# Presentation: 351730

## Observation of Diffractively Produced  W- and Z-Bosons

_**Observation of Diffractively Produced  W- and Z-Bosons**_

- LISHEP02
- February 5, 2002
- Rio de Janeiro, Brazil

**Andrew Brandt**
**University of Texas, Arlington**

****

****

- E

## Diffraction

## Slide 3

## CDF Diffractive W

## CDF Diffractive W

- CDF {PRL 78 2698 (1997)} measured RW = 1.15 ± 0.55%
- where RW = Ratio of diffractive/non-diffractive W
- a significance of 3.8

## DØ Detector 

- Central Calorimeter

- End Calorimeter

- Hadronic Calorimeter

- EM Calorimeter

- L0 Detector

**beam**

- nL0 = #  hit tiles in L0 detector

- ncal = # cal towers with energy above threshold

- 		**Energy Threshold****	****	******** coverage** EM Calorimeter	150 MeV			2.0<||<4.1 Had Calorimeter	500 MeV			3.2<||<5.2

_**DØ Detector**_** **

## Data Samples

**Central and Forward electron W Event Selection:**
-          Start with Run1b W    e candidate sample

**Z  Event Selection:**  Start with Run1b Z   ee candidate sample

- DØ Preliminary

## Signal Measurement

**Use topology of events to look for the  diffractive W and Z signal**
**Measure forward calorimeter tower multiplicities above energy threshold in the range 3.0<|********|<5.2**
**	****EM cal threshold = 150 MeV**
**	****HAD threshold = 500 MeV**
**	**
- Look for low multiplicity events on minimum
- multiplicity side of detector** - Rapidity Gap**
**(**electron not necessarily opposite side of
- detector from rapidity gap)

- W POMPYT electron  distribution (mean= -0.43)
-  with a gap at positive 

## Central W Multiplicity

**Peak at (0,0) indicates diffractive W-boson**
**Signal:  ****68 of 8724 events in (0,0) bin**

**n****cal**

**L0**

**n****L0**

**n****cal**

**n****cal**

**n****L0**

- DØ Preliminary

## Forward W Multiplicity

**Peak at (0,0) indicates forward  diffractive **
**W-boson in forward electron sample:**
**23 of 3898 events in (0,0) bin**

**L0**

**       ********        ****-2.5   -1.1 0 1.1    3.0        5.2**

****

- Minimum side

_**Forward W Multiplicity**_

**n****L0**

**n****cal**

- DØ Preliminary

## Z Multiplicity

**Peak at (0,0) indicates diffractive Z-boson:**
**9 of 811 events in (0,0) bin**

_**Z Multiplicity**_

**n****cal**

**n****L0**

- DØ Preliminary

## Central W Event Characteristics

- DØ Preliminary

**E****T****=35.27**

## Signal Measurement 

**2-D Fitting of multiplicity**

**Fitting Method**:
-     1) Fit 2D multiplicity distribution once with
- simultaneous signal and background fit.

- 2) Combine several individual fits into
-  **RANGE FIT** method - systematically vary
- bins used in fit and average results
- 3) Use **RANGE FIT** method in 2 ways:
-     a) signal and background from **same sample**
-     b) **Alternate Background Sample** -
-        background shape from high statistics sample
-        ***Important for low statistics samples***

## W Multiplicity Fit

- Data

- Fit

- Fit Background

- Fit Signal

- Residuals

- DØ Preliminary

## Use High Statistics Background

**Solid line:**** Z****             Dashed: ****Cen+Fwd W**** sample**

**Solid line: ****Central W****    Dashed: ****Cen+Fwd W**** sample**

**Background shapes agree, but fit more reliable with higher stats**

**n****cal**

**n****cal**

- DØ Preliminary

## Multiple Interaction Contamination Correction

- Difference between number of events after single interaction
- cuts and number of predicted single interaction events is
- Residual contamination correction:
-  **9.2% + 5.4% - 5.7% **(increases gap fraction)

- = Predicted SI events

- DØ Preliminary

## Data Results

**Sample****	****         Diffractive                  Probability Background**
**                                        ****All                  Fluctuates to Data **
- Central W       (1.08 + 0.21 - 0.19)%      1 x 10-13     **7.7******
- Forward W     (0.64 + 0.19 - 0.16)%      6 x 10-7      **5.3******
- All W	         (**0.89 + 0.20 – 0.19)%**
- Z	         (1.44 + 0.62 - 0.54)%      5 x 10-5      **4.4******

*******Observed**** clear ****Diffractive W and Diffractive Z**** signals**
*******Measured**** Diffractive W/All W and Diffractive Z/All Z**

- DØ Preliminary

## Monte Carlo Rates

**Calculate diffractive W and Z fractions predicted by the Monte Carlo to compare to data:**
- 1) Determine diffractive W and Z fraction for each pomeron model independent of detection efficiency.
- Pomeron Structure: Quark, Hard Gluon, Soft Gluon
- 2) Combine MC diffractive fractions with appropriate gap efficiencies to get visible fraction.
- 3) Compare final visible fractions to data fractions.

- f visible = f predicted ·gap

## MC Diffractive Fractions

-  Find predicted rate  POMPYT·2 / PYTHIA
- 	*Factor of 2 since only antiproton allowed to diffract
- 	*Apply same cuts as data
- 	*Full detector simulation 	(error statistical)

**Sample             Quark ****	****      Hard Gluon      Soft Gluon**
**Central  W**      (20  1)%    (0.45  0.02)%    (0.10  0.01)%
**Forward W****     (**21  2)%    (0.61  0.04)%    (0.38  0.02)%
**Z****	**** **           (17  1)%   (0.45  0.02)%    (0.13  0.01)%

**(Pion exchange **predicts diffractive fraction on the order of 10-31 ** ****for W and Z)**

**NOTE:****	*****Quark pomeron model highest fraction**
**	*****Soft gluon model smallest fraction**
**	*****Pion exchange zero**
**	**

- DØ Preliminary

## Rate Comparison

- Correct MC for gap efficiency 20-30% for quark and hard gluon (soft gluon fractions <0.02%)

**	****           ****FINAL GAP FRACTION**
**Sample              ****Data****	****	**** ****Quark ****	****        ****Hard Gluon****       Cen W**  (1.08 + 0.21 - 0.19)%  (4.1  0.8)%  (0.15  0.02)%
**For W   **(0.64 + 0.19 - 0.16)%  (7.2  1.3)%  (0.25  0.04)%
**Z ****	**(1.44 + 0.62 - 0.54)%  (3.8  0.7)%  (0.16  0.02)%

**NOTE****: Observe well-known normalization problem for all structure functions, also different dependence on ******** for data and MC, as in dijet case**

- DØ Preliminary

## W+Jet Rates

**  ****Jet ****E****T**** ****	****Data****        ****Quark      Hard Gluon**
- >8GeV** **              (10 ± 3)%       14-20%                89 %
- >15GeV	             (9 ± 3)%         4-9 %                  53 %
- >25GeV              (8 ± 3)%         1-3 %                  25 %

- 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.**

- DØ Preliminary

##  Extraction

- Determine  distributions using calorimeter :

******data**** ******

**E****T****i****e****y****i**
**  ****2E**

- *Sum over all particles in event - those with largest ET 	and closest to gap given highest weight in sum.
- * Rapidity gap defined to be at **+******
- *Test method in MC first
- 	-first at particle level
- 	-next after detector simulation
- *Finally, apply method to DATA

## Particle Level MC  Test

- Compare ******calc** measured from all particles ** to ********true** from proton

**Z**

**W**

**Slope = 1.0 +- 0.1****   ****=>    ********calc**** = ********true**

- Quark

- Hard gluon

- Quark

- Hard gluon

- DØ Preliminary

## Detector Level MC  Test

**Z**

**W**

**Slope****W**** = 1.5 +- 0.3           Slope****W**** = 1.6 +- 0.3**
- Note: values >1 compensate for missing energy of neutrino

**Slope****Z**** = 1.0 +- 0.1**

_**Detector Level MC**__****__**  Test**_

- After detector simulation: use same calorimeter method available in data

- Quark

- Hard gluon

- DØ Preliminary

## Diffractive W DataDistribution

**Calculate **** ******** for W-boson data events****:**
***only use  events with rapidity gap ****{****(0,0) bin}   **
**minimizes non-diffractive background**
*******correction factor**** ****1.5+-0.3****  derived from MC used to calculated data ******

**Most  events have****  ********<0.1 (note MC used only to calculate correction factor)**

****

- DØ Preliminary

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

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

**Gap Region 2.5<|********|<5.2**

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

- DØ Preliminary

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

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

**Gap Region 2.5<|********|<5.2**

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

- DØ Preliminary

## Summary

**  ****New definitive observation of Diffractive **
**   ****W-boson signal:   R****W**** = (0.89 + 0.20 – 0.19)%**
**  ****First observation of Diffractive Z-bosons**
**   **
-    Diffractive W shows similar characteristics to
-     non-diffractive ones.
-   Pomeron based MC does not predict **magnitude **
**   ****or ******** dependence of results**
** **Double gap events observed at 630 and 1800
-   GeV, final results soon