# Presentation: 480427

## Higgs Physics at LEP

- Jesús Marco
- IFCA (CSIC) Santander, SPAIN
- Workshop on the Future of Higgs Physics
- Fermilab  3 May 2001

## Higgs Physics at LEP

- Search for the SM Higgs at LEP 200
- Experimental *personal* reflections:
- Achievements!
- Collaborating
- Results, Exciting Events & Discussions
- Latest references
- Work going on!

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Search for the SM Higgs at LEP 200

- LEP: e+e- collider at CERN
- Four experiments: ALEPH, DELPHI, L3, OPAL
- LEP1: 1989-1995, s=91 GeV, measurement of  Z boson parameters
- LEP2: 1995-2000, s =130-209 GeV
- Objective in LEP200: “Find the Higgs”
- Which “Higgs”? SM Higgs
- After LEP precision measurements, only “way”?
- Mass range:
- After LEP1, from direct searches (Z mH>60 GeV/c2
- Delimited by triviality and vacuum stability arguments
- Indication from precision measurements

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## SM Higgs production at LEP200

- Higgstrahlung
-    (dominant)

**e****+**

**e****-**

**Z*******

**H**

**Z**

**b**

**b**

**f**

**f**

*Kinematical limit: *
*m** H**< ****s - M**Z*

-   +  Fusion
- (WW, no kinematical limit)

- Range explored: 60115 GeV/c2

- 50 fb x 200 pb-1~10 events/experiment

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Signal topologies

- Four-jet channel:
- Zqq  Hbb
- 70% x 80% > 50%
- Kinematics & b-tag

- & background...

- ZZ “irreducible background”
- Zqq//ll  Zbb
- 1 pb (x 30% if bb)
- “The Reference”

- Missing energy channel:
- Z  Hbb
- 20% x 80% > 15%
- Energy flow & b-tag

- Leptonic channels:
- Zll(e/  Hbb
- Zqq  H    (70% x 7%  ~ 5% )
- Lepton id (& b-tag)

- WW background
- WWqqqq   WWqq
- 18 pb
- “No b-tag (except Vcb)”

- Two fermion (Z background
- ISR (single/double)
- QCD:  qq (g)(g)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Achievements!

- Performance of the LEP accelerator
- The detectors (in particular Si vertex detectors)
- B-tagging
- Kinematical reconstruction
- ZZ reference
- Background modelling
- Multidimensional techniques
- Statistical methods

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## LEP accelerator

- Excellent performance!
- Very clean environment

| year | ‘95 | ‘96 | ‘97 | ‘98 | ‘99 | 2000 |
| --- | --- | --- | --- | --- | --- | --- |
| s | 130-136 | 161-172 | 183 | 189 | 192 196 200 202 | 204 205 207 208 |
| Lum
(pb–1) | 3    3  | 11  11 | 55 | 160 | 25  80  80  40  | 9   72  130  8   |
| Lum 
x4 exp | 24 | 88 | 220 | 640 | 900 | 875 |

- >2.5 fb-1  @ Ecm>180 GeV

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## B-tagging

- From “offsets” to the use of secondary vertex information in multidimensional methods
- Excellent WW rejection reached, but tail control is critical!

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## ZZ: irreducible background, but also reference channel

- First events observed at s=183 GeV
- Good agreement with SM prediction
- B-tag involved

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Analysis example: four-jet channel

- Preselection:
- Hadronic events and rejection of ISR
- “Spherical” (not two jets back to back)
- Jet reconstruction
- Jet Algorithm (Durham)
- Jet b-tag
- Kinematical fits ( 4-C, imposing E,p conservation, or 5-C including constraint on MZ, (M)~3 GeV/c2)
- Discriminant variables:
- antiQCD: Emin x min  sphericity (H2 +H4), min(Mij) min(cos ij +cos kl)
- antiWW: b-tagging, ideogram probability PWW (used in WW analysis)
- antiZZ:  PZZ
- Build a single discriminating variable (NN)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Pairing and mass reconstruction

- six possible pairings:

| Z dijet | (1,2) | (1,3) | (1,4) | (2,3) | (2,4) | (3,4) |
| --- | --- | --- | --- | --- | --- | --- |
| H dijet | (3,4) | (2,4) | (2,3) | (1,4) | (1,3) | (1,2) |

- For each pairing, make a 5C fit with Mij =MZ and build a likelihood  including the probability that the two other jets come from the Higgs.
- Most likely combination is selected!
- A unique mass value is  defined

- Could we gain by using the mass info
- of all possible pairings?

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- 3 May 2001

## 2-D estimation of s/b

- Discriminating variable (NN/likelihood/btag) vs Mass
- Distributions for signal (given hypo) and background

- M H

- M H

- NN

- Signal

- Background

-   Careful binning and smoothing

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Confidence Level Estimation

- Counting vs weighted counting:
- Counting (Poisson): select a subset after relevant cuts in the input variables, assuring “good” signa/background separation; estimator is “number of events”
- Weighted counting: assign a weight to each event, and “add” them to get the estimator (no need for relevant cuts!)
- Likelihood ratio:
-               Log(Q) = - S + log ( 1 + s/b )

-  B only

-  S+B

-    Inputs: DATA and MC (background and given signal hypothesis) sets of events (or pdf distributions:)

- wi

- Confidence levels are defined from these distributions, for the background only (discovery estimator) and signal+background hypothesis. Conservatively, CLs=CLs+b/CLb used for limits.

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Collaborating

- A complex and delicate chain!:
- Detectors (hardware) operating at 90-95% eff.
- Realignment/reprocessing/b-tag tuning needed
- Hot-line chain operating in parallel
- Delivering “preliminary” results...
- ...but discussing on its significance and analysing event by event features (including stability)
- Higgs research teams involved many people:
- From the MC generation...
- ...to the final CL computation

- It takes some time until, at some stage, the team is more interested in knowing the truth than in finding the Higgs...

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Combining: the LEP Higgs WG

- The competition/collaboration with other experiments is even more complex
- Statistical combination:
- Inputs preparation: pdf for data/MC background and signal for each mH hypo
- Independent experiment/channel/Ecm set
- Inclusion of systematic errors
- ALRMC method used
- Cross-checks and extra info (1999 & 2000):
- Mass plots (after cut on the discriminant variable when possible, i.e., if the reconstructed mass not used, to use a common integrated S/B in the plot)
- List of relevant candidates
- “Control” plots of variables used in the analysis

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- 3 May 2001

## 2000 Results

- Combined limit on SM Higgs from previous  years:
- 						mH>108 GeV/c2
- Results by 20th July (LEPC):
- 50 pb-1 @205 GeV / expt
- 25 pb-1 @207 GeV / expt
- combined limit mH>113.3 GeV/c2
- Only the kinematical limit zone (“high mass”) not excluded...

| Ecm | (m H=115) | #events/expt |
| --- | --- | --- |
| 205 | 30 fb | ~ 1.5 |
| 207 | 64 fb | ~ 1.5 |
| 208 | 86 fb | ~ 0.5 |
| TOTAL |  | ~  4 events |

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## The first candidate

- Four-jet (bbqq)
- M=114 GeV/c2

- s/b|mH=115=4.7
- bbgg (QCD)? unlikely:
- cosmin  = -1.98 (but NN correlated to mass!)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## four-jet bbbb candidates

- Two strong candidates, m=113, 110 GeV/c2
- “Only” possible background: ZZ (+wrong pairing/undetected ISR)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Evolution of new candidates

- LEPC 5th sept:
- Excess seen by ALEPH for m~114, reflected in combination (2.2 )
-  Right after LEPC, DELPHI reevaluated s/b weights thanks to increased MC statistics, and a 4j candidate with m~114 increased s/b due to its b-tag (even if kinematically compatible with WW)
- LEPC fest (october)
- New combination, excess is 2.6 .
- Reevaluated inputs from DELPHI + new inputs from L3 & Opal
- LEPC 3rd November
- Last public combination, excess is 2.9 
- ALEPH: increased  luminosity
- DELPHI: after realignment/reprocessing the candidate drops after loosing SV
- OPAL: no strong candidate but many low s/b
- L3: new H candidate: m~114, s/b~2
- LEP stop decision

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## H candidate

- Two well b-tagged jets
- m~114.4 GeV/c2
- ~3 GeV/c2)

- Two fermion background is critical at the kinematical limit:
- double ISR, and bb events (when the neutrinos take most of the energy), give collinear topologies; for an event at rest, the mass recoiling to a Z is pushed to s-MZ
- While, due to the Z width, even in the Higgstrahlung close to the kinematical limit, the H is not usually produced at rest

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Publications 

- Individual experiments results with 2000 data:
**ALEPH** (CERN-EP/2000-138): “Observation of an excess in the search for the Standard Model Higgs Boson at ALEPH”
**excess of events compared to what expected from SM background, suggesting a Higgs boson with mass m~114 GeV/c****2**
**DELPHI** (CERN-EP/2001-4): “Search for the Standard Model Higgs Boson at LEP in the year 2000”
**No evidence for any Higgs signal, limit set m****H****>114.3 GeV/c****2**
**L3** (CERN-EP/2000-140): “Higgs Candidates in e+e- interactions at s=206.6 GeV”
**few candidate events compatible with the Higgs hypothesis**
**OPAL** (CERN-EP/2000-156): “Search for the Standard Model Higgs Boson in e+e- collisions at s=192-209 GeV”
**Excess over a broad mass range**

-  LEP combined result (3rd Nov. presentation)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## LEPHWG

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Event weights distribution

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Estimators for mH=115 GeV/c2

- By experiment

- By channel

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Combination

- 2. 9 effect
- mH>113.5 GeV/c2
-          (115.3 expected)

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## Which Higgs?

- Lightest MSSM Higgs boson  “h”
- Results in the MSSM framework released as Winter Conference notes (LHWG note 2001/2)
*LEP 220 (additional cavities) would have covered the whole plane and solve this hard region *

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## MSSM 

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001

## What is being done...

- Other channels (no experimental discrepancy observed?):
- H+H-
- invisible Higgs
- fermiophobic Higgs
- flavour independent analysis
- Prevision for final “papers”:
- ALEPH ending SM paper.
- DELPHI final LEP2 data reprocessing started.
- L3 data reprocessed + additional MC statistics
- OPAL: reoptimization of analysis
- Preparing the “final” LEP combination:
- LEP Higgs Workshop (Evian 11-12 may)
- Channels/models/scans  revisited

- J.Marco   HIGGS SEARCHES AT LEP

- 3 May 2001