Status of LHC
Kajari Mazumdar
Department of Experimental High Energy Physics
Tata Institute of Fundamental Research
Mumbai
Proton-on-proton collision at LHC
(sx1x2) = (sx)
proton
x1p
x2p
[ “Hard scattering partons” ]
proton
proton beams
IACS Kolkata,
September 27, 2010
Plan
• Introduction to LHC, motivation
• Experiments at LHC
• Over all detector performance
• Initial Physics plots
• Expected Physics at 7 TeV with maximum
luminosity.
Today’s, emphasis is on analyses done already with real data.
Results shown are mostly prepared for ICHEP Conference, July, 2010, with data
collected till almost mid-July.
Eternal Questions
What principles govern energy, matter, space
and time at the most elementary level?
• What is the world made up of?
• How does it work?
High Energy Physics tries to answer them all!
What lies within…?
The probe wavelength should
be smaller than the distance
scale to be probed:
x 
E
1020 m  1013 eV
 10 TeV
(1 TeV = 1012 electronVolt
= 1.6 * 10 -7 Joule)
Tool at hand: Large Hadron Collider (LHC @ CERN)
LHC is the biggest and the most
expensive scientific endeavour
Price tag ~ USD 9.1 billion
No. of scientists involved ~ 10K
1232 dipole magnets
+ 400 quadrupole magnets
+ Various other types of magnets
SC coils: 12000 tonnes/7600 km
100 – 150 m under the surface
27 km at 1.9 K (superfluid He)
Vaccuum ~ 10-13 Atm.
Technological progress pushes frontiers of basic science research and there are
important spin offs.
Eg. World Wide Web was born to meet the needs of avaiilability of scientific info
for HEP experiments of 1990s.  today it is a household item which had changed
our lifestyle.
Cosmic Recipe: How we understand the universe Energy =kT
Length scale = hc/E
• LHC takes us back in time towards the beginning of the universe!
at an epoch of about 10-12 sec. after the big bang.
•Earlier experiments have probed the prevailing situation upto a time
t ≈ 10-10 s, when the world was as hot as T=1015 K
•Protons and neutrons formed around: t ≈ 10-4 s, T=1013 K
•Nuclei are formed after t = 3 minutes, T=109 K
(equivalent energy density ~0.1 MeV, distance scale ~10-12 m)
•Today: t = 13.7 Billion years since the beginning, T=3 K
Present wisdom: Behaviour of matter particles can be explained in
terms of very few fundamental interactions, which might have
evolved over time as the universe cooled down from a single unified
one.
GRAND UNIFIED THEORY!
Standard Model (SM) of Particle Physics as of today
• 4 types of basic forces :
Relative strengths:
10 -40: 10 -5: 10-2: 1
Gravitational, Weak, Electromagnetic, Strong.
• 2 types of fundamental particles : fermions & bosons.
• Almost all the predictions of SM match very well, till date, with
experimental observations.
SM is still not a satisfactory Theory!
• One of the most disturbing feature: can’t explain the origin and the
mass patterns of elementary particles, both fermions and bosons.
• Theoretically, weak interaction involving massless carrier particles
can be described very well, similar to electromagnetism.
• But, in nature, we do encounter short range of weak interaction 
carriers of EW (as opposed to photon)!
Introduction of mass in the theory causes complications!
We got to unravel the mystery of mass!
Most plausible: all fundamental particles acquire mass by interacting with an all
pervading field, as a consequence, this idea also evokes another fundamental
particle, the Higgs boson!
Higgs particle not yet seen most uncomfortable situation. E
==> may be it is too heavy to be produced in the experiments?
Strategy: Heavy particles (by nature unstable) of interesting properties should
show up if enough energy is gathered to produce them in the experiment,
provided they existed when the universe was hotter.
Need accelerators!
LHC is an exploratory, high energy, high intensity machine which can produce heavy
particles of mass upto few TeV.
The primary goal of the LHC is to find the Higgs boson…
… if it isn’t found, to find out why it isn’t there!
Mass spectrum in Standard Model
•All mass values are obtained
from experiments.
• Mass of the Higgs boson
not known theoretically.
can be anything between
114 to 750 GeV as we
understand today.
Higgs particle has to be
hunted out!
 experimentalists’ job.
The high energy machine
should have the capability to
produce the Higgs over whole
range  input to the
requirement of LHC energy
Quantum corrections to Higgs boson mass require some
New Physics at TeV energy scale.
Further problem with SM
even with a single Higgs The nature of this Beyond Standard Model physics is not
boson!
known, though several contenders are more favourable,
eg., SuperSymmetry.
New Physics also invokes new set of additional particles!
Symmetry Breaking
Nature has various symmetries (translational, rotational, ..) and related
conservation laws: guiding principles in theoretical formulations.
Some of the symmetries are also broken, sometimes spontaneously.
Eg. behaviour of ferromagnet wrt temperature: above Curie point, the spin
alignments are all random. Below critical temperature, the alignment
direction is degenerate.
Below C point
Above C point
In particle physics, symmetry considerations required, carriers of weak
interaction (W, Z bosons) also to be massless as photon, carrier of EM interaction.
Spontaneously broken electroweak symmetry endows masses to W,Z bosons and
various fundamental particles, via Higgs mechanism  this is still a postulate.
LHC Timeline
•
First studies in 1982, project approved in 1994 ; final decision in 1996.
•
Construction started in 2002
•
First beams sent around the storage ring: September 10, 2008.
•
First collisions in November, December 2009 at energy 900 GeV, 2,360 GeV
•
March 30, 2010, saw the first collisions at 7 TeV.
•
The design bunch intensity of 8e11 protons/bunch is already achieved.
Current instantanous luminosity ~ 1030 cm-2 s-1 .
•
Data size increasing steadily. Integrated lumi @ 7 TeV expected: 1 fb-1
•
At present there are 48 bunches/beam (target : 2808 bunches/beam)
•
LHC will contnue to operate till end of 2011 with proton on proton collions
and 2 short runs of Heavy Ion collisions.
•
Energy upgrade to 14 TeV expected by 2014.
Several Indian groups working for LHC accelerator and experiments for last 15
to almost 20 years  we are lucky to be part of LHC family!
Experiments at
LHC
• ATLAS (46m X 25m X 25 m)
• CMS (21m X 15 m X 15 m)
ATLAS, CMS: general
purpose p-p experiment.
ALICE: study of quarkgluon plasma in heavy
ion collisions.
• ALICE (26m X 16 m X 16 m)
• LHCb: (21m X 10m X 13m)
• LHCf: 2x( 0.3m X 0.8 mX0.1 m)
• Totem
• LHCb: dedicated experiment for studying B-physics and CP violation
• The LHCf experiment uses forward particles created inside the LHC p-p
collision as a source to calibrate cosmic ray
• Totem is meant for studying diffractive events, measure luminosity delivered
in CMS.
What happens in LHC experiment
Mammoth detectors register signals for
Energetic, mostly (hard) inelastic collisions
involving large momentum transfer.
Basic features in hadron collisions
Hadron machine  large rates for QCD processes!
Most are reasonably soft interactions.
We are interested in hard, collisions.
Interesting processes are very rare, by nature.
accumulate enough luminosity over time.
 Background has to be discriminated from
Signal  Monte Carlo studies essential as
preparation.
Most sophisticated and complex detectors for general-purpose
physics, planned, constructed by thousands since last 20/15 years
107 electronic readout channels, to be ready every 25 ns
Higgs event in CMS
Operating conditions:
one “good” event (e.g Higgs in 4 muons )
+ ~20 minimum bias events)
All charged tracks with pt > 2 GeV
Reconstructed tracks with pt > 25 GeV
Event size:
Processing Power:
~1 MByte
~X TFlop
The GRID: the new information Super highway.
LHC employs a novel computing technology, a distributed computing and
data storage infrastructure: to meet the unprecedent challenge of data
processing.
• tens of thousands of standard PCs collaborate worldwide
• much more processing capacity than a single supercomputer
• access to data to thousands of scientists all over the world.
250 Million events simulated
at TIFR during April ‘09 to
March.’10. ~ 300 TB storage
GRID computing centres for
regional scientists at : TIFR for
CMS experiment and VECC for
ALICE experiment.
First collisions at 7 TeV and the euphoria: 30/3/2010
before
First 5 minutes!
Prologue for physics from collision
• Years of test beam activities,
• increasingly realistic simulations,
•
commissioning with cosmics
to understand and optimize the
detector performance : 10 billion
events analysed.
• validation of the software tools
were fundamental to achieve
results within few hours of data
taking.
Luminosity
need high instantaneous luminosity
to have enough number of even rarer
events produced within a relatively
short time period  higher the
integrated luminosity, quicker we can
probe with greater significance.
Even at low luminosity, collision
data has been extremely important
for studying various features of
hadron interactions  led to paper
publications by various
collaborations, sometimes within
few days of data collection!
Strategy of LHC experiements
• Experiments mostly preparing for high-pT physics, needed for discoveries.
• However experiments first need to establish that analyses are done correctly by
confirming predictions of Standard Model as we know already.
• Start by studying most abundant processes, given lower energy available.
• Study soft interactions at LHC in detail need tracking capabilities of very soft
charged particles. Special effort gone in to tap particles of transverse momentum as low
as 30 MeV.
•Soft physics needs improvement in phenomenological description.
• With high luminosity several soft interactions gets piled up along with the hard
interaction during the same bunch crosssing  experiments must identify and know the
nature of event pile up in detail.
• Check the basic event reconstruction: muon, electron, photon, jet, missing transverse
energy: are they behaving the same way in real data as we assumed in our monte carlo
simulation?
• experiments must be ready to perform b-physics, jet physics as early as possible.
• Study W, Z production rate at high energy : standard candles to assess detector.
Quality of performance on the way to tap top, Higgs, New Physics.
The experiments for pp collision also play a role in the study of heavy ions.
Charged Hadron Multiplicity
Important to study particle production model,
mostly non-perturbative regime
Steep rise in average number
with energy
Charged particle multiplicity grows faster
than predicted in most of the models.
Rapidity distribution, expected to be flat
at h = 0.Rapidity and pseudo-rapidity
values are numerically different for pions,
kaons with momentum of few hundred MeV
Charged Hadron Momentum distribution
• Constrain generator tune, better
understanding of pQCD+PDF etc.
Reference spectrum for observation
Of dense QCD medium effects in
Heavy Ion collisions.
2-particle correlations
Different pieces in the cartoon are
indeed connected.
2-particle correlations can be studied in
terms of independent clusters, whose
size and density have energy
dependence.
Correlations between 2 particles are
stronger than described in Monte
Carlo.
Bose-Einstein Correlations
Correlations between identical bosons (pions) due to constructive interference of
multi-particle wave function.
Effect observable in regions of phase space populated by bosons of similar momenta.
Construct a double
Ratio: MC/data
Monte carlo events do not have
correlations
Inclusive diffraction in minimum bias events at LHC
Diffractive events correspond to large fraction of
minimum-bias dataset.
Modelling of soft diffraction is often arbitrary!
Definining and constraining, as well as energy
dependence of diffractive events: important ingradient
For minimum bias events at LHC
Diffractive events observed by looking at the
absence of forward hadronic activity due to
presence of large rapidity gap look at
opposite directions, require low activity on one side.
 Important tuning needed for generators.
Low mass resonances
• Tracks displaced from primary
vertex (d3D > 3σ)
• Common displaced vertex
(L3D > 10σ)
PDG Mass:
1672.43 ± 0.29
Ω-  ΛK-
Invariant mass distribution
for different combinations
(Ω±  ΛK± or ±  Λ± ) fit to
a common vertex.
PDG Mass:
1321.71 ± 0.07
-  Λ-
Jets and Missing ET
The highest mass dijet event in the first 120nb-1 of data
Dijet mass: 2.130 TeV
Highest ever produced in
any hadron collision
Missing energy measurement in various methods is highly reliable missing
Energy characteristics can be utilised to search for interesting events, eg. SUSY
Inclusive jet cross section
All results are in good agreement with NLO theory: success of QCD!
Various jet reconstruction algorithms produce matching results.
Particle Flow approach the distributions can be extended to a low pT value of 18 GeV.
Inclusive b-jet cross section
Important test of our capability to master the b-tagging tools (eg.High Purity version of
the Secondary Vertex Tagger).
Reasonable agreement with NLO but discrepancies in h and pT shapes.
Physics with Dijet final state
At LHC, jet production rate offers enough statistics to probe new energy regimes
fast!  sensitive to a variety of SM extensions:
• Compositeness at high energy scale (L) : do quarks have substructures?
• TeV Scale gravity: large extra dimensions (eg. ADD)
warped extra dimensions (eg. RS)
•New Strong dynamics: Technicolour, Chiral colour/axi-gluons
•Contact interactions: dijet angular distributions search for non-resonant production of
new physics at high mass
At 95% CL, non-resonant New Physics excluded : L < 930 GeV
Resonant New Physics (excited quark) excluded: 400<m q⃰ <1290 GeV
Search for narrow resonances in di-jet final states
Dijet mass differential cross section distribution is sensitive to the coupling of any new
massive object from New Physics to quarks and gluons.
If no bump in mjj, set limit on excited quark production
Latest published limit:
CDF: 260 < M (q*) < 870 GeV
LHC latest
0.4 < M (q*) < 1.29 TeV
excluded at 95% C.L.
95% CL exclusion limits for
• String resonances with mass <1.67TeV;
. excited quarks of mass < 0.59TeV;
• axi-gluons of mass <0.52TeV
Stopped gluinos and Heavy, Stable Charged Particles
Search for long living particles decaying in the detector after the end of
each LHC fills (special trigger to record important release of energy in “no
beam condition”) and for heavy particles releasing anomalous signals in
CMS while traversing the tracking system (high momentum, highly ionizing
“muons”).
Gluino masses are excluded <229GeV (t=200ns) and <225GeV (t=2.6ms).
Limits on gluinos from HSCP analysis at 271 and 284 GeV (with muon id).
Prompt Photon Measurement
Prompt photon production at hadron colliders:
• include hard scattering sub process, QED
radiation off quarks, quark/gluon
fragmentation
• Can be studied with modest luminosity since
the rate is very high ~ mb at 7 TeV
• A testing ground for perturbative QCD
• Constraint of gluon parton distribution fn
High pT photon identification: important signal for many search physics:
test SM Gauge Boson couplings at
high energy: V1 V2 g
Higgs boson: H  gg
gauge-mediated SUSY breaking model
exotics: graviton decay G  gg, excited
fermion decay f⃰  f g
But huge background from hadron decays, dominated by 0 and h decays to
photons  demand isolated photons
Here is the Compact Muon Solenoid!
Resonances
Heavy Stable Charged particles
Prospect of discovery of Higgs boson at LHC at CM energy 7 TeV
Current Tevatron exclusion limit: for Higgs mass: 158 to 175 GeV
For Mx> 140 GeV, S/N ratio better  LHC competitive to Tevatron.
Comparable to the sensitivity of Tevatron with lumi ~300-500 pb-1 by 2011
Prospects at 7 TeV with 1 fb –1
Higgs discovery sensitivity:
160-170 GeV
Higgs mass which can be
excluded: 145 to 190 GeV
LHC is indeed a big deal to High Energy Physicists!
• LHC opens up a completely new, vast world, in terms of the physics
possibilities.
• LHC is a voyage into Terra Incognita: we may see completely
unexpected physics anticipation of The Discovery is tremendous.
• Initial phase of LHC  validation of the experiment.
Several “first studies” have been made in the context of the experiment.
• There very good reasons to believe that LHC will settle the issue of Higgs
mechanism, shed light on physics at TeV scale.
Stay tuned!
• Reaping the physics harvest at LHC is a challenge zillion times tougher than
finding a needle in the hay stack.
The status of LHC at this moment is similar to asking what a new continent is
going to be like when we can just glimpse the shore….
Conclusions
LHC project is the unbelievable in pursuit of the unimaginable.
LHC machine started colliding protons in November, 2009.
LHC will continue colliding protons at energy 7 TeV till 2011
LHC will also have heavy ion collisions during 2010 and 2011.
All the experiments are successfully collecting and analysing the collision
data.
■ First LHC data indicate that the performance of the detector, simulation
and reconstruction (including the understanding of material and control of
instrumental effects) is far better than expected.
This is only the beginning of an exciting physics phase and a major
achievement of the worldwide LHC Collaboration after > 20 years of efforts
to build a machine and detectors of unprecedented technology, complexity
and performance.
a la` Newton: To me there has never been a higher source of earthly honour
or distinction than that connected with advancement in science.
42
Back up
TeV resonance Z’  e+e-
Prospect for SuperSymmetry
• Projections for capabilities are biased, experiments having resorted to
specific model (mSUGRA).
• Unknowns: preferred SUSY model with all features for physics based
experimental implications, all possible backgrounds and their sizes, ...
• Mainly 2 channels suitable for early analysis: all hadronic and same-sign
dileptons
• All hadronic channel weakly dependent on tanb value!
 large rate for background, but good efficiency for signal
 Interesting sensitivity beyond current experimental limits already
achievable with data corr. to 100 /pb
SUSY Higgs  tt @1 /fb
Large region of m A – tan b plane, much beyond current limits can be
excluded
J/y→μ+μ- differential and total cross section
Signal events: 17156  569
Sigma: 43.3 0.5 (stat.) MeV
M0 : 3.0927  0.0005 (stat.) GeV
S/B= 6.4 ; c2/ndof = 1.7
Signal events: 710  29
Sigma: 20.3 0.7 (stat.) MeV
M0 : 3.0945  0.0008 (stat.) GeV
S/B= 64 ; c2/ndof = 1.1
Differential cross section as a function of pT for the two different rapidity intervals and
in the null polarization scenario. The total cross section for inclusive J/ψ production in
the di-muon decay channel is
BR(J/ψ→µ+µ−)·σ(pp→J/ψ + X) = (289.1 ± 16.7(stat) ± 60.1(syst)) nb
(4 ≤pT≤30GeV/c and |y| <2.4; the systematic uncertainty is dominated by the
statistical precision of the muon efficiency determination from data).
Max peak luminosity: L~1.6 x 1030 cm-2s-1
 average number of pp interactions per bunch-crossing: up to 1.3
 “pile-up” (~40% of the events have > 1 pp interaction per crossing)
Event with 4 pp interactions in the same bunch-crossing
~ 10-45 tracks with pT >150 MeV per vertex
Vertex z-positions : −3.2, −2.3, 0.5, 1.9 cm (vertex resolution better than ~200 μm)
Particle Identification
Multiplicity distribution 7 TeV
reasonably described by negative-binomial distributions
comparison with different models – not satisfactory
arXiv:1004.3514[hep-ph]
05/05/2010, ALICE status and first physics, Karel Safarik, CERN
LHC open presentation
arXiv:1004.3514[hep-ph]
52
CMS detector
LHC is a giant!
27 km at 1.9 K (superfluid He)
700K litres of liq. He + 12M litres
of liq. nitrogen
Vaccuum ~ 10-13 Atm.  100
times more tenuous than the
space of communication satellite
SC coils: 12000 tonnes/7600 km
Top physics
jet
b-jet
q
b
p
t

W- t
b 
l-
b-jet


Measurement of top quark production rate
Light quark content of top: Br(tWb)/ Br.(tWq)
Search for high mass ttbar resonance
Single top measurement
jet
W+ q
p
top pair to µµ +Jets Candidate
y [cm]
Multiple primary vertices 
multiple pp collisions (“pile-up”)
Jets & muons originate from
same primary vertex
Very clean candidate sitting in a
region where we expect very
little background!
z [cm]