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.


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.

 


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.


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


SUSY was not round the corner

July 21, 2011

ATLAS have produced new exclusions limits for SUSY (Taffard) models using jets plus missing transverse energy from 1/fb of data. These go well beyond previous limits leading to the conclusion that “SUSY” was NOT “just rounnd the corner” as theorists hoped.

Where does this leave SUSY? Well SUSY is a many parameter theory with a lot of variations and it remains the most plausible explanation for many observations. More searches may find it.

Ultimately it will be the Higgs searches that have the final say. SUSY predicts a light Higgs with higher mass partners. If the Higgs is found to be something different SUSY will be much harder to motivate. Until the Higgs sector is resolved, SUSY lives on.


ATLAS squark and gluino search with 165/pb extends SUSY exclusions

June 4, 2011

A new conference note from ATLAS using 165/pb of data has extended exclusions for SUSY. Previous published results used 36/pb of 2010 data. These new findings will be reported at the Physics at LHC conference next week. With 691/pb delivered, further search results will be possible very soon.

Gluino masses below 725 GeV are excluded in the simplest models. The new limit of the excluded region is the thick red line on this plot. The odler limit using just the 2010 data is the thinner black line. The no-go zone has extended about 200 GeV to the right.

It is disappointing for many phenomenologists that no SUSY signal has yet appeared, but SUSY has a large parameter space and it will take much more data to really rule out the theory completely. It seems likely that if SUSY is not found then something else will prove to be the solution for physics at the TeV scale. The data from the LHC to be accumulated over the next two years will tell us how the Higgs sector looks. Whether it is SUSY or not, it is likely to be something of interest.


Suzy at Last?

July 30, 2010

The first time I went to a lecture on supersymmetry the auditorium was so packed that many people could not get in. I was pleased I had anticipated the high demand and arrived very early. In his talk entitled “Is the End in Sight for Theoretical Physics?” The speaker explained to us that supersymmetry was the greatest hope for theoretical physics because it offered the possibility to unify the gauge theories of particle physics with a quantum theory of gravity in a way that might avoid the infinities of quantum field theory.

The speaker was of course Stephen Hawking and the occasion was his inauguration as Lucasian Professor in Cambridge. The version of supersymmetry that had him so excited was N=8 Supergravity in 4 dimensions. Cautiously he predicted that a complete theory of particle physics could be worked out in 20 years time using this new superunified theory.

30 years have passed and we know that things did not work out quite as Hawking has hoped. He thought that N=8 supergravity might be a unique candidate for a fully unified theory of physics, although the particles we now know as fundamental would have to be composite. He did not consider higher dimensional theories because he thought that details such as the number of spacetime dimensions could be explained by anthropomorphic arguments.

A few years later, supergravity was replaced by superstring theories and higher dimensions became mandatory. The underlying theory still possesses a similar uniqueness but now anthropomorphic arguments are needed to  select the real world vacuum from a vast landscape of possibilities that superstring theory offers. Hawking has now retired as Lucasian professor to be replaced by one of superstrings’ pioneers,  Michael Green. Supersymmetry and superstrings face a skeptical backlash from a large section of the younger generation who are disillusioned by its failure to provide clear predictions for particle physics or cosmology after so much time.

Now the table may be turning full circle and this time support for supersymmetry comes not just from theory, but from experiment too. The version of supersymmetry that has come to the fore is the Minimal Supersymmetric Standard Model – an extension of the well established Standard Model of particle physics that includes an additional broken supersymmetry. This leads to one superpartner for every familiar particle that we know already, plus an alternative Higgs sector with fives Higgs particles, two of them charged.

The MSSM first appeared just a year after Hawking’s lecture. Since its early days it has been understood that it improves the naturalness of low energy particle physics due to anomaly cancellations that help keep the Higgs sector light. With the addition of supersymmetry the three running coupling constants converge at one energy point, suggesting a dessert of new physics up to a more complete unification at the GUT scale. The model also provides a natural R-parity symmetry that would make its lightest particle stable. This offers a unique candidate for dark matter whose stability would otherwise be very hard to explain.

For the last decade or perhaps more, theorists have been anticipating the imminent discovery of supersymmetry in the world’s highest energy particle accelerators. Fermilab was thought to have a chance of discovery with the Tevatron and there were even some false starts that faded away as the statistics grew. Now their hopes turn to the Large Hadron Collider but the Tevatron is not finished yet. In recent months we have seen some tantalising results reported by Fermilab that support the MSSM.  Nothing is conclusive yet, but the combined evidence all seems to point in the right direction.

For those of us who grow up with the idea that supersymmetry is the final move in a game of unification that leads inevitably to a complete theory, these reports are too hard to dismiss. After the ICHEP conference we drool over the results that should have been shown, but weren’t. Plots which show inconclusive signals of less than 3-sigmas statistical significance are quick and easy to approve for publication. They don’t lead to big headlines. Anything above three sigmas would count as an observation and that puts it in a different league of results. With some history of failed observations from the past, Fermilab are likely to put off publication until the next round of data is seen to add rather than subtract from the result. For us the outsiders, the mere absence of certain plots starts to look like a sign to get excited about.

For the supersymmetry skeptics the conclusions to be drawn are different. Any signal below 3 sigma is to be dismissed as noise. They can even dismiss the exclusion of the Higgs mass range that now strongly supports a light Higgs sector as predicted by supersymmetry. It is indirect and still inconclusive.

If supersymmetry is indeed just below the surface, what will happen next? The Tevatron will continue to analyse the data they have while collecting some more until about 2013. The signal will grow until it is clear that something new has been seen. The LHC will not have the luminosity to see the low mass Higgs sector before the Tevatron, but supersymmetry will offer other new particles of higher mass. The LHC might pick out some of those very quickly and start to study their properties. Very soon the parameter space of supersymmetric models will be narrowed down. There will be a huge spurt of activity amongst theorists as they figure out how particle physics works at this scale. If there really is a desert of new physics beyond supersymmetry it may be possible to work out a convincing scenario for physics right up to the GUT scale. Possibly the next generation of accelerators will be needed to pin down most of the coupling constants. If they are clever enough, there may be enough information to figure out the mechanism for breaking supersymmetry at the GUT scale. That could reveal a perfectly supersymmetric world at higher energies with far fewer free parameters.

It will not stop there. If supersymmetry is part of gauge field unfication then its unbroken gauge form will include supergravity. The experimenters will have had their day again as theory pushes into higher energies with renewed confidence. How far it will run is hard to say but the connection between supersymmetry and quantum gravity is hard to pull apart. Knowing the details of supersymmetry at the electroweak scale could be enough to lead us to the end of theoretical particle physics in the sense that Hawking predicted 30 years ago. Perhaps even superstrings will suddenly look right again. Until we have the next results from experiment we cannot be sure, but that is what makes the current situation so exciting. In just a few years – perhaps even just months – a renaissance of  particle physics merging experiment and theory might be well underway. It might pan out in a less predictable way than I have suggested here, but it is sure to be revealing, if it happens at all.

Update: see also the discussion on Lubos blog, and of course his many detailed pages extolling the virtues of supersymmetry.