Download Trigger Studies with Minimum Bias data samples

Workshop on Forward Physics at the LHC
December 13, 2010, Manchester, UK
Soft QCD at LHC: Overview
Edward Sarkisyan-Grinbaum
(University Texas – Arlington)
1
The Large Hadron Collider
• Proton-proton collider
• 27 km circumference
• At 4 interaction points
detectors measure the
outcome of the collisions
- ALICE, ATLAS,
CMS, LHCb
Design
Start up (12/2009)
Current (12/2010)
Energy (c.m)
14 TeV
900 GeV (2.36 TeV)
7 TeV
Luminosity
1034 cm-2s-1
~7 × 1026 cm-2 s-1
~2 × 1032 cm-2 s-1
Bunches/Beam
2808
4
368
2
Soft QCD at the LHC (I)
• Minimum Bias measurements
- soft multihadron production process
- needs better understanding, model-sensitive
- non-perturbative QCD process
• Study of underlying events
- hadronic final states in presence of a “hard” process
- allows perturbative QCD calculations to be applied,
sensitive MC test
- crucial for precision measurements and search for
new physics
• Angular correlations in Minimum Bias events
- measurement of Φ-distributions of track density
- sensitive to tuning of phenomenological models in
MC simulations
3
Soft QCD at the LHC (II)
• Strange particle production
- requires high statistics
- crucial MC tests
• Heavy-ion measurements
- formation of new matter under extreme conditions
- further understanding of particle production
mechanism(s) and distinguishing among models
- looking for new phenomena
• Study of correlations
- understanding dynamics of multiparticle production
- goes beyond studies of averaged/global variables
& spectra
- interesting in all aspects of soft QCD
• Diffraction studies: covered in other talks
4
Soft QCD Motivation (I)
PYTHIA
900 GeV/7TeV
/(~49mb)
/(~14mb)
/(~9mb)
σtot = σel + (σND + σSD +σDD)
The processes are not well predicted, Monte Carlo
models need to be tuned to the data
5
Soft QCD Motivation (II)
Modeling in soft QCD necessary for high pT physics
Improves our understanding of QCD effects, total cross
section, saturation, jets, mass reconstruction
Collision energy dependence is of great interest,
asymptotic regime of QCD partonic (quark-gluon) picture
1.96 TeV
●
Phys. Rev. D79 (2009) 112005
7 TeV?
6
Measurements for minimum bias analysis
Distributions of hadrons (primaries) within a phase-space
accessible to the (tracking) Inner Detector
Data collected at 900 GeV, 2.36 TeV in December
2009, and at 7 TeV starting in March 2010
7
Comparison of pT for different experiments:
900 GeV
pT>500 MeV, nch≥1
• First ATLAS publication:
Phys. Lett. B 688 (2010) 21
• Measurements at high
pT threshold
• Good agreement for
the same selection
criteria
• Now have many new results with low-pT threshold
and higher energies
8
ATLAS: nch spectra @ 900 GeV vs 7 TeV
900 GeV
low nch
7 TeV
full nch
low nch
ATLAS-CONF-2010-046
full nch
• At low nch data differs from MC and the models differ
between each other due to diffraction, see later discussion
• At nch>40 data exceed PYTHIA tunes by 50%
• PHOJET fails to describe data above nch~20
9
CMS: nch spectra
CMS PAS QCD-10-004
7 TeV
2.36 TeV
900 GeV
• More PYTHIA tunes exist (P0, ProQ20,DW), omitted for clarity
• No Monte Carlo is able to describe multiplicities at all energies
• In general, all MCs generate too many hight-pT particles
• Energy dependence stronger than at lower energies, better
described by Regge-inspired model than by parton saturation model
10
ALICE: nch spectra @ three energies
Eur. Phys. J. C68 (2010) 89 & 345
• At low nch data differs from MC and the models differ
between each other due to diffraction
• At high multiplicities PYTHIA ATLAS tune agrees best with data
although exceeds by 20-50%, PHOJET and PYTHIA D6T behave
differently, while Perugia-0 underestimates the data
11
ALICE & CMS: KNO & NBD
Eur. Phys. J. C68 (2010) 89, CMS PAS QCD-10-004
Koba-Nielsen-Olesen (KNO) scaling
Negative Binomial Distribution fit
• KNO scaling holds for
small central η-bins,
while violated as bin
width increase (MPI?)
• NBD fits the data in
small |η|-interval but
deviates for larger
intervals
12
ATLAS: η spectrum @ 900 GeV vs 7 TeV
ATLAS-CONF-2010-046
7 TeV
900 GeV
• Mid-rapidity density (η≈0), pT>100 MeV:
3.486 ± 0.008(stat) ± 0.077(syst) at 900 GeV
5.635 ± 0.002(stat) ± 0.049(syst) at 7 TeV
• Measurements 5-20% higher than Monte Carlo predictions
13
CMS: η spectrum @ three energies
JHEP02 (2010) 041
Phys. Rev. Lett. 105 (2010) 022002
Good agreement
within LHC and
with earlier data
• Three method of charged particle reconstruction used:
clusters in pixel barrel (pT>30 MeV) , pixel tracklets (pT>50 MeV),
tracks in full tracker (pT>100 MeV). The methods in good agreement
• Mid-rapidity density (η≈0), pT>0 MeV (extrapolation, 2% correct.):
3.48 ± 0.02(stat) ± 0.013(syst) at 900 GeV,
4.47 ± 0.04(stat) ± 0.016(syst) at 2.36 TeV
5.78 ± 0.01(stat) ± 0.023(syst) at 7 TeV
14
ALICE: η spectrum @ three energies
Eur. Phys. J. C68 (2010) 89 & 345
• Very low momentum sensitivity, high particle identification capability
• Monte Carlo models PHOJET & PYTHIA D6T underestimate the data
• η-density (|η|<1) for inelastic colls. with at least 1 charged particle (INEL>0):
3.81 ± 0.01(stat) ± 0.07(syst) at 900 GeV,
4.70 ± 0.01(stat) +0.11 -0.08 (syst) at 2.36 TeV
6.01 ± 0.01(stat) +0.20-0.12 (syst) at 7 TeV
15
Mid-rapidity density energy-dependence
• MCs do not describe
the low-pT data
• Power-law increase
with c.m. energy: s0.1
• Good agreement
among all three
experiments
16
ATLAS: pT spectrum @ 900 GeV vs 7 TeV
ATLAS-CONF-2010-046
900 GeV
• Monte Carlo predictions
agree with the data at
intermediate pT from
0.5 to 3 GeV
• All Monte Carlo differ
from the data by 10-50%
7 TeV
17
ALICE: pT spectrum @ 900 GeV
Phys. Lett. B 693 (2010) 53
• Monte Carlo do not describe the data, except Perugia gives a fair
description while is ~20% below the data
• The modified Hagedorn function well describes the spectra, allowing
extrapolation to pT=0. Hard part is well described by a power law
• The spectrum seems to be harder at mid-rapidity
18
CMS: pT spectrum @ three energies
JHEP02 (2010) 041, Phys. Rev. Lett. 105 (2010)
022002
|η|<2.4
• Average transverse momentum ‹pT›:
0.46 ± 0.01(stat) ± 0.01(syst) @ 900 GeV,
0.50 ± 0.01(stat) ± 0.01(syst) @ 2.36 TeV,
Used for extrapolations to pT=0
0.545 ± 0.005(stat) ± 0.015(syst) @ 7 TeV,
19
ATLAS: ‹pT› vs. nch @ 900 GeV vs. 7 TeV
ATLAS-CONF-2010-046
900 GeV
7 TeV
• Change in MC description with energy increase
• Breaking down by sub-processes clarifies the differences
20
ALICE: ‹pT› vs. nch and √s
Phys. Lett. B 693 (2010) 53
• PYTHIA Perugia0 describes well the data at large pT, while none of
the models succeed at low momentum.
• ‹pT› dependence confirms harder spectrum in at mid-rapidity
21
CMS: ‹pT› vs. nch and √s
JHEP02 (2010) 041
CMS PAS QCD-10-004
• No MC describes the energy & multiplicity dependences
• ‹pT›-n scaling with energy is remarkable
22
CMS @ 7 TeV: ‹pT› and ‹nch› vs models
N. Van Remortel @ 2nd MPI@LHC (2010)
• Most PYTHIA tunes overestimate ‹pT› and underestimate
mid-rapidity density
• Some analytical models are spot on
23
Underlying event (UE)
• Hadronic final states in presence of a “hard” process
• Formed by beam-beam remnants, multiple parton
interactions
• Cannot be uniquely separated from initial and finlal
state radiation
• Allows perturbative calculations to be applied, sensitive
MC model test
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
Proton
Underlying Event
Underlying Event
“Minimum Bias” Collisions
Proton
Proton
Outgoing Parton
Final-State
Radiation
24
Underlying Event Analysis
Consider charged tracks in minimum bias events
- Align event leading pT track at Φ=0
Three equal azimuthal regions in |ΔΦ|
- “Transverse” region most sensitive to UE,
perpendicular to hardest scattering axis
- “Toward” and “away” regions expected to
be dominated by hard parton-parton scattering
and radiation
Measure track-based observables:
- Charged particle number density/multiplicity vs. pT lead
- Scalar pT-sum vs. pT lead
- ‹pT› vs. p T lead
- ‹pT› vs. Nch
- ΔΦ distribution of number density
- ΔΦ distribtion of scalar pT sum
25
ATLAS, ALICE: UE Multiplicities
7 TeV
900 GeV
S.Vallero @ 2nd MPI@LHC (2010)
• Twice high UE activity in 7 TeV, predicted by MC at Tevatron
• 10-15% excess in data, PYTHIA DW (ATLAS) & Perugia 0 (ALCE) closest
• Rising densities of jet-like activity in toward/away regions (not shown)
compared to fast saturation with pT lead in UE
26
CMS: UE Multiplicities
900 GeV
7 TeV
Eur. Phys. J. C 70 (2010) 555
CMS PAS QCD-10-010
Ratio
• Fast rise at low pT lead and the following saturation is attributed to MPI
• MCs brackets the data at 900 GeV but slower and saturates at 7 TeV
• All MC predict ratio 7TeV/900GeV smaller than in data for pT<8 GeV
27
ATLAS, ALICE: UE Scalar pT sum
900 GeV
7 TeV
S.Vallero @ 2nd MPI@LHC (2010)
• Higher UE density in 7 TeV expected from higher multiplicity
• 10-15% deficit in Monte Carlo models, PHOJET furthest from the data
• Rising densities of jet-like activity in toward/away regions (not shown)
compared to fast saturation with pT lead in UE
28
CMS: UE Scalar pT sum
7 TeV
Eur. Phys. J. C 70 (2010) 555
CMS PAS QCD-10-010
900 GeV
Ratio
• Fast rise at low pT lead and the following saturation is attributed to MPI
• MCs reproduce the behaviour at 900 GeV but fail at 7 TeV
• All MC predict ratio 7TeV/900GeV smaller than in data for pT<8 GeV
29
ATLAS: UE Multiplicities vs ΔΦ
arXiv:1012.0791
900 GeV
7 TeV
• No leading charged particle at ΔΦ = 0, positive half reflected about 0
• Sharper rise in the transverse region compared to MCs, stronger
correlations in PYTHIA than measured in the toward region
30
ALICE: UE Multiplicities vs ΔΦ
900 GeV
S.Vallero @ 2nd MPI@LHC (2010)
7 TeV
• Perugia-0 favoured tune @ 900 GeV, and @ 7 TeV for pT< 2 GeV
• CMS D6T best tune @ 7 TeV for higher transverse momentum
31
ATLAS: UE Scalar pT sum vs ΔΦ
arXiv:1012.0791
900 GeV
7 TeV
• No leading charged particle at ΔΦ = 0, positive half reflected about 0
• Sharper rise in the transverse region compared to MCs, stronger
correlations in PYTHIA than measured in the toward region
32
CMS: UE Scalar pT sum vs ΔΦ
900 GeV
Eur. Phys. J. C 70 (2010) 555
CMS PAS QCD-10-010
7 TeV
• Activity at |ΔΦ|<60 and |ΔΦ|>120 is
understood due to both partons
fragmentation, in the transverse
region activity is depleted but nonzero attributed to MPI
• In the toward region, PYTHIA tunes above the data, except P0 at
pT lead>2 GeV @ 900 GeV, while @ 7 TeV PYTHIA 8 best
• In the transverse region, the best are PYTHIA DW & CW for 900 GeV
data, while for 7 TeV data, all modes significantly underestimate data
for low pT lead. All models on top of the data as pT lead increases
33
ATLAS: UE ‹pT› vs pT lead
arXiv:1012.0791
900 GeV
7 TeV
• ‹pT› increases by ~20%
with energy increase
• Higher ‹pT› in
toward/away region as
expected; same trend of
jet-like rising at both
energies
• All MC, except PITHYA
DW, underestimate
data in the plateau of
the transverse region,
and overestimate in the
toward/away regions
34
900 GeV
ATLAS: UE ‹pT› vs Nch
arXiv:1012.0791
7 TeV
• Transverse region: MC
brackets data, PYTHIA
overshoots, HERWIG
undershoots, DW the
worst
• Toward/Away region:
MCs tend to overshoot
data
• No firm conclusion can
be made
35
Angular Correlations in MB:
Crest shape variable
ATLAS-CONF-2010-082
36
Angular Correlations in MB:
Same-opposite observable
ATLAS-CONF-2010-082
37
ATLAS: Angular Correlations in MB:
pT-ordered showers, Perugia
900 GeV
7 TeV
ATLAS-CONF-2010-082
Very small
systematic errors
allow detailed
analysis
38
ATLAS: Angular Correlations in MB:
virtuality-ordered showers, Perugia
900 GeV
7 TeV
ATLAS-CONF-2010-082
Sensitive to tuning
of models in MC
simulations
39
ATLAS: Angular Correlations in MB:
colour-reconnection models
900 GeV
7 TeV
ATLAS-CONF-2010-082
Sensitive to tuning
of models in MC
simulations
40
ATLAS: Angular Correlations in MB:
|η|<1 region
900 GeV
7 TeV
ATLAS-CONF-2010-082
Better match for
small η-space,
while expected:
CDF tuning data
available in this
region
41
CMS, LHCb: Strangeness production
CMS PAS QCD-10-007
Pythia underestimates production yield of strange particles at all energies
and large increase of the production cross section between 0.9 & 7 TeV
900 GeV
Φ-analysis
7 TeV
7 TeV
42
T.M. Karbach @ 2nd MPI@LHC (2010)
42
ALICE, ATLAS: Strangeness production
E.Sicking @ 2nd MPI@LHC (2010)
• Data yields slightly harder than MC predictions at pT>1 GeV/c
• Φ described well
ATLAS-CONF-2010-033
Λ0
Ks0
7 TeV
43
• Preliminary results. Acceptances and efficiencies yet under study
43
ALICE: Strangeness production
E.Sicking @ 2nd MPI@LHC (2010)
ALICE preliminary
stat. error only
Phojet
Pythia ATLAS-CSC
Pythia D6T
Pythia Perugia-0
• Models underestimate the data – factor of 2!
• K/π data fairly energy independent
44
44
Ridge in nuclear collisions
2-particle correlations in η-Φ phase-space
45
CMS: Ridge effect in pp
JHEP09 (2010) 091
• Long-range Near-side 2-particle correlations!
• Not reproduced by Monte Carlo models
• Waits for its explanation
46
Two-particle correlations
JHEP09 (2010) 091
•Two-particle η-Φ correlations: cluster structure
Δη
CMS
• “Cluster” fit extracts size and range of
cluster emission
ALICE, ATLAS: analysis in progress
47
Bose-Einstein Correlations
• Bose-Einstein (two- & three-particle) correlations: enhancement
in identical particle (boson) correlation function at near momenta
• Provides estimate on particle emission source size (radius),
coherency vs. chaoticity in the hadroproduction process
PRL 105, 032001 (2010)
 dN signal / dQ 

R(Q)  
 dN

/
dQ
reference


• Main problems: reference sample
of non-correlated particle pairs,
suitable fit function (usually
Gaussian)
• Main interest: radius (R) &
coherency strength (λ) energy
dependence, their particle mass
and multiplicity dependences
• ATLAS, LHC 7 TeV: work in
progress
Phys. Rev. D82, 052001 (2010)
48
ALICE: First heavy-ion data
arXiv:1011.3916
•
Mid-rapidity density: dNch/dh ~ 1600 ± 76(syst)
Phys. Rev. Lett.,
in press
• Somewhat on high side of expectations
• Increase with c.m. energy faster than
in pp data
PbPb @ 2.76 TeV per nucleon
pp extrapol.
MC
Saturation
models
hydro
pp extrapol.
• Particle flow,azimuthal anisotropy: almost no change compared to RHIC
arXiv:1011.3914, Phys. Rev. Lett., in press
49
ATLAS: Jet quenching
• Expected in medium, e.g. in
quark-gluon plasma
• Not observed in pp collisions
PbPb @ 2.76 TeV per nucleon
U. Wiedemann
• Use R = 0.4 anti-kt jets (calibrated using
energy density cell weighting)
• Select events with leading jet, E > 100 GeV, |η| < 2.8
• Sub-leading: highest E jet in opposite hemisphere, Δφ >
T1
Peripheral
T
π/2 with ET2 > 25 GeV, |η| < 2.8
• Introduce new robust variable to quantify dijet imbalance
A = (ET1-ET2)/(ET1+ET2)
arXiv:1011.6182
Phys. Rev. Lett.
in press
• First observation of large dijet asymmetry
Most central
50
CMS: dijet imbalance &
Peripheral
0
Z
Most central
B. Wysloukh
@ LHCC meeting
2nd Dec. 2010
• Use R = 0.5 anti-kt jets
• Select events with leading jet, E > 120 GeV, |η| < 2
• Sub-leading: highest E jet in opposite hemisphere, Δφ >
PbPb @ 2.76 TeV per nucleon
T1
T
2.5 with ET2 > 50 GeV,
• Study new robust variable to quantify dijet imbalance
A = (ET1-ET2)/(ET1+ET2)
Z0→µ+µ- in PbPb @ 2.76 TeV per nucleon51
Conclusions and Outlook
• Great work done by all LHC experiments
• Huge amount of extremely interesting data
collected in a very short period
• Many analyses being published, much more
in pipeline
• New phenomena seen and crucial observations
made, Monte Carlo models require more
understanding
• 2011 looks very promising, stay tuned!
• Let
us glance at the schedule…
52
The LHC in 2011
•
•
•
Beam back around on February
21st!
2 weeks re-commissioning with
beam (at least)
• 4 day technical stop every 6 weeks.
• 4 weeks ion run
Thanks
• &
End of run – December 12
Approximately 200 days proton
Beam back Happy•New
physics!
Year!
around 21st
• Maybe 8 TeV (4 TeV/beam)
February.
th
Peak
luminosity
6.4 x 1032
Integrated per
day
11 pb-1
200 days
2.2 fb-1
Stored energy
72 MJ
R.Field
53
Back-up slides
54
Monte Carlo generators/tunes used
• PYTHIA 6, actually 6.4.21 (P6): pT-ordered parton shower, MRST LO* p.d.f.,
multiple parton-parton scattering, string fragmentation
• PYTHIA ATLAS AMBT1: P6 tuned by ATLAS to the low-multiplicity data
• PYTHIA ATLAS MC09 (reference) P6 tune: parameters tuned to underlying
events and minimum bias data from Tevatron at 630 GeV to 1. 8 TeV
(ATLAS optimization), used to determine ATLAS detector acceptances and
efficiencies, to correct the data
• PYTHIA ATLAS MC09c tune: MC09 optimizing0 the strength of the colour
reconnection to describe pT dependence on nch in the CDF data at 1.96 TeV
• PYTHIA Perugia0 P6 tune: soft QCD part is tuned using only minimum bias
data from Tevatron and CERN ppbar data
• PYTHIA DW/CW P6 tune: uses the virtuality-ordered showers and used to
describe the CDF II underlying events and Drell-Yan process data
• PYTHIA 8: includes new features such as hard scattering in diffractive
systems, up-to-date LO p.d.f. set, possibility to use one p.d.f. set for hard
scattering and another one for the rest, more underlying-event processes
(J/ψ, DY,…)
• PHOJET: two-component Dual Parton Model with soft hadronic processes by
Pomeron exchange and semi-hard processes by perturbative parton
scattering
55