# So how will CERN's LHC detect a Higgs boson

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1. Jun 2, 2012

### calgarian

So how will CERN's LHC produce a Higgs boson

I saw a video where a professor said

"So you do it by ... using e=mc$^{2}$ ... you collide some protons at huge energies, so that's giving you energy, and that energy gets converted into the mass of all possible new particles that there can be"

I'm not a physicist and I thought what the LHC was doing was colliding protons in order to break them into their constituent parts, one of which is a Higgs boson. But the quote above says that the collision is intended to create energy which somehow converts to mass. How exactly does energy spontaneously convert into mass?

Last edited: Jun 2, 2012
2. Jun 2, 2012

### Staff: Mentor

The kinetic energy of the colliding protons can be used to create new massive particles - they are not present in the protons before!*
The energy is not created in the collision, it is put into the protons before by accelerating them.

*at least not in a relevant way. Just ignore this comment.

3. Jun 3, 2012

### Drakkith

Staff Emeritus
Energy has mass. So when colliding particles together at high energies you already have mass there to use.

Note: I wrote the following before realizing the question was just about the energy and mass thing. I'm keeping it up here for now since it relates to the OP's thread title unless its blatantly wrong.

Protons are made up of Quarks, and the force that holds these quarks together inside a proton is the color force. It turns out that when you try to pull two quarks apart the color force DOESN'T get weaker with distance. Because of this, the farther you pull them apart the more energy you need to do so. At a certain point it is more favorable for two other quarks to be created using the energy you are pulling with than for the two quarks to continue to get further away from each other. These newly created quarks are bound by the strong force to the original two quarks, so instead of pulling two quarks apart you end up creating two more quarks and you come away with two hadrons, which are a class of particle that is made up of quarks.

Now, because of this effect, smashing protons together at huge energies results in not just two new quarks, but a LOT of new quarks. These new quarks all bind together and form various particles of various masses, most of which decay extremely quickly. During the various decay processes, even more types of particles can be created, such as leptons, photons, and bosons.

Now, if the higgs boson has a mass that is within the energy range of the LHC, then sometimes these various particles and decay processes can lead to the creation of a Higgs boson. By looking at the various particle tracks in the detectors and by following all the decay processes we can piece together where a higgs might be created. The problem is that this isn't a black and white picture where we can go "Bingo! There it is!". The particle creation and decay processes are inherently random in certain ways which forces us to collide HUGE numbers of protons together and look at all of their tracks in hopes of sorting out where the Higgs might be from the rest of the bunch, which we can refer to as "noise".

The entire process takes years to bear fruit, if at all, and currently the LHC has done over 1 trillion collisions in search of the Higgs boson.

4. Jun 3, 2012

### calgarian

Yeah I realized afterwards that my title sucks. I guess I am asking a more general question of how energy gets converted into specific types of mass like a Higgs boson.

As I say above I am having difficulty of how you go from a "general" form of mass in energy to, from the CERN professor's quote, "all possible new particles that there can be." How do specific "types" of mass just decide to show up out of nowhere?

5. Jun 3, 2012

### Parlyne

This statement is nonsense. It's about as meaningful as saying that color has greasiness. Energy and mass are both properties that an object can have. A correct statement of the relationship between them is that mass is a type of energy. Specifically, it is the energy that the object has in the frame of reference where its momentum is 0.

Colliding objects allow the conversion of some or all of their kinetic energy into mass specifically because the two objects together have less momentum than either one individually.

Most of the interesting processes at high energy colliders aren't at all related to the fragmentation and hadronization processes you're describing here. The high energy (partonic) collisions that lead to the production of heavy particles not present in the initial state actually happen on a much shorter time scale than the processes you're discussing. The Higgs (and other interesting heavy particles) are directly produced in the collisions of the quarks and gluons that are already present in the two protons. The remaining quarks and gluons which don't participate in the direct collision undergo the processes you've described.

6. Jun 3, 2012

### Staff: Mentor

It is random which particles are produced in a specific collision. Actually it depends on your favorite interpretation of QM, but for practical purposes it is just a random event. A higgs (if it exists with ~125GeV mass) is produced about once every 10 billion collisions.
In general, lighter particles are much more frequent - a charm anticharm quark pair (~1.5 GeV per quark) is produced about every 10 collisions, and single collisions can easily produce something like 30 light quarks (<0,1 GeV).

7. Jun 4, 2012

### Drakkith

Staff Emeritus
I disagree with mass being a "form" of energy. But given the lack of a concrete definition of the term "mass" let's agree to disagree for now.

Ok, that makes sense.

8. Jun 23, 2012

### calgarian

I came across this quote in an article about CERN's search for the Higgs boson:
Based on the responses ITT, am I correct in concluding that the bolded part is misleading or poorly worded at best? My new understanding is that they will not be looking at debris, as that term is normally used, but rather at particles generated from the energy of the collision. Is my new understanding correct?

Last edited: Jun 23, 2012
9. Jun 23, 2012

### Drakkith

Staff Emeritus
Pretty much.

10. Jun 25, 2012

### kloptok

But it is also a fact that a lot of the collision products are uninteresting for the process searched for and actually an obstacle in the experiment, hence the use of the term "debris".

11. Jun 25, 2012

### calgarian

The author of the article used the term "debris" to describe something that the experiment is designed to record, as opposed to being uninteresting and obstacles as you describe them.

12. Jun 25, 2012

### kloptok

Really, this is about how one would define the word "debris". I came across the following:

debris
1.
a. The scattered remains of something broken or destroyed; rubble or wreckage.
b. Carelessly discarded refuse; litter.

Looking at the second definition (b), I see your point. Not everything is "litter" in the collision, in that case it would be of no point doing the experiment. However, the first definition (a) certainly applies, at least the first part. The protons are "broken" or "destroyed" and the scattered remains are then the "sub-atomic debris". And it was not my intention to say that the experiment is only designed to record that which is not labeled as "debris" (and thus ignore the "debris"). You want to detect as much as possible to get a full understanding of each event.

But this is all just semantics. What I meant with "uninteresting" and "obstacle" is that when colliding two hadrons there are a lot of things happening which do not belong to the hard process. In the forward and backward direction a large number of particles are created from what is left of the hadron after partons have collided in the hard process. This is what I alluded to when I wrote "uninteresting". Of course, you should not discard this entirely as you need as much information as possible on the event to be able to sum up momenta etc.

The point is that in a hadron collision, there are lots of things happening which do not directly belong to the hard process and it is a lot of work to isolate what you are looking for. Hence an "obstacle to the experiment".

13. Jun 25, 2012

### calgarian

I'm really not sure what point you are arguing, kloptok. The point I am making is that as a non-physicist I had been confused as to what the LHC is doing because of articles like the one I quoted. The author makes it sound as if we will find the Higgs boson in "the scattered remains" of the collided protons, which implies that when a proton is broken into pieces one of those pieces is a Higgs. I recently learned the correct understanding of the process, which is that the Higgs will arise from the energy released in the collision of the two protons.

14. Jun 25, 2012

### kloptok

Alright, I didn't note that you were a non-physicist, sorry about that. And you are right in your understanding, I didn't mean to make it sound like you had misunderstood it. Rather I wanted to supplement your understanding.

Anyway, let's take it from the beginning: At the LHC protons are collided. The large kinetic energy of the colliding protons creates a multitude of particles, among which can be the Higgs. But in every collision there will in the end also be hundreds, maybe thousands, of other particles also produced, and a lot of them coming from the particles inside the proton which didn't collide. These particles make it difficult to see if the Higgs was there in the first place, as you have to isolate the few particles that actually come from the decay of the Higgs from all the other particles. This is I would say the reason why you will often encounter terms such as "debris" in articles about collisions with protons (or other composite particles), such as at the LHC. As a physicist looking for the Higgs, there is a huge amount of work to work through all this other information from each collision. An electron-positron collider for example will provide much "cleaner" collision with a lot less particles to keep track of, since the electron/positron is point-like. Hence these collisions are in this sense easier to analyse. Again, my point was not to confuse, just to try to clarify the use of the word "debris" and why one would use it in the context of proton collisions, which are very complicated to analyse due to the proton's compositeness. The important thing is obviously to understand what the author's mean when writing "sub-atomic debris" and why they would write such a thing, and not exactly how the word debris is defined.

You can for example look at this picture, which shows a collision where four muons are produced (the blue lines, the rest in form of white lines is the "debris"): http://www.atlas.ch/photos/atlas_photos/selected-photos/events/FourMuon.jpgphotos/events/1112301_01.jpg [Broken]

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15. Jun 25, 2012

### Drakkith

Staff Emeritus

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16. Jun 25, 2012

### kloptok

That's odd. It's from www.atlas.ch-->Multimedia-->Images-->Events-->Proton Collision Events ( http://www.atlas.ch/photos/events-collision-proton.html [Broken] ). The fourth one from the left, entitled "Event with 4 muons" (but not the first one called this though).

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17. Jun 25, 2012

### Dickfore

Wikipedia has an entry. Look at the two Feynman diagrams in the figure on the right of this paragraph.

18. Jun 26, 2012

### Staff: Mentor

http://www.atlas.ch/photos/atlas_photos/selected-photos/events/FourMuon.jpg [Broken] is the image, I think the link has some copy&paste-error.

Thousands is a bit much (that corresponds to heavy ion collisions), but ~100 are not uncommon together with high-energetic processes. However, keep in mind that most collisions to not produce higgs/top/W/Z, and tend to produce less particles.

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19. Jun 26, 2012

### calgarian

Can someone give a brief primer on what information is conveyed by an image like that? I've seen similar images but I have no clue what the images are saying. I imagine the length and direction of the lines means something?

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20. Jun 26, 2012

### calgarian

Hmm ... so does this mean that the Higgs will be produced from a combination of the energy released in the collision and stuff that makes up the protons? I was getting the understanding that that they would just pop up "magically" out of pure energy (again, I am not a physicist :tongue:)