Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

145 submissions , 122 unreviewed
3,930 questions , 1,398 unanswered
4,862 answers , 20,637 comments
1,470 users with positive rep
502 active unimported users
More ...

High pT and high Q^2 in deep inelastic hadronic collisions.

+ 3 like - 0 dislike
123 views

When reading about high energy collisions (for example proton-proton collisions at LHC), I always find that deep inelastic scattering (DIS) means high transverse momentum (pT). So that usually at the LHC experiments one look at high pT jets of particles. What is the relation between DIS and pT? 

I also find the relation \(Q \sim p_T\), which, for me, is hard to demonstrate. Can someone please shed light on this? Moreover, I found statements about the interaction time \(\tau\), which is related to Q and pT in this way: \(\tau \sim \frac{1}{Q} \sim \frac{1}{p_T}\), where again \(Q\sim p_T \) is assumed. To understand, I would really like someone to define Q for a proton-proton (parton-parton) interaction. Is this Q the same if I take an s channel or a t channel process? Thanks!

asked Jun 12, 2016 in Experimental Physics by anonymous [ no revision ]

1 Answer

+ 2 like - 0 dislike

Q^2 is the Mandelstam variable t, i.e. the four momentum transfer squared where the s channel is the x axis in the feynman diagram.

t channel

where p1 p2 are incoming.

In the same link it is seen that at the relativistic limit :

stu

The dot product for t (Q^2) is p transverse to the incoming beam direction of p1 , p2 are incoming.

etc.

Is this Q the same if I take an s channel or a t channel process

No, Q is only defined in the t channel, with incoming p1 p2.  

p transverse  to the beam is a very useful variable, as it does not change with the Lorenz transformation to the Center of Mass System. It is an easy trigger variable for experiments that indicates the size of the  mass break up at the CMS . The higher the p transverse of the event the more deeply inelastic the specific event ( think Rutherford scattering). The approximation with Q allows comparison estimates with appropriate analytic expressions of crossections.


This post imported from StackExchange Physics at 2016-06-16 13:40 (UTC), posted by SE-user anna v

answered Jun 15, 2016 by anna v (1,785 points) [ revision history ]
edited Jun 16, 2016 by anna v
Hi @anna, thanks for your answer. What I don't understand is why all people say: "At LHC we look at high $p_T$ events, as these events are due to deep inelastic scattering processes". This statement works for a process with a photon exchanged in the $t$ channel. But take a Drell-Yan process: $q\bar{q}\rightarrow \gamma^*\rightarrow q\bar{q}$, which happens in the $s$ channel. In this case the two out-coming quarks (which I'll see as two jets) won't have a preferred direction (i.e. they won't be necessarily at high $p_T$). Am I wrong?

This post imported from StackExchange Physics at 2016-06-16 13:40 (UTC), posted by SE-user Marco
One chases high ptransverse events because they are the true picture of the center of mass interaction, the transverse component does not change with the Lorenz transformation. It is sufficient for an interesting event to have a high pt because it shows high break up at the cms. Identifying the pt with Q2 is a mathematical definition which in terms of Feynman diagrams may be useful or not.

This post imported from StackExchange Physics at 2016-06-16 13:40 (UTC), posted by SE-user anna v

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar$ysicsOv$\varnothing$rflow
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
To avoid this verification in future, please log in or register.




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...