General Question That Every One Should Know

Goals of this page:

Here we will find the general questions on LHC, CMS, physics at CMS, and about GEM detectors.

General Physics

1. Why wave-function in the lagrangian should be a complex quantity?
   • kdfk

Large Hadron Collider

1. Brief introduction about LHC
   • Show Answer Hide

           * It is a particle accelerator and collider.
           * First started in 10 september 2008 with 900 GeV CoM later on in 2010-11 it collided protons with 7 TeV CoM and in 2012 at 8 TeV CoM.
           * Circumference : 27 km
           * It consists of superconducting magnets to bend the high energy proton beam.
           * And RF cavity for accelerating charged particles.  *Write more about it*
           * Accelerator consists of 2 separate beam pipe (kept at ultrahigh vacuum, in which particles moves in opposite direction
           * vacuum  is about 10^6 mbar
           * The superconducting magnets require chilling the superconductor at temperature  -271.3C using liquid helium.

2. Make list of various parameters of LHC at 8 TeV and 14TeV
3. LHC general parameters: Link
4. LHC Beam Parameters
5. Why not we just started with 14 TeV energy at the start of LHC in 2008?
6. What is the formula for instantaneous luminosity?
7. Summary of Different Experiments at LHC:
   • Show Answer Hide

           * It is a particle accelerator and collider.
           * First started in 10 september 2008 with 900 GeV CoM later on in 2010-11 it collided protons with 7 TeV CoM and in 2012 at 8 TeV CoM.
           * Circumference : 27 km
           * It consists of superconducting magnets to bend the high energy proton beam.
           * And RF cavity for accelerating charged particles.  *Write more about it*
           * Accelerator consists of 2 separate beam pipe (kept at ultrahigh vacuum, in which particles moves in opposite direction
           * vacuum  is about 10^6 mbar
           * The superconducting magnets require chilling the superconductor at temperature  -271.3C using liquid helium.

8. Detectors
   • Show Answer Hide

          * CMS
          * ATLAS 
             * It is one of main purpose detector of LHC, because it investigates wide range of physics.
             * Six different detecting subsystems arranged in layers around the collision point record the paths, momentum, and energy of the particles, allowing them to be individually identified.
             * A huge magnet system bends the paths of charged particles so that their momenta can be measured. * ALICE : Detect quark-gluon plasma, a state of matter thought to have formed just after big-bang.
             * It is one of heavy ion detector on the LHC ring.
             * designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms. * LHCb This will shed light on why we live in a universe that is composed of only matter but no antimatter.
             * This will investigate the slight difference between matter and antimatter by studying a type of particle called the b-quark.
             * Instead of surrounding the entire collision point with an enclosed detector as do ATLAS and CMS, the LHCb experiment uses a series of sub detectors to detect mainly forward particles.
             * first sub-detector is mounted close to the collision point, with the others following one behind the other over a length of 20 meters. 

9. What parameters we need to change to increase the COM energy from 7 TeV to 13/14 TeV?
   • Value of Lattice magnet (responsible for keeping the beam stable and aligned in the circular beam pipe) *Value of Insertion magnet (it squeezes particle before they collide. Three quadrupole magnet used to create a system called inner triplet. Two inner triplets are needed near each collision point to tighten the beam; about 12.5 times narrower - from 0.2mm down to 16 micrometer)
   • Frequency of Radiofrequency cavity (to increase the energy of beam)
10. Life time of LHC beam?
    • It is around 10-12 hours. But whenever the instantaneous lumi goes below some threshold like 7-8 x 10^32 then they dump the beam and fill it again.
11. Brief note on Accelerating chain:
12. Why we have many accelerators in LHC or why can't I accelerate my particle from low energy( few Mev's to TeV) in the LHC only?
13. Why on accelerator in LHC is called BOOSTER?
14. Why we collide the two proton beams at certain angles not headon?
15. Can we measure energy of muons in CMS or ATLAS?
    • No. By the bending of muons we can only measure momentum not energy.
    • Energy can only be measured in calorimeters but muons doesn’t deposit there energy in calorimeters.

Compact Muon Solenoid

1. What is the direction of magnetic field in the CMS?
2. Why we are using Lead-Tungstate crystal in ECAL?
3. Difference between radiation length and interaction length.
4. Why HCAL is sampling calorimeter?

Physics at CMS

1. Branching ratio:
   • W to hadrons : 67.6%
   • W to leptons : 10.8%
   • Z to hadrons : 69.91%
   • Z to leptons : 3.36%
2. List Cross-section of all possible important Physics processes at LHC?
3. List reconstruction efficiency of various particles.
4. Difference between Helicity and Chirality.
5. How you measure or distinguish b/w different polarization of particles ?
6. On what factors the cross-section depends?
7. How many different types of magnets used at LHC and what are their purposes?
8. How Longitudinal and transverse W are recognized experimentally? (From helicity its theoretical)
9. What is the measure of W_L and W_T?
10. How do you identify the experimental signatures of forward jets?
11. Why the branching ratio of H->WW is higher than H->ZZ even though Z boson is heavier than that of W?
12. Why is Higgs coupling to boson (K_b) can not take negative values while Higgs coupling to fermion (K_f) can be +ve as well as -ve?
    • find the plot

Detectors

#GenDet

General Detectors

1. Why muons are minimum ionizing particles?
   • It does not brem in ECAL because it is massive (and brem depends inversely on the mass^4). Write the correct formula.
2. Which gases used in the gaseous detectors at CMS?
   • 96.2% (C_2H_2F_4 ) : 3.5 (C_4H_10) : 0.3% (SF_6)
3. What are the purposes of each gas used?

Gas Electron Multiplier

1. What are the specifications of GEM ?

Links of various school materials

1. fermilab school lectures and videos
2. European School in Instrumentation for Particle and Astroparticle Physics
3. CERN accelerator school

General Articles about HEP

1. Five sigma and all that by Jon Butterworth

General Research Papers

1. The CMS experiment at LHC
2. The CERN Large Hadron Collider: Accelerator and Experiments
3. Luminosity Determination at Proton Colliders

Research Papers on Statistics

1. Statistical Issues in Searches for New Physics by Louis Lyons

Papers on WW Scattering

#GenProbCMSSW

General Problems Faced in CMSSW

#GenProbCpp

General Problems Faced in C++

Error messages/codes

Error Messages/Codes page

Last reviewed by: -- RamKrishnaSharma - 17 Sep 2014