1. Searches for New Physics in Photonic Final States at the LHC with CMS
APS April Meeting
Denver CO, May 03, 2009
Andy Yen
California Inst. of Technology
CMS Collaboration

2. Andy Yen (Caltech/CMS)
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.
First I would like to present a quick overview of the CMS detector.
CMS is a general purpose detector designed to have good sensitivity towards Higgs and other new physics signatures.
These include SUSY, MSSM, compositeness, and extra dimensions.
The CMS features a high precision crystal ECAL which is the source of much of its discovery potential especially in signatures involving photonic final states.
Andy Yen (Caltech/CMS)

3. The CMS ECAL (76k PbWO4 crystals, 90 tons)
Barrel: 61,200 crystals in 170 φ-rings of 360 (|η| < 1.48)
Two Endcaps: 7,324 crystals each (1.48<|η| < 3)
Barrel Crystals
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.
EndCap(s)
The CMS ECAL consists of 76 thousand PbWO4 crystals and weighs a total of 90 tons.
The ECAL is divided into a barrel and endcap region.
After calibration, the CMS ECAL is capable of achieving energy resolution of 0.5% for photons and electrons of over 100 GeV.
This has been verified in several test beam experiments.
Active work needs to be done to maintain this resolution as the individual crystal’s performance will change over time through radiation damage.
We see from the equation for energy resolution that for high energies, the constant term dominates so achieving the design calibration precision will be important.
Andy Yen (Caltech/CMS)

4. Searching for Higgs at CMS
H→γγ is the most promising decay channel at CMS.
Direct Signal
Relatively low SM background.
The Higgs is produced at the LHC primarily through gluon gluon fusion and vector boson fusion.
Recent Tevatron results have indicated that the Standard model Higgs should be light with a mass under 160 GeV.
At CMS, within this most probable mass range, the most promising decay mode is H->gamma gamma.
Despite its small branching ratio, the H->gamma gamma channel is very promising because
1) The high precision CMS ECAL should allow higgs peak reconstruction with resolution below 1%
2) it has a relatively low Standard Model background.
BR ~ 2 x – 140 GeV
Combined LEP, CDF and D∅, Mar 09 MH < 160 GeV (95% CL)
Andy Yen (Caltech/CMS)

5. H Signal and Backgrounds
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 g
box g
born
brem q g q g g g g q g g q
The H->gamma gamma signature consists of two isolated photons with high transverse energy or ET.
There is an irreducible background from prompt diphoton production through quark annihilation and gluon annihilation, also known as the born and box processes.
The main reducible backgrounds are from jets and drell-yan electrons which are misidentified as photons.
A jet fake can occur when a jet contains a fairly isolated neutral meson such as a pizero. If this neutral meson has sufficiently large transverse momentum, the Lorentz boost causes the two photons from its decay to be nearly collinear and the energy collected within a single ECAL cluster.
Although CMS has a very high rejection factor for these jet fakes, the sheer size of their cross sections makes them one of the dominant background for H->gamma gamma searches.
Andy Yen (Caltech/CMS)

6. CMS: Optimized Hgg Analysis
Integrated luminosity for 5s discovery (CMS NOTE-2006/112)
Higgs Signal and Backgrounds
The CMS Optimized Analysis uses a neural net to discriminate between the signal and background. Photon isolation and event kinematics variables are used as the inputs.
As we can see, the Higgs invariant mass peaks are very small compared to the SM background.
In the figure, they are scaled by a factor of 10. Thus, improvements in the ECAL energy resolution can have a large impact on discovery potential.
The jet fakes contribute the majority of the background so achieving clean photon identification is key.
The QCD diphoton background is not full understood so it will need to be measured when first data become avaliable.
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.
Andy Yen (Caltech/CMS)

7. Andy Yen (Caltech/CMS)
Search for RS Gravitons Ggg/e+e-
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
Another new physics search using photons at CMS is Randall-Sundrum gravitons.
In the Randall-Sundrum model, it was demonstrated that by adding a single extra dimension, it is possible to rigorously solve the hierarchy problem. In particle physics, the hierarchy problem is essentially the problem of why many fundamental couplings and masses are so different, for instance, why is the weak force 10^32 times stronger than gravity.
The randall-sundrum model solves this problem with a planck brane connected to our SM brane through a curved extra dimension.
There are two parameters that control graviton couplings and widths, the graviton mass and a coupling constant c which is a function of the curvature.
RS graviton can decay into a dilepton or diphoton pair.
Andy Yen (Caltech/CMS)

8. Andy Yen (Caltech/CMS)
RS Gravitons Ggg
Fully Simulated with Backgrounds;
ECAL Saturation Corrected (for Eg > 2 TeV)
CMS NOTE-2006/051
Bkgds: QCD Jets, Direct Photons, Drell-Yan
Can Directly Measure the Width
Because of the high mass of these gravitons, typically 1 TeV or higher, there is very little SM background for this signal allowing for distinct resonance peaks.
The main backgrounds are the same as for the Higgs search.
Because of the low backgrounds, it is possible to directly measure the width.
The discovery potential is quite high for both small and large couplings. With 30fb-1 of integrated luminosity, we expect to discover RS gravitons up to 3.5 and 1.5 TeV, depending on the value of c.
Discovery Reach 10 fb-1: MG > 3.14 TeV for c=0.1; MG > 1.32 TeV for c= fb-1: MG > 3.54 TeV for c=0.1; MG > 1.59 TeV for c=0.01
Andy Yen (Caltech/CMS)

9. ADD Graviton Emission in γ+MET Channel
The γ+(Z → νν) background can be estimated by studying the γ+(Z→μμ) process
CMS NOTE-2006/129
Signature:
A single high-pT photon in the central η region.
High missing pT back-to-back with the photon in the azimuthal plane
L=60 fb-1
MD= 1 – 1.5 TeV for 1 fb-1
TeV for 10 fb-1
The ADD model is a large extra dimension model which also provides a solution to the hierarchy problem.
Here, a graviton is produced along with a photon leading to a signature that consists of a single high Pt photon in the central eta region and high missing et back-to-back with the photon.
The primary background comes from gamma + Z -> two neutrinos and a photon.
The normalization of the single photon background can be obtained by measuring the rates of Z to two muons or Z to two electrons and a photon and using the known Standard Model branching ratios.
CMS has fairly good discovery potential in this channel, with 1fb-1, we expect to be able to make a discovery for MD, the new scale of gravity, up to 1.5 TeV.
Andy Yen (Caltech/CMS)

10. Andy Yen (Caltech/CMS)
ECAL Calibration
For early CMS runs, using π⁰ mass peaks is optimal.
L=2x1033 gives average π0γγ rate of 1.5 kHz or 2,100 π0/crystal/day
As I have highlighted several times in this talk, the CMS ECAL calibration will be crucial to the success of the CMS experiment.
Fortunately for CMS, several calibration techniques have been developed that will allow us to reach design precision.
One of these strategies makes use of pi0 decays. The advantage of this strategy is that it is both precise and fast.
At the LHC luminosity, we expect to collect pi0->gamma gamma decays at the rate of 1.5 KHz which translates to 2100 Pi0’s per crystal per day.
The Z->ee width is used to monitor improvements and the position of the Z mass peak can be used to set the absolute energy scale.
Andy Yen (Caltech/CMS)

11. Barrel Pi0 Callibration (Work in Progress)
Data after L1 Trigger
Online Farm
p0 Calibration
~1 kHz
>10 kHz
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.
Here we can see the effectiveness of the Pi0 calibration in the ECAL barrel.
The pi0 calibration algorithm can give a precision of 0.5% after corrections are made for cracks in the ECAL and pileup events.
The calibration algorithm converges rapidly which means this calibration can be done in real time as data is being collected.
Thus, within 100 hours at full luminosity, the entire barrel can be calibrated to 0.5% precision.
In practice, the startup luminosity will be lower so it will take a couple weeks.
Pi0 calibration will also work in the endcaps, but it will take more time.
Andy Yen (Caltech/CMS)

12. Effect of ECAL Calibration
Crystals Pulse Amplitudes
in a clustering algorithm
Particle Energy
Achieving a precise in situ crystal-by-crystal calibration of the CMS ECAL will be crucial for the Hγγ search (CMS NOTE-2006/021)
Calibration Target
On this slide, we can see the large impact calibration will have on searches in the H->gamma gamma channel.
We can see that the effect of calibration is dramatic on the Higgs mass peak and that calibration greatly improves the Higgs mass resolution leading to much improved discovery potential.
January 09, 2008
Andy Yen (Caltech/CMS)

13. Conclusion The most exciting times are just beginning!
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.
Photonic events also serve as a useful test of SM QCD predictions.
As we can see, there are numerous early discovery opportunities available.
However, CMS is well prepared and ready to meet this challenge.
and much work is currently being done on that area.
In addition to searching for new physics, photonic events…
…which is an interesting subject in itself.
CMS will restart this September so the most exciting…
I hope many of you will follow along as we enter what will be a new era for particle physics.
The most exciting times are just beginning!

14. Extra Slides Follow

15. Ggg Discovery Potential
CMS NOTE-2006/051
This slide is optional, depends on time.
Andy Yen (Caltech/CMS)

16. 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
qq  ee*  eeg
qq  μμ*  μμg
Especially for the case of excited electrons where the final state consists of 3 electromagnetic objects.
Excited leptop decaying directly into two regular leptons is the optimal decay channels for both muons and electrons because it allows for direct reconstruction. For e*, the high Pt of eeg gives a very distinctive event topology.
For high excited lepton masses, the SM background becomes negligible against the potential signal.
The end result is that discovery is possible during early LHC running. (read e* discov pot)
Both both e* and u*, CMS can cover parameter space far beyond Tevatron.
Discovery Potential (1 fb-1): Me* to ~2 TeV, Λ up to ~10 TeV
Andy Yen (Caltech/CMS)