elastic scattering, total cross section and luminosity measurements with atlas

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 Current status

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


)(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%


Physics Interest in L

Relates the cross section s 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


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) (ALPHA 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...


+

CNI region challenge

y*

y*

parallel-to-point focusingydet

IP Leff

Experimental technique: Large * optics beam divergence ~ 0.2 rad, low luminosity independence of vertex position: parallel to point focusing ALFA detector (scintillating fibers) at 10-15beam in Roman Pots @

240m (after 2009)

RP

IP240m 240m

RPRP RP

RP RP RP RP

* = 2625 m

L ~ 1027 cm-2s-1


Details in

dedicated talk

Intrinsic Beam divergence during physics run ~ 30 rad

ALPHA 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


L/L ~ 3% - Available 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:

inelLN

M

inelN

ML

A detector able to count the number of inelastic interactions per BXLinel

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


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


Installation in summer 2007

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


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 (after

2009)

Calibration with ALPHA goal precision on L

2-3%

Phase 1-low lumi L < ~ 1033 cm-2 s-1 (up to 2009)

Approved Feb. 2007Calibration initially from

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


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 run < 1%


Main methodFor phase 1

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

acceptance inel

MC expectation

Zero Degree Calorimeter

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

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

Primary goal: to measure spectator neutrons in heavy ion interactions (centrality)Installation of hadronic module in fall 2007

housed in the shielding unit that protects the S.C magnets from radiation (TAN)


ZDC as a beam monitor

ZDC (lower curve) bkg free over 4 orders of magnitude

Van derMeer scan (ZDC coincidence rate vs. relative beam position )

ZDC at RHIC as an accelerator tool (in pp)

PHENIX horizontal vernier scan Feb. 12, 2003

y = 0.7003x

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2

Beam displacement (mm)

SMD

Read

ings

(mm

)

also measures beam displacement (red points) Useful for crossing angle commissioning


Summary & Conclusions

LUCID at 17m (summer 2007) dedicated luminosity monitor

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

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


Eager to get first beam

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

elastic scattering, total cross section and luminosity measurements with atlas

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