# How low is low?

Whenever there’s talks on low-x stuff here at HERA, a lot of my colleagues skive out. Diffraction is spooky, phenomenological, and in general not well understood. I mean, we’re still using pre-parton model technology (regge stuff) for the predictions. For resolved PHP, the uncertainty on the photon PDFs is tragic, so its not really clear if your testing pQCD or universal factorisation or what. Jets above pseudorapidity of 3.5 or so are usually poorly reconstructed, and people have forgotten why its important. Not a fun place to be in general if you ask me. The reason I mention it here is that there’s some cool stuff out on the arXive today, and LHC physics will be swamped with low-x exchanges. Like it or not, the theory and experimental communities will need to work on this stuff to get a decent signal at LHC.

Problem goes like this: All the stuff below the factorization scale like p-PDFs and structure functions are dependant on the renormalization scale and the Bjorkjen $x$ as well. If you measure, say $F_2^p$ with, HERA, and take this to TeVatron, no problem. QCD tells you explicitly how to evolve these non perturbative functions, but that has to be done with an approximation. The DGLAP evolution equations are re-summed in $log(Q^2)$, and fixed order in $log(1/x_{Bj})$. TeVatron has higher $Q^2$, and their $log(x_{Bj})$ range is twice higher than ours, so if DGLAP rolls here, then it’s cool at TeVatron too, and that’s basically what we observe. Look at page 11 of this this pdf for a plot. On the other hand, the LHC will have a comparable reach in $x$ to HERA ($xapprox 10^{-6}$), but they’ll see alot more low-x schenanigans. While the measureents of pDFS etc are fine, you try to evolve them down to “low” $x_{Bj}$ and you might get nonsense. How low is low? Don’t know.

The forward jets people try to enrich their samples with BFKL ( resummed in $log(1/x_{BJ})$) dynamics, but they usually end up only seeing CCFM (gluan radiation angular ordered) dynamics. This in itself is cool, since we’re talking takeover of new dynamic regimes but it still might not bee good enough at the Major Leagues. Anyhow, some pheno folk just fit some BFKL-Pomeron stuff (I want to see this stuff in DIS, but, oh well) to ZEUS Photoproduction data. They don’t show comparisons to the DGLAP-Pomeron, but these objects are different enough that I’m not complaining. It’s not definite proof of BFKL-dynamics, but its cool. Paper’s here.

In other news relating to useful gauge theories: This paper shows how you can modify an old model developed for $F_2^p$ at low-x from HERA color dipole stuff, and add in BFKL dynamics of the scaling. At high pseudorapidities, where we expect new dynamics, it works significantly better than the old style. In the central region the two models work the same. They even do a back check to HERA at moderate $x_{Bj}$, and it looks ok, except that the plot is cluttered, I’d like to see a re-fit and a ratio plot. I’m no RHIC guy, but it looks cool to me.