jan/15/04 su donglow energy r measurements with isr1 su dong stanford linear accelerator center...

Jan/15/04 Su Dong Low Energy R Measurements with ISR 1

Low Energy R Measurements with ISR Su Dong

Stanford Linear Accelerator Center

CLEO-c/BES c//QCD workshop, Beijing, Jan/15/2004


Content

• The need for improved R measurements for muon g-2 and running QED.

• Initial State Radiation (ISR) basic characteristics and experimental issues.

• Current measurements at KLOE and BaBar.

• Radiative corrections.

• Prospects.


R measurements

R = (e+e- Hadrons) / (e+e- +-)

A large number of measurements scattered at various energy

ranges, over the last 3 decades.


R Measurements: the last progress

BES

Focus down runningQED

=> mHiggs from precision EW

CMD-2

Prediction for muong-2=> Test physics beyond SM

Foundation of the SM: prediction for precision EW tests relies on R


The Need for R in Muon g-2

a= (g-2)/2

a x1011

QED 11658406 + 3 Hadronic (LO) ~7000 + ~60Hadronic (NLO) 101 + 6Hadronic (light-by-light) 80 + 40

Weak 152 + 4

The leading order hadronic correction cannot be calculated from perturbative QCD => Need experimental R(s) measurement.

Most contributions and uncertainty come from s’< 3 GeV2. Low energy measurements traditionally done by summing exclusive modes, dominated by mode contribution.


The status of a

Muon g-2 measurement is still 2.7 from e+e- based prediction.e+e- and data agree better after CMD2 rad corr bug fix ( up ~3%, while quoted error was +0.6% !), but still has discrepancy at s = 0.7-0.9 GeV2.New e+e- R measurements would be very desirable !

Aug/03 review by: Davier, Eidelman, Hoecker, Zhang hep-ph/0308213

Jan/04 !


The running QED

s = (s)]Most useful at s=Mz

2 to confront the ensembleof precision electroweakmeasurements.

(Mz) x104

Leptonic 314.98 Top -0.70 + 0.05 Hadronic 276.1 + 3.6 * (* Burkhardt &Pietrzyk)The hadronic vacuum polarization

again mostly requires experimentalR measurements at low-medium energiesMost uncertainty from s1/2 ~ 1-5 GeV still.

Note: Point to point R measurements would be great, but the primary need is a precise INTEGRAL.


R Measurements: The New approach with ISR

Operating at a fixed CM energy to simultaneously explore the whole lower energy range below with initial state radiation (ISR)

(don’t have to fight over when to operate on what energy. They are there all the timeparasitic to whatever else you want to do !)

Rapid rise in both theoretical and experimental interests.

(the possibility of R measurement with ISR actually first emerged from CLEO data in 1995 as a background to the b->s analyses…)

Becoming truly competitive with the luminosity of the B/-c/factories. BaBar and Dane already started working at s1/2=10.6 GeV and s1/2=~1 GeV respectively.

It’s still in the early days and there are more questions than answers.


ISR cross section at Ecm=(4s)

ISR photon mostly along beamline. Only using ~5-10% events with photon in calorimeter fiducial (~|cos*|<0.8 for good containment),but still integrate to ~0.05nb of +had events below s1/2~7 GeV.(compare to the non-radiative ~1nb BB and 3.4nb udsc at s1/2=10.58GeV)


The ISR Experimental Approaches

KLOE @ Ecm=1.02 GeV : untagged >165o OR <15o

and in 45o<<135o

BaBar @ Ecm=10.58 GeVTagged ISR |cos*|<0.85

Inclusive OR exclusive final states recon.

cos*= - 0.92 cos*= 0.72

cos*= 0.89


Features of Measurements with ISR • Simultaneous collection of data at all CM energy s½ below with no

point-to-point normalization (same luminosity at all s½), and really `sees’ all s½ integrated (c.f. scan points can miss peak)

• Tagged ISR photon eliminates many background sources, particularly beam-gas/beam-wall and cosmics which do bother experiments at low energies. Hopefully will allow more efficient hadronic selection with less bias.

• Selecting ISR photon well within fiducial also forces recoil hadronic system aiming at fiducial detector region (and events more collimated due to recoil boost) with no bias on the hadronic system itself. This should reduce hadronic event signal blur due to acceptance losses.

• The recoil boost to the hadronic system hardens particle momentum spectra to reduce loss due to detection cut off at low momentum (useful for tuning MC model).


ISR statistics compared to low energy e+e-

CMD2: published L ~ 200 pb-1 (has x5 more data now being analyzed)BaBar ISR: current L ~ 150 fb-1 + efficiency estimate, |cos*|<0.80 tag KLOE ISR: current L ~ 140 pb-1 , |cos*|>0.966 (no tag) actual analysis c- factory ISR: for L ~ 3 fb-1

, |cos*|<0.90

Ecm= 0.61—0.96 GeV:

Luminosity HadronsCMD2 180K Bhabha 114K (more data on peak)BaBar ISR 350K 1540K (more data at s½=0.8-1 GeV vs CMD2)KLOE ISR 20M Bhabha 1500K (untagged) c-factory 250K

Ecm= 2—5 GeV: HadronsBES-I ~85KBaBar ISR 2810K (=30 points at 100 MeV s½ step with 0.35% stat.) c-factory 193K in range s’1/2=2.0-2.7 GeV for s1/2 =3.77 GeV


Experimental Issues

Because the rather special topology of the ISR events and we are aiming at precision measurements, it is important to examine detector design, especially trigger and filter strategies to ensure the data preservation.

BaBar + KKMC ISR


Monte Carlo Simulation for ISRNo generic MC which can do everything yetJETSET: Can generate down to CM energy of 2 GeV but no resonances and only LO ISR. EVA/AFKQED: LO ISR + FSR + Leading Log structure function,

for [Binner et al.; Arbuzov et al.]; 3,4, and other modes without FSR [Czyz,Kuhn + BaBar extensions] PHOKHARA: ISR with NLO for +FSR with latest V3.0 [G.Rodrigo et al.]. BaBar has incorporated generic qq mode with JETSET. KKMC: LO ISR + structure function, empirical tabulation of resonances and exclusive final state (still a development version). [S.Jadah et al.] LUND AreaLawMC: Not an ISR MC, but low energy hadronic

decays match BES data. [Anderson/BES] Cooperation between B/c-/ factories is highly desirable !


BaBar: Exclusive R ratio measurementsFully reconstruct the final state and use e+e- -> events as a `luminosity’ normalizer to do a classical R ratio measurement:

Advantages:• Fully reconstructed final state give good resolution for s’. This is very

important for the a integral for g-2 with strong s’ dependence.• The ISR photon efficiency cancels out.• The initial state radiation corrections and ISR spectrum shape

uncertainties largely cancel out. • Some tracking efficiency systematic partially cancel out especially for

the 2 track modes.Disadvantage: • Need to determine efficiency separately.• Statistical precision dominated by the lower stat. sample.• Unlikely to be workable for high multiplicity modes.

)'()1(

)1()'( s

dN

dNs

eefradf

radff


BaBar Radiative : Luminosity

Topology: two charged tracks + hard photon + Muon ID

1 tag2 tag

Compare data cross section with MC simulation after efficiency and radiative corrections.

Calibrate muon efficiency from data itself: tag 1 and check efficiency of the other.


BaBar Radiative : Resolution

Use J/ signal to monitor the resolution directly

Apply energy and momentum conservation using a 1C kinematic fit to improve final state reconstruction resolution.

After fit

16 MeV

8 MeV

Before fit


BaBar +- Pion Form Factor with Pion Form Factor with - - interference interference

BaBar data covering the full mass range. Systematic study in progress (tough goal at ~1%)

89 fb-1

BaBar preliminary

Interferencewith


BaBarBaBar Preliminary

Publication being reviewed within BaBar


BaBar

BaBar covers the full mass range. Publication (also include KK, KKKK) submission imminent.

BaBar Preliminary89 fb-1


KLOE analysis scheme• Untagged ISR in beam pipe.• Use 2 track momentum and E,P

conservation for single ISR to define MTRK from

• MTRK cut: Each of ee0 background < 5%. Also suppress FSR effect.

• Correct for selection efficiency, background and radiative correction vs s’ (up to 7%

• Normalize to Bhabha Lumi for final spectrum.

• Issues remain: more FSR checks; Bhabha lumi generator discrepancy (2% vs BHWIDE). Try ?

ee

signalregion

M

(GeV2)

MTRK (MeV)

tail

0)( 22

21

222

222

1

qpp

MpMpM trktrk


KLOE preliminary resultS

elec

tion

ef

fici

ency

80%

60% 0(e+

e-

)

(nb

)

KLOE result: a (0.37-0.93) = 378.4 0.8stat 4.5syst 3.0theo 3.8FSR

CMD-2 revised: a (0.37-0.93) = 378.6 2.7stat 2.3syst+theo

M2GeV2 M2GeV2


BaBar: Inclusive R MeasurementsIt is necessary to approach the higher s’ final states with inclusive measurement, but partial final state does not determine reduced CM energy. What about using the ISR photon energy ? Resolution enough ? Does it matter ? The goal is < +5% (BES total err ~7%).

Assume this is OK, the procedure would be:• Make inclusive hadronic final state selection with rather forgiving

cuts for high efficiency (ISR tag defeat many background without biasing hadronic final state).

• Using either standard luminosity measurement and use theoretical ISR spectrum shape, or as normalization.

• Include in the final state selection and subtract out using theoretical cross sections.

• Final state efficiency from MC calibrated by special data samples (the high efficiency hopefully squeeze down room for systematic uncertainty.)

Can work up to s’1/2~ 5-6 GeV before photon background from Normal non-radiative hadronic events becoming significant.


ISR photon resolution relevance

had integrand fortunately similar to ISR cross section at low s½

~just counting ISR photons and s½ resolution hardly matters ! but not so lucky with g-2 a…


ISR photon resolution effects

Low s’½ sensitive to photon energy resolution.

High E* range covers wide range of s’.

Use e+e- -> events (and rad Bhabha) as calibration source


ISR photon resolution test: Smeared spectrum

Using the R

spectrum in

KKMC

as a toy model.

Inject data

resolution.

Detector

fiducial

|cos|<0.8


ISR photon resolution test: integral

The absolute total effect of integral at 6 GeVcompared to perfect detector:

~7% All clusters

~3% Crystal center

~3% Rough edge corr.

~0.5% Gaussian

We can surely calibratefrom data to a small fraction of the 3% !


ISR inclusive analysis: background

BaBar MC test with KKMC for inclusive e+e- +hadron selection, after photon cluster shape cut and 0->veto, but before any final state event shape cuts. Background mainly from high momentum 0 in uds events.

S’ (GeV2)

ISR signal

backgr


NLO Radiative Corrections

G. Rodrigo et al. hep-ph/0106132: Higher order corrections to

radiative spectrum at low s’ is smaller and flatter if e+e- Ecm is higher.


FSR effect magnitude in

S’1/2 (GeV) S’1/2 (GeV)

S1/2=10.58 GeV S1/2=4 GeV

Phokhara 3.0 generator: compare turning FSR on/off


Final State Radiation

Who is radiating ?

Hadron or the quark ?

(Collinear soft radiation are probably mostly from hadrons, but what about wide angle hard FSR ?)

ISR+FSR interference significant at Dane, but may be not for BaBar at low s’ ?

Not everyone agrees…

But there are various ways to test

this from data. We are looking into them…


Initial and Final State Radiation

Data MC

BaBar QED charge asymmetry due to initial state and final state interference

- N ()

- N ()

Pion polar angle Asymetry

• data • MC

-50%

50%

Showing 20% discrepancy with MC (phokhara 3.0)

KLOE

Learning from data


Prospects• Very promising prospects for R measurements with ISR events.

Statistics at BaBar, KLOE are significantly exceeding existing energy scan measurements. CLEO-c and BES-III ISR will also have more statistics than current scan data.

• The ISR measurements also have very different characteristic from the fixed CM energy measurement environment, and many clear systematic advantages, yet each approach also has some weakness.

• Even among ISR measurements, there are very different approaches for different experiments designed to suite the different needs of a and had.

• With all measurements statistically strong, it will all come down to systematics. Measurements from more varieties of environment and methods are essential to establish a convincing picture.

The fun has just begun in a new arena …


Backup Slides


The status of aAug/02


Uncertainty dominated by R measurementerrors in 1-7 GeV region still.


Many different evaluations.

Red solid circles: Data driven – only use perturbative QCD at >10 GeV. Black open circles: Some additional assumptions to allow use of perturbative QCD at intermediate and low energy range.

The recent BES measurementsof R (at ~+7% precision) hadsignificant impact (more so for

pure data driven calculations)

had shift –0.00030 Electroweak fit from sin2w

mH + ~23 GeV

Current (data driven, inc. BES)

had) ~ +0.00036

Equivalent to sin2w +0.00011

Compare to current experimental

measurements: sin2w +0.00017Burkhardt & Pietrzyk PLB B513, 46 (2001)


The companion spectral function measurements

Conserved Vector Current (CVC)

(e+e- +-) v(- -0) Measurement of the decay -0spectral function at LEPand CLEO. Similar comparison also work for 4 modes.

Some isospin symmetry violating effects need to becorrected (difference in radiative corrections, charged vs. neutral masses, mixing, EM decays etc.), amount to -157+28 x10-11 on a.

( See Aug/02 review by DEHS:

Davier, Eidelman, Hoecker, Zhang, hep-ph/0208177 )


BaBar Radiative : Efficiency Obtain the efficiencies directly from the DATA

11

22

BINS in P,,

MUONIDID

MUONTAGTAGBARREL: Z, FORWARD ECAP: X,Y

Example: Radiative DimuonsExample: Radiative DimuonsCan be selected with very high purity ~98%

Other test samples:Other test samples:Pi: events 0 K: B events D0 K


ISR photon resolution test

Use e+e- -> data:CM energy and cluster directions => predict energy E0 and examinemeasured E/E0.

All cluster E/E0 tail dominated by crystaledge effects. Can check using clusters hitting center of crystal.


KLOE analysis details

BKG/TOT e e

M2 (GeV2)

5%BKG/TOT +

1

1.02

1.04

1.06

1.08

0.2 0.4 0.6 0.8 1.0

(s) from F. Jegerlehner

(s)

(corr

ecti

on

tos

))

M2(GeV2)

• without TrackMass cut• with TrackMass cut

(ISR+FSR) – ISR ISR

jan/15/04 su donglow energy r measurements with isr1 su dong stanford linear accelerator center...

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