
#37
Mar2105, 05:02 AM

P: 4,008

I have had many questions on virtual particles and vacuum fluctuations. That is why i decided to write down this text in my journal...
Enjoy: http://www.physicsforums.com/journal...90&action=view Suggestions and comments are always welcome, as usual I also wrote a text on grouptheory in QFT, please let me know your thoughts as to whether the content is clear enough Grazie mille marlon 



#38
Mar3005, 12:26 AM

Admin
P: 21,637

Found this  http://www.benbest.com/science/standard.html
The Standard Model of Particle Physics A Simplified Summary by Ben Best maybe a bit redundant. 



#39
Mar3005, 11:20 AM

P: 4,008

Thanks Astronuc for the link. I also want to give this link to these introductory lectures notes on nuclear physics, provided by Humanino :
http://hitoshi.berkeley.edu/129A/strong1.pdf marlon ps : here is some additional info on both gluons and pions : http://www.physicsforums.com/journal...age=10&page=10 and http://hyperphysics.phyastr.gsu.edu...les/expar.html 



#40
Mar3005, 04:37 PM

P: 4,008

Hi,
I Just wanted to point out this very interesting thread on the parityconservation of the neutral pi meson decay. http://www.physicsforums.com/showthread.php?t=68715 regards marlon ps : if there are any questions, please don't hesitate... 



#41
Mar3005, 04:42 PM

P: 4,008

The beta decay really announced the advent of QFT. In the beginning some scientists thought that the electron really came out of the nucleus, ofcourse this is wrong. What happens is this : the electron is created "out of nothing". This means that the energy involved in the beta decay is used to create this electron out of the vacuum. This kind of process is only possible in QFT and that is why beta decay was one of the first major breakthroughs of QFT.
Also, keep in mind that negative beta decay (neutron > proton) is a particle decay mode while the positive beta decay (proton > neutron) is a nuclear decay mode because the neutron is more heavy then the proton. This proton can only decay (due to energyconservation) when it is surrounded by many other protons in a nucleus...Part of the energy coming from protonprotoninteractions can also account for mass via E=mc˛. Beta plus decay commonly means the basic process p>n + e++v. It is a nuclear decay mode in that it can only happen if the proton is inside a heavier nucleus and the final state nucleus is more tightly bound; the process is forbidden in free space by energy conservation since a neutron alone is heavier than a proton marlon More info here : http://www.physicsforums.com/showthread.php?t=66287 



#42
Mar3005, 04:51 PM

P: 4,008

I have posted this in my journal but i thought it might be nice to put this text in this thread too...
Many students have difficulties understanding what 'transforming like a vector or tensor' really means in physics. Here is the solution...Before you begin, be sure that you know really well what a tensor is...if you do not, check out my 'what is a tensor' entry... A spinor is a special kind of vector. I mean, it has the property that if you rotate it 360° you get the exact opposite (A > A) of what you originally rotated. Rotate another 360° and you get where you started off in the first of the two rotations (A> A>A). Now let us look at the rotationgroup SO(3) or even any other group, it don't matter : An object v transforms as a vector if you can write v' = Uv where U is a representation for the group in question, U represents a rotation. Another way to say this is if you transform an object under a certain group, the 'image' of this transformation will be a linear combination of the object that you transformed. So transforming like a vector really means that the object you transform will be written out as a linear combination of it's components after the transformation. An object transforms as a tensor if you can write v'=UU'U''v So this means that v transforms 'as a product of vectors' because of the multiple Umatrices. Now, transforming like a spinor really means that the object tranforms like a vector (you know what that means) but not just any vector. This is a special case, where the Umatrix does not represent just any transformation but a transformation that gives you the opposite of the initial object after a rotation of 360°. Rotations are generated by the Joperator. J = 1 for example means that the quantity at hand transforms like a vector under three dimensional rotations. And the other way around, if an object transforms like a vector under these 3D rotations, you know it will have spin J =1 and thus three degrees of freedom. YES, because the fundamental representations will be (3*1)matrices which have three components...Spin 2 is a tensor and Spin 0 is a scalar...ODD SPIN IS A SPINOR One can recognize a spinor by the way it transforms under a group. If the generator is a Paulimatrix you are done...Just like in the case of SU(3), if you now the generator is a GellMann matrix, you know you are working with anobject in the adjoint representation and these objects are GLUONS. Let us look into gluons : There are 3 colors. Why ? Well, becauseSU(3) is the group of 3 x 3 unitary matrices with determinant 1. The most easy matrix such an SU(3) matrix can work on is the 3*1colummatrix (ythis one has three components and is called the fundamental representation). SU(3) is the symmetry group of the strong force. What this means is that, as far as the strong force is concerned, the state of a particle is given by a vector in some vector space on which elements of SU(3) act as linear (in fact unitary) operators. We say the particle "transforms under some representation of SU(3)". For example, since elements of SU(3) are 3 x 3 matrices,like i already said before , they can act on column vectors by matrix multiplication. This gives a 3dimensional representation of SU(3). The quarks are represented by this 3*1matrix. The antiquarks can be represented by row vectors because we can multiply a 3*3matrix with a row vector on the LEFT side of the matrix. The gluons are represented by the socalled adjoint representation which consits out of traceless 3*3matrices. It can be seen that a row of such a matrix represents one quark colour and a colom of such a matrix represents a anticolour. each gluon is therefore constructed out of a colouranticolour combination. Given that there are 3 such colours and anticolours, you would expect 9 gluons. However there are only eight . Can you see why ??? ps : you know that the colours are red green and blue and it is the postulate of QCD that the sum of these three represents colourneutrality !!! This is the main law that needs to be respected : in interactions : the sum of all involved colours must be WHITE regards marlon ps : maybe others can add or correct ??? 



#43
Apr305, 06:14 PM

P: 3

all these particles are great, but let's ask one basic question.
When you see those particle tracks, specifically the electron and positron slowing down in a spiral ... is this occurring in a plane, or is there a "z" component to the motion ? Is it really a helix ? with gradually decreasing radius? Seems like this motion is fundamental to understanding particle physics ... 



#44
Apr405, 12:48 AM

P: 4,008

marlon 



#45
Apr805, 08:51 AM

P: 4,008

Quarks have spin 1/2 and a proton has spin 1/2. There are three quarks in one proton, so you'd think that a proton spin must be 3/2. Ofcourse this is NOT the case.
Ever wondered why that is ? Here is the answer : http://www.physicsforums.com/showthread.php?t=70409 Ps : it's this 'system' that also solves the apparent parityviolation in the neutral pion decay...I refer to the previous made post in this thread, dealing with that matter... marlon 



#46
Apr1005, 06:17 AM

P: 1

Hi, I am programming a computer model of atom nucleus at the moment. Could you help me with the following two questions:
Does standard model give a forumla that describes strong force between two quarks as a function of distance? Given a mass of each quark in a nucleon, is there a formula that gives out the mass of nucleon itself? 



#47
Apr1205, 10:03 AM

P: 4,008

regards marlon 



#48
Apr1205, 10:10 AM

P: 4,008

Ofcourse,seeing as how our most powerful supercomputers struggle just to model a single proton or neutron, I don't think full QCD is going to be an efficient way to model the nucleus.
As for links, how about something like this: HartreeFockBogoliubov Mass Formula Just Google the name... Here is a nice overview with various references : http://www.apsidium.com/number/nuclear_masses.pdf marlon 



#49
Apr1405, 02:38 PM

P: 2,828

Here is a very nice introduction to instantons by Diakonov. He points out that, surprisingly enough, the linear rising potential between quarks probably is not the key to the problem of confinement. This simple picture of the gluetube string is probably far too simple to account for what is realized in Nature.
Instantons and baryon dynamics 



#50
Apr1505, 08:32 AM

P: 4,008

Wanna know more about the words intrinsic and helicity ? :
http://hyperphysics.phyastr.gsu.edu...trino3.html#c1 Attention : the spin S of a particle is called the intrinsic angular momentum. After reading this text, i hope you have a clear understanding of what that means. Also a particle with certain spin does NOT actually rotate around its axis, this is a common misconception. The rotationpart is an abstract grouptheoretical formulation that arises because if certain symmetries that need to be respected. This thread explains everything on orbital angular momentum and it's connection to spin : http://www.physicsforums.com/showthread.php?t=71642 It's important to realize how an atom can interact with an extern magnetic field. This is why the concept of magnetic moment has been invented. it's all in this thread, check it out people.... regards marlon 



#51
Apr1705, 07:19 AM

P: 4,008

Hey, guys and girls,
I have written a little and simple text in order to eliminate some common misconceptions of QFT (well QM too ofcourse). NUMBER ONE: Besides, i would like to add this : people always ask how do electrons orbiting the nucleus prevent from falling in ? Well, this question itself contains incorrect formulations. the electron does NOT orbit the nucleus. Just look at the lowest energy orbital : the sorbital. It is a sphere around the nucleus. So , prior to any kind of measurement, the electron is basically everywhere around the nucleus. Same goes for any other orbital (ofcourse the have another shape) QM proves us that the kinetic energy is higher when being closer to the nucleus, but the potential energy is lower (more negative). The sum of these two really yields a stable equilibrium throughout all energy levels. NUMBER TWO: People always mix QFT with QM. A wave function that describes an electron is not a wave that IS an electron. It just contains the electronproperties. This is QM In QFT, particles arise as fluctuations of fields but the fields themselves ARE NOT particles. Particles arise (and also forces) as actual vibrations of these fields. Just think of the mattress analogy that i have used throughout my entire journal NUMBER THREE: An electron has a spin (intrinsic angular momentum), but it does NOT actually rotate around its axis, guys. The 'rotational nature' of spin comes from the behavior of the Dirac wavefunction (this is a matrix that represents a physical state and arises when solving the Dirac equation. This equation describes a fermion : a particle with noninteger spin) under coordinatetransformations (which are called the rotations). With behavior i mean : how does the physics change if we interchange the components of this Dirac spinor, if we change the parity, if we apply coordinate transformations to the wavefunction and so on....For example, if we rotate the wavefunction 360°, do we still get the same physical laws...You see the pattern ??? It is this specific behavior that yields the name SPINOR because if you rotate it 360°, you get the opposite value. Now, changing coordinates (represented by rotations) and looking how the physics changes or not, is NOT THE SAME as actually rotating. So, spin arises thanks to symmetries involved but there is no actual rotation. ps : be sure that you know what 'intrinsic' means NUMBER FOUR: Finally : there ain't two ways of describing light : The particle waveduality exists only because of our 'classical minds' ; we wanna think in terms of either particles or waves. There is no problem with that but we do need to keep the correct perspective on things here. First of all 'particles' in this case does not mean little objects with finite boundaries. It means little finite pieces of energy (this is the actual quantization , right ?) Secondly, in QM we have experiments that are better explained with the wavelike notion (eg the double slit experiment) and we have those experiments that are better described with the particlelike notion (eg photoelectric effect). However in the end both descriptions are just ONE SINGLE way of describing the physical properties of light....that is all. NUMBER FIVE: Another common misconception is the fact that the photoelectric effect proved the existence of photons. That is not true because this photoelectric effect can be described in terms of the wavelikenotion of the incident EMradiation too. It is only the atoms of the target electrode that are treated with QM. However, the particlelike notion of light is suggested by this experiment. If you wanna read more, check out my journal and find the article on creating an entangeled photonstate in an undergrad lab Here's the article : Create entangled photons yourself in an undergrad laboratory : http://marcus.whitman.edu/~beckmk/Q...r/Thorn_ajp.pdf regards marlon 



#52
Apr2105, 05:43 AM

P: 4,008

Standard Model : SU(3)xSU(2)xU(1) gauge theory
Gauge bosons : 8 SU(3) colorcoupled gluons, 3 SU(2) 2 W\'s and 1 Zboson, 1 U(1) B the foton In the unbroken SU(2)xU(1) gauge theory of the electroweak interactions, there are four fields. These fields are usually called W1, W2, W3, (from SU(2)) and B (from U(1)) After spontaneous symmetry breaking due to the Higgs mechanism, you still have four fields, but three of them gain masses. The W1 and W2 mix to form the W+ and W and the W3 mixes with the B to form the Z0. The rest of the W3B mix remains massless and is the photon (often called A and belonging to the remnant U(1) symmetry of QED). Three generations of spinor fermions divided into: 1) colored doublet leftquarks (ups and downs) 2) doublet leftleptons (electrons and neutrinos) 3) colored rightups 4) colored rightdowns 5) rightelectrons with various hypercharges The singlet states correspond to particles that don't feel the weak force like 3,4 and 5... Or you can classify like this : Mass Particles A. Six quarks 1. Up, down, strange, charm, top, bottom 2. Combine to form Hadrons in two varieties: baryons, mesons B. Six leptons 1. Three with charge (Tau, muon, electron) 2. Three neutrinos each corresponding to a charged lepton 3. Decay, don't combine II. Three types of interactions mediated by force particles A. Strong (gluons) B. Electroweak 1. Electromagnetic (photon) 2. Weak (Z, W+, W bosons) C. Gravity (graviton?) Can you understand "the why" of this classification ? marlon 



#53
Apr2105, 08:56 AM

PF Gold
P: 2,885

I can not understand why the leptons are classified according electric charge, but the quarks are not. 



#54
Apr2105, 01:24 PM

Sci Advisor
HW Helper
P: 1,937

The quarks are classified like the leptons.
The quarks come in 3 doublets. (u,d),(c,s),(t,b). In each doublet the upper quark has a charge +e more than the lower quark. This is just like the leptons. Details are related to the groups involved. The group structure is S(3)XSU(2)XU(1). Why the quarks are +2/3 and 1/3 is involved with hypercharge and group details. If the GUT is SU(5), the breakdown into this structure is unique, but th proton decays. 


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