, in a system that is nearly full of electrons, you can talk about "holes" (the place where an electron ought to be, but isn't), and treat it mathematically like a particle, even when really it is the absence of a particle.

 

[freyar]

This is sadly, a pretty crappy article in some respects. In particular, this quote:

Majorana fermions are so special because they are different from other fermions, which have antiparticles — particles that have the same mass but opposite charge. An electron is negatively charged, and its antiparticle is a positron. When a particle such as an electron comes into contact with its antiparticle (in this case, a positron), the two annihilate, turning into energetic photons in this example.
Bosons, however, are particles that are their own antiparticle, and they don't annihilate when they touch each other. Majorana fermions are like photons in that respect, as they act as their own antiparticles. But unlike photons, Majoranas will still annihilate when they meet their antimatter cousins.

is 100% incorrect on the difference between fermions and bosons. A bit above this, they give a partially correct explanation in saying that fermions follow the Pauli exclusion principle and bosons don't. I believe the BBC article on this was a bit better, but I don't have time to search for it right now.

Here is the correct story:
Fermions have what we call 1/2-integer spin (like 1/2, 3/2, etc), so they are always rotating in some sense. For technical reasons, this means that two identical fermions can never have the same state --- meaning two electrons can't be in the same place, for example.
Bosons have integer spin (0,1,2, etc). Two identical bosons are allowed to have the same state --- this phenomenon is what allows us to make lasers, since photons (spin 1) are bosons.
That's the difference between bosons and fermions. Note that I said nothing about antiparticles.

Now, here's the thing about antiparticles. It really comes down to the difference between real and complex numbers. A particle and its antiparticle are the same if the particle is associated with a real number. A photon can be described by real numbers, so its antiparticle is still a photon. An electron, which is a Dirac fermion like Umbran says, must be described by complex numbers, so its antiparticle, the positron, is different. A W-boson is like a photon in lots of ways, but it is a complex number particle, so it has a different type of antiparticle (the W and its antiparticle are called W+ and W-). Majorana fermions are like electrons except they can be described by real numbers, so they are their own antiparticles.

None of this really talks about annihilation of particles. In principle, a particle and its antiparticle can always annihilate into something else. W+ and W- bosons can annihilate, for example. Photons would be able to annihilate if there were something lighter for them to annihilate into, but they're massless, so there's nothing lighter! That's why photons can't annihilate, not particle-antiparticle business.

It is important to note, however, that these people did not find a new fundamental particle, but a "quasiparticle" - an arrangement in matter that you can think of acting like a particle, but isn't a physical object. In solid state physics, for example, in a system that is nearly full of electrons, you can talk about "holes" (the place where an electron ought to be, but isn't), and treat it mathematically like a particle, even when really it is the absence of a particle.

And this is a really important point that the article just glossed over.

 

[The Red King]

Well, the detector isn't a person, so I think it doesn't apply.

Also, the description text for the image is inconsistent with the explanation of the article itself.





Okay, so they are their own anti-particle. But do they destroy each other (like Fermions do) or not (like Bosons do).
I believe they destroy each other, though, since further Wikipedia research suggests that Neutrinos are a possible candidate for being Majorana Fermions, and if so, there were possible forms of beta decay that would be neutrino-less. Which to me indicates that the neutrino and anti-neutrino destroy each other, leaving no neutrinos behind.

They say in the article that the particles do not in fact destroy each other.


So how are they their own anti-particle?

Is that like parking on the driveway and driving on the parkway?

 

[Umbran]

None of this really talks about annihilation of particles. In principle, a particle and its antiparticle can always annihilate into something else.

Can, but don't have to, which is kind of the operative part. Since bosons don't obey the Pauli Exclusion Principle, they can sit in the same place and *not* annihilate.

As for photons, in some situations I recall the math working out to be - photons actually can annihilate, and do. But the results of the annihilation ends up to be a pair of photons of the same energy, so for the most part you don't notice it happened. Two photons go in, two photons of the same wavelength come out.

And this is a really important point that the article just glossed over.

Yes. "Rare particle discovered" really isn't an accurate description. "Rare state we can describe with math as if it were a particle," would be more correct.

 

[freyar]

They say in the article that the particles do not in fact destroy each other.


So how are they their own anti-particle?

Is that like parking on the driveway and driving on the parkway?

See my post just above. Majorana fermions can annihilate themselves; current experiments testing if neutrinos are Majorana rely on this fact.

In the case of this article, they don't have fundamental particles but rather "quasiparticles," which are conglomerate excitations of a bunch of electrons. It's entirely possible that these Majorana quasiparticles could annihilate each other, but since it's not even confirmed that they've been found in this experiment, it's way too early to talk about observing them annihilating each other.

 

[freyar]

Can, but don't have to, which is kind of the operative part. Since bosons don't obey the Pauli Exclusion Principle, they can sit in the same place and *not* annihilate.

I'm sorry to say that this just isn't right. Think about electrons. Electrons cannot annihilate electrons (since they have conserved charge), but they still cannot sit in the same place. Electrons and positrons are allowed to be in the same place because they are not identical particles (Pauli exclusion does not apply to different types of particles), and they have a certain probability to annihilate if the are "in the same place" (that is, there is a nonzero annihilation cross section when electrons and positrons come together). The same is true of bosons and their antiparticles.

As for photons, in some situations I recall the math working out to be - photons actually can annihilate, and do. But the results of the annihilation ends up to be a pair of photons of the same energy, so for the most part you don't notice it happened. Two photons go in, two photons of the same wavelength come out.

What you've described is a scattering process, not annihilation. If you want to talk about photons annihilating, you need to talk about photons in a plasma, which are effectively heavy. If they are heavy enough, then they could annihilate into something else, like an electron-positron pair. Or, even in a vacuum, two photons can collide and produce an electron-positron pair, but we tend to call this "pair production" rather than annihilation since the photons have to have a minimum energy to do this.

 

[Umbran]

I'm sorry to say that this just isn't right.

It is and it isn't - I'm mixing my voice a bit, for the non-technical audience.

Sitting in the same quantum state is, for purposes of the layman, equivalent to being in the same place at the same time (it is also more than that, but for these purposes, that's not relevant). Two things cannot get closer than being in the same quantum state. It is the ultimate in "closeness". Sure as anything, if two particles were in the same state, if annihilation were *required*, it would happen.

Since bosons can be in the same state, annihilation is not strictly required, even if it is possible. That's all I mean.

What you've described is a scattering process, not annihilation.

By my recollection (and, admittedly, I've not done the math in a long time), if you want the full cross section for "two photons in, and two of the same out" you must include the diagrams for the scattering process *and* for annihilation. I recall there being a singularity in there that needs renormalization.

And, if you want the cross section for, "two photons in, and two of *something* out," then it isn't distinguishable from an annihilation process that reduces to a scattering under the energy required for electron pair production.

I'm taking the "looks, walks, and quacks like a duck" line here.