Status of LHC Kajari Mazumdar TIFR, Mumbai WHEPPXI, PRL, Ahmedabad 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: few examples from CMS, ATLAS Expected Early Physics performance at 10 TeV: few examples from CMS
The LHC at CERN, Geneva Proton-on-proton collision at LHC (sx 1 x 2 ) = (sx) [ Hard scattering partons ] proton x 1 p x 2 p proton proton beams
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!
Main events towards start of LHC machine in 2009 August, 2009: CERN announces LHC will deliver (only proton-onproton) 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.
Golden orbits, for 1.18 TeV beam on 1.12.2009 Horizontal position of beam, mm It took only ~500 sec. to ramp up beam energy from 450 GeV to 1.18 TeV! History in the making: 4x4 bunches higher luminosity 16 bunches/beam on 16 December. CMS experiment is on twitter! Beam is highly sensitive to stray fields, LEP tradition!
Deviation of beam from accelerator ring in horizontal and vertical directions r ~ ± 30 m CMS solenoid 3.8 Tesla
Highly precise beams ramped upto 1180 GeV Beams affected by earth s tides!
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: 440m X 5m X 5m ATLAS and CMS are 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. The LHCf experiment uses forward particles created inside the LHC p-p collision as a source to calibrate cosmic ray interaction with earth s atmosphere. Totem is meant for measurement of total cross-section, elastic scattering and diffractive events, in effect also measures luminosity delivered (to CMS).
ATLAS cavern, October 2005 Length : ~ 46 m Radius : ~ 12 m Weight : ~ 7000 tons ~10 8 electronic channels 14
Prologue for physics from collision Main factors to achieve results within few hours of data taking: Years of test beam activities increasingly realistic simulations commissioning with cosmic rays to understand and optimize the detector performance validation of the software tools Experiments have been ready for data!
Status on the experimental front 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. 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 soon. Real Physics exploitation part will auger in 2010
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 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 Before RF cogging After RF cogging Applied shift of 900 ps providing vertex shift of +13.5 cm Beam pickup scope shots, beam 1 & 2 Bunches stable within 20 ps (RMS)!
The ALICE fill (ca 16:35) on 23.11.2009 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 afew 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 Estimated integrated luminosity ~ 8 mb 1 21
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) Mon 23 Nov 14:46 Apparent vertex at -60 cm Difficult to conclude that we see collisions
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,..
Total cross-section: Elastic, inelastic and diffractive. Diffraction: one or both of beam particles excited to higher mass state Fraction of total events in pythia simulation: 22% SD + 12% DD + 78% NSD. Only the inelastic component of total cross-section is measured by CMS, ATLAS. CMS, ATLAS detectors are meant for physics with high momentum particles.
Rates of various processes in hadron collision Reduction in CM energy from 14 to 10 TeV degraded 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 to un-explored physics reduces further for 7 TeV Uncertainty in measurement of various cross-sections is mostly dominated by that in Parton density function. Several measurements at LHC will estimate the pdfs accurately and consistently.
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 Event selection eff.: 16.5%, 30.6%, 79.5% for Single diffractive, Double and nonsingle diffractive events
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 duration Average rate Average inst. luminosity Integrated luminosity Sample Number DAQ Average Average inst. Integrated A of events 61 duration 54 mins rate 0.03 Hz luminosity 0.5 10 24 cm 2 s 1 luminosity 1.5 mb 1 B 136 46 mins 0.07 Hz 1.2 10 24 cm 2 s 1 3.4 mb 1 Cross checks: Assuming that =0% for SD and DD increases luminosity by 10%. change inelastic cross section to 34 mb increases luminosity by 15%.
Data taken with no magnetic field in tracking detector Track counting possible No momentum measurement. Paper has charged density distribution based on 284 events. dn(ch)/ d ±0.15 (stat.) ± 0.25(syst.) For NSD interactions, consistent with previous measurements in p pbar experiment at the same energy. CMS unpublished result : dn(ch)/ d ±0.06 (stat.) ± 0.21(syst.), Author list and associated institutes run for 5.5 pages, before the abstract! Published in EPJC
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 crosssection and charged multiplicity distribution in terms of some parameters determined from lower energy experiments goes into the complete event simulation /generation Need to re-tune these parameters at new energies. Multiplicity in pp and p-pbar collisions differ at lower energies. At 900 GeV, difference ~ 0.1 %
ATLAS: π 0 2 photon candidates with E T (γ) > 300 MeV E T (γγ) > 900 MeV Shower shapes compatible with photons No corrections for upstream material Note: soft photons are challenging because of material in front of EM calorimeter (cryostat, coil): ~ 2.5 X 0 at η=0 Data and MC normalised to the same area 34
Sunday 6 December: machine protection system commissioned stable (safe) beams for first time full tracker at nominal voltage whole ATLAS operational 36
Rapid analysis in CMS, preliminary results Charged particle spectra Excellenet performance of CMS detector
Onia yield in CMS collision data Most of the J/ψ in forward region, being with low pt, upto about 0.5 GeV! Analyzed total minimum bias events: ~12k at 2360 GeV, 321,500 at 900 GeV Expect in CMS, at 2360 GeV, signal to background ratio~ 16:1, after selection, in mass range 3.0 to 3.2 GeV Dimuon event in CMS, it is J/ψ!
First Dijet event in ATLAS 2 jets back-to-back in both with (uncalibrated ) E T ~ 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 E T
3-jet event in CMS, all of reasonably high transverse energy
Photon + jet event at 2360 GeV
ATLAS Jet measurements events with 2 jets p T > 7 GeV Uncalibrated EM scale 43 Monte Carlo normalized to number of jets or events in data
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 (360 0 in φ, η < 5, ~ 200000 cells) METx METx / METy indicate x/y components of missing E T vector METx METy 44
NSD: non-single diffractive Preliminary result: Average charged hadron transverse momentum = 0.46 ±0.01 (stat) ±0.02 (syst.) GeV/c
Physics prospect in LHC, near future 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. Large rates helps many QCD and EW studies Involving W, Z. Physics potentials at 7 TeV LHC energy are currently being evaluated. Hope of 100 pb -1 of integrated luminosity at 7 TeV during 2010. 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 Rediscovering Standard Model is high on the agenda instead of searches
Asymmetries provide constraint on parton density function Z-asymmetry at 10 TeV, electron channel W-asymmetry at 10 TeV, muon channel 10 pb -1 100 pb-1
Early physics with leptons
Predicted jet yield: limited reach at lower energy Many QCD analysis can be done early eg., Azimuthal decorrelation in dijet events, Central transverse thrust, dijet mass Distribution, dijet rates in two regions of Pseduorapidity, etc.
Early Top studies at LHC With few 10 pb -1 Top 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 top First search for high mass tt-resonance. Allows direct measurement of Wtb 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
Conclusions 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. 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. 53
Backup
Search for resonance in ttbar pair (dimuon+x) 10 TeV, 100 pb -1 Many possibilities, Tevatron limit for lepto-phobic resonance > 700 GeV Assume 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, combining H ZZ, H WW, channels mass range 150-190 GeV.
Jet rate and contact interaction at high scale Jet reconstruction and higher order corrections Cone vs. recombination algorithms Discrepancies between theory and experiments using midpoint algorithm where more partons are allowed 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: 5 o 10 o from penguins: 10 o B s mixing phase: 0.023 s eff from penguins: 0.11 Rare Decays: 2 fb 1 (1 year)* Bs K* s 0 : 0.5 GeV 2 B s A dir, A mix : 0.11 A : 0.22 B s BR.: 6 x 10 9 at 5 59
Flavour Tagging Performance of flavour tagging: Efficiency Wrong tag w Tagging power Tagging power: D 2 1 2w 2 B d ~50% B s ~50% 33% ~6%
Particle Identification in LHCb Bs Ds K, distinguish from Bs Ds,K Primary vertex B s b t D s K K
B Vertex Measurement in LHCb Example: Bs Ds K 47 m 144 m B s D s K Primary vertex d 440 m Decay time resolution = 40 fs K K ) ~40 fs Vertex Locator (Velo) Silicon strip detector with ~ 5 m hit resolution 30 m IP resolution Vertexing: Impact parameter trigger Decay distance (time) measurement B mass Measurement Primary vertex B s b t D s K K K 2000 1500 1000 500 0 m Bs σ Bs = 5.37 GeV/c = 13.8 MeV/c 5.3 5.35 5.4 5.45 5.5 mass [GeV/c 2 2 m Bs σ Bs = 5.42 GeV/c = 24.0 MeV/c B s Bs Ds K Bs Ds 2 2 2 ]
Flavour Tagging in LHCb 1000 800 Time dependent decays Perfect reconstruction 1000 800 Perfect reconstruction + flavour tagging B s Primary vertex D s b tag K K Flavour tagging: D 2 = (1 2w) 2 6% Events 600 400 200 Events 600 400 200 0 0 0 1 2 3 4 5 0 1 2 3 4 5 Proper time (ps) Proper time (ps) 1000 800 Perfect reconstruction + flavour tagging + proper time resolution 1000 800 Events Perfect reconstruction + flavour tagging + proper time resolution + background 1000 800 600 400 200 Perfect reconstruction + flavour tagging + proper time resolution + background + acceptance Events 600 400 Events 600 400 0 0 1 2 3 4 5 Proper time (ps) 200 200 0 0 1 2 3 4 5 Proper time (ps) 0 0 1 2 3 4 5 Proper time (ps)
Max peak luminosity seen by ATLAS : ~ 7 x 10 26 cm 2 s 1 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: 64 ~ 90%
Inner Detector Silicon strips Pixels p K 180k tracks π Transition Radiation Tracker Transition radiation intensity is proportional to particle relativistic factor γ=e/mc 2. Onset for γ ~ 1000 65
Background separation in ATLAS ATLAS has taken data before and after the RF cogging Must observe shift in z 0 of tracks if indeed we select collision events! Track z 0 distribution of collision candidate events taken before and after RF cogging Observed shift: +12 cm
Beam injection, record collision events. HLT algos off. HLT active after LHC declares stable beam Rejection factor of ~10 4 looking for space points in the Inner Detector at Level 2 trigger ~20 BPTX prescaled by x20 as input to L2
Muon Spectrometer ( <2.7) : air core toroids with gas based chambers Muon trigger and measurement with momentum resolution < 10% up to E ~ TeV 3 level trigger reducing the rate from 40 MHz to ~200 Hz Length : ~ 46 m Radius : ~ 12 m Weight : ~ 7000 tons ~10 8 electronic channels Inner Detector ( <2.5, B=2T): Si Pixels and strips (SCT) + Transition Radiation straws Precise tracking and vertexing, e/ separation (TRT). Momentum resolution: /p T ~ 3.4x10 4 p T (GeV) 0.015 EM calorimeter: Pb LAr Accordion e/ trigger, identification and measurement E resolution: ~ 1% at 100 GeV, 0.5% at 1 TeV HAD calorimetry ( <5): segmentation, hermeticity Tilecal Fe/scintillator (central), Cu/W LAr (fwd) Trigger and measurement of jets and missing E T E resolution: /E ~ 50%/ E 0.03 68
3-jet event in CMS
K s 0 + -, p, p + p T (track) > 100 MeV MC signal and background normalized independently K 0 S Λ 71
Measurement of electron and photon Mass of is low in both data and MC, due to readout thresholds (100 MeV/crystal) and conversions of g in the material
Performance of CMS detector
LHCb Detector, ready aligned, mostly tested with cosmics
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 We have to wait for few years still for this to become a reality