What Next for Particle Physics? Author(S): Jon Butterworth Source: American Scientist, Vol

Sigma Xi, The Scientific Research Honor Society

What Next for Particle Physics? Author(s): Jon Butterworth Source: American Scientist, Vol. 103, No. 2 (March–April 2015), pp. 144-147 Published by: Sigma Xi, The Scientific Research Honor Society Stable URL: https://www.jstor.org/stable/43707797 Accessed: 09-06-2021 12:23 UTC

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms

Sigma Xi, The Scientific Research Honor Society is collaborating with JSTOR to digitize, preserve and extend access to American Scientist

This content downloaded from 86.59.13.237 on Wed, 09 Jun 2021 12:23:12 UTC All use subject to https://about.jstor.org/terms What Next for Particle Physics?

The discovery of the Higgs boson at the Large Hadron Collider was a triumph for the Standard Model. Now the hunt is on for a deeper theory of reality.

Jon Butterworth

Quarks, electrons, and neutrinos, as When bosons clump together they of trouble when people re- fermions, all have a half unit of spin. do some fascinating stuff too. The port on the discovery of the This distinction causes a huge differ- condensate they form is responsible for The port Higgs of trouble Higgsword boson, on boson, the master the bosonpar- when discovery the causes master people no of par- end the re- ence in their behavior. the superconductivity in the magnets ticle that allows the other fundamen- The best way we have of under- at the Large Hadron Collider (LHC), tal paticies to have mass. It regularly standing fundamental particles is where we found the Higgs. But it's gets pronounced "bosun," and once, quantum field theory. In this theory, a hard to beat being responsible for the while being interviewed on TV, I saw state is a configuration describing all whole of chemistry, and therefore biol- it spelled as "bosom." To clarify: Boson the particles in a system. The math- ogy. Some theories extend the Standard is the name for a generic class of par- ematics is such that if you swap the Model by relating force-carrying bo- ticles. The Higgs boson is one, but so places of two identical fermions, with sons to matter-particle fermions. They are many other particles. In the current identical energies (say, two electrons), do this by introducing a new symme- Standard Model of particle physics, all then you introduce a negative sign try between them. This symmetry is the particles that carry forces - gluons in the state. If you swap two bosons, so mathematically compelling that it is (strong nuclear force), the W and the Z there is no negative sign. called "super" - super symmetry. (weak nuclear force), photons (electro- Because swapping two identical magnetism), plus the graviton (grav- particles of the same energy makes no Reasons for Supersymmetry ity), if there is one - are bosons. physical difference to the overall state, For a variety of reasons it is not pos- Quarks, electrons, and neutrinos, on you have to add up the two differ- sible to discuss LHC physics for long the other hand, are fermions. (Quarks ent cases (swapped and unswapped) without talking about supersymmetry. are the constituent particles of protons when calculating the actual probability It is a bit weird that there have been 21 and neutrons.) The difference between of a physical state occurring. Adding annual supersymmetry meetings, even them is just spin. But in this context, the plus and the minus in the fermion though there is as yet no experimental spin is a quantum of angular momen- case gives zero, but in the boson case evidence for supersymmetry playing tum. It is a bit like the particle is spin- they really do add up. This means any any role in actual particle physics. Per- ning, but that is really just an analogy, state containing two identical fermions haps it's excusable. At least before the because pointlike fundamental particles of the same energy has zero probabil- LHC switched on, supersymmetry was could not spin, and anyway fermions ity of occurring, whereas a state with arguably the best way to improve the have a spin such that in a classical two identical bosons of the same en- Standard Model of particle physics. analogy they would have to go round ergy has an enhanced probability. Recall that all the matter particles twice to get back to where they started. This fairly simple bit of mathemat- (quarks and leptons) are fermions, and Quantum mechanics is full of semi- ics is responsible for the periodic table all the forces are carried by bosons. You misleading analogies like this. and the behavior of all the elements. might (especially if you are a physi- Regardless, spin is important. Bo- Chemical elements consist of an atom- cist) ask whether this is really a rule of sons have, by definition, integer spin. ic nucleus surrounded by electrons. nature, or a coincidence. What if you The Higgs has zero; the gluon, photon, Because electrons are fermions, not all swapped all the bosons and fermions W and Z all have one; and the postu- the electrons can be sucked into the over, would the world be very different lated graviton has two units of spin. lowest energy level around the nucle- or not? This is a very good question, us. If they were, the probability of that by which I mean that asking questions state happening would be zero, by the similar to this has led us over the years Jon Butterworth is Head of Physics and As- argument above. So as more electrons to some very important and interest- tronomy at University College London and are added around a nucleus, they have ing answers. It is a symmetry question. a physicist at the Large Hadron Collider in to sit in higher and higher energy lev-Symmetry is probably the single most Switzerland. Excerpted from Most Wanted els - less and less tightly bound to the important concept in physics. One of Particle: Tl>e Inside Story of the Hunt nucleus. The behavior of a chemical for the Higgs, the Heart of the Future of the most important theorems we have, element - how it reacts with other ele- Physics by Jon Butterworth. Reprinted by which applies to the classical and quan- ments and binds to form molecules - permission of The Experiment Publishing. tum regimes of physics, is Noether's Copyright © 2015 by Jon Butterworth. All is driven by how tightly bound its out- theorem, named after mathematician rights reserved. ermost electrons are. Emmy Noether. This states that for

144 American Scientist, Volume 103

This content downloaded from 86.59.13.237 on Wed, 09 Jun 2021 12:23:12 UTC All use subject to https://about.jstor.org/terms every continuous symmetry in nature, there is a conservation law. Although symmetry is firmly estab- Hüll ! IF JIB ¡i lished as a useful principle in physics - and in particle physics in particular - supersymmetry has yet to prove itself. Why, then, have there been (at the time of writing) 21 conferences on the topic? As far as I can see there are three big arguments in its favor: It helps with an important problem in the Standard Model. It sort of predicts dark matter, the invisible mass component of the universe. It looks nice. kip. ļ4 iffifc. ^ The first of these reasons has to ■ ■ mH» BflÉKUB KImHHMD do with the Higgs boson. Unlike supersymmetry, the Higgs boson is an integral part of the Standard Model, without which it doesn't work. There is a subtle problem with this, though. Because the Higgs boson, uniquely amongst all Standard Model particles, has no spin, its mass picks up a particular kind of quantum correction. If left alone to do their thing "natural- JmjL ¡I jjj^F ly," these quantum corrections tend to make the Higgs boson millions of times heavier than it has to be in the Standard Model. This was (and is) a real worry for the credibility of the theory. From one point of view, it makes the Standard Model look like a coincidence on the level of one in ten thousand million million. This is about a hundred times less likely than winning the lottery jackpot two weeks running if you buy a single ticket each week. Supersym- metry gets around this because fermions give negative corrections and bosons give positive ones, so if there is an (even approximate) symmetry The ATLAS detector, where the author works, is one of two large devices at the Large Hadron between the two, most of the correc- Collider that gathered the historic evidence for the Higgs boson. The Higgs is crucial to the tions cancel each other out and the reigning Standard Model of particle physics because it explains how particles get their masses, Higgs mass can be sensible without but it leaves major gaps in understanding. (Photograph by Claudia Marcelloni De Oliveira.) fine-tuning things to achieve such a crazy coincidence. There are two types of symmetries although it obviously turns one kind The second argument is to me (technically, the the Poincaré group of ex- of particle into another, it also involves most compelling. Astronomical obser-ternal, space-time symmetries, and a space-time transformation, because vations tell us there is probably internalsome symmetries such as charge), spin is actually angular momentum. dark matter out there (or else we and re- there is a theorem that states that Supersymmetry is therefore a special ally do not understand gravity). Many external and internal symmetries can- loophole in the theorem that says inter- supersymmetry models predict a par-not mix up with one another. Internal nal and external symmetries can't mix. ticle that would be an ideal candidate symmetryfor operations turn one kind In fact it is the only such loophole in dark matter. It may be right behind of you. particle into another (for example, a four-dimensional theory sūch as the When two different branches of science the matter-antimatter symmetry op- one we need to describe our universe. have problems that seem to converge on eration turns electrons into positrons), Because all the other available sym- the same solution, look out for progress. whereas external symmetry operations metries are exploited in nature, with The third argument is essentially the move you around in space-time (for elegant and far-reaching consequences, fact that supersymmetry is a way of example, the translation symmetry it is very attractive to suppose this last pushing ideas about symmetry, which operation just moves an electron to available a symmetry should appear too. have been shown to be a great way of different place). But swapping a bo- Those are three quite strong reasons understanding nature, even further. son for a fermion does both, because for taking supersymmetry seriously. www.americanscientist.org 2015 March-April 145

This content downloaded from 86.59.13.237 on Wed, 09 Jun 2021 12:23:12 UTC All use subject to https://about.jstor.org/terms But they all have their weaknesses too. data, and our conclusions, hit me hard. the universe and sort of sticks to some For the first one, maybe the universe I had seen some of the slides already, particles to give them mass. just got lucky? Or maybe we're miss- and the documentation and analyses That is indeed quite an extreme leap ing something subtle in the Standard behind them. These were the work of to make, based on some fairly esoteric Model that might force these cancel- hundreds of colleagues, many of them mathematics. The only way of proving lations, so they happen without that more directly involved than me in this whether we'd done the right thing or fine-tuning, a bit like cheating on the particular analysis. Years and years of not, whether the field is real or not, lottery. For the second of those rea- work lay behind the results. Seeing Fa- was to make a wave, an excitation, in sons, well, there are other theories that biola declare to all of us what we had the field. This wave is, or would be, can also produce dark-matter candi- done was surprisingly emotional. the Higgs boson. And it has to show dates. And for the third, we know that At the end of the talk, we decided up at the LHC or the field is either not many beautifully symmetric math- to stop calling it an "excess of events" there or is very different from what we ematical ideas have wrecked them- and call it a new boson. After all the expected. There was nowhere to hide. selves on the rocks of data. We shall rumors and the hints, all the projec- Inventing a whole-universe-filling have to wait and see. tions and the hows and whys, finally field to make your math come out we had, beyond reasonable doubt, right is pretty radical. But it was The Big Announcement discovered something fundamentally looking as though it might just have The crucial moment for me, for new. Pretty much anything could worked.in On July 4, 2012, we had seen the LHC's ATLAS detector where principle have turned up at the LHC, something fundamentally new, which I worked, and for the LHC as a because no one had done this before. fit the description of the particle pre- whole started on July 3, 2012. I wasBut if the Higgs boson had not shown dicted by mathematical understanding in Salle Curie, one of the conference up, our understanding of fundamental of previous data, coupled with some rooms below Building 40 at CERN. physics, as encapsulated in the Stan- prejudices about aesthetics, symmetry There are four of these rooms, and dard Model, would have been shown and how a decent universe ought to the weekly meetings of the Standard to be incomplete. Well, let's be frank, hangit together. I don't know about you, Model group were usually held in would have been wrong. but this still amazes me. Salle Curie. However, this morn- The chain of reasoning is amazing. ing Fabiola Gianotti, the spokes- We knew that the origin of mass oc- Where Do We Go from Here? person (meaning boss) for ATLAS, curs at LHC energies. We knew this As I write this, the restart of the LHC would be rehearsing a talk. It was becauseen- two fundamental forces, elec- with higher-energy beams for physics titled, with studied neutrality, "Status tromagnetism and the weak nuclear is expected early in 2015 - April Î, in of Standard Model Higgs Searches force, unify at these energies. The rea- fact. One thing we will definitely do in ATLAS." It would be given on theson these forces look different to us in with that upgraded LHC, and hopeful- morning of the following day, with everyday, a low-energy life is that the ly with other machines, too, is examine webcast around the world. force-carrying particles for the weak very closely how well the Standard It had become increasingly clear, ini- force, the W and Z, have mass and Model works above the electroweak tially to us and gradually to the media, the photon does not. We had, in the symmetry-breaking scale. This energy that this was likely to be the big one. Standard Model, come to the conclu- regime is qualitatively different from But even knowing what was coming, sion that this mass can only happen anything we have looked at before. the moment when Fabiola showed our if a certain kind of quantum field fills In this regime, the electromagnetic and weak forces are in some sense uni- fied. Certainly their strengths are now comparable. Without the discovery of the Higgs boson, this would have been a no-go area for the Standard Model. The theory would have been unable to make predictions for these energies, and would have been relegated to a low-energy "effective theory," stun- ningly accurate for energies below a couple of hundred GeV (billion electron volts), but out of its depth above the electroweak symmetry-breaking scale. With the discovery of the Higgs, the Standard Model has a new lease of life. It can make predictions for very high-energy physics, covering everything even an upgraded LHC is able to reach. This is a bold claim, and putting it to the test will be intrigu- The Large Hadron Collider, housed in a 27-kilometer-long tunnel near Geneva, smashes par- ing. One area I find fascinating is the ticles at 99.999964 percent the speed of light. Four major detectors measure the results of col- theoretical activity stimulated by the lisions; ATLAS and CMS were key to finding the Higgs boson. (Image by Philippe Mouche.) fact of observing a new boson with

146 American Scientist, Volume 103

This content downloaded from 86.59.13.237 on Wed, 09 Jun 2021 12:23:12 UTC All use subject to https://about.jstor.org/terms O35: m Background zz ATLAS Preliminary WÊ Background Z+jets, H-+ZZ^-*4I ¿2 30 " H Background Signal (mH=1 Z+jets, 25QeV) HT7T f 4> 6 - m Signai (m, =150 GeV) Hi« |¿< If lìl 25 7 ■ Sj3nai Cv190 GfcV)

20 is = 7 TeV: jldt = fb'l TeV: /Ldt = 5.8 fb~'H

250 iria [GeV]

A particle event recorded at ATLAS in 2012 tracks the creation and decay of a Higgs boson (left). An energy signal from particle collisions establishd the Higgs's mass (middle). A graphical depiction shows the Higgs field, which binds to other particles and establishes their masses. Your personal weight is a result of that field. (Left two images courtesy of ATLAS col- laboration; right image courtesy of Fermilab.)

a definite mass. A lot of this work is supersymmetry is never likely to go There still must be something beyond very technical, but one general theme away. The beauty and elegance of the it, supersymmetry or no. is a re-examination of symmetries and mathematics behind it, coupled with The most glaring omission is grav- quantum corrections already in the the fact that it is required by string ity. We have, thanks to Einstein, a very Standard Model to see if they contain theory, or M-theory, or most likely good theory of gravity, but it is not more physics than we first thought. any other attempt to bring gravity and a quantum theory. Space-time is the There are all kinds of (possibly mis- stage on which quantum field theory leading) clues scattered around and plays its part, but at some high energy games that can be played. For exam- the idea of a classical space-time comes ple, consider a numerical coincidence. The Standard Model into conflict with quantum field theory, The sum of the masses squared of the and we don't know what happens then. fermions is very close to the sum of of particle physics Other problems and omissions in- the masses squared of the bosons. To is clearly not the clude the small point of the missing 85 put it another way, if you had found percent or so of matter in the universe a symmetry that imposed a condition full story, there still - the dark matter that is visible only that the sum of the fermion masses by its gravitational effects on galaxies must equal the sum of the boson mass- must be something and other astrophysical objects. Is it a es, you could have predicted a Higgs beyond it. new fundamental particle? It certainly mass of about 123 GeV. Not too far off doesn't seem to be explainable by any what we have measured! Standard Model particle. Worse, there is The catch is that there is no symme- dark energy, which makes up 68 percent try we know of that imposes this, so at quantum field theory together, will en- of the stuff (matter plus energy) in the present it is just a curiosity. There are sure, I guess, that it remains an impor- universe. From one point of view, dark other ways one could make "predic- tant part of the toolbox of theoretical energy is just a label for the fact that tions" or hunt for coincidences, and physics, cosmology and mathematics the rate of expansion of the universe is the more ways of looking for a coin- more or less indefinitely increasing, for reasons that are unclear. cidence, the less significant a coinci- What is at stake is whether super- While we are at it, why are we made dence is if you find it. Look a million symmetry has anything to do with of matter and not antimatter? And different places, and you'll probably electroweak symmetry-breaking, or why are there three copies, three gen- find a million-to-one chance turning with dark matter, or indeed whether erations, of the fundamental particles? up. Equally, although a bit of numerol- it has anything to do with any phe- And why does the weak force see only ogy might give a clue, it is only useful nomenon ever likely to be measured particles with left-handed spin, ignor- if it is a clue to a real dynamical theory. in a particle-physics experiment. And ing the right-handed ones? And then The way to go is to make measure- supersymmetry is only (currently) the what about the neutrinos in all of this, ments and do real calculations, not most popular extension to the Stan- and why are they so light when the play number games. dard Model. Cock-a-hoop though the top quark is so heavy? The LHC data, from the LHCb ex- Standard Model may be with its latest There are a lot of seemingly arbitrary periment and CMS (two other LHC success in predicting a fundamental features of nature here that, to a certain experiments) as well as ATLAS, have scalar boson and extending its region type of mind (e.g. mine), plead for a ruled out huge swathes of previously of applicability well above the elec- more elegant explanation than "just be- possible variants of supersymmetric troweak energy scale, the Standard cause." The LHC, and particle physics theories. Yet despite this, as an idea Model is clearly not the full story. more broadly, has a lot on its plate. www.americanscientist.org 2015 March-April 147

This content downloaded from 86.59.13.237 on Wed, 09 Jun 2021 12:23:12 UTC All use subject to https://about.jstor.org/terms