Mass effect: Maybe Higgs, maybe not

By Phil Plait | December 13, 2011 9:29 am

Today, scientists at CERN in Geneva announced their results for their search for the Higgs boson, a subatomic particle that, if it exists, is thought to be responsible for giving other particles mass. It’s no exaggeration to call it a keystone in quantum mechanics, and finding it for sure will be a huge accomplishment for particle physicists.

So, did they find it?

Maybe. Then again, maybe not.

Um, what? OK, this’ll take a wee bit of explaining.

Last things first

I said Higgs, Magnum. HIGGS.

First, the conclusion, so at least you have that in mind as you read the rest. There are two experiments running at CERN looking for the Higgs particle. They don’t smash particles together, look around with magnifying glasses and tweezers, and then yell “AHA!” when they find one. Instead, they build up a picture of it after doing gazillions of particle collisions. After a year of runs, both experiments see something that might be Higgs, but they’re not 100% sure. One sees something at about the 94% confidence level, the other at 98%. That’s pretty good, but it’s not enough to be completely sure. It seems likely they’ve found something, but it’s like a fuzzy picture: it looks like Higgs, but it still might be something else.

So why can’t they be sure one way or another?

CSI: Geneva

Basically, what the Large Hadron Collider at CERN does is whip protons around at nearly the speed of light, then smashes them into each other. At that speed they have huge energies, and when they collide that energy gets converted into matter: other particles. Like shrapnel, these new particles explode away from the collision site. Many of these new particles aren’t stable; they decay into yet lower energy particles after incredibly short time intervals. For example, electron and protons are almost certainly stable over long times (like the lifetime of the Universe), but neutrons decay after only a few minutes, turning into a proton, and electron, and a particle called an antineutrino.

So these daughter particles from the proton collisions in LHC decay, and they have daughter particles, and some of those decay, and so on. At the LHC there are two ginormous detectors called ATLAS and CMS. Both of these, in essence, measure the energy of the particles that hit them; like forensics team, they look at the aftermath of the collision and try to work backwards to figure out what happened.

We know to some extent how much energy is expected from these collisions due to all the particles that are currently known, so those can be accounted for. But if there’s some excess of energy, that could very well indicate a new particle. And we have theories as to how much energy the Higgs particle should have. So the energies are measured, calibrated for known particles, and the excesses are examined.

What both experiments found is an excess of energy — a bump in the graph — indicating a particle that has an energy* about 125 times that of a proton — right in the expected range for the Higgs particle. That’s exciting! But what they’re doing is counting up things statistically, so they can’t be 100% sure. The bump in the graph is still fuzzy.

Rolling loaded dice

An article on Ars technica gave a great analogy, which I’ll paraphrase: imagine you have a pair of dice. One of them is not normal: instead of the numbers 1 – 6, it replaces the 6 with a 5. If you roll the dice once, you might get 2-3, or 1-5, or 6-4. It’s random. But because one of the die is missing a 6 and has an extra 5, if you roll them enough times that starts to become apparent. After a bunch of rolls, you see too many 5s and not enough 6s. If you roll them, say, three times you might not see anything, but roll them 10,000 times and you’ll definitely know something is up. The more you roll, the more confident you’ll be.

It’s the same thing at CERN. Every run in the collider is a roll of the dice. Do it once and you might see something, but your confidence is low. Do it again and you get better statistics. Do it thousands or millions or billions of times, and you get more confident. In fact, you can calculate your confidence level. For one experiment, the bump at 125 times the energy of the proton has a confidence level of about 94%, the other experiment sees it at about 98%.

Here’s the plot from the ATLAS experiment at LHC:

The horizontal axis is energy — it’s measured in a weird unit called GeV, but a proton has an energy of very close to 1 GeV, so think of the axis as being in proton units — and the vertical axis is a measure of how certain they are the measurement is real, which depends on how many times they saw a particle at that energy. The units are called sigma, and are calculated statistically. 1 sigma means your confidence level is about 68%, 2 sigma is about 95%, and so on. The bigger sigma is, the more confident you can be.

The dashed line is what they would expect to see if Higgs doesn’t exist, and the solid line connecting the dots are what they actually saw. You can see the bump around 125 GeV; it’s way above what you’d expect if Higgs doesn’t exist, and that’s what has everyone so hot and sweaty. The bump tops out at about 2.4 sigma, which is where the 98% confidence comes from. The results from the CMS experiment are a bit noisier, and top out at 1.9 sigma, or 94%.

Exuding confidence

That sounds good, and it is, but in physics we want even better confidence than that. After all, a 94% confidence means there’s a 6% chance of being wrong (as the CERN press release notes, there’s a 3% chance of rolling two sixes with a pair of normal dice on the first try, so you have to be careful here). So while these confidence levels are good, they’re not great. Physicists start getting excited around the 99.99% (4 sigma) confidence level, and start celebrating at 99.9999% (5 sigma). Seriously. After all, at that point the odds of being wrong are literally one in a million!

Some good news too is that this works the other way as well: looking at higher energies, they don’t see any evidence for the Higgs particle. So while they can’t be sure they see it at 125 GeV, they are in fact very confident they don’t see it at energies higher than 125 GeV or so. That’s good: it’s always nice to eliminate possibilities when you’re looking for something.

OK, so what does this mean?

So the grand conclusion here is:

Scientists at CERN cannot claim with enough confidence they have found the Higgs particle, but neither can they rule it out. There’s a good chance they have have found something, and it very well may be real, but they cannot say with complete confidence that it’s the Higgs.

They will continue to run more experiments and try to bump up that confidence level a few more notches. In other words, they have to keep rolling those dice, building up the numbers, and get better statistics. As they do, those confidence levels will change, and hopefully move into the “5-sigma-we-have-a-winner” stage. But that takes time, and it’ll be 2012 at least before we know more one way or another.

If you want to read more about this, I suggest Dennis Overbye’s article at the NYT, and this nice overview by Jon Butterworth at The Guardian.

* In quantum/particle physics, energy and mass are two sides of the same coin. All these experiments measure energy, but that’s pretty much the same as the rest mass of the particle. So if energy makes you uncomfortable, think of it as mass and you’ll be fine. So in this case, what they seem to have found is something that has a mass about 125 times that of a proton.

Related posts:

- LHC smacks some protons! (Includes a video I made when I toured the LHC a few years ago)
- Brian Cox calls ‘em like he sees ‘em
- Breaking: LHC still will not destroy the Earth
- Get your mass handed to you

CATEGORIZED UNDER: Cool stuff, Science, Top Post
MORE ABOUT: ATLAS, CERN, CMS, Higgs, Higgs boson, LHC

Comments (108)

  1. Tomo

    Thanks Phil!

    This explains it oh so much better than the other reports I’ve been reading!

    Also, relevant:

  2. Its looking a lot like higgsmass.

  3. Ari

    How can this subatomic particle have a mass 125 times greater than a proton?

    Quantum Headache?

  4. Navneeth

    I’m sure you’re aware, Phil that it was an update on their progress and not their (final) results as your wording in the first line might suggest to some (it did to me!).

  5. Navneeth

    3. Ari,
    Why shouldn’t it? In fact the most massive elementary particle known — the top quark — is a cousin, so to speak, of the constituents of protons and neutrons, each of which incidentally is also slightly more massive than the proton.

  6. Aaron

    How would they know exactly what it is (i.e. Higgs) and not something else they had not been expecting? I get that theory says something should reside within that range but how do they know it’s not something theory has not yet predicted?

  7. Slim

    Thanks, this is the best explanation I’ve read in years! Amazing how graphs are terribly useless if you have no idea what you’re looking at.

  8. “So, did they find it? Maybe. Then again, maybe not.”

    Hooray perhaps! Eureka possibly!

  9. Great explanation! Although I have to say, 2 sigma is not enough to write home about. Give me 5 sigma and we can talk!

  10. Relativity

    What chance that 125GeV particle is not the Higgs? Is there another particle we are not aware of at that high energy level?

  11. If it pans out, presumably Peter Higgs will get a Nobel Prize. Should one be awarded for the experiments as well? Discuss.

  12. Chris

    it’s measured in a weird unit called GeV
    I guess a GeV is weird for astronomers, but perfectly normal for the rest of us. Although in your footnote you should mention E=mc^2, that is the origin of mass/energy equivalence.

    @9 Relativity
    There is a possibility it is another particle, there is the supersymmetric particles that have been predicted, although no one is saying that’s a possibility. But I think the uncertainty is not so much that there is a different particle popping up here, but counting statistics could be making a false signal. If you look at a lot of random noise sometimes you’ll see something that looks like a signal, but with more integration it goes away. So they are being cautiously optimistic that the signal is real and not some fluctuation.

  13. So the Bayesian in me can’t help but point out that — formalities of 5 sigma aside — this is just about exactly where a plurality of scientists expected to see the Higgs, so from a Bayesian perspective, wouldn’t that make it more likely?

    Obviously this is only an informal argument, but it seems to me that finding a ~3 Sigma result right where we expected to see a bump is fairly convincing — probably more convincing than seeing a 4 Sigma result at a much higher or lower energy level.

  14. Gary Ansorge

    3. Ari

    ,,,because mass and energy are just different ways of looking at the same thing. Remember E=MC^2?

    Then recall that E also equals 1/2MV^2, so by virtue of its TOTAL energy, which is simply rest mass (E=MC^2) plus its energy of motion(E=1/2MV^2), these will add up to an energy/mass 25 times larger than a proton at rest.(fair notice: I left out the mass increase due to velocity equation,,,just keeping it simple, Silly)

    9. Relativity

    “Is there another particle we are not aware of at that high energy level?”

    Maybe, but if it walks like a duck and quacks like a duck,,,it’s probably a duck,,,or a Higgs,,,

    Gary 7

  15. Relativity

    John Butterworth of the Guardian summed this news up best:

    A physicist saw an enigma
    And called to his mum “Flying pig, ma!”
    She said “Flying pigs?
    Next thing you’ll see the Higgs!”
    He said “Nah, not ‘til it’s five sigma!”

    Awesome! ;)

  16. Relativity

    Another thing…if this “discovery” becomes 4+ sigma by end of 2012, what chance is that to be the Higgs and not some “background fluctuations” as Fabiola Gionatti of the Atlas detector demurred? Do we know what that background fluctuation is?

    When the Opera group announced early this year about those superluminal neutrinos, we had to wait for peer review confirmation from the Japanese and American neutrino detectors and that will take some time to happen, if at all. Then we can all celebrate.

    There is no other high-energy particle accelerators besides the LHC (now that the Tevatron is permanently shutdown). How are we to believe Geneva’s future announcement if we can’t even perform a similar experiment elsewhere?

  17. Ari

    5 & 13 …

    I understand the Mass to Energy Part … its just the mass to mass part that *confuses*

    I smash a proton into particles one of which has 125 times the mass of a proton?

    Or is what I am missing associated with motion vs rest mass?

  18. Chris

    @16 Ari
    You have 2 protons colliding. The total energy is > 1 TeV (1 TeV = 1000 GeV), out of that collision you have a bunch of new particles created. Energy has been converted into mass. Now that collision had plenty of energy to make your Higgs or what ever else. The reason you need such an excess is that protons are made of quarks, so really the energy is divided between the 3 quarks. With a possible muon collider you could get all the energy in one particle.

  19. Michael Swanson

    When I was twelve I knew that molecules were made of atoms, atoms were made of protons, neutrons and electrons, and I think I had just heard that quarks were what made protons and neutrons. I liked it. Now there is such a dizzying array of subatomic particles and energies that I find to be an unfathomable mess. When I try to comprehend it I just end up feeling like an old man yelling about how the stuff kids listen to these isn’t even music!

    Get off my lawn!

  20. “… a subatomic particle that … is thought to be responsible for giving other particles mass.”

    Is that because it has plenty to spare?

  21. Quatguy

    Great description, thanks Phil. Most of the science reporting on this has been crap so it is great to have it spelled out by someone who sounds like they know what they are talking about using every day analogies.

  22. Joel Green

    Wait…does this mean schools are going to have to start teaching gravity as more than just a theory?

  23. Chris

    @19 Bill
    If the Higgs didn’t exist then all particles would be massless and travel at the speed of light (or we’d need a major new theory). But there are virtual Higgs popping into and out of existence in ~10^-26 seconds (Variation of Heisenberg Uncertainty Principle). They couple to particles like the electron and proton slowing them down and giving what we view as mass. Think of the Higgs as the floor covered with the tennis ball flufff. Have little kids run around. Now put Velcro bottoms on their shoes and they slow down because they are coupling to the floor.

  24. Ari

    Maybe its the description of the particle being “subatomic” that’s causing my brain to hurt!

    Not sure about density but something 125 *more massive* than a proton makes me think LARGE. I guess not large on the scale of an atom but certainly large on the scale of a nucleus.

    Guess I don’t think of stuff bigger than protons/neutrons as being subatomic.

    Now something small moving really fast (the same thing) on the other hand I get :)

  25. Marianne Brooks

    Nice post and it’s exciting news but maybe also a justification for this hugely expensive, billion dollar, CERN project. So far the actual results of CERN didn’t really live up to the expectations… I’ve read the main popular science book about the Higgs boson ( and will follow the news closely in the coming months to see if they have finally found it (or not).

  26. Pete

    Ari: It might help you to think of a proton as mostly empty as well. The quarks it is made up from are *way* smaller than the proton itself is (or maybe “appears to be”).

    It’s like an atom, just smaller…

    Also, the concept of “density” doesn’t really apply any more at those scales.

  27. Chief

    21. chris

    I liked the explanation on the use of velcro in relation to the Higgs. I’m still trying to get my head around how the higgs does this with such a short life span. or am I wrong and it is not popping in and out very rapidly to create a so called smooth field.

  28. Tom

    Ari #24, I think I understand your question, but I don’t have the answer. Maybe a different way to ask it would be, what is the mass of a Higgs at rest? Can it be more massive than a proton at rest, if it is actually a part of the proton?

  29. Jeff


    they’ve pinned down the expected Higgs mass now and that’s within it.

    A major prof. of mine, David Cline was on Rubio’s CERN team that made the last big experimental discovery in particle physics, the W,Z bosons and I am reminded I still have the paper. This would be great if they discovered the Higgs at last.

    I really wonder if officialdom isn’t just hedging because they don’t obviously want to get caught offguards with something this big. but everything I’ve read makes me very optimistic about this and they’ll confirm next year. I’ve worked in colleges for 30 years and I know how very, very conservative their official pronouncements are, and how many times I scratch my head and say, that isn’t what I would have announced.

    If this isn’t the Higgs, then what, do they have to rework the whole standard theory?

  30. brett

    Hey what’s the big deal,has been a guy on ebay selling quality used Higgs’ bosuns for ages. Come nicely packaged too.

  31. abadidea

    22 Joel Green: I’m sure you’re kidding, but there’s an extremely widespread misconception that “scientific theory” is the same thing as “scientific hypothesis”. Calling it a “theory” is already the highest accolade we can give it. In other words, when creationists say “evolution is only a theory,” they’re saying “evolution is only an incredibly widely tested, evidenced, and explanatory model of biological reality”

  32. QuietDesperation

    That does it, CERN. We’re splitting up. I tire of the teasing. Call me when you’re ready to commit to something solid.

  33. Jess Tauber

    Given the paucity of females in physics, one thing you WON’T be hearing is that old song about the girls of sigma five…

    Which won’t matter anyhow since the signal is really the Old Ones burrowing into our plane of reality. Lovecraft saw this coming. So did Sponge-Bob.

  34. Turing E

    If the Higg’s boson is responsible for giving mass to other particles, why does it have mass? Legitimate question. I don’t understand the physics of the Higg’s boson very well, but it seems counter-intuitive.

  35. MNP

    Since I dislike the implications of physics as it stands, I hope they don’t find it because that means a rethinking of large parts of physics. But either way, I hope they keep experimenting.

  36. OtherRob


    If the Higgs didn’t exist then all particles would be massless and travel at the speed of light (or we’d need a major new theory).

    The answers to these questions may simply be above my head, but I’m gonna ask anyway….

    Why would all particles be massless and travel at the SoL without the Higgs? And if the Higgs is true, what makes it special? Why does it have mass while all the other sub-atomic particles don’t?

    Or am I looking at this the wrong way? I think that in my head I’m seeing the Higgs as a bunch of tiny spheres clinging to the spheres of other sub-atomic particles weighing them down. And I’m wondering why the other particles don’t just “have mass” on their own.

    Any help anyone could give me would be greatly appreciated. Just be aware that I have a Liberal Arts degree. ;)

  37. Woolfman

    Can someone (Phil?) explain why the Higgs needed the ultra-high energies provided by CERN to find it? A 130 odd GeV is less than those particles shown further up the graph. I suspect I am either incorrectly understanding the Higgs relationship of mass to energy or we’re talking about the total energy of the collision needing to be higher to allow for the individual “sums” to show up.

  38. Nate

    @ James #13: It is my understanding that previous theories have limited the energy range of the Higgs and that multiple experiments over time have gradually shrunk that range down (because they didn’t find it at those other energy levels) to what we have now. Think of it as “it’s always in the last place you look” rather than knowing a priori. Basically, if they don’t find it in the range they are currently looking at, they’ve more or less exhausted the current theories and we are missing something fundamental – an equally interesting discovery, if you ask me.

  39. Wayne Robinson

    Lisa Randall described how the Large Hadron Collider works in her book ‘Knocking on Heaven’s Door’ by noting that nucleons (protons and neutrons) consist of 3 quarks in a sea of virtual particles, which flash into and out of existence. If you add the masses of the 3 quarks together, they come nowhere near to the actual mass of the nucleon. The virtual particles make up much of the mass of the nucleons.

    If you collide together two protons traveling at almost light speed in opposite directions, the two protons, which consist mostly of empty space, pass through each other like clouds or flocks of birds, and it’s the occasional collision of the virtual particles which are of interest since they have the mass and energy to produce novel particles, the rest of the protons proceeding onwards, unimpeded.

    Whether the Higg’s boson is virtual or real is irrelevant. The force of repulsion between two electrons is mediated by virtual photons, not real ones.

    I wonder, if Higg’s bosons are verified, would they be the basis of dark matter? Would they clump in the right distribution or would they even respond to gravity?

    Particle physics, quantum physics and cosmology are too much for my Newtonian brain …

  40. Justin

    @ Turing E #35:

    The Higgs boson isn’t actually the thing that gives particles mass… it’s just a marker that’s left behind. It’s the massless Nambu–Goldstone bosons which are absorbed into particles causing symmetry breaking which really gives particles mass.

    So the Higgs boson is a byproduct of the Higgs Mechanism. That’s why it has mass.

  41. Justin (41): The article was already very long, and I didn’t want to stat talking about Higss fields. :) At some point you have to simplify to get to the main, more important point.

  42. Marc JX8P

    Thanks for your great explanation! For someone like me who has no background in these matters (pun not intended – but still funny…) but who is very interested in what these discoveries actually mean this is so much more helpful than the short blurbs you get in the news and the half-hearted analogies that you get there.

  43. Justin

    @ Phil #42:

    I wasn’t criticizing your article. Quite the contrary. Your article was great. I was only responding to Turing E’s question, “Why does it [the Higgs Boson] have mass?” That’s all.

  44. Tara Li

    So what is that -2 sigma dip before the “Higgs Bump” (Roughly 114 GeV)? Or the excess starting at about 138 GeV? Are they so focused on this one thing that maybe they’re missing a whole forest of other exciting species?

  45. Julius.Mazzarella

    I am a little confused. How did we know when quarks were discovered that they were quarks? There was no name on it that said. I am a quark. Does that mean that all the particles we have discovered there is some chance they are not what we think. If not then what makes the Higgs different. Maybe someone can explain if there is any difference between looking for any other particle we have found to date and the Higgs.

  46. Wow, fantastic explanation of what’s going on. The best I’ve seen yet! Thanks, Phil!!

    I was slightly disappointed that an actual “mass effect” was not discovered; when I saw that title I was looking forward to interplanetary spaceships and personal forcefields ;)

  47. Heh, found this on teh Wiki for the Higgs, thought it was cute:

    The Higgs boson is often referred to as “the God particle” by the media, after the title of Leon Lederman’s book, The God Particle: If the Universe Is the Answer, What Is the Question? Lederman initially wanted to call it the “goddamn particle,” because “nobody could find the thing.”; but his editor would not let him.

  48. chris j.

    does this mean that the earth is in imminent danger of being crushed down to the size of a pea?

    in terms of the science involved here, i won’t be able to understand it unless it’s presented in rap form by people dancing and wearing hard hats. paging alpinekat…

  49. VinceRN

    This was a great explanation. Thanks.

  50. Brad

    Chris J.: Possibly, but in that timeline the Space Shuttle was still flying, so who knows?

    (Pity most of that season sucked, however.)

  51. George Kopeliadis

    Thanks Phill. Now I start understanding the situation.

  52. James

    @37, OtherRob:

    Particles can’t have mass on their own because it violates something called SU(2)L symmetry (you don’t need to worry about what this is, but it’s somewhat analogous to the rotational symmetry we see around us). This symmetry must be preserved for the theory to make sense (to explain why would go far beyond what you need to know). The way the Higgs field gives particles mass is essentially via the back door – it couples to particles and then in its lowest energy states, it assumes a constant value everywhere, instead of being zero as we’d expect (most fields vanish in their lowest energy state). This mimics “real” mass, but it’s actually just a dynamical field that happily preserves SU(2)L symmetry.

    As has been indicated further up, it’s the Higgs field which gives particles mass, not the Higgs boson – the boson (particle) itself is essentially a ripple in the Higgs field, which acquires mass in much the same way as all the other particles.

  53. James

    @47 Julius.Mazzarella:

    The quark model was suggested in the 1960s. It said that hadrons (protons, neutrons, etc) were composite particles, consisting of smaller particles we chose to call “quarks” (well, Murray Gell-Mann did, George Zweig called them “aces”, but that didn’t catch on).

    So we’ve never seen an individual quark, but the model makes various predictions – we’ve done numerous experiments, found very good agreement with the predictions, so we conclude that protons do indeed have particles with them. They match the properties of what we call “quarks”, so lo and behold, we declare that we have found quarks!

    As for the Higgs, there will be various consistency checks to perform to verify that it is indeed the Standard Model Higgs – ie. it will have to decay into other particles in the correct ratios predicted by theory. I would also assume they’ll need to check that it is what’s called a “scalar” particle, but they’ll need a lot more data than they currently appear to have to verify that.

  54. James

    @ 40 Wayne Robinson:

    No, the Higgs is not dark matter, principally for the reason that is highly unstable – when produced in the LHC, it wouldn’t even make it to the detectos before decaying, so it couldn’t hang around long enough to be dark matter.

  55. Jim

    Chris @12: “I guess a GeV is weird for astronomers, but perfectly normal for the rest of us”

    The rest of us except me :-)

    I think I understood this briefly, when I was studying physics in secondary school, but now I find it hard to grasp discussing mass in terms of (what looks like) electrical potential difference!

    Wikipedia reminds me that it’s technically a unit of energy, so we’re back to m = E/c^2 again ;-)

  56. complex field

    @ari: it has been about thirty years since i have messed with qm, but i think part of your headache is the equating of mass with physical extent. basically (iirc) due to the heisenberg uncertainty principle, knowing the mass or extent to any degree of certainty reduces the certainty of the other. so, if you know the mass of some particle to some exacting degree, eg 5 sigma, then its extent becomes essentially unknowable. wikipedia has an excellent treatment. also, as pointed out above, when you have a 1 TeV collision, you are bound to get some very massive particles as a result of converting the kinetic energy to mass.

  57. #57 Jim:
    Moving an eletrically charged particle through a potential difference requires ( or releases ) energy. The electron-volt ( eV ) is defined as the energy required to move the charge of an electron through a potential difference of one volt. ( In dimensional analysis terms, charge times potential difference equals energy. )
    The eV, and multiples thereof such as MeV and GeV, is used to measure the energy of subatomic particles, because measuring it in more conventional units of energy, such as ergs or joules, would give us impractically tiny numbers.

  58. Timmy

    You are just trying to confuse me with all this statistical claptrap. I just want to know one thing: Did those CERN scientists kill God or what? I need to know if I have to get up early for church on Christmas.

    The best quote I read so far:
    “We don’t call it the ‘God particle’, it’s just the media that do that,” a senior U.S. scientist politely told an interviewer on a major European radio station on Tuesday.

    “Well, I am the from the media and I’m going to continue calling it that,” said the journalist – and continued to do so.

  59. Svlad Cjelli

    Why can’t this boson behave in an orderly and lawful manner?

    Because it’s a




  60. OtherRob

    @James, #54

    Thanks for the explanation. I’m slowly trying to wrap my head around these concepts. Earlier in the thread Wayne mentioned his “Newtonian brain”. I think I have one of those too.

    Anyhow, I see that Phil posted some more pretty pictures so I’m going to go look at those for a while. ;) Shiny…

  61. @59 Neil Haggath: Noob question here, but am I right in assuming that the energy of a particle (in eV) is unrelated to its mass? That is, a more massive particle takes more energy to accelerate to a given speed, but its inertia can carry it through a greater charge differential, so the point is moot?

    Also, I think it’d be easier for laypeople like me to wrap our minds around particle energy as speed. Is it possible to make that conversion, or is it the sort of thing where the particles are moving at such high relativistic speeds that the difference between 10 GeV and 10 TeV is like the difference between 99.96% C and 99.99% C?

  62. @54 James: I think you just blew my mind. Also, the inevitable Trekkie question: if mass is not an intrinsic property of matter, but rather changes in the topology of the Higgs field, does this mean that it might conceivably be possible to create “mass modification” devices (and all the cool stuff that comes with it)?

    @61 OtherRob: I think I have a Newtonian brain too. Like right now, for instance, I really want to eat a fig Newton. Or three.

  63. scott

    well, however it all plays out, when and if they ever figure it all out (if that can be done and then there is nothing else to look for…boring) we will know everything and then i guess we will just work on ways to manipulate it.

    at any rate, it’s all just an “empty” space illusion of all these particles clumping together – out of something from somewhere..?

  64. alex

    I new mass effect wasn’t a lie :D

  65. missy

    loved your explanation! very clear and concise.

  66. Brian Too

    @16. Relativity,

    The goal of experimental replication is a fundamental part of the design of the LHC. That’s why there are 2 detectors, Atlas and CMS, along with 2 completely different teams running each one. Atlas and CMS also have completely different designs and were constructed separately.

    Which makes it all the more significant that both detectors found a signal.

  67. Mike Saunders

    This is a really good post

  68. James

    64. Joseph G:

    The mass of an particle is determined by two things: a) the coupling of that particle to the Higgs field and b) the Higgs “vev” – the value of the Higgs field that it assumes in its lowest energy state (the “vacuum”). From the standpoint of modern physics, both of these things are wholly determined by the inputs of the Standard Model field theory (i.e. they’re simply constants we measure, like the charge on an electron or Newton’s gravitational constant, and they’re not up for change by us).

    Now, that’s not to say that in some more fundamental theory (like string theory) we won’t find these parameters determined by some dynamical physics (indeed I suspect we will), but from the standpoint of modern physics, they’re simply numbers we’re given with no scope for manipulation. Sorry.

  69. @James, many thanks for being patient with folks like me who don’t get it. 8-)
    So my current understanding is that, what seems like mass to middle-sized minds like mine is really just the Higgs field which, alone among fields, has a non-zero value in its lowest energy state. Is that a reasonable precis?

    Is there an anti-Higgs field with a value less than zero?

  70. James

    @73 Bill:

    Yep, that’s a pretty good précis of the idea!

    There are two ways of interpreting your second question. One is the straightforward question of whether or not there is a Higgs with a negative vev. Well, you could try constructing one if you like (though in practical terms, it would be equivalent to introducing a negative coupling of quarks/leptons to the Higgs). There are many different theories of the Higgs and hopefully the LHC will allow us to discard most of them, but I’m not aware of any that have negative couplings. Now, if a matter particle had a purely negative Higgs coupling, it would render the vacuum unstable (the particle would have no minimum energy state, so it would cascade off into oblivion). So it’s not something we consider.

    The second way of interpreting your question is in terms of an anti-Higgs field (ie. the anti-matter partner of the Higgs) – I suspect you may have meant the question in both ways. The short answer is that the Higgs has no antiparticle (or it is its own antiparticle, like the photon). The longer answer is…

    In quantum field theory, charged particles/fields are described by complex numbers (if you’re not familiar with these, a complex number can be thought of as a pair of ordinary real numbers). The two degrees of freedom correspond to particle and anti-particle, so uncharged particles, which are described by just one real number, are their own antiparticle (the photon is one example).

    The Higgs is somewhat complicated. Technically, it’s a complex doublet field – it consists of two complex numbers (the two comes from the SU(2)L symmetry I mentioned in post 54). It therefore has four degrees of freedom; one from each of the real numbers that describe it. When the SU(2)L symmetry is broken (something that is also referred to as “electroweak symmetry breaking”), three of those numbers get absorbed into the force carriers of the weak interaction (the W+, the W- and the Z vector bosons), turning them from massless bosons (like the photon) to massive ones (the reasons why massive bosons have an extra degree of freedom are fairly esoteric).

    This all leaves one degree of freedom for the Higgs. It is thus a neutral, uncharged particle and hence it is its own antiparticle. So, no, there is no anti-Higgs!

  71. @72 James: Thanks for the clarification :)

  72. The higgs mass (125 GeV) Was calculated in our article, see paper published in Pacific Journal of Science and Technology 12(1),214-236)2011
    (Exact value is 125,39 GeV).

  73. @74 James: Please, please tell me that you’re a particle physicist and that all of this stuff is FAR more advanced then the stuff I’m supposed to have learned (had I been paying attention) in Physics 2 :)

    *pressing two brain cells together with my hands* So, does the Higgs field have a lower value where it’s coupled with a particle, or in the “vacuum,” where all the virtual particles hang out? Do particles simply have mass because they displace all the virtual particles which don’t have (net) mass?

    Feel free to give up on me at any time, I won’t blame you :D

  74. #63 Joseph:
    The electron-volt is simply a unit of energy, like an erg or a joule, but far smaller. It’s defined in terms of charges and potential difference, but can be used to measure any kind of energy, such as the kinetic energy of a particle. Similarly, the joule is defined as the energy expended in moving a force of one newton through a distance of one meter, and the calorie is defined as the energy required to raise the temperature of one gram of water by one Celsius degree – but there is no reason why you can’t measure potential or kinetic energy in calories, or heat energy in joules. They are just units of energy, which are defined in different ways, but are interchangeable. One calorie is equal to 4.2 joules. One eV is equal to some very small fraction of a joule.
    So the energy of a particle, whether measured in eV, joules, ergs or calories, is not unrelated to its mass. Its “rest energy” is proportional to its mass, according to E = mc**2; mass and energy are interchangeable. Its kinetic energy is related to its mass and its velocity, i.e. mv**2 / 2.
    When we’re talking about relativistic velocities, it becomes more complicated, as a particle’s mass increases with velocity, according to the Lorentz or gamma factor. ( This effect is negligible at velocities much less than c. ) This is part of the reason why no massive particle can travel exactly at the speed of light – because if it did, it would have infinite mass.
    So regarding your question about energy and speed, you got it in one! The particles in the LHC are travelling at 99.99…% of the speed of light. At those sort of speeds, the kinetic energy increases exponentially; it actually takes more energy to accelerate a particle from 0.99 c to 0.999 c than it does to accelerate from zero to 0.99 c! In fact, for each two 9′s you add to that fraction, the value of the Lorentz factor increases by a factor of 10.
    So talking about the difference in speed between 0.99 c and 0.9999 c becomes pretty meaningless; it makes much more sense to talk about the particle’s energy ( which increases by a factor of 10 in that example ). This is why cosmic ray particles, which also travel at highly relativistic velocities, are also described in terms of their energy.
    Hope this helps.

  75. #63 Joseph:
    I hope I’m not insulting your intelligence; I obviously don’t know the extent of your physics knowledge. Mine is degree level, but I’ve forgotten most of it. I’m OK with Special Relativity, but when it comes to General Relativity or quantum mechanics, forget it!

  76. James

    @76, Joseph G:
    Yeah, I’m a particle physicist (well, I suppose I was – I’ve just finished my PhD, so now I’m unemployed).

    The Higgs field adopts the same value everywhere; it’s the fluctuations on top of that constant field which are what we refer to as Higgs bosons.

    In for a penny, in for a pound…

    To make it clear how the Higgs mechanism works, this is how it appears in the theory:

    In our equations (specifically, in an important quantity called the Lagrangian), there is a term representing a quantum interaction that looks like:


    One of those Qs represents the annihilation of a quark and the other represents the creation of one (I won’t distinguish between quarks and antiquarks in the following). The m is a number that corresponds to the quark’s mass. So, as a quark floats through space, this interaction inherent in its physics causes it to vanish and immediately reproduce itself, with an interaction strength determined by m. In other words, you can think of mass as a self-interaction of a particle.

    The interaction of the Higgs with a quark looks similar:


    Here, the H represents the destruction of the Higgs field and g is a number giving you the strength of the coupling. So this is an interaction which, with strength g, represents a Higgs and a quark annihilating each other and leaving behind a quark. But since we know the fluctuations of the Higgs field happen on top of a constant value, we can split this up as:

    gvQQ + ghQQ

    where v is the constant value (the “vev”) and h is the fluctuation (the Higgs boson). Now, we see that the quark gets a mass from this, where m = gv. So a light particle like the electron has a small coupling g, and a heavy particle like the top quark has a much larger coupling.

    We still have the Higgs-quark interaction hQQ. We can actually interchange creation/annihilation as much as we like, so this interaction can also represent the destruction of two quarks and the creation of a Higgs boson, and vice versa. So one of the ways a Higgs could be produced in the LHC is that two quarks annihilate, produce a Higgs, which then turns back into two quarks, or two leptons, or two W bosons, etc.

    Hope this was at least vaguely intelligible! It’s a somewhat pedagogical explanation, but essentially right.

  77. Jeff

    James, thanks, understood your explanation.

    I was in the same boat as you 30 years ago, and for some reason I took a left turn into community college teaching, and I for one am glad I did, and am near pension time now. Best wishes, and I hope you guys get experimental verification of all your ideas, one way or the other.

  78. Nigel Depledge

    Michael Swanson (19) said:

    When I was twelve I knew that molecules were made of atoms, atoms were made of protons, neutrons and electrons, and I think I had just heard that quarks were what made protons and neutrons. I liked it. Now there is such a dizzying array of subatomic particles and energies that I find to be an unfathomable mess. When I try to comprehend it I just end up feeling like an old man yelling about how the stuff kids listen to these isn’t even music!

    The Standard Model goes something like this (please note this is my own very approximate understanding):

    There are 6 types of quark : Up, Down, Strange Charmed, Truth (Top) and Beauty (Bottom). There are 6 types of lepton, of which three are neutrinos (the others include electrons and positrons).

    Then there are the bosons (particles that mediate forces): the photon (electromagnetic force), W+, W-, Z0 (weak nuclear force), Gluons (strong nuclear force) and maybe Higgs (erm . . . giving the other particles a fundamental property, namely mass – I don’t really understand how).

    So, anyhow, even though you were only made aware of about half of this lot when you were 12, the foundations for it all were laid down by the 1960s.

  79. Nigel Depledge

    Other Rob (37) said:

    Why would all particles be massless and travel at the SoL without the Higgs? And if the Higgs is true, what makes it special? Why does it have mass while all the other sub-atomic particles don’t?

    Don’t forget that Higgs particles must also travel through the HIggs field.

    Or am I looking at this the wrong way? I think that in my head I’m seeing the Higgs as a bunch of tiny spheres clinging to the spheres of other sub-atomic particles weighing them down. And I’m wondering why the other particles don’t just “have mass” on their own.

    What is mass?

    The Standard Model explains the existence of mass as our perception of the interaction of fundamental particles with the HIggs field. I have no idea what alternatives might be plausible.

  80. Nigel Depledge

    Julius Mazzarella (47) said:

    How did we know when quarks were discovered that they were quarks? There was no name on it that said. I am a quark. Does that mean that all the particles we have discovered there is some chance they are not what we think. If not then what makes the Higgs different. Maybe someone can explain if there is any difference between looking for any other particle we have found to date and the Higgs.

    Our names for these “particles” are really just labels for a set of measureable properties.

    So, something might be a proton because it possesses properties A, B and C; whereas a neutron has properties C, D and E. And so on. Our minds insist on considering these things as little billiard balls, but really they are just clouds of probability. It’s all quantum.

  81. OtherRob

    @Nigel Depledge, #83:

    Don’t forget that Higgs particles must also travel through the HIggs field.

    Don’t other particles travel through the Higgs field? Or do they just not interact with it?

    I don’t know how much of all of this I’m “getting”, but I do appreciate Phil’s article and especially the efforts of everyone on this thread who are trying to explain it to us “Newtonians”. Even if it’s only a little bit, my knowledge of the way the universe works is increasing. And that’s a good thing. :)

  82. So when can we visit the Prothean Citadel? :D

    (Oh wait, not that kind of Mass Effect)

  83. @78 Neil Haggath: Lorentz factor! Thaaat’s the phrase I was looking for. The degree to which an object’s mass appears to increase to an observer due to relativistic velocity (assuming you want to really boil it down without using a bunch of math to define it), right? I kept thinking “time dilation,” but that’s not quite it.
    So a particle’s energy is a function of both rest mass and speed, then? Which is presumably yet another reason it makes more sense to use electronvolts to measure energy rather then just speed?

    @ 79 Neil Haggath: I hope I’m not insulting your intelligence; I obviously don’t know the extent of your physics knowledge.
    Hardly!!! As far as the extent of my knowledge, it’s pretty much limited to whatever I pick up from blogs like this and whatever books are on sale. In a nutshell, my education was interrupted by certain circumstances, so I’m probably the equivalent of a high school dropout in terms of a formal education level.
    Anyway, I really do appreciate you and the others here trying to explain these things to laypeople like myself.

  84. They smash a beam of protons into another beam of protons?

    I’m surprised they don’t do what some of the smaller synchrotrons do, and circulate a beam of protons in one direction around the ring and a beam of antiprotons in the other direction.

  85. So the Higgs field is sort of like a force? That surrounds us and penetrates us? It binds the galaxy together? :D
    Seriously though, this Higgs field sounds an awful lot like the “fabric of spacetime” metaphor that you used to see in science textbooks (that has since fallen out of favor). If the coupling of particles to the Higgs field gives them mass, is this also what warps space in the vicinity of mass?

    @80 James: Thanks! I think I’ll read it a few times before even trying to formulate any questions :D

    @86 Katherine Lorraine: Given what I learned in Mass Effect 1 and 2, I want to stay as far away from the friggin’ Citadel as humanly possible. In fact, I’m amazed it wasn’t simply evacuated [spoiler warning] when the Reaper connection was discovered.

  86. Joseph G wrote:
    >Lorentz factor! Thaaat’s the phrase I was looking for. The degree to which an object’s mass appears to increase to an observer due to relativistic velocity (assuming you want to really boil it down without using a bunch of math to define it), right? I kept thinking “time dilation,” but that’s not quite it.

    I tend to refer to this factor as the “gamma factor”, since the Greek letter (lowercase) gamma is usually used in the equations to represent 1 / sqrt(1 – v^2/c^2).

    Unless you’re Poul Anderson, in which case you take the inverse and call it “tau”.

  87. Scott

    In the year 2011, physicists at CERN discovered a previously unknown subatomic particle.

    In the decades that followed, these mysterious particles revealed startling new new technologies, enabling travel to the furthest stars. The basis for this incredible technology was a force that controlled the very fabric of space and time.

    They called it the greatest discovery in human history.

    The civilizations of the galaxy call it…


  88. James

    @90 Joesph G:

    No, the Higgs and gravity are not linked in that way, though I can understand why many people jump to that thought. In general relativity, spacetime is warped in the presence of energy. Mass is just a property of a particle that indicates that there exists a frame of reference in which the particle is at rest. It is the energy associated with the particle when it is at rest – but energy itself is more general.

  89. Nigel Depledge

    Other Rob (85) said:

    Don’t other particles travel through the Higgs field? Or do they just not interact with it?

    If I understand correctly, Higgs bosons are the mediators of the HIggs field, and any particle (Higgs bosons included) that travels through the Higgs field acquires mass.

    I could be wrong.

  90. James

    The Higgs field is everywhere. It is, by definition, constant. Every particle travels through it; most interact with it; some do not (photons, gluons… neutrinos are a different matter).

  91. Darrin

    I’m Commander Shepard, and this is my favorite blog on the Citadel.

  92. @Joseph G:

    True… maybe one of those off-planet worlds then like the Asari homeworld whatever it was called. I’d like to meet a Turian, they’re so cute :3


    I giggled.


    *ROFL* Thank you, thank you. I was gladly not drinking anything when I read that or you’d owe me a new keyboard.

  93. Joseph G

    @Darrin: Heh, took me a second. I’m not sure how I handled that encounter in the game. I’m pretty sure I punched someone in the face :D

    @Katherine Lorraine: You know how I can tell you’re a geek? You think Turians are “cute” :D

    No offense, I’m a geek too :-P

  94. #88 Joseph:
    The Lorentz or Gamma Factor is indeed also connected with time dilation. It’s the factor by which time slows down as measured by a relativistic traveller, as well as the factor by which mass increases. The time measured by an observer at rest is gamma times that measured by the traveller. Theoretically, by travelling at very highly relativistic speeds, a spacecraft could travel any arbitrary distance within the lifetime of its crew; at 0.9999999c ( IIRC ), it could cross the Galaxy and back in 50 years, as measured by its crew – but of course, they would return to find that 200000 years had elapsed on Earth! The energy required would make this rather impractical, though…
    It’s also the factor by which lengths are contracted. Gamma comes up a lot in Special Relativity!
    So we have two reasons why no material object can reach the speed of light. If it did, its mass would become infinite, and time from its point of view would stop. If you look at the definition of gamma in comment #91, you can see that when v is equal to c, gamma becomes equal to infinity, as the bottom term becomes zero. You can also see why , when v is 0.99… c, adding two more 9′s increases gamma by a factor of 10.
    Note that Lorentz discovered the gamma factor 20 years before Einstein formulated Special Relativity, which explained it. It was initially thought that it was a consequence of everything travelling through the hypothetical “Ether”, which was supposed to be an absolute frame of reference; e.g. the contraction of lengths was thought to be a physical contraction caused by motion through the ether. The famous Michelson-Morley Experiment in 1889 was intended to prove the existence of the Ether, but of course proved the opposite.
    If you would like any further explantion of all this ( within my limited ability; many others could do a lot better ), then feel free to drop me an e-mail via my web site ( click on my name ).

  95. @Joseph G:

    Guilty as charged. Massive geek here. And yes, Turians are cute, Garrus is my favorite Turian ♥

  96. Yavor


    Correct me if I am wrong but will not this Higgs be a light one? And if it is what about the vacuum decay?


  97. Mass is an intrinsic property of matter, so is inertia. So, it cannot be due to any external field.
    Therefore the Higg’s field and particle is not an option. Strangely mass is to be explained
    by another particle having mass? Anyway nobody can explain what fields are made of either.
    I conclude that all fundamental particles are made of tiny strings that vibrate to produce intrinsic energy and therefore mass, that is all there is to mass.

  98. christina knight

    Thank you for being more honest about what has actually been found than I have found on other sites. I am still betting that the Higgs does not exist. However, whatever it is that has been discovered will probably lead to new physics, and that is important.


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