Thursday, 27 October 2011

What if they don't find the Higgs?

(...No I didn't cut my wrists after the Tevatron shutdown, contrary to what you might have concluded from my blogging history...)

So, the 2011 run of the LHC is coming to a close, I mean the interesting part ;-). A 5 inverse femtobarn stash of data has been collected by each ATLAS and CMS. These data will by fully analyzed and scrutinized by the late winter 2012, while rumors should start popping up on blogs before the end of this year. One thing that is already clear is that new physics did not jump in our faces, which is hardly a surprise. And neither did the Higgs boson, which is more intriguing. Contrary to what I expected, the 2011 data may not yield a conclusive statement about the Higgs: neither a clear cut discovery nor excluding the entire low mass range appears likely at this point. We can now at least entertain the option, which as recently as last summer was unthinkable, that the LHC will not find the Higgs particle with the properties predicted by the Standard Model. What then?

First of all, it will be fun to watch the CERN management explaining the public that *not* discovering the Higgs is a success. For theorists, on the other hand, the best of all worlds will have been granted. In fact, we already have a deck of cards to play for that occasion, each very interesting as each pointing to exciting new physics within our reach. Here are the 3 main broad scenarios (not mutually exclusive):
  • Higgs exists but has a smaller production cross section.
    In the Standard Model the Higgs is produced mostly in gluon fusion, via a loop diagram with top quarks. One can easily imagine new particles meddling into Higgs production via a similar loop process; all they need is a color charge and a significant coupling to the Higgs. Thus, in every major new physics scenario modifying the Higgs production rate is possible without stretching the parameters too much. One interesting case is the composite Higgs, where the Higgs cross section is almost always suppressed, typically down to 70-90% of the Standard Model value. For experimentalists this is the simplest scenario, all they need to do is sit and wait a bit longer, and the Higgs will eventually show up. The matter should be sorted out after the 2012 data are analyze.
  • Higgs exists but has non-standard decays.
    For a low mass Higgs, around 120 GeV, the main discovery channel is the decay into 2 photons. Again, this is a loop process in the Standard Model so it's very easy for new physics to modify the branching fraction for that decay. As in the previous case, one may just sit and wait for the Higgs to eventually show up. However Higgs decays can be easily modified in a far more dramatic fashion than the production rate. For example, Higgs may be invisible, that is decaying into very weakly interacting particles whose only signature is the unbalanced momentum in the event. Or Higgs may dominantly decay into multiparticle final states (some popular model predict decays to 4 tau leptons or 4 b-quarks) and we'll never see the bastard in the diphoton channel. That would be a very interesting scenario not only for theorists but also experimentalist, as it would require clever new methods to spot the Higgs on top of the QCD background.
  • There is no Higgs.
    That would mean that the mechanism of electroweak symmetry breaking is inherently strongly coupled, somewhat resembling breaking of the chiral symmetry in QCD. This is the most challenging scenario for theorists and experimentalists, and the one that may require a lot of patience. In the optimistic case, the 14 TeV LHC run we will spot a number of resonances analogous to QCD mesons and little by little we'll understand the structure of the underlying gauge theory. But these resonances may well be too heavy or too wide to be efficiently studied at the LHC. Ultimately, we may need to probe the properties of the scattering amplitudes of W and Z bosons where, according to theory, these strong interactions must leave an imprint. The problem is that such a measurement is very non-trivial in the dirty LHC environment (the SSC or a linear collider would be a different story), so we may need some new theoretical or experimental ideas to make the progress. It's probably too early to bet large amounts on this scenario (which is currently disfavored by electroweak precision data) but if no hint of the Higgs is seen by the end of 2012 that will become the most promising direction.
In summary, discovering the Higgs would be a big news, a huge achievement of the whole community, one small step for mankind, et cetera. But not discovering it would be more exciting by a lot, a lot, a lot. Of course, assuming that eventually we will find something ;-)

31 comments:

Kea said...

It won't change anything. If the establishment theorists can brazenly lay claim to FTL neutrinos, they won't have any problem throwing out the fairies.

Anonymous said...

Unfortunately I'm not quite convinced that the tax payers will honor the "more exciting case" of "no Higgs".

Andrew Oh-Willeke said...

It seems as if the LHC is pretty relentlessly and systemmatically ruling out any BSM physics in the sub-TeV region.

The more theoretically inclined folks seem to think that we're simply in a small desert and at single digit TeV that there will be almost a "phase change" where everything suddenly gets dramatically weird. In this view the blowup of the equations at high energy levels isn't a bug in the equations, it is a profound truth.

The other possibility seems to be that we don't find a Higgs but also don't find anything that is the least bit unexpected even as energies climb higher and higher - and LHC seems to have quite a bit of room to extend its "boring streak." Suppose LHC runs for a decade and we haven't found any new particles, haven't any new forces, haven't any tweaks to Standard Model equations, nada. Is that even conceptually possible? Is it possible to do SM predictions that predict anything at that many inverse fbs at such high energies?

Sure, the Particle Data Group can add a few significant digits to its existing physical constants and move exclusion thresholds a few orders of magnitude out. That is pretty much the default answer. Maybe more data provides better guidance on how to model QCD backgrounds where they had been some differences of opinion as to how to handle some find points of the calculations.

But, at some point, the eye of the needle that theorists hve to fit gets awfully constrained - and maybe the interesting finds start coming from big deep space telescopes instead of colliders, and from neutrino physics and precision beta decay measurements.

The default possibility, it seems to me, is that we see no Higgs, but the Standard Model equations continue to work despite themselves for reasons we don't fully understand.

Anonymous said...

But what if the Higgs boson does not exist because the general paradigm underlying the standard model has one or more fundamental flaws?

New ideas that are radically different from those of the general particle physics paradigm would be needed to make sense of the lack of a Higgs, the vacuum energy density crisis, the complete absence of "WIMPs", etc.

Would particle physicists ever start from scratch and only build on what is empirically well-founded?

Not any time in the near future, given the reactions to the initial LHC results, the undeterred enthusiasm for strings, branes and SUSY, and the credulity regarding the superluminal neutrinos.

Sigh.

rescolo said...

Another possibility is to have an extended scalar sector which allows for the SM-like Higgs to be heavier while being consistent with electroweak observables.

Jester said...

Right, one may put it as a separate scenario #4: a very heavy Higgs m~1 TeV, plus some new physics that fixes the electroweak observables(doesn't have to be an extended scalar sector, new fermions, or gauge bosons could also work).

chris said...

Andrew,

without the Higgs the SM has effective terms in the Lagrangean that will lead to unitarity violations in WW scattering at the TeV scale. Now i'm quite open to new scenarios, but unitarity violation just means that the theory becomes inconsistent. There is simply no way that the "SM equations continue to work without the Higgs".

Anonymous said...

Well educated tax payers can certainly understand that no Higgs (for the time being) is more exciting. Kea, what kind of personality problem you suffer, dear?

Joel Rice said...

Neither Higgs nor the CKM are algebraically straightforward in the sense that spin is, and the SM does not explain fermion generations, so something is kludgy about it. The linear combinations make sense for getting probabilities, but at a cost of obscuring what should be straightforward algebraic relationships.

Robert L. Oldershaw said...

There is an interesting piece in Nature today on this subject.

http://www.nature.com/news/2011/111028/full/news.2011.619.html

Two quick comments.

(1) We have been through such snipe hunts before with "strings", "magnetic monopoles" and "WIMPs". Decades of efforts, billions of dollars, and only a long depressing series of false-positives to show for all of it. If the standard model Higgs is not found in the 114-145 GeV range, then they will probably tell us the real Higgs is much more massive and ask for a colossal new collider to continue the snipe hunt. Sigh. If one does not learn from history, ...

(2) Before the initial no-show results from the LHC were available, such no-show scenarios were regarded as the "worst case scenario" by the leaders of the particle physics community. Now we start to hear quite a different tune. Why, pray tell?

Robert L. Oldershaw
Discrete Scale Relativity

DB said...

Why would the SSC be a less "dirty" environment than the LHC?

Anonymous said...

From a recent Nature article,

"Tommaso Dorigo, a particle physicist at the University of Padua in Italy and a member of the CMS team, says he is "willing to bet a few bucks" that the small excess of events indicating that the Higgs boson has a mass of around 120 gigaelectronvolts, reported in July, really are due to the Higgs. And he estimates that, if is he right, then by the time the 2011 data are analysed early next year, the significance of the excess will have grown to about 3 sigma (a significance value of 5 sigma is the minimum at which discovery of the Higgs boson could definitely be claimed). "That would not be enough to claim a discovery," he says, "but it would be enough to convince most physicists that the effect is real.""

Coming from someone who has been a consistent Higgs skeptic, this suggest that the Higgs does, in fact, exist with a mass around 120 GeV.

Jester said...

@DB: the SSC wouldn't be less dirty, but the much higher CM energy would be advantageous for studying WW scattering (the interesting amplitude grows as the CM energy squared when there's Higgs is absent or has coupling departing from the SM).
@Anon: right, it's definitely the most like outcome. But last summer we had ATLAS and CMS combined had a 4 sigma excess and it went away. So not all hope is gone yet...

Alex said...

If I were the person having to do the press release on the LHC discovery of some weird strongly coupled or otherwise strange mechanism of EWSB, I would simply write the headline:

"Higgs Mechanism Discovered - thanks to a quirk of nature, even deeper insights are possible than originally thought"

And there you have it.

Andrew Oh-Willeke said...

"without the Higgs the SM has effective terms in the Lagrangean that will lead to unitarity violations in WW scattering at the TeV scale. Now i'm quite open to new scenarios, but unitarity violation just means that the theory becomes inconsistent. There is simply no way that the "SM equations continue to work without the Higgs"."

In other words, if we simply plug a zero into the equations were a Higgs boson mass belongs, we'd have trouble at the TeV range with non-unitarity in WW scattering (which is a bad thing because something that has to add up to 100% in all real world observations is probabilities).

But, the notion I am getting at would be simply putting some number for a Higgs boson mass at the right order of magnitude into the Langrangian equations for calculation purposes (perhaps the Z boson mass, or perhaps 119 GeV), even though experimentally we know for a fact that the Higgs boson isn't there, in the same way that we put numbers in for a renormalization scale that we have no observational basis for including. After all, if there were a Higgs boson at that mass, the equations wouldn't break down, right?

The equations don't know if our Higgs boson mass constant is fake or real when we are calculating with them and that would be the obvious mathematical trick to use to make meaningful predictions even if our Higgs hunt comes up empty.

The equations shouldn't blow up calculationally at the TeV scale if we don't put a suitale number in them where the Higgs boson mass belongs. Suppose that some sort of trickery like that works to produce results that accurately predict the experimental output, even though we are using a fake number for a Higgs boson mass of a suitable order of magnitude, rather than a real one. This is a slightly more nuanced description of the sort of experimental limbo that I could see as happening even at several TeV.

And, I suspect that if that happened the headlines would probably first read: "Higgs boson not found" sometime in 2011 or 2012, with the follow up story of "fake Higgs boson mass works like renormalization" not making many more headlines outside scientific journals than renormalization did when it was invented.

Kea said...

Anonymous cowards telling real female physicists that they have personality disorders? Not very original.

Chris Austin said...

@ rescolo and Jester, with regard to scenario #4: a very heavy Higgs m~1 TeV, is there some way this can be achieved without the electroweak sector becoming quite strongly coupled below the Higgs mass? i.e. so that this case really is distinct from #3, no Higgs, in that the 1 TeV Higgs somehow keeps the electroweak sector weakly coupled above 600 GeV?

LukaszB said...

The standard model of particles with Higgs boson concept suffers from a kind of conceptual schizophrenia. My reasoning is simple (correct me if I am wrong):

The premis: both spin and mass of elementary particles are eigenvalues of Casimir invariants of the Poincare group.

The conclusion: Casimir spin and Casmir mass have the same conceptual origin and therefore should be treated in a similar way. Thus, there should be a kinematical scheme which generates the spectrum of masses akin to the mechanism of the spectrum of spin (spin quantisation is the conclusion from the theory of representations of the group of rotations). Similarly, the mechanism of 'generating' masses should be geometric in nature, that is, not based on a Hamiltonian.

Spin of all particles is always a multiple of 1/2, irrespectively of their composition, because
the particles 'live' in the same spacetime. There is no 'Higgs-like' field which induces Casimir spin. One does not need any field to model Casimir spin. Why then people think the Casimir mass of quantum particles should be induced by other quantum fields like Higgs boson?

Could anybody enlighten me?

Jester said...

Chris, indeed if they discovered a very heavy Higgs I would find it most natural if it were a composite resonance of some strong sector, something like Sigma in QCD. But formally one can imagine a heavy and wide Higgs (not much heavier than TeV because of unitarity constraints) and nothing else in the electroweak breaking sector.

Alex said...

LukaszB,

that's an interesting thought. I think the problem you have here is that you don't want to describe the kinematics of particles only - for that doing Poincaré representation theory would be sufficient - but also the dynamics. For that you need Quantum Field Theory (apparently...), and in Quantum Field Theory the Poincaré representations in the Fock space are only determined after solving the dynamics of the theory - in particular there are as you know symmetries which forbid nonvanishing P^2 for most particles. You could try to construct a purely geometric origin of all the Poincaré reps that constitute the known particles, but I don't see how one would be able to obtain the correct electroweak physics predictions if this construction is not compatible with the electroweak symmetries which are their foundation (apparently...). I'm not saying you shouldn't try, but that's the point that you would have to address imho.

Peter Woit said...

Anonymous,

I don't think it's accurate to describe Tommaso Dorigo as a Higgs skeptic, much less a "consistent Higgs skeptic". I suspect you're thinking of his views on supersymmetry.

Bryan Sanctuary (Dr.) said...

Failures of experiments are just as important as successes (Michelson Morley), so I am perfectly happy if they do not find the Higgs Boson, in fact I think it would encourage some new thinking because the Standard Model does not answer all the questions.

LukaszB said...

Alex,
Thank You for answering my question.

We see that the problem of mass is indeed complex.

It seems we lack the precise notion of what mass is. Another notion of mass is the charge of the gravitational field, an integral over a sphere at infinity, eg. the parameter M in Schwarzschild spacetime is such a charge. We know that the charge of another filed, namely, of the electromagnetic field, is quantised. We know this, but we do not know why e^2/hc=1/137.., we take this value from experiment. The standard model of particles does not answer the question why 1/137... and not 1/130. Something fundamental is missing from our understanding of mass and electric charge.
There must be a quantum theory of the electric charge and it must be based on electromagnetism (however, we know that quantum electrodynamics as we know it today does not suffice to answer this question). Similarly, it could well be that for explaining the spectrum of masses we need a kind of quantum theory of the gravitational charge (G*M^2/hc is also a dimensionless number quite analogous to e^2/hc). However, the standard model of particles completely ignores gravitation.

This great hole in our understanding of mass, suggests that Higgs boson will have not been discovered. The mechanism of acquiring mass by quantum particles is probably quite different.

In the worst case, if they find the Higgs, we will probably never find answer to above questions. Surely, without Higgs boson, physics will be much more interesting.

chris said...

LukaszB,

mass is not the charge of the gravitational field - energy is (or, more to the point, the energy-momentum tensor). that is quite a substantial difference.

Paolo Valtancoli said...

Finding just a desert is the best cure against the pseudo science of string theory....

SpearkMarktheSecond said...

Nice post. If the production rate is suppressed by new (destructively interfering) couplings through new colored particles, would any of the Higgs' powers to renormalize the theory be modified?

I thought there was a theorem of sorts that no Higgs<150 GeV or so `rules out' an SUSY model tied to the weak scale. Don't know how easy that is to evade.

It will be great, however, if a Higgs does who up in the 115-150 GeV range. Wonderful drama.

Anonymous said...

"This great hole in our understanding of mass". You mean to say this great hole in *your* understanding.

Anonymous said...

Kea, I have no problem with your feminity, just with the ilimited range of your pointless observations. Don't play the gender card.

Axis said...

From what I've read the main problem with no higgs boson is that perturbative methods break down for longitudal WW scattering. But how is that an argument for higgs? This to me looks like a practical problem - the approximation is simply no longer valid.

The fact that adding certain terms restores consistency in no way implies that those terms describe anything real.

Tony K said...

"So, the 2011 run of the LHC is coming to a close, I mean the interesting part ;-)."

As a heavy ion grad student, I think I am obliged to say, "Ouch!" :-D

Oh, and this is (some of) what we can pull off with 3 weeks/year of data... users2011-LHC_v2_cole.pdf

Anonymous said...

A thought after seeing the Nova special "The Fabric of Whatever"

Kaku, Nielsen, Greene, Carroll, Susskind, Page, etc.

I wonder if nature abhors string theory so much that it is causing its proponents to wander off into la-la land babbling incoherently?

Albert Z