CMS Luminosity Measurement(s)

Cornell Journal Club

Daniel MarlowPrinceton UniversitySeptember 30, 2005

Talk Outline

• Goals & General Strategy• Real time Techniques

– HF – Pixel Telescopes– TAN-region Fast Ionization Chamber

• Offline Techniques– Total cross section measurement (TOTEM)– W & Z Counting

• Bookkeeping• Other Issues

Design Goals: General Desirables

• Absolute calibration, based on a known cross section with a reliably calculated acceptance.

• Temporal stability against gain changes and other drifts: “countable objects” or self calibrating signals (e.g., MIP peak).

• Linearity over a large range of luminosities.

• Real time operation independent of full DAQ.

• Redundancy– There is no perfect method– Applies to both real time monitoring and to

offline absolute normalization

Design Goals: Specific Issues

• Real time monitoring– Bunch by bunch (yes)– Update time: 0.1 s to 1.0 s or slower

• Offline – Robust logging– Easy access to luminosity records– Dynamic range (1028 ~ 1034 cm–2s–1)

• Absolute Calibration– Target from previous workshops 5% (or better)

General Strategy

• Use TOTEM* measurement of total cross section at low luminosity as a reference point.

• Use real time techniques (HF, Pixel Telescopes, FIC) to extrapolate/interpolate to design luminosity

• Renormalize at design luminosity using processes of ~known cross section (e.g., W’s and Z’s)

*TOTEM is a forward detector that will measure the total p-p cross section, thus providing a normalization point.

Lumi Basics

• Apologies to the experts!

• Basic equation

• We are often interested in the mean number of interactions per bunch crossing:

LdtN event

MHz 403564

2835

BXmb

f

f

LN

total BX

filled BX

Zero Counting• If μ<<1 measuring the luminosity is

straightforward, since the probability of two events in a single BX is ~μ2 . It is enough just to count hits.

• For μ~1, one must either be able to distinguish between single and double interactions (not generally possible in this context), or, one must “count zeroes”

)]0(log[

);0( !

);(

p

epn

enp

n

Zero Counting

• For μ>>1, one starts to run out of zeroes to count. There is no hard limit, but requiring at least 1% zeroes seems reasonable.

This corresponds to

6.4)01log(.

General Strategy

Zero feast

Zero famine

Extrapolation of the total cross section as measured by TOTEM involves six orders of magnitude,* over which the number of min bias interactions goes from ~10-5 to ~25 per BX.

TOTEM

High L

*The extrapolation is not quite so bad, since TOTEM running will be done with fewer filled bunches.

Importance of Bunch-by-Bunch Measurements

• Presumably bunch-by-bunch luminosity information is of interest to the machine people.

• But it may also be relevant to physics simulations. In particular, if there are significant bunch-to-bunch variations in the luminosity, then the distribution of the number of underlying interactions will no longer be Poisson.

Importance of Bunch-by-Bunch Measurements

• To see the effect, we consider the case where the bunch-to-bunch variations in the mean number of expected interactions is Gaussian distributed.

• The cases considered range from no smearing (all bunch luminosities the same) to 50% bunch-to-bunch variations.

Importance of Bunch-by-Bunch Measurements

pure Poisson

10% smearing

20% smearing

50% smearing

The “HF”

The simulation results to be presented here will deal exclusively with the HF.

HF

HAD (143 cm)

EM (165 cm)

5mmTo cope with high radiation levels (>1 Grad accumulated in 10 years) the active part is Quartz fibers: the energy measured through the Cherenkov light generated by shower particles.

This is the cause of two of the peculiar features of this calorimeter:

The visible energy is carried by relativistic particles, i.e. electrons: the calorimeter is sensitive to the EM component (0) of the hadronic shower. Shower size depends on Moliere radius not i

The light is generated preferentially at 45 degrees: light propagation is far from ‘usual’ meridian one.

Iron calorimeter Covers 5 > > 3 Total of 1728 towers, i.e.2 x 432 towers for EM and HAD x segmentation (0.175 x 0.175)

HF is fast and transverse shower size is small

from Greg Snow Dec. 2002

Signals From HF

Minimal add’l hardware requirements

•Mezzanine board to tap into HF data stream and forward bits to a PC via Ethernet•Autonomous (mini) DAQ system to provide “always on” operation

Iron fiber calorimeter.

3 < η < 5HF

T1 & T2 are elements of TOTEM

HF Energy Depositions

The energy depositions in single interactions are typically quite sparse.

Simulation details:

• PYTHIA w. diffractive events added.

• DC04 (GEANT)

• Extract HF depositions to Rootuple.

Energy Depositions

At design luminosity, there are typically 25 interactions per BX.

ET Depositions

Total in one endcap.

Single interaction BX’s only.

An ET threshold of 1 GeV will detect most interactions.

Diffractive interactions

HF Methods & SimulationMethods:• Count minimum bias events at low luminosity• Count “zeroes” at design luminosity

• Use also linear ET sum, which scales directly with luminosity.

Simulation Information:• Recent work by Chris Rogan, Princeton undergrad.• Thanks to Monika Grothe & Wisconsin group for

providing MC samples with diffractive events included.• MC details

– Full GEANT & ORCA– 0.25 p.e./GeV– QIE FADC scale realistically included– Thermal noise included– Tested performance over a range of assumptions

• Defeat the zero famine at high luminosity by counting zeroes in a much smaller solid angle.

• There are 864 HF “physical” towers. • In effect these provide 864 quasi-independent

measurements of the luminosity. • Average to arrive at final result.

HF Zero Counting

MC Results: Physical Tower Zero Counting

Line is determined by value at 1034 , not a fit.

3 FADC count hit threshold

Zero starvation

Deviation from linearity

<N

>/B

X<

N>

/Nexp

ect

ed

Physical Tower Zero Counting w. Increased Threshold

Thresholds ~2X previous plot

Superlinearity typical of high thresholds

<N

>/B

X<

N>

/Nexp

ect

ed

MC Results: ET Sum Method

Line is determined by value at 1034 , not a fit.

Deviation from linearity

<E

T>

/BX

(G

eV

)<

ET>

/ETexp

ect

ed

This is the average ET summed over the HF for each BX. Nominal noise and threshold (ADC least count effects included).

Effect of noise at low luminosity.

MC Results: ET Sum Method

Deviation from linearity

<E

T>

/BX

(G

eV

)<

ET>

/ETexp

ect

ed

Noise Subtracted

HF Luminosity Readout Path• 9 HTRs for HF+ and HF- • Each HTR has 1 output with luminosity info

– 100Mbps raw ethernet packets sent to router

• Router to computer over Gigabit ethernet• Dead time, throttle, etc. info from GCT sent to CPU• This computer will feed LHC, luminosity DB, etc.

HTR

HTR

HF

9 HTRs/VME crate

HTR

HTR

HF

ROUTER

CPU

Global Trigger

Luminosity consumers

Maryland/Princeton/Virginia

HF Luminosity Card

• Initial prototype board built. Next version due back soon.– 32MBytes of SDRAM– Virtex2PRO/VP7 has 722kbits block ram– Embedded processor

• Function:– Receive data @ 40MHz from each Xilinx– Keep running sum of tower occupancy per bucket– Keep ethernet blasts in memory for transmission

TOP

BOT

Pixel Luminosity Telescope (PLT)

• The HF method is based on an existing detector, and thus has the advantage of being inexpensive and relatively easy to implement.

• It does not, however, really fit the bill when it comes to providing a luminosity measurement based on “countable objects.”

• Motivated by the CDF approach of counting MIPs using Cherenkov telescopes, we are proposing a charged-particle telescope system based on single crystal diamond detectors.

• This system is not yet fully funded.

Pixel Luminosity Telescope (PLT)Measure luminosity bunch-by-bunch

• Small angle (~1o) pointing telescopes

• Three planes of diamond sensors (8 mm x 8 mm)

• Total length 20 cm

• Located at r = 4.5 cm, z = 175 cm

• Diamond pixels bump bonded to CMS pixel ROC

Count 3-fold coincidences on bunch-by-bunch basis.

• Eight telescopes per side

• Form 3-fold coincidence from ROC fast out signal

Rutgers/Princeton/UC Davis

Simple, stand alone detector, operating independently of main CMS DAQ

Single Crystal (SC) CVD Diamond

• Full charge collection at 0.2 V / m ― 18,000 e― for 500 m diamond

• Availability of 8 mm x 8mm pieces ― 2 pieces end of last year― 8 pieces in May― ready for production by Fall

― Landau 60% narrower than Si

• Test performance of irradiated diamond pixel detectors

― efficiency― spatial resolution― radiation hardness Fall Fermilab test beam

• No need for cooling

• Radiation hard (few x 1015 cm-2)

Polycrystalline Diamond

Polycrystalline:

Single Crystal:

• wide distribution• low pulse height tail

crystal boundaries

• distribution well separated from zero

very difficult to achieve 1% stability of efficiency using polycrystaline sensors

• charge spreading over several pixels

CMS Pixel Readout Chip

CMS pixel chip has “fast” multiplicity counting built in

8 mm

8 mm

80 x 52 pixels100 m x 150 m

active area

Fast output level (each bunch crossing)

• 0, 1, 2, 3, 4 double column hits

• individual pixel thresholds adjustable

• individual pixels can be masked

Full pixel readout (every L1 trigger)

• pixel address and pulse height of each hit

• diagnostic of fast out signal

• determination of track originIP* σxy~35 μm σz~700 μmScattering Beam halo• determination of IP location

*statistical accuracy on relative position assuming 1033 for 1 s

Location

• End of Be section of beam pipe (~ 1.7 m from IP)

• Just outside of beam pipe (~ 4 cm from beam line)

Luminosity telescopes

IP

Rates

for L = 1033

• tracks per telescope per BC: 0.053

• interactions per BC: 2.5

• tracks per telescope per 1 s: 1.7 x 106

• tracks per array per 1 s: 2.7 x 107

• tracks per array per BC: 0.84

• number of buckets per orbit:number of buckets per orbit: 3564• filled buckets per orbit: 2835

• tracks per array per bucket per 1 s: 9500

1.0% statistical luminosity accuracy per bucket per 1 s @ L = 1033

• ratio of coin./tracks for 18 int. per BC: 0.81

Pythia 6.227 -- CMS Min. Bias productiin

Backgrounds

almost all coming from photons from pi0’s interacting in beam pipe

accidental hits per plane per interaction: 0.02

24% fakes in 3-fold coincidences @ L =1034

increase when beam pipe and CMS detector material put in

Coincidences: 11%

Singles rate: 77%sources mostly from beam pipesome from pixel detector

(assuming accidentals not correlated)

Sources of Hitshadron coincidences

electron coincidences singles

• hadrons from physics (IP)

• electrons from physics (0 photon conversions in beam pipe)

• singles from IP, beam pipe, pixel detector, telescope planes

8 telescopes per side φ=0Top and Bottom of the 3rd plane reduced by

0.5mm

8 telescopes per side φ=22.5Top and Bottom of the 3rd plane reduced by

0.5mm

4 telescopes per side φ=45Top and Bottom of the 3rd plane reduced by

1mm

PLT Status

• The PLT is an ideal real-time luminosity monitor.

• A detailed design showing the PLT to be compatible with CMS has been worked out, simulation studies are well advanced, and work on verifying the single-crystal diamonds is in progress.

• The required funding of ~$300K has not been secured, but we are pursuing various avenues and remain optimistic.

• The BCM mechanical mount will accommodate the PLT.

LHC Luminosity Monitor• The LHC accelerator project* plans to

incorporate fast ionization counters (FICs) in the TAN region, which is ±140m from the IP.

D1 triplet TAS TAS triplet D1

TANTAN

IP140 m140 m

nL R

*US LARP project in particular.

LHC Luminosity Monitor

to IP

Instrumented Cu bar absorber

FIC, which is integrated with the ZDC, is located at the position of the beam septum.

LHC Luminosity MonitorTarget specifications:

• <0.5% relative precision• Long term stability (~1 month) for calibration with detectors• High radiation environment (100 MGy/year)• Bunch-by-bunch capability

Solution• Segmented, multi-gap, pressurized ArN2 gas ionization chamber constructed of rad-hard materials

Quadrant segmentation provides sensitivity to beam position.

LHC Luminosity MonitorThe drift speed in the FIC is such that resolving individual bunches is an issue. The strategy being pursued consists of analog pulse shaping combined with digital deconvolution.

-0.2

0

0.2

0.4

0.6

0.8

1

-20 0 20 40 60 80 100

Cha

nnel

out

put [

V]

t [ns]

Out @ 20 PP int

Out @ 20 PP int + 1 PP int

0 50 100 150 200 250 300 350

0

1

2

3

4

5

6

Step 25k

mV

olts

nsec

1 to 10 2 to 10 3 to 10 4 to 10 5 to 10 6 to 10 7 to 10 8 to 10 9 to 1010

LHC Luminosity Monitor

•Project is part of LARP, a consortium of US labs participating in LHC accelerator work.

•This project is led by LBL.

•The current plan is to provide information to the LHC machine group.

•I am in contact with the LBL people and I am working toward getting the information for incorporation into the CMS real time luminosity database.

TOTEM

Luminosity Independent Method

Measure elastic scattering in Roman Pots and inelastic in T1 and T2 (see next slide). Should give result good to ~1% or so.

Normalization Using W’s and Z’s

• Lots of rate• Well understood

theoretically• Readily detectable

LHC event rates at 'nominal luminosity'CMS Trigger TDR

M. Dittmar et al.

Basic idea is to use

ZppWpp &

Normalization Using W’s and Z’s

• For a typical process we have

• If we take as a reference process, with

• The ratio does not depend on the luminosity or the pp cross section.

proton-proton2

21Xpartons ),,(PDF LQxxN Xpp

protonproton2

21ZqqZpp ),,()ho( LQxxPDFN

),,(PDF

),,(PDF2

21

221

qq

XqqZppXpp Qxx

QxxNN

Z

Zpp

Consider a process j, whose cross section we wish to measure

where is the event yield in the final sample (background removed by cuts and fitting), is the raw number of triggers, εj is the selection efficiency (trigger plus offline), and L0 is the luminosity, uncorrected for either deadtime or prescaling.

Bookkeeping and Normalization

yieldjN

triggeredjN

0

yield

L

N

j

jj

A CMS Note on this is in preparation.

triggered

0

1jj

j

NL

triggeredtriggered

yield

0

1j

j

j

j

NN

N

L

. . .General Considerations

Note that

which is the fraction of all events that pass the analysis cuts, can be determined from any unbiased sample of triggers. Note that all one needs to know is the raw number of triggers. Dead times and prescale factors do not explicitly enter (of course, one would want to know these as cross checks).

A similar analysis (too complicated for a short talk) shows that multiple overlapping triggers with arbitrary prescales can also be accommodated.

,triggered

yield

j

jj N

N

Technical Requirements

• A hardware scaler that counts L1A’s before any prescaling or deadtime losses are applied.

• We need to know the fraction of events rejected by the HLT.

• This information must be “coordinated” with the scaler and luminosity information.

Summary and Conclusions

• Various techniques are being pursued for online luminosity monitoring.

• HF

• PLT

• FIC

• The combination should provide redundancy and cross checks.

• A simple and robust strategy for obtaining offline normalization has been formulated.

Extra Slides

ET Depositions

Most of the energy depositions in the physical towers are below 1 GeV

Location of IP CentroidRelative location of IP

assume readout rate of 30 kHz• L = 1033

about 1200 tracks per telescope per 1 s

• 4 telescopes involved in x (or y) measurement

• σx = 2.4 mm

35 m centroid precision in x (or y)

Absolute location of IP• assume 300 um relative alignment of planes

• 16 telescopes involved in z measurement

• σz = 90 mm

700 m centroid precision in z

1.5 mm in x (or y) 30 mm in z

@ L = 1033 in 1 s

IP Distributions

X

Y Z

x = 2.4 mm

y = 5.9 mm

z = 89 mm

smeared because of track curvatureand longitudinal beam length (σ~7.5 cm)

What’s Needed

• ROC (pixel readout chip)• TBM (readout control chip)• FEC (VME control link module) • FED (VME flash ADC module)• Optical links • Port Card (board for housing lasers, photodiodes,power distribution)• Fan in/out chip (chip for fan out/in of control lines)

• HDI (circuit on which sensors are mounted)• Bump bonded sensors• Custom chip • Support structure • DAQ board (circuit for histograming of fast out signals)

Many parts developed for CMS pixel detector

A few need to be custom developed

Bump Bonding

• Bumping of readout chips done at PSI as part of pixel project

• Metallization of diamond Ti/W pixel electrodes, capability at Rutgers

• Bumping of individual diamond pieces UC, Davis has successfully done this in the past

• Flip chipping UC, Davis has capability

• Total of 48 pieces need to be bump bonded

• 100 % pixel yield not required

• Diamond sensor can be reworked if bumping fails

TOTEM & CMS

HF

T2 CASTORT1

•Need common TOTEM/CMS running to cross normalize inelastic cross section.

•Effect of removing T1?