# String theorists, will the LHC see SUSY and if not, will this be a huge blow?

## string theorists, will the LHC see SUSY and if not, will this be a huge blow?

• ### I'm a string theorist, the LHC will not see SUSY but string theory research should continue on

• Total voters
6
ensabah6
string theorists, will the LHC see SUSY and if not, will this be a huge blow?

http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/03/25/scibigbang125.xml

NIMA ARKANI-HAMED “My hunch is that there’s a better than evens chance that supersymmetry will show up at the LHC”

EVA SILVERSTEIN “I’d be extremely puzzled if they don’t find the Higgs, but wouldn’t be devastated if they didn’t come up with evidence for supersymmetry.

‘Some of my intuition comes from string theory, an appealing candidate for a theory of all the forces of nature. According to many - perhaps most - versions of string theory, supersymmetry does not hold good at the energies probed by the LHC, so its discovery might require further explanation from this point of view.”

What are the ramifications to string theory if LHC does not find SUSY

g33kski11z
I'm not a "string theorist" .. or even a physicist for that matter, but I do, however, hope that the LHC will discover great things.. maybe not the Higgs particle or {provide evidence} for SUSY. I think it would be just as exciting {if not more so} for them to discover something that they didn't expect.

{by the way, I didn't vote}

Riogho
The problem with supersymmetry is that it has 125 adjustable parameters. And even if the LHC doesn't discover the superpartners, they can just be modified and say that they are just out of reach of the LHC. Same with string theories.

BenTheMan
The problem with supersymmetry is that it has 125 adjustable parameters.

Well...not really. It is true that the soft susy breaking lagrangian has a large number of free parameters, but these can be economized by only a few parameters at a large scale. These few parameters, for example, then evolve down to the TeV scale. So you are right, IF you don't want to think about WHERE those parameters come from.

string theorists, will the LHC see SUSY and if not, will this be a huge blow?

What WILL the LHC see, then? Large extra dimensions? Universal extra dimensions?

I think any of these will be MORE evidence for string theory.

If the LHC only sees a single higgs, my feeling is that high energy theory (be it strings, loops, or whatever) will be more or less over. We'll all have to get jobs in finance or spend our days trying to figure out why the cosmological constant is small.

humanino
If the LHC only sees a single higgs, my feeling is that high energy theory (be it strings, loops, or whatever) will be more or less over. We'll all have to get jobs in finance or spend our days trying to figure out why the cosmological constant is small.
I shared more or less the same feeling, and was talking about it to a colleague earlier this week. His opinion is that, no matter what comes out of the LHC, it will be presented as a success. He backed up his claims mentioning several other large facilities which never brought the physics they were built for, and are indeed presented as successes. History having a tendency to repeat itself...

In addition, there is still quite some work to be done in the non-perturbative regime of QCD for what matters. It is quite an interesting problematic : you know you have the right equations (QCD), but you cannot calculate much from it. You know you can expect lattice guys to provide more and more significant results, triggering themselves more understanding in the analytical models. Besides, medium-energy hadronic physics is backed up by nuclear power and weapons money, making it almost impossible to shut down. So, high-energy theorists are more than welcome to come help medium-energy theorists before loosing there soul in finance. And, you don't need to give up science to make money in finance BTW

BenTheMan
So, high-energy theorists are more than welcome to come help medium-energy theorists before loosing there soul in finance.

Sure we could all do top quark spectroscopy.

I think you're right that it will be PRESENTED as a success, but it's hard to motivate the NEXT experiment if we only find a single higgs. Basically, we'll lose all ability to make a prediction about larger energy scales, because it will mean that the SM is more or less completely decoupled from higher energy physics, and that Nature is finely tuned. If that's the case, then we have lost all constraints on model building, except fitting the low energy data.

Theorists will be the first to tell you that it is experiment that drives theory, and with no new experiments, it's hard to imagine that there can be any theory.

Well there will still be the little problem of having a suitable darkmatter particle, since the astrophysics is now pretty much under control. We sort of know we need one somewhere not too far from what we've already measured on general grounds hence just a scalar Higgs would imply something fishy going on.

BenTheMan
We sort of know we need one somewhere not too far from what we've already measured on general grounds hence just a scalar Higgs would imply something fishy going on.

Yeah, but our best hope is the direct detection experiments. They're not going to build the ILC in the hope that we turn something up. And if the hierarchy JUST IS a hierarchy (i.e. it's tuned), there's no reason that we could rule out some axion dark matter or something living in some impossible to probe corner of parameter space that they can't get to.

I mean, the only hope that we WILL have is neutrino masses. But the see saw scale is typically too big to probe directly. Also, if you're willing to live with a finely tuned higgs mass, why not a finely tuned Yukawa coupling for the neutrino?

humanino
Sure we could all do top quark spectroscopy.
That is not what I was referring to, that is perturbative and (sorry) boring.

In the (improbable) case LHC only find one scalar Higgs, I bet you one bottle of the Champagne of your choice that ILC will still be built
We already have instances of big projects failures presented as major advances and accepted as such by deciding hierarchies. Smart people always find a way to pretend their work is important. In this respect, it seems history serves nothing, or maybe it is too young to be called "history".

It might also as well be that our current understanding needs major re-fondations.

I agree that the ILC will go through regardless, but it will probably be delayed and altered a good twenty years if the LHC finds nothing.

Of course I tend to agree with Ben that it wouldn't make much sense. Most of the parameter space is covered by the LHC with enough statistics. The possibility that the LHC finds no evidence for say SuSY and then the ILC reverses that is smallish. Moreover with the hierarchy problem in full blast, its not clear how you even proceed to continue modeling. Traditional beyond the SM phenomology would be dead in favor of the more theoretical type, or at least requiring a much higher degree of astrophysics input than currently is the norm (and its already pretty packed)

BenTheMan
Traditional beyond the SM phenomology would be dead in favor of the more theoretical type, or at least requiring a much higher degree of astrophysics input than currently is the norm (and its already pretty packed)

I dunno... Right now pretty much the only guideline we have is naturalness. So if that dies, what's left? I don't think there can be ANY model building if you toss naturalness.

As for bets on the ILC, I'll take em all. You two are the first people who I've heard say that the ILC will be built regardless. Somehow the US decided we needed ant farms in space instead of a particle smasher in Texas---do you really think poiticians will understand the need for an ILC if the LHC finds nothing?

But I'm not worried. It would be nice if some of the FCNC/proton decay signatures started showing up, but I still think SUSY is the only way to go---at least, that's where my money is. The rest of the models just aren't that compelling. What would be REALLY nice is for someone to find a way to fix the minimal SUSY SU(5) model in a nice way, but that's just a dream :)

bilha nissenson
I am not sure I should be in this thread, I studied physics many years ago, quit my PhD, and went into high tech, lately I got interested in Physics again, I was surprised by how little things changed in 25 years. While technology changed inside out, I (I was mainly in image processing) it seemed to me that nothing really new happened in the understanding of foundation of physics.
So perhaps now that string theory is on the way out, people will have time to go back and really think what they are talking about when they speak about elementary particles.

BenTheMan
it seemed to me that nothing really new happened in the understanding of foundation of physics.

D Branes?
Black holes and information loss?

I'm late, otherwise I could rattle off about 30 things that string theory has taught us about Nature singe 1983...

So perhaps now that string theory is on the way out, people will have time to go back and really think what they are talking about when they speak about elementary particles.

On the way out?

Hardly.

Only if you listen to marcus, who seems to only be interested in Smolins Amazon.com sales figures.

jal
The world is changing
Let us say that the results from CERN validate an extended Standard Model.
Okay!
So what!
There is a lot more going on than finding another symbol to put into a theoretical formula.
What CERN will be doing is “making” a QGP (quark-gluon plasma ) ball.
Now!…. That is interesting …and promissing

I didn't want to boooooor you so I made a blog entry.
https://www.physicsforums.com/blogs/jal-58039/cern-and-fusion-power-1428/ [Broken]
CERN and fusion power
--------
jal

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humanino
lately I got interested in Physics again, I was surprised by how little things changed in 25 years. While technology changed inside out, I (I was mainly in image processing) it seemed to me that nothing really new happened in the understanding of foundation of physics.
If you are referring to major discoveries confirmed by observation and making it down the re-writing of textbooks, then you are right to say we did not have that during the last 25 years, but this is no suprise. It takes slightly more than a fourth of century to bypass Einstein or Bohr & Co.

But at the same time, I myself keep being impressed how fast paradigms change over. I have been interested in physics only since the early 90s, and the landscape (no irony) in fundamental physics has evolved constantly. In the line of what BenTheMan was mentionning, you had the so-called first and second string revolutions, the developpement of Loop Quantum Gravity, hints of links between the standard model and non-commutative geometry... Cosmology has made it from uncertainties in the exponent to precision measurements. We have a list of new outstanding problems to resolve...

I don't have somuch time myself right now to make a decent contribution to such a broad question. Maybe you can read Review of Modern Physics volume 71 special issue 1999 "Historic perspective on modern physics" ?

ccdantas
I'm late, otherwise I could rattle off about 30 things that string theory has taught us about Nature singe 1983...

Many developments, yes, but what makes you certain that they have anything to do with Nature?

Before someone thinks that I'm trying to flame this discussion, I must say that this is a genuine question of mine. If someone said that LQG has taught us # things about Nature, I would ask the same.

BenTheMan
Many developments, yes, but what makes you certain that they have anything to do with Nature?

This is definitely an honest question:) I guess I should have tempered my response a bit. The other poster seemed to think that string theory has been stagnant since 1983, which is certianly not the case. Since then, we've had three revolutions. 1985, 1995 and 1998---Green-Schwarz, Witten-Polchinski, and Maldacena, respectively. The notion that string theor is dying is a myth perpetrated by some who post regularly here, and some who publish derogatory books towards our field.

jal
Will the LHC see SUSY?
No!
They will see
http://aliceinfo.cern.ch/static/HIF/Talks/Gelis0606.pdf
Color Glass Condensate and forward physics at the LHC
Franc¸ois Gelis
CEA / DSM / SPhT
-------
This is just another way of saying a Quark-Gluon Plasma ball (QGP)
jal

Gold Member
Sure we could all do top quark spectroscopy.
Except that toponium does not exit.

Gold Member
This is definitely an honest question:) I guess I should have tempered my response a bit. The other poster seemed to think that string theory has been stagnant since 1983, which is certianly not the case. Since then, we've had three revolutions. 1985, 1995 and 1998---Green-Schwarz, Witten-Polchinski, and Maldacena, respectively.

Well, Maldacena is more evolution than revolution. Or both; it was a revolution when it happened, and it keeps returning to the top all the years, specially after the (very slight) fading of large dimensions.

All these stats from spires top cites are useful to understand how the topic evolves. For instance in the last four years you can see Maldacena attacting more work than Randall-Sundrum, Antoniadis paper losing some interest, and a pair of papers, of Gubser and Witten, regaining positions each year. Also, you can see that the total number of citations to the top five theoretical papers does not decrease, marking constant work.

If you look in the full range of arxived statistics, you can see that the top cites of the field are really stable since the Maldacenian age. They lost some attention when other topic reclaims work hours: flux compactifications, m-theory, matrix models, randall-sundrum... but then when the focus on this topic dissappears, they escalate positions again.

Coin
I would like to ask a slightly different version of the OP question:

1. If the LHC does not find SUSY-- do people continue to expect SUSY is true? Of course we all know SUSY could still be true even without evidence of such from the LHC, but I'm asking here more from a social standpoint-- would people continue to take SUSY seriously if the LHC does not support it?

2. Let us say that for one reason or another the LHC results lead people to rule out SUSY as realistic. How does String Theory research continue on past this point? Will changes to the String Theory program be necessary for it to survive in a post-SUSY era?

Micha
Honest discussion. Interesting to read. But I think, a little overly pessimistic.

Even if the LHC finds a SM Higgs only, there is still a job to do for theoretical physicists:
a unification of GR and QFT.

In the past progress in physics has come from successful unifications of existing
physical theories, which themselves were backed up by empirical data.
Since a lot of emperical knowledge is encoded in the two theories GR and QFT and since we all believe that nature can not manifestly be built from two incompatible frameworks, there should be one consistent theory which we can find, which has these two theories as a limit. Looking for this theory, that is the point here, is not a search in the wild, but a search backed up by empirical data, which is encoded in the mathematical structure of the two theories.

Some (not me) might argue, that string theory is this unified theory. Well, if this is true, then the task is, bring string theory under control to make predictions.

To be honest, I stole this view from Rovelli (in my own words, so I screwed it up
probably).

It is very nice to listen:
http://www.fuw.edu.pl/~kostecki/school.html

If SuSY is not found at the LHC, it will still be utilized by theorists in wide contexts. Pretty much like every good idea from particle physics, that may or may not be part of the real world (Peccei Quin symmetry, seesaw mechanism, etc)

It just won't be seen as something necessarily physical and more a toy model that has improved analytic properties.

BenTheMan
Even if the LHC finds a SM Higgs only, there is still a job to do for theoretical physicists:
a unification of GR and QFT.

With what data? Beter values of Lambda? A higgs mass to three decimal places?

The point is that people make very strong arguments based on naturalness---in fact, this is one of the main reasons that people hate string theory. String theorists are willing to accept some UN-naturalness if the end result is natural (i.e. fine tuning the cosmological constant is ok if you can derive the Standard Model). Also, string theorists are willing to TUNE parameters that their theory should explain---this is called moduli fixing. (I know because I'm writing a paper on it now, and hopefully finishing the damned thing soon.) This is one thing that the non-string QG community HATE---they want no tunings. In my mind, this is the best argument AGAINST string theory, that it seems to require tons of tuning to get the SM out.

So, if we only find a single higgs and nothing else, you MUST accept that the weak scale is fine tuned. Period. SUSY stabilizes the hierarchy, as do half a dozen other theories---so we THINK we can explain things. If you are willing to accept that the weak scale is fine tuned, though, then there is no reason to apply naturalness arguments any more. And this will definitely be the case if we only find a single higgs.

So finding just the higgs will be bad for everybody---experimentalists, theorists, model builders, lab managers, the guy who mops the floors at CERN---everybody. There will be no more data---the higgs mass will be measured accurately over 20 years of LHC and (hopefully) vLHC, and that will be that. We won't be able to distinguish models except on the basis that they describe the measurements that we have made, and if one is willing to accept tunings, then you can ALWAYS introduce parameters in your Lagrangian to make things work.

So unifying GR and QFT turns into kind of a moot point until some government decides to take a VERY risky gamble on a new particle accelerator. And with looming oil shortages, and energy and food prices soaring, no government would risk the billions of dollars needed to build a new collider.

So it's not just string theorists who should be hoping to find something interesting other than just a bunch of top quarks and a few higgses at LHC. The coffee shops in Geneva should be saying their prayers, too!

BenTheMan
2. Let us say that for one reason or another the LHC results lead people to rule out SUSY as realistic. How does String Theory research continue on past this point? Will changes to the String Theory program be necessary for it to survive in a post-SUSY era?

I think it's an interesting point that nobody builds models that don't have low energy SUSY. Well, let me say that very few people build models that don't have low energy SUSY---I've never read a paper that describes a non-supersymmetric compactification, but that doesn't mean they don't exist.

But in principle, it is possible to do. I think a generic stack of D branes in some Type IIA background breaks all of the SUSYs (correct me Haelfix or anybody if I'm wrong). Also, you can take a torus and mode out by some discrete symmetries until you kill all of the SUSYs (i.e. project out all of the gravitinos), then compactify a heterotic string on it.

I think if we don't see SUSY very soon, then string model building will die for about a month, and then there will be a HUGE flux of string models without low energy SUSY. And I'll be close behind, I hope :) The point is, not seeing SUSY won't necessarily harm string theory---it just means that its broken at a higher scale.

Coin
The point is, not seeing SUSY won't necessarily harm string theory---it just means that its broken at a higher scale.

Hm, okay. So would you say then that string models without SUSY are just not in the cards, period?

Micha
With what data? Beter values of Lambda? A higgs mass to three decimal places?

You are probably right, as far as experimental high energy physics is concerned. Also I am not saying, I particularly would like this outcome.

I was talking about theoretical physics.

As an example:
What data did Einstein need to develop GR?
Zero. He unified Newtons gravity with SR.

What data did he have to develop SR?
Zero. (Arguable, ok)
He in fact noticed that the transformation properties of classical mechanics
and Maxwells theory were inconsistent.

What data did Newton use to develop classical mechanics?
Zero. He unified Keppler's theory of the planitary motion with Galilei's theory
of falling bodies.

Staff Emeritus
Gold Member
Hm, okay. So would you say then that string models without SUSY are just not in the cards, period?

From Becker, Becker, and Schwarz:

"The third general feature of string theory is that its consistency requires supersymmetry, which is a symmetry that relates bosons to fermions. There exist nonsupersymmetric bosonic string theories (discussed in Chapters 2 and 3), but lacking fermions, they are completely unrealistic. The mathematical consistency of string theories with fermions depends crucially on local supersymmetry. Supersymmetry is a generic feature of all potentially realistic string theories. The fact that this symmetry has not yet been discovered is am indication that the characteristic energy scale of supersymmetry breaking and the masses of supersymmetry partners of known particles are above the experimentally determined lower bounds."

BenTheMan
You are probably right, as far as experimental high energy physics is concerned. Also I am not saying, I particularly would like this outcome.

I was talking about theoretical physics.

As an example:
What data did Einstein need to develop GR?
Zero. He unified Newtons gravity with SR.

What data did he have to develop SR?
Zero. (Arguable, ok)
He in fact noticed that the transformation properties of classical mechanics
and Maxwells theory were inconsistent.

What data did Newton use to develop classical mechanics?
Zero. He unified Keppler's theory of the planitary motion with Galilei's theory
of falling bodies.

Ahh...but who would have believed GR if it weren't for the experimental predictions. Almost immediately, Eddington tested GR's predictions that light follows geodesics, and the perhelion of mercury was derived.

Writing something down isn't the problem. Getting everybody to believe it's correct is always the issue.

From Becker, Becker, and Schwarz:

"The third general feature of string theory is that its consistency requires supersymmetry, which is a symmetry that relates bosons to fermions. There exist nonsupersymmetric bosonic string theories (discussed in Chapters 2 and 3), but lacking fermions, they are completely unrealistic. The mathematical consistency of string theories with fermions depends crucially on local supersymmetry. Supersymmetry is a generic feature of all potentially realistic string theories. The fact that this symmetry has not yet been discovered is am indication that the characteristic energy scale of supersymmetry breaking and the masses of supersymmetry partners of known particles are above the experimentally determined lower bounds."

This string scale supersymmetry, which we could never test at an accelerator. If SUSY is broken at a very high scale, then there is no way we'd know about it.

Theres nothing preventing SuSy from being broken at higher scales, other than naturalness and minimalism. It so happens (maybe a historical accident) that its a good model for stabilizing the electroweak scale (the original motivation).

If naturalness goes out the window, people will need to figure out better constraints from somewhere else (probably astrophysics/cosmology), unfortunately it is unlikely we will see something as strong and scale independant in our lifetime absent some unforseen theoretical breakthrough.

So like I said, high energy physics in the LHC disaster scenario will become the poorly funded domain of theorists. The ILC will be built b/c there is still some residual political will (a remant imo of the cold war) to do such a thing, but its unlikely another will be built after that unless there really is a compelling breakthrough.

Homework Helper
Gold Member
So like I said, high energy physics in the LHC disaster scenario will become the poorly funded domain of theorists. The ILC will be built b/c there is still some residual political will (a remant imo of the cold war) to do such a thing, but its unlikely another will be built after that unless there really is a compelling breakthrough.

I am surprised by this. What countries do you feel stil have this will? After the cancellation of the SSC (which was planned to be even more powerful than the LHC and therefore seemed like a good bet for some form of new physics or at least dicovery of the Higgs) it seemed to me that the US will not be a major source of funding for any future large accelerator. I am not sure that even a consortium of countries will have the will and means to invest billions and billions of dollars. But that's just an impression I have and I may be completely off target.

BenTheMan
The ILC will be built b/c there is still some residual political will (a remant imo of the cold war) to do such a thing, but its unlikely another will be built after that unless there really is a compelling breakthrough.

Seriously? You're talking about the same Democrats that just CUT funding for high energy physics?

After the cancellation of the SSC (which was planned to be even more powerful than the LHC and therefore seemed like a good bet for some form of new physics or at least dicovery of the Higgs) it seemed to me that the US will not be a major source of funding for any future large accelerator.

I think that the best that we can hope for is something like SUSY at the LHC. Honestly---there will then be a strong case to make for the ILC, which is a precision experiment, not an energy experiment. If this can be couched in a way that US politicians (thick skulled as they can be) can actually COMMIT to funding, then we MAY get an ILC in 30 years.

Either that, or we'll put more ant farms in space.

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jal
I think that the best that we can hope for is something like SUSY at the LHC.
http://arxiv.org/abs/0707.2923v1
The Odderon at RHIC and LHC
Authors: Basarab Nicolescu
(Submitted on 19 Jul 2007)
Abstract: The Odderon remains an elusive object, 33 years after its invention. The Odderon is now a fundamental object in QCD and CGC and it has to be found experimentally if QCD and CGC (Color Glass Condensate) are right. In the present talk, we show how to find it at RHIC and LHC. The most spectacular signature of the Odderon is the predicted difference between the differential cross-sections for proton-proton and antiproton-proton at high s and moderate t. This experiment can be done by using the STAR detector at RHIC and by combining these future data with the already present UA4/2 data. The Odderon could also be found by ATLAS experiment at LHC by performing a high-precision measurement of the real part of the hadron elastic scattering amplitude at small t.
=====
QGP (quark-gluon plasma ) Is still a phase that needs experimentation if we ever want to be able to tap into this kind of energy.

BenTheMan
QGP (quark-gluon plasma ) Is still a phase that needs experimentation if we ever want to be able to tap into this kind of energy.

I don't know what kind of energy'' there is to tap into.