CSC note on DC3 Samples W →( e/ μ ) ν at NLO: Validation

CSC note on DC3 Samples W→(e/μ)ν at NLO: Validation Laura Gilbert 20-02-07

Introduction In December 2005 the rulers of the planet ATLAS CSC ordered one million of each of W→e±ν, W→μ±νMC@NLO events to be generated. More than a year on, in the farthest-flung reaches of Grad Student Hell, the validation effort still continues...

Why the MC CSC Note? • To provide detailed description of how and why the DC3 sample was generated. • Eg. explain choices of generators and filters, give details of processes used and methodology. • To provide basic validation of both generator level and full sim samples before official release.

Why MC@NLO? • MC@NLO is slower and more cumbersome than some of the LO generators. • Gauge boson + jet production are the dominant backgrounds to many Higgs and BSM processes. • Need eg. 10% theoretical accuracy for important processes such as Higgs production, → similar for W backgrounds. • LO predictions for W + heavy flavour processes suffer from large uncertainties (non-negligible mass of parton compared with boson) • NLO diagrams needed to reduce uncertainties esp. in the heavy flavour jet distributions (generally NLO corrections amount to 20-100%) • → W Parton shower models at LO are not sufficiently accurate in describing LHC hard jets, re-scaling an LO would not be acceptable experimentally for these studies.

Event Generation (again…) • Generator level plots for CSC note on W->e/mu nu at NLO • The MC@NLO \cite{CTEQ6m} event generator with the CTEQ6m \cite{CTEQ6m} • PDF set was used for this dataset. One million events were generated for • each channel with MC@NLO (see \cite{myMC@NLO} for a detailed • description), and then input to the the Jimmy \cite{Jimmy} interface to • HERWIG \cite{HERWIG} for fragmentation and hadronisation (is this how it • works? lmg). Photos \cite{Photos} was used to deal with final state QED • radiative corrections, and Tauola \cite{Tauola} for with $\tau$-lepton • decays. See section \ref{genpars}for details of the commands used to • drive the various event generation$\backslash$simulation packages. • Finally the DC3 simulation skeleton was used to fully simulate the • sample using GEANT \cite{GEANT}.

Generator Level Validation Plots • At NLO two processes contribute: • PROCESS A: qq→eν: 77% of events. • PROCESS B: qq→Wg; W→eν: 23% of events. • NB we are looking at a subset of each sample here: ~200k of each of W+, W-. (199324 W+,199290 W-)

υl l+ υl l+ u l- -l W- d Reminder: Rapidity Distributions of Ws and leptons Generally in proton x(u)>x(d)>x(sea) W+ production: x(u)>>x(d) W+ VALENCE QUARK SEA QUARK W- production: x(d)≥x(u) Nb. x1,2=(M/14TeV)e±y so for - W at y=0: x1,2≈5x10-3 - W at y=2.5: x1≈0.07, x2≈5x10-4.

W e q q ν Generator Level Validation Plots • At NLO two processes contribute: • PROCESS A: qq→eν: 77% of events. There is a W intermediate but it is not written out in the event record. Clearly the W will have no transverse momentum. e, ν produced back-to-back in transverse plane.

e, ν generator level sanity plots electronandpositron antineutrinoand neutrino φ Φevenly distributed η η much as expected

e, ν generator level sanity plots electronandpositron antineutrinoandneutrino ET ET distribution sensible, cuts off around MW/2 pz pz distribution sensible when compared to η

What does the W look like from this process? Nb. Ws from heavy quarks produced at slightly lower rapidities • ET=0 → φ, η undefined, but mass, pz and y can be reconstructed from e, ν • y and pz look as expected • masses:not very Gaussian, width a bit narrow? • PDG: Mw=80.425±0.038 ; Γ=2.124±0.41 GeV • Here (W-): Mw=80.42 ; Γ≈1.8 GeV

W q q g Generator Level Validation Plots • At NLO two processes contribute: • PROCESS B: qq→Wg; W→eν: 23% of events. W is written into event record, produced back-to-back with a gluon in the transverse plane → jets. e ν

W generator level sanity plots • masses:not very Gaussian, left skewed, width a bit wide? • PDG: Mw=80.425±0.038 ; Γ=2.124±0.41 GeV • Here (W-): Mw=80.31 ; Γ≈2.4 GeV

At first glance… wargh!

Phew…

Let’s take a quick look at the W decay products… electronandpositron antineutrinoand neutrino φ Φevenly distributed η η much as expected

e, ν generator level sanity plots electronandpositron antineutrinoandneutrino ET ET distribution sensible, cuts off around MW/2 pz pz distribution sensible when compared to η

For interest let’s add the two • Differs noticably from LO plots.

Fwhm~2300 so looks better

Generator level stats • NB generator level filter was applied to charged lepton: pt>10GeV, |eta|<2.7

Validation of fully simulated sample • Constrained to work with 11.0.42 since v12 samples not yet available • The good news is you’ve seen all these plots already • I intend to include…

Conclusions • Still need to check muons gen level + full sim properly (plots created, need to look at them more closely) • Will finish write-up and email off this week • Any volunteers for proof reading and constructive criticism? • Much valuable experience gained…

CSC note on DC3 Samples W →( e/ μ ) ν at NLO: Validation