When should rapidity or pseudorapidity be used for particle measurements?

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SUMMARY

In particle physics, rapidity (y) and pseudorapidity (η) serve distinct purposes in measurements. Rapidity is defined as y = 0.5 ln((E + p_z)/(E - p_z)) and is primarily used for massive particles, particularly in experiments like the LHC's ATLAS, where it is applicable to particles such as J/ψ. Pseudorapidity, defined as η = -0.5 ln(tan(θ/2)), is a geometric measure that remains consistent across all particles within a detector's coverage. For massive objects like jets and vector bosons, rapidity is preferred, while pseudorapidity is suitable for massless particles.

PREREQUISITES
  • Understanding of particle physics concepts, specifically rapidity and pseudorapidity.
  • Familiarity with the LHC and its experiments, particularly ATLAS and CMS.
  • Knowledge of Lorentz invariance and its implications in particle measurements.
  • Basic understanding of particle mass thresholds in experimental physics.
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  • Research the specific applications of rapidity in LHC experiments, focusing on massive particles.
  • Study the geometric implications of pseudorapidity in particle detection and analysis.
  • Examine the differences in measurement techniques for jets and vector bosons at ATLAS and CMS.
  • Explore the concept of Lorentz invariance and its relevance to rapidity and pseudorapidity.
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Particle physicists, experimental physicists, and researchers involved in high-energy physics experiments, particularly those working with data from the LHC and its detectors.

ChrisVer
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I have one question, when should someone use rapidity y= 0.5 \ln \frac{E + p_z}{E - p_z} and when pseudorapidity \eta = -\frac{1}{2} \ln \tan \theta/2?
I've read that rapidity is used for "massive" particles... for experiments like LHC (or more specifically ATLAS), what threshold is considered massive?
I am sure that tau leptons (~1.7GeV) are not considered massive... however I recently read that for particles like J/ψ they use the rapidity... https://arxiv.org/abs/1104.3038
 
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Rapidity is almost Lorentz invariant - it shifts by a constant with a boost. Pseudorapidity is geometric - a detector that covers a particular psedorapidity region does so for all particles.
 
For massless particles, both are equivalent. For massive objects there is a difference. At ATLAS and CMS, massive objects are mainly jets and the vector bosons, hadrons only if the measurement happens at very low energy like in your linked example.
 

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