Arkani-Hamed & Dimopoulos on high energy SUSY [Archive]

Urs Schreiber

May28-04, 02:01 PM

Lubos Motl

May28-04, 02:39 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 Fri, 28 May 2004, Urs Schreiber wrote:\n\n> As you have all seen, in\n> N. Arkani-Hamed & S. Dimopoulos:\n> hep-th/0405159\n\nThis paper has excited many of us, and even though it is very hard to\nestimate how likely their scenario is, I like the way how Nima and Savas\ndivided the features of the MSSM to successes and failures, and simply\nconstructed a model that preserves almost everything they call a success,\nand eliminates almost everything they call a failure - which is possible\nsimply because they sacrificed the assumption of naturalness, which, as\nthey claim, has already been sacrificed anyway because of the cosmological\nconstant.\n\nThis direct and bold approach to kill problems and preserves victories is\nan "Einsteinian" solution that has succeeded many times in history of\nphysics.\n\n> Now I am afraid that I have a somewhat heretic question, which will\n> probably demonstrate the vastness of my ignorance, but anyway: Are these\n> two potential advantages all that keeps people from considering models\n> with _no_ susy in 4 dimensions?\n\nFirst of all, I don\'t think that the whole phenomenology is supersymmetric\nphenomenology. In fact, I believe that most papers in phenomenology\nconsider non-supersymmetric models - or models that can (but don\'t need\nto) be supersymmetrized. In string theory, the approach is slightly\ndifferent because the supersymmetric (and highly supersymmetric)\nbackgrounds are the natural starting points, and reducing SUSY is the\nnon-trivial next step. But in fact, my impression is that today, too many\nstring theorists are studying various non-supersymmetric models, too. ;-)\n\nSUSY has many potential successes and many potential failures. Each of us\ngives them some weight, and the sum of these weights determines the total\nopinion of anyone whether she believes in SUSY and/or finds it a good\nidea.\n\nLet me start with the advantages.\n\n1.\nFirst advantage is that supersymmetry is a nice symmetry, the only\npossible extension of Poincare symmetry that does not commute with it, and\nit is very natural from the string theory viewpoint, and string theory is\nthe only known deeper theory underlying QFTs in particle physics.\nObviously, this argument is important for those who find string theory\nimportant, and less important for others. This argument does not\nnecessarily imply that SUSY must survive to low energies. Nima and Savas\nare applied string theorists, so they count this argument #1 as moderately\nrelevant.\n\n2.\nLow energy SUSY is still the most canonical framework to explain why the\nelectroweak scale is much smaller than the GUT or Planck scale. Nima and\nSavas sacrifice this ability of SUSY because they take high energy SUSY,\nand they argue that things can be unnatural anyway as seen in the case of\nthe cosmological constant. If we allow an unnatural value for the C.C.,\nwhy should not we allow a slightly less tiny, but still unnatural Higgs\nmass? Incidentally, I fully sympathize with this argument, that puts the\nC.C. and other parameters on more equal footing, even though I still\nprefer to believe in the existence of a scientific explanation of *both*\nof these small numbers (and others). ;-)\n\n3.\nSupersymmetry added to the Standard Model modifies the running of the\ncoupling constants so that they unify much better than in the case of the\nnon-supersymmetric Standard Model. By preserving a similarity of their\nspectrum with the MSSM, they save this advantage of SUSY.\n\n4.\nSupersymmetry offers a good dark matter candidate - the lightest\nsuperpartner (LSP). Nima and Savas continue to have it. A special\nmechanism must be used to keep the gauginos light - the light gauginos are\nalso necessary for the gauge coupling unification. The fermions\nessentially come in full SU(5) multiplets, and therefore they don\'t\ndestroy the unification. The gauginos don\'t come in full SU(5) multiplets\n- because the new SU(5) gauginos are heavy and don\'t contribute to loops -\nand therefore Nima and Savas must keep light gauginos. They also sacrifice\nthe second Higgs doublet, but they argue that numerically the scalars\ncontribute very little anyway (2/3 vs. 11/3 etc.).\n\n---------------------------\n\nOn the other hand, SUSY has many potential disadvantages - the possible\nappearance of dimension 5 operators violating the baryon number -\noperators that would typically cause a speedy proton decay that is\nincompatible with observations. The Standard Model only has dimension 6\nB-violating operators. Because the superpartners have not yet been seen,\nthe SUSY breaking scale is slightly higher than what would be most natural\nfor SUSY to fully solve the hierarchy (light Higgs) problem; there is\nsome not-too-fine tuning (1%) necessary anyway.\n\nIn the most natural model with the obvious values of coupling constants,\nSUSY would introduce many new interactions that are known to be very\nsmall, and some of them are guaranteed by the non-supersymmetric Standard\nModel to be very small (or zero) - flavor changing neutral currents, CP\nviolating phases, and so on. The SUSY Standard Model is therefore less\nnatural or worse in this sense, which might be troubling and is a\npotential circumstantial evidence against SUSY - at least against low\nenergy SUSY. By using high energy SUSY breaking scale, Nima and Savas\nwould be able to suppress at least some of these unwanted numbers.\n\n> I am asking because I seem to recall having been told that in\n> particular in intersecting brane models it is easy to have neither susy\n> nor for instance gauge coupling unification.\n\nSure. Gauge coupling unification is something that you don\'t expect if you\ndon\'t unify the groups - and the intersecting brane models can have the\nStandard Model without GUT (without unification). Nevertheless, the\nunification is something nice. A simple gauge group - such as SO(10) -\nwith one coupling constant is more economical, predictive, and beautiful,\nthan the Standard Model with 3 different groups and 3 couplings. You seem\nto be suggesting that it is a goal or a virtue not to have any\nunification. It is certainly not a virtue! From a purely scientific\nperspective, unification means that you explain at least one more coupling\nconstant. It\'s OK not to have unification, but you are then losing a part\nof a nice story, and it can only be an attractive solution if your model\nhas something else to offer (well, for example, if it is the correct model\nthat allows you to find all fermionic masses haha). And as we said,\n(high-precision) unification requires SUSY or something very similar;\nSUSY and GUT collaborate.\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\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 Fri, 28 May 2004, Urs Schreiber wrote:

> As you have all seen, in
> N. Arkani-Hamed & S. Dimopoulos:
> http://www.arxiv.org/abs/hep-th/0405159

This paper has excited many of us, and even though it is very hard to
estimate how likely their scenario is, I like the way how Nima and Savas
divided the features of the MSSM to successes and failures, and simply
constructed a model that preserves almost everything they call a success,
and eliminates almost everything they call a failure - which is possible
simply because they sacrificed the assumption of naturalness, which, as
they claim, has already been sacrificed anyway because of the cosmological
constant.

This direct and bold approach to kill problems and preserves victories is
an "Einsteinian" solution that has succeeded many times in history of
physics.

> Now I am afraid that I have a somewhat heretic question, which will
> probably demonstrate the vastness of my ignorance, but anyway: Are these
> two potential advantages all that keeps people from considering models
> with _no_ susy in 4 dimensions?

First of all, I don't think that the whole phenomenology is supersymmetric
phenomenology. In fact, I believe that most papers in phenomenology
consider non-supersymmetric models - or models that can (but don't need
to) be supersymmetrized. In string theory, the approach is slightly
different because the supersymmetric (and highly supersymmetric)
backgrounds are the natural starting points, and reducing SUSY is the
non-trivial next step. But in fact, my impression is that today, too many
string theorists are studying various non-supersymmetric models, too. ;-)

SUSY has many potential successes and many potential failures. Each of us
gives them some weight, and the sum of these weights determines the total
opinion of anyone whether she believes in SUSY and/or finds it a good
idea.

Let me start with the advantages.

1.
First advantage is that supersymmetry is a nice symmetry, the only
possible extension of Poincare symmetry that does not commute with it, and
it is very natural from the string theory viewpoint, and string theory is
the only known deeper theory underlying QFTs in particle physics.
Obviously, this argument is important for those who find string theory
important, and less important for others. This argument does not
necessarily imply that SUSY must survive to low energies. Nima and Savas
are applied string theorists, so they count this argument #1 as moderately
relevant.

2.
Low energy SUSY is still the most canonical framework to explain why the
electroweak scale is much smaller than the GUT or Planck scale. Nima and
Savas sacrifice this ability of SUSY because they take high energy SUSY,
and they argue that things can be unnatural anyway as seen in the case of
the cosmological constant. If we allow an unnatural value for the C.C.,
why should not we allow a slightly less tiny, but still unnatural Higgs
mass? Incidentally, I fully sympathize with this argument, that puts the
C.C. and other parameters on more equal footing, even though I still
prefer to believe in the existence of a scientific explanation of *both*
of these small numbers (and others). ;-)

3.
Supersymmetry added to the Standard Model modifies the running of the
coupling constants so that they unify much better than in the case of the
non-supersymmetric Standard Model. By preserving a similarity of their
spectrum with the MSSM, they save this advantage of SUSY.

4.
Supersymmetry offers a good dark matter candidate - the lightest
superpartner (LSP). Nima and Savas continue to have it. A special
mechanism must be used to keep the gauginos light - the light gauginos are
also necessary for the gauge coupling unification. The fermions
essentially come in full SU(5) multiplets, and therefore they don't
destroy the unification. The gauginos don't come in full SU(5) multiplets
- because the new SU(5) gauginos are heavy and don't contribute to loops -
and therefore Nima and Savas must keep light gauginos. They also sacrifice
the second Higgs doublet, but they argue that numerically the scalars
contribute very little anyway (2/3 vs. 11/3 etc.).

---------------------------

On the other hand, SUSY has many potential disadvantages - the possible
appearance of dimension 5 operators violating the baryon number -
operators that would typically cause a speedy proton decay that is
incompatible with observations. The Standard Model only has dimension 6
B-violating operators. Because the superpartners have not yet been seen,
the SUSY breaking scale is slightly higher than what would be most natural
for SUSY to fully solve the hierarchy (light Higgs) problem; there is
some not-too-fine tuning (1%) necessary anyway.

In the most natural model with the obvious values of coupling constants,
SUSY would introduce many new interactions that are known to be very
small, and some of them are guaranteed by the non-supersymmetric Standard
Model to be very small (or zero) - flavor changing neutral currents, CP
violating phases, and so on. The SUSY Standard Model is therefore less
natural or worse in this sense, which might be troubling and is a
potential circumstantial evidence against SUSY - at least against low
energy SUSY. By using high energy SUSY breaking scale, Nima and Savas
would be able to suppress at least some of these unwanted numbers.

> I am asking because I seem to recall having been told that in
> particular in intersecting brane models it is easy to have neither susy
> nor for instance gauge coupling unification.

Sure. Gauge coupling unification is something that you don't expect if you
don't unify the groups - and the intersecting brane models can have the
Standard Model without GUT (without unification). Nevertheless, the
unification is something nice. A simple gauge group - such as SO(10) -
with one coupling constant is more economical, predictive, and beautiful,
than the Standard Model with 3 different groups and 3 couplings. You seem
to be suggesting that it is a goal or a virtue not to have any
unification. It is certainly not a virtue! From a purely scientific
perspective, unification means that you explain at least one more coupling
constant. It's OK not to have unification, but you are then losing a part
of a nice story, and it can only be an attractive solution if your model
has something else to offer (well, for example, if it is the correct model
that allows you to find all fermionic masses haha). And as we said,
(high-precision) unification requires SUSY or something very similar;
SUSY and GUT collaborate.
__{_______________________________________________ _____________________________}
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)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^