Status of LHC

Kajari Mazumdar

TIFR, Mumbai

LHC Seminar Series, TIFR January, 2010

Plan of the talk

• Recent highlights of LHC machine operation

• Status of experiments during LHC startup

• Over all detector performance

• Initial Physics plots

• Expected Early Physics performance at 10

TeV

-- few examples

x1p x2p(sx1x2) = (sx)

proton beams

proton proton

[ “Hard scattering partons” ]

Proton-on-proton collision at LHC

The LHC at CERN, Geneva

• Proton Beams started circulating in LHC ring in September 2008, beam energy 450 GeV from SPS celebrations everywhere.

• Subsequently, magnet currents ramped up to energize injection beams in

LHC ring accident! LHC delayed by one year!

• August , 2009: CERN announces LHC will deliver (only proton-on-proton) collisions at 7 TeV in 2009.

• End-October, 2009: LHC machine is cold (27 Km @ 4 Kelvin)

• 20 November, 2009: proton beams in orbit, well-tuned

• 21 November, quiet beam

Beam size ~ 300 m in transverse, 10.5 cm in horizontal direction

Impressively small dispersion, lifetime upto several hours.

• Beam intensity < 5. 10 9 protons/bunch

• Experiments observe beam halo muons, beam splashes during November

20,21, 22

• 23 November: first collisions at LHC at 900 GeV (Pilot Run)

Nov.30: collisions at 2360 GeV LHC sets the world record in

energy for hadron collision. Tevatron reach 1960 GeV.

Main events towards start of LHC machine in 2009

Golden orbits, for 1.18 TeV beam on 1.12.2009Horizontal position of beam, mm

CMS experiment is on twitter!Beam is highly sensitive to stray fields, LEP tradition!

It took only ~500 sec . to ramp up beam energy from 450 GeV to 1.18 TeV !

History in the making: 4x4 bunches and even 16 bunches/beam on 16 December.

Deviation of beam from accelerator ring in horizontal and vertical directions r ~ ± 30 m

CMS solenoid 3.8 Tesla

Beams affected

by earth’s tides!

Highly precise beams ramped upto 1180 GeV

Beam Intensity (10 10 protons/bunch), 6.12.2009

Long beam lifetimes

In all aspects LHC machine operations have been impressive

Steve Meyers on December 7, 2009

Expect collision to start with CM energy 7 TeV after mid-Feb., 2010

Experiments at LHC

• 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 mX 0.1

m)

• Totem

• The LHCf experiment uses forward particles created inside the LHC p-p collision as a source to calibrate cosmic ray

• ATLAS, CMS general purpose p-p experiment.

• ALICE is meant for study of quark-gluon plasma in heavy ion collisions.

• LHCb: CP violation studies, using forward spectrometer to detect B-decays and measure the daughter particles.

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

Prologue for physics from collision

• Years of test beam activities, • increasingly realistic simulations,• commissioning with cosmics to understand and optimize the detector performance • validation of the software tools were fundamental to achieve results within few hours of data taking.

• All the experiments collected collision data during December 2009.

• During pilot run, experiments had magnetic field turned off.

• The subdetector electronic channels are operational with > 99%

efficiency.

• Data taking efficiency on average ~ 90% during stable beam

conditions.

• Data can be analysed rapidly, using Grid computing facility .

• Detector performances are according to design.

Real Physics exploitation part will auger in 2010

Status on the experimental front

ALICE put up on arXive first paper on charged multiplicity within 2 days of first collision (pilot run) based on 284 events!

ATLAS, CMS have almost final versions of similar study. Should be out in about 10 days.

Published in EPJC

Author list and associated institutes run for 5.5 pages, before the abstract!

Data taken with no magnetic field in tracking detector Track counting possibleNo momentum measurement.

Paper has charged density

distribution based on 284

events.

Transverse momentum distribution, higher tail hard part of collision

Pseudorapidity distribution Dominated by softer part of collision

Charged particle density increases by factor 1.7 to 1.9 for cm energy increase of 900 GeV to 7 TeV to 14 TeV

Phenomenological models describe energy-dependence of total –cross-section and charged multiplicity distribution in terms of some parameters determined from lower energy experiments goes into the complete event simulation /generationNeed to re-tune these parameters at new energies .

Multiplicity in pp and p-pbar collisionsdiffer at lower energies.At 900 GeV, difference ~ 0.1 %

18

ATLAS cavern, October 2005Length : ~ 46 m Radius : ~ 12 m Weight : ~ 7000 tons~108 electronic channels

Events at ATLAS experimental site:First beam circulates on November 20 See beam halo muons

Beam Splashes during Nov. 20,21, 22, 23• Avalanche of scattered particles from beam-on-

collimator hits 450 GeV protons, ~ 10K in number,

hits heavy metal foil few 10s of m

upstream of the detector.

• Detectors fully lit, typically

few hundred thousands hits in

central detector.

• About 3000 TeV energy recorded in

calorimeter part of the detetctor.

Beams collide in ATLAS, Nov. 23, 14.22 CET

Background separation in ATLAS

Before RF cogging After RF coggingApplied shift of 900 psproviding vertex shift of +13.5 cm

Beam pickup scope shots, beam 1 & 2

• The ATLAS beam pickups showed a phase inconsistency of 900 ps causing the primary vertex to be shifted by 13.5 cm in z

• Based on this information, at around 14:50, the LHC operators performed an RF cogging to correct the z positioning of the beam spot at IP1

Bunches stable within 20 ps (RMS) !

• Sequence of events:– beam 1 injected, captured, circulating– data taking started– at 16:38 beam 2 injected on “P2” bucket, captured, circulating– as soon as beam 2 injected, the ALICE trigger rate jumps from a few 10-3 s-1

(with beam 1 bunch only) to ~ 10-1 s-1 no further adjustment needed

– within seconds, the first event popped up on the display– at 17:21 the beams were dumped and the run closed with 284 events

• for an estimated integrated luminosity of about 8 mb-1

The “ALICE fill” (ca 16:35) on 23.11.2009

23

“Splash” Event in CMS (calorimeter on, tracker, magnet off)

Beam circulating, Halo Muon in CMS

23.11.09 afternoon: While ATLAS and Alice started recording collisions at

the centre of their detector, beam2 needed better steering at CMS (straight

charged tracks seen without magnetic field on)

Difficult to conclude that we see collisions

Mon 23 Nov 14:46 Apparent vertex at -60 cm

Main goals of 2009 collision data

• Commission of various worksflows, like, Data quality monitor, Monte carlo tuning, ..

• Retune selections and understanding of physics objects, like: jets ,

photons, electrons, muons, missing transverse energy,.. in minimum

bias data.

• Perform early physics analyses: charged hadron multiplicity,

transverse moemntum spectrum, jet spectrum, underlying events, low

mass resonances in muons, photons

• Prepare for higher energy runs in 2010: study of fake rates, b-

tagging,..

Estimation of cross-section

uncertainty mostly dominated by

that in Parton density function.

Various measurements at LHC will

estimate the pdfs accurately and

consistently.

Note heavy quark pdfs play important roles at LHC.Reduction in CM energy from 14 to

10 TeVdegraded sensitivity for discoveries: 200 GeV Higgs down by 50%2 TeV Z’ reduces by 30%New Physics with scale > 4 TeV reduced by order of magnitude! Sensitivity reduces further for 7 TeV.Also machine is expeted to deliver data for ~ 200 pb -1

• Only the inelastic component of total cross-section is measured by CMS, ATLAS.• Detectors are meant for physics with high momentum particles.

• Physics potentials at 7 TeV LHC energy are currently being evaluated.• Hope of 100 pb-1 of integrated luminosity at 7 TeV during 2010.

•Rediscovering Standard Model is high on the agenda, instead of searches

Event Trigger and analysis in CMS

Use beam monitoring systems

1. Beam Scintillator Counters @ ~ ± 11m from IP, 3.23 ≤|| ≤ 4.65

measure hit and coincidence rates, resolution 3 ns.

mip detection eff. ~ 96%

2. Beam pick-up devices @ ± 175 m from IP

precise information on bunch structure and timing of incoming beams

Total triggered event ~ 240k.

Primary vertex reconstruction: use tracks with P_T > 900 MeV/c

_xy : 0.5 mm

Prob. of multiple collision in same event ~ 10 -4

Fraction of total events in pythia simulation: 22% SD + 12% DD + 78% NSD.

Event selection eff.: 16.5%, 30.6%, 79.5% for charged tracks with || ≤ 2.5

Luminosity determination @ ATLAS• 197 golden collision candidates from data of Nov 23, in 2 parts.

• From Monte Carlo (solenoid field on) , the selection efficiency, including trigger, for inelastic and diffractive minimum bias events is about 70%

• Using as total minimum bias cross section of 58 mb

40 mb inelastic, 12 mb Single Diffractive (SD), 6 mb DoubleDiffractive(DD)

L = N /

Sample Number

of

events

DAQ

duratio

n

Averag

e rate

Average inst.

luminosity

Integrated

luminosity

Sample Number

of

events

DAQ

duratio

n

Averag

e rate

Average inst.

luminosity

Integrated

luminosity

A 61 54 mins 0.03 Hz 0.5 × 1024 cm2 s 1 1.5 mb1

B 136 46 mins 0.07 Hz 1.2 × 1024 cm2 s1 3.4 mb1

Cross checks: • Assuming that =0% for SD and DD increases luminosity by 10%.• change inelastic cross section to 34 mb increases luminosity by

15% .

34

ATLAS: π0

■ 2 photon candidates with ET (γ) > 300 MeV

■ ET (γγ) > 900 MeV■ Shower shapes compatible with photons■ No corrections for upstream material

Data and MC normalised to the same area

Note: soft photons are challenging because of material in front of EM calorimeter(cryostat, coil): ~ 2.5 X0 at η=0

36

Sunday 6 December: machine protection system commissioned

stable (safe) beams for first time full tracker at nominal voltage whole ATLAS operational

Rapid analysis in CMS, preliminary results

Charged particle spectra

Onia yield in CMS collision data

Expect in CMS signal to background ratio ~ 16:1, after selection

Dimuon event in CMS, it is J/ψ !

Most of the J/ψ in forward region, being with low pt, upto about 0.5 GeV!

First Dijet event in ATLAS

2 jets back-to-back in both with (uncalibrated ) ET

~ 10 GeV expect actual/calibrated energy ~ 20

GeV

Probability of seeing such a event within a short while= 1 %

They have been lucky! of 1.3 and 2.5, ~ no missing ET

3-jet event in CMS, all of reasonably high transverse energy

Photon + jet event at 2360 GeV

43Uncalibrated EM scaleMonte Carlo normalized to number of jets or events in data

events with2 jets pT> 7 GeV

44

Missing transverse energy

■ Sensitive to calorimeter performance (noise, coherent noise, dead cells, mis-calibrations, cracks, etc.) and backgrounds from cosmics, beams, …■ Measurement over full calorimeter coverage (3600 in φ, |η| < 5, ~ 200000 cells)

METy

METx / METy indicate x/y components of missing ET vectorMETx

METx

NSD: non-single diffractiveTotal inelastic NSD + SD + DDSingle/double diffractive: one/both beam particles excited into higher mass state

Physics prospect in LHC, near future

Large rates helps many QCD and EW studiesInvolving W, Z.

Gluon density falls more rapidly at lower energy in general signal rate is lower compared to background at lower energy signal to background ratio lower.

All the values for reaches have to reestimated at 7 TeV, at 100 pb -1.

Need heavy quark contents of proton at LHC energies

W,Z production Fundamental benchmark process at hadron collider.

• Processes are well understood theoretically.

• Luminosity reaction with potential accuracy of ~ 1%, finally

• measure cross-section at new energy regime

• Basic event signature: charged lepton, missing energy

Starting point for detailed analysis:

• Boson Pt spectrum

• additional accompanying jets

• asymmetries

• W-mass and width

At startup

• Calibration source knowing Z mass

• Validate lepton isolation criteria

• evaluate reconstruction, trigger, selection efficiencies .

• use tag-n-probe method on Z events

• Measure fake rates

Z-asymmetry at 10 TeV, electron channel

Provides constraint on parton density function

W-asymmetry at 10 TeV, muon channel

10 pb-1

100 pb-1

Early physics with leptons

Predicted jet yield: limited reach

Many QCD analysis can be done early eg.,Azimuthal decorrelation in dijet events,Central transverse thrust, dijet mass Distribution, dijet rates in two regions ofPseduorapidity, etc.

Early Top studies at LHC

Allows direct measurement of Wtb

With few 10 pb-1Top rediscovery,Measurement of top-pair production rate.

With 100 – 300 pb-1 Br(t Wb)/ Br(t Wq)Light quark content in top,Re-evidence of single topFirst search for high mass tt-resonance.

For 100 pb -1 and of energy10 TeV, @ 95% CL Br exclusion:8.3 pb for M(tt) = 2TeV.

LHCb early analysis

Clean beam-gas events ~ 1/min, as expected

53

All the experiments have successfully collected first LHC collision data.

■ The experiments operated efficiently and fast, from data taking at the pit, to data transfer worldwide, to the production of first results (on a very short time scale … few hours to few days).

■ 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 at this (initial) stage of the experiment and in an energy regime ATLAS and CMS was not optimized for.

Conclusions

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.

Backup

Search for resonance in ttbar pair (dimuon+x)

10 TeV, 100 pb -1

Many possibilities, Tevatron limit for lepto-phobic resonance > 700 GeVAssume simplest possibility of Z’ tt, width = 1% of mass experimental resolution dominates

Potential for Higgs discovery With decreasing cm energy signal goes

down faster, since Higgs is mainly produced

via gg fusion, compared to background

• Standard Model Higgs boson can be

Discovered in the range 140-450 GeV, by

Both experiments with 5 fb -1.

Exclusion : with 1 fb -1 at 10 TeV, combiningH ZZ, H WW, channels mass range 150-190 GeV.

Jet rate and contact interaction at high scaleJet reconstruction and higher order corrections

Cone vs. recombination algorithmsDiscrepancies between theory and experimentsusing midpoint algorithm where more partons areallowed in the cone.

LHCf experiment

• Dedicated for Astroparticle physics, aimed to operate upto 14 TeV• LHCf will be able to measure the flux of neutral particles (pions as

well as neutrons) produced in p-p collisions at LHC in the very forward region calibration of air-shower Monte Carlo codes currently used for modeling cosmic rays interactions in the Earth atmosphere, and hence the primary energy of ultra-high energy cosmic rays.

capable of addressing the issue of constituents which contribute to knee region of energy spectra.

• 2 small calorimeters, each placed 100 m away from the ATLAS IP.• Sampling and imaging calorimeters inserted inside neutral absorbers• Inside TAN, the 2 proton beam lines separate out LHCf measures particles produced upto pseudo-rapidity = infinity• Capable of measuring photons upto few TeVs

LHCb is a heavy flavour precision experiment searching for New Physics in CP Violation and Rare Decays

A program to do this has been developed and the methods, including calibrations and systematic studies, are being worked out..

CP Violation: 2 fb-1 (1 year)*• from trees: 5o - 10o

• from penguins: 10o

• Bs mixing phase: 0.023• s

eff from penguins: 0.11

Rare Decays: 2 fb-1 (1 year)* • BsK* s0 : 0.5 GeV2

• Bs Adir , Amix : 0.11 A : 0.22• Bs BR.: 6 x 10-9 at 5

59

Flavour Tagging

Performance of flavour tagging:

Efficiency

Wrong tag w

Tagging power Bd ~50%

Bs ~50% 33% ~6%Tagging power:

22 1 2D w

Particle Identification in LHCb

bt

Bs K

K

,K

Ds

Primary vertex

Bs → Ds K, distinguish from Bs Ds

B-Vertex Measurement in LHCb

Vertex Locator (Velo)Silicon strip detector with ~ 5 m hit resolution 30 m IP resolution

Vertexing:• Impact parameter trigger• Decay distance (time) measurement

Ds

Bs K

K

K

d

47 m 144 m

440 mPrimary vertex

Decay time resolution = 40 fs

) ~40 fs

Example: Bs → Ds K

bt

Bs K

K

K

Ds

Primary vertex

Bs→ Ds KBs →Ds

B-mass Measurement

Flavour Tagging in LHCb

btag

Bs KK

K

Ds

Primary vertexTime-dependent decays

• Flavour tagging: D2 = (1-2w)2 6%

64

Recorded data samples Number of Integrated luminosity events (< 30% uncertainty)

Total ~ 920k ~ 20 μb-1

With stable beams ( tracker fully on) ~ 540k ~ 12 μb-1

At √s=2.36 TeV (flat top) ~ 34k ≈ 1 μb-1

Average data-taking efficiency: ~ 90%

Max peak luminosity seen by ATLAS : ~ 7 x 1026 cm-2 s-1

65

Inner Detector

p

K

π

180k tracks

Pixels

Silicon strips

Transition Radiation Tracker

Transition radiation intensity is proportional to particle relativistic factor γ=E/mc2. Onset for γ ~ 1000

• ATLAS has taken data before and after the RF cogging

• Must observe shift in z0 of tracks if indeed we select collision events!

Background separation in ATLAS

Track z0 distribution of

collision candidate events taken before and after RF cogging

Observed shift: +12 cm

Rejection factor of ~104 looking for space points in the Inner Detector at Level 2 trigger

Beam injection, record collision events.HLT algos off.

HLT active after LHC declares stable beam

BPTX prescaled by x20 as input to L2

~20

68

Inner Detector (||<2.5, B=2T): Si Pixels and strips (SCT) + Transition Radiation straws Precise tracking and vertexing,e/ separation (TRT).Momentum resolution: /pT ~ 3.4x10-4 pT (GeV) 0.015

Length : ~ 46 m Radius : ~ 12 m Weight : ~ 7000 tons~108 electronic channels

Muon Spectrometer (||<2.7) : air-core toroids with gas-based chambersMuon trigger and measurement with momentum resolution < 10% up toE ~ TeV

EM calorimeter: Pb-LAr Accordione/ trigger, identification and measurementE-resolution: ~ 1% at 100 GeV, 0.5% at 1 TeV

HAD calorimetry (||<5): segmentation, hermeticityTilecal Fe/scintillator (central), Cu/W-LAr (fwd)Trigger and measurement of jets and missing ET

E-resolution:/E ~ 50%/E 0.03

3-level triggerreducing the ratefrom 40 MHz to~200 Hz

3-jet event in CMS

71

pT (track) > 100 MeVMC signal and background normalized independently

Ks

0p p , L pp , L p p

K0S

ΛL

Mass of islow in both data and MC, due to readout thresholds (100 MeV/crystal) and conversions of g in the material

Measurement of electron and photon

Performance of CMS detector

LHCb Detector, ready aligned, mostly tested with cosmics

All charged tracks with pt > 2 GeV

Reconstructed tracks with pt > 25 GeV

Operating conditions:one “good” event (e.g Higgs in 4 muons )

+ ~20 minimum bias events)

Event size: ~1 MByte Processing Power: ~X TFlop

Higgs event in CMS

We have to wait for few years still forthis to become a reality