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Searches for New Physics in Photonic Final States at the LHC with CMS
1
APS April Meeting
Denver CO, May 03, 2009
Andy Yen California Inst. of Technology
CMS Collaboration
The CMS Detector
• CMS is general purpose detector with good sensitivity towards Higgs and other New Physics signatures.
• High precision crystal ECAL is the source of much of its discovery potential.
2Andy Yen (Caltech/CMS)
The CMS ECAL (76k PbWO4 crystals, 90 tons)
• Capable of energy resolution of < 0.5% for ~100 GeV photons (testbeams).
• Goal is to achieve and maintain this resolution in situ at the LHC.
3Andy Yen (Caltech/CMS)
Barrel Crystals
EndCap(s)
Barrel: 61,200 crystals in 170 φ-
rings of 360 (|η| < 1.48)
Two Endcaps: 7,324 crystals each
(1.48<|η| < 3)
Searching for Higgs at CMS
• H→γγ is the most promising decay channel at CMS.
• Direct Signal
• Relatively low SM background.
4Andy Yen (Caltech/CMS)
Combined LEP, CDF and D∅, Mar 09
MH < 160 GeV (95% CL)
BR ~ 2 x 10-3
115 – 140 GeV
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H Signal and Backgrounds
g
g
gq
q q
q
SIGNAL (NLO) 2 Isolated, High ET Photons
Gluon Fusion
Vector Boson Fusion
Associated Production
with Z, W, tt
BACKGROUND ‘Irreducible’ backgrounds
(two real photons)
qq→ γγ (born diagram)
gg→ γγ (box diagram)
‘Reducible’ backgrounds
(at least one fake photon
pp→ g +jets or isolated π0
pp→ jets (2 fake g)
pp→ ee (Drell Yan), e’s
mis-ided as photons
box born brem
Andy Yen (Caltech/CMS)
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CMS: Optimized H Analysis
• We see that photon energy resolution is very important in this analysis.
• Photon ID is also key.• Diphoton QCD background will need to be measured when
first data become available.
mH (GeV) L fb-1
115 8.5
120 7.7
130 11
140 16
150 ~30
Integrated luminosity for 5s discovery
(CMS NOTE-2006/112)
Higgs Signal and Backgrounds
Andy Yen (Caltech/CMS)
Gravity scale = MPl exp(-krc) ~TeV;
for krc ~11-12, no hierarchy problem
Graviton resonances mn = xn k exp(-krc), J1(xn)=0
Two parameters control graviton couplings and widths:
mass mG and constant c=k/MPl
Signals: Narrow, high-mass resonance states in
di-lepton and di-photon systems
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Search for RS Gravitons G/e+e-
Andy Yen (Caltech/CMS)
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Fully Simulated with Backgrounds;
ECAL Saturation Corrected (for E > 2 TeV)
Discovery Reach10 fb-1: MG > 3.14 TeV for c=0.1; MG > 1.32 TeV for c=0.0130 fb-1: MG > 3.54 TeV for c=0.1; MG > 1.59 TeV for c=0.01
Bkgds: QCD Jets, Direct
Photons, Drell-Yan
Can Directly Measure the
Width
RS Gravitons G
Andy Yen (Caltech/CMS)
CMS NOTE-2006/051
ADD Graviton Emission in γ+MET Channel
9Andy Yen (Caltech/CMS)
Signature:
A single high-pT photon in the
central η region.
High missing pT back-to-back with
the photon in the azimuthal plane
MD= 1 – 1.5 TeV for 1 fb-1
2- 2.5 TeV for 10 fb-1
The γ+(Z → νν)
background can be
estimated by studying
the γ+(Z→μμ) process
CMS NOTE-2006/129
L=60 fb-1
ECAL Calibration
• For early CMS runs, using π⁰ mass peaks is optimal.
• L=2x1033 gives
average π0γγ rate
of 1.5 kHz or 2,100
π0/crystal/day
10Andy Yen (Caltech/CMS)
Barrel Pi0 Callibration(Work in Progress)
11Andy Yen (Caltech/CMS)
• Good results after correcting for ECAL cracks and pileup π⁰’s• Calibration algorithm converges rapidly.• Can calibrate most of ECAL to 0.5% in under 100 hours.
Data after L1 Trigger Online Farm p0 Calibration
>10 kHz~1 kHz
Effect of ECAL Calibration
12January 09, 2008
Crystals Pulse Amplitudes
in a clustering algorithmParticle EnergyCalibration
Achieving a precise in situ crystal-by-crystal calibration of the CMS
ECAL will be crucial for the Hγγ search (CMS NOTE-2006/021)
Calibration Target
Andy Yen (Caltech/CMS)
Conclusion
• Photonic events also serve as a useful test of SM QCD predictions.
• Only a small sampling of CMS physics with photonic final states presented.
• Many early discovery opportunities.
• Detector performance, especially achieving ECAL calibration will be crucial to success of CMS physics program.
• Jet fake rates and Photon ID are also very important due to the large SM backgrounds.
The most exciting times
are just beginning!
Extra Slides Follow
G Discovery Potential
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CMS NOTE-2006/051
Andy Yen (Caltech/CMS)
Excited Leptons(Preliminary Results)
• e* and μ* can be produced copiously at the LHC (via contact interactions), and then decay via contact and electroweak interactions into ordinary photons and leptons
• Analysis benefits from good resolution of ECAL
• Discovery Potential (1 fb-1): Me* to ~2 TeV, Λ up to ~10 TeV
16Andy Yen (Caltech/CMS)
qq ee* ee qq μμ* μμ