[No Caption]

Olena Shmahalo/Quanta Magazine

Save Save Save Save Save Save Save Save Save Save Save Save Save Save

Physicists at the Large Hadron Collider (LHC) in Europe have explored the properties of nature at higher energies than ever before, and they have found something profound: nothing new.

It’s perhaps the one thing that no one predicted 30 years ago when the project was first conceived.

The infamous “diphoton bump” that arose in data plots in December has disappeared, indicating that it was a fleeting statistical fluctuation rather than a revolutionary new fundamental particle. And in fact, the machine’s collisions have so far conjured up no particles at all beyond those catalogued in the long-reigning but incomplete “Standard Model” of particle physics. In the collision debris, physicists have found no particles that could comprise dark matter, no siblings or cousins of the Higgs boson, no sign of extra dimensions, no leptoquarks — and above all, none of the desperately sought supersymmetry particles that would round out equations and satisfy “naturalness,” a deep principle about how the laws of nature ought to work.

“It’s striking that we’ve thought about these things for 30 years and we have not made one correct prediction that they have seen,” said Nima Arkani-Hamed, a professor of physics at the Institute for Advanced Study in Princeton, N.J.

The news has emerged at the International Conference on High Energy Physics in Chicago over the past few days in presentations by the ATLAS and CMS experiments, whose cathedral-like detectors sit at 6 and 12 o’clock on the LHC’s 17-mile ring. Both teams, each with over 3,000 members, have been working feverishly for the past three months analyzing a glut of data from a machine that is finally running at full throttle after being upgraded to nearly double its previous operating energy. It now collides protons with 13 trillion electron volts (TeV) of energy — more than 13,000 times the protons’ individual masses — providing enough raw material to beget gargantuan elementary particles, should any exist.

Lucy Reading-Ikkanda for Quanta Magazine

So far, none have materialized. Especially heartbreaking for many is the loss of the diphoton bump, an excess of pairs of photons that cropped up in last year’s teaser batch of 13-TeV data, and whose origin has been the speculation of some 500 papers by theorists. Rumors about the bump’s disappearance in this year’s data began leaking in June, triggering a community-wide “diphoton hangover.”

“It would have single-handedly pointed to a very exciting future for particle experiments,” said Raman Sundrum, a theoretical physicist at the University of Maryland. “Its absence puts us back to where we were.”

The lack of new physics deepens a crisis that started in 2012 during the LHC’s first run, when it became clear that its 8-TeV collisions would not generate any new physics beyond the Standard Model. (The Higgs boson, discovered that year, was the Standard Model’s final puzzle piece, rather than an extension of it.) A white-knight particle could still show up later this year or next year, or, as statistics accrue over a longer time scale, subtle surprises in the behavior of the known particles could indirectly hint at new physics. But theorists are increasingly bracing themselves for their “nightmare scenario,” in which the LHC offers no path at all toward a more complete theory of nature.

Some theorists argue that the time has already come for the whole field to start reckoning with the message of the null results. The absence of new particles almost certainly means that the laws of physics are not natural in the way physicists long assumed they are. “Naturalness is so well-motivated,” Sundrum said, “that its actual absence is a major discovery.”

Missing Pieces

The main reason physicists felt sure that the Standard Model could not be the whole story is that its linchpin, the Higgs boson, has a highly unnatural-seeming mass. In the equations of the Standard Model, the Higgs is coupled to many other particles. This coupling endows those particles with mass, allowing them in turn to drive the value of the Higgs mass to and fro, like competitors in a tug-of-war. Some of the competitors are extremely strong — hypothetical particles associated with gravity might contribute (or deduct) as much as 10 million billion TeV to the Higgs mass — yet somehow its mass ends up as 0.125 TeV, as if the competitors in the tug-of-war finish in a near-perfect tie. This seems absurd — unless there is some reasonable explanation for why the competing teams are so evenly matched.

Courtesy of Maria Spiropulu

Maria Spiropulu of the California Institute of Technology, pictured in the LHC’s CMS control room, brushed aside talk of a nightmare scenario, saying, “Experimentalists have no religion.”

Supersymmetry, as theorists realized in the early 1980s, does the trick. It says that for every “fermion” that exists in nature — a particle of matter, such as an electron or quark, that adds to the Higgs mass — there is a supersymmetric “boson,” or force-carrying particle, that subtracts from the Higgs mass. This way, every participant in the tug-of-war game has a rival of equal strength, and the Higgs is naturally stabilized. Theorists devised alternative proposals for how naturalness might be achieved, but supersymmetry had additional arguments in its favor: It caused the strengths of the three quantum forces to exactly converge at high energies, suggesting they were unified at the beginning of the universe. And it supplied an inert, stable particle of just the right mass to be dark matter.

“We had figured it all out,” said Maria Spiropulu, a particle physicist at the California Institute of Technology and a member of CMS. “If you ask people of my generation, we were almost taught that supersymmetry is there even if we haven’t discovered it. We believed it.”

Hence the surprise when the supersymmetric partners of the known particles didn’t show up — first at the Large Electron-Positron Collider in the 1990s, then at the Tevatron in the 1990s and early 2000s, and now at the LHC. As the colliders have searched ever-higher energies, the gap has widened between the known particles and their hypothetical superpartners, which must be much heavier in order to have avoided detection. Ultimately, supersymmetry becomes so “broken” that the effects of the particles and their superpartners on the Higgs mass no longer cancel out, and supersymmetry fails as a solution to the naturalness problem. Some experts argue that we’ve passed that point already. Others, allowing for more freedom in how certain factors are arranged, say it is happening right now, with ATLAS and CMS excluding the stop quark — the hypothetical superpartner of the 0.173-TeV top quark — up to a mass of 1 TeV. That’s already a nearly sixfold imbalance between the top and the stop in the Higgs tug-of-war. Even if a stop heavier than 1 TeV exists, it would be pulling too hard on the Higgs to solve the problem it was invented to address.

Lucy Reading-Ikkanda for Quanta Magazine

“I think 1 TeV is a psychological limit,” said Albert de Roeck, a senior research scientist at CERN, the laboratory that houses the LHC, and a professor at the University of Antwerp in Belgium.

Some will say that enough is enough, but for others there are still loopholes to cling to. Among the myriad supersymmetric extensions of the Standard Model, there are more complicated versions in which stop quarks heavier than 1 TeV conspire with additional supersymmetric particles to counterbalance the top quark, tuning the Higgs mass. The theory has so many variants, or individual “models,” that killing it outright is almost impossible. Joe Incandela, a physicist at the University of California, Santa Barbara, who announced the discovery of the Higgs boson on behalf of the CMS collaboration in 2012, and who now leads one of the stop-quark searches, said, “If you see something, you can make a model-independent statement that you see something. Seeing nothing is a little more complicated.”

Particles can hide in nooks and crannies. If, for example, the stop quark and the lightest neutralino (supersymmetry’s candidate for dark matter) happen to have nearly the same mass, they might have stayed hidden so far. The reason for this is that, when a stop quark is created in a collision and decays, producing a neutralino, very little energy will be freed up to take the form of motion. “When the stop decays, there’s a dark-matter particle just kind of sitting there,” explained Kyle Cranmer of New York University, a member of ATLAS. “You don’t see it. So in those regions it’s very difficult to look for.” In that case, a stop quark with a mass as low as 0.6 TeV could still be hiding in the data.

Experimentalists will strive to close these loopholes in the coming years, or to dig out the hidden particles. Meanwhile, theorists who are ready to move on face the fact that they have no signposts from nature about which way to go. “It’s a very muddled and uncertain situation,” Arkani-Hamed said.

New Hope

Many particle theorists now acknowledge a long-looming possibility: that the mass of the Higgs boson is simply unnatural — its small value resulting from an accidental, fine-tuned cancellation in a cosmic game of tug-of-war — and that we observe such a peculiar property because our lives depend on it. In this scenario, there are many, many universes, each shaped by different chance combinations of effects. Out of all these universes, only the ones with accidentally lightweight Higgs bosons will allow atoms to form and thus give rise to living beings. But this “anthropic” argument is widely disliked for being seemingly untestable.

Béatrice de Géa for Quanta Magazine

Nima Arkani‐Hamed discussing theoretical physics with a colleague at the Institute for Advanced Study in Princeton, N.J.

In the past two years, some theoretical physicists have started to devise totally new natural explanations for the Higgs mass that avoid the fatalism of anthropic reasoning and do not rely on new particles showing up at the LHC. Last week at CERN, while their experimental colleagues elsewhere in the building busily crunched data in search of such particles, theorists held a workshop to discuss nascent ideas such as the relaxion hypothesis — which supposes that the Higgs mass, rather than being shaped by symmetry, was sculpted dynamically by the birth of the cosmos — and possible ways to test these ideas. Nathaniel Craig of the University of California, Santa Barbara, who works on an idea called “neutral naturalness,” said in a phone call from the CERN workshop, “Now that everyone is past their diphoton hangover, we’re going back to these questions that are really aimed at coping with the lack of apparent new physics at the LHC.”

Arkani-Hamed, who, along with several colleagues, recently proposed another new approach called “Nnaturalness,” said, “There are many theorists, myself included, who feel that we’re in a totally unique time, where the questions on the table are the really huge, structural ones, not the details of the next particle. We’re very lucky to get to live in a period like this — even if there may not be major, verified progress in our lifetimes.”

As theorists return to their blackboards, the 6,000 experimentalists with CMS and ATLAS are reveling in their exploration of a previously uncharted realm. “Nightmare, what does it mean?” said Spiropulu, referring to theorists’ angst about the nightmare scenario. “We are exploring nature. Maybe we don’t have time to think about nightmares like that, because we are being flooded in data and we are extremely excited.”

There’s still hope that new physics will show up. But discovering nothing, in Spiropulu’s view, is a discovery all the same — especially when it heralds the death of cherished ideas. “Experimentalists have no religion,” she said.

Some theorists agree. Talk of disappointment is “crazy talk,” Arkani-Hamed said. “It’s actually nature! We’re learning the answer! These 6,000 people are busting their butts and you’re pouting like a little kid because you didn’t get the lollipop you wanted?”

This article was reprinted on The Atlantic.com.





View Reader Comments (42)

Leave a Comment

Reader CommentsLeave a Comment

  • As always, an excellent article by N.W. The author writes: "A white-knight particle could still show up later this year or next year, or, as statistics accrue over a longer time scale, subtle surprises in the behavior of the known particles could indirectly hint at new physics."

    I'm curious to learn what further experimental or data post-processing work has been carried out to confirm the existence of the 125 GeV Higgs bump originally observed in 2012, and if it is at all possible that the Higgs bump might wash away into the background if further data were to be acquired? Can such Higgs data be acquired and extracted simultaneously with the current experiments at 13 GeV?

  • Not nightmare at all. This is an adventure. And we need more people interested in fundamental physics and an anti-dogmatic attitude.

  • I don't like that it is being called a nightmare scenario. There have been many instances in the history of physics where experiment has disagreed with theory. The aether did not exist and these supersymmetric particles might not exist either. There are other theories independent of supersymmetry and string theory that have not been ruled out. That being said, it is still disappointing not to have more answers.

  • A gem of an article. Making subjects like this fully understandable, is quite a feat. Outstanding work Natalie. Looking forward to the next post.

  • That there are no experimental News for solving old theoretical problems emphasizes again and more the wrong attitude of the arXiv mediators by suppressing basically New Physical Ideas without at least some instructive explanation or counter reasoning. Extensions of flavor-geometric duality idea of SM mass and mixing hierarchies is one example.

  • It's a nightmare scenario because you're no closer to the truth. It's like spending 40 years building a bridge only to discover that the bridge will never support the weight of the train.

    You've learned how NOT to build a bridge but you still don't know how TO build the bridge correctly.

    Super-Symmetry and "naturalness" offered perfect explanations for all the formulas. It made prediction possible. Now we're literally back to square 1. We're back to coming up with theories and spending decades (and trillions of dollars) trying to test them, if they even can be tested.

    I know it's science and we have to move ahead, but this is the biggest kick in the balls for everyone whose been working on it. That's why there's such a depression about it.

    If you're not a part of it (I am not) it's just ho hum…whatever. But for people I know who are a part of it…it's the apocalypse. Imagine believing that 1 + 1 = 2 only to discover that 1 is not a constant and can sometimes be 9 or 351 or anything.

    For a mathematician that would be a disaster.

  • TG3D — Many further searches for the Higgs Boson have been performed and the evidence has gotten stronger and stronger. At the same ICHEP conference, analyses "rediscovering" the Higgs Boson in the new dataset were presented. The accumulated evidence for the 125 GeV Higgs is very strong, and there is no real chance that it will fade away (the chance would be extremely small). In contrast, the accumulated evidence for this hypothetical particle was much lighter than the evidence for the Higgs now is. (Though, in hindsight it appears that the early Higgs announcement might have jumped the gun a little bit, because it seems like the signal from the real Higgs boson was boosted by a statistical fluctuation in the initial data).

  • If the bump in the 2015 data was a random fluctuation, why the errors bars didn't cross the standart model fitting curve in the first place.

  • @Jack, you say that the Higgs announcement might have jumped the gun a little bit, but isn't that exactly what the 5 sigma requirement is for? Even if the actual signal is smaller than the mean signal at that time, the chance of it being *entirely* due to chance was very low.

    By the way, I find it amusing that in the new data, the diphoton excess seems to have turned into something of a *deficit* around 750GeV (in both the ATLAS and the CMS data). The error bars are much larger than before though, so it's probably just a statistical anomaly again.

  • @Pierre, if the error bars are properly constructed and represent +/- one sigma, only 68% of them should cross the predicted curve.

  • The experimental path forward is the HL-LHC; a higher intensity version of the LHC and in the same location, and at about the same energy – 14 TeV. Is this wise? Isn't much higher energy now the appropriate experimental goal?

  • I am a little confused. Since the standard model has all the particles in what should the next one be? What if there are no more to be found? I that possible given the current theoretical framework?

  • I'd say the excitement has, and the media emphasis should, begin to shift to astrophysics where things actually have been and actually are being discovered: MACHO dark matter, LIGO's gravitational waves and the possibility of primordial black hole dark matter, an estimated 6,000 fast radio bursts per day from unknown sources, and plenty of discoveries in the gamma ray part of the spectrum.
    Why are we so obsessed with particles? Maybe strict reductionism has been leading us down a dead-end rabbit hole. Maybe it is time to come up for air and see the light.

  • Maybe you can illuminate for me what exactly in the realm of MACHO dark matter has been discovered, because apparently I missed out on that. In fact, I don't see anything at all on MACHOs except more unmotivated nonsense. Astrophysics is great but it's not going to get you any motivation on this. Condensed matter physics totally rules now.

  • @rick

    Roughly speaking, naturalness means the following. We believe that the phenomena we measure are encoded by laws of nature that define what happens at very short distances (= high energies). What we actually measure is encoded by a number of short-distance parameters and a rule that tells us how to translate these to observable data. This recipe teaches us the following: if some other particle interacts with the Higgs, the Higgs mass changes drastically:

    m_Higgs = something + g*E_high

    where g is a parameter and E_high is the maximal energy available to the other particle. E_high may be something like the Planck scale – much, much higher than familiar particle masses. Since g is just some parameter one expects it to be not too small – maybe 0.1 or 0.01. But that means that the measured Higgs mass should be extremely high. The only way to avoid this is to very finely adjust g so that m_Higgs lands precisely at 125 GeV. The necessity of this fine-tuning is known as the naturalness problem. There are other ways out, for example by introducing some new structure (supersymmetry) or by making the Higgs not a fundamental particle but "composite", like a molecule.

    Note: for most other particle masses we measure, there is only a very weak dependence on E_high, typically log(E_high). It's really because m_Higgs is proportional to E_high that we're in trouble.

  • Even though it may not be falsifiable at this point, this just adds more circumstantial weight to the many-worlds hypothesis. So many of the phenomena we have observed, from waveform collapse to the seemingly arbitrary gravitational constant, can be explained as illusions caused by the existence of a multiverse.

  • The "anthropic" multiverse theory posits that of the numerous universes, ours is the one with a low low .125 GeV force Higgs boson by random outcome. If this mass-conferring particle varies from universe to universe, it follows that all other properties could vary as well. Thus, why would our laws of physics resemble those of other universes at all…..from a probability perspective.

  • "You shall not find any new physics, because all physical events are interpreted well-known particles (leptons, quarks, and gauge bosons) and forces which have long known (electroweak, gravity, strong interactions)."
    Quznetsov G 2013 Logical foundation of fundamental theoretical physics (Lambert Academic Publ.)

  • Well, Tom, I assume you know how to do a search on "MACHOs" at the arxiv preprint repository. The papers by Hawkins (2010-2016) give the best case scenarios.

    The minimum number of MACHOs generally agreed upon by knowledgeable astrophysicists is of the order of 100,000,000,000. That is an estimate based on several poorly-tested assumptions. Hawkins has shown in a published paper (MNRAS) that reasonable alternative models of our Galaxy allow up to 100% of the dark matter to be in the form of stellar-mass and planetary-mass primordial black holes, i.e., MACHOs.

    We would do well to educate ourselves on these observational realities, and open our minds to new possibilities in dark matter research.

  • Failure is not such a bad outcome. Failure can open your mind to new thinking or new alternatives. Failure may cause ineffective leaders or institutions that have had power to fade and allow more creative minds to take hold.

  • @ Gerd: Thank you for your clear explanation of the concept of naturalness; yours is probably the clearest explanation for non-HEP types I've found on the Internet.

    @Jack: And thanks to you for an update on the current status of the Higgs boson bump at 125 GeV.

  • Perhaps Thomas S Kuhn …his book The Structure of Scientific Revolutions ….could lend a hand We do have a tendancy to be possessed by our possesions and treasure them inordinately perhaps it is a time for ….twisting whiskers. Maybe it's a problem with our imaginations ie: "is the universe made of/like this" …or maybe not

  • An entire generation of physicists have spent their lives and careers on String theory and super-symmetry. All wasted. Why should the laws of physics ever be "beautifully balanced"? Everything breaks down in singularities.

  • @rick, TG3D

    I actually messed up my explanation slightly (it was late at night, sorry!). Actually the "something" in my formula needs to be fine-tuned, not g. However, the idea stays the same.

  • @Gary A Smith,
    I am not sure if yours is a rhetorical question or joke . But assuming it is a serious question, stop quark is supposed to be supersymmetric partner of top quark if it exists!

  • One new approach to getting out of the theoretical doldrums that we have been floundering in for 45 years would be to explore the possibility that fractal geometry offers new ideas and fundamental principles to guide us. A more currently in vogue terminology might be discrete (i.e., "broken") conformal geometry and symmetry.

    Everywhere we can reliably observe nature we find the self-similarity that defines fractal structures and processes. I would be happy to furnish a list of 80 examples of fractal self-similarity from the microcosm, macrocosm and cosmocosm to anyone who requests it.

    Shouldn't scientists observe and take guidance from nature?

  • Physicists are very lucky people. They are always happy. It doesn't matter whether they find something or nothing.

  • Maybe, since the LHC energies do not scale with gravity (as what is measured is in the Minkowski space-time) the results might not include interactions being looked for, e.g. Supersymmetry.

  • «The medium is the message», said McLuhan. But who is the medium of knowledge of nature? It is not only our communication means, but also our observation means. The physical properties of a telescope or particle accelerator determine a priori all physical realities observable through them. So when an instrument of observation does not offer anything new, it means that he has reached the limits of his own powers of penetration into the mysteries of nature. Physics is not an encyclopaedic science, which only observe and classify objects in nature, but is a hermeneutics of nature, ie, an art to interrogate and interpret the responses of nature. Physical objects do not exist in and of itself, but they are created by our own faculty of imagination. Kant said that, two hundred years ago. So if we want to see something new, we must first imagine a different kind of existence and then another way of looking at things.

  • Kudos to Natalie Wolchover. I look forward to reading more of her writing.

    More generally, Quanta Magazine is excellent, and that means QM has a very good editor.

  • What a nightmare it must be. I mean, it is good that we now know more about what we don't have to look for than before, it's still good, but it must be really frustrating for those physicist that have spent their lifetimes on those theories. Anyway, Nature is a heartbreaker so we will just continue admiring its beauty and trying to understand her. She's just such a diva.

  • Nice article! But probably, in relation to the reported disappointment, the broad label of "physicists" should be replaced either by "particle physicists" or "physicists with a vested interest". In particular, those who have worked hard on the beautiful idea of supersymmetry, and haven't given up in the light of many years of negative results (including no proton decay), are seeing their field reduced from physics to mathematics – at least until the next breakthrough in observational particle physics comes along. Funding and the field will decline, at least for now.

    At least they were fighting a good fight, with potential physical relevance, so there is no disgrace in their disappointment (and it must be remembered that the LHC data is certainly not disappointing per se – the LHC team should be rightly ecstatic about having nailed the Higgs!). In contrast, string theorists have only been doing mathematics for a couple of decades now, sitting well outside the physics spectrum.

    There is still plenty of good particle physics data coming in via astronomy, and hopefully from cosmic rays in the future, so the broader field is not yet moribund 🙂

  • "Anyway, Nature is a heartbreaker so we will just continue admiring its beauty and trying to understand her. She's just such a diva."

    Well, Diego, that's the way biology looked until the new paradigm of evolution came along. And that's the way physics looked until the new paradigm developed by Galileo and Newton came along.

    Take home lesson: when things look confusing and/or bleak, search for a new paradigm that can unify and explain what we clearly do not understand. Just tweaking the failed models or resorting to untestable pseudo-science has not and will not get us out of the fog.

  • Natural, unnatural… thankfully it wasn't "supernatural".
    I'm just glad there isn't a black hole over Europe. I have an appointment over there in September.

  • James,

    Not to worry about "a black hole over Europe", but there is a bit of a black cloud regarding societal ferment in Europe these days. Of course this is even more dramatically evidenced here in the good ol' USA. Strange days indeed! Keep a low profile.

  • Another excellent article by Natalie Wolchover, one of the finest science writers of our day! Similar "nightmare scenarios" exist for SETI (still nothing at the 21 cm line), practical fusion (always 30 years away), and dark matter searches; or maybe I am just too impatient.

  • As I tried to argue in my Aug. 10 (6:15 PM) post, maybe the nightmare scenario in the dark matter search is due simply to the fact that we are insisting that it be hypothetical subatomic particles, and there is no dark matter to be found in that mass range.

    There are other candidates that are better motivated and have much more empirical support. Primordial black holes in the stellar-mass range would be one such candidate.

  • I am not surprised nothing was found.
    20 years of "advanced" electronics is going to show you something we can't think of? I mean yes 20 years of electronics has produced smart phone wich allows almost instant communication but the universe we experience has morphed for 13.8 b years (earth years at the speed of light). The truth of the matter is that as long as we can't know what dark matter and dark energy are this science will be on a hold.

Comments are closed.