The LHC for Dummies. Jamie Boyd (CERN) acknowledgments to many colleagues!

Transcription

1 The LHC for Dummies Jamie Boyd (CERN) acknowledgments to many colleagues! 1

2 The LHC high energy, high luminosity Physics goals of the LHC dictate High energy (design 14 TeV), to maximize sensijvity to heavy new parjcles High luminosity (design Hzcm - 2 ) to maximize sensijvity for rare processes Both drive design of the LHC dipole magnet the key technological challenge of the LHC. Energy given by magne&c field and accelerator radius (defined by LEP ring). 8.3 T field needed for 14 TeV. Requires super conducjng magnets operated at 1.9K, cooled with superfluid liquid He cryogenics a key challenge (worlds biggest fridge!). 2

3 The LHC high energy, high luminosity Physics goals of the LHC dictate High energy (design 14 TeV), to maximize sensijvity to heavy new parjcles High luminosity (design Hzcm - 2 ) to maximize sensijvity for rare processes High luminosity requirements drives LHC as a p- p collider (cant make enough anj- protons for p- pbar). Drives design of dipole unit with 2 beams pipes with opposite fields in single cryogenic container. 3

4 Pileup High luminosity requirement gives muljple pp interacjons in each bunch crossing ( event ) Called pileup Pollutes detector signals from interesjng interacjon with signals from pileup Huge effort in detector design, and reconstrucjon/analysis to mijgate the effect of pileup Currently run with up to 50 inelasjc interacjon per bunch crossing at the beginning of fills (design 25) Future luminosity upgrades will push the pileup to ~150 and drive the need for upgraded detectors (with improved granularity) 4

5 pp collisions 5

6 Luminosity formula Number of colliding bunches limited by structure of LHC orbit (number of BX, necessary gaps for kicker magnets ) Number of protons per bunch (~10 11 ) Geometric Factor: Reflects lumi reducjon due to length of bunch and crossing angle crossing angle needed to avoid long- range collisions Transverse size of the beam at the InteracJon point Normalised emijance: how much space the beam occupies in the phase space: defined in Injectors Size of the beam at the IP. Defined by LHC opjcs (= magnejc layout). Play with this formula here: hjp://lpc.web.cern.ch/lumi2.html 6

7 Beta funcjon /CMS 7

8 Beta funcjon /CMS The LHC experts are constantly trying to squeeze the beams more in ATLAS/ CMS to give more luminosity. For example β* was 80cm in 2015, 40cm in 2016 and 30cm at the end of

9 Luminosity formula For high luminosity: High bunch intensity (max: 1.25e11p/bunch limited by injectors) High number of bunches (limited by bunch spacing of 25ns) Small beams at interacjon points: - small β* (squeeze from triplet magnets around IP) (limited by aperture in triplet) - low emijance (from injectors) Small crossing angle, short bunches (F- >1) Increasing the number of bunches has the advantage that the pileup does not go up! Play with this formula here: hjp://lpc.web.cern.ch/lumi2.html 9

10 How to make LHC Beams Accelerator chain A single accelerator cannot bring a beam from 0 to 7 TeV. Dynamic range of accelerators is limited (technical reasons) At CERN a serious of accelerators were already present when LHC was designed These accelerators became Injectors for the LHC but also used for other tasks such as providing beam for the fixed target physics programme Beams are prepared in various steps from a LINAC to the injecjon into the LHC In the LHC beams are Accumulated from the injectors unjl the ring is full Accelerated (from 450 GeV to 6.5 TeV) Squeezed at the experiment to reach a small Beta* Brought to collisons A typical (good) physics fill is ~12h long Then the intensity has dropped so much that it is more efficient to re- fill (which takes 3-4 hours on a good day much more if problems occur) Luminosity usually dropped by a factor of ~2 aser 12hrs due to burn- off 10

11 The Accelerator Chain 11

12 The Source (+ RF Quadrupole: 750keV ) 12

13 Linac (II) 50 MeV 180mA 30m 13

14 PSB (The Booster) rings (157m) 1.4 GeV p/ring 14

15 PSB: InjecJon and ExtracJon ExtracJon RecombinaJon - 1 RecombinaJon - 2 To PS 15

16 1959 PS (Proton Synchrontron: 25GeV, 628m) 25 GeV, 628m RF GymnasJcs: Bunches are compressed merged and split (BCMS) In order to get long high intensity low emijance trains in the LHC 16

17 SPS (450GeV, 6.9km)

18 LHC: 26.7km, ~7 TeV

19 How does the LHC fit in this? Time 6.5 TeV 450 GeV InjecJon Ramp Squeeze & Adjust Stable beams for physics Dump & Ramp down = Field in main magnets = Beam 1 intensity (current) = Beam 2 intensity (current) The LHC is built to collide protons at 7 TeV per beam, which is 14 TeV centre of Mass In 2012 it ran at 4 TeV per beam, 8 TeV c.o.m. Since 2015 it runs at 6.5 TeV per beam, 13 TeV c.o.m 19

20 Concepts of bunch trains The bunches are prepared in the injector chain The smaller the accelerator, the less bunches in one ring The next accelerator in the chain usually accumulates the trains or batches of the previous accelerator and packs them as close as possible to longer trains. Bunches need to be transferred from one accelerator to the next. During InjecJon and ExtracJon they are kicked by very fast kicker magnets These magnets have rise and fall- Jmes larger than the space between bunches. During extracjon the magnet needs to have Jme to ramp up without parjcle passing through it During injecjon the InjecJon Kicker needs to ramp up without parjcles already in the larger ring passing through it This implies gaps between the batches and injecjons. In the LHC: 200ns between batches in the SPS. 800ns between injecjons in the LHC. This leads to rather complex filling schemes, especially when collisions should be distributed fairly among all experiments 20

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22 Enemies of the LHC 22

23 Where to get informajon LHC Page 1: hjps://op- webtools.web.cern.ch/vistar/vistars.php?usr=lhc1 Accelerator mode Beam mode Protons per beam Beta* Beam energy (Dipole current) Beam Intensites Beam 1 Beam 2 Comment from Shis crew Various Flags When preparing a physics fill 23

24 Where to get informajon LHC Page 1: hjps://op- webtools.web.cern.ch/vistar/vistars.php?usr=lhc1 Compressed display of previous slide Instant luminosijes of experiments Filling Scheme When the beams collide: Stable Beams 24

25 Decoding the filling scheme name LHC Page 1: hjps://op- webtools.web.cern.ch/vistar/vistars.php?usr=lhc1 min. bunch spacing total number of bunches/ beam number of bunches colliding in ATLAS/CMS number of bunches colliding in ALICE max. LHC bunch train length number of bunches colliding in LHCb number of injecjons/beam Same bunches always collide in ATLAS/CMS but not true for LHCb/ALICE who always get less 25

26 More informajon LPC Website: hjp://lpc.web.cern.ch/ Filling Scheme Display/Info/ Editor Performance Plots A lot of informagon relevant for the daily business of the experiments Luminosity calculator Schedule informajon 26

27 The future 27

28 The future 28

29 The future 29

30 Thank you and have fun with the LHC data! In case of further quesjons: 30

31 Useful reading: HL- LHC: Mike Lamont Summer student lecture (2017): hjps://indico.cern.ch/event/634016/ajachments/ / /hl- LHC- SummerStudents- July17.pdf Hadron colliders: Rende Steerenberg CERN/Fermilab summer school (2017): hjps://indico.cern.ch/event/598530/contribujons/ /ajachments/ / /hadronacc- 1_2017.pdf hjps://indico.cern.ch/event/598530/contribujons/ /ajachments/ / /hadronacc- 2_2017.pdf 31

32 Backup 32

33 Emijance limitajons and BCMS? The limitajon is found at the beginning of the Injector chain: Space charge effects become smaller: Injector Chain: LINAC2! Booster (PSB)! PS! SPS! LHC Space charge lead to a large tune spread in accelerators with low energies The Booster is operajng from 50 MeV to 1.4 GeV: The space charge induced tune spread leads to a emijance blow up since the parjcles approach resonances in the tune diagram. At higher energies (! upgrade of injector complex, but nothing can be done now) At lower bunch intensijes ( this is always possible ) Comparison of Standard and BCMS beam- producjon for LHC (RF gymnasjcs in PS) Standard: 4+2 bunches from PSB injected to PS: split into 3 aser injecjon Net rajo: 6/72 = 1/12 Bunches split twice into 2 before extracjon to SPS BCMS: 4+4 bunches from PSB injected to PS: merged pairwise before split into Net rajo: 8/48 = 1/6 HALF inigal intensity needed 33

34 Where to get informajon LHC Logbook: hjps://op- webtools.web.cern.ch/elogbook/index.php 34

35 Measuring the luminosity Luminosity measurements Need to be done by experiments, crijcal for cross- secjon measurements Desire precise measurements (%- level) Basis are dedicated Van Der Meer scans These are supposed to measure the beam parameters necessary to calculate the luminosijes with high precision and to record the data from dedicated luminosity detectors which can then be calibrated. Difficulty: extrapolajon to normal running condijons An art by itself One of the most discussed topics in the experiments online world Scan beams through each other in x- y and look at rate in dedicated lumi detectors. Use this to determine beam size, which combined with a precise measurement of the beam current gives the luminosity. Calibrate detectors with this. 35

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37 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 superconducjng cables (Cu- clad Nb- Ti) A provides a nominal field of 8.33 Tesla OperaJng in superfluid helium at 1.9K 37

38 Filling the LHC and SaJsfying Fixed Target users = Field in main magnets = Proton beam intensity (current) = Beam transfer To LHC clock- wise or counter clock- wise 450 GeV SPS 26 GeV PS PSB 1.4 GeV 1.2 seconds Time 38

39 Emijance Phase space, occupied by the beam In pracjce: Difficult to generate a beam with a low emijance, and even more difficult to preserve a low emijance in a chain of accelerators with transfers, accelerajon, measurement devices, Momentum (u ) Coordinate (u) 39

40 InjecJon zone (Beam 1) of LHC InjecGon Septum (MSI): A special magnet generajng a strong magnejc field for the incoming beam without disturbing the circulajng beam: Brings injected beam close to circulajng beam InjecGon Kicker (MKI): Kicker magnets have to have very fast rise/fall Jmes, in order not to disturb already injected beam 40

41 Bunch structure much smaller at interacjon point 41

42 Idea of having a hadron collider in the LEP tunnel first discussed in 1984 Brief history of the LHC hjps://jmeline.web.cern.ch/jmelines/the- Large- Hadron- Collider Dec 1994 : ConstrucJon approved April 2007 : Last dipole magnet goes underground Sept 2008 : LHC startup / Incident Nov 2009 : Beams back aser consolidajon Year Energy species bunch spacing delivered lumi TeV pp no bunch trains ~9/ub TeV pp/pbpb 150ns 35/pb TeV pp/pbpb 75/50ns 5/~ TeV pp/ppb 50ns 25/~ LS TeV pp/pbpb 25ns 5/~ TeV pp/ppb 25ns 40/~ TeV pp 25ns ~25/~+ LHC also delivers special runs for luminosity measurements (vdm scans), and for diffracjve measurements (high β*, low- luminosity). Extremely flexible accelerator! 42

43 LHC performance today The luminosity in LHCb and ALICE is much reduced by having a larger β* (3m in LHCb and 11m in ALICE). The luminosity is then levelled by separajng the beams to give a constant pileup (~1 in LHCb, ~0.001 in ALICE). dashed lines indicate design values! 43

44 An example of the 2016 LHC schedule: ~20weeks of high luminosity pp physics ~4 week ion run ~2.5 weeks technical stops ~3 weeks machine developments 44

45 The Energy in the LHC beam The energy in one LHC beam at high energy is about 320 Million Joules This corresponds to the energy of a TGV engine going at 150 km/h... but then concentrated in the size of a needle 45

46 Machine ProtecJon The LHC is (probably) the first accelerator that has the power to destroy itself! HUGE effort in the design and operajon for machine protecjon. Many interlocks to dump the beam if anything looks like its going wrong Relying on expansive beam instrumentajon (beam posijon monitors, beam loss monitors) Complex set of collimators to clean the beam halo (off- momentum or off- orbit protons) and to protect crijcal machine/experiment components. Requires an ultra reliable beam- dump system. Most feared scenario is an asynchronous beam dump when the beam extracjon kicker fires at the wrong Jme, spraying the high energy beam around. LHC designed for 1/year but never occurred at high energy/intensity. 46

47 How well do we know the beam energy? Uncertainty on the beam energy can be relevant for precision physics Precise cross secjon for jbar, W, Z producjon ElasJc scajering measurements (ALFA) Recent paper outlining the uncertainty for this Determined from magnejc model of the LHC Cross checked (with less precision) using p- Pb run (different orbits for p and Pb can be used to check the magnejc model) Subtle effects like Jdes negligible effect on the energy (but visible on the orbit) 47

48 How well do we know the beam energy? Effect of the Jdes on the beam energy blue = predicted Precise cross secjon for jbar, W, Z producjon red = measured Uncertainty on the beam energy can be relevant for precision physics ElasJc scajering measurements (ALFA) Recent paper outlining the uncertainty for this Determined from magnejc model of the LHC Cross checked (with less precision) using p- Pb run (different orbits for p and Pb can be used to check the magnejc model) Subtle effects like Jdes negligible effect on the energy (but visible on the orbit) 7.8 magnitude earthquake in New Zealand! 48