# Presentation: 658515

## Search for Leptoquark at D  using Neural Networks

- Silvia Tentindo-Repond
- Florida State University
- for the
- D**** Collaboration
- Workshop on Neural Computation in
- High Energy Physics
- Maale Hachamisha, Israel
- Oct 13-15, 1999

## Outline

- Introduction
- Brief overview of Tevatron and DØ
- Leptoquark Phenomenology
- Leptoquark Searches
- First Generation Searches
- Second Generation Searches
- Results and Conclusions

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Leptoquarks (LQ)
Fractional electric charge
Color charge
baryon and lepton number
can be scalar or vector particles

- Fractional electric charge
- Color charge
- baryon and lepton number
- can be scalar or vector particles

- First Generation

- Second Generation

- Third Generation

- Experimental limits on rare decays and flavor changing neutral currents imply that leptoquarks don’t mix generations

**Introduction**

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Leptoquark Searches at Dzero

- DØ has conducted several searches for Leptoquarks for the last several years
- Searches considered :
- electron channels (eejj ; eυjj)
- muon channels  (μμjj ; μνjj)
- neutrino channels (ννjj)
- scalar and vector couplings
- Motivation for recent searches
- HERA 97 - Excess reported

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## HERA Excess

- Observation of high Q2 events in e+p collisions at HERA, by both H1 and ZEUS
- H1: 7 events (1 pred.) *Z.Phys.****C74**** (1997) 191*
- ZEUS: 4 events (0.9 pred). *Z.Phys.****C74**** (1997) 207*
- Possible interpretation as LQ1
- LQ1 mass ~ 200 GeV

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Experimental Apparatus  The Tevatron

- Fermilab Tevatron
- proton - antiproton collider
- center-of-mass energy        = 1.8 TeV

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## The DØ Detector

_**Tracking**_
- || < 3.5
- (vertex) = 6mm
- (r) = 60m (VTX)
-           = 180 m (CDC)
-           = 200 m (FDC)

_**Muon Spectrometer**_
- || < 3.3
- (1/*p*)=0.18(*p*-2)/*p*2  0.003

_**Calorimetry**_
- || < 4.0
-  = 0.1  0.1
- (EM) = 15%/E
- ()     = 50%/E
- (jet)   = 80%/E

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Leptoquark  Phenomenology

**Leptoquark  Phenomenology**

- At the Tevatron, Leptoquark would be dominantly pair-produced via strong interactions
- decay within the detector
- Pair production process insensitive to the coupling 
- decay parametrized by β  ( branching ratio to charged leptons)

- pp  LQLQ 	(-j)(+j) 	2
- 		 (±j)(j)  	2(1 - 
- 		 (j)(j)   	(1 - 2

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Leptoquark Cross Sections

- NLO Scalar

- LO Vector

- at 200 GeV/*c*2 :* *slq  = 0.16 pb	 YM =  10 pb
-                           MV =  2 pb	 Min =  0.6 pb

- Krämer, *et. al., *
- PRL **79**, 341
-  (1997)

- J. Blümlein, *et. al.,*
- Z. Phys. C** 76**, 137 (1997)

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Search Strategy 

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Neural Networks

- Neural networks used in this search:
- feed-forward
- back-propagation
- supervised learning
- single output
- single hidden layer
- sigmoid transfer function
- JETNET 3.0 (Lund)
- adapted to D0 Software environment
- ( P. Bhat, H.Prosper, C.Stewart )

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarksee + jets

- Accept events with
- ETem > 20GeV, |eem|  1.1 OR 1.5  |e|  2.5)
- nj(ET  15 GeV, |e|  2.5)  2
- R(ej)  0.7
- VETO (82 GeV/c2  mee  100 GeV/c2 )
- electron ID
- 101 events remaining from 123 7 pb-1

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First generation Leptoquarks (Conventional Analysis) 

- 101 candidate events - “base sample”
- Backgrounds:
- Drell-Yan    66.8 **± **13.4
- QCD            24.3 **±**  3.6
- ttbar               1.8 **±**  0.7
- Total            92.8 **±** 13.8
- Optimization - use kinematic parameters of MC events for signal and MC/data events for background to develop optimal set of cuts
- Best results obtained with cuts on ST:
- ST = HTjet + HTe
-     HTjet=  ETjet( if > 15 GeV) ; HTe = ETe
- ST > 350 GeV maximizes the signal for LQ masses of 200 to 220 GeV/c2 above a total background of 0.44 +/- 0.06 events.

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarks (Conventional Analysis) 

***Signal Systematic Errors***

- Most effective cut

- Main Backgrounds:  Drell-Yan + 2j, multijet, and Top

- LQ Signal Efficiency 36% at 225 GeV/c2

**Events / 25 GeV**

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First generation Leptoquarks Neural Network Analysis

***Training:***
***Signal: LQ(200 GeV) MC***
***Mixture of QCD, D-Y and Top backgrounds***
***Set desired output  -> 1 for signal***
***                               ******-> 0 for background***

**H****T****e**** = ********E****T****e**
**H****T****j  ****= ********E****T****j**

**First Generation Leptoquarks****(Neural Network Analysis)**

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarks ee + jets

**First Generation Leptoquarks**** ****ee + *****jets***

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarks ee+jets Mass Fit (MC) 

-  3 constraint (3c) mass fits:
- Energy balance and equal LQ masses
- Monte Carlo studies
-   MLQ1 = 225 GeV

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarks ee+jets Mass Fit (data)

-  ST > 0 GeV

- blue points - data
- solid black line - background
- red dashes  - 225 GeV/c2 LQ sample

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation LeptoquarksST vs Mfit

- ST > 0 GeV
-     B                                       S
-     Data

**LQ1 Mass    #events expected**
**                     ****with S****T****>350 GeV**
**     ****160                     20**
**     ****200                       6**
**     ****225                       3**
** ****No evidence for LQ signal is  seen in mass analysis as well**

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarks ee+jets Mass Limit

**M****LQ1**** > 225 GeV/c****2**** ,********=1.0**

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarkse + jets

- Final selection made using a 2 input Neural Network

- Network inputs

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarkse + jets

*D**NN* > 0.85 gives 0.41  0.270 to 0.54  0.25 expected background.

- gives 0.60  0.27 expected background

- _Signal Efficiencies_

- 14.7  1.1 % mslq = 180 GeV/*c*2

- Backgrounds W+2j, multijet, tt

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## First Generation Leptoquarkseejj + eν + νν combined results

***D0*** Limits on First Generation leptoquarks

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Second Generation Leptoquarks + jets 

- Separate signal and background
*S*CM  1.5 (1 + 2)
- 1 < 2 < 3 are normalized eigenvalues of
*D*NN selection

*D*NN = output of a 7151 Neural Network

- Inputs :

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Second Generation Leptoquarks + jets 

- Again
- careful examination of the distributions shows no evidence for a signal
- apply a selection on a neural network which maximizes the significance

- 491 NN with inputs

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Second Generation Leptoquarks Exclusion Contours

**Summary of 2****nd**** Generation Leptoquark Mass limits at DØ**

**Second Generation Leptoquarks ****Exclusion Contours**

- Combine limits at various values of 
- obtain mass limits at each point
- exclusion contours in  vs. *m*slq plane

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999

## Conclusions

- DØ has performed LQ searches in all 3 decay channels
-  + *jets* with complete data in progress
- Tevatron Combined Limit (DØ & CDF)
- (ee + *jets*   = 1)  242 GeV/*c*2

- First Generation

- Second Generation

- Phys. Rev. Lett. **79** 4321 (1997)
- Phys. Rev. Lett. **80** 2051 (1998)
- PRD in progress for complete search

- 2 PRLs nearing submission

- Silvia Tentindo-Repond  NCHEP99

- Oct 13-15, 1999