Higgs – Page 5 – Sean Carroll

Beam Seen in LHC’s CMS Experiment

November 8, 2009 / 19 Comments

Mischievous baguette-dropping birds be damned! The LHC had another milestone this weekend, as the CMS experiment detected “splash” events.

They’re not quite to the promised land yet (even remembering that the beam energies are a lot lower than we eventually want them to be). A little while ago we had beam traveling through the accelerator, which is obviously a big step. These splash events happen when the beam collides into something “upstream,” creating a splash of particles that are then detected by the experiment. The big step will be when beams moving in opposite directions actually collide with each other inside the detector. I predict you’ll hear soon when that happens.

You can follow CMS at its Facebook fan page. 528 fans, I’m sure we can boost that number.

I already have a bet with Brian Schmidt that we will fine at least 3-sigma evidence for the Higgs within five years (either at Fermilab or the LHC). Feeling pretty optimistic right now.

Beam Seen in LHC’s CMS Experiment Read More »

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Spooky Signals from the Future Telling Us to Cancel the LHC!

October 14, 2009 / 119 Comments

A recent essay in the New York Times by Dennis Overbye has managed to attract quite a bit of attention around the internets — most of it not very positive. It concerns a recent paper by Holger Nielsen and Masao Ninomiya (and some earlier work) discussing a seemingly crazy-sounding proposal — that we should randomly choose a card from a million-card deck and, on the basis of which card we get, decide whether to go forward with the Large Hadron Collider. Responses have ranged from eye-rolling and heavy sighs to cries of outrage, clutching at pearls, and grim warnings that the postmodernists have finally infiltrated the scientific/journalistic establishment, this could be the straw that breaks the back of the Enlightenment camel, and worse.

Since I am quoted (in a rather non-committal way) in the essay, it’s my responsibility to dig into the papers and report back. And my message is: relax! Western civilization will survive. The theory is undeniably crazy — but not crackpot, which is a distinction worth drawing. And an occasional fun essay about speculative science in the Times is not going to send us back to the Dark Ages, or even rank among the top ten thousand dangers along those lines.

The standard Newtonian way of thinking about the laws of physics is in terms of an initial-value problem. You specify the state of the system (positions and velocities) at one moment, then the laws of physics tell you how it will evolve into the future. But there is a completely equivalent alternative, which casts the laws of physics in terms of an action principle. In this formulation, we assign a number — the action — to every possible history of the system throughout time. (The choice of what action to assign is simply the choice of what laws of physics are operative.) Then the allowed histories, the ones that “obey the laws of physics,” are those for which the action is the smallest. That’s the “principle of least action,” and it’s a standard undergraduate exercise to show that it’s utterly equivalent to the initial-value formulation of dynamics.

In quantum mechanics, as you may have heard, things change a tiny bit. Instead of only allowing histories that minimize the action, quantum mechanics (as reformulated by Feynman) tells us to add up the contributions from every possible history, but give larger weight to those with smaller actions. In effect, we blur out the allowed trajectories around the one with absolutely smallest action.

Nielsen and Ninomiya (NN) pull an absolutely speculative idea out of their hats: they ask us to consider what would happen if the action were a complex number, rather than just a real number. Then there would be an imaginary part of the action, in addition to the real part. (This is the square-root-of-minus-one sense of “imaginary,” not the LSD-hallucination sense of “imaginary.”) No real justification — or if there is, it’s sufficiently lost in the mists that I can’t discern it from the recent papers. That’s okay; it’s just the traditional hypothesis-testing that has served science well for a few centuries now. Propose an idea, see where it leads, toss it out if it conflicts with the data, build on it if it seems promising. We don’t know all the laws of physics, so there’s no reason to stand pat.

NN argue that the effect of the imaginary action is to highly suppress the probabilities associated with certain trajectories, even if those trajectories minimize the real action. But it does so in a way that appears nonlocal in spacetime — it’s really the entire trajectory through time that seems to matter, not just what is happening in our local neighborhood. That’s a crucial difference between their version of quantum mechanics and the conventional formulation. But it’s not completely bizarre or unprecedented. Plenty of hints we have about quantum gravity indicate that it really is nonlocal. More prosaically, in everyday statistical mechanics we don’t assign equal weight to every possible trajectory consistent with our current knowledge of the universe; by hypothesis, we only allow those trajectories that have a low entropy in the past. (As readers of this blog should well know by now; and if you don’t, I have a book you should definitely read.)

To make progress with this idea, you have to make a choice for what the imaginary part of the action is supposed to be. Here, in the eyes of this not-quite-expert, NN seem to cheat a little bit. They basically want the imaginary action to look very similar to the real action, but it turns out that this choice is naively ruled out. So they jump through some hoops until they get a more palatable choice of model, with the property that it is basically impotent except where the Higgs boson is concerned. (The Higgs, as a fundamental scalar, interacts differently than other particles, so this isn’t completely ad hoc — just a little bit.) Because they are not actually crackpots, they even admit what they’re doing — in their own words, “Our model with an imaginary part of the action begins with a series of not completely convincing, but still suggestive, assumptions.”

Having invoked the tooth fairy twice — contemplating an imaginary part of the action, then choosing its form so as to only be relevant where the Higgs is concerned — they consider consequences. Remember that the effect of the imaginary action is non-local in time — it depends on what happens throughout the history of the universe, not just here and now. In particular, given their assumptions, it provides a large suppression to any history in which large numbers of Higgs bosons are produced, even if they won’t be produced until some time in the future.

…

Spooky Signals from the Future Telling Us to Cancel the LHC!Read More »

Spooky Signals from the Future Telling Us to Cancel the LHC! Read More »

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The Race for the Higgs

February 17, 2009 / 19 Comments

The Large Hadron Collider should be lurching back to life this year, but the Tevatron at Fermilab might yet have a last hurrah. While the LHC is still fixing itself after last fall’s explosions, the Tevatron has been collecting data like mad, and hopes to continue for another couple of years. At the American Association for the Advancement of Science meeting in Chicago, Fermilab scientists said they have quite a good shot at collecting “evidence for” (although not quite “discovery of”) the Higgs boson before all is said and done.

Adam Yurkewicz at US/LHC Blogs has the scoop, and you should go there for more info. But this graph tells the basic story. It’s the probability that Fermilab will be able to find “three-sigma” evidence for the Higgs, depending on what its mass is, if the Tevatron gets to run through 2011.

Due to complicated background events, finding a particle like the Higgs isn’t just a matter of smacking together protons and antiprotons at higher and higher energies. Some possible values of the Higgs mass make it easier to find than others, since the reactions that produce it aren’t as swamped by boring known events. That’s why the Tevatron has a shot, even if LHC opens with substantially larger energies later this year. The BBC story portrays the whole thing as a race, which is fine, but to the rest of the world it’s more important to just find the darn thing than which continent gets there first. (Given that the Higgs is a boson, the smart money would seem to be on Europe.)

The Race for the Higgs Read More »

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Live-Blogging the LHC Startup!

September 9, 2008 / 156 Comments

9:20 am Pacific Time: Let’s be clear. Tonight’s start-up is a symbolic event, not a physics event; as I understand it, the beam will only be circulating in one direction, so there won’t even be any collisions. Still, it’s a very important symbolic event! The first time the beam goes through the entire machine. So, just for fun, here will be a running commentary throughout the day, with links and musings and all that makes the blogosphere special. Co-bloggers are welcome to chime in, and any particle physicists out there who want to say something about the LHC are welcome to comment or email.

9:45 am (Pacific), Sean: Feel free, in the comments, to make predictions about what the LHC will discover (ultimately, not today). Here are mine. Crackpots not welcome. And seriously, folks — black-hole/world-ending jokes are only funny the first million times.

1:14pm (EST), Mark: Here at Cornell there’s going to be a public forum this evening with refreshments, chats with physicists, two talks (by Yuval Grossman and Peter Wittich) and with various instruments and components of the detector on display.

10:26am (PDT), JoAnne: Actually, it is the end of the world as we know it. I will never again have to write a paper detailing the signatures of some crazy new Terrascale theory, wondering if there is any chance of connection to reality. I will never again have to plot a cross section as a function of the Higgs mass. In fact, I will never again have to do a loop over the Higgs mass in a code. I will never again wonder how electroweak symmetry is broken, how the hierarchy between the electroweak and gravity fundamental scales is maintained, whether there is a WIMP dark matter particle, or whether supersymmetry or extra spatial dimensions actually exist. Fundamental questions and roadblocks that have plagued us for literally decades will finally be answered and we will at last be able to move forward instead of spinning our wheels. Yes, indeed, the world will be truly different.

10:47am (Pacific), Sean: Of course we are not the only blog covering this. The US/LHC Blogs have lots of information, and Tommaso Dorigo offers some inside scoop. There is also main CERN page for the event, and one for press releases.

12:02pm (Pacific), Sean: The real excitement of the LHC startup is, of course, that it’s an excuse to party. Mike in comments already mentioned the Fermilab pajama party. Here at Caltech, where it’s not quite so ridiculously late at night, we’re having pizza and beer. And (for the wimps who can’t stay up), a lunch BBQ tomorrow. Everyone should feel free to put together their own party! Suggested soundtrack. (Dammit, I’m violating my own rules.)

12:54pm (Pacific), Sean: I’ve asked some experts to chime in. Here is Gordy Kane, University of Michigan:

The Standard Model(s) of particle physics and cosmology are wonderful established descriptions of the world we see. They leave out a lot we would like to understand, from dark matter and the matter asymmetry of the universe, to WHY the forces and particles (quarks and leptons) are what they are. LHC won’t tell us much more about the world we see and how it is made, but the discoveries there will point the way to “WHY”. It’s a WHY machine.

The discovery that makes sense is supersymmetry, i.e. the superpartners of some of the Standard Model particles. There’s a lot of indirect phenomenological evidence that indeed some superpartners will be seen at LHC, such as the unification of the forces at very short distances, the absence of large new effects at the LEP and Tevatron colliders, and the very good indirect evidence for a light Higgs boson. A supersymmetric world is also one where we can understand how the electroweak symmetry is broken and how the matter asymmetry arises, and it has a dark matter candidate. I estimate ten or twenty gluinos and a lot of Higgs bosons will be produced in October this year (but not seen unless we are very lucky about the decay signatures). IF the LHC indeed establishes the world is supersymmetric, there is a great bonus – we can write string theories at the Planck scale where the laws of nature should be written and calculate predictions for LHC experiments and dark matter from them, and we can extrapolate data from LHC and dark matter experiments to the Planck scale to see what theories are suggested. Without that window we might never learn the underlying theory from which everything emerges.

It’s very lucky that our technologies and our society allowed us to afford and to build the LHC to study nature so deeply (another anthropic idea?). It’s very unlikely (because of technological and financial and cultural limits) that we can ever have a further facility to extend this study, so we’re very lucky that a framework like string theory has emerged, one that addresses all the basic questions, at the same time we may be able to get from LHC the data that can test and establish it.

1:24 pm (PDT), JoAnne: The History Channel (US cable TV) is airing
The Next Big Bang at 8 PM this evening. The show details our expectations for the LHC and features David E. Kaplan of Johns Hopkins as well as many other of your favorite physicists, so don’t forget to tune in!

1:58pm (Pacific), Sean: Ph.D. Comics weighs in.

6:53pm (EST), Mark: BBC World News America, starting in a few minutes on the East coast, and repeated later, will have a piece on the LHC.

4:05pm (Pacific), Sean: Prize for the best paper title goes to Mihoko Nojiri, arXiv:0809.1209.

The Night before the LHC
Authors: Mihoko M. Nojiri

Abstract: I review recent developments on the use of mT2 variables for SUSY parameter study, which might be useful for the data analysis in the early stage of the LHC experiments. I also review some of recent interesting studies. Talk in SUSY08.

4:25pm (Pacific), Sean: There will be a live webcast from CERN beginning at 11pm Pacific, with the actual beam scheduled for half an hour later. But right now you can click the link, and listen to a pre-packaged CERN video. You can also watch the startup on EVO, if you know what that means (or care to learn).

4:50 pm (PDT), JoAnne: Yours truly has just been recruited for a 5 minute live radio interview on KCSB (the station is on the UC Santa Barbara campus) at 7:30 tomorrow morning. I guess David Gross has the good sense to be asleep at that hour! In any case, I’ll be sure to drink some coffee first, lest I spew some gibberish on blackholes.

6:55pm (Pacific), Sean: Sorry, the “live” blogging took a hiatus while I was talking to Hal Eisner, a TV reporter (“extraordinaire,” he asks me to add) from the local Fox affiliate. He, quite rightly, was hectoring me mercilessly in an attempt to explain the purpose of the LHC at a level accessible to six-year-olds. (He also tried very hard to get me to say “God particle,” which I mostly resisted.)

What is the purpose? It’s to discover the laws of nature, of course, or at least extend our knowledge of them. But that doesn’t always quite cut it for people. I think it would suffice to the aformentioned six-year-old; kids are naturally curious, but adults have it beaten out of them by a relentlessly pragmatic world. Among other things, the LHC represents a tremendous triumph of the basic inquisitiveness of the human species.

7:20 pm (PDT), JoAnne: There’s a host of First Beam Day activities planned for tomorrow across the US. Check the listings for an event near you. Here in SF Bay area, swissnex, the annex of the Consulate General of Switzerland in San Francisco, is throwing a party tomorrow night in coordination with SLAC and LBNL. Much fun will be had by all!

7:53pm (Pacific), Sean: If you’re wondering whether the Large Hadron Collider has destroyed the world yet, see here.

If you’re wondering whether physics is more or less tawdry than politics, see here.

8:17pm (Pacific), Sean: The right response to end-of-the-world chatter is to change the subject — it’s just crackpottery, not a legitimate scientific debate. But damn, you have to be impressed with the vigor of the meme. Far and away the first thing that comes to mind when a person on the street hears “giant atom-smasher in Switzerland” is “might destroy the world.” How do we combat that? What is the one idea we would like to pop into people’s minds when they hear that phrase, and how do we get it there?

11:52pm (EST), Mark: Gotta sleep, but will try to tune into BBC Radio 4’s Big Bang Day when I wake up!

9:26pm (Pacific), Sean: Reporting now from the High Energy Physics conference room here at Caltech. In an hour and a half we’ll open a live feed to our colleagues at CERN, who will be updating us on what happens. Of course, the best answer is simply “all systems nominal.” The only way a detector will actually see anything (as I understand it) is if the beam is not focused perfectly from the start, which is perfectly possible. If the beam is well-behaved, it will just zip through.

But of course, there are many steps along the way, and “first protons circumnavigating the accelerator” is as good a “turn on” event as any. Folks in the know have assured me that CERN will not be hosting multiple “trust us, this is the real start” events — this is it.

9:48 pm (PDT), JoAnne:
From looking at our comments, it’s clear that some folks are still genuinely frightened by the LHC. This should not have happened. The LHC is one of the most exciting scientific journeys in our lifetimes! We should all watch it in wonder and be amazed at its discoveries.

Many a thoughtful, carefully analyzed and written scientific treatise has appeared which thoroughly disproves the claim that the LHC will destroy the Earth. But these aren’t published or mentioned or taken seriously by the press…. (HELP – I’m sounding like a Republican!)

So, let me present a different, non-scientific, but emotional argument. We physicists are human beings too. We have children, parents, siblings, friends, etc, that we care deeply about. We care about this planet and its future and the future of our families. There are literally thousands of physicists, worldwide, involved in the LHC. If there was a serious concern, the scientists themselves would have stepped forward.

As for me, one of my best arguments is that my bottle of 1990 LaTour remains in my cellar. I’m going to pull it out when we achieve collisions at the next accelerator after the LHC! Oh – and the fact that I’ve just spent the last 8 months undergoing intensive, arduous treatment for cancer so that I too can have a future and be a part of the LHC.

10:00 pm (PDT), John:

B minus two hours. Oh yeah! We’ve waited a long time for this.

11:03pm (Pacific), Sean: Action is heating up, although the pizza has yet to arrive. So I’m going to start paying attention to the “real world.” I’ll come back if any disasters occur.

11:30 pm (PDT), John:

Looks like CERN has stuck a PR video in place of the live webcast…not too surprising…maybe the site got hammered, or they have that up until it starts.

The SPS is cycling nicely. That’s what they’ll use to inject the beam in 30 minutes.

11:37 pm (PDT), JoAnne: This is the error message I’m getting:

Due to a huge interest for this live video feed of the LHC First Beam day, you may not be able to see the live video stream and we apologise for this.
Please try reloading the page, come back later, or check the other connection options available on this page.
Many thanks for your interest in CERN and the LHC!

The folks at CERN should have planned for heavy traffic – I’ve waited 25 years for this and I’m disappointed.

11:48 pm (Pacific), Sean: Getting updates from CERN. No disasters, but there was apparently a tiny glitch with one of the collimating magnets, which has now been fixed.

The current beams are low energy (450 GeV, lower than the Tevatron at Fermilab). They want to ramp up to 5,000 GeV (5 TeV) by the end of October — on October 21st, there is a get-together featuring heads of state, and they would love to have actual high-energy collisions by then.

They will be circulating the beam in both directions — just not at the same time, at least today.

The computing system involves about a hundred thousand processors — soon to be upgraded to a few hundred thousand. Data flies from CERN to Caltech at about 40 GB per second, which they also want to upgrade by a factor of ten.

11:58 am (Pacific), Sean: The webcast is limited to 2000 connections! Who’s the rocket scientist behind that?

Midnight (Pacific), Sean: ~~First beam! Or so they say.~~ (See below.)

12:03 am (PDT), John:

Woo hoo! Did it work?

I think it actually starts in a few minutes. The press kit says

9:00 Live satellite broadcast and webcast begin with an introduction from the commentators in the CERN Control Centre, an animation showing the passage of a beam through the LHC, and highlights of the LHC operators’ daily meeting where they lay out the procedure for getting the first beam circulating in the LHC.

9:06 Coverage begins of the first attempt to circulate a beam in the LHC. Lyn Evans, LHC project leader, will narrate the proceedings from the CERN Control Centre. Video of accelerator operators at work in the CCC will alternate with views of the LHC apparatus in its tunnel 100 meters underground.

12:08 (Pacific), Sean: Well, there was a video countdown. No human being has actually confirmed yet…

12:11 am (PDT), JoAnne: Only 2000 connections? No wonder nobody can get on! With all the hype they should have planned better than this….

12:22am (Pacific), Sean: Robert Aymar, CERN Director General … is speaking in French. Translation: in a few minutes we will let the beam zip through the LHC, sector by sector. (They stick absorbers in the way of the beam at certain points, just to check things in each sector before letting it go.) Sounds like the whole thing will take some time.

Liveblogging closer to the source from Adam Yurkewicz, and from David Harris.

I can’t update our blog because too many people are trying to read it!

12:33am (Pacific), Sean: First beam for real! We saw it! Not yet all the way around, as per previous update.

12:36am (Pacific), Sean: BBC reporter: “Ooh! This is exciting!”

12:38am (Pacific), Sean: Okay, I think the beam they had was … actually still in the injector, not the LHC. Because now there is really beam in the LHC! Still not all the way around.

12:40am (Pacific), Sean: Carlo Rubbia seen wandering around the LHC control room.

12:46am (Pacific), Sean: They removed another absorber, and now the beam has reached CMS! I think that’s 3 octants from the beginning.

1:02am (Pacific), Sean: They’ve made it about half way around, and are preparing a beam dump. Sadly, our reserved time on the videoconference has run out, as has my stamina, so I’m heading home. They’re predicting that a full circle will be achieved in the next half-hour or hour.

See you tomorrow!

1:12 am (PDT), JoAnne: The beam is at Point 8, which is 3/4 of the way around! Thanks to SkyNews for the feed!

1:18 am (PDT), JoAnne: Now the beam is at ATLAS, 7/8 of the way through. They are giving ATLAS some events (not collisions, but beam halo and beam gas). Lyn Evans, LHC project manager, was heard to say that he’s going to win his bet, whatever that is.

1:23 am (PDT), JoAnne: BEAM! We have BEAM! All the way round! Now they’re doing it again.

1:43 am (PDT), JoAnne: SkyNews has just interviewed folks in the control rooms for each of the 4 experiments. All of the detectors turned on without trouble and are excited to be getting beam halo and beam gas events. LHCb and ATLAS saw the muons from the beam dump!

Now that the beam has safely travelled through the full accelerator, it’s time for some shut-eye.

7:38 am (PDT), JoAnne: Turns out that the live radio interview was with KCBS here in the Bay Area (which makes much more sense than KCSB in Santa Barbara – our communications department got that wrong!) and just finished. They mainly asked questions about the operation of the accelerator, what comes next, etc. They did ask if the research was open and if all the results would be public or if some of it would be kept secret. And, yes, the subject of those pesky blackholes came up…

9:34 am (Pacific), Sean: As commenters have noted, Google has caught the fever:

But here is something better: the signal from ATLAS when beam first went through.

Click for the full glory!

Live-Blogging the LHC Startup! Read More »

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Guest Post: David E. Kaplan on the LHC on the History Channel

September 7, 2008 / 17 Comments

You may have heard that there’s some sort of big science machine scheduled to turn on in Europe. Very soon, in fact: first (official) beam at the Large Hadron Collider is supposed to occur around 9:30 Central European Summer Time (3:30 a.m. Eastern, if I have done the math correctly) on Wednesday. Call it Tuesday night, for us West Coasters.

The folks at the History Channel recognize the importance of the event, and they’ve recruited Friend of CV David E. Kaplan, a particle theorist at Johns Hopkins, to host a special show entitled the Next Big Bang. And we, of course, have recruited David to tell you a little about the show. (In the picture, David is the one wearing glasses.)

(p.s. This LHC game is surprisingly educational. Via DILigence.)

Update: Hey, I guess this is a preview? Well, not of the History Channel documentary in particular, but closely related (and see David struggling with a bad hair day). Via symmetry breaking.

—————————————————-

Hello All. This post represents shameless advertising for a television program which I am hosting on the History Channel this week. The show is a one-hour program about the Large Hadron Collider (LHC) experiment outside of Geneva and will air the day before the first proton bunch circulates the entire 27 km ring (September 9th, 8pm/midnight EDT/PDT, 7pm/11pm CDT, 6pm/10pm MDT). The show will visually describe the complexity and scale of the experiment and some of the potential discoveries we hope to make (the Higgs particle, supersymmetry, dark matter, extra dimensions).

For many reasons this is an amazing moment in the history of science (many which have probably been repeated on this blog before). [Indeed — ed.] There are roughly 75 countries with at least one institution (university or lab) which has contributed to the construction of this machine. The list includes strange bedfellows: India and Pakistan, Israel and Iran and the United States, Greece and Turkey, Russia and Georgia, all of western Europe, most of eastern Europe, some of northern Africa and south America, Japan, China, S. Korea, etc. This unlikely team has constructed the biggest single machine in the history of the planet after over 20 years since the first plans were laid. At 10,000 scientists, this project represents the modern day pyramids.

What gets me though is that high-energy physics have not really seen a discovery that has directly shaken the standard model of particle physics for thirty years. The discovery of neutrino masses were a surprise, but fit nicely in the standard model if there is new physics at (unreachably) high energies. Dark matter was certainly a surprise, but could potentially only couple to us gravitationally, and again not uproot the standard model. The same can be said about dark energy to an even greater extreme. However, an unexpected particle has not been discovered since the seventies. The seventies were the time that not only the standard model was discovered experimentally, but its underpinnings, quantum field theory, was confirmed as the correct underlying description of all matter interactions (other than gravitational). The (perhaps, not so) amazing thing is, the surprising discoveries stopped by the end of the seventies, and we have been confirming the standard model even since.

The implication is that almost the entire particle physics community, both theorists and experimentalists, who are actively working on LHC physics have never been involved in a surprising discovery. This large community of scientists have been building up to this moment for their entire careers. The scale of these experiments are such that one can really only expect one discovery per generation, and this one is ours.

The show is not perfect, but there are some stunning analogies. I did not write the show, but I fact-checked most of it. There is no attempt to scare the viewer with ‘disaster scenarios’, but simply an attempt to cover what the physicists are constructing and what they expecting or hoping to discover. There is also a bit of history of particle physics.

Enjoy the show. I’ll stay connected so I can answer any questions that come up.

Insertion of the tracker into the CMS detector. Photo by Maxmillion Price, copyright CERN. Click for full size.

Guest Post: David E. Kaplan on the LHC on the History Channel Read More »

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What Will the LHC Find?

August 4, 2008 / 202 Comments

With the Large Hadron Collider almost ready to turn on, it’s time to prepare ourselves for what it might find. (The real experts, of course, have been preparing themselves for this for many years!) Chad Orzel was asked what we should expect from the LHC, and I thought it would be fun to give my own take. So here are my judgments for the likelihoods that we will discover various different things at the LHC — to be more precise, let’s say “the chance that, five years after the first physics data are taken, most particle physicists will agree that the LHC has discovered this particular thing.” (Percentages do not add up to 100%, as they are in no way exclusive; there’s nothing wrong with discovering both supersymmetry and the Higgs boson.) I’m pretty sure that I’ve never proposed a new theory that could be directly tested at the LHC, so I can be completely unbiased, as there’s no way that this experiment is winning any Nobels for me. On the other hand, honest particle phenomenologists might be aware of pro- or con- arguments for various of these scenarios that I’m not familiar with, so feel free to chime in in the comments. (Other predictions are easy enough to come by, but none with our trademark penchant for unrealistically precise quantification.)

The Higgs Boson: 95%. The Higgs is the only particle in the Standard Model of Particle Physics which hasn’t yet been detected, so it’s certainly a prime target for the LHC (if the Tevatron doesn’t sneak in and find it first). And it’s a boson, which improves CERN’s chances. There is almost a guarantee that the Higgs exists, or at least some sort of Higgs-like particle that plays that role; there is an electroweak symmetry, and it is broken by something, and that something should be associated with particle-like excitations. But there’s not really a guarantee that the LHC will find it. It should find it, at least in the simplest models; but the simplest models aren’t always right. If the LHC doesn’t find the Higgs in five years, it will place very strong constraints on model building, but I doubt that it will be too hard to come up with models that are still consistent. (The Superconducting Super Collider, on the other hand, almost certainly would have found the Higgs by now.)
Supersymmetry: 60%. Of all the proposals for physics beyond the Standard Model, supersymmetry is the most popular, and the most likely to show up at the LHC. But that doesn’t make it really likely. We’ve been theorizing about SUSY for so long that a lot of people tend to act like it’s already been discovered — but it hasn’t. On the contrary, the allowed parameter space has been considerably whittled down by a variety of experiments. String theory predicts SUSY, but from that point of view there’s no reason why it shouldn’t be hidden up at the Planck scale, which is 10¹⁵ times higher in energy than what the LHC will reach. On the other hand, SUSY can help explain why the Higgs scale is so much lower than the Planck scale — the hierarchy problem — if and only if it is broken at a low enough scale to be detectable at the LHC. But there are no guarantees, so I’m remaining cautious.
Large Extra Dimensions: 1%. The idea of extra dimensions of space was re-invigorated in the 1990’s by the discovery by Arkani-Hamed, Dimopolous and Dvali that hidden dimensions could be as large as a millimeter across, if the ordinary particles we know and love were confined to a three-dimensional brane. It’s a fantastic idea, with definite experimental consequences: for one thing, you could be making gravitons at the LHC, which would escape into the extra dimensions. But it’s a long shot; the models are already quite constrained, and seem to require a good amount of fine-tuning to hold together.
Warped Extra Dimensions: 10%. Soon after branes became popular, Randall and Sundrum put a crucial new spin on the idea: by letting the extra dimensions have a substantial spatial curvature, you could actually explain fine-tunings rather than simply converting them into different fine-tunings. This model has intriguing connections with string theory, and its own set of experimental predictions (one of the world’s experts is a co-blogger). I would not be terribly surprised if some version of the Randall-Sundrum proposal turned out to be relevant at the LHC.
Black Holes: 0.1%. One of the intriguing aspect of brane-world models is that gravity can become strong well below the Planck scale — even at LHC energies. Which means that if you collide particles together in just the right way, you could make a black hole! Sadly, “just the right way” seems to be asking for a lot — it seems unlikely that black holes will be produced, even if gravity does become strong. (And if you do produce them, they will quickly evaporate away.) Fortunately, the relevant models make plenty of other predictions; the black-hole business was always an amusing sidelight, never the best way to test any particular theory.
Stable Black Holes That Eat Up the Earth, Destroying All Living Organisms in the Process: 10^-25%. So you’re saying there’s a chance?
Evidence for or against String Theory: 0.5%. Our current understanding of string theory doesn’t tell us which LHC-accessible models are or are not compatible with the theory; it may very well be true that they all are. But sometimes a surprising experimental result will put theorists on the right track, so who knows?
Dark Matter: 15%. A remarkable feature of dark matter is that you can relate the strength of its interactions to the abundance it has today — and to get the right abundance, the interaction strength should be right there at the electroweak scale, where the LHC will be looking. (At least, if the dark matter is thermally produced, and a dozen other caveats.) But even if it’s there, it might not be easy to find — by construction, the dark matter is electrically neutral and doesn’t interact very much. So we have a chance, but it will be difficult to say for sure that we’ve discovered dark matter at the LHC even if the accelerator produces it.
Dark Energy: 0.1%. In contrast to dark matter, none of the energy scales characteristic of dark energy have anything to do with the LHC. There’s no reason to expect that we will learn anything about it. But again, maybe that’s because we haven’t hit upon the right model. It’s certainly possible that we will learn something about fundamental physics (e.g. supersymmetry or extra dimensions) that eventually leads to a breakthrough in our understanding of dark energy.

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What Will the LHC Find?Read More »

What Will the LHC Find? Read More »

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