With the default parameters, in mass region 8 < m < 11 GeV/c 2 and for |y|<0.5 the Pythia prediction is for a cross section of 29.5 pb. Using the STAR HF Tune, the parameters are: The fit parameters for the blue line are: This is inspired by using a QCD tree-level power-law distribution multiplied with a phase-space factor in the numerator. The distributions are fit to a function to parameterize the shape, shown in the black lines. The blue histogram is for pairs with |y_pair|<0.5, which is the region of interest. The red histogram is for all e+e- pairs generated by Pythia. Invariant mass spectrum of e-e+ pairs originating from b-bbar pairs. The distribution is well approximated by a Gaussian with mean ~ 0 and width close to 1 (off by 4.3%). 4: Rapidity distribution of the e+e- pairs from b decay.
XSECTION 4.8 FULL
The full rapidity distribution of the e+e- pairs is shown below:įig. However, these are generated in all of phase space, and we mainly have acceptance at mid-rapidity. When an event has both an electron and positron from the b-bbar pair this can generate a trigger. 3: pT distribution of the e+ e- daughters of the b quarks. We search for the e+e- daughters of the beauty hadrons, their pT distribution is shown below.įig. The of the B mesons is 3.055 GeV/c, one can estimate the peak of the Z distribution (most of the momentum of the b quark is carried by the meson, so it should be close to 1) as 3.055/3.331=0.92.Īfter the beauty hadrons are produced, they can decay producing electrons and positrons. 2:Pt distribution of B0 and B0-bar mesons. As an example, we plot here the B0 and B0-bar pT distribution, below.įig. These then fragment into B mesons and baryons. The pT distribution of the genrated b quarks is shown below. The selection of e+e- in the final state is done by following the fragmentation of the b or bbar quark into a B meson or baryon, and then looking at its decay products to find an electron or positron.
XSECTION 4.8 PDF
The main changes to the default parameters are the reduction of the bottom quark mass from 4.8 (default) to 4.3 GeV/c 2, the change of PDF from CTEQ5L (default) to the LHAPDF set MRSTMCal.LHgrid, and the choice of renormalization and factorization scales. The cross section increases to 1.023 ub with the STAR HF Tune v1.1. Using the STAR HF Tune, the cross section statistics reported by Pythia change to the following: The sum for both subprocesses pp -> b bbar is 0.830 ub. So gg initiated subprocess has a 0.697 ub cross section and the q-qbar initiated subprocess has a 0.133 ub cross section. *- End PYTHIA Event and Cross Section Statistics -* | | Tried Selected Accepted | (estimated) (mb) |
XSECTION 4.8 CODE
| Subprocess Code | Number of events | sigma +- delta | *- PYTHIA Event and Cross Section Statistics -* The cross-sections reported by Pythia (numbers after 5M events) using the default parameters: The macro for running with the STAR HF tune is here. We then used the STAR Heavy Flavor tune v1.1 for Pythia 8. Macro for running with default parameters is here. We used Pythia 8 to produce b-bbar events.
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