Few comments on B-production sensitivity to (some) PDFs at LHC energy within ATLAS sensitive volume

M.Smizanska

18.2.2009

LHC experiments, p-p collisions: parton kinematics for b-events

ATLAS/CMS LHCb

Bjorken x1 vs x2 plots for events that passed trigger&offline selections for one of B hadrons and no constraints for anti B in the same event.

ATLAS/CMS needs down to 10-4 Starts from 10GeV – the picture shows also lines for lower pT just to demonstrate that minimal x doesn’t change below 10GeV.

LHCb most sensitive to knowledge of structure functions at x as low as 10-5

CDF x-range for b-production within 10-3 LHC 1-2 orders lower although not all the LHC statistics at the limiting x

ATLAS x-range for b-events after trigger - well covered by HERA measurements, so all modern PDFS should be able correctly predict!

Uncertaintes in b-cross section at NLQCD

P.Nason, CERN 2000-004

•Large uncertaintes for LHC predicitons – dominantly due to higher order effects

R and F scale factors in the running coupling constant s(R 2 /2) and the PDF functions Fi(x, F

2 )

•Sensitivity to variations of gluon density is small - constraints form HERA DATA are strong enough in the region where most of B-production takes place

with mb = 4.75 GeV and R = F = 0

MRST HO   0.43 mbCTEQ5M1   0.55 mb

Calculated by A.Dainese, using NLQCD MNR code

Example of Pythia small sensitivity to PDFs for b-production in phase space covered by L1 muon trigger

The ratio of the integrated muon pT spectra for CTEQ3 and EHLQ

a) all Pythia ‘production mechanisms’ summed together

b) Pythia ‘production mechanisms’ shown separately:

• full line flavour excitation g+b→ g+b

• dashed gluon gluon fusion g+g →bb

• dotted gluon gluon scattering followed by gluon splitting g+g →g+g, g →bb

The most sensitive is b-flavour excitation mechanismPythia at LHC energy and ATLAS –

weak dependence of muon rates on PDFs, even very old ones as EHQL.

ATLAS muon rates with PYTHIA5: ratio for CTEQ3L/EHLQ

Dependency on PDF may appear in PY despite QCD stable vrt PDF change.

“Unstability” of Pythia to PDF may be a consequence of other PY parameters, not properly understood..