Hermes Recoil Detector Status Ignazio Vilardi On behalf of the HERMES Collaboration

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


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;


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

qq

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


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,(


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


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


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


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


The end of polarized targets at HERMES Improve Exclusivity Selection Detect recoiling proton

modify target region

Detector to measure recoiling proton


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


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


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


Target cell

Target cell inside beam pipeTarget cell inside beam pipeThickness = 75 Thickness = 75 m and length = 150 mmm and length = 150 mm


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 ();


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;


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;


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


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


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


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;


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;


Residuals for the SFT from cosmic data

SFT Paralleltop

SFT ParallelBottom

= 0.28mm

1mm fibers

= 0.31mm


Residuals for SSD (magnet-off data)

= 0.28strip

= 0.26strip

SI inner

SI outer


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;


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


Demonstration of recoil principle

p

+-

Use Recoil Detector to remove Use Recoil Detector to remove background!!!background!!!


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)



HERA: eHERA: e++/e/e- - (27.6 GeV) - proton (920 GeV) (27.6 GeV) - proton (920 GeV) collidercollider

HERA @ DESYHERMES @ HERA


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


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


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

Documents

Hermes Recoil Detector Status Ignazio Vilardi On behalf of the HERMES Collaboration