ALICE detector and trigger strategy for diffractive and electromagnetic processes

1
ALICE detector and trigger strategy for
diffractive and electromagnetic processes

• ALICE experiment
• ALICE detector systems
• Diffractive gap trigger
• Signatures of Odderon
• Conclusions, outlook

2
The ALICE experiment
Acceptance central barrel -0.9 lt h lt 0.9
HMPID
TRD
MUON SPEC.
ITS
TPC
Acceptance muon spectr. 2.5 lt h lt 4.
TOF
PHOS
3
The ALICE magnet
LHC IP2 (LEP L3)
B 0.2 - 0.5 T
2001
2006
4
The inner tracking system (ITS)
Inner Tracking System 10 m2 Si detectors, 6
layers Pixels, Drift, double sided Strips
Pixels
Strips
Drift
5
Time projection chamber (TPC)

• largest TPC tracking and PID by dE/dx
• l 5 m, Ø 5.6m, 90 m3, 20 tons570 k
  channels, 500 x 106 space-time pixelsup to 80
  Mbyte/event (after 0 suppression)
• very thin lightweight FC 3 X0
• 3 mm Al, only 5x more than the drift gas !
• very delicate fragile
• high track density small space charge
• drift gas, chamber lay-out, field strips, ..
• HV100 kV (400V/cm)
• novel digital electronics (ALTRO)
• highly integrated, digital shaping many other
  features

drift gas Ne - CO2 N2 (86/9/5)
6
TPC in operation
16 May 2006 First cosmic and laser tracks !
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Transition Radiation Detector (TRD)

• Electron-pion separation, tracking, trigger on pt
  gt 1.5 GeV/c

gas
TRD Supermodule assembly
Xe/CO2 (85-15)
supermodule 6 layers radially 5 segments
z-dir. g 30 chambers
TRAP chip 300,000 RISC processors distributed
over 700 m2
18 supermodules
total 540 chambers
8
Time of Flight Detector (TOF)

• MRPC, multi-gap-resistive plate chambers, s lt 100
  ps

TOF strip production 150 m2, 150,000 channels
9
ALICE central barrel tracking performance

• Robust, redundant tracking
• from lt 100 MeV/c to gt 100 GeV/c
• Very little dependence on
• dN/dy up to dN/dy 8000

• ?p/p lt 5 at 100 GeV with careful control of
  systematics

10
ALICE central barrel particle identification

• - decay topologies (K0, K, K-, ?, D)
• - K and L decays beyond 10 GeV/c
• leptons (e,µ ), photons, p0
• electrons TRD p gt 1 GeV/c

• excellent particle ID up to 50 to 60 GeV/c

11
ALICE central barrel comparison to other LHC
detectors
h-pt acceptance

• Relatively low magnetic field

12
ALICE diffractive gap trigger
g additional forward detectors for event
classification (no particle
identification)
1 lt h lt 5
-4 lt h lt -1
g definition of gaps h , h_
Luminosity L 5x1030cm-2s-1
g one interaction/ 80 bunches
diffractive L0 trigger (hardware)
gap h 3 lt h lt 5 g Dh 0.5
gap h- -2 lt h lt -4 g Dh 0.5
high level trigger (software)
-3.7 lt h lt 5
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ALICE forward calorimeter

• neutron calorimeter on each side
• Placed at 116 m from interaction region
• Measures neutral energy at 0o
• trigger signal at L1 (L0 in special runs)
• Diffractive events
• pp g ppX no energy in zero degree
  calorimeter
• pp g pNX energy in one calorimeter
• pp g NNX energy in both calorimeters

( no Roman pots for proton tagging )
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ALICE diffractive physics

• ALICE acceptance matched to diffractive central
  production
• g-pomeron, double pomeron, odderon-pomeron

central barrel
Dh 4
Dh 3
f
Data taking pp _at_ L 5x1030 cm-2s-1 pPb _at_ L
1029 cm-2s-1 PbPb _at_ L 1027 cm-2s-1
gap
gap
had
h
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The Odderon

• Consider processes a b g a b, amplitude
  Aab(s,t)
  a b g
  a b, amplitude Aab(s,t)
• Define A(s,t) ½ ( Aab(s,t) Aab(s,t))
• g Aab(s,t) A(s,t) A (s,t)
• Aab(s,t) A(s,t) A (s,t)
• A identical for both processes, positive
  C-parity Pomeron
• A changes sign, negative C-parity Odderon,
  (Photon)
• g mesonic reggeon contributes to A
• g Odderon is part of A which doesnt vanish
  rapidly with s

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Signature Odderon cross section

• Look at processes with rapidity gaps
• Examples

diffractive pseudo scalar and tensor meson
production C 1 states
diffractive vector meson
production C -1 states
g measure cross sections
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The hunt for the Odderon

• Production cross sections in pp at LHC energies
• diffractive production p0,h,hc(JPC0 ),
  f0(0), a2(2)
• g contributions from Photon-Photon,
  Photon-Odderon, Odderon-Odderon
• Look for diffractive J/Y production JPC 1
• g Photon-Pomeron, Odderon-Pomeron
  contributions
• g such an experimental effort is a continuation
  of physics programs carried out at LEP (gg) and
  HERA (g-Odderon)

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Diffractive J/Y production in pp at LHC

• First estimates by Schäfer, Mankiewicz, Nachtmann
  1991
• pQCD estimate by Bzdak, Motyka, Szymanowski,
  Cudell
• Photon t-integrated 15 nb
  (2.4 - 27 nb)
• Odderon t-integrated 0.9 nb
  (0.3 - 4 nb)

ds
dy
y0
ds
dy
y0

• At L 5x1030 cm-2s-1
• 0.15 J/Y in ALICE central barrel in 1 s,
  150k in 106 s
• 9000 in ee- channel in 106 s

g identify Photon and Odderon contribution by
analysing
pT distribution ( Odderon harder pT spectrum )
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Odderon in diffractive J/? production

• Photon and Odderon contribution have different
  t-dependence

g different pT distri- bution of J/?
L.Motyka, L.Szymanowski
20
Signature Odderon interference

• Cross sections contain squared Odderon
  amplitudes
• g Odderon-Pomeron interference !

ds Ag(AP AO) 2 dNq AP 2
2Re(APAO) AO 2

• look at final states which can be produced by
  Odderon or Pomeron exchange
• find signatures for interference of C-odd and
  C-even amplitude

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Interference signal

• Interference effects (relative contribution C
  1)
• Photon-pomeron amplitude C 1
• Photon-odderon amplitude C 1
• Asymmetries in pp and KK pairs (C 1) in
  continuum
• charge asymmetry relative to polar angle of p in
  dipion rest frame

Brodsky, Rathsman, Merino 1999 asymmetry
fractional energy open charm in diffractive
photoproduction 15
Hägler, Pire, Szymanowski, Teryaev 2002
forward-backward charge asymmetry in pion
production 10 , 1 GeV/c2 lt mpp- lt 1.3 GeV/c2
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Conclusions, outlook

• ALICE has opportunity for diffractive/photon
  physics
• Diffractive trigger defined by two rapidity gaps
• Neutron tagging at zero degree
• Phenomenology of Pomeron/Odderon
• Photon-Photon physics