Wide angles
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by
DiNapoli
Why is a particle leavinig at a wide angle weird (In this case) ? I see that this is said a lot but I don't understand it.
Posted
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by
andy.haas
scientist
I'm not sure which particle you mean. I don't see one at a large angle?
There something sort of interesting in this event, the two white tracks look to be pointing to a photon object (yellow) at 2 o'clock. This is most likely an electron, which has emitted a photon (gamma ray) in the tracker, which has then converted into an e+e- pair in some material in the tracker.Posted
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DiNapoli
The question came up after reading the response of Whoandwhatitis on the left, I think he means that the photons are opposite to eachother because there's also a signature of a photon at aprox. 9 o'clock. That's how I explain wide angle in this case.
Posted
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markbakovic
there's a bit of ambiguity in what people tag as "#wideangle": sometimes it's azimuthal angle (6 o'clock/12 o'clock in normal view), sometimes its divergence from the beam axis/pseudorapidity/etc. (as seen in slice view). sometimes it applies to tracks sharing a vertex, sometimes not so much.
at the end of the day any project like this can't be too pedantic about what people tag/consider/think is any given feature. there's value in letting people tag whatever they think falls into that category and following up on them later (eg Dr. Andy's answer to "what constitutes 'Something weird'?", basically "whatever you think is weird") and on the other hand the scientists have way too little time to police any tagging standard anyway.
I'm probably as bad as anyone tagging things incorrectly, but as far as my understanding reaches I tag what I'm confident I can be consistent about and don't worry about what I'm less clear on (eg I don't use #wideangle, I'm occasionally circumspect/flippant enough to tag "#blue things" 😉 rather than the b-jets they represent)
Posted
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DiNapoli
in response to markbakovic's comment.
Ok thank you that explains a lot.
But 😃 ,after carefully reading your post I want to rephrase my question this way; if a pair of particles travel In a azimuthal angle (as you say in the first alinea of your post above) from the core of the collider, does that mean anything?
I'll ask an extra question about this I think it makes it clearer; Can the angle of the tracks wich particles make when they seperate from eachother tell anything about wich particles it are/could be?
Posted
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markbakovic
absolutely. the angles between tracks are taken into account when calculating invariant mass and certain angle relationships hint at the decay process from which they result (eg muon tracks that are greater than Phi=90 degrees apart vs less than 90 degrees) (and hinting is all we can go by without the momentum measurements of each track).
Consider if you like a particle made of a quark and the same flavour antiquark (I like Upsilon mesons personally, but there are others just as illustrative) which decays into its two constituent quarks*: since the quarks inherit momentum equally, have the same mass and are exactly equally but oppositely charged they tend to part ways either side of the parent particle's trajectory. So their trajectories will diverge from the parent particle's in proportion to its momentum, parent particle moving fast = the quarks move a long way in that direction for each bit they move apart, particle moving slow = a "wide angle" 😉 between their tracks. If they turn out to be too angularly close together for their momentum, they likely didn't come from the same particle, but from two different ones. Jet tagging algorithms thus also use dijet angles as part of their logic.
this is also why there's a tag for #diametric 😉
*[EDIT] Well that was dumb, and I was wrong. No b/bbar decays for Upsilon mesons as far as I (now) know, and I may have oversimplified in an effort to simplify. Electric charge doesn't come into it, but other quantum numbers (which are opposite for quark antiquark pairs) do. And they repel, or "mirror momentum vectors" at least. But I can't make it really clear how 😃
Posted
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DiNapoli
in response to markbakovic's comment.
Hello @markbakovic and thnx for the response, I can see you try to eplain it as good as possible in normal terms I really appericiate that 😉
And because of that I think I mostly understand what you mean after carefully reading your answer.So I've put down some notes beneath to see if I really get the point on certain things or if I'm terribly off.
(I might be saying things twice or repeating your answers).1-This also means that the faster the particle goes the less it gets affected by the magnetic field in the collider, right?
2-Faster parent particles will travel further in the detector before they decay.
3-So the slower the particle the more it gets curved/bent by the magn. field (thus wider angle) in their opposite directions?
4-Slower parent particles will mostly decay close to the centre/collision point.5-So this means that if I see a vertex/decay near the centre and the constituent quarks of the parent particle travel al long way into the detector at a close angle I can classify this as weird because a slow parent particle would have a wide angle and wouldn't pass so much momentum over to the quarks who come out of it.
6-As In point 5 above I can understand that the two quarks likely don't come from the same parent particle but "likely" means that there are some other possiblities also like an exotic/unknown particle with strange/different behaviour? Or am I dreaming now?
If you're into it take your time answering I'm not In a hurry and we all got things to do 😉
Somebody else might enlighten me also, no prob.
Thanx in advance..Posted
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by
markbakovic
I'll try and answer what I think I can (and leave the rest to the team):
1- Yes but also no. Sort of. This is sometimes true when comparing particles of the same invariant mass and different speeds (different momenta), but not always true when comparing particles with different invariant mass and different speeds (with potentially the same momenta) or different amounts of electric charge. It's a "Force due to B vs Inertia due to Momentum" relationship, where you can get the same deflection with several different combinations of mass, velocity and charge. To make things worse even the "fixed" field strength is subject to fluctuations, though it's pretty high so any error is a small percentage.
Even worse, technically it's actually not true at all, because the force is proportional to the particle's velocity, (so "The faster a particle goes the more it gets affected by the magnetic field") but in a practical sense time of flight comes into play (so the less time the particle spends in the field the less its trajectory deviates). However that also means that angles come into consideration too, as a very transverse particle spends a lot less time in the field than one that travels half the length of the detector and exits through the endcap. This difference is reduced somewhat by the magnetic field force's dependence on the direction of the particle's velocity relative to the orientation of the field, but not entirely if the particle slows down over its journey (which again depends on its mass, which can even change in flight if it emits a decay particle).
Even further complicating things is bremsstrahlung: a charged particle moving very fast and changing its trajectory loses momentum in the form of photons (this is a big engineering problem for circular particle accelerators, and part of why, for example, SLAC is still a big deal even though laymen often see it as "old fashioned"). Whether it is possible for particles to travel fast enough to emit bremsstrahlung but slow enough to curve enough to emit bremsstrahlung in ATLAS I don't know, but it's a possibility I guess.
2- Depends on the parent particle: particles have "mean lifetimes", so don't all live the same length of time, and again there may be intermediate decays, especially involving neutrinos or uncharged particles which we can't see.
3- Again, I'm not clear on what #wideangle should signify, but I don't think it's anything to do with track curvature (due to magnetic field interaction). Rather it seems sensible to consider vertex angle, ie the angle at the decay point from which two or more tracks start.
4- Not exactly: slow long lived particles will not. Heavy particles are generally short lived and so will decay in a short amount of time. A lot of the beam energy that went into making them contributed to their mass so they may not travel fast. But there is no reason they can't travel fast: they may have inherited enough momentum for both mass and speed from the proton collision. Heavier particles decaying near the collision point: ok, that might be a fair generalisation because heavier particles tend to be slow...
5- ...but likewise they may travel slow but decay all their momentum into lots of light particles: there's no reason these can't inherit a lot of forward momentum from the mass of their parent. Not all heavy particle decays produce quarks and other heavy decay products, they just can (whereas light particle decays can't). Remember that the blue marker isn't foolproof.
6- Possibly. A great example would be your other discussion thread about AHH000pyo. Although Normal view shows plenty of azimuthal angle separation between the two b-jets I'll risk guessing from the tracks in Slice view that they have quite large z components of momentum (or high pseudorapidity if that means more to you). So in this sense there's not much angle between them and they "should be from a high speed particle" if they come from the same parent. If we saw charged particle tracks coming from near the centre to suggest they both came from slow moving particles (and we do see a few possibles) then this would suggest a two particle parent explanation. Indeed this is what Dr. Andy offered by suggesting ttbar ->WWbb because that decay would be each top quark decays into one bottom quark and one W boson; so each b-jet comes from a different parent particle (either the top or the antitop quark).
In short: no you're not dreaming and are correct in your reasoning: even "known" particles like t quarks can be rare finds (actually bottom quarks are noteworthy enough, and I'd like to see some preliminary statistics on how many we've all observed now that b-jets are getting tagged in so many events...).
It is a good idea to maintain some scepticism though: there are a lot of explanations for anything we observe here, and even with all the analysis tools nobody is shouting "EYPHKA" over one event. That's why they have us to sift through thousands of events and put some in the "ocv" pile for statistical analysis. In physics it's an "anomaly" or a "tension" or an "excess" until it's been seen thousands of times.
That said If you're keen by all means "tag anything as weird that you think is weird"; you've already found some interesting ones for sure.
P.S. I just realised while proofreading that I may have misled you slightly by mentioning in my Y meson example that both quarks are the same but for their opposite electric charges, and saying that it decays into two quarks etc. I was pretty much completely wrong to use that as an example (Y mesons don't decay into b quarks), but I'll come back to that as an aside.
The field is a means of detecting and measuring momenta of more stable particles, and particles whose charge is high compared to their mass are mostly the ones affected by it (like electrons/positrons, lower energy muons and maybe charged pions etc.). It doesn't affect vertex angles between decay products, as these angles are due to forces between the products themselves (which at the small distance scales of particle decay can be far higher than anything externally applied).
Now to come back to what is wrong with what I said (why I'm wrong should be fairly obvious: I'm not a particle physicist). I suggested that bbbar particles decay into a b quark and an antib quark; they don't, as far as I've found. In fact I should have chosen a Z boson, which can decay into b/bbar. Next is that I specifically mentioned the oppposite "charge" of the quarks (and I assume this is what suggested they diverge because of the B field, because otherwise they'd attract, right?) I honestly didn't think about that at all. They do indeed have opposite electric charge, otherwise the mesons in question wouldn't be electrically neutral. They also have opposite "colour" charge (see here for more on QCD), spin (more), and intrinsic parity (more and a little more) and as far as I understand it it's one or the other of those last two that determines the divergence of quarks from a parent particle, but I hope you'll forgive me for being at about the limit of my understanding before I can give a clear and simple reason why.
😃
Posted
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by
DiNapoli
in response to markbakovic's comment.
Haha I saw the [edit] 😉 But with helping me out a lot I won't blame you for anything 😉 (As for the rest I've met here at the forum) You now more about the subject than me and at least you help me to look in the right direction (but believe me you help a lot more).
I will look at the links you added In your response plus some other things.Thanks again!
Posted
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Whoandwhatitis
moderator
in response to andy.haas's comment.
This makes sense. What I was referring to is in Normal / Zoom views, where the long white particle track meets the dashed red line. They meet at about a 90° angle. However, we don't see this in Slice view so I don't think I checked this out when I tagged it as "wideangle".
This is most likely an electron, which has emitted a photon (gamma ray) in the tracker, which has then converted into an e+e- pair in some material in the tracker.
Neat!
Posted