Transcript Physics with CMS Paolo Meridiani (INFN Roma1) Lectures at the IX International School

Physics with CMS
Paolo Meridiani (INFN Roma1)
Lectures at the IXth International School
“The actual problems of Microworld Physics”
Gomel - Belarus
Paolo Meridiani - INFN Roma1
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Outline
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Lecture 1
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Lecture 2
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Is SM satisfactory? Open questions in the SM?
LHC: the answer to unanswered questions?
CMS Detector: a challenging detector for a challenging machine
CMS Commissioning: how much time is required to make it work?
CMS early physics: what should be done at the beginning?
SM physics with CMS: known SM physics can be done better in CMS?
Higgs Physics with CMS: if it’s there we will catch it!
Lecture 3
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Beyond the SM physics at CMS: hunting new theories
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First questions
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Is Standard Model satisfactory?
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LEPEWWG
SM is consistent with all experimental data for
E<100 GeV
But: theorists say it’s “theoretically
unsatisfactory”
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It does not explain its bizarre spectrum in
quantum numbers, why 3 generation
It does not include quantum gravity
And most of all..
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Mass in Standard model
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Nature of mass in SM: Spontaneous Simmetry Breaking
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Mass can be accounted for in SM
only with Spontaneous Simmetry Breaking
(mass terms violate gauge invariance)
But spontaneous simmetry breaking
does not predict masses!
SM EW sector is tested at extreme precision,
but Higgs is not yet observed
P.W. Higgs, Phys. Lett. 12 (1964) 132
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Quadratic divergence in Higgs mass requires fine tuning
Higgs mass sensitive to physics >> EW scale (hierarchy problem). There is
the possibility of having  close to EW scale
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And still...
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Fermion masses are just parameters (Yukawa couplings)
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Analogy: hydrogen spectrum lines before Bohr...
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And if this is not sufficient...
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Appeal for Grand Unified Theories. In SM gauge coupling strengths
does not unify at any scale.
aEM  a1  1/128  0.008
aWEAK  a2  0.03
aS  a3  0.12
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at s = 100 GeV
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Evidence for dark matter
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No DM candidate is present in the SM. Evidence for non-relativistics
cold matter
Cosmic Microwave Background: Observations from WMAP:
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A new machine: LHC
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Hints that new physics could be present between the EW
and TeV scale
So let’s build a new machine able to explore this mass
range
Which type of collider? LEP e+e- s 200 GeV. Better pp. Why
pp?
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Other requirements?
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pp interaction more complex, but...
Syncrotron radiation (mp/me)4  1013
Other possibility is e+e- linear collider, but which s?
High luminosity, need to search for rare processes
(N = L )
Answer is: LHC (Large Hadron Collider) @ CERN
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LHC @ CERN
We have a 27km tunnel already used for LEP, TeV energies can be
reached with superconducting magnets
Operation temperature 1.9 K: LHC will be the largest
cryogenic system in the world
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LHC machine parameters
Nominal luminosity: 1034 cm-2s-1
40 Mhz is the frequency: bunches are
distant less than 25 ns
The energy of the
proton beam in LHC is
equivalent to the
kinetic energy of 100
trucks of 10 Tons at
100 Km/h
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2008 LHC schedule
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LHC commissioning
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Beam commissioning
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Phase A
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Should start May 2008
2 months to get first
collisions
First collisions - low
intensity, un-squeezed.
No crossing angle
Gradual increase in current up to 156 bunches/beam
Pilot physics: un-squeezed to
partial squeeze
≤ 1032 cm-2s-1
Bunches
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Ib
Luminosity
Event rate
1x1
18
1010
1027
Low
43 x 43
18
3 x 1010
3.8 x 1029
0.05
43 x 43
4
3 x 1010
1.7 x 1030
0.21
43 x 43
2
4 x 1010
6.1 x 1030
0.76
156 x 156
4
4 x 1010
1.1 x 1031
0.38
156 x 156
4
9 x 1010
5.6 x1031
1.9
156 x 156
2
9 x 1010
1.1 x1032
3.9
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LHC: what happens when proton collides?
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Protons are composite objects made of
quark and gluons
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Large momentum interaction happens
between quark-antiquark, quark-gluon,
gluon-gluon which retains a fraction of the
proton momentum (described by parton
distribution function PDF)
   dxa dxb f a ( xa , Q 2 ) f b ( xb , Q 2 )ˆ ab ( xa , xb )
ˆ ab
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f i (x,Q2 )
a ,b
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s different from center-of-mass energy of
the scattering process (sxaxb)
 hard scattering cross section But most of them are interactions with low
 parton distribution function
transferred momentum
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Total inelastic xsec at 14 TeV  70 mb
Small scattering angle (minbias): <pT>  700
MeV
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What do we know about PDF?
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How PDF look like for a proton?
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Highest energy machine for such purposes is HERA an ep collider
• Electrons of 30 GeV on 900 GeV protons
u and d quarks dominate at large x
values
Gluons dominate at small x (bigger
uncertainty at small x)
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PileUp
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At each bunch crossing on average 20 minbias events overlap with the
much less probable interesting events...
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This is the so called pile-up (important especially at nominal high
luminosity)
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Rates of various processes at LHC
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How a typical LHC event look like?
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In a typical low momentum
(minimum bias) interaction
dN
7
d
charged particles
uniformly distributed in 
On average <1400>
particles with <pT> 700 MeV
Good old LEP
Z →+-
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…
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How to get rid of pileup?
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How to select the interesting part of the event?
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Fundamental tool is to cut on the high transverse momentum
particles to search for the results of a large momentum transfer
interaction
This is the main principle that is applied also in the triggering step
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PT > 25 GeV
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Typical signatures at LHC
No hope to observe the fully-hadronic final
states  rely on , 
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Fully-hadronic final states only with hard
O(100 GeV) pT cuts
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Lepton with high PT
Signature: Lepton & photons
Missing energy
•Mass resolutions of ~ 1% (10%)
needed for ,  (jets)
• Excellent particle identification:
e.g. e/jet ratio pT > 20 GeV is 10-5
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LHC: requirements for good detectors
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Maximum possible coverage/hermeticity
Detectors require fast response, otherwise integrate signal over »1 bunch
crossings
• Typical response time: 20-50 ns
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LHC detectors require high granularity to minimise the probability of pile-up
signals in the same detector channel
 high cost
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LHC detectors must be radiation resistant:
• high flux of particles from pp collisions  high radiation environment,
particularly in forward detectors
up to 1017 neutrons/cm2
up to 107 Gy
• Requires also radiation-hard electronics (military-type technology)
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Detector & electronics must survive > 10 years of operation!
For Physics:
• Good measurement of leptons and photons with high pT
• Good measurement of transverse missing energy
• Capability of identify b quarks and  leptons
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CMS
btw (CMS aka Compact Muon Solenoid) 
CALORIMETERS
SUPERCONDUCTING
COIL
ECAL
Scintillating
PbWO4 crystals
HCAL
Plastic scintillator/brass
sandwich
IRON YOKE
TRACKER
Silicon Microstrips
Pixels
Total weight : 12,500 t
Overall diameter : 15 m
Overall length : 21.6 m
Magnetic field : 4 Tesla
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MUON BARREL
Drift Tube
Chambers
Resistive Plate
Chambers
MUON
ENDCAPS
Cathode Strip Chambers
Resistive Plate Chambers
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The CMS collaboration
Austria
Institutions
Member States
Non-Mem. States
61
64
USA
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Total
174
USA
Member States
Non-Mem. States
USA
Total
1055
428
547
2030
Associated Institutes
Number of Scientists
Number of Laboratories
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CERN
Bulgaria
Finland
France
Russia
Scientists
Belgium
Germany
Greece
Hungary
Italy
Uzbekistan
Ukraine
Slovak Republic
Georgia
Belarus
Poland
UK
Armenia
Portugal
Turkey
Brazil
Serbia
China, PR
Spain
Korea
PakistanMexico Iran
China (Taiwan)
Switzerland
Colombia
New-Zealand Ireland
Croatia
India Cyprus
Estonia
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Tracker
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Muon system
250 Chambers
200K Channels TDC
200μm Resolution
468 Chambers
240K strips
150μm Resolution
Course position, fine
timing
Barrel 80K channels
Endcap 92K channels
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Performance of Tracking + Muon system
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ECAL
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Maximum resolution:
homogeous crystal calorimeter
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75000 crystals
Inside the solenoid
PbW04:
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ECAL
25 X0 in 22 cm
High granularity:
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fast
Radiation resistant
Compact detector:
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lead tungstate: PbW04
Crystal front face: 22 x 22 mm2
Lateral containment: RMolière= 22 mm
preshower :
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endcap: 1.653<||<2.6
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HCAL
Had
Had
Had
Had
Barrel: HB
Endcaps: HE
Forward: HF
Outer: HO
HB & HF: Brass
Absorber and
Scintillating tiles.
HO: Scintillator
“catcher”.
HF: Iron and Quartz
fibers
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HCAL performance
Jet ET resolution
Mjj resolution at 120 GeV
Mjj resolution ≤ 15%
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CMS Status
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CMS has being built all on surface and pieces are then lowered into the
cavern
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BIG and HEAVY objects
Tracker has been fully integrated in surface and is recording cosmic
tracks since Mar07
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Big engineering achievements lowering very
Dead or noisy channels < 0.3%
Half of Ecal Barrel is now installed inside HCAL. The second half is
being installed right now. Fully commissioned on surface with cosmics
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Also here excellent quality > 99.9%
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HCAL is fully installed and being commissioned
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Muon system (DT+RPC+CSC) fully installed and being commissioned
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Some pictures from CMS
installation...
Ecal Barrel installation
Ecal inside Hcal
End of first lowering
phase (half CMS +
central wheel)
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Heavy lowering...
Lowering of the 2nd endcap disk
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Media are interested too...
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Next big step: CMS Commissioning
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Commissioning with physics data proceeds in four phases:
 Phase 1 : Cosmics running before LHC (before May 2008)
 initial physics alignment / calibration of the detector
 debugging electronics, DAQ, trigger systems, magnet
 Phase 2 : One beam in the machine
 beam-halo muons and beam-gas events
 more detailed alignment (especially for internal detectors)
 Phase 3 : First pp collisions : prepare the trigger and the detector
tune trigger menus/measure efficiencies
begin to measure reconstruction efficiencies, fake rates, energy scales,
resolutions etc.
Improve calibrations and alignments
 Phase 4 : Commissioning of physics channels
begin to understand backgrounds to discovery channels …
Start first measurement of known processes
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CMS commissioning schedule before LHC
starts
GOAL
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Tracker and Muon alignment
Alignment strategy
before collisions: construction+optical alignment, cosmics and
beam halo muons
after collisions: muons from Z,W to achieve alignment goal
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Tracker and Muon alignment
Impact on pt=100 GeV/c muons of misalignment
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ECAL calibration
Before data taking:
Pre-calibration using test beam, light yield
meas., cosmics: ~1.5%
Early collisions:
Few hours of min. bias events (1kHz calib.
Stream): 1..2%
Phi symmetry, pi0
Nominal luminosity:
Isolated electrons from W,Z using E/p ~ 0.5%
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HCAL calibration
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At the startup
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ADC/GeV conversion evaluated with radiocative sources (all
channels) and test beams. Precision 4%
Timing measured within 1nsec using laser and LED pulsers
After startup: MinBias + Di-Jet balance to reach 2%
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CMS: from RawData to Physics Results
ON-line
OFF-line
LEVEL-1 Trigger
Hardwired processors (ASIC, FPGA)
Pipelined massive parallel
HIGH LEVEL Triggers
Farms of
processors
1s of LHC running 
•1 day by the present CERN
network system
•the amount of information
exchanged by WORLD
TELECOM (100 000 000
phone calls)
•the data exchanged by the
WWW in the whole of
January 2000
Reconstruction&ANALYSIS
TIER0/1/2
Centers
25ns
10-9
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3µs
10-6
ms
10-3
sec
10-0
Giga
hour
103
Tera
year
106 sec
Petabit
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End of Lecture 1
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