New LEP tune for hadronisation and shower changes

[Frank Siegert]

Recent changes in the shower (alpha_s scale factor) and the hadronisation make it necessary to retune the hadronisation parameters.

[Frank Siegert]

Hendrik, do you have any news on this?

[hoeth]

There is a working tune with ORDER_ALPHAS=0 in trunk since last week, and I made one with ORDER_ALPHAS=1 last weekend. Doesn't look any better, but wants a smaller ALPHAS(MZ). Handed this to Steffen and Stefan for them to decide what they prefer (and to look into the question whether the soft enhancement factor in the gluon splitting needs to be modified for ORDER_ALPHAS=1). Still waiting for feedback.

Upshot is: We have two working setups and the experts have to make up their mind which they prefer.

[Stefan Hoeche]

I would prefer a tune for ORDER_ALPHAS=1, this is usually much better for DIS. From my side there will be no further changes in the shower alorithm for rel-1.3.0, as I have worked almost exclusively on the CSS during the last ~6 weeks. If we now decide for any major changes we will enter an infinite loop of tests and tunes. This has happened in the past and I am not particularly keen on repeating the exercise.

[Stefan Hoeche]

Results from a benchmark on trunk r17369 are here: ​http://www.ippp.dur.ac.uk/~dph3sh/ded/benchmark/r17369/ The hadronisation seems to work much better for LEP, but for HERA things don't look quite that good. Especially the transverse energy flow (one of our show-off plots) is pretty bad. Note that I made the LEP plots with CSS_AS_FAC=2.46, the ones with default settings are here ​http://www.ippp.dur.ac.uk/~dph3sh/ded/benchmark/r17334_nll/ HERA setups have CSS_AS_FAC=2.46 in any case.

[hoeth]

The "ORDER_ALPHAS=1, CSS_AS_FAC=2.46, anything else is default" LEP plots (I assume that's what they are?) look somewhat worse than the pure default (which is ORDER_ALPHAS=0) or the ORDER_ALPHAS=1 tune (which has an embarrassingly low ALPHAS(MZ)). But I think it's fine for now and we should use it like that.

[hoeth]

We will release now and work on further tuning (MPI, LEP) along the way.