gluex luminosity limits

GlueX Luminosity LimitsRichard Jones, University of Connecticut

GlueX Collaboration Meeting, Newport News, May 8-10, 2008

1. Design luminosity

2. Physics possibilities at higher luminosities

3. Limiting factors in current design


2

Design Luminosity

Goal – produce sufficient samples of exclusive reations to

be systematics-limited (maximum sensitivity to weak exotic

waves) in amplitude analysis for key channels.

Translation – when that occurs depends on the final state,

ie. specific backgrounds, PID demands, …

Rule of thumb: 107 events is sufficient for a decent PWA

Consider a hypothetical case:

= 50 nbBR = 30% = 25%


3

Design Luminosity

L = .071 x 30 cm x 6.0 1023 x 10-33 x Ibeam gcm3

1g

cm2

nb

= 1.3 10-9 nb-1 x Ibeam

At Ibeam = 107 /s, it would take 57 khr (~ 20 years) to

collect these statistics.

At Ibeam = 108 /s, it would take 6 khr (~ 2 years) to collect

these statistics.

Result: 108 /s is sufficient to complete the hybrid

spectroscopy program. But is it optimal ?


4

Design Luminosity

Define: tagger figure of merittagger figure of merit

•Factor that rescales the amount of run time needed to reach a given level of statistical error in a tagged histogram.

•Reference for FOM shown is the GlueX tagged beam under nominal conditions at 9 GeV, but with no mistags.

1.1. Assumes detector identifies correct Assumes detector identifies correct

beam bucket 100% of the time.beam bucket 100% of the time.

2.2. Shows some gains up to 3 10Shows some gains up to 3 1088 Hz. Hz.

3.3. Gains are only about 25% for factor 3 Gains are only about 25% for factor 3

in backgrounds.in backgrounds.


5

Design Luminosity

For 25% more statistics, what do we lose? x3 radiation damage in FCal

x3 accidentals in the TOF and Start

x3 pileup in the FDC, extra tracks, etc.

x3 in channel count in the microscope – $$$

x3 in radiator thickness – reduced polarization


6

Design Luminosity

If this argument was not made before, what

was the basis of the design goal of 108 /s? the intuitive criterion of 50% accidental tags

evidence from Monte Carlo simulation that detector backgrounds are going to preclude higher luminosities

1. FCal radiation damage – already an issue at 108

2. TOF occupancy – within a factor of 3-5 of ceiling

3. FDC pileup and extra tracks – within factor of 3-5


7

Physics at Higher Luminosity

What physics might make this interesting? inverse DVCS – looks feasible

threshold J/– statistically difficult

Cascade baryons – needs kaon PID


8

Beam Limiting Factors

Tagging near the end-point no polarization

no significant collimation

amorphous radiator – factor 100 more luminosity available (if untagged)

current tagger design has full coverage over 9-11.4 GeV, designed to run up to 50 MHz / GeV.

at 50 MHz / GeV in end-point region, detector backgrounds are comparable to nominal conditions

with polarized beam at 108 /s.


9

Detector Limiting Factors

FCal radiation damage Inner blocks could be shielded, giving up

low-angle acceptance, ok for some physics.

FTOF occupancy ditto.

FDC pile-up – will be ultimate limiting factor. essential for just about any physics no effective means to shield them


10

Conclusions

Design luminosity is optimized for carrying out the hybrid spectroscopy program.

Nominal high-intensity running conditions are consistent with tagging at 50 MHz / GeV at the end-point.

The photon source will produce as much intensity as the experiment can handle in any scenario.

With a dedicated end-point tagger, one can tag effectively up to 250 MHz, provided the detector can trigger.

With 250 MHz on 11 < E < 12 GeV, detector background

would be x5 nominal, probably an upper limit. FDC pile-up will be the limiting factor – how to estimate it?

gluex luminosity limits

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