# Presentation: 014201

## WW scatteringDec. 17 LBL ATLAS analysis meeting

- Motivation
- Theoretical Introduction
- Review of MC analyses

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Why?

- It is an electroweak measurement using well known SM particles, not a search.
- A well defined problem, can extensively study with simulation before data without worrying about choosing some phase space point or other.
- A model independent probe of mass and EW symmetry breaking, independent of Higgs, SUSY, extra-D, etc.
- Remains an interesting measurement even if a Higgs particle is observed or SUSY particles are discovered.
- But note this is NOT a quick result. Needs luminosity of order 100fb-1 (>~3 years)

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## A cynic’s view of the Higgs mechanism

- Problem: EW Lagrangian with simple mass terms is not gauge invariant.  Bad m2AA
- Solution 1: Add a gauge invariant part that contains multiple terms, one of which is the desired simple mass term. The other terms are required for gauge invariance but not observable at low E
- The new physics question then is, what are the other terms?- we already know that **W,Z** have mass, that’s not really new physics (the Higgs mechanism claims the other terms are fundamental scalar fields and couplings, solution 1.1)
- Solution 2: Forget local gauge invariance, the EW SM is just a low E effective theory and none of the terms are valid in some limit (non-perturbative at some point)
- The new physics questions then is, when does perturbation theory break down- dynamics become strong?

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## EW precision fits

- Low mass Higgs favored, but there are assumptions:
- Same mass mechanism for **W,Z** and fermions
- No additional new particles in loop corrections of EW parameters
- With added complexity many options are possible
- No Higgs
- Higgs present but not responsible for EW symmetry breaking
- More complex sector producing Higgs-like degrees of freedom
- No obligatory reason yet for a more complicated theory (except for Higgs self interaction maybe...), but no reason to rule out either- only data will tell.

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Can we forget Higgs and study phenomenologically the W,Z mass?

- Yes
- If W,Z bosons were massless they would be just like the photon and have 2 polarization states (both transverse to the propagation direction)
- Longitudinal polarization is impossible without mass, so one can guess that by studying W**L** and Z**L** we can learn something about mass.
- Recall that in the Higgs mechanism the W,Z acquire longitudinal polarization (third degree of freedom) by eating 3 Goldstone Higgs bosons.
- There is actually a formal Equivalence Theorem, that equates W**L**,Z**L** processes, to those of scalar Goldstone bosons.
- Even more interestingly, we already know very well a system of 3 (almost) Goldstone bosons: **-**0.
- So W**L****+**** **W**L****-**** **Z**L****0** can be viewed as a reincarnation of **-**0.

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Review of Pion Scattering

- Cannot calculate low E pion scatering in QCD
- But there are low energy theorems that actually predate QCD
- Knowledge from pion scattering helped uncover the underlying QCD physics
- Analogy is that knowledge of W**L**,Z**L** scattering will help uncover the underlying physics.
- What do we see in 2->2 pion scatering?
- Can look at  +-, ++,+0, 00
- All different, but related by isospin.
- Rich resonant structure as unitarity bound is approached
- Exactly the same can happen in EW Goldstone boson scattering
- But, if resonance(es) are light (below WW mass) this can be the good old light Higgs, with no structure in real WW, WZ final states
- This is a crucial prediction of the Higgs mechanism.
- Finding a light scalar particle does NOT prove the Higgs mechanism. **It’s finding a light scalar PLUS the absence of structure in ****WW****, ****WZ**** at higher mass!**
- W+W-, W+W+, WZ final states are complementary (remember isospin). As one x-section falls the others rise => there is no “blind spot”.
- (But pions are really composite.
- Can the “Higgs” be composite? => Technicolor)

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## The scattering process

**Hard, forward jets**
**No primary hadronic interaction => clean event (in principle)**

**W,Z**

**W,Z**

**q**

**q**

**W,Z**

**W,Z,********,g**

**q**

**q**

**jets**

- Signal

- Backgrounds

- (plus others, like ttbar)

**High Pt central jets**
**Hadronic interaction => messy event**

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Forward jets from Davide’s Talk

- HWW 2l 2ν

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## What to look for?

- Forward jets and clean central detector are a must
- Central all leptonic final states
**WZ -> l****+****l****- ****l******
**W****+****W****+**** -> l****+ ****l****+******** **
- Central Semileptonic
**WW or WZ -> l********jj**
**ZZ of WZ -> lljj**
- Some things get a little murky here
- Everyone seems to drop the **L** subscript at this point
- Are the W’s polarized?
- Do the kinematics of **2->2** scattering select longitudinal states only?
- Does an admixture of transverse states simply add a known background (and do we know what that admixture is?)

- From CMS

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## What about W+W- -> l+ l- ?

- This is favored mode for H->WW because the spin 0 H decay has very different kinematics from background
- 				from Ian’s talk
- But for non-resonant WW the backgrounds are not well separated

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## W+W+->l+ l+(ATLAS)

**W****+****W****+****->l****+ ****l****+****(ATLAS)**

- Luminosity requirement given by statistics needed to exclude non-resonant strong WW scattering (K-matrix)

- Complementarity with **WZ**

- Pixel B-layer is pretty dead here

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## ZZ + WZ -> l+l-jj              (CMS)

- This is missing the WW channel, which has higher rate
- But the background separation is pretty clean
- Any resonances would be easy to see (good Minv resolution

- Minv [GeV]

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Di-jet reconstruction (CMS)

- I) **well separated**
**    ****jets**

- II) **Merged Jets **

- Reconstruct
- as one cluser

**What algorithm to choose? ...... **depends on MWW

- 500                              61%                           27%

- 1000                             23%                          75%

- MH, GeV                  2Jets (R=0.5)             1Jet (R=0.7)

**M****CL****= (****∑|E****CELL****|)****2  - ****(∑E****CELL****)****2**

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004

## Conclusion

- A low or intermediate mass peak discovery during initial running would not be the end of the story
- High energy behavior has to be known to get the full picture => full luminosity results remain interesting.
- Some studies in both ATLAS and CMS give a feel for luminosity range required in different channels
- Use of fast simulation
- Ultimate reach combining all modes not fully evaluated
- should also consider combined ATLAS/CMS result
- Can ultimately get by with <100pb-1?

- M. Garcia-Sciveres  -- WW Scattering

- Dec. 17, 2004