Quarks and proof

1. May 29, 2005

ion

Could someone tell me if the quark has been experimentally proved to exist?
I mean in a particle accelerator. Or indirectly inferred.

2. May 29, 2005

dextercioby

You have to be joking,right...?The existence of 6 flavors of quarks has been experimentally proven so far.Theory admits 16.

Daniel.

3. May 29, 2005

ion

No,I genuinely did not know.
When and where were this discoveries made?
Could you give some references.
It looks like I need an awful lot of catching up!

Thanks for the response,dextecioby.

4. May 30, 2005

Staff: Mentor

Check out the Particle Data Group.

As for a timeline (going from memory here, so these are only approximate), the up, down and strange quarks were part of the original quark model in the 1960s, to explain patterns of properties among the particles that had been discovered up till then. More direct evidence came from high-energy scattering experiments (electrons and neutrinos off of protons and neutrons) beginning around 1969.

The charm quark was "invented" in the early 1970s basically for purely theoretical reasons. The first particles containing charm quarks were observed in 1974 (the J/psi), and came pretty much as a surprise. This happened during my senior year in college, right before I visited some universities that I was considering for graduate school. At one place I went out to lunch with a bunch of people who spent the whole time talking about the J/psi.

The bottom quark (or rather particles containing them) was observed around 1977, again pretty much as a surprise, although with the experience of the J/psi behind them, a new quark was one of the first things people thought of.

Pretty soon after that most people were convinced that a top quark had to exist to go along with the bottom quark, for symmetry reasons, but it wasn't actually observed until...um, OK Google, help me out here... aha, 1995.

Something that sticks in my mind from my late grad school period is seeing a preprint (preliminary draft of a paper) titled "Topless Bottom Models" which sounds somewhat pornographic. :rofl:

5. May 30, 2005

arivero

6. May 30, 2005

ion

Thank you jtbell and arivero for your inputs.
Much appreciated.

7. May 30, 2005

Meir Achuz

"... if the quark has been experimentally proved to exist?"
This really depends on what any individual means by
"experimentally proved to exist?".

Before quarks, it usually meant observing an interaction of the particle.
For instance, the neutrino was inferred in beta decay, but the community did not consider it "experimentally proved to exist" until Cowan and Reines observed an inverse beta decay caused by neutrinos. I personally am satisfied with the experimental evidence for the existence of quarks, but "evidence" is not "proof".
It is a clever theory that includes the prediction that the core particles can never be liberated. One thing that can be said in favor of evidence for quarks is that numerous experiments trying to disprove their existence have failed.
The best evidence for quarks include, using quarks to:
--correlate baryon and meson masses and magnetic moments.
--understand point-like structures seen inside the proton in deep inelastic scattering of electrons by protons. As I said, the best argument for quarks is a lack of evidence against their existence, despite numerous eforts.

8. May 30, 2005

Staff: Mentor

There are also "quark jets": bunching of outgoing hadrons in high-energy collisions, with properties that agree with what is expected from the "hadronization" of a single quark. These were first studied in the early to mid 1980s as I recall.

9. May 30, 2005

James Jackson

I'm no QFT expert, but how does this hold with the measurement of the $Z^0$ width?

The width is dependent on the lifetime, which in turn is dependent on the number of decay channels. By looking for the individual decay channels, the partial widths of the visible decay channels can be determined. This leaves the invisible neutrino decay channels to make up the rest of the total width, so measuring the width of the $Z^0$ allows a count of the number of neutrino families.

Then, to ensure the theory of electroweak interactions can be renormalisable, the condition on the electric charges of the quarks and leptons is applied:

$$\sum Q_l+3\sum Q_a =0$$

where the first sum is over the leptons, the second over the quark flavours (the three is due to the colour charge).

This sum is only satisfied if the number of quark and lepton families are the same.

10. May 30, 2005

dextercioby

Incidentally,the #16 appears in considerations of QCD renormalization only.It would be fair to discuss the renormalizability of a GUT model,to include quarks and leptons.

Daniel.

11. May 30, 2005

James Jackson

So would it be fair to say that by looking at QCD normalisation only there is something missing from QCD if 16 quark families are allowed, which is clearly disallowed by QED + experimental results?

P.S. Do you know what the current state of play with QCD in general is, mainly what mathematical tools are being developed (if any) in place of pertubation theory in the QCD case?

12. May 30, 2005

dextercioby

You got the lattice version which is used in numerical calculations.But still,in the SM,QCD is the thorn as,due to confinement,experiments don't work that easily and even the calculations are extremely complex.

I wouldn't say QCD is missing something,just because it makes a possibly falsifiable prediction.It's a partial theory.If u prefer,it misses electroweak interactions and gravity.

Daniel.

13. May 31, 2005

curvedlogic

I just happen to be reading and article which states... ‘The Bc meson contains a charm quark and a bottom anti quark...' and it goes on to say that ‘its mass, the new findings reveal, is about six times that of a proton.'

This seems ambiguous. Does it mean that the afore mentioned quarks are just part of the proton or that they produce less than the sum of their parts?

14. May 31, 2005

James Jackson

See Daniel's post above - confinement produces some strange results, such as quark masses seeming to vary from particle to particle. Of course, it's impossible to get a singular quark to measure its mass...

15. May 31, 2005

Nicky

What does string theory say about quarks? Take a meson, for example. Is the meson considered composed of two strings, one for each quark, or is the meson a single string with two oscillation modes excited?

16. May 31, 2005

Miserable

Just to point out, there's no problem with extra generations of particles as long as they all have masses greater than M_Z/2, so that the Z decays are kinematically forbidden.

17. May 8, 2011

486DX

Hi Meir Achuz
I'm very interested in what you said "the best argument for quarks is a lack of evidence against their existence, despite numerous efforts"
Could you elaborate on this, or provide additional references on such a topic?

Thank You!

18. May 9, 2011

Meir Achuz

I meant that as a general statement that no measurement of the magnetic moment or mass of any octet or decuplet baryon is in significant disagreement with the quark model.