The New HERAPDF

Nov26 2008 HERA SFgroup

AM Cooper-Sarkar

Appears compatible with HERAPDF0.1 when doing fits at Q20=4.0 GeV2

But humpy gluon is Chisq favoured and there are two humpy mimima

Fitting with Q20=1.9 GeV2 is tricky

Straight gluon is favoured but shape of d-valence has changed a lot and is softer than d-bar at high-x

This situation is improved if GMVFN rather than ZMVFN dynamical heavy quark scheme is used

Moving from QCDNUM16 to QCDNUM17 gives the possibility of greater accuracy at high-x

A combination of QCDNUM17 quadratic spline interpolation on the high-x grid, and the use of GMVFN give results compatible with HERAPDF0.1 (and by implication with CTEQ MRSTW etc)

Fit to new post-Oct20 2008 data set, using formalism of HERAPDF0.1 gives very compatible results

19 extra points brings total to 592 data points: Chisq 573

100errors added in quadrature- no procedural as yet

Chisq/ndp for NCe+ data increases to 1.10

Let’s see the comparison of new to old in more detail for various flavours

Conclusion: compatible

Fit to new post-Oct20 2008 data set, using ZEUS-JETS parametrization gives total chisq 574 and very compatible results:

both to old ZEUS-JETS

and to new fit using HERAPDF0.1 form of parametrization

Now look at alternative parametrizations: first ZEUS-JETS

optimization gives the same form of parametrization as in April 2008

The PDF uncertainties for the inbetween parametrization (left) are more conservative than those for the zeus-jets parametrization (middle) and than the H1 parametrization (right) just as we found earlier.

Inbetween style Zeus-jets style H1-style

Let’s see the comparison of new inbetween to new zeus-jets in more detail for various flavours

Conclusion: broadly compatible, need model dependence to really judge

Now look at alternative parametrizations: humpy version of inbetween

Fit to new post-Oct20 2008 data set, using humpy gluon parametrization with ‘inbetween’

Humpy-2 Chisq 559 is now preferred

Humpy-2 is preferred by NC e+ data:

chisq/ndp 1.06 compared to 1.10

It is not preffered by CCe+: chisq/ndp 0.82 compared to 0.76.

Other data set Chisq do not change much

Let’s see the comparison of Humpy-2 to straight in more detail for various flavours

Conclusion: more or less as we saw it when investigating HERAPDF0.1

But I have not been following strict historical order

First I found another humpy solution with an identical Chisq=559

Humpy-1 is preferred by NC e+ data:

chisq/ndp 1.05 compared to 1.10

It is not preffered by CCe+:

chisq/ndp 0.90 compared to 0.76.

This change in the CCe+ fit is what is giving the big change in d-valence

(HERA-II CCe+ data could change this).

The resulting d-valence is very soft- softer than dbar at high-x

Now look at alternative parametrizations: another humpy version of inbetween

It’s not the humpiness it’s the valence that worries me it’s the valence shapes

Let’s see the comparison of Humpy-1 to straight in more detail for various flavours

Conclusion: not at all like we saw when investigating HERAPDF0.1

MUCH more model dependent

NOTE: Joel found only Humpy-1, Gang-li found Humpy-2, I can find both if my start parameters are shifted

This is a fit with Q20=1.9 using the ‘inbetween’ parametrization.

For Q20=1.9 the straight gluon solution is preferred.

Now we see that the alternative behaviour of the valence has nothing to do with Humpyness of the gluon.

It is hard to get the original HERAPDF0.1 valence shapes with low Q20.

In fact low Q20 fits are much less stable

Again d-valence is softer than d-bar at high-x

Now start looking at jobs with low Q20, so that we can move to dynamic heavy quark treatment and to the NNLO QCDNUM programme

Remember HERAPDF NEW means fitting with Q20=4 exactly as for HERAPDF0.1 AND it is compatible with HERAPDF0.1

Let’s see the comparison of Q20=1.9 to Q20=4.0 straight gluon solutions for fits to the new data in more detail for various flavours

Conclusion: very large model dependence- and some amount of embarassment wrt what we have already said to the world- also if we use this Q20=1.9 minimum for our central value we’ll be way out of line with CTEQ/MSTW

Also if you look in detail you will see that dbar > dvalence at large x> 0.7

NOTE the problem is not just the low start point

We used to get compatible shapes from q20=2.0 before the new data were added

This shows the OLD Q20=2.0 compared to HERAPDF NEW with Q20=4.0

Let’s see the comparison of Q20=2.0 to Q20=4.0 straight gluon solutions for fits to the older data in more detail for various flavours

Conclusion: only modest model dependence for older data

The new data is now emphasing the preferences of the NCe+ sample such that the ‘needs’ of the CCe+ sample are downgraded- we have thus relatively less information on d-valence.

We have find a way to live with this - until we combine HERA-II (or possibly assign larger weight CCe+ - CTEQ have done this sort of thing in the past).

Of course the newer type of solution could be the correct one but since it has some nasty features I have tried various ways to recover the old one. Many unsuccessful tries with penalty Chisq, setting limits on parameters-even with fully hessian error treatment

Some success with moving to Robert Thorne’s heavy quark variable flavour number scheme (2008 version more compatible to ACOT)

Since the point of the move to low Q20 was to implement these schemes this is relatively good news.

I remain somewhat uneasy about all this and the d-valence remains somewhat soft. The work comparing this to similar jobs with Q20=4.0, alternative parametrizations etc. has yet to be done.

Now move to a dynamically generated heavy quark variable flavour number scheme

Early problems with beta-00, beta-01 are now solved

NOTE this programme is NOT BACKWARD COMPATIBLE you have to completely re-write your user routine.

Motivation to use it at NLO: Botje pays more attention to accuracy at high-x

Linear and quadratic spline interpolation on the x grid is available.

GOOD NEWS: linear is compatible to QCDNUM16– I have recovered a similar unpleasant soft d-valence solution (see next page)

BAD NEWS: quadratic is significantly different – it has somewhat harder d-valence (but still not harder than d-bar)

Now move to NNLO QCDNUM:QCDNUM17-beta-02 BUT USE it at NLO

Compare the Q20=1.9 solution with QCDNUM16– red line

To the Q20=1.9 solution with QCDNUM17 linear spline interpolation –black line and yellow band

Compatibility of QCDNUM 16/17:

Well compatibility between QCDNUM versions is GOOD, though still getting the soft d-valence doesn’t thrill me

Comparison of linear and quadratic spline interpolation in more detail

Incompatibility of quadratic and linear is BAD since it means we’ve probably never been sufficiently accurate at high-x

Finally move to a dynamically generated heavy quark variable flavour number scheme

But using QCDNUM17 linear and quadratic

Compare the Q20=1.9 solution with QCDNUM16– red line

To the Q20=1.9 solution with QCDNUM17 linear spline interpolation –black line and also note that the quadratic red-line d-valence is harder

Comparison of linear and quadratic spline interpolation for dynamical heavy quark variable flavour number in more detail

Conclusions

Everything looks OK (definition: similar to what we saw before) if we stick to Q20=4.0

(except that we have a double minimum in humpy solutions!)

We could stick to Q20=4 even for dynamic heavy quark generation since it is POSSIBLE to evolve backwards

However we’ll have to face low Q20 for NNLO eventually. Low Q20 fits are tricky.

For Q20~2.0 we have a ‘new look’ for the d-valence (and a bit for u-valence) which is not compatible to what we saw before and has dbar> dvalence at high-x

(Because the CCe+ data don’t have so much relative weight)

Moving from ZMVFN- GMVFN partly mitigates this –but much checking back to all types of jobs must be done and H1/ZEUS fitters must agree on the results for this type of job and its very slow

Moving from QCDNUM16 to QCDNUM17 quadratic interpolation also mitigates this further (if combined with GMVFN) – but much checking of all types of jobs must be done and H1/ZEUS fitters must agree on the results for this type of job

NOT QUICKLY DONE

Use of RTVFN in HERAPDF0.1 was compatible with the standard fit

But for HERAPDFNEW it seems to be pushing us towards an alternative and nicer minimum than the one found in ZMVFN for Q20=1.9

EXTRAS