Elastic scattering, total cross section

and luminosity measurements with ATLAS

C. Sbarra INFN and University of Bologna

On behalf of the ATLAS Luminosity and Forward Physics Working Group

DESY, Hamburg, 21-25 May 2007

Introduction Atlas strategy Experimental techniques

Elastic Scattering

s

2

2

||totEM2

)(4t

2 tb

NCCNIt

eiLffLdt

dN

momentum transfer -t ~ (p)2

= beam scattering angle p = beam momentum

= 14 TeV prediction of BSW model

d/d

t (

mb/

GeV

2 )

L, tot,b, and from FIT in CNI

region (UA4)

CNI region: |fC| ~ |fN| @ LHC: -t~6.5 10-4 GeV2; min~3.4 rad

(min~120 rad @ SPS) DESY, Hamburg, 21-25 May 2007 C. Sbarra

Im

Re

0

tel

el

tf

tf

Total Cross-Section

)(log stot

Luminosity-independent measurement via optical-theorem

simultaneous evaluation of forward elastic and inelastic rate

(TOTEM)

02

2

1

16L

t

tot dt

dN

Inversely:

tot + dN/dt|t=0) (L/L > ~ 2 tot/tot)

(L + dN/dt|t=0) tot/tot > ~ ½

L/L)

inelasticelastictot NN L

inelel

ttot NN

dtdN

0

2

)/(

1

16

DESY, Hamburg, 21-25 May 2007 C. Sbarra

)(log stot tot (LHC) ~ 110 mb (=2; best-fit)

tot(LHC)95 mb (=1)

elastic rate down to |t|=10-

3 GeV2 to keep extrapolation error small (1-2%)

Sufficient coverage to access Nel+Ninel

The LHC Luminosity

*

44 surfaceImpact

2

**1 211 21

N

b

yx

k

i ii

k

i ii NkfNNfNNfL

bb

Nxi = number of protons in bunch i of beam x; f=revolution frequency; x,y=transverse beam dimensions at the IP; Kb = number of bunches; *= function at IP; N=*x*y/* normalized emittance; =E/mp (~7460)

Accuracy limited by

Precision in measurement of bunch currents Extrapolation of xy from measureament point

to IP Beam-beam effects at IP, beam crossing

angle, ...Typical accuracy from machine 5-10%

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Physics Interest in L

Relates the cross section of a given process to its event rate N=L overall normalization of physics analysis; monitor of LHC

performances

Systematic error dominated by luminosity (ATLAS TDR )

Higgs coupling tan measurement

DESY, Hamburg, 21-25 May 2007 C. Sbarra

L/L

L/L

L/L

ATLAS Strategy

Elastic scattering in CNI region to get L, b, and tot at L ~ 1027 cm-2s-1 (optical theorem as a back-up solution) (ALFA detector in Roman Pots)

Luminosity monitor calibrated at low lumi but working up to L ~ 1034cm-2s-

1 (LUCID)

Goal precision on L ~ 2-3%

Absolute L from QED (pp pp) and QCD (Wl, Zll) processes (need to control PDF)

Improve Luminosity from machine with ZDC

Further luminosity/beam monitoring with BCM, MBTS...

DESY, Hamburg, 21-25 May 2007 C. Sbarra

+

CNI region challenge

y*

y*

parallel-to-point focusingydet

IP Leff

Experimental technique: Large * optics beam divergence ~ 0.2 rad; * ~ 600 m independence of vertex position: parallel to point focusing ALFA detector at 10-15beam in Roman Pots @ 240m from IP on each side

RP

IP240m 240m

RPRP RP

RP RP RP RP

* = 2625 m

L ~ 1027 cm-2s-1

DESY, Hamburg, 21-25 May 2007 C. Sbarra

ALFA detector

Scintillating fiber trackerScintillating fiber tracker Kuraray 0.5 mm × 0.5 mm

fibers 10 layers per coordinate

Absolute Luminosity For Atlas

Overlap detector (alignment)

Trigger scintillator

DESY, Hamburg, 21-25 May 2007 C. Sbarra

ALFA performance

Systematics on L

beam divergence and optics

detector acceptance, resolution & alignment

background from halo (beam-gas, off-momentum, betatron oscillations)

Background from non-elastic interactions

Fit to simulated dN/dt data corresponding to ~ 1 week (10M events) of running at L = 1027 cm-2s-1

DESY, Hamburg, 21-25 May 2007 C. Sbarra

L/L ~ 3% - after 2009

(H. Stenzel ATL-LUM-PUB-2007-001)

“Luminosity independent” calibration constant A (determined by simultaneous absolute L measurement same precision as L )

Luminosity Monitor

Calibrated at low luminosity where the average number of particle per detected interaction <N> is measured (small probability of more than 1 interaction per BX)

If is the efficiency to detect one interaction:M

LN

M

LN

A detector able to count the number of interactions per BX

L

by measuring <M> = mean number of charged particles per BX.

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Needed dinamic range in (bunch by bunch L) @ LHC : 2.5 10-6 - 25

Calculated and measured tot only used for consistency cross checks

(S. Ask – ATL-LUM-PUB-2006-001)

LUCID LUminosity monitor using Cerenkov Integrating Detector

Cherenkov threshold (10 MeV for e, 2.8 GeV for to limit background

Pointing geometry (limit back.) No landau fluctuations (counting

particles) Good time resolution (2-3 ns)

bunch by bunch & on-line luminosity Light, rad-hard

Array of polished aluminum tubes in C4F10 Cherenkov radiator (P=1bar)light emitted at 3° and read-out after ~ 3

reflections directely (or via optical fiber) by PMT

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Similar to CLC in CDF at Tevatron

6-7 MRad/y at L=1034 cm-2s-1

.5-.7 MRad/y at low luminosity

LUCID location

Pseudorapidity coverage 5.4<||<6.1 (5.6<||<6.0)

Beam pipe

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Radiation test with gammas no problems with PMT up to 20 MRad

front of tubes at ~ 17 m from IP

Phase I

Phase 2–high lumi L ~ 1034 cm-2 s-1

Installation after 2009Calibration with ALPHA

goal precision on L 2-3%

Phase 1-low lumi L < ~ 1033 cm-2 s-1

Installation summer 2007Calibration initially from LHC

(10%), then W/Zl/ll+QED (5-10%)

DESY, Hamburg, 21-25 May 2007 C. Sbarra

LUCID Phases

16 tubes per side directely read-out by PMT; 4 tubes per side with

fibers

168 tubes & winston cones per side read-out via optical fibers to MaPMT

Outer layer R=114.7 mm; Inner layer R=96.3 mm Tube diameter=15mm

<~7 ~23

Collision (zero) Counting - fine in in phase I

Hit Counting - no saturation in phase I

Particle Counting - linear by construction - sensitive to gain fuctuations

(Side coincidence / single side)

Phase I Measurement

Methods:

Systematics (under study)

Optics differences between calibration and physics <~ 1%

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Main methodFor phase 1

CLC/CDF experience: L/L~ 2% + 4% + 4% ~ 6%

acceptance

cross-section

MC expectation

Zero Degree Calorimeter

Tungsten-quartz fiber calorimeters at ~140 m from the IP on each side

LOI presented in January 2007 (CERN-LHC-2007-001)

Versatile device to study both HI and pp physics (>8.3); effective beam-tuning device

Installation of hadronic module in fall 2007

housed in transverse aperture of neutral particle absorber (TAN)

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Surviving at most 3 years in pp at L=1033cm-2s-1 due to radiation

ZDC as a beam monitor

ZDC at RHIC as an accelerator tool (in pp): Van der Meer scan (ZDC coincidence rate vs. relative beam position)

Useful to measure the beam crossing angle

Useful to locate longitudinal IP position in the early days

Further luminosity monitor at low luminosity

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Useful to tune machine parameters in the early days

Summary & Conclusions

LUCID at 17m (summer 2007) dedicated luminosity monitor

ZDC at 140m (fall 2007) LHC parameter calibration/beam monitor

ALFA in Roman Pots at 240m (after 2009) absolute L, tot, , b

DESY, Hamburg, 21-25 May 2007 C. Sbarra

Eager to get first beams

Three detectors in forward region

LHC luminosity at the start-up ~10% Rate of known QED/QCD processes at mid-term ~5% if PDF

under control ALFA detector after 2009 ~2-3%

LUCID Calibration