Luminosity measurement in ATLAS with Diamond Beam Monitor

Transcription

1 University of Ljubljana Faculty of Mathematics and Physics Luminosity measurement in ATLAS with Diamond Beam Monitor PhD topic defense Supervisor Candidate Prof. dr. Marko Mikuž Luka Kanjir October 14th, 2014

2 The Introduction Luminosity general LHC overview absolute luminosity & luminosity monitoring luminosity detectors of ATLAS online luminosity calculator Beam Conditions Monitor geometry and results Diamond Beam Monitor geometry sensors two data acquisition streams algorithms and geometry adjustment for high luminosity measurements future plans Van der Meer scan summary 2

3 Luminosity Typical approach in analysis is to count the number of scattered particles in a given solid angle per unit of time proportionality factor (experimental conditions) used for analysis dn dσ =L dωdt dω aim of analysis dσ differential cross section describes physics dω L luminosity describes experimental conditions 2 1 usually expressed in units cm s measure of interaction rate by collider (for a given physical process) integrated luminosity gives the number of interaction that occur per unit cross-section b 1 (barn) 3

4 Why measure luminosity? luminosity is needed to determine absolute cross-sections measurements the uncertainty of luminosity measurement contributes significantly to the total uncertainty for cross-section dn dσ =L dωdt dω 4

5 Delivered and recorded luminosity delivered luminosity quantitative measure of collisions produced by the collider recorded luminosity produced when the detector was active and recording data 5

6 Characteristics of LHC accelerator protons from hydrogen atoms first accelerated in Linac, then PSB,PS,SPS and finally LHC in SPS protons reach 450 GeV after that injected into LHC Radio-frequency (RF) cavity is the accelerating structure which provides sinusoidal electric potential RF works at approximately 400MHz In one bunch ~ protons it gives about 25 proton- proton collisions per bunch crossing which collides in 4 detectors placed in LHC: ATLAS and CMS 6

7 Single bunch pair luminosity N 1, N 2 proton population of the bunches f rev 2 π r 1 ) frequency of the bunch revolution = khz = ( c γ relativistic factor for LHC=7461= me ϵn normalized emittance- average spread of particle p coordinates in position-and-momentum phase space property of the proton source optical betatron function- defined by the arrangement of the magnets along the design orbit value at Interaction Point (IP) property of magnets used for focusing the beams high luminosity: highly populated bunches of low emittance, colliding at high frequency at location with optical betatron function as low as possible ( strong focusing very close to the IP ) to achieve smaller emittance the area of ellipse in position-momentum phase space should be as small as possible 7

8 Absolute luminosity absolute luminosity can be determined in a number of ways: well calculable processes analyzing recorded data but for a specific processes with known cross-section calculable to highest precision : production of W and Z bosons using collider beam parameters poor precision on ϵ and beam parameters and van der Meer scan van der Meer scan or luminosity scan during the scan, one of the beams is displaced and the rate of interactions measured as a function of displacement this method measures beam parameters, effectively absolute luminosity used for calibration not always useful well calculable processes used after data is recorded ϵ and van der Meer scan special beam parameters not optimal other methods used for measuring luminosity during data taking 8

9 Luminosity monitoring the method is needed to have prompt information about luminosity to control experimental conditions methods for absolute luminosity could be used to luminosity monitoring, but are very unpractical that is why other methods are used for luminosity monitoring: event counting event is a single bunch crossing proton-proton interactions are uncorrelated number of interactions described by the Poisson distribution particle counting relation between luminosity and particle multiplicity proportionality between number of produced particles and number of proton collisions 9

10 Luminosity detectors in ATLAS pseudorapidity is defined as η= ln [tan(θ /2)] minimum bias trigger scintillators (MBTS) - large scintillators, 2.1< η <3.8 large acceptance saturates at high L FCAL used for energy deposition - integration over longer periods of time, 2.5< η <4.9 Zero Degree Calorimeter (ZDC) neutral particle detector with small acceptance, η >8.3 LUCID (Luminosity measurement Using a Cherenkov Integrating Detector) measure individual bunches, 5.6< η <6.0 high background BCM it's main function is safety, η ~

11 Online Luminosity calculator runs with frequency of 1 Hz with three main functions collection of data from LHC and ATLAS sub-detectors calibration of the raw luminosity information, time integration and normalization result publication, used as permanent storage it sends the current luminosity to ATLAS Control Room, preferred luminosity measurement is sent from it directly to LHC 11

12 Beam Conditions Monitor (BCM) 2 2 diamond sensors ( 1x1 cm, contact 0.8x0.8 cm and 500 μm thick) in highly radioactive area ~ 500 kgy in 10 years diamonds are high resistivity, radiation hard, low leakage current (at 1000V ~100 pa) (low noise) and with low dielectric constant 4 modules at the each side of detector (A and C) z=±1.84 m, Δt=6ns, r=5.5 cm and η~4.2 the whole detector is one pad fast and short signal (~2ns,rise time <1ns) timing detector from BCM coaxial cable goes to digitization electronics (NINO circuits) in the toroids 12

13 BCM results here are few plots as the result of BCM luminosity measurements used 1. total integrated luminosity 2. instantaneous luminosity for individual bunches 3. luminosity as a function of BCID 13

14 Diamond Beam Monitor (DBM) diamond and silicon sensors for beam spot, background monitoring and bunch-by-bunch luminosity monitor (aim <1% per BC per LB) 8 telescopes, 6 diamond, 2 silicon, 3+1 at each side of ATLAS with 3 modules per telescope, each module with FEI4B readout chip 3.2< η <3.5, first diamond based tracking detector ~90 cm from IP on both sides, ~70 mrad angle detector will be able to recognize if the particle is from background or from IP trigger, vertex resolution better than 1mm part of pixel detector 2 the active area of module is ~89% (20x16.8 mm ) pixels (80x336), μm pixel size position sensitive detector with time resolution of 25 ns 14

15 DBM, diamond and silicon sensor poly crystalline chemical vapour deposition (pcvd) diamond sensor advantages compared to silicon low leakage current works at room temperature better performance in terms of signal-to-noise ratio made in microwave reactors on non-diamond substrate average grain size increases across pcvd thickness charge collection properties are improved FE-I4 front end chips are used for read out planar silicon pixel sensor single chip (DC) same size 1 n doped silicon with resistivity of 2 to 5 k Ωcm 200μm thick 15

16 DAQ for DBM same read-out drivers (ROD) and back-of-crate (BOC) as insertable beam layer (innermost pixel barrel) ROD-BOC readout and control processor unit data with hit location and TOT value come from FE-I4 to Readout Buffer Input cards in readout system data transfer 160 (320) MB/s 0.5 (1) Mevent/s random L1 trigger DBM data in the IBL (ATLAS-Pixel) event record two possible data acquisition streams: 1 direct from FE chip TDAQ-L1 2 hit OR through HITBUS chip 16

17 1 direct from FE-14 complete information about pixels hit dead time of detector (simple and complex) amount of energy deposited more information prolonged period to track counting possible analyze this stream gives possibility to use track counting method for luminosity measurement possible to distinguish background from particles from IP development aimed for later stage due to complexity 17

18 DAQ for DBM 2 hit OR HITBUS - if there is a hit in maskable FE-I4 region, output is 1, and when there is no hit, output is 0 (simple event counting) no dead time hit no hit information only from each FE-I4 masking of pixels possible chip potentially fast (40 MHz) logical operations between modules of same telescope measure the hit rate or their combinations further used in algorithms to calculate average number of proton collisions per Bunch Colliding IDentifier similar way of determining luminosity as in BCM better for measuring higher expected luminosity due to adaptable regions 18

19 Algorithms use simple event counting the outcome of each event can be either 0 or 1 measurement, r average probability to get 1 (for an event to satisfy algorithm) find out the average number of interactions, μ (Luminosity) per Beam Collision IDentifier (BCID) with detector efficiency, ϵ r and μ are connected with statistical relations called μ- corrections dependent on chosen algorithm conditions for example event OR algorithm: lots of event condition algorithms, but in BCM four of them are used (limited by FPGA resources) these four parallel measurements are called luminosity algorithms: OR counts the number of events that have any hit regi stered in second half of the bunch crossing period each hit marks event as OR event it has high statistics 19

20 Pixel mask masking of pixels (taking just part of active detector plane) possible to adjust the size according to luminosity this means that in hit OR stream we get 1 when the masked pixels are hit, else we get 0 for higher luminosity the size of mask can be smaller μ average number of proton collisions per (BCID) r A probability that the detector is hit (on A or C side) for higher μ,or logic saturates (whole area of detector) reason why do the masking 20

21 Plans What are the next steps? simulation to evaluate the acceptance of the detector estimate the time needed for accumulate enough statistics determine which of the presented algorithms is optimal the size of pixel mask the luminosity data stream will have to be created, originating from BOC extensive work will have to be done to calibrate the detector the useful method for this is the Van der Meer scan mostly used by ATLAS luminosity detectors 21

22 Van der Meer scan the method is used to calibrate the detector determine the visible cross section from L= μ vis f r σ vis beams are moved and for different beam positions, event rate is measured (for both transverse directions, x and y) the scan curves are fitted with Gaussian distributions and effective beam width, Σ x, y, extracted from this and number of protons in bunches we can calculate the luminosity from f rev N 1 N 2 L= 2 π Σx Σy number of protons per bunch is measured by bunch current detectors (FBCT and DCCT) 22

23 Summary better luminosity determination precise physical analysis diamond sensors radiation hardness, low leakage current, fast signal DBM position detector luminosity detector two data streams two ways of measuring tracking detector DBM adjusted to higher luminosity physically and functionally 23

24 Backup 24

25 Luminosity limitations and reduction effects beam beam effects every bunch is a large number of charges represent an electromagnetic potential for other charges appear in head-on collisions emittance growth crossing angle introduced to restrict collisions only to the IP (to avoid parasitic collisions) for crossing angle θ c, luminosity is reduced by factor total beam current luminosity proportional to product N 1 N 2 so that stored energy ~( N 1 + N 2 )must be safely absorbed at the end of each run beam dumping system and the magnet system provide additional limits (max. beam energy and intensities) hourglass effect β function and beam sizes have minimum at the IP and grow, so if collisions occur away from the IP the luminosity is reduced effect is significant if β is small compared to bunch length beam offset if colliding with a small transverse offset, luminosity is reduced 25

26 Bunch population product There are two type of detectors which measure bunch population: DCCT total current measurement with high accuracy (4 detectors) FBCT bunch-by-bunch current measurement (4 detectors) 26

27 Calibration uncertainties beam centering if not centered correctly in vdm scan, luminosity is not observed at the peak, and not equal to the maximum head-on luminosity beam-position jitter real beam separation may be effected by random deviations of the beam position- this induce fluctuations at each scan point vis bunch-to-bunch consistency of σ measurements calibrated σ of machine conditions vis value should be independent emittance growth vdm scan assumes that the transverse emittance of both beams is constant in reality there is a small emittance growth which is visible with slight increase of measured value Σ and in the same time decreasing the peak specific luminosity during the scan Σ increase reduction in the peak interaction rate 27

28 Position of DBM 28

29 Dead time simple dead time after triggering the detector, the DAQ record that event (it takes some time) detector is blind for other three BCID complex dead time if time for recording one event is very long, in can happen that in this time new events occur if number of new events smaller than 16 then each of them will be record one after another, else the detector will not be able to record more than 16 event 29

30 FE-I4 chip specification FE-I4 & FE-I3 30