Charm and intermediate mass dimuons in In+In collisions

R. Shahoyan, IST (Lisbon)on behalf of the NA60 collaboration

Quark Matter 2005, Budapest

Motivation (NA38/NA50 results)

NA60 concept and data analysis

Intermediate mass region (IMR) analysis

(preliminary results)

Summary and outlook

2

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 < cosCS < 0.5, etc) required charm production cross-sections

higher than the “world average”

NA38/NA50 proton-nucleus data

IMR dimuons in p-A collisions: the reference

cc

3

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

peripheralcollisions

centralcollisions

IMR dimuons in heavy-ion collisions: the excess

M (GeV/c2)M (GeV/c2)

4

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

Thermal dimuon production or charm enhancement?

The data collected by NA38/NA50 cannot distinguish among these two alternatives. We need to measure secondary vertices with ~ 50 m precision to separate prompt dimuons from D meson decays

5

hadron absorber

and trackingmuon trigger

magnetic field

iron w

all

muonother

Concept of NA60

targets

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

2.5 T dipole magnet

beam tracker vertex tracker

6Muon 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… but

a 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

7Vertex resolution (along the beam axis)

Beam Trackersensors

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

8Vertex resolution (in the transverse plane)

The interaction vertex is identified witha resolution of 10–20 m accuracy in the transverse plane

Dispersion between beam track andVT vertex

Vertex resolution (assuming BT=20 m)

10

20

30

0

(

m)

Number of tracks

Beam Tracker measurement vs. vertex reconstructed with Vertex Telescope

BTBT

The BT measurement (with 20 m resolution at the target) allows us to control the vertexing resolution and systematics

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J/

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)

Offset resolution

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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

2/)2( 11212

xyyyxx VyxVyVx

2/)( 22

21

Offset resolution

J/

Weighted Offset () 100

Offs

et r

esol

utio

n (

m)

11Background Subtraction: method

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 backgroundare subtracted by

Event Mixingin narrow bins in centrality, for each target,

and magnetic field polarities (~6000 samples)

wrongly matched combinatorials (fakes)

12Background 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.

CBmixing

13Background Subtraction: method

CBmixing

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.

14Background Subtraction: method

CBmixing

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…

Fakesmixing

15Background Subtraction: method

CBmixing

Fakesmixing

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

FakesCB

mixing

16Background Subtraction: method

CBmixing

Fakesmixing Fakes

CBmixing

Subtracting the combinatorial fakes from all fakes we obtain the fake signal

17Background Subtraction: method

CBmixing

Fakesmixing Fakes

CBmixing

Subtracting the fake signal from the total matched signal leads to the correct signal spectrum

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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 eventevent 1

event 2

Background Subtraction: method (offsets)

(All masses)

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

Background Subtraction: accuracy

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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

Accounting for residual background

21Background Subtraction: resulting mass distribution

Data integrated over centrality

Matching 2 < 1.5

SignalTotal Background

22Background Subtraction: resulting offset distribution

Signal Fake Matches

Dimuon weighted offsets

1.2 < M < 2.7 GeV/c2

0 < yCM < 1|cos| < 0.5

Kinematical domain where the analysis is performed:

23Offset 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

24NA60 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

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

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

The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data

25Is 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

Answer: Yes, an excess is clearly present !

(Even if we use the higher charm yield)

“world average” “NA50 p-A ”

26Is 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)

27Is 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

Dimuon weighted offsets

Answer: No, the fit fails: Charm is too flat to describe the remaining spectrum…

we need more prompts

28How many more prompts do we need?

Dimuon weighted offsets

Leave both contributions free,and see if we can describe the offset distribution

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)

29Is 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

Answer: No, both options require two times more prompts than the expected Drell-Yan !

(the Charm contribution is too small to make a difference)

Dimuon weighted offsets

“world average” “NA50 p-A ”

30Mass shape of the excess with respect to DY (or Charm)

The mass spectrum of the excess dimuons is steeper than DY(and flatter than Open Charm)

Fix the DY and Charm contributions to expected yields

31

Relative excess:(Data – Sources) / Sources

(Data – Sources) / Nparticipants

Faster than linear increase with Nparticipants

Centrality dependence of the Excess = [Data - Sources]

very

preliminary

1.There is an excess of intermediate mass dimuons in Indium-Indium collisions

2.The offset distribution requires a factor 2 more prompts than expected from DY The excess is not due to open charm enhancement

3.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

Summary and Outlook