Changes between Version 1 and Version 2 of SingleTopNLO
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 04/06/12 16:33:02 (7 years ago)
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SingleTopNLO
v1 v2 6 6 7 7 === Motivation === 8 There are two ways to calculate tchannel single top production. The first is using a 2>2 process where the bquark is taken in the initial state. By taking the bquark from the PDF and setting the factorization scale equal to the top mass, the logarithms %$\log(m_b/m_t)$%will be resummed to all orders.8 There are two ways to calculate tchannel single top production. The first is using a 2>2 process where the bquark is taken in the initial state. By taking the bquark from the PDF and setting the factorization scale equal to the top mass, the logarithms $\log(m_b/m_t)$ will be resummed to all orders. 9 9 Tchannel single top production can also be calculated without the bquark PDF by including the gluon splitting into the diagram. The process becomes then a 2>3 process with also a massive bquark in the final state. The goal is to calculate the the NLO corrections to the 2>3 process and provide in this way reliable predictions for the kinematics of this bquark. 10 10 … … 15 15 === Results === 16 16 The parameters used are 17 * %$m_t=172$%GeV18 * %$m_b=4.7$%GeV19 * %$m_W=80.419$%GeV20 * %$m_Z=91.118$%GeV21 * %$\mu_R=\mu_F=m_t$%17 * $m_t=172$ GeV 18 * $m_b=4.7$ GeV 19 * $m_W=80.419$ GeV 20 * $m_Z=91.118$ GeV 21 * $\mu_R=\mu_F=m_t$ 22 22 * PDF is CTEQ6L (LO) and CTEQ6M (NLO) 23 23 * Jet definition: kT jet algorithm, with ptjetmin=15 GeV, etajetmax=8, Rcut=0.7, Inclusive. … … 27 27 ==== total cross section ==== 28 28 Default scale choices: 29 * For %$2\to 2$% we choose %$\mu_R^l=\mu_F^l=\mu_R^h=\mu_F^h=m_t$%.30 * For %$2\to 3$% we choose %$\mu_R^l=\mu_F^l=m_t/2$% and %$\mu_R^h=\mu_F^h=m_t/4$%.31 32  %$\sigma(2\to 2)$% '''LO'''  '''NLO'''  '''kfactor''' 29 * For $2\to 2$ we choose $\mu_R^l=\mu_F^l=\mu_R^h=\mu_F^h=m_t$. 30 * For $2\to 3$ we choose $\mu_R^l=\mu_F^l=m_t/2$ and $\mu_R^h=\mu_F^h=m_t/4$. 31 32  $\sigma(2\to 2)$  '''LO'''  '''NLO'''  '''kfactor'''  33 33 Tevatron (fb)  809.1 + 0.6  891.9 + 0.7  1.10  34 34 LHC top (pb)  142.1 + 0.1  146.8 + 0.1  1.03  35 35 LHC antitop (pb)  81.93 + 0.05  86.43 + 0.06  1.05  36 36 37  %$\sigma(2\to 3)$% '''LO'''  '''NLO'''  '''kfactor''' 37  $\sigma(2\to 3)$  '''LO'''  '''NLO'''  '''kfactor'''  38 38 Tevatron (fb)  595.8 + 0.7  800 + 1  1.34  39 39 LHC top (pb)  132.8 + 0.3  132.0 + 0.5  0.99  … … 44 44 ==== scale dependence ==== 45 45 As the central choices for the renormalization and factorization scales we have chosen: 46 * for the heavy line: %$\mu_{F,R}^{h}=m_t/4$%. This is chosen from the previous runs, where for %$m_t/4$%the total cross section for the 2>3 is closest to the 2>2 and also the LO is very close to the NLO.47 * for the light line: %$\mu_{F,R}^{l}=m_t/2$%. This is close to the average value for the %$Q^2$%of the W boson, which would be the natural choice for the massless line if we could use eventbyevent scale choises. Also here the LO is very close to the NLO.46 * for the heavy line: $\mu_{F,R}^{h}=m_t/4$. This is chosen from the previous runs, where for $m_t/4$ the total cross section for the 2>3 is closest to the 2>2 and also the LO is very close to the NLO. 47 * for the light line: $\mu_{F,R}^{l}=m_t/2$. This is close to the average value for the $Q^2$ of the W boson, which would be the natural choice for the massless line if we could use eventbyevent scale choises. Also here the LO is very close to the NLO. 48 48 49 49 The results for the scale dependence can be found in the tables below: 50 50 51 51 52  light quark line %$\mu_0=m_t/2$%  heavy quark line %$\mu_0=m_t/4$% '''2>3 at NLO, LHC top'''  '''2>3 at NLO, LHC antitop'''  '''2>3 at NLO, Tevatron top''' 52  light quark line $\mu_0=m_t/2$  heavy quark line $\mu_0=m_t/4$  '''2>3 at NLO, LHC top'''  '''2>3 at NLO, LHC antitop'''  '''2>3 at NLO, Tevatron top'''  53 53  '''ren. scale'''  '''fac. scale'''  '''ren. scale'''  '''fac. scale'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  54 54  1  1  1  1  LHCt_89  132.0 + 0.5  LHCa_89  76.0 + 0.3  tev_89  800 + 1  … … 83 83 84 84 85  light quark line %$\mu_0=m_t/2$%  heavy quark line %$\mu_0=m_t/4$% '''2>3 at LO, LHC top'''  '''2>3 at LO, LHC antitop'''  '''2>3 at LO, Tevatron top''' 85  light quark line $\mu_0=m_t/2$  heavy quark line $\mu_0=m_t/4$  '''2>3 at LO, LHC top'''  '''2>3 at LO, LHC antitop'''  '''2>3 at LO, Tevatron top'''  86 86  '''ren. scale'''  '''fac. scale'''  '''ren. scale'''  '''fac. scale'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  87 87  1/8  1/8  1/4  1/4  LHCt_1  194.8 + 0.5  LHCa_1  108.4 + 0.2  tev_1  1368 + 2  … … 107 107 108 108 109  For both quark lines %$\mu_0=m_t$%, massive bquark  '''2>2 at NLO, LHC top'''  '''2>2 at NLO, LHC antitop'''  '''2>2 at NLO, Tevatron top''' 109  For both quark lines $\mu_0=m_t$, massive bquark  '''2>2 at NLO, LHC top'''  '''2>2 at NLO, LHC antitop'''  '''2>2 at NLO, Tevatron top'''  110 110  '''ren. scale'''  '''fac. scale'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  111 111  1/16  1/16  LHCt_1  195.7 + 0.5  LHCa_1  110.3 + 0.2  tev_1  1241 + 2  … … 123 123 124 124 125  For both quark lines %$\mu_0=m_t$% '''2>2 at NLO, LHC top'''  '''2>2 at NLO, LHC antitop'''  '''2>2 at NLO, Tevatron top''' 125  For both quark lines $\mu_0=m_t$  '''2>2 at NLO, LHC top'''  '''2>2 at NLO, LHC antitop'''  '''2>2 at NLO, Tevatron top'''  126 126  '''ren. scale'''  '''fac. scale'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  127 127  1/16  1/16  LHCt_1  167.6 + 0.1  LHCa_1  99.02 + 0.07  tev_1  1257.3 + 1.0  … … 138 138 139 139 140  For both quark lines %$\mu_0=m_t$% '''2>2 at LO, LHC top'''  '''2>2 at LO, LHC antitop'''  '''2>2 at LO, Tevatron top''' 140  For both quark lines $\mu_0=m_t$  '''2>2 at LO, LHC top'''  '''2>2 at LO, LHC antitop'''  '''2>2 at LO, Tevatron top'''  141 141  '''ren. scale'''  '''fac. scale'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  '''tag'''  '''cross section'''  142 142  1/16  1/16  LHCt_1  51.98 + 0.03  LHCa_1  29.28 + 0.02  tev_1  512.9 + 0.4  … … 181 181 182 182 183 scale dependence at the LHC for independent scale variations for light and heavy fermion lines: <br />183 scale dependence at the LHC for independent scale variations for light and heavy fermion lines: [[br]] 184 184 <img src="%ATTACHURLPATH%/scaleLHLHCtop.png" alt="scaleLHLHCtop.png" width='300'/> 185 185 186 The above plot shows the scale dependence for the 2>3 process at NLO. In this plot the renormalization scale is set equal to the factorization scale %$\mu_R=\mu_F$%, but the scale for the heavy fermion line is varied independently from the scale of the light fermion line. The ''black'' curves show the scale variation of the heavy fermion line, i.e. the scales for the light line are fixed, and vice versa for the ''red'' line. The value to which the scales are fixed for a particular curve can be read of from the point where this black (or red) line crosses a red (or black) line in a point.186 The above plot shows the scale dependence for the 2>3 process at NLO. In this plot the renormalization scale is set equal to the factorization scale $\mu_R=\mu_F$, but the scale for the heavy fermion line is varied independently from the scale of the light fermion line. The ''black'' curves show the scale variation of the heavy fermion line, i.e. the scales for the light line are fixed, and vice versa for the ''red'' line. The value to which the scales are fixed for a particular curve can be read of from the point where this black (or red) line crosses a red (or black) line in a point. 187 187 * It is obvious that the total scale dependence, i.e. the ''blue'' curve (which is the same as the blue curve in the plots above), is totally coming from the scale variations in the '''heavy''' fermion line. 188 188 … … 197 197 198 198 In the above plots the cross section is plotted as a function of the mass of the (anti)b quark (for the 2>3 process). From these plots it is clear the cross section is quite sensitive to the mass of the bquark. In particular, using the running mass at the scale of the top quark or at the scale of the bottom quark could enhance the cross section by over 1015%. We should figure out to which mass we should let the bmass run. Note that the CTEQ6 PDF sets assume a bottom mass of 4.5 GeV. 199 * The NLO plots lie perfectly on a straight line (within the statistical MC errors). This suggests that the 'large logarithms' at NLO (that would be the terms proportional to %$\alpha_s^2\log^2(m_b/m_t)$%) do not play an important role and that using the resummed calculation with the bquark PDF is not the best estimation for this process.200 201 202 203 ===== scale dependence for %$m_b=m_c$%=====199 * The NLO plots lie perfectly on a straight line (within the statistical MC errors). This suggests that the 'large logarithms' at NLO (that would be the terms proportional to $\alpha_s^2\log^2(m_b/m_t)$) do not play an important role and that using the resummed calculation with the bquark PDF is not the best estimation for this process. 200 201 202 203 ===== scale dependence for $m_b=m_c$ ===== 204 204 <br /> 205 205 <img src="%ATTACHURLPATH%/scalemb=mcTeV.png" alt="scalemb=mcTeV.png" width='300'/> … … 207 207 <img src="%ATTACHURLPATH%/scalemb=mcLHCantitop.png" alt="scalemb=mcLHCantitop.png" width='300'/> 208 208 209 In the above plots the cross section as a function of the scales is plotted. The mass of the bottom quark is set equal to the mass of the charm. The scale dependence for the NLO calculation is still very small, but the kfactors are much larger as compared to the %$m_b=4.7\textrm{ GeV}$%.209 In the above plots the cross section as a function of the scales is plotted. The mass of the bottom quark is set equal to the mass of the charm. The scale dependence for the NLO calculation is still very small, but the kfactors are much larger as compared to the $m_b=4.7\textrm{ GeV}$. 210 210 211 211 Unfortunaly the 'wrong' charm mass has been used for the 2>3 process: for the 2>2 process the charm PDF has been used which has a charm mass of 1.3 GeV. For the 2>3 process the a charm mass of 1.5 GeV was used. Using a charm mass of 1.3 also for the 2>3 process would increase this cross section slightly. The wobbly lines for the 2>3 at NLO at the LHC are entirely due to lack of statistics. Due to the small bottom numerical instabilities start playing a role and numerical convergence becomes more difficult. … … 222 222 223 223 === To do === 224 1. '''Analytic Check of Born amplitude''' <br>224 1. '''Analytic Check of Born amplitude''' [[br]] 225 225 We need to understand what is the analytic dependence of the short distance cross section as a function of the bmass. 226 1. '''Check of the calculation''' <br>226 1. '''Check of the calculation''' [[br]] 227 227 We check the calculation inderectly by evaluating the schannel top qq>W*>tbg at NLO. The idea is then to set mb=mt and compare with Oleari's NLO calculation for e+e> Z/gamma>bb~ g. 228 1. '''Check the mass effects in the 2>2 calculation''' <br>Just to be sure, it would be useful to also compare with the 2>2 calculation at NLO, where the 2>3 contribution is calculated with a finite mass for the b. This can be done by using the collinear subtraction in the massive MSbar scheme, as we have done for W+jets and in the W+c calculation.229 1. '''Allow for different eventbyevent scales in the process''' <br>228 1. '''Check the mass effects in the 2>2 calculation''' [[br]] Just to be sure, it would be useful to also compare with the 2>2 calculation at NLO, where the 2>3 contribution is calculated with a finite mass for the b. This can be done by using the collinear subtraction in the massive MSbar scheme, as we have done for W+jets and in the W+c calculation. 229 1. '''Allow for different eventbyevent scales in the process''' [[br]] 230 230 We need to be able to check all possible factorization and renormalization scales. The main point is that we can treat the light quark line and the heavy quark line independently, since as in the 2>2 there is no talking between the lines. The aim is to have four scales: muf_light, mur_light, muf_heavy, mur_heavy. So first thing is to assess the real scale dependence of the results by varying these scale independently. Scott suggested that we used a dynamical scale for the heavy line. After some thought I think that min(mT(b),mT(t)) =~ mT(b) should be used. By doing so we will slightly overestimate the diagrams where the initial gluon splits into a ttbar pair. However this contribution is anyway very small and should have no impact on the final result. This is also similar to the choice made by PS MC. 231 231 … … 280 280 281 281 282