Qcd Definition and 169 Threads

Hello! Could anybody help me? My wondering seems so trivial, but I can't skip it. They say that since u and d quarks are much lighter than QCD scale(~200MeV), in reality we can consider the QCD Lagrangian has an approximate global chiral symmetry with respect to these two flavors. At first, it...

The pole masses of the heavy quarks (c, b and t) are relatively well defined in QCD (i.e. the solution of m²(p²) = p² extrapolated using the beta function and the available data from other values of µ usually obtained based upon model dependent decompositions of hadron masses that include these...

Hi there, I'm currently working on a relatively simple code to do some lattice simulations. I have access to a computing cluster at school and have been learning how to use OpenMP to parallelize my code (each node has 16 cores). I'm currently not planning to use MPI. My main question is...

I will soon start a Master thesis in Lattice QCD. There I will spends lots of time developing C++ code and running it on different supercomputers. After that I consider doing a PhD if the Master thesis runs well. For the experimentalists I can see a lot of almost-engineering jobs where they...

Homework Statement I want to diagonalize the quadratic form $$ m_0((m_u+m_d)\pi^3\pi^3+\frac{2}{\sqrt{3}}(m_u-m_d)\pi^3\pi^8+\frac{1}{3}(m_u+m_d+4m_s)\pi^8\pi^8)$$ which can be found under equation 5.47, in order to get the mass of the η and ##\pi^0## pions. This quadratic form is produced by...

Hello all, I hope you can give me a hand with a QFT homework I'm working on. We are to compute the beta equation of a Non-abelian SU(N) theory with: Complex scalars (massless), bosons, ghosts. My question is referring to the Boson self-energy scalar loop correction. 1. Homework Statement We...

I have the opportunity to pursue a research project in either lattice QCD or QED. In the case of QED, I'd be writing the code myself, will be able to understand the whole thing, et cetera. There shouldn't be any mystery in this. It almost seems like a long exercise, and there isn't anything I'd...

I am considering the following virtual correction to the DIS process between a quark and photon. The process on the left in the attachment is one such QCD correction I can make but the diagram on the right is another (I just changed where the gluon vertices lie). It seems to me that both...

Hi, everyone, I am now looking for a good textbook on QCD. I am especially want to learn the derivation of DGLAP evolution equations. So, it will be better if the book contains the detailed derivation of evolution equations step by step for all the diagrams using Feynman rules. I looked through...

How to understand QCD vacuum? What is the model independent expectation value for it? Looking for some basic literature on it. Thanks in advance!

So I've been trying to figure this out, and I've come to 2 possible conclusions, either QCD is broken, or it behaves completely different, with different characteristics than I am used to (such as possibly not obeying superposition?) If blue is attracted to both red and green, then shouldn't a...

Hi physicists , I'm in high-school yet, and since I've started study physics for myself my aim is to understand more complex fields of physics, and one of these fields in QED(Quantum Eletrodynamics) and QCD(Quantum Chromodynamics), could anyone tell me what the path to go for a good basis for...

Someone please explain what the colors mean in QCD. Are different colors different quarks? Are different colors the same quark with different amounts of energy? Is it something else entirely? Does it apply to leptons?

QCD is asymptomatic safe well known to grow weaker at higher energies. if gravity is also asymptotic safe would it grow weaker at higher energies instead of stronger as predicted by classical GR? if not, is it possible there is a quantum gravity theory that like QCD grows weaker at higher...

Hi all, In former threads, namely: "Color factors of color -- octet scalars", and "The double line notation and the adjoint representation" I were asking about the difference between the interaction vertices among: * three gluons (GGG), as in SM, * three colored octet scalars, call ## S =...

Since I am a beginner in particle physics I was wondering if anyone could explain to me the concepts of Quantum Chromo Dynamics rather than saying it is the study of the colour force. 1. What are the concepts of QCD? 2. If QCD has more to it than colour, what are the other concepts? 3. Can you...

I was reading this document from CMS: https://cds.cern.ch/record/2002036?ln=en But I have some problems understanding how they calculated the bckg coming from the QCD. My questions come from Sec.6 Background, so you can jump and have a quick glimpse over it (?). They used the tight-to-loose...

Homework Statement Consider the fermionic part of the QCD Lagrangian: $$\mathcal{L} = \bar\psi (\mathrm{i} {\not{\!\partial}} - m) \psi \; ,$$ where I used a matrix notation to supress all the colour indices (i.e., ##\psi## is understood to be a three-component vector in colour space whilst...

Can someone give me a brief overview of QCD, I'd like to know famous physicists that worked in the field, the main theory's it includes, when did it begin and why do we study it? Thank you.

I have found this paper, it seems interesting although i haven't read it all yet arXiv:1507.00460 [pdf] Dark matter, Mach's ether and the QCD vacuum Gilles Cohen-Tannoudji Subjects: History and Philosophy of Physics (physics.hist-ph); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Guys...help me out to understand this 3-vertex factor of non-abelian and or QCD fields. it is from peskin & schroeder page 507.

Does anyone have a good reference to a more modern introduction to lqcd? By modern I mean including unquenched, staggered quark, etc. Most intros I find are quite dated and I'm interested in learning about newer technology and methods/improved. Figured there may be some lattice theorists here...

Can anyone explain to me what "twist" refers to in the context of QCD? More specifically, I have come across the terms "leading twist" and "higher twist".

Hello, everyone! I have a short question. I think everybody knows the Furry theorem for QED, which tells, in particular, that fermion loops with odd number of fermions always give 0. Does the same true for QCD of weak theory? Can one have non-zero contribution to 3-gluon vertex (or 3-boson...

This thread involves a little general relativity and a little particle physics. Observations indicate that space is expanding at an accelerated rate. Given this would continue, even atomic nuclei should be torn apart leaving single particles, isolated from one another by causal horizons. What...

I stumbled across a paper which stated that the relation between statistical mechanics and field theory is exploited to recast lattice QCD in terms of a "microcanonical ensemble" of sorts. I was curious to know if this was still a commonly used technique. The paper in question...

I am trying to calculate differential cross-section for partonic collisions (QCD) like q + q \rightarrow q +q q + \bar{q}\rightarrow q + q g + g \rightarrow g + g I can't find those calculations done anywhere, just the results and maybe some middle tips, that's all. As you may know those...

What is the difference between Yang-Mills and QED theories? Yukawa and QCD? specially in terms of the lagrangians. I really want to get into this subject with a previously first sight.

I am studying this paper about the strong CP problem and axions by R.D. Peccei: http://arxiv.org/abs/hep-ph/0607268 At the moment I read through the introduction and I have some questions about some points, so please could you help me clear out some things?. [In general I'll try to feed back...

Say we try and calculate the ground state energy of the bound state of a quark antiquark meson via lattice QCD. Say I look at one space time lattice point of one path. Do the fermi fields "live" on the lattice points? Do the boson fields "live" on the legs between the space time lattice points...

This isn't a homework problem. I am preparing for a particle physics exam and although I understand the theoretical side of field theory, I have little idea how to approach practical scattering questions like these. THE PROBLEM: Dark matter might be observed at the LHC with monojet and...

Hello Community! I can't find a good answer(if there is) to my question. When in statistical mechanics we can define the order parameter for to study some phase transition. we need to define a order parameter. Now, I want to know if we can to define/find some "order parameter" for to...

So I've been set a computational problem regarding applying the Shroedinger equation to quarkonium and I'm just trying to do some research around the subject because it's nothing like I've ever covered before and quite fascinating. There's quite a few questions that I can't find answers to...

Hello, I am a first year graduate student in physics who is interested in getting involved in the field of lattice QCD. I purchased the text "Lattice methods for quantum chromodynamics" by DeGrand and Detar. I have never taken a course on quantum field theory, but I hoped that having...

Hello to all When we study the Confinement of QCD, the coupling strength increases as the momentum transfer decreases (i.e, p→0) and the coupling strength decreases as the momentum transfer increases (i.e, p→∞). My question is: can we say ω =m; where ω is the energy which is given as...

http://d.kankan3d.com/file/data/bcs/2014/0508/w65h1446064_1399517186_873.jpg 1.How to deal with the delta functions in eq.2.3.153 to obtain the eq.2.3.154 by integrating over q'? 2.How to caculate the integral from eq.2.3.154 to eq.2.3.156, especially the theta function?

Hi all, I am currently trying to calculate the beta function for scalar QCD theory (one loop for general su(n)). I therefore need to calculate the Feynman rules in order to apply them to the one loop diagrams. Unfortunately I am getting very confused with what the Lagrangian for scalar QCD...

I found a few curious arxiv papers by Alexandre Deur (via Twitter). Deur works in QCD. He explains QCD strings (flux line between quark and antiquark) as due to attraction between gluon field lines, ultimately due to gluon-gluon interactions. Phenomenologically this adds a linear term to the...

Hello, there are several papers on QCD in Hamiltonian formulation, especially in Coulomb gauge. Unfortunately the Hamiltonian H is rather formel and highly complex. Question: is there a paper discussing the contribution of individual terms of H to the nucleon mass?

the number stand for the index of particles (quarks and gluons) M=\bar{v}(p_2) ig_sT_{12}\gamma^\mu(12)u(p_1)\frac{-i}{p_7^2}\bar{u}(p_5) ig_sT_{56}\gamma_\mu(56)\bar{v}(p_6)\frac{-i}{m-\gamma^\mu p_{9\mu}}\bar{v}(p_3)...

Hi every one, This time i am studying Quantum field theory and some times we call also high energy physics. But when we study QCD, more we focus on the confinement part, i.e non-perturbative part. That means low energy or large distance. My question is, can we consider non-perturbative QCD as...

QCD has 3 "colors". I'm wondering whether there is something special about the number 3, or whether it is possible to generalize to N colors and get a very similar theory.

Hi pf, i have been wondering what differentiates QED Vacuum from QCD Vacuum? How would u explain its implications? I mean, how can u define pure vacuum in 2 ways?

This isn't really a homework question but it's from a past paper for revision. It is a short question so it shouldn't require lengthy mathematics. It asks you to show that a quark and an anti-quark of the same colour will attract. The problem gives the 8 generator matrices of SU(3) and...

Is it feasible to calculate a three-point correlation on the lattice? Say, I have two quark fields separated at z_1+z_2 and 0, and a gluon field inserted at z_2. Also I need two gauge links to make this expression gauge invariant: \bar{\psi}(z_1+z_2) \Gamma(z_1+z_2; z_2) F^{\mu\nu}(z_2)...

Hi guys! A serious doubt just passed through my mind. It is probably a silly question. In writing the Feynman rules for QCD, we know that the quark-gluon vertex is given by: $$-ig\gamma_\mu T^a_{ij}$$ where T is the SU(3) generator and i and j are the colors of the incoming and outgoing...

A few years ago, Tamar Friedmann (then at MIT) wrote two papers proposing a new interpretation of the hadron spectrum, and some new phenomenological laws governing hadron size (and see this talk for a synopsis). According to yesterday's hep-ph update, they're finally going to appear in...

There was a thread about how to describe the proton in string theory that was deleted while I was writing an answer, so I will post my answer here in a new thread. How the proton, and processes inside the proton, are described in string theory, is an open research question, part of the area...

Form factors in QCD are given in following form "f(q^2)=f(0)/(1-q^2/m^2)". This expression has a pole at q=m. How do we plot these form factors vs center of mass energy and deal with the pole.

Coupling constant of strong interaction is only finitely greater than coupling constant in QED if the quarks insides hadron not very far apart each other,so I do not understand why there are not exist color multiplet states of hadrons,other hand I do not understand why the singlet states have...