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Tests of QCD in pp Interactions at 14 TeV

Manoj Kumar Jha

5th March, 2007Centre for Detector and Related Software

TechnolgyDepartment of Physics & Astrophysics

University of Delhi

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Outline

Large Hadron Collider (LHC) Compact Muon Solenoid (CMS) Experiment The Preshower Detector Motivation The CMS Software

Action on DemandStages of Reconstruction

Results Optimization of transverse shaping of Lead absorbers in

the CMS Preshower Direct Photons Production at LHC Parton kT Smearing Effects in Direct Photon Production

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Large Hadron Collider (LHC), CERN

p-p Collider Centre of Mass

Energy = 14 TeV Luminosity = 1034 /cm2/s

Experiments at LHC Compact Muon Solenoid (CMS)

ATLAS

ALICE

LHC-B

Physics at LHC

Search for SM Higgs Boson

SUSY Physics

B-Physics

New Physics

Test of QCD

Quark Gluon Plasma

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CMS Detector

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The main function of the PSD is to provide /0 separation in Endcap.

Light Higgs : HMajor Bgd. : 0

The Preshower of the CMS Detector

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Why?In Endcap region (1.65 < ||<2.6), the important background to the “gold-plated” H decay is 0 (closely spaced), faking as real photon.

The main function of the PSD is to provide -0 separation

Components PSD contains two thin lead ‘absorbers’ followed by Si micro-strip detector planes

The CMS Preshower

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Isometric view of the array of PSD, square arrays correspond to micro-module and Si detector

Micromodule assembly

Si Strip Detector for PSD

Individual strips (32)

Individual strips (32)

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The CMS Software

• COBRA: Framework & architecture

implementation.• Geometry: Geometry description of the

CMS • OSCAR: Object Oriented Simulation

for CMS.• ORCA: Object Oriented

Reconstruction for CMS Analysis.

• IGUANACMS: Interactive Graphics for User

Analysis for CMS.

The main projects

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OSCAR (Object Oriented Simulation for CMS Analysis and Reconstruction)

OO design. Use Geant 4. Detector description

is handled by DDD which is a COBRA subsystem.

XML files with the actual detector configuration are under version control, managed by the Geometry project.

DDD Basic Architecture

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Stages of Reconstruction

SimHitsProduced by MC, stored in DB

Digis Include Pileup, some stored in DB (Tk)

RecHitsPre-processed digits,some stored in DB (Calo)

RecObjTracks, Clusters etc,will be stored in DB

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Optimization of Transverse Shaping of Lead absorbers in the CMS Preshower

has been cited in CMS Physics TDR Vol. I, Page No. 150

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Outline

Motivation Level-1 trigger algorithm Level-1 trigger efficiency in presence of Preshower How to optimize the lead shape ? Optimization of lead shape around η=1.653 Optimization of lead shape around η=2.6 Summary

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Motivation: Transverse Shaping of Lead Absorbers

Preshower fiducial coverage 1.653 < η < 2.6

ECAL trigger towers in endcap are not in η & Ф.

Simple radial lead shaping does not match trigger tower boundaries (or any other physical boundary).

Partial coverage of crystals by Lead

Limitations within the ORCA Level 1 Et are scaled without the

knowledge of crystals covered by lead

Loss in L1 trigger efficiency at preshower boundaries.

Motto of Work Minimize loss of L1 efficiency

while maintaining (as much as possible) full ES fiducial coverage.

Layout of Preshower, Supercrystals and Trigger towers of ECAL endcap in fixed phi region

eta = 1.653

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CMS Level-1 Trigger Algorithm

Based on a sliding windows technique which spans the complete η - φ plane.

Et cut = Et in central towers + highest Et in 4 towers which are adjacent to central towers.

Cuts for isolation H/E < 0.05 Sum of ECAL energy in remaining

8 towers < 2 GeV Sum of Et in one of the 4 L-

shaped regions about the central ECAL crystal < 1 GeV

Leve1-1 trigger system detects signature of electrons/photons, taus, jets, and missing transverse energy and total transverse energy.

Electron/Photon Algorithm

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Level-1 trigger efficiency in the presence of the Preshower

Level-1 trigger efficiency decreases by 1% after introduction of Preshower with circular lead in the ECAL endcap. The loss in the Level-1 trigger efficiency around η = 1.653 may be recovered if

the shape of lead is optimized in the transverse direction.

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Reasons for dip in Level-1 trigger efficiency

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Distribution of number of brem candidates in an event

Distribution of brem for events which have Level-1 isolated clusters

Distribution of brem for events which have Level-1 non-isolated clusters

Number of brem candidates are same for events which have Level-1 isolated and non-isolated clusters.

No. of Brem ~ 6No. of Brem ~ 6

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Distribution of number of basic clusters

Distribution of basic clusters for events which have Level-1 isolated clusters.

Distribution of basic clusters for events which have Level-1 non-isolated clusters.

No. of basic clusters is large for events which have Level-1 non-isolated clusters Events are depositing energies in more than one trigger tower.

It is failing the isolation cut of there being at least one quiet corner of electron/photon algorithm of Level-1 trigger.

OR Failing one of the four five-tower corners has all towers below a programmable threshold.

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Distribution of ET of Seed Clusters for Superclusters

Distribution of ET of seed clusters for events which have Level-1 isolated clusters

Distribution of ET of seed clusters for events which have Level-1 non-isolated clusters

Falling PT distribution for non-isolated case is due to fact that events are depositing energies in several trigger towers.

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Reasons for low Level-1 trigger efficiency around η = 1.653

Self-vetoing by the isolation cut: If electron energy is split across non-

adjacent towers then it might be suspected that this would lead to electrons appearing as non-isolated deposits because some of the electron’s own energy falls in the L-shaped region.

Splitting of Deposits:If the electron is incident on the tower

corners where a shower can be almost equally split between four trigger towers resulting in an trigger Et much less the incident Et and may therefore fall below the trigger cuts.

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How to Optimize the lead shape ?

Crystal in the ECAL endcap

Extrapolated front face of crystal at the second plane of the lead

Extrapolated front face at the first plane of the lead

Dimension of the crystals Length = 22.0 cm Rear Side = 3.0 cmFront Side = 2.8618 cmTaper Angle = 0.360

Each crystals in ECAL endcap are off pointing from Z = 0 and angle of “off pointing” depends on the location of the crystal in the ECAL endcap .

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Constraints Conditions

All lead must be covered by silicon sensors

Mechanical constraints Horizontal &

vertical cutting of Lead are easier

Things need to be considered

Thickness of the Lead Spread of event vertex

Could shape according to trigger towers ?

Loss of preshower fiducial coverage due to variable size of trigger towers

Trigger tower geometry is not frozen

How to Optimize the lead shape (cont.)?

eta = 1.653

eta = 1.8

eta = 1.6

phimin = 28.3530

phimax=34.16610

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L1 Trigger Efficiency for Different Shapes of Lead in the Preshower

There is a clear improvement of L1 trigger efficiency at the boundary region when the lead is shaped according to crystal dimensions, even though the lead does not exactly match the crystal dimensions. Lead should be shaped roughly according to the dimensions of the crystals around η = 1.653.

20 GeV Pt Electrons

10K Events/point

Electrons fired in a fixed region of phi ( from 28.300

to 34.170) in ECAL Endcap

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Shaping of the Lead absorbers around η = 2.6 to follow

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Layout of region used in simulation around η = 2.6

η = 2.6 represents the inner boundary of the Preshower.

Number in pink color represents the supercrystals ID of ECAL endcap available in ORCA.

Red rectangular blocks represents the layout of simulated Lead according to dimensions of the crystals in the ECAL endcap.

Events consist of single electron of PT of 20 GeV fired in η from 2.8 to 2.322 and in φ from 24.940 to 44.890 .

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Leve-1 trigger efficiency as a function of the pseudorapidity

CMS Level-1 trigger algorithm finds isolated and non-isolated electron/photon candidates in the region that extends in η from -2.5 to 2.5.

Level-1 isolated trigger efficiency is independent of shape of Lead absorbers.Shape of the Lead should be circular around η = 2.6 otherwise it will incur additional cost and manpower for shaping the Lead absorbers according to dimension of crystal in ECAL endcap.

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Summary

Around η =1.653, the Level-1 trigger efficiency decreases by ~ 3% even in absence of the Preshower.

Level-1 trigger efficiency decreases by further 1% around η =1.653 after introduction of Preshower with circular lead in the ECAL endcap.

Shape of lead has been optimized around η =1.653 Lead should be shaped roughly according to

the dimensions of the crystals in the endcap. Loss in Level-1 trigger efficiency has been

recovered after shaping the Lead according to dimensions of the crystals.

Shape of Lead should be circular around η =2.6.

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Direct Photon Production at the LHC

Published in Phys. Rev. D., 67, 014016, Jan. 2003

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Outline

Introduction Production Mechanisms CDF Data Analysis Expectations at LHC regime -- Event kinematics -- LO & NLO Cross-sections -- Theoretical uncertainties: QCD scales & PDF’s -- Rapidity & Cone size dependence -- Background due to 0 decay

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Physics Motivations: Why study Direct Photons?

Ideal testing ground for the essential validity of pQCD formalisms.

-- photons are simple, well measured EM objects-- emerge directly from the hard scattering

without fragmentation

Important to understand QCD of photon production in order to reliably search for Higgs -- direct photon pairs as major irreducible

background to --Intermediate Mass Higgs boson discovery in the channel at LHC

H

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Physics Motivations: pros & cons of direct γ study

Advantages over Jet/Hadron Study Theoretical Simplifications -- Fewer subprocesses -- point-like coupling of the photon makes the higher-

order QCD calculations relatively simple. Smaller Systematic Uncertainties -- Energy resolution of ECAL is generally better than

HCAL --Precise measurement of direction & energy of photons directly without the need for an algorithm to

reconstruct a jetDisadvantages Backgrounds copiously produced from neutral meson decays. Lower event rate compared to jet production. Complications from photons produced during jet fragmentation.

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Introduction

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pQCD Predictions for Inclusive Cross-sections

Kinematical region pp→ γX at √s=14 TeV

20 GeV < pT < 400 GeV & –3 < η < 3

LO QCD : PYTHIA 6.2 PDF—CTEQ5M1 Events Generated--105 Subprocesses: qg→qγ , qqbar →gγ & gg →gγ

NLO QCD Uses CDF Run 1B isolation cut which rejects events with a

jet of ET > 1 GeV in a cone of radius 0.4 around the photon.

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Direct Photons at LHC : LO Cross Section

Compton scattering provides the dominant contribution in the entire kinematical region

Contribution from annihilation scattering ↑ with ↑ pT ...

Gluon-gluon scattering contributes negligibly.

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Direct Photons at LHC : LO & NLO Cross Section

• NLO QCD contribution higher than the LO in the whole pT range under analysis.

• K-factor = σ(NLO)/ σ(LO) of upto 2 at low pT

• NLO contribution decreases in importance with rise in pT.

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Pseudorapidity Spectrum of Direct Photons at LHC(High pT region)

• η distribution is almost insensitive to the large-x behaviour of gluon distribution.

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Direct Photons at LHC in Different Pseudorapidity Windows

• As expected, direct photon cross section is more for larger η interval.

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Cone Size Dependence of Direct Photons at LHC

• Cross-section ↓ almost uniformly over the whole pT region with ↑ in cone size.

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Quark-Gluon Compton scattering provides the dominating contribution in the entire kinematical region at LO QCD.

NLO contribution dominates at low pt and decreases in importance considerably at high pT.

NLO predictions depend only marginally on the choice of scales.

pT distribution –insensitive to PDFs at high pT, but quite sensitive at low pT

At low pT, rapidity distribution is much more sensitive to the small-x behaviour of gluon distribution.

Direct photons are produced fairly copiously in the central rapidity region.

Direct photon cross-section decreases with increasing cone size.

Summary

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Parton kT Smearing Effects in Direct Photon Production

Published in Phys. Rev. D, Vol. 68, July 2003.

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Highlights: Data vs Theory

-- kT Phenomenology -- Modelling kT effects

Data Analysis -- CDF & DØ Run 1b Data at 1.8 TeV & 630 GeV-- NLO QCD (CTEQ6M)

Simulation of multiple soft-gluon effects (kT)

-- NLO QCD + kT Enhancement vs Data

Initial-State Soft Gluon Effects expected at LHC

Conclusions

Outline

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Highlights: Data vs Theory

Global QCD analysis of direct production process :

A pattern of disagreements of data from NLO QCD predictions, both at collider and fixed-target regime.

Data display steeper pT dependence than theory in the low pT region

Deviation occurs at different x-values for expts. at different energies

New parton distribution functions, improved photon fragmentation functions & change of theory scales can not be expected to resolve the low pT discrepancy

Proposed Explanation: NLO calculation lacks kT-kick due to multiple soft-gluon emission

J.Huston et. al., PRD 51, 6139 (1995)

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KT Phenomenology

Conventional QCD hard-scattering formalism: interacting partons collinear with the incoming hadrons & outgoing partons are back-to-back kT = 0

The incident partons may have some transverse momentum => net pT imbalance among the outgoing particles,

If the outgoing particles are photons or leptons, then QT should provide a good measure of kT:

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Modeling kT Effects

Fully resummed PQCD description of soft-gluon emission effects in inclusive direct production still awaited.

Phenomenological kT-smearing model:

-- Soft-gluon radiation is parameterized in terms of an effective <kT>

that provides additional transverse impulse to the outgoing partons.

-- because inclusive cross sections fall rapidly with pT, net effect

of kT is to increase the yield – by factor K(pT)

-- LO QCD calculations model incident-parton kT using Gaussian smearing.

No such smearing is available in current NLO calculations.

Approx. Treatment:

Double counting of contributions is expected to be small.

<kT> values for calculations of K(pT) are consistent with observations.

)0(

)()(

TLO

TLO

TLO

k

kpK

)(* TLONLO pK

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Isolated Photon Cross section at s=1.8 TeV

NLO calculations (Vogelsang) agree qualitatively with data over a wide range of pT.

Excess of photons over theory in

the low pT region

DØ and CDF data are consistent

with each other

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Simulation of Soft-gluon Effects at s=1.8 TeV

PYTHIA: Default primordial kT distribution of 1 GeV & ISR on. No. of ISR gluons is significantly larger than the NLO PQCD approx. of 0 or 1.

Net pT of remnant ISR gluons: ~2.2 GeV at pT(min)=10 GeV 4.5 GeV at pT(min)=50 GeV .

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DØ Inclusive Cross section: kT Effects

Parton kT-smearing produces strong enhancement of cross section at low pT .

NLO theory + kT effects: successful to a great extent in describing the Data

kT per parton ~ 3 GeV for s=630 GeV & ~ 4 GeV at s =1.8 TeV).

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CDF Inclusive Cross section: kT Effects

Parton kT-smearing produces strong enhancement of cross section at low pT .

NLO theory + kT effects: successful to a great extent in describing the Data

kT per parton ~ 3 GeV for s=630 GeV & ~ 4 GeV at s =1.8 TeV).

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Conclusions

Data sets in the central region exhibit a steeper slope than the theoretical predictions at low pT .

Theoretical uncertainties from scale variations & choice of PDFs fail to explain the discrepancy at low pT.

Reasonably good agreement between data & theory in the Forward region.

NLO predictions + kT effects yield significantly better description of measured cross sections.

Better theoretical understanding of soft gluon effects would clearly benefit global determinations of G(x), especially at medium and large x values.

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Backup Slides

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CMS Softwares

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COBRA (Coherent Object Oriented Base for Reconstruction, Analysis)

Insulate user code from services

Manage persistent data

transparently

Manage Collections, Runs etc

Manage the order of reconstruction

Ensure a uniform interface to all

CMS code

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Action on Demand Action on demand and implicit invocation are powerful tools to

manage the order in which things are done, and to avoid doing things that you don’t need to do

Algorithms register with the framework “I can produce Tracks of type T1”

They do nothing unless triggered Connections between algorithms (i.e. data objects required) are

handled by CARF (subsystem of COBRA) User asks for Tracks of type T1 CARF determines they are not already in persistent store (and valid) CARF triggers (previously registered) T1 algorithm T1 algorithm asks CARF for Tracker RecHits CARF serves them from DB or triggers the Tracker RecHit algorithm

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ORCA (Object Oriented Reconstruction for CMS Analysis)

Flagship oo software project Combination of Signal & Pile-up events Detector digitisation and reconstruction of

detector objects Tracks, Clusters, Vertices

Reconstruction of physics objects Jets, Electrons, Photons, Muons

Simulation of L1 Trigger decisions The Higher Level Trigger algorithms

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Direct Photons

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Scale Dependence of NLO Predictions for Inclusive Cross-sections at LHC

LO QCD : strong scale dependence particularly at low pT.

NLO QCD: less sensitivity .

( normalization uncertainty of at most 14% over the whole range.)

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Sensitivity of NLO Predictions to the Choices of PDF’s

almost insensitive to the choice of PDF at high pT >300 GeV.

more and more sensitivity in the lower pT region.

pT spectrum is slightly sensitive to the small-x behaviour of gluon distribution.

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Pseudorapidity Spectrum of Direct Photons at LHC(Low pT region)

• η distribution is quite sensitive to the small-x behaviour of gluon distribution.

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Direct Photons at LHC in Different Pseudorapidity Windows

Direct photons are produced fairly copiously in the central region. The production rate decreases in the high η domain, particularly at high pT.

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Isolated Photon Cross section at s=630 GeV

NLO calculations (Vogelsang) agree qualitatively with data over a wide range of pT.

Excess of photons over theory in the low pT region

DØ and CDF data are consistent with each other

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Theoretical Uncertainty: Scale Dependence

NLO calculations are quite sensitive to the choice of QCD scales

Importance of still higher order contributions.

Different choices of theory scales fail to explain the discrepancy at low pT.

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Theoretical Uncertainty: Sensitivity to PDFs

Uncertainty from PDFs is small, smaller than that from scale dependence : 9% at s=1.8 TeV & 7% at s =630 GeV.

Not possible to cure the low pT discrepancy by fine tuning of parton distributions.