Experimental search for the Higgs Boson
Kajari Mazumdar
Department of High Energy Physics
Tata Institute of Fundamental Research
Mumbai.
B.P.Baria Science Institute, Navsari August 24, 2012
Visit http://cms.web.cern.ch/content/cms-education
http://www.tifr.res.in/~mazumdar
For further questions, mail to [email protected]
Our Universe
Today 13.7 Billion Years
1028 cm
1. Quantum gravity era: t≈ 10-43 s 1032 K (1019 GeV, 10-34m)
3. Protons and neutrons formed: t ≈ 10-4 s, 1013 K (1 GeV, 10-16 m)
•4. Nuclei are formed t = 3 minutes, • 109 K (0.1 MeV, 10-12 m)
Energy =kT
Length scale = hc/E
Big Bang
(30 K)
2. LHC 10-12 s, 1017 K
(104 GeV, 10-20 m)
Atom Proton
Big Bang
Radius of Earth
Radius of Galaxies
Earth to Sun
Universe
Super-Microscope
LHC
Hubble ALMA
VLT AMS
Dimensions in Physics
The probe wavelength should be smaller than the distance scale to be probed:
xE
LHC takes us
backward in time,
almost near the
beginning.
Is there more inside?
Electron-Volt=energy gained be an electron in a potential difference of 1 Volt. 1 GeV ~ mass of a proton = 109 electronVolt = Thousand Million eV = 1.6 * 10 -10 Joule
1 TeV = 1012 eV =1000 GeV
Probing shorter than ever length scales
• To probe structure of an atom (10 -10 m) need energy probe, E = 10 keV
• For nucleon structure (10 -15 m) : E = 100 MeV
• For probing new territory of 10 -20 m we need energy ~10+13 eV = 10 TeV!
Motivation for LHC • Search for the Higgs boson • Explore physics at TeV energy scale many theoretical ideas • be ready for the unexpected as well
20 years to plan, build the LHC machine and the experiments. 20 more to work with opportunity for you!
• Presently, LHC provides energy upto 8 TeV, equivalent to ~ 1.2 micro Joule.
•A 100 watt light bulb, for an hour consumes 0.36 Mega Joule.
LARGE HADRON COLLIDER (LHC)
The earliest accelerator
made by humans
Accelerate charged particles using electric field
Bend them in circular arc using magnetic field
LHC accelerator complex
From Aerial view to 100 m underground
LHC: The Giant Marvel of Technology
• 100-150 m under the surface
• 27 km at 1.9 K (super-fluid Helium)
• Vacuum ~ 10-13 Atm.
• SuperConducting coils for very high magnetic field : 12000 tonnes/7600 km
• thousands of magnet components aligned with precision of 0.1 mm
• Temperature generated at LHC due to proton-proton collision ~1016 0c, compare with sun: 5506 0c, a matchstick: 250 0c
LHC machine to be maintained at -271 0c vs. Home freezer: -8 0c Boomerang nebula: -272 0c, antarctica: -89.2 0c,
Largest ever human endeavour, required huge resources to be put in. To be passed on to younger generations of today and tomorrow: YOU!
Airial view CMS experimental site in 1988
One of the 8 service points for LEP accelerator in the same tunnel (1989-2001)
Gallo-Roman villa of 4h century AD discovered While preparing the site for CMS experiments. Roman coins during archeological excavations
LHC motivations: explore, search, measure
Each constituent of proton carries only a fraction of the proton’s energy Effective energy in a violent collision varies possibility of producing various new particles of different masses at LHC Higgs of any mass within allowed range could be produced at LHC
•One of the fastest race tracks: protons zipping past with 99.999999% of velocity of light around the LHC ring 11000 times/sec. • Collides these protons to accumulate sufficient energy so that heavy particles could be produced in the lab ( E = mc2 )
What happens in LHC experiment
Proton-Proton 1600 bunch/beam Protons/bunch 2. 1011
Beam energy 4 TeV Luminosity 7.1033 /cm 2/s Crossing rate 20 MHz Collisions 108 Hz
Summer, 2012
ATLAS detector
Length: 50m, Diameter: 30m, weight: , 10Million electronics channels
CMS Collaboration: 1740 Ph.D.s + 1535 students (845 for Ph.D.) + 790 engineers from 179 institutes in 41 countries. ATLAS collaboration: 3000 signing authors (including 1000 students) from 174 institutes in 38 countries
Only a small fraction of 4300 people who made CMS possible
CMS Detector
CMS detector
Electronic Eye
100 thousand times stronger than earth’s field
The missing piece
we have been after
Slice of CMS detector
The detector can only “see” g, e±, m ±, p±, n, p, K !
Measure the position and momentum of g, e±, m ±, with high resolution.
10 million electronic channels record data every 50 ns (rate 20 MHz)
• The largest silicon based detector
• Total area ~ 205 sq.m
• 76 Million electronic channels
• To be operated at -200 C
• Innermost layer: 100 X 150 mm pixels
• 74,000 crystals: 24 X 2 X 2 cm3
• Compact inorganic, scintillators
transparent but 96% metal by mass,
supported by 0.4 mm thick glass/carbon
fibre structure.
Some of the subsystems of CMS detector
1. At the core is a device called the inner tracker detects and analyzes
the momentum of charged particles passing through the detector, eg., e-, e+.
2. Surrounding the inner tracker is a calorimeter measure the energy of
particles by absorbing them, eg., e, p.
3. The outermost subdetector is muon spectrometer measures muon
position and momentum.
Scientists look at the path the particles took
and extrapolate information about them.
Reconstruct 20K charged tracks in a
single event (lead-lead collisions at LHC)
Essential components of a detector
Presently event size ~ 1MB, event rate: 20 MHz data collection rate ~ 400 Hz
Data production at LHC : several Petabytes/year Novel computing technology evolved naturally from internet. • distributed computing and data storage and management infrastructure. • tens of thousands of standard PCs collaborate worldwide. • Scalable hardware, open-source software.
• much more capacity than a single supercomputer.
2 GRID computing centres in India: Mumbai, Kolkata
LHC computing GRID
Key is the high speed connectivity
Data accessible to anybody, anytime
For MH= 125 GeV, G H= 4.2 MeV Branching ratios (%)
H WW* : 23
H ZZ* :2.9
H bb : 56 H cc: 2.8
H tt : 6.2
H mm: 0.021
H gg : 8.5 H gg : 0.23
H g Z : 0.16
Detection of the Higgs boson
• Higgs boson decays within ~ 10 -24 s
• Decay modes of a heavy particle X which is
unstable and heavier than A,B,C
X A : a% of total decay events
X B : b% of total decay events
…
Branching ratio for X A is a%
Since we do not know the mass of the Higgs,
we have to take into account all possible decay
modes at different values of the Higgs mass.
• signal is simple and easy to identify: final state with 2 energetic photons. •Narrow peak to be identified on top of huge continuous background in the invariant mass distribution of the photons.
Crucial for mass resolution: • individual energy measurement •angle between 2 g s.
The calorimeter material has to be of high Z Photons , electrons, positrons loose energy completely by electromagnetic interaction with the detector material. Excellent mass resolution ~1%
m2γγ= 2 E1 E2 (1-cosα)
Higgs decaying to a pair of photons
Event rate
To have a good chance of producing rare particles (small s), like the Higgs
boson, a large number of collisions are required.
• Cross-section X Branching ratio (to 2photon final state) ~ 50 fb = 50 X 10 -39 cm2, for m H = 125 GeV. • The data used corresponds to production of about 500 Higgs events of this mass (But the detector is not fully perfect, can only measure a fraction of these events)
• Total number of 2-photon events with similar properties including background (processes which give similar signature in the experiment) more than 60,000.
• First collision of protons at high energy: 30 April 2010.
• Experiments have been busy to establish their authenticity by measuring the
standard processes first
• The benchmark measurements are precursor for claiming a discovery.
Foundation for the discovery
CMS result for 2 photon final state
Observed significance 3.7 s
Area under m ± 1 s = 68.269%
Area under m ± 2 s = 95.450%
Area under m ± 3 s = 99.730%
Area under m ± 4 s = 99.994%
Area under m ± 5 s = 99.999%
Significance of a result
Probability for the background to fluctuate upwards to create the
observed excess is 0.2 in a million, corresponding to better
than 5 sigma effect.
Observed significance 3.2 s
CMS result from 4lepton final state
Characterization of excess near 125 GeV
CMS experiment has observed
clear evidence of a resonance with
Combined significance: 5 s
p-value: 2 X 10-7
Fitted mass: 125.3 ±0.4 (stat) ±0.5(sys.) GeV
ATLAS experiment:
Combined significance: 5.9 s
p-value: 1.9 X 10-9
Fitted mass: 126 ±0.4 (stat) ±0.4(sys.) GeV
on the web: hep-ex arXiv: 1207.7235
on the web: hep-ex arXiv: 1207.7214
About 3000 authors in each paper.
India and the LHC: partners in the discovery
India signed agreement with CERN to participate in the LHC in 1991. • One of the first countries to offer partial financial support became observer state in CERN Council in recognition.
• Contribution in LHC project in all aspects: accelerator, experiments (CMS & ALICE), computing. • Supplied magnet related components for LHC machine
• Participation in CMS and ALICE experiments 30 Ph.Ds, 12 engineers & 35 Ph.D students contribution in detector fabrication, maintenance, monitor , calibration physics studies Software development • Grid computing has been the backbone of quick turnover of physics at the LHC. • LHC Tier2 Grid computing centre for CMS and ALICE
Conclusion
• The LHC project has been planned and prepared meticulously over long time the harvest is excellent and is very fast! • The Higgs boson particle has been discovered by the experiments at LHC fulfilling its raison de etre. • This is considered as the proof of the idea proposed almost 50 years ago why weak interaction is short-ranged and electromagnetic interaction is long-ranged.
• This development is very significant for the whole human kind since it gives us the confidence about our way of understanding the evolution of the universe at the first moments.
• However, we have miles to go before we sleep! We learnt Higgs boson exists We need to know its properties
• LHC will also explore many more interesting physics in coming decades Stay tuned!
Thank you for giving me an opportunity to share the excitement.
Higgs production at LHC
gluon-gluon fusion Vector boson fusion Associated productions with W, Z, top
• Nature of dark matter: we know only 4% of the constituent of the
universe
• A good 25% of the rest is
massive enough to dictate the motion of galaxies
non-luminous, and hence “dark”
LHC can tell us the nature of this dark matter!
LHC will also shed light on:
• why there is only matter and no antimatter
• properties of the 4th state of matter: Quark-Gluon-Plasma which
existed 1 pico sec. after the big bang.
….
Grand menu from LHC
All this is possible because LHC is essentially a microscope
AND a telescope as well!
observed
Expected from visible
Distribution of matter
