1. Muon detection in NA60  Experiment setup and operation principle  Coping with background R.Shahoyan, IST (Lisbon)

2. hadron absorber magnetic field iron wall targets 2.5 T dipole magnet beam tracker vertex tracker muon spectrometer NA60 setup and operation principle Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them to the muon measured in the spectrometer  Improved kinematics (~20 MeV/c 2 at  instead of 80 MeV/c 2 in NA50) Origin of muons can be accurately determined

3. hadron absorber magnetic field iron wall targets 2.5 T dipole magnet beam tracker vertex tracker muon spectrometer NA60 setup and operation principle 40 cm BeO 25 cm Al 2 O 3 440 cm C 40 cm Fe  ~50 X 0 and 14  I Together with 120 cm Iron wall imposes ~4.5 GeV cut on triggered mouns 7 Indium subtargets with 8 mm spacing l =1.5mm, Ø =1mm, except the first one with Ø =12mm  0.16 In-In interaction probability Note: muons suffer most of the multiple scattering in the end of the absorber  allows partially to correct the measured angles (Branson plane correction).

4. hadron absorber magnetic field iron wall targets 2.5 T dipole magnet beam tracker vertex tracker muon spectrometer NA60 setup and operation principle 8 MWPC (24 planes) with 2mm pitch 4 scintillator trigger hodoscopes  highly selective dimuon trigger: 4.5  10 7 ion per 6s burst (1.2 MHz int.rate)  ~5000 triggers  ~50% lead to reconstructed dimuon toroidal magnet tracking chambers iron wall

5. Dimuon trigger Azimuthal acceptance is split to 6 windows of 42 o (because of the magnet coils) Trigger requires at least 2 sextants with “single muon” condition: similar slabs are fired in 2 homothetic hodoscopes (R1,R2) before the magnet (selects the particles coming from the target region, measures polar angle) fired slabs in 2 hodoscopes (R3,R4) after the magnet are allowed by the special r1,2:r3:r4 slabs coincidences matrix accounting for the deflection (~1/P T ) of the muon with acceptable momentum in the toroidal magnetic field (rough momentum measurement) Trigger efficiency (~90%) is monitored by extra hodoscopes during special runs

6. hadron absorber magnetic field iron wall targets 2.5 T dipole magnet beam tracker vertex tracker muon spectrometer NA60 setup and operation principle ×8 pixel (ALICE1) planes (~ 2%X 0 per plane) Beam Tracker meausers incoming ion with  x,y~20  m

7. hadron absorber magnetic field iron wall targets 2.5 T dipole magnet beam tracker vertex tracker muon spectrometer Matching is done by selecting muon - VT track associations with small enough weighted distance (  2 matching ) in x,y-slopes and inverse momentum. VT tracks  p /p ~ 6%    1 mrad  impact ~50  m measured muons  p/p ~ 2%  ~ 10 mrad impact ~ 5 cm (at 10 GeV/c) NA60 setup and operation principle

8. Effect of the matching on the mass resolution Mass spectrum from the Muon Spectrometer only (  M ~80 MeV/c 2 at  ) Mass spectrum after matching (  M ~20 MeV/c 2 at  ) Dimuon matching rate ~70% at J/ , decreases towards low masses (difference between the detector acceptances, VT reconstruction eff…

9. Each muon may be associated with VT tracks made of clusters generated by:  correct muon only (or its parent for ,K  ) :correct match  completely alien tracks : fake match  correct muon and some extra clusters from other tracks if the track is present only because the muon left enough hits:correct if the track would exist even in the absence of the muon hits:fake The dimuon is “correctly matched” if none of the 2 muons is fake Matching between muons and tracks in vertex region correct matches purely fake matches (no cluster due to the muon in the matched track)

10. By varying the cut on the matching  2 we can change the signal / fakes ratio Since the muons from ,K decays have typically higher matching  2 than the prompt ones (due to decay angle between the parent in VT and muon in the MS ), we can reduce the combinatorial background contribution by tightening the matching condition. Matching between muons and tracks in vertex region Mean number of candidates per matched muon (matching  2 <3) Multiplicity in the Vertex Tracker

11. Acceptance for matched dimuons (enhanced at low M/Pt wrt NA50 thanks to the dipole magnet )

12. Beam Tracker sensors windows Good target identification even for the most peripheral collisions (  4 tracks) The interaction vertex is identified with better than 200  m accuracy along the beam axis Vertex resolution (along the beam axis)

13. Vertex resolution (in the transverse plane) The interaction vertex is identified with a resolution of 10–20  m accuracy in the transverse plane Dispersion between beam track and VT vertex Vertex resolution (assuming  BT =20  m) 10 20 30 0  (  m) Number of tracks Beam Tracker measurement vs. vertex reconstructed with Vertex Tracker BT The BT measurement (with 20  m resolution at the target) allows us to control the vertexing resolution and systematics

14. Weighted Offset for J/ψ XY The tail is gone ! Offset resolution (and its dependence on alignment) J/  at SPS energies is always prompt: use it to monitor the offset resolution For real analysis use offsets weighted by the track and vertex fir error matrix, to minimize the dependence of the cuts on the muon momentum Data presented at QM05 After fine alignment (2006)

15. Background Subtraction Our measured dimuon spectra consist of: correctly matched signal signal muons from the spectrometer are associated with their tracks in the Ver.Tel. wrongly matched signal (fakes) at least one of the muons is matched to an alien track correctly matched combinatorial pairs muons from ,K decays are associated with their tracks or with the tracks of their parent mesons association between the ,K decay muon and an alien track All these types of background are subtracted by Event Mixing in narrow bins in centrality, for each target, and magnetic field polarities (~6000 samples) wrongly matched combinatorials (fakes)

16. Background Subtraction Combinatorial Background (mainly from uncorrelated  and K decays) Subtracted by building a sample of  pairs using muons from different Like Sign events. Mixing procedure accounts for correlations in the data due to the dimuon trigger. CB mixing

17. Background Subtraction CB mixing Subtracting the Mixed CB from the data we obtain the Signal (correct and fake) in  +  - sample and zero (or residual background) in the Like Sign dimuons sample.

18. Background Subtraction CB mixing The Fake Matches Background is subtracted by Monte Carlo (used for the Low Mass Analysis) or by matching the muons from one event to tracks from another one; a special weighting procedure is used to account for the mixed fake matches… Fakes mixing

19. Background Subtraction CB mixing Fakes mixing In order to extract from the fake matches the signal contribution we repeat the Combinatorial Mixing procedure for the generated fakes sample, obtaining the combinatorial fake matches Fakes CB mixing

20. Background Subtraction CB mixing Fakes mixing Fakes CB mixing Subtracting the combinatorial fakes from all fakes we obtain the fake signal

21. Background Subtraction CB mixing Fakes mixing Fakes CB mixing Subtracting the fake signal from the total matched signal leads to the correct signal spectrum

22. Background Subtraction Comb.bckg. generation quality is monitored by comparison to Like-Sign spectra. Fake matches bckg. generation quality is monitored by comparison to MC overlaid by real data

23. The “mixed” background sample (fake matches and combinatorial) must reproduce the offsets of the measured events: therefore, the offsets of the single muons (from different events) selected for mixing must be replicated in the “mixed” event. mixed event event 1 event 2 Background Subtraction (offsets) (All masses) “Dimuon weighted offset”( ) from vertex

24. Data integrated over centrality (with old alignment!) Matching  2 < 1.5 signal total background “Dimuon weighted offset”( ) from vertex Final spectra