LHCb Wiki Pages

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
====== Calculation of the Cross Section ======
===== Introduction =====
Note that the follwing numbers all depend on the settings of the Monte Carlo generator (Pythia, in this case), as none of the quantities has been measured to a high accuracy at the LHC energy.

===== 1. b bbar Cross Section =====
Note that the numbers for the b bbar cross section differ quite a lot, depending on where you look them up.
Often, **500 microbarn** is quoted. However, for the DC06 production, **698 microbarn** was used, where the MC09 production uses **1040 microbarn** for 14 TeV and **699 microbarn** for 10 TeV.

===== 2. Hadronization =====
A b quark hadronizes with a certain probability into different sorts of B mesons. In **DC06**, these are:
  * B+/B-: 40.5%
  * B0/B0bar : 40.5%
  * Bs0/Bs0bar: 10%
  * b Baryons: 9%

In **MC09**, these are:
  * B+/B-: 41%
  * B0/B0bar : 41%
  * Bs0/Bs0bar: 10.5%
  * b Baryons: 7.5%

===== 3. Factor of 2 =====
As your b-quark as well as your bbar-quark may hadronize into your decay, you have to multiply the decay with a factor of 2.

===== 4. Branching Fraction of your Decay =====
The get the cross section for your decay, you have to know the branching fraction of the corresponding B-meson to the desired final states (which can be found in the PDG or the EvtGen decay table).

===== 5. Acceptance / Efficiency =====
Depending on what you want to calculate you also have to take into account the acceptance of the detector and the efficiency of the reconstruction.
The cut that is applied in Pythia to generate events that mostly decay in the LHCb detector (angular acceptance of 400mrad) is about 34.7% for B mesons and 43.2% for b bbar inclusive.

===== Summary =====
The formula then is:
b bbar cross section * 2 * f_had * branching fraction * efficiency and acceptance.

The yield can the be calculated by multiplying it with the integrated luminosity.


===== Links =====
The numbers used for DC06 production can be found [[https://twiki.cern.ch/twiki/bin/view/LHCb/SettingsDc06 | here ]], the corresponding numbers for MC09 [[https://twiki.cern.ch/twiki/bin/view/LHCb/SettingsMc09 | here ]]. Some information is also in the LHCb Technical design report (chapters "Simulation" and "Physics Performance").
Physik-Institut

LHCb Wiki Pages

User Tools

Site Tools

Page Tools
