Higgs boson discovery confirmation

Factual Questions
121

The Large Hadron Collider at CERN sends clumps of protons around a 17-mile circular path in both directions. When each clump of protons approaches each experiment’s location, the protons are deflected so as to cross paths exactly in the middle of the detector (CMS shown in the link).

Each proton that runs into a proton heading in the other direction is an independent trial. Protons are made up of quarks and the gluons that bind them together, so each trial is really a collision of a whole mess of particles at once, and the result is generally rather messy. The most interesting cases are where a single quark or gluon from each proton takes the brunt of the collision, and these two particles interact to produce a heavier particle. This heavier particle is usually an unstable one that decays almost immediately, and the decay products are detected (along with any spray from the “spectator” pieces of the protons) by the surrounding detector elements.

The production of a simple Standard Model Higgs boson at the LHC should happen in about 1-billionth of the proton collisions. The fact that it happens rarely is annoying enough, but it’s not the real issue. The real issue is that for every Higgs you produce, there have been a billion other collision producing other things that could mimic the Higgs’ decay pattern. The detectors and the analyses are designed to minimize this identification confusion, but in the end, almost all of the decays that look just like a Higgs decay are in fact not a Higgs decay. (The jargon is that these are “background events”.) As you say, you need to build up a very large number of trials to notice the tiny extra decays due to the presence of the occasional Higgs particle on top of all the uninteresting background.

Nature throws us a bone, though. If you had to just count Higgs-like decays and try to tell if you got more than you “should”, then we’d be miles from discovery. However, if you can measure the momenta of all the decay products, then you can calculate the mass of the particle that decayed into them. You can then look to see not just that you got extra Higgs-like decays but that these extra ones pile up at a particular calculated mass. This is very powerful is demonstrating that a new particle (and not a statistical fluctuation) is the source of the extra decays.

Here are the CMS and ATLAS plots that show the number of Higgs-like decays they see as a function of the mass they calculate for the parent particle using the observed decay products. Of note is that the background events lead to a wide smear of calculated masses. This isn’t because it’s a bunch of differently massed particles producing them but rather that the daughter particles detected in each background case are usually an incomplete set, which causes you to calculate a sort of random answer for the mass. The little bump of extra events you can see in the plot tells you both that there is something extra happening and what the mass of the parent particle is. And the figure shows that CMS and ATLAS see completely consistent values for the mass of this new particle.

These figures are showing only the cases where the Higgs was seen decaying into two photons. This is the easiest decay mode for removing background events since there isn’t as much that can mimic it. (Although as the plots show, there’s still plenty of background!) But unfortunately, it’s also a very unlikely way for the Higgs to decay. Take a look at this plot. It shows all the different ways a Standard Model Higgs can decay (colored lines) and the fraction of time it would decay to each (vertical axis), as a function of the mass of the Higgs. We now know the mass to be around 125 GeV, so we can read off how often it should decay to, say, a charm/anticharm quark pair (labeled “cc”, with a line over one of the c’s) or anything else. The two-photon case is labeled “gamma-gamma” and is way at the bottom. At 125 GeV, it is only 0.2% of the decays. But, it’s still the easiest one to use since all the others have tons of unrelated processes that mimic them, making them very hard needles to find in their respective haystacks.

Each search for one of these decay channels is an independent effort requiring somewhat different approaches. And, while the two-photon case is the most powerful, the others do help (with ZZ being the next best), and so the experiments combine the statistical power from each to get the best final answer possible. Also, a key question is whether the observed rate of decay into each of these different daughter particles follows the pattern expected for a Standard Model Higgs. So far it is all consistent, but the statistical power is very low in most of the decay modes so it is too early to say for sure.

122

First, let’s be clear again that we all appreciate “iamnotbatman’s” incredibly thoughtful and lucid posts (I have some ideas for a book I think they should write, but another thread…) It’s not trying to prove anyone wrong…

To me the discussion is what constitutes fruitful scientific work. No one can really understand a lot of things about physics (time dilation, what’s a probability wave exactly, what does it mean to bend space), but we accept them because they “fit” with our basic conception of the world (e.g., I can sort of imagine that as you approach speed of light, you increasingly enter a different realm).

MWI, to me, doesn’t do that. Main sticking point is I can accept probability waves when talking about a subatomic particle, but not in larger scale systems. We can observe probabilistic effects in subatomic particles, but not in larger scale systems.

So, again, where are the aspects of the wave equation that live on, after a measurement is made? (Before the measurement, no problem that they exist, since we’ve observed that that sort of weirdness goes on in the subatomic realm. Not so in macroscopic…)

I’m not so bothered by the philisophical inconsistencies of the Copehagen Interpretation because they accord with experiment, and they seem no less arbitrary as why particles have certain charges, why we live in a world with 4 forces and not 42, etc.

123

I would support the hugtrino or hugton.

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They exist, but there is no sense in asking “where”. That’s a category mistake, like asking what is the sound of the color red. We can describe them mathematically as continuing to exist and evolving along the same position and time coordinates used before the measurement, but they can no longer interact with us, and therefore we cannot measure them.

The MWI accords with experiment as well. It is predictively equivalent to the Copenhagen interpretation. I have argued that the MWI is less arbitrary than the Copenhagen interpretation, and that the Copenhagen interpretation is internally inconsistent. The charges etc of particles seem arbitrary, but if we came up with a theory that post-dicted those values with less arbitrariness, we would hopefully value it more than the old theory.

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I like ‘hugtron’

126

Thanks, that’s what I was looking for and now it makes sense

127

I have to say, this message board is really great sometimes.

Thanks for this explanation, I feel like I have a decent handle on things now. If I might ask another question, what is it that causes the Higgs boson to appear in the few cases where it does? That is, if we had an extreme level of control over the conditions of the collisions, could we always produce a Higgs boson? Or is this one-in-a-billion rate just a property of the universe, something like a half-life?

128

It’s random. In the same way that decay products are quantum mechanically random, so too are production products. Indeed, the same formalism that one uses to calculate the probabilities of particle X decaying into a, b, or c is also used to calculate the probabilities of a collision producing X, Y, or Z.

(A side point: some of the production randomness at the LHC is due to the fact that complex objects (protons) are being collided, and you could perhaps consider that an experimentally improvable situation. But, you can’t have isolated quarks or gluons, so you can’t have a pure quark or gluon collider. Thus, you are stuck with messy protons, which have many uninteresting ways of interacting. There is design work underway for a next-generation electron-positron collider that would provide a much cleaner environment for studying detailed properties of the Higgs or anything else that may be discovered at the LHC. But even though elementary particles would be collided there, production is still probabilistic. At a 500-GeV e[sup]+[/sup]e[sup]-[/sup] collider, a Higgs would be produced in about one collision out of every hundred.)

129

@iamnotbatman If you had any more patience, I had one more clarifying question on “Many Worlds”

Just wondering if there is not an experiment that could be done to test one aspect of the theory. My understanding of MWI as you described it is (and please bear with me if I get terminology wrong): if a subatomic particle that has two states (“up” and “down” let’s say) collides with an aircraft carrier we’re standing on that measures the particle in the “up” state, then there is also a system consisting of the aircraft carrier and the particle in the “down” state that continues existing in the same location and time as the aircraft carrier we’re on (but which we can’t observe).

So, one might wonder why that aircraft carrier is not detectable because of its effects on gravity, etc., and I assume the answer is the same reason that all the possible states of an “undetermined” subatomic particle don’t have effects on gravity. So, to test the possibility of both states of the aircraft carrier existing in the same place, could you run an experiment where you shoot millions of paths of “undetermined” particles so that they intersect (i.e., occupy the same time and place), and see whether the results of the experiment are equivalent to if there were no intersections? (Because, if there were any interference between the particle paths, wouldn’t that say you can’t have different wave equations/aircraft carriers occupying the same space?)

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Re: Higgs Boson Humor

Apropos of this discussion, here’s http://iowahawk.typepad.com/iowahawk/2012/07/dnc-scientists-disprove-existence-of-roberts-taxon.htmlsome theoretical physics satire (definitely from a somewhat right-wing perspective, but I’m a lefty and find it very funny):

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So those LHC doomsday predictions might come true after all…

132

So again to try the patience, just another clarifying question.

Am I right in understanding that the Higgs boson doesn’t normally exist in nature, but producing it was the best (maybe only) method of proving that the Higgs field does exist in nature?

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On the profession, which has now entered the public consciousness since, well, forever, and some basics questions.

If you showed the CMS and ATLAS plots to any able particle physicist, would he/she shout “eureka, praise the lord,” etc., or is Higgs stuff more specialized?

In those plots of decay paths, did figuring each path take lifetimes of research and ultimately testing? And rate a Nobel or two?

Do people know–or what results are people entertaining–of what happens when you keep smashing protons at higher and higher speeds?

Why were protons chosen?

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What wonderful responses to my original question! Is it any wonder that I love this message board? That comment by iamnotbatman on Schrodinger’s equation upthread just blew me away. And the religious types talk of scientists taking the mystery and wonder out of the universe! The reality that lies at the root of ourselves and the whole universe is proving to be orders of magnitude more awesome, wondrous, breathtaking, staggering than a whole host of creation myths.

By the way I suppose one could date the beginning of science to the first caveman curiously breaking something to pieces to see what it was made of. And isn’t that what we’re still doing with our super colliders? With a teeny bit more force though. :slight_smile:

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The reason goes back to quantum mechanics, regardless of which interpretation you subscribe to. When a particle is in a superposition of states, those states do not interact with each other as though they are separate particles; they are states representing one particle, not multiple different particles. For example, forget gravity (because it is so weak, let’s discuss something more dramatic), let’s talk about a charged particle. A charged particle can become in a quantum superposition so that its wave function is spread a bit out in space. But if you treated each part of that wave function as a separate particle that can interact with the other parts of the wave function, then they would all fly apart, exploding at near the speed of light. Because charged particles repel, very strongly. So when a particle is in a quantum superposition, you don’t treat it as a bunch of separate particles; the wave function represents one particle only, just different possible states for the one particle to be in. In the MWI each of these possible states are not just possible, but real. But they don’t interact with each other. In the Copenhagen interpretation each possible state is only possible, but not real until measurement. But still, each possible state does not interact with the others.

Now, when I say “does not interact”, I should clarify one thing. The states do “interact” in the sense that they are described by a wave function that, when added together, can interfere constructively or destructively, the same way light can interfere constructively or destructively. But the states are not actually interacting, they are just adding linearly on top of each other, which mathematically can have the effect of causing interference. This is one of the main sources of interesting quantum effects, again regardless of which philosophical interpretation you subscribe to.

In spirit you seem to be describing an experiment that gets to the heart of quantum mechanics (again, regardless of any philosophical interpretation). It is called the double-slit experiment. It means that a single particle can “interfere with itself” because it is described by a wave (note again that there is no interaction, gravitational or otherwise). Again this is described within ordinary quantum mechanics, and is true in the MWI or the Copenhagen interpretation.

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You are right.

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If they were competent, and if the plots had well labelled axes and titles.

Yes. A tremendous of both experimental and theoretical work went into being able to make those plots. And yes, nobels were won for a number of decay channels in there, for example the discovery of the W and Z bosons.

No one knows, but that’s why we build bigger particle accelerators, so we can find out. There may be new particles that are so massive they can only be discovered with higher energy particle beams.

Because they are charged and heavy. Charged allows us to accelerate them using electric fields. Heavy allows us to move them along a circular path without them radiating away too much of their energy.

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@iamnotbatman

Thanks. I guess by bringing an aircraft into the mix, I’m proposing a variation of Shroedinger’s Cat. If you set up a detector so that if an “up” particle hit it, the aircraft carrier disintegrated, then it seems in the MWI interpretation, you have superimposed states where in one the aircraft carrier is still there and its gravity is bending space-time, and another where there’s no more aircraft carrier and so space-time is not bent.

So, space itself seems to have to become part of the wave equation in MWI. Hope that clarifies where I’m still mystified…

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Well, if you insist on including gravity in the mix, I have to qualify anything I say with “we don’t have a theory of quantum gravity yet”. But yes, space itself must be part of the wave equation, if you start talking about gravitational stuff (otherwise you can ignore the fact that space is part of the wave equation, because in each “world” the space is an exact duplicate).

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The Higgs boson exists in nature just the same as any other unstable particle. It is created in the same sorts of collision processes that the LHC uses. You are correct in the sense that you won’t find a bunch of them just sitting around somewhere, but the same can be said about almost every other particle we have a name for, so this isn’t special about the Higgs.

Any particle physicist worth his or her salt should have no problem interpreting and appreciating the results. A young grad student in a separate area of particle physics that hasn’t gotten to the point in their career where they notice the bigger picture might have to ask a question or two, but for the Higgs even that would be the exception. (I bring grad students into it simply because a significant fraction of the people working in particle physics (or any scientific research field) at any one time are grad students.)

Obviously the deeper you dive into the specifics of how the measurements were made, the more you will get into collider physics jargon, collider detector jargon, LHC-specific jargon, analysis-specific jargon, … But even then, a particle physicist working in another arena should have no problem understanding how pretty much any nitty-gritty part of the experiment was done if they could talk with an expert over a long lunch.

Once you specify what sort of Higgs field you are introducing into the theory, then the calculations of the decay rates are fairly routine (if cumbersome). The basic features of the curves (how they trend, where they go up or down) is qualitatively explanable without much math at all. To be sure, most particle physicists can’t calculate this stuff at the drop of a hat because it’s a chore and it’s been done, so these calculations are generally out-sourced to those that specialize in it (in practice by using the software tools they have created). To be sure, a decent fraction of experimentalists would have some difficultly doing these calculation, but most could probably pull it off if given ample time and a particle physics library.

This all assumes we have a Standard Model in place and we’re just asking “What happens if you put in a Higgs field.” In actually getting to the basics of the Standard Model was a major feat of science in the last century, and numerous Nobel prizes came from all that. But, I get the impression that you are asking a more Higgs-specific question and aren’t looking that far back (or further, to the first quantum mechanics and relativity work, or even the electromagnetism of the 1800’s before that, … Lots of giants’ shoulders.)

Nope, and the LHC is the most recent step in the “higher and higher speeds” sequence. The Standard Model has many ugly properties that point to something new happening at some not-too-much-higher energy. There are many intriguing, and generally well-motivated, theories around about new stuff that might be seen, but so far nothing new has appeared.

(There are also reasons to expect new stuff happening many orders of magnitude higher in energy, but this physics can’t be accessed directly just by ramping up the energy, so one uses various indirect means.)

In addition to being charged and heavy, they are cheap and plentiful. The accelerator complex literally starts with a gas cylinder of hydrogen. Proton/antiproton collisions were used at the Tevatron at Fermilab, and this has some distinct engineering and analysis advantages, but making antiprotons takes a lot of resources (off all kinds: people, energy, accelerator time, raw numbers of protons, …), and you wouldn’t have been able to get anywhere near the number of collisions per second at the LHC with proton/antiproton collisions.

(One engineering challenge, as an example: to keep a proton moving around in a circle you use magnets, since charged particles are deflected when they move through a magnetic field. Positively charged particles are deflected one way, and negatively charges particles are deflected the other way. In a proton/antiproton collider, the protons are going (say) clockwise around the ring and the antiprotons are going counterclockwise, which is perfect, since it means the protons need to be deflected to the right and antiprotons to the left. The two counter-circulating beams can thus share a beam pipe and can use the same magnetic fields around the ring. In the proton/proton case, this breaks down, and you need two oppositely directed magnetic fields, one for each beam. This requires two independent beam pipes and two magnets sitting very close to one another, each producing 8.3 tesla to boot! (Pictures?) But the engineering complexity is worth it for a collision rate that is much higher than you could get with protons and antiprotons.)