Diffractive W and Z Production at CDF

We measure the fraction of events with a W or Z boson which are produced diffractively at CDF using the Roman-pot detectors in the range of momentum fraction 0.03 < xi < 0.10 and 4-momentum transfer squared |t| < 1 to be

R_W = [0.97 +/- 0.05(stat) +/- 0.11(syst)]%
R_Z = [0.85 +/- 0.20(stat) +/- 0.11(syst)]%

Roman-pot tracking is used to determine the momentum fraction xi and 4-momentum transfer squared t of the diffractive exchange
As in diffractive dijet production, we can calculate xi from the energy in the calorimeter
However, in the case of diffractive W production, the missing energy from the neutrino yields a difference compared to the true xi determined from the RP track
The difference between the calorimeter and RP xi allows us to determine the neutrino kinematics, and therefore the W kinematics:
For example, the W mass can be reconstructed as shown below.
This also allows the rejection of events with multiple pbar-p interactions, e.g., an event consisting of a non-diffractive interaction producing a W plus a soft diffractive interaction, as shown in the plot below of xi reconstructed in the calorimeter for all events with a Roman-pot track, those consistent with a single interaction and missing energy due to a neutrino, and those with reconstructed W mass in the range of 50-120 GeV.

reconstructed W mass (eps) (gif)
momentum fraction xi reconstructed using calorimeter (eps) (gif)

Kinematic selection of diffractive and non-diffractive W events

W events are selected by requiring

central electron with E_T > 25 GeV or muon with p_T > 25 GeV
missing E_T (MET) > 25 GeV
W transverse mass in the range 40-120 GeV
event z vertex within 60 cm of nominal IP

electron E_T (eps) (gif)
MET in W -> e nu (eps) (gif)
W transverse mass in W -> e nu (eps) (gif)
electron eta (eps) (gif)

muon E_T (eps) (gif)
MET in W -> mu nu (eps) (gif)
W transverse mass in W -> mu nu (eps) (gif)
muon eta (eps) (gif)

Reconstructed kinematics of diffractively-produced W

W rapidity (eps) (gif)
W p_T (eps) (gif)

Reconstructed kinematics of diffractive and non-diffractive Z bosons

Z events are selected by requiring

central electron with E_T > 25 GeV or muon with p_T > 25 GeV
second electron (central or plug) with E_T > 25 GeV or muon with p_T > 25 GeV
reconstructed Z mass in the range 66-116 GeV
event z vertex within 60 cm of nominal IP

Diffractive Z candidates are those which have a Roman-pot track and calorimeter-reconstructed xi < 0.10

Of the 36 diffractive Z candidates, we estimate 11 (using the non-diffractive calorimeter xi distribution) to be background e.g. due to a non-diffractive Z interaction overlapping a soft diffractive interaction

reconstructed Z mass (eps) (gif)
momentum fraction xi reconstructed using calorimeter (eps) (gif)
(The mass distribution seems to be shifted for diffractive relative to non-diffractive events; the Kolmogorov-Smirnov probability that the two distributions are compatible is 14%.)

Z rapidity (eps) (gif)
Z p_T (eps) (gif)