Jornadas LIP 2008 – Pedro Ramalhete

17 m hadron absorber vertex region 8 MWPCs 4 trigger hodoscopes toroidal magnet dipole magnet hadron absorber targets 40 cm CERN

hadron absorber Beam Tracker targets Vertex Tracker dipole field 2.5 T beam IC not to scale beam hadron absorber QB and ZDC target area 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 collision

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 p T and rapidity distributions

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

7  Beam ion fragments ◦ ZDC measures 115x158 = 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 = 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: 49 2 In the second case, the beam nucleus fragments after the UPC and the QB sees a signal smaller than 49 2 (it sees the sum of Z i 2, where Z i 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 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: E ZDC = 115x158 = TeV ; Blade = 49x49 = 2401 E ZDC Blade

16  Projection on the Blade axis using E ZDC in the interval [12 : 24] TeV  Peak at 2382(49x49=2401)  Sigma = 216(9%)

17  Projection on the E ZDC axis using Blade in the interval [1800 : 3000]  Peak at 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 = TeV ◦ The number of charges of a full ion: 49 protons; if there is no beam fragmentation, the Quartz Blade measures 49 2 = 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 49 2 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 target s beam tracker vertex tracker hadron absorber muon other magnetic field iron wall 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 toroidal magnet tracking chambers iron wall previously used by NA10, 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