# Presentation: 529368

## Low-mass Higgs Searches at the Tevatron

**Ben Kilminster**
**Ohio State University/CDF**
**for CDF/D0**

**4 New results**** in H->bb channels :**
**ZH-> ********bb**
**	****D0  - 0.3 fb****-1**
**	****CDF - ****Update**** ****from 0.3 to 1 fb****-1**
**WH-> l********bb**
**	****D0  - 0.4 fb****-1**
**	****CDF - ****Update**** from 0.8 to 1 fb****-1**
**ZH->llbb**
**	****D0  - ****New ****Channel !****   ****0.4 fb****-1**
**	****CDF - ****New ****Channel !****   1 fb****-1**

- L=**1 fb****-1**

- L=**1 fb****-1**

- L=**1 fb****-1**

## Mass ~ Inertia : how hard it is to move free quark or lepton

Mass caused by transition between left-handed fermion to right-handed particle via Higgs field, H0
For instance, top quark mass, Mt :

- Mass caused by transition between left-handed fermion to right-handed particle via Higgs field, H0
- For instance, top quark mass, Mt :

- Standard Model  mass generation via Higgs

- Mt = tR <H0> tL

## What we know about Higgs

- Required Higgs boson not yet discovered !!

- “Standard Model”   (SM)
- Simplest Higgs mechanism possible
- Higgs is 1 particle
- H
- spin 0
- electrically neutral
- interacts with all SM particles
- couples more strongly with higher mass particles

- SM not wrong yet !

- Expected Higgs Mass

- LEP Direct :
-  MH > 114 GeV @ 95%
- New CDF/D0 top mass (174.1  2.1 GeV)  & new LEP W mass (80.392  0.029 GeV)
-  MH  = 85 +39 -28 GeV
-  MH < 166 GeV @ 95 % CL

- LEP EWWG

- Low mass Higgs  Favored !!

## What we know about Higgs

- Production (pp @ 1.96 TeV c.o.m.)

- Low mass region:
- MH < 135 GeV
- H  bb dominates
- WH & ZH - easier to identify than gg -> H

- Decay by mass [GeV]

- Decay

- Excluded

**68% CL **

**95% CL **

- Most
- likely
- MH

## Fermilab’s Tevatron

- World’s highest-energy particle collisions
- ~4 miles circumference protons-antiprotons
- 2 multi-purpose detectors:
-             D and CDF
- Run I (1992-1996)
-   s = 1.8 TeV
- Integrated luminosity 120 pb-1
- Run II (2001-present)
-   s = 1.96 TeV
- Integrated luminosity by July ‘06:
- Delivered > 1.6 fb-1
- Higgs analyses use up to 1 fb-1
- Design goal of 8 fb-1 by 2008

- 1 fb-1  delivered May 2005

- Good slope
- after
-  shutdown!

- July, 2006

## Review of low mass Higgs channels

## B-Tagging Techniques

- Positive tag (right side)

- Interaction point
- primary vertex

**L****xy ****> 0**

-  All channels have 2 jets originating from b quarks
-  Require one or both to be “b-tagged”

- 2nd vertex

**L****xy ****< 0**

- 2nd vertex

- Interaction point

- Charm-jets and mistagged jets can be controlled by
- strictness of cut on  **L****XY**** / ********XY**

- “Mistags” of tagged light-quark jets can be understood from “negative tags”

- Algorithm exploits long b lifetime and large mass to look for displaced vertices or tracks with impact parameter

## B-Tagging Techniques at CDF

- Can improve purity with a
- Neural Network trained to
- discriminate b from c and light jets

- B-Tag Efficiency

- Light quark mistag rate

- (Positive Tag)

- (Negative Tag)

## Identifying bb resonances : D0

- Z-> bb
- H->bb benchmark
- Can be used to determine b-jet energy scale
- New D0 analysis finds evidence for Z->bb in dijet data
- Background derived from data
- 1168 Events in peak  (300 pb-1)
- MZ = **81.0 ******** 2.2 GeV** measured
-  **83 ******** 2 GeV** expected (from MC)

## ZH ->  bb

- Di-jet QCD

**Most difficult background:**

- Tev’s most sensitive Channel

## ZH MET+bb at CDF

- ZH MET+bb at CDF

- MET in QCD control region:
-  no leptons
-  met close to second jet

- Mjj in EWK control region:
-  one lepton
-  met away from second jet

-  Mjj Signal region:
-  no leptons
-  met away from second jet

- Improvements : (S/√B)2=6.3 gain in Lum.
-   Includes WH -> lbb   ( lepton not detected)
-   Improved EWK lepton veto
-   Dijet mass fit separately 1-tag, 2-tags
- ZH / SM  = 14   for  MH : 115 GeV

- #
- Leptons

- (MET, J2)

- L=**1 fb****-1**

## ZH->  bb D0

- ZH->  bb D0

- Instrumental background (from energy mismeasurement) in signal region understood by parameterization of Met

- Result:  Dijet mass fit
- in 1 b-tag & 2 b-tags
-     L = 261 pb-1
- ZH   < 3.4 pb
- for MH : 115 GeV

## WH -> l  bb

- WH lbb

- 2 b jets  ~ 50 GeV each
- 1 lepton  ~ 40 GeV each
- Missing ET ~ 40 GeV
*WH Highest production X-sec*

**Most difficult background:**
**W+bb jet production **

## WH->l bb CDF

- Variety of b-jet identification scenarios
- optimized to find the best a priori limit
-  BEST :  Separate 1-tag + NN-tag
-                 and 2-tag scenario

- Result:  Dijet mass fit
- WH   < 3.4 pb
- for MH : 115 GeV

- L=1 fb-1

## WH->lbb D0

- Result:  Dijet mass fit
- in 1 b-tag & 2 b-tags
-       L = 378 pb-1
- ZH   < 2.4  pb
- for MH : 115 GeV

## ZH -> l+l- bb

- ZH l+l- bb

- 2 b jets  ~ 50 GeV each
- 2 leptons  ~ 40 GeV each
- Z mass constraint
*Cleanest signal*

## ZH->llbb D0

- New analysis with 389 pb-1 (Z->ee), 320 pb-1 (Z->+-)

- Dijet mass before b-tagging

- Dijet mass after 2 b-tags

| Process | before b-tag | after 2 tags |
| --- | --- | --- |
| Z+bb | 17 | 3 |
| Z+jj | 937 | 5 |
| ttbar | 11 | 4 |
| ZZ+ZW | 26 | 0.6 |
| QCD | 44 | 0.6 |

- ZHM=115    = 0.1 evts

- Total BKG  : 13 evts

- Result:  Dijet mass fit
- ZH   < 7.9 pb  (Z->ee)
- ZH   < 11 pb  (Z-> )
- for MH  = 115 GeV

## 2D Neural Network trained to separate Signal from  Background
Z+jets  vs.  ZH “x” axis   (85% BKG)
ZH  vs.  ttbar “y” -axis      (8% BKG)
Optimized design with 9 inputs

- Z+jets  vs.  ZH “x” axis   (85% BKG)
- ZH  vs.  ttbar “y” -axis      (8% BKG)
- Optimized design with 9 inputs

- ZH->llbb CDF Method

## ZH->llbb  CDF results

- (1,0)

- (1,1)

- (0,0)

- ZH

- TT

- Z+j

- ZZ, ZW

- Fakes

- ZH vs TTBAR axis

- Results in Data : ee,  combined

- Expected :  103 +- 17
- Observed :  104 events

- Result:  Entire 2D distribution  fit
-     Brand new result :  1 fb-1
- ZH   < 2.2 pb  @ 95% CL
- for MH : 115 GeV

- L=**1 fb****-1**

- Note: ZH * 5  !

## Summary of Observed limits

| Analysis | CDF limit  @ MH = 115 GeV
(factor above SM) | D0 limit 
(factor above SM) |
| --- | --- | --- |
| ZH -> bb | Includes WH (miss lep)
(14) | 3.4 pb
(41) |
| WH -> lbb
 | 3.4 pb
(23) | 2.4 pb
(16) |
| ZH -> llbb | 2.2 pb
(27) | 6.1 pb
(75) |

## Summary

- CDF/D0 fully exploring all Low Mass Higgs
- ZH -> l+ l- bb channel added by both CDF and D0
- CDF has updated WH-> l bb, ZH -> llbb with 1 fb-1
- Experimental techniques providing factors of equivalent luminosity

- 4

- 8

- fb-1

- 0

- Now

- Projected Luminosity

- 2009

-  ***400***

- Limits will improve with luminosity and smarts !
- 	4 - 8 fb-1  can find us a light Higgs

## BACKUPS

## Higgs: ZHllbb

- Higgs: ZHllbb

## Summary

- CDF & D0 Preliminary

## SM / MSSM Compatibility

- If MSSM is theory, is it worth looking for SM Higgs ?
- For  MA   > 200 GeV
-  light MSSM Higgs  h behaves like SM Higgs
- Wh and Zh couplings same as WH and ZH
- H branching ratios same as h
- SM searches valid
-  If only one Higgs accessible at Tevatron/LHC, LC may be required to distinguish SM from MSSM  (Carena, Haber, Logan, Mrenna Phys.Rev.D65 055005, 2002)
- MA < 200 GeV
- For large tan   (> 3),  SM-like Higgs is suppressed
- Discovery potential mainly in MSSM
- Small tan , SM searches valid

## CDF sees Zbb decays in Run 2

- 	Double b-tagged events with no extra jets and a back-to-back topology are the signal-enriched sample: Et3<10 GeV, 12>3
- 	Among 85,784 selected events CDF finds  3400±500 Zbb decays
- 		- signal size ok
- 		- resolution as expected
- 		- jet energy scale ok!
- 	This is a proof that we are in business with small S/N jet resonances!
- 	***CDF expects to stringently constrain the b-jet energy scale with this dataset***