# Presentation: 491006

## Precision Electroweak Measurements at the LHC

*Thomas J. LeCompte*
- High Energy Physics DivisionArgonne National Laboratory

## My Personal Definition of “Precision Electroweak”

- The Standard Model is remarkable, in that at tree level, only three parameters are needed to predict everything.
- GF,  and sin2(w) would be one choice
- Precision electroweak measurements are those that attempt to look for deviations from this simple three-parameter theory
- Loop effects:
- Couplings that differ from SM values
*i.e. evidence that three parameters are not enough*

- “Precision EWK” is usually a code word for “W Mass”.  I would like not to focus exclusively on that, and devote more time for anomalous gauge couplings.

## Status of the Simulation

- CMS released its Physics Technical Design Report in the summer
- ATLAS released its Physics TDR in 1999 (!)
-  Since then
*More up-to-date generators*
*Parametric simulation & GEANT3 replaced by GEANT4*
*Digitization allows for both in-time and out-of-time pileup*
*Misalignments and miscalibrations are supported*
*Reconstruction has gone through several release cycles*
- ATLAS will be releasing updates on many TDR analyses throughout the year

- I’m surprised the forthcoming and public numbers are not more different.  Increasing realism is often just about balanced by more intelligent reconstruction and analysis.

## Part I. The W Mass

- I am not going to try and sell you on the idea that the LHC will reach a precision of 15 or 20 MeV.Instead, I want to outline some of the issues involved.

## Rapidity – of getting m(W) results published

- No hadron collider experiment has published an uncertainty of 100 MeV in less than 1400 days.

- See Ashutosh Kotwal’s talk for details on the new and exciting Run II m(W) measurement.

- The trend is for later runs to be on a curve which begins lower and to the right of earlier runs.

## Measuring m(W) – Why It Takes so Long

- Set Momentum Scale
- Use known states like Z0, J/, and  family
- As this is done, removing tracking systematic problems:
*Misalignments, miscalibrations, twists, distortions, false curvatures, energy loss...*
- Set Energy Scale
- Use electrons and “known” material and momentum scales
- Recoil & Underlying Event Characterization
- Modeling, Modeling, Modeling
- Transverse mass vs. lepton pT vs. missing energy, QCD radiation, QED radiation, production models, underlying event, residual nonlinearities...

- It’s not unusual for >1000 plots to appear in the (complete) set of internal notes for this analysis

## Difficulty 1: The LHC Detectors are Thicker

***Difficulty 1: The LHC Detectors are Thicker***

- Detector material interferes with the measurement.
- You want to know the kinematics of the W decay products at the decay point, not meters later
- Material modeling is tested/tuned based on electron E/p
- Thicker detector = larger correction = better relative knowledge of correction needed

- CMS material budget

- ATLAS material budget

- X~16.5%X0

- (red line on lower plots)

## Difficulty 2 – QCD corrections are more important

- No valence antiquarks at the LHC
- Need sea antiquarks and/or higher order processes
- NLO contributions are larger at the LHC
- More energy is available for additional jet radiation

- At the Tevatron, QCD effects are already 1⁄4 of the systematic uncertainty
- Reminder: statistical and systematic uncertainties are comparable.
- To get to where the LHC wants to be on total m(W) uncertainty is going to require **continuous** effort on this front.

## Major Advantage – the W Rate is Enormous

- The W/Z cross-sections at the LHC are an order of magnitude greater than the at the Tevatron
- The design luminosity of the LHC is ~an order of magnitude greater than at the Tevatron
- I don’t want to quibble now about the exact numbers and turn-on profile for the machine, nor things like experimental up/live time
- Implications:
- The W-to-final-plot rate at ATLAS and CMS will be ~1⁄2 Hz
- Millions of W’s will be available for study – statistical uncertainties will be negligible
- Allows for a new way of understanding systematics – dividing the W sample into N bins (see next slide)* *
- The Z cross-section at the LHC is ~ the W cross-section at the Tevatron
- Allows one to test understanding of systematics by measuring m(Z) in the same manner as m(W)
- The Tevatron will be in the same situation with their femtobarn measurements: we can see if this can be made to work or not
- One can consider “cherry picking” events – is there a subsample of W’s where the systematics are better?

## Systematics – The Good, The Bad, and the Ugly

- Masses divided into several bins in some variable
- Masses are consistent within statistical uncertainties.

- Clearly there is a systematic dependence on this variable
- Provides a guide as to what needs to be checked.

- Point to point the results are inconsistent
- There is no evidence of a trend
- Something is wrong – ***but what?***

- Good

- Bad

- Ugly

## W Mass Summary

- ATLAS and CMS have set themselves some very ambitious goals in a 15 or 20 MeV W mass uncertainty
- This will not be easy
- This will not be quick
- It might not even be possible
*For example, suppose the PDF fits of the time simply have spreads that are inconsistent with better than a 25 MeV uncertainty.*
- Personal view: given time, the LHC will be competitive with the Tevatron.  I wouldn’t want to speculate on how much or how little time this would take.
- Even after the Higgs is discovered, this measurement is important
- Finding one Higgs is not necessarily the same as finding all of them.
- Indirect constraints will be important in interpreting the discovery

## Part II: Multiboson production

- Zzzzzzzzzzz

## What is the Standard Model?

- The (Electroweak) Standard Model is the theory that has interactions like:

- but not

- but not:

- Only three parameters - GF,  and sin2(w) - determine all couplings.

## Semiclassically, the interaction between the W and the electromagnetic field can be completely determined by three numbers:
The W’s electric charge
Effect on the E-field goes like 1/r2  
The W’s magnetic dipole moment
Effect on the H-field goes like 1/r3 
The W’s electric quadrupole moment
Effect on the E-field goes like 1/r4 
Measuring the Triple Gauge Couplings is equivalent to measuring the 2nd and 3rd numbers
Because of the higher powers of 1/r, these effects are largest at small distances
Small distance = short wavelength = high energy

- The W’s electric charge
*Effect on the E-field goes like 1/r**2**  *
- The W’s magnetic dipole moment
*Effect on the H-field goes like 1/r**3** *
- The W’s electric quadrupole moment
*Effect on the E-field goes like 1/r**4** *
- Measuring the Triple Gauge Couplings is equivalent to measuring the 2nd and 3rd numbers
- Because of the higher powers of 1/r, these effects are largest at small distances
- Small distance = short wavelength = high energy

***The Semiclassical W***

## Triple Gauge Couplings

- There are 14 possible WW and WWZ couplings
- To simplify, one usually talks about 5 independent, CP conserving, EM gauge invariance preserving couplings: g1Z, , Z, , Z
- In the SM, g1Z =  = Z = 1 and  = Z = 0
- Often useful to talk about g,  and  instead*.*
- Convention on quoting sensitivity is to hold the other 4 couplings at their SM values.
- Magnetic dipole moment of the W = e(1 +  + )/2MW
- Electric quadrupole moment = -e( - )/2MW2
- Dimension 4 operators alter g1Z, and Z: grow as s1⁄2
- Dimension 6 operators alter  and Z and grow as s
- These can change either because of loop effects (think e or  magnetic moment) or because the couplings themselves are non-SM

## Why Center-Of-Mass Energy Is Good For You

- The open histogram is the expectation for  = 0.01
- This is 1⁄2 a standard deviation away from today’s world average fit
- If one does just a counting experiment above the Tevatron kinematic limit (red line), one sees a significance of 5.5
- Of course, a full fit is more sensitive; it’s clear that the events above 1.5 TeV have the most distinguishing power

- From ATLAS Physics TDR: 30 fb-1

- Approximate Run II Tevatron Reach

- Tevatron kinematic limit

## Not An Isolated Incident

- Qualitatively, the same thing happens with other couplings and processes
- These are from WZ events with g1Z = 0.05
- While not excluded by data today, this is not nearly as conservative as the prior plot
*A disadvantage of having an old TDR*

- Plot is from ATLAS Physics TDR: 30 fb-1  Insert is from CMS Physics TDR: 1 fb-1

## Not All W’s Are Created Equal

- The reason the inclusive W and Z cross-sections are 10x higher at the LHC is that the corresponding partonic luminosities are 10x higher
- No surprise there
- Where you want sensitivity to anomalous couplings, the partonic luminosities can be hundreds of times larger.
- The strength of the LHC is not just that it makes millions of W’s.  It’s that it makes them in the right kinematic region to explore the boson sector couplings.

- From Claudio Campagnari/CMS

## Confession Is Good For The Soul

- There is a built-in swindle to this formalism
- Effects that grow as s1⁄2 or s are easy to see at large m(VV)
- As m(VV) grows, these effects eventually violate unitarity
*Of course those are easy to see!  They’re impossible to miss!** *
- It may be useful to think about turning this around
- If there is new physics at high Q2/short distances, it can manifest itself at lower Q2/longer distances as changes in the couplings: g1Z, , Z,  and Z
- While the effects will be largest at high Q2, they might not look like exactly like these predictions – it all depends on what this new physics is.
- NLO effects tend to increase the VV cross-section at high Q2 – this reduces sensitivity somewhat.

- “That’s not confessin’ – that’s advertisin!” – Cole Porter

## TGC’s – the bottom line

- Not surprisingly, the LHC does best with the Dimension-6 parameters
- Sensitivities are ranges of predictions given for either experiment

| Coupling
 | Present Value | LHC Sensitivity (95% CL, 30 fb-1 one experiment) |
| --- | --- | --- |
| g1Z |  | 0.005-0.011 |
|  |  | 0.03-0.076 |
| Z  |  | 0.06-0.12 |
|  |  | 0.0023-0.0035 |
| Z  |  | 0.0055-0.0073 |

## Early Running

- Reconstructing W’s and Z’s quickly will not be hard
- Reconstructing photons is harder
- Convincing you and each other that we understand the efficiencies and jet fake rates is probably the toughest part of this

- We have a built in check in the events we are interested in
- The Tevatron tells us what is happening over here.
- We need to measure out here.
- At high ET, the problem of jets faking photons goes down.
- Not because the fake rate is necessarily going down – because the number of jets is going down.

## Things I Left Out and Really Shouldn’t Have

- Angular distributions have additional resolving power
- Remember, the W decays are self-analyzing
- Different couplings yield different angular distributions
*Easiest to think about in terms of multipole moments*
- Neutral Gauge Couplings
- In the SM, there are no vertices containing only ’s and Z’s
- At loop level, there are ~10-4 corrections to this
- It is vital that these be explored

## Quartic Couplings

- These are special – this is where the Higgs mechanismtouches the Standard Model
- Signature is three bosons in the final state
- Yields are small, but not ridiculous
- many dozens of events
- this will not be an early measurement!
- It is difficult to generate a model where thetrilinear couplings are at their SM values, but the quartics are not
- It is still important to check
- If something new is seen in the trilinears, one might need the quartics to sort things out.

- From Azuelos et al. hep-ph/0003275
- 100 fb-1, all leptonic modes inside detector acceptance

## Summary

- An early, competitive W Mass measurement from the LHC is improbable
- Today, I’m not willing to defend any number for the ultimate uncertainty
- There are simply too many unknowns
- I’m still optimistic, more so because of recent Tevatron results
- One should expect early results on TGCs, particularly  and Z
- 4 fb-1 at the Tevatron corresponds to ~ 0.4 fb-1 at the LHC
- There will be an LHC-long program on gauge couplings
- First  and Z
- Next g1Z,  and Z,
- Finally quartic couplings

- I’d like to thank the organizers, the ATLAS and CMS collaborations and particularly their physics TDR editors