# IntrinsicKtDetails

This is a more complete version of Detector_Design_Requirements#Measuring_Intrinsic_kT.

Contact person: Mark Baker and Elke Aschenauer

## The Physics Idea

The intrinsic kT parameter in programs like LEPTO and PYTHIA was originally intended to "take into account the motion of the quarks inside the original hadron". Consider an ep collision in the hadronic center-of-mass system (HCMS) with the positive z-axis as the γ* direction and the negative z-axis as the proton direction. To leading order, the struck parton will have a pT equal to the intrinsic kT and the target remnant jet will have an equal and opposite (2-vector) recoil kT. Effectively, there will be a two jet system rotated by an angle arctan(2kT/W). After fragmentation, the impact of the intrinsic kT is to increase the pT for high |xF| compared to xF near 0. This can be seen in the next plot which shows the "seagull plot", <p2T> vs. xF, for three different values of intrinsic kT for two different programs all for 15x100 GeV ep and for 30x250 GeV ep. The biggest pT and biggest difference is at high |xF| and especially at negative xF. The other advantages to negative xF compared to positive are that the model dependence is much smaller for negative xF and the beam energy dependence is much smaller as well. Essentially, the negative xF region is just significantly less sensitive to the effects of QCD and is more reflective of the initial parton intrinsic kT.

 15 GeV x 100 GeV Seagull Overlay 30 GeV x 250 GeV Seagull Overlay

Of course we don't just have to rely on seagull plots (moments of pT); we have more information if we look at the spectra themselves. The plots below contain the spectra for positively charged particles for ep 15x100 GeV for 3 different intrinsic kT values: 0.44, 0.88, 1.3 GeV and for two models: LEPTO-PHI and PYTHIA (EIC-modified version) for an ideal detector given in bins of x-Feynman.

 -1.0 < xf < -0.95 -0.95 < xf < -0.90 -0.90 < xf < -0.85 -0.85 < xf < -0.80 -0.80 < xf < -0.75 -0.75 < xf < -0.70 -0.70 < xf < -0.65 -0.65 < xf < -0.60 -0.60 < xf < -0.55 -0.55 < xf < -0.50 -0.50 < xf < -0.45 -0.45 < xf < -0.40 -0.40 < xf < -0.35 -0.35 < xf < -0.30 -0.30 < xf < -0.25 -0.25 < xf < -0.20 -0.20 < xf < -0.15 -0.15 < xf < -0.10 -0.10 < xf < -0.05 -0.05 < xf < 0.0 0.0 < xf < +0.05 +0.05 < xf < +0.10 +0.10 < xf < +0.15 +0.15 < xf < +0.20 +0.20 < xf < +0.25 +0.25 < xf < +0.30 +0.30 < xf < +0.35 +0.35 < xf < +0.40 +0.40 < xf < +0.55 +0.45 < xf < +0.60 +0.50 < xf < +0.55 +0.55 < xf < +0.60 +0.60 < xf < +0.65 +0.65 < xf < +0.70 +0.70 < xf < +0.75 +0.75 < xf < +0.80 +0.80 < xf < +0.85 +0.85 < xf < +0.90 +0.90 < xf < +0.95

In order to focus the discussion, let's choose a few representative xF bins (counting from 1). Bin 5 (-0.80<xF<-0.75) has the largest model-independent discrimination between the 3 values of intrinsic kT. Bin 11 (-0.50<xF<-0.45) still has a good discrimination but tends to have better acceptance for non-ideal detectors. Bin 8 (-0.65<xF<-0.60) was chosen because it's intermediate between bins 5 & 11. Finally, bin 30 (+0.45<xF<+0.50) was chosen because it's the mirror image of bin 11 and because it has among the best discrimination for the positive x-Feynman bins, so it's the fairest comparison.

The plots below compare LEPTOPHI & PYTHIA for 3 intrinsic kT values for ep, but as spectra in the four regions: -0.80<xF<-0.75, -0.65<xF<-0.60, -0.50<xF<-0.45 and 0.45<xF<0.50 for two different sets of beam energies (15x100 GeV and 30x250 GeV). Note that all of the negative-xF bins show a clean distinction between different values of intrinsic kT with minimal model dependence. In contrast the positive xF bin is much more difficult to disentangle.

 15 GeV x 100 GeV ep -0.80 < xf < -0.75 15 GeV x 100 GeV ep -0.65 < xf < -0.60 15 GeV x 100 GeV ep -0.50 < xf < -0.45 15 GeV x 100 GeV ep +0.45 < xf < +0.50
 30 GeV x 250 GeV ep -0.80 < xf < -0.75 30 GeV x 250 GeV ep -0.65 < xf < -0.60 30 GeV x 250 GeV ep -0.50 < xf < -0.45 30 GeV x 250 GeV ep +0.45 < xf < +0.50

The reason for this has to do with basic physics of QCD. The impact of higher order (hard) QCD diagrams and of final state parton showering (soft QCD) is to add extra sources of pT for xF>0 and near 0, with very little impact for xF<-0.3. This is because it is the struck quark or gluon which radiates. The impact of initial state parton showering is less obvious, but kinematically, it turns out that it also primarily adds pT for non-negative xF. This means that the target remnant jet recoil may have the better "memory" of the original intrinsic kT. This is illustrated in the figure below which shows LEPTO-PHI for the standard intrinsic kT value of 0.44 GeV (rms) and 4 different modes of QCD: QCD on (standard), pQCD but no Parton Shower, Parton Shower but now hard pQCD, no QCD at all (just LO no parton shower). Clearly the impact of QCD (including the strong beam energy dependence) is all in the current jet hemisphere of the HCMS (xF>0). Intrinsic kT shows up and is cleanest for xF<-0.3.

 15 GeV x 100 GeV QCD Overlay 30 GeV x 250 GeV QCD Overlay

The plots below compare the three different intrinsic kT values for ep, for the same four modes of QCD shown above, but as spectra in the four xF regions for two different sets of beam energies (15x100 GeV and 30x250 GeV). Looking at the plots we see that all of the negative-xF bins are nearly independent of QCD effects and show a clean distinction between different values of intrinsic kT. And, again, the positive xF bin is much more difficult to disentangle.

 15 GeV x 100 GeV ep -0.80 < xf < -0.75 15 GeV x 100 GeV ep -0.65 < xf < -0.60 15 GeV x 100 GeV ep -0.50 < xf < -0.45 15 GeV x 100 GeV ep +0.45 < xf < +0.50
 30 GeV x 250 GeV ep -0.80 < xf < -0.75 30 GeV x 250 GeV ep -0.65 < xf < -0.60 30 GeV x 250 GeV ep -0.50 < xf < -0.45 30 GeV x 250 GeV ep +0.45 < xf < +0.50

In practice, the target remnant jet in ep and the target remnants (high ${\displaystyle |x_{F}|}$ particles) in pp are often not measured, and rarely correlated with the particles more directly involved in the hard scattering. This means that you can crank up the intrinsic kT parameter in PYTHIA, say, and increase the intrinsic pT of most particles while ignoring the presumably non-physical effect of very high target remnant recoil. If you only compare PYTHIA to mid-rapidity in pp and the current jet region in ep, you cannot easily distinguish between the effects of higher order QCD (or parton showers) vs. intrinsic kT. So, as the PYTHIA manual says "any shortfall in shower activity ... has to be compensated by the primordial kT source, which thereby largely loses its original meaning."

Turning this around, if we have substantial acceptance of the target jet in the hadronic center-of-mass frame (negative x-Feynman particles, especially xF<-0.3), we CAN distinguish intrinsic kT from collision QCD effects - and we can keep the model-builders honest. More details about this idea can be found at http://skipper.physics.sunysb.edu/~abhay/2014/EICUM/Talks/06262014-PM//3_Baker.pdf.

## Defining the Fiducial Acceptance

There are two fundamentally different kind of acceptance gaps to consider in the lab frame: azimuthal gaps and polar angle (θ or equivalently η). The easiest one is the case of partial azimuthal acceptance in the lab frame. If we are considering unpolarized (or polarization averaged) ep collisions, and simple single-particle measurements (not two-particle correlations), then all observables should, on average, be azimuthally symmetric and this type of acceptance gap only affects the statistics, not the overall result. It is much easier to correct for than the gaps in η.

The Roman Pot stations themselves are nearly azimuthally symmetric, but they will be located after the DX beam-bending magnet in the forward going (proton beam) direction and that introduces a significant asymmetry. In order to hit the Roman Pots, the forward-going (p-direction) particles from the collision must first pass through the DX bending magnet. In order to make it, they have to be positively charged and have a momentum of at least about 30% of the beam momentum (technically beam momentum/charge) and a polar angle θ<14mr. If they have a non-zero θ angle (but still less than about 14 mr) then they will have a better chance of making it if the lab angle φ is such that it opposes the DX magnet kick. In this case, the DX magnet will kick it back into the acceptance. This introduces a significant, but completely predictable, azimuthal asymmetry. J.H. Lee simulated this effect. See for example some of his talks: [1] or [2]. His result (rescaled for a 100 GeV beam) is shown below on the left. This plot shows the fractional acceptance ranging from 0 to 1 for bins of θ and p in the lab, where θ=0 is the proton beam direction. For use in our simulations, we interpolated to fill in some 0/0 gaps in the histogram and cleaned up the edges. The result is shown on the right.

 Roman Pot Acceptance 100 GeV beam (J.H. Lee) Smoothed Roman Pot Acceptance 100 GeV beam

In most of cases in the above plot where the acceptance is neither 0 or 1, the acceptance is really a fractional azimuthal acceptance - for certain φ angles, the acceptance is near 1 while for others it is near 0. For particles with momentum greater than about 84% of the beam momentum and θ<3.5 mr, there is an additional hole in the acceptance. The proton beam itself is at p=100 GeV and θ=0. Particles which are too close to the beam in momentum space don't reach the inner edge of the Roman Pot detector which is placed far enough from the beam to avoid direct contact.

The following set of plots shows, as an example, the stages that we progress through to determine the fiducial acceptance mask for positively charged particles in the region -5.0 < η < +5.0 or in the Roman Pots for 15x100 GeV ep. We first consider the Roman Pots and apply an azimuthal acceptance correction for those regions of lab p, θ that have a φ acceptance of at least 40 mr. Then we mapped the azimuthally-corrected Roman Pot and the main detector acceptances to (xF,pT). The first (leftmost) plot below shows the fractional acceptance (ranging from 0-1) in (xF,pT) before the azimuthal correction, while the second plot shows the acceptance after the correction. Note: we removed the region for xF>0.95 to avoid low statistics. Any part of the azimuthally-corrected acceptance with a coverage of at least 25% was considered measurable. The third plot is the Fiducial Mask plot, which flags as 1 (red) any bin which is measurable and 0 (white) any bin which is not. Note: we tightened the xF cut to xF<0.9, some 0/0 bins have been interpolated. The last (rightmost) plot shows the fiducially masked acceptance. It is the product of the first and third plots and represents the actual (not azimuthally corrected) acceptance for those regions which are considered measurable.

 Fractional Acceptance 5<η<+5 + Roman Pots ep 15x100 GeV Fractional Acceptance 5<η<+5 w/ Azimuthally Corrected Roman Pots ep 15x100 GeV Fiducial Mask plot 5<η<+5 + Roman Pots ep 15x100 GeV Fiducial Masked Acceptance plot 5<η<+5 + Roman Pots ep 15x100 GeV

## The Impact of Imperfect Detector Acceptance

Let's consider a variety of detector acceptances and a variety of xF bins and compare the spectra for positively charged particles for the three intrinsic kT values for the two models for 15x100 GeV ep.

ep 15x100 GeV -0.80 < xF < -0.75 -0.65 < xF < -0.60 -0.50 < xF < -0.45 +0.45 < xF < +0.50
-5.4 < eta < +5.4 w/ RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -5.4 < eta < +5.4 w/ RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -5.4 < eta < +5.4 w/ RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -5.4 < eta < +5.4 w/ RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -5.4 < eta < +5.4 w/ RP
-5.0 < eta < +5.0 w/ RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -5.0 < eta < +5.0 w/ RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -5.0 < eta < +5.0 w/ RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -5.0 < eta < +5.0 w/ RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -5.0 < eta < +5.0 w/ RP
-4.75 < eta < +4.75 w/ RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -4.75 < eta < +4.75 w/ RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -4.75 < eta < +4.75 w/ RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -4.75 < eta < +4.75 w/ RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -4.75 < eta < +4.75 w/ RP
-4.5 < eta < +4.5 w/ RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -4.5 < eta < +4.5 w/ RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -4.5 < eta < +4.5 w/ RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -4.5 < eta < +4.5 w/ RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -4.5 < eta < +4.5 w/ RP
-3.0 < eta < +4.0 w/ RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 w/ RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 w/ RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 w/ RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 w/ RP
-3.0 < eta < +4.0 NO RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 no RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 no RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 no RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 no RP
-3.0 < eta < +5.0 NO RP
15 GeV x 100 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +5.0 no RP
15 GeV x 100 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +5.0 no RP
15 GeV x 100 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +5.0 no RP
15 GeV x 100 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +5.0 no RP

Looking at the above plots, we can see:

• An excellent measurement with the first two cases: |η|<5.4 w/ RP and |η|<5.0 w/ RP. Note: the optimized ePHENIX (-3.0<η<5.0 w/ RP) would look identical to the |η|<5.0 w/ RP case.
• A solid measurement with the next two cases: |η|<4.75 w/ RP and |η|<4.5 w/ RP
• A poor measurement with the next case: -3.0<η<4.0 w/ RP (default ePHENIX + RP). Note BeAST with |η|<4.0 w/ RP would look identical.
• No measurement at all with default ePHENIX: -3.0<η<4.0 w/o RP
• A tail-only measurement with an improved ePHENIX, but no RP: -3.0<η<5.0 w/o RP.

If at all possible, we should avoid tail-only measurements since the tail of the pT distribution will be much more model dependent than one which includes the peak for two reasons: 1. If the kT distribution itself isn't Gaussian, it can be difficult to interpret the overall intrinsic kT distribution just from the tail. 2. If there are any residual higher order QCD effects in the target jet, they very well may show up as tails in the pT distribution for negative xF.

The message is that a measurement with Roman Pots and with charged particle measurement out to at least η of 4.5 is important to make this measurement.

From a physics point of view, any plot in terms of HCMS variables like xF and pT should be the same for 15x100 or 30x50 GeV ep because they have the same ${\displaystyle {\sqrt {s}}}$. However, the acceptance could be quite different, and in particular, in the 30x50 GeV case, the target jet is not quite as far forward in the laboratory. So below, we repeat the set of detector acceptances and the set of xF bins and compare the spectra for positively charged particles for the three intrinsic kT values for the two models for 30x50 GeV ep.

ep 30x50 GeV -0.80 < xF < -0.75 -0.65 < xF < -0.60 -0.50 < xF < -0.45 +0.45 < xF < +0.50
-5.4 < eta < +5.4 w/ RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -5.4 < eta < +5.4 w/ RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -5.4 < eta < +5.4 w/ RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -5.4 < eta < +5.4 w/ RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -5.4 < eta < +5.4 w/ RP
-5.0 < eta < +5.0 w/ RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -5.0 < eta < +5.0 w/ RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -5.0 < eta < +5.0 w/ RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -5.0 < eta < +5.0 w/ RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -5.0 < eta < +5.0 w/ RP
-4.75 < eta < +4.75 w/ RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -4.75 < eta < +4.75 w/ RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -4.75 < eta < +4.75 w/ RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -4.75 < eta < +4.75 w/ RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -4.75 < eta < +4.75 w/ RP
-4.5 < eta < +4.5 w/ RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -4.5 < eta < +4.5 w/ RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -4.5 < eta < +4.5 w/ RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -4.5 < eta < +4.5 w/ RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -4.5 < eta < +4.5 w/ RP
-3.0 < eta < +4.0 w/ RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 w/ RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 w/ RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 w/ RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 w/ RP
-3.0 < eta < +4.0 NO RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 no RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 no RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 no RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 no RP
-3.0 < eta < +5.0 NO RP
30 GeV x 50 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +5.0 no RP
30 GeV x 50 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +5.0 no RP
30 GeV x 50 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +5.0 no RP
30 GeV x 50 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +5.0 no RP

Looking at the above plots for 30x50 GeV, we see that this energy setting is more forgiving than the 15x100 setting, primarily because the target jet is not quite as far forward (along the proton beam direction) in the lab frame.

• An excellent measurement with the first three cases: |η|<5.4 w/ RP, |η|<5.0 w/ RP and |η|<4.75 w/ RP.
• A solid measurement with |η|<4.5 w/ RP.
• A borderline measurement with the next case: -3.0<η<4.0 w/ RP (default ePHENIX + RP). This measurement is missing the peak of the distribution, but is a significant improvement over the 15x100 version.
• A tail-only measurement with default ePHENIX: -3.0<η<4.0 w/o RP. This is a significant improvement over the 15x100 version.
• A good measurement with an improved ePHENIX, but no RP: -3.0<η<5.0 w/o RP. Again a very big improvement over the 15x100 version.

Finally, let's look at the set of detector acceptances and xF bins and compare the spectra for positively charged particles for the three intrinsic kT values for the two models for 30x250 GeV ep. In this case, the target jet is focused even more far forward and the Roman Pot acceptance becomes even more important.

ep 30x250 GeV -0.80 < xF < -0.75 -0.65 < xF < -0.60 -0.50 < xF < -0.45 +0.45 < xF < +0.50
-5.4 < eta < +5.4 w/ RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -5.4 < eta < +5.4 w/ RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -5.4 < eta < +5.4 w/ RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -5.4 < eta < +5.4 w/ RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -5.4 < eta < +5.4 w/ RP
-5.0 < eta < +5.0 w/ RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -5.0 < eta < +5.0 w/ RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -5.0 < eta < +5.0 w/ RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -5.0 < eta < +5.0 w/ RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -5.0 < eta < +5.0 w/ RP
-4.75 < eta < +4.75 w/ RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -4.75 < eta < +4.75 w/ RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -4.75 < eta < +4.75 w/ RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -4.75 < eta < +4.75 w/ RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -4.75 < eta < +4.75 w/ RP
-4.5 < eta < +4.5 w/ RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -4.5 < eta < +4.5 w/ RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -4.5 < eta < +4.5 w/ RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -4.5 < eta < +4.5 w/ RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -4.5 < eta < +4.5 w/ RP
-3.0 < eta < +4.0 w/ RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 w/ RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 w/ RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 w/ RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 w/ RP
-3.0 < eta < +4.0 NO RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +4.0 no RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +4.0 no RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +4.0 no RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +4.0 no RP
-3.0 < eta < +5.0 NO RP
30 GeV x 250 GeV ep, -0.80 < xf < -0.75, -3.0 < eta < +5.0 no RP
30 GeV x 250 GeV ep, -0.65 < xf < -0.60, -3.0 < eta < +5.0 no RP
30 GeV x 250 GeV ep, -0.50 < xf < -0.45, -3.0 < eta < +5.0 no RP
30 GeV x 250 GeV ep, +0.45 < xf < +0.50, -3.0 < eta < +5.0 no RP

Looking at the above plots for 30x250 GeV, we see that this energy setting puts the target jet even more forward in the lab frame, making the Roman Pots necessary and to a large extent sufficient for this measurement. We have:

• Again an excellent measurement with the first two cases: |η|<5.4 w/ RP and |η|<5.0 w/ RP.
• A solid, but tailless measurement for any other case including the Roman Pots.
• No measurement with default ePHENIX: -3.0<η<4.0 w/o RP.
• A very poor tail-only measurement with an improved ePHENIX, but no RP: -3.0<η<5.0 w/o RP.

The tailless (Roman Pot only) measurement is a very solid measurement of the bulk of the intrinsic kT distribution, and a big improvement over a tail-only measurement. It is better to have a more complete measurement so that we can see possible effects of non-Gaussian tails to the kT and/or residual higher order QCD effects as well as the basic kT.