Elastic Scattering at s=1.96 TeV Using the DØ Forward Proton Detector Andrew Brandt University of Texas, Arlington on behalf of DØ Collaboration An FPD Quadrupole castle with four detectors installed Andrew Brandt UTA DPF 2011 Brown University

Elastic Scattering The particles after scattering are the same as the incident particles =p/p=0 for elastic events; The cross section can be written as: This has the same form as light diffracting from a small absorbing disk, thus processes with one or more intact protons are referred to as diffraction Characterized by a steeply falling |t| distribution and a dip where the slope becomes much flatter Elastic “dip” Structure from Phys. Rev. Lett. 54, 2180 (1985). t = -(pi-pf)2 Andrew Brandt UTA DPF 2011 Brown University

Forward Proton Detector There are 8 quadrupole spectrometers 4 each (Up, Down, In, Out) on the outgoing proton (P) and anti-proton (A) sides, with each spectrometer comprised of two detectors (1, 2) Use Tevatron lattice and scintillating fiber hits to reconstruct  and |t| of scattered protons (anti-protons) The acceptance for |t|>|tmin| where tmin is a function of pot position: for standard operating conditions |t| > 0.8 GeV2 Andrew Brandt UTA DPF 2011 Brown University

FPD Detectors 3 layers in detector: U and V at 45o degrees to X, 90o degrees to each other Each layer has two planes (prime and unprimed) offset by ~2/3 fiber Each channel contains four fibers Two detectors in a spectrometer Scintillator for timing (primariliy used for halo rejection) 17.39 mm V’ V Trigger X’ X U’ U 17.39 mm 1 mm 0.8 mm 3.2 mm Andrew Brandt UTA DPF 2011 Brown University

Large β* Store In 2005 DØ proposed a store with special optics to maximize the |t| acceptance of the FPD In February 2006, the accelerator was run with the injection tune, β* =1.6m (about 5x larger than normal) Only 1 proton and 1 anti-proton bunch were injected Separators OFF (no worries about parasitic collisions with only one bunch) Integrated Luminosity (30 ± 4 nb-1) was determined by comparing the number of jets from Run IIA measurements with the number in the Large β* store A total of 20 million events were recorded with a special FPD trigger list Andrew Brandt UTA DPF 2011 Brown University

Track Finding Alignment Hit Finding Track Reconstruction. Use over-constrained tracks that pass through horizontal and vertical detectors to do relative alignment of detectors and use hit distributions to align detectors with respect to the beam A1U A1I A1O A1D Hit Finding Require less than 5 hit fibers per layer (suppresses beam background) Use intersection of fiber layers to determine a hit Track Reconstruction. For events with good hits in both detectors, use the aligned hit values and the Tevatron lattice transport equations to reconstruct the proton track Andrew Brandt UTA DPF 2011 Brown University

Elastic Spectrometer Combinations Elastic events have tracks in diagonally opposite spectrometers A1U A2U P2D P1D LM Proton Anti-proton Momentum dispersion in horizontal plane results in more halo (beam background) in the IN/OUT detectors, so concentrate on vertical plane AU-PD and AD-PU to maximize |t| acceptance while minimizing background AU-PD combination has the best |t| acceptance Andrew Brandt UTA DPF 2011 Brown University

Detector Positions after Alignment Andrew Brandt UTA DPF 2011 Brown University

Hit Finding Combination of fibers in a plane determine a segment Need two out of three possible segments to get a hit U/V, U/X, X/V (or U/X/V) reconstruct x and y position in detector use alignment to go from detector to beam coordinates Can also get an x directly from the x segment (can compare these x measurements to measure resolution) X U V beam Andrew Brandt UTA DPF 2011 Brown University

Fiber Correlations within a Detector Correlations between fibers in the primed and unprimed plane of each layer in AU-PD elastic candidates Fiber hit correlations are a useful debugging tool to demonstrate that the detectors are operating as designed. The figure shows the fiber correlation of primed and unprimed layers in the A1 and P1 detectors, respectively, for elastic trigger events with single hits in the three different planes, U, V, and X. From the plots one can confirm the correct cable mapping (given the expected correlation between primed and unprimed layers) as well as determine dead or hot channels (there were very few instances of either). Andrew Brandt UTA DPF 2011 Brown University

Layer Multiplicity for Elastic Candidates This figure shows the fiber multiplicity distribution for a couple representative layers (out of the six fiber layers) of the A1U detector for elastic candidate events selected with the SDPRO and XTXT triggers, standard hit requirements were applied with the exception of the number of struck fibers <=4, this plot just shows that elastic events have low fiber multiplicity and that the cut of number of struck fibers throws away a very small fraction of events which are in any case later corrected by the hit selection efficiency Elastic events have low fiber multiplicity Andrew Brandt UTA DPF 2011 Brown University

Detector Resolutions xuv-xx=2 Andrew Brandt UTA The figure shows distributions of the difference between the x coordinate obtained from UV fiber intersections and from the X fiber segment, for events with all 3 fiber planes ON in the vertical quadrupole detectors. The detector resolution can be estimated by dividing the width of this distribution by the square-root of 2, since the difference depends on uncertainties from both the UV and the X measurement fibers. xuv-xx=2 Andrew Brandt UTA DPF 2011 Brown University

Halo Rejection Proton Halo -78 -109 nsec 109 nsec A1U A2U P2D P1D LM Proton Halo -78 nsec -109 The in-time bit is set if a pulse detected in the in-time window (consistent with a proton originating from the IP) The halo bit is set if a pulse is detected in early time window (consistent with a halo proton) We can reject a large fraction of halo events using the timing scintillators (depending on the pot locations) Andrew Brandt UTA DPF 2011 Brown University

Correlations Between Detectors No Early Hits fiducial region No Early Hits In addition to providing a time measurement for particles that pass through the detector at the expected time and setting an in-time bit, the TDC system also set a halo bit for early time hits. Halo particles are particles traveling far enough outside of the main beam core that they can pass through the detectors and be mistaken for elastic or diffractive protons. If these halo particles arrive at the proton detectors at the time expected for an in-time proton, frequently they have passed through the diagonally opposite antiproton detectors at an earlier time (and vice versa). The first column of the figure shows the y coordinate correlations for the first detector of the proton and anti-proton spectrometers for the two elastic combinations AU-PD and AD-PU. Although the expected linear correlation for elastic events can be observed, there is clearly some uncorrelated background. A likely source of this background is halo contamination, which can be reduced through the use of the FPD timing system. The second column shows the y correlations for events with one or more halo bits set in the elastic configuration. Column 3 shows the elastic candidate events remaining after removal of the halo events from Column 2. There is still some residual background, partially due to inefficiency of the trigger scintillator, but mostly due to the different acceptances of the different spectrometers (if one spectrometer is not inserted as far as its elastic partner, it will not only have lower acceptance for elastic events, but it will also have less halo acceptance, reducing the effectiveness of the halo veto). The red box shows the fiducial region kept for the elastic analysis, which equalizes the acceptance for elastic tracks in both spectrometers. residual halo due to different acceptances of the two spectrometers -> fiducial cuts Elastic Halo Andrew Brandt UTA DPF 2011 Brown University

Measuring Cross Section Count elastic events Divide by luminosity Correct for acceptance and efficiency Unsmearing correction for |t| resolution Subtract residual halo background Take weighted average of four measurements (2 elastic configurations each with two pot positions) Andrew Brandt UTA DPF 2011 Brown University

Correcting Cross Section Corrections: f acceptance (geometrical loss due to finite size of opposite spectrometer) Unsmearing correction due to beam divergence, |t| resolution (standard approach using ansatz function) Efficiency: use triggers requiring A1-P1 or A2-P2 hits, offline demand 3rd hit, then measure efficiency of 4th detector Use side bands to measure and subtract background “observed” “true” “smearing” “unsmearing” or “unfolding” Andrew Brandt UTA DPF 2011 Brown University

|t| Observe expected colinearity between proton and anti-proton Resolution ranges from 0.02 at low |t| to 0.045 at high |t| Observe expected colinearity between proton and anti-proton DPF 2011 Brown University Andrew Brandt UTA

Measurement of Elastic Slope (b) Systematic error dominated by trigger efficiency correction (this is likely to be significantly smaller in final result, which just missed the DPF deadline) First measurement of b at s=1.96 TeV Andrew Brandt UTA DPF 2011 Brown University

d/d|t| Compared to E710+CDF E710/CDF for s=1.8 TeV; expect logarithmic dependence with s Andrew Brandt UTA DPF 2011 Brown University

d/d|t| Compared to UA4 Comparison of D0 data to UA4 data. We observe the expected “shrinkage” effect with increasing center of mass energy, namely the slope is steeper and the dip moves to lower |t| values. Slope steeper and slope change earlier for higher s (shrinkage) Andrew Brandt UTA DPF 2011 Brown University

Conclusions We have measured d/d|t| for elastic scattering over the range: 0.2< |t| <1.2 GeV2 the first such measurement at s=1.96 TeV For 0.2<|t|<0.6 GeV2 we have measured the elastic slope b=16.5 ± 0.1 ± 0.8 GeV-2 We observe that the elastic slope is steeper and changes slope earlier than lower energy data such as UA4 Extra Points Paper is in final review stages to be submitted to PRD in next few weeks Arnab Pal (UTA Ph. D. student) just defended his thesis on single diffractive cross section, analysis in early review stages Andrew Brandt UTA DPF 2011 Brown University