# Presentation: 500771

## Z-primes at the Tevatron & LHC

**Tim M.P. Tait**
**Fermi National Accelerator Laboratory**

- Fermilab TeV4LHC
- 9/17/04

- Marcela Carena
- Alejandro Daleo
- Bogdan Dobrescu

- With:

## Outline

- Introduction: Why Z’s ?
- What We Know Today
- Theoretical Framework
- LEP II bounds
- What We Hope To Find Tomorrow
- Model Independent Studies
- Bridges between Modelers & Experiment
- Outlook

- Tim Tait

- Fermilab, 9/10/04

## Introduction: Why Z’?

- The SM is based on local gauge invariance under: *SU(3)**C** x SU(2)**W** x U(1)**Y*.
- It is natural to ask: are these all of the fundamental interactions of nature?  What are the limits on more?
- How well has LEP already explored them?
- How do hadron machines increase our knowledge?
- Good theoretical reasons point to extra gauge bosons.
**GUT**s with “large” gauge groups (*S0(10), E**6*, ...)
**Extra dimensions** with gauge fields in the bulk have copies of ******, Z, which are massive, neutral vector particles.
- Theories like Topcolor use them to drive **EWSB**.
- The **little Higgs** theories use massive vector particles to cancel quadratic divergences in the Higgs mass induced by W and Z.
- New **SUSY** theories use them to survive the LEP II bound on **m****h**.

- Tim Tait

- Fermilab, 9/10/04

## Theory Framework

- We extend the SM: *SU(3)**C** x SU(2)**W** x U(1)**Y  **x ****U(1)******Z***.
- There are Z’s coming from larger groups like *SU(2).*  We can usually describe their properties with a ***U(1)***, though we don’t capture all of the physics.
- Each SM fermion **** (and the usual Higgs) is assigned a charge **z****** under ***U(1)******Z***.  The **Z-********-****** coupling is: **g****Z**** z******.
- A new scalar **** is charged under ***U(1)******Z***, (**z******** = 1**).  Its VEV **u** determines the **Z’** mass: **M****Z’** = **g****Z**** z********u**** .**
** ******** **is neutral under the SM.
- Theory can further relate the **z****** to each other:
- SU(2)W invariance:  zuL = zdL.
- Yukawa interactions: zuL + zuR + zH = 0.
- Anomaly cancellation.
- FCNCs and family universality: zdL = zsL= zbL.

- Yukawas from higher dimensional operators?

- New Fermions?

- Tim Tait

- Fermilab, 9/10/04

## Specific “Model-lines”

- Subject to theoretical motivations, we find a set of 4 “model lines”.
- Each has a free parameter, ***x***, specifying all of the couplings of the Z’.
- The **E****6** Z’s are certain values of ***x*** for certain model lines.

|  | B-xL | Q+xu | 10+x5 | d-xu |
| --- | --- | --- | --- | --- |
| qL=(uL,dL) | +1/3 | +1/3 | +1/3 | 0 |
| uR | +1/3 | +x/3 | -1/3 | -x/3 |
| dR | +1/3 | (2-x)/3 | -x/3 | +1/3 |
| lL=(eL,L) | -x | -1 | +x/3 | (x-1)/3 |
| eR | -x | -(2+x)/3 | -1/3 | +x/3 |

- Tim Tait

- Fermilab, 9/10/04

## e+e- colliders constrain Z’s in two important ways.
Precision measurements of Z physics
SM predictions in agreement at roughly few per mil level.
A Z’ which mixed with Z would distort these predictions.

If there is a single (SM) Higgs, this requires zH very small.
For multiple Higgses, one can cancel the mixing, by fine-tuning the Higgs potential against the zHs (to the sub-% level).
This is how the famous E6 models survive LEP-II limits.
I will fix zH = 0.

- e+e- colliders constrain Z’s in two important ways.
- Precision measurements of Z physics
- SM predictions in agreement at roughly few per mil level.
- A Z’ which mixed with Z would distort these predictions.
- If there is a single (SM) Higgs, this requires zH very small.
- For **multiple Higgses**, one can cancel the mixing, by fine-tuning the Higgs potential against the zHs (to the sub-% level).
- This is how the famous **E****6** models survive LEP-II limits.
- I will fix zH = 0.

**e****+****e****-**** Limits: Z-Z’ Mixing**

- SM Higgs VEV

- J Hewett, T Rizzo, **Phys.Rept.183, 193 (1989)**

- Tim Tait

- Fermilab, 9/10/04

## Contact Interactions

**Contact Interactions**

- Searching for evidence of physics beyond the SM in e+e-**** contact interactions.
- At **E<< m****Z**’, the Z’ effects look like four fermion interactions:
- These depend quite sensitively on how Z’ couples to e, and also to other fermions.
- LEP-II has searched for evidence of contact interactions in almost all conceivable channels.
- Bounds vary from channel   to channel, complicating    the analysis.

- Tim Tait

- Fermilab, 9/10/04

## Hadron Colliders

- Hadron colliders look for Z’s most effectively in the decay into charged leptons (**e****±****, ********±****, ********±**).
- Unlike LEP, hadron colliders are sensitive to Z’s which do not couple to **e****± **: unexplored territory!
- The signal appears as a resonance above the (smooth) lepton pair backgrounds.  Interference effects are tiny.

- Tim Tait

- Fermilab, 9/10/04

## Common Ground

- Run I and existing Run II analyses have presented limits on **** x BR.  Specific **E****6** models are usually overlaid to illustrate the results.
- This is good because **** x BR is fairly model-independent: results can in principle be applied to other theories with Z’s.
- However, it is inconvenient because computing a hadronic cross section is not a completely trivial task : one has to know about QCD, PDFs, etc..
- These are important for ****, but don’t have anything to do with Z’s.
- An alternate approach tries to factor out the universal QCD dependence, and bound only the Z’ quantities themselves.
- This works perfectly at LO and NLO in QCD; at NNLO there is some (very) small model-dependence.
- Dominant NLO EW corrections are model-independent (and probably important at the few % level).

- Tim Tait

- Fermilab, 9/10/04

## Bounds & Projections

- Projections scaled by √L

**Bounds & Projections**

- CDF Run II Data!

- Tim Tait

- Fermilab, 9/10/04

## TeV4LHC ?

- LHC will eventually reach into the TeV range for Z’ masses.
- Equivalently, for lower masses the larger luminosity should eventually allow one to probe **smaller couplings**.
- Of course, our theoretical results for a “common ground” between experiments and model-builders are also of value at the LHC.
- Once the Z’ is discovered, we will need more observables to pin down its properties.
- Tevatron inputs for high *x* PDFs will be important for Z’ and all high mass search processes.
- Especially high *x* gluon PDFs are constrained by jet data, and are particularly important at LHC (LHC k-factors can be ~100%).
- Techniques such as **** ID are being developed and can be refined using real data at Tevatron.

- Tim Tait

- Fermilab, 9/10/04

## Outlook

- Models with massive neutral bosons, Z’s, are of great interest for a variety of reasons.  Many different kinds of models of physics beyond the SM contain Z’s.
- e+e- colliders put strong bounds, and favor a Z’ which does not mix with the Z.  This implies either zH=0 or some kind of tuning.
- We have introduced several new “model lines” which contain popular **E****6** inspired models, but also generalizations of them.
- A new proposal is that we directly bound quantities which contain Z’ physics, but none of the QCD inputs, bringing experiment and modelers together easily (for both sides!).
- Tevatron can provide important inputs to LHC, including high *x* PDFs and new experimental technique.
- Let’s find nature’s next force!

- Tim Tait

- Fermilab, 9/10/04