# Presentation: 353483

## Standard Model Higgs searches at Tevatron

- DooKee Cho
- Boston University
- For the CDF and DØ collaborations
- Aspen Winter Conference 2007
- Jan. 08 - Jan. 13 2007

## Tevatron Performance

- Record High:
**Peak luminosity: 227.5x10****30**** cm****-2****s****-1**
**Weekly integrated luminosity: 42.3 pb****-1****/week**
- Total delivered integrated luminosity: 2.2 fb-1

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## CDF & DØ  Detector Upgrades

- New central drift chamber and silicon tracker
- New forward calorimeters ("plug")  (1<||<3)
- New TOF, extended muon coverage
- improved tracking & secondnary vertex trigger

- New Silicon and Fiber tracker  in 2T magnetic field
- Added preshower detector in front of calorimeter
- Improved muon system
- New DAQ and trigger system
- Layer 0 detector is added
- New calorimeter track trigger system

**CDF**

**Upgrades enhance the ability to make sensitive searches**

**CDF**

- RunIIb upgrade

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Experimental Constraints

- The Higgs mechanism is the key of Electro-Weak symmetry breaking and gives masses to elementary particles, with its own mass unpredicted
- Direct searches at LEP2: mH > 114.4 GeV at 95% C.L.
- Indirect limit from fits to precision EW measurements from LEP, SLC, and Tevatron: mH < 166 GeV at 95% C.L. (< 199 GeV if LEP2 limit included)
- Indirect best fit value: 85 +39 -28 GeV at 68% C.L.
- A light Higgs is favored

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Higgs sensitivity

- The integrated luminosity (fb) required per experiment, to either exclude a SM Higgs at 95% C.L. or discover it at the 3s or 5s level; no systematics

-  No single channel guarantees  success
-  Combine two experiment’s  results
-  Improved understanding of   detectors is necessary
-  Need advanced analysis   techniques to maximize   sensitivity

- Tevatron entered sensitive region with 2 fb-1 already by the end of 2006

**LEP**

- 2009

- 2006

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## SM Higgs Production and Decay

**Dominant Decays**
****** ****bb     for M****H**** < 135 GeV**
****** ****WW********* for M****H**** > 135 GeV**

**Search strategy:**
**    ****M****H**** <135 GeV: ****associated production WH and ZH with H********bb decay**
**    ****Backgrounds: Wbb, Zbb, top, WZ,QCD****       ****complement with WWW********* & WW*******
**    ****M****H**** >135 GeV: ****gg ********H production with decay to WW*******
**    ****Backgrounds:  WW, DY, W/ZZ, tt, tW, ********    ****complement with WWW*******

**Production cross section (m****H**** 115-180)**
****** ****in the 0.8-0.2   pb range for gg ******** H**
****** ****in the 0.2-0.03 pb range for WH  associated vector boson production**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## WH  lbb

- Best Channel for low mass higgs
- Event signature:
**A high p****T**** lepton (e,********)**
**Large missing E****T**
**Two***** b*****-jets (********1 *****b*****-tag)**
- Backgrounds:_x0004_Wbb,Wcc,W+jets (mistags),Diboson,Top,Multijet

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## WH  lbb

- (pp -> WH)BR(H->bb) < 3.9 - 1.3 (pb)
- Higgs masses from 110GeV/c2 to 150GeV/c2 (95% C.L.)

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZHbb(or WH  lbb)

- Low mass higgs channel
- Event signature:
**Large missing E****T**
**Two***** b*****-jets (********1 *****b*****-tag)**
- Backgrounds:_x0004_W+h.f,Z+h.f,WZ,ZZ,top production, multijets,W+jets(mistag)
- Optimization: Kinematics
******(Leading Jet,E****T****)**
**H****T****/H****T****>0.45**
**Leading Jet E****T****>60 GeV**
**E****T****>75 GeV**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZHbb(or WH  lbb)

| Process | Signal Region(1tag) | Signal Region(2tag) |
| --- | --- | --- |
| Multijets | 10425 | 3.51.7 |
| Mistag | 11019 | 4.80.9 |
| Top | 284 | 5.51.1 |
| W+h.f | 368 | 2.91.4 |
| Z+h.f | 245 | 2.30.6 |
| Diboson | 7.11.1 | 0.60.1 |
| ZH(115 GeV) | 0.770.09 | 0.250.05 |
| WH(115 GeV) | 0.640.07 | 0.220.04 |
| Total Expected BG | 31040 | 19.63.1 |
| Observed | 333 | 24 |

- Double Tagged Candidate Event
- Jet Et1 = 100.3 GeV - TAGGED
- Jet Et2 = 54.7 GeV  - TAGGED
- Missing Et = 144.8 GeV
- Dijet mass = 82.1 GeV

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZHbb(or WH  lbb)

| Higgs Mass(GeV) | ZH Exp. | WH Exp. | Combined | Obs. |
| --- | --- | --- | --- | --- |
| 110 | 25.5 | 34.8 | 14.9 | 17.8 |
| 115 | 28.4 | 34.0 | 15.4 | 16.0 |
| 120 | 31.7 | 35.1 | 16.8 | 15.6 |
| 125 | 40.5 | 39.2 | 20.0 | 21.4 |
| 130 | 50.0 | 41.5 | 22.6 | 22.8 |

- (pp -> ZH) / SM < 17.8 - 22.8 (pb)
- Higgs masses from 110GeV/c2 to 130GeV/c2 (95% C.L.)

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZH  l+lbb

- Low mass higgs channel
- Event signature:
**A pair of high P****T**** lepton with an invariant mass constraint**
**At least two***** b*****-jets ****(********1 *****b*****-tag)**
- Backgrounds:_x0004_Z+jets,Zbb,top,WZ,                       ZZ,multijet

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZH  l+lbb

| 1 b-tagged jet (Mjj 60~110 GeV) |  |  |
| --- | --- | --- |
|  | e+e | + |
| Data | 126 | 99 |
| Bckg. | 111 22 | 88 18 |
| QCD
Z + jets
Z + bb
tt
ZZ
WZ | 4.4
82.4
17.8
2.0
2.2
2.3 | 0.4
75.5
16.0
1.8
1.4
2.0 |
| ZH 115 | 0.243 0.011 | 0.21 0.009 |

| 2 b-tagged jets (Mjj 60~110 GeV) |  |  |
| --- | --- | --- |
|  | e+e | + |
| Data | 8 | 10 |
| Bckg. | 9.8 3.4 | 8.7 3.1 |
| QCD
Z + jets
Z + bb
tt
ZZ
WZ | 0.25
3.7
3.8
1.3
0.11
0.75 | 1.8
3.7
3.4
1.2
0.07
0.65 |
| ZH 115 | 0.169 0.014 | 0.14 0.012 |

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## ZH  l+lbb

- No excess observed, so set cross section limits
- Modified frequentist approach (CLS), using di-jet mass distribution
- 95% C.L. on *ZH* cross section: 3.3-1.6 pb for mH=105-155 GeV
- Advanced Analysis (NN) and 12% more data in CDF improved the limits

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## WHWWW*ll’’+X (2 like-sign leptons)

**WH********WWW*********l************l********’********’+X ****(2 like-sign leptons)**

**Event Signature**
**Two high momentum isolated leptons of the same charge**
**Large missing E****T**
**Backgrounds:**
**physical: WZ/ZZ**
**instrumental: charge flips,multijet**
**Estimation of background composition**
**ee/********: fit invariant mass of two leptons Mjj to a weighted sum of distributions for WZ.ZZ,charge flips and multijet**
**e********:obtain track probability from ee/********, then multiply by the number of unlike sign**** events to get the number of like sign events due to charge flips**

**Low BR, but  channel important in the “intermediate region”    125-145 GeV**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## WHWWW*ll’’+X (2 like-sign leptons)

**WH********WWW*********l************l********’********’+X ****(2 like-sign leptons)**

**Effect on global combination significant in the intermediate region (125-145 GeV)**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## SM “Heavy” Higgs: H  WW*  ll

**H**

**4****th**** generation?**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## H  WW*  ll

** ****Selection Strategy:**
** ****Presection:****                               ****lepton ID, trigger, opposite charge leptons**
** ****Remove Multijet and Z********l****+****l****-****:****       ****E****T**** > 20 GeV **
** ****Reject events with mismeasured jet energy**
** ****Higgs Mass Dependent Cuts:**** ****Invariant Mass (M****l+l-****); Min. Transverse Mass            ****	****	****	****            Sum of lepton p****T****l**** and E****T**** (********p****T****l**** + E****T****)**
** ****Anti tt(bar) cut:****                       H****T**** = ******** P****T****jet**** < 100 GeV**
** ****Spin correlation in WW pair:****  ************(l,l) < 2.0**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## H  WW*  ll

**Expected/Observed # of events for m****H**** = 160 GeV (L ~ 950 pb****-1****):**

**SM  only a factor 4 away **
**We exclude 4****th**** generation models,  for which       m****H****=150-185 GeV **

|  | Data | Sum BG | WW | W+jet/ | Z/ | top | WZ/ZZ | Multijet | HWW* |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| e,e | 10 | 10.30.6 | 7.00.2 | 1.40.6 | 0.00.0 | 1.10.1 | 0.80.1 | 0.060.02 | 0.415 |
| e, | 18 | 24.41.5 | 16.40.1 | 5.31.5 | 0.020.01 | 2.10.1 | 0.60.1 | 0.10.05 | 0.97 |
| , | 9 | 9.80.8 | 6.60.1 | 1.00.4 | 0.60.4 | 0.50.1 | 0.50.1 | 0.60.6 | 0.35 |

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Combining Higgs boson Searches

**Combining Higgs boson Searches**

**	****D****Ø ****uses the CLs (LEP) Method**
**	****     **the CLS confidence interval is a normalization of CLS+B
- 	      CLS+B = signal + bkgd hypothesis,  CLB = bkgd only hypothesis
- 	         CLS = CLS+B/CLB :      CLS+B & CLB are defined using a “test statistic”
-     Test statistic used is the Log-Likelihood Ratio (LLR=-2 ln Q)
-     generated via Poisson statistics** ****(Q=e****-(s+b)****(s+b)****d****/e****-b****b****d****) s,b,d=sig.,bkd,data)**
**   ****Tevatron Higgs combination is done with CLs and cross checked with Bayesian approach******** they give results compatible within 10%.**

## CDF & DØ SM Higgs Limits

- We present limits in terms of R=95% CL limit/SM
- R1 indicates model exclusion

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Tevatron SM Higgs Limits

- At the time of this combination,CDF added:
-                 /               /                 at 1 fb-1
- DØ added: HWW(ee,e,) at 1 fb-1
- Systematics very similar in size, most treated as uncorrelated between DØ and  CDF
- Robs:10.4 at mH=115         3.8 at mH=160
- Rexp:  7.6 at mH=115         5.0 at mH=160
- With asymmetric inputs(0.3-1.0 fb-1), we can extrapolate limits to 1 fb-1:
**R****exp****: 6.0 at m****H****=115****        4.0 at m****H****=160**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Sensitivity Prospects 

- Advanced analysis techniques (NN,ME) provide factor of ~1.7 in equivalent luminosity
- New channels (taus,HZZ) will increase the sensitivity
- Many systematics currently limited with statistics

| Ingredient (D0) | Equiv Lumi
Gain@115 | Xsec Factor
mH=115 GeV | Xsec Factor
mH=160 GeV |
| --- | --- | --- | --- |
| Today with 1fb-1 | - | 6.0 | 4.0 |
| Lumi=2 fb-1 | 2 | 4.2 | 2.8 |
| NN b-tagging | 3 | 2.4 | 2.8 |
| NN Analyses | 1.7 | 1.9 | 2.1 |
| Improved mass resolution | 1.5 | 1.5 | 2.1 |
| New Channels | 1.3 | 1.3 | 1.8 |
| Reduced systematics | 1.2 | 1.2 | 1.7 |

- R~1 when integrated lum     ~3 fb-1         ~5.5 fb-1

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Conclusion

- At Tevatron, SM Higgs analyses are in very exciting time period
**Increasing dataset (2 fb****-1**** now and expecting more data) **
**Better understanding background description**
**Both experiments understood detectors very well**
**More advanced analyses**
- The combination limits of CDF and DØ are very encouraging
-   Tevatron is the best place for Higgs search right now and we are getting close to the sensitivity needed to have evidence for alight Higgs boson !!!

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Backup

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07

## Systematic Uncertainties @ CDF

**Systematic Uncertainties @ CDF**

- DooKee Cho/Boston University - Aspen 2007

- 1-9-07