The LHC machine in Run-2, and physics highlights from the ATLAS experiment. Jamie Boyd (CERN) Erice School (14-23 June 2018)

Transcription

1 The LHC machine in Run-2, and physics highlights from the ATLAS experiment Jamie Boyd (CERN) Erice School (14-23 June 2018) 1

2 Preamble I am a CERN scientist working on the ATLAS experiment for the last >10 years Trigger, data-preparation, SUSY, For 2016/2017 I was appointed as the LHC programme coordinator Coordinating the LHC schedule Acting as an interface between the LHC experiments and the accelerator teams In this talk I try to take advantage of these two items to give a brief overview of the highlights and prospects for the LHC and for ATLAS 2

3 The LHC machine in Run-2 3

4 4

5 Luminosity given by: Luminosity L = n b N 1 N 2 g f rev 4p b * e n F(f, b *,e,s s ) Maximize by: -increase number of bunches (n b ) -increase number of protons per bunch (N 1/2 ) -decrease beam size at collision point (emittance: ε n, squeeze: β*) -increase geometric factor F pileup related to lumi/bunch size of beam at collision point ~15μm crossing angle needed to avoid parasitic collisions and long range beam-beam effects reduces luminosity. 5

6 Brief summary of LHC Run-2 In 2015 the LHC re-started operations after a 2 year shutdown Nearly doubled collision energy 8 TeV -> 13 TeV Reduced the bunch spacing from 50 ns -> 25 ns Run-2 of the LHC scheduled for

7 Year Brief summary of LHC Run-2 peak luminosity (x10 34 cm -2 s -1 ) integrated luminosity (/fb) Stable beams fraction (%) main challenges ~4 ~30% cryogenics electron cloud radiation to electronics ~50% leak in SPS beam dump (levelled)** 50 ~50% losses in cell 16L2 due to accidental air intake. mitigated Peak luminosity by running ~0.5x10 34 with cm -2 fewer s -1 (half bunches. design) (so far), 60 (expected) ~50% (so far..) losses in cell 16L was a challenging year for the accelerator many systems needed commissioning cryogenics much more strained due to electron cloud induced heating with 25 ns bunch spacing radiation effects to electronics reduced machine availability for part of the year Year ended with 1 month very successful Pb-Pb run 7

8 Year Brief summary of LHC Run-2 peak luminosity (x10 34 cm -2 s -1 ) integrated luminosity (/fb) Stable beams fraction (%) main challenges ~4 ~30% cryogenics electron cloud radiation to electronics ~50% leak in SPS beam dump (levelled)** 50 ~50% losses in cell 16L2 due to accidental air intake. mitigated Peak luminosity by running ~0.5x10 34 with cm -2 fewer s -1 (half bunches. design) (so far), 60 (expected) ~50% (so far..) losses in cell 16L2 In 2016 many improvements to the system allowed an impressive increase in the availability of the machine, and in the fraction of the time in stable beams (30%->50%) Reaching same availability as the best year of LEP, with much more complex (super conducting) machine! Very impressive! Design luminosity reached and passed up to 1.5x10 34 cm -2 s -1 (50% above design) Year ended with 1 month p-pb run covering two collision energies! 8

9 Year Brief summary of LHC Run-2 peak luminosity (x10 34 cm -2 s -1 ) integrated luminosity (/fb) Stable beams fraction (%) main challenges ~4 ~30% cryogenics electron cloud radiation to electronics ~50% leak in SPS beam dump (levelled)** 50 ~50% losses in cell 16L2 due to accidental air intake. mitigated Peak luminosity by running ~0.5x10 34 with cm -2 fewer s -1 (half bunches. design) (so far), 60 (expected) ~50% (so far..) losses in cell 16L2 luminosity leveled at 1.5e34 by beam separation crossing angle reduced in discrete steps during fill In had to run with fewer, but higher intensity bunches due to beam losses. High luminosity, but too high pileup for experiments. Luminosity levelled (by separating beams) to keep pileup at a high but manageable level. 9

10 Year Brief summary of LHC Run-2 peak luminosity (x10 34 cm -2 s -1 ) integrated luminosity (/fb) Stable beams fraction (%) main challenges ~4 ~30% cryogenics electron cloud radiation to electronics ~50% leak in SPS beam dump (levelled)** 50 ~50% losses in cell 16L2 due to accidental air intake. mitigated Peak luminosity by running ~0.5x10 34 with cm -2 fewer s -1 (half bunches. design) (so far), 60 (expected) ~50% (so far..) losses in cell 16L2 β* reduced at end of fill: 30cm -> 27cm -> 25cm increases the luminosity at the end of the fill. 10

11 Comparison of key LHC parameters Design Run Energy (TeV) 14 7/8 13 Bunch Spacing (ns) Bunch Intensity (x10 11 ppb) # bunches Emittance (μm) β* (cm) >25 Crossing angle (μrad) Peak Lumi (cm - 2 s -1 ) > > x x x x x10 34 (levelled) Peak pileup Performance steadily increasing, main parameters better than design except energy and number of bunches. 2.0x

12 High Luminosity (HL)-LHC Design Run-1 Run-2 Energy (TeV) 14 7/8 13 Bunch Spacing (ns) Bunch Intensity (x10 11 ppb) # bunches Emittance (μm) β* (cm) >25 Crossing angle (μrad) Peak Lumi (cm - 2 s -1 ) > x x x10 34 Peak pileup HL-LHC > >? 5.0x10 34 (7.0x10 34 ) 150 (200) nearly double bunch intensity, following complete upgrade of injector chain significantly more squeezed beam, due to replacement of final focusing magnets luminosity levelled to give acceptable pileup level. HL-LHC project designed to give 3000/fb in ~10 years. Very large pileup requires major upgrade of ATLAS/CMS detectors in LS3 ( ). 12

13 Incredible flexibility of the LHC In Run-2 the LHC has carried out a number of special runs on top of the usual high luminosity, high energy p-p, p-pb, andpb-pb collisions Forward physics runs with special optics * (~parallel beam at IP, no focusing) for elastic scattering measurements in the coulomb-nuclear interference region A pilot physics run colliding Xe ions (~6hrs of stable beam from ~16hrs of machine time) Taking advantage of having Xe ions in the injector chain (for the fixed target programme) Useful for both heavy ion physics and machine physics Lower energy run 5 TeV as reference data for Pb-Pb collisions (same energy as nucleon collision energy in Pb run) Low pileup pp running for ATLAS/CMS Important for precision W-boson physics, such a W-mass measurement Very long fills for efficient use of machine (>70% time in stable beams) 13

14 An example of the precision of the LHC In November 2016 during the p-pb run, we had very long fills (>24hrs). This allowed the effect of the tidal forces on the LHC ring to be observed (red) and compared to the old model from LEP (blue) magnitude 7.8 earthquake in New Zealand! The beam energy is known to ±0.1% from the magnetic model, and validated to 0.7% using the orbit in p-pb collisions. Phys. Rev. Accel. Beams 20, (2017) 14

15 (my) Key messages on LHC LHC is running extremely well, exceptional availability, and pushing luminosity performance Every year, unexpected challenges. But (so far) mitigated by clever ideas/tricks Design and construction of the LHC and injector complex very high quality, allows a lot of flexibility to go beyond the design in many ways! Testament to the many brilliant people who worked on this We are lucky to have such an amazing tool Bodes well for the future of our science 15

16 ATLAS Run-2 Highlights Briefly covering: - Higgs physics - Searches for New Physics - Standard Model measurements 16

17 The ATLAS detector 17

18 The ATLAS detector Different detectors allow to efficiently reconstruct and identify: - electrons, muons, photons, - hadronic jets (initiated from quarks or gluons) (tagging those from b-quarks), - hadronic tau decays -missing transverse energy (MET), can come from SM neutrino, weakly interacting new physics particles or detector effects Online selection of interesting events (trigger) very big challenge. Bunch crossing rate 40Mhz and write ~1kHz of events to tape for offline analysis. Sophisticated hardware and software trigger needed to select events of interest. 18

19 Run-2 pileup in ATLAS The average pileup in the Run-2 dataset is ~33. But with a peak above 60, mostly coming from 2017 running with fewer, high intensity bunches. In 2018 running with more bunches allows higher luminosity, but with less pileup (much better for physics!) LHC design peak pileup A zoom in of an event in ATLAS showing 30 reconstructed vertices. This event contains a Z->μμ process and many soft QCD interactions. 19

20 ATLAS physics performance at high pileup A huge effort has gone into making the reconstruction of particles in ATLAS robust against pileup. Keeping high efficiency, and good resolution even at a pileup of 60 is very challenging. Muon efficiency very high, and stable with pileup, and well reproduced by simulations. Electron energy scale stable with pileup and well reproduced by simulation. 20

21 ATL-PHYS-PUB Run-2 pileup in ATLAS a new regime 21

22 Brief reminder of LHC Higgs physics Production: 22

23 Brief reminder of LHC Higgs physics Decay: Mass resolution very good (1-2%) for: H->ZZ*->4l H->γγ Much worse(10-20%) for: H->bb H->ττ H->WW* A. Hoecker (CERN) 23

24 Brief history of Higgs physics in ATLAS Run-1 ( s = 7 and 8 TeV) Discovery of a Higgs like particle 24

25 Brief history of Higgs physics in ATLAS Run-1 ( s = 7 and 8 TeV) Higgs discovery 25

26 Brief history of Higgs physics in ATLAS Run-1 ( s = 7 and 8 TeV) 5σ in individual channels (including H->WW*), and first detailed studies 26

27 Brief history of Higgs physics in ATLAS Run-1 ( s = 7 and 8 TeV) First differential measurements, precise mass measurement (combined with CMS), and spin/cp studies 27

28 Brief history of Higgs physics in ATLAS Run-1 ( s = 7 and 8 TeV) First global coupling analysis (ATLAS+CMS). Overall μ-value 1.09±

29 Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) re-discovering the Higgs at 13 TeV 29

30 Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) re-discovering the Higgs at 13 TeV 30

31 Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) b b e, μ, ν e, μ, ν Evidence for H->bb with VH production 31

32 Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) NEW!! Precise differential cross sections (H->γγ/ZZ*) 32

33 NEW!! arxiv: Precise differential measurements Measure the differential cross section as function of H pt, y, Njets and jet pt using the two high purity channels H->γγ, H->ZZ*->4l. Combining these channels to give the best precision. Results compared to various theoretical predictions. Generally data well described by predictions at current level of precision. 33

34 ATLAS-CONF Candidate W(->eν) H(->4μ) event. The 4 muons (red lines) have a mass of GeV, with the electron (green) and MET consistent with a W decay. S/B~7 for such an event! μ μ e μ μ 34

35 NEW!! Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) Observation of H->ττ in ATLAS CMS observed H->ττ in

36 NEW!! ATLAS-CONF H->ττ Observation H->ττ crucial for demonstrating Higgs coupling to leptons, and second generation fermions. Complex analysis using both leptonic and hadronic τ-decays (had-had, lep-had, lep-lep channels). To control backgrounds have selections targetting VBF production and high-pt Higgs production. Main background is Z->ττ, Higgs signal is ~x1000 smaller and is close in di-tau mass. Robust cut-based analysis making use of MVA to estimate the Higgs mass. Combined with Run-1 this gives a significance of 6.4σ(5.4σ exp.). Measured cross section compatible with SM. 36

37 NEW!! ATLAS-CONF H->ττ Observation H->ττ observation along with the fact that we do not see evidence for H->μμ, implies that the Higgs does not couple equally to leptons. Very different coupling from what has been seen so far arxiv: Search for H->μμ No sign of an excess of events at the Higgs mass 37

38 Inspired by G. Salam s talk at LHCP 2018 Observation of H->ττ The Standard Model Gauge interactions, well studied in last 50 years Yukawa coupling (Higgs to fermions) not a gauge interaction. Never studied before. H->ττ observation, first time we can really study this! Higgs potential. Need HH production to study this! Gauge interaction with scalars. Similar to what we have seen before 38

39 NEW!! Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) Observation of tth production in ATLAS CMS observed tth production at similar time 39

40 NEW!! arxiv: Discussed in talk of M. Llacer on Friday Observation of tth production The tth final state is very heavy, so the cross section is low (~1% of inclusive Higgs production), and it is a complex final state. To search for this a large number of channels are combined with semi-leptonic and dileptonic decays of the tops, and considering Higgs decays to: bb, ZZ*, WW*, ττ, γγ Analysis uses sophisticated techniques (BDTs) to improve sensitivity for the different challenging final states, with very small signals. Using 36/fb of data for H->WW*,ττ,bb and 80/fb for H->γγ,ZZ*, combining these gives 5.9σ (Obs.), 4.9σ (Exp) significance. Combining with Run-1 gives 6.3σ/5.1σ. Measured μ-value = 1.3±0.3 consistent with SM (1σ high). (First ATLAS paper including the full 2017 dataset) 40

41 NEW!! arxiv: Observation of tth production (mostly) Indirect measurement of top-higgs coupling from contributions in loop (mostly from gluon fusion production) top 41

42 NEW!! arxiv: Observation of tth production Direct measurement of top-higgs coupling from tth production measurement (μ=1.3±0.3 =>κ t /κ t SM =1.15±0.15) (added by me for illustration) (mostly) Indirect measurement of top-higgs coupling from contributions in loop (mostly from gluon fusion production) top With current precision fully consistent, constraining possible new physics contributions to the loop in gluon fusion Higgs production. 42

43 Higgs physics in ATLAS: Current/future status A. Hoecker (CERN) Next steps in Higgs physics at ATLAS: - Observe H->bb (largest Higgs branching fraction) - Evidence for rare decay H->μμ (probe 2 nd generation coupling) - More precise differential measurements (new physics can show up as deviation at high pt) -Constrain H->invisible branching ratio (Higgs portal to Dark Matter) A very important measurement is the Higgs self coupling (measured by di-higgs production), first studies suggest with the full 3000/fb at the HL-LHC HH production could be measured but without enough precision to determine the self-coupling more study and innovation needed here! 43

44 New physics searches in ATLAS ATLAS has a huge programme of searches for new physics. Covering: - Supersymmetry - New gauge bosons - Vector-like quarks - Dark matter particles - Extra Higgs bosons - Unconventional signatures (long lived particles etc..) Can only show two examples here. Bottom line, despite a very thorough search programme, no significant excess observed. From these searches, strong limits have been placed on new physics models. For some models with strongly interacting new particles we reach a point where more luminosity will not improve limits (or discovery potential), higher energy needed. However for many cases (especially for weakly interacting new particles) much more data is needed to get to the ultimate reach. In addition new techniques and clever ideas are needed to increase sensitivity in many places. 44

45 ATLAS-CONF Searches for heavy gauge bosons (W ) Search for a heavy W-like boson (W ) decaying to an e+ν or μ+ν. Key is efficient reconstruction of very high pt leptons with good momentum resolution. Analysis looks for excess of events at high transverse mass (mt) (calculated from lepton pt and missing transverse momentum. Distribution of mt for electron and muon channel. Example signals shown. Considerably worse resolution for muon channel limited by muon detector alignment, position resolution and magnetic field very high pt tracks too straight to measure curvature well. Exclude SSM W with masses up to 5.6 TeV. 45

46 ATLAS-CONF High mt event in electron channel Searches for heavy gauge bosons (W ) Electron pt=1.1 TeV MET = 1.1 TeV mt = 2.2 TeV 46

47 arxiv: Search for Higgsino production Higgsino, SUSY partner of Higgs, expected to be light in natural SUSY models. Triplet of states with very small mass splitting: Decay via virtual Z or W can give leptons and MET, but very low pt. Look for events with high pt ISR jet to boost signal system increase lepton pt and MET in detector. Lepton pt still very low push detector to limits to utilize lowest possible pt electrons and muons. Χ ± 1 Χ 0 2 Δm ~ a few GeV Z* W* Χ 0 1 For pure Higgsino mass splitting from radiative corrections and a few hundred GeV. Even small mixing with wino/bino gives bigger splitting up to ~10 GeV even for higgsino dominated states. 47

48 arxiv: Search for Higgsino production Higgsino, SUSY partner of Higgs, expected to be light in natural SUSY models. Triplet of states with very small mass splitting: Decay via virtual Z or W can give leptons and MET, but very low pt. Look for events with high pt ISR jet to boost signal system increase lepton pt and MET in detector. Lepton pt still very low push detector to limits to utilize lowest possible pt electrons and muons. pt> 4 GeV pt> 4.5 GeV Exclude higgsino with mass up to ~140 GeV, going beyond LEP limit of ~100 GeV for the first time. Sensitive for mass splittings down to 3 GeV. 48

49 Standard Model Measurements ATLAS has a large programme of SM cross section measurements, including very precise W,Z and top cross-sections, di-boson, jet cross sections etc Luminosity precision ~2.5% from detailed van der Meer scan analysis, much better than originally thought possible. 49

50 arxiv: Precise W/Z cross sections (7 TeV) Impressive sub-% level experimental uncertainty with 1.8% luminosity uncertainty. Ratios (where luminosity cancels) are powerful tests, can constrain PDFs, and test lepton universality. Source W x-sec Z x-sec Lepton 0.30% 0.34% MET 0.25% - Background 0.38% 0.08% Theory Modeling 0.18% 0.22% Data statistics 0.04% 0.08% Total experimental 0.60% 0.43% Luminosity 1.8% 50

51 arxiv: Precision W-boson mass measurement W-boson mass fundamental SM parameter, with important sensitivity in EWK fit. Measure the W mass using the pt spectra of the lepton (e,μ), or the mt spectra, by fitting templates with different mass hypotheses to the corrected-data. Need very precise understanding of experimental systematic uncertainties, lepton efficiency, scale and resolution, but also resolution on the hadronic recoil which gives MET). Pileup has a big effect on this. Important theory systematics from PDFs (extrapolating W pt from measured Z pt) Example distribution (pt of ve muons) used to extract W-mass. Comparison of the fitted W mass in different channels (+/-/e/μ) /methods (mt/pt) 51

52 arxiv: Precision W-boson mass measurement W boson mass fundamental SM parameter, with important sensitivity in EWK fit. Measure the W mass using the pt spectra of the lepton (e,μ), or the mt spectra, by fitting templates with different mass hypotheses to the corrected-data. Need very precise understanding of experimental systematic uncertainties, lepton efficiency, scale and resolution, but also resolution on the hadronic recoil which gives MET). Pileup has a big effect on this. Important theory systematics from PDFs (extrapolating W pt from measured Z pt) Result with precision at same level as best single experiment (CDF) (19MeV), compatible with 52 world average, but slightly lower, improves compatibility with expectation from EWK fit.

53 arxiv: Precision W-boson mass measurement W boson mass fundamental SM parameter, with important sensitivity in EWK fit. Measure the W mass using the pt spectra of the lepton (e,μ), or the mt spectra, by fitting templates with different mass hypotheses to the corrected-data. Need very precise understanding of experimental systematic uncertainties, lepton efficiency, scale and resolution, but also resolution on the hadronic recoil which gives MET). Pileup has a big effect on this. Important theory systematics from PDFs (extrapolating W pt from measured Z pt) Short term prospects: Hope to use low-pileup data taken in 2017/8 to improve precision to ~10-15 MeV level! Result with precision at same level as best single experiment (CDF) (19MeV), compatible with 53 world average, but slightly lower, improves compatibility with expectation from EWK fit.

54 ATLAS Summary ATLAS experiment is running very well, and able to produce high quality physics with the high pileup Run-2 data Large luminosity, high quality dataset allows: More and more precise probing of the Higgs boson Searches for new physics, no surprises seen yet Searches cover a large range of signatures ranging from ultra low pt to ultra high pt Very precise Standard Model measurements Increased dataset, improved detector and clever new ideas will allow us to push the Standard Model further in the coming years! 54

55 Backup slides 55

56 LHC 16L2 : Air inlet as most probable cause Situation at the end of first BS thermal cycle to 80 K (No pumping though pumping port) Cold bore (2 K) Beam screen (80 K) Plug-in module (80 K) Cold bore (2 K) Beam screen (80 K) Surface covered with N2, O2, Ar H2O coverage nof affected by thermal cycle Thermal anchor (50 K) Pumping port valve (293 K) Results from gas analysis in 16L2 Clear signature of atmospheric gas

57 Some LHC innovations introduced in Run-2 BCMS (low emittance) beam Clever tricks in the injectors to produce beam with lower emittance - ~30% gain in luminosity ATS optics Clever optics using quadropoles in the arc to help with the squeeze of the beam at the IP needed for HL-LHC but already being used Reducing the crossing angle during the fill Crossing angle to avoid long range beam-beam effects close to IP. As beam intensity falls during the fill the crossing angle can be reduced to keep the same dynamic aperture. Increases luminosity at end of fill. Complex procedure: to do this in a safe way, some collimators need to move with the crossing angle Needed for HL-LHC scheme Squeezing the beam during the fill (β* levelling) Squeeze the beams more as the fill progresses. As beam intensity falls during the fill, gain margin to squeeze more, and increase luminosity. Complex procedure: to do this in a safe way, some collimators need to move, and need to make sure the orbit in LHCb and ALICE not effected Needed for HL-LHC scheme 57

58 LHC 1232 main dipoles of 15 m each that deviate the beams around the 27 km circumference 858 main quadrupoles that keep the beam focused 6000 corrector magnets to preserve the beam quality Main magnets use superconducting cables (Cuclad Nb-Ti) A provides a nominal field of 8.33 Tesla Operating in superfluid helium at 1.9K 58

59 Electron cloud in the LHC Avalanche effect with 25ns beam. Causes beam induced heating, and hence can limit bunch intensity. Running with 8b4e (gaps in 25ns trains) dramatically reduces the heat load from e-cloud 59

60 Incredible flexibility of the LHC In Run-2 the LHC has carried out a number of special runs on top of the usual high luminosity, high energy p-p, p-pb, andpb-pb collisions Forward physics runs with special optics 1km and 2.5km β* (~parallel beam at IP, no focusing) for elastic scattering measurements in the coulomb-nuclear interference region Precise measurements of the cross-section and ρ-parameter by TOTEM and ATLAS(ALFA) detectors very close to the beam (~3σ) A pilot run colliding Xe ions (~6hrs of stable beam from ~16hrs of machine time) Taking advantage of having Xe ions in the injector chain (for the fixed target programme) Used as a machine development for running with lighter ion species (e.g. for collimation studies) Useful physics for all experiments despite low luminosity 5 TeV pp run as reference data for Pb-Pb collisions (same energy as nucleon collision energy in Pb run) Low pileup pp running for ATLAS/CMS Important for precision W-boson physics, such a W-mass measurement Very long fills for efficient use of machine TOTEM result shows surprising drop of ρ-parameter at increasing energy 60

61 Incredible flexibility of the LHC In Run-2 the LHC has carried out a number of special runs on top of the usual high luminosity, high energy p-p, p-pb, andpb-pb collisions Forward physics runs with special optics 1km and 2.5km β* (~parallel beam at IP, no focusing) for elastic scattering measurements in the coulomb-nuclear interference region Precise measurements of the cross-section and ρ-parameter by TOTEM and ATLAS(ALFA) detectors very close to the beam (~3σ) A pilot run colliding Xe ions (~6hrs of stable beam from ~16hrs of machine time) Taking advantage of having Xe ions in the injector chain (for the fixed target programme) Used as a machine development study for running with lighter ion species (e.g. for collimation studies) Useful physics for all experiments despite low luminosity (many Xe results shown at Quark matter conference in May) 5 TeV pp run as reference data for Pb-Pb collisions (same energy as nucleon collision energy in Pb run) Low pileup pp running for ATLAS/CMS Important for precision W-boson physics, such a W-mass measurement Very long fills for efficient use of machine 61

62 Incredible flexibility of the LHC In Run-2 the LHC has carried out a number of special runs on top of the usual high luminosity, high energy p-p, p-pb, andpb-pb collisions Forward physics runs with special optics 1km and 2.5km β* (~parallel beam at IP, no focusing) for elastic scattering measurements in the coulomb-nuclear interference region Precise measurements of the cross-section and ρ-parameter by TOTEM and ATLAS(ALFA) detectors very close to the beam (~3σ) A pilot run colliding Xe ions (~6hrs of stable beam from ~16hrs of machine time) Taking advantage of having Xe ions in the injector chain (for the fixed target programme) Used as a machine development study for running with lighter ion species (e.g. for collimation studies) Useful physics for all experiments despite low luminosity (many Xe results shown at Quark matter conference in May) 5 TeV pp run as reference data for Pb-Pb collisions (same energy as nucleon collision energy in Pb run) Low pileup pp running for ATLAS/CMS Important for precision W-boson physics, such a W-mass measurement Very long fills for efficient use of machine (70% time in stable beams) 62

63 HL-LHC luminosity profile Luminosity leveled at 5e34 to limit pileup to manageable level for experiments. 63

64 64

65 65

66 66

67 Heavy ion physics R AA is the yield in PbPb scaled by pp, normalized to take into account the geometry. For event with low activity (Npart) scaled yields are similar (R AA ~1), but for more energetic events a suppression in yield in PbPb compared to pp is observed for hadrons, jets, and J/Ψ, but not for Z bosons. This is attributed to interactions and energy loss in the medium (QGP) jet quenching for jets. Z bosons are not strongly interacting and therefore do not show quenching effects. 67

68 ATLAS detector upgrades In order to be able to cope with the pileup and radiation that HL-LHC will provide, large parts of the detector will need to be upgraded in LS3. Aim is to keep the same performance (or improve) compared to the original detector operating at low pileup. Large scale ~250M CHF project, dominated by replacing the full tracker. 68

69 A. Hoecker (CERN) 69

70 ATLAS physics performance at high pileup A huge effort has gone into making the reconstruction of particles in ATLAS robust against pileup. Keeping high efficiency, and good resolution even at a pileup of 60 is very challenging. Missing energy and jet reconstruction particularly effected by the contribution from soft pileup interactions. Missing transverse energy a key variable in SUSY and Dark Matter searches. Resolution starts to degrade at highest pileup (>50). Jet multiplicity grows with pileup. But requiring most of the jet energy comes from the same vertex, makes the multiplicity much 70 more stable with pileup.

71 ATL-PHYS-PUB Run-2 pileup in ATLAS With the 2017 data we reached a regime where the probability of multiple hard scatter processes, from separate pp interactions, in the same event becomes relevant. Look for events with 2 leptonically decaying Z bosons, and plot difference in production point (z0). See 13 events where the 2 Z candidates originate from points separated by at least 5mm. Broadly consistent with expectations. Such events can present a background for certain physics analyses, and need to be taken into account. Using tracking information to require the hard-scatter processes come from the same vertex reduces such backgrounds by a factor of ~100. But for processes involving photons this can not be done. 71

72 Rare Higgs decays ATLAS searches for rare Higgs decays not expected to be seen so far. Examples include: H->μμ, Zγ H->ργ,Φγ (constraining coupling to 1 st and 2 nd generation quarks) H->cc Setting limits Mode Interest SM x-sec Limit (Obs. / Exp.) H->μμ 2 nd generation lepton coupling /2.9 x SM H->Zγ New particles in loop smilar to γγ before Z->ll requirement 6.6 / 5.2 x SM H->invisible Higgs portal to Dark Matter H->ZZ->4ν - only BF <28% / 31% H->ργ H->Φγ 1 st generation quark coupling 2 nd generation quark coupling H->cc 2 nd generation quark coupling / 150 x SM 72

73 Brief history of Higgs physics in ATLAS Run-2 ( s = 13 TeV) NEW!! Higgs mass measurement with Run-2 data 73

74 NEW!! arxiv: Higgs Mass measurement Measured in the 2 high resolution channels H->γγ, H->ZZ*->4l Many detailed studies carried out for Run-1 analysis (using Z->ll/llγ and J/ψ decays) demonstrated excellent understanding of γ, e, μ energy scale and resolution. Reconstructed muon momentum scale from Z, ϒ and J/ψ decays. Understood to 1 per-mille. Reconstructed electron momentum scale from Z and J/ψ decays. Understood to better than 5 per-mille. 74

75 NEW!! arxiv: Higgs Mass measurement Measured in the 2 high resolution channels H->γγ, H->ZZ*->4l Many detailed studies carried out for Run-1 analysis (using Z->ll/llγ and J/ψ decays) demonstrated excellent understanding of γ, e, μ energy scale and resolution. Tension between 2 channels with Run-1 data gone in Run-2. Result is compatible with Run-1 ATLAS/CMS combined result, with similar precision. 75

76 Discussed in talk of M. Llacer on Friday tth production why this is interesting Yukawa coupling (like H->ττ), demonstrates coupling to quarks, but also In SM gluon fusion production of Higgs boson goes through a loop diagram with the top quark the main contribution in the loop: t Therefore, in the SM, the top Yukawa coupling can be indirectly inferred from a combined fit to Higgs measurements in modes dominated by gluon fusion. However, in BSM heavy new particles in the loop can also play a role. Directly measuring the top coupling and comparing with the indirect measurement is therefore a powerful test of the SM. Since the Higgs is too light to decay to top, this is measured in tth production. The tth final state is very heavy, so the cross section is low (~1% of inclusive Higgs production), and it is a complex final state. To search for this a large number of channels are combined with semi-leptonic and di-leptonic decays of the tops, and considering Higgs decays to: bb, ZZ*, WW*, ττ, γγ 76

77 NEW!! arxiv: Discussed in talk of M. Llacer on Friday Observation of tth production A candidate event from the tth(γγ) analysis the mass of the γγ system is GeV The event contains 6 jets, one of which is b-tagged. 77

78 arxiv: Searches for strongly interacting SUSY particles Classic search for heavy squarks and gluinos, look at events with high pt jets and large missing transverse momentum. Main background from Z(νν)+jets and leptonic W/top+jets decays (where lepton is missed in reconstruction or is a hadronic tau). Example signal model: gluino production decaying to jets and MET. Main background Z+jets with Z decaying to neutrinos 78

79 arxiv: Searches for strongly interacting SUSY particles Classic search for heavy squarks and gluinos, look at events with high pt jets and large missing transverse momentum. Main background from Z(νν)+jets and leptonic W/top+jets decays (where lepton is missed in reconstruction or is a hadronic tau). Example signal region plot showing observed data, expected SM background, and what a signal could look like. Example limit plot showing signal parameter space excluded by this search. Exclude gluinos with mass up to ~2 TeV, and neutralinos with mass up to ~0.9 TeV in simplifed model considered. 79

80 SUSY searches 80

81 non-susy searches 81

82 Standard Model Measurements ATLAS has a large programme of SM cross section measurements, including very precise W,Z and top cross-sections, di-boson, jet cross sections etc Luminosity precision ~2.5% from detailed van der Meer scan analysis, much better than originally thought possible. 82