1. B-Production in PYTHIA Nick Brook University of Bristol Review of current use Review of current use Comparisons Comparisons B-asymmetries B-asymmetries Summary Summary

2. Current status currently LHCb use PYTHIA 6.134 tuned to UA5 data using multiple interaction model use CTEQ4L and running p T cut off with  s b events are selected from min. bias events PYTHIA 6.157 is the latest release of PYTHIA PYTHIA 6.134 is 18 months old – latest version of code has changes that effect b-production

3. codeSub-process6.134  B  b) 6.155  B  b) 0 all sub processes 411.9638.3 11 3.356.3 12 1.1 13 0.0 28 69.0300.4 53 122.0108.8 68 216.5171.7 gluon splitting source of b-production “old” version of PYTHIA excitation process excluded Comparison of “old” & “new” PYTHIA

4. Comparison of b-events from different sub-processes Different sub-processes have different colour configurations  different properties of the hadronic final state and/or kinematic properties of B-mesons

5. Why haven’t we moved to later versions of PYTHIA? Answer: 1. Problems with implementation of p T min in the QCD 2  2 process 2. Changes on heavy meson asymmetries in hadroproduction  address these issues 

6. codeSub-process(i) % b-evts (ii) % b-evts 11 10.09.1 12 0.2 13 0.0 28 50.051.0 53 10.810.2 68 28.629.4 Investigation of p T min cut in PYTHIA Method (i) – Generate events with p T min > 10 GeV/c Method (ii) - Generate events with p T min > 1 GeV/c discard events with p T min > 10 GeV/c process breakdown now consistent – previous problem cured

7. Investigation of p T min cut in PYTHIA -alternative breakdown of sub-processes: 1. g-splitting (no b-quark in hard process) 2. q-excitation (1 b-quark in hard process) 3. pair production (2 b-quarks in hard process) Production Process (i) %b-evts (ii) %b-evts g-splitting36.138.1 q-excitation 52.951.3 pair prod.11.010.6

8. Heavy meson asymmetries in hadroproduction -essentially a leading quark effect – dependent on behaviour of (string) fragmentation Can think of 3 contributions: i.Low mass – colour singlet system forms a cluster that undergoes 2-body decay into 2 hadrons ii.Very low mass – inv. mass of cluster too small for 2- body decay, cluster forced to collapse into a single hadron reflecting flavour content iii.Beam remnant drag – hadron inherits energy & momentum from end-point of string envisage (i) & (ii) not so important at LHC energies – play important role at fixed target energies

9. Comparison of PYTHIA with asymmetry data E791, Phys. Lett. B371 (1996) 157. Fixed target expt. 500 GeV   beam on proton trgt. Studied asymmetry as a function of x F & p T 2 investigated charged D’s (D - is leading particle) At these energies it is expected that LO diagrams (fusion processes) dominate production mechanism

10. Version of PYTHIA used by LHCb does have hadroproduction asymmetries asymmetries at fixed target energies are overestimated

11. Aside:  Aside: - can reset parameters to reproduce original result 

12. Asymmetries at LHC generate with just the fusion processes(!) at moment Differences between versions occur in the most forward region of LHCb For p T > 5.0 the differences between versions maybe less. Asymmetry at the 2% level

13. … try to convince you there are differences Different versions with the parameters reset in later version to earlier values really need more stats. & correct processes !!!

14. Does our tuning “survive” the PYTHIA upgrade ? YES !

15. Summary PYTHIA 6.134 (current LHCb version) does NOT generate the correct sub-processes and hence string configuration PYTHIA 6.134 DOES have heavy meson asymmetries in hadroproduction PYTHIA 6.134 overestimates asymmetries at fixed target energies Problems related to p T min cut implementation in later versions of PYTHIA seems cured Later versions of PYTHIA still reproduce same min. bias sample as earlier version Move to latest version of PYTHIA