Why I Still Like String Theory

May 16, 2013

There is a new book coming up by Richard Dawid “String Theory and the Scientific Method. It has been reviewed by Peter Woit and Lubos Motl who give their expected opposing views. Apparently Woit gets it through a university library subscription. I can’t really review the book because at £60 it is a bit too expensive. Compare this with the recent book by Lee Smolin which I did review after paying £12.80 for it. These two books would have exactly the same set of potential readers but Smolin is just better known which puts his work into a different category where a different type of publisher accepts it. I dont really understand why any author would choose to allow publication at a £60 price-tag. They will sell very few copies and get very little back in royalties, especially if most universities have free access. Why not publish a print-on-demand version which would be cheaper? Even the Kindle version of this book is £42 but you can easily self publish on Kindle for much less and keep 70% of profits through Amazon.

My view is equally predictable as anyone elses since I have previously explained why I like String Theory. Of the four reasons I gave previously the main one is that it solves the problem of how quantum theory looks in the perturbative limit about a flat space-time with gravitons interacting with matter. This limit really should exist for any theory of quantum gravity and it is the realm that is most like familiar physics so it is very significant that string theory works there when no other theory does. OK, so perturbative string theory is not fully sewn up but it works better than anything else. The next best thing is supergravity which is just an effective theory for superstrings.

My second like is that String Theory supports a holographic principle that is also required for quantum gravity. This is a much weaker reason because (a) it is in less well known territory of physics and requires a longer series of assumptions and deductions to get there (b) It is not so obvious that other theories wont also support the holographic principle.

Reason number three has not fared so well. I said I liked string theory because it would match well with TeV scale SUSY, but the LHC has now all but ruled that out. It is possible that SUSY will appear in LHC run 2 at 13 TeV or later, or that it is just out of reach, but already we know that the Higgs mass in the standard model is fine-tuned. There is no stop or Higgsino where they would be needed to control the Higgs mass. The only question now is how much fine-tuning is there?

Which brings me to my fourth reason for liking string theory. It predicts a multiverse of vacua in the right quantities required to explain anthropic reasoning for an unnatural fine-tuned particle theory. So my last two reasons were really a hedge. The more evidence there is against SUSY, the more evidence there is in favour of the multiverse and the string theory landscape.

Although I dont have the book I know from Woit and Motl that Dawid provides three main reasons for supporting string theory that he gathered from string theorists. None of my four reasons are included. His first reason is “The No Alternatives Argument”, apparently we do string theory because despite its shortcomings there is nothing else that works. As Lee Smolin pointed out over at NEW, there are alternatives. LQG may succeed but to do so it must give a low energy perturbation theory with gravitons or explain why things work differently. Other alternatives mentioned by Smolin are more like toy models but I would add higher spin gravity as another idea that may be more interesting. Really though I dont see these as alternatives. The “alternatives theory view” is a social construct that came out of in-fighting between physicists. There is only one right theory of quantum gravity and if more than one idea seems to have good features without them meeting at a point where they can be shown to be irreconcilable then the best view is that they might all be telling us something important about the final answer. For those who have not seen it I still stand by my satirical video on this subject:

A Double Take on the String Wars

Dawid’s second reason is “The Unexpected Explanatory Coherence Argument.” This means that the maths of string theory works surprisingly well and matches physical requirements in places where it could easily have fallen down. It is a good argument but I would prefer to cite specific cases such as holography.

The third and final reason Dawid gives is  ”The Meta-Inductive Argument”. I think what he is pointing out here is that the standard model succeeded because it was based on consistency arguments such as renormalisability which reduced the possible models to just one basic idea that worked. The same is true for string theory so we are on firm ground. Again I think this is more of a meta-argument and I prefer to cite specific instances of consistency.

The biggest area of contention centres on the role of the multiverse. I see it as a positive reason to like string theory. Woit argues that it cannot be used to make predictions so it is unscientific which means string theory has failed. I think Motl is (like many string theorists) reluctant to accept the multiverse and prefers that the standard model will fall out of string theory in a unique way. I would also have preferred that 15 years ago but I think the evidence is increasingly favouring high levels of fine-tuning so the multiverse is a necessity. We have to accept what appears to be right, not what we prefer. I have been learning to love it.

I dont know how Dawid defines the scientific method. It goes back many centuries and has been refined in different ways by different philosophers. It is clear that if a theory is shown to be inconsistent, either because it has a logical fault or because it makes a prediciton that is wrong, then the theory has to be thrown out. What happens if a theory is eventually found to be uniquely consistent with all known observations but its characteristic predictions are all beyond technical means. Is that theory wrong or right? Mach said that the theory of atoms was wrong because we could never observe them. It turned out that we could observe them but what if we couldn’t for practical reasons? It seems to me that there are useful things a philosopher could say about such questions and to be fair to Dawid he has articles freely available on line that address this question, e.g. here, so even if the book is out-of-reach there is some useful material to look through. Unfortunately my head hits the desk whenever I read the words “structural realism”, my bad.

update: see also this video interview with Nima Arkani-Hamed for a view I can happily agree with

 https://www.youtube.com/watch?v=rKvflWg95hs

69 Comments | String Theory, Supersymmetry | Permalink
Posted by Philip Gibbs

Evidence for a charged Higgs Boson?

October 12, 2012

Last week Upsala was home to a specialised HEP workshop about the search for a charged Higgs bosons. Such particles are predicted in some beyond standard model theories such as supersysmmetry. There is not much direct evidence yet for such charged scalar bosons but the searches as described at the workshop have not looked beyond the 2011 data using 5/fb at most. There is still a lot of room left for them to appear.

The best hope for BSM observations in the data so far comes from anomalies in the Higgs decay rates. In particular the decay to two tauons has not been observed where expected and the rate for decay to two photons is too large. In my opinion the tau decay is not a very convincing discrepancy yet because the stats are low, especially because ATLAS has not yet done the analysis with 2012 data. The diphoton excess is also not fantastically convincing with a combined significance of about 2.2 sigma according to Joe Incandela (CMS spokesperson) but it has persisted since 2011 and is seen by both ATLAS and CMS. It is probably too big to be explained by theory errors from the analysis of the standard model so some BSM explanation is a real possibility. Both observations will be considerably clarified at the Hadron Collider Physics conference in Kyoto next month.

Meanwhile there is little to stop theorists thinking about what could account for such anomalies if they turn out to be real. This is not just idle speculation. Any theory that might explain the anomalies could make unique predictions for new physics that could prioritize the searches to help the collaborations home in on new physics more quickly. This is crucial to plan future accelerators.

The diphoton decay channel is especially sensitive to new physics because the basic Higgs boson is not charged. Photons only interact with charged particles so the Higgs can only decay to photons via loop diagrams that include massive charged particles. We know of several such particles in the standard model and the ones that contribute the most in this case are the W bosons and the top quark. If you know anything about the type of Feynman diagrams involved you will know that bosons and fermions in loops interfere deconstructively. In this case the W bosons have the larger amplitude and the top quark reduced it by about 40%. This means that to increase the decay rate and explain the tentative excess you would need to postulate the existence of (at least) a new heavy charged boson, such as a charged Higgs scalar. It has to be heavier than about 105 GeV otherwise it would have been observed at LEP, but upper limits depend on its properties.

As it happens there are phenomenologists who are too skilled at their job so that they can explain the excess in many other ways, e.g. using “vector-like” fermions or a fermiophobic Higgs or even just QCD corrections. I am simply going to be skeptical and suggest that they are thinking wishfully about their pet theories. To the unbiased mind the new charged boson is the most obvious explanation for an excess. That still leaves open the question of what spin ( and other properties) the boson has. A spin one charged boson would have to be very similar to a W gauge boson and would mediate new forces. The limits on such new particles is already good.  Higher spin would make it a charged graviton. Let’s not go there.

Another major parameter for a new particle to determine is its lepton number. If the particle had lepton number one (like a scalar lepton) then its R-parity would be odd. All standard model particles have even R-parity so if lepton number is conserved our mystery particle would either have to be stable or decay to another new stable particle. Heavy charged particles are easy to detect and lighter stable particles would be hard ot miss at the LHC. ATLAS and CMS were designed with missing energy searches in mind so that they could look effectively for supersymmetry. Indeed a scalar tau would be a good candidate except that SUSY searches have already gone a long way to exclude them.

So there are many possible explanations for the diphoton excess, if it is real physics, but the scalar charged boson with zero lepton number is the simplest case that still has a good chance of being around still. Any such scalar charged boson would immediately be identified as a likely charged Higgs if it was found.

Coming back to last weeks workshop, it is good to see that the charged Higgs as an explanation for the diphoton excess was indeed the subject of a talk. The speaker Stefano Moretti concentrated on the Higgs triplet model which has charged and doubly charged Higgs bosons. The doubly charged Higgs would be particularly effective in explaining the diphoton excess because doubling the charge quadruples the extra cross-section since there are two gamma vertices. Of course some next to minimal SUSY models have a similar feature. Here is the set of Feynman diagrams involved

With so many contributions all adding to the diphoton excess the charged Higgs can comfortably be heavier than limits set by direct searches so far. Soon we will get more information with a better determination of the excess and better charged Higgs searches. The 2012 data at 8 TeV will be much more penetrating than the 2011 data heavy new particles and by now we have three times as much of it. Of course this story could go in many directions from here. The diphoton excess may fade or be explained by better standard model calculations. It might even be some systematic error symptomatic of a less than perfect understanding of the detectors. If it does hold up there are lots of new physics possibilities, but if I had to put my chips down at this point I think the charged Higgs has the best odds all things considered.

51 Comments | Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

SUSY 2012

August 13, 2012

The SUSY 2012 conference starts in Beijing today. It is the biggest supersymmetry conference of the year and we expect to see the latest results using the 5/fb gathered in 2012 at 8 TeV before the last technical stop. Actually at least some of the results have already appeared with three new conference notes from ATLAS this morning here, here and here. CMS released their results earlier, see their twiki page .

Because of the high masses being searched for the extra TeV of energy over last year’s 7 TeV actually provides 2 to 3 rimes as much sensitivity, so even without combining the new results with the similar amount of data collected last year we get significantly better depth. Sadly there is nothing yet observed in these notes beyond standard model expectations. This is disappointing but there may be other searches released later and there are always places for SUSY to hide from the LHC.

The most promising anomaly at this time is the 1.8 times SM excess in the diphoton channel seen in the Higgs search which currently has 2.5 sigma significance BSM in ATLAS and 1.5 sigma in CMS. If the peaks coincided the combined significance would be about 2.8 sigma but they are at slightly different masses so the combined result is actually no better than ATLAS on its own. You could argue that this might be a callibration error and the 2.8 sigma is good. In any case there will be twice as much data available in a few weeks and we will see if the excess is a statistical fluctuation or not. Looking at the four individual results from the two experiments and last year vs this year they can be plotted on a mass vs signal scale roughly as follows

The green line is the standard model expectation, blue circles are CMS and red are ATLAS. Black is the unofficial combination. The results are comparable to throwing 4 dice and getting four sixes. Was it a fluke or were the dice loaded, and if so, how?

If the effect is not statistical it could easily be a combination of systematic errors. This would most likely be due to errors in the theoretical calculations that would affect both experiments. (TS pointed out this paper which fingers QCD uncertainties) Many people would suggest we wait for the dice to be rolled again and then look at systematics more carefully before taking this too seriously. However, by time that has happened the long shutdown will be on us. If there is a possibility for something to be seen here it makes sense to look at what it could be. Theorists might then make predictions that could be tested this year if triggers can be adjusted in time.

I am assuming that the excess in the diphoton channel is due to extra particles that affect the Higgs decay loop and that the production rate via gluon fusion is close to SM predictions. This may be wrong but it is what the data looks like so far. That being the case, the Higgs diphoton loop can most easily be enhanced if there is a new charged particle that adds to the loop. A boson would probably add to the cross-section while a fermion would subtract from it but some knowledgeable theorists say that “vector-like” fermions are also a possibility and who am I to argue. It must be colourless to avoid spoiling the gluon fusion production rate. It could carry lepton number which would affect its decay possibilities. Mass would be greater than 105 GeV otherwise it would be produced via mediated photons at LEP, but less than about 300 GeV to have a significant affect on the loop. Best candidates are scalar leptons like the stau or charged scalars like a charged Higgs, but vectors such as a W’ are also possible. These things have been searched for and already excluded in the required mass range, but only under model specific assumptions. Hadron colliders ahve big blind spots especially when particles decay via jets. There is still hope that something is being missed.

19 Comments | Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

Bayes and String Theory

June 12, 2012

If Supersymmetry is found or excluded at the Large hadron Collider, how will it affect your opinion on string theory as unification of gravity and particle physics? This is a hard question and opinions differ widely across the range of theorists, but at the least any answer should be consistent with the laws of probability including Bayes Law. What can we really say?

A staunch string theorist might want to respond as follows:

“I am confident about the relevance of superstring theory to the unification of gravity and the forces of elementary particles because it provides a unique way to accomplish this that is consistent in the perturbative limits (Amongst other reasons.) Unfortunately it does not have a unique solution for the vacuum and we have not yet found a principle for selecting the solution that applies to our universe. Because of this we cannot predict the low energy effective physics and we cannot even know if supersymmetry is an observable feature of physics at energy scales currently accessible. Therefore if supersymmetry is not observed at the TeV scale even after the LHC has explored all channels up to 14 TeV with high integrated luminosities, there is no reason for that to make me doubt string theory. On the other hand, if supersymmetry is observed I will be enormously encouraged. This is because there are good reasons to think that supersymmetry will be restored as an exact gauge symmetry at some higher scale, and gauged sypersymmetry inevitably includes gravity within some version of supergravity. There are further good reasons why supergravity is not likely to be fully consistent on its own and would necessarily be completed only as a limit of superstring theory. Therefore if supersymmetry is discovered by the LHC my confidence in string theory will be greatly improved.” 

On hearing this a string theory skeptic would surely be seen shaking his head vigorously. He would say:

“You cannot have it both ways! If you believe that the discovery of supersymmetry will confirm string theory then you must also accept that failure to discover it falsify string theory. Any link between the two must work equally in both directions. You are free to say that supersymmetry at the electro-weak scale is a theory completely Independent of string theory if you wish. In that case you are safe if suppersymmetry is not found but by the same rule the discovery of supersymmetry cannot be used to claim that superstring theory is right. If you prefer you can claim that superstring theory predicts supersymmetry (some string theorists do) but if that is your position you must also accept that excluding supersymmetry at the LHC will mean that string theory has failed. You can take a position in between but it must work equally in both directions.”

  The Tetrahedron of Possibilities

What does probability theory tell us about the range of possibilities that a theorist can consider for answers to this problem? Prior to the experimental result he will have some estimate for the probability that string theory is a correct theory of quantum gravity and for the probability that supersymmetry will be observed at the LHC. In my case I assign a probability of PST = 0.9 to the idea that string theory is correct and PSUSY = 0.7 to the probability that SUSY will be seen at the LHC. These are my prior probabilities based on my knowledge and reasoning. You can have different values for your estimates because you know different things, but you can’t argue with mine. There are no absolutely correct global values for these probabilities, they are a relative concept.

However, these two probabilities do not describe everything I need to know. There are four logical outcomes I need to consider altogether:

  • P1 = the probability that both string theory is correct and SUSY will be found
  • P2 = the probability that string theory is correct and SUSY will not be found
  • P3 = the probability that string theory is wrong and SUSY will be found
  • P4 = the probability that string theory is wrong and SUSY will not be found

You might try to tell me that there are other possibilities, such as that SUSY exists at higher energies or that string theory is somehow partly right, but I could define my conditions for correctness of string theory and for discovery of SUSY so that they are unambiguous. I will assume that has been done. This means that the four possible outcomes are mutually exclusive and exhaustive. We can conclude that P1 + P2 + P3 + P4 = 1. Of course the four probabilities must also be between 0 and 1. These conditions map out a three-dimensional tetrahedron in the four-dimensional space of the four probability variables with the four logical outcomes at each vertex. This is the tetrahedron of possible prior probabilities and any theorists prior assessment of the situation must be described by a single point within this tetrahedron.

So far I have only given two values that describe my own assessment so to pinpoint my complete position within the three-dimensional range I must give one more value. If I thought that string theory and SUSY at the weak scale were completely independent theories I could just multiply as follows

P1 = PST .PSUSY = 0.63
P2 = PST .(1 – PSUSY) = 0.27
P3 = (1 – PST) .PSUSY = 0.07
P4 = (1 – PST) .(1 – PSUSY) = 0.03

The condition that the two theories are independent fall on a surface given by the equation P1 . P4 = P2 . P3 that neatly divides the tetrahedron in two.

As I already explained I do not think these two things are independent. I think that SUSY would strongly imply string theory. In other words I think that the probability of SUSY being found and string theory being wrong is much lower than the value of 0.07 for P3 . In fact I estimate it to be something like P3 = 0.01. I must still keep the other probabilities fixed so P1 + P2 = PST = 0.9 and P1 + P3 = PSUSY = 0.7. This means that all my probabilities are now known

P1 = 0.69
P2 = 0.21
P3 = 0.01
P4 = 0.09

Notice that I did not get to fix P1 separately from P3. If I know how much the discovery of SUSY is going to affect my confidence in string theory then I also know how much the non-discovery of SUSY will affect it. It is starting to sound like the string theory skeptic could be right, but wait. Let’s see what happens after the LHC has finished looking.

Suppose SUSY is now discovered, how does this affect my confidence? My posterior probabilities P’2 and P’4 both become zero and by the rules of conditional probabilities P’ST = P1/PSUSY = 0.69/0.7 = 0.986. In other words my confidence in string theory will have jumped from 90% to 98.6%, quite a significant increase. But what happens if SUSY is found to be inaccessible to the LHC? In that case we end up with P’ST = P2/(1-PSUSY) = 0.21/0.3 = 0.7 . This means that my confidence in string theory will indeed be dented, but it is far from falsified. I should still consider string theory to have much better than level odds. So the skeptic is not right. The string theorist can argue that finding SUSY will be a good boost to string theory without it being falsified if SUSY is excluded, but the string theorists has to make a small concession too. His confidence in string theory has to be less if SUSY is not found.

Remember, I am not claiming that these probabilities are universally correct. They represent my assessment and I am not a fully fledged string theorist. Someone who has studied it more deeply may have a higher prior confidence in which case excluding SUSY will not make much difference at all to him even if he believes SUSY would strongly imply string theory.

33 Comments | Large Hadron Collider, String Theory, Supersymmetry | Permalink
Posted by Philip Gibbs

Bayes and Susy

May 10, 2012

Here’s a puzzle. There are three cups upside down on a table. You friend tells you that a pea is hidden under one of them. Based on past experience you estimate that there is a 90% probability that this is true. You turn over two cups and don’t find the pea. What is the probability now that there is a pea underneath? You may want to think about this before reading on.

Naively you might think that two-thirds of the parameter space has been eliminated, so the probability has gone from 90% to 30%, but this is quite wrong. You can use Bayes Theorem to get the correct answer but let me give you a more intuitive frequentist answer. The situation can be models by imagining that there are thirty initial possibilities with equal probability. Nine of them have a pea under the first cup, nine more under the second and nine more under the third. The remaining three have no pea under any cup. This distribution models correctly the 90% that a pea is there since 27 out of 30 do. If you now eliminate the cases where the pea is under the first or second cup you are left with nine instances of it under the third cup and three that it not there. So the correct probability is 9 out of 12 or 75%, much better than the naive 30% guess.

I mention this because I saw a comment over at NEW pointing to this paper about applying Bayesian statistics to the probability of finding SUSY at the TeV scale. The puzzle illustrates that Bayesian rules do not reduce the probability of something existing by as much as you would think if you eliminate a large chunk of the parameter space. Before experiments started to have their say I felt that SUSY at the TeV was a well motivated theory and I like the maths of supersymmetry, so I might have estimated the probability of it being there as 90%. By the time that paper was written LEP had eliminated lower mass SUSY just as you might turn over a couple of cups and not find the pea. At the start of 2011 before the LHC started to have much say I estimated the probability at 75%.

You might argue that another two-thirds of the parameter space has been eliminated since then. By the same analysis this would reduce the probability for SUSY at the TeV scale to 50%. However, we also now know that the mass of the Higgs is around 125 GeV with 4 sigma confidence (actually the mass region around 115 GeV - 120 GeV is still wide open so the story is not concluded yet) If the mass had been 115 GeV it would have been a good indicator for SUSY and at 140 GeV it would have been a strong eliminator. At 125 GeV it still “smells” like SUSY but the aroma is not so sweet. This can’t be quantified but for me it pushes the probability for SUSY back up to about 70%

If you are a SUSY sceptic I know what you are thinking. You think that LEP eliminated much more than two-thirds of the parameter space and the LHC eliminated much more than two-thirds of what was left. Is this really the case? All the diagrams from ATLAS and CMS which show large chunks of the parameter space being eaten up are misleading. Firstly there is no uniform measure of probability that can be assigned to the area of the plot. Secondly and more importantly all these plots rely on highly constrained versions of SUSY to reduce the parameter space to two dimensions so that it can be analysed and plotted. If SUSY phenomenologists have made a mistake it was to think that using these simplified models would be a good way to search for SUSY. This was not well motivated and has been shown wrong. If SUSY is to be found she will be seen in direct searches for particles such as the stop or stau. The Higgs is only starting to be seen in the data now so why should we think that heavier particles would already have shown up? The Higgs was in a place where it was not easy to find but this could also be the case for the stop especially if its mass is near the top (see also Stealth Supersymmetry) Higgs searches are relatively straight forward to analyse because if we know its mass we also know its cross-sections and decay rates (assuming the standard model). This is not the case for the stop, tau or gluinos. We have to keep searching until the limits placed on cross-sections are so small that all possibilities are excluded. The LHC is nowhere near that point yet.

As a curious footnote it is amusing to see that my Stop Rumours post is gradually making its way towards being the most read article on this blog. Why so much interest?  Looking into it I found that hit counts on most posts reduce to a trickle after a few days but this post keeps collecting hits at about a hundred a day, even after three months. The stats show that this is because of people searching for the single word “stop” on google. When I do the search myself I find that the post does indeed appear at the bottom of the first page. The “Stop Rumours” title must be enticing enough to lure people to click their way in. I suspect they are a bit baffled by what they find but maybe they will learn something about physics. It is very unusual to get a first page ranking for a single common word like “stop” so why is this happening? A clue is that the Google entry has an attached note saying that “Cliff Harvey shared this”. This is a feature of Google plus where Harvey maintains an excellent column commenting on people’s blog posts. If I log out of Google plus I no longer see my post in the Google search listing but once logged in I notice that a whole load of my search results are there because Harvey has shared them. Judging by the steady trickle of hits on my post this must be the same for a large number of people. If you are interested in SEO you will find this fact quite interesting and perhaps useful until Google tweak their parameters back to something more sensible.

47 Comments | Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

Stop rumours!

February 7, 2012

Meaning that there are rumours going round about stops or scalar tops, not that we should stop spreading rumours. In SUSY theories stops are the lightest sfermions (scalar fermions are bosons not fermions) related to top quarks which are the heaviest leptons and indeed the heaviest particle in the standard model. If stops exist they would help stabilise the Higgs vacuum which could be too unstable if the Higgs mass is around 125 GeV as now expected, but noone has seen one yet and the situation for theorists has been getting a bit desperate because they had expected to see them at the LHC and the so the anti-SUSY bloggers have been poking fun and saying I told you so.

Now rumours have been squarked to the blogotwittersphere via Motl at TRF and Jester of Resonaances that a signal for the stop has been seen in the data. the story so far has been summed up by Cliff Harvey on Google+ so look there for the details. There is a seminar next week that could be relevant to the rumour but Jester’s last tweet says knowingly  “Caution: theorists rumoring about stops is fact, but what is now out on blogs is 100% false. Dont jump unless more reliable rumors appear” so what is going on?

Sooner or later someone is going to start a rumour just to catch us out. So is the greatest news story in the history of science about to break or have we been duped by a revengeful experimentalist who saw the next seminar as an opportunity to get back at us for all those earlier leaks on the theory blogs? Is it indeed a slepton or something we should have slept on?

By the way there is an LHCb seminar about to be webcast and they are the only ones with plausible BSM signals so far so let’s slide back to reality and enjoy that, until next week.

29 Comments | Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

What would a Higgs at 125 GeV tell us?

December 4, 2011

13 December: please follow the live blog for up-to-date news

The rumours tell us that next week ATLAS and CMS will announce a strong but inconclusive signal for the Higgs boson at about 125 GeV. This may be wrong and even if it is right there may be other candidate signals to think about, and it will take much more data to verify that the signal is indeed correct for the Higgs, but if it is right, what then are the implications of the Higgs at this mass?

This question will be the subject of much discussion in the coming months and I can only touch on it here. Certainly the central topic of the debate will be the stability of the vacuum and whether it implies new physics, and if so, at what scale?

It has been known for about twenty years that for a low Higgs mass relative to the top quark mass, the quartic Higgs self-coupling runs at high energy towards lower values. At some point it would turn negative indicating that the vacuum is unstable. In other words the universe could in theory spontaneously explode at some point releasing huge amounts of energy as it fell into a more stable lower energy vacuum state. This catastrophe would spread across the universe  at the speed of light in an unstoppable wave of heat that would destroy everything in its path. Happily the universe has survived a very long time without such mishaps so this can’t be part of reality, or can it?

As it turns out a Higgs mass of 125 GeV is quite a borderline case. The situation was analysed taking into account the best recent valued for the top mass and weak coupling constants by Ellis et al in 2009. Here is their most relevant graphic with a line running across at 125 GeV (plus or minus 1 GeV) added by me. The horizontal axis tells us the energy at which the coupling constant goes negative. The yellow band indicates the limit for vacuum stability. Because of uncertainty in the top mass and the weak coupling, and also due to some theoretical unknowns, the exact point at which this limit is reached is not known exactly. The yellow band covers the range of possibilities.

The second plot taken from Quiros shows the scale of instability as a function of Top and Higgs mass. I have added a green spot where we now seem to live.

At 126 GeV the vacuum might remain stable up to Plank energies (see e.g. Shaposhnikov and Wetterich). If this is the case then there is nothing to worry about, but depending on the precise values of the standard model parameters, instability could also set in at energies around a million TeV. This is well above anything we can explore at the LHC but such energies are found in the more extreme parts of the universe and nothing bad has happened. The most likely explanation would be that some new unknown physics changes the running of the coupling to avert it from going negative. Examples of something that could do this include the existence of a Higgsino or a stop as predicted by supersymmetry, but there are other possibilities.

It is also possible that some amount of vacuum instability could really be present. If there is meta-stability the vacuum could remain in its normal state. There would be the possibility of disaster at any moment but the half-life for the decay of the vacuum would have to be  more than about the 13 billion years that it has survived so far. In the plot above the blue band indicates the region where a more immediately unstable vacuum is reached. It is unlikely that this case is realised in nature.

As the plot shows, if the mass of the Higgs turns out to be 120 GeV despite present rumours to the contrary then the stability problem would be a big deal. This would be a big boost for SUSY models that stabilize the vacuum amd mostly prefer the light Higgs mass. If on the other hand the Higgs mass was found at 130 GeV or more, then the stability problem would be no issue. 125 GeV leaves us in the uncertain region where more research and better measurements of the top mass will be required. It will still encourage the SUSY theorists as work such as that of Kane shows, but the door will still be open to a range of possibilities.

There are other things apart from the stability of the vacuum that theorists will look at. What is the nature of the electro-weak phase transition implied by this Higgs mass? Can it play some role in inflation or other phenomenology of the early universe? How does the result fit with electro-weak precision measurements and what else would be required to reconcile theory with experiment in such tests, especially the muon magnetic anomaly? 2011 has been a great year for the experimenalists but next year the theorists will also have a lot of work to do.

51 Comments | Higgs Hunting, Supersymmetry | Permalink
Posted by Philip Gibbs

A Typical LHC plot

August 28, 2011

Here is a typical LHC plot :)

As you can see, with 1.1/fb CMS has observed one event in a channel that may give a signal of a Higgs through decay to two Z bosons which in turn decay to two tau leptons and two other leptons. This is consistent with standard model backgrounds shown.

It will require about 100 times as many events for this channel to make any real impact on the search for the Higgs boson. Luckily the LHC will eventually record a few thousand /fb so this channel will be very useful.

There are other channels with better cross sections but results ao far shown have still used just a few events, or they are swamped by thousands of background events. It is possible to combine several channels and compare with what is expected from a particular theoretical model such as a standard model Higgs boson or MSSM supersymmetry, but such models tend to work in a reduced parameter space and may not match reality well. In the case of supersymmetry they look at models where a stable lightest particle is in reach of the LHC so that it shows up in missing energy searches. It would have been nice if this led to a quick discovery but it hasn’t.

Int ime each of these channels will be populated with lots of events and can be compared with standard model backgrounds. Bumps could appear anywhere leading to the discovery of some new particle. Once its properties are mapped through its different decay modes it can be fitted into a new model, which may or may not correspond to a supersymmetric multiplet.

People are starting to say that supersymmetry is in a corner, or even that the LHC seems to be incapable of producing new physics. It is far too early for any such conclusions. We need to be patient.

 

1 Comment | Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

Has the LHC seen a Higgs Boson at 135±10 GeV?

August 13, 2011

Once again rumours are circulating that the Higgs Boson has been seen and now they are more stronger than ever. At the EPS conference it was seen that both ATLAS and CMS have an excess of events peaking at around 144 GeV. Fermilab had a signal in the same place but much weaker. At the Lepton-Photon conference starting 22nd August ATLAS and CMS will unveil their combined plot. The question is, will the combined signal at 144 GeV be enough to announce an observation over 3-sigma significance?

Needless to say some early versions of the combined plot have already been leaked but rather than show results that may change I am just going to discuss my own unofficial combinations that are not very different. So here again is my combined plot for CMS, ATLAS and the Tevatron.

This shows a brought excess peaking at 144 GeV where it is well over 3-sigma significance. It extends from 120 GeV to 170 GeV above 2-sigma most of the way but it shows an exclusion above 147 GeV at 95% confidence. The signal is the expected size for a standard model Higgs boson from 110 GeV up to 145 GeV but is excluded by LEP below 115 GeV. What could it be, a Higgs boson, two Higgs bosons or something else?

The width of the Higgs boson is determined by its lifetime and at this mass it should be no more than 10 GeV. However there is a lot of uncertainty in the measured energy in some of the dominant channels. Some useful plots shown at Higgs Hunting 2011 by Paris Sphicas show what a simulated signal looks like in the WW channels and it is clear from these that a Higgs boson at 130 GeV or 140 GeV is perfectly consistent with the broad signal now observed.

There is also a hint of a signal around 120 GeV but it is not strong enough for a claim. I would say that overall this plot is consistent with a single Higgs boson with mass between about 125 GeV and 145 GeV or more than one Higgs boson in the range 115 GeV to 150 GeV. Whatever it is, the significance is enough to claim that a Higgsless model is now unlikely to be right unless some other particle is mimicking the Higgs boson in this plot and it is probably a scalar. Afterall, we can’t really say that the signal is definitely a Higgs boson until we can confirm that it has the right cross-section in some of the individual channels.

What does this say for SUSY and other models? The MSSM requires a Higgs boson below 140 GeV. In detail the signature would be different from the standard model Higgs boson. If there were a Higgs below about 130 GeV the vacuum would be unstable (but perhaps metastable) I think something as light as 120 GeV would be hard to accept as a standalone Higgs boson and would have to be stabilised with something that looks like either a SUSY stop or a Higgsino. On the other hand a 140 GeV Higgs can easily exist on its own and requires no new physics even at much higher energy scales. At this point we cannot rule out either MSSM or a lone Higgs boson.

Earlier I said that the electroweak fits could kill the standard model and that is still the case. At Higgs Hunting 2011 Matthias Schott from the gfitter group told us that a Higgs at 140 GeV has just a p-value of 23% in the fit which includes the Tevatron data. This is far short of what is required to rule it out but it tends to suggest that there may be something more to be found if the gfitter data is good (count the caveats in that sentence.) So just how good is the gfitter data?

This plot shows the effect on the electroweak fit of leaving out any one of the measurements used.

The green bar shows the overall preferred fit for the Higgs boson mass giving it a mass of 71 GeV to 122 GeV. But anything below 114 GeV is excluded by LEP. Anything below 122 GeV would certainly favour SUSY which is why this plot has been encouraging for theorists who prefer the BSM models. Indeed it is possible to get a much better fit to the data with just about anything other than the standard model.

How seriously should we take this? To get back some sanity have a look at the effect of the Al measurement. The fit includes two separate measurements of this parameter, one from LEP and one from SLD (SLAC Large Detector). The reason for using the two is that they disagree with each other at about 2-sigma significance. This could just be statistical error in which case we should use the combination of them both, but suppose it is a systematic error in one or other of the experiments, such as a mismodelled background? Removing the SLD measurement would push the preferred Higgs mass up and widen the error bars so that anything up to 160 GeV becomes a reasonable fit.  This is just one example of how a measurement could compromise the fit. That being the case I think we should not take the fit too seriously if we have good direct evidence for something different, and now we do.

In conclusion

From reliable sources I am expecting CERN to issue a press release about the status of the search for the Higgs Boson next week in advance of the LP2011 conference. If the official Higgs combination is similar to my version (the leak shows that it is) then they have the right to claim an observation (but not a discovery) of a strong signal consistent with a Higgs boson at 144 GeV (or soewhere else nearby). They cannot excluded other BSM signals including MSSM. I don’t know exactly how they will spin it but they will want the media to take notice.

For more details we will need to await the next analysis. Given present results and the extra data already recorded I am sure we will not have to wait too long.

85 Comments | Higgs Hunting, Large Hadron Collider, Supersymmetry | Permalink
Posted by Philip Gibbs

What is Dead?

July 26, 2011

There is a lot of interesting talk around the blogs about the fate of SUSY and even the whole field of phenomenology. It is a fascinating debate.

The CERN DG had some words of caution to give us during yesterday’s press conference. These are early days for the LHC and we should not imagine that it has already given a definitive report, but it has made some good points along with the Tevatron.

The Higgs sector does not look like what the standard model predicts. There are hints of something in the light mass window but it does not look like the SM Higgs. It does not have sufficient cross-section and may be spread out over too wide a mass range. It is too early to say what that is, or even if anything is really there. Much more data must be collected so that each experiment can separately say what it sees. That could take until the end of next year, but we will certainly have more clues at the end of this year. If the Standard Model is out, then we cannot be sure that some heavier Higgs is not another possibility. It just wont be the SM Higgs.

SUSY predicts a light Higgs but all the searches for missing energy events predicted by SUSY have been negative so far. Does this mean SUSY is dead? Of course is doesn’t. Some of the simpler SUSY models such as MSSM are looking very shaky, but there are other variants. We need some SUSY based fits using all the available data including the Higgs searches. Hopefully the phenomenologists will provide some updates for those soon to let us know what the conclusions are. I have explained in the past that SUSY is a well motivated theory. Many phenomenoligists have put a lot of work into it,  but if the LHC rules it out I am sure they will be the first to give us the right reasons to think so.

I don’t agree that the work of phenomenologists has been a waste of time. Without their research the experiments would not have been able to set up the model based tests that have told us so much. A lot of different ideas apart from SUSY are being tested. They can’t all be right. Following the EPS conference there will be a number of follow-up meetings to discuss the implications (see the Calendar). This will be the time for the theorists to come back and tell us what is left on the table. It will help the experimenters to prioritize the searches they want to put most effort into as more data becomes available.

The parameter space of SUSY is large and flexible but everywhere it describes a Higgs sector that is different from the standard model. That is why I think the Higgs sector is crucial to understanding whether SUSY at the electroweak scale will live or die. That part of the story is still at an early stage. The next chapters in this gripping tale will unfold in the next few months. There could be several unexpected twists on the way.

Update 27-Jul-2011: Tommaso Dorigo has a relevant article about SUSY fits with a pointer to some updates from the MasterCode project

28 Comments | Higgs Hunting, Large Hadron Collider, Physics, Supersymmetry, Tevatron | Permalink
Posted by Philip Gibbs

« Previous Entries
Follow

Get every new post delivered to your Inbox.

Join 192 other followers