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CLAS RTPC for 4He Experiment & Light Nuclei Tagging

S. Stepanyan (JLAB) For CLAS/eg6 group

Exploring Hadron Structure with Tagged Structure Functions,

Jefferson Lab, January 16-18, 2014

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Outline

n  Experiments in CLAS/EG6 run – DVCS and meson spectroscopy in coherent scattering

n  Second generation RTPC n  RTPC calibrations with elastic scattering off 4He n  Preliminary results on coherent DVCS off 4He

n  New Low-Energy Recoil Detector for CLAS12

S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

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CLAS/Eg6 Run n  Meson Spectroscopy and DVCS (coherent and incoherent) on 4He n  Both experiments are making use of zero spin and isospin of the target n  Coherent DVCS on 4He will allow to extract real and imaginary parts of

Compton amplitude in model independent way from BSA measurment

ALU =!0 "( )!A

!1 "( )+!2 "( )!A +!2 "( ) !A2 +!A2( )n  Production of π0η and π0η’ off 4He limits quantum numbers of exchange

state and simplifies PWA analysis

n  Both experiments required detection and identification of recoil α-particles q  2nd Generation Radial Time Projection Chamber with 20 cm long, 6 atm, 4He

gaseous target located inside of the Hall-B superconducting solenoid (used for RTPC momentum analysis and Møller reduction)

q  Production data with 6.6 GeV polarized electrons, 1 GeV run for RTPC calibration

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Target & RTPC assembly for eg6

New target cell, 6 mm ID & with 27 µm wall thickness, target gas He @ 6 atm. (Bonus run with 50 µm thick target cell)

He @ 1 atm

Working gas, NeDME @ 1 atm

Open geometry (has not been utilized effectively)

Improved backend readout and increase the trigger rate to 2.5 kHz

Total material thickness on the way of particles at 90o is ~39 µm

2 aluminized mylar foils, 2 and 6 µm thick


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Δ, N*

Elastic scattering on 4He at 1.2 GeV

RTPC calibration


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Selection of elastic events


Inclusive electron W-distribution of all events

W = MHe

2+ 2!M

He

2"Q

2

! = Ee"E

e'

Q2= 2E

eEe'(1" cos#

e')

φ- vs. Z-RTPC distribution of elastic electron in CLAS and a track in RTPC

Difference between measured and calculated from elastic kinematics polar angle of recoil 4He

Difference between measured and calculated from elastic kinematics polar angle of recoil 4He

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Δϕ CLAS-RTPC (°)

RTPC Efficiency: Detect Elastic 4He n  With electron is selected, look for RTPC track

with matching vertex and scattering angles q  Each Δ-quantity below has a cut on the others’

peaks q  Resolutions are roughly 8mm, 2°,3° on z,φ,θ

n  Test cut sensitivity, particularly Δθ n  Result: ~400K Exclusive Elastic Events


ΔZ CLAS-RTPC (cm) Δθ CLAS-RTPC (°)

W (GeV)

Pass

Fail Elastic Cuts

Inclusive

e 4He! "e 4He

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RTPC Efficiency: Measure Yields

n  The ration of the number of Exclusive and Inclusive Elastic events ( i.e. with and without 4He detection) will be the RTPC efficiency

n  Shown here is Gaussian signal and backgrounds q  Physical models of background (quasi-

elastic) and signal (radiated elastic peak) were also tried

n  Efficiency is not highly sensitive to yield extraction method so long as it is consistent for exclusive and inclusive final states


W (GeV)

Inclusive

Exclusive

Example Fits

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RTPC Efficiency: Result

n  What is the best variable(s)? q  Ideally p,θ,ϕ,z simultaneously q  Very limited p/θ range for elastics (50

MeV/c, 5°) q  ϕ is tricky due to CLAS+RTPC

acceptance n  Z-vertex is also important

q  Calibration is more difficult at ends of the detector

q  Sensitive to field effects q  We find similar tracking efficiencies

for LEFT/RIGHT, although we know noise and # dead channels is different


Z-Vertex (cm)

RTP

C E

ffic

ienc

y

Q2 (GeV2)

Yiel

d

LEFT

RIGHT

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RTPC gain calibration with elastic events


PID in RTPC is based on a track energy loss in the drift volume of the detector – calibration of gains of individual pads are very important Large gain variation from pad-to-pad is due to non-uniformity of space between GEM plains

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DVCS Exclusivity Variables: Simulation vs. Data


θγX (°)

Missing PT (GeV) Coplanarity θγγ*He (°)

M2X (eγ4He) (GeV2)

EX (eγ4He) (GeV)

e4He è eγ4He

Full Exclusivity Cuts |θcop| < 2° |θγX| < 2°

PtX < 0.2 GeV |M2X| < 0.02 GeV2

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BSA in coherent DVCS -


0

ALU =α sinφ

1 + β cosφ

n  1-d t-bins n  83.7% beam polarization

is taken into account n  1-σ MINOS uncertainties

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–  Significant trends in t and xB, similar to Guzey’s calculation •  Model from EG6 proposal •  Kinematics are close but not matched to data

Compton Form Factor HHe, Re and Im parts


EG6 Range 1 < Q2 < 2.3 GeV2

0.05 < -t < 0.2 GeV2 0.1 < xB < 0.25

GS: Guzey & Strikman, Phys. Rev. C 68 (2003) 015204

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Results: BSA and Generalized EMC Ratio Beam Spin Asymmetry @ 90

q  Significantly non-zero and relatively flat ~25%

q  Consistent with HERMES ((eγX, no 4He detection)

n  A. Airapetian et al, Phys. Rev. C 81 (2010) 035202


GS: Guzey & Strikman, Phys. Rev. C 68 (2003) 015204 LT: Liuti & Taneja, Phys. Rev. C 72 (2005) 032201

AHe

LU

/A

p LU

@90

◦

AHeLUApLU

@ 90◦

Generalized EMC Ratio –  Binning chosen to match published

e1dvcs kinematics for the denominator –  We only cover eg1dvcs’s lowest t-bin –  A hint of the predicted behavior

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New LERD for CLAS12


N. Baltzel (ANL)

•  Low-pressure wire chamber (LPWC), surrounded Si-strip layers, and scintillator fibers

•  Insensitive to MIP, LPWC provides fast signal for triggering and PID

•  All inside low-pressure vessel, vessel inside the solenoid (for simulation CLAS12 solenoid at 60% field has been used

•  Momentum from tracking, PID from energy loss and time information

Lessons learned from GEM RTPCs: o  big gain variations from pad-to-pad (two mechanically different detector

designs, in both the main problem is uniformity of diisdance between GEM planes) - limits PID based on dE/dX

o  no triggering capability, limits trigger rates, target windows have big effect in the trigger

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Low energy recoil detector and CLAS12 CD

Silicon Tracker

Scintillator Counters, δt=60ps

5T SC Solenoid Magnet

Replace CLAS12 barrel tracker with LERD Use CTOF to extend momentum range of LERD


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Particle ID


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Summary q  CLAS experiment for coherent photo- and electro-production on a spin and

isospin zero target, 4He, run in 2009 (with shorter than PAC approve beam time) q  Second generation RTPC à la Bonus detector has been deployed for detection

and identification of low energy recoil particles The new RTPC had few improvements – open geometry for a drift volume, lower material budget on a way particles, a much faster trigger rate …

q  Although Bonus and eg6 RTPCs have very different mechanical design, the non-uniformity of distances between GEM planes remind a main issue for pad gain uniformity and hence was the limiting factor in PID reach

q  For reliable detection and identification of wide verity of low energy particles - p, d, 3H, 3He, and 4He, a fast timing information from the detector is desirable

q  Fast timing information can be used in the trigger as well – trigger rate was a limiting factor for high luminosity running in eg6, since with only CLAS trigger the 60% of events did not have RTPC track, and 40% come from target windows

q  A new detector concept has been developed for CLAS12 based on LPMWC, Si-strips and scintillator fibers, simulations and conceptual design is in progress

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Backup slides

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Forward Central Detector Detector

Angular range Tracks 50 – 400 350 – 1250 Photons 20 – 400 ---

Resolution

δp/p (%) < 1 @ 5 GeV/c < 5 @ 1.5 GeV/c

δθ (mr) < 1 < 10 - 20

Δφ (mr) < 3 < 5

Photon detection Energy (MeV) >150 ---

δθ (mr) 4 @ 1 GeV --- Neutron detection

Neff < 0.7 (EC+PCAL) n.a. Particle ID

e/π

Full range ---

π/p

< 5 GeV/c < 1.25 GeV/c

π/K < 2.6 GeV/c < 0.65 GeV/c K/p

< 4 GeV/c < 1.0 GeV/c

π(η)gγγ

Full range ---

CLAS12 – Design Parameters Forward Detector

Central Detector

!

L =1035cm

"2s"1


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hist1_0013

Entries 8876

Mean 27.8

RMS 9.573

0 20 40 60 80 100 120 1400

50

100

150

200

250

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400

450

hist1_0013

Entries 8876

Mean 27.8

RMS 9.573

THETA (deg) itrk0

hist1_0012Entries 8876

Mean 1.1

RMS 0.5973

0 0.5 1 1.5 2 2.5 30

20

40

60

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100

120

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160

180

200

220


Mean 1.1

RMS 0.5973

P (GeV) itrk0

hist1_0015

Entries 8876

Mean -58.93

RMS 20.11

-100 -80 -60 -40 -20 0 200

50

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hist1_0015

Entries 8876

Mean -58.93

RMS 20.11

Zv (cm) itrk0


Mean 167.9

RMS 102.5

0 50 100 150 200 250 300 3500

20

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120

140

160


Mean 167.9

RMS 102.5

PHI(deg) itrk0

hist1_0158

Entries 32

Mean -55.62

RMS 24.1

-1000-800 -600 -400 -200 0 200 400 600 800 10000

2

4

6

8

10

12

hist1_0158

Entries 32

Mean -55.62

RMS 24.1

Zv (cm) long range


Mean -0.2179

RMS 2.507

-10 -8 -6 -4 -2 0 2 4 6 8 100

200

400

600

800

1000

1200


Mean -0.2179

RMS 2.507

Yv (cm) itrk0

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Helium bag downstream window (15 µm Al)

Target downstream window (15 µm Al)

Target upstream window (φ 4mm, 15 µm thick Al)

4He at 1 atm

Target gas

Passage of 100 nA 6 GeV through 30 cm long “straw” target

RTPC and high pressure helium target


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