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Status of LHC. Proton-on-proton collision at LHC.  ( sx 1 x 2 ) =  ( sx ). [ “Hard scattering partons” ]. x 1 p. x 2 p. proton. proton. proton beams. Kajari Mazumdar Department of Experimental High Energy Physics Tata Institute of Fundamental Research Mumbai.

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Status of LHC

Proton-on-proton collision at LHC

(sx1x2) = (sx)

[ “Hard scattering partons” ]





proton beams

Kajari Mazumdar

Department of Experimental High Energy Physics

Tata Institute of Fundamental Research


IACS Kolkata, September 27, 2010

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  • 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.

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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:

(1 TeV = 1012electronVolt

= 1.6 * 10 -7 Joule)

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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

27 km at 1.9 K (superfluid He)

Vaccuum ~ 10-13 Atm.

100 – 150 m under the surface

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.

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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.


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Standard Model (SM) of Particle Physics as of today

  • 4 types of basic forces :

  • 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!

Relative strengths:

10 -40: 10 -5: 10-2: 1

  • 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!

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We got to unravel the mystery of mass! today

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 yetseen 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.

LHC is an exploratory, high energy, high intensity machine which can produce heavy particles of mass upto few TeV.

Need accelerators!

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!

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Mass spectrum in Standard Model today

  • 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.

The nature of this Beyond Standard Model physics is not known, though several contenders are more favourable, eg., SuperSymmetry.

New Physics also invokes new set of additional particles!

Further problem with SM

even with a single Higgs


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Symmetry Breaking today

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.

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  • LHC Timeline today

    • 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!

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Experiments at LHC today

  • Totem is meant for studying diffractive events, measure luminosity delivered

  • in CMS.

ATLAS, CMS: general

purpose p-p experiment.

ALICE: study of quark-gluon plasma in heavy ion collisions.

  • ATLAS (46m X 25m X 25 m)

  • CMS (21m X 15 m X 15 m)

  • 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

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What happens in LHC experiment today

Mammoth detectors register signals for

Energetic, mostly (hard) inelastic collisions

involving large momentum transfer.

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Basic features in hadron collisions today

  • 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.

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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

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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: ~1 MByte Processing Power: ~X TFlop

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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.

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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.

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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!

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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.

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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

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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.

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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.

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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


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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.

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Low mass resonances

Invariant mass distribution

for different combinations

(Ω±ΛK± or ±Λ± ) fit to a common vertex.

  • Tracks displaced from primary vertex (d3D > 3σ)

  • Common displaced vertex

  • (L3D > 10σ)

PDG Mass:

1672.43 ± 0.29

PDG Mass:

1321.71 ± 0.07



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Jets and Missing E T

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

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I nclusive 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.

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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.

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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

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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

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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 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 V2g

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 p0 and h decays to photons  demand isolated photons

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Here is the Compact Muon Solenoid!

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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

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Prospects at 7 TeV with 1 fb energy 7 TeV –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 is indeed a big deal to High Energy Physicists! energy 7 TeV

  • 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….

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Conclusions energy 7 TeV

  • 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.


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Back up energy 7 TeV

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TeV resonance Z’ energy 7 TeV  e+e-

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Prospect for SuperSymmetry energy 7 TeV

  • 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

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J/ energy 7 TeV 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).

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  • Max peak luminosity: L~1.6 x 10 energy 7 TeV 30 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)

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Multiplicity distribution 7 TeV energy 7 TeV

reasonably described by negative-binomial distributions

comparison with different models – not satisfactory



05/05/2010, ALICE status and first physics, Karel Safarik, CERN LHC open presentation


Cms detector
CMS detector energy 7 TeV

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LHC is a giant! energy 7 TeV

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

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Top physics energy 7 TeV













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

Top pair to jets candidate
top pair to µµ +Jets Candidate energy 7 TeV

Multiple primary vertices  multiple pp collisions (“pile-up”)

Jets & muons originate from same primary vertex

y [cm]

Very clean candidate sitting in a region where we expect very little background!

z [cm]