# Presentation: 752122

## Diffractive Studies at the Tevatron

**Diffractive Studies at the Tevatron**

**Gregory R. Snow**
**University of Nebraska**
**(For the D0 and CDF Collaborations)**

** ****Introduction**
** ****Hard Color Singlet Exchange (CSE)**
**  ****(forward jet - gap - forward jet)**
** ****Hard Single Diffraction**
**  ****(forward gap - opposite dijets or W)**
** ****Double Pomeron Exchange Signatures**
**   ****(forward gap - jets - forward gap)**
** ****Improvements for Tevatron Run II**
** ****Conclusions**

**ICHEP98**
**Vancouver, BC, Canada**
**July 23 - 29, 1998**

**Workshop on Forward Physics and**
**Luminosity Determination at the LHC**
**Helsinki, Finland**
***November 2, 2000***

**International Europhysics Conference**
**Tampere, Finland**
**July 15 - 21, 1999**

**CMS Collaboration Meeting**
***March 8, 2001***

## D0 Run I Diffraction Results

**D0 Run I Diffraction Results**

**Gregory R. Snow**
**University of Nebraska**
**(For the D0 Collaboration)**

** ****Introduction**
** ****Hard Color Singlet Exchange (CSE)**
**  ****(forward jet - gap - forward jet)**
** ****Hard Single Diffraction**
**  ****(forward gap - opposite dijets or W)**
** ****Double Pomeron Exchange Signatures**
**   ****(forward gap - jets - forward gap)**
** ****Diffractive W and Z Production**
**   ****(W/Z plus gap)**
** ****Conclusions**

**Small-x Workshop**
**Fermilab**
**September 17 - 20, 2003**

**New**

**New-ish**

**Nice result,**
**need to publish**

## So why am I here?

** ****Dino**** was my Ph.D. thesis advisor at RockU, somehow**
**  ****diffraction gets in your blood**
** ****Oversaw some of this work as D0 Run I QCD co-convener **
**  ****1998-2001 (with Schellman, Brandt, Elvira) and Editorial**
**  ****Board member/chair**
** ****I have great appreciation for ****Andrew Brandt’s**** invention**
**  ****and leadership of the D0 diffraction effort**
** ****Expanded D0’s Run I physics menu**
** ****Several unique Ph.D. theses and publications resulted**
** ****Run I pioneering work has led to ****Run II**** ****Forward **
**  ****Proton Detector**** – more comprehensive studies**
**  ****to come (see upcoming talks)**
** ****I said “yes” to this talk**

## Conclusion before the talk

**So you’re at a cocktail party, and someone asks you:**
**“****What fraction of ****(insert your favorite QCD dijet, W/Z, **
**direct photon, heavy flavor process here)**** events are**
**accompanied by a striking rapidity gap in the calorimeter?”**
**Your answer: “About ****ONE PERCENT****. Come here often?”**

## QCD Physics from DØ

**           ****QCD publications represent 25% of the 125 papers published by**
**                     ****D******** from its Run 1 data sample (100 pb****-1****)**
_**Jet physics**_
- “Transverse Energy Distributions within Jets at 1.8 TeV”
- “Studies of Topological Distributions of Inclusive Three- and Four-Jet Events at 1800 GeV”
- “Measurement of Dijet Angular Distributions and Search for Quark Compositeness”
- “Determination of the Absolute Jet Energy Scale in the D Calorimeters”
- “The Dijet Mass Spectrum and a Search for Quark Compositeness at 1.8 TeV”
- “The Inclusive Jet Cross Section in Collisions at 1.8 TeV”
- “Limits on Quark Compositeness from High Energy Jets in Collisions at 1800 GeV”
- “The ratio of Jet Cross Sections at 630 GeV and 1800 GeV”
- “Ratios of Multijet Cross Sections at 1800 GeV”
- “High-pT Jets at 630 and 1800 GeV”

_**Direct photon physics**_
- “The Isolated Photon Cross Section in the Central and Forward Rapidity Region at 1.8 TeV”
- “The Isolated Photon Cross Section at 1.8 TeV”
_**QCD with W and Z**_
- “A Study of the Strong Coupling Constant Using W + Jets Processes”
- “Measurement of the inclusive differential cross section for Z bosons as a function of transverse momentum produced at 1.8 TeV”
- “Evidence of Color Coherence Effects in W+Jets Events at 1.8 TeV”
- “Differential production cross section of Z bosons as a function of transverse momentum at 1.8 TeV”
- “Differential Cross Section for W Boson Production as a Function of Transverse Momentum in Collisions at 1.8 TeV
_**Rapidity gaps, hard diffraction, BFKL dynamics**_
- “Rapidity Gaps between Jets in Collisions at 1.8 TeV”
- “The Azimuthal Decorrelation of Jets Widely Separated in Rapidity”
- “Jet Production via Strongly-Interacting Color-Singlet Exchange”
- “Color Coherent Radiation in Multijet Events from Collisions at 1.8 TeV”
- “Probing Hard Color-Singlet Exchange at 630 Gev and 1800 GeV”
- “Hard Single Diffraction in Collisions at 630 and 1800 GeV”
- “Probing BFKL Dynamics in Dijet Cross Section at Large Rapidity Intervals at 1800 and 630 GeV”
- “Observation of Diffractively Produced W and Z Bosons in Collisions at 1800 GeV”

## D0 Hard Diffraction Studies

- Understand the Pomeron via ......

**Or W/Z**

## DØ Detector (Run I)

- EM Calorimeter

- L0 Detector

**beam**

*(**n*L0 = # tiles in L0 detector with signal
- 	2.3 < || < 4.3)

- Central Drift Chamber

*(**n*trk = # charged tracks with || < 1.0)

- End Calorimeter

- Central Calorimeter

*(**n*cal = # cal towers with energy above threshold)

- Hadronic Calorimeter

**Central Gaps**
-      EM Calorimeter
- 	ET > 200 MeV
-           	|| < 1.0
**Forward Gaps**
-      EM Calorimeter
- 	E > 150 MeV
- 	2.0 < || < 4.1
-      Had. Calorimeter
- 	E > 500 MeV	3.2 < || < 5.2

**Calorimeter tower thresholds**
_**used in rapidity gap analyses**_

**Three different rapgap tags**

****** ******** ******** ****= 0.1 ******** 0.1**

## Central Gaps between Dijets

** ****Two forward jets with |******** | > 1.9, ******** > 4.0**
**   ****and rapidity gap in |******** | < 1.0**
** ****Signature for color singlet exchange**
** ****Study gap fraction f****S**** as function of**
** ****Center of mass energy (1800, 630)**
** ****Jet E****T**
** ******
** ****Compare with  Monte Carlo color singlet models **
** ****Low energy run (630 GeV) very illuminating**

## DØ Central Gaps between Dijets

** ****2-D multiplicity (n****cal**** vs. n****trk****) between two leading**
**   ****jets (E****T**** > 12) for (a) 1800 GeV and (b) 630 GeV**
** ****Redundant detectors important to extract**
**   ****rapidity gap signal**
** ****Dijet events with ****one interaction**** per bunch crossing **
**   ****selected using multiple interaction flag**

- Negative binomial
- fit to “QCD”
- multiplicity

***f******S****** = (N******data******- N******fit******)/N******total***
- Rapidity gap fraction

## {

**Fraction of events with central gap**
**f****S****630**** =  1.85 ******** 0.09 ******** 0.37 %**
**f****S****1800**** = 0.54 ******** 0.06 ******** 0.16 %**
**                       ****(stat)   (sys)**
**Sys. error dominated by background**
**fit uncertainties**
_**Ratio (630/1800)**_
**f****S****630****/f****S****1800**** = 3.4 ******** 1.2**

- DØ Central Gaps between Dijets

**Gap fraction vs. second-highest jet E****T**

**Example of comparison to different**
**color singlet models**

- s = 1800 GeV

**Data**** ****consistent***** *****with a ****soft color rearrangement **
**model preferring initial quark states,**** ****inconsistent**** **
**with ****two-gluon, photon, or U(1)**** models**

**In the ****soft-color**** scenario, one can**
**extract a ratio of gap survival**
**probabilities **

- 1.5  0.1

## Hard Single Diffraction

-    -4.0        -1.6               3.0           5.2

- Measure minimum multiplicity here

- -5.2           -3.0  -1.        1.    3.0       5.2

- Two topologies

- Measure multiplicity here

**Central jets**

**Forward jets**

** ****Gap fractions (central and forward) at ********s = 630 and 1800 GeV**
** ****Single diffractive ********distributions**
** ****Comparisons with Monte Carlo to investigate Pomeron structure**
** ****Physics Letters B531, 52 (2002) **

**Jet E****T****’s > 12, 15 GeV**

## 2-D Multiplicity

**Forward jets**

**Forward jets**

**Central jets**

**Central jets**

**L0 scintillator**
**tiles used now**

**2-D Multiplicity**
**distributions**

## Hard Single Diffraction

- 1800 Forward Jets

-  Solid lines show show HSD candidate events
-  Dashed lines show non-diffractive events

-     _Event Characteristics_
-  Fewer jets in diffractive
-   events
-  Jets are narrower and
-   more back-to-back
-   (Diffractive events have
-    less overall radiation)
-  Gap fraction has little
-   dependence on average
-    jet ET

## Hard Single Diffraction

**Signal**

**Data**

**Background**

-          _Signal extraction_
-  Forward 1800 GeV example
-  Signal: 2D falling exponential
-  Background: 4 parameter
-   polynomial surface

-                                    _**Lessons**_
** ****Forward Jets Gap Fraction > Central Jets Gap Fraction**
** ****630 GeV Gap Fraction > 1800 GeV Gap Fraction**

- Publication compares data with POMPYT M.C. using different quark and gluon structures.
- This data described well by a Pomeron composed dominantly of quarks, or a reduced flux
- factor convoluted with a gluonic Pomeron with both soft and hard components.

## Hard Single Diffraction

-   0.2 for s = 630 GeV

- Prescription based on
- calorimeter energy deposition

- used to extract  distributions,
- fractional energy loss of
- diffracted beam particle

- Shaded bands indicate
- variance due to calorimeter
- energy scale uncertainties

## Double Gaps at 1800 GeV|Jet | < 1.0, ET > 15 GeV

**Double Gaps at ****1800 ****GeV****|Jet ********| < 1.0, E****T ****> 15 GeV**

- Demand gap on one side,
- and measure multiplicity
- on opposite side

**Gap Region 2.5<|********|<5.2**

## Double Gaps at 630 GeV|Jet | < 1.0, ET > 12 GeV    

**Double Gaps at ****630**** GeV****|Jet ********| < 1.0, E****T ****> 12 GeV    **

- Demand gap on one side,
- and measure multiplicity
- on opposite side

**Gap Region 2.5<|********|<5.2**

## Diffractively Produced W and Z

**Publication (hep-ex/0308032) accepted by Phys. Lett. B**

- Electron from W decay, with missing ET

- Rapidity gap

- 

- 

- Process probes quark
- content of Pomeron

- W  e
- Z  e+e-
- considered
- _and_
- require single
- interaction to
- preserve possible
- rapidity gaps
- (reduces available
- stats considerably)

## Diffractively Produced W’s

**L0**

**n****L0**

**n****cal**

- Plot multiplicity in 3<||<5.2 (minimum side)

- _Central_ and _forward_ electrons considered

**L0**

**n****cal**

**n****L0**

- 68 of 8724
- in (0,0)

- 23 of 3898
- in (0,0)

## Diffractively Produced Z’s

**n****cal**

**n****cal**

**L0**

**n****L0**

**n****cal**

- Plot multiplicity in 3<||<5.2
- (minimum side)

- 9 of 811
- in (0,0)

## Compare diffractive W characteristics to all W’s

**Standard W Events     Diffractive W Candidates**

- Electron ET

- Missing ET

- Transverse mass

- Good agreement
- given lower
- diffractive statistics

## {

-  Diffractive W/Z signals extracted from fits to the 2-D multiplicity distributions,
-   similar to hard single diffraction dijet analysis
-  Small correction to fitted signal from residual contamination from multiple interaction
-   events NOT rejected by single interaction requirement (based on # vertices, L0 timing)
-  Corrections due to jets misidentified as electrons and Z’s which fake W’s very small

**{**

- Opposite
- trend compared
- to hard diffractive
- dijet case

**Notes:**
Partonic models don’t describe the entirety of the data (give more Forward than Central)
Forward W isn’t described well by any normalization.
Soft Color seems to do best, but not evolved enough
CDF was 3.8sig signal, significant existence for Z even with few events

## W+Jet Rates

- It is instructive to look at W+Jet rates for rapidity gap
- events compared to POMPYT Monte Carlo, since we expect
- a high fraction of jet events if the Pomeron is
- dominated by the hard gluon NLO process.

**The W+Jet rates are consistent with a quark dominated**
**pomeron and inconsistent with a hard gluon **
**dominated one.**

## W/Z Cross Section Ratio

- where WD/W and ZD/Z are the measured gap fractions from
- this analysis
- and
- R=(W)/ (Z) = 10.43 ± 0.15(stat) ± 0.20(sys) ± 0.10(NLO)
- B. Abbott *et al.* (D0 Collaboration), Phys. Rev **D** 61, 072001 (2000)
- Substituting in these values gives
**                                  ****R****D ****= 6.45 + 3.06 -**** ****2.64**
**This value of R****D**** is somewhat lower than, but consistent with, **
**the non-diffractive ratio.**

## Diffractive W Boson 

- Diffractive W Boson ****

-  Sum over all particles in event:  those
-   with largest ET and closest to gap given
-   highest weight in sum (particles lost
-   down beam pipe at **–****** do not contribute
-  Use  only events with rapidity gap
-    {(0,0) bin} to minimize
-    non-diffractive background
-  Correction factor 1.5 ± 0.3 derived from
- Monte Carlo used to calculate 

- Most events  < 10%, mean is 5%

## OK, so it’s right – about 1%

**OK, so it’s right – about 1%**

**1.0%**

**Central**
**1800 GeV**

**Forward**
**630 GeV**

**Hard Single Diffraction**

**Central gap**
**1800 GeV**

**Central gap**
**630 GeV**

**Forward**
**1800 GeV**

**Central**
**630 GeV**

**All W**

**Forward W**

**Central W**

**All Z**

- Tomorrow, I’ll have a few words to say about
- augmenting this plot at LHC (CMS) energies