---
title: Physics Questions
tags: questions
---

# Basic Particle Physics

1. Why SM is a successful theory?
1. How many input parameters are there in the SM?
2. How many couplings it have other than $\alpha_s,~\alpha_{EM},~\alpha_{weak}$?

1. What is Higgs mechanism? How a particle gets their mass?
3. Is SU(3) gauge symmetry broken or not?
4. Do quark get mass from Higgs mechanism?
5. Coupling constant:
1. What is the relative strength of coupling constant?
2. How these parameters varies with energy?
3. $\alpha = \frac{1}{137}$, How it is defined?

1. What will happen if the total number of quaks and total number of leptons are different in the SM?
2. What do you mean by ***Asympotatic freedom in the quark sector***?
3. When an atom absorbs a photon - does this changes their spin? why or why not?
20. Why do gauge bosons have to be massless to preserve gauge invariance?

1. Cross-Section

- It is one of most important measurement in particle physics.
- It tell us the probability of interaction of two particles or beam of particles in a given way[^1].
- Parameters on which cross-section depends are:
- Size of particles,
- impact parameter,
- number of particles in beam,
- angle at which the beam hit the target,
- In other word, cross-sections provides the picture of of the properties of fundamental particles.
- The measure of the one or more aspects of the interaction are called as the ***differential cross-section***.
- And the summaries of all possible differenntial cross-section are called the cross-section.

[^1]:Link: https://www.symmetrymagazine.org/article/speak-physics-what-is-a-cross-section

1. Should cross-section after hadronization / showering change?

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In general, without matching/merging, the cross section after parton shower and hadronization should be equal to the initial cross section. This is a fundamental aspect of what the parton shower represents, namely that in the soft/collinear limit the soft emission or splitting can be factorized from the core matrix element and interpreted as a probability for emission. So, the parton shower and hadronization should not change the cross section but rather tell how it "spreads" to different jet multiplicities. A good reference for more information is the presentation in [^2] . It's a 4 part lecture with video on CDS if you really want to get into the gory details :)
</span>

[^2]: Indico Link: https://indico.cern.ch/event/226365/contributions/1533462/

Ref: [https://hypernews.cern.ch/HyperNews/CMS/get/generators/3540/1/1.html](https://hypernews.cern.ch/HyperNews/CMS/get/generators/3540/1/1.html)

10. Is the Fermi-Kurie plot useful for the Cs decay?
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11. Why is the instrument response to the K-conversion electrons broad? What is that telling you about the instrument?
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14. What is the absorption cross-section for beta particles vs energy when they traverse a piece of material? (Melissinos, pg 150 et seq.)
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15. What is the error bar per point?
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16. What is a statistically sound way to search for a neutrino mass with your data?
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17. How susceptible is your result for the endpoint of the Na decay to background subtraction?
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1. What do you mean by abelian and non-abelian nature of a group?

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If the generator of the group commutes then it is kown as the abelian else it is known as the non-abelian.
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2. How do you know that muon, electrons, etc are spin half particles?

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They are measured quantity by the experiment.

Electrons: Two experiment evidence for this in 1920 and 1922. First evidence was back in 1920, the closely spaced splitting of the hydrogen spectral lines, also known as fine structure. Another experimental evidence observed in 1922 in the **Stern-Gerlach experiment**. In Stern-Gerlach experiment when a beam of silver atoms passes through the inhomogeneous magnetic field it splitted into two beams.

Muons:
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3. Why there is asymmetry between production of W+ and W- bosons in pp colliders?

<span style="color:blue">
The asymmetry in the $W^+$ and $W^-$ production is because of the proton contains two up and one down quarks.
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4. What is the difference between minbias and zero bias events in terms of MC production?

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MinBias requires at least one hit on each side of the detector. In ZeroBias there is no such requirement (the event is recorded whenever there is a collision). It depends on the physics being studied to decide whether you can use MinBias instead of ZeroBias.
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5. What is the nature of confinment interaction in compositness? (or Binding mechanism)?

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6. What is tha value of $\Lambda_{QCD}$?

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7. In LHC going from 8 TeV to 14 TeV, should the number of proton per bunch also increases or decreses and why?

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Going from 8 TeV to 14 TeV, the per proton radiation will increase. So, total radiation will increase.
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Also 8 TeV to 14 TeV; 50ns to 25ns less bunch spacing so inter bunch and intra bunch interaction will increase. But, magnetic field for focusing is same. So, bunches will deform. Thus, we need to decrease the number of protons to keep the focusing same at same magnetic field.
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8. What are uses of low pt photon trigger?

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anomalous coupling, triple and quartic gauge boson coupling.
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9. Why Delta-R is taken for isolation study?

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To isolate a particle (say electron) from the QCD background or the pile-up.
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10. How proton distribution and QCD cross-section for for sample look like?

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11. What are the detector components in CMS and their individual resolution?
Why?

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12. Why b-quark has large life time?

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14. How do you define pile-up?

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- It is defined as the multiple bunch crossing in the same proton-proton bunch crossing.
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- Theoritically it can be estimated as the product of the proton-proton cross-section ($\sigma_{inel}$), the instantaneuos luminosity ($L$) and the mean time interval between the two collisions, ($<t>$): $$mean~pile-up = \sigma_{inel} \times L \times <t> $$
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15. Why use fit function?

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To model a curve, i.e. to get a parametric form so that it can be estimated at every point and also may be extraplotted. Also, sometime it helps in case of less statistics at tails.
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16. What is the uncertanity associated with each term and how you calculate
them?

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17. After what energy in LHC we can say we are colliding quark-gluons not
protons?

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18. What is missing transverse energy?

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This is defined as the magnitude of the missing transverse momentum vector, defined as the projection on the plane perpendicular to the beams of the negative vector sum of the momenta of all reconstructed particles in an event.
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19. Transverse mass?

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$M_{T} = \sqrt{2*p_{T,lep} * E_{T,miss} [1 - cos(\Delta \phi (l, E_{T,miss}))] }$ </span>

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To reject multijet background we generally apply higher value of MET cut, like $M_T > 40 GeV$</span>

20. Assymetric W-boson production?

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W-boson production is known to be charge asymmetric due to the structure of the proton, with more w+ boson produced than w- boson.</span>

21. Gauge Invariance?

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Gauge invariance is the central feature of quantum field theory, because they insures that the calculated observable are finite (i.e. the amplitudes in a perturbative expansion are renormalizable).
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23. Explain screening and anti-screening effect?

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24. Why we require infrared and colliniear safe jets?

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25. What is unitarity invariance? Why do you want to preserve it upto 1 TeV only?

1. Are all possible triple and quartic interaction of $W,~\gamma,~and ~Z$ allowed in the SM? Why or why not?
1. What is the motivation for using the *Zeppenfeld variable*?
1. For the Anomalous Quartic Gauge Coupling (aQGC) limits why one generally use only dimension-4 operators not the higher one?
1. What is the significance of the width of higgs mass ratio? or If a particle is decaying through weak interaction and another particle decaying through the strong interaction. Then in which case decay width will be broader?
1. Why Z-width is narrower compared to W-width?
1. Why luminosity systematic uncertanity considered on signal sample but not on others?

# Other question

1. Shape of the Higgs branching ratio to ZZ: [https://physics.stackexchange.com/questions/3773/shape-of-the-higgs-branching-ratio-to-zz](https://physics.stackexchange.com/questions/3773/shape-of-the-higgs-branching-ratio-to-zz)