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Physics at the LHC (1) Kerstin Tackmann (DESY) Herbstschule Maria Laach September 4, 2013 Kerstin Tackmann (DESY) Physics at the LHC (1) 1 / 38

Particle Physics and the Large Hadron Collider The Standard Model very successfully describes (laboratory-based) measurements Sometimes to amazing accuracy: the limit on the electron dipole moment has been verified to O(10 27 )...but we also have hints that it may not be the end of the story... What is the origin of the matter/antimatter asymmetry in the universe? What is dark matter? How do neutrinos acquire mass?...and would like to understand more... Are the forces unified at high energies? What did the early universe look like? Do we (really) understand why elementary particles have mass?... The LHC is studying physics at high energies (and high luminosities), trying to answer these questions Kerstin Tackmann (DESY) Physics at the LHC (1) 2 / 38

Physics at the LHC menu Wednesday, September 4 The Large Hadron Collider, proton collisions and the experiments Thursday, September 5 Standard Model (SM) measurements at the LHC Friday, September 6 Higgs physics at the LHC Saturday, September 7 Searches for physics beyond the SM Many thanks to Peter Jenni, Andreas Hoecker and Sandra Kortner, from whom I have borrowed a few slides and plots. Kerstin Tackmann (DESY) Physics at the LHC (1) 3 / 38

Why a hadron collider? Energy loss from synchrotron radiation in a circular collider (per turn) ( ) E = q2 E 4 ( ) E 4 e mp = 10 13 3Rɛ 0 mc 2 E p m e Higher energies much easier to reach with proton collisions But protons also have disadvantages......only part of the protons energies is available for the partonic collision...unkown boost along the beam direction (incomplete kinematic information)...large probability for low-energy processes...strong interaction makes theoretical predictions more complicated Another way around: linear collider, length determines possible collision energy u u d Kerstin Tackmann (DESY) Physics at the LHC (1) 4 / 38

Large Hadron Collider (@CERN, Geneva) LHC uses LEP tunnel Circumference 26.7 km 100 m below the surface Design: pp collisions at s = 14 TeV pp collisions at 2009 s = 900 GeV 2010/11 s = 7 TeV 2012 s = 8 TeV and lead-lead collisions at s = 2.76 TeV per nucleon 2013/2014 shutdown: machine and detector consolidation 2015- pp collisions at 13-14 TeV Kerstin Tackmann (DESY) Physics at the LHC (1) 5 / 38

The LHC (pre-)accelerator chain Linac 60 MeV Booster 1.4 GeV PS 25 GeV SPS 450 GeV LHC 3.5-7 TeV >50 years of CERN history still operational Previous main accelerator turns into pre-accelerator for the next step Kerstin Tackmann (DESY) Physics at the LHC (1) 6 / 38

LHC magnets 6700 magnets controlling the beams Dipoles bending 1232 Quadrupoles focusing 400 Sextupoles 2464 Octupoles/decapoles 1568 Orbit correctors 642 Others 376 Quadrupoles (inner triplet) Kerstin Tackmann (DESY) Physics at the LHC (1) 7 / 38

Technical challenge: dipole magnets Keeping a proton with momentum p on circular orbit p(tev) = 0.3B(T)R(km) For p = 7 TeV and R = 4.3 km need B = 8.4 T 12 ka need superconducting magnets (normal conducting magnet would melt): Nb-Ti cooled to 1.9 K with pressurized superfluid Helium Kerstin Tackmann (DESY) Physics at the LHC (1) 8 / 38

LHC design parameters beam energy 7 TeV instantaneous luminosity 10 34 cm 2 s 1 integrated luminosity/year 100 fb 1 dipole field 8.4 T dipole current 11700 A circulating current/beam 0.53 A number of bunches 2808 bunch spacing 25 ns protons per bunch 10 11 rms beam radius at IP1/5 16 µm rms bunch length 7.5 cm stored beam energy 360 MJ crossing angle 300 µrad number of events per crossing 20 luminosity lifetime 10 h...but this is not how LHC has been operating so far Kerstin Tackmann (DESY) Physics at the LHC (1) 9 / 38

LHC operations On September 19, 2008 one superconductor joint failed, damaged the vaccum enclosure, caused a catastrophic release of Helium and collatoral damage (up to 50 cm displacement of several quadrupoles, beam pipe contaminated with soot,...) 1 year of repairs and improvements Highest safe energy for operation was determined to be 3.5 TeV per beam (given measured resistance in joints) Operation with 1400 bunches, i.e. 50 ns bunch spacing Kerstin Tackmann (DESY) Physics at the LHC (1) 10 / 38

First collisions at 900 GeV (end of 2009) Kerstin Tackmann (DESY) Physics at the LHC (1) 11 / 38

Luminosity and event rate Rate of events N produced for a process with cross section σ dn/dt = Lσ Luminosity drives our ability to study rare processes with f revolving frequency n bunch number of bunches N p number of protons/bunch L = fn bunchn 2 p 4πσ x σ y 4πσ x σ y effective cross section area of beams Integrated luminosity L = Ldt Kerstin Tackmann (DESY) Physics at the LHC (1) 12 / 38

Instantaneous luminosity at LHC CMS Peak Luminosity Per Day, pp Peak Delivered Luminosity (Hz=nb) 10 8 6 4 2 0 1 Jun Data included from 2010-03-30 11:21 to 2012-12-16 20:49 UTC 2010, 7 TeV, max. 203.8 Hz=¹b 2011, 7 TeV, max. 4.0 Hz=nb 2012, 8 TeV, max. 7.7 Hz=nb 10 1 Sep 1 Dec 1 Mar 1 Jun 1 Sep Date (UTC) 1 Dec 1 Mar 1 Jun 1 Sep 1 Dec 10 8 6 4 2 0 Significant increase of instantaneous luminosity from 2010 to 2012 Increase of number of bunches, protons per bunch, more tightly focused beam Maximum instantaneous luminosity reached close to 8 10 33 cm 2 s 1 Not far from design, but with rather different conditions Kerstin Tackmann (DESY) Physics at the LHC (1) 13 / 38

Integrated luminosity ] 1 Delivered Luminosity [fb 35 30 25 20 15 10 5 0 Jan ATLAS Online Luminosity 2010 pp 2011 pp 2012 pp Apr s = 7 TeV s = 7 TeV s = 8 TeV Jul Oct Month in Year At the beginning of 2011, we were hoping for 1 fb 1 LHC has outperformed expectations! Efficiency (delivered by LHC analyzed) 90% 2012 23.3 fb 1 2011 5.6 fb 1 2010 0.048 fb 1 Kerstin Tackmann (DESY) Physics at the LHC (1) 14 / 38

The price of the high luminosity: pileup Many pp collisions for each bunch crossing many events overlayed in the detector CMS peak interactions per crossing, pp 45 Data included from 2010-03-30 00:00 to 2012-12-16 20:50 UTC 45 Peak interactions per crossing 40 35 30 25 20 15 10 5 2010, 7TeV ¾=68.0mb 2011, 7TeV ¾=68.0mb 2012, 8TeV ¾=69.4mb 40 35 30 25 20 15 10 5 0 1 Jan 1 May 1 Sep 1 Jan 1 May 1 Sep Date (UTC) 1 Jan 1 May 1 Sep 0 ATLAS was designed to operate with 23 interactions overlayed Kerstin Tackmann (DESY) Physics at the LHC (1) 15 / 38

Pileup Challenge to trigger, software and analyses Large amount of data to process and store Identification and measurement of the interesting objects Especially for jets, E miss T and τ Z µµ with 25 interaction vertices /0.1] 1 Recorded Luminosity [pb 180 ATLAS Online Luminosity 160 s = 8 TeV, 1 Ldt = 20.8 fb, <µ> = 20.7 140 s = 7 TeV, 1 Ldt = 5.2 fb, <µ> = 9.1 120 100 80 60 40 20 0 0 5 10 15 20 25 30 35 40 45 Mean Number of Interactions per Crossing Kerstin Tackmann (DESY) Physics at the LHC (1) 16 / 38

Proton collisions are a bit messy... u u u u d Kerstin Tackmann (DESY) d Physics at the LHC (1) 17 / 38

Proton collisions in detail......or rather, how we simulate them... + Decay Hadronization of partons to hadrons, nonperturbative model Parton shower: splitting of partons modeling initial and final state radiation Hard scatter described by matrix element (perturbative) Proton structure: partons inside the proton Underlying event: induced by interaction of remaining partons in protons Kerstin Tackmann (DESY) Physics at the LHC (1) 18 / 38

Parton distribution functions (pdfs) Proton content: valence quarks, sea quarks, gluons Momentum distribution of partons described by pdfs, function of Momentum fraction (of proton momentum) x Q 2 (scale of hard process) CM energy for parton collision ŝ = x 1 x 2 s(= m 2 X ) For m X = 100 GeV Tevatron ( s = 2 TeV) x = 0.22 (if x 1 = x 2) LHC ( s = 14 TeV) x = 0.08 (if x 1 = x 2) Larger cross sections at LHC LHC: cross section dominated by gg Kerstin Tackmann (DESY) Physics at the LHC (1) 19 / 38

Cross sections Cross section (σ) is the probability that a process a + b X occurs when a and b collide within a given area Differential cross section dσ/dy is the probability for final state with given dy Example: jet transverse momentum spectrum dσ/dp T Proton collisions: For inclusive processes σ(pp X) can be computed via factorization theorem, separating the short distance and long distance σ pp X = a,b 1 0 dx 1 dx 2 f a (x 1, Q 2 )f b (x 2, Q 2 )ˆσ ab X (x 1, x 2, Q 2 ) Hard scattering: production of W, Z, top, Higgs,..., computed in perturbative QCD at scale Q 2 Parton distribution functions nonperturbative structure of the proton p p x 1 x 2 X Note: strictly speaking only proven for inclusive processes Kerstin Tackmann (DESY) Physics at the LHC (1) 20 / 38

Kinematic variables Transverse momentum p T and missing transverse momentum ET miss Transverse momentum is conserved p i T = 0 Large missing transverse momentum ET miss = p miss T invisible particle escaped detection (e.g. neutrino) Longitudinal momentum p z and (visible) energy E Boost of partonic CM unknown cannot use p z and E conservation Polar angle θ Not Lorentz invariant Pseudorapidity η and rapidity y η = 1 p + pz ln = ln 2 p p z ( tan θ 2 y = 1 E + pz ln = 1 x1 ln 2 E p z 2 x 2 ) (= y if m = 0) y and p T are invariant under longitudinal boosts Particle production in hadron colliders is roughly constant in y Kerstin Tackmann (DESY) Physics at the LHC (1) 21 / 38

Implication of running at lower s Lower s need larger x to have the same available energy Production of high-mass objects more difficult at lower s More luminosity needed for discovery of new particles In particular for gg induced processes (like Higgs production) Relative behavior of signal and background processes also important Ratio of parton luminosities at 7/14 TeV (from James Stirling) 1 Integrated Luminosity, fb 12 10 8 6 4 ATLAS Preliminary (Simulation) 5 σ s=7 TeV 5σ s=8 TeV 3 σ s=7 TeV 3σ s=8 TeV 95% CL s=7 TeV 95% CL s=8 TeV 2 0 100 120 150 200 300 400 500 m H [GeV] Kerstin Tackmann (DESY) Physics at the LHC (1) 22 / 38

Production cross sections at hadron colliders Many orders of magnitude between Higgs/New Physics and QCD backgrounds Kerstin Tackmann (DESY) Physics at the LHC (1) 23 / 38

The World of LHC... CMS multipurpose 3000 physicists 184 institutions 38 countries + smaller earldoms: LHCf, TOTEM, Moedal LHCb b physics 730 physicists 54 institutions 15 countries ATLAS multipurpose 3000 physicists 177 institutions 38 countries Alice heavy ion physics 1300 physicists 130 institutions 35 countries Kerstin Tackmann (DESY) Physics at the LHC (1) 24 / 38

Alice Study of quark-gluon plasma Dedicated to heavy ion collisions, but also taking data during pp collisions L3 solenoid Tracking: Large TPC, Si pixel, drift and microstrip detectors PbWO 4 crystal and Pb/scint calo Particle id: RICH, TRD and TOF 16 26 m 10 ktons Kerstin Tackmann (DESY) Physics at the LHC (1) 25 / 38

LHCb b physics and CP violation (indirect search for new physics) forward spectrometer dipole magnet Tracking: 21-layer Si microstrip, straw tubes Pb/scint and Fe/scint calorimeters Particle id: 2 RICH systems multiwire proportional chamber µ system 10 13 21 m 5.6 ktons Kerstin Tackmann (DESY) Physics at the LHC (1) 26 / 38

CMS 4 T solenoid magnet Tracking: Si pixel, microstrip PbWO 4 crystals and Fe/scint calos µ chambers in return yoke 15 22 m 12.5 ktons Kerstin Tackmann (DESY) Physics at the LHC (1) 27 / 38

ATLAS 2 T solenoid magnet Tracking: Si pixel, microstrip, straw tubes Transition radiation for e ± id Pb/LAr and steel/scint, Cu/LAr calos µ chambers in 0.4 T toroid field 24 45 m 7 ktons Kerstin Tackmann (DESY) Physics at the LHC (1) 28 / 38

Particle identification (at ATLAS and CMS) Electron Track in tracking system Shower in electromagnetic calorimeter No (little) energy in hadronic calorimeter Kerstin Tackmann (DESY) Physics at the LHC (1) 29 / 38

Particle identification (at ATLAS and CMS) Photon No track or conversion vertex in tracking system 40% of photons convert into e + e due to high material budget Shower in electromagnetic calorimeter No (little) energy in hadronic calorimeter Kerstin Tackmann (DESY) Physics at the LHC (1) 30 / 38

Particle identification (at ATLAS and CMS) Hadron (Jet = hadronic shower) Tracks in tracking system (from charged component) Shower in hadronic and electromagnetic calorimeter b-jet Origin of tracks offset with respect to pp interaction vertex Kerstin Tackmann (DESY) Physics at the LHC (1) 32 / 38

Particle identification (at ATLAS and CMS) Hadron (Jet = hadronic shower) Tracks in tracking system (from charged component) Shower in hadronic and electromagnetic calorimeter τ -jet Collimated, 1 or 3 tracks in tracking system 1 track: can be an electron or muon in case of τ e(µ)νν Kerstin Tackmann (DESY) Physics at the LHC (1) 33 / 38

Particle identification (at ATLAS and CMS) Muon Track in tracking system Little energy deposited in electromagnetic calorimeter Track in muon system Kerstin Tackmann (DESY) Physics at the LHC (1) 34 / 38

Particle identification (at ATLAS and CMS) Neutrino (E miss T ) No signal in any subdetector Transverse energy imbalance in the event Kerstin Tackmann (DESY) Physics at the LHC (1) 35 / 38

Trigger Only a small fraction of events can be saved and processed with available CPU and space. Trigger system identifies interesting events in two to three steps: Level-1: hardware-based trigger using specially designed electronics, slower subsystems store data in pipeline High-level (in ATLAS Level-2 and Event Filter): software-based using large computing farms, fast algorithms and/or algorithms close to offline reconstruction Dedicated trigger chains for different types of objects (leptons, photons, jets,... often combining different objects in the high-level trigger) Kerstin Tackmann (DESY) Physics at the LHC (1) 36 / 38

Computing Reconstruction, analysis and simulation of data is performed on a worldwide grid of computing centers, organized as a hierachical system Tier-0 Tier-1 Tier-2 Data recording Initial data reconstruction Data distribution Permanent storage Reprocessing Analysis Simulation Simulation End-user analysis Kerstin Tackmann (DESY) Physics at the LHC (1) 37 / 38

Future LHC upgrades and data taking Shutdowns for upgrade of accelerators and detectors scheduled High luminosity (HL) LHC (after LS3) currently under discussion in Europe, the US,... Kerstin Tackmann (DESY) Physics at the LHC (1) 38 / 38