# Presentation: 950478

## Search for Leptoquark Production at  DÆ

**Vladimir Sirotenko**
- Northern Illinois University
- for
- The DÆ Collaboration
- QFTHEP’97, Samara, Russia

## Outline

**   ****(LQ) at Hadron Colliders**
**Previous DØ Measurements**
**Results on:**
**3rd Generation LQ (LQ3)**
**2nd Generation LQ (LQ2)**
**Results on 1st Generation LQ,**
**    ****new, optimized for high masses : **
**(LQ1) (LQ1) ****® ****eeqq**** (****b****=1)**
**(LQ1) (LQ1)**** ****®**** ****e****n****qq**** (****b**** =1/2)**
**Mass limit vs Beta -**** ****m****LQ1****(****b****)**
**Summary/Conclusions**

**Outline**

## Introduction

**Leptoquark  Phenomenology**
**Leptoquarks (LQ) appear in many SM **
**  ****extensions to explain symmetry between**
**  ****leptons and quarks**
**LQ are coupled to both leptons and quarks**
**  ****and have SU(3) color, fractional electrical**
**  ****charge, baryon B and lepton L numbers**
**LQ interactions are fixed by the effective**
**  ****Lagrangian in the assumptions of B and L**
**  ****numbers conservations, gauge invariance,**
**  ****renormalizability and chiral couplings**
**LQ can have spin 0 (scalar) or 1 (vector)**
**LQ appear in 3 generations corresponding **
**  ****to 3 lepton/quark generations, mixing is**
**  ****restricted by FCNC data**
**LQ BR into charged leptons can be 0, 1/2 **
**  ****or 1 only   **

## LQ Production at the Tevatron

**LQ Production at the Tevatron**

***At the Tevatron leptoquark pair production via gluon splitting (strong interactions) dominates***

***Pair production is insensitive to the value of ******l****** since it enters only in the LQ lifetime and for   ******l****** > 10******-12****** LQ’s decay within a collider detector***
***It depends only on *****b (μ b****2 *****for lljj, *****μ 2b(1-b) *****for l*****n*****jj and *****μ (1-b) ****2**** *****for *****nn*****jj)***
***It has very little model dependence (such as PDF choice, etc.) similar to top pair production***

## Next-to-Leading Order Theory

**Next-to-Leading Order Theory**

***NLO theory - add three diagrams to include *****a*****S******3***

***Krämer, Plehn, Spira, and Zerwas, hep-ph/9704322***

***LO previously ***
***used by DO***
***Blümlein et al.***

***+15 GeV***
***shift***

**s*****NLO******(200 GeV) = 0.18 pb***

## Past Results: 1st Generation

**PR D48, 3939 (1993):   113 GeV for ****b ****= 1            **
**                                        ****80 GeV for ****b ****= 1/2**
**     **_**DO**_
**PRL 72, 965 (1994):     130 GeV for ****b**** = 1 **
**                                      ****116 GeV for ****b**** = 0.5**
**  **_**HERA (H1)**_
**   ****Z.Phys. C64, 545 (1994):  230 GeV for ****b****=0.5****                                 **
**Recently presented:**
**    **_**DO**_**  ****(red #’s are vector LQ with Y-M couplings)**
**Moriond ‘97: ****	****     175/****298**** GeV for ****b**** = 1        **
**                                  ****147/****270**** GeV**** ****for ****b**** = 1/2**
**                                           ****81 GeV**** ****for ****b**** = 0**
**    **_**CDF**_**   ****(switched to NLO cross section)**
**Vanderbilt ‘97,**
**Fermilab seminar 5/31/97: 210 GeV for ****b ****= 1**

**Past Results:**** 1**_**st**_** Generation**

**eejj**

**eejj**
**e****n****jj**

**eejj**

**eejj**
**e****n****jj**
**nn****jj**

## Past Results:2nd Generation

**PRL 75, 1012 (1995):   131 GeV for ****b**** = 1            **
**                                        ****96 GeV**** ****for ****b**** = 1/2**
**   **_**DO**_
**PRL 75, 3618 (1995):   119 GeV for ****b ****= 1 **
**                                        ****97 GeV for ****b ****= 1/2**
**Recently presented:**
**   **_**CDF**_
**Moriond ‘97:                197 GeV/c****2**** for ****b**** = 1 **
**                                    **
**   **_**DO**_** ****(with different Q****2****-scale for LO cross section)**
**Moriond ‘97:                167 GeV/c****2**** for ****b**** = 1**
**                                    **

**mm****jj**

**Past Results:****2**_**nd**_** ****Generation**

**mm****jj**
**mn****jj**

**mm****jj**

**mm****jj**

## Past Results:3rd Generation

**PRL 78, 2906 (1997):  99/****225**** GeV for q=4/3 or 2/3**** **
**                                      **
**Recently presented:**
**	****   **_**DO**_** ****(with different Q****2****-scale for LO cross section)**
** ****Moriond ‘97:           80 GeV for q=1/3**
**                                          **
**                                    **
**   **

**tt****jj**

**bb****n****t****n****t**

**Past Results:****3**_**rd**_** Generation**

## D0 Detector

***D0 Detector***

**Pseudorapidity:**
**h**** = -ln tan (****q****/2)**

***3 Level ***
***Trigger System***

## Triggers:   2m + 2jets    (ptm > 3.5)   L=60.1 pb-1

**	**** ****Run 1      1****m + 2****jets    (pt****m**** > 3.5)     ****L=19.5 pb****-1**
**	****	****	****       ****1****m + 2****jets****	**** (pt****m**** > 15 )     ****L=92.4 pb****-1**
**Jets tagged with ****m**
**D****R(****m-****jet) < 0.5  and  E****T****(jet) > 10 GeV**
**jet cuts for 1****m ****sample**
**Z****m**** ****= ****pt****m**** / H****T ****< 0.2****	**** H****T****=****S(****E****T****J****+ E****T****m****)**
**F****m**** = E(R****m**** ****< 0.4) / E(R****m**** ****< 0.6) **
**Jet untagged **
**E****T****(jet) > 25 GeV**
**Topological cuts:**
**“****nn****”, E****T**** > 35 GeV**
**DF ****( jet-E****T**** ) > 0.7**

**3rd Generation Leptoquarks**

**charge = 1/3, ****n****t****n****t****bb**

## 3rd Generation Leptoquarks

- no DF cut

## 3rd Generation Leptoquarks

- Data

## 3rd Generation Leptoquarks

**Signal Acceptance**
**2% at M****LQ3 ****= 100 GeV**
**3-5% at higher LQ3 masses**
**Major Background**
**Top :       ****	****  1.5 **±** 0.5****	**** **
**W/Z + 2jets :     1.1**±** 0.5**
**QCD multijets :  negligable **
**Data:****	****                     2 events****Expected Background:  2.6+/- 0.7**

## 3rd Generation Leptoquarks

**M****LQ3**** > 98 GeV/c****2**** , M****VLQ3**** > 202 GeV/c****2**

**NLO Theory Paper: **
**  ****Krämer, Plehn, Spira, and Zerwas, hep-ph/9704322 (1997)**

- V

- S

## ZH Production Limit

**The same data as were used for LQ3 search,**
**the same signature (****nn****bb) :  ****Z ****->**** ****nn, H -> ****bb**
**acceptance is about 3%**

***Current limit is “only” 500 times higher, ***
***next Run II can catch up  this factor***

## 2nd Generation Leptoquarks

**mm****jj search**
**Data: 1994-1996 Tevatron run (94.4 pb****-1****)**
**Event Selection**:

- 2 isolated muons:
- pT > 15 GeV, |h| < 1.0, DR(m,j)>0.5

- Df(m1,m2)  < 160o  if
- | hm1 + hm2 | < 0.5

- HT(jets) > 100 GeV

**0 events survive**

- 2 Jets: ET > 15 GeV, |h| < 2.5

- Z-kinematic fit  P(c2)< 1%

- Maximum f gap < p
- between leading muons and jets

- Mmm > 10 GeV

-  Standard
- tt**®**dimuon
-  selection

-   Top
- rejection

## 2nd Generation Leptoquarks

**Data Sample = 94.4 pb****-1**
**Data : 0 events ; Exp Bkgd: 0.9 ± 0.2 events**

**NLO Theory: Krämer, Plehn, Spira, and Zerwas, **
**hep-ph/9704322 (1997)**

**M****LQ2**** > 184  GeV/c****2**** at   ****b = 1.0**

**M****LQ2**** > 140  GeV/c****2**** at****   ****b**** ****= 0.5**

## First Generation Leptoquarks (LQ1)

**eejj search**
**Changes since Moriond ‘97:**
**New NLO theory**** ***
**The Moriond 175 GeV lower limit for**** ****b****=1 ****becomes 190 GeV**
**Improved particle ID**
**Use TRD info and 5-parameter e likelihood**
**Loose initial cuts, e.g. one “loose” electron**
**2 times the previous acceptance**
**Optimization at higher m****LQ****(200 GeV)**
*******Krämer, Plehn, Spira, and Zerwas,  hep-ph/9704322 (‘97)**

## Electron ID Efficiency Improvement

**Electron ID Efficiency Improvement**

**Factor of two improvement in efficiency by**
**using both ****“tight”**** and ****“loose”**** electrons**

***“******tight”-“tight” sample***

***“******tight”-“loose” sample***

## Kinematic Properties of LQ decay

**Kinematic Properties of LQ decay**

***LQ Kinematic properties (M******LQ****** = 225 GeV)***

## LQ1 - Search Strategy 

## LQ1 - Event Selection

**                    ****eejj search**
**123 pb****-1**** of data ****(entire Tevatron Run 1)**
**Trigger is 99.5**±**0.5% efficient for m****LQ1****=200 GeV**
**Offline:**
**Two EM objects,  E****T**** > 20 GeV, **
**  ****|****h****EM****| ****< 1.1 or 1.5 <**** |****h****EM****| ****< 2.5**
**EM objects used to recalculate the vertex **
**event objects E****T****’s recalculated from new vertex**
**Two jets with E****T**** > 15 GeV,**** |**** h****|****< 2.5,**** ****cone radius = 0.7**
**Exclude events in Z boson mass window: 82 - 100 GeV**
**                ****101 events**

## LQ1 - Optimization

**                     ****eejj search**** **
**101 candidate events - “base sample”**
**Backgrounds:**
**Drell-Yan    66.8 **± **13.4**
**QCD            24.3 **±**  3.6**
**Top                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 were obtained for S****T****:**
**S****T ****= H****T****jet**** ****+ H****T****e**
**   ****H****T****jet****= ****S**** E****T****jet****( if > 15 GeV) ; H****T****e**** =****S**** E****T****e**** **
**S****T ****> 350 GeV maximizes the signal for LQ masses of 200 to 220 GeV/c****2**** above a total background of 0.44 +/- 0.06 events.**

## LQ1 - Optimization

- Take **each point** of
- the signal class as
**a cut-point**

- Number of cut-points
- equals the number of
- Monte Carlo signal
- events available.

**Random Grid Search (RGS)**

## Optimal Cuts

**Optimal Cuts**

***The S******T  ******cut is about 20% more efficient for the signal compared to the *****d*****M/M(LQ) variable***

***About 50 different combinations of variables were tried. The S******T****** was shown to be the single most effective variable for backgrounds of about 0.4 events***

***Background***

***Signal***

***The best cut:***
***S******T****** > 350 GeV***

**                     ****eejj search**** **
**(101 events)**
**                                        ****integral****                **
**                ****cut                                                    cut****                      ****LQ****differential**** **
** **
**S****T****>350 leaves no events in our candidate sample**

## LQ1 - Signal Efficiency

**                    ****eejj search**** **
**Derived from ISAJET Monte Carlo + **
**DØ detector GEANT simulation.**
**36% at m****LQ1****=225 GeV**

##                eejj search

-                **eejj search**
**For m****LQ1****=(120,260) GeV**
**Energy scale                         ****2-11%**
**Electron identification          ****5%**
**Acceptance                             ****5%**
**Gluon radiation                     ****7%**
**PDF & Q****2 ****scale                      ****7%**
**Luminosity                              ****5%**
**Monte Carlo statistics            ****2%**
**total                               13-16%**

## Neural Network Analysis

***Training:***
***Signal: LQ(200 GeV) MC***
***Mixture of QCD, D-Y and Top backgrounds***

***Output: ***
***NN discriminant 0 < D******NN****** < 1 maximized for the signal***

***The ******D******NN****** > 0.95****** cut corresponds to similar to the RG analysis background. Overall efficiency is slightly better than that of the RG analysis. Both analyses are highly correlated with each other and result in essentially the same LQ mass limits***

**H****T****e**** = ****S****E****T****e**
**H****T****j  ****= ****S****E****T****j**

## Neural Network Analysis

**Neural Network Analysis**

***Results of the NN-based optimization***

***D******NN****** = 0.95***

***D******NN****** = 0.90***

***D******NN****** > 0.95***

***RG Analysis***
***S******T****** > 350 GeV***

***D******NN****** = 0.80***

***D******NN****** = 0.70***

***D******NN****** = 0.60***

***Last event has***
***D = 0.92***

***H******T******j***

***H******T******j***

***H******T******j***

***H******T******e***

***H******T******e***

***H******T******e***

## eejj search

**To be published in Phys. Rev. Letters**
**hep-ex/9707033 and Fermilab-Pub-97/252-E**

**eejj search**

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

## LQ1 - Mass Fit (MC) 

**3 constraint (3c) mass fits:**
**Energy balance and equal LQ masses**
**Monte Carlo studies**
** ****m****LQ1 ****= 225 GeV**

## LQ1 - Mass Fit (data)

**S****T**** > 0 GeV**** **

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

## LQ1 - ST vs mLQ1

**S****T**** > 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**

**eejj search**

## LQ1 - one LQ®nj

**e****n****jj search****   **
**Changes since Moriond ‘97:**
**New NLO theory**
**Improved particle ID**
**Optimization at higher m****LQ1****(180 GeV)**
**115 pb****-1**** of data (entire Tevatron run 1)**
**Trigger is 99.5**±**0.5% efficient**
**Offline:**
**1e,**** ****E****T****>20 GeV****, |****h****|****<1.2 or 1.5< ****|****h****|****<2.5**
**2j, E****T****>20 GeV****, |****h****|****<2.5**
**E****T**** > 30 GeV, min**** DF****(j,E****T****) < 0.25**** **
**	****	****	****(reject QCD “fakes”)**
**M****T****e****n ****>110 GeV (reject W’s)**
**Exclude****	**** events with isolated muons**
**RGS: confirms E****T ****and M****T****e****n ****cuts**
**14 events are left after those cuts**

## LQ1 - one LQ®nj

**E****T**** accolinearity cut effect on the**
**(a) LQ signal; (b) QCD background**
**dots - before M****T****e****n**** cut, boxes - after**

## enjj search

**e****n****jj search**
**Additional variables - optimization:**
**S****T****12**** ****= H****T****jet12**** ****+ E****T****e****+E****T**
**  ****where H****T****jet12**** = E****T****jet1**** +  E****T****jet2**
**d****M = min(| M****ej**** - m****LQ1**** | / m****LQ1**** ) for jets 1,2 and m****LQ1 ****= 120 - 220 GeV**** with 20 GeV step**
**New since HCP’97 - use Neural Network with cut D****NN ****> 0.85**
**Strategy - maximize the signal for LQ masses of 120 to 220 GeV/c****2**** above a total background of about 0.4 events**
**Results: ****no events left**
**Expected background (Top,W+2jets,QCD):**
**0.30**** **±** ****0.26**
**Signal efficiency:**
** ****3.9% **±** 0.9%   for     m****LQ1****=140 GeV**
**14.7% **±** 1.1%    for    m****LQ1****=180 GeV**
**18.9% **±** 3.8%    for    m****LQ1****=220 GeV**

## LQ1 - one LQ®nj

**Comparison of the (a) M****T****e****n**** and (b) ST****12**
**distributions for data (solid) and total**
**MC background (dashed)**

##                enjj search

-                **e****n****jj search**
**For m****LQ1****=(100,220) GeV**
**Energy scale                         ****2-16%**
**Electron identification          ****5%**
**Acceptance/Smearing           ****5%**
**Gluon radiation                     ****4%**
**PDF & Q****2 ****scale                      ****5%**
**Luminosity                             ****5%**
**Monte Carlo statistics           ****2%**
**total                               10-19%**

## enjj search

**e****n****jj search**

**Data events:****	****14**
**Background events:****	****18 ****±**** 2**
** ****D****NN   ****dependance**

## enjj search

**e****n****jj search**

## enjj search

**e****n****jj search**

**(Preliminary)**
**   ****M****LQ1**** > 175 GeV/c****2****  for  ****b**** ****= 0.5**

## enjj search

**e****n****jj search**

***Basic idea: to use known probability***
***densities of several variables to estimate***
***“******LQ mass” in a candidate events***

## enjj search

**e****n****jj search**

***Comparison of kinematic properties of 14***
***events in a base sample with that expected***
***for signal and background***

## eejj+ enjj search

**eejj+**** ****e****n****jj search**

**(Preliminary)**
**M****LQ1**** > 204 GeV/c****2****  for  ****b**** ****= 0.5**

## First Generation LQ1b vs mLQ1

**eejj+e****n****jj+****nn****jj search**
**(Preliminary)**
** **
** **
** **
** **
** **
** **
** **
** **
** **
** **
**eejj - ****b****2**** ; e****n****jj - 2****b****(1-****b****) ; ****nn****jj - (1-****b****)****2**

## LQ1 - Mass Limit

**Tevatron Search**
**(Unofficial!)**
**Combining DØ and CDF results      needs to be done carefully by experts from both collaborations taking into account correlations**
**Unofficially the limits are:**
**~240 GeV     for ****b****=1**
**~220 GeV    for ****b****=1/2**

## Summary

**DØ has set new lower limits on scalar Leptoquark masses using NLO theory and new optimized searches at 95% CL:**
**First Generation (LQ1):**
**225 GeV****       ****for**** ****b=1****(eejj) ****- submitted PRL**** **
**204 GeV****        ****for**** ****b=1/2**** ****(****eejj/e****n****jj****) ****- ****prelim.**
**Second Generation (LQ1):**
**184 GeV     for**** ****b=1**
**140 GeV     for**** ****b=1/2**
**Third Generation (LQ1):**
**98 GeV       for**** ****b=0**
**Our lower limit on m****LQ1**** rules out a LQ1 interpretation of HERA data within (most) existing models**