| Latest Blessed Date: | March 10, 2000 |
| Contact Person: | M.G. Albrow1, K. Borras2, K. Goulianos2, K. Terashi3 (e-mail) |
1 Fermilab, Batavia, IL, 60510
2 Rockefeller University, New York, New York 10021
3 University of Tsukuba, Tsukuba, Ibaraki 315, Japan
| ANALYSIS NOTES | TITLE | LAST UPDATE |
| CDF 4711 (version 3.3)
(Part I) (Part II) (Part III) |
Observation of Dijet Production in Double Pomeron Exchange in pbar-p Collisions at sqrt(s)=1800 GeV | March 7, 2000 |
| CDF 5023 (version 2.2) | Further Studies of Double Pomeron Exchange | March 7, 2000 |
In this note, we report a study of dijet production in Double Pomeron Exchange (DPE) events, in which both the beam
hadrons are quasi-elastically scattered and large rapidity gaps, defined as a region of rapidity (or pseudorapidity) devoid
of particles, are left in both the forward directions. (Illustration of event topology)We used the data triggered with the forward antiproton spectrometer in pbar-p collisions at sqrt(s)=1800 GeV.
A leading antiproton in the diffractive scattering was observed in the forward spectrometer. For the sample of events
containing the leading antiproton with 0.035<xi<0.095 and |t|<1.0 GeV2, where xi is the fractional momentum loss of
the antiproton and t is the four momentum transfer squared, we required the presence of at least two jets with transverse
energy ET>7 GeV or ET>10 GeV. In the selected sample of single diffractive (SD) dijet events, we observed the DPE
dijet events as an excess of events with a rapidity gap in the forward pseudorapidity range 2.4<eta<5.9 on the outgoing
proton direction. Taking into account the efficiencies for the event selection, fractions of the background contributions
and the forward spectrometer acceptance, we estimated the preliminary cross sections of dijet production in DPE, SD
and non-diffractive (ND) events, and their relative cross section ratios. In comparisons of the kinematical distributions
of dijets, we found that the jet ET spectra were similar in DPE and SD events, but they were slightly steeper than that of
ND events. The DPE events were cleaner with the two leading jets being more back-to-back in phi.We compared our results with the predictions of the Monte Carlo simulation based on the Ingelman-Schlein hard
diffraction model. The ratio of DPE to SD dijet event rates and the kinematical distributions of the date were in
agreement with the Monte Carlo predictions.
The analysis of dijet production in Double Pomeron Exchange events using the Roman Pot data has been developed.
In this note, we summarize the results of recent studies for a determination of the fractional momentum loss xi of the
proton, and the diffractive structure functions of the proton and antiproton. From the xi for the proton and antiproton
in DPE events, we estimated the mass fraction of dijet to central DPE system. From this measurement, we give a
production limit of Non-Factorizable DPE for our kinematical range.
The following plots and numbers were blessed or re-blessed at QCD meeting on March 10, 2000. Results with yellow
background will be presented in the PRL paper.
| PLOTS | |
| Observed
DPE signal and the xi
distributions of the DPE events. |
(a) Beam-Beam Counter hit multiplicity on the proton side, NBBCp, versus forward calorimeter tower multiplicity, NFCALp: the peak in the (0,0) bin contains the DPE signal; (b) multiplicity distribution along the diagonal bins in (a) with NBBCp = NFCALp; (c) xipbar measured by the RPS for SD events (histogram) and for the "DPE" events of the (0,0) bin in (a); (d) xip of the DPE events. (summary slide) |
| Kinematical
distributions of
DPE dijet events. |
Distributions for "DPE" events [(0,0) bin in Fig. 2] (points) compared with corresponding single diffractive (non-diffractive) distributions shown as solid (dashed) histograms: (a) mean jet ET; (b) mean eta of the dijet system; (c) azimuthal angle difference between the leading two jets; (d) ratio of dijet mass obtained from the energies within jet cones of eta-phi radius 0.7 to total central system mass. The shaded histogram shows the expected dijet mass fraction for events in which the dijet mass accounts for the total central system mass. (summary slide) |
| DPE/SD
(SD/ND) dijet event
rates as a function of Bjorken-x of partons in the (anti-)proton. |
Ratios of DPE to SD (SD to ND) dijet event rates per unit xip (xipbar), shown as open (filled) circles, as a function x-Bjorken of partons in the p (pbar). The errors are statistical only. The SD/ND ratio has a normalization systematic uncertainty of +/-20%. The insert shows tilde{R}(x) per unit xi versus xi, where the tilde over the R indicates the weighted average of the R(x) points in the region of x within the vertical dashed lines, which mark the DPE kinematic boundary (left) and the value of x = xipmin (right). (summary slide) |
| Dijet
mass and dijet mass
fraction to the central DPE system mass. |
Dijet mass Mjj(cone) reconstructed from the towers within the cones (R=0.7) and fraction of dijet mass to the central DPE system mass, given by Rjj = Mjj(cone) / MDPEX, where MDPEX = (xipX xipbarRP s)1/2. The DPE events (single diffractive background) with dijets of ETjet1,2>7 GeV in the (0,0) bin are shown as the black points (shaded histograms). (summary slide) |
| Dijet
mass fraction for DPE
events with dijets. |
Fraction of dijet mass within the cones of R=0.7 to the central system mass for DPE events with (a) ETjet1,2>7 GeV and (b) ETjet1,2>10 GeV dijets (points). The histograms represent the ratios of dijet mass within the cones to the total dijet mass. All the plots are for DPE events in the (0,0) bin corrected for the Roman Pot acceptance. The SD dijet background in the (0,0) bin is subtracted. (summary slide) |
| Bjorken-x of partons in the
proton and antiproton for DPE, SD and ND dijet events. |
xpbar (upper) and xp (lower) distributions of the DPE dijet candidates in the (0,0) bin (black points), SD (dashed histogram) and ND (dotted histogram) dijets with ETjet1,2>7 GeV. If the third jet ET is greater than 5 GeV, it is included in the x evaluation. The shaded histograms show the shapes of the SD background in the DPE candidates. (summary slide) |
| NUMBERS | |
| Absolute DPE Dijet Production
Cross Section |
The absolute cross sections of dijet production in DPE events
are measured to be
43.6 +/- 4.4(stat) +/- 21.6(syst) nb for dijets with ETjet1,2>7 GeV, andin the kinematical region 0.035<xipbar<0.095, 0.01<xip<0.03, |tpbar|<1.0 GeV2 and pseudorapidity of jets -4.2<etajet<2.4. The systematic uncertainties are dominated by the uncertainties on jet energy calibration (40%), underlying event energy to be subtracted from the jets (17-26%), and the inclusive diffraction cross section in normazation (20%). (summary slide) |
| Upper Limit of Non-Factorizable
DPE Contribution |
From the measured xip and the Roman Pot xipbar,
we have evaluated the mass fraction of dijet to central DPE system. From
the distribution of dijet mass fraction, we observed one event which has
the characteristics being consistent with the Non-Factorizable DPE process.
From 95% confidence limit (C.L.) upper bound of 5.1 NF-DPE candidate events
and the measured DPE dijet cross section, we obtained the upper limit of
NF-DPE dijet cross section at 95% C.L. to be
3.7 nb for dijets with ETjet1,2>7 GeV,in the kinematical region 0.035<xipbar<0.095, |tpbar|<1.0 GeV2 and pseudorapidity of jets -4.2<etajet<2.4. This NF-DPE cross section from the data is an order of three lower than a theoretical prediction based on the model proposed by A. Berera. (summary slide) |
| Ratio of DPE to SD (SD to ND)
Dijet Event Rates per Unit xi |
Ratio of DPE to SD (SD to ND) dijet event rates per unit xip
(xipbar) as a function of Bjorken-x of partons
in the p (pbar) was examined. By selecting the data points in the relevant
Bjorken-x region 10-2.8<x (=xp=xpbar)<0.01 for
both ratios, we obtained the weighted averages of the DPE/SD and SD/ND
ratios to be
RDPESD = 0.80 +/- 0.26 and,in the kinematical region 0.035<xipbar<0.095, |tpbar|<1.0 GeV2 for DPE and SD and 0.01<xip<0.03 for DPE. The RSDND ratio was estimated by performing a straight line fit to the SD/ND ratio as a function of xi for six xi intervals between 0.035 and 0.095, and extrapolating a fit to the value xi = 0.02. The ratio of these two ratios, denoted by D, is D = RDPESD / RSDND = 0.19 +/- 0.07.The deviation of D from unity represents a breakdown of factorization. (summary slide) |