Diffraction at DØ: Past, Present, and Future E   Andrew Brandt, University of Texas at Arlington Future QCD Workshop May 20, 2004 Fermilab A1UA2U P2DP1D P Pbar LM VC

Diffraction/Color singlet exchange Exchange of quantum numbers of the vacuum (no charge or color), often referred to as Pomeron exchange Experimental signatures: rapidity gap - absence of particles or energy above threshold in some region of rapidity in the detector intact proton - p or p scattered at small angle from the beam Single Diffraction search for rapidity gap in forward regions of DØ Luminosity Monitor Calorimeter Elastic Scattering search for intact protons in beam pipe Forward Proton Detector proton track rapidity gap proton track either p or p intact p and p intact, with no momentum loss no other particles produced Hard Diffraction (UA8), SD+high P T

Learning about the Pomeron QCD is theory of strong interactions, but 40% of total cross section is attributable to Pomeron exchange -- not calculable and poorly understood Does it have partonic structure? Soft? Hard? Super-hard? Quark? Gluon? Is it universal -- same in ep and ? Is it the same with and without jet production? Answer questions in HEP tradition -- collide it with something that you understand to learn its structure Note: variables of diffraction are t (momentum transfer) and  ~ M 2 (fractional momentum loss) with FPD measure without FPD just measure 

Luminosity Monitor North (η<0) South (η>0) p p Luminosity Monitor (LM) Scintillating detector 2.7 < |η| < 4.4 Charge from wedges on one side are summed: Detector is on/off on each side, North and South Run II LM (Run I: n l0 = # tiles in L0 detector with signal 2.3 < |  | < 4.3) (Run I L0)

Calorimeter FH EM CH LM 2.7 LM range 4.4 Cells arranged in layers: electromagnetic (EM) fine hadronic (FH) coarse hadronic (CH) 2.6 Esum range Sum E of Cells in EM and FH layers above threshold: E EM > 100 MeV E FH > 200 MeV Liquid argon/uranium calorimeter (Run I: n cal = # cal towers with energy above threshold)

Hard Diffractive Jets  s = 1800 GeV  s = 630 GeV Peak at zero multiplicity striking indication of rapidity gap (diffractive) signature in events with hard scattering (two jets with E T >12 GeV) Data gap fractions ~1%, higher for  s=630 than 1800 GeV Rates smaller than expectations based on HERA data Process also studied at H1, Zeus, and CDF Phys. Lett. B 531, 52 (2002)

Hard Color-Singlet Exchange (Central Gap) Count tracks and EM Calorimeter Towers in |  | < 1.0 jet    (E T > 30 GeV,  s = 1800 GeV) Measured fraction (~1%) rises with initial quark content : Consistent with a soft color rearrangement model preferring initial quark states Inconsistent with two-gluon, photon, or U(1) models Measure fraction of events due to color-singlet exchange Phys. Lett. B (1998) Cox&Forshaw + Ingleman&Enberg found ways to flatten BFKL prediction!

Why Study Diffractive W Boson?

Observation of Diffractive W/Z lObserved clear Diffractively produced W and Z boson signals lEvents have typical W/Z characteristics lBackground from fake W/Z gives negligible change in gap fractions Sample Diffractive Probability Background All Fluctuates to Data Central W( )% 7.7  Forward W( )% 5.3  All W( – 0.17)% 7.5  All Z ( )% 4.4  n cal n L0 Diffractive W and Z Boson Signals Central electron WForward electron W All Z n cal n L0 n cal n L0 Phys. Lett. B 574, 169 (2003)

DØ/CDF Diffractive W Boson Comparison CDF {PRL (1997)} measured R W = (1.15 ± 0.55)% for |  |<1.1 where R W = Ratio of diffractive/non-diffractive W (a significance of 3.8  ) This number is corrected for gap acceptance using MC giving 0.81 correction, so uncorrected value is (0.93 ± 0.44)%, consistent with our uncorrected data value: We measured ( –0.17)% for |  |<1.1 Uncorrected measurements agree, but corrections derived from MC do not… Our measured(*) gap acceptance is (21 ± 4)%, so our corrected value is 5.1% ! (*) : derived from POMPYT Monte Carlo Comparison of other gap acceptances for central objects from CDF and DØ using 2-D methods adopted by both collaborations: DØ central jets 18% (q) 40%(g) CDF central B 22%(q) 37% overall CDF J/  29% It will be interesting to see Run II diffractive W boson results!

Run II Improvements Larger luminosity allows search for rare processes Integrated FPD allows accumulation of large hard diffractive data samples Measure , t over large kinematic range Higher E T jets allow smaller systematic errors Comparing measurements of HSD with track tag vs. gap tag yields new insight into process

  E Soft Diffraction and Elastic Scattering: Inclusive Single Diffraction Elastic scattering (t dependence) Total Cross Section Centauro Search Inclusive double pomeron Search for glueballs/exotics Hard Diffraction: Diffractive jet Diffractive b,c,t, Higgs Diffractive W/Z Diffractive photon Other hard diffractive topics Double Pomeron + jets Other Hard Double Pomeron topics Rapidity Gaps: Central gaps+jets Double pomeron with gaps Gap tags vs. proton tags Topics in RED were studied with gaps only in Run I 1000 tagged events expected in Run II DØ Run II Diffractive Topics In this talk highlight diffractive Z boson and elastic analysis

Search for diffractive Z → μμ DØ Run II preliminary Summer 2003 Inclusive Z→μμ sample well understood: 2 muons, p T > 15GeV, opposite charge at least one muon isolated in tracker and calorimeter anti-cosmics cuts based on tracks: displacement wrt beam acolinearity of two tracks RunI publication ”Observation of diffractively produced W and Z bosons in pp Collisions at sqrt(s)=1.8 TeV”, Phys. Lett. B 574, 169 (2003) Nine single diffractive Z→e+e- events. No result in muon channel. RunII: first search for forward rapidity gaps in Z→μ+μ- events M μμ (GeV)

cosmics shape expected from inclusive sample WORK IN PROGRESS First Step Towards Gap: LM Only Separate the Z sample into four groups according to LM on/off: Expect worst cosmic ray contamination in sample with both sides of LM off no evidence of overwhelming cosmics background in LM off samples No GAP South GAP North Two GAPs

Calorimeter Energy Sum Areas are normalised to 1 empty events physics samples Compare 'empty event' sample with physics samples: Empty event sample: random trigger. Veto LM signals and primary vertex, i.e. mostly empty bunch crossings Physics samples: minimum bias (coincidence in LM), jet and Z→μμ events Log(energy sum) on North side: 10 GeV Esum cut of 10GeV was chosen for current study Final value will be optimised using full data sample Use energy sum to distinguish proton break-up from empty calorimeter:

Z Mass of Rapidity Gap Candidates Invariant mass confirms that these are all Drell-Yan/Z events Will be able to compare Z boson kinematics (p T, p z, rapidity) 89.8 ± 0.1 GeV 89.6 ± 1.0 GeV 89.3 ± 2.0 GeV 90.2 ± 1.3 GeV WORK IN PROGRESS Add Esum<10 GeV requirement to gap samples: Gap North & Gap South combined No GAP One GAP South GAP North

17 Diffractive Z→μμ candidate outgoing proton side outgoing anti-proton side muon

18 Z→μμ with rapidity gaps: Summary Preliminary definition of rapidity gap at DØ Run II Study of Z→μ+μ- events with a rapidity gap signature (little or no energy detected in the forward direction) Current status: Evidence of Z events with a rapidity gap signature Quantitative studies of gap definition, backgrounds, efficiency in progress (effects could be large) No interpretation in terms of diffractive physics possible yet Plans: Measurement of the fraction of diffractively produced Z events Diffractive W→μν, W/Z→electrons, jets and other channels Use tracks from Forward Proton Detector

Forward Proton Detector Forward Proton Detector (FPD) Quadrupole Spectrometers surround the beam: up, down, in, out use quadrupole magnets (focus beam) - a series of momentum spectrometers that make use of accelerator magnets in conjunction with position detectors along the beam line Dipole Spectrometer inside the beam ring in the horizontal plane use dipole magnet (bends beam) also shown here: separators (bring beams together for collisions) A total of 9 spectrometers composed of 18 Roman Pots

Quadrupole ( p or ) Dipole ( only) M X (GeV) GeV M X (GeV)   Geometric  Acceptance Acceptance Dipole acceptance better at low |t|, large  Cross section dominated by low |t| Combination of Q+D gives double tagged events, elastics, better alignment, complementary acceptance Actual acceptance more limited due to higher than expected halo backgrounds

All 6 castles with 18 Roman pots comprising the FPD were constructed in Brazil, installed in the Tevatron in fall of 2000, and have been functioning as designed. A2 Quadrupole castle installed in the beam line. Castle Status

FPD Detector Design l6 planes per detector in 3 frames and a trigger scintillator lU and V at 45 degrees to X, 90 degrees to each other lU and V planes have 20 fibers, X planes have 16 fibers lPlanes in a frame offset by ~2/3 fiber lEach channel filled with four fibers l2 detectors in a spectrometer 0.8 mm 3.2 mm 1 mm mm U U’ X X’ V V’ Trigger

Detector Construction At the University of Texas, Arlington (UTA), scintillating and optical fibers were spliced and inserted into the detector frames. The cartridge bottom containing the detector is installed in the Roman pot and then the cartridge top with PMT’s is attached.

20 detectors built over a 2+ year period at UTA. In , 10 of the 18 Roman pots were instrumented with detectors. Funds to add detectors to the remainder of the pots have recently been obtained from NSF (should acknowledge funding from UTA REP, Texas ARP, DOE, and Fermilab as well). During the shutdown (Sep-Nov. 2003), the final eight detectors and associated readout electronics were installed. All 18 pots are routinely inserted near the beam. A2 Quadrupole castle with all four detectors installed Detector Status

Elastic Scattering A2UA2UA1UA1U P2DP1D p p p P Measure dN/dt for elastic scattering using partially instrumented FPD: Quadrupole acceptance: t > 0.8 GeV 2 (requires sufficient scattering angle to leave beam) all ξ (no longitudinal momentum loss necessary) proton side: quadrupole ‘down’ spectrometer full detector read-out antiproton side: quadrupole ‘up’ spectrometer trigger only Elastic scattering: ξ = 0

After alignment and multiplicity cuts (to remove background from halo spray):  Events are peaked at zero, as expected, with a resolution of   = Elastic Data Distributions dN/dt  =  p/p Acceptance loss Residual halo contamination The fit shows the bins that will be considered for corrected dN/dt

Preliminary Elastic Scattering Results The dσ/dt data collected by different experiments at different energies A factor of must be applied to each curve New DØ dN/dt distribution has been normalized by E710 data Compare slope with model: Block et al, Phys. Rev. D41, pp 978, 1990.

What’s Getting Done Now lSeven Ph.D. students in diffractive group based at Fermilab+2 remote lAnalyses in progress on lLarge samples of SD and DPE jet events with gaps, some of which with FPD tracks lW/Z +gap and/or track (electron and muon channels) lJ/ψ +gap data (χ c ) lAnalyses attempt to measure diffractive structure as f(ξ,t) in many channels lSearch for exclusive production in double pomeron to jets or χ c  Updating expectations based on current experience  Improving trigger capabilites to allow selection of double track events in global trigger list  Working on halo background reduction  Quadrupole detector commissioning

What Else Will Likely Get Done lSpecial run to map out |t| down to and perhaps measure total cross section (when detectors fully commissioned) lInclusive double pomeron/glueball searches, (when trigger is ready)

A Measurement of the Total Cross Section? With beam injection lattice: MC

Uncertainty on the measurement of  _TOT due to extrapolation to t=0 (50000 elastic events): Position|t|_rangeSlope  _T(mb) 88 0.05<|t|< ± 9 0.12<|t|< ±  0.16<|t|< ±  0.20<|t|< ± This error is only due to error in extrapolation. Still need to include error in the measurement of N inelastic Extrapolation Error

What GTeV Could/Should Do in Diffraction lfocus on double pomeron topics lbest possible acceptance for low+high mass ldiffractive measurements in regions of phase space without acceptance in Run II, or where statistics insufficient

Summary lEarly FPD stand-alone analysis shows that detectors work, will result in elastic dN/dt publication (already 1 Ph.D.) lFPD now integrated into DØ readout (detectors still work) lCommissioning of FPD and trigger in progress lFPD should provide diffractive structure results in many topics and establish (or not) exclusive double pomeron production l Tune in next year for first integrated FPD physics results lMany double pomeron topics should be pursued further