STAR task force for tuning PYTHIA

[Raghav Kunnawalkam Elayavalli <kunnawalkamraghav AT gmail.com>]

Hi Renee and Zilong,

The difference in the x-secs between pythia6 and pythia8 is something that we also observed and my understanding is that its primarily due to the PDF thats used.  see slide 6 here https://drupal.star.bnl.gov/STAR/system/files/jetcorr_embedding_update_raghav_11Feb2021_v0.pdf especially the inset plot which shows the ratio of jet spectra for pythia8 vs 6. As you said, i think this is primarily producing the shape differences in jet spectra and this cannot be explained by differences in jet shower algorithm etc… 

Cheers

Raghav 

**************************************

Raghav Kunnawalkam Elayavalli

email: raghav.ke AT yale.edu 

Associate Research Scientist  

Yale University and 

Brookhaven National Lab

<he/him/his>

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On Apr 24, 2021, at 11:57 AM, Chang, Zilong via Star-tf-tunepy-l <star-tf-tunepy-l AT lists.bnl.gov> wrote:

Hi Renee,

Yes I’m also surprised to see that the partonic cross-section from PYTHIA 8 is close to PYTHIA 6 at low partonic p_T, but larger at high partonic p_T. The difference could almost make up the difference in the jet cross-section at high p_T. I looked at the Pythia 8.2 webpage, and it says the default factorization and normalization scales are different between PYTHIA 8 and PYTHIA 6. I tried to set the scales to match PYTHIA 6 but the PYTHIA 8 cross-section remain unchanged.

mode  SigmaProcess:renormScale2   (default = 2; minimum = 1; maximum = 6)

option 3 : the arithmetic mean of the squared transverse masses of the two outgoing particles, i.e. (mT_3^2 + mT_4^2) / 2 = pT^2 + 0.5 * (m_3^2 + m_4^2). Useful for comparisons with PYTHIA 6, where this is the default. 

mode  SigmaProcess:factorScale2   (default = 1; minimum = 1; maximum = 6)

option 3 : the arithmetic mean of the squared transverse masses of the two outgoing particles. Useful for comparisons with PYTHIA 6, where this is the default.

http://home.thep.lu.se/~torbjorn/pythia82html/Welcome.html

I don’t know if it is a sqrt{s} scaling to cause this difference, I’m also suspecting it could be due to PDF difference (NNPDF2.3 vs CTEQ6l1). I’m trying to test this.

Zilong

On Apr 23, 2021, at 11:46 PM, Fatemi, Renee <rhfate2 AT g.uky.edu> wrote:

Hi Zilong,

This is really interesting. Clearly we need to tune away these shape changes. What I am really surprised by is the huge shift in absolute Xsec from PYTHIA 6->8.  I wonder if this is also due to a sqrt(s) scaling?

Thanks,

Renee

On Fri, Apr 23, 2021 at 5:24 PM Chang, Zilong via Star-tf-tunepy-l <star-tf-tunepy-l AT lists.bnl.gov> wrote:

Hi guys,

I used the PYTHIA8.2.35 default tune installed under StRoot (first link below) to generate the inclusive jet cross-section and compared it to the run12 pp510 GeV preliminary results. You can find my results at the second link below. The PYTHIA8 seems to under-predicate the measured cross-section at low jet p_T, but over-estimate it at high jet p_T.

https://www.star.bnl.gov/cgi-bin/protected/viewvc.cgi/cvsroot/StRoot/StarGenerator/Pythia8_2_35/

https://drupal.star.bnl.gov/STAR/system/files/Pythia8tune_pp510.pdf

Thanks,

Zilong

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