Testing of CMS Endcap RPCs and the Determination of Top Quark Mass Using High Pt Jets at LHC
1 Jets and High-pt Physics with ALICE at the LHC Andreas Morsch CERN.
-
date post
15-Jan-2016 -
Category
Documents
-
view
220 -
download
0
Transcript of 1 Jets and High-pt Physics with ALICE at the LHC Andreas Morsch CERN.
1
Jets and High-pt Physics with ALICE at the LHC
Andreas Morsch
CERN
2
Outline
Introduction Jets at RHIC and LHC: New perspectives and challenges
High-pT di-hadron correlations
Reconstructed Jets Jet Structure Observables -Jet Correlations
3
Jets in nucleus-nucleus collisions
Jets are the manifestation of high-pT partons produced in a hard collisions in the initial state of the nucleus-nucleus collision.
These partons undergo multiple interaction inside the collision region prior to fragmentation and hadronisation.
In particular they loose energy through medium induced gluon radiation and this so called “jet quenching” has been suggested to behave very differently in cold nuclear matter and in QGP.
The properties of the QGP can be studied through modification of the fragmentation behavior
Hadron suppression Jet structure.
4
Jet Physics at RHIC
In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (125 GeV in R< 0.7) and the relatively low accessible jet energies (< 20 GeV).
Use leading particles as a probe.
p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV
5
Quantities studied
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
pT (trig)
pT(assoc)
Hadron Suppression
Similar RCP: Ratio central to peripheral
Hadron Correlations:
pT(trig) – pT(assoc)(trig, assoc)…
“away side”
“same side”
6
Evidence for Jet Quenching
In central Au+Au Strong suppression of inclusive hadron yield in Au-Au collisions Disappearance of away-side jet
No suppression in d+Au Hence suppression is final state effect.
Phys. Rev. Lett. 91, 072304 (2003).
Pedestal&flow subtracted
STARSTAR
7
Surface emission bias
RHIC measurements are consistent with pQCD-based energy loss simulations. However, they provide only a lower bound to the initial color charge density.
Eskola et al., hep-ph/0406319
RAA~0.2-0.3 for broad range of q
8
Jet Physics at LHC: Motivation
Study of reconstructed jets increases sensitivity to medium parameters by reducing Trigger bias Surface bias
Using reconstructed jets to study Modification of the leading hadron Additional hadrons from gluon
radiation Transverse heating.
From toy model
= ln(Ejet/phadron)
Reconstructed Jet
s = 5500 GeV
A. Dainese, C. Loizides, G. Paic
9
Jet Physics at LHC: New perspectives
ET > Njets
50 GeV 2.0 107
100 GeV 1.1 106
150 GeV 1.6 105
200 GeV 4.0 104
Jet rates are high at energies at which they can be reconstructed over the large background from the underlying event.
Reach to about 200 GeV Provides lever arm to measure the
energy dependence of the medium induced energy loss
104 jets needed to study fragmentation function in the z > 0.8 region.
Pb-Pb
O(103) un-triggered (ALICE) => Need Trigger
10
Jet Physics at LHC: New challenges
More than one jet ET> 20 GeV per event More than one particle pT > 7 GeV per event 1.5 TeV in cone of R = 2+2 < 1 ! We want to measure modification of leading hadron and
the hadrons from the radiated energy. Small S/B where the effect of the radiated energy should be visible: Low z Low jT Large distance from the jet axis
Low S/B in this region is a challenge !
11
New Challenges for ALICE
Existing TPC+ITS+PID || < 0.9 Excellent momentum
resolution up to 100 GeV Tracking down to 100 MeV Excellent Particle ID
New: EMCAL Pb-scintillator Energy resolution ~15%/√E Energy from neutral particles Trigger capabilities
central Pb–Pb
pp
12ALICE Set-up
HMPID
Muon Arm
TRD
PHOS
PMD
ITS
TOF
TPC
Size: 16 x 26 meters
Weight: 10,000 tons
13
Di-hadron Correlations:from RHIC to LHC
Di-hadron correlations will be studied at LHC in an energy region where full jet reconstruction is not possible (E < 30 GeV).
What will be different at LHC ? Number of hadrons/event (P) large
Leads to increased signal and background at LHC Background dominates, significance independent of multiplicity
Increased width of the away-side peak (NLO) Wider -correlation (loss of acceptance for fixed -widow) Power law behavior d/dpT ~ 1/pT
n with n = 8 at RHIC and n = 4 at LHC Changes the trigger bias on parton energy
PNBS
SPp
N
BS
SPp
PN
BS
S
PB
S
NPB
NPS
T
T
1: high RHICFor
1 :LHC and low RHICFor
P1 and
1
2
PYTHIA 6.2
See also, K. Filimonov, J.Phys.G31:S513-S520 (2005)
14
Scaling From RHIC to LHC
S/B and significance for away-side correlations Scale rates between RHIC and LHC
Ratio of inclusive hadron cross-section N(pT) ~ pT
4
pTtrig > 8 GeV
RHIC/STAR-like central Au-Au (1.8 107 events)
LHC/ALICE central Pb-Pb (107 events), no-quenching
From STAR pTtrig = 8 GeV/c
15
Di-hadron Correlations
STAR LHC, ALICE acceptanceHIJING Simulation
“Peak Inversion”
O(1)/2
4 105 events
M. Ploskon, ALICE INT-2005-49
16
The biased trigger bias
hep-ph/0606098
pTtrig > 8 GeV
<pTpart> is a function of pT
trig but alsp pTassoc, s, near-side/away-side, E
See also, K. Filimonov, J.Phys.G31:S513-S520,2005
17
From di-hadron correlations to jets
Strong bias on fragmentation function … which we want to measure
Low selectivity of the parton energy Very low efficiency, example:
~6% for ET > 100 GeV 1.1 106 Jets produced in central Pb-Pb collisions (|| < 0.5) No trigger: ~2.6 104 Jets on tape ~1500 Jets selected using leading particles
18
Reduction of the trigger biasby collecting more energy from jet fragmentation…
Unbiased parton energy fraction production spectrum induced bias
19
Reconstructed Jets: Objectives
Reduce the trigger bias as much as possible by collecting of maximum of jet energy Maximum cone-radius allowed by background level Minimum pT allowed by background level
Study jet structure inclusively Down to lowest possible pT (z, jT)
Collect maximum statistics using trigger.
20
Jet Finder in HI Environment:Principle
Loop1: Background estimation from cells outside jet conesLoop2: UA1 cone algorithm to find centroid
using cells after background subtraction
Rc
21
Jet Finder based on cone algorithms
Input: List of cells in an grid sorted in decreasing cell energy Ei
Estimate the average background energy Ebg per cell from all cells. For at least 2 iterations and until the change in Ebg between 2
successive iterations is smaller than a set threshold: Clear the jet list Flag cells outside a jet. Execute the jet-finding loop for each cell, starting with the highest cell energy.
If Ei – Ebg > Eseed and if the cell is not already flagged as being inside a jet: Set the jet-cone centroid to be the center of the jet seed cell (c, c) = (i, i) Using all cells with (i-)2+(i-)2 < Rc of the initial centroid, calculate the new
energy weighted centroid to be the new initial centroid. Repeat until difference between iterations shifts less than one cell. Store centroid as jet candidate.
Recalculate background energy using information from cells outside jets.
22
Optimal Cone Size
Jets reconstructed from charged particles:
Need reduced cone sizes and transverse momentum cut !
Ene
rgy
cont
aine
d in
sub
-co
ne R
E ~ R2
Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1).R and pT cut have to be optimized according to the background conditions.
23
Background Fluctuations
Background fluctuations limit the energy resolution. Fluctuations caused by event-by-event variations of
the impact parameter for a given centrality class. Strong correlation between different regions in plane ~R2
Can be eliminated using impact parameter dependent background subtrcation.
Poissonian fluctuations of uncorrelated particles E = N [<pT>2 +pT
2] ~R
Correlated particles from common source (low-ET jets) ~R
Out-of-cone Fluctuations
24
Background Fluctuations
Evt-by-evtbackground energy
estimation
25
Signal fluctuationsResponse function for mono-chromatic jets
ET = 100 GeV
E/E ~ 50%
E/E ~ 30%
26
Putting things together:Intrinsic resolution limit
pT > 0 GeV1 GeV2 GeV
Resolution limited by out-of-conefluctuations common to all experiments !
Ejet = 100 GeV
Background included
27
Expected resolution including EMCAL
Jet reconstruction using charged particles measured by TPC + ITS And neutral energy from EMCAL.
28
Trigger performance
Trigger on energy in patch xBackground rejection set to factor of 10=>HLT
Centrality dependent thresholds
29
Reference systems
Jet trigger
Compare central Pb+Pb to reference measurements• Pb+Pb peripheral: vary system size and shape• p+A: cold nuclear matter effects• p+p (14 TeV): no nuclear effects, but different energy• p+p (5.5 TeV): ideal reference, but limited statistics
Includes acceptance, efficiency, dead time, energy resolution
All reference systems are required for a complete systematic study
30
Jet yields: one LHC year
Jet yield in 20 GeV bin
Large gains due to jet trigger
Large variation in statistical reach for different reference systems
31
Resolution buys statistics
32
ALICE performanceWhat has been achieved so far ?
Full detector simulation and reconstruction of HIJING events with embedded Pythia Jets
Implementation of a core analysis frame work Reconstruction and analysis of charged jets. Quenching Studies on fragmentation function.
33
Energy spectrum from charged jets
Cone-Algorithm: R = 0.4, pT > 2 GeV
Selection efficiency ~30% as compared to 6% with leading particle !No de-convolution, but GaussE-n ~ E-n
34
Jet structure observables
Low z (high ): Systematics is a challenge, needs reliable tracking. Also good statistics (trigger is needed)
35
Hump-back plateau
Bias due to incomplete reconstruction.
Erec > 100 GeV
Statistical error
2 GeV
104 events
36
Systematics of background subtraction
Background energy is systematically underestimated (O(1 GeV))Corrections under study (thesis work of R. Dias Valdez)
37
jT-Spectra
Bias due to incomplete reconstruction.
Erec > 100 GeV
Statistical error
104 events
jT
38
Estimate quenching at LHC:
/fmGeV50~ˆ7~ˆ 2RHICLHC qq
Quenching Studies
Compare distributions with and without quenching
The measurement: ratio of dashed over solid= Pb+Pb(central)/p+p
fm/GeV50ˆ 2q
Solid: unquenched (p+p)
Pythia-based simulation with quenching
Large R, no pT cut
Dashed: quenched jet (central Pb+Pb)
39
Toy Models
Pythia hard scattering Initial and Final State Radiation
Afterburner A
Afterburner B
Afterburner C
.
.
.
Pythia Hadronization
Two extreme approaches Quenching of the final jet system and radiation of 1-5 gluons.
(AliPythia::Quench using Salgado/Wiedemann - Quenching weights) Quenching of all final state partons and radiation of many (~40) gluons
(I. Lokhtin: Pyquen)*
Nuclear Geometry(Glauber)
)*I.P. Lokhtin et al., Eur. Phys. J C16 (2000) 527-536 I.P.Lokhtin et al., e-print hep-ph/0406038http://lokhtin.home.cern.ch/lokhtin/pyquen/
Jet (E) → Jet (E-E) + n gluons (“Mini Jets”)
40
ALICE+EMCal in one LHC year
ratio
BBS 002.0
41
Benchmark measurement:p+Pb reference
With EMCal: jet trigger+ improved jet reconstruction provides much greater ET reach
42
Benchmark measurement:Peripheral Pb+Pb reference
Without EMCal, significant quenching measurements beyond ~100 GeV are not possible
43
Summary of statistical reach
Ratio >4 With EMCAL W/O EMCAL
RAA 225 165
RpA 225 125
RAA(5.5 TeV) 225 100
RAA() 150 110
RCP 150 (70)
Ratio z>0.5 With EMCAL W/O EMCAL
RAA 150 100
RpA 150 (70)
RAA(5.5 TeV) 140 (60)
Large : ~10% error requires several hundred signal events (Pb central) and normalization events (pp,pA).
Large z>0.5 requires several thousand events
The EMCAL • extends kinematic range by 40–125 GeV• improves resolution (important at high z)
Some measurements impossible w/o EMCAL
44
More to come …
Dijet correlations “Sub-jet” Suppression ?
Look for “hot spots” at large distance to jet axis ~10 GeV parton suppression within 100 GeV jets ?
R0 = 1fm
tform = 1/(kT)tsep = 1/
45
Photon-tagged jets
Dominant processes:
g + q → γ + q (Compton)
q + q → γ + g (Annihilation)
pT > 10 GeV/c
-jet correlation E = Ejet
Opposite direction Direct photons are not perturbed by the medium Parton in-medium-modification through the fragmentation function
min max
IP
PHOS
EMCal
TPC
46
Identifying prompt in ALICE
x5signal
Statistics for on months of running:2000 with E > 20 GeV
E reach increases to 40 GeV with EMCAL
47
Fragmentation function
quenched jet
non-quenched
Pb-Pb collisions
Background
Signal
HIC background
48
Summary
Copious production of jets in Pb-Pb collisions at the LHC < 20 GeV many overlapping jets/event
Inclusive leading particle correlation Background conditions require jet identification and reconstruction in
reduced cone R < 0.3-0.5 At LHC we will measure jet structure observables (jT, fragmentation
function, jet-shape) for reconstructed jets. High-pT capabilities (calorimetry) needed to reconstruct parton energy Good low-pT capabilities are needed to measure particles from medium
induced radiation. EMCAL will provide trigger capabilities which are in particular needed
to perform reference measurements (pA, pp, ..) ALICE can measure photon tagged jets with
E > 20 GeV (PHOS + TPC) E > 40 GeV (EMCAL+TPC) Sensitivity to medium modifications ~5%