Q&A with a LHCb expert
Posted: Thu Dec 17, 2009 1:30 pm
From the chat...
[20:53] <photino> CMS has also rediscovered the pion http://cms.web.cern.ch/cms/Media/Public ... es/K0s.png
[20:53] <photino> lhcb was faster...
[20:54] <Anitusar> lhcb also found the Ks and lambda
[20:54] <photino> sorry that's actually the K_0 production in the plot above, from pairs of pions
[20:57] <photino> it's 1967 all over again at the CMS !! --
http://www.springerlink.com/content/676q2904uvn70825/
[20:55] <photino> anitusar: they should have a bingo where they mark off what's been seen
[20:55] <robcrazee> lol lhc bingo
[20:55] <robcrazee> need a higgs to win
[20:57] <Anitusar> ahh, the Higgs is bad for the lhcb
[20:57] <robcrazee> how so?
[20:57] <Anitusar> unless its between 125 and 140 GeV
[20:58] <Anitusar> CMS and ATLAS are build to find the Higgs, LHCb not really
[20:58] <photino> lhcb is optimised to detect kaons and pions, isn't it
[20:59] <Anitusar> and low momentum muons
[20:59] <sAxion> LHCb will measure CP violations = very important for early universe
[21:00] <Anitusar> we also search for new physics the indirect way, complementary to the big ones
[21:00] <photino> which detector grabs low energy muons? ie do they make it to the muon detectors at the end?
[21:01] <Anitusar> at least M1, which is before the calo
[21:01] <Anitusar> we also have less material, so muons are not stopped
[21:01] <photino> why don't electrons trigger the muon detectors?
[21:02] <bd_> I'd expect electrons to interact strongly with the rest of the machine and be lost before they get there...
[21:03] <Anitusar> electrons are stopped pretty fast when they see material
[21:04] <photino> but why wouldn't the same interactions apply to muons?
[21:04] <photino> ie in what way does the extra mass change things
[21:05] <Anitusar> the electrons make distinct shower in ECAL, also different rings in the RICH2, so one can separate electrons from muons pretty easy
[21:07] <sAxion> dont they bend more in mag?
[21:07] <Anitusar> the extra mass changes changes the energy loss per volume element.
[21:07] <Anitusar> the bending depends also on the momentum
[21:08] <photino> what's the main CP-violating decay lhcb is built to observe?
[21:09] <Anitusar> Bs -> J/Psi Phi is one important channel
[21:14] <photino> will lhcb have trouble if new physics comes into play because they can't capture everything produced in a collision?
[21:15] <Tau> photino: I thought ATLAS was supposed to catch everything.
[21:15] <photino> tau: sure, i'm asking about lhcb
[21:15] <Anitusar> no, what we test is the standard model predictions. So if there are not correct, we also "measure" new physics
[21:16] <Anitusar> we also look "only" for b-quarks, which fly always in forward direction
[21:17] <photino> but in order to determine the energy of the b-quarks, aren't you making standard model assumptions about their production?
[21:17] <photino> i suppose that's what you meant before
[21:18] <photino> or maybe you work backwards from the decay products?
[21:19] <Anitusar> yes, you always measure the decay products
[21:19] <Anitusar> b-quarks have the nice feature, that they fly for about a 1cm, so one can reconstruct them very well
[21:20] <Anitusar> and what we look for is the difference between the b-quark and anti-b
[21:21] <Tau> Anitusar: I almost get it. What difference is there between them? Energy?
[21:23] <Anitusar> concerning the difference, this is complicated, but you look for asymmetries in the decay of the both particles
[21:21] <photino> ie the CKM matrix?
[21:22] <Anitusar> we measure 4 entries in the CKM matrix
[21:23] <Tau> Yes, I looked up the CKM matrix. I can read the math, although I'm not familiar with it. A bit confusing, but I think I get it a bit.
[21:24] <photino> tau: basically the particles are vectors in a hilbert space, and there are 2 relevant bases which enter interactions. the CKM matrix translates between them
[21:25] <Anitusar> yes, the mass eigenstates are not equal to the weak eigenstates, if this helps
[21:26] <Tau> Yes, I get an idea. So normally, you see mixtures of quarks, instead of exactly a top, strange and so on, as they like everybody to believe? I'll have to remove the idea from my brain that a particle consists always of a nice triple of quarks; there are all kind of mixtures (quantum interpolations, to use the right term).
[21:30] <Tau> With this new knowledge, I am going to try to read the Wikipedia page on CKM matrix. Wish me luck
[21:29] <Anitusar> all i care is that are 3 parameter and one phase, which leads to CP-violation
[21:29] <photino> anitusar: i still don't think i understand how you can get away with not knowing exactly how the b's were produced? (or maybe you do know but don't need to measure?)
[21:30] <Anitusar> we do not care, how the b was formed. We measure from the point of production onward
[21:31] <photino> and you determine the energy/momentum of those b's entirely working backwards from the decay product? wow!!
[21:32] <Anitusar> an example: you get two muons
[21:32] <Anitusar> combine them to a J/Psi
[21:33] <Anitusar> next two Kaons and build a phi
[21:33] <Anitusar> then you build the bs by combinening those two
[21:34] <Anitusar> measure where the primary vertex and you know, how long the Bs lived
[21:34] <Anitusar> the position of the primary vertex
[21:35] <photino> the primary vertex being the J/psi phi production vertex?
[21:35] <Anitusar> no the bs decay is the secondary vertex
[21:35] <Anitusar> no primary vertex is constructed using all the other tracks
[21:38] <Tau> I've seen those tables with decay probabilities: you probably need these for the reconstruction. Does that imply you're never sure what happened?
[21:39] <Tau> Or is it almost always possible to figure it out?
[21:40] <Anitusar> as we go backwards, we do need the tables for reconstruction
[21:41] <Anitusar> but we get the Standard model prediction for a decay, and then we look if we see diviations
[21:42] <Anitusar> e.g. we should see 190 decays of a certain kind and find 400, we know the Standard model misses something
[21:42] <Tau> Anitusar: OK, so that's how they made the tables.
[21:44] <Anitusar> sorry, backwards misses a "not" in the sentence
[21:48] <Tau> Well, I learned a lot this far. Thanks.
[21:49] <photino> yes thanks for your time anitusar!
[21:50] <Anitusar> no problem, but i am more of an electronic guy, so the theories are not my best topic
[21:52] <photino> i've never understood the CKM matrix. i mean, where did it come from? looks so ad-hoc...
[21:52] <photino> hope the lhc finds out for us
[21:53] <Anitusar> you need the CKM mechanism to get CP-violation, got the nobel prize 2008 for predicting the third family
[21:55] <Anitusar> we will just measure the elements of the matrix with more precision to see how large the CP-violating effects are in the Standard model
[21:57] <photino> oh, i meant to ask how does the VELO detect the vertex position?
[21:59] <Anitusar> the beams are tuned for lhcb in such a way, that we have only one interaction per collistion in average
[22:00] <Anitusar> so most of the track are coming from that collision, therefore the primary vertex is where the most tracks intersect
[22:02] <photino> interesting, does that mean later at high intensities they will squeeze the beem very finely to get single interactions?
[22:03] <photino> whereas the other experiments only care about luminosity?
[22:03] <Anitusar> the opposite, the beam is not as much focused as for the other experiments
[22:04] <Anitusar> thats why lhcb has no problem with low luminosity in the beginning
[22:04] <photino> so not only is the velo really close to the beam, but the beam is wider than for the other experiments as well
[22:04] <photino> if i understand you right, it is designed for relatively low luminosity
[22:05] <Anitusar> yeap
[22:05] <photino> still lots of collisions but over a wider area
[22:06] <Anitusar> atlas and cms have over 20 interactions per collision, wher ewe will have 1
[22:07] <photino> but they want to make quark-gluon plasma unlike you
[22:07] <photino> (or does the term only apply to lead ion collisions)
[22:08] <Anitusar> usually ion collisions
[22:08] <Anitusar> but they want to create as much particles as possible
[22:10] <photino> a tradeoff with vertrex tracking precision
[22:11] <Anitusar> yes, we need to the position of the primary vertex very well and also in which primary vertex the b was produced
[22:11] <Anitusar> so having only one makes things easier
[22:12] <photino> i think i'm beginning to see now why the design is the way it is
[22:12] <photino> thanks lots!
[22:15] <spencer> thx here too
[22:15] <Anitusar> glad i can help
[22:16] <photino> at higher energies, will you rename the experiment LHCt?
[22:16] <spencer> now my universe is inside-out
[22:17] <Anitusar> No, the t is bad. It cannot commit to other quarks to form a meson. Wants to be alone
[22:19] <Axion> Surely it cant be alone due to colour confinement
[22:20] <Anitusar> it decays before there is any chance to form a meson (quark anti-quark pair)
[22:20] <Axion> ok
[22:21] <photino> the top is VERY heavy for a quark
[22:22] <photino> it weighs as much as a whole heavy metal atom
[22:22] <Axion> top heavy
[22:22] <photino> haha
[22:26] <Tau> I've been following the discussion. Thanks for the explanations.
[22:27] <homolupus> so have I, it has been most interesting
[22:28] <Axion> I liked it too
[23:03] <spencer> BTW did LHC get the 1 million events ?
[23:04] <Anitusar> lhcb was at 423000 yesterday
[23:06] <Anitusar> well everyone wants the J/Psi for which you will need about a million events
[20:53] <photino> CMS has also rediscovered the pion http://cms.web.cern.ch/cms/Media/Public ... es/K0s.png
[20:53] <photino> lhcb was faster...
[20:54] <Anitusar> lhcb also found the Ks and lambda
[20:54] <photino> sorry that's actually the K_0 production in the plot above, from pairs of pions
[20:57] <photino> it's 1967 all over again at the CMS !! --
http://www.springerlink.com/content/676q2904uvn70825/
[20:55] <photino> anitusar: they should have a bingo where they mark off what's been seen
[20:55] <robcrazee> lol lhc bingo
[20:55] <robcrazee> need a higgs to win
[20:57] <Anitusar> ahh, the Higgs is bad for the lhcb
[20:57] <robcrazee> how so?
[20:57] <Anitusar> unless its between 125 and 140 GeV
[20:58] <Anitusar> CMS and ATLAS are build to find the Higgs, LHCb not really
[20:58] <photino> lhcb is optimised to detect kaons and pions, isn't it
[20:59] <Anitusar> and low momentum muons
[20:59] <sAxion> LHCb will measure CP violations = very important for early universe
[21:00] <Anitusar> we also search for new physics the indirect way, complementary to the big ones
[21:00] <photino> which detector grabs low energy muons? ie do they make it to the muon detectors at the end?
[21:01] <Anitusar> at least M1, which is before the calo
[21:01] <Anitusar> we also have less material, so muons are not stopped
[21:01] <photino> why don't electrons trigger the muon detectors?
[21:02] <bd_> I'd expect electrons to interact strongly with the rest of the machine and be lost before they get there...
[21:03] <Anitusar> electrons are stopped pretty fast when they see material
[21:04] <photino> but why wouldn't the same interactions apply to muons?
[21:04] <photino> ie in what way does the extra mass change things
[21:05] <Anitusar> the electrons make distinct shower in ECAL, also different rings in the RICH2, so one can separate electrons from muons pretty easy
[21:07] <sAxion> dont they bend more in mag?
[21:07] <Anitusar> the extra mass changes changes the energy loss per volume element.
[21:07] <Anitusar> the bending depends also on the momentum
[21:08] <photino> what's the main CP-violating decay lhcb is built to observe?
[21:09] <Anitusar> Bs -> J/Psi Phi is one important channel
[21:14] <photino> will lhcb have trouble if new physics comes into play because they can't capture everything produced in a collision?
[21:15] <Tau> photino: I thought ATLAS was supposed to catch everything.
[21:15] <photino> tau: sure, i'm asking about lhcb
[21:15] <Anitusar> no, what we test is the standard model predictions. So if there are not correct, we also "measure" new physics
[21:16] <Anitusar> we also look "only" for b-quarks, which fly always in forward direction
[21:17] <photino> but in order to determine the energy of the b-quarks, aren't you making standard model assumptions about their production?
[21:17] <photino> i suppose that's what you meant before
[21:18] <photino> or maybe you work backwards from the decay products?
[21:19] <Anitusar> yes, you always measure the decay products
[21:19] <Anitusar> b-quarks have the nice feature, that they fly for about a 1cm, so one can reconstruct them very well
[21:20] <Anitusar> and what we look for is the difference between the b-quark and anti-b
[21:21] <Tau> Anitusar: I almost get it. What difference is there between them? Energy?
[21:23] <Anitusar> concerning the difference, this is complicated, but you look for asymmetries in the decay of the both particles
[21:21] <photino> ie the CKM matrix?
[21:22] <Anitusar> we measure 4 entries in the CKM matrix
[21:23] <Tau> Yes, I looked up the CKM matrix. I can read the math, although I'm not familiar with it. A bit confusing, but I think I get it a bit.
[21:24] <photino> tau: basically the particles are vectors in a hilbert space, and there are 2 relevant bases which enter interactions. the CKM matrix translates between them
[21:25] <Anitusar> yes, the mass eigenstates are not equal to the weak eigenstates, if this helps
[21:26] <Tau> Yes, I get an idea. So normally, you see mixtures of quarks, instead of exactly a top, strange and so on, as they like everybody to believe? I'll have to remove the idea from my brain that a particle consists always of a nice triple of quarks; there are all kind of mixtures (quantum interpolations, to use the right term).
[21:30] <Tau> With this new knowledge, I am going to try to read the Wikipedia page on CKM matrix. Wish me luck
[21:29] <Anitusar> all i care is that are 3 parameter and one phase, which leads to CP-violation
[21:29] <photino> anitusar: i still don't think i understand how you can get away with not knowing exactly how the b's were produced? (or maybe you do know but don't need to measure?)
[21:30] <Anitusar> we do not care, how the b was formed. We measure from the point of production onward
[21:31] <photino> and you determine the energy/momentum of those b's entirely working backwards from the decay product? wow!!
[21:32] <Anitusar> an example: you get two muons
[21:32] <Anitusar> combine them to a J/Psi
[21:33] <Anitusar> next two Kaons and build a phi
[21:33] <Anitusar> then you build the bs by combinening those two
[21:34] <Anitusar> measure where the primary vertex and you know, how long the Bs lived
[21:34] <Anitusar> the position of the primary vertex
[21:35] <photino> the primary vertex being the J/psi phi production vertex?
[21:35] <Anitusar> no the bs decay is the secondary vertex
[21:35] <Anitusar> no primary vertex is constructed using all the other tracks
[21:38] <Tau> I've seen those tables with decay probabilities: you probably need these for the reconstruction. Does that imply you're never sure what happened?
[21:39] <Tau> Or is it almost always possible to figure it out?
[21:40] <Anitusar> as we go backwards, we do need the tables for reconstruction
[21:41] <Anitusar> but we get the Standard model prediction for a decay, and then we look if we see diviations
[21:42] <Anitusar> e.g. we should see 190 decays of a certain kind and find 400, we know the Standard model misses something
[21:42] <Tau> Anitusar: OK, so that's how they made the tables.
[21:44] <Anitusar> sorry, backwards misses a "not" in the sentence
[21:48] <Tau> Well, I learned a lot this far. Thanks.
[21:49] <photino> yes thanks for your time anitusar!
[21:50] <Anitusar> no problem, but i am more of an electronic guy, so the theories are not my best topic
[21:52] <photino> i've never understood the CKM matrix. i mean, where did it come from? looks so ad-hoc...
[21:52] <photino> hope the lhc finds out for us
[21:53] <Anitusar> you need the CKM mechanism to get CP-violation, got the nobel prize 2008 for predicting the third family
[21:55] <Anitusar> we will just measure the elements of the matrix with more precision to see how large the CP-violating effects are in the Standard model
[21:57] <photino> oh, i meant to ask how does the VELO detect the vertex position?
[21:59] <Anitusar> the beams are tuned for lhcb in such a way, that we have only one interaction per collistion in average
[22:00] <Anitusar> so most of the track are coming from that collision, therefore the primary vertex is where the most tracks intersect
[22:02] <photino> interesting, does that mean later at high intensities they will squeeze the beem very finely to get single interactions?
[22:03] <photino> whereas the other experiments only care about luminosity?
[22:03] <Anitusar> the opposite, the beam is not as much focused as for the other experiments
[22:04] <Anitusar> thats why lhcb has no problem with low luminosity in the beginning
[22:04] <photino> so not only is the velo really close to the beam, but the beam is wider than for the other experiments as well
[22:04] <photino> if i understand you right, it is designed for relatively low luminosity
[22:05] <Anitusar> yeap
[22:05] <photino> still lots of collisions but over a wider area
[22:06] <Anitusar> atlas and cms have over 20 interactions per collision, wher ewe will have 1
[22:07] <photino> but they want to make quark-gluon plasma unlike you
[22:07] <photino> (or does the term only apply to lead ion collisions)
[22:08] <Anitusar> usually ion collisions
[22:08] <Anitusar> but they want to create as much particles as possible
[22:10] <photino> a tradeoff with vertrex tracking precision
[22:11] <Anitusar> yes, we need to the position of the primary vertex very well and also in which primary vertex the b was produced
[22:11] <Anitusar> so having only one makes things easier
[22:12] <photino> i think i'm beginning to see now why the design is the way it is
[22:12] <photino> thanks lots!
[22:15] <spencer> thx here too
[22:15] <Anitusar> glad i can help
[22:16] <photino> at higher energies, will you rename the experiment LHCt?
[22:16] <spencer> now my universe is inside-out
[22:17] <Anitusar> No, the t is bad. It cannot commit to other quarks to form a meson. Wants to be alone
[22:19] <Axion> Surely it cant be alone due to colour confinement
[22:20] <Anitusar> it decays before there is any chance to form a meson (quark anti-quark pair)
[22:20] <Axion> ok
[22:21] <photino> the top is VERY heavy for a quark
[22:22] <photino> it weighs as much as a whole heavy metal atom
[22:22] <Axion> top heavy
[22:22] <photino> haha
[22:26] <Tau> I've been following the discussion. Thanks for the explanations.
[22:27] <homolupus> so have I, it has been most interesting
[22:28] <Axion> I liked it too
[23:03] <spencer> BTW did LHC get the 1 million events ?
[23:04] <Anitusar> lhcb was at 423000 yesterday
[23:06] <Anitusar> well everyone wants the J/Psi for which you will need about a million events