Transcript GlueX Luminosity Limits

GlueX Collaboration Meeting, Newport News, May 8-10, 2008
GlueX Luminosity Limits
Richard Jones, University of Connecticut
1. Design luminosity
2. Physics possibilities at higher luminosities
3. Limiting factors in current design
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, …
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 Rule of thumb: 10 events is sufficient for a decent PWA
 Consider a hypothetical case:
s = 50 nb
BR = 30%
e
= 25%
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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Design Luminosity
L
= .071 g 3
cm
x
30 cm
x
6.0 1023 1
g
x
2
10-33 cm
nb
x
Ibeam
= 1.3 10-9 nb-1 x Ibeam
 At Ibeam = 10 g/s, it would take 57 khr (~ 20 years) to
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collect these statistics.
 At Ibeam = 10 g/s, it would take 6 khr (~ 2 years) to collect
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these statistics.
 Result: 10 g/s is sufficient to complete the hybrid
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spectroscopy program. But is it optimal ?
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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Design Luminosity
Define: tagger 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. Assumes detector identifies correct
beam bucket 100% of the time.
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2. Shows some gains up to 3 10 Hz.
3. Gains are only about 25% for factor
3 in backgrounds.
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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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
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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Design Luminosity
 If this argument was not made before, what
was the basis of the design goal of 10 g/s?
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
the intuitive criterion of 50% accidental tags

evidence from Monte Carlo simulation that detector
backgrounds are going to preclude higher luminosities
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1. FCal radiation damage – already an issue at 10
2. TOF occupancy – within a factor of 3-5 of ceiling
3. FDC pileup and extra tracks – within factor of 3-5
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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Physics at Higher Luminosity
 What physics might make this interesting?

inverse DVCS – looks feasible

threshold J/Y – statistically difficult

Cascade baryons – needs kaon PID
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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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 g/s.
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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Detector Limiting Factors
 FCal radiation damage

Inner blocks could be shielded, giving up lowangle g 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
GlueX Collaboration Meeting, Newport News,
May 8-10, 2008
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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 < Eg < 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 Collaboration Meeting, Newport News,
May 8-10, 2008
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