Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at √s =7 TeV with the ATLAS detector

CERN-LHC. A measurement of charged-particle distributions
sensitive to the properties of the underlying event is presented for
an inclusive sample of events containing a Z-boson, decaying to an
electron or muon pair. The measurement is based on data collected using
the ATLAS detector at the LHC in proton--proton collisions at a
centre-of-mass energy of 7 TeV with an integrated luminosity of 4.6/fb.

Distributions of the charged particle (excluding the two leptons from Z decay)
multiplicity and of the charged particle transverse momentum are measured
in regions of azimuthal angle delta(phi) = |phi - phi(Z)| defined with respect
to the Z-boson direction:

- the toward region: |delta(phi)| < 60 degrees
- the transverse region: 60 < |delta(phi)| < 120 degrees
- the away region: |delta(phi)| > 120 degrees
- the trans-max region: one of the two transverse sectors having
a maximum scalar sum of transverse momenta of charged particles
on event-by-event basis
- the trans-min region: one of the two transverse sectors having
a minimum scalar sum of transverse momenta of charged particles
on event-by-event basis

The fiducial volume is defined as follows:

- the two oppositely charged leptons from Z decay are required to have
pT > 20 GeV, |eta| < 2.4 and 66 < M(ll) < 116 GeV
- charged particles are required to have a proper lifetime tau > 0.3*10**-10 sec,
be primary (i.e. produced directly in pp interaction or being decay products of
prompt particles with the lifetime less than tau),
have pT > 0.5 GeV and |eta| < 2.5

The observables:

- pT(Z): transverse momentum of the Z-boson

- N(ch)/delta(eta)/delta(delta(phi)): number of stable charged particles per eta-phi unit, averaged over eta-phi range, where -2.5<eta<2.5 and delta(phi) ranges are defined above

- SumPT/delta(eta)/delta(delta(phi)): scalar pT sum of stable charged particles per eta-phi unit, averaged over eta-phi range,
where -2.5<eta<2.5 and delta(phi) ranges are defined above

- mean pT: average pT of stable charged particles (for each phi region)

Corrections to the raw data and presentation of the results:

- The data are fully corrected for all instrumental effects:
finite track reconstruction efficiency and resolution;
Z -> mu+ mu-, e+ e- identification efficiency and Z momentum resolution;
background from pileup and final states yielding the same experimental
signature as the Z -> mu+ mu-, e+ e- signal.

- Published distributions can be directly compared to a particle level
MC generator output with the two definitions of Z -> lepton+ lepton- final state
(as indicated in the tables):

- Born approximation:
Z momentum is defined as the sum of leptons' and FSR photons' momenta,
in neglect of interference between photon radiation off the leptons and
other charged legs in the process.
The particle level fiducial volume can be viewed as the one obtained
in an imaginary experiment without FSR, e.g., with Z -> nu nubar
final state where both leptons have pT>20 GeV and |eta|<2.5.

- Dressed leptons:
dressed lepton momentum is defined as a sum of the lepton's momentum and
the momenta of all photons of non-hadronic origin within DeltaR<0.1 cone
around the direction of the lepton.
The particle level fiducial volume is defined using dressed lepton momenta.
Material referenced as `auxiliary' in the tables is available at
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2011-42/.