1. Hermes Recoil Detector Status
Ignazio Vilardi
On 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. The Hunt for Lq
Study of hard exclusive processes leads to a new class of PDF’s
Generalized Parton Distributions
a theoretical framework that provides the
most complete description of the nucleon
possible access to Lq
Ji’s sum rule
exclusive:
all reactions are reconstructed
missing energy (DE) and missing Mass (Mx) = 0
from incl.-DIS:
HERMES ~0.3
A. Airapetian et al, Phys. Rev. D 75 (2007)
Ignazio Vilardi
DUBNA SPIN 07, September 2007

4. GPD’s Introduction AUT,sp+ AUT,sr,F,w AC,ALU, AUT, AUL
What do GPD’s characterize?
unpolarized polarized
conserve nucleon helicity
flip nucleon helicity
not accessible in DIS
DVCS
pseudo-scalar mesons
vector mesons
Observables:
AUT,sp+
AUT,sr,F,w
AC,ALU, AUT, AUL
Data from HERMES on tape
Ignazio Vilardi
DUBNA SPIN 07, September 2007

5. Deep Virtual Compton Scattering
HERMES kinematics:
BH c.s. >> DVCS c.s.
two experimentally indistinguishable processes:
DVCS
Bethe-Heitler
isolate BH-DVCS interference term non-zero azimuthal asymmetries
beam helicity asymmetry:
HERA (polarised electrons and positrons) only place in the world so far to measure the complete Compton amplitude
beam charge asymmetry:
Ignazio Vilardi
DUBNA SPIN 07, September 2007

6. DVCS event selection at HERMES
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:
Overall background
contribution  15% in exclusive region
Ignazio Vilardi
DUBNA SPIN 07, September 2007

7. A factor of 9 more data on tape!!
Beam Spin Asymmetry (BSA)
A. Airapetian et al., Phys. Rev. Lett. 87 (2001)
Expected sin dependence in exclusive region  Im H
sin amplitudes small and positive above exclusive region
A factor of 9 more data on tape!!
Ignazio Vilardi
DUBNA SPIN 07, September 2007

8. Expected cos() dependence  Re H
Beam Charge Asymmetry (BCA)
A. Airapetian et al., Phys. Rev. D75 (2007) 11103
Symmetrizised BCA in exclusive bin
  ||  Cancel sin dependence
Solid curve – 4 Parameter fit
P1 + P2 cos + P3 cos2 + P4 cos3
cos - amplitudes zero for
higher missing masses
Expected cos() dependence  Re H
A factor of 9 more data on tape!!
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
Ignazio Vilardi
DUBNA SPIN 07, September 2007

10. 3D Model of the Recoil Detector
1T Magnet not shown
Scintillating
Fibers
Tracker
(SFT)
10 cm
BEAM
Photon Detector (PD)
Silicon Strip
Detector
(SSD)
Ignazio Vilardi
DUBNA SPIN 07, September 2007

11. Performance of the Recoil Detector
detection of the recoiling proton
p: MeV/c;
76% f acceptance (because of
layout of SSD);
p/p PID via dE/dx;
MONTECARLO
Ignazio Vilardi
DUBNA SPIN 07, September 2007

12. Benefits of the Recoil Detector
background suppression
semi-incl. DIS: 5% -> <<1%
associated BH: 11% -> ~1%
Ignazio Vilardi
DUBNA SPIN 07, September 2007

13. Target cell inside beam pipe Thickness = 75 m and length = 150 mm
Ignazio Vilardi
DUBNA SPIN 07, September 2007

14. 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 MeV/c;
PID /p from dE/dx
for p < 250 MeV/c;
76% azimuthal
acceptance ();
Ignazio Vilardi
DUBNA SPIN 07, September 2007

15. 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 MeV/c;
PID /p from dE/dx for p < 700 MeV/c;
100% azimuthal acceptance;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

16. Photon Detector:  from + (+p0p)
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
(+  p0  p  );
Reconstructs 0 if both 
are detected;
Contributes to PID /p
(together with SFT) for
p > 600 MeV/c;
can provide
cosmic trigger;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

17. Magnet: Møller e- suppression and track bending
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;
Removes Møller (and Bhabha) electrons by letting them spiral forward and in this way protects the silicon detector from background;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

18. Data Taking and general performance
Data taking started in February 06 with electron beam
Fiber tracker fully operational
2.5 Mio. DIS from hydrogen target;
0.5 Mio. DIS from deuterium target;
Switch to positron beam
in July 06
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;
Recoil Detector ran stable
for 10 months
Ignazio Vilardi
DUBNA SPIN 07, September 2007

19. DATA 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);
<SRIM>
Protons
106 MeV/c
stopped in
2nd layer
135 MeV/c
Energy deposition in
inner Silicon module
vs. outer Silicon
module OK;
DATA
500 MeV/c
Deuterium target OK;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

20. Momentum reconstruction with Recoil D.
Low momentum protons (stopped in outer Silicon)
 Sum of energy deposits;
Higher momentum protons
 dE/dx (Bethe-Block formula);
High momentum particles
 Bending in magnetic field;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

21. 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;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

22. Residuals for the SFT from cosmic data
Parallel
top
 = 0.28mm
SFT
Parallel
Bottom
 = 0.31mm
1mm fibers
Ignazio Vilardi
DUBNA SPIN 07, September 2007

23. Residuals for SSD (magnet-off data)
inner
 = 0.26strip SI
outer
 = 0.28strip
Ignazio Vilardi
DUBNA SPIN 07, September 2007

24. Tracking SFT DATA SI DATA
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;
Ignazio Vilardi
DUBNA SPIN 07, September 2007

25. e-p elastic scattering Clear correlation can be observed;
First recoil-spectrometer correlation
Z 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
Ignazio Vilardi
DUBNA SPIN 07, September 2007

26. Demonstration of recoil principle
Mx < 1.7 Gev
The Detector works!!! p - +
Use Recoil Detector to remove background!!!
Ignazio Vilardi
DUBNA SPIN 07, September 2007

27. Outlook ... see you next year with physics results from
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)
... see you next year with physics results from
HERMES with Recoil Detector
Ignazio Vilardi
DUBNA SPIN 07, September 2007

28. Ignazio Vilardi
DUBNA SPIN 07, September 2007

29. HERA: e+/e- (27.6 GeV) - proton (920 GeV) collider
HERA
DESY
HERA: e+/e- (27.6 GeV) - proton (920 GeV) collider
Ignazio Vilardi
DUBNA SPIN 07, September 2007

30. HERMES: DIS e+/- (27.6 GeV) on p
Internal Gas Target: He , H , D , H unpol: H2,D2,He,N2,Ne,Kr,Xe
Particle ID: EM-Calorimeter: energy measurement for leptons and g
Preshower, TRD, a1997: Cherenkov, 1998a: RICH + Muon-ID
Momentum: measured by deflection of particle in magnetic field
Particle trajectories: measured by hits in tracking chambers
Ignazio Vilardi
DUBNA SPIN 07, September 2007

31. The spin structure of the nucleon
Gluons
are important !!
Sea quarks Dqs DG
Don’t forget the
orbital angular
Momentum!!
Naïve parton model
BUT
1989 EMC measured
S =
Spin Puzzle
Quark orb. ang. mom.
Gluon
orb.
ang.
mom.
Proton
spin
Gluon
spin
Quark spin
Ignazio Vilardi
DUBNA SPIN 07, September 2007

32. BCA – Comparison to Model calculations
M. Vanderhaegen et al., Phys. Rev. D 60 (1999)
K. Goeke et al., Prog. Part. Nucl. Phys. 47 (2001)
P1 = ± 0.019
P2 = ± 0.027
P3 = ± 0.026
P4 = ± 0.027
Large contribution from associated
production in last t-bin (not included in models)
GPD’s Model (only protons)
Regge-inspired with D-term disfavored
Ignazio Vilardi
DUBNA SPIN 07, September 2007