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Sparsh Navin – University of Birmingham
for the ALICE collaboration
Trigger Efficiencies in ALICE
andDiffraction in PYTHIA
Diffractive and electromagnetic processes at the LHC, Trento 2010
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Outline
ALICE and pp trigger efficiencies• ALICE detectors for first physics with protons
• Process Types
• First physics trigger efficiencies
Diffraction and PYTHIA• Kinematic comparison – PYTHIA 6.4 and PHOJET
• Diffraction in PYTHIA – then and now
• Comparison of kinematic plots – PYHTIA 8.310
• Summary
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A Large Ion Collider Experiment • Primary purpose – heavy ion detector for Pb-Pb collisions at 5.5 TeV
• First physics programme is pp at 0.9, 2.36, 7 (maybe 10) and 14 TeV
- Time and space alignments
- pp bench mark to study genuine Pb-Pb effects
• Special features for pp collisions:
- Low magnetic field => low pT cut-off, tracking resolution from 100 MeV/c
- Excellent PID over broad range of momenta
- Primary vertex resolution (100μm for pp, 10 μm for Pb-Pb)
• pp Physics contributions:
- Physics cross checks with previous results
- Good description of underlying event and high multiplicity collisions
• 1st measurements: , multiplicity and momentum distributions
and
chdN
dch
T
dN
dp
vs T chp N
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A Large Ion Collider Experiment - 2
52 m underground 16 m high 26 m long
10,000 tonnes 0.5 T magnetic field
V0A z = 3.3m
ITS (Inner
Tracking System)
TPC(Time
Projection Chamber)
• SPD – Inner layers - 3.9cm 7.6cm radii - better than 100μm resolution - provide the SPD trigger signal
• Main gas-filled tracking detector
2.8 5.1
3.7 1.7
• Provide the V0A and V0C trigger signals• Time resolution better than 1ns
V0C z=-0.9m
2 1.4
ZDC at 116m
0.9
8.7 ZN 8.4 ZP
ZEM at 7 m 4.8 5.7
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Process Types
•True cross sections at LHC energies are not known • Scaling of cross sections with energy is model dependent
PHOJETDefault
fractionsPYTHIA
0.134 SD 0.187
0.063 DD 0.127
Figure from Torbjörn Sjöstrand, MCnet school, 2008.
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V0A V0C
Outgoing undiffracted p beam
Diffractive system
Single Diffraction (SD)0
Pseudorapidity gap
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V0A V0C
Diffractive system (2)
Diffractive system (1)
Double Diffraction (DD)0
Pseudorapidity gap
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8888
V0A V0C
Non Diffractive (ND)
No pseudorapidity gap
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First Physics Triggers
Minim
um B
ias Triggers
MB1 SPD or (V0A or V0C)
MB3 SPD and (V0A and V0C)
•Different efficiencies for different
Triggers (MB1, MB3)
Type of process (SD, DD, ND)
Event generator (Pythia and Phojet)
Global Fast Or (GFO) is the trigger from the
Silicon Pixel Detector (SPD)
Actual trigger used – at least 2 pixels in coincidence with beams
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Trigger Efficiencies and corrections
processprocesstotaltriggered efNN /Efficiency
• Need to know the fraction (f) and the efficiency (e) for each process. • Efficiency is process, trigger and generator dependent
Eg: MB1 = SPD or V0A or V0C
Process SD DD ND
Fraction (f) 0.187 0.127 0.686
Efficiency (e) 0.714 0.864 0.999
NDNDDDDDSDSD efefef
Process SD DD ND
Fraction (f) 0.134 0.063 0.803
Efficiency (e) 0.767 0.938 0.999
MB1 efficiencies:
Pythia: 92.9%
Phojet: 96.4%
Reason for difference: f – uncertainty in
fractionse – uncertainty in
kinematics
Major difference is in diffractive events
~2-4% effect each
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Kinematics – eta ND 7 TeV
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Kinematics – eta SD 7 TeV
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Kinematics – pT ND 7 TeV
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Kinematics – pT SD 7 TeV
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Kinematics – multiplicity ND 7 TeV
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Kinematics – multiplicity SD 7 TeV
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Diffraction in PYTHIA – “old”
Event Generation:
• Diffractive cross sections given by model by Schuler and Sjöstrand (Phys. Rev. D 49, 2257 (1994))
• Diffractive mass ( ) and momentum transfer (t) generated according to:
2
2
1~ b|t|
2X X
d se
dtdM M
Particle Production:
• above mass of incoming particles => isotropic decay into 2-body state
• More massive system treated as a string with quantum numbers of the original hadron
21 XM GeVc
XM
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Pomeron couples to valence quarks
Dominates at small
Diffraction in “old” PYTHIA – stretching the string
Version 6.214 (Fortran)
• q and g contributions are set by a user-defined fixed ratio
Version 8.1 (C++)
• Slope p and normalisation N set by user
• mass ( ) dependence
• Gluonic domination at large
Has a gluon (g) and quark (q) contribution
XM
P( )
P( ) pX
q N
g M
XM
XM
Pomeron couples to gluon
Dominates at large
XM
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Diffraction in PYTHIA 8.130 – “new”Event Generation:• Cross sections - same way as before
• Diffractive mass ( ) and momentum transfer (t) picked by Pomeron flux model
Particle Production: • Pomeron-p collisions
• Pomeron PDF with - dependence from H1 data
- H1 2007 DPDF Fit Jets and H1 2006 Fits A and B
- Pion PDF also available
• Standard PYTHIA machinery for multiple interactions, parton showers, hadronization
Mass separation:• For non-perturbative description (as before)
- longitudinally stretched strings
• For perturbative
XM
2Q
1.2 < M 10 XGeV GeV
M 10 X GeV
Work done with Torbjörn Sjöstrand - MCnet
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• Energy dependent
• Only SS provides separate t spectrum for DD
Pomeron Flux factor
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PYTHIA “old” vs “new” - eta 7 TeV
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PYTHIA “old” vs “new” - pT 7 TeV
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PYTHIA “old” vs “new” - multiplicity7 TeV
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Comments
PP
proton
Ex
E g
gP
Ex
E
2X PM x s
Diffractive hard scattering cross section:
2/ ( )/ ( )2 2
Pomeron flux Pomeron PDF
ˆ( ) ( , ) P proton P q g P q g
d df x f x Q
dQ dQ
In the massless limit1p1p
2p
Px
gx
X
Not known from first principles
Multiple interactions => screening of diffractive rates
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• Does cut-off depend on the diffractive mass?
• Introduce a screening factor to go from ep to pp collisions
• Momentum sum of PDFs
• H1 2006 DPDF LO fit
• Tuning to data
Future Plans - PYTHIA
0Tp
• Include Central Diffraction
1p1p
2p2p
X
LRG
LRG
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Summary
• The ALICE detectors and trigger for the first pp physics programme were discussed
• Trigger efficiencies are model dependent - PYTHIA and PHOJET
• Earlier versions of PYTHIA had a primitive description of diffraction
• No hard diffraction – caused the difference in pT and multiplicity tails compared to PHOJET
• New version (8.130) has Pomeron description of diffraction
• Hard collisions can be simulated
• Better agreement with PHOJET
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• ALICE first paper
• Extraction of fractions from data - ALICE
• Diffraction in Phojet
Back up slides
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ALICE first paper - arXiv: 0911.5430 [hep-ex]
• Trigger – atleast 2 fired chips in coincidence with bunches – 284 events
• Magnetic field off; SPD used for analysis; SDD, SSD and V0 used for cross checks and background ID and removal.
• In the region
• Trigger efficiency obtained from MC PYTHIA 6.141 D6T and PHOJET 1.12:
SD – 48% to 58%, DD – 53% to 76%, ND – 98% to 99%
• Fractions taken from UA5:
SD – , DD - , ND –
• Trigger efficiencies: Inelastic - 87% to 91%, NSD – 94% to 97%
• Results obtained with PYTHIA, difference between PYTHIA and PHOJET used to estimate systematic uncertainty.
0.5 3.10 0.13( ) 0.22( )chdN
stat systd
3.51 0.15( ) 0.25( )chdNstat syst
d
-Inelastic
-NSD
0.153 0.031 0.08 0.05 0.767 0.059
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Extraction of fractions• Trigger on bunch crossing• Define 8 uncorrelated trigger types using SPD, V0A and V0C• Meausre
NININDNDDDDDSDSDgentrig
gen
NIgen
NIgen
NItrig
gen
NDgen
NDgen
NDtrig
gen
DDgen
DDgen
DDtrig
gen
SDgen
SDgen
SDtrig
gentrig
NItrig
NDtrig
DDtrig
SDtrigtrig
efefefefNN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NNN
NNNNN
calcmeasured
2
2
))((Error
)()(
trig measuredtrig
measuredtrigcalctrig
iN
iNiN
• Program works out combinations of fractions to generate so as to minimise
2 calctrigN
trigN
- Z.Matthews 20/03/09, ALICE first physics meeting
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Central elements of Phojet- R. Engel workshop on soft diffraction at LHC 26/6/09
Two component Pomeron
Only one pomeron with soft and hard contributions
Topological identification of different terms (Dual parton model)
Soft and hard partons differ in impact parameter distribution
Application of existing parton density parametrisation
Initial and final state radiation (leading logQ^2 parton showers)