Which are the three particles? If tau- decays to muon/electron and two neutrinos, how can there be three particles that's seen there? So here are we looking for tau decaying into hadrons and a tau-neutrino? Even in case of tau decaying into hadrons, during the vertex reconstruction wouldn't the missing momentum carried by the tau-neutrino affect the uncertainty? PS: Maybe the next video explains this.
Hi! Good questions! There are many experiments, whose results are compared in the 2nd video: ruclips.net/video/xrLllaBiSWM/видео.html (I'd encourage you to watch it, although it doesn't address your questions.) These different experiments used different methods, and some of them used more than one way to measure the tau lifetime. It would be way too hard to summarize them all, so I'll just make a few general comments. Usually, the experiments used tau decays to hadrons + an invisible tau neutrino. The example I drew here involved the tau decaying to 3 hadronic particles, plus the tau neutrino. So, the 3 particles that are actually seen in the detector and used to get the vertex are hadrons. However, I should note that this is not what every experiment did. Some experiments also looked for tau decays that involved only a single visible particle. These analyses, I think, would have included the cases where the tau decayed to a mu/e and two neutrinos. (It's also possible to have a tau decay to a single hadron and a tau neutrino. Naively, I would assume that these analyses included both the one-track hadronic decays and the decays to e and mu, but I haven't checked.) Going back to the 3-track decays to hadrons: As for neutrino and the vertex reconstruction, this is a bit tricky. The neutrino and the hadronic tracks all point back to the same decay vertex, so, in principle, if you could measure the hadronic tracks perfectly, the neutrino wouldn't add any more information about where the vertex occurred. But, we don't live in an ideal world, and the trajectories of these tracks have uncertainties. So, as a very general statement, in an alternate universe where the neutrino was visible and also pointed back to the vertex, you would have more information to reconstruct the vertex, and thus your vertex reconstruction would presumably be better. But there is another issue. In addition to there being an uncertainty on where the tau decayed, there is also an uncertainty on where it was produced. (The e+ and e- collide over a small volume, but you don't know exactly where in that volume a particular collision occurred and produced the tau+tau- pair.) So, you might try to figure out where a particular tau was produced, in order to calculate the distance it traveled before it decayed. And, doing this might involve trying to reconstruct the direction the tau was traveling with high precision. (From the vertex, you know where it decayed. So, if you know the tau's direction of travel, that can tell you about where it was produced.) This would be easy if you knew the momenta of all the particles that the tau decayed to. But, as you can't see the neutrino, you can't directly measure the neutrino momentum. This can feed into an uncertainty on the direction of travel of the tau, which then gives an uncertainty on where the tau was produced. So, the missing momentum of the neutrino can affect the uncertainty of the location where the tau was produced, instead of where it decayed! A complete answer to your question, I would guess, probably depends on the specific experiment in question. I'm sure that there are lots of details and nuance that I don't know without taking a long look at the papers. But I hope that I've been able to give you at least some flavor of the answer to your question. Does that help?
@@ThinkLikeaPhysicist That helps! Absolutely! Especially the argument that neutrino track isn't important for vertex reconstruction. But it's only as long as the three hadrons are produced at the vertex itself, right? I assume the Feynman diagram will be such that a tau decays into tau neutrino and a virtual boson which splits into a q-qbar meson. This meson could travel a little bit before it emit gluons which splits into two more mesons (the three observed particles), then the vertex reconstruction won't be easy, right? So the lifetime of meson produced in the tree level Feynman diagram becomes important, I guess. Am I making sense? I'll watch the second part some time soon! :)
Hi! So, yes, the Feynman diagram has a tau split into a tau neutrino and a W boson, which then splits into q-qbar. But, the gluons are actually radiated off of the original q-qbar pair, not the meson. So, after the W splits to q-qbar, one or both of those quarks can emit a gluon. The gluon(s) produced can then split into additional q-qbar pairs. (For high-energy processes, this process can continue for many steps, producing perhaps dozens of q-qbar pairs in the end. This produces the phenomenon seen in colliders known as "jets".) So, eventually one ends up with a bunch of q-qbar pairs. After that happens, though, the quarks and antiquarks start assembling into mesons and baryons. This entire process happens extremely quickly however--far too quickly for us to see any of these details. (As far as we can see on macroscopic distance scales, all of that looks like it happens at a point.) So, what we see in the detector are just the mesons and baryons produced at the end of that process. It is possible that some of those mesons/baryons may decay afterward; depending on the lifetime of the meson/baryon in question, we may be able to see it decay in the detector.
@@ThinkLikeaPhysicist sounds good. So the assumption is that all the jet formation happens almost at the vertex itself. I guess that's because the strong force is strong? :D
Hi, The short answer to your question is yes. Here's a slightly longer answer: In principle, one can measure g-2 for any of the leptons. In the case of the muon, this is determined by seeing how the direction of the muon's spin changes with time in a magnetic field. The tau, however, has a very short lifetime, and it decays far too quickly to do this measurement. So, other methods have to be used. The magnetic moment of the tau describes a particular way in which the tau can interact with photons. In particular, it allows a process where, if you collide 2 photons, you can annihilate the photons and produce a tau+tau- pair. The DELPHI experiment (reference below) studied this process. Actually, the DELPHI experiment collides e+e-. But, if each of the e+ and e- radiates (in other words, emits) a photon, those photons can interact and produce the tau+tau- pair. That is the process that they used. The precision of this, though, is not so impressive compared to the Standard Model prediction. The Standard Model prediction is (g-2)/2 = 0.00117721+-0.00000005 and the limit DELPHI was able to set was -0.052 < (g-2)/2
Hi! I'm planning to do a series of videos on muon g-2. Since you asked a question related to muon g-2, I was thinking about mentioning your comment at the beginning of the first video, kinda like what I did on this video: ruclips.net/video/pfgQS5kK70w/видео.html Would you like me to? I could show your comment as-is, or I could show it but black out your name, or I could just not show it at all, depending on your preference. Would you prefer any of those options? Thanks!
Hi, Usually, the experiments are done with e+ and e- of the same energy. In the sequel video to this one, I'll mention results from more than a dozen experiments. I think that only one of them--Belle--used beams of different energies. There is also an experiment, Babar, not discussed in my video, which did a tau lifetime measurement; they also used asymmetric beams. As far as I know, the rest used beams of the same energy.
I'm so glad I found your channel! I've tapped out most other sources I can find on RUclips, and it's difficult to come across detailed information on specific particle properties. I'm gonna watch all your videos now!
Oh, I saw your question about tau g-2 when you posted it. (I don't see the comment anymore, but I'll give you a quick answer nonetheless.) You are right in that the tau lifetime is too short to use the same techniques. That makes measuring tau g-2 difficult. But, it doesn't make a measurement totally impossible. Instead of looking at how the tau interacts with an applied magnetic field, we instead look at its interactions with photons in collider experiments. To the best of my knowledge, the best limits are still from the Delphi experiment at LEP (an electron-positron collider that ran in the same tunnel as the current LHC back in the 1990s). Their result is -0.052 < (g-2)/2
@@ThinkLikeaPhysicist Thanks for the reply! I ended up deleting the comment because I saw you had other videos on the Tau, and I wanted to check them out first in case you already answered my question there. I've got to imagine the increased mass would allow for even more rare interactions to occur more often than with the muon.
You're right, in fact. Actually, the precision on electron g-2 is much better than that of the muon. But, because the electron mass is so much smaller, it's less useful for new physics searches. I'll try to have a video on this sometime soon. (But it won't be the next video. Maybe the one after that. Not sure.) Thanks!
Any questions?
Which are the three particles? If tau- decays to muon/electron and two neutrinos, how can there be three particles that's seen there? So here are we looking for tau decaying into hadrons and a tau-neutrino? Even in case of tau decaying into hadrons, during the vertex reconstruction wouldn't the missing momentum carried by the tau-neutrino affect the uncertainty?
PS: Maybe the next video explains this.
Hi!
Good questions!
There are many experiments, whose results are compared in the 2nd video:
ruclips.net/video/xrLllaBiSWM/видео.html
(I'd encourage you to watch it, although it doesn't address your questions.) These different experiments used different methods, and some of them used more than one way to measure the tau lifetime. It would be way too hard to summarize them all, so I'll just make a few general comments.
Usually, the experiments used tau decays to hadrons + an invisible tau neutrino. The example I drew here involved the tau decaying to 3 hadronic particles, plus the tau neutrino. So, the 3 particles that are actually seen in the detector and used to get the vertex are hadrons.
However, I should note that this is not what every experiment did. Some experiments also looked for tau decays that involved only a single visible particle. These analyses, I think, would have included the cases where the tau decayed to a mu/e and two neutrinos. (It's also possible to have a tau decay to a single hadron and a tau neutrino. Naively, I would assume that these analyses included both the one-track hadronic decays and the decays to e and mu, but I haven't checked.)
Going back to the 3-track decays to hadrons: As for neutrino and the vertex reconstruction, this is a bit tricky. The neutrino and the hadronic tracks all point back to the same decay vertex, so, in principle, if you could measure the hadronic tracks perfectly, the neutrino wouldn't add any more information about where the vertex occurred. But, we don't live in an ideal world, and the trajectories of these tracks have uncertainties. So, as a very general statement, in an alternate universe where the neutrino was visible and also pointed back to the vertex, you would have more information to reconstruct the vertex, and thus your vertex reconstruction would presumably be better.
But there is another issue. In addition to there being an uncertainty on where the tau decayed, there is also an uncertainty on where it was produced. (The e+ and e- collide over a small volume, but you don't know exactly where in that volume a particular collision occurred and produced the tau+tau- pair.) So, you might try to figure out where a particular tau was produced, in order to calculate the distance it traveled before it decayed. And, doing this might involve trying to reconstruct the direction the tau was traveling with high precision. (From the vertex, you know where it decayed. So, if you know the tau's direction of travel, that can tell you about where it was produced.) This would be easy if you knew the momenta of all the particles that the tau decayed to. But, as you can't see the neutrino, you can't directly measure the neutrino momentum. This can feed into an uncertainty on the direction of travel of the tau, which then gives an uncertainty on where the tau was produced. So, the missing momentum of the neutrino can affect the uncertainty of the location where the tau was produced, instead of where it decayed!
A complete answer to your question, I would guess, probably depends on the specific experiment in question. I'm sure that there are lots of details and nuance that I don't know without taking a long look at the papers. But I hope that I've been able to give you at least some flavor of the answer to your question.
Does that help?
@@ThinkLikeaPhysicist That helps! Absolutely! Especially the argument that neutrino track isn't important for vertex reconstruction.
But it's only as long as the three hadrons are produced at the vertex itself, right? I assume the Feynman diagram will be such that a tau decays into tau neutrino and a virtual boson which splits into a q-qbar meson. This meson could travel a little bit before it emit gluons which splits into two more mesons (the three observed particles), then the vertex reconstruction won't be easy, right? So the lifetime of meson produced in the tree level Feynman diagram becomes important, I guess. Am I making sense?
I'll watch the second part some time soon! :)
Hi!
So, yes, the Feynman diagram has a tau split into a tau neutrino and a W boson, which then splits into q-qbar. But, the gluons are actually radiated off of the original q-qbar pair, not the meson.
So, after the W splits to q-qbar, one or both of those quarks can emit a gluon. The gluon(s) produced can then split into additional q-qbar pairs. (For high-energy processes, this process can continue for many steps, producing perhaps dozens of q-qbar pairs in the end. This produces the phenomenon seen in colliders known as "jets".)
So, eventually one ends up with a bunch of q-qbar pairs. After that happens, though, the quarks and antiquarks start assembling into mesons and baryons. This entire process happens extremely quickly however--far too quickly for us to see any of these details. (As far as we can see on macroscopic distance scales, all of that looks like it happens at a point.) So, what we see in the detector are just the mesons and baryons produced at the end of that process. It is possible that some of those mesons/baryons may decay afterward; depending on the lifetime of the meson/baryon in question, we may be able to see it decay in the detector.
@@ThinkLikeaPhysicist sounds good. So the assumption is that all the jet formation happens almost at the vertex itself. I guess that's because the strong force is strong? :D
Motivation,and overview of the measuring equipments
In muons decay there is a precision measure called " g-2 " also used for any violation in leptonic sector, in tau decay is there any role of "g-2" ?
Hi,
The short answer to your question is yes.
Here's a slightly longer answer:
In principle, one can measure g-2 for any of the leptons. In the case of the muon, this is determined by seeing how the direction of the muon's spin changes with time in a magnetic field. The tau, however, has a very short lifetime, and it decays far too quickly to do this measurement. So, other methods have to be used.
The magnetic moment of the tau describes a particular way in which the tau can interact with photons. In particular, it allows a process where, if you collide 2 photons, you can annihilate the photons and produce a tau+tau- pair. The DELPHI experiment (reference below) studied this process. Actually, the DELPHI experiment collides e+e-. But, if each of the e+ and e- radiates (in other words, emits) a photon, those photons can interact and produce the tau+tau- pair. That is the process that they used.
The precision of this, though, is not so impressive compared to the Standard Model prediction. The Standard Model prediction is
(g-2)/2 = 0.00117721+-0.00000005
and the limit DELPHI was able to set was
-0.052 < (g-2)/2
@@ThinkLikeaPhysicist thanks for a detailed discussion
Hi! I'm planning to do a series of videos on muon g-2. Since you asked a question related to muon g-2, I was thinking about mentioning your comment at the beginning of the first video, kinda like what I did on this video:
ruclips.net/video/pfgQS5kK70w/видео.html
Would you like me to? I could show your comment as-is, or I could show it but black out your name, or I could just not show it at all, depending on your preference. Would you prefer any of those options?
Thanks!
@@ThinkLikeaPhysicist Hi!... Please do what ever suitable for the contents.. I'm Okay with all...
We are in wait for g-2 series
Thanks!
OK, thanks! I'll put it in.
At 11:45 ee+ collision happened with same momenta or with a asymmetric momenta?
Hi,
Usually, the experiments are done with e+ and e- of the same energy. In the sequel video to this one, I'll mention results from more than a dozen experiments. I think that only one of them--Belle--used beams of different energies. There is also an experiment, Babar, not discussed in my video, which did a tau lifetime measurement; they also used asymmetric beams. As far as I know, the rest used beams of the same energy.
I'm so glad I found your channel! I've tapped out most other sources I can find on RUclips, and it's difficult to come across detailed information on specific particle properties.
I'm gonna watch all your videos now!
Thanks! Much appreciated!
Oh, I saw your question about tau g-2 when you posted it. (I don't see the comment anymore, but I'll give you a quick answer nonetheless.)
You are right in that the tau lifetime is too short to use the same techniques. That makes measuring tau g-2 difficult. But, it doesn't make a measurement totally impossible. Instead of looking at how the tau interacts with an applied magnetic field, we instead look at its interactions with photons in collider experiments.
To the best of my knowledge, the best limits are still from the Delphi experiment at LEP (an electron-positron collider that ran in the same tunnel as the current LHC back in the 1990s). Their result is
-0.052 < (g-2)/2
@@ThinkLikeaPhysicist Thanks for the reply!
I ended up deleting the comment because I saw you had other videos on the Tau, and I wanted to check them out first in case you already answered my question there.
I've got to imagine the increased mass would allow for even more rare interactions to occur more often than with the muon.
You're right, in fact.
Actually, the precision on electron g-2 is much better than that of the muon. But, because the electron mass is so much smaller, it's less useful for new physics searches.
I'll try to have a video on this sometime soon. (But it won't be the next video. Maybe the one after that. Not sure.)
Thanks!
Lifetime
Carroll where's Carroll, Christmas Carroll