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A search for exclusive Z→ll events and a measurement of the pp →pγγp→pllp cross section for dilepton invariant mass > 40 GeV/c^2

 
Michael Albrow (Fermilab) and Emily Nurse (University College London)
 Abstract
We present a search for exclusive Z boson production in proton-antiproton collisions at sqrt(s) = 1.96 TeV, using the CDF II detector at Fermilab. We observe no exclusive Z→ll candidates and place the first upper limit on the exclusive Z cross section in hadron collisions, σ(excl) < 0.96 pb at 95 % C.L. In addition, we observe a continuum of exclusive lepton pairs consistent with the quantum electrodynamic process pp →pγγp→pllp. The cross section of the continuum for M(ll) > 40 GeV/c2 and |η(l)| < 4 is σ = 0.24^{+0.13}_{-0.10} pb, which is the first measurement for this mass range and is consistent with the standard model prediction.
  
Public note available here
 
 Introduction

We present a search for exclusive Z production with the Z decaying to an ee or μμ pair and a measurement of the pp →pγγp→pllp (denoted γγ→ll in the following) cross section for dilepton invariant mass > 40 GeV/c^2. These processes are shown here:
The standard model cross section for exclusive Z production at the Tevatron is predicited to be 0.3 fb, therefore a signal would imply beyond the standard model physics. The signature for high mass γγ→ll events is very similar to that for exclusive Z→ll events, apart from the difference in the lepton kinematics. Since the γγ→ll process is well predicted by QED, measuring a cross section in agreement with the prediction gives us confidence in our sensitivity to detecting exclusive Z events. In addition, high mass γγ→ll events can in principle be used to calibrate forward (anti)proton detectors as the event kinematics can be found from the final state leptons as well as the final state (anti)protons.
 
 
  
 Analysis Technique

First we select samples of dilepton events with M(ll) > 40 GeV/c^2 for the γγ→ll candidates and a sub-sample with 82 < M(ll) < 98 GeV/c^2 for the Z→ll candidates. We then require that no other particles were produced in the collision, ensuring that the event is exclusive. We require no additional tracks in the event. In addition we cut on the sum of the energy in the calorimeters (this is done separately for five sub-regions of the calorimeters). In order to decide the correct energy threshold we plot the sum energy distributions for empty events (where no collision occured, this should be similar to that for exclusive events) and for W→lν events with no additional tracks (which should be very similar to inclusive Z→ll events with no additional tracks). The plots are shown below, the red arrow indicates where we decide to make the cut. Note that no cut is made on the CENTRAL HADRONIC calorimeter.
   

  
 Candidate Events

We select 8 events that pass the exclusive γγ→ll selection criteria and 0 that pass the exclusive Z→ll selection criteria. Event display pictures of the entire detector, the Central Outer Tracker and a lego plot of the calorimeters for an example candidate γγ→μμ event (top) and an example candidate γγ→ee event (bottom) are shown here:


The M(ll), 180 - Δφ and Δ|pT| distributions of the 8 candidates are plotted on top of the LPAIR MC prediction:


 
 
  
 Backgrounds

The backgrounds to γγ→ll events are inclusive Z/γ*→ll events that pass the exclusivity cuts (0.28 +/- 0.19 events) and γγ→ll events where the proton or anitproton dissociates and the products are not detected (1.45 +/- 0.61 events). The backgrounds to exclusive Z events are inclusive Z/γ*→ll events that pass the exclusivity cuts (0.163 +/- 0.099 events) and γγ→ll events that have M(ll) in the Z mass region (0.492 +/- 0.061 events). Cosmic ray events are eliminated by a dedicated tagger that uses the timing of the COT tracks. In order to demonstrate that the candidate events do not contain any cosmic ray events the plot below shows the difference in the time measured by the COT track for the two leptons.
 
  
 Results

With the eight candidate events we measure a cross section for exclusive dilepton production of 0.24^{+0.13}_{-0.10} pb for M(ll) > 40 GeV/c^2 and |η(l)| < 4. This is in excellent agreement with the theoretial prediction from LPAIR of 0.256 pb. With no candidate events in the Z mass window we place an upper limit on the exclusive Z production cross section of 0.96 pb at 95% C.L.