# Presentation: 827127

## Electroweak Sector of the Standard ModelEPS Conference, HEP2007 23rd July 2007Presented byTerry Wyatt (University of Manchester)

- With my thanks to:
- EW parallel session organizers
- Giuseppe Degrassi, Jorgen D'Hondt, Chris Hays
- >40 speakers in 14.5 hours of EW parallel sessions
- special thanks to Martin Gruenewald
- Scientific secretaries
- Tim Coughlin, Tammy Yang

## Introduction

- At tree level EW theory determined by three “input” parameters
- Most precisely known:
- , GF, mZ

-    Add QCD: s

## Loops

- EW observables then depend on:
- , GF, mZ, mt, mH
- Basic programme:
- Measure precisely L and R couplings of each fermion to , Z, W
- Measure precisely boson self-interactions
- Measure precisely s, , GF, mZ, mt, mW
- Test consistency of measurements with SM predictions
- Find the Higgs!

## Running of 

- Uncertainty dominated by had(q2)
- effect of qq loops at low q2
- Cannot be calculated from first principles in pQCD
- Can be related to 0had(s) by dispersion relation
- Experimentally accessible by:
- direct scans
- radiative return

## Data on 0had(s) 

- CMD-2 at Novosibirsk                    •   KLOE at Frascati
- CLEO at Cornell: Rhad = had/
-                                                                                      + new results from Babar
-                                                             expected soon
-                                                             (radiative return)

-  new!

-  new!

## had(MZ2)

- _Precise_ determination of had(MZ2) is a tricky business!
- combination of results from many (sometimes old) experiments
- treatment of correlated systematics, radiative corrections
- close collaboration between expt. and theory essential!
-                                                                currently used by LEPEWWG

## g-2

- Hadronic corrections also dominate uncertainty in g-2
- a = (g-2)/2
- Comparison of a**
- in the range 0.630-0.958 GeV
- Recent analyses confirm discrepancy at ~3.4 sigma level
- aexpt - atheory = (27.6 ± 8.1) ·10-10
- theory uncertainty (slightly) smaller than expt.

-  new!

## 30th June 2007: The end of an ERA!

## 30th June 2007: The end of an (H)ERA!

- Integrated luminosity

- polarized electron beam
- Pe typically 30-40%

- up quark couplings

## Results with Polarized Beams at HERA 

- Charged Current (CC)                            Neutral Current (NC)
-         exchange of W±   (e± p   X)                      exchange  and Z0   (e± p  e± X)
- left-handed e+        and     right-handed e-
-          do not interact via W±!
- Directly tests the EW model at large _negative_ Q2
- Still a factor ~2 more HERA-2 data to be analyzed

## The Fermilab Tevatron Collider

_**The Fermilab Tevatron Collider**_

- 1992-95 Run I:
-      ~ 100 pb-1, 1.8 TeV
- Major accelerator/detector upgrades
- 2002-06  Run IIa:
-       ~ 1.6fb-1, 1.96 TeV
- Further upgrades
- 2006-09  Run IIb:
-       ~ 6-8 fb-1

**CDF**

**DØ**

- ~85% data taking efficiency

## Signatures of W and Z Production at the Tevatron

- Z: pair of charged leptons:
- high pT
- isolated
- opposite-charge
- W: single charged lepton:
- high pT
- isolated
- ETmiss (from neutrino)

## W Mass and Width from CDF

_**W Mass and Width from CDF**_

- 200 pb-1 data set
- Many handles to calibrate tracker and calorimeter pT scale and resolution
- J/,, Z, E/p in We
-     dead material

## W Mass and Width from CDF

- mW = 80413 ± 34(stat) ± 34(syst) MeV
- Major systematics:
- lepton pT scale and resolution, QED, PDFs
-                    CDF chooses to treat CTEQ6 error sets as 1.6 uncertainty

## W Mass and Width from CDF

******W**** = 2032 ******** 71 (stat + syst) MeV    (350 pb-1)**
**c.f. indirect measurement from cross sections:**
**       ********W**** = ****2092 ± 42 MeV  ****(stat + syst) MeV (72 pb-1)**

-    Major systematics:
-   lepton pT resolution ~30 MeV
-    recoil model ~50 MeV
-    backgrounds ~30 MeV
-    uncorrelated between electron
-       and muon channels

## Watch Out for the Theoretical Uncertainty!

## W Mass

- Final LEP2 MW awaits:
- final combination of colour reconnection limits
- final MW combination

- mW = 25 MeV

## Di-Boson Physics at the Tevatron

- Data consistent with SM
- do not yet require “radiation zero”

- W

- DØ

## Di-Boson Physics at the Tevatron

- Z

## WZ and ZZ

## Top Quark Pair Production and Decay at Tevatron

- Production:      85%                             15%
- Decay
- Final state determined by decay of the two Ws
-                                   “lepton+jets”
-                                              (e or ),  ,  Wqq,  two b-jets
-                                                   30% of tt decays
-                                                   moderate background
-                                                      usually yields most precise measurements

- CDF measurement

## A Rich Programme of Top Physics to Explore!

- a

## Production Cross Section and Br(tbX)

- Use additional kinematic discriminant in 0-tag bins to enhance tt
- Simultaneous fit to tt and R

**0.9 fb****-1**

## Production Cross Section and Br(tbX) 

-                                                                                  tt / tt (%)
- DØ preliminary (900 pb-1)
- tt **= 8.08****+0.85****-0.82**** (stat+syst) ****±**** 0.49 (lumi) pb       ****               ****12%**
- CDF preliminary (1120 pb-1)
- tt **= 8.2 ****±**** 0.5 (stat) ****± 0.8 (syst)**** ****± 0.5 ****(lumi) pb  ****                ****13%**
- SM prediction
- tt **= 6.7 ****±**** 0.8 pb  ****                                                                  ****12%**
**                                                     **tt and R consistent
-                                                      with SM expectations

## W Helicity in Top Decays

- Test left-handed tWb coupling
- *: angle between l± momentum in W rest frame and W momentum in top rest frame

**left handed**
**SM:    f****-****=0.3**

** ****longitudinal**
**           ****f****0****=0.7**** **

**right handed**
**f****+****~10****-4**

## W Helicity in Top Decays

**Simultaneous fit:**
**f****0**** = 0.74 ± 0.25(stat) ± 0.06(syst)**
**f****+**** = -0.06 ± 0.10(stat) ± 0.03(syst)**
**Fixing f****0**** = 0.7**
**f****+**** = -0.06 ± 0.06(stat) ± 0.03(syst)**

-  l+jets
- di-lepton

**Fixing f****0**** = 0.7**
**f****+**** = 0.017 ± 0.048(stat) ± 0.047(syst)**

-  l+jets

## Top Mass Measurement

- Example Technique: Matrix Element (ME)
- Form event probability *P**evt*
- Where *P**sgn* is the probability to observe *x* for given values of *m**top* and *JES* (Jet Energy Scale calibration factor)
- Integrate over all unmeasured quantities and experimental resolutions
- Fit simultaneously *m**top* and *JES *
- using mW constraint

- transfer
- function

- from ME

- PDFs

- b-tag
- weights

## Top Mass Measurement

- Dominant systematics
- ISR
- FSR
- PDFs
- b-jet energy scale
- b fragmentation
-                                              ***m**top **/** **m**top**~1%*
- Is *m**top* in Monte Carlos used by experiments the same as *m**top* (pole) used in the EW fits?
- e.g., colour reconnection effects?

- CDF
- l+jets

## Evidence for Single Top Production

- Top pairs:                         tt **= 6.7 ****±**** 0.8 pb **
- Single top:
- Motivation
- Top pairs
- can measure only ratio of couplings to kinematically allowed final states
- Single top
- s+t  |Vtb|2
- |Vtb|2 can be determined with assumptions of 3 generations, unitarity

- Vtb

## Backgrounds to Single Top 

_**Backgrounds to Single Top **_

- s+t only a factor of two lower than tt
- signal event signature less pronounced
- fewer high pT objects
- Backgrounds much more of a challenge!
- W+jets poorly understood
- especially W+heavy flavour
- considerable tuning of MC to data required
-                   (after b-tag)

## Kinematic Discriminants

- Even after b-tagging signal swamped by background
- Use multivariate kinematic discriminants
- e.g., Likelihood, Matrix Element, Neural Networks, Boosted Decision Trees
- Validate on “background-enriched” sub-samples
- “W-like” (low total visible ET)       “tt-like” (very high total visible ET)

- DØ

## DØ Evidence for Single Top

-        Decision Tree          Matrix Element          Bayesian NN
- Consistent excess seen in all three analyses

## DØ Evidence for Single Top

- Combined result
- s+t consistent with SM
- 3.6 significance
- 2.4 expected significance
- First direct measurement of |Vtb|
- Assuming standard model production:
- Pure left-handed coupling
- |Vtd|2 + |Vts|2 << |Vtb|2
- Additional theoretical errors needed (top mass, scale, PDF etc...)
- Measurement does not assume 3 generations or unitarity
**                 ****|V****tb****| = 1.3 ± 0.2      or        0.68 < |V****tb****| < 1 at 95%CL**

## CDF Single Top Results

- Likelihood

- Neural Networks

- No evidence of signal
- s+t<2.7pb at 95% C.L.
- Expected signal significance 2.0

- No evidence of signal
- s+t<2.6pb at 95% C.L.
- Expected signal significance 2.6

- p-value = 1.0% (2.3)
- s+t=2.7(+1.5/-1.3)pb
- Expected signal significance 2.5

- Matrix Element

- 1.2% of pseudo-experiments fluctuated as unluckily as the observed data

## mW and mt (direct)

- Important parameters of SM
- Real interest is to compare with other information

## Precision EW Data from LEP/SLC

- Asymmetries measure:
- Longstanding issue:
- “leptonic” and “hadronic” asymmetries consistent only at ~3 level
- (may not be resolved until the ILC!)

## mW and mt (direct and indirect)

- Powerful consistency check of SM
- Decreasing mW is the priority!

## mW and mt (compared to mH)

- Data prefer light Higgs
- Decreasing mW is the priority!

## Limits on mH

- mH = 76+33-24 GeV
- mH < 144 GeV (95%CL)
- Direct search limit (LEP-2):
- mH > 114 GeV (95%CL)
- Probability MH>114 GeV:
- 15%
- Renormalise probability
-   for mH>114 GeV to 100%:
-  mH < 182 GeV (95%CL)
**So let’s find it!**

## The Crucial Observables

- Fit to all measurements but excluding:
-              had(MZ)
-                                                      Mtop
-                                                                                         MW
- Yields MH < 140 GeV (95%CL)
- cf MH < 144 GeV (95%CL) including external had(MZ)

## Searches for the SM Higgs Boson at the Tevatron

- Higher cross section
-  - but can only distinguish from backgrounds with HWW decay
- “The Higgs is underneath the needle in the haystack”

_**Gluon Fusion:**_** **Most interesting at intermediate to high masses

## Associated Production

- Many improvements:
- increased lepton acceptance
- improved b-tagging
- improved bb mass resolution

**ZH********llbb**

## High Mass

- New DØ search in H WW 

## DØ Combined 1 fb-1 Limit

- Single experiment sensitivity
- within factor 6 (4) of SM for mH=~115 GeV  (mH=~160 GeV)
- Many improvements in pipeline
- more data, combine experiments, more channels, get cleverer
- expect sensitivity to continue to improve ~L and not L !
- Success with low cross section, high background SM signals:
- WZ, ZZ, single top
- Direct limits, or direct hints, from Tevatron with ~2-4 fb-1?!

- WH WWW will help
-  fill gap in sensitivity
-  around mH= 130 GeV

## Outlook for EPS2009?

## LHC Sensitivity

- Intense theoretical activity in this area
- QCD is under control at 10% level
- Need to consider EW corrections ~5-8%

- Huge EW event samples crucial to commission detector, trigger and event reconstruction
- in 1 fb-1
- 250k l+jet tt events
- 1M Zll events

## Summary

- Precise EW physics requires close interplay
- precise experiment   precise Monte Carlo  precise theory
- Stay tuned for many new results from Tevatron experiments
- 1 fb-1  6 fb-1
- With the start up of the LHC the next year or two will be exciting times for EW physics!
- Longer term prospects
- mt ~ 1 GeV, mW ~ 10 MeV
- will require a heroic effort!

## Outlook for EPS2009?