# Presentation: 390799

## Some Studies of the G3X Matching 2 for CMU

- Thomas J. LeCompte
*Argonne National Laboratory*

## Outline

- Introduction
- Dataset description and warnings for the unwary
- General procedure
- Exploration of the x (only) 2
- Wild speculation
- Exploration of the x (only) 2
- More wild speculation
- Discussion

## Dataset Description

- I used ~25,000 CMU-CMU J/’s
- Started from the “Beata sample”
- Production as of some time in the past
- Dropped CdfMuon objects and reran the 4.7.1 linker
- Warning!  Not every plot shown is of the full data sample or has exactly the same selection
- If I draw a comparison on a slide, I was careful to make sure that it was valid
- Taking a number from one slide and comparing to one on another may not be.
- I tried to spare you pages and pages of identical looking plots

## General Procedure

- For efficiencies
- Fit the J/ sample to a single Gaussian on a flat background
- Split into “passing” and “failing” samples.  Refit, holding the mass and width fixed from the previous step.
- For distributions
- Divide into “signal + background” (3.0-3.2 GeV) and “background” (2.8-3.0 and 3.2-3.4) samples
- The plots shown have the normalized background subtracted off

## Chi-squared for x

- Try to duplicate the cuts in CDF-1986 (Run1)
- The calorimeter material is unchanged
- Minor changes to tracking and muons
- Should be close to Run 1
- Measured () = 99.19 ± 0.17% (w/Run 1 cuts)
- Corresponds to () = 99.6 ± 0.1%
- Run 1A with an equivalent z-cut was 99.3 ± 0.1%
- So, the agreement is pretty good.
***	******	******	******I should have stopped there!***

## pT dependence of the x-position 2 

- Divding this in bins of pT() shows that this is not quite flat.
- My first thought was “Maybe the chi-squared is not entirely flat in pT().

## Fitted widths vs. pT(mu)

- The signed square root of the 2 (the “pull”) should be Gaussian
- It is
- I’ll spare you the plots
- It should also be flat in pT()
- It’s not
- This is not a reflection of the 30 cm cut
- I see that cut in the data and it’s beyond the fit range
- Changing the fit range doesn’t do much (except improve the 4th point)

## Flattening the 2 variable

- Below 2 GeV
- Scale linearly in pT() from 0.94 at 1.4 GeV to 1.06 at 2 GeV
- This is almost certainly not the best way to fix this
- Above 2 GeV
- Scale by 1.06
- Results
- Below 2 GeV,  = 0.98
- Above 2 GeV,  = 1.00
- Therefore, we’ve removed about 80% of the pT dependence.

- J/ efficiency change
- Recovers about 35% of the J/’s that were lost to the 2 cut.
- () was 98.7 ± 0.2%
- () becomes 99.1 ± 0.2%
- The difference is real
- Remember, I look at the subtracted distributions
- Background rejection change
- 15% of the background was rejected
- This becomes 13%

## pT dependence of the x-position 2 (after the fix)

- The trend is unchanged
- The first three bins are consistent with what they were before the fix
- The last bin is as well, but the uncertainties are too large to say much of anything
- The overall efficiency is slightly higher
- i.e. the inefficiency is 35% smaller
- This fix did good things, but didn’t change this at all

## Conclusions on G3X

- G3X gives us the extrapolation uncertainty “out of the box” to within ±5 or 6%.
- That’s 2mm on the multiple Coulomb scattering of a 3 GeV muon 3.5m from the production point
- We probably do not know the detailed composition of the calorimeters to ±5 %
- GEANT will do no better than its input data
- A crude rescaling tunes away 80% of this
- Recovers 35% of the rejected J/s
- Lets in ~15% more background
- How to react to this probably takes some discussion

## Speculations on pT

- We’ve tuned the 2 distribution to be flat in pT
- That the efficiency is not flat implies the problem is in the tails of the distributions, not the core
- Most mass distributions of the events that fail 2 cuts have the J/ shifted 1⁄2 towards higher mass.
- Could this just be a selection bias?
- Tracks that are misreconstructed towards higher *p**T* will have a matching 2 window that is too small, and more failures
- These will also give you a mass that’s too high
- If they are misreconstructed towards lower *p**T*, the matching 2 window will be too big, and the events will likely pass.

## Chi-squared for z

- This is the first real look at the z-position 2 (it was not filled in production)
- I started with the simple question – “Does this look like a 2?”
- The answer appears to be “no”
- The distribution is a factor of 1.7 too wide
- Ken points out that this calculation does not include the intrinsic CMU *z*-resolution, which is comparable to the MCS term.

## pT dependence of the z-position 2 

- If that weren’t bad enough...
- This momentum dependence is stronger than in the x-view
- Fit to a 1/ *p**T* multiple scattering term and a CMU resolution term:
- 17 ±  2 cm/ *p**T* for MCS
- 11.1 ± 0.8 cm for CMU

## Concocting a better 2

- Start with G3X
- Add a CMU resolution term
- 11.1 cm
- Returned by the fit
- Makes a 2 that is too narrow: 0.81 instead of 1
- Not flat in *p**T*
- 8.0 cm
- Pulled out of thin air
- Makes the 2 have the right overall width
*Still* not flat in *p**T*

## What’s Going On?

- The problem can’t be with G3X
- It gets x right to within ±5%
- The multiple scattering is the same (within geometry) for x and z
- I have verified this: it’s not just how it’s supposed to work
- COT stereo failures unlikely
- Functional form is wrong
- J/ mass still looks okay
- CMU z-resolution the best candidate
- This can be explained by having this  resolution vary from 14 cm at low *p**T* to 10 cm at high *p**T*
- In fact, 10 cm at high *p**T* is what is expected

## Why a Change in CMU z-resolution?

- As far as I know, it’s never been validated that this is independent of *p**T*
- It’s not a problem with 3-hit stubs at low *p**T*
- Except in the 1.4-1.5 GeV bins, this distribution is flat
- Low *p**T* muons are even lower when they hit the CMU
- Could dE/dx be responsible for the resolution change?
- It looks like we can see the relativistic rise – but could a 20% change in ionization cause a 40% change in resolution?
- Are *p**T* and *z* uncorrelated?
- If not, is this effect big enough?

## Does a z 2 cut do any good?

- Maybe we’re spinning our wheels.
- Maybe there simply aren’t any events with a good x  2 and a bad z  2!
- That turns out not to be the case
- Consider dimuon events with x-position 2 < 9 (both muons)
- 1.5% of J/ events have a x-position 2 < 9 x (1.7)2
- 6% of sideband events have a z-position 2 < 9 x (1.7)2
- Perhaps its not a cut you would make, but there *are* events in this category.

## Summary

- The x position 2 looks pretty good
- The value is right to ±5%
- There are ±1-2% peculiarities still remaining in the efficiencies
- Today, I’d say it’s ready for use for anyone except for cross-section measurements
- e.g. improving purity for B lifetimes
- We have a z position 2
- It’s more problematic
- It’s construction doesn’t include CMU resolution
- ~70% effect
- The CMU resolution itself looks like it has a 40% problem
**Could we think about a stub-by-stub z uncertainty?**