probing new physics with dijets at cms
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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
Robert Harris, Fermilab 3
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
Robert Harris, Fermilab 4
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
Robert Harris, Fermilab 5
ATLAS 22 x 44 meters, 6000 tons 2000 physicists, 34 nations
CMS 15 x 22 meters, 12500 tons 3000 collaborators, 37 nations
Robert Harris, Fermilab 6
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.
Robert Harris, Fermilab 7
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.
Robert Harris, Fermilab 8
Introduction toJets
Robert Harris, Fermilab 9
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
Robert Harris, Fermilab 11
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)
Robert Harris, Fermilab 12
Introduction toNew Physics with Jets
Robert Harris, Fermilab 13
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
Robert Harris, Fermilab 14
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)
Robert Harris, Fermilab 15
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
Robert Harris, Fermilab 16
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
Robert Harris, Fermilab 17
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
Robert Harris, Fermilab 18
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
Robert Harris, Fermilab 19
CMS Search Plans for Contact Interactions
Robert Harris, Fermilab 20
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
Robert Harris, Fermilab 21
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.
Robert Harris, Fermilab 22
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
Robert Harris, Fermilab 23
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
Robert Harris, Fermilab 24
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
Robert Harris, Fermilab 25
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
Robert Harris, Fermilab 26
CMS Search Plans for Dijet Resonances
Robert Harris, Fermilab 27
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
Robert Harris, Fermilab 28
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
Robert Harris, Fermilab 29
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
Robert Harris, Fermilab 30
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
Robert Harris, Fermilab 31
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
Robert Harris, Fermilab 32
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
Robert Harris, Fermilab 33
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
Robert Harris, Fermilab 34
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
Robert Harris, Fermilab 35
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.
Robert Harris, Fermilab 36
Backup Slides
Robert Harris, Fermilab 37
The CMS Detector
Hadronic
Electro-magnetic
Calorimeters
Protons
Protons
Robert Harris, Fermilab 38
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
Robert Harris, Fermilab 39
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
Robert Harris, Fermilab 40
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.
Robert Harris, Fermilab 41
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
Robert Harris, Fermilab 42
||<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
Robert Harris, Fermilab 43
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
Robert Harris, Fermilab 44
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
Robert Harris, Fermilab 45
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
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