Photon Pair Production Cross Sections at 7 TeV from CMS at LHC | LHC Preprint

[TrickyDicky]

http://arxiv.org/abs/1110.6461
Abstract
"The integrated and differential cross sections for the production of pairs of isolated photons is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 36 inverse picobarns is analysed. A next-to-leading-order perturbative QCD calculation is compared to the measurements. A discrepancy is observed for regions of the phase space where the two photons have an azimuthal angle difference, Delta(phi), less than approximately 2.8."

This is kind of strange. Any thoughts?

 

[TrickyDicky]

And I thought anything about the LHC was a hot topic here in the hep subforum...

 

[Parlyne]

I think, perhaps, no one's quite sure what to make of this. My first thought was that it probably just meant that they'd mismodeled some of the low(ish) energy QCD garbage, giving a slightly higher occurrence of slightly boosted neutral pions. But, I guess the invariant mass and transverse momentum distributions would rule those out as too light.

 

[Parlyne]

Matt Strassler has taken a stab at discussing this over at his blog.

http://profmattstrassler.com/articles-and-posts/lhcposts/two-photons-data-and-theory-disagree/

 

[TrickyDicky]

Matt Strassler has taken a stab at discussing this over at his blog.

http://profmattstrassler.com/articles-and-posts/lhcposts/two-photons-data-and-theory-disagree/

Thanks for the link, Parlyne.
What Matt Strassler says is almost as hard to believe as the LHC results, that the experimentalists doing the experiment weren't able to implement correctly the theoretical predictions. This has several interpretations, a few of them are that either the theory is not very good making predictions, or they are very hard to implement which would limit their predictive power too, or the experimentalist performing this at the LHC (wich is actually a collaboration of many people from many countries) are, let's put it like this, not very clever.
But you're right that apparently people don't know what to make of this.

 

[Vanadium 50]

TrickyDicky, is there any reason why you seem to go out of your way to be as offensive as possible? It's easy to be a sideline quarterback and say "I could do a better job".

This is a particularly difficult calculation, and had you bothered to read Reference 8 in the paper, you would have learned that two calculations can differ by factors of at least 3. This is comparable to the discrepancy seen by the Tevatron and LHC experiments. (CMS is the fourth experiment to see this)

 

[TrickyDicky]

TrickyDicky, is there any reason why you seem to go out of your way to be as offensive as possible?

Huh? why would you say that? Have I really offended you? I thought personal judgements were better left out of the science forums according to PF rules.

It's easy to be a sideline quarterback and say "I could do a better job".

Where do you get the impression I think that? Certainly I haven't written that anywhere.

This is a particularly difficult calculation, and had you bothered to read Reference 8 in the paper, you would have learned that two calculations can differ by factors of at least 3. This is comparable to the discrepancy seen by the Tevatron and LHC experiments. (CMS is the fourth experiment to see this)

Thanks for your opinion on this.