# Presentation: 018454

## The case for another run of FOCUS (E831) in retrospect

- What might an old FOCUS experimenter do now with more photons?

## FOCUS: Photoproduction of Charm with an Upgraded Spectrometer*

- * I preferred SPOCK (Some People On a Charm Kick...)

- -Photons created from the “0.8 Tevatron” (We got 800 GeV protons in the fixed target areas)

- -Reconversion reduced hadron contamination

- -Can measure incoming Beam momentum, and “tag” before Brem.

- -Versatile beam! Photons, electrons, pions, protons, muons

## Liquid Deuterium optimized yield of high energy photons:

- Uranium 0.6Xo to get more Z/A

- Both e+ and e- give 1.7 x e-:
- FOCUS used lower energy    (250-300 GeV) than E687            (350 GeV) to use e+ beam.

- Production target in an active (sweeping) dump. Important to sweep up to, and bend quickly after.

- Have an adjustable lead collimator inside a 2 piece dipole in the photon beam.

- Experimental target is much bigger than nominal beam to help shield vertex detectors from soft photons

- Pb = High Z

## Why Lower Energy Photons for FOCUS?

- Photon flux really craps out at higher energies
- Neutral Hadron background has a more persistent harder component
- Worse for e+ side from Lambda decays
- Charm cross section doesn’t fall as quickly: (vs photon E)

- 200

- 400

- 600

- 800

- 300

- 100

- 200

- (nb)

- GeV

- Idea was to get more rate!

- p.s. We have never seen evidence for B’s in E687 or FOCUS

## Why photons at all?

- Good: Higher ratio of charm/hadronic interaction for comparable energy hadron beams:
-  ~1/100 vs ~1/1000
- Pair production easier to filter than hadrons
- And pions/kaons make muons!
- Good: Events Cleaner (only glue on one side of charm pair = less hadronization, cleaner interaction point)
- Accidentally Good: beam is spread out (~1cm)
- Embedded extra pairs likely NOT confused with Charm Vertices
- Radiation damage effects are spread out
- Bad: Lots of pairs to deal with (>10 MHz)
- Bad: tougher to get high charm rate (<1 Hz)
- Bad: tougher to get higher energy photons

## FOCUS: Triggering Concepts

- Make sure charged tracks produced by a photon interacting in the experimental target pass through the vertex detector
- Tracks that are not in a region you expect from pairs (sweep in a thin swath after magnets)
- In separated parts of the detector
- When there is ~35GeV or more energy deposited in the Hadronic trigger OR there were separated hits in muon detectors (muon trigger often needed prescale)
- Slower triggers were refinements

- Separates e+e- from Hadronic

- Enhances charm that CAN be reconstructed

## Magnets bend in and out of the page in this view. Notice the Gap for pairs! 

- Magnets bend in and out of the page in this view. Notice the Gap for pairs!

## Detector imposes acceptances that can be reflected in the trigger.

**Magnets have 0.4 GeV/c and 0.8 GeV/c kick repectively**

**Cerenkov Detectors separate K/pi from 4.5 – 60 GeV/c**

- Muon ID above 10 GeV/c

- No need to take the lower energy events!

- Generated
- Photons

- Energy

- Energy

- Charm
- Acceptance

- Energy trigger
- Rule of thumb is
- ~75% of Photon
- Energy for Charm

-  Reconstructed!

## Let’s expand this region

- Neutral Vee Reconstruction

**Electromagnetic Shower Reconstruction**

**Let’s expand this region**

## Reconstructing Particle Decays outside of target material is a winner!

- After out of target cut, data background dominated by Charm (green is overlay of a quickish simulation which has no min-bias)

## Verifying a D track with Embedded Silicon works too (but lose x10 in stats!)

- Silicon Plane hits in target region improve resolution, help overcome effects of extended target

**No Xtra**

- Interaction vertices in the Target Silicon can be constrained.

## FOCUS was the best we could do with what we had. Otherwise...

- The target region would have been populated with diamond strip detectors that were both targets and detectors.
- We would have had a finely segmented hadron calorimeter to do neutral hadron reconstruction.
- We would have run longer...

## The Case (then) for an Extension to FOCUS

- Re-arrange the targets to emphasize charged D tracking and measure fD via the decay
- Replace the PB converter for the final photon beam with a crystal (Coherent Brem)

## The D reconstruction technique

- Gives D direction resolution of 1 mr

## Then we look at the muon transverse momentum 

**Then we look at the muon transverse momentum **

**But we felt we could do better with the resolution**

- Study showing that the D
- Track resolution in this technique has big contributions from the interaction vertex

## The bottom diagram is the best we could do given time/money constraints

- But keep in mind:
- D tracking is very do-able
- 2) More active tracking improves the resolution
- 3) Thinner target segments allow a constraint

- FOCUS Extension Configuration

- E831/FOCUS Configuration

## So what else is D tracking good for?

- 3pi object

- How about

- Energy resolution is not real good in the Hadron Calorimeter, but *where* the shower occurred is!

- D direction

## First Public Showing!

- Calorimeter Energy only

- Using Pperp info only

- Tighter Cuts

- A peek at:

- (from a partial FOCUS sample)

- Can be done for any neutral leaving a position in the calorimetry.
- Again, resolution is dominated by determining the D direction

## Hardening the Photon beam

- One of the last tests we did with the FOCUS spectrometer was to replace the 0.2 Xo Pb radiator with a 1.1 cm Silicon crystal (0.12 Xo)

## Got a lot more Bang for the electron

-    Pb
- (e+/e-)

-    Xtal
- (e+/e-)

- Xtal

- Pb

- Normalized Hadronic Trigger Rate

- Crystal Angle

- > 30 GeV in Hadron Calorimeter

- > 100 GeV in Hadron Calorimeter

- Crystal Angle

- Saw a bigger angular dependence
- at lower photon energies.
- Effects more pronounced in beam with less angular spread!

- Need a more complete study if there is more interest.

-       Xtal
- electron only

## A new FOCUS in retrospect

- What is likely to be left to do beyond cross section measurements in the Post Cleo-c, BaBar, Belle, LHCb era?
- I claim that what made the FOCUS extension unique was the emphasis on improving the resolution of reconstucted charm, and in particular in reconstructing *D tracks*
- There is a lot of research now on pixel detectors, (for the ILC in fact!) and the hope for a “digital emulsion” experiment may not be so far fetched anymore. This is where I would concentrate.