B-Production in PYTHIAB-Production in PYTHIANick BrookNick Brook

University of BristolUniversity of Bristol

• Review of current useReview of current use

• ComparisonsComparisons

• B-asymmetriesB-asymmetries

• SummarySummary

Current status

• currently LHCb use PYTHIA 6.134

• tuned to UA5 data using multiple interaction model

• use CTEQ4L and running pT 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

code Sub-process 6.134Bb)

6.155Bb)

0 all sub processes

411.9 638.3

11 3.3 56.312 1.1 1.113 0.0 0.028 69.0 300.453 122.0 108.868 216.5 171.7

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gluon splitting source of b-production

“old” version of PYTHIA excitation process excluded

Comparison of “old” & “new” PYTHIA

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

Why haven’t we moved to later versions of PYTHIA?

Answer:

1. Problems with implementation of pTmin in

the QCD 2 2 process

2. Changes on heavy meson asymmetries in hadroproduction

address these issues

code Sub-process (i)% b-evts

(ii) % b-evts

11 10.0 9.112 0.2 0.213 0.0 0.028 50.0 51.053 10.8 10.268 28.6 29.4

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Investigation of pTmin cut in PYTHIA

Method (i) – Generate events with pTmin > 10 GeV/c

Method (ii) - Generate events with pTmin > 1 GeV/c

discard events with pTmin > 10 GeV/c

process breakdown now consistent – previous problem cured

Investigation of pTmin 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-splitting 36.1 38.1q-

excitation52.9 51.3

pair prod. 11.0 10.6

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

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 xF & pT2

investigated charged D’s (D- is leading particle)

At these energies it is expected that LO diagrams (fusion processes) dominate production mechanism

NLL

NLLTF pxA

),( 2

Version of PYTHIA used by LHCb does have

hadroproduction asymmetries

asymmetries at fixed target energies are

overestimated

Aside:Aside:

- can reset parameters to reproduce original result

Asymmetries at LHCAsymmetries at LHC

generate with just the fusion processes(!) at moment

Differences between versions occur in the most forward region of LHCb

For pT> 5.0 the differences between

versions maybe less. Asymmetry at

the 2% level

… … try to convince you there are differences 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 !!!

Does our tuning “survive” the PYTHIA upgrade ?Does our tuning “survive” the PYTHIA upgrade ?

YES !

SummarySummary• 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 pTmin 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 PYTHIAMove to latest version of PYTHIA