View Full Version : Decay question
Creighton Hogg
Jul22-04, 10:04 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Hi guys, this is another dumb question from me.\nIn string theory I know that interactions are described thru the the\nworldsheets of the strings: scattering, splitting, merging. Also, I know\nthat in the real world we don\'t observe decays modes such as \\mu -> e +\n\\gamma. This is assumed as a conservation of lepton family number. What\nprevents this from happening in the context of string theory? It seems\nlike there\'s little room for an ad-hoc conservation law in string theory,\nso how do you prevent this from happening?\n\nThanks\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Hi guys, this is another dumb question from me.
In string theory I know that interactions are described thru the the
worldsheets of the strings: scattering, splitting, merging. Also, I know
that in the real world we don't observe decays modes such as \mu -> e +\gamma. This is assumed as a conservation of lepton family number. What
prevents this from happening in the context of string theory? It seems
like there's little room for an ad-hoc conservation law in string theory,
so how do you prevent this from happening?
Thanks
Lubos Motl
Jul24-04, 12:48 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>On Thu, 22 Jul 2004, Creighton Hogg wrote:\n\n> Hi guys, this is another dumb question from me.\n> In string theory I know that interactions are described thru the the\n> worldsheets of the strings: scattering, splitting, merging. Also, I know\n> that in the real world we don\'t observe decays modes such as \\mu -> e +\n> \\gamma. This is assumed as a conservation of lepton family number. What\n> prevents this from happening in the context of string theory? It seems\n> like there\'s little room for an ad-hoc conservation law in string theory,\n> so how do you prevent this from happening?\n\nThat\'s a fair question, I would say. String theory leads to realistic\nlow-energy effective field theories, and if such a theory happens to\nsuppress the muon decay to e+gamma, it would follow from calculations in\nstring theory that such a process does not occur - or occurs very slowly\n- regardless whether you would expect such an interaction to be allowed\nby the naive picture of splitting strings.\n\nBut the real question is whether string theory naturally suppresses such\ndecays. The more general problem of flavor changing neutral currents\n(FCNCs) is a tough problem in phenomenology. Even if you study\npoint-particle quantum field theory, the FCNCs will occur in various\nmodels of supersymmetry breaking; technicolor etc. and they are\npotentially scary signals for these models because their required\nsuppressions seems to require some degree of fine-tuning.\n\nYou are right that you could suppress such processes by requiring the\nseparate generation lepton numbers to be conserved. String theory tends to\nsay that such conservation laws cannot be exact. Continuous symmetries in\nstring theory are never global symmmetries - this is not a theorem, but it\nseems to be the case in the contexts we know. Therefore, such a new\nconservation law should correspond to a local, gauge symmetry, but then it\nalso leads to new kind of forces.\n\nIt seems very likely that we don\'t want such new exact symmetries, and\ntherefore the generation lepton number must be approximate only. (You can\nconstruct more complicated processes where it\'s violated anyway.) String\ntheory can give you various tools how such processes can be suppressed -\ne.g. sequestering. Imagine that the muon field and the electron field are\nexcitations of two different branes, separated in one of the additional\n(hidden) dimensions. Then the geometric separation will simply suppress\nthe interactions between the different generations.\n\nThe question of suppressing these unwanted processes is a large topic - in\nfact, a significant part of current phenomenology, and you should ask a\nmore specific question if you want it to be answered in one posting.\n________________________________________ ______________________________________\nE-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/\neFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>On Thu, 22 Jul 2004, Creighton Hogg wrote:
> Hi guys, this is another dumb question from me.
> In string theory I know that interactions are described thru the the
> worldsheets of the strings: scattering, splitting, merging. Also, I know
> that in the real world we don't observe decays modes such as \mu -> e +
> \gamma. This is assumed as a conservation of lepton family number. What
> prevents this from happening in the context of string theory? It seems
> like there's little room for an ad-hoc conservation law in string theory,
> so how do you prevent this from happening?
That's a fair question, I would say. String theory leads to realistic
low-energy effective field theories, and if such a theory happens to
suppress the muon decay to e+\gamma, it would follow from calculations in
string theory that such a process does not occur - or occurs very slowly
- regardless whether you would expect such an interaction to be allowed
by the naive picture of splitting strings.
But the real question is whether string theory naturally suppresses such
decays. The more general problem of flavor changing neutral currents
(FCNCs) is a tough problem in phenomenology. Even if you study
point-particle quantum field theory, the FCNCs will occur in various
models of supersymmetry breaking; technicolor etc. and they are
potentially scary signals for these models because their required
suppressions seems to require some degree of fine-tuning.
You are right that you could suppress such processes by requiring the
separate generation lepton numbers to be conserved. String theory tends to
say that such conservation laws cannot be exact. Continuous symmetries in
string theory are never global symmmetries - this is not a theorem, but it
seems to be the case in the contexts we know. Therefore, such a new
conservation law should correspond to a local, gauge symmetry, but then it
also leads to new kind of forces.
It seems very likely that we don't want such new exact symmetries, and
therefore the generation lepton number must be approximate only. (You can
construct more complicated processes where it's violated anyway.) String
theory can give you various tools how such processes can be suppressed -
e.g. sequestering. Imagine that the muon field and the electron field are
excitations of two different branes, separated in one of the additional
(hidden) dimensions. Then the geometric separation will simply suppress
the interactions between the different generations.
The question of suppressing these unwanted processes is a large topic - in
fact, a significant part of current phenomenology, and you should ask a
more specific question if you want it to be answered in one posting.
__{_______________________________________________ _____________________________}
E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
eFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Creighton Hogg
Jul26-04, 02:33 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nOn Sat, 24 Jul 2004, Lubos Motl wrote:\n\n> On Thu, 22 Jul 2004, Creighton Hogg wrote:\n>\n> > Hi guys, this is another dumb question from me.\n> > In string theory I know that interactions are described thru the the\n> > worldsheets of the strings: scattering, splitting, merging. Also, I know\n> > that in the real world we don\'t observe decays modes such as \\mu -> e +\n> > \\gamma. This is assumed as a conservation of lepton family number. What\n> > prevents this from happening in the context of string theory? It seems\n> > like there\'s little room for an ad-hoc conservation law in string theory,\n> > so how do you prevent this from happening?\n\n<snip>\n> String theory can give you various tools how such processes can be suppressed -\n> e.g. sequestering. Imagine that the muon field and the electron field are\n> excitations of two different branes, separated in one of the additional\n> (hidden) dimensions. Then the geometric separation will simply suppress\n> the interactions between the different generations.\n\nThis might just be my normal density, but if you have the muon and\nelectron be excitations on two different branes, then aren\'t you going to\nhave a different "photon" for each of them? Since, as far as I\nunderstand, photons are the lowest level of excitations of open strings on\na single brane, then having multiple non-stacked branes will give you a\nbunch of U(1)\'s won\'t it? That doesn\'t seem good to me, so what am I\nmissing?\n\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>On Sat, 24 Jul 2004, Lubos Motl wrote:
> On Thu, 22 Jul 2004, Creighton Hogg wrote:
>
> > Hi guys, this is another dumb question from me.
> > In string theory I know that interactions are described thru the the
> > worldsheets of the strings: scattering, splitting, merging. Also, I know
> > that in the real world we don't observe decays modes such as \mu -> e +
> > \gamma. This is assumed as a conservation of lepton family number. What
> > prevents this from happening in the context of string theory? It seems
> > like there's little room for an ad-hoc conservation law in string theory,
> > so how do you prevent this from happening?
<snip>
> String theory can give you various tools how such processes can be suppressed -
> e.g. sequestering. Imagine that the muon field and the electron field are
> excitations of two different branes, separated in one of the additional
> (hidden) dimensions. Then the geometric separation will simply suppress
> the interactions between the different generations.
This might just be my normal density, but if you have the muon and
electron be excitations on two different branes, then aren't you going to
have a different "photon" for each of them? Since, as far as I
understand, photons are the lowest level of excitations of open strings on
a single brane, then having multiple non-stacked branes will give you a
bunch of U(1)'s won't it? That doesn't seem good to me, so what am I
missing?
Thomas Dent
Jul27-04, 10:30 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Creighton Hogg <wchogg@hep.wisc.edu> wrote\n\n> > > (...) Also, I know\n> > > that in the real world we don\'t observe decays modes such as \\mu -> e +\n> > > \\gamma. This is assumed as a conservation of lepton family number. What\n> > > prevents this from happening in the context of string theory? It seems\n> > > like there\'s little room for an ad-hoc conservation law in string theory,\n> > > so how do you prevent this from happening?\n>\n> <snip>\n> > String theory can give you various tools how such processes can be suppressed -\n> > e.g. sequestering. Imagine that the muon field and the electron field are\n> > excitations of two different branes, separated in one of the additional\n> > (hidden) dimensions. Then the geometric separation will simply suppress\n> > the interactions between the different generations.\n>\n> This might just be my normal density\n\n1.2 g/cm^3?\n\n> but if you have the muon and\n> electron be excitations on two different branes, then aren\'t you going to\n> have a different "photon" for each of them? Since, as far as I\n> understand, photons are the lowest level of excitations of open strings on\n> a single brane, then having multiple non-stacked branes will give you a\n> bunch of U(1)\'s won\'t it? That doesn\'t seem good to me, so what am I\n> missing?\n\nThere are intersecting branes in such a picture. Imagine several\n\'lepton-number branes\' along one dimension and a \'hypercharge brane\'\nalong another, then they intersect at points where the e, mu, etc.\nlive.\n\n/ /\n---/---/--- U(1)\ne mu\n\nsomething like that.\n\nOf course, there will be Kaluza-Klein modes of the U(1) gauge field\nthat mediate lepton flavour violation, also it\'s not possible to gauge\nall lepton numbers as might be implied by giving each lepton a brane\n(actually in most models the e, mu and tau all live on the same L\nbrane that wraps multiple times round a T^2), and there is other stuff\nlike string instantons that mediate interactions between distant\nbranes, but they can be suppressed by high scales (K-K) and large\nvolume (instanton) respectively. There is also a funny thing where\nanomalous U(1)\'s like L can exist and their gauge bosons get massive\nby coupling to axions but the symmetry remains as an unbroken global\nsymmetry (unlike in the heterotic case).\n\nAnyway, these intersecting brane models are a few years old now. Of\ncourse, any observation of mu -> e gamma would be a big boost for BSM\nstuff like this.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Creighton Hogg <wchogg@hep.wisc.edu> wrote
> > > (...) Also, I know
> > > that in the real world we don't observe decays modes such as \mu -> e +
> > > \gamma. This is assumed as a conservation of lepton family number. What
> > > prevents this from happening in the context of string theory? It seems
> > > like there's little room for an ad-hoc conservation law in string theory,
> > > so how do you prevent this from happening?
>
> <snip>
> > String theory can give you various tools how such processes can be suppressed -
> > e.g. sequestering. Imagine that the muon field and the electron field are
> > excitations of two different branes, separated in one of the additional
> > (hidden) dimensions. Then the geometric separation will simply suppress
> > the interactions between the different generations.
>
> This might just be my normal density
1.2 g/cm^3?
> but if you have the muon and
> electron be excitations on two different branes, then aren't you going to
> have a different "photon" for each of them? Since, as far as I
> understand, photons are the lowest level of excitations of open strings on
> a single brane, then having multiple non-stacked branes will give you a
> bunch of U(1)'s won't it? That doesn't seem good to me, so what am I
> missing?
There are intersecting branes in such a picture. Imagine several
'lepton-number branes' along one dimension and a 'hypercharge brane'
along another, then they intersect at points where the e, \mu, etc.
live.
/ /
---/---/--- U(1)
e \mu
something like that.
Of course, there will be Kaluza-Klein modes of the U(1) gauge field
that mediate lepton flavour violation, also it's not possible to gauge
all lepton numbers as might be implied by giving each lepton a brane
(actually in most models the e, \mu and \tau all live on the same L
brane that wraps multiple times round a T^2), and there is other stuff
like string instantons that mediate interactions between distant
branes, but they can be suppressed by high scales (K-K) and large
volume (instanton) respectively. There is also a funny thing where
anomalous U(1)'s like L can exist and their gauge bosons get massive
by coupling to axions but the symmetry remains as an unbroken global
symmetry (unlike in the heterotic case).
Anyway, these intersecting brane models are a few years old now. Of
course, any observation of \mu -> e \gamma would be a big boost for BSM
stuff like this.
Creighton Hogg
Jul27-04, 12:13 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>On Tue, 27 Jul 2004, Thomas Dent wrote:\n\n> Creighton Hogg <wchogg@hep.wisc.edu> wrote\n> >\n> > This might just be my normal density\n>\n> 1.2 g/cm^3?\n\nOnly on good days. :-)\n\n> > but if you have the muon and\n> > electron be excitations on two different branes, then aren\'t you going to\n> > have a different "photon" for each of them?\n\n> There are intersecting branes in such a picture. Imagine several\n> \'lepton-number branes\' along one dimension and a \'hypercharge brane\'\n> along another, then they intersect at points where the e, mu, etc.\n> live.\n>\n> / /\n> ---/---/--- U(1)\n> e mu\n\nOkay, this makes more sense now. So the single brane intersecting both\nthe e brane and the mu brane is the U(1)_Y, and U(1)s on the e-brane and\nmu-brane are related to lepton family number?\n\n> Of course, there will be Kaluza-Klein modes of the U(1) gauge field\n> that mediate lepton flavour violation, also it\'s not possible to gauge\n> all lepton numbers as might be implied by giving each lepton a brane\n> (actually in most models the e, mu and tau all live on the same L\n> brane that wraps multiple times round a T^2)\n\nWait, so in models with the charged leptons on the same brane (whose\ncompactified directions intersect with U(1)_Y right?), the e would\ncorrespond to 0 winding number, mu one wrap around, and the tau two wrap\narounds? I guess what I\'m asking is if winding number is the quantum\nnumber used to prevent flavor changing?\nOr am I completely missing the way this works?\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>On Tue, 27 Jul 2004, Thomas Dent wrote:
> Creighton Hogg <wchogg@hep.wisc.edu> wrote
> >
> > This might just be my normal density
>
> 1.2 g/cm^3?
Only on good days. :-)
> > but if you have the muon and
> > electron be excitations on two different branes, then aren't you going to
> > have a different "photon" for each of them?
> There are intersecting branes in such a picture. Imagine several
> 'lepton-number branes' along one dimension and a 'hypercharge brane'
> along another, then they intersect at points where the e, \mu, etc.
> live.
>
> / /
> ---/---/--- U(1)
> e \mu
Okay, this makes more sense now. So the single brane intersecting both
the e brane and the \mu brane is the U(1)_Y, and U(1)s on the e-brane and
\mu-brane are related to lepton family number?
> Of course, there will be Kaluza-Klein modes of the U(1) gauge field
> that mediate lepton flavour violation, also it's not possible to gauge
> all lepton numbers as might be implied by giving each lepton a brane
> (actually in most models the e, \mu and \tau all live on the same L
> brane that wraps multiple times round a T^2)
Wait, so in models with the charged leptons on the same brane (whose
compactified directions intersect with U(1)_Y right?), the e would
correspond to winding number, \mu one wrap around, and the \tau two wrap
arounds? I guess what I'm asking is if winding number is the quantum
number used to prevent flavor changing?
Or am I completely missing the way this works?
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