May 14 th 2008 averaging meeting A M Cooper-Sarkar Look at the HERA-I PDFs in new ways Flavour break-up High-x Compare to ZEUS data alone/ H1 data alone

May 14th 2008 averaging meeting A M Cooper-Sarkar

• Look at the HERA-I PDFs in new ways• Flavour break-up

• High-x

• Compare to ZEUS data alone/ H1 data alone fitted in the same way

• Look at predictions for W/Z production at the LHC

Flavour break-up

Flavour break-up ubar, dbar, sbar. cbar

The model uncertainty in sbar and cbar is quite big- not suprisingly

Fc varies 0.15±0.05 (30%) and fs varies 0.33±0.08 (24%)

The blue lines are just showing that ‘humpy’ param and massive heavy quarks (rtvfn) don’t make much difference

Flavour break-up ubar, dbar, sbar. cbarThe blue lines are just showing that zeus-jets style or h1-style parametrizations don’t make much difference

I have not shown the variations with different alphas values- because this really does just affect the gluon

Just vary mc and mb no effect on sea flavours

Just vary the low Q2 cut again no effect on sea flavours

Flavour break-up ubar, dbar, sbar. cbarNow considering what contributes to the model dependence

Change fs: mostly affects sbar Change fc: mostly affects cbar

Remember fs and fc are used for normalising ubar and dbar, so there is some small cross-talk


Now vary the value of Q2_0

This affects all flavours:

ubar and dbar because it amounts to a change in parametrization and

sbar and cbar because it amounts to changing fs and fc-

(what we have done is vary Q2_0 keeping the same fs and fc when these fractions would obviously change with Q2)

should we do something about this?


Look at high-x make the y axis log!

Here are our PDFs at high-x compared to those of MSTW08

Red is our experimental error, yellow is the model band, green is MSTW08.

You can see pretty good agreement of sea quark and d-valence, and of u-valence until x > 0.7, where there’s no data so either of us could be right

BUT our gluon at x > 0.2 is softer and its uncertainties do not reflect our lack of knowledge in this region. Also it is softer than our sea for x > 0.3 which seems a bit weird.

Here MSTW08 is on topHere MSTW08 is underneath

Here’s the same comparison for CTEQ65, where I haven’t separated our model uncertainty from our experimental error

The comments are identical as for MSTW08

Here’s the same comparison for ZEUS-JETS 2005. ZEUS-JETS is much closer to the HERA PDF, but it is a BIT harder and a BIT less precise, such that it is in striking distance of the CTEQ65 error band.

This means that ZJ2005 can almost fit Tevatron jet data- χ2/d.p = 122/82 (for Run-I D0) for the central value of ZJ2005. IF I take the extremal values of the ZJ2005 high-x gluon then I get 156/82 (soft) and 87/82(hard)

So our HERA PDF will have something like 156/82 because its on the soft edge of ZJ

Here’s just a plot of our PDFs which has on only our experimental uncertainties.

So far our model uncertainties have not succeeded in making the high-x gluon significantly larger.

We will be criticized for being unable to fit Tevatron jet data well.

But the point is not that we have to fit it, it is that our present uncertainties seem a bit unrealistically low.

So (a la Pumplin at DIS08) can one

Make it more uncertain by adding parameters?

So far I have tried looking at ‘humpy’ which has more gluon parameters, and at a fit with Cg.ne.0 but NOT the humpy solution. Neither of these solve the problem. Do we need to do something about this?

Comparison of new fit to fits to ZEUS data only or H1 data onlyfits done in the same way

i.e. optimized ‘inbetween’ parametriation with all the same assumptions.

New HERA-I PDFs experimental error only

Fit to ZEUS data only fitted in the same way. Errors OFFSET (as for ZEUS-JETS)

Fit to ZEUS data only fitted in the same way. Errors in quadrature Norms fitted.

There is still the choice as to how to do the errors, I have chosen OFFSET but also show quadrature. I think our message is best illustrated by the comparison to OFFSET


Fit to H1 data only fitted in the same way. Errors OFFSET hence Not as for H1PDF2K


Fit to ZEUS and H1 data as two separate data sets, but fitted in the same way.

Errors OFFSET

I am quite aware that I could have chosen to do the errors Hessian on these ZEUS only, H1 only and ZEUS+H1 as separate data set fits.

I did this exercise before on an earlier version of our combination and our fit. See hep-ph/0508304 from one of the HERALHC workshops. The conclusion was that by putting ZEUS and H1 data separately through a Hessian fit you can get a fit with the same impressively small errors as our combination fit BUT the central values of the gluon and the d-valence were very different. The QCD fit imposes many assumptions when setting the correlated experimental shift parameters, by contrast our combination is ‘assumption free’. Hence I prefer to make the comparisons to the more conservative OFFSET method. This gives a clearer message about improvement to the outside world.

Predictions for W/Z production at the LHC

W+ from ZEUS-S PDFW+ pre HERA PDF W+ from ZEUS-J PDF

In these plots there are experimental errors only

No model dependence

What has HERA data ever done for us? A little history…

So looking at the predictions for W+ rapidity distributions (NLO code J.Stirling) we see a terrific improvement in putting in the HERA data (these are ZEUS-S style global fits without and with the ZEUS 97/97 data). The ZEUS-JETS fit gives more or less as good a precision as the ZEUS-S global fit because at high scale (Q2=MW2) in the central rapidity region the W+ (and W-, Z distributions) are driven by the low-x gluon (by g→qqbar splitting)

W+ from HERA-II projections

W+ from HERA-I PDF

Ta-Dah!!

A couple of years ago we even made a plot of how good it could get with HERA-II data.

But we were pessimistic

We were not expecting the improvement in systematic error that our combination has made.

The predictions are very precise ~1% error

PDF set σW+ BW→lν (nb) σW- BW→lν (nb) σz Bz→ll (nb)

ZEUS-2005 11.87±0.45 8.74±0.31 1.97±0.06

MRST01 11.61±0.23 8.62±0.16 1.95±0.04

HERA-I 12.13±0.13 9.13±0.15 2.01±0.025

CTEQ65 12.47±0.47 9.14±0.36 2.03±0.07

CTEQ61 11.61±0.56 8.54±0.43 1.89±0.09

But wait.. this does NOT have model dependence

PDF set σW+ BW→lν (nb) σW- BW→lν (nb) σz Bz→ll (nb)

HERA-I 12.13±0.13 9.13±0.15 2.01±0.025

Fs=0.25 12.12 9.09 2.00

Fs=0.4 12.15 9.16 2.02

Fc=0.10 12.26 9.23 2.04

Fc=0.20 12.00 9.03 1.99

Q2min=2.5 12.13 9.12 2.01

Q2min=5.0 12.17 9.17 2.01

Q2_0=2 11.77 8.85 1.95

Q2_0=6 12.37 9.29 2.06

αs=0.1156 12.02 9.01 1.98

αs=0.1196 12.26 9.19 2.04

humpy 11.95 9.00 1.98

Zeus-style 12.45 9.36 2.07

Model dependences

Varying mc and mb (not shown) gives results well within errors, similarly for fs

Fc variation is on the edge of the errors

Q2min variation is well within

Q2_0 variation is the biggest effect outside errors

Varying αs is on the edge of the errors

Varying the parametrization is also outside errors.

W+ experimental only Variation of alphas ~ same size as experimental error

Variation of fc ~ same size as experimental error

Variation of parmetrization ZJ or humpy style

Variation of Q2_0 is the most significant model error in the measurable range

Look at plots of W+ (W- and Z have the same features see EXTRAS)

The total production cross-sections do not tell the full story about the shape in rapidity. Errors tend to be slightly larger in the central region

The pattern repeats in the lepton sector with small differences in detail

e+ experimental only Variation of alphas ~ same size as experimental error

Variation of fc- ~ same size as experimental error


And we actually measure leptons: let’s look at lepton+


The pattern repeats in the lepton sector with small differences in detail

And we actually measure leptons: let’s look at lepton-

e+ experimental only Variation of alphas ~ same size as experimental error




First AW = (W+ - W-)/(W+ + W-)

The model dependences cancel out at central rapidity

See EXTRAS for the full rapidity range where you can see that ‘humpy’ does give differences at the edges of y.

AW experimental only Variation of alphasVariation of parmetrization ZJ or humpy style

Variation of Q2_0

Variation of fc

Now let’s look at ratios

For the lepton asymmetry the wash out of model dependence in the measurable region is not quite so perfect but it is still quite impressive

See EXTRAS for full rapidity range

And we actually measure leptons: let’s look at lepton asymmetry

Alep experimental only Variation of alphas Variation of fcVariation of parametrization ZJ or humpy style

Variation of Q2_0

Another important ratio is

Z/(W+ + W-)

The experimental error on this is VERY small.

For model dependences: fc, Q2_0 and parametrization do not matter much but

Alphas has a noticeable effect

Z/W experimental only Variation of alphas Variation of fcVariation of parametrization ZJ or humpy style

Variation of Q2_0

Now let’s look at ratios

In the Z to leptons ratio the same features appearThe experimental error is VERY small. For model dependences: fc, Q2_0 and parametrization do not matter much butAlphas has a noticeable effect

And we actually measure leptons: let’s look at Z/(e+ + e-)

Z/leptons experimental only

Variation of alphas Variation of fcVariation of parametrization ZJ or humpy style

Variation of Q2_0

Comparsion to other PDFs just CTEQ for now

Hera-I pdfs exp cteq61 cteq65

Now we need some comparsion to other PDFs: Z (W+,W- in extras)

Hera-I pdfs: Q2_0 model dependence

Note CTEQ61 is lower and less precise than CTEQ65

HERA-I PDFs are very precise BUT model dependence IS significant.

Still winning wrt CTEQ

But we measure leptons: comparison to other PDFs: lepton+ (lepton- in EXTRAS)

Hera-I pdfs exp


cteq61 cteq65


HERA-I PDFs are very precise BUT model dependence IS significant.

Still winning wrt CTEQ

cteq61 Mrst01(4)cteq65

Now we need some comparison to other PDFs: ratios: AW

Negligible model dependence in HERA PDF.

Below see AW across full kinematic range

Hera-I pdfs exp

cteq61 cteq65

Lepton asymmetry in full kinematic range is in EXTRAS

But we measure leptons: comparison to other PDFs: lepton asymmetry

Mrst01(4)

Small model dependence in HERA PDF from both Q2_0 and alternative parametrizations


Hera-I pdfs: alternative parametrization

Hera-I pdfs exp

Mrst01(4)

This is a bit wider than 65 because of strangeness uncertaintyHera-I pdfs:

alphas model dependence

Now we need some comparison to other PDFs: ratios: Z/W

Hera-I pdfs exp cteq61 cteq65 cteq66

Small model dependence in HERA PDF from alphas

No matter what their other discrepancies all PDFs are agreed on this ratio.

Recently strangeness uncertainty has been introduced and this affects it- but it is NOT a big deal, see CTEQ66

Summary• Flavour break up behaves as expected, not sure whether to ‘go public’ with

plots. Probably should sort out Q2_0, fc, fs double counting in model dependence.

• High-x gluon is soft, but worse than this is that it does not have a large enough uncertainty- work on this more?

• Comparison to ZEUS-ONLY or H1-ONLY seems OK, probably won’t ‘go public’

• Prediction of W/Z at LHC:

1. Very small experimental errors.

2. Model uncertainty from choice of Q2_0 (effectively parametrization) is significant for W,Z and decay lepton spectra.

3. Model uncertainty cancels out of W asymmetry in central rapidity range, small model uncertainty is left in lepton asymmetry

4. Small model uncertainty from alphas in Z/W ratio and Z/lepton ratio.

5. Interesting for PDF4LHC.

extras

The pattern repeats in W-

W- experimental only Variation of alphas ~ same size as experimental error




The pattern repeats in Z

Z experimental only Variation of fc- ~ same size as experimental error

Variation of alphas ~ same size as experimental error



AW asymmetry over the full rapidity range

alphas has no effect

Fc has no effect

Zj/humpy has effect only at high-y

Q20 is not a big effect..model dependences cancel out at central rapidity.

Variation of alphas Variation of fcVariation of parmetrization ZJ or humpy style

Variation of Q2_0

AW experimental only

the lepton asymmetry over the full rapidity range

Alep experimental only Variation of alphas Variation of fcVariation of parmetrization ZJ or humpy style

Variation of Q2_0

cteq65Hera-I pdfs exp


Now we need some comparsion to other PDFs: W+

cteq61


Hera-I pdfs exp


cteq61 cteq65

Now we need some comparsion to other PDFs: W-


Hera-I pdfs exp


cteq61 cteq65

But we measure leptons: comparison to other PDFs: lepton-

HERA-I PDFS cteq61 cteq65

Lepton asymmetry across full kinematic range

But we measure leptons: comparison to other PDFs: Z/leptons

Hera-I pdfs exp cteq61 cteq65

Small model dependence in HERA PDF from alphas

Hera-I pdfs: alphas model dependence

Documents

May 14 th 2008 averaging meeting A M Cooper-Sarkar Look at the HERA-I PDFs in new ways Flavour break-up High-x Compare to ZEUS data alone/ H1 data alone