g-2 accelerator and cryo needs

Mary ConveryMuon Campus Review

1/23/13

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The anomolous magnetic moment and g-2

• g2 but higher-order corrections– QED, EW, hadronic, new physics?

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proposed exp. precision

• Currently ~3s discrepancy between theory and experiment

• New muon g-2 experiment at Fermilab expected precision could yield ~5s

𝒂= 𝒈−𝟐𝟐

Measuring g-2• Polarized muons in magnetic field precess with Larmor spin

precession frequency

• Measure g-2 using cyclotron

• Requires precise measurements of and of the magnetic field4

g = 2 g > 2

�⃑� 𝒔=−𝒆 𝑩𝜸𝒎𝒄 −

𝒆𝒎𝒄 𝒂�⃑� 𝒂=𝒈−𝟐

𝟐

�⃑� 𝒄=−𝒆 �⃑�𝜸𝒎𝒄

�⃑�𝒂¿�⃑� 𝒔−�⃑�𝒄=−𝒆 �⃑�𝟐𝒎𝒄 (𝒈−𝟐)

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Magic momentum• One more trick:– Polarized muons in storage ring with vertical focusing by

electrical quadrupole field

– At magic momentum pm = 3.094 GeV/c (g = 29.3), g-2 precession frequency wa independent of electric field

�⃑� 𝒂=−𝒆𝒎𝒄 [𝒂 �⃑�−(𝒂− 𝟏

𝜸𝟐−𝟏 ) �⃑�× �⃑� ]

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Measuring • Distribution of decay electrons as function of time

𝑵 (𝒕 )=𝑵𝟎𝒆− 𝒕 /𝜸𝝉 [𝟏− 𝑨cos (𝜔𝑎𝑡+𝜑 ) ]𝝎𝒂

𝝎 𝒄

Intensity at a single detector station shortly after injectionPhys. Rev. D73 (2006) 072003

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g-2 apparatus

• Reusing storage ring from BNL g-2 experiment

• New calorimeters and straw-tube tracking

Planned improvements

• Rebunch high-intensity beam into multiple bunches to lower the instantaneous rate

• Increase the detector segmentation to reduce the instantaneous rate in a given cell

• Modify secondary beamlines to store as many muons from pion decays as possible

• Remove pions and protons from muon beam to prevent hadronic flash in calorimeters– Allows analysis of more (earlier) decay e+– Longer beamline for pion decay– Let heavier protons separate in time from pions/muons and kick them

out• Improve beam dynamics in storage ring• Improve storage ring field uniformity and the measurement and

calibration system8

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g-2 schedule

• Preparing for CD1 review this spring• MC1 building complete early FY14• g-2 storage ring ships early FY14• Ring reassembly starting FY14• Cryo ready to cool ring early FY15• Ring magnetic field shimming starting mid FY15• Recycler and Muon beamline work FY14-15• New beamline enclosure beneficial occupancy mid FY15• Beam to g-2 early 2016

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Protons on target

• 4x1020 POT in two-year running period in order to detect at least 1.8x1011 e+ from m+ decay in the g-2 storage ring

• Minimize pile-up of multiple e+ in a single detector channel by keeping single pulse <~1012 POT

• Time between beam pulses > 10ms for DAQ• Bunch length less than revolution time of muons around

storage ring (149ns)

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Estimation of POT needed

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2

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Secondary beam

• Create 3.1 GeV secondary pions off a target– forward decay kinematics require pp=1.005pm

• Capture 3.094 GeV (“magic momentum” muons)• Increase muon flux by accepting pions with momentum ±2%,

small b functions to reduce pion beam size so that muons with larger decay angles accepted– Needed in decay region, i.e. most important in beamline

immediately downstream of target (M2 line)• Beamline long enough for ~all pions to decay in order to

maximize number of muons• Aim for beamline acceptance of 40p mm mrad

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Hadronic flash

• Long enough beamline for ~all pions to decay in order to minimize pions into storage ring

• Prevent secondary protons from making it into storage ring

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• Correlation between spin angle and muon momentum• Pion decays in straight sections give no correlations, by

symmetry• Pion decays in regions with non-zero dispersion do give

correlations

• Want to minimize and/or be able to calculate systematic effects

• Limit momentum spread of beam

Systematic effects

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Lost muons systematic effect

• Muons in g-2 storage ring may be lost before decaying, depending on where they are in phase space

• If average spin direction of “lost muon” sample is different than that of “muon decay” sample, this introduces a systematic error on the measurement

• 1) If muons born in the Delivery Ring fill phase space differently from straight section born muons, different lost muon vs time in the muon storage ring for DR and straight-section born muons

• 2) Muons born in the DR have different average spin direction because the muon spin precesses in the DR, but the pion spin doesn’t (no spin)

• Effect = 1) x 2) • “Average bending field of DR is about one tenth that of the muon

storage ring dipole magnetic field. This helps a lot. In fact, skipping the DR might give a larger lost muon systematic error, due to more pion decays in the muon storage ring!”

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Differential decay systematic effect

• Low momentum muons decay earlier on average than high momentum muons – tlab = gt0

• If high and low momentum muons have different average spin direction, then average spin direction changes with time in storage ring, which leads to a mis-measurement of the spin precession rate

• Correct for effect due to turns in Delivery Ring, estimates suggest ~10 turns is small enough effect

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Summary table of accelerator requirements

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g-2 storage ring magnet

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g-2 ring superconducting coils / cryostats

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• Cryo Refrigeration plant will be built by Fermilab Accelerator Division using AIP funds

• Design should provide two dedicated refrigeration systems for g-2

• AD Cryo system design should include a method to trap contamination

• AD to provide Transfer Line for LHe and LN2 to the g-2 Hall

1000 liter Liquid Helium dewar from BNL

g-2 expectations for cryogenics

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Compressor SystemRefrigeration Plant

Experimental Hall

g-2 cryogenic system at BNL

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g-2 Cryo Requirements

FLUID PRESSURE STATEFLOWRATE

normal operations

Liquid Helium 28 psia saturated liquid 25 g/s

Helium Vapor Return 21 psia cold vapor 23.6 g/s

Helium Gas Return -compressor suction 16 psia assumed warm gas 1.4 g/s

Helium Gas - compressor discharge

compressor discharge pressure warm gas incidental use

Liquid Nitrogen 20-25 psig saturated liquid 1.6 g/s

Nitrogen Gas 20-25 psig warm gas incidental use

Cryogenic system requirements

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Cryogenic heat leaks in the BNL system

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Cryo schedule

• Cryogenics for g-2 storage ring is on critical path for g-2• Need to have ring cold in FY15 to allow ~9 months for field

shimming

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g-2 schedule priorities

• Critical path:– MC-1 building complete– Ship ring– Cool ring– Field shimming

• Advantages to completing Recycler extraction and primary beamline improvements early to commission in stages

• Secondary beamline, DR, extraction line work less priority