Jornadas LIP 2008 – Pedro Ramalhete

17 mhadron absorber

vertexregion

8 MWPCs4 trigger hodoscopes

toroidal magnet

dipole magnet

hadron absorber

targets

40 cm

NA60 @ CERNNA60 @ CERN

hadronabsorberBeam

Tracker

targets

Vertex Tracker

dipole field2.5 T

beam

IC

not to scale

beam

hadron absorber

QB and ZDC

targetarea

Besides the muon spectrometer and the silicon pixel vertex tracker, NA60 had several other detectors which are very useful to have a clean tagging of the centrality of the In-In collision

- Zero Degree Calorimeter (ZDC)on the beam line (just before the beam dump)

- Quartz Blade (QB)on the beam line, just upstream of the ZDC

- Beam Tracker (BT)on the beam line, upstream of the targets

- Interaction Counter (IC)between the vertex tracker and the hadron absorber

Centrality of the In-In Centrality of the In-In collisioncollision

5

Get the mass distribution of the VTdimuons produced in Ultra-Peripheral In-In Collisions

After accounting for backgrounds, acceptances and efficiencies, extract the absolute production cross sections of and J/ photo-production, as well as their pT and rapidity distributions

6

Beam ion remains intact◦ ZDC measures 115x158 = 18.17 TeV◦ Quartz Blade measures 49x49 = 2401◦ Only Opposite-Sign muon pairs are created

Beam

Target

7

Beam ion fragments◦ ZDC measures 115x158 = 18.17 TeV◦ Quartz Blade measures less than 49x49 = 2401◦ Only Opposite-Sign muon pairs are created

Beam

Target

8

Hadronic collision◦ ZDC measures less than 115x158 = 18.17 TeV◦ Quartz Blade measures less than 49x49 = 2401◦ Opposite-Sign and Like-Sign muon pairs are created

Beam

We are interested in considering two different cases within the UPCs:

1) In + In In + In +

2) In + In In* + In* +

In the first case, the beam nucleus remains intact and the QB detector should see a signal corresponding to the square of the charge of the In nucleus: 492

In the second case, the beam nucleus fragments after the UPC and the QB sees a signal smaller than 492 (it sees the sum of Zi

2, where Zi is the

charge of fragment i)

So, within the resolution of the QB detector, NA60 should be able to separate the UPC events in these two interesting sub-classes

What happens to the beam What happens to the beam nucleus ?nucleus ?

11

Run and Burst Selection◦ 2003 Indium-Indium data◦ Only the runs with 4000 A in the ACM toroidal magnet◦ NA60 standard run & burst selection for dimuon physics

analyses

Event Selection◦ Both muons must be matched

(from the Muon Spectrometer to the Vertex Tracker)◦ Both matched muons must belong to the same VT-Tracks vertex◦ The VT-Tracks vertex must be in target region

([-4 : +4] cm, to exclude vacuum windows)◦ Only Dimuon T0J triggers

(dimuon triggers stabilized in time by the Beam Tracker signals)

12

ZDC (Zero Degree Calorimeter)◦ Measures the energy of the “beam spectator” nucleons◦ Is sensitive to Beam Pileup (and Interaction Pileup)◦ Readout gate of ~19 ns

Quartz Blade◦ Measures the (sum of squared) number of charges of the “beam spectators”◦ Is sensitive to Beam Pileup (and Interaction Pileup)◦ Readout gate of ~30 ns

Beam Tracker (or “beamscope”, BS)◦ Counts the beam ions and measures their time in relation to the trigger;

it is also used to time-stabilize the dimuon trigger Interaction Counter

◦ Counts the interactions and measures their time in relation to the trigger◦ Is sensitive to Interaction Pileup

Vertex Tracker◦ Tracks charged particles and counts the number of interactions (Vertices)◦ Is sensitive to Interaction Pileup◦ Readout gate of ~200 ns

13

The 7 Indium targets are easily recognized

Two peaks on the edges: windows of the vacuum box

Selection: 4.0 to 4.0 cm;keeps only dimuons produced in In-In collisions

Opposite-sign:

Like-sign: and

15

Indium beam peak: EZDC = 115x158 = 18.17 TeV ; Blade = 49x49 = 2401

EZDC

Blade

16

Projection on the Blade axis using EZDC in the interval [12 : 24] TeV

Peak at 2382 (49x49=2401) Sigma = 216 (9%)

17

Projection on the EZDC axis using Blade in the interval [1800 : 3000]

Peak at 18.59 TeV (18.17) Sigma = 1.93 TeV (10%)

18

Using only events with ZDC triggers we obtain the two plots (log scale for entries): Left: ZDC vs Quartz Blade Right ZDC vs Quartz Blade (with BeamScope time selection to eliminate beam

pileup)

19

Using only events with ZDC triggers we obtain the following 3D plots (linear scale): Left: ZDC vs Quartz Blade Right ZDC vs Quartz Blade (with BeamScope time selection to eliminate beam pileup)

20

These figures show the ZDC vs. QB distributions for Dimuon triggers, before and after Beam Tracker Time Selection

This selection provides reliable measurements in the ZDC and Quartz Blade (with only very little remaining beam pileup)

BT Time Selection

21

In order to select ultra-peripheral events we use the ZDC and the Quartz Blade information and look for events with the following caracteristics:◦ The ZDC energy of a full ion: 115 nucleons x 158 GeV per nucleon = 18.17 TeV◦ The number of charges of a full ion: 49 protons;

if there is no beam fragmentation, the Quartz Blade measures 492 = 2401

We can use a 2D Gaussian to select events in a 2σ range around {18.17, 2401}, where σ is the resolution; this selection will reject 14% of good events

Given the resolutions of the ZDC and Quartz Blade detectors, the QB is better than the ZDC to distinguish “very peripheral collisions” from “no collisions at all”

The next slides show how the resolutions of the ZDC and QB were determined and how they are used in the Event Selection of this analysis

22

Animated 2D sequence of the plots shown on the previous slides.

The elipse shows the area of the Blade/ZDC 2σ cut

Important things to notice:◦ Disappearance of the “peak” at 492 for

VTTracks>6 ◦ Lowering of the mean ZDC value

(Mean y) from 18TeV (2 tracks) to 16TeV (30 tracks)

◦ Beam ion fragmentation increases with the number of tracks

◦ After 15 tracks, almost no event is inside the Blade/ZDC cut

Some slides with extra information

The challenges

dipole field

targets

beam tracker vertex tracker

hadron absorbermuonother

mag

netic fi

eld

iron w

all

muon trigger and tracking

• The vertex tracker has to handle very high charged-track multiplicities granularity silicon pixels !

• And it has to survive a very high number of interactions (much higher interaction rate than trigger rate) radiation hard detectors !

• And it should be as fast as possible, to reduce the interaction pile-up problem

radiation hard silicon pixel detectors... operating at 10 MHz

The muon spectrometer: some photos

hadron absorber

toroidalmagnet

trackingchambers

ironwall

previously used byNA10, NA38 & NA50

beam

added by NA60

The NA60 target region in the 2003 Indium run

2.5 T dipole magnet

Beam Tracker

Pixel detectors

operated at 130 K

(improves radiation hardness)

eight 4-chip and eight 8-chip planes