What is to gain from colliding protons?

[jonohashmo]

hi,
im new to the whole subject of particle physics. I've searched for a long time and couldn't find an answer to a very simple question. WHY PROTONS?? why did they spend 8 billion dollars to build a collider to collide protons? why not electrons for example? why do they think that colliding protons (and not electrons or antiprotons) will find the higgs particle?

 

[The_Duck]

It turns out to be much harder to accelerate electrons to the same energies as we can do for protons. Because electrons are about 2000 times lighter than protons, when we use electrons in circular accelerators they lose much more energy to what is called synchrotron radiation. So in accelerators like the Large Hadron Collider, where the goal is to attain very high energy collisions, we prefer protons to electrons.

 

[jonohashmo]

so basically the only reason is that itll be easier to accelerate? other than that i wouldn't have made a difference if it was electrons?

and thanks for the quick reply:)

 

[Orodruin]

Given the previous answer (which is correct), one may also ask the reverse question of why anyone would want to build an electron/positron collider. The answer to that is that electrons are elementary particles and their energy would be very well controlled unlike that of the quark constituents of a proton (which at these energies can be considered as free particles and will carry a fraction of the proton momentum). The result of this is that an electron/positron collider (such as the previous LEP at CERN or the planned International Linear Collider) is generally better at making precision measurements. Because of these distinctions (proton collider good at getting high energy, electron/positron for making precise measurements), the proton collider is sometimes referred to as a "discovery machine" while the electron/positron collider would be a "precision machine".

In some sense they are complimentary approaches. Ideally you would discover something at a proton collider and then use the information you got out in order to guide the design of an electron/positron collider to make precise measurements of the discovered new physics. (The idea of ILC at the moment is to make precision measurements of how the Higgs boson interacts with other particles.)

 

[Einj]

There is also another reason. Machines like the LHC that accelerate protons are called "discovery" machines while electron accelerators are called "precision" machines. The reason is that when you collide two electrons, you are colliding two single elementary particles and you know exactly (or within the experimental uncertainty) what the energy of these particle is. On the other hand, when you collide two protons you are actually colliding two composite objects, each of them containing 3 valence quarks and a lot of sea quarks.

Now, the point is that you don't know exactly what the energy of the single quarks is and it turns out that it can belong to a pretty wide range. Therefore, proton (or hadron in general) machines are built to basically perform simultaneous experiments with very different energies. Hence, if you are looking for something but you don't know what energy is required to produce it (e.g. the Higgs boson), proton machines are much more suitable. You don't need to manually change the energy of the beam every time to span the whole range of energies.

 

[jonohashmo]

great answers thank you!:)

i have another question. what is suspected to happen when you collide two protons together at these energies? are they expecting to separate the quarks as well as to find the higgs boson?

 

[Einj]

Quarks are never really separated. This phenomenon is called confinement and it tells you that, at the end, quarks will always be bound in colorless hadrons. However, when you collide two protons at these energies you are actually performing very high energy collisions between the quarks and gluons composing the protons and then you are studying the products of these collisions like, for example, W bosons, heavy mesons, Higgs boson and (hopefully) new unknown particles (e.g. supersymmetry).

 

[ChrisVer]

What is happening when you collide two protons at those energies?
Well first of all, the protons as already pointed out, are not elementary particles, but they consist of quarks and gluons...
The sea+valance quarks and gluons all together are called partons. Then at those energies you expect to have partons interactions, and with high enough center of mass energy to get the result..
Think that from a proton of 7TeV energy, a parton will have only a fraction of it...

 

[mfb]

Most of the collisions are quite boring for high-energy physics - the protons make a very soft collisions, maybe one or two other light particles are created, but nothing else happens.

The interesting collisions are those where a lot of energy is used to make new particles. This is typically the collision of a quark or gluon from one proton with a quark or gluon from the other proton. At those energies, the binding energy in the proton is negligible - all the interesting physics happens in this collision, the rest of the protons does not contribute to it. "Interesting physics" includes the possible creation of Higgs bosons or other heavy particles. Those particles decay too fast to detect them directly, so you observe their decay products as they fly through the detector, together with many other particles that were created in the collision.

why do they think that colliding protons (and not electrons or antiprotons) will find the higgs particle?

It did!

An electron-positron collider at the energy and luminosity (a measurement of the collision rate) values of the LHC would probably be better, but with current technology that would need a linear accelerator with a length of about 500 kilometers - completely impractical.

 

[jonohashmo]

thank you all:)
some more questions:

how can one find or measure something that you don't know anything about? how does the detector know to separate them if it doesn't know what its looking for?

as i am in switzerland now, i scheduled a tour of the LHC. does anyone have any suggestions as to what should i look for and what should i ask to make my trip more interesting?

 

[TumblingDice]

how can one find or measure something that you don't know anything about? how does the detector know to separate them if it doesn't know what its looking for?

There are different methods to make observations and put together the big picture. A few of the earliest include cloud chambers to track the paths of what's produced in each collision. How far and how fast a new particle travels can determine its mass. Adding a magnetic field will deflect particles with charges one way or another.

The LHC at CERN has seven different experiments, each with their own array of detectors:
http://home.web.cern.ch/about/experiments

Here's a link that explains types of detectors and how they work/are used:
http://home.web.cern.ch/about/how-detector-works

 

[Orodruin]

how can one find or measure something that you don't know anything about? how does the detector know to separate them if it doesn't know what its looking for?

Very good question. The point is that we generally did have a pretty good idea of what we were looking for as long as the standard model was correct. The theory made a prediction and the effects were observed - thus giving credence to the theory.

With the Higgs, it lives far too short to be registered in the detector. Instead, what we observe is a certain behavior of the decay products remaining after its decay. The properties of the Higgs are all very well aligned with the standard model expectation.

But not always do you find what you are searching for, it could be that the theory used to make a prediction is wrong and needs to be rejected. And in some rare joyous moments, you could also discover something you were not expecting ...

 

[mfb]

how can one find or measure something that you don't know anything about? how does the detector know to separate them if it doesn't know what its looking for?

Most searches are for specific particles predicted by various theories - the Higgs was one of them (now found), supersymmetric particles are the most prominent other example. Black holes are one of the more exotic objects to look for. Theory makes some predictions how the decays should look like, so the experiments can see if they find that. If not, they set exclusion limits ("if this theory is true and this particle exists, then it has to have a mass of at least x, otherwise we would have seen it").

Apart from that, there are also very general searches for heavy particles. Those particles tend to decay to very high-energetic other particles (often more than one detectable particle), and high-energetic particles (especially in groups) are rare. So just by comparing the total number of high-energetic particles to the predictions, you can see if there has to be something else.

as i am in switzerland now, i scheduled a tour of the LHC. does anyone have any suggestions as to what should i look for and what should i ask to make my trip more interesting?

Do you have some organized tour to the LHC itself (= probably to one of the detectors)? There are tours, but you have to organize them in advance, otherwise you won't see the LHC at all. The experimental control centers are easier to visit (at least from the outside), there is the CERN globe, and you can walk around at CERN if someone invites you. If you want to see hundreds of physicists at the same place, the area around the main canteen is the right spot.

 

[jonohashmo]

Do you have some organized tour to the LHC itself (= probably to one of the detectors)?

I logged on to their website and ordered a tour for 3 people its not private but I'm not sure exactly where they'll take us...

 

[Orodruin]

Do you have some organized tour to the LHC itself (= probably to one of the detectors)?

I logged on to their website and ordered a tour for 3 people its not private but I'm not sure exactly where they'll take us...

So I am guessing this is what you signed up for:
http://outreach.web.cern.ch/outreach/visites/individuelle.html

Based on that, you will not be taken underground to any of the detectors or to the accelerator itself. Even in the group tours you are not guaranteed an underground visit. You should however be able to visit a control room for one of the experiments.

 

[ChrisVer]

Well if LHC is running, then there is nothing special in going to ATLAS detector [I was lucky I visited it during the maintenance era, and the detector was "open" and you could see its parts ]. If you want to see the size of it, well the building of the control room outside has a paint of ATLAS detector and the size is equal (so you can get an idea of the magnitude). Entering inside you are just going to see a cylindrical metal.
Probably (by the 2hrs time tour) they will get you for a presentation talk (near the canteen building) where then you can buy souvenirs, and maybe a visit to the ATLAS control room...
Personally, although I prefer the physics searched in ATLAS, I really liked the COMPASS tour (you get a closer feeling to what is actually going on there).

 

[mfb]

The best time to visit the detectors was a few months ago, when they were fully opened for maintenance. I think now they are closing them again for the next run (to start in early 2015). LHCb is an exception, it is always open by design ;).
Seeing them won't get easier over the years - with an increasing number of collisions, the detector parts become more and more radioactive, so getting access gets more and more problematic.

 

[enorbet]

@mfb - Now you have my curiosity piqued. Why the "winkie" after LHCb? I find that experiment particularly interesting.

 

[mfb]

LHCb looks like a fixed-target experiment, with the different subdetectors in a row - you can easily see the different parts from the side.
The other detectors surround the collision point with multiple detector layers and in their closed state you do not see the inner parts any more.