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Ignazio Vilardi DUBNA SPIN 07, September 2007 1
Hermes Recoil Detector Status
Ignazio VilardiOn behalf of the HERMES Collaboration
DUBNA-SPIN-07 September 3 – 7, 2007
Ignazio Vilardi DUBNA SPIN 07, September 2007 2
Outline GPD’s and hard exclusive processes;
Hermes results: Beam Spin Asymmetry (BSA) and Beam Charge Asymmetry (BCA);
The Hermes Recoil Detector: Motivation and Design;
Hermes Recoil Detector Status;
Outlook;
Ignazio Vilardi DUBNA SPIN 07, September 2007 3
Study of hard Study of hard exclusive processesexclusive processes leads to a new class of leads to a new class of PDF’sPDF’s
GGeneralized eneralized PParton arton DDistributionsistributions, , ,q qq qH H EE
possible access to Lpossible access to Lqq
1
1 0
1
2q
t
HJ Exdx
1
2q qLJ exclusive:exclusive: all reactions are reconstructedall reactions are reconstructedmissing energy (DE) and missing Mass (Mmissing energy (DE) and missing Mass (Mxx) = 0) = 0
from incl.-DIS:from incl.-DIS:HERMES ~0.3HERMES ~0.3
The Hunt for Lq
A. Airapetian et al, Phys. Rev. D 75 (2007) 012007
a theoretical framework that a theoretical framework that provides the provides the most complete description of the nucleonmost complete description of the nucleon
Ji’s sum ruleJi’s sum rule
Ignazio Vilardi DUBNA SPIN 07, September 2007 4
What do GPD’s characterize?What do GPD’s characterize?unpolarized unpolarized polarizedpolarized
, ,qH x t , ,qH x t
, ,qE x t , ,qE x t
conserve nucleon helicityconserve nucleon helicity
flip nucleon helicity flip nucleon helicity not accessible in DISnot accessible in DIS
Data from HERMES on tapeData from HERMES on tape
DDVVCCSS pseudo-scalar mesonspseudo-scalar mesons vector mesonsvector mesons
AACC,A,ALULU,, AAUTUT, A, AULUL AAUTUT,,++ AAUTUT,,
Observables:Observables:
,q qH E ,q qH EqHqEqH
GPD’s Introduction
qxHqxH qq )0,0,(~
;)0,0,(
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two experimentally indistinguishable processes:HERMES kinematics:BH c.s. >> DVCS c.s.
* 2 2*~ | | | |BH DVCS DVC BS HB DVCSHd isolate BH-DVCS interference term non-zero azimuthal asymmetries
beam helicity asymmetry:
~ Im( )BH DVCSe ed d 89
beam charge asymmetry:~ Re( )
e BH De VCSd d
HERA (polarised electrons and positrons) only place in the world so far to measure the complete Compton amplitude
Deep Virtual Compton Scattering
DVCSDVCS Bethe-HeitlerBethe-Heitler
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Exactly 1 DIS lepton in the spectrometer and 1 photon in the calorimeter;
Recoiling proton undetected Exclusive reactions via the missing mass technique ;
exclusive region:
DVCS event selection at HERMES
222x
2 GeV 7.1M5.1 'epeM x
peep ' 'eep
Xeep 0'
Overall background contribution 15% in exclusive region
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Beam Spin Asymmetry (BSA)
1)()(
)()(
||
1)(
FNN
NN
PA
b
LU
1
Expected sin dependence in exclusive region Im Hsin amplitudes small and positive above exclusive region
sinHIm
A. Airapetian et al., Phys. Rev. Lett. 87 (2001) 182001
Ignazio Vilardi DUBNA SPIN 07, September 2007 8
Beam Charge Asymmetry (BCA)
1)()(
)()()(
FNN
NNAC
1 cosRe H
Symmetrizised BCA in exclusive bin || Cancel sin dependenceSolid curve – 4 Parameter fitP1 + P2 cos + P3 cos2 + P4 cos3
cos - amplitudes zero for higher missing masses
027.0060.0cos CA
A. Airapetian et al., Phys. Rev. D75 (2007) 11103
Expected cos() dependence Re H
Ignazio Vilardi DUBNA SPIN 07, September 2007 9
The end of polarized targets at HERMES Improve Exclusivity Selection Detect recoiling proton
modify target region
Detector to measure recoiling proton
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10 cm10 cm
3D Model of the Recoil Detector
Photon Detector (PD)Photon Detector (PD)
ScintillatinScintillating g
FibersFibersTracker Tracker
(SFT)(SFT)
1T Magnet not shown1T Magnet not shown
Silicon StripSilicon StripDetector Detector
(SSD)(SSD)
BEAM
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detection of the recoiling protondetection of the recoiling proton p: 135 -1200 MeV/c;p: 135 -1200 MeV/c; 76% 76% acceptance (because of acceptance (because of
layout of SSD);layout of SSD); /p PID via dE/dx; /p PID via dE/dx;
Performance of the Recoil Detector
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background suppressionbackground suppression
Benefits of the Recoil Detector
semi-incl. DIS: 5% -> semi-incl. DIS: 5% -> <<1%<<1%
associated BH: 11% -> associated BH: 11% -> ~1%~1%
Xeep 0'
0,
'
pn
eep
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Target cell
Target cell inside beam pipeTarget cell inside beam pipeThickness = 75 Thickness = 75 m and length = 150 mmm and length = 150 mm
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Silicon Detector: protons < 0.5 GeV/c 16 double sided Silicon sensors, size 99 x 99 mm2, 300 µm thickness; 128 strips per sensor side, 8192 read out channels in total, 758 µm strip pitch; High and Low Gain read out; strip orientation for space point reconst.; p-measurements from dE/dx 135 - 500 MeV/c; PID /p from dE/dx for p < 250 MeV/c; 76% azimuthal acceptance ();
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SFT Detector: protons > 0.3 GeV/c 2 barrels with each parallel and stereo layer (10°) for space point reconstruction and tracking;
4992 fibers (1 mm diameter) in total; p - measurements 300-1200 MeV/c; PID /p from dE/dx for p < 700 MeV/c; 100% azimuthal acceptance;
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Photon Detector: from + (+p0p) 3 layers of scintillators after tungsten converter: the first
parallel and the last 2 stereo (± 45°); 60 strips in the parallel one and 44 in the other ones; Detects photons from intermediate resonance (+ p0 p ); Reconstructs 0 if both are detected; Contributes to PID /p (together with SFT) for p > 600 MeV/c; can provide cosmic trigger;
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Removes Møller (and Bhabha) electrons by letting them spiral forward and in this way protects the silicon detector from background;
Provides a 1T solenoid longitudinal superconducting magnetic field for tracking. For this reason the homogeneity of the field has to be better than 20% in either direction;
Magnet: Møller e- suppression and track bending
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Data Taking and general performance• Fiber tracker fully operational• 2.5 Mio. DIS from hydrogen target;• 0.5 Mio. DIS from deuterium target;
• Fiber tracker and Photon detector fully operational;• Finished commissioning of Silicon Detector in September 06;• 28 Mio. DIS events from hydrogen target;• 7 Mio. DIS events from deuterium target;
Switch to positron beam in July 06
Data taking started in February 06 with electron beam
Recoil Detector ran stablefor 10 months
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First Recoil Detector Results (SSD) Signal from the Silicon detector divided by a coupling
capacitor (10 pF) into HG and LG readout channels: its ratio OK; Correlation between inner and outer Silicon modules OK (different disposition of Silicon strips); Energy deposition in inner Silicon module vs. outer Silicon module OK;
<SRIM>Protons
Deuterium target OK;
stopped instopped in22ndnd layer layer
106 MeV/c106 MeV/c
135 MeV/c135 MeV/c
500 MeV/c500 MeV/c
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Momentum reconstruction with Recoil D.
Higher momentum protons dE/dx (Bethe-Block formula);
Low momentum protons (stopped in outer Silicon) Sum of energy deposits;
High momentum particles Bending in magnetic field;
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Internal Alignment (magnet-off data)
Six parameters (three translations and three rotations) which are common for all tracks are fitted;
Each track is fitted with a straight line taking into account alignment parameters at current iteration;
After iterative procedure converges it is repeated with new initial values of alignment parameters to be sure that alignment procedure does not depend on initial approach;
Residuals and dependence of residuals on coordinates used as a tool to check alignment procedure;
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Residuals for the SFT from cosmic data
SFT Paralleltop
SFT ParallelBottom
= 0.28mm
1mm fibers
= 0.31mm
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Residuals for SSD (magnet-off data)
= 0.28strip
= 0.26strip
SI inner
SI outer
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Tracking
Full tracking is in production including alignment; Efficiency of the tracking algorithm studied on MC and found to be 98.4 %; Starting to study ghost tracks;
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First recoil-spectrometer correlation
e-p elastic scattering
Selection of single recoil tracks by making cuts on
the momentum of the lepton
detected by the forward
spectrometer.
Clear correlation can be observed;
correlation
correlation
Z correlation
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Demonstration of recoil principle
p
+-
Use Recoil Detector to remove Use Recoil Detector to remove background!!!background!!!
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Outlook Analysis with Recoil Detector;
Conventional analysis (without Recoil Detector);
Once background contribution is measured: refine analysis of pre-recoil DVCS data;
• DVCS Beam Charge Asymmetry (BCA) challenging as only Fiber Tracker operational during e- running
• DVCS Beam Spin Asymmetry (BSA)
• Hard exclusive meson production (, 0, 0)
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Ignazio Vilardi DUBNA SPIN 07, September 2007 29
HERA: eHERA: e++/e/e- - (27.6 GeV) - proton (920 GeV) (27.6 GeV) - proton (920 GeV) collidercollider
HERA @ DESYHERMES @ HERA
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Particle ID: EM-Calorimeter: energy measurement for leptons and Preshower, TRD, 1997: Cherenkov, 1998: RICH + Muon-ID
Momentum: measured by deflection of particle in magnetic field
Particle trajectories: measured by hits in tracking chambers
HERMESHERMES:: DIS eDIS e+/-+/- (27.6 GeV) on p (27.6 GeV) on p
Internal Gas Target: He , H , D , H unpol: H2,D2,He,N2,Ne,Kr,Xe
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1 1( )
2 2 v svu q Gd
( ) s su d u d s s
1 1( )
2 2 v gsv qLd LGqu
The spin structure of the nucleonThe spin structure of the nucleon
Naïve parton modelNaïve parton model43 vu 1
3 vdBUTBUT
1989 EMC measured = 0.120 0.094 0.138± ±
Spin PuzzleSpin Puzzle
Gluons Gluons
are important !!are important !!
Sea quarks Sea quarks qqss
GG
Don’t forget theDon’t forget theorbital angular orbital angular
Momentum!!Momentum!!
1 1( )
2 2 v vu d
Quark orb. ang. mom.
Gluonspin
Gluonorb.ang.
mom.
Protonspin
Quark spin
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BCA – Comparison to Model BCA – Comparison to Model calculationscalculations
PP11 = - 0.011 = - 0.011 ± 0.019± 0.019PP22 = 0.060 = 0.060 ± 0.027± 0.027PP33 = 0.016 ± 0.026 = 0.016 ± 0.026PP44 = 0.034 ± 0.027 = 0.034 ± 0.027
M. Vanderhaegen et al., Phys. Rev. D 60 (1999)K. Goeke et al., Prog. Part. Nucl. Phys. 47 (2001)
• Large contribution from associated production in last t-bin (not included in models) • GPD’s Model (only protons)• Regge-inspired with D-term disfavored