LHC Lumi Days 201229-Feb-2012CERNMassimiliano Ferro-Luzzi 1 Overview and relative bunch population...

LHC Lumi Days 2012 29-Feb-2012 CERN Massimiliano Ferro-Luzzi 1

Overview and relative bunch Overview and relative bunch population measurementspopulation measurements

Massimiliano Ferro-Luzzi, on behalf of the BCNWGMassimiliano Ferro-Luzzi, on behalf of the BCNWG

An overview– definitions, methods

what is a van der Meer scan (VDM),

what is the beam-gas imaging method (BGI)

– brief historical reminder of Bunch Current Normalization effort The relative bunch population measurements

– FBCT , vs others (BPTX, DCCT, LDM, Beam-Gas (BG) rates

– Effects on cross section

– Analysis of bunch-by-bunch cross-section data Conclusions

Thanks to the LHC groups and to the ALICE, ATLAS, CMS and LHCb collaborations for their support

Special thanks to Vladik Balagura for his detailed work on the bunch-by-bunch cross section analysis

Special thanks to Vladik Balagura for his detailed work on the bunch-by-bunch cross section analysis


What is luminosityWhat is luminosity

R = L

Rate Luminosity Cross section

L = f N1 N2 "the beam overlap"

revolution bunch population in

frequency beam1 and beam2

address with VDM scans or BGI, see later

address with VDM scans or BGI, see later

An absolute bunch charge measurement is needed


Van der Meer’s trickVan der Meer’s trick

Consider single circulating & colliding bunch pair with zero crossing angle

R = L = f N1 N2 1(x,y) 2(x,y) dx dy

With transverse displacements x , y of one beam w.r.t. the other:

R (x , y) = L(x , y) = f N1 N2 1(x-x , y-y) 2(x,y) dx dy

R (x , y) dx dy = f N1 N2 1(x-x , y-y) 2(x,y) dx dy dx dy

= f N1 N2 2(x,y) [ 1(x-x , y-y) dx dy ] dx dy

= f N1 N2 2(x,y) dx dy = f N1 N2

=1

z

=1

x

See refs. [1,2,3,4]See refs. [1,2,3,4]


residual gas

Beam-gas imaging methodBeam-gas imaging method

Again, luminosity

L = fN1N2 "overlap"

Beam interacts with residual gas around the interaction region

Reconstruct beam-gas interaction vertices

=> sample transverse beam profile

measure individually the 1 and 2 and rebuild the overlap

– even much better if combined with beam-beam interaction vertex distributions

– can measure also , bunch charges, hourglass effect, etc...

Strength with respect to van der Meer method:

(a) non disruptive, do not affect the beams !

(b) can run fully parasitically during physics running time

=> potentially smaller systematics uncertainties

Requires: (1)vtx detector resolution smaller (or at least comparable) to the beam sizes(2)residual pressure & acceptance must be adapted to this method

Requires: (1)vtx detector resolution smaller (or at least comparable) to the beam sizes(2)residual pressure & acceptance must be adapted to this method

pioneering measurement900 GeV, in 2009 [6]pioneering measurement900 GeV, in 2009 [6]

See refs [5,6,7]See refs [5,6,7]


A bit of historyA bit of history

20092009 1st lumi calib at Pt8, pioneering BGI at 900 GeV

20102010 1st flat top energy calibrations (VDM and BG) in Apr-May 2010:

BCN systematics dominate the cross section uncertainty ~ 10%

itself dominated by DCCT total current normalization

itself dominated by DCCT baseline uncertainty

started BCNWG, and a detailed analysis 4-6% Another lumi calib in Oct 2010

large currents (reduced baseline effects) down to 3-4 % And another in Nov 2010 (PbPb)

– Much more ghost/satellites !! Meanwhile, advanced solid ground work to tackle satellites/ghost

charge issues, in anticipation of DCCT uncertainty reduction


20112011

Large effort to understand BCN:– DCCT: BE-BI + Colin Barschel (CERN-PH, Aachen Univ.)

– Relative bunch populations, satellites, ghost: BCNWG

Three notes (being) written:– DCCT (Total current normalization): "RESULTS OF THE LHC DCCT CALIBRATION

STUDIES", CERN-ATS-Note-2012-026 PERF

– Relative populations: "STUDY OF THE RELATIVE LHC BUNCH POPULATIONS FOR

LUMINOSITY CALIBRATION", CERN-ATS-Note-2012-028 PERF

– Ghost and satellites: "STUDY OF THE LHC GHOST CHARGE AND SATELLITE

BUNCHES FOR LUMINOSITY CALIBRATION", CERN-ATS-Note-2012-029 PERF

The two main topics of this session :

I) The results of these BCN studies

II) The results of the expts' lumi calibrations


The scan seriesThe scan series


Beam and bunch populations Beam and bunch populations NN

Ntot = Nmain + Nghost ( + Npilots )

For each beam: (we suppress here the beam index j=1 or 2)

Nmain = Ni iM

sum over the set of all the main bunches (colliding or not)

total beam population (every charge in the ring)

NiPi = Nmain

population fractions: ( Pi = 1 ) iM


Main devices used in BCNMain devices used in BCN

Total current: Total current: DCCT

Individual bunch charge: Individual bunch charge:

Fast Bunch Current Transformers (FBCT)

IP1 button pickups (BPTX) Longitudinal Density Monitor (LDM) Beam-Gas rates in IP8

the main device:the main device:

but also:but also:

See the talk of

<= Colin Barschel

<= here + Jean-Jacques Gras (session 3)

<= Gabriel Anders

<= Adam Jeff

<= Jaap Panman


Definitions: slots, RF bins and all thatDefinitions: slots, RF bins and all that

25ns 25nsslot i+1slot i

responseRF bin

2.5ns

LHC = 3564 slots = 35640 RF bins (1 bin contains 1 RF bucket)

RF bin i10-9

LHCb beam-gas rate allows us to measure the "ghost" charge in the nominally non-filled slots (40 MHz electronics, integrating over 25ns...), but cannot discriminate in the nominally filled slots

FBCT integrates over ~24ns and applies a threshold (cut noise)

phase undefined ??

typical ghost/satellites in protonsand in Pb runs

... i-2 i-1 i i+1 i+2 ...

threshold


Definition of ghost and satellite charges Definition of ghost and satellite charges

Given that:– Total beam population is measured with the DCCTs

– The relative populations are measured (mainly) with the FBCTs, which which have a ~25ns integration time, while LHC RF buckets are 4 ~ 1ns

long, contained inside RF bins of 2.5ns

which have a 25ns slot threshold below which the slot is assigned 0 charge

– The main tool for measuring the particles invisible to the fBCT has been the LHCb BG rates, which also have a 25ns granularity

we choose to define...


... (among many possible definitions)... (among many possible definitions)

Ghost charge: charge in all slots which are not nominally filled (total charge below the FBCT threshold it is not "seen" by the FBCT )– can be estimated from LHCb beam-gas rate

caveat: trigger efficiency over 25 ns !

– can be estimated from LDM, <0.1ns time resolution ! caveat: normalization to nominal bunches! (emittance, position...)

definitions of phase relative to FBCT

Satellites: the charge in those neighbouring RF bins which are within 25ns "across" a nominally filled RF bin– they participate negligibly in luminosity (if crossing angle >0)

– are integrated in the "slot population" by FBCT what is the integration efficiency ?

what is the definition of phase (is the nominal bin centered ?)


The FBCTThe FBCT

The data for the relative bunch population measurements come from a ‘Slot Sum’ mode of acquisition with a specified number of 900 samplings for averaging (taken in 3600 turns with one sampling every fourth turn).

The Slot Sum returns 3564 bunch slot intensities on the high bandwidth channel each averaged over the specified number of turns.

The electronics were configured such that the gain was switched at around 2.3·1010p per bunch (averaged over all populated bunch slots)– 2011 pp scans = LO GAIN, while PbPb scans = HI GAIN

Block schematicssee Ref. [11]Block schematicssee Ref. [11]

thanks to D. Belohrad/ JJ Grasthanks to D. Belohrad/ JJ Gras

toroid trafo


FBCT subtletiesFBCT subtleties

FBCT uses two integrators (odd and even slots)

Gain was equalized by BE-BI experts

Possible systematic effects between odd/even channel not further investigated in BCNWG

Phase is regularly adjusted to have an optimum between good signal response and low "spill-over" effect

During VDM scans, this is generally done after reaching flat top (before STABLE BEAMS)

signal of the same bunch with the 2 integrators (skipping one clock cycle)

signals while change the fine phase delay


Typical example of FBCT measurements (Oct 2011)Typical example of FBCT measurements (Oct 2011)

fill 2234fill 2234

courtesy of V. Balaguracourtesy of V. Balagura


FBCT vs BPTX comparisonFBCT vs BPTX comparison

More of this in Gabriel Anders' presentation Compared also to:

– LDM => Adam Jeff

– BG rates => Jaap Panman

– DCCT , see next

BPTX is a completely different device, in a different location, with different electronicsBPTX is a completely different device, in a different location, with different electronics

fill 1653 mar 2011fill 1653 mar 2011 fill 2234 oct 2011fill 2234 oct 2011


Comparing FBCT sum vs DCCTComparing FBCT sum vs DCCT(Pb-Pb Nov 2010)(Pb-Pb Nov 2010)

Total current (DCCT) changes by a factor ~0.8 over the duration of the fill

How about the ratio R ?

R = FBCTsum / DCCT

R0 = R at time 0 (~arbitrary)

summing signals of all nominally filled slotssumming signals of all nominally filled slots

PbPb Nov 2010PbPb Nov 2010

sumFBCT(squares)

DCCT A and B(circles and triangles) 1%


Comparing FBCT sum vs DCCTComparing FBCT sum vs DCCT(Pb-Pb Nov 2011)(Pb-Pb Nov 2011)

PbPb Nov 2011PbPb Nov 2011


Comparing FBCT sum vs DCCT Comparing FBCT sum vs DCCT (pp 3.5TeV)(pp 3.5TeV)

pp May 2011pp May 2011 pp Oct 2011pp Oct 2011


Comparing FBCT sum vs DCCTComparing FBCT sum vs DCCT(pp 1.38TeV)(pp 1.38TeV)

pp Mar 2011pp Mar 2011

Klystron B2 trip

Here: more pronounced change of R .

Why ? Non-optimized longitudinal plane (RF) increasing ghost charge (see J. Panman) Beam2: RF klystron trip

Klystron B2 trip

1%

FBCT response depends on bunch length !FBCT response depends on bunch length !


Fill 1795, check on FBCT bunch length sensitivityFill 1795, check on FBCT bunch length sensitivity

Analyzed the start of ramp– strong change

of bunch length

– stable orbit Evaluate the

slope (sensitivity) of FBCT vs bunch length

energyenergyDCCTDCCT

DCCTDCCT

FBCTsumFBCTsum

FBCTsumFBCTsum

bunchlengthbunchlength

bunchlengthbunchlength

thanks to N. Bacchetta / JJ Grasthanks to N. Bacchetta / JJ Gras


FBCT response sensitivity on bunch lengthFBCT response sensitivity on bunch length

(dN/N)Slope:

dl

Slopes :

B1 ~ 0.37 % / 0.1ns

B2 ~ 0.48 % / 0.1ns

around nominal bunch intensity

Future: could be refined: make a bunch-by-bunch analysis

ll

thanks to N. Bacchetta / JJ Grasthanks to N. Bacchetta / JJ Gras


Typical bunch length spread during STABLE BEAMS Typical bunch length spread during STABLE BEAMS

Spread of ~ 0.02 ns (envelope) => expect ~0.1% max effect on FBCT-measured populations

NB: this effect was further reduced in the end-of-June 2011 technical stop by addition of a 70MHz filter

May 2011 VDM scans, fill 1783May 2011 VDM scans, fill 1783 thanks to N. Bacchetta / JJ Grasthanks to N. Bacchetta / JJ Gras


Position dependence of FBCT responsePosition dependence of FBCT response

Special MD (machine development): – 30 June 2011, fill 1910, 12 bunches, 450 GeV only

beam1 Y move

beam1 Y move

beam1 X movebeam1 X moveb

ea

m1

FB

CT

be

am

1 F

BC

T

be

am

2 F

BC

Tb

ea

m2

FB

CT


Worst beam/plane combination from this studyWorst beam/plane combination from this study

Beam 1 horizontal: FBCT slope (sensitivity) = ~ 1%/mm (at ~1e11 p)


Effect on lumi calibration experiments ?Effect on lumi calibration experiments ?

Minimal ! beams stay within 0.1mm at FBCT

during VDM scans

=> effect must be at most 0.1% NB: did not measure the individual

bunch positions at FBCT (only beam average)

Beam2 Y X Beam1 Y

X


Bunch-by-bunch cross section analysisBunch-by-bunch cross section analysis

Since Nov 2010, usually, (many) more than one colliding pair per experiment were available during VDM scans

=> each pair "i" gives one cross section measurement

Inspect the "consistency" between all bunch pairs If one observes an "inconsistency" (beyond statistics), one may try

to model and apply a correction


Response of bunch charge measuring deviceResponse of bunch charge measuring device

Ideal

Nmeasured

Ntrue

Nmeasured

Ntrue

Real ??

(overly exaggerated here)

still, can be considered locally linear (affine) to a good approximation

overestimation of population fractionoverestimation of population fraction

underestimation of population fractionunderestimation of population fraction

pure

pro

porti

onal

ity lin

e

pure

pro

porti

onal

ity lin

e


Simple fit modelSimple fit model

Define the "correct(ed)" populations:

Define the deviations from a local (arbitrary) point:

Assume errors are:

Because of , one of the parameters can be fixed:

=> Minimize:

"errors"measured

e.g.

set of all Main bunches (1...m)

set of all Colliding main bunches

1 par (), 3-par (12 ) or 5-par (121 2)

beam j=1,2i= index for BCID


Example: LHCb Mar 2011 (1.38 TeV)Example: LHCb Mar 2011 (1.38 TeV)

each point is one colliding bunch pair (IP8)each point is one colliding bunch pair (IP8)

PRELIMINARY


Other Mar 2011 scansOther Mar 2011 scans

PRELIMINARY

PRELIMINARY

PRELIMINARY

PRELIMINARY


ATLAS/CMS May 2011 (pp 3.5 TeV, 1.5m)ATLAS/CMS May 2011 (pp 3.5 TeV, 1.5m)

PRELIMINARY

PRELIMINARY

PRELIMINARY

PRELIMINARY


ALICE/LHCb May 2011 (pp 3.5 TeV)ALICE/LHCb May 2011 (pp 3.5 TeV)

PRELIMINARY

PRELIMINARY PRELIMINARY


Table of fit resultsTable of fit resultsthese are 1 by definition


ObservationsObservations

Affine fit generally improves the 2/ndf for pp, but it remains large for the higher-stat experiments (ATLAS, CMS) – improves especially in Mar 2011, not so in May 2011 ...nor Nov 2010

(PbPb, CMS)

Quadratic fit improves a further bit the 2/ndf for Mar 2011, but does nothing good for May 2011

For May 2011: have also a complete data set from BPTX– similar analysis done with BPTX data, similar observations

Cross section results are stable: change by less than 0.4%

Questions: How do the extracted fit parameters (1, 2 for the affine fit) compare

from IP to IP ? Especially: how do they compare between CMS and ATLAS in the

same fill ?


Comparing fit parameters...Comparing fit parameters...

"moderate" agreement – NB: only IP1 and IP5 use the same

bunch pairs!

But interesting observation on the difference FBCT - BPTX when compared with direct FBCT vs BPTX fit ...

shaded bands from direct FBCT vs BPTX fitshaded bands from direct FBCT vs BPTX fit

Hypothesis:slope - 1 FBCT - BPTX Hypothesis:slope - 1 FBCT - BPTX


Comparing IP1/IP5Comparing IP1/IP5

See a correlation between IP1/5 in both Mar and May 2011 In Mar 2011, the affine (and even quadratic) fits reduce correlation In May 2011, they don't

r = unweighted Pearson’s sample correlation coefficient

B1 (B2) bunch collides in IP1&5 and...noneIP8 (IP2)IP2 (IP8)


Bunch-by-bunch responseBunch-by-bunch response

Nmeasured

Ntrue

Remember: here, for illustration, the effects are disgustingly exaggerated

variations of the individual bunch properties can introduce changes ("fluctuations") in the response (bunch-by-bunch)

In our analysis, we did not (could not) take into account (and even less correct for) variations in bunch length, positions, and ...

This may (?) explain why we can't model perfectly with an affine fit

But the fluctuations could also be unrelated to the population fraction measurements!

bunch with different bunch length ?

bunch with different position at FBCT ?


ConclusionsConclusions

The FBCT provide accurate measurements of the relative bunch populations– however, scrutinizing the high precision data, we start seeing some

systematic effects at the permil level several small effects observed: dependence on bunch length, beam

positions, non-linearity, phase ? (satellites?), ...

Our detailed studies give us confidence that the effect on the cross section normalization are small (<0.5%, see table)– valid for the VDM scans from Nov 2010 onward

Given our current understanding*, we recommend using the "fudged" uncertainties (from the 1-par or 3-par b-by-b fits) as a measure of the systematic uncertainty on the cross section due to the relative bunch population measurements.

*we cannot explain convincingly the non-statistical fluctuations of the

bunch-by-bunch cross section data


Literature Literature (more here: http://lpc.web.cern.ch/lpc/lumicalib.htm)(more here: http://lpc.web.cern.ch/lpc/lumicalib.htm)

[1] "Calibration of the effective beam height in the ISR" , S. van der Meer, ISR-PO/68-31, 1968 (CERN). link

[2] "Measurement of the luminosity of the p-pbar collider with a (generalized) Van der Meer method", C. Rubbia, CERN p-pbar note 38. link

[3] "Absolute Luminosity from Machine Parameters", H. Burkhardt, P. Grafstrom, LHC-PROJECT-Report-1019 ; CERN-LHC-PROJECT-Report-1019; link

[4] "Notes on Van der Meer Scan for Absolute Luminosity Measurement", V. Balagura, Nucl. Instr. and Meth. A (2011), doi:10.1016/j.nima.2011.06.007. arXiv:1103.1129 [physics.ins-det] , see http://arxiv.org/abs/1103.1129

[5] "Proposal for an absolute luminosity determination in colliding beam experiments using vertex detection of beam-gas interactions", MFL, Nucl. Instrum. Methods Phys. Res., A 553 , 3 (2005) 388-399. link

[6] "Prompt production in pp collisions at s=0.9 TeV", LHCb Collab., R. Aiij et al., Physics Letters B Vol. 693, Issue 2, 27 Sep.2010, Pages 69–80.

[7] "Absolute luminosity measurements with the LHCb detector at the LHC", LHCb Collaboration, 2012 JINST 7 P01010 doi:10.1088/1748-0221/7/01/P01010. link arXiv.

[8] "LHC Bunch Current Normalisation for the October 2010 Luminosity Calibration Measurements" A. Alici et al. (BCNWG note2), CERN-ATS-Note-2011-016 PERF link

[9] "LHC Bunch Current Normalisation for the April-May 2010 Luminosity Calibration Measurements" G. Anders et al. (BCNWG note1), CERN-ATS-Note-2011-004 PERF link

[10] "LHC Lumi Days: LHC Workshop on LHC Luminosity Calibration", CERN-Proceedings-2011-001. link

[11] "The LHC Fast BCT system: A comparison of Design Parameters with Initial Performance" D. Belohrad, L.K. Jensen, O.R. Jones, M. Ludwig, J.J. Savioz, CERN-BE-2010-010, link


Backup slidesBackup slides


Error on cross section ought to be small!Error on cross section ought to be small!

Relative error on cross section due to wrong measurement of population fractions: correct

measured

depends on

- correlations between errors in one beam and population fractions in

the other beam, and

- in correlations between errors in population fraction in each beam


some (semi-quantitative) coarse boundsome (semi-quantitative) coarse bound

Define

Then:

Using as an indicative value, one can evaluate

In case then

unlikely and really bad


Other stuffOther stuff

Also tried to look at bunch families: no convincing structure observed

For example, May 2011

different "families" according to collision scheduledifferent "families" according to collision schedule

offset is related to slopeoffset is related to slope

group of 6 points: 6 VDM scansgroup of 6 points: 6 VDM scans

courtesy of V. Balaguracourtesy of V. Balagura

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LHC Lumi Days 201229-Feb-2012CERNMassimiliano Ferro-Luzzi 1 Overview and relative bunch population...

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