Probing New Physics with Dijets at CMS

1

Probing New Physics with Dijets at CMS

Robert M. Harris

Fermilab

HEP Seminar

Johns Hopkins University

March 26, 2008

Robert Harris, Fermilab 2

Outline

Motivation

Introduction Jets at CMS New Physics with Jets Best limits on new physics from Tevatron

CMS Search Plans for Contact Interactions Jet Rate: Inclusive Jet PT

Jet Angle: Dijet Ratio

CMS Search Plans for Dijet Resonances Jet Rate: Dijet Mass Jet Angle: Dijet Ratio

Conclusions


Credits Study done at LHC Physics Center (LPC)

A center of CMS physics at Fermilab

CMS approved, publicly available Initial study completed in April 2006 and

published in CMS Physics TDR Vol II J. Phys. G: Nucl. Part. Phys. 34: 995-1579 CMS Notes (2006 / 069, 070 and 071)

Update study completed in December 2007 http://cms-physics.web.cern.ch/cms-

physics/public/SBM-07-001-pas-v3.pdf

Demonstration of physics at LPC Working within the CMS collaboration

Thanks to my many CMS colleagues! Involved postdocs and grad students: M. Cardaci, S. Esen, K. Gumus, M. Jha, K.

Kousouris, D. Mason.

P T D R

U p d a t e


CMS

ATLAS

In 2008 science will start to explore a new energy scale

14 TeV proton-proton collisions will allow us to see deeper into nature than ever before

Two large detectors will observe the collisions

Large Hadron Colliderat CERN

Geneva Switzerland


ATLAS 22 x 44 meters, 6000 tons 2000 physicists, 34 nations

CMS 15 x 22 meters, 12500 tons 3000 collaborators, 37 nations


What will the LHC detectors see?

But we hope to see more than the standard model !

We expect to see the “standard model”:

The particles and forces already catalogued . . .

. . . & perhaps a Higgs particle that remains to be discovered.


Questions in the Standard Model

Can we unify the forces ? , Z and W are unified already. Can we include gluons ? Can we include gravity ? Why is gravity so weak ? ?

?

Why three nearly identical generations of quarks and leptons? Like the periodic table of the elements,

does this suggest an underlying physics? Is it possible that quarks and leptons are

made of other particles?

These & other questions suggest new physics beyond the standard model. We can discover this new physics with simple measurements of jets at the LHC.

The simple picture of the standard model raises many fundamental questions.


Introduction toJets


Jets at LHC in Standard Model

Proton

q, q, g

Proton

q, q, g

The LHC collidesprotons containingcolored partons: quarks, antiquarks & gluons.

q, q, g q, q, g

q, q, g

q, q, g

The dominant hard collision process is simple 2 2 scattering of partons off partons via the strong color force (QCD).

Jet

Jet

Each final state parton becomesa jet of observable particles.

The process is called dijet production.

Robert Harris, Fermilab 105.022 R

Experimental Observation of Jets

CalorimeterSimulation

ET

01

-1

Jet 1 Jet 2

Dijet Mass = 900 GeV2

212

21 )()( ppEEm

Dijets are easy to find Two jets with highest PT

in the calorimeter. A jet is the sum of calorimeter

energy in a cone of radius

CMS Barrel & Endcap Calorimeters

proton

=-1

proton

Jet 1

Jet 2

=1

Transverse

=0


Jet Response & Corrections Jets in Barrel have uniform response

vs & are sensitive to new physics Jet response changes smoothly and

slowly up to | jet | = 1.3

Measure relative response vs. jet in data with dijet balance Data will tell us what is the region of

response we can trust.

Measured jet pT in the calorimeter is less than true jet pT (particles in cone)

Measured jets are corrected so pT is the same as true jet pT Scales Jet (E,px,py,pz) by

~1.5 at pT = 70 GeV ~1.1 at pT = 3 TeV for jets in barrel region

||<1.3

||<1

Jet Response vs relative to Barrel

CMS Preliminary

Jet Response vs pT in Barrel

Jets we use

(GeV)


Introduction toNew Physics with Jets


Quark Compositeness and Scattering

Three nearly identical generations suggests quark compositeness. Compositeness is also historically motivated.

Molecules Atoms Nucleus Protons & Neutrons Quarks Preons ?

Scattering probes compositeness.

In 1909 Rutherford discovered the nucleus inside the atom via scattering. Scattered particles off gold foil. Too many scattered at wide angles to

the incoming beam Hit the nucleus inside the atom!

A century later, we can discover quark compositeness in a similar way ! Rate: more jets at high pT than QCD. Angle: more dijets in center of the CMS

barrel than at the edge. Measured with dijet ratio (more later)

Today we model quark compositeness with contact interactions.

Gold

DetectorDiscovery of Nucleus

More of this Than of this

Quark Compositeness Signal

q q

q

q

q q

q

q


Quark Contact Interactions

New physics at large scale Composite Quarks New Interactions

Modeled by contact interaction Intermediate state collapses to a

point for dijet mass << . For example, the standard

contact interaction among left-handed quarks introduced by Eichten, Lane and Peskin

Excluded for < 2.7 TeV (D0)

Observable Consequences Effects at high pT & dijet mass.

Rate: Higher rate than QCD Angle: Angular distributions

can be very different from QCD.

Quark Contact Interaction

M ~

Composite Quarks New Interactions

M ~

Dijet Mass <<

q

q

q

q

q

q q

q

L = ± [2/ ] (q q) (q q)


Dijet Resonances

X

q, q, g

q, q, g

q, q, g

q, q, g

Mass

Rat

e

M

New particles, X, produced in parton-partonannihilation will decay to 2 partons (dijets).

They are observed as dijet resonances: mass bumps.

Tevatron has searched but not found any dijet resonances so far:

D0 Run 1

time

spac

e

CDF Run 1

( D

ata

– F

it )

/ F

it


Why Search for Dijet Resonances?

Experimental Motivation LHC is a parton-parton resonance factory in a previously unexplored mass region

With the higher energy we have a good chance of finding new physics. Nature may surprise us with previously unanticipated new particles.

We will search for generic dijet resonances, not just specific models. One search can encompass ALL narrow dijet resonances.

Resonances narrower than jet resolution produce similar mass bumps in our data. We can discover dijet resonances if they have a large enough cross section.

Theoretical Motivation Dijet Resonances found in many models that address fundamental questions. Why Generations ? Compositeness Excited Quarks Why So Many Forces ? Grand Unified Theory W ’ & Z ’ Can we include Gravity ? Superstrings & GUT E6 Diquarks Why is Gravity Weak ? Extra Dimensions RS Gravitons Why Symmetry Broken ? Technicolor Color Octet Technirho More Symmetries ? Extra Color Colorons & Axigluons


Dijet Resonance Details

1

1

2

1

1

1+

½+

0+

J P

qq0.01SingletZ ‘Heavy Z

q1q20.01SingletW ‘Heavy W

qq,gg0.01SingletGR S Graviton

qq,gg0.01OctetT8Octet Technirho

qq0.05OctetCColoron

qq0.05OctetAAxigluon

qg0.02Triplet q*Excited Quark

ud0.004Triplet DE6 Diquark

Chan/ (2M)Color XModel Name

mainly t - channel

QCD Background

X

q, q, g

q, q, g

q, q, g

q, q, gDijet Resonance

s - channel

Signal and backgroundwill have very different

angular distributions


CDF Dijet Resonance Search in Run II

Preliminary Results Just Released Best limits on dijet resonances Thanks to Ken Hatakeyama !

Model Excluded (GeV) Model Excluded (GeV)

A or C 260 - 1250 D 290 - 630

T8 260 - 1110 W ’ 280 - 840

q* 260 - 870 Z ’ 320 - 740


CMS Search Plans for Contact Interactions


Rate: Inclusive Jet pT

Inclusive jet pT is a QCD measurement that is sensitive to new physics. Counts all jets inside a pT bin and interval, and divides by bin width and luminosity.

Corrected calorimeter jets (CaloJets) agree with particle level jets (GenJets). CaloJets before corrections shifted to lower pT than GenJets Ratio between corrected CaloJets and GenJets is “resolution smearing”: small at high pT.

Simple correction for resolution smearing in real data is to divide rate by this ratio.Resolution SmearingInclusive Jet Cross Section


Rate: Jet PT and Contact Interactions

Contact interactions create large rate at high PT and immediate discovery possible Error dominated by jet energy scale (~10%) in early running (10 pb-1)

E~ 10% not as big an effect as = 3 TeV for PT>1 TeV. PDF “errors” and statistical errors (10 pb-1) smaller than E scale error

With 10 pb-1 we can see new physics beyond Tevatron exclusion of < 2.7 TeV.Rate of QCD and Contact Interactions Sensitivity with 10 pb-1

Sys Err.

PDF Err.


Dijet Ratio: Simple Angular Measure

Dijet angular distributions are sensitive to new physics. Contact Interactions & Resonances

Dijet Ratio = N(||<0.7) / N(0.7<||<1.3)

Number of events in which each leading jet has ||<0.7, divided by the number in which each leading jet has 0.7<||<1.3

Numerator is sensitive to new physics at low cos *.

Denominator is dominated by QCD at high cos *.

Simplest measurement of angular distribution Uses detector variable Uses same mass bins as resonance

search in rate vs. mass.

Jet 1

Jet 2

Numerator

Sensitiveto New Physics

|cos ~ 0

Denominator

Dominated By QCD

|cos *| ~ 0.7,usually

Jet 1

Jet 2 Jet 2(rare)

or

= -1.3 - 0.7 0.7 1.3

z

z


Angle: Dijet Ratio from QCD We have optimized the dijet ratio for a contact interaction search in barrel

Old dijet ratio used by D0 and PTDR was N(||<0.5) / N(0.5<||<1.0) New dijet ratio is N(||<0.7) / N(0.7<||<1.3)

Dijet ratio from QCD agrees for GenJets and Corrected CaloJets Flat at 0.6 for old ratio, and flat at 0.5 for new ratio up to around 6 TeV.

Old Dijet Ratio New Dijet Ratio


Optimization dramatically increases sensitivity to contact interactions. Raising the signal and decreasing the QCD error bars.

Old Dijet Ratio (D0 and PTDR Cuts)

3

5

10

+ (TeV)

QCD

New Dijet Ratio (Optimized in Barrel)

3

5

10

+ (TeV)

QCD

Angle: Dijet Ratio from Contact Interactions


Dijet Ratio and Systematic Uncertainties

Systematics (red) are small They cancel in

the ratio.

Relative Energy Scale Energy scale in

center vs edge of barrel in .

Estimate +/- 0.5 % is achievable in barrel.

Determined with dijet balance

Parton Distributions We’ve used

CTEQ6.1 uncertainties

P T D R


CMS Search Plans for Dijet Resonances


Dijet Mass Resolution First high statistics study of CMS dijet

resonance mass resolution.

Gaussian core of resolution for ||<1 and ||<1.3 is similar.

Resolution for corrected CaloJets 9% at 0.7 TeV 4.5% at 5 TeV Better than in PTDR 2 study.

2 TeV Z’

|η| < 1.3

Corrected CaloJets

GenJets

Natural Width

Resolution


Rate vs. Dijet Mass and Resonances

Measure rate vs. corrected dijet mass and look for resonances. Use a smooth parameterized fit or QCD prediction to model background

Strongly produced resonances can be seen Convincing signal for a 2 TeV excited quark in 100 pb-1

Tevatron excluded up to 0.87 TeV.

QCD Backgound Resonances with 100 pb-1


Rate: Systematic Uncertainties

Jet Energy CMS estimates +/- 5 %

is achievable by 1 fb-1

Changes dijet mass cross section between 30% and 70%

Parton Distributions CTEQ 6.1 uncertainty

Resolution Bounded by difference

between particle level jets and calorimeter level jets.

Systematic uncertainties on the cross section vs. dijet mass are large. But they are correlated vs. mass. The distribution changes smoothly.

P T D R


Rate: Sensitivity to Resonance Cross Section

Cross Section for Discovery or Exclusion Shown here for 1 fb-1

Also for 100 pb-1, 10 fb-1

Compared to cross section for 8 models

CMS expects to have sufficient sensitivity to Discover with 5

significance any model above solid black curve

Exclude with 95% CL any model above the dashed black curve.

Can discover resonances produced via color force, or from valence quarks.

P T D R


Rate: Discovery Sensitivity for Models Resonances produced by the valence

quarks of each proton Large cross section from higher

probability of quarks in the initial state at high x.

E6 diquarks (ud D ud) can be discovered up to 3.7 TeV for 1 fb-1

Resonances produced by color force Large cross sections from strong force With just 1 fb-1 CMS can discover

Excited Quarks up to 3.4 TeV Axigluons or Colorons up to 3.3 TeV Color Octet Technirhos up to 2.2 TeV.

Discoveries possible with only 100 pb-1

Large discovery potential with 10 fb-1

Mass (TeV)

E6 Diquark

Excited Quark

Axigluonor Coloron

Color OctetTechnirho

CMS100 pb-1

CMS1 fb-1

CMS10 fb-1

5 Sensitivity to Dijet Resonances

0 1 2 3 4 5

P T D R


Rate: Exclusion Sensitivity to Models

E6 Diquark

Excited Quark

Axigluonor Coloron

Color OctetTechnirho

W ’

R S Graviton

Z ’

Tevatron Exclusion (Dijets)

CMS100 pb-1

CMS1 fb-1

CMS10 fb-1

Mass (TeV)

95% CL Sensitivity to Dijet Resonances

0 1 2 3 4 5 6

Resonances produced via color interaction or valence quarks. Wide exclusion possibility

connecting up with many exclusions at Tevatron

Resonances produced weakly are harder. But CMS has some sensitivity

to each model with sufficient luminosity.

Z’ is particularly hard. Weak coupling and requires

an anti-quark in the proton at high x.

P T D R


Angle: Dijet Resonances with Dijet Ratio

All resonances have a more isotropic decay angular distribution than QCD Spin ½ (q*), spin 1 (Z’), and spin 2 (RS Graviton) all flatter than QCD in dN / dcos*.

Dijet ratio is larger for resonances than for QCD. Because numerator mainly low cos*, denominator mainly high cos *

QCD

Dijet Ratio vs MassDijet Angular Distributions


Angle: Dijet Resonances with Dijet Ratio Dijet ratio from signal + QCD compared to statistical errors for QCD alone

Resonances normalized with q* cross section for ||<1.3 to see effect of spin.

Convincing signal for 2 TeV strong resonance in 100 pb-1 regardless of spin.

Promising technique for discovery, confirmation, and eventually spin measurement.

Dijet Ratio for q* Dijet Ratio for Spin ½, 1, 2


Conclusions We’ve described CMS search plans for new physics with dijets.

Inclusive jet pT could give a convincing contact interaction signal at startup! Can discover + = 3 TeV in 10 pb-1 even if jet energy errors are 10%.

Dijet ratio will probe contact interactions in dijet angular distributions Can discover + = 4, 7, 10 TeV in 10, 100, 1000 pb-1 with small systematics.

Dijet mass can be used to discover a dijet resonance up to many TeV. Axigluon, Coloron, Excited Quark, Color Octet Technirho or E6 Diquark Produced via the color force, or from the valence quarks of each proton.

Dijet ratio can discover or confirm a dijet resonance with small systematics. Gives a convincing signal for a 2 TeV q* with 100 pb-1. Eventually it will be used to measure the resonance spin.

CMS is preparing to discover new physics at the TeV scale using dijets.


Backup Slides


The CMS Detector

Hadronic

Electro-magnetic

Calorimeters

Protons

Protons


CMS Barrel & Endcap Calorimeters(r-z view, top half)

HCAL BARREL

ECAL BARREL

SOLENOID

HCALENDCAP

ECALENDCAP

HCALENDCAP

ECALENDCAP

Z

HCAL OUTER

3 m

HCAL > 10 IECAL > 26 0


Trigger and Luminosity Collision rate at LHC is expected to be 40 MHz

40 million events every second ! CMS cannot read out and save that many.

Trigger chooses which events to save

Two levels of trigger are used to reduce rate in steps Level 1 (L1) reduces rate by a factor of 400. High Level Trigger (HLT) reduces rate by a factor of 700.

Trigger tables are intended for specific luminosities We’ve specificied a jet trigger table for three luminosities L = 1032 cm-2 s-1. Integrated luminosity ~ 100 pb-1.

LHC schedule projects this after ~1 months running. L = 1033 cm-2 s-1. Integrated luminosity ~ 1 fb-1.

LHC schedule projects these after ~ 1 year of running. L = 1034 cm-2 s-1. Integrated luminosity ~ 10 fb-1.

One months running at design luminosity.

4 x 107 Hz

1 x 105 Hz

1.5 x 102 Hz

Event Selection

CMS Detector

L1 Trigger

HLT Trigger

Saved for Analysis


Path

L1 HLT ANA

ET

(GeV)

Pre-

scale

Rate

(Hz)

ET

(GeV)

Rate

(Hz)

Dijet Mass

(GeV)

Low 25 2000 146 60 2.8

Med 60 40 97 120 2.4 330

High 140 1 44 250 2.8 670

Super 450 1 14 600 2.8 1800

L = 1032

100 pb-1

Ultra 270 1 19 400 2.6 1130

L = 1033

1 fb-1

Add New Threshold (Ultra). Increase Prescales by 10.

Mass values are efficient for each trigger, measured with prior trigger

L = 1034

10 fb-1

Add New Threshold (Super). Increase Prescales by 10.

Jet Trigger Table and Dijet Mass Analysis

CMS jet trigger saves all high ET jets & pre-scales the lower ET jets. Prescale means to save 1 event out of every N events.

As luminosity increases new trigger paths are added

Each with new unprescaled threshold.


Trigger Rates & Dijet Cross Section(QCD + CMS Simulation)

Include data from each trigger where it is efficient in dijet mass. Stop analyzing data from trigger where next

trigger is efficient

Prescaled triggers give low mass spectrum at a convenient rate. Measure mass down to 300 GeV Overlap with Tevatron measurements.

Trigger without any prescaling saves all the high mass dijets

Expect the highest mass dijet event to be ~ 7.5 TeV for 10 fb-1

~ 5 TeV for 100 pb-1

LHC will open a new mass reach early!

Put the triggers together to form a cross section.

|jet |<1

PrescaledTriggerSamples

P T D R

P T D R


||<1.3

||<1

Jet response vs relative to ||<1.3

CMS Preliminary

Jet Region Barrel jets have uniform response & sensitive to new physics

Jet response changes smoothly and slowly up to | jet | = 1.3 CaloTowers with ||<1.3 are in barrel with uniform construction. CaloTowers with 1.3<||<1.5 are in barrel / endcap transition region

Some of our analyses use | jet |<1.3, others still use | jet |<1 All are migrating to | jet |<1.3 which is optimal for dijet resonances

Measure relative response vs. jet in data with dijet balance Data will tell us what is the region of response we can trust.

Barrel Jet(||<1.3)

Probe Jet(any )

Dijet Balance

= 1.3

HBHE

Hcal towers and cuts

TransitionRegion

= 1


Dijet Event Cleanup Dijet events do not usually contain large missing ET

A cut at MET / ET < 0.3 is >99% efficient for PT > 100 GeV Won’t change the QCD background to new physics.

Most unphysical background contain large missing ET Catastrophic detector noise, cosmic ray air showers, beam-halo backgrounds A simple cut at MET / ET < 0.3 should remove most of these at high jet PT. This cut is our first defense, simpler and safer than cutting on jet characteristics.

99% Efficiency Cut & Chosen CutMET / ET for QCD Dijets and Cut


Dijet Resonances: Optimization of cut

QCD cross section rises dramatically with || cut due to t-channel pole. Z’ signal only gradually increases with || cut optimal value at low ||.

Optimal cut is at || < 1.3 for a 2 TeV dijet resonance. Optimization uses Pythia Z’ angular distribution for the resonance.

cut and cross section cut and sensitivity


CDF Run II Resonance Search

Dijet resonance shapes are similar

Separate limits for W’ and Z’ models

Systematic uncertainties reduced from run 1. Much lower jet energy scale error

Documents

Probing New Physics with Dijets at CMS