Charm and intermediate mass dimuons in In+In collisions

1. Charm and intermediate mass dimuons in In+In collisions • Motivation (NA38/NA50 results) • NA60 concept and data analysis • Intermediate mass region (IMR) analysis (preliminary results) • Summary and outlook R. Shahoyan, IST (Lisbon)on behalf of the NA60 collaboration Quark Matter 2005, Budapest

2. IMR dimuons in p-A collisions: the reference • NA38/NA50 was able to describe the IMR dimuon spectra in p-A collisions as the sum of Drell-Yan and Open Charm contributions • However, the yield needed to describe the NA38/NA50 spectra(with PYTHIA’s kinematical distributions, after B.R., acceptances, in the window 0 < yCM < 1, -0.5 < cosCS < 0.5, etc) required charm production cross-sections higher than the “world average” NA38/NA50 proton-nucleus data

3. centralcollisions peripheralcollisions IMR dimuons in heavy-ion collisions: the excess The yield of intermediate mass dimuons seen in heavy-ion collisions (S-U, Pb-Pb)exceeds the sum of DY and Open Charm decays, extrapolated from the p-A data M (GeV/c2) M (GeV/c2)

4. Thermal dimuon production or charm enhancement? The intermediate mass dimuon yields in heavy-ion collisions can be reproduced: • by scaling up the Open Charm contribution by up to a factor of 3 (!) • or by adding thermal radiation The data collected by NA38/NA50 cannot distinguish among these two alternatives.  We need to measure secondary vertices with ~ 50 mm precisionto separate prompt dimuons from D meson decays

5. muon trigger and tracking iron wall magnetic field targets hadron absorber muon other Concept of NA60 2.5 T dipole magnet vertex tracker beam tracker Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorberand match them to the muon measured in the spectrometer Improved kinematics (~20 MeV/c2 at  instead of 80 MeV/c2 in NA50)Origin of muons can be accurately determined

6. Muon Matching Muons from the Muon Spectrometer are matched to the Vertex Telescope tracks by comparing the slopes and momenta. Each candidate passing a matching 2 cut is refitted using both track and muon measurements, to improve kinematics. Most background muons from  and K decays are rejected in this matching step… buta muon might be matched to an alien track (or to its proper track which picked too many wrong clusters)  Fake matches, additional source of background  By varying the cut on the matching 2 we can improve the signal/background ratio

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

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

9. J/ Offset resolution Using the muons from J/ decays (no background, from the interaction point) we determine the resolution of the impact parameter of the track at the vertex (offset) : 40–50 m The non-Gaussian tails are caused by imperfect alignment (to be improved)

10. J/ Weighted Offset ()  100 Offset resolution (m) Offset resolution Good enough to separate prompt dimuons from Open Charm off-target decays !vertex  impact < c (D+ : 312 m, Do : 123 m) To eliminate the momentum dependence of the offset resolution, we use the offset weighted by the error matrix of the fit: for single muons and for dimuons

11. 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 pairsmuons 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 backgroundare subtracted byEvent Mixingin narrow bins in centrality, for each target, and magnetic field polarities (~6000 samples) wrongly matched combinatorials (fakes) Background Subtraction: method

12. CBmixing Background Subtraction: method 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.

13. CBmixing Background Subtraction: method 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.

14. CBmixing Fakesmixing Background Subtraction: method 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…

15. CBmixing Fakesmixing FakesCBmixing In order to extract from the fake matches the signal contribution we repeatthe Combinatorial Mixing procedure for the generated fakes sample, obtaining the combinatorial fake matches Background Subtraction: method

16. CBmixing Fakesmixing FakesCBmixing Subtracting the combinatorial fakes from all fakes we obtain the fake signal Background Subtraction: method

17. CBmixing Fakesmixing FakesCBmixing Subtracting the fake signal from the total matched signal leads to the correct signal spectrum Background Subtraction: method

18. mixed event event 1 event 2 Background Subtraction: method (offsets) 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. (All masses)

19. Background Subtraction: accuracy The quality of combinatorial subtraction can be controlled by comparing the built mixed event Like Sign dimuon spectra to the corresponding measured data.

20. Accounting for residual background The Like-Sign spectra should be similar to the background on the OS dimuon spectrum use residual LS background as an estimate of the unsubtracted OS background It is accounted as a systematical error: the errors on the background are globally scaled upto guarantee that the residual LS background is zero within 3 standard deviations Because of the high background level, a ~1% error in the background estimate leads to ~10% systematical error on the extracted signal

21. SignalTotal Background Background Subtraction: resulting mass distribution Data integrated over centrality Matching 2 < 1.5

22. Signal Fake Matches Background Subtraction: resulting offset distribution Dimuon weighted offsets 1.2 < M < 2.7 GeV/c20 < yCM < 1|cos| < 0.5 Kinematical domain where the analysis is performed:

23. Offset distributions of the expected sources • To account for residual misalignments in the real data, the offsets of the reconstructed MC muons were smeared until they reproduce the weighted offsets measured for J/ muons. • Prompt contribution: use an average of the J/ and  measured offset distributions • Open Charm contribution: use the MC distribution, after smearing Dimuon weighted offsets

24. NA60 Signal analysis: simulated sources Charm and Drell-Yan contributions are calculated by overlaying Pythia events on real data (using CTEQ6M PDFs with EKS98 nuclear modifications and mc=1.3 GeV/c2) The fake matches in the MC events are subtracted as in the real data Relative normalizations: • for DY: K-factor of 1.8; to reproduce DY cross-sections of NA3 and NA50 • for charm: we use two options for the expected cross-section: a) 6.3 b/nucleon: suggested by a “world average” of direct charm measurements b) a factor 2 higher: needed to reproduce the NA50 p-A dimuon data 450 GeV Absolute normalization: The expected DY contribution, as a function of the collision centrality, is obtained from the number of observed J/ events and the  suppression pattern; see talk by Roberta Arnaldi  A 10% systematical error is assigned to this normalization The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data

25. Is there an excess in In-In collisions? Fix the Charm and DY contributions to the expected yields,and see if their Sum describes the measured Data “NA50 p-A mm” “world average” Answer: Yes, an excess is clearly present ! (Even if we use the higher charm yield)

26. Is it compatible with the NA50 observation? Can we describe the measured mass spectrum by leavingthe Charm normalization as a free parameter? NA50 could, with up to a factor of 3 Charm enhancement in central Pb-Pb collisions… ~ 2 in terms of NA50p-A normalization Answer: Yes, leaving the Charm contribution free describes the In-In data, with a “charm enhancement” factor around 2 in “NA50 units” (but with a poor 2)

27. Dimuon weighted offsets Is this validated by the offsets information? Fix the prompt contribution to the expected DY,and see if we can describe the offset distribution with an enhanced Charm yield Answer: No, the fit fails: Charm is too flat to describe the remaining spectrum…  we need more prompts

28. How many more prompts do we need? Leave both contributions free,and see if we can describe the offset distribution Dimuon weighted offsets Answer: Two times more prompts than the expected Drell-Yan provides a good fit (and the Charm yield is as expected from the NA50 p-A dimuon data)

29. Is the prompt yield sensitive to the Charm level? Fix the Charm contribution to either of the two references,and see how the level of prompts changes “NA50 p-A mm” “world average” Dimuon weighted offsets Answer: No, both options require two times more prompts than the expected Drell-Yan ! (the Charm contribution is too small to make a difference)

30. Mass shape of the excess with respect to DY (or Charm) Fix the DY and Charm contributions to expected yields The mass spectrum of the excess dimuons is steeper than DY(and flatter than Open Charm)

31. Centrality dependence of the Excess = [Data - Sources] Relative excess:(Data – Sources) / Sources very preliminary (Data – Sources) / Nparticipants  Faster than linear increase with Nparticipants

32. Summary and Outlook • There is an excess of intermediate mass dimuons in Indium-Indium collisions • The offset distribution requires a factor 2 more prompts than expected from DY  The excess is not due to open charm enhancement • The excess grows faster than linearly with the number of participants • Results are very robust with respect to variations of the matching 2 cut: changing the Signal / Background ratio by a factor of 2 changes the results by less than 10%  The excess cannot be due to a bias in the background subtraction • For the moment, our offset distribution cannot discriminate between the two expected charm yields (which differ by a factor of two)  • Reprocess already analyzed data after improving the detector’s alignment • Explore full Indium-Indium statistics (~ 50% of the data not yet reconstructed) • Analyze high statistics p-nucleus 2004 data