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[QUE/QFT-15009] QFT-PROBLEMNode id: 4399page$\newcommand{\Lsc}{\mathscr L}$ Assuming interactions of charged pions to be of the form \(\Lsc_\text{int} (x)= (g/4)(\pi(x)^+\pi(x)^-)^2\) find the \(S\) matrix element for \(\pi-\pi\) scattering \[\pi^+ + \pi^- \longrightarrow \pi^+ + \pi^-\] transition probability per unit time per unit volume for \(\pi - \pi\) scattering. Compute the total cross section for the scattering process and show that \[ \frac{d\sigma}{d\Omega}= \frac{g^2}{64\pi^2 E_\text{cm}^2}\] |
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22-02-01 19:02:32 |
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[QUE/QFT-15008] QFT-PROBLEMNode id: 4398page\(\newcommand{\dd}[2][]{\frac{d #1}{d #2}}\) Assuming \(\frac{g}{4!} \pi^4(x)\) interaction for neutral pions of mass \(m\), find
- transition probability per unit time per unit volume for \(\pi - \pi\) scattering.
- Compute the total cross section for the scattering process and show that \[ \dd[\sigma]{\Omega}= \frac{g^2}{64\pi^2 E_\text{cm}^2}\]
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22-02-01 19:02:44 |
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[QUE/QFT-15007] QFT-PROBLEMNode id: 4395pageWrite Klein Gordon equation and the corresponding Lagrangian for a spin zero charged particle in presence an external electromagnetic field. Obtain the Hamiltonian. To the lowest order in \(e\), compute the scattering cross section for Coulomb scattering of a charged pion taking the vector potential to be that of a nucleus of charge density \(\rho(\vec{x})\). Write your answer for cross section in terms in of Fourier transform of \(\rho(\vec{x})\). |
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22-02-01 19:02:28 |
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[QUE/QFT-15006] QFT-PROBLEMNode id: 4394page $\newcommand{\dd}[2][]{\frac{d#1}{d#2}}$ Write Dirac equation in external electromagentic potential \(A_\mu(x)\). Assume the potentials for Coulomb interactions with a nucleus of charge \(Ze\) to be \[ \vec{A}=0, \quad A_0 = \frac{Ze}{4\pi |x|}. \] Show that the differential cross section for electron scattering from nucleus is given by \[\dd[\sigma]{\Omega}= \left(\dd[\sigma]{\Omega}\right)_\text{R} \Big(1-v^2\sin^2(\theta/2)\Big)\] where \[\left(\dd[\sigma]{\Omega}\right)_\text{R} = \frac{Ze^2}{64\pi m^2v^4 \sin^4(\theta/2)}\] is the Rutherford cross section for nonrelativistic Coulomb scattering. |
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22-02-01 19:02:28 |
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[QUE/QFT-15005] QFT-PROBLEMNode id: 4393page$\newcommand{\Hsc}{\mathscr H}$
- The interaction Hamiltonian for pion decay \(\pi^-\longrightarrow e^- + \bar{\nu} \) can be written as \[\Hsc_\text{int} = \frac{g}{\sqrt{2}}\bar{\psi}_e(x)\gamma_\mu(1-\gamma_5)\psi(x)_\nu \partial^\mu \phi_\pi^-(x) + h.c. \] Show that the decay rate is given by \[\Gamma = \frac{g^2}{8\pi} \frac{m_e^2(m_\pi^2-m_e^2)^2}{m_\pi^3}.\]
- Assuming that the coupling constant for \(\pi^-\longrightarrow \mu^- + \bar{\nu} \) is equal to that for the electron decay, calculate the branching ratio \[\frac{\Gamma(\pi^-\longrightarrow \mu^- +\bar{\nu})} {\Gamma((\pi^-\longrightarrow e^- + \bar{\nu})} \]
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22-01-31 08:01:39 |
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[QUE/QFT-15003] QFT-PROBLEMNode id: 4066pageConsider the pion decay process \( \pi^- \longrightarrow e^- + \bar{\nu}.\) Let the fields for \(\pi^-, e^- \text{ and }\nu \) be denoted by \(\phi_{\pi^-}(x), \psi_{e}(x) \text{ and } \psi_\nu (x)\) respectively.
- Which of the following interactions will contribute to the pion decay process in the first order?
- [(i)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(ii)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- [(iii)] \(\phi_{\pi^-}(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(iv)] \(\phi_{\pi^-}(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- Is there a term which will contribute to the decay \(\pi^+ \longrightarrow e^+ \nu \)? If yes, which one?
- Consider processes allowed by different terms (i)-(iv) and check if any of known selection rules is violated by any of the four expressions (i)-(iv).
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22-01-31 08:01:21 |
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[QUE/QFT-15004] QFT-PROBLEMNode id: 4392pageThe original four fermion interaction for beta decay of neutron \[n \longrightarrow p + e^- + \bar{\nu} \] is of the of form \[ \bar{\psi}_p(x)\gamma_\mu\psi_n(x) \bar{\psi}_\nu(x) \gamma^\mu \psi_e(x) + h.c.\] Now consider other processes given below. Which of these processes (real or virtual) are permitted and which ones are not permitted by the above interaction in the first order?
- \( \bar{p} \longrightarrow \bar{n} + e^- +\bar{\nu} \);
- \( \bar{p} \longrightarrow \bar{n} + e^- +\nu \);
- \( n \longrightarrow p + e^+ + \nu \);
- \( p \longrightarrow n + e^+ + \bar{\nu} \);
- \( \bar{n} \longrightarrow \bar{p} + e^+ + \nu \);
- \( \bar{n} \longrightarrow \bar{p} + e^+ + \bar{\nu} \).
Give brief reason in each case. |
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22-01-31 08:01:18 |
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[QUE/QFT-15002] QFT-PROBLEMNode id: 4390page$\newcommand{\Hsc}{\mathscr H}$ The interaction of \(\Lambda^0\) hyperon, responsible for decay into a proton and a \(\pi^-\), is given by \[ \Hsc_\text{int} = \bar{\psi}_p(x) ( g - g^\prime\gamma_5)\psi_\Lambda(x) \phi_\pi ^\dagger + h.c. \]
- Give examples of three virtual processes allowed in the first order of this interaction term.}
- Show that the partial decay rate of \(\Lambda^0 \longrightarrow p + \pi^-\) is given by \[ \Gamma = \frac{1}{4\pi}\frac{|\vec{p}|}{M_\Lambda}\left(|g|^2(E_p+M_p) + |g^\prime|^2 (E_p-M_p) \right)\]
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22-01-31 08:01:08 |
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[QUE/QFT-15001] QFT-PROBLEMNode id: 4389pageProve that the probability per unit volume per unit time that the external potential \[ \vec{A}= (0,0,a\cos\omega t), A_0=0\] creates an electron-positron pair in the vacuum is given by \[ R = \frac{2}{3} \frac{e^2}{4\pi} \Big(\frac{|a|^2}{2} \Big) \omega^2 \Big( 1+ \frac{2m^2}{\omega^2}\Big) \sqrt{1- \frac{4m^2}{\omega^2}} \] |
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22-01-31 08:01:18 |
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[QUE/QFT-15010] QFT-PROBLEMNode id: 4400page$\newcommand{\Lsc}{\mathscr L}$ $\newcommand{\dd}[2][]{\frac{d#1}{d#2}}$ Assuming interactions of charged pions to be of the form \(\Lsc_\text{int}(x)= (g/4)(\pi(x)^+\pi(x)^-)^2\) find
- the \(S\) matrix element for \(\pi-\pi\) scattering \[\pi^+ + \pi^- \longrightarrow \pi^+ + \pi^-\]
- transition probability per unit time per unit volume for \(\pi - \pi\) scattering.
- Compute the total cross section for the scattering process and show that \[ \dd[\sigma]{\Omega}= \frac{g^2}{64\pi^2 E_\text{cm}^2}\]
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22-01-31 08:01:07 |
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[QUE/QFT-15012] QFT-PROBLEMNode id: 4401page$\newcommand{\Lsc}{\mathscr L}$ Consider a system of two real scalar fields \(\phi_1, \phi_2\) described by the Lagrangian density \begin{equation} \Lsc = \frac{1}{2} \partial_\mu \phi_i \partial^\mu \phi_i - \frac{1}{2} m^2 \phi_i\phi_i -\frac{1}{4} \lambda (\phi_i\phi_i)^2. \end{equation} Compute the scattering cross section to the lowest order in \(\lambda\). Find the cross sections for the three processes
- \(\phi_1 + \phi_2 \longrightarrow \phi_1 +\phi_2\)
- \(\phi_1 + \phi_1 \longrightarrow \phi_1 +\phi_1\)
- \(\phi_1 + \phi_1 \longrightarrow \phi_2 +\phi_2\)
Write your answers as a constant times \(\sigma_0\equiv \frac{\lambda^2}{64\pi s}\) where \(s\) is the total energy in the center of mass frame |
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22-01-31 08:01:50 |
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[QUE/QFT-15013] QFT-PROBLEMNode id: 4402pageShow that the Rutherford scattering cross section for a second quantized Dirac particle in an external Coulomb field \((Ze^2/r)\) is given by \begin{equation} \frac{d\sigma}{d\Omega} = \frac{Z^2\alpha^2(1-v^2\sin^2(\theta/2))}{4|\vec{k}|^2 v^2 \sin^4(\theta/2)}. \end{equation} where \(\vec{k}\) is the momentum of the incident particle and \(\theta \) is the angle of scattering. |
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22-01-31 08:01:01 |
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[QUE/QFT-15014] QFT-PROBLEMNode id: 4403pageThe matrix element for the decay process \(\pi \to \mu + \nu\) is given by
\[ m_{fi} =\frac{g}{\surd 2} \bar{u}^{(r)}(p) (i \gamma^\mu Q_\mu) (1-\gamma_5)v^{(s)}(k)\]
for the life time computation we need to take absolute square, to average over initial spin and sum over final spin states. You may assume neutrino to be have a mass \(m_\nu\) and take limit \(m_\nu \to 0\). Compute the life time of pion decay. |
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22-01-31 07:01:39 |
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[QUE/QFT-15003] QFT-PROBLEMNode id: 4391pageConsider the pion decay process \( \pi^- \longrightarrow e^- + \bar{\nu}.\) Let the fields for \(\pi^-, e^- \text{ and }\nu \) be denoted by \(\phi_{\pi^-}(x), \psi_{e}(x) \text{ and } \psi_\nu (x)\) respectively.
- Which of the following interactions will contribute to the pion decay process in the first order?
- [(i)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(ii)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- [(iii)] \(\phi_{\pi^-}(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(iv)] \(\phi_{\pi^-}(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- Is there a term which will contribute to the decay \(\pi^+ \longrightarrow e^+ \nu \)? If yes, which one?
- Consider processes allowed by different terms (i)-(iv) and check if any of known selection rules is violated by any of the four exprressions (i)-(iv).
- Which of the following interactions will contribute to the pion decay process in the first order?
- [(i)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(ii)] \(\phi_{\pi^-}^\dagger(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- [(iii)] \(\phi_{\pi^-}(x)\bar{\psi}_e(x) \gamma_\mu(1+\gamma_5) \psi_\nu(x) \)
- [(iv)] \(\phi_{\pi^-}(x)\bar{\psi}_\nu(x) \gamma_\mu(1+\gamma_5) \psi_e(x) \)
- Is there a term which will contribute to the decay \(\pi^+ \longrightarrow e^+ \nu \)? If yes, which one?
- Consider processes allowed by different terms (i)-(iv) and check if any of known selection rules is violated by any of the four exprressions (i)-(iv).
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22-01-24 13:01:25 |
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[QUE/SM-09001] SM-PROBLEMNode id: 5087pageEven though there is high density of electrons in a metal ( a large fraction are free to move, the mean free paths are long, of the order of $10^{_6}$m. Give a qualitative argument for such a long mean free path. Will the mean free path increase or decrease with the increase of temperature? |
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22-01-24 13:01:55 |
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[QUE/SM-09001] SM-SOLUTIONNode id: 5228pageANSWER :
In collision of electrons only a fraction close to the Fermi surface will undergo a change in energy. That fraction is \(\sim n\Big({\frac{kT}{\epsilon_F}}\Big)\) of the electrons. Thus \[ \text{Mean free path } \lambda = \frac{\epsilon_F}{n(k)\sigma^2} >> \frac{1}{n\sigma^2}.\] Further \(\lambda \propto \frac{1}{T}\), as \(T\) increases \(\lambda\) will decrease. |
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22-01-24 13:01:16 |
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[QUE/SM-04026] SM-PROBLEMNode id: 5227pageConsider a perfect gas having N particles obeying Maxwell- Boltzmann statistics and is in equilibrium at temperature $T$. Find the average of $\epsilon^r$ ( $r$ is a real number) where $\epsilon $ is the energy of a single particle. |
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22-01-23 20:01:26 |
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[QUE/SM-5001] SM-PROBLEMNode id: 5222pageConsider a system in touch with a reservoir with which it exchanges energy and volume. ( a movable piston is attached to the system and the piston is in touch with the reservoir). Find the partition function $Z_v$ and show that the thermodynamical function $-k\,T\,{\rm{ln}}Z_v$ is the Gibb's free energy $G$. |
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22-01-23 20:01:12 |
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[QUE/SM-04024] SM-PROBLEMNode id: 5223pageConsider a system having 3 non-interacting particles, each having three possible energy levels with energies $0\,,\,2\Delta\,,\,4\Delta$. Let $N_1\,,\,N_2\,,\,N_3$ be the number of particles occupying the energy levels Find the average of $N_1N_2$ when the system is at temperature $T$. |
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22-01-23 20:01:46 |
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[QUE/SM-05003] SM-PROBLEMNode id: 5224pageConsider free fermions at temperature $T\,=\,300^o$ K. The Fermi energy is $5$ electron volt. Give a qualitative argument why the specific heat for such a system is linearly proportional to the temperature. |
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22-01-23 20:01:23 |
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