Search for New Physics in events with two jets in Proton

Search for New Physics in events
with two jets in Proton-Proton Collisions
at √s=13 TeV
Candidata: Giulia D'Imperio
Supervisor: Shahram Rahatlou
Seminario dei dottorandi del XVIII ciclo
25/02/14
Giulia D'Imperio – INFN Roma – Università La Sapienza
Outline
◈ Field of research and motivations
◈ Hadronic collisions
◈ Hadronic jets in CMS
◈ Jet calibration with events with une photon (γ) + jet
◈ Present results of dijet searches
◈ Work in progress: cross section measurement of γ + jet
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Giulia D'Imperio – INFN Roma – Università La Sapienza
The energy frontier
The Large Hadron Collider (LHC) is
the world’s largest and most powerful
particle accelerator
◎ 2010-2011
@ 7 TeV
2012 @ 8 TeV
2013-2014 shutdown
2015 restart @ 13 TeV
◎ four particle detectors – ATLAS, CMS,
ALICE and LHCb
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Why still search new physics at the LHC?
◈ Particle Physics experiments never observed anything in
disagreement with the Standard Model (including the recent
discovery of the Higgs boson)
BUT
◈ Standard Model appears to be incomplete
◈ Many questions are still open:
How to solve hierarchy problem?
What is dark matter?
Why the asymmetry between matter and antimatter in the
universe?
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Why search heavy hadronic resonances?
◈ The LHC will reach the energy in the center of mass of 13 TeV
Since the 60s physicists discovered new particles raising the
energy in the collisions!
◈ Many models predict the existence of heavy particles that
could be observed in a final state with two jets at the LHC
◈ This search will be one of the most important analysis in the
very first months of data-taking (Ph.D. thesis timescale)
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Hadronic collisions
This is what happen in a proton-proton collision
Asymptotic freedom → at high energy
interaction between massless partons
A lot of energy avaiable in the
collision
Most of the interactions
→ low energy, many soft
particles forward
proton
proton
Quark confinement
→ hadronization: jets of
collimated particles
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Heavy resonances
The simplest way to observe a new particle is to search this type of
processes in the “S channel”
◎ Quarks and gluons hadronize: 2 energetic jets back-to-back in the
transverse plan
◎ Invariant mass is the mass of the particle X
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Giulia D'Imperio – INFN Roma – Università La Sapienza
The CMS detector
Hadronic jets are reconstructed using informations of tracker and calorimeters
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Giulia D'Imperio – INFN Roma – Università La Sapienza
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Giulia D'Imperio – INFN Roma – Università La Sapienza
A “bump” hunt
Simulation with PYTHIA of the signal for different masses of a graviton G
Basically a Breit-Wigner
shape with a low energy
tail (QCD radiation and )
Resolution of the mass peak depends on the resolution of the transverse energy
of jets
Giulia D'Imperio – INFN Roma – Università La Sapienza
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Jet energy calibration with γ + jet events
The “raw” energy measured from the CMS detector needs to be corrected
Detector effects (misalignment, calibration of the calorimeters,...)
Physics effect (multiple collisions)
HCAL
ECAL
Energy balance in events with
one photon and one jet is
used to correct the jet energy
◎ photon and jet back-to-back in
the transverse plan
q,g
◎ photon energy resolution
negligible with respect to jets
γ
pT balancing
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Energy balancing
Example of pT balancing without residual corrections for data and simulation
100 < pT(γ) < 125 GeV
|eta| < 0.8
Center of the gaussian for data and simulation is slightly different
→ calculation of residual corrections of the energy scale
(function of jet energy and eta)
Giulia D'Imperio – INFN Roma – Università La Sapienza
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Dijet mass with background
Main background is QCD
This plot is with full statistics of 2012, 19.6 fb-1 @ 8 TeV and after selection
No “bumps” in the dijet mass spectrum.
◎ | | < 1.3
◎ |eta| < 2.5
◎ Mjj > 890 GeV
Central jets, high invariant
mass, cut in deltaEta to remove
most of the background from
qq, gg scattering in T channel
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Present results for dijet searches at 8 TeV
No discovery, but we can put upper limits.
Limits at 95% CL on the expected cross-section for several models
◎ Dashed lines are the predicted
cross sections for different models
of heavy resonances
◎ Solid lines are observed upper
limits of signal cross section in
different final states (gg, qg, gg)
◎ Where dashed lines cross solid
lines defines the lower limit on
resonance mass for the considered
model at 95% CL
◎ We don't know the energy scale of new physics but with 13 TeV cross sections will rise
of a factor 100-1000 for ~TeV mass resonances
◎ LHC should collect some fb in few months
◎ Discovery or exclusion very soon
-1
Giulia D'Imperio – INFN Roma – Università La Sapienza
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Events with photon and jet(s)
◎ Studied events with photon + jet for the jet calibrations
◎ Now working at the cross section measurement
◎ precision measurement: different from the “search” approach
◎ Useful for experimental purpose: background modeling in some new physics
searches
◎ Useful for theoretical purpose: possible constraints on gluon pdf
SIGNAL
BACKGROUND
Fragmentation photons ( from the decay
of η, π0)
+ radiation of isolated and energetic photons
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Differential cross section measurement
Nsig
Systematic uncertainties for the moment are not studied but in this plot we use an
estimate of ~15% (from previos studies)
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Road map
◈ 2013-2014
γ+jet cross section measurement
(finalization and expected publication in spring-summer 2014)
◈ 2014
jet calibration with events with γ + jet
◈ 2014-2015
preparation for the analysis with jets @ 13 TeV
◈ 2015 first search for new physics with first data @ 13 TeV
(defense of the Ph.D. thesis and expected publication at the end of
the year)
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Giulia D'Imperio – INFN Roma – Università La Sapienza
B
K
C
A
P
U
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Hadronic collisions
Asymptotic freedom → at high energy interaction between massless partons
◎ Two energetic jets back-to-back in the transverse plan
◎ Invariant mass is the mass of the particle X
Jets are composed object and definition depends on the algorithm used in
recostruction:
◎
Radius parameter DeltaR = 0.5 defines the width of the cone
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Events with photon and jet(s)
SIGNAL
Isolated photon cross section measurement
+ QCD with ISR or FSR of isolated and energetic photons
BACKGROUND
Fragmentation photons ( from the decay of η, π0)
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Events with photon and jet(s)
SIGNAL
Isolated photon cross section measurement
+ QCD with ISR or FSR of isolated and energetic photons
Gluon in the initial state:
a precise measurement of the cross section can constraint the gluon pdf inside the
proton [1]
[1] David d'Enterria, Juan Rojo “Quantitative constraints on the gluon distribution function in the 21
proton from collider isolatedphoton data”, arXiv:1202.1762 [hepph] Giulia D'Imperio – INFN Roma – Università La Sapienza
Cross section measurement
General relation between cross section and number of events
Precise measurement of
the number of signal
events in data to extract a
measurement of the cross
section
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Giulia D'Imperio – INFN Roma – Università La Sapienza
Differential cross section measurement
Signal template
Background template
13 bins in pT
Fit on data
pT 99-105 GeV
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Giulia D'Imperio – INFN Roma – Università La Sapienza

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